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Thin film chalcogenide photovoltaic materials

The Thin Film Chalcogenide Photovoltaic Materials symposium 2018 will closely follow the research in the field of chalcogenide materials for photovoltaic applications. The field is in fast progress, especially considering the emerging field of new materials, such as Cu2ZnSn(S,Se)4 in addition to the more mature materials CdTe, and Cu(In,Ga)(S,Se)2. The symposium has a long tradition on attracting the most successful researchers in the world within this field. It has grown over the years and become one of the largest symposia at E-MRS Spring Meetings.


Chalcogenides are highly interesting for their use as light absorber layers in solar cells due to their uniquely high absorbance. These materials include CdTe and the system CuInSe2 – CuGaSe2 – CuInS2 – AgInSe2 -CuAlSe2 and their alloys. The most commonly used alloy of the latter system is Cu(In,Ga)(S,Se)2. An emerging material with a growing research interest is Cu2ZnSn(S,Se)4. Examples of chalcogenide materials used as components in this type of solar cells are CdS and Zn(O,S), which are used as buffer layers.

In the recent past, industrial activities have expanded, especially in the field of CdTe and Cu(In,Ga)Se2 and a production capacity of above 2 GW per annum has been built up for CdTe and above 1 GW per annum for CIGS. All of the aforementioned materials are complex and further fundamental research is needed to improve the electrical and material properties and thus enhance the quality of solar cells and modules. The findings will lead to increased efficiencies of the solar cell devices and thereby improved cost structures of the photovoltaic systems. Presently, record efficiencies of 22.6% for Cu(In,Ga)Se2, 22.1% for CdTe and 13.6% for Cu2ZnSn(S,Se)4 are obtained, which outperform for example multicrystalline silicon, which has 21.3% efficiency, whereas monocrystalline silicon is at challenging 26.6%.

New concepts for further increasing the performance and reducing the costs by, for example, improved solar cell architectures and processing will be discussed together with new and improved characterization methods involving the latest development in microscopy and electrical measurement equipment.

In previous E-MRS conferences the research communities in these highly productive research fields have met to discuss and learn from each other. In addition to oral presentations and poster sessions, discussion sessions with thematic topics have also been included at the end of each day. A young scientist tutorial has been a very popular event among the PhD students in the field. We strongly believe that this successful series will attract the leading researchers in the field also in the next E-MRS Spring Meeting 2018.

The Thin Film Chalcogenide Photovoltaic Materials Symposium at 2018 E-MRS Spring meeting will include a scientific workshop on Cu2ZnSn(S,Se)4 co-organized by the coordinators of the EU Horizon 2020 projects SWInG (Dr. Bart Vermang) and STARCELL (Dr. Edgardo Saucedo).

Hot topics to be covered by the symposium:

  • Processes for film synthesis
  • Thin film growth, theory and experimental aspects
  • Material combinations and heterostructures
  • Chalcogenide PV materials, theory and modeling
  • Novel/ alloyed chalcogenide materials
  • Material characterization methods
  • Electrical characterization methods, device analysis
  • New understanding of defects in chalcogenide-based PV materials
  • Research related to upscaling and manufacturing
  • Diagnostic tools
  • Chalcogenide-based solar cells in tandem devices
  • The role of alkaline elements in chalcogenide-based solar cells
  • Passivation of interfaces and surfaces
  • Novel device concepts
  • Advanced light management concepts

List of confirmed invited speakers:

  • Maria Malitckaya, Aalto Univ.: DFT calculations on role of alkalines in chalcopyrite based solar cells
  • Matthias Maiberg, Univ. Halle: 2D simulations of chalcogenide thin film solar cells
  • Marika Edoff, Uppsala Univ.: Silver diffusion and interface properties in high efficiency (Ag,Cu)(In,Ga)Se2 solar cells
  • Katsumi Kushiya, Showa Shell Sekiyu K.K.: Interface optimization in the commercialized CIS-based thin-film PV modules
  • Susanne Siebentritt, Univ. Luxembourg: Quasi fermi level splitting in CIGS thin-film solar cells
  • Norio Terada, Kagoshima Univ.: Spectroscopic study of electronic structure of CIGSSe-based cells
  • Markus Gloeckler, First-Solar: High efficiency CdTe solar cells and modules
  • Lydia Helena Wong, NTU: High efficiency Cu2ZnSn(S,Se)4 solar cells from non-toxic hybrid inks
  • Thomas Schnabel, ZSW: Band gap tuning of Cu2ZnGeSxSe4-x absorbers for thin-film solar cells
  • Aron Walsh, ICL: Killer defects in kesterite thin-film solar cells
  • Byoung Koun Min, KIST: Solution-processed CIGS for power generating windows

Tentative list of scientific committee members:

  • D. Lincot (CNRS), France
  • C. Ferekides (University of South Florida) USA
  • T. Wada, (Ryokoku Univ.) Japan
  • A.N. Tiwari (Empa), Switzerland
  • R. Noufi (NREL) USA
  • J.F. Guillemoles (CNRS) France
  • H.W. Schock (Helmholtz) Germany
  • D. Cahen (Weizmann Institute of Science) Israel
  • U. Rau (Jülich) Germany
  • R. Scheer (University of Halle) Germany
  • A. Romeo, (University of Verona) Italy
  • M. Edoff, (University of Uppsala) Sweden


Selected papers will be published in a special issue of the journal "Thin Solid Films" (Elsevier).

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08:50 Opening remarks    
Kesterite Growth : Thomas Schnabel, Antonio Cabas-Vidani
Authors : Lydia Helena WONG
Affiliations : Nanyang Technological University, Singapore

Resume : With tunable bandgap and high absorption coefficient, Cu2ZnSn(S,Se)4 or CZTSSe, is an attractive candidate for abundant, non-toxic, high efficiency thin film solar cells. However despite having a Shockley?Queisser limit of around 30%, the world record efficiency cell to date has an efficiency of around 13%. The low efficiency is attributed to the low Voc which may be attributed to many factors, including but not limited to bulk antisite defects, interface defects and band-alignment, and secondary phases. One way to alter the defect characteristics without changing the optimized cation stoichiometry is via cation-substitution. In this approach, cations in CZTS are substituted with other isovalent elements, for example, partial substitution of Cu with Ag or Li; Zn with Cd or Ba; and Sn with Ge have been shown to improve some of the device characteristics in CZTSSe. In this talk, a summary of our group?s activity in cation substitution of CZTSSe thin film (using Ag, Cd, Mn, etc) will be presented. The thin films have been fabricated by solution processed methods using non-toxic precursors followed by sulfurization or selenization. The solar cell device characteristics of the high efficiency device ( > 10%) will be discussed and correlated with the physical, optical and electrical properties of the absorber.

Authors : Devendra Pareek, Teoman Taskesen, David Nowak, Wenjian Chen, Mohamed H. Sayed, Ulf Mikolajczak, Jörg Ohland, Jürgen Parisi, Levent Gütay
Affiliations : Laboratory for Chalcogenide Photovoltaics, Department of Energy and Semiconductor Research, Institute of Physics, University of Oldenburg, D-26129, Oldenburg, Germany

Resume : The often-reported reaction pathway for kesterite Cu2ZnSnSe4 formation involves a multi-molecular reaction, i.e. Cu2Se SnSe ZnSe 1/2Se2 --> Cu2ZnSnSe4. Due to the high volatility of SnSe at usual annealing temperatures (500 ? 550 °C), the reaction pathway and composition of the materials are impacted. In this study, we report a strategy to drive the pathway of kesterite formation towards a bi-molecular reaction, i.e. Cu2SnSe3 ZnSe --> Cu2ZnSnSe4, by suppressing the formation of binary Sn-chalcogenides. In our view, any kesterite formation reaction which starts from metallic precursors consists of two parts, i.e. metal alloying regime (T < TSe-onset) and chalcogenide regime (T > TSe-onset), where the temperature of selenization onset (TSe-onset) can be a direct or indirect process parameter. We have investigated the time and temperature dependent formation of metal alloys and chalcogenide phases in our fabrication procedure, which starts from sputtered precursors and yields to regular cell efficiencies of > 10%. We tracked the phase formation at various temperatures by means of XRD and depth-dependent Raman measurements by interrupting the process at multiple durations. Our results indicate that the formation and preservation of Cu-Sn alloys in combination with the suppressed formation of Cu-Zn phases before selenization onset plays a key role in avoiding the generation of volatile Sn-chalcogenides. This drives the formation pathway towards the bi-molecular route.

Authors : A. Hernández-Martínez1, S. Giraldo1, L. Arqués1, X. Alcobé2, M. Placidi1, V. Izquierdo-Roca1, P. Pistor1,3, A. Pérez-Rodríguez1,4, E. Saucedo1
Affiliations : 1Catalonia Institute for Energy Research (IREC), 08930 Sant Adrià de Besòs (Barcelona), Spain; 2Centres Científics i Tecnològics de la Universitat de Barcelona (CCiTUB), Lluís Solé i Sabarís 1-3, 08028 Barcelona, Spain;3Martin Luther University Halle-Wittenberg, Universitätsplatz 10, 06108 Halle (Saale), Germany;4IN2UB, Departament d?Electrònica, Universitat de Barcelona, Martí i Franquès, 1-11, 08028 Barcelona, Spain.

Resume : Historically, kesterite photovoltaic devices fabricated with precursors that already contain chalcogen, have shown better performances than those that does not contain this early chalcogen in its structure. After an exhaustive literature review, we have observed that there are two main formation pathways for kesterites: one that involves the ternary compound Cu-Sn-X and Zn-X phases (X =S and/or Se) and a second one that involves binary compounds as Cu-X, Zn-X and Sn-X. In this work, we present a unified model about the formation pathways demonstrating the impact of the ?chalcogen availability? on the preferred synthesis route, by implementing a specially designed experiment to demonstrate the strong interrelationship between the reaction pathways and the parameter defined as ?chalcogen availability?. For this purpose, in IREC?s kesterite baseline we performed a detailed study about the dependence of the reaction pathways on the chalcogen vapour pressure. Combining multi-wavelength Raman spectroscopy, XRD, XRF and SEM we found that for low chalcogen vapour pressures (low chalcogen availability), the tri-molecular reaction is preferred against the bi-molecular one, in the middle range both mechanisms can compete and for high vapour chalcogen pressures, the bi-molecular pathway starts to dominate the kesterite formation. Finally, the advantages of the last mechanism in terms of less secondary phases formation and better devices performance will be presented and discussed.

10:00 Coffee Break    
Kesterite Bulk Properties & Composition I : Aron Walsh, Hannes Hempel
Authors : Thomas Schnabel, Ingmar M. Kieferle, Erik Ahlswede
Affiliations : Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg, Meitnerstraße 1, 70563 Stuttgart, Germany

Resume : For its potential future use as an interesting top cell material for multijunction solar cells, a precise control of the band gap is crucial for the wide-gap kesterite material Cu2ZnGeSxSe4-x (CZGSSe). The most straightforward way to achieve this is to control the [S]/([S]+[Se])-ratio of the absorber. CZGSSe absorbers were prepared in a two-step process based on a metal salt solution that is deposited onto a Mo-coated substrate and subsequently annealed in Se-containing nitrogen atmosphere.[1] Without additional incorporation of sulfur, this leads to a [S]/([S]+[Se])-ratio of 0.3 and a band gap of 1.5 eV with efficiencies of up to 6 %. [1] However, for application in multijunction solar cells the band gap is still too low. Therefore, different pathways to further increase the [S]/([S]+[Se])-ratio and thus the band gap were investigated. One approach was adding different amounts of GeS during the annealing process in Se-atmosphere. The additional S incorporation into the kesterite lattice was confirmed by X-ray diffraction and Raman spectroscopy and the band gap could be fine-tuned in a range between 1.5 and 1.7 eV. [2] Although the open-circuit voltage of the fabricated solar cells could be considerably increased, the efficiency strictly decreased due to a reduced fill factor and short-circuit current density. Therefore a second approach for additional sulfur incorporation was pursued. Here an additional annealing process in S atmosphere was conducted. Different annealing temperatures and durations were used to tune the [S]/([S]+[Se])-ratio. Acknowledgement: This project has received funding from the European Union?s Horizon 2020 research and innovation program under grant agreement No 640868. [1] T. Schnabel et al., RSC Advances 7 (2017), 26-30. [2] T. Schnabel et al., Energies 10 (2017), 1813.

Authors : Erika V. C. Robert a ; René Gunder b ; Finn Babbe a ; Conrad Spindler a ; Teoman Taskesen c; Levent Gütay c ; Susan Schorr b ; Phillip J. Dale a
Affiliations : a Physics and Materials Science Research Unit, University of Luxembourg, Belvaux, L-4422, Luxembourg b Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn Meitner Platz 1, D -14109 Berlin c Laboratory for Chalcogenide Photovoltaics, Department of Energy and Semiconductor Research, Institute of Physics, University of Oldenburg, D-26111 Oldenburg

Resume : Cu2(Sn,Ge)S3 is a p-type semiconductor made from abundant materials which is being currently investigated for use in thin film solar cells. By changing the group IV element from pure Sn to pure Ge the band gap increases from 0.93 eV to 1.56 eV. The optimum band gap for a single junction solar cell is between about 1.10 and 1.40 eV, which could be achieved by an alloy containing both Sn and Ge. Here we report on several routes to control the alloy composition and also measure their band gaps. Cu2(Sn,Ge)S3 alloys are formed by annealing sputtered Cu-Sn-Ge containing precursor films in a SnS and GeS atmosphere. We find the precursor stacking order Cu:Sn/Cu:Ge has no influence on the final alloy composition. The final alloy is single phase, with a uniform composition in depth, and a monoclinic crystal structure at annealing temperatures above 500°C. The study highlights the quick intermixing of Sn and Ge, while it is known instead that group III elements In and Ga leave depth gradients in Cu(In,Ga)Se2 films for similar annealing times and temperatures. By repeating the study for several Ge compositions we can extract lattice parameters from Le Bail refinement of X-ray diffraction data and bandgaps from photoluminescence allowing us to find the optimal composition range for single junctions. Finally, we show how a band gap gradient can be engineered by annealing Ge-free precursors in a GeS atmosphere, which is important for future high efficiency devices.

Authors : Antonio Cabas-Vidani1, Stefan G. Haass1, Christian Andres1, Raquel Caballero2, Yaroslav E. Romanyuk1, Ayodhya N. Tiwari1
Affiliations : 1 Laboratory for Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstr. 129, 8600 Duebendorf, Switzerland 2 Departamento de Física Aplicada, Universidad Autónoma de Madrid, C/ Francisco Tomás y Valiente 7, 28049 Madrid, Spain

Resume : Contrary to heavier alkali elements, lithium is expected to alloy with the kesterite phase leading to a solid solution (LixCu1-x)2ZnSn(S,Se)4, hence the bandgap can be tuned by varying the ratio Li/(Li+Cu). Previous experiments report contrasting results on lithium influence in CZTSSe solar cells and only trace amounts of lithium were incorporated [1][2]. Here, we report an experimental series of solution-processed (LixCu1-x)2ZnSn(S,Se)4 with lithium content ranging from Li/(Li+Cu) = 0 to 0.12 in the selenized absorber as detected by ICP-MS. Lithium is added to the precursor solution in the form of LiCl, however, only 20% of the nominal lithium content remains in the selenized absorber because of re-dissolution losses at the spin-coating stage. The proportional increase in lattice parameter a and bandgap from 1.05 to 1.18 eV, confirm the alloying with lithium inside the kesterite phase. Increase in grain size is observed through SEM cross-sections for lithium contents up to Li/(Li+Cu) = 0.07, whereas a higher lithium content leads to a more porous absorber morphology due to inhomogeneous distribution of Li-containing compounds in the kesterite layer. IV parameters are correlated with the absorber morphology evolution and a champion device exhibits an efficiency of 11.5% (active area 12.3%) for Li/(Li+Cu) = 0.06. [1] H. Xin et al., Phys. Chem. Chem. Phys., 17 (2015), 23859. [2] Y. Yang et al., ACS Appl. Mater. Interfaces, 9 (2017), 23878-23883.

Authors : Jose Marquez Prieto1, Galina Gurieva1, Charles Hages1, Sergej Levcenko1, Susan Schorr1,2, and Thomas Unold1
Affiliations : 1 Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany 2 Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstr. 74-100, 12249 Berlin, Germany

Resume : Cu2ZnSn(S,Se)4 (CZTSSe) based solar cells are presently limited by low open-circuit voltage (Voc) values. One strategy to overcome this limitation is through partial incorporation of additional elements such as Ge or Ag. To experimentally assess the potential of such new compounds as a photovoltaic absorber, detailed studies of the crystal structure and the optoelectronic properties are needed. For precise determination of the crystal structure of CZTSSe derived compounds, neutron diffraction is required and large volume powder samples are needed. However, limited optoelectronic characterization of CZTSSe derived powders is found in literature. In this study, we present the characterization of (AgxCu1-x)2ZnSnSe4 powders with variable Ag composition (x = 0.17, 0.26 and 0.46) with quantitative photoluminescence (PL) and diffuse reflectance (DR) hyperspectral imaging techniques. From a combination of X-ray and neutron diffraction it was concluded that as soon as Cu is substituted by Ag, (AgxCu1-x)2ZnSnSe4 powders crystallize in the stannite type structure. A combination of time-resolved PL, DR and quantitative PL imaging has been performed taken into account the potential contribution of secondary phases such as ZnSe and Ag8SnSe6. From the quantitative PL analysis we estimate a quasi-Fermi level splitting value above 0.5 eV in this sample (with an excitation equivalent to 1 sun), being able to produce solar cells with Voc deficits among the lowest for CZTSSe derived materials.

Authors : Sigbjørn Grini, Nils Ross, Clas Persson, Charlotte Platzer-Björkman, Lasse Vines
Affiliations : Department of Physics, Centre for Materials Science and Nanotechnology, University of Oslo, P.O.Box 1048, Blindern, N-0316 Oslo, Norway; Ångström Solar Center, Division of Solid State Electronics, Uppsala University, Uppsala, Uppland, 75120, Sweden

Resume : The conversion efficiency in Cu2ZnSn(S,Se)4 (CZTSSe) solar cells may be improved by varying the [S]/([S] [Se]) ratio with depth, where one approach is to diffuse Se into sulfide Cu2ZnSnS4 (CZTS). However, it has proved difficult to diffuse Se into CZTS grains at temperatures below the nucleation temperature, which can be explained by a low diffusivity in CZTS at typical processing conditions or a barrier preventing Se to be incorporated into the CZTS grains. Therefore, to understand the incorporation of Se in CZTS, its migration up to crystallization temperatures should be investigated. In the present study, the grain boundary (GB) diffusion of Se in CZTS and its interplay with nucleation are revealed by varying the average grain size between samples. Se was diffused into CZTS with heat treatments in Se atmosphere and measured by secondary ion mass spectrometry (SIMS), while a metallic Se cap did not supply sufficient Se to act as a source. Indeed, Se is observed in the back contact at 360 °C (30 minutes) and its accumulation depends on grain size. Moreover, an increase of Se in the CZTS layer is observed, which may be ascribed to bulk diffusion. At 410 °C, Se nucleation fully dominates the process and re-crystallization of CZTSSe is supported by Na and Se diffusion from the GBs, limiting the control over a potential band gap gradient. The GB diffusion and approaches to promote bulk diffusion while limiting nucleation will be further discussed.

Authors : J. Andrade-Arvizu1*, Y. Sánchez1, I. Becerril-Romero1, M. Placidi1, A. Hernández-Martínez1, S. Giraldo1, F. Oliva1, V. Izquierdo-Roca1, A. Pérez-Rodríguez1-2, and E. Saucedo1
Affiliations : 1 Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, Barcelona, Spain; 2 IN2UB, Departament d?Electrònica, Universitat de Barcelona, Martí i Franquès, 1, 08028 Barcelona, Spain.

Resume : Kesterite solar cells were fabricated by a sequential process based on the sulfo-selenization of sputtered Cu/Sn/Cu/Zn metallic precursors. A novel two stage chalcogenization process in a multi-zone tubular furnace was developed to induce a complex graded anionic composition profile. In this way, the precursor was selenized/sulfurized under a (Se/S + Sn) atmosphere in a posterior zone of the furnace, and then submitted to an ulterior superficial chalcogenization stage in which S/SeS2 was situated in a third zone, allowing this for a customized temperature control. Various chalcogenide amounts were employed in order to control the S and Se content of the final CZTSxSe(1-x) films. A multi-wavelength Raman analysis from the front and back surface of the films (lift-off) was performed to investigate the composition, allowing for a bandgap estimation of the surface and back regions. With the optimized SeS2/Se amounts, we achieved a graded composition of the CZTSSe phase with high S content at the top and low content at the bottom. The fabricated devices showed values of VOC > 440 mV and efficiencies of up to 7.3 %. Further characterization (XRD, SEM, EDX, XPS and XRF) confirmed the complex graded bandgap profile. Preliminary results in which the samples are first sulfurized and then superficially selenized show that it?s possible to create an inverted grading profile with higher bandgap towards the bottom of the cell, opening perspectives for ?U? Eg grading in kesterites.

12:15 Lunch    
Kesterite Bulk Properties & Composition II : Lydia Helena Wong, Yaroslav E. Romanyuk
Authors : Aron Walsh
Affiliations : Department of Materials, Imperial College London, UK

Resume : The term ?killer defects? was coined to describe those imperfections that cause rapid electron-hole recombination even at low concentrations [1]. Solar cells based on materials with the kesterite structures - including Cu2ZnSnS4 (CZTS) and Cu2ZnSnS4 (CZTSe) - suffer from a notorious open-circuit voltage deficit, even in champion devices. Identification and mitigation of detrimental killer defects in kesterite materials would result in a step change of photovoltaic performance [2]. The modern theory of defects and doping in semiconductors has become increasingly predictive, with the ability to describe non-equilibrium processes, include finite-temperature effects, and a first-principles description of electron-phonon (non-radiative) interactions [3]. I will discuss our latest findings from the application to kesterite semiconductors. These include analysis of extended defects (stacking faults and grain boundaries)[4], as well as a re-investigation of point defects, in a hunt for the origin of the low-voltage in kesterite solar cells. [1] A. M. Stoneham, ?Non-radiative transitions in semiconductors? Rep. Prog. Phys. 44, 1251 (1981) [2] S. K. Wallace, D. B. Mitzi and A. Walsh, ?The steady rise of kesterite solar cells? ACS Energy Lett. 2, 776 (2017) [3] A. Walsh and A. Zunger ?Instilling defect tolerance in new materials? Nature Mater. 16, 964 (2017) [4] J. Park, S. Kim, A. Walsh ?Opposing effects of stacking faults and antisite domain boundaries on the conduction band edge in kesterite quaternary semiconductors? Phys. Rev. Mater. (2018); arXiv:1801.01044

Authors : K. Nagaya,1 S. Fujimoto,1 H. Tampo,2 S. Kim,2 H. Shibata,2 S. Niki,2 and H. Fujiwara1
Affiliations : 1 Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan. 2 AIST, Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan

Resume : Tail state formation in solar-cell absorber materials is quite detrimental for the performance of photovoltaic devices, and the sharp absorption feature near the band gap region is critical for improved Voc. In this study, we have analyzed the experimental tail absorption of Cu2ZnGeSe4 (CZGSe), Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS) particularly by applying density functional theory (DFT), in an attempt to clarify the tail state formation in the quaternary compounds. Quite interestingly, the absorption-coefficient spectrum of CZGSe obtained experimentally shows remarkable agreement with that obtained from DFT assuming a single kesterite phase, confirming a very sharp absorption edge with a very small Urbach energy of 25 meV. In contrast, coevaporated CZTSe and CZTS layers exhibit very large absorption tails with the Urbach energies of ~80 meV, which cannot be explained by the single-phase kesterite formation. Our detailed DFT analysis reveals that the quite extensive tail-state formation observed in CZTSe and CZTS originates from Cu-Zn cation disordering, which generates trap states near the conduction band edge. Quite remarkably, such cation mixing is suppressed almost completely in CZGSe because of the thermal instability of Cu-Zn substituted phases, as confirmed by DFT calculations. Since CZGSe has an ideal band gap of ~1.4 eV with suppressed tail state formation, we concluded that CZGSe is one of the most favorable material for photovoltaic devices.

Authors : Susan Schorr, Galina Gurieva
Affiliations : Helmholtz-Zentrum Berlin for Materials and Energy

Resume : We have demonstrated [1], that kesterite type CZTSe, CZTS and CZGeSe can self-adapt to Cu-poor and Cu-rich compositions without any structural change except the cation distribution. The ability to accept deviations from stoichiometry is correlated to the formation of intrinsic point defects. Besides Cu/Zn disorder (Cu-Zn anti sites in lattice planes at z=1/4 and ¾) known to occur in stoichiometric compounds [2], is present as well. The latter depends on the composition of the kesterite phase and thermal treatment of the sample [3]. Based on our systematic neutron diffraction studies of point defects in these kesterites we were able to identify existence regions of point defects in the ternary phase diagram, showing correlations between chemical composition of the kesterite phase and specific point defects. This gives not only the possibility of predicting defect types from the chemical composition of the kesterite phase, but also to identify a compositional region where only shallow defects occur. Nevertheless the formation of off-stoichiometric kesterites is most often accompanied by the occurrence of secondary phases, thus there is a discrepancy between the integral layer composition and the composition of the kesterite phase present. [1] Gurieva et al., J. Appl. Phys. 213 (2018) 161519; Gurieva et al. J. Phys. Chem. Sol. 99 (2016) 100 [2] Schorr et al., Eur. J. Mineral. 19 (2007) 65; Sol. En. Mat. Sol. Cells 95 (2011) 1482 [3] Toebbens et al., Phys. Stat. Sol. B 253 (2016) 1890

Authors : Hannes Hempel1, Matthew. C. Beard2, Ingrid Repins2, Rainer Eichberger3, Thomas Unold1
Affiliations : 1 Department Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109, Berlin, Germany 2 National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO 80401-3305, U.S.A. 3 Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

Resume : The photovoltaic efficiency of polycrystalline Cu2ZnSnSe4 is suspected to be limited by band tails. Therefore we study the effect of tail states on charge carrier dynamics in detail and reveal the interdependence of thermalization, transport and recombination over the full lifetime of charge carriers from femtoseconds to microseconds. To this end we combine complementary time resolved techniques including transient absorption, terahertz spectroscopy and photoluminescence as well as their temperature dependence. Using this analysis, we directly observe how charge carrier transport and recombination slow down while charge carriers get trapped and localized into tail states. We reveal simultaneous band thermalisation and partial tail capture on the ultra-fast time-scale followed by the temperature dependent transport and recombination. Above room temperature the later charge carrier dynamics are similar to a tail-free semiconductor and can be described by a single Shockley-Read-Hall life time and a relatively fast transport in extended states. For lower temperatures carriers are frozen into deeper and more localized tail states. Therefore, trapping increases and results in reduced mobilities and apparent long lifetimes. At even lower temperatures charge carrier mobilities and recombination rates become time-dependent similar to the situation in amorphous semiconductors and we model this behavior by multiple trapping of carriers in localized tail states.

Authors : Maurizio Ritzer (1), Sven Schönherr (1), Philipp Schöppe (1), Sergio Giraldo (2), Gerardo Larramona (3), Laure Giuliano (3), Christophe Choné(3), Galina Gurieva (4), Konrad Ritter (1), Gema Martínez-Criado (5), Susan Schorr (4)(6), Gilles Dennler (3), Edgardo Saucedo (2), Carsten Ronning (1), Claudia S. Schnohr (1)
Affiliations : (1) Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany; (2) Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, Spain; (3) IMRA Europe S.A.S., 220 rue Albert Caquot, BP 213, 06904 Sophia Antipolis, France; (4) Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner Platz 1, 14109 Berlin, Germany; (5) Instituto de Ciencia de Materiales de Madrid (CSIC), 28049-Cantoblanco, Madrid, Spain; (6) Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany

Resume : Kesterite Cu2ZnSnSe4 (CZTSe) absorbers often feature compositional fluctuations and secondary phases that are believed to limit the solar cell efficiency. We therefore applied spatially resolved X-ray fluorescence analysis using a synchrotron nanobeam to study local variations of the material composition in kesterite absorbers with an integral composition of 0.75 ≤ Cu/(Zn+Sn) ≤ 0.94 and 1.04 ≤ Zn/Sn ≤ 1.25. Complete glass/Mo/Cu2ZnSnSe4/CdS/ZnO/ITO solar cells were prepared by selenization of a sputtered metallic Cu-Zn-Sn precursor topped with a Ge nanolayer, providing conversion efficiencies of more than 10%. Furthermore, glass/Mo/Cu2ZnSn(S,Se)4 absorbers were grown by annealing and selenization of a spray-coated Cu-Zn-Sn-S colloid precursor, showing efficiencies in the order of 8 to 10% for complete solar cells. A stoichiometric, single-phase Cu2ZnSnSe4 powder sample was taken as reference. Thin cross section lamellas were prepared using a focused ion beam system. The measurements were performed at the ID16B station at the ESRF with a beam energy of 29.6 keV and a spot size of ~50 nm. The sputtered samples show no indication for inhomogeneities, whereas the solution-grown samples exhibit fluctuations of the Se/S ratio, small secondary phase precipitates, and slight compositional variations at some grain boundaries depending on the layer composition. Furthermore, the presence and distribution of germanium remaining in the sputtered samples after selenization is quantified.

Authors : Florian oliva 1, Sergio Griraldo 1, Laia Arques 1, Lorenzo Calvo 2,3, Alejandro Perez-Rodriguez 1,3, Edgardo Saucedo 1, Victor Izquierdo-Roca 1
Affiliations : 1 - Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, Spain 2 - Centres Científics i Tecnològics (CCiTUB) de la Universitat de Barcelona, C/ Lluis Solé i Sabaris 1-3, 08028 Barcelona (SPAIN) 3 - Departament d'Enginyeria Electrònica i Biomèdica, IN2UB, Universitat de Barcelona, C/ Martí i Franqués 1, 08028 Barcelona (SPAIN)

Resume : Main current challenge in thin film kesterite solar cells is the urgent need for the reduction of the large VOC deficit systematically observed in these devices. The true origin of this deficit is still unclear, although among others, it has been associated with the presence of band fluctuations (BF). In this context, UV-Raman spectroscopy (RS) and photoluminescence (PL) have becoming very valuable tools to characterize the presence of different defect types as well as their concentrations, and can be applied for understanding the BF origin. In this work a detailed analysis by RS, PL, and Auger spectroscopy (AES) at the mm/µm-scale of high efficiency (Eff. >9%) Cu2ZnSnSe4 (CZTSe) solar cells prepared by a sequential process is presented. The mm/µm-scales analysis using macro (70µm) and micro (2µm) spots for excitation show spectral variations in the range of 10% at both scales. This indicates the presence of defect/composition inhomogeneities in the absorber that further correlates with the band tails observed by EQE. A comparative analysis between CZTSe final absorber and incomplete synthetized sample (stopped at 400ºC), suggests the inhomogeneities are correlated with the inhomogeneous distribution of the Cu-related phases at the surface in the first annealing steps. Finally, a comparative analysis with reference devices prepared with different compositions/defects suggest that the inhomogeneities observed in the cells can be responsible of about 2% reduction in the Eff.

15:30 Coffee Break    
Poster 1: Kesterite Solar Cells : Bart Vermang, Edgardo Saucedo
Authors : Negar Gheshlaghi (a); Hadi Sedaghat Pisheh (a)
Affiliations : (a) Nanscience and Nanoengineering Programme, Institute of Science and Technology Istanbul Technical University, Maslak Istanbul 34469 TURKEY

Resume : Abstract : Earth abundant copper-zinc-tin-chalcogenide (CZTSSe) thin films as an important class of material have attracted increasing attention for the development of low cost and sustainable thin film solar cells. On the other hand, the possibility of tuning the electrical and optical properties with adjustment of size and composition of quantum dots (QDs) offers great prospects in photoactive applications. In particular, CdSe nanocrystal (NC) covered either with ZnS or CdS results in core/shell system where the band gap of the core lies energetically within the band gap of the shell material. ?n this survey, CZTSSe nanoparticles is coated on Molybdenum coated glasses substrate. Then CdSe/Cd(Zn)S and CdSe/Cd(Zn)S/Zn(Cd)S QDs disperesed in n-Hexane spray coated on CZTSSe film and then the system coated with ZnO.The characteristics for the best performing,CZTSSe solar cell coated with CdSe based core/shell measured in the dark, in order to evaluate influence of CdSe based core/shell QDs on power conversion efficiency of the solar cell.

Authors : Sunghyun Kim; Ji-Sang Park; Aron Walsh
Affiliations : Department of Materials, Imperial College London, UK

Resume : Cu2ZnSn(S,Se)4 (CZTS) has attracted much attention as a replacement of the commercial light absorber of Cu(In,Ga)(S,Se)2, consisting of only earth-abundant elements. However, to realize the high efficiency of CZTS enough to support a Terawatt photovoltaic industry, the large open-circuit voltage (VOC) deficit need to be reduced. Non-radiative carrier recombination is the likely origin of large VOC deficit. We have performed first-principles calculations within the framework of density functional theory to search for killer defects in CZTS. In CZTS, the sulfur vacancy is electrically benign without any donor levels in the band gap. However, we find that the sulphur vacancy can capture electrons very efficiently, and act as killer non-radiative recombination center. We point out that trap-assisted recombination does not necessarily accompany a charge transition level deep in the bandgap of a semiconductor.

Authors : Makbule Terlemezoglu1,2,3; Özge Bayrakl1,2,4; Tahir Çolakolu2; Musa Kurtulu Abak2; Hasan Hüseyin Güllü2,5; Rait Turan1,2; Mehmet Parlak1,2
Affiliations : 1 Department of Physics, Middle East Technical University, 06800 Ankara; Turkey; 2 The Center for Solar Energy Research and Applications (GÜNAM), Middle East Technical University,06800 Ankara, Turkey; 3 Department of Physics, Namk Kemal Ünivesity,59030 Tekirdag, Turkey; 4 Department of Physics, Ahi Evran University, 40100 Krehir, Turkey; 5 Central Laboratory, Middle East Technical University, 06800 Ankara, Turkey

Resume : Cu2ZnSn(S,Se)4 (CZTSSe) is a promising alternative absorber material to achieve high power conversion efficiencies, besides involving low-cost and earth-abundant elements compared to the used ones in conventional solar technology. In this study, we developed a novel fabrication technique utilizing RF sputtering deposition of CZTSSe thin films having a surface decorated with vertically aligned self-assembled nanoflakes without needing any other auxiliary/complementary processes like post-selenization or sulphurisation. TEM studies revealed that the randomly distributed nanoflakes were composed of nanocrystals. The size of nanoflakes is controlled by the material flux and substrate temperature during magnetron sputtering. In addition, the effects of substrate temperature on the formation and size of vertical nanoflakes were also investigated in detail. One of the main advantages arising from the film surface with vertically aligned nanoflakes having about 10µm height is efficient light trapping to decrease the surface reflectance. As a result of the detailed production and characterization studies it was revelaed that we developed a repeatable controlled fabrication sequence to prepare CZTSSe thin films with self-textured surfaces yielding low surface reflectance.

Authors : Jin-Kyu Kang, Dae-Ho Son, Jun-Hyoung Sim, Kee-Jeong Yang, Juran Kim, William Jo, Dae-Hwan Kim
Affiliations : Convergence Research Center for Solar Energy, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu, Korea 42988; Department of Physics & New and Renewable Energy Research Center (NREC), Ewha Womans University, Seoul, Korea 03760

Resume : Abstract : Recently, Cu2ZnSn(S,Se)4 (CZTSSe) compounds are currently attracting a great deal of attention as an alternative for well-developed CIGS absorbers due to the its earth abundant composition and low cost. Among different routes for fabricating these CZTSSe materials, two-stage process, which involves deposited precursors and annealing seems to be the most promising. Both precursor deposition and annealing play important roles for the high efficiency CZTSSe. In this research, solar cells with a metal and a chalcogenide stack structure as a light absorbing precursor layer of CZTSSe were studied. The samples were characterized by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, Kelvin probe force microscopy (KPFM), conductive atomic force microscopy (c-AFM) and solar simulator. The optimum annealing temperature using metal precursor is lower than that using chalcogenide precursor due to the different reaction mechanism. Raman spectroscopy confirms that CTS and Zn alloy make CZTSSe easily when using metal stack structure. The low annealing temperature makes it possible to reduce MoS(e) thickness and secondary phases. KPFM and c-AFM confirm that the downward band bending at the grain-boundaries for the metal precursor and upward band bending for the chalcogenide precursor. The CZTSSe device with metal stack structure precursor exhibited a maximum conversion efficiency of 12.3%, which is almost 20% higher than that with chalcogenide stack structure precursor. Acknowledge: This work was supported by the DGIST R&D Program of the Ministry of Science and ICT (18-BD-05) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20173010012980).

Authors : C. Guillot-Deudon1, M T. Caldes1, P. Bais1, M. Boujtita2 and A. Lafond1
Affiliations : 1-Institut des Materiaux Jean Rouxel (IMN), Universite de Nantes, CNRS, 2 rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 3, France 2- Université de Nantes, CNRS, CEISAM, UMR CNRS 6230, Faculté des Sciences et des Techniques, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France.

Resume : Copper Zinc Tin chalcogenides have been extensively studied as PV absorbers. Both cationic (ZnCu) and anionic disorder could impact the efficiency of the solar cells (1). A careful chemical crystallographic study of Cu2-2aZn1+aSn(SxSe1?x)4 series (2) has shown that the presence of Cu vacancies limits the disorder. Therefore, the correlation between the chemical composition and the optoelectronic properties of these materials was investigated, combining diffuse reflectance, photoelectrochemical and XPS measurements. The nature of the majority charge carriers and band position diagram have been determined for all the compounds. The bandgap value varies from 1.05 to 1.48 eV from CZTSe to CZTS. XPS study shows a significant difference in the surface composition of the stoichiometric compounds and that of the Cu-poor ones. For the latter, the surface composition is very depleted in Cu and enriched in Zn. Furthermore, a more pronounced transition in the diffuse reflectance spectra was observed for Cu-deficient compounds. It therefore seems Cu-poor compounds exhibit a better light absorption. It is the first time that this difference of behavior between stoichiometric and Cu-deficient compounds is described on powdered samples. This feature could probably be one of the reasons for the best performances of the non-stoichiometric CZTSSe-based solar cells. (1) T. Gokmen, et al., Appl. Phys. Lett. 2013, 103, 103506. (2) P. Bais et al., Inorg. Chem. 2017, 56, 11779?11786

Authors : Zong Zhi, Lan Huang, TK Lau, Lishu Liu, Jinlong Gao, and Xudong Xiao
Affiliations : The Chinese University of Hongkong

Resume : In the past few years, CZTS(Cu2ZnSnS4) was regarded as a promising alternative to CIGS(CuInGaSe2) due to the elemental abundance, optimal bandgap, and high absorption coefficient. In the very beginning, the one-step co-evaporation technique was employed to fabricate CZTS due to its agreed success in CIGS. Until 2010 when IBM managed to make decent CZTS solar cells with the efficiency of 6.7% using the sulfurization of sputtered precursors, the two-step method(deposition and sequential sulfurization of precursors) was widely acknowledged as a better technique considering the difficulties in precisely controlling of quaternary composition and volatile secondary phases. However, in some way, the research on CZTS was retarded as the reported efficiency was improved from 6.7% to only 9.2% from 2010 to 2016. It is indicated that the two-step method did solve some problems but also brought some new ones, such as the inadequate capability of controlling the reaction process. Therefore, we restarted the research on the one-step co-evaporation method which has the merits of better film quality and atomic level control of growth process. Our preliminary studies show that an optimal growth profile alleviates the segregation of ZnS secondary phase and then contributes to a better efficiency. Besides, Sn content in CZTS absorber shows a saturation behavior which indicates the excessive Sn content would re-evaporate as volatile phases. This is predictable according to the principle of chemical reaction and decomposition of CZTS at a high temperature. Therefore, even with the same composition, a series of CZTS fabricated under different Sn partial pressures shows totally different appearance and solar performance. Our best efficiency is 4.3% (w/o ARC) which is comparable to the best-reported cells using one-step co-evaporation, and it is achieved under the optimal Sn partial pressure. The one-step method has preliminarily shown its potential on precise control of film growth, and it is also where the front and rear interfaces would benefit from. Further experiments are going to be implemented in the near future.

Authors : Sarra Mahjoubi, Nabila Bitri, Isabelle LY, Fatma Chaabouni
Affiliations : Sarra Mahjoubi: Laboratoire de Photovoltaïque et matériaux semi-conducteur (LPMS), Ecole Nationale d?Ingénieurs de Tunis (ENIT), Université Tunis El Manar, Tunis, Tunisie; Nabila bitri: Laboratoire de Photovoltaïque et matériaux semi-conducteur (LPMS), Ecole Nationale d?Ingénieurs de Tunis (ENIT), Université Tunis El Manar, Tunis, Tunisie; Isabelle LY: Centre de Recherche Paul Pascal, Université Bordeaux 1, Bordeaux, France; Fatma Chaabouni: Laboratoire de Photovoltaïque et matériaux semi-conducteur (LPMS), Ecole Nationale d?Ingénieurs de Tunis (ENIT), Université Tunis El Manar, Tunis, Tunisie.

Resume : Kesterite Cu2ZnSnS4 (CZTS) is one of the most promising candidates as the absorber materials in solar cells with ideal characteristics, such as the low cost and high efficiency due to its direct band gap and to its high absorption coefficient. In this paper, we report the study of Cu2ZnSnS4 CZTS thin films prepared by spray sandwich method for different copper concentration (0.01, 0.02, 0.03 and 0.04M). XRD pattern of the prepared samples has revealed the formation of polycrystalline pure CZTS thin films for 0.04M copper concentration. Moreover, this sample shows no secondary phases and exhibits kesterite structure with preferential orientation along (112) direction. The crystallite sizes of CZTS thin films were found to vary from 30.85nm to 59.84 nm. Morphological of the samples reveals controlled growth. Optical analysis indicates broad absorption in the visible region for all samples and the estimated optical band gap of CZTS thin films were found to vary from 1eV to 1.8 eV. The Nyquist impedance spectra exhibited depressed semicircles with equivalent circuit function as typical parallel RC. The activation energy value calculated from angular frequency relaxation and DC conductivity proved that the conduction mechanism was thermally activated by hopping between localized states. Moreover, the analysis of frequency and temperature dependence of conductivity support the correlated barrier hopping (CBH) model.

Authors : Maarja Grossberg a), Taavi Raadik a), Jüri Krustok a,b), Marit Kauk-Kuusik a), Kristi Timmo a), Reelika Kaupmees a), Valdek Mikli a), Arvo Mere a,b)
Affiliations : a) Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; b) Division of Physics, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : The quaternary Cu2CdGeSe4 (CCGSe) is a potential absorber material for thin film solar cells. This study presents the results of photoluminescence (PL), Raman scattering and X-ray diffraction (XRD) analysis of high-temperature and low-temperature CCGSe monograins with orthorhombic (Pmn21) and tetragonal (I-42m) crystal structure, respectively. Change in the vibrational properties of CCGSe with the change in the crystal structure was studied by Raman spectroscopy in combination with XRD. Photoluminescence study revealed different dominating radiative recombination mechanisms for tetragonal and orthorhombic CCGSe, showing the low-temperature (T=10K) PL bands at 1.06 eV and 1.12 eV, respectively. Detailed analysis of the recombination mechanisms and related defects is presented.

Authors : Juran Kim, William Jo, Jayeong Kim, Seokhyun Yoon; Jun-Hyoung Sim, Kee-Jeong Yang, Dae-Hwan Kim, Jin-Kyu Kang,
Affiliations : Department of Physics, and New and Renewable Energy Research Center (NREC), Ewha Womans University, Seoul, Korea 03760; Convergence Research Center for Solar Energy, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu, Korea 42988

Resume : Flexible Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells were fabricated on Mo-foil substrates, with four-different precursor stack orders. To make up the performance of Cu(In,Ga)Se2 (CIGS), whose power conversion efficiency (PCE) on polymer substrates is already 20.4%, several critical issues ought to be resolved. As known, one of the biggest challenges in fabrication of high-efficient CZTSSe on Na-free substrates is to inject alkali ions into the light absorption layer. Unlike soda lime glass (SLG), Mo-foil needs extrinsic Na supply and the optimal doping condition. In this research, CZTSSe thin-films on Mo-foil substrates were compared and examined depending on the Na-incorporation. Scanning probe microscopy (SPM) was utilized for surface and interface electrical characterization. Moreover, the samples were illuminated by 640, 532, and 405-nm wavelength laser lights during the SPM measurement to inquire the conduction variation under diverse light condition. As a result, we were able to observe the drastically enhanced carrier transportation on the Na-doped CZTSSe thin films. Depending on the precursor stack order, CZTSSe thin films without Na displayed different PCE and thin-film properties. However, the thin films were sharply improved up to the similar extent, once NaF layers were applied. With micro-Raman scattering spectroscopy, the thin film homogeneity was shown to be uniformed, but also the signal of secondary phases on the surface was detected. In this way, we can ascertain the beneficial effects of Na on the CZTSSe thin films. Therefore, more adequate post-deposition process and/or optimal element ratio can be suggested for better performance of CZTSSe thin-film solar cells.

Authors : * Na Kyoung YOUN1, 2, Jihye GWAK1,3, Jae Ho YUN1,3, Young-Joo EO1,3, Kihwan KIM1, Seung Kyu AHN1,3, Joo Hyung PARK1, Se Jin AHN1,3, Jin Su YOO1,3, Jun Sik CHO1,3, Dong Hwan KIM2, and ** Ara CHO1,3
Affiliations : 1 Photovoltaic Research Center, Korea Institute of Energy Research, DaeJeon, 34125, Korea 2Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea 3 University of Science and Technology (UST), Daejeon, South Korea

Resume : CuIn1?xGaxSe2 (CIGS) is the most promising absorber material to fabricate solar cells with high conversion efficiency. Recently the CIGS thin film solar cells showed over 22% efficiency in 2017. However, the production of CIGS is ultimately limited due to the scarcity of In and Ga elements. For this reason, another quaternary compound semiconductor such as Cu2ZnSnSe4 (CZTS) has been studied as an alternative to the CIGS absorber. CZTS is less toxic nature and contains more earth abundant elements comparing to CIGS. Also, CZTS compound has a direct band gap (~1.5 eV) and high absorption coefficient (>104cm-1). Until now, the world-best record of CZTS has been reported 12.6% conversion efficiency so it shows the potential of high efficiency thin film solar cell. However this CZTS absorber of high efficiency was produced with toxic hydrazine. Thus, we tried to fabricate high quality CZTS absorber without such toxic unstable precursors. In this study, CZTS thin film was formed using 2-step RTA process for grain growth. After post annealing process, high quality CZTS thin films were prepared. Various analyses were performed on the post-annealed CZTS film, and the CZTS thin film solar cells based on this absorber layer were also fabricated to investigate photovoltaic performances.

Authors : Cora Preiß (1), Konrad Ritter (1), Stefanie Eckner (1), Philipp Schöppe (1), Thomas Bischoff (2), Silvana Botti (2), Susan Schorr (3,4), Claudia S. Schnohr (1)
Affiliations : (1) Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany; (2) Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany; (3) Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany; (4) Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany

Resume : Substituting Zn by Fe in Cu2ZnSnS4 changes the band gap of the material from about 1.5 eV to about 1.8 eV [T. Shibuya et al., Appl. Phys. Lett. 104, 021912, 2014]. Interestingly, the transition from Cu2ZnSnS4 to Cu2FeSnS4 is accompanied by a change of the crystal structure from kesterite type to stannite type via a complex rearrangement of the cation species [S. Schorr et al., Eur. J. Mineral. 19, 65, 2007]. Furthermore, the S anions in the mixed system are surrounded by different local cation configurations. In order to determine the element-specific bond lengths of the alloy material, Cu2(Zn,Fe)SnS4 powder samples with 0 ? Fe/(Zn+Fe) ? 1 were investigated with extended X-ray absorption fine structure spectroscopy. All bond lengths are nearly independent of the alloy composition, yet they differ substantially for the different elements. While the Cu-S and Fe-S bond lengths are identical, the Zn-S and Sn-S bond lengths are larger by about 0.03 and 0.12 Å, respectively. Based on these experimental results, the S anion position is modelled for different cation configurations and is found to be clearly different in the Zn or the Fe containing environment. This leads to an intrinsic structural inhomogeneity of the alloys on a subnanometer scale. The impact of the S anion displacement on the band gap energy was determined by density functional theory based calculations revealing a strong correlation between local atomic arrangements and electronic properties of Cu2(Zn,Fe)SnS4.

Authors : Nabila BITRI, Sarah DRIDI, Ghada El FIDHA, Sarra MAHJOUBI, Fatma CHAABOUNI, Isabelle LY
Affiliations : * Nabila BITRI, Sarah DRIDI, Ghada El FIDHA, Sarra MAHJOUBI, Fatma CHAABOUNI : Université Tunis El Manar, Ecole Nationale d?Ingénieurs de Tunis (ENIT),Laboratoire de Photovoltaïque et matériaux semi-conducteur (LPMS), 1002 Tunis, Tunisie. * Isabelle LY : Centre de Recherche Paul Pascal, Université Bordeaux 1, Bordeaux, France.

Resume : Quaternary-structured semi-conductors Cu2XSnS4 (X = Fe, Ni and Zn) are important materials due to indium free content and consisting of earth abundant elements, which can significantly decrease the cost of devices. Here, we have successfully produced Cu2FeSnS4, Cu2NiSnS4 and Cu2ZnSnS4 thin films by a simple and an easy technique « Spray ». The resulting Cu2XSnS4 thin films were characterized by XRD, Raman, SEM, UVeVis and impedance spectroscopy to investigate the crystal structure, morphology, band gap, resistivity, and conductivity. This work demonstrates that Cu2XSnS4 thin films have p-type conductivity and are promising absorber materials for energy-conversion applications.

Authors : Nishant Saini, Jes Larsen, Nils Ross, Charlotte Platzer-Björkman
Affiliations : Ångström Solar Center, Division of Solid State Electronics, Uppsala University

Resume : The performance of Cu2ZnSnS4 (CZTS) solar cells lags behind Cu(InxGa1-x)Se2 solar cells because of low open circuit voltage (Voc) and fill factor, due to absorber-buffer band mismatch, in the case of sulfide kesterite, and intrinsic defects in the bulk. Germanium incorporation in CZTS has been reported to improve the Voc deficit, increase minority carrier lifetime and alter the grain growth. In this work, we prepare Cu2ZnGexSn1-x(S,Se)4 (CZGTSSe) films and devices with a two-stage process, which includes compound sputtering and S/Se annealing in an Argon atmosphere. This is different to the more common route of annealing of metal precursors. One of the better preforming CZGTSSe devices with an efficiency of 9.7%, Voc = 0.529V, Jsc = 28.9mA/cm2 and FF = 63.4%, was fabricated during our initial attempts. This is comparable to the best efficiencies made in our group with more optimized processing of Ge-free CZTSSe. In this work, we report routes for Ge incorporation in CZTSSe based on compound co-sputtering and limitations set by sputtering power range of different targets. We compare the effect from small Ge addition to the process with larger substitution of Sn in the sulfide CZTS. Materials characterization includes Raman scattering, x-ray diffraction and scanning electron microscopy. Compositional measurements include x-ray fluorescence calibrated with Rutherford backscattering.

Authors : Stephen Campbell*, Yongtao Qu*, Jonathan D Major**, Vincent Barrioz*, Neil S Beattie*, Guillaume Zoppi*
Affiliations : * Northumbria University UK, ** University of Liverpool UK

Resume : Kesterite solar cells based on chalcogenide Cu2ZnSn(S,Se)4 (CZTSSe) are a viable approach to thin film photovoltaics, utilising Earth-abundant, non-toxic elements. CZTSSe films produced from nanoparticle inks offer a low cost solution-based method of fabrication. However, improving efficiency in these devices has proved challenging, in part due to the presence of detrimental complex defects within the bulk of the CZTSSe absorber. In this study, the behaviour of nanoparticle-based CZTSSe absorbers and solar cells made from low and high quality grade chemicals is studied. Photoluminescence (PL) spectroscopy revealed the presence of a shallow acceptor defect state and similar shallow acceptor plus shallow donor states in the low and high purity precursor absorbers, respectively. In addition, the low purity precursor absorber had a higher total defect density resulting in a 10 meV increase in electrostatic potential fluctuations. Deep level transient spectroscopy (DLTS) indicated the presence of detrimental deep defect states in both types of CZTSSe device. Notwithstanding the high purity precursor absorber with lower defect density, the power conversion efficiencies of both types of CZTSSe solar cells were similar (~5%), implying an issue other than defects in the absorber bulk inhibits device performance as evidenced by biased quantum efficiency analysis and temperature dependent current voltage measurements.

Authors : Christian Andres, Antonio Cabas-Vidani, Yaroslav E. Romanyuk, Ayodhya N. Tiwari
Affiliations : Laboratory for Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstr. 129, 8600 Duebendorf, Switzerland

Resume : As an option of tuning the bandgap of kesterite thin films, Ge can be used to replace Sn on the cation site thus enabling tuning the bandgap from 1.0 eV for Cu2ZnSnSe4 to 1.5 eV for Cu2ZnGeSe4. First, we use co-sputtering of Cu, Zn, Sn and Ge targets to obtain a mixed metal precursor, which is converted into a homogenous 5x5 cm2 absorber by rapid thermal annealing with a Selenium capping layer. In contrast to the previously reported stacked metal precursor [1], no preferential accumulation of Ge is observed at the back regardless of Ge content. Using the same processing conditions, the bandgap could be varied between 1.0 and 1.35 eV without substantial deterioration of the solar cell efficiency and thereby offering a possibility for design of solar cells with bandgap graded absorbers. In a second approach, metal gradients are induced in the precursor layer by changing sputtering rates of Ge and Sn in order to obtain a Ge back grading in the absorber. SIMS depth profiling confirmed the persistence of the Ge gradient in the selenized absorber. A combination of XRD, SEM, and XRF is used to characterize changes in crystal structure, morphology, and the formation of secondary phases upon grading at precursor stage. J-V and EQE measurements of the graded Cu2ZnSn1-xGexSe4 solar cells will be presented in order to reveal if the back grading improves photovoltaic parameters as compared to the non-graded reference cells. [1] Márquez, J. et al., Chem. Mater. 29, 9399?9406 (2017).

Authors : Ignacio Becerril-Romero[1], Jacob Andrade-Arvizu[1], Markus Neuschitzer[1], Yudania Sánchez[1], Paul Pistor[2], Edgardo Saucedo[1], Marcel Placidi[1]
Affiliations : [1] IREC ? Catalonia Institute for Energy Research, Sant Adrià de Besòs, Spain; [2] Martin?Luther?University Halle?Wittenberg,Halle (Saale), Germany

Resume : Much of the success of Mo as back contact (BC) for CIGS and Kesterite-based solar cells lies in the formation of a MoSe2 layer at the absorber/Mo interface during the synthesis of the absorber, which provides an ohmic contact mainly by improving band alignment. Despite the important role of this layer, little work has been done on researching alternative materials that could offer more suitable characteristics to fulfil this role. This work explores the use of thin (25-150 nm) W sacrificial layers deposited on a Mo bilayer as an innovative BC for CZTSe solar cells fabricated by a sequential process (sputtering of a metallic precursor followed by a reactive annealing). The term sacrificial is used to describe that W is completely selenized. W is found to greatly improve the absorber/BC interface eliminating the typical voids associated to the Mo-CZTSe detrimental reaction. In addition, EQE measurements show an electron reflector behaviour through an improved collection towards the back contact that increases with the increasing thickness of the W layer. This translates into a boosted Jsc of the devices rising from 28.9 (Mo reference) up to 36.1 mA/cm2 using a 125 nm W layer. On the other hand, the Voc of the cells shows a slight degradation with the increasing W thickness. Nevertheless, the highest efficiency (~8%) was achieved using a 50 nm W layer. Further work is being carried out to determine the potential of this promising W-Mo back contact.

Authors : Wenjian Chen, Teoman Taskesen, Ulf Mikolajczak, Devendra Pareek, Jörg Ohland, Jürgen Parisi, Levent Gütay
Affiliations : Laboratory for Chalcogenide-Photovoltaics (LCP), Energy and Semiconductor Research Laboratory (EHF), Institute of Physics, Carl von Ossietzky University of Oldenburg, Germany

Resume : To date molybdenum (Mo) is the most commonly used back contact material for CZTSe based solar cells. During the selenization of the absorber an interfacial MoSe2 layer is usually formed for most fabrication procedures with thicknesses ranging from a few tens of nm up to >1 µm. In order to improve the back contact interface of the solar cells, the control of MoSe2 formation provides a great potential. In this study, we show the influence of plasma pre-treatments on the as-grown Mo surface prior to our CZTSe fabrication procedures which consist of the deposition of stacked metallic layers and the annealing in Se atmosphere. The pre-treatments include the application of pure Ar and mixed gas Ar-N2 plasma. We observe a significant impact of the process parameters on the obtained MoSe2 thickness and CZTSe absorber quality. These results are compared with those from depositing back contact barriers such as TiN and SiOxNy onto the Mo. We discuss the obtained results in terms of MoSe2 formation and absorber quality, thus showing the consequences on fabricated kesterite solar cells.

Authors : Xinya Xu, Stephen Campbell, Yongtao Qu, Vincent Barrioz, Guillaume Zoppi, and Neil S. Beattie*
Affiliations : Department of Mathematics, Physics and Electrical Engineering, Ellison Building, Northumbria University, Newcastle upon Tyne NE1 8ST, UK

Resume : Flexible substrates have the potential to create lightweight solar cells that offer a wide range of applications including automotive and architectural. In this work, Earth abundant Cu2ZnSnS4 nanoparticle inks were deposited on molybdenum foil substrates and subsequently converted to high quality thin film Cu2ZnSn(S,Se)4 photovoltaic absorbers. Integration of these absorbers within a thin film solar cell device structure yields a solar energy conversion efficiency of 3.8%, which is comparable to identical devices processed on rigid glass substrates. Importantly, this is only achieved when a thin layer of molybdenum is first sputtered directly onto the foil. The layer limits the formation of a thick Mo(S,Se)x layer resulting in a substantially reduced series resistance. In addition to the device performance, results of experiments on the morphology of the photovoltaic absorber are also presented. Relative to rigid devices on soda lime glass, the average Cu2ZnSn(S,Se)4 grain size is smaller for the flexible devices and this is attributed to the absence of Na in the Mo foil. Strategies to develop larger grains are presented including the introduction of dopants. As well as flexible Mo foil substrates, Cu2ZnSnS4 nanoparticle inks were also deposited on polyimide and the results of thermal processing to convert these films to Cu2ZnSn(S,Se)4 photovoltaic absorbers are presented.

Authors : M.A. Sulimov 1,2, M.V. Yakushev 1,2,3,4, J. Márquez-Prieto 5, I. Forbes 5, P.R. Edwards 3, V.D. Zhivulko 6, O.A. Borodavchenko 6, A.V. Mudryi 6, J. Krustok 7, R.W. Martin 3
Affiliations : 1 Institute of Metal Physics UB RAS, 620108, S. Kovalevskaya Street 18, Ekaterinburg, Russia; 2 Ural Federal University, Mira 19, 620002 Ekaterinburg, Russia; 3 Department of Physics, SUPA, University of Strathclyde, Rottenrow 107, G4 0NG Glasgow, UK; 4 Institute of Solid State Chemistry of Ural Branch of the RAS, Pervomaiskaya 91, 620990 Ekaterinburg, Russia; 5 Northumbria Photovoltaic Application Group, Faculty of Engineering and Environment, Northumbria University, Ellison Place, Newcastle upon Tyne NE1 8ST, UK; 6 Scientific-Practical Material Research Centre of the National Academy of Science of Belarus, P.Brovki 19, 220072 Minsk, Belarus; 7 Tallinn University of Technology, Departments of Physics and Materials Science, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : Cu2ZnSnSe4 films on Mo/glass, produced by selenising metallic precursors at 450°C (1), 500°C (2) and 550°C (3), were used to make solar cells with efficiencies ? of 3.2% (1) and 8.1% (2), whereas Mo was completely selenised in (3). The bandgap Eg, measured at 4.2 K by photoluminescence excitation, was of 1.06 eV (1), 1.02 eV (2) and 1.02 eV (3). Analysis of the photoluminescence (PL) spectra suggest that the origin of the dominant band at 0.95 eV, 0.96 eV and 0.9 eV for these cells, respectively, is likely to be free-to-bound (FB) recombination mechanism influenced by band-tails due to red spectral shifts of the band at rising temperature and strong shifts of 12 meV, 14 meV and 16 meV per decade of excitation intensity change. The PL spectra also contain a broad band at 1.3 eV (cut off by the detector sensitivity limit) assigned to defects in CdS. The intensity of FB at 6 K at first increases with selenisation temperature (ST) rise to 500°C by a factor of two, suggesting a reduction in the concentration of deep non-radiative traps, followed by the sharp rise of ? despite a small decrease in Eg. However the average depth of potential fluctuations (? = 24 meV) remains unchanged. Further increase of ST reduces the FB intensity by a factor of 4, red shifts it increasing the width and compensation level. These signs of deteriorating electronic properties are followed by a decrease of ? to 21 meV suggesting that ? is not directly related to ?.

Authors : Elif PEKSU, Hakan KARAA?AÇ
Affiliations : Istanbul Technical University, Department of Physics, Istanbul, Turkey

Resume : In this study, Cu2ZnSnS4 (CZTS) thin films, employed as an absorber layer for photovoltaic applications, were successfully deposited onto both soda-lime glass and Mo-foil substrates. Structural, electrical, optical and morphological properties of the deposited films obtained by these techniques were discussed and compared with each other in detail. The elemental composition of the as-grown CZTS films (without resorting to the sulfurization treatment) prepared by sol-gel method was found to be nearly stoichiometric. In other words, Cu/Zn+Sn and Zn/Sn ratios were calculated to be 1.09 and 1.06, respectively. However, it was not the case for the films prepared by thermal evaporation method, respect ratios of which were found to be around 0.5 and 2.36. Structural analysis has revealed that the as-grown CZTS films deposited by these two technique have an amorphous matrix and it transform into a polycrystalline form with a kesterite phase having (112), (200), (220), (312) (JCPDS 26-0575) reflection peaks following the post-annealing process. A significant improvement in intensity of the resolved peaks was also observed following the increase in annealing temperature. From the recorded SEM and AFM measurements, it was deduced that the thermally evaporated films possess comparatively smoother surfaces. That is, while the surface roughness of the as grown films prepared sol-gel method was around 27.403nm, it was only 1.619 nm for that prepared by thermal evaporation method. To determine the absorption coefficient and optic band gaps of the deposited CZTS thin films the transmittance and reflectance measurements were performed in the wavelength range of 350-1000 nm. The band gaps of the films were found to be ranging from 1,44 to 1,56 eV depending on the applied post-annealing temperature.

Authors : H. Oueslati1, M. Ben Rabeh*,1 and M. Kanzari2
Affiliations : 1Université de Tunis El Manar, Ecole Nationale d?Ingénieurs de Tunis, Laboratoire de Photovoltaïques et Matériaux Semi-conducteurs, BP 37 Le Bélvédère 1002, Tunis, Tunisie; 2Université de Tunis, Institut préparatoire aux études d?ingénieurs de Tunis, Laboratoire de Photovoltaïques et Matériaux Semi-conducteurs, BP 37 Le Bélvédère 1002, Tunis, Tunisie.

Resume : Multicomponent chalcogenide Cu2ZnxFe1-xSnS4 (CZFTS) powders with varying x to 0 from 1 were synthesized by direct melting of the constituent elements taken in stoichiometry compositions. The polycrystalline Cu2ZnxFe1-xSnS4 thin films were fabricated by thermal evaporation under vacuum method on no-heated glass substrates and then annealed under nitrogen flux in sulfur atmosphere. In this study, the structural and morphological properties of CZFTS multi ternary compounds were investigated. X-ray diffraction patterns and Raman spectra of CZFTS powders indicated that only CZFTS phase was present with a stoichiometric composition and the transformation of the stannite to kesterite phase is occurred by decreasing the Fe content. The structural properties of post-sulfurized CZFTS thin films was also studied and some microstructural parameters were estimated. In addition the surface morphology of the as-deposited and post-annealed thin films was analyzed by atomic force microscopy AFM. Moreover, CZFTS ingots and thin films exhibit an obvious p type semiconductor. Keywords: Cu2ZnxFe1-xSnS4; Thin films; Thermal evaporation; Sulfurization process; Structural properties.

Authors : Deepak Kumar Kaushik and A. Subrahmanyam
Affiliations : Semiconductor Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai, India, 600036.

Resume : Copper zinc tin sulphide (Cu2ZnSnS4), quaternary chalcogenide semiconductor, is a potential absorber material among its contemporary contenders for high efficiency and low-cost thin film solar cells. The main challenge of CZTS is associated with the Cu-Zn cation disordering due to its similar ionic radii and consequently, leads to the large open circuit voltage deficit (Eg ? VOC) where Eg and VOC are optical band gap and open circuit voltage respectively. Among the several techniques to describe the Cu-Zn cation disorder, temperature-dependent electrical transport measurement is a viable approach to describe the conduction mechanism in the vicinity of Fermi level and the disorder in the material. In the present work, the role of Cu-Zn cation disordering is characterized by temperature dependent (25-300 K) electrical transport phenomena in reactive DC magnetron sputtered CZTS thin films. It shows that impurity band lies in the range of 2 meV to 19 meV away from the valence band edge which corresponds to the copper vacancy (VCu). The broad impurity band (due to a high degree of disorder) is merged into the valence band which reflects the degenerate behaviour of the CZTS thin films even though the hole density is less than the critical value < 10^18 cm^-3. The merging of broadened impurity band is one of the reasons for the large open circuit voltage deficit and consequently, limits the photovoltaic efficiency of CZTS based solar cells. Some possible solutions have been proposed in the article to reduce the open circuit voltage deficit.

Authors : Sara Niedenzu, Galina Gurieva, Susan Schorr
Affiliations : Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany, Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstr. 74-100, 12249 Berlin, Germany; Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany; Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany, Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstr. 74-100, 12249 Berlin, Germany

Resume : The quaternary chalcogenide semiconductors Cu2-BII-CIV-X4 (BII-Zn; CIV-Ge, Si, X-S, Se) contain only earth abundant, non-toxic elements and have drawn wide attention for their potential applications in many fields. For example Cu2ZnGe(S1-xSex)4 are interesting candidates for top cells in multi-junction solar cells due to its band gap of 1.4 eV to 1.7 eV [1]. However instead of tuning the band gap by different anion ratios, also cationic substitution like Ge ? Si leads to a wider band gap. Thus Cu2ZnSiSe4, which have a band gap of 3.0 eV and 3.2 eV [2], are proven to be promising nonlinear optical materials for use in the infrared region [3]. In this work polycrystalline bulk samples of Cu2-BII-CIV-X4 (BII-Zn; CIV-Ge, Si, X-S, Se) have been synthesized primarily by solid state reaction from pure elements. The chemical homogeneity and composition of the synthesized polycrystalline powders were proven by WDX. The phase content and basic structural characterization were retrieved from the X-ray powder diffraction data by Rietveld refinement [4]. The results of the chemical composition in combination with structural characterization of Cu2-BII-CIV-X4 will be presented. Furthermore the type and the percentage of potential secondary phases will be pointed out. [1] T. Schnabel et al., Energies (2017) 10, 1813 [2] Rosmus et al., Z. Anorg. Allg. Chem. (2012), 2578?2584 [3] Rosmus et al., Inorg. Chem. (2014), 53, 7809?7811 [4] J. Rodriguez-Carvajal and T. Roisnel,

Authors : M. Kauk-Kuusik, X. Li, M. Pilvet, K. Timmo, M. Grossberg, T. Raadik, M. Danilson, V. Mikli, J. Krustok, J. Raudoja, A. Mere
Affiliations : Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : New quaternary compound Cu2CdGeSe4 was synthesized by molten salt method for photovoltaic applications. The effect of salt material (CdI2 and KI), synthesis temperature and time on the structural, morphological, compositional and optoelectronic properties was investigated. Phase analysis by Raman spectroscopy and X-ray diffraction showed that Cu2CdGeSe4 powder synthesized at 500 oC had a tetragonal structure and powders synthesized at temperatures 600 oC and 700 oC had orthorhombic structure. The band gap obtained from EQE measurements was found to be 1.27 eV for orthorhombic structured powders and 1.14 eV for tetragonal structured powders. The best solar cell fabricated from the Cu2CdGeSe4 powder synthesized in KI at 700 oC showed the conversation efficiency of 4.21% (active area), with an open-circuit voltage of 0.46 V, a short-circuit current density of 23.3 mA/cm2 and fill factor of 39%.

Authors : Souhaib Oueslati1 and 2, Kaia Ernits 2, Dieter Meissner1 and 2, Maarja Grossberg1
Affiliations : 1Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; 2crystalsol OÜ, Akadeemia tee 15a, 12618 Tallinn, Estonia

Resume : The open circuit voltage (Voc) of Cu2ZnSn(S,Se)4 Kesterite solar cells is lower than in the closely related Cu(InGa)Se2 solar cells. Several studies show that partial substitution of tin (Sn) by germanium (Ge) can reduce the Voc deficit significantly and increase the minority carrier lifetime, demonstrating 9.9% efficiency with Cu2ZnGexSn1-x(S,Se)4 CZGTSSe thin film [1]. Our contribution provides a first investigation on the influence of Ge incorporated in CZTSSe monograins synthesized in molten flux. The compositional characterization by using Energy Dispersive X-ray spectroscopy and Raman spectroscopy as well as time resolved PL measurements confirmed homogeneous and high quality single crystals. The effect of Ge on the electronic transitions was revealed by PL measurements at room and low temperatures (T =10K). Solar cell devices were completed in a standard process using a CdS buffer layer, i-ZnO/ZnO:Al front contact and applying a graphite back contact. Initial best preforming CZGTSSe monograins solar cell has an efficiency of 5.8%, Voc= 648mV, Jsc= 13.9mA/cm2 and FF= 64.5%. More results of ongoing optimization of the processes will be presented. [1]: S.Giraldo et al, Energy Environ. Sci., 2017, 10.1039/C7EE02318A.

Authors : M. Gansukh1, S. Engberg1, R. Küngas2, S. Dahl2, S. López-Marino3, J. B. Pedersen1, O. Hansen3, N. Pryds4, S. Canulescu1 and J. Schou1
Affiliations : 1 DTU Fotonik, Technical University of Denmark, DK-4000 Roskilde, Denmark 2Haldor Topsoe A/S, Haldor Topsøes Allé 1 DK-2800 Kgs. Lyngby, Denmark 3DTU Nanotech, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark 4DTU Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark

Resume : Pulsed Laser Deposition (PLD) is a method that can produce highly reproducible thin films with controlled composition and thickness. Previous studies on CZTS solar cells were mostly based on sulfide precursors. In this study, we compare precursors obtained with multicomponent oxide- and sulfide-based targets for Cu2ZnSnS4 (CZTS) solar cells using different PLD conditions. In addition, CZTS absorbers are made by annealing in a sulfur atmosphere. Sn losses during annealing are well known for increasing the occurrence of highly detrimental defects in solar cells. Oxide precursors can minimize these Sn losses during the annealing when compared to their sulfide counterparts. By using oxide precursors, it is possible to achieve high quality CZTS absorbers and explore new alternative processing routes for solution based methods that require pre-air annealing for burning the residual carbon. Preliminary results from EDX analyses on the annealed CZTS absorbers showed that the oxygen amount in oxide and sulfide precursors is comparable. Raman, photoluminescence and XRD analyses showed that high quality films were formed from oxide precursors after the annealing. Precursors with Cu/(Zn+Sn) metallic ratios varying from 0.7 to 1.0 have been made by changing the laser fluence on both oxide and sulfide targets. The best films and solar cells produced from both oxide and sulfur precursors routes will be compared in terms of structural, optical, electrical and morphological properties.

Authors : Valdek Mikli, Sergei Bereznev, Jaan Raudoja, Maris Pilvet, Maarja Grossberg, Marit Kauk-Kuusik
Affiliations : Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : The formation of Cu2ZnSnS4 (CZTS) thin films prepared by pulsed laser deposition (PLD) from single phase monograin target is investigated. The study is focused on the modification of the structural, compositional, and electrical properties of the films by varying deposition and post-growth annealing conditions, namely temperature and annealing atmosphere. The compositional analysis by Raman and Energy Dispersive X-ray Spectroscopy (EDX) showed Cu-rich multiphase composition of the as-deposited films. After KCN etching, single phase CZTS thin films were obtained. The CZTS films were then heat-treated in the selenium atmosphere resulting in 20% of sulfur replacement by selenium. The electrical properties of the CZTS films were analyzed by using Kelvin Probe measurements. The detailed discussion of the results is presented.

Authors : Özge Bayrakl?, Makbule Terlemezoglu, Hasan Hüseyin Güllü,Gökhan Sürücü, Mehmet Parlak
Affiliations : Department of Physics, Middle East Technical University,Ankara 06800, Turkey- Center for Solar Energy Research and Applications (GÜNAM), METU, 06800 Ankara, Turkey- Department of Physics, Ahi Evran University,40200, K?r?ehir, Turkey; Department of Physics, Middle East Technical University,Ankara 06800, Turkey- Center for Solar Energy Research and Applications (GÜNAM), METU, 06800 Ankara- Turkey-Department of Physics Namik Kemal University, 59030, Tekirdag, Turkey; Center for Solar Energy Research and Applications (GÜNAM), METU, 06800 Ankara; Department of Electric and Energy, Ahi Evran University,40200, K?r?ehir, Turkey- Department of Physics, Middle East Technical University,Ankara 06800, Turkey; Department of Physics, Middle East Technical University,Ankara 06800, Turkey- Center for Solar Energy Research and Applications (GÜNAM), METU, 06800 Ankara

Resume : Quaternary kesterite compounds (Cu2ZnSnS4(CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnSn(S,Se)4 (CZTSSe) )become the promising candidates for chalcopyrite absorbers since they include non-toxic, more abundant elements and low cost materials. Moreover, these materials have high absorption coefficient in the order of 104cm-1, direct band gap in the range from 1.0 to 1.5eV and p-type conductivity. In this work, CZTSe polycrystalline thin films were analyzed under the aim of photovoltaic applications. Firstly, CZTSe thin films were fabricated in stacked layer onto soda lime glass substrates using pure elemental sources in thermal evaporation system. Then, the deposition was improved with in-situ selenization process. Deposited films were analyzed in terms of structural, optical and electrical properties. The structural and the morphological evolution were analysed by means of X-ray diffraction (XRD), Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM). Moreover, transmission measurements were performed to determine the optical characteristics of these samples by using Kubelka Munk Theory. To complete the material characterization processes, the electrical and photoelectrical properties of the films have been investigated by carrying out temperature dependent conductivity, photoconductivity under different illumination intensities in the temperature range of 100?400 K. It is determined due to the characterization results that in-situ selenization process have a certain effect on the fabrication of CZTSe thin films with the intended properties as an absorber layer.

Authors : Teoman Taskesen, Devendra Pareek, David Nowak, Janet Neerken, Johannes Schoneberg, Wenjian Chen, Vincent Steininger, Levent Gütay
Affiliations : Laboratory for Chalcogenide Photovoltaics, Energy and Semiconductor Research Laboratory (EHF), Institute of Physics, Carl von Ossietzky University of Oldenburg, Germany

Resume : Cu2ZnSnSSe4 (CZTSSe) has emerged as a promising absorber material for PV applications due to its high absorption coefficient, tunable band gap and low cost constituents. However, the record efficiency of CZTSSe based solar cells is still pending below 13%, mainly due to high open circuit voltage deficit (Vocdef). In this study, we discuss our recent results on Cu2ZnSnSe4 (CZTSe) based solar cells, which have shown efficiencies of ? 11%. The samples are prepared by sputtering of metallic precursors and subsequent annealing under a selenium atmosphere. Two outstanding behaviours for kesterite has been observed: i) the photon energy of the photoluminescence (PL) emission and ii) the activation energy extrapolated from Voc(T) are both matching the band gap region of the absorber, underlining the high quality of absorber and device. The impact of various processing strategies on the formation of the absorber and quality of the solar cells was investigated in this work by scanning electron microscope, energy-dispersive X-ray spectroscopy, Raman spectroscopy, PL, current-voltage, and external quantum efficiency measurements. Moreover, the long-term stability of a CZTSe solar cell was studied and the results reveal a high stability of the solar cell as the device efficiency remained almost unchanged after the air-annealing at 85 °C for 200 hours. The best device in this work reveals a Vocdef below 340 mV (relative to Shockley-Queisser limit).

Authors : Lewis D. Wright1, Jamie Lowe12, Mustafa Togay1, Fabiana Lisco1, Sona Ulcina1, Andrei Malkov2, Jake Bowers1
Affiliations : 1 Centre for Renewable Energy Systems Technology, Wolfson School of Mechanical, Electrical, andManufacturing Engineering, Loughborough University, Loughborough, Leicestershire, LE113TU 2 Department of Chemistry, School of Science, Loughborough University, Loughborough, Leicestershire, LE113TU

Resume : The kesterite Cu2ZnSnS(e)4 (CZTS(e)) is a strong contender for TW-scale photovoltaic deployment with its tunable direct band gap, high absorption coefficient (>10^4 cm^-1) and Earth-abundant material components. Fortunate also is the apparent preference for non-vacuum/solution processing as shown by its spin-coated 12.6% champion device. The device in question uses the solvent hydrazine, both incredibly reactive and unstable. As a processing tool, both for researchers and potential industrial upscale, hydrazine is unsafe. The Brutchey group introduced an amine-thiol mixture for use as a solvent; it has been shown that combining an amine (a molecule with functional group R-NH2) with a thiol (R-SH) produces a solvent capable of complexing a range of elements, oxides, sulfides, selenides, and tellurides which are recoverable when heated. Whilst significantly less dangerous than hydrazine (aka rocket fuel), combinations such as ethylenediamine/ethanedithiol bring their own set of risk phrases making them hazards in their own right. Here we present work that utilises the amine/thiol route using water as the main medium. Combining the commonly used thiourea with cysteamine we produce a molecular solution from metal oxides, thus avoiding the use of binary halides, which we deposit by ultrasonic spray deposition. The best efficiency to date using this solution is 4.1%. This process lays the groundwork for solution processing without the need for excessively dangerous solvents and further strengthens the case for CZTSSe as a TW-scale absorber material.

Authors : M. Espindola-Rodriguez1,2, D. Sylla2, F. Oliva2, Y. Sánchez2, V.Izquierdo-Roca2, A. Pérez-Rodríguez2,3, J. Schou1, E. Saucedo2 and M. Placidi2.
Affiliations : 1. Technical University of Denmark, Fotonik, DK-4000 Roskilde, Denmark. 2. Catalonia Institute for Energy Research, IREC, Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs (Barcelona), Spain. 3. IN2UB, Departament d?Electrònica, Universitat de Barcelona, C. Martí i Franquès 1, 08028 Barcelona (Barcelona), Spain.

Resume : The scalable, low?cost and environment?friendly electrodeposition technique was used in this work to sequentially electrodeposit the metallic stacks precursors: Cu?Zn?Sn (CZT) for CZTSe?based solar cells, followed by a thermal annealing under Se atmosphere in order to form the Cu2ZnSnSe4 kesterite absorber material onto semitransparent substrates. We report on the process optimization of the sequential elemental-layer deposition by controlling the composition of the precursors adjusting the thickness of each metallic layer (Cu/Sn/Zn), i.e. by controlling precisely the charge (Qi) applied to each individual metal component onto semitransparent FTO (Sn2O:F)/Mo-nano (30nm) substrates for advanced applications such as bifacial solar cells concepts among others. The compositional optimization studies conducted in this work by adjusting QCu allowed us to obtain solar cells with bifacial (front) power conversion efficiencies of up to 4.2% (4.0%) with 325 mV (328 mV), 29 mA/cm2 (27 mA/cm2) and 44% (45%) of Voc, Jsc and FF respectively, without any intentional doping nor ARC/metallic grind. Replacing the standard soda-lime glass/Mo opaque back contact opens new application possibilities to this photovoltaic technology produced by a scalable deposition technique.

Authors : Katharina Rudisch, W. F. Espinosa-García, J. M. Osorio-Guillén, C. Moyses Araujo, Charlotte Platzer-Björkman, Jonathan Scragg
Affiliations : Solid State Electronics, Ångström Solar Center, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden; Institute of Physics, University of Antioquia UdeA, Calle 70 No 52-21, Medellin, Colombia; Institute of Physics, University of Antioquia UdeA, Calle 70 No 52-21, Medellin, Colombia; Materials Theory Division, Department of Physics and Astronomy, Uppsala University, P.O Box 516, S75120, Uppsala, Sweden; Solid State Electronics, Ångström Solar Center, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden; Solid State Electronics, Ångström Solar Center, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden

Resume : Researchers around the world have struggled to reduce the large open circuit voltage deficit in solar cells with Cu2ZnSnS4 absorber layers. One of the proposed causes for the large deficit is Cu-Zn disorder, which seems inevitable in typical production routes. It has been proposed that Cu-Zn disorder occurs in kesterite Cu2ZnSnS4 because Cu and Zn share a common crystal plane, and that the related stannite structure might be ?immune? from cation disorder since in that structure, the similar-sized cations are in different planes. In this work we explore if cation disorder can be avoided in the compound Cu2MnSnS4, which crystallizes in the stannite structure. In thin film Cu2MnSnS4 samples, the formation of the compound Cu2MnSnS4 and the co-existence of several secondary phases can be verified by XRD and Raman. We discuss the crystal structure of Cu2MnSnS4 based on results from DFT-calculations and confirm the stannite crystal structure for the compound. The bandgap of thin film Cu2MnSnS4 was found by spectrophotometry to be around 1.6 eV. After further low temperature treatments, the absorption shows reversible changes: a blue shift of the band gap and a reduction of the Urbach tail energy. We conclude from this observation that, just like the kesterite case, disorder can also occur in stannite compounds. We discuss the different implications of our findings and relate them to the current understanding of the cation disorder in Cu2ZnSnS4 and related compounds.

Authors : Asma Hannachi, Nawfel Khemiri, Mounir Kanzari.
Affiliations : Photovoltaic and Semiconductors Materials Laboratory - ENIT BP 37, 1002 Tunis, Tunisia

Resume : In the present paper, optical properties of Cu2ZnxFe1-xSnS4 (with x= 0, 0.25, 0.5, 0.75 and 1) thin films grown by vacuum evaporation method were studied. The as deposited films were sulfurized for 30 min at sulfurization temperature Ts=400°C. Optical measurements showed that CZFTS films sulfurized at 400°C exhibited an optical transmittance between 60 and 80%. The optical constants such as refractive index, absorption and extinction coefficients were calculated using the optical measurements in the spectral range of 300nm-1800nm. The dispersion of the refractive index is discussed in terms of the single oscillator model proposed by Wemple and DiDomenico. By using the model by plotting (n2 ? 1)?1 against (h?)2 and fitting a straight line, oscillator parameters, E0 the single oscillator energy and Ed the dispersion energy of the oscillator and measure the average strength of the inter-band optical transition, were directly determined. The optical absorption coefficient of all compounds between 9.104 and 2.105 cm-1 in the visible and NIR spectral range and the calculated refractive index was found to be in the range of 2.08 - 3.16. The electrical free carrier susceptibility (?e) and the carrier concentration on the effective mass ratio (N/m*) were evaluated according to the model of Spitzer and Fan.

Authors : Md abdul aziz SUZON, Louis GRENET, Fabrice EMIEUX, Henri MARIETTE
Affiliations : CEA-Liten, France, Université Grenoble Alpes, France;CEA-Liten, France;CEA-Liten, France;CEA-Liten, France, Université Grenoble Alpes

Resume : CZTSSe material is a promising candidate to replace CIGS in thin film solar cell technology as it uses only earth-abundant constituents. Despite having similar properties, CZTSSe based device is still suffering from low efficiency compared to CIGS-based devices. Absorber doping with alkali and crystallization control with Sb (antimony) are some of the reasons explaining the high performances of CIGS solar cells, due to the defect passivation and surfactant ability of those dopants. Therefore, this study focuses on strategies to improve sulfur based CZTS absorber synthesis with the incorporation of Na and Sb to the process. Two different routes have been tested to incorporate Na in the absorber layer by evaporating NaF before and after synthesis of the precursor. The effect of Na (by two routes) and the joint effect of Sb and Na on CZTS based material and solar cell have been assessed via different material and device characterization. The beneficial effect of Na and the negative effect of excess Na on material and photovoltaic properties has been demonstrated. On Contrary, the incorporation of Sb does not clearly improve the devices, despite having improved crystal growth in the presence of Na. This new approach of Na doping on the sequential process for CZTS solar cells allow to substantially improve the efficiency from 2% (w/o doping) to 5% (with doping) and above all improve the reproducibility of the process through a precise control of the alkali in the absorber.

Authors : Md abdul aziz SUZON, Louis GRENET, Fabrice EMIEUX, Henri MARIETTE
Affiliations : CEA-Liten, France, Université Grenoble Alpes, France;CEA-Liten, France;CEA-Liten, France;CEA-Liten, France, Université Grenoble Alpes

Resume : Thin film photovoltaic material CZTSSe became popular to the scientific committee to replace CIGS material as it uses only earth-abundant constituents. Despite having similar properties as CIGS, CZTSSe material is still suffering from low efficiency. The impact of bandgap grading (change of In/Ga ratio throughout the absorber) was observed in CIGS material by efficiency increment due to minimization of recombination. Similarly, by changing the ratio of S/Se throughout absorber it is possible to tune the bandgap of a CZTSSe absorber. Works have been done recently that showed bandgap gradient, which is process dependent. But there is no work to establish a general reaction mechanism of these processes and proper nano-characterization to understand if this grading exists within a single grain locally or in bulk globally. Therefore, this study focuses on applying single grading and double grading to find a clear reaction mechanism towards controllability of this process with nano-characterization. It has been demonstrated that replacement of Se by S or S by Se can be controlled with proper control of temperature and duration. Also, these reactions promote some secondary phases such as ZnS, SnS2. Nano-characterization is underway using ToF-SIMS tomography (for the global distribution of S/Se in bulk) and Auger mapping (for the local distribution of S/Se in a single grain), which will provide us some in-depth information towards bandgap grading process of CZTSSe material.

Authors : Jamie Lowe, Andrei Malkov, Jake Bowers, Lewis Wright, So?a Uli?ná, Fabiana Lisco, Mustafa Togay
Affiliations : Centre for Renewable Energy Systems Technology, Wolfson School of Mechanical, Electrical, and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire, LE113TU; Department of Chemistry, School of Science, Loughborough University, Loughborough, Leicestershire, LE113TU.

Resume : Kesterite (Cu2ZnSn(S,Se)4) solar cells are becoming increasingly popular due to their tuneable band gap, relative affordability of the constituent elements, and the ability to produce high efficiency devices from solution processes. This has prompted further research into producing efficient kesterite solar cells from cheap and non-toxic materials. Current kesterite solar cells with the highest efficiency used hydrazine as a solvent, which is highly toxic. In an attempt to reduce the toxicity of the process, a variety of groups have shown that an amine-thiol solvent system can dissolve Cu, Sn, and Zn. However, amine-thiol systems can still be highly toxic and malodorous. Therefore we have aimed to produce kesterite solar cells from less hazardous amine-thiol solvents in order to improve commercially viability. Here, we selected cysteamine and ethanolamine as the solvent system due to their low toxicity. Together they form a versatile non-toxic amine-thiol solvent mixture which can be used to dissolve Cu, Sn, Zn (and their oxides), Se, and S. Various solutions were produced and used to fabricate kesterite solar cells via spin coating, producing devices with a PCE of 3.35%. Experiments were performed to optimise spin coating parameters, selenisation conditions, and which precursor solution produced the best devices. Further optimisation of the deposition and annealing process will lead to efficiency improvements.

Authors : Vanira Trifiletti, Silvia Mostoni, Roberto Scotti, and Simona Binetti.
Affiliations : Department of Materials Science and Solar Energy Research Center (MIB-SOLAR), University of Milano-Bicocca, Via Cozzi 55, Milan (Italy)

Resume : Thin photovoltaics based on chalcopyrite film, Cu(In,Ga)Se2 (CIGS) and related alloys, have exhibited so far efficiency over 22,3% on the laboratory scale. However, the CIGS solar cells scale-up is expected to be hampered because of the low availability of In and Ga in the Earth crust. The most promising next leader in the chalcogenide thin film technology seems to be the kesterite compound Cu2ZnSnS4 (CZTS), as it is based on earth-abundant elements and it shows the better results when low-cost solution processable techniques are employed in the fabrication. Solution-processed inorganic semiconductors, in fact, offer a rising route for the low-cost mass production of solar cells. In this contest, we fine tune the precursor-solution, sustained by means of the Electron Paramagnetic Resonance and Raman spectroscopy, for proposing a stable kesterite precursor ink, which does not require sulphur addiction to complete the phase formation. The high-quality CZTS thin films, obtained by spin coating deposition onto fluorine doped tin oxide coated glass, were characterized by means of Raman spectroscopy, X-ray diffraction and scanning electron microscopy equipped for energy-dispersive spectroscopy. Preliminary devices have been tested.

Authors : H. Oueslati1, M. Ben Rabeh1 and M. Kanzari2
Affiliations : 1Université de Tunis El Manar, Ecole Nationale d?Ingénieurs de Tunis, Laboratoire de Photovoltaïques et Matériaux Semi-conducteurs, BP 37 Le Bélvédère 1002, Tunis, Tunisie. 2Université de Tunis, Institut préparatoire aux études d?ingénieurs de Tunis, Laboratoire de Photovoltaïques et Matériaux Semi-conducteurs, BP 37 Le Bélvédère 1002, Tunis, Tunisie.

Resume : Multicomponent chalcogenide Cu2ZnxFe1-xSnS4 (CZFTS) powders with varying x to 0 from 1 were synthesized under vacuum by direct melting of the constituent elements taken in stoichiometry compositions using high-purity elements. Structural and morphological properties of CZFTS multi ternary compounds were studied by X-ray diffraction (XRD), Raman spectroscopy and electronic scanning microscope (SEM) equipped with energy dispersive spectroscopy (EDS). X-ray diffraction patterns and Raman spectra of CZFTS powders showed that the as-synthesized CZFTS nanocrystal had a single phase and the transformation of the stannite to kesterite phase is occurred by decreasing the Fe content. Also, the lattice parameters of CZFTS powders were estimated. Elemental composition indicate that the as-synthesized CZFTS had a stoichiometric composition. In addition, the SEM images of CZFTS powders revealed that a small grain were present in the surface which became large due to higher Zn content. Moreover, the hot probe method showed that all CZFTS ingot exhibited an obvious p type semiconductor.

Authors : M. Distaso,1 R. Ahmad,1 M. Wu,2 ‎M. Prato,3 E. Spiecker,2 W. Peukert1
Affiliations : 1Institute of Particle Technology, FAU Erlangen Nürnberg, Germany; 2 CENEM, FAU Erlangen Nürnberg, Germany 3 Italian Institute of Technology, Genova, Italy

Resume : Kesterites (Cu2ZnSnS4 and Cu2ZnSnSe4, CZTS(e)) emerged as a favourable photovoltaic material for the low toxicity, high abundance of its constituents and efficiency in solar cells.1 However, the phase purity of CZTS phase plays a fundamental role as secondary phases hinder the device efficiency. In the current contribution, the formation of CZTS nanoparticles was studied as function of time and temperature using different molecular precursors under otherwise identical experimental conditions. The as-obtained structures were characterized by electron microscopy, Raman and Energy Dispersive X-ray Spectroscopy (EDX), powder X-ray diffraction (XRD), X-ray-Photoelectron Spectroscopy (XPS). The outcome of the work leads to the development of general strategies for the rational design of crystalline CZTS nanoparticles free of detectable secondary phases and crystallographic defects such as twins and stacking faults.2 It further extends the understanding on the order – disorder transitions in CZTS materials.2 The critical role of the Zn-precursor counter-ion towards phase purity is highlighted and rationalized. 1 W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu, and D. B. Mitzi, Adv. Energy Mater. 2014, 4, 1301465. 2 R. Ahmad, M. Brandl, M. Distaso, P. Herre, E. Spiecker, R. Hock, W. Peukert, CrystEngComm 2015, 17, 6972.

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Kesterite Interfaces : Susan Schorr, Christian Andres
Authors : G. Gurieva1*, A. Franz1 and S. Schorr1,2
Affiliations : 1 Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany 3 Free University Berlin, Institute of Geological Sciences, Malteserstr. 74-100, Berlin, Germany

Resume : Cu2ZnSnSe4 solar cells, containing only abundant elements, with Ag alloying recently reached efficiency of 10.2% [1]. The open-circuit voltage in CZTSSe devices is believed to be limited by absorber band tailing caused by the exceptionally high density of Cu/Zn antisites. By replacing Cu in CZTSe with Ag, the density of I–II antisite defects (e.g Cu/Zn disorder) is predicted to drop. It was shown by neutron diffraction that CZTSe crystallizes in the kesterite type structure (space group I4 ̅) [2]. So far, only X-ray diffraction used for structural characterization of Ag2ZnSnSe4 was reported in the literature, and it suggests that it also shows the kesterite type structure [3]. The aim of this study was to clarify the crystal structure of the room temperature modification as well as to deduce possible cationic point defects, paying a special attention to the presence of Cu/Zn disorder in these compounds. The results allow us to suggest that in spite of both end members of the (Ag1-xCux)2ZnSnSe4 solid solution being reported to crystallize in kesterite type structure, at least the solid solution with x=0.17 and 0.46 crystallize in the stannite type structure (space group I4 ̅2m). Here the site 2a is completely occupied by the divalent cation (Zn), whereas the site 4d is occupied by Cu and Ag randomly. The latter corresponds to the sites 2c and 2d in the kesterite type structure, in which the Cu/Zn disorder in kesterite type CZTSe occurs. In this way the possibility for the formation of Cu/Zn disorder is completely blocked in (Ag1-xCux)2ZnSnSe4. [1] T. Gershon, et al Adv. Energy Mater. 2016, 1502468 [2] S.Schorr, Solar Energy Materials and Solar Cells, 95 (2011)1482. [3] W. Gong et al, phys. stat. sol C., 1-4(2015)

Authors : G. Brammertz1,2, T. Kohl1,2, J. De Wild1,2, M. Meuris1,2, B. Vermang2,3, J. Poortmans2,3,4
Affiliations : 1 Imec division IMOMEC (partner in Solliance & EnergyVille), Wetenschapspark 1, 3590 Diepenbeek, Belgium. 2 Institute for Material Research (IMO), Hasselt University (partner in Solliance & EnergyVille), Agoralaan gebouw H, Diepenbeek, 3590, Belgium. 3 imec (partner in Solliance & EnergyVille), Kapeldreef 75, Leuven, 3001, Belgium. 4 Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium.

Resume : We have studied the fabrication of polycrystalline Cu2ZnGeSe4 solar cell absorbers by H2Se selenization of sequentially deposited metal layer stacks. The fabricated layers are polycrystalline with grain size of the order of 1 µm and a band gap of about 1.4 eV. We have executed a stop experiment, stopping the crystallization reaction at different times during the process, then analyzing the subsequent XRD spectra. We have found that mainly Cu3Ge and ZnSe phases form very rapidly at temperatures below 350°C. The formation of Cu2ZnGeSe4 happens through the reaction of Cu9Se5, Cu3Ge and ZnSe phases. Depending on the order of the sequentially deposited metal layer stack, the formation reaction proceeds at different speeds and yields layers with different levels of grain size and adhesion to the Mo back contact. Based on these results we have then designed an optimized layer stack. Solar cells were fabricated with this optimized absorber layer and these cells showed a maximum power conversion efficiency of 5.7 %. The resulting solar cells show good series and shunt resistance behavior, an ideality factor of 1.9 and a saturation current density of 2 10-9 A/cm2, but seem to be limited by bad minority carrier collection under forward bias conditions, leading to a relatively low fill factor. Admittance spectroscopy results seem to indicate that a surface defect is at least partially responsible for the low lifetime of the absorber. This project has received funding from the European Union?s Horizon 2020 research and innovation program under grant agreement No 640868.

Authors : W. Kogler, J. Hanisch, T. Schnabel, A. Bauer, S. Spiering and E. Ahlswede
Affiliations : Zentrum für Sonnenenergie- und Wasserstoff-Forschung, Meitnerstrasse 1 70563 Stuttgart, Germany

Resume : Cu2ZnSn(S,Se)4 (CZTSSe) based solar cells are assigned to the second generation of photovoltaic devices. This thin film technology stands out by the use of cheap and earth abundant elements as well as its tunable direct band gap and high absorption coefficient. However, the record efficiency for this type of solar cell was obtained using CdS as the buffer material [1]. Since Cd is a toxic element and also causes parasitic absorption in the short wavelength region of the spectrum, the substitution of CdS by an environmentally friendly, high band gap material is one of the key challenges for CZTSSe based solar cells. In this work In2S3 and Zn(O,S) were investigated as alternative buffer materials using different chemical and physical deposition techniques, namely chemical bath deposition (CBD), atomic layer deposition (ALD) and sputtering to figure out the optimal deposition process. First investigations on In2S3 and Zn(O,S) buffered devices resulted in solar cells with similar open circuit voltage (Voc) and short circuit current densities (JSC) compared to CdS buffered devices. The difference in the VOC values of In2S3 and Zn(O,S) is most likely caused by a better band alignment of the atomic layer deposited In2S3 to the absorber materials, which is indicated by JV-T measurements. However, Zn(O,S) was deposited by sputtering, which could be the reason for the worse band alignment due to the introduction of sputtering damage on the surface of the absorber. A comparative investigation of Zn(O,S) from chemical deposition techniques is subject of ongoing work. Furthermore, Raman microscopy is used to determine the S/S+O ratio by carefully evaluating the positions of the ZnS- and ZnO-like peaks according to the method reported by Guc et al. [2]. Additionally, Time-of-flight secondary ion mass spectrometry (ToF SIMS) measurements were performed on the different investigated devices to obtain information about the diffusion behavior of the different buffer materials. Finally, the complete devices were characterized by JV-measurements and UV-Vis spectroscopy to obtain the characteristic solar cell parameters and information about losses inside the device.

Authors : L. Choubrac(1), S. Harel (1), G. Brammertz (2,3), X. Kozina (4), L. Arzel (1), M. Bär(4,5), R.G. Wilks (4,5), M. Meuris (2,3), B. Vermang(2,6), and N. Barreau (1)
Affiliations : (1) Institut des Matériaux Jean Rouxel (IMN)-UMR 6502, Université de Nantes, CNRS, 2 rue de la Houssinière, 44322 Nantes Cedex 3, France (2) imec division IMOMEC – partner of Solliance, Wetenschapspark 1, 3590 Diepenbeek, Belgium (3) Institute for Material Research (IMO) Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium (4) Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 14109 Berlin, Germany (5) Energy Materials In-situ Laboratory Berlin, Albert-Einstein Straße 21, 12489 Berlin, Germany (6) Faculty of Engineering Technology, Hasselt University, 3590 Hasselt, Belgium

Resume : Several positive impacts have been reported when Ge is incorporated in tin-kesterite [Cu2ZnSn(S,Se)4] absorber films, including better crystallization of the material, reduced Voc-deficit, and reduced Cu2ZnSn(S,Se)4/CdS interface recombination. The findings suggest that Ge-kesterites [Cu2ZnGe(S,Se)4] may be a promising absorber material for thin film photovoltaic devices, however the highest efficiency reported so far is 5.5%, for a Cu2ZnGeSe4 solar cell. In this study, Cu2ZnGeSe4 was grown by Selenization of metal stacks. Reference solar cells prepared with a CdS buffer layer exhibit efficiencies of 5%, mostly limited by interface recombination [according to electrical (IV, EQE) characterization]. Characterization of the absorber layer by complementary techniques (XPS, Raman, XRD, SEM, photoluminescence) reveals the presence of ZnSe at the absorber surface and a non-optimal Cu2ZnGeSe4/CdS energy level alignment. In order to address these issues, we employed two parallel strategies, i.e. removal of secondary surface phase and replacing the standard CdS with a Zn(O,S) buffer layer. Removing ZnSe by HCl etching improves Voc, but reduces the overall efficiency because of FF and JSC (due to collection length) reduction. After HCl etching oxidized Se phases are found at the surface. For their removal different wet chemical (passivation) routes have been studied. Resulting (etched/passivated) Cu2ZnGeSe4/CdS-based solar cells reach efficiencies > 7%, thanks to an improved Voc (+15 %) and current (+10%). The optimization of the preparation route for the alternative Zn(O,S) buffer finally results in efficiencies >7% for non-etched/passivated Cu2ZnGeSe4. This work describes significant improvement in the efficiency of Cu2ZnGeSe4-based thin film solar cells above the state of the art. Further optimization based on the utilization of both strategies is expected to lead to further improvements in device performance. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 640868

Authors : A. Crovetto (1), R. Ettlinger (2), A. Cazzaniga (2), J. Schou (2), O. Hansen (1)
Affiliations : (1) DTU Nanotech, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark (2) DTU Fotonik, Technical University of Denmark, 4000 Roskilde, Denmark

Resume : Two major issues of pure-sulfide Cu2ZnSnS4 (CZTS) solar cells are their unfavorable interface properties and the strong dependence of their performance on process conditions. In this contribution we will present details of the CZTS/CdS interface band diagram as a function of process parameters. The interface band edge positions are measured by x-ray photoemission spectroscopy (XPS) and the near-interface band gaps are extracted by spectroscopic ellipsometry using a newly developed method. The additional ellipsometry measurement is a crucial step for detecting the effect of interdiffusion on the materials’ band gaps in the near-interface region. The interface properties are found to be unfavorable under all investigated process conditions, either due to a cliff-like conduction band offset or to substantial band gap narrowing of CZTS near the interface. The least harmful process conditions for the interface are a low CdS deposition temperature, either with or without post-annealing. Even under such conditions, a decrease of at least 200 mV in the Shockley-Queisser limit for the open circuit voltage is expected due to a narrowed band gap at the interface or in the region of interdiffusion. Cd interdiffusion into CZTS appears therefore to be a detrimental effect for the interface. A buffer layer with a different cation than Cd could solve the problem. Finally, we will show how the proposed measurement method can be applied to other types of chalcogenide solar cells.

Authors : Christian Neubauer1,2, Ali Samieipour1, Souhaib Oueslati1,2, Mati Danilson1, Kaia Ernits2, Dieter Meissner1,2
Affiliations : 1Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; 2crystalsol OÜ, Akadeemia tee 15a, 12618 Tallinn, Estonia

Resume : The p-type semiconductor material Cu2ZnSn(SxSe1-x)4 (CZTSSe) with its beneficial properties such as high absorption coefficient, tunable bandgap and abundant raw materials gained attention in recent years as an absorber material in photovoltaic applications. The monograin technology in combination with a cost efficient roll-to-roll application has the potential for a large scale industrial production. Material efficiencies of around 12 % were measured. However the stability in photovoltaics is crucial for a reliable and economic implementation in the photovoltaic market. Despite the importance of the degradation mechanism, very few papers were published so far. In this paper we investigated differently stable CZTSSe monograin layer (MGL) devices and the impact of thermal degradation on the back and frontside of the devices and its parameters. Diffusion processes, especially of copper, was identified as a major driver of the degradation. Temporary barriers at the CZTSSe-graphite interface of the back contact are observed and investigated by current-voltage (I/V), X-ray photoelectron spectroscopy (XPS) and Kelvin probe (KP) measurements. External quantum efficiency (EQE) and photoluminescence (PL) measurements show a change in wavelength dependent charge carrier extraction, which suggests a change at the p-n junction.

10:00 Coffee Break    
CdTe and CIGS Solar Cells and Modules : Ayodhya N. Tiwari, Alessandro Romeo
Authors : Gang Xiong, Gloeckler Markus
Affiliations : First Solar Inc 1035 Walsh Ave, Santa Clara CA, 95050 USA

Resume : First Solar?s thin-film CdTe solar cells have been certified at 22% cell efficiency. Fundamental work by others has demonstrated that a shift away from copper-doped CdTe to a dopant from the 5th column of the periodic table (?Group-V?) should be possible and it may enable pathways to even higher efficiency. Over the last two years, First Solar has made steady progress on implementing Group-V doping in polycrystalline CdTe. The first milestone was the consistent demonstration of greater than 10e16 cm-3 hole concentration with a flat doping profile and narrow depletion width. Following this success, we produced >900 mV Voc on CdTe devices free of Se. It has been more challenging to produce similar Voc with Group-V doped CdSexTey devices. The failure mechanism is not well understood, however, a primary suspect is increased front junction recombination. Despite the challenges, we were able to fabricated devices >19% efficiency and >850mV Voc on Group-V doped CdSexTey devices. High-temperature long-term stability tests demonstrated stable doping and performance level. First Solar?s leading thin-film manufacturing capabilities are in process of ramping the new utility-power optimized Series 6 module. Series 6 modules will have initial power up to 440 Watts with an optimized design to lower installation and other balance of system cost. It will preserve First Solar existing CdTe module power density advantage: superior temperature coefficient, spectra and shading response.

Authors : O. Oklobia, G. Kartopu, and S. J. C. Irvine
Affiliations : Centre for Solar Energy Research, College of Engineering, Swansea University, OpTIC Centre, St. Asaph Business Park, LL17 0JD, UK

Resume : Copper dopant incorporation near the surface of CdTe absorber is a common approach of fabricating low resistive back contacts for CdTe solar cells. However, it is difficult to carefully control this process, which often results in electrical shunts, and therefore poor device performance [1]. P-type ZnTe as a back-contact layer provides a suitable alternative, as it is chemically stable and has a near-perfect valence band alignment with CdTe [1, 2]. We used metal organic chemical vapour deposition (MOCVD) to grow arsenic – doped polycrystalline ZnTe layers as a back-contact layer on CdCl2 passivated CdTe absorber for the first time. Different arsenic dopant flows (ranging between 0-10 sccm) were used in the growth process for ZnTe thin film on uncoated glass, yielding varying carrier concentrations. High carrier concentrations of up to ~9×1018 cm-3 with a corresponding low resistivity of ~1.42Ω. cm was achieved as a result of arsenic doping (10 sccm), as determined from Hall-effect measurements. CdTe cells with undoped ZnTe back-contact showed very poor J-V characteristics with conversion efficiencies of ~0.2%. The insertion of a ZnTe:As back-contact layer resulted in a conversion efficiency of >~10% (for 3-10 sccm flow), with particular improvement in JSC, correlating to the improved red response observed in device’s EQE. References [1] S. Uliĉná, P. J. M. Isherwood, P. M. Kaminski, J. M. Walls, J. Li, C. A. Wolden, Development of ZnTe as back contact material for thin film cadmium telluride solar cells, Vacuum, 139, 159-163 (2016), references therein [2] N. Amin, A. Yamada, M. Konagi, Effect of ZnTe and CdZnTe Alloys at the Back Contact of 1-µm-Thick CdTe thin Film Solar Cells, Japan Journal of Physics, 41 (2002).

Authors : Martina Lingg, Amit Munshi, Christian Andres, Walajabad S. Sampath, James R. Sites, Stephan Buecheler, Ayodhya N. Tiwari
Affiliations : Martina Lingg; Christian Andres; Stephan Buecheler; Ayodhya N. Tiwari: Laboratory for Thin Films and Photovoltaics, Empa - Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, 8600 Duebendorf Switzerland Amit Munshi; Walajabad S. Sampath; James R. Sites: Next Generation Photovoltaics Center, Colorado State University, Fort Collins, CO 80523, USA.

Resume : As one of the most promising absorber materials for high-efficiency and low-cost solar cells, CdTe holds the highest market share among thin film photovoltaics technologies. Recent improvements of CdTe solar cells have involved a CdTe(1-x)Se(x) layer as part of the front side of the absorber. The used alloy has a lower band gap than CdTe, which increases the photocurrent. However, the different band gap of CdTe(1-x)Se(x) can result in band misalignments when it replaces CdTe in the traditional device structure. In this work we investigate the weaknesses in solar cells with CdTe(1-x)Se(x) absorbers. With temperature-dependent current-voltage measurements we attempt to identify barriers in homogeneous CdTe(1-x)Se(x) absorbers caused by band offsets between CdTe(1-x)Se(x) and the MgZnO window layer as well as the back contact. Additionally, we investigate the impact of a CdTe(1-x)Se(x) layer on the size of the space charge region, as with copper doping CdTe(1-x)Se(x) has a lower achievable charge carrier concentration than CdTe. We compare these results to high-performance solar cells with a CdTe(1-x)Se(x) absorber layer at the front of a CdTe layer.

Authors : Katsumi Kushiya
Affiliations : Energy Solution Business Center, Showa Shell Sekiyu K.K.

Resume : Based upon the product catalogues of CIS-based thin-film PV modules commercialized by the companies, such as Solar Frontier, Hanergy-Solibro and AVANCIS, FF is currently lower than that of other PV technologies. If CIS-based thin-film PV modules achieved the same level of FF (FF≧0.756) as other PV technologies, the nominal output power of 200Wp and higher would be achieved. Although at this moment, we do not have any definitive solution for the question why FF of commercialized CIS-based thin-film PV modules is lower than that of other PV technologies, detailed analyses of device structure and various resistances in the device will help to enhance the FF over 0.8. FF of 0.8 had already been achieved in the small-area (less than 0.5 cm2 as a cell size) CIGS solar cells in which CIGS absorber was prepared by a coevaporation (i.e. three stage process) and a CdS buffer layer by a chemical bath deposition was applied. In the current champion small-area (1.04 cm2) CIS-based thin-film solar cell prepared by Solar Frontier and measured by AIST, FF was 0.797, in which CIS-based absorber was prepared by a sulfurization after selenization and Cs doping as a post-deposition treatment and a CdS buffer by a chemical bath deposition were applied. This was the highest ever in Solar Frontier as well as a selenization process group for CIS-based absorber formation. This also indicates that CIS has high potential to achieve the FF of over 0.8 by optimizing the interface or improving the junction quality. So far, it was not a common goal to focus on achieving the FF of over 0.8. In the present contribution, it will be discussed how to optimize the interface mainly through consideration for the sensitivity to light soaking effect of commercialized products by Solar Frontier. It would be a guideline to understand the production process more deeply and a focusing point what we should do. Furthermore, it would also contribute to enhance the competitiveness of CIS-based thin-film PV technology.

Authors : Dmitry Poplavskyy, Rouin Farshchi
Affiliations : MiaSolé Hi-Tech Corp, Santa Clara, CA 95051, USA

Resume : We report accelerated heat degradation studies on fully encapsulated CIGS modules as a function of film growth parameters, in particular back contact selenization (pre-Se), as well as the impact of bias (light/voltage) during heat degradation. We show that pre-Se conditions have a profound effect on the heat stability of the device, whereby reduced pre-Se, while increasing initial efficiency, results in strong heat degradation, mostly driven by reduced minority carrier lifetime (as evident from EQE) in the light-soaked state. This is also accompanied by a stronger increase in the shallow acceptor concentration (as measured by CV) in the degraded state suggesting that the Cu-Se divacancy mechanism is likely responsible. In this case, appearance of a high concentration of deep states accompanies increased doping, with the former reducing bulk lifetime and the latter further affecting electron collection due to narrow depletion width. This result suggests that bulk structural properties of the absorber film are strongly impacted by the back contact selenization conditions, making the film more susceptible to heat degradation. In the second part of this paper we show that electrical or light bias during heat exposure reduces degradation, in particular almost fully eliminating the above Jsc loss. This is a surprising result as usually the positive effects of bias are attributed to interfacial changes, while our results demonstrate that bulk properties can be improved as well.

12:15 Lunch    
CIGS: New Device Concepts : Jan Keller, Daniel Siopa
Authors : Byoung Koun Min
Affiliations : Clean Energy Research Center, Korea Institute of Science and Technology, Hwarangro 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea

Resume : Building-integrated photovoltaics (BIPV) are considered to be the most suitable PV installation feature particularly for global trends of urbanization (e.g. distributed and on-site electricity generation system). Among the various physical requirements for being BIPV, colorful and aesthetic characteristics have become very important because a coloration of building is considered as a work of art in most modern city. Chalcopyrite thin-film PVs (e.g. CuInxGa1-xSe2, CIGS) will be an attractive candidate because they are durable enough and cost effective with high efficiency. Therefore, it would be very promising for BIPV applications if the authentic aspect is successfully satisfied with the least energy loss. In this study, to fabricate light-transmitting, colorful, and cost-effective power-generating windows, we introduced solution-processed CIGS photovoltaics combined with 1D photonic crystal (PC) dichroic filter/mirror films. By the application of various colored 1D PC dichroic films to the outside or inside of the CIGS photovoltaic modules, colorful and aesthetic value-added CIGS thin-film photovoltaic modules were realized. The details of the fabrication method and characterization of the solution processed colorful CIGS photovoltaics will be discussed in the presentation.

Authors : Hee-sun Yun(1,2), Jang Mi Lee(1,2), Doh-Kwon Lee(1,2)
Affiliations : 1) Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Korea; 2) Division of Nano and Information Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Korea

Resume : CuIn1-xGaxSe2 (CIGS in general and CISe with x = 0) photovoltaic (PV) cell has enough potential, such as long-term stability, non-toxicity, high efficiency and so on, to be commercially used for eco-power generation. Nevertheless, it is still challenging to increase solar cell market share of CIGS as much as expected probably due to slump in the unit cost of silicon solar cell. In other words, it might be hard for CIGS to draw attention in cost aspect so that it is needed to consider other applications of CIGS that silicon solar cell can’t realize. As PV technologies have been improved over the past decades, the public starts to require not only efficient but also the beautiful solar panel. However, it is hard to satisfy consumer’s needs with heavy and indistinctive Si-based solar panel. In this study, in an attempt to realize aesthetics of solar cells, we applied the photoelectric effect in a one-pot electrodeposition process using Se aqueous solution containing Cu and In chlorides. As a result, CISe area exposed to light was approximately 1.4 times thicker in thickness than the other part that is not illuminated. In addition, [Cu] to [In] ratio, which is a pivotal factor for high performance, was decreased which is attributed to facilitated In ion reduction reaction. After the annealing process, the morphology of each part is different upon Cu to In ratio, which realized the surface pattern of CISe. Thus far, photo-electrochemically deposited CISe with the pattern attained 8.62 % efficiency.

Authors : J. Lorthioir, N. Barreau, L. Arzel
Affiliations : Institut des Matériaux Jean Rouxel, 2 rue de la Houssinière - BP 32229 - 44 322 NANTES CEDEX 3

Resume : Our study focuses on the structure of the photovoltaic modules based on Cu(In,Ga)Se2 thin films. Passing from a solar cell to a photovoltaic module using the current monolithic interconnection decreases the conversion efficiency about 5% abs. In fact, the architecture of the modules induces optical and resistive losses due to the thick ZnO layer. Additionally the P1 P2 P3 scribings reduce the active area about 10%. Thus, we put forward an alternative architecture of modules consisted of a serial interconnection between cells from the front contact to the back contact directly. For this reason, the Molybdenum (Mo) back contact scribing is carried out using a photolithography process and optimized to minimize the dead zone. Then, the thickness of the thin film is also adjusted. This new concept allows to decrease 1) the number of scribes, 2) the ZnO thickness, 3) the air exposure of Mo and 4) the inactive area. Using the same deposition process, we synthesize solar cells and alternative modules consisting of a series of 2 cells. Our preliminary results are very promising and show a high fill factor of 71% for modules and 75% for cells. This suggests a small loss caused by series resistance. 17% efficiency has been attained for cells while for modules it reached 12%. This decrease may be due to the difference of Voc: 700mV for cells vs 1.2V for modules. Future work will concentrate on better understanding how these alternative modules can improve the photovoltaics efficiency.

Authors : Nasim Rezaei *, Olindo Isabella *, Zeger Vroon **, Miro Zeman *
Affiliations : * Delft University of Technology, Photovoltaic Materials and Devices group, Mekelweg 4, 2628 CD Delft, the Netherlands; ** TNO-Brightlands Materials Center, Urmondsebaan 22, 6167 RD Geleen, PO BOX 18, 6160 MD Geleen, the Netherlands

Resume : Light management and rear surface passivation are essential for high efficiency ultra-thin Copper indium gallium (di)selenide (CIGS) solar cells. While light scattering with efficient rear reflectance and surface passivation can be achieved by the passivated emitter and rear cell- (PERC-) like technique [1] in combination with a bi-layer stack of dielectrics [2], antireflection coatings (ARC) boost light in-coupling. In this work, we first propose a multi-ARC based on MgF2 applied to a 750-nm thick CIGS cell. Deploying HFSS, a 3D finite element method Maxwell’s equation solver [2, 3], the refractive index of three porous MgF2 layers has been graded for minimizing reflection between 300 nm and 800 nm, improving the photo-generated current density (Jph) by 6.34% (from 28.37 to 30.17 mA/cm2). Then, we report on our 3D-to-2D hybrid opto-electrical modelling framework [4] for simulating the performance of an ultra-thin CIGS solar cell endowed with the abovementioned MgF2-based multi-ARC at the front and a previously optimized PERC-like structure at the back [2]. The resulting modelled 3D optical situation has been fed to TCAD Sentaurus [5] for solving the inherently-2D semiconductor equations. The current density-voltage characteristics have been studied as function of rear dielectric to total area cover ratio. [1] B. Vermang et. al., IEEE JPV (2014) [2] N. Rezaei, OPEX (2018) [3] ANSYS white paper, “ANSYS HFSS” [4] M. Zeman et al., SOLMAT (2013) [5] N. Guerra et al., SOL ENERGY (2017)

Authors : Benjamin J. Morgan
Affiliations : University of Bath

Resume : Halide-substituted lithium-argyrodites, Li6PS5X (X=Cl, Br, I) are a promising family of lithium-ion solid electrolytes, with potential applications in all-solid-state lithium-ion batteries. Changing X from I to Cl produces a strong increase in lithium-ion conductivity, which has been attributed to increased crystallographic disorder for X and S ions across 4a and 4c sites. A previous molecular dynamics study [1] has predicted that efficient long-ranged Li-ion transport is only achieved for such X/S disordered systems, with this attributed to changes in the rates of lithium-ion jumps between lattice sites. A microscopic explanation for this change in lithium-ion dynamics, however, is lacking. To study this behaviour, we have performed a series of ab initio molecular dynamics simulations of ordered and disordered Li6PS5X. In contrast to previous computational studies, we have analysed the lithium-ion dynamics in terms of transitions between local potential minima (inherent structures) in lithium-ion configuration space, which allows us to resolve non-trivial lithium motion [3]. We find that in fully ordered Li6PS5X, the lithium ions form six-coordinate coordination octahedra around the (4a/4c) S anions. Interestingly, this is the case for X ordered over the 4a sites and over the 4c sites, indicating a strong preference for separated (4a/4c) S ions to be octahedrally coordinated by Li. Lithium motion consists of highly correlated processes within individual octahedra; octahedral rotations, and reorganisations via trigonal prismatic configurations; neither of which give long ranged lithium diffusion. For anion-disordered Li6PS5X, this preferred octahedral coordination is disrupted. We observe a number of LixS coordination environments with x ? 6, and 6-fold coordination environments increasingly deviate from ideal octahedral symmetry. This increased Li- coordination disorder facilitates long-ranged Li-ion diffusion between LixS polyhedra. We note that for pairs of S ions in adjacent 4a / 4c sites, it is not possible for both S ions to simultaneously achieve ideal octahedral coordination. We propose, therefore, that the capacity for long-ranged lithium transport in anion-disordered Li6PS5X arises from geometric frustration of preferred octahedral lithium configurations, which produces a highly disordered network of lithium-ion polyhedra, with fast lithium-ion diffusion. References: [1] Rao et al. Sol. Stat. Ionics (2013), 230, 72. [2] de Klerk et al. Chem. Mater. (2016), 28, 7955. [3] Stillinger and Weber, Science (1984), 225, 983.

Authors : Louis Gouillart, Andrea Cattoni, Julie Goffard, Frederique Donsanti, Marie Jubault, Negar Naghavi, Stéphane Collin
Affiliations : Centre for Nanosciences and Nanotechnology (C2N/CNRS), 91460 Marcoussis, France; IPVF, Institut Photovoltaïque d’Ile de France, 30 RD 128, 91120 Palaiseau, France

Resume : Cu(In,Ga)Se2-based (CIGS) solar cells with an ultra-thin absorber layer (<500 nm) allow manufacturing cost reduction but exhibit low efficiencies due to poor light absorption and back-contact recombination. To overcome this efficiency drop, we develop new back-contact layers based on a reflective and passivating nanostructured mirror for CIGS solar cells. First, numerical electromagnetic simulations of complete solar cells with different CIGS thicknesses and various mirror materials have been performed. We demonstrate the possibility to reach a short-circuit current of Jsc = 36.3 mA/cm² for a 150-nm-thick CIGS absorber with a Ag nanostructured mirror. Experimentally, we have first replaced the conventional Mo back-contact with flat mirrors encapsulated in transparent conducting layers. This reflective back mirror leads to an improvement of the Jsc of 5.2 mA/cm² (from 20.8 to 26.0 mA/cm²) for 480-nm-thick CIGS solar cells, and is fully compatible with nanopatterning for further light-trapping improvement. Regarding rear passivation, we have developed a cost-effective process to pattern nanoholes in an Al2O3 layer on 5x5 cm². Using nanoimprint lithography and wet etching, we demonstrate a Voc increase from 606 to 627 mV compared to bare molybdenum. We will show that a combination of these building blocks could lead to 20% efficiencies for CIGS solar cells with absorbers thinner than 500 nm.

Authors : Romain Carron [1], Enrico Avancini [1], Thomas Feurer [1], Benjamin Bissig [1], Paolo A. Losio [2], Renato Figi [3], Claudia Schreiner [3], Melanie Bürki [3], Emilie Bourgeois [4,5], Z. Remes [6], Milos Nesladek [4,5], Stephan Buecheler [1], Ayodhya N. Tiwari [1]
Affiliations : [1] Laboratory for Thin films and Photovoltaics, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, 8600 Dübendorf, Switzerland [2] Institute of Computational Physics, Zurich University of Applied Sciences (ZHAW), Technikumstr. 9, 8401 Winterthur, Switzerland [3] Laboratory for Advanced Analytical Technologies, Empa – Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, 8600 Dübendorf, Switzerland [4] Institute for Materials Research (IMO), Hasselt University, Wetenschapspark 1, B-3590 Diepenbeek, Belgium [5] IMOMEC division, IMEC, Wetenschapspark 1, B-3590 Diepenbeek, Belgium [6] Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10/112, 162 00 Prague, Czech Republic

Resume : CuIn(1-x)GaxSe2 (CIGS) solar cells tend to show non negligible losses in the spectral response close to the bandgap which can have different causes. Possible charge carrier collection losses can be distinguished from incomplete optical absorption by comparison of optical simulations of solar cell devices with experimental data, leading to insights into devices limitations such as recombination at the back contact or an imperfect collection function. In this contribution we present refractive indices for all layers present in a CIGS solar cell with high efficiency, and especially absorption coefficients for CIGS over the full composition range for high-efficiency devices. The reliability of the CIGS absorption data is assessed by the good match of light absorption simulations with optical measurements of the absorbers. The charge carrier collection losses are estimated from the difference between devices optical simulations and the experimental EQE. Little to no significant wavelength-dependent collection losses could be evidenced in high-efficiency devices. By contrast, a significant current loss is observed in the case of Ga-free CuInSe2 absorbers, especially for carriers generated deep in the absorber layer. For such low bandgap absorbers the introduction of a Ga-containing graded layer at the back of the absorber efficiently reduces the collection losses, and leads to a large improvement in the device open-circuit voltage.

15:30 Coffee Break    
Poster 2: Kesterite, CdTe, and Tandem Solar Cells : Markus Gloeckler, Katsumi Kushiya
Affiliations : University Mohammed V,Faculty of Sciences, Physics Department, LPM, B.P. 1014, Rabat, Morocco

Resume : XS (X=Cd, Sn) thin films with molar concentration (MX = 0.05M, MS=0.5M) are deposited by spray pyrolysis technique on heated glass substrate at 350°C. The physical properties of the film are characterized by several techniques in order to study their structural, optical and electrical properties. It is observed from X-ray diffraction (XRD) analysis that the CdS film is mainly composed with hexagonal Wurtzite structure with a preferred grain orientation along (101) plane and SnS film is mainly composed with orthorhombic crystal structure with a preferred grain orientation along (111) plane. From optical measurements, the average optical transmission is 80 % (CdS) and 70% (SnS). The band gap value is found 2.4 eV (CdS) and 1.8 eV (SnS). The Hall Effect electrical measurements show that the sample is n-type for CdS and p-type for SnS. The values of the electrical resistivity 3x10-2(?.cm) for CdS and 2.75x10-2 (?.cm) for SnS are obtained.

Authors : Ji-Sang Park, Sunghyun Kim, and Aron Walsh
Affiliations : Thomas Young Centre and Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK

Resume : Usually, in chalcogenide thin film solar cells such as CdTe, Cu(In,Ga)Se2, and Cu2ZnSn(S,Se)4, the absorber layers are made of polycrystalline materials. Planar defects like stacking faults can be formed easily in the absorber layers in such cases, and the device efficiency can be significantly influenced accordingly. Some recent experimental studies show that another type of planar defects called antisite domain boundaries can be formed in Cu2ZnSnS4. However, our knowledge of their effect on the band structure was far from complete. In this regard, we quantitatively investigated the stability and the electrical properties of stacking faults and antisite domain boundaries in Cu2ZnSnS4 and Cu2ZnSnSe4 using the first-principle calculation method. We found that the stacking faults act as electron barriers, whereas the antisite domain boundaries are electron captures in the materials. Both types of defects do not significantly affect the valence band.

Authors : J.K. Larsen, F. Larsson, Y. Ren, N. Ross, C. Platzer-Björkman
Affiliations : Ångström Solar Center, Division of Solid State Electronics, Ångström Laboratory, Uppsala University, Uppsala 752 36, Sweden

Resume : We have previously observed that the band gap of Cu2ZnSnS4 (CZTS) is increased when annealing Cu-Zn-Sn-S precursors in an atmosphere with a high sulfur partial pressure for a short time. Since the sulfur partial pressure drops during the annealing a lower band gap was observed in CZTS annealed for longer durations. In this work we investigate the effect of the modified band alignment at the CZTS/Zn1?xSnxOy (ZTO) interface as a result of different CZTS annealing conditions and ZTO growth conditions. The conduction band of ZTO can be increased by reduction of the growth temperature. We investigate three different ZTO growth temperatures on CZTS with two different apparent band gaps to obtain insights into the band alignment. The results are compared to CdS reference devices where a cliff-like band alignment is expected. Contrary to the expectation it was observed from JV characteristics that the CZTS/ZTO devices with the widest absorber band gap were more blocked than devices with narrower band gaps. This indicates that both conduction and valence bands were downshifted relative to ZTO in CZTS annealed in high sulfur partial pressure. The result implies that the band gap widening observed in high sulfur pressure annealed CZTS is caused by a larger downshift of the valence band edge relative to the conduction band.

Authors : N. Khemiri*, A. Hannachi, M. Kanzari
Affiliations : Université Tunis El Manar, Institut Prepatoire aux Etudes d?Ingénieurs El Manar (IPEIEM), Laboratoire de Photovoltaïque et Matériaux Semi-conducteurs 1002, Tunis, Tunisie.

Resume : In this work, we present experimental studies of three non toxic and earth abundant thin film solar cell materials: Cu2ZnSnS4, SnSb4S7 and Zn(S,O). Cu2ZnSnS4 and SnSb4S7 are among the most promising candidates to substitute CIGS as absorbers in solar cells, whereas Zn(S,O) is the most interesting candidate to replace CdS as a buffer layer. Using different characterization techniques such as UV-Vis-NIR spectroscopy, X-ray diffraction, atomic force microscopy, we demonstrate that all these materials exhibit structural, morphological and optical properties that are suitable for solar cell applications. The optical constants of the films were calculated from the analysis of the transmittance and reflectance data in the spectral range 300-1800 nm. The calculated band gap energies were 1.55, 1.45 and 3.08 eV for Cu2ZnSnS4, SnSb4S7 and Zn(S,O) films, respectively. X-ray diffraction patterns show that Cu2ZnSnS4, SnSb4S7 and Zn(S,O) films were polycrystalline in nature with a preferential growth along (112), (-21-3) and (002) planes, respectively.

Authors : Naoufel Khemiri*, A. Abdelnadher, I. Trabelsi, M. Kanzari
Affiliations : Université Tunis El Manar, Institut Prepatoire aux Etudes d?Ingénieurs El Manar (IPEIEM), Laboratoire de Photovoltaïque et Matériaux Semi-conducteurs 1002, Tunis, Tunisie.

Resume : Sodium doped Cu2ZnSnS4 (Cu2ZnSnS4: Na) thin films were successfully deposited by vacuum thermal evaporation method. Na concentration varied from 0 to 7 at.%. The effect of Na doping on the optical and electrical properties of the Cu2ZnSnS4 thin films was investigated. The optical, structural and electrical properties were studied using UV-Vis-NIR spectroscopy, X-ray diffraction (XRD) and AC impedance spectroscopy, respectively. The optical properties of Cu2ZnSnS4: Na thin films were calculated the transmittance and reflectance data. With the increase of the Na concentration, the band gap of the films increased gradually from 1.52 to 1.90 eV for sodium doping of 0-7%. The XRD analysis confirms the existence of Cu2ZnSnS4 phase with the preferential plane (112). Electrical properties have been investigated by AC impedance spectroscopy over a wide range of temperature up to 285 °C starting from room temperature in the frequency range 5 Hz?13 MHz. The complex impedance plots display one semicircle with equivalent circuit functions as typical parallel RC.

Authors : F. Lafond (1-2), Ph. Baranek (1-2), A. Postnikov (3)
Affiliations : (1) EDF R&D Department EFESE, EDF Lab Paris ? Saclay 3 rue Gaspard Monge F-91120 Palaiseau France, (2) Institut Photovoltaïque d?Ile de France (IPVF) 30 Route départementale 128 91120 Palaiseau France, (3) University of Lorraine Jean Bariol Institute Laboratoire de Chimie et Physique ? Approche Multi-échelle des Milieux Complexes (LCP-A2MC) 1 Bd Arago F-57078 Metz France

Resume : A promising technology in photovoltaics, aimed at improving the efficiency of solar cells, is the use of ?tandem? cells, which combine layers of different absorbers to achieve a wider spectral range of the sunlight conversion. Chalcopyrite-type Cu(In,Ga)(S,Se)2 mixed compounds can make a good complement to Si-based bottom cells insofar as their band gaps are sufficiently wide and their structural parameters adapted to that of silicon. The most obvious way of tuning to reach these objectives is to change the chalcopyrite?s chemical composition. However, the defects and dopants, either intentionally added or technically unavoidable, bring about unexpected disturbances. They may, e.g., displace the equilibria of crystalline phases, induce variations of the band gap, create "parasite" energy levels in the latter, and thus strongly influence the performance of materials. In this work, the effects of the chemical composition on the thermodynamic and optoelectronic properties of chalcopyrites are investigated at the first-principles level (within the density functional theory). Hybrid functional methods combined with supercell approach are used to study the structural and electronic properties of CuGaxIn1-x(SySe1-y)2 and CuxX1-xAB2 (with X = Na, K, Rb and Cs, A = Ga and In, and, B = S and Se). For a number of commensurable trial compositions, the equilibrium structural parameters, substitution energies, bang gap variations, as well as thermodynamic, dynamic and dielectric properties are determined and discussed, and conclusions concerning the prospective use of such mixed chalcopyrite cells in tandem with silicon are drawn.

Authors : Elif Peksu, Hakan Karaa?aç
Affiliations : Istanbul Technical University, Department of Physics, Istanbul, Turkey

Resume : Today, silicon is the leading material used in photovoltaic solar cells based on its various advantages such as cost efficiency, stability, high reliability, well balanced electrical and optical properties and well established processing techniques. As an alternative to single crystal based solar cells, a search for highly efficient low cost inorganic solar cells has occurred over the last two decades. Research has been focused on using various materials in solar cells, such as; amorphous Si, organic, II-VI and I-III-VI2 materials. Most of today?s thin-film solar cells are based on CuInSe2 (CIS), CuInxGa1-xSe2 (CIGS) and CdTe semiconductor materials, which are consisted of rare, expensive and toxic elements ( In, Te, Ga, and toxic Cd). Although these solar cells have reached reached the commercialization stage with high power conversion efficiencies, the requirement of using rare and expensive elements severely limit the mass production and deployment of them. Therefore, in recent years, a considerable research effort has been focused on development of new photovoltaic absorber materials that can embody earth-abundant, low-cost, and environmentally benign constituent elements for the the realization of higly efficient thin film solar cells at lower cost. Cu2ZnSnS4 (CZTS) compound has recently emerged as a potential photo-absorber material for thin-film solar cells due to their superior physical and chemical properties, which addresses all the issues related to the drawbacks of the aforementioned materials. In this study, high quality CZTS thin films for the realization of highly efficient low cost core-shell like Si-nanowire /CZTS structured solar cell were successfully obtained by using sol-gel technique. For the fabrication of stoichiometric high quality thin films, a production route based on a combination of spin-coating and thin film deposition by sol-gel processes was tested, which is believed to address many issues associated with difficulties reported so far in literature in obtaining highly stoichiometric CZTS thin films. The combination of an absorber thin film layer (CZTS) with a high absorption coefficient and the technology of silicon enables the realization of cost-effective high efficient solar cells. CZTS thin films were deposited on both n-Si-wafer and n-Si nanowires (NWs) fabricated through the electroless etching technique. Post-annealing process at 400-550 oC under inert gas flow was applied to the deposited films to obtain stoichiometric mono-phase CZTS thin films. The structural (XRD), optical (Reflection), morphological (AFM and SEM) and electrical properties (resistivity) of the CZTS thin films deposited on planer-Si was compared with that of deposited onto Si-NW arrays. Results have shown that there is significant modification in these properties following the incorporation of Si nanowires into CZTS thin film matrix for the construction of a third generation solar cell. In other words, it couples the efficiency of the 1st generation solar cells with the benefits of the 3rd generation solar cells. Silver dot contacts were evaporated by thermal evaporation to form the ohmic top-contacts of the solar cells structure by using a copper-shadow mask. As a back contact of the solar cells, the back side of the n-Si wafer was coated with a 150-nm thick silver thin film layer via thermal evaporation technique. The photovoltaic behaviors of the constructed prototype solar cells were examined under standard test conditions (AM 1.5G). From the recorded Current ? voltage characteristics of the solar cells under dark and light illumination conditions, the solar parameters including open-circuit voltage (Voc), short-circuit current (Isc), series-/-shunt resistances and power conversion efficiency were calculated.

Authors : Juran Kim, William Jo, Jayeong Kim, Seokhyun Yoon; Jun-Hyoung Sim, Kee-Jeong Yang, Dae-Hwan Kim, Jin-Kyu Kang; Chan-Wook Jeon
Affiliations : Department of Physics, and New and Renewable Energy Research Center (NREC), Ewha Womans University, Seoul, Korea 03760; Convergence Research Center for Solar Energy, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu, Korea 42988; School of Chemical Engineering, Yeungnam University, Gyeongsan, Korea 38541

Resume : Owing to its suitable electrical and optical properties, Cu2ZnSn(S,Se)4 (CZTSSe) is one of the most potent candidates to be substituted for Cu(In,Ga)Se2 (CIGS) material. Many research groups are trying to keep up with the performance of CIGS solar cells, however, the best power conversion efficiency (PCE) of CZTSSe solar cells (12.6%) is still below that of CIGS (22.6%). In this research, high-performing CZTSSe thin films were successfully fabricated and their optoelectrical properties on surfaces were examined using Kelvin probe force microscopy (KPFM) under diverse light-illumination conditions: red (640 nm), green (532 nm), and blue (405 nm) laser lights. Since the latest researches revealed that upward energy band barrier in grain boundaries (GBs) in high-efficient CZTSSe thin-film surfaces, we have suggested different conduction behaviors. Furthermore, the surface potential variation of the thin-film inside indicates downward potential bending, while that of the surface shows upward potential bending at the GBs. Under illumination, as the surface can be charged by photons, the energy band bending can be affected to be reduce barrier at the GBs. Though the GB barrier decreases, the conduction band of the material can be lower to help the carrier separation between the interfaces. Therefore, we can propone the presence of interfacial states altering the band alignment to assist the carrier transport in the solar cell devices.

Authors : Shahara Banu1, 2, Yunae Cho1, Ara Cho1, 2*
Affiliations : 1New and Renewable Energy Research Division, Photovoltaic Laboratory, Korea Institute of Energy Research (KIER), Daejeon, South Korea 2 University of Science and Technology (UST), Daejeon, South Korea

Resume : CuSbS2 (CAS) is an emerging absorber material for thin film solar cell applications with the chalcostibite structure, which shows high optical absorption co-efficient and favorable energy band gap. It also has low cost, low toxic and earth abundant constituent. Theoretical investigations shows that CAS has similar spectroscopic limited maximum efficiency (SLME) with CuInSe2 (CISe). Despite these promising characteristics, photovoltaic performance depends on some other factors such as the proper band alignment at the interfaces in particular with the buffer and the back contact, defect concentrations and carrier recombination. In this study, to fabricate non-toxic and low cost CuSbS2 solar cells, we used non-vacuum methods using hybrid inks through the sulfurization process. Sulfur ratio was varied during the sulfurization process to investigate the effect of sulfur flux on the structural and electrical properties of the CAS thin films while Cu/Sb were kept constant. In our study, we found that CAS solar cell performances was strongly correlated to the sulfur flux condition during the sulfurization. Therefore, optimum sulfur flux is needed to get better solar cell performance. The different sulfur ratios samples showed notable differences in surface morphology and interficial properties. The structural and electrical properties of CAS thin film solar cells with the change of sulfur ratios are analyzed by means of EDS, XRD, SEM analysis as well as external quantum efficiency, temperature dependance IV and capacitance-voltage measurements.

Authors : Yi Zhang, Li Wu, Jianjun Li, Xiaoru Liu
Affiliations : Nankai University

Resume : Zn(O,S) film is a promising low-cost and environment-friendly Cd-freebuffer layer for chalcopyrite and kesterite thin film solar cells. However,CZTSSe/CBD-Zn(O,S) solar cells with general thickness buffer layer (20?50 nm) have suffered from the poor interface performance,which may arise from the large positive conduction band offset(CBO) between CZTSSe and Zn(O,S), the low conductivity ofZn(O,S) layer, and the ZnO and Zn(OH)2 secondary phases inthe Zn(O,S) layer. In this study, the band fluctuation caused by ZnO secondary phase inZn(O,S) layer is identified as the main reason deteriorating the deviceperformance. By a concentrated ammonium etching and subsequent softannealing treatment, the detrimental ZnO and Zn(OH)2 secondary phasesare eliminated from the Zn(O,S) layer and the hetero-junction performanceis improved significantly. The Zn(O,S)/CZTSedevices showed quite small series resistance of 0.23Vcm2, andrelatively high fill factor of about 60%.Consequently, the power conversion efficiency ofthe Zn(O,S)/CZTSe solar cells was improved from 1.17% to a favorablevalue of 7.2%. Temperature dependent J?V properties reveal a defect levelassisted charge carrier transport mechanism across the Zn(O,S)/CZTSeinterface. These encouraging results imply that Zn(O,S) buffer layer is apromising substitution for toxic CdS in future manufacturing of highperformance thin film solar cells.

Authors : Y. Sánchez 1, M. Neuschitzer 1, S. Giraldo 1, M. Espíndola-Rodríguez 1, L. Calvo-Barrio 2,3, M. Placidi 1, V. Izquierdo-Roca 1, E. Saucedo 1
Affiliations : 1. Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, Spain 2. Centres Científics i Tecnològics de la Universitat de Barcelona (CCiTUB), Lluís Solé i Sabarís 1-3, 08028 Barcelona, Spain 3. IN2UB, Departament d?Electrònica, Universitat de Barcelona, 08028 Barcelona, Spain

Resume : The development of Cu2ZnSnSe4 (CZTSe) solar cells has been driven by the know-how accumulated for the related compound Cu(In,Ga)Se2 (CIGS). In terms of device structure, and similarly to CIGS, the CdS buffer is the preferred n-type semiconductor counterpart, in spite of the environmental concerns about this material (Cd toxicity), and the non-well known band alignment between CZTSe and CdS. In this comparative work, we explore different strategies to substitute/reduce CdS in the CZTSe technology including: Cd-free buffer layers (Zn(O,S), In2S3), bi-layer structures (ZnS/CdS, In2S3/CdS) and ternary compounds (ZnxCd1-xS). All the buffer layers were grown by chemical bath deposition onto Glass/Mo/CZTSe, with a careful optimization of the deposition parameters. Solar cell devices made with the different buffer layers were then fully characterized using illuminated J-V curves, EQE, CV-T and JV-T methods, and the different complex buffers using ICP-OES, SEM, XPS and T-R measurements. Our results show that Cd-free buffer layers perform systematically worse than CdS reference, most probably due to a large spike band-alignment. Nevertheless, when these layers are combined with small quantities of CdS in a bilayer structure or ternary compound, devices with efficiencies comparable to the CdS reference are obtained. The relevance and characteristics of this more complex buffer structures will be discussed for the improvement of CZTSe based solar cell properties with reduced Cd content.

Authors : Sergio Griraldo1, Eric García-Hemme2, Tariq jawhari3, Xavi Alcobé3; Lorenzo Calvo3,4, Victor Izquierdo-Roca1, Edgardo saucedo1
Affiliations : 1 - Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, Spain 2 - Dpto. De Estructura de la Materia, Física Térmica y Electrónica, Universidad Complutense de Madrid, 28040 Madrid, Spain. 3 - Centres Científics i Tecnològics (CCiTUB) de la Universitat de Barcelona, C/ Lluis Solé i Sabaris 1-3, 08028 Barcelona (SPAIN) 4 - Departament d'Enginyeria Electrònica i Biomèdica, IN2UB, Universitat de Barcelona, C/ Martí i Franqués 1, 08028 Barcelona (SPAIN)

Resume : The improvement of kesterite based solar cells efficiency over 13% requires to finding technological solutions for relevant problems, among them the optimization of the buffer/absorber interface. This optimization includes the reduction of the surface recombination, the passivation of the surface, and the correct band alignment between the absorber and the buffer layer. In this context, in Si based solar cells technologies, ion implantation has proved to be a powerful tool to modify the surface properties and improve the devices performance. However, in the Cu2ZnSnSe4 (CZTSe) case these methodologies have not been well implemented and studied yet. This work presents the physico-chemical characterization of CZTSe layers implanted with different ions, using two distinct implantation doses and energies to obtain box-like depth profiles, and including: Cl to induce a CZTSe n-type buried layer, Er to improve the transport properties, and Si/S to modify the CZTSe surface band-gap and in consequence the CZTSe/CdS band alignment. Afterwards, the implanted layers were treated under different soft annealing conditions, and characterized using multi-wavelength Raman spectroscopy, SEM, EDX, XRD, and XPS. We will show how the implantation process induces different grades of structural damage, regardless of the implanted ion, and can be partially recovered after a soft annealing. Finally, a correlation of the surface properties with the devices characteristics will be performed, in terms of the effects of the different implanted ions.

Authors : P. Vidal-Fuentes, F. Oliva, M. Neuschitzer, I. Becerril, Y. Sánchez, M. Espíndola-Rodríguez, V. Izquierdo-Roca, E. Saucedo
Affiliations : Catalonia Institute for Energy Research (IREC), Jardins de les dones de negre 1 2pl, 08930 Sant Adrià del Besòs, Barcelona, Spain

Resume : Scarcity of key elements (In, Ga, Te) in most relevant chalcogenide thin film photovoltaic technologies (CdTe and Cu(In,Ga)Se2), has driven the investigation in the photovoltaic field towards other materials, formed exclusively by more abundant elements. One of these new candidates is the chalcostibite compound (CuSbS2) as an alternative to CuIn(S,Se)2, due to the relative abundance (thus, lower cost) and nearly equal ionic radius of Sb in comparison to In. This makes very similar both compounds opening interesting options for the possible partial replacement of In by Sb in the thin film photovoltaic field. In this work, CuSbS2 thin films where prepared by a sequential process based in the reactive thermal annealing under S atmosphere of Cu-Sb evaporated precursors, exploring different annealing conditions. Both, Cu and Sb layers where deposited onto Mo coated SLG substrates by thermal evaporation. A systematic study was done by varying the Cu layer thickness in relation to the Sb one, then selecting the most promising Cu/Sb ratios and carrying out different sulfurization processes. The photovoltaic cells where completed by chemical bath deposition of CdS buffer and sputtered i-ZnO/ITO as top contact. The final devices SLG/Mo/CuSbS2/CdS/i-ZnO/ITO were characterized by X-ray fluorescence, X-ray diffraction, AM1.5 solar light simulator, external quantum efficiency, Raman spectroscopy and scanning electron microscopy, reporting a record device with 1.9% efficiency.

Authors : Ji-Eun Lee , and Han-Ki Kim*
Affiliations : School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Kyunggi-do 16419, Republic of Korea

Resume : The transparent electrodes of optoelectronic devices such as solar cells, organic light-emitting diode (OLEDs), require low resistive and high transmittance. We investigated InTiON/Ag-Ti/InTiON multilayer films with a low resistance and a high transmittance by co-sputtering system at room temperature. The electrical and optical properties of InTiON/Ag-Ti/InTiON multilayer films depended on the insertion of a nano-size Ag-Ti layer. The optimized InTiON/Ag-Ti/InTiON multilayer films that Ag-Ti thicknesses of 14 nm and InTiON thickness of 40 nm showed a sheet resistance of 4.13 Ohm/square, an average transmittance of 87.68 % at visible range (400 nm~800 nm) and work function of 4.69 eV. In addition, the InTiON/Ag-Ti/InTiON multilayer films had a very smooth surface morphology without surface defects. Furthermore, we examined the mechanical properties. The InTiON/Ag-Ti/InTiON multilayer films had a constant resistance change(ᅀR/R0) within an outer bending radius of 6 mm. All properties are better than Sn-doped In2O3(ITO) film that have mainly been used as transparent electrodes. Consequently, InTiON/Ag-Ti/InTiON multilayer films are promising alternative to ITO films for transparent electrodes.

Authors : Mohamed H. Sayed, Erika V. C. Robert, Phillip J. Dale, Levent Gütay
Affiliations : Laboratory for Chalcogenide Photovoltaics, Department of Energy and Semiconductor Research, Institute of Physics, University of Oldenburg, 26111 Oldenburg, Germany; Physics and Materials Science Research Unit, University of Luxembourg, L-4422 Belvaux, Luxembourg

Resume : Various chemical deposition techniques have been widely employed for the fabrication of chalcogenide thin films including spin coating, doctor blading and chemical spray pyrolysis. Chemical spray pyrolysis deposition technique offers a lot of advantages for the processing of thin films due to its simplicity, cost effectiveness, extremely large choice of precursors and its application for large area deposition. The processing conditions such as spray parameters (e.g. type of salts, solvent, deposition temperature etc.) and annealing parameters (e.g. annealing temperature, annealing atmosphere, amount of chalcogen etc.) play a critical role and have a significant impact on the properties of the chalcogenide thin films and hence the device performance. In this study, chemical spray pyrolysis was employed to prepare environmental benign and earth abundant ternary Cu2SnS3 thin films. The Cu2SnS3 layers were processed under ambient air by using only water as a solvent. This deposition step was followed by an annealing step in chalcogen atmosphere. The influence of different processing conditions on the structural and morphological properties of the prepared thin films were investigated using SEM, XRD and Raman spectroscopy. Cu2SnS3 based thin film solar cells were fabricated and the resulting efficiencies are discussed in the context of the varied parameters. Highest reached efficiencies were in the neighborhood of 2%.

Authors : M. Marzougi1, M. Ben Rabeh*,1 and M. Kanzari2
Affiliations : 1Université de Tunis El Manar, Ecole Nationale d?Ingénieurs de Tunis, Laboratoire de Photovoltaïques et Matériaux Semi-conducteurs, BP 37 Le Bélvédère 1002, Tunis, Tunisie; 2Université de Tunis, Institut préparatoire aux études d?ingénieurs de Tunis, Laboratoire de Photovoltaïques et Matériaux Semi-conducteurs, BP 37 Le Bélvédère 1002, Tunis, Tunisie.

Resume : Quaternary chalcogenide Cu2ZnSnS4 (CZTS) compound, a potential material for application as absorber layer in thin film solar cells, is successfully synthesized by direct melting of the constituent elements taken in stoichiometry compositions. Alkali element Na was incorporated into CZTS thin films synthesized by thermal evaporation method, in order to further improve the structural and optical properties. X-Ray diffraction, Raman spectroscopy and optical spectrophotometry were used to characterise the phase purity and optical properties. It showed that the diffusion of Na ions is uniform in the films after annealing. The XRD analysis showed that the films possess polycrystalline structure and the preferred orientation being in (112) direction. Na ions incorporation in CFTS thin films could improve the cristallinity, the graine size and the absorption coefficient. For CZTS: Na 5%, optical results revealed an direct optical band gap of 1.6 eV with p-type conductivity. Keywords: CZTS; Semiconductor; Thin films; Thermal evaporation; Na-doped; Optical properties.

Authors : Ara Cho1,2*, Shahara Banu1,2, SeJin Ahn1, Jae Ho Yun1, Jihye Gwak1, Seung Kyu Ahn1, Young-Joo Eo1, Jun Sik Cho1, Ju Hyung Park1, Jin Su Yoo1, Kihwan Kim1
Affiliations : 1 New and Renewable Energy Research Division, Photovoltaic Laboratory, Korea Institute of Energy Research (KIER), Daejeon, Republic of Korea 2 Renewable Energy Engineering, University of Science and Technology (UST), Daejeon, Republic of Korea

Resume : In recent years, Cu-Sn-S system has been investigated in several research areas and they have several phases such as Cu2SnS3, Cu2Sn3S7, Cu4SnS4, etc. Among them, Cu2SnS3 (CTS) has been spotlighted as a potential absorber material for thin film solar cell applications because of its high absorption coefficient (> 104 cm-1) and lack of Fermi level pinning. It also has low cost, low toxic and earth abundant constituent. Until now, 4.63% conversion efficiency was reported by 2 step process with vacuum processed precursor. In this study, we used cost-effective non-vacuum process using hybrid inks through the sulfurization to obtain CTS thin films. The structural and electrical properties of CTS thin films were analyzed by EDS, XRD, SEM, TEM and the fabricated CTS solar cell showed 2.943% conversion efficiency. Finally, other photovoltaic performances (such as EQE, IVT and CV) will be presented.

Authors : M.A. Jafarov, E.F. Nasirov, S.A.Jahangirova,
Affiliations : Baku State University, Baku, Azerbaijan,

Resume : II?VI semiconducting materials have emerged with high potential in applications for fabricating optical devices which include short wavelength emitting laser diodes and light emitting diodes operating in the blue region due to their wide direct band gap properties. It is a subject of great interest in regard to chalcogenide-based semiconducting materials which are used in making devices like photodetectors and photovoltaic devices. ZnS layers were electrodeposited from an aqueous electrolyte containing 0.3 M ZnCl2 and 0.03 M (NH4)2S2O3 in 800 mL of de-ionized water. Electropurification of the ZnCl2 was carried out for 48 h prior to the addition of (NH4)2S2O3 in order to remove any possible impurity ions present in the solution. Finally, the pH of the electrolyte containing both precursors was adjusted to 3.00 ± 0.02. The temperature of the electrolyte was 30°C. Uniform and transparent ZnS layers were cathodically deposited on cleaned glass/?TO substrates using a simple two-electrode deposition system at a cathodic potential of 1550 mV established using a cyclic voltammogram. The deposited layers using an average deposition current density of ~65 ?A?cm?2 and deposition time of 60 min have thickness of ~150 nm. These were then annealed in air at 350 °C for 10 min. Prior to the deposition of Cu2ZnSnS4, the glass/?TO/ZnS substrates were cleaned with methanol and deionised water. The deposition of Cu2ZnSnS4 layers was also done using a two-electrode system at a cathodic deposition potential of 1450 mV also established using a cyclic voltammogram. The Cu2ZnSnS4 deposited on glass/?TO had a thickness ~300 nm while that deposited on glass/?TO/ZnS had a thickness ~150 nm. This therefore brings the total thickness of the ZnS/Cu2ZnSnS4 bi-layer to ~250 nm comparable to the ~300 nm of Cu2ZnSnS4 grown on glass/?TO. The CdTe deposition electrolyte contained 1 M CdSO4 (99.0%) and 1 mM TeO2 (99.999%) in 800 mL of de-ionized water. To do this, a cyclic voltammogram was recorded using the two-electrode system, to determine the reduction potential of Cd2 . The TeO2 was first dissolved in H2SO4 and then added into the bath after the electro-purification of CdSO4, and the pH of the electrolyte adjusted to 2.00 ± 0.02. Optical absorption and transmittance measurements on the various deposited thin film layers were carried. The result of using ZnS as the buffer/window layer is directly reflected in the improved high short-circuit current density (Jsc) as well as improved open-circuit voltage (Voc), fill factor (FF) and ultimately, the conversion efficiency (?) of the 3-layer device, compared to the device as shown in Figure 3a,b. The measured Voc values of 640 mV and 630 mV are not as large as expected and are indicative of the presence of leakage paths which is also evident in the low fill factor values obtained. However, to ensure that the observed high Jsc values are genuine, the diodes producing them were isolated by carefully removing the CdTe material around them and repeating the I-V measurements. It is therefore possible in these solar cells for photons with energy lower than the energy bandgap of CdTe to create useful electron-hole pairs that contribute to photo-generated current. These capacitance values suggest that the glass/ITO/ZnS/Cu2ZnSnS4/CdTe/In solar cell has a wider depletion region compared to the glass/?TO/ Cu2ZnSnS4/CdTe/?n 2-layer solar cell. Both figures display a slow response of 1/C2 with applied reverse bias voltage.

Authors : Kihwan Kim, Jin Su Yoo, Jun-Sik Cho, Jihye Gwak, Seung Kyu Ahn, Young-Joo Eo, Joo Hyung Park, Sejin Ahn, Ara Cho, In Young Jeong, Dong Hyeop Shin, and Jae Ho Yun
Affiliations : Photovoltaic Laboratory, Korea Institute of Energy Research, Daejeon 34129, Republic of Korea

Resume : In this work, Ag incorporation into wide-bandgap CIGS solar cells (Eg ? 1.4 eV) was chosen to a way to improve device performance. CIGS (or ACIGS) absorber layers were prepared with a conventional three-stage process; Ag and Cu were simultaneously evaporated for the ACIGS absorbers. The Ag alloying turned out to cause enhanced recrystallization and bandgap widening. By examining various compositions of Ag/(Ag Cu) and Ga/(Ga In) ratios, favorable compositions for bandgaps (Eg ? 1.4 eV) were explored. The results indicated specific compositions according to bandgaps were effective to improve efficiency along with Voc; however, Ag alloying, in general, helped to enhance Voc in wide bandgap CIGS solar cells. With this work, 17.5%-, 14.8%- and 12.2%-efficient ACIGS solar cells were demonstrated from absorbers? Eg = 1.4 eV, 1.5 eV, and 1.6 eV, respectively. We also examined potential of chalcopyrite-based tandem cells. Various kinds of chalcopyrite-based tandem (two-junction and two-terminal) solar cells were demonstrated, where beam-splitting mechanical tandem, mechanically stacked tandem, and monolithic tandem designs were included. The beam-splitting tandem cells employed a beam-splitting mirror, and thus, the two subcells fairly shared the incident photons. With this configuration, achievable device performance of the tandem cells, approximately 14% with Voc = 1.429 V, could be determined. The mechanically stacked tandem and monolithic tandem cells are considered to exhibit more potential for practical use; however, the device performance from those types turned out to be inferior to that of the beam-splitting tandem cell by a significant margin. Even though light management (i.e., current matching) and securing thermal robustness of the bottom subcell proved to be challenging, viable operations between the two subcells could be achieved. The more details will be discussed in the conference.

Authors : C. Dogru*, M. Terlemezoglu, Ö. Bayrakl?, H.H. Güllü and M. Parlak
Affiliations : Department of Physics, Middle East Technical University (METU), Ankara 06800, Turkey Center for Solar Energy Research and Applications (GUNAM), METU, Ankara 06800, Turkey Central Laboratory, Middle East Technical University (METU), Ankara 06800, Turkey

Resume : CdZnTe thin film is a promising material for photovoltaic applications since it has a tunable direct band gap properties and it is used as an absorber layer due to its high intrinsic mobility, high absorption coefficient, and high atomic number. CdZnTe thin films are deposited by techniques like physical evaporation, chemical vapor deposition, sputtering, close-spaced sublimation (CSS), molecular beam epitaxy and electrodeposition. However, the physical evaporation technique has many advantages such as low cost, good reproducibility and environmentally friendly. In this study, the effect of substrate temperature on CdZnTe thin films deposited by thermal evaporation have been investigated. Optical properties is studied using UV-VIS spectrophotometer. Crystallographic properties and chemical compositions are analyzed by X-ray Diffraction (XRD) and Energy Dispersive X-ray analysis (EDX). In addition, Morphological properties are examined using Scanning Electron Microscopy (SEM). Finally, the rectification properties using I-V measurements and capacitance-voltage (C-V) measurements have been also performed. As a result, an optimization process has been carried out for substrate temperature of CdZnTe thin films.

Authors : Elisa Artegiani, Daniele Menossi, Mauro Leoncini, Marco Cavallini, Alessandro Romeo
Affiliations : LAPS-Laboratory for Photovoltaics and Solid State Physics, Department of Computer Science, University of Verona, Verona, 37134, Italy.

Resume : CdTe thin film solar cells are the most successful thin film photovoltaic devices in terms of production yield, as attested by the remarkable market performance. Typically the junction is made with n-CdS which, however, absorbs some of the light spectrum due to its band-gap at 2.4 eV. So, in order to exceed 20% efficiency it is crucial to increase the spectrum of light convertible by the absorber. One possible alternative buffer that, at the same time, allows a larger transparency and a proper band alignment with CdTe is Magnesium Zinc Oxide (MZO). We have already shown that Magnesium Zinc Oxide (MZO), deposited by RF magnetron sputtering, can be used as an alternative high resistance transparent (HRT) layer for CdS/CdTe thin film solar cells. The application of a MZO layer on Indium Tin Oxide (ITO) has delivered an increase of more than 2 % in absolute efficiency compared to only ITO contacted cells and however they result in higher efficiency also compared to ZnO/ITO-contacted devices . MZO results in higher transparency not only compared to CdS but also to ZnO, and, more important, it gives a better band alignment with CdS: MZO band gap can be tuned changing the substrate temperature during the deposition. This results in an increase in efficiency, in particular in terms of current density as well as fill factor. An analysis of MZO properties and their electrical behaviour with CdS will be presented, moreover CdTe devices without CdS buffer layer will be shown and characterized.

Authors : Elisa Artegiani, Daniele Menossi, Alessia Delle Donne*, Simona Binetti*, Mauro Leoncini, Marco Cavallini, Alessandro Romeo
Affiliations : LAPS-Laboratory for Photovoltaics and Solid State Physics, Department of Computer Science, University of Verona, Verona, 37134, Italy. * *MIBSOLAR CENTER and Department of Materials Science, University of Milano-Bicocca, I-20125, Italy

Resume : One of the major issues for CdTe solar cells is the activation treatment step, generally done by deposition of CdCl2 and subsequent annealing of the stacks in air. It is very well known that without this process-step no high efficiency devices can be obtained. However CdCl2 is carcinogenic and highly soluble in water, making it quite dangerous during the fabrication process, for this reason MgCl2 has been recently introduced. In this work we have studied devices (exceeding 14% efficiency) prepared by low-substrate temperature CdTe deposition and activated with MgCl2 treatment deposited by methanol solution. It is very interesting the fact that MgCl2 treated cells perform high current density without post deposition annealing of the back contact; in our process this step is generally needed for efficiencies above 10% because it is crucial to diffuse copper into the CdTe bulk to improve the carrier concentration. For this reason finished cells have been analyzed in terms of deep level transient spectroscopy and photoluminescence and the results compared with our previous work where cells were analyzed by capacitance voltage and drive level capacitance profiling. Moreover quantum efficiency has been also used for comparison. The effect of magnesium in the CdTe activation treatment will be presented and discussed.

Authors : Mauro Leoncini, Elisa Artegiani, Marco Cavallini, Alessandro Romeo
Affiliations : LAPS-Laboratory for Photovoltaics and Solid State Physics, Department of Computer Science, University of Verona, Verona, 37134, Italy.

Resume : One of the several keys to increase efficiency of CdTe solar cells is the improvement of light transmission of the buffer layer. CdS, with a band gap of 2.4 eV, results to be opaque at low wavelength regions, for this reason, in order to gain in blue and UV light, extremely thin CdS has to be applied, otherwise the buffer has to be substituted with wide band gap materials. On the other hand, by thinning the CdS layer we give place to pinholes and shunts also due to the wide intermixing of CdTe and CdS layers and subsequently consumption of CdS. For this reason we have applied and compared different CdS post-deposition treatments in order to stabilize the layer and avoid post treatment shunts. . Treatments at temperatures above 500 °C in argon and chlorine atmosphere on 80 nm thick CdS have performed higher stability compared to not treated CdS, resulting in improved performance with a 10% increased current density and a 5% increased open circuit voltage and fill factor, with an overall efficiency above 14%.. Moreover light transmission of the CdS treated layers from 300 to 450 nm is increased of about 10%. In this paper a study of treated thin CdS with novel recrystallization treatments will be presented and analysed by means of X-ray diffraction, atomic force microscopy, micro-Raman. Finished CdTe devices made on thin CdS will be shown and their performance characterized in terms of current-voltage, capacitance-voltage and quantum efficiency

Authors : S. Lopez-Marino, M. Gansukh, S. Engberg, F. Martinho, A. Hajijafarassar, N. C. Schjødt, O. Hansen, S. Canulescu, J. Schou
Affiliations : S. Lopez-Marino; A. Hajijafarassar; O. Hansen DTU Nanotech, Technical University of Denmark, Ørsteds Plads Building 344-345B, DK-2800 Kgs. Lyngby, Denmark M. Gansukh; S. Engberg; F. Martinho; S. Canulescu; J. Schou DTU Fotonik, Technical University of Denmark, Frederiksborgvej 399, Risø Campus Byuilding 130, DK-4000 Roskilde, Denmark N. C. Schjødt Haldor Topsoe A/S, Haldor Topsøes Allé 1 DK-2800 Kgs. Lyngby

Resume : Earth abundant and low toxicity CZTSSe based photovoltaics offers potential of covering future energy demands in the TW level. Currently, a strong voltage deficit seems to hinder its desired marketability. Multiple charged defects and lateral composition inhomogeneities resulting in band tailing are identified as triggers. Among the most detrimental defects, the Sn related ones stand out due to their deep electronic nature within the CZTSSe bandgap and easy formation linked to highly volatile SnxS(e)y species. Recently, addition of Ge resulted in a more efficient balance of the Sn presence during the annealing, changing the CZTSSe reaction mechanism from tri to bi-molecular and improving devices performance. Thus, deeper understanding of the absorber synthesis seems clearly still necessary. Oxide precursor based routes have not been as much explored as well as established Cu-Zn-Sn w/o chalcogen based ones. An intrinsic more stable nature of oxide containing species when compared to their chalcogen counterparts, in particular SnOy species, could shed more light on current absorber limitations. In this work, CZTO and CZTS precursors are prepared using single quaternary targets and PLD, which offers the possibility of fine composition tuning. Multiple reactive annealing variations are investigated by analysing absorber microstructure, composition and optoelectronic features of absorbers and solar cells. Results obtained with SEM, Raman, XRD, EDX, EQE, J-V and PL will be reported

Authors : Alireza Hajijafarassar, Andrea Crovetto, Thomas Pedersen, Simón López Mariño, Jørgen Schou, Ole Hansen
Affiliations : DTU Nanotech, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark; DTU Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark; DTU Danchip, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark; DTU Nanotech, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark; DTU Fotonik, Technical University of Denmark, DK-4000 Roskilde, Denmark; DTU Nanotech, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark;

Resume : Silicon-based tandem solar cells are one of the promising approaches to increase the efficiency of photovoltaic devices beyond the single-material detailed-balance limit while benefiting from the well-established technology of the conventional silicon solar cells. However, an inexpensive, earth abundant, stable and non-toxic material with a proper band-gap has not yet been identified for the upper cell. We suggest CZTS, a chalcogenide with a bandgap of 1.5 eV, as a promising material for the top absorber. The presence of a high temperature (500-600 oC) annealing process in a sulfur-rich atmosphere during synthesis of CZTS makes monolithic integration challenging. Thus, a barrier layer is necessary between the two absorbers. This barrier layer must act effectively as a diffusion barrier and allow exchange of charge carriers between the two cells. In this research, we investigated various layers of TiN and TiO2, deposited with (PE)ALD, as potential materials for the barrier layer. For this purpose, simple stacks of Cu/(~25nm)TiN/Si and Cu/(~80nm)TiO2/Si were prepared, and systematic isothermal annealing experiments at 550 oC in vacuum were performed to simulate the CZTS synthesis conditions. Subsequently, the samples were characterized by XRD, XPS, and sheet resistance measurements to evaluate the effectiveness of TiN and TiO2 as diffusion barriers. Furthermore, optical simulations were done to determine the optical performance of these layers in the near infrared spectrum.

Authors : Daniel Hirsch, Olaf Zywitzki, Thomas Modes, Torsten Kopte, Bettina Späth, Bastian Siepchen, Christian Drost, Krishnakumar Velappan, Christoph Metzner
Affiliations : Fraunhofer FEP; Fraunhofer FEP; Fraunhofer FEP; Fraunhofer FEP; CTF Solar GmbH; CTF Solar GmbH; CTF Solar GmbH; CTF Solar GmbH; Fraunhofer FEP

Resume : One strategy to enhance the photocurrent of a CdTe solar cell is increasing the transparency of the CdS window layer. Using an alternative window material with a higher bandgap is a common method. In this study we reduce the thickness of the CdS layer in order to increase the photocurrent and we passivate the front-contact by means of an oxygen plasma in the CSS-process to maintain the open circuit voltage. In general thin CdS-layers agglomerate during the chlorine activation which causes a defect-rich interface between CdTe and SnO2. Those ?Pinholes? reduce the open circuit voltage dramatically due to charge carrier recombination. We show that the use of the oxygen Plasma Enhanced CSS-technology during the CdS-deposition leads to less recombination at this interface. EBIC-measurements show a bright signal at the front contact for cells with oxygen plasma treated CdS layer. Indeed, the measured open circuit voltage increases with raising oxygen flow into the plasma. It is assumed that oxygen can passivate the interface between CdTe and the front contact and counteracts the loss in voltage. Simultaneously, the photocurrent stays high, which results in an improvement of the cell efficiency. Consequently, the EQE signal shows an almost rectangular behavior. This new PECSS-technology for depositing CdS affords the opportunity to produce CdTe solar cells with a photocurrent of more than 27 mA/cm² and an open circuit voltage above 800 mV.

Authors : Torsten Schwarz,1 Alex Redinger,2 Susanne Siebentritt,2 Baptiste Gault,1 Dierk Raabe1, and P.Choi3
Affiliations : 1. Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany. 2. Laboratory for Photovoltaics, University of Luxembourg, Belvaux, Luxembourg. 3. Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea.

Resume : Cu2ZnSnS(e)4 (CZTSSe) is an emerging material as absorber for solar cells, which have recently attracted substantial interest. Currently, the record efficiency of CZTSSe solar cells is 13.6%. However, this value is still far below the 22.9% record efficiency of Cu(In,Ga)S(e)2 and further development is needed. Apart from secondary phases, grain boundaries (GB) and other extended defects, such as stacking faults and dislocations, can also significantly affect the electrical and optical properties of the absorbers. Detrimental deep traps within the band gap (non-radiative recombination centers) may be associated with them. Hence, a better understanding of the chemistry of structural defects is desired. In a previous study, we observed by atom probe tomography (APT) for a Cu-poor CZTSe low temperature precursor that the GBs are Cu-enriched and no impurity segregation. For the annealed precursor the GBs were decorated by Na and K impurities and showed (almost) no change in the concentration of the matrix elements. Here, we investigate Cu-poor CZTSe thin films by APT. Thanks to a doubled atom detection efficiency we detect a larger variety of chemical fluctuations. Na impurities can also decorate the mainly Cu-enriched GBs in the precursor. For the absorber, we also clearly observe fluctuations of the matrix elements for the mainly by Na decorated GBs. The fluctuations vary from one GB to another. We will discuss possible effects of the observed phenomena on the cell performance.

Authors : Neha Bansal1, Anies Mutiari1,2,3, Clare Conboy4, Martin Artner5, Josef Weber5, Veronika Mayer6, Juergen Roth6, Matthias Weil3, Rachmat Adhi Wibowo1*
Affiliations : (1) AIT Austrian Institute of Technology GmbH, Center for Energy, Photovoltaic Systems, Giefinggasse 2, 1210 Vienna, Austria (2) Center for Materials and Technical Products, Ministry of Industry Republic of Indonesia, Jl. Sangkuriang 14, Bandung 40135, Indonesia (3) Institute for Chemical Technologies and Analytics, Technical University of Vienna, Getreidemarkt 9/164-SC, 1060 Vienna, Austria (4) Printed Electronics Ltd. Hedging Lane,Tamworth, Staffordshure, UK, B77 5HH (5) Frimeco Produktions GmbH, Aspernbrückengasse 2, 1020 Vienna, Austria (6) PMT-Jetmill GmbH, Industriepark 1, 8773 Kammern im Liesingtal, Austria

Resume : The Cu-poor and Zn-rich kesterite Cu2ZnSnS4 (CZTS) thin films were sintered on Molybdenum-coated glass substrates from CZTS-based ink. The main identified challenge to prepare photovoltaic device-relevant CZTS film from CZTS-based ink was to produce dense CZTS film with large and uniform CZTS grain size. The present work was deliberately aimed to study the microstructural evolution of CZTS film as a function of sintering parameter and type of sintering additive. Based on structural phase analysis, it was found that sintering of CZTS films from 450 °C to 620 °C in low vacuum quartz tube furnace leads to severe CZTS decomposition into its binary phase constituents. On a contrary, sintering of CZTS film under 1 atm. nitrogen gas and sulfur vapor successfully prevents CZTS from detrimental decomposition. The tetragonal kesterite phase of CZTS film was stable up to sintering temperature of 620 °C, just below the softening point of glass substrate. Nevertheless, the effective densification and recrystallisation of CZTS film from CZTS-based ink takes place only when alkalis of Na and /or K-sources were added into CZTS ink, as revealed by electron microscopy. Na-acetate gradual addition up to 10 mol. % enhances as-sintered CZTS film grain size as well as suppresses CuS secondary phase growth. On the other hand, the presence of K-carbonate as small as 2 mol. % dramatically densifies and recrystallize CZTS film with much less of CuS secondary phase. This suggests K superiority over Na in mediating microstructural evolution of CZTS film. The alkalis in the sintering of CZTS film may act as sintering fluxes that densifies CZTS films, leading to the tetrahedrally-shaped CZTS grains.

Authors : N. Spalatu, J. Hiie and M. Krunks
Affiliations : Laboratory of Thin Film Chemical Technologies, Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : CdCl2 activation treatment is an indispensable technique for producing efficient CdTe solar cells. However, the process is complicated due to presence of oxygen, resulting in segregation of CdCl2·2CdO residual phases on the GBs and high Cl concentration (over 1019 cm?3) in the CdTe lattice. High concentration of residuals strongly limits the density and mobility of holes in CdTe by self-compensation and cause hygroscopicity of the cells. In a previous study we proved that controlled thermal annealing in closed evacuated ampoules and in a process tube at 500 - 600 oC is feasible tool to remove residual phases from CdTe:Cl films grown by close spaced sublimation (CSS), and as a result significant improvement in film optoelectronic properties has been observed. Here we report a systematic investigation of the impact of those processing steps on properties of CSS CdTe/CdS cells by analysing I-V and EQE measurement results. An improvement in conversion efficiency from 11% characteristic of the cell with classical CdCl2 activation in air, to 14 % for annealed cell was recorded. This enhancement is mainly caused by improvements in VOC from 780 mV to 820 mV, and FF from 60 to 72 %. The EQE showed enhanced response in the long-wavelength region, implying better collection efficiency in CdTe. This is in good agreement with increased p-type carrier concentration previously observed in annealed CdTe:Cl films. C-V measurements are in progress to determine the doping concentration.

Authors : Stefano Pisoni, Thierry Moser, Thomas Feurer, Marc Padrutt, Shiro Nishiwaki, Ayodhya N. Tiwari, Stephan Buecheler
Affiliations : Laboratory for Thin Films and Photovoltaics, Empa ? Swiss Federal Laboratories for Material Science and Technology, Ueberlandstrasse 129, Duebendorf, Switzerland

Resume : Photovoltaic conversion efficiency of single-junction solar cells is intrinsically limited by Shock-ley-Queisser limit, with significant energy losses due to thermalization. A viable way to overcome this limit is to develop multijunction solar cells. Perovskite solar cell represents an ideal candidate for wide band gap top cell in combination with low band gap Cu(In,Ga)Se2 (CIGS) bottom cell. In the last years, we observed a significant improvement in perovskite/CIGS tandem performances with efficiencies approaching 24%. Those achievements refer to devices grown on glass substrates. The possibility to develop highly efficient perovskite and CIGS solar cells on flexible substrates lays the foundations to lightweight flexible tandem devices by high-throughput roll-to-roll manufacturing. Here, we report a multi-stage perovskite deposition approach by combining vacuum- and solution-based techniques. We develop NIR-transparent perovskite solar cells with efficiency approaching 14%, grown on a flexible front sheet which is used for flexible CIGS module encapsulation. Eventually, we demonstrate flexible perovskite/CIGS tandem with 19.5% efficiency measured in four-terminal configuration. Our work represents a step forward towards new concepts of flexible thin film tandem devices, and provides insights into engineering of PbI2 growth, which underlines the versatility of hybrid vacuum-solution perovskite deposition.

Authors : Ali Samieipour(1), Christian Neubauer(1,2), Souhaib Oueslati1(,2), Valdek Mikli(1), Dieter Meissner(1,2)
Affiliations : (1)Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia (2)crystalsol OÜ, Akademia tee 15 a, 12618 Tallinn, Estonia

Resume : To our best knowledge as yet results of ageing experiments of Kesterite Solar Cells have not been published. However, at least in crystalsol detailed investigations of monograin-based solar cells have been performed proving very different stabilities depending on the composition of the Kesterite. Although completely stable solar cells even without any sealing have been demonstrated internally, in this paper we report on ageing effects observed for internally called ?highly unstable? Cu2ZnSnS4 solar cells aged for 8.5 and 95.5 hours at 105 °C hours in air. During these times the initial efficiency of 6.4 % decreased by 84 % and another 93 %, respectively. The photocurrent generation in cross sections of the CZTS / CdS / ZnO active junctions was investigated by electron beam induced current measurements in an electron microscope. Besides a pronounced decrease of the EBIC signal also a shift of the signal much further into the Kesterite material was observed, which is explained by diffusion of a recombination center from the surface into the CZTS (probably out-diffusion of Cu or Sn atoms into the contacting CdS buffer layer). After the main signal observed close to the interface has vanished, a minimum in the EBIC signal in a depth of around 400 nm becomes visible, which can be explained by the depth at which the Fermi level crosses the position of a main recombination center, maximizing the loss of charge carriers in this depth.

Authors : Khaoula Nefzi, Adel Rabhi, Mounir Kanzari
Affiliations : Université de Tunis El Manar (UTM), Ecole Nationale d'Ingénieurs de Tunis Laboratoire de Photovoltaïque et Matériaux Semi-conducteurs (LPMS), BP 37 le Belvédère 1002, Tunis, Tunisie Faculté des Sciences de Bizerte, Département de Physique, 7021 Jarzouna, Bizerte, Tunisie Université de Tunis, IPEITunis Montfleury, Tunis, Tunisie

Resume : Thin films of Cu3SbS3 were deposited on non-heated glass substrates by vacuum thermal evaporation technique. The as-deposited Cu3SbS3 thin films were placed into programmable furnace (type Nabertherm-Germany) and annealed at different annealing temperatures (Ta). The as-deposited films were annealed in air atmosphere for 2 h in the temperature range 100 - 350°C. Their structure and chemical compositions are studied by X-ray diffraction (XRD), dispersive X-ray spectroscopy analysis (EDAX) and scanning electron microscopy (SEM). The variations of the microstructural parameters, such as crystallite size (D), dislocation density (?) and strain (?), with annealing temperature were investigated. Optical measurements show that all the Cu3SbS3 thin films have relatively high absorption coefficient between 105 and 106 cm-1 in the spectral range 1.4 ? 2.4 eV with p-type conductivity. Electrical properties have been investigated by ac impedance spectroscopy over a wide range of temperature up to 593 K starting from room temperature in the frequency range 5Hz?13 MHz. The complex impedance plots display one semicircle with equivalent circuit functions as typical parallel RC. By increasing the temperature, the Impedance spectroscopy analysis shows that the resistance decreases from 105 ? to 103 ?. In addition, the analysis of conductivity indicates that both AC and DC conductivities of materials increase with increasing temperature. The activation energy values calculated from DC conductivity and angular frequency relaxation are almost identical, indicating that the conduction mechanism was thermally activated and was assured by hopping between localized states.

Authors : Christian Neubauer1,2, Souhaib Oueslati1,2, Ardi Loot3, Jaan Aarik3, Ali Samieipour1, Kaia Ernits2, Dieter Meissner1,2
Affiliations : 1Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; 2crystalsol OÜ, Academia tee 15a, 12618 Tallinn, Estonia; 3Institute of Physics, University of Tartu, W. Ostwaldi tn. 1, 50411 Tartu, Estonia

Resume : Currently material solar cell efficiencies above 12 % are obtained for Kesterite monograin solar cells in crystalsol. Although the monograin powder particles are surprisingly homogenous with a standard deviation of less than 14%, all grains show pronounced variations in photocurrent (PC) and in luminescence intensity for both, electrical as well as light generation. Therefore highly spatially resolved photocurrent measurements of single grains can directly be correlated to photoluminescence (PL) and electroluminescence (EL) measurements. Whereas PL and PC measurements reveal regions of high photovoltaic activity, corresponding EL measurements show the opposite effect. Regions of high PL and PC activity show a low EL signal and vice versa. These results can be understood by assuming different barrier heights at the CZTS/CdS interface as will be shown in the paper. This investigation method allows the characterization of powder materials even before solar cells are prepared, avoiding variations due to the differences in solar cell making.

Authors : Adrien Rivalland (1), Nicolas Barreau(1), Ludovic Arzel (1), Sébastien Dubois (2)
Affiliations : (1) Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS-UMR 6502, Nantes ; (2) Université Grenoble-Alpes CEA/LITEN, INES, 50 Avenue du Lac Léman, 73375 Le Bourget-du-Lac, France

Resume : To surpass 30% conversion efficiency, tandem solar cells with crystalline silicon (c-Si) sub-cell seem to be one of the most promising architectures regarding the theoretical efficiency. Furthermore a monolithic two-terminal approach with wide bandgap thin films does not require a significant modification of the solar modules fabrication. CuIn1-xGaxSe2 is a promising candidate thanks to its good efficiency around 22% for x = 0.3 (single junction). Nevertheless its bandgap is lower than the optimum value for a top-cell, which is around 1.7 eV. Such a value corresponds to x = 1. However the CuGaSe2 record efficiency is too low (? = 11.9%) for a tandem integration with c-Si. Hence, an optimization is needed to improve CuGaSe2 co-evaporation process and adapt it for the substrates used on tandem applications. We have chosen a CuPRO (Copper Poor / Copper Rich / Copper Off) process to grow CuGaSe2 absorbers on SLG/Molybdenum. During the process, the second stage allows the formation of large grains and the third reduces the defects concentration. In our work, we focus on the morphological and electrical impacts of the copper content during the first stage. However, since the CuGaSe2 growth depends on the nature of the substrate, further work is required in order to adapt the CuPRO process for silicon and oxides (SiO2, ZnO ?) substrates, used for tandem solar cells.

Authors : M.A. Jafarov, E.F. Nasirov, S.A.Jahangirova,
Affiliations : Baku State University, Baku, Azerbaijan,

Resume : The semiconductor compound CdZn(S,Se) (CZSSe) is considered as one of the ideal photovoltaic absorber layer materials for low-cost thin film solar cells, since CZSSe has a large absorption coefficient and all the constituent elements are naturally abundant. In this work, we investigate the influence of the bath temperature (40-80°C) on the chemical bath deposited CdZn(S,Se) (CZSSe) nanoparticles and films. The films have been characterized by X-ray diffraction and scanning electron microscopy for the structural and morphological study, respectively. The mixture was cooled to 00C with stirring until the mixture turned into a clear solution and then purged with argon. Na2S2O3 (1.85 mmol), SeO2 (1.85 mmol), and Na2S2O3 (0.925 mmol) with Na2Se (0.925 mmol) were applied as chalcogenide sources to fabricate CdZnS (CZS), CdZnSe (CZSe), and CZSSe nanoparticles. The solutions of Na2S2O3, SeO2, and Na2S2O3 with SeO2 were also purged with argon and cooled to 00C, respectively, followed by a quick transfer into the metal sources to react with CdCl2 and ZnCl2, for 2 min. The CZSSe films coated on Al substrates fabricated were applied to the preparation of CZSSe solar cells. The CZSSe solar cells with a structure of Al/p-CdS/CZSSe/In were fabricated.

Authors : L. Ion,1 S. Iftimie,1 A. Radu,1 O. Toma,1 V.A. Antohe,1 N. Vasile,1 and S. Antohe1,2
Affiliations : 1University of Bucharest, Faculty of Physics, 405 Atomistilor, P.O. BOX MG-11, 077125, Magurele, Ilfov, Romania; 2Academy of Romanian Scientists, 54 Splaiul Independentei, 030167, Bucharest, Romania

Resume : In this study we report the grown of zinc selenide (ZnSe) thin films by radio frequency magnetron sputtering onto indium tin oxide (ITO) coated glass substrates, used as window layer for cadmium telluride (CdTe) based photovoltaic structures, for space applications. The structural, optical, morphological and electrical properties of fabricated ZnSe layers were analyzed by X-ray diffraction, optical spectroscopy and spectroscopic ellipsometry, scanning electron microscopy (SEM), and temperature dependence of the electrical resistivity measurements, respectively. The prepared films are polycrystalline with a marked (111) texture and optical band gap values of 2.83 ? 2.84 eV. CdTe absorber layer was obtained by thermal evaporation (TVE) and its thickness values were larger than 2 ?m. The photovoltaic structures were completed by copper (Cu) and gold (Au) commixture thin films deposited by TVE and acting as back electrode. Specific parameters together with the electrical and photo-electrical behavior of ITO/ZnSe/CdTe/CuAu devices was analyzed before and after irradiation with protons (3 MeV, fluency 10-13 cm-2), and the effect of irradiation was discussed. Keywords: ZnSe films, CdTe, ZnSe/CdTe heterojunction, rf-magnetron sputtering

Authors : Yousaf Hameed Khattak, Faisal Baig,Shafi Ullah, Bernabé Marí , Tanveer Ahmed ,Hanif Ullah
Affiliations : Departament de Física Aplicada, ETS de Enginyeria del Disseny, Universitat Politécnica de Valencia, Camí de Vera València, (Spain) Department of Electrical Engineering, Federal Urdu University of Arts, Science and Technology Islamabad (Pakistan)

Resume : CdTe/CGS/CdS/ZnO solar cell is proposed in this research work. Numerical analysis of single and multi-junction solar cell based on cadmium telluride (CdTe) and copper gallium sulfide (CGS). Performance of a cell is analyzed by using SCAPS (Solar Cell Capacitance Simulator) software. By changing the physical parameters using SCAPS software, electrical characteristics of CdTe/CGS/CdS/ZnO based multijunction solar cell is analyzed. Under AM1.5 illumination simulation of J-V (current versus voltage) and quantum efficiency results are to be done. The cell structure is, the top cell is a CGS solar cell and the bottom cell is CdTe cell used in tandem configuration. Efficiency of CGS/CdS/ZnO solar cell is 10.578 % (with FF=83.697%, Voc= 0.820V and Jsc= 15.403 mA/cm2) and efficiency of CdTe/CGS/CdS/ZnO is 20.701%. (With FF=83.650%, Voc= 0.873V and Jsc= 28.318 mA/cm2).

Authors : Jun Zhang, Shuwen Xue, Lexi Shao
Affiliations : School of Physical Science and Technology, Lingnan Normal University, Zhanjiang 524048, People’s Republic of China

Resume : It is known that most record-efficiency kesterite solar cells have been typically fabricated using a high-resistivity cadmium sulfide (CdS) buffer layer deposited by chemical bath deposition (CBD) in order to avoid the formation of undesirable shunt paths due to sputtering damage during the deposition of the subsequent zinc oxide (ZnO) window layer. However, the use of cadmium is undesirable from the viewpoint of environmental safety. Moreover, the quantum efficiency of the kesterite solar cell drops at short wavelengths due to the optical absorption loss from the CdS layer. This implies that further improvement in the short-circuit current can be achieved by replacing CdS with other appropriate wider bandgap buffer materials. In recent years, Zinc sulfide (ZnS), an important II-VI compound semiconductor with a wide direct band gap (Eg = 3.6~3.8 eV), has have attracted intensive attention for its potential application as Cd-free buffer layer material in kesterite solar cells. For photovoltaic applications, ZnS thin film is nearly transparent in all wavelengths of solar spectrum and has high absorption for the wavelength below 520 nm as compared to CdS. Many techniques including sputtering, molecular beam epitaxy, pulsed laser deposition, chemical vapor deposition, successive ionic layer adsorption and reaction, spray pyrolysis, chemical bath deposition (CBD), and photochemical deposition (PCD) have been proposed to fabricate the ZnS thin films. Among these methods, PCD is most attractive because it can be employed as the large-area growth without vapor deposition related to physical techniques and free of some inherent problems associated with high temperature fabrication. Moreover, this technique can be used to deposit ZnS in selected areas. In this study, Zn(O,S) thin films were deposited on ITO glass substrates by pulsed UV photochemical deposition (PCD). In the deposition process, ozone is fed into the deposition solution to incorporate oxygen into ZnS to form Zn(O,S) film. Oxygen content is easily controlled by modulating ozone flow rate. The structure, morphology, transmittance and optical bandgap of Zn(O,S) thin film were investigated by X-ray diffraction, scanning electron microscope and UV-vis spectrophotometer. Our results showed that the as-prepared samples exhibited high transparency in the visible regions. The optical bandgaps of Zn(O,S) films, ranging from 3.2 to 3.7 eV, can be readily tuned by modulating oxygen content in the samples. This study confirmed that Zn(O,S) is suitable as Cd-free buffer layer material in the kesterite solar cells.

Authors : H. Y. Ueng*, H. L. Weng
Affiliations : *Department of Electronic Engineering, Chang Gung University, 333 ; Taiwan Institute of Electronics Engineering, National Chiao Tung University, Hsinchu, 300 Taiwan

Resume : CdTe with direct energy gap of 1.45 eV is well adapted to the spectrum of solar radiation. Several studies have been reported on the electrical conduction of CdTe films for photovoltaic application. Investigations deal with the effect of film’s composition by varying growth parameters leading to influence the electrical conduction of CdTe films. The as deposited n- or p-type CdTe thin films shows lower conductivity, is due to non-stoichiometric effect as well as the heavy compensation between native defects. Hence, the higher n-type or p-type conductivity of CdTe films can be obtained under control by deviating from their stoichiometric composition as well as alternating the deposited potential [2]. In this study, the p-i-n structure of homojunction CdTe solar cell was fabricated by electrodeposition. The kinetic model based on the Butler-Volmer equation has been developed for a variety of deposited conditions, and numerical simulation is used for the deposited process of CdTe thin film by varying the electrodeposited parameters. The potential of perfect stoichiometric (PPS) can be obtained. Therefore, the main strategy is easily achieved to improve the intrinsic quality of i-layer resulting in higher efficiency of CdTe solar cell. In addition, based on the kinetic model, theoretical calculation and numerical simulation, the intrinsic CdTe thin film with near-stoichiometric and the self doping n-type Cd-rich and p-type of Te-rich CdTe with non-stoichiometric can be prepared successfully. By the precise potential-control of the electrodepositing process, the properties of electrodeposited CdTe thin films are consistent with the theoretical prediction, to be confirmed experimentally. Technologically, the defect engineering process, based on the theoretical approach of non-stoichiometric effect, is not limited only to optimize the effective type-conversion but also to improve the quality of each electrodeposited CdTe layers as well as higher the performance of solar cell. Actually, the defect engineering was designed to improve the effective self-doping of n-type and p-type, and also the as-deposited near-stoichiometric (PPS) layer. By preceding the type conversion process sequentially, the efficiency of p-i-n CdTe photovoltaic cell is enhanced. In short, the electrodeposited CdTe thin film with its process of special defect engineering leads to very well commercial advantages such as lower cost and larger area size on CdTe thin-film solar cells [1-2]. Finally, the near stoichiometric and non-stoichiometric compositions were obtained and electrodeposited by following with the results of simulation. Film’s conductivity by can be improved by defect engineering process, which can be achieved better by effective type conversion, and optimized by correlating with the non-stoichiometric effects [1-2]. [1] R.D. Engelken and T.P. Van Doren, J. Electro -chem. Soc., I32 (1985) 2910. [2] H.Y. Ueng et al., J. Electron. Mater. 37, p.1821, 2008.

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CIGS: Alkaline Treatment I : Jiro Nishinaga, Enrico Avancini
Authors : M. Malitckaya, H.-P. Komsa, V. Havu, and M. J. Puska*
Affiliations : Department of Applied Physics, Aalto University, P.O. Box 11100, Finland

Resume : Solar cells based on the Cu(In, Ga)Se2 (CIGS) absorber are among the most promising candidates to replace costly silicon solar cells, thanks to high efficiencies and low costs of CIGS solar cells. The recent progress in their efficiency is mainly due to incorporation of different alkali metal atoms into the absorber layer [1]. However, the microscopic mechanisms how the alkali metal post deposition treatment (PDT) improves the efficiency of CIGS solar cells are, despite extensive research, not yet clear [2]. It has been found experimentally that solar cells after heavy alkali (K, Rb, and Cs) PDT have (i) smoother morphologies of the absorber layer, (ii) higher open circuit voltages, and (iii) higher efficiencies than those after light alkali (Li, Na) PDT. In the present study we explain why the effects of different alkali metal atoms vary remarkably. Our first principles investigations show that light alkali metal atoms prefer to accumulate on the Cu sublattice as impurities and incorporation of heavy alkali metal atoms contributes to the phase separation [3]. Starting from thermodynamics of impurities and extending to their kinetic parameters, our aim is to gain a comprehensive understanding of the absorber microstructure after the PDT treatment. Formation of secondary phases during the PDT process has been predicted by considering which are the possible reactions and calculating their enthalpies. Some of the phases can appear only at high temperatures and certain impurity concentration ranges. Therefore, we discuss also which are the conditions for preferable formation of mixed and separated phases. The properties of these phases have been analyzed and conclusions regarding the ensuing materials and device characteristics are drawn. 1. Jackson, P., Wuerz, R., Hariskos, D., Lotter, E., Witte, W. and Powalla, M., Phys. Status Solidi RRL 10, 583–586 (2016).  2. P. Reinhard, B. Bissig, F. Pianezzi, et al. Chemistry of Materials 27, 5755 (2015) 3. M. Malitckaya, H.-P. Komsa, V. Havu, and M.J. Puska. J. Phys. Chem. C 121, 15516–15528 (2017)

Authors : Roland Wuerz, Wolfram Hempel, Philip Jackson
Affiliations : Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden Württemberg (ZSW), Meitnerstrasse 1, 70563 Stuttgart

Resume : Recent record cell efficiencies of Cu(In,Ga)Se2 (CIGS) solar cells have been achieved by post-deposition treatment (PDT) with the heavy alkali elements Rb and Cs. So far it is unclear if this is caused mainly by a bulk or a grain boundary (GB) effect of these heavy alkali elements. The diffusion of the heavy alkali element rubidium (Rb) in CIGS layers was investigated over a temperature range from 148 °C to 311 °C by outdiffusion from a rubidium fluoride (RbF) layer deposited on a finished CIGS layer. The diffusion profiles were measured by secondary ion mass spectrometry. By using CIGS layers with different grain sizes, diffusion along GBs could be distinguished from diffusion into the grain interior. Based on this data, the slower diffusion coefficient in the volume can be described by the Arrhenius equation D(V, Rb) = 3.2 ∙ 10-8 exp(-0.43 eV/kBT) cm2 s-1 and the fast diffusion along the GBs by D(GB, Rb) = 7.3 ∙ 10-9 exp(-0.30 eV/kBT) cm2 s-1. Additionally, the ion exchange mechanism with Na was investigated. In Na doped CIGS layers the diffusion of Rb into the bulk of the grains is hindered and Rb mainly diffuses along GBs. In contrast, in case of alkali free CIGS layers Rb diffuses into the grains as well as into the GBs. Finally, the effect of Rb on the solar cell parameters of CIGS thin-film solar cells was investigated. RbF PDT was found to enhance the open-circuit voltage, the fill factor and the carrier density in a similar way as observed for K and Na.

Authors : Philipp Schöppe 1, Sven Schönherr 1, Roland Wuerz 2, Philip Jackson 2, Wolfgang Wisniewski 3, Gema Martínez-Criado 4 5, Maurizio Ritzer 1, Konrad Ritter 1, Carsten Ronning 1, Claudia S. Schnohr 1
Affiliations : 1 Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany; 2 Zentrum für Sonnenenergie‐ und Wasserstoff-Forschung Baden-Württemberg, Meitner-strasse 1, 70563 Stuttgart, Germany; 3 Otto-Schott-Institut, Friedrich-Schiller-Universität Jena, Fraunhoferstr. 6, 07743 Jena, Germany; 4 Instituto de Ciencia de Materiales de Madrid (CSIC), 28049 Cantoblanco, Madrid, Spain; 5 European Synchrotron Radiation Facility (ESRF), 38043 Grenoble, France

Resume : Cu(In,Ga)Se2 solar cells are the most efficient ones among all thin film photovoltaics. The recent push in record efficiencies was mainly realized by applying a RbF post deposition treatment (PDT) to the absorber. However, it is not yet fully clear why the introduced Rb improves the solar cell performance. In order to investigate the beneficial effect of Rb, a Cu(In,Ga)Se2 absorber was grown on a Mo coated alkali free substrate and subjected to a RbF PDT. This pure RbF PDT leads to a significantly higher conversion efficiency. A thin cross sectional lamella was cut out of the layer stack and investigated via a combination of different electron microscopy techniques and synchrotron based X-ray fluorescence analysis. It is evident that Rb segregates at random grain boundaries and dislocation cores, where it likely passivates defects. In contrast, Rb does not segregate at benign Σ3 twin boundaries. Additionally, Rb agglomerates at the interface between the absorber and the MoSe2 layer. Our results thus provide clear indications of the origin of the beneficial effect of Rb in Cu(In,Ga)Se2 solar cells [Schöppe et al., Nano Energy 42 (2017) 307]. Subsequently, we investigated a high efficiency solar cell grown on a glass substrate and subjected to a RbF PDT, thus providing a conversion efficiency of over 20 %. Applying the same combination of analysis techniques, we clearly demonstrate that our conclusions for a pure RbF PDT are also valid for state of the art devices.

Authors : A. Villanueva-Tovar, T. Kodalle, R. Klenk, C.A. Kaufmann, R. Schlatmann
Affiliations : Helmholtz–Zentrum Berlin für Materialien und Energie, Competence Centre Thin-Film- and Nanotechnology for Photovoltaics Berlin, 12489 Berlin, Germany.

Resume : It has been widely demonstrated, that the performance of Cu(In,Ga)Se2 (CIGSe) thin film solar cells can be increased by post deposition treatments using alkali-fluoride compounds. Several models have been given in the literature to explain the beneficial effects in RbF-treated samples. However, there is no general agreement with respect to which recombination mechanism is connected to the improvement in efficiency. In this study, temperature dependent IV measurements (IVT) together with supporting measurements (GDOES, CV-profiles, EQE) have been performed to analyze the recombination mechanisms in samples with varied RbF incorporation, bandgap profiles and composition (CGI ratio). The results demonstrate that the values of the bandgap energy (Eg), the activation energy of the bucking current (EA) extracted from the extrapolation of the open circuit voltage (Voc) at T = 0 K and from the slope of the Arrhenius plot are all in reasonable agreement. The diode factors were generally around 1.5 for T > 200 K, thus showing no evidence of a fundamental change in recombination mechanism with RbF-PDT. While a gain in Voc is generally observed with the RbF-PDT, maintaining a high fill factor is more challenging. The latter effect is connected to an increasing deviation between dark and illuminated (Isc-Voc) characteristics in the IVT measurement with RbF.

Authors : Shogo Ishizuka, Noboru Taguchi, Shingo Tanaka, Jiro Nishinaga, Yukiko Kamikawa, Paul Fons, Hajime Shibata
Affiliations : National Institute of Advanced Industrial Science and Technology (AIST)

Resume : Evaporation of relatively heavy alkali-metal fluorides such as KF, RbF, or CsF onto Cu(In,Ga)Se2 (CIGS) thin films, a process often referred to as alkali-metal post-deposition treatment (KF-, RbF-, or Cs-PDT), has been recognized to be a promising technique to boost CIGS solar cell performance. Use of these techniques is also known to lead to morphology modifications manifested by wormhole formation at CIGS film surfaces. In this study, we examined the effects of RbF-PDT on ternary CuInSe2 (CIS) films. It was found that there were specific facets where granular particles (RbF or related compounds) were preferably deposited and as a consequence a large number of wormholes were formed. It was also found that granular particles concentrated in the grain boundaries present at CIS film surfaces as well as the specific facets. Electron backscatter diffraction (EBSD) measurements revealed that the preferred facets where granular particles tend to concentrate are on other than (111)-planes, namely the preferred facets were (220)/(204)-related planes rather than the (112)-planes of chalcopyrite CIS. CIS solar cells fabricated using RbF-PDT CIS films showed a substantial enhancement in device performance, similar to the case for quaternary CIGS devices. These trends were, however, not present for In-free CuGaSe2 films and devices, implying that the effect of relatively heavy alkali-metals strongly depends on the group III elemental composition in CIGS.

10:00 Coffee Break    
CIGS: Alkaline Treatments II : Romain Carron, Torsten Schwarz
Authors : J. de Wild1,2,, M. Simor3, D. Buldu1,2, T. Kohl1,2, G. Brammertz1,2, M. Meuris1,2, J. Poortmans1,4,5, B. Vermang1
Affiliations : 1 Institute for Material Research (IMO), Hasselt University (partner in Solliance & EnergyVille), Agoralaan gebouw H, Diepenbeek, 3590, Belgium 2 Imec division IMOMEC (partner in Solliance & EnergyVille), Wetenschapspark 1, 3590 Diepenbeek, Belgium. 3 TNO Solliance, High Tech Campus 21, 5656 AE Eindhoven, The Netherlands 4 imec (partner in Solliance & EnergyVille), Kapeldreef 75, Leuven, 3001, Belgium 5 Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium

Resume : To increase the cost efficiency and reliability of CIGS solar cells we aim to make thinner CIGS layers of ~ 500 nm. This will not only reduce the amount of material, but also the number of defects in the bulk that may cause performance instabilities. One of the instabilities in CIGS is caused by alkali elements, which are required to achieve high performance, but are also mobile and their concentration should be controlled. To investigate the effects of alkali of thin CIGS layers, we deposited the CIGS by 1-stage coevaporation on Mo/Si(O,N)/SLG substrates, with Si(O,N) as alkali barrier. The alkali are deposited by spincoating or evaporation, either before or after CIGS deposition. Photoluminescence (PL) and time resolved PL (TR-PL) were performed on CIGS/CdS samples and electrical characterization was done on finished solar cells in standard AZO/i-ZnO/CdS/CIGS/Mo configuration. There was no significant change in solar cell performance between the different Na treatments. K treatment however showed significant changes in PL, TR-PL and solar cell performance. Generally, a larger blueshift and broader emission peak was measured for the K treated sample, implying higher degree of compensation. Higher lifetime was measured as well. Efficiencies of K treated solar cells increased more than 2% due to higher Rshunt and Voc. This work received funding from the European Union’s H2020 research and innovation programme under grant agreement No. 715027.

Authors : M. Balestrieri (1), V. Achard (2-1), T. Hildebrandt (2-1), L. Lombez (3-1), M. Jubaut (2-1), D. Lincot (3-1), F. Donsanti (2-1)
Affiliations : (1) IPVF, Institut Photovoltaïque d'Ile de France, 30 RD128 91120 Palaiseau, France; (2) EDF R&D, IPVF, 30 RD128 91120 Palaiseau, France; (3) CNRS, IPVF, 30 RD128 91120 Palaiseau, France

Resume : Cu(In,Ga)Se2 photovoltaic technology has become very attractive because of the very high efficiencies achieved (close to 23%) compared to other thin film materials. The use of flexible substrates paves the way to roll-to-roll deposition techniques, and to new PV applications. Among flexible substrates, polyimide holds the record efficiency (20.4%). Recent research showed that a combination of Na with a heavier alkali produces the highest efficiencies mainly through an improvement of the Voc, but the exact mechanisms are still under investigation. On polyimide, only few studies exist on NaF and KF post deposition treatments (PDT), despite the key importance of understanding the mechanisms and the effectiveness of these processes at low temperature. In this work, the electrical performances solar cells prepared on polyimide with different PDTs will be analyzed and discussed in light of structural (GD-OES, XPS, XRD …) and optical (Raman, PL, EQE) characterization data, supported by physical modeling. In particular, we show that strong improvement of the Voc can be obtained by adding indium during the alkali PDT. The absorber modification obtained using indium in combination with KF leads to an improvement of Voc of about 20 mV with respect to NaF/KF PDT and of almost 50 mV with respect to NaF-PDT only. Our results show a record efficiency of 18.1 % (without anti-reflective coating), a value which is close to the highest reported value of 20.4% (with ARC). Other alkalis are currently under investigation and the latest results will be presented.

Authors : Fredrik Larsson, Olivier Donzel-Gargand, Jan Keller, Marika Edoff, Tobias Törndahl
Affiliations : Ångström Solar Center, Solid State Electronics, Uppsala University, SE-751 21 Uppsala, Sweden

Resume : The beneficial effect of KF-PDT for CIGS solar cells has been frequently reported. However, the buffer layers used in most of these studies have been grown conventionally by a wet CBD method. In this contribution, we have applied ALD grown Zn(O,S) buffer layers. The intention is to study the impact of KF-PDT on devices with buffer layers grown by a dry, vacuum-based deposition process. Devices were fabricated with the structure: SLG/Mo/CIGS-KF/[CBD CdS or ALD Zn(O,S)]/i-ZnO/ZnO:Al/metal grid. It was found that a wet-chemical surface treatment of the CIGS-KF surface is required before growing the ALD Zn(O,S) layer. Different surface treatments could be attributed to changes in the I-V characteristics. Furthermore, the effect of KF-PDT was evaluated for both buffer layer processes by varying the KF flux during the PDT process. Regardless of buffer layer, the KF-PDT led to a significant increase in Voc. However, the presence of a K-In-Se rich surface phase resulted in lower FF values for the ALD Zn(O,S) devices. It was found that a comparable device efficiency to CdS (η ≈ 18%) for the Zn(O,S) buffer layer could only be attained by etching away the K-In-Se rich phase prior to ALD growth, followed by a low-temperature anneal of the finalized device. While the mechanism behind the low-temperature anneal is not fully understood, it indicates that the K-In-Se rich surface phase in itself is not responsible for the KF-induced Voc gain observed for the Zn(O,S) devices.

Authors : Tim Kodalle1, Léo Choubrac2, Ludovic Arzel2, Rutger Schlatmann1&3, Nicolas Barreau2, Christian A. Kaufmann1
Affiliations : 1 PVcomB / Helmholtz-Zentrum Berlin für Materialien und Energie GmbH Berlin, Germany; 2 Institut des Matériaux Jean Rouxel Nantes, France; 3 Hochschule für Technik und Wirtschaft Berlin, Germany

Resume : Both, RbF and KF post deposition treatments (PDTs) have been proven to enhance the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells by increasing VOC and FF. Here we compare the impact of both PDTs on the material and device properties of CIGS thin films and corresponding solar cells. Thereby we spotted different dependencies of the PDTs’ effectiveness on the copper content ([Cu]/([Ga]+[In]) = CGI). Compared to untreated references, KF-treated solar cells show higher efficiency gain at lower CGI, while RbF-treated devices exhibit best performances close to stoichiometry. An additional benefit of the PDT can be achieved by adapting the CdS buffer layer: typically its deposition can be made shorter, leading to higher JSC. Therefore we investigated the growth of chemical bath deposited CdS on alkali-treated CIGS layers. To do so we carried out a combined study of Raman scattering, analyzing the PDTs effect on the CdS growth stages and its growth rate, and scanning electron microscopy (SEM). Latter reveals e.g. that both PDTs lead to a closer coverage of the CdS layer compared to the reference. Finally we used standard device analytics at four selected times during the CdS-growth to correlate the observed modifications with device properties. It turns out, that both PDTs lead to strongly improved VOC and FF especially after short CBDs validating the results of the Raman/SEM-study. All results will be discussed with respect to the recently reported models regarding alkali-induced effects.

Authors : Jiro Nishinaga, Takashi Koida, Shogo Ishizuka, Yukiko Kamikawa, Hajime Shibata, Shigeru Niki
Affiliations : AIST

Resume : The chalcopyrite compound Cu(In, Ga)Se2 (CIGS) has the potential to be used as a semiconductor material in thin-film photovoltaic devices with high conversion efficiencies. In 2013, Swiss Federal Laboratories for Material Science and Technology (EMPA) demonstrated the effectiveness of subjecting the CIGS surface to KF post-deposition treatment (KF-PDT) and many studies on high-efficiency CIGS solar cells with KF-PDT have been reported. However, CIGS solar cells with KF-PDT cannot always retain these high conversion efficiencies for a long time; that is, the degradation of conversion efficiency frequently occurs. Although the degradation mechanism of CIGS solar cells has not yet been clarified, the acceptor concentrations in CIGS layers easily decrease when the cells are kept in the dark, even at room temperature. Light soaking and current injection by positive bias have been reported as an effective method to increase the acceptor concentrations of CIGS layers. Light or current induced acceptors are thought to be metastable, and high acceptor concentrations are generally considered beneficial for high conversion efficiency. In this study, we examine whether heat-light soaking and current injection at high temperature are useful to enhance the conversion efficiency of CIGS solar cells with KF-PDT, and the best cell efficiency of 22% is achieved after 25 hours of heat-light soaking.

Authors : Harvey Guthrey, John Moseley, Jiro Nishinaga, Hajime Shibata, Yukiko Kamikawa, Hitoshi Tampo, Mowafak Al-Jassim
Affiliations : National Renewable Energy Laboratory, Colorado, U.S.;Advanced Institute of Industrial Science and Technology (AIST), Tsukuba, Japan

Resume : The addition of the alkali metals to CuInGaSe2 (CIGS) thin films through post deposition treatments (PDTs) or incorporation during film growth has been of interest for quite some time as a means of increasing photovoltaic device performance. Alkali metals alter recombination processes within these devices and an in-depth knowledge of these changes is critical to optimizing PDTs such that recombination losses are minimized. Device measurements indicate significant changes in both interface and bulk recombination after PDT application. Cathodoluminescence (CL) microscopy is a powerful technique to probe changes in charge carrier recombination behavior at sub-micron scale features such as grain boundaries. In this contribution, CL analysis of CIGS thin films exposed to various PDTs will be presented with a detailed discussion of how alkali PDTs alter recombination behavior at grain boundaries and in grain interiors. In addition to standard CL analysis, temperature and injection level are also included as experimental variables during acquisition of spectrum imaging data. This provides another layer of detail related to the spatial variations in recombination behavior to further clarify how PDTs alter recombination process in CIGS photovoltaic devices.

Authors : Florian Werner, Conrad Spindler, Susanne Siebentritt, Boris Veith-Wolf, Jan Schmidt
Affiliations : Laboratory for Photovoltaics, Physics and Materials Science Research Unit, University of Luxembourg, L-4422 Belvaux, Luxembourg; Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, D-31860 Emmerthal, Germany

Resume : We employ ac and dc electrical measurements and photoluminescence to investigate aluminum oxide passivation layers deposited by plasma-assisted atomic layer deposition (ALD) on polycrystalline Cu(In,Ga)Se2 (CIGS) for application in thin-film solar cells. Already a few monolayers of aluminum oxide result in non-ohmic highly resistive current-voltage characteristics, which indicates a controlled ALD process and formation of a closed and compact dielectric layer on the rough CIGS surface. Despite the excellent growth, as-deposited aluminum oxide layers do not provide any meaningful surface passivation. Bias-dependent admittance spectra show clear evidence for a broad distribution of interface defects with defect concentrations around (1–5)e12 eV-1cm-2. The capacitance step associated with these interface defects however vanishes below our detection limit after annealing for a few minutes, even at moderate temperatures of 350 °C. As a result, the photoluminescence yield improves drastically, and the effective carrier lifetime increases from 2 ns to 20–30 ns. For such moderate annealing conditions, we do not detect any evidence for the formation of fixed charges in the aluminum oxide, and the improved interface passivation is thus purely related to a chemical passivation. Nevertheless, the photoluminescence yield of the annealed sample, even without field-effect passivation by fixed charges, is comparable or superior to reference samples with a state-of-the-art CdS buffer layer.

12:15 Lunch    
Poster 3: CIGS Solar Cells : Marika Edoff, Norio Terada
Authors : Zhiyu Xiong1, Junbo Gong2, Min Yin2, Zejun Ding2, Zengming Zhang2, Xudong Xiao1
Affiliations : 1. Department of Physics, Chinese University of Hong Kong, Hong Kong, Hong Kong. 2. School of Physical Sciences, University of Science and Technology of China, Anhui, China.

Resume : Spectral conversion has been widely applied in the field of luminescence and is potentially beneficial in photovoltaics. Among different approaches, down conversion shows a theoretical potential to enhance the limit of energy conversion efficiency of a single-junction solar cell from about 32% to 40%. For CIGS solar cells, the main current loss in the short wavelength region (< 550 nm) comes from absorption of the CdS buffer layer with a relative small bandgap. As the spectral response of CIGS has an optimal efficiency in the longer wavelength (550-1000 nm) region, spectral down converter that converts photons with wavelength shorter than 550 nm to longer wavelength at a high luminescent efficiency has the potential to reduce the above current loss for CIGS solar cells. In this work, transparent ceramics of Y3Al5O12: Ce3 have been fabricated and applied as the down convertor for CIGS solar cells. Their absorption and photoluminescence properties have been investigated. The device conversion efficiencies of CIGS solar cells with and without the down converter have been measured and compared. The results show that optimized Y3Al5O12: Ce3 ceramics may bring about 9% enhancement to the current density, which enhanced the efficiency of CIGS solar cells from 17.3% to 18.9%. Compared with a pure antireflection coating with an enhancement of ~6%, the great enhancement in down-converter device demonstrates that the Y3Al5O12: Ce3 ceramics has a potential to further improve the performance of CIGS solar cells.

Authors : Hossam Elanzeery, Michele Melchiorre, Finn Babbe and Susanne Siebentritt
Affiliations : Laboratory for Photovoltaics, Physics and Materials Science Research Unit, University of Luxembourg, Belvaux, L-4422, Luxembourg

Resume : Cu rich Cu(In,Ga)Se2 has less defects & better transport properties than Cu-poor. Still, Cu-rich Cu(In,Ga)Se2 cells suffer from low open circuit voltage. To understand the difference, we performed admittance measurements (ADM) on Cu-rich CuInSe2 cells, which revealed the presence of an admittance step with activation energy of around 200meV. This step could be a bulk or an interface defect or a transport barrier. To investigate more about this step, ADM were performed on Schottky contacts & results showed the presence of this 200meV step, indicating it is an effect of the bulk of the absorber and not at the interface. IV(T) was performed & activation energy of the series resistance as function of temperature was extracted showing values much less than 200meV observed by ADM indicating that this step is not a barrier, but is a defect in the bulk of the absorber. This defect disappeared with introducing post-deposition treatments (PDT): In-Se & KF treatments. To further understand the nature of this defect, a PDT was performed using only Se on Cu-rich absorbers. ADM were performed and the 200meV defect disappeared indicating that the 200meV step is a bulk defect related to Se. Interestingly, Se PDT succeeded in increasing efficiency of Cu-rich CIS from 8 to 9.4% with increase in VOC by nearly 30mV & increase in FF by 10% absolute. Se PDT was able to increase efficiency due to removing the 200meV Se-related bulk defect, which appears to be one of the problems of Cu-rich CuInSe2.

Authors : Jan Keller, Wei-Chao Chen, Lars Riekehr, Tomas Kubart, Marika Edoff
Affiliations : Uppsala University (Sweden), Ångström Solar Center

Resume : This contribution presents first results on bifacial Cu(In,Ga)Se2 (CIGS) solar cells that use hydrogen-doped In2O3 layers (IOH) as a transparent back contact. In order to enable sufficient carrier collection and at the same time allow for reasonable light absorption, an absorber layer thickness of only 600-700 nm was chosen. The thin absorber layer results in an increased impact of the back contact recombination velocity (i.e. interface defect density) and emphasizes the need for a back surface field. The back contact properties affect the photovoltaic performance even more at illumination from the back side. A reduced collection efficiency for electrons generated close to the CIGS/IOH interface is usually the major limitation for back-side illuminated cells. Here, we investigate the passivation effect of Al2O3 films deposited on the IOH layers in order to mitigate the corresponding losses in short circuit current. Furthermore, the effect of the Na supply technique (from glass substrate or NaF pre-deposition) and of a variation in IOH thickness are studied. The cell with the best combined efficiency (?) shows values of ? = 11.0% at front side illumination and ? = 6.0% at back side illumination. Standard electrical characterization (IV, EQE) is supported by optical measurements and microstructural/chemical investigations on the cross-sections (SEM & TEM). The observed trends will be evaluated and the potential of further modifications discussed.

Authors : Ch.Nicolaou1, A. Zacharia2, G. Itskos2 and J. Giapintzakis1
Affiliations : 1 Department of Mechanical and Manufacturing Engineering, University of Cyprus, 75 Kallipoleos Av., PO Box 20537, 1678 Nicosia, Cyprus; 2 Experimental Condensed Matter Physics Lab, Department of Physics, University of Cyprus, 75 Kallipoleos Av., PO Box 20537, 1678 Nicosia, Cyprus

Resume : CuIn1-xGaxSe2-based thin film solar cells have attracted considerable attention by the research community due to the promising potential for the fabrication of highly efficient thin film solar cells and modules. A tremendous evolution of the efficiency has been reported through the years, reaching the record efficiency of 22.6% in laboratory scale, exceeding the record value of multi-crystalline Si-based solar cells. CuIn1-xGaxSe2 (CIGS) absorber is p-type, direct semiconductor with tunable energy gap and superior absorptivity of ~105cm-1 in the visible spectrum. CdS buffer layer is n-type, wide-bandgap semiconductor. CdS is widely investigated as it has continuously yielded high-e?ciency cells. Many deposition methods are being utilized to grow CIGS and CdS thin films; however, only few works on the pulsed laser deposition (PLD) technique have been reported. In this presentation, the growth of CuIn0.7Ga0.3Se2/ CdS/ Mo multi-layer structure on soda-lime glass (SLG) using PLD will be presented. The influence of the growth conditions of CIGS and CdS films will be investigated through a complete characterization of structure, composition and morphology. Also, electrical and optical measurements will be performed in order to study the properties of the films and the Mo/CIGS/CdS interfaces. In this presentation, the optimum PLD growth conditions for CIGS and CdS films will be presented based on an overall assessment of the films and interfaces characteristics.

Authors : A. Zacharia1, C. Nicolaou2, J. Giapintzakis2, G. Itskos1
Affiliations : 1 Experimental Condensed Matter Physics Lab, Department of Physics, University of Cyprus, 75 Kallipoleos Av., PO Box 20537, 1678 Nicosia, Cyprus 2 Department of Mechanical and Manufacturing Engineering, University of Cyprus, 75 Kallipoleos Av., PO Box 20537, 1678 Nicosia, Cyprus

Resume : CuIn1-xGaxSe2 (CIGS) thin films have been employed as absorber layers in highly efficient solar cells, owing to an optimum for photovoltaics energy gap, high absorption coefficients and attractive electrical properties. High quality CIGS thin films for solar cell devices are nowadays almost solely fabricated via co-evaporation or sputtering processes. Pulsed laser deposition (PLD) is a fabrication method that combines simple usage and good transfer of stoichiometry from multinary target materials. These advantages appear beneficial towards the production of high-quality, uniform and low cost CIGS thin films. Yet the fabrication and investigation of PLD-based CIGS films appears heavily underexplored. In the present study we report on a comprehensive study of the influence of PLD growth parameters on the optical properties of CuIn0.7Ga0.3Se2 thin films grown on soda-lime glass. The material is spectroscopically investigated using a variety of steady-state and time-resolved optical techniques across the 10-300 K range. The produced films possess attractive light harvesting properties, comparable to evaporated/sputtered material, with high absorption coefficients approaching ~10^5 cm-1 and reduced surface light scattering. A clear improvement of the emission intensity and linewidth is observed as the PLD growth temperature increases up to 500 K, accompanied by a sharpening of the absorption edge consistent with an increased film quality and crystallinity. The broad multi-component luminescence is analysed using excitation- and temperature-dependent measurements revealing distinct band-edge and defect-based transitions, yielding a model of the competing recombination mechanisms in the PLD-grown CIGS films.

Authors : Dong-Hwan Jeon, Young-Il Kim, Dae-Kue Hwang, Ki-Jeong Yang, Jin-Kyu Kang, Dae-Hwan Kim*
Affiliations : Convergence Research Center for Solar Energy, Daegu Gyeongbuk Institute of Science and Technology (DGIST)

Resume : We have studied annealing process at temperatures less than 500°C in the two-step CIGS process, which is essentially required for the use of polymer films. The annealing process higher than 550°C has a limitation in substrate selection and needs to be lowered under 500°C for flexible CIGS solar cells based on polymer films. Before RTP annealing process, the Se film was deposited on the sputter-deposited In/CuGa layer by a thermal evaporator. In a CIGS absorber layer annealed at 480°C, Ga was found to be severely present more near Mo/CIGS interface, which lowered the open circuit voltage characteristic in the device. In order to solve this problem, Sb was added to the precursor, and it was confirmed that the Ga distribution in the thin film was changed according to the amount of Sb added. As the thickness of Sb increased to 10nm, Ga was observed to diffuse more easily to the surface and Voc was also increased in the device. The solar cell containing Sb exhibited better device performance with an efficiency of 9.13%.

Authors : J. P. Teixeira(1), P. M. P. Salomé(2)(3), J. M. V. Cunha(2), M. A. M. Cardoso(1), V. Fjällström(4), P. A. Fernandes(2)(5), M. Edoff(4), J. P. Leitão(1)
Affiliations : (1) Departamento de Física and I3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal (2) International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal (3) Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal (4) Ångström Laboratory, Solid State Electronics, Ångström Solar Center, Uppsala University, SE-751 21 Uppsala, Sweden (5) I3N, CIETI and Departamento de Física, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugal

Resume : The effects of Na on the electrical properties of Cu(In,Ga)Se2 (CIGS) based solar cells are well known to improve the values of power conversion efficiency by increasing the open-circuit voltage (Voc) and fill factor (FF). Many reports have investigated the effects of Na in CIGS pointing out that its incorporation increases the free carrier density, due to a decrease in compensation. However, the exact influence of Na is still debated in the CIGS community. In this work, we studied two samples: a reference one (15% efficiency), prepared according to standard industrial procedures and a sample completely alkali-free grown on an Al2O3 substrate (7% efficiency). Completed solar cells were characterized by J-V and, as expected, significant losses in Voc and FF for the Na-free sample, were observed. Additionally, the lack of Na doping leaded also to a very low free majority carriers density and very large depletion regions, as shown by C-V measurements. Photoluminescence (PL) measurements on the reference sample showed a single, broad, and asymmetric band, whereas on the Na-free one, a second component at lower energies is identified. The impact of the decrease in the free majority carriers density on the PL and external quantum efficiency results, is discussed in the scope of the role of Na changing the compensation ratio.

Authors : Jakob Bombsch1, Enrico Avancini2, Romain Carron2, Thomas Kunze1, Evelyn Handick1, Raul Garcia-Diez1, Yufeng Zhang1,4, Roberto Félix1, Regan G. Wilks1,3, Stephan Buecheler2, Ayodhya N. Tiwari2, and Marcus Bär1,3
Affiliations : 1 Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB), Berlin, Germany 2 Laboratory of Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials and Science and Technology, Dübendorf, Switzerland 3 Energy Materials In-Situ Laboratory Berlin (EMIL), Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany 4 Department of Physics, Xiamen University, Xiamen, China

Resume : The performance of chalcopyrite-based thin-film solar cells has recently been improved by performing alkali post-deposition treatments (PDT), where recent records have been reached using Rb [1] and Cs [2,3]. We used hard and soft x-ray photoelectron spectroscopy to study the impact of NaF/RbF PDTs on the chemical and electronic properties of low temperature processed CIGSe absorbers as a function of RbF PDT deposition rate. We find a stronger copper depletion at the absorber surface with increasing Rb content in agreement with Ref. [4]. Furthermore, we were able to identify a second Se species, scaling with Rb content. To monitor the formation of the CdS buffer layer/CIGSe absorber interface, we studied a buffer layer thickness series deposited with chemical bath deposition. The observed difference in the attenuation of the absorber lines suggests a faster and/or more compact growth of the CdS layer for the RbF treated absorbers. We will present and discuss the impact of the NaF/RbF PDT on the chemical and electronic structure of the CdS/CIGSe interface as a function of RbF PDT deposition rate. Ultimately, we aim to link our findings to the increase in efficiencies of respective CIGSe solar cells. [1] Jackson et al., Phys. Status Solidi RRL 10, No 8, 583?586 (2016) [2] Solar frontier (, 2018-01-12) [3] PVSEC 27, session 2ThO4.3, talk by Jyh-Lih Wu (Solar Frontier) [4] Avancini et al., Chem. Mater. 29 (22), 9695?9704 (2017)

Authors : Mingqing Wang*, Md. Anower Hossain, Giovanni Altamura, and Kwang-Leong Choy** *Presenting author?s email: **Corresponding author?s email:
Affiliations : UCL Institute for Materials Discovery, University College London (UCL), Roberts Building, Malet Place, London WC1E 7JE, United Kingdom

Resume : In order to lower the cost of chalcogenide based solar cells, a novel and non-vacuum Electrostatic Spray Assisted Vapour Deposition (ESAVD) process has been developed to deposit CIGS and CZTS absorber layers for thin film solar cells [1-4]. During ESAVD process, the aerosol containing a mixture of chemical precursor is charged and directed towards a heated substrate where it would undergo decomposition and chemical reaction to deposit a stable solid film onto the substrate [3]. ESAVD is a scalable process for industrial level and it can be operated in open atmosphere. It is also very easy to be adapted for large area deposition with the deposition uniformity as high as 90% under optimised conditions. In our work, CIGS and CZTS absorber thin films with high uniformity (? ±5%) have been achieved after optimizing the deposition conditions of ESAVD process. The composition, purity and grain size of absorber have significant influence on the device performance of the fabricated solar cells. The composition ratio and absorber thickness were further optimized based on the combined results of XRF, SEM, Raman, EDX, and photovoltaic properties of devices. The optimized CIGS solar cell with Cu/In+Ga= 0.78 and In/In+Ga=0.29 showed the best efficiency of 9.55% with Voc = 0.518V, jsc= 28.79 mA cm?2, and FF = 64.02%. Further improvement of films quality is crucial to obtain enhanced carrier collection efficiency in solar cells. Thus, copper(Cu) deficient CIGS films were intentionally designed to avoid the dangerous KCN post-treatment for the removal of extra high conductive minor phase of CuxSe. Subsequently, the as-deposited absorber was dipped into aqueous NaCl solution before selenization to increase the grain size and p-type conductivity of the absorber. Photovoltaic results from the j-V curve have demonstrated that dipping in 0.2M NaCl for 20minutes resulted in an increased Voc and jsc due to the enhanced grain size and p-type conductivity, which showed the jsc, Voc, FF and ? of the solar cell of 29.30mA/cm2, 0.564V,65.59% and 10.83%, respectively. ESAVD has also been used to fabricate earth abundant absorber CZTSSe. Different alkali metals such as Na, Li and Rb were incorporated in ESAVD deposited CZTSSe compounds to further improve the photovoltaic performances of the related devices. Photovoltaic properties results showed that Li, Na and Rb incorporation can increase power conversion efficiency of CZTS devices up to 5.5%. The introduction of a thiourea treatment, has further improved the quality of the absorber/buffer interface, pushed the device efficiency up to 6.3%.

Authors : Juran Kim, William Jo, Jayeong Kim, Seokhyun Yoon; Jihye Gwak, Jae Ho Yun
Affiliations : Department of Physics, and New and Renewable Energy Research Center (NREC), Ewha Womans University, Seoul, Korea 03760; Photovoltaic Laboratory, Korean Institute of Energy Research, Daejeon, Korea 34129

Resume : Cu(In,Ga)(S,Se)2 (CIGS) solar cells have reached to power conversion efficiency (PCE) of 22.6% on rigid substrates. Taking a step forward with a commercialization stage, CIGS solar cells are expanding its application field, employing bendable substrates. CIGS solar cells on polymer substrates have already achieved the PCE of 20.4%. To catch up with the performance of conventional CIGS solar cells, it should be important to figure out the energy loss mechanism and propose a suitable condition for post-deposition treatment (PDT). Especially, the role of Na ions in CIGS thin-films is well-known, and its incorporation in Na-free flexible substrates would be critical. Hereby, NaF layers were applied as PDT and the annealing temperature of CIGS thin-films was varied. In consequence, we were able to create CIGS thin-films with four-different PCE of 0, 6, 12, and 15%, and they were optically and electrically probed by micro-Raman scattering and Kelvin probe force microscopy (KPFM), respectively. As a result, high-efficiency CIGS thin-film displays enhanced crystallinity and higher energy band barrier near the grain boundaries (GBs). This can ameliorate carrier transportation, improving device performance. Furthermore, 640, 532, and 405-nm wavelength laser modules were used for investigating the effects of surface photo-voltage, analyzing the recombination process in the materials indirectly. Also, the phase distribution on the surfaces were determined by the difference among other phases; the work function of CIGS (?CIGS) is approximately 5.0 eV.

Authors : M. Schuster, T. Osterrieder, D. Stapf, V. Barthel, P.J. Wellmann
Affiliations : Materials Department 6 (i-MEET), Martensstr. 7, 91058 Erlangen, Germany, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)

Resume : Dense, coarse grained CuInSe2 thin-films are manufactured with vacuum-free processing based on nanoparticulate precursors. Bimetallic copper-indium-, elemental selenium, and binary selenide (Cu2-xSe and In2Se3) nanoparticles were synthesized by wet-chemical methods and dispersed in organic solvents as nano-inks. Different combinations and layer-stacks from these nano inks were printed on molybdenum coated float-glass-substrates via doctor-blading. The temperature treatment to transform these layer-stacks into dense CuInSe2-films was also investigated, using a variation of temperatures, a face-to-face technique and mechanically applied pressure to overcome issues like high porosity, oxidation, or selenium- and indium-loss. All thin-films were characterized with, SEM, EDX and XRD. CuInSe2 formation takes place between 320 °C and 390 °C and MoSe2 is observed at 550 °C. Dense layers with a thickness of about 3 µm were formed in this work which show potential to be used as solar cell absorber.

Authors : I. Majumdar1,2*, V. Parvan1,2, D. Greiner1, C. A. Kaufmann1, M. Ch. Lux-Steiner2, R. Schlatmann1, and I. Lauermann1
Affiliations : 1PVcomB / Helmholtz-Zentrum Berlin für Materialien und Energie,Schwarzschildstr. 3, D-12489 Berlin, Germany 2Freie Universität Berlin, Fachbereich. Physik, Arnimallee 14, D- 14195 Berlin, Germany

Resume : Over the years, a significant increase in Cu(In,Ga)Se2 absorbers (CIGSe)-based thin film solar cell efficiencies has been as a result of both (i) Na diffusion from soda-lime glass substrates (SLG), as well as (ii) external Na incorporation by a post-deposition treatment (PDT). In the present work, we have studied the absorber surface for these two cases. For this purpose, CIGSe absorbers were evaporated on Mo-coated SLG substrates by the standard three-stage co-evaporation process for studying case (i). CIGSe absorbers with Na diffusion barrier layers were also prepared on which Na treatment was done by evaporating Na metal on CIGSe absorbers from a Na metal dispenser for studying case (ii). From X-ray Photoelectron Spectroscopy (XPS) analysis, a chemically different Na surface species for the CIGSe on SLG substrate absorber was noted in terms of the Modified Auger Parameter (MAP) of Na as compared to the Na-treated CIGSe absorber, showing a difference of about 2 eV. In both cases, a blue shift in binding energy of only the XPS core signal Ga 2p has been observed with a change of the excitation energy from MgK? to AlK?. From Ultra-violet Photoelectron Spectroscopy (UPS) analysis, both valence band maximum to Fermi level distance and work function at the absorber surface has been found to be significantly greater for the CIGSe on SLG substrate absorber as compared to the Na-treated one. Band alignments at sputtered Zn(O,S)/CIGSe heterojunctions have also been studied for both cases.

Authors : Eduard Bailo1, Marcel Placidi2, Philip Jackson3, Dimitrios Hariskos3, Lorenzo calvo4,5, Xavier Alcobe4, Francisco Ramos1, Alejandro Perez-Rodriguez2,5, Victor Izquierdo-Roca2
Affiliations : 1 - Francisco Albero S.A.U. (FAE), Carrer Rafael Barradas, 08908, L'hospitalet de Llobregat, Barcelona, Spain 2 - Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, Spain 3 - Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstr. 1, 70563 Stuttgart, Germany 4 - Centres Científics i Tecnològics (CCiTUB) de la Universitat de Barcelona, C/ Lluis Solé i Sabaris 1-3, 08028 Barcelona (SPAIN) 5 - Departament d'Enginyeria Electrònica i Biomèdica, IN2UB, Universitat de Barcelona, C/ Martí i Franqués 1, 08028 Barcelona (SPAIN)

Resume : The synthesis of high efficiency Cu(In,Ga)Se2 (CIGSe) devices requires Cu-poor stoichiometries, which could induce different mechanisms for the Cu deficit accommodation, thus impacting in the final devices opto-electrical performance. The thin film Cu poor chalcopyrites structural characterization is not evident due to the similitude between the CIGS itself and their related Ordered Vacancies Compounds (OVC). On the other hand in the case of low concentration of Cu vacancies, the detection of defective CIGS is not obvious. In this context, Raman spectroscopy (RS) using resonant conditions has demonstrated being a suitable tool for the characterization and detection of both OVC and defective CIGS since it is highly sensitive to crystal quality, punctual defects, and OVC phases formation. In this work, a complete RS analysis (including resonant and non resonant conditions), supported by XRD, XPS and PL, is performed on samples with compositions from Cu-rich to Cu-poor CuInSe2 (CIS) (Cu/In=1.4-0.4) thin films. This analysis allows detecting the presence of a gradual transition between the stoichiometric CIS, the defective CIS and the formation of OVC phases, based on the asymmetry of the 173 cm-1 peak and intensity increase of the 232 cm-1 peak and the XRD changes. A similar RS characterization is then applied on high efficiencies CIGS samples (18.5-20%) and a correlation of the intensity of the CIGS related defects peak with the cell performance is founded and discussed.

Authors : Hasan A. Yetkin(1,2), Tim Kodalle(1), Marc D. Heinemann(1), Bernd Szyszka(2), Rutger Schlatmann(1,3), Christian A. Kaufmann(1)
Affiliations : (1) PVcomB / Helmholtz-Zentrum Berlin für Materialien und Energie GmbH Berlin, Germany (2) Technical University Berlin, Germany (3) Hochschule für Technik und Wirtschaft Berlin, Germany

Resume : The thermal stability of a bottom device is paramount for reliable application in tandem devices. Ideally, it will permit processing of a top device at required optimum process temperatures. Here we investigate the degradation behavior for CdS-buffered Cu(In,Ga)Se2 (CIGS) thin film solar cells with and without Na-incorporation under thermal stress in ambient air and vacuum with the aim to gain a more detailed understanding of the degradation mechanisms. Previous studies of CdS/CIGS devices primarily attributed the degradation after application of thermal stress to Cd diffusion into the CIGS absorber layer. On the other hand potential-induced degradation of CIGS devices has been associated with the migration of alkali elements (mostly Na) within the device. It is further known that air and humidity exposure will affect Na diffusion within CIGS devices. For the devices studied, we observe severe degradation after annealing at 250 °C in air and at 300 °C in vacuum. The electrical, morphological and compositional properties of the samples before and after defined application of thermal stress are studied. Unlike literature reports, we find no pronounced Cd-diffusion into the CIGS absorber layer. For Na-containing samples the observed degradation can be explained by the formation of Na-induced acceptor states in the TCO front contact. Thus, supported by 1-D numerical device simulation using SCAPS, various possible degradation models are discussed and correlated with our findings.

Authors : S. Suresh1,2 , J. de Wild3,4 , T. Kohl3,4, G. Brammertz3,4 ,D. G. Buldu3,4 , M. Meuris3,4, J. Poortmans1,3,5 , O. Isabella2 , M. Zeman2, B. Vermang3,4
Affiliations : 1 imec (partner in Solliance & EnergyVille), Kapeldreef 75, Leuven, 3001, Belgium. 2 Photovoltaic Materials and Devices Laboratory, Delft University of Technology, Mekelweg 4, Delft, 2628 CD, The Netherlands 3 Institute for Material Research (IMO), Hasselt University (partner in Solliance & EnergyVille), Agoralaan gebouw H, Diepenbeek, 3590, Belgium 4 imec division IMOMEC (partner in Solliance & EnergyVille), Wetenschapspark 1, 3590 Diepenbeek, Belgium 5 Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium

Resume : Having an absorber layer thickness below 1 µm for a regular CIGS solar cell reduces the pathways for electron collection which leads to reduced recombination losses in the bulk absorber layer. Additionally, it reduces material costs and production time. Yet, having such a thin absorber decreases the cell efficiency significantly, mainly due to incomplete light absorption and high Mo/CIGS rear-surface recombination. The aim of this paper is to implement some innovative rear surface modifications for a 500-nm thick CIGS absorber layer to reduce these affects: a passivation layer to reduce the back-surface recombination and point contact openings using nano-particles (NPs) (CdS, Ag, etc.) to create electrical contacts, with some NPs also improving the optical confinement. For example, one approach is to use a layer of thermally evaporated and uniformly deposited Ag, which is then annealed to create Ag NPs with typical diameters of 100 to 150 nm. Ag NPs create the electrical contacts and improve light scattering. An Al2O3 layer is used to passivate the Mo/CIGS rear surface which also prevents the NPs from completely dissolving into the absorber during the absorber deposition. The impact of the implementation of all these rear-surface modifications on the opto-electrical properties of the CIGS solar cell will be discussed and analysed in this paper. This work received funding from the European Union?s H2020 research and innovation program under grant agreement No. 715027.

Authors : Solène Béchu 1,2, Anaïs Louba t1,2, Matteo Balestrieri 1, Muriel Bouttemy 1,2, Thibaud Hildebrandt 1,3, Valentin Achard 1,3, Mathieu Frégnaux 1,2, Damien Aureau 1,2, Jackie Vigneron 1,2, Frédérique Donsanti 1,3, Marie Jubault 1,3, Neghar Nagavi 1,4, Laurent Lombez 1,4, Daniel Lincot 1,4, Arnaud Etcheberry 1,2
Affiliations : 1. Institut Photovoltaïque d?Ile-de-France (IPVF), 30 RD 128, 91120 Palaiseau, France; 2. Institut Lavoisier de Versailles (ILV), UMR 8180 CNRS-UVSQ, 45 av. des Etats-Unis, Versailles, 78035, France; 3. EDF, IPVF, 30 RD 128, 91120 Palaiseau, France; 4. CNRS UMR 9006 IPVF, 30 RD128, 91120 Palaiseau, France

Resume : One of the recent evolutions of CIGS technology is thin absorber layer deposited on flexible substrates. Contrary to ?classical? CIGS solar cells, those substrates don?t provide an alkali source, which has proven to be a factor for efficiency increase. This lack of alkali source is replaced by alkali post deposition treatment (PDT) right after the absorber deposition on the front rear side of the cell. As the deposition process evolves, it becomes necessary to study the alkali elements diffusion over the CIGS absorber layer but also to understand the surface chemistry right after the post-deposition treatment. This work addresses those two critical issues by implementation of XPS measurements as an efficient tool to perform this control by the determination of the composition and chemical environments in constancy and complementary of GD-OES measurements bringing information on alkali depth distribution. The comparison between NaF and KF PDT will be emphasized. Both as deposited samples, to conserve the initial CIGS chemical surface, and freshly rinsed, to reproduce the process stage before the buffer layer deposition, are investigated. Specific spectroscopic signatures and alkali incorporation are observed depending on the nature of the alkali. With air ageing unusual surface chemistries are evidenced, with the absence of oxidized Se contribution and different alkali mobility toward the surface are shown. These process particularities will be discussed here.

Authors : Daniel Abou-Ras, Norbert Schäfer, Aleksandra Nikolaeva, Maximilian Krause, Charles J. Hages, Sergej Levcenko, José Marquez, Thomas Unold
Affiliations : Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

Resume : Inhomogeneities in Cu(In,Ga)Se2 thin films have been reported to lead to band-gap or electrostatic potential fluctuations, which may reduce the photovoltaic performance of the corresponding solar cells via enhanced recombination. The issue of where these inhomogeneities occur in the Cu(In,Ga)Se2 absorber has so far not been discussed in detail in literature. The present work gives an overview of spatial variations in composition, net doping, and lifetime on various scales, also with respect to their occurrences at interfaces and extended structural defects. Impacts of these spatial variations on the device performance of the corresponding solar cells are discussed. It can be shown that compositional inhomogeneities possibly affecting the device performance are only present at (partial) dislocations and at the Cu(In,Ga)Se2/buffer interface, and that inhomogeneous distributions of excess charges at line/planar defects as well as of net doping concentrations affect considerably the potential landscape within Cu(In,Ga)Se2 thin films.

Authors : Aleksandra Nikolaeva1, Maximilian Krause1, Jose Marquez1, Charles Hages1, Sergej Levcenko1, Thomas Unold1, Wolfram Witte2, Dimitrios Hariskos2, Daniel Abou-Ras1
Affiliations : 1 Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany; 2 Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstr. 1, 70563 Stuttgart, Germany

Resume : Electrostatic potential fluctuations in Cu(In,Ga)Se2 solar absorbers may enhance the recombination of excited charge carriers and therefore may be one origin of the limited open-circuit voltage in the corresponding solar cells. The present contribution reports on lateral fluctuations of the widths of the space-charge region, w_SCR (proportional to the net doping density N_A^-0.5), and of the diffusion lengths L_D (proportional to the lifetime of electrons ?^0.5) in Cu(In,Ga)Se2 solar cells, which were accessed by means of electron-beam induced current (EBIC) measurements in a scanning electron microscope appliedusing on cross-sectional specimens. Solar cells with low ([Ga]/([Ga] [In])=0.30) and high ([Ga]/([Ga] [In)=0.66) Ga content and with various chemical-bath-deposited buffer layers (CdS, Zn(O,S), and InxSy) were analyzed . Complementary energy-dispersive X-ray spectroscopy (EDX) were carried out, which show that apart from the Ga/In gradients, no substantial, spatial changes in composition are present in the investigated Cu(In,Ga)Se2 thin films. Therefore, we rule out the presence of relevant band-gap fluctuations. By extracting EBIC profiles acrossperpendicular to the p-n junction, it was revealed that w_SCR and L_D exhibit substantial fluctuations depending on the utilized buffer layer, while for devices with the same buffer layer, the Ga concentration in the Cu(In,Ga)Se2 thin film has no influence on the average fluctuation amplitudes. The average values for w_SCR and L_D (about 400-500 nm and 1-2 µm) were confirmed by capacitance-voltage and quantum efficiency measurements.

Authors : So?a Uli?ná1, Mustafa Togay1, Lewis D. Wright1, J. Lowe1,2, Andrei V. Malkov2, John M. Walls1 and Jake W. Bowers1
Affiliations : 1Centre for Renewable Energy Systems Technology (CREST), Wolfson School of Mechanical, Electrical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK; 2Department of Chemistry, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK

Resume : CIGS is one of the most efficient thin-film PV technologies, with highest performing devices deposited using vacuum processes such as co-evaporation. However, atmospheric processes utilising molecular solutions and simple deposition methods such as spraying have the potential to reduce capital costs considerably. Currently, the highest performing solution-based CIGS solar cell uses hydrazine as a solvent combined with metal chalcogenide precursors. Our group has focused on the use of amine-thiol solvents as a medium to dissolve metal chalcogenides, retaining the excellent starting precursor whilst removing the hazardous aspect of the hydrazine method. Typically, hydrazine-free CIGS absorbers form a bilayer structure consisting of large grains on the top and poorly crystallised bottom. The imperfect absorber morphology represents a barrier in achieving high VOC in solution-processed CIGS. Attempts to make the absorber fully crystallised were made by optimising the selenisation conditions; however the fine-grain layer was never removed completely. Intentionally incorporating a dopant such as Sb into the CIGS bulk was shown as an effective way to overcome the crystallisation issues in low temperature-processed CIGS In this work, antimony in the form of various precursors was added into the precursor solution of metal chalcogenides dissolved in a mixture of ethylenediamine/ethanedithiol. The solution was sprayed onto Mo-coated substrates resulting in solar cells with PCEs over 10%.

Authors : Yunae Cho, Kihwan Kim, In Young Jeong, Young-Joo Eo, Seung Kyu Ahn, Dong Hyeop Shin, Soomin Song, Ara Cho, Joo Hyung Park, Se Jin Ahn, Jin Su Yoo, Jun-Sik Cho, Jae Ho Yun, and Jihye Gwak
Affiliations : Photovoltaic Laboratory, Korea Institute of Energy Research, Daejeon, South Korea

Resume : There have been increasing interests of Cu(In,Ga)Se2 (CIGS) thin film solar cells due to their high energy conversion efficiency among thin film solar cells. CIGS solar cells consist of several layers and resulting interfaces, whose optical and electrical properties can greatly affect the performance of the devices. The structural and compositional properties of CIGS absorber depend on characteristics of molybdenum (Mo) back contact. Although properties of Mo thin films have been widely studied for CIGS solar cells, a step toward deeper understanding of effects of Mo on CIGS solar cell is necessary for ensuring further progresses. . In this work, we have investigated influences of Mo back contact on CIGS solar cells. Mo layers with two different orientations, 110- and randomly orientations, were deposited on the soda-lime glass using DC magnetron sputtering. CIGS absorbers were prepared by a three-stage co-evaporation. To confirm effects of the orientations of Mo layers on the CIGS absorbers and resulting devices, we performed structural and compositional analyses on both of the Mo layers and CIGS absorbers. With in situ thermo-Raman spectroscopy and secondary ion mass spectroscopy, the randomly oriented Mo back contact was found to release a greater amount of alkali ions (i.e., Na and K) into the CIGS absorber than the 110-oriented one. Accordingly, the different Na incorporations induced by the orientation of Mo back contact turned out to significantly influence performances of the resulting devices, indicating the CIGS solar cells with randomly-oriented Mo exhibited superior device performances to the other. Device characterizations, including temperature-dependent IV, CV, admittance spectroscopy, and drive-level capacitance profiling, revealed that the alkali ions? release by the orientation of Mo back contact affected not only carrier transport at the CIGS/Mo interface but also recombination mechanism in the space charge region. The more details will be discussed in the conference.

Authors : Mishael Stanley, Marie Jubault, Fréderique Donsanti, Negar Naghavi
Affiliations : EDF ? R&D, IPVF, 30 Route Départmentale 128, 91120 Palaiseau, France -Mishael Stanley; Marie Jubault; Frédérique Donsanti Institut Photovoltaïque d?Ile de France (IPVF), 30 Route Départmentale 128, 91120 Palaiseau, France -Mishael Stanley; Marie Jubault; Frédérique Donsanti; Negar Naghavi CNRS, 30 Route Départmentale 128, 91120 Palaiseau -Negar Naghavi

Resume : An advantage of metallic substrates over polyimide foils is the excellent temperature tolerance due to their high melting point. Hence they are well suited for the deposition conditions of CIGS solar cells. However, while Mo foil can be used as both the substrate and back contact in this system, the stainless steel despite the necessity to include a diffusion barrier layer remains economically more interesting. In this work we will compare the impact of flexible molybdenum and stainless steel foils on the properties of the CIGS solar cells. In previous work, efficiencies up to 14 % with Mo foil were reached while the preliminary tests on stainless steel without an impurity diffusion barrier layer yielded cell efficiencies up to 12.2%. In order to have a better understanding of these low efficiencies and to improve them, 4 different stainless steel substrates were characterized and compared to Mo foil. The CIGS absorber layers were deposited at 480°C and 550°C. Both substrates and CIGS absorber layers were characterized by XRD, GD-OES, SEM, AFM, and optical measurements. It will be shown that regardless of the benefits of alkali treatment, such as Na and K, or Ga gradient, which can improve cell performances, a crucial factor influencing the increase of the efficiencies of our cells remains the impact of the back contact. The impact of the different characteristics of the substrates on the cell performance will be discussed.

Authors : Tomoya Ishida, Tsuyoshi Maeda, Takahiro Wada
Affiliations : Department of Materials Chemistry, Ryukoku University

Resume : We studied crystallographic and optical properties of CuInSe2, CuIn3Se5, and CuIn5Se8 in Cu2Se-In2Se3 system [1]. The crystal structure changed from chalcopyrite to stannite-type with increasing In2Se3 content. CuIn3Se5 and CuIn5Se8 had a wider band gap than that of CuInSe2 and had a deeper valence band maximum (VBM) from the vacuum level. In Ag2Se-In2Se3 system [2], AgIn5Se8 exists in the Ag-poor side of AgInSe2. In this study, we prepared (Cu1-xAgx)InSe2 and (Cu1-xAgx)In5Se8 solid solutions and characterized their crystallographic and optical properties. Chalcopyrite-type (Cu1-xAgx)InSe2 and stannite-type (Cu1-xAgx)In5Se8 were successfully synthesized by mixing the elemental powders and sequential heating in N2 gas. Tetragonal lattice constants a and c of (Cu1-xAgx)InSe2 and those of (Cu1-xAgx)In5Se8 increased with increasing Ag content. The band-gap energy of the (Cu1-xAgx)InSe2 system increased from 1.00 eV for x=0.0 to 1.22 eV for x=1.0. For the (Cu1-xAgx)In5Se8 system, the band-gap energy increased from 1.21 eV for x=0.0 to 1.40 eV for x=1.0. Energy level of the VBM from vacuum level was measured by photoemission yield spectroscopy (PYS) and that of the conduction band minimum (CBM) was also determined by adding the band-gap energy to the VBM level. The VBM of the (Cu1-xAgx)InSe2 system decreased from -5.29 eV for x=0.0 to -5.80 eV for x=1.0. The VBM of the (Cu1-xAgx)In5Se8 system also decreased with increasing Ag content, while the CBM was almost constant. [1] T. Maeda, W. Gong, and T. Wada, Jpn. J. Appl. Phys. 55, 04ES15 (2016). [2] G. Petzow and G. Effenberg, Ternary Alloys, VCH Verlagsgesellschaft, Weinheim, Geamany 2, 308 (1988).

Authors : Sung-Min Youn, Jin Hyeok Kim, Chaehwan Jeong
Affiliations : Applied Optic & Energy Research Group, Korea Institute of Industrial Technology, Gwangju 500-480, South Korea; Department of Material Science and Engineering, Chonnam National University, Gwangju, 500-757, South Korea

Resume : CuIn1-xGaxSe2(CIGS) has been considered as one of the best candidates in opto-electronic devices for photovoltaics with high-conversion efficiency. For decades, many efforts have been conducted to increase the efficiency of CIGS solar cells by improving the major factors which are short-circuit current density(Jsc), fill factor(F.F) and open-circuit voltage(Voc). Another research is also underway to develop CIGS solar cells on flexible substrates like a stainless steel, polyimide, etc. In the case of SUS, impurities (Ni, Cr and Fe) diffused from the substrate cause deterioration of the characteristics of the device. Therefore, it is very important to deposit a diffusion barrier onto substrate to prevent the diffusion of impurities, and also to analyze the defect of the devices due to impurities. Deep-level transient spectroscopy (DLTS) is a very useful method for defect analysis in the devices. In this research, a defect analysis study of the device was performed using DLTS equipment with or without diffusion barrier.

Authors : Takahiro Kusumoto, Akinobu Kai, Tsuyoshi Maeda, Takahiro Wada
Affiliations : Department of Materials Chemistry, Ryukoku University

Resume : We reported alkali-metal effects of Li, Na, and K in CuInSe2 (CISe) and CuGaSe2 (CGSe) studied by first-principles calculation [1]. The substitution energy of Li for Cu (LiCu) was much lower than those of NaCu and KCu. Then, we successfully synthesized (Cu,Li)InS2 [2] and (Cu,Li)In(S,Se)2 [3] solid solutions. Chalcopyrite-type samples were obtained for the (Cu1-xLix)InS2 with 0.0 ? x ? 0.1 and the (Cu1-xLix)In(S0.5Se0.5)2 with 0.0 ? x ? 0.2. In this study, we synthesized (Cu1-xLix)GaS2 samples and characterized their crystallographic and optical properties. Single-phase chalcopyrite-type (Cu1-xLix)GaS2 solid solution could be obtained for 0.0 ? x ? 0.35. Crystal structures were analyzed by Rietveld refinement using XRD data. The tetragonal lattice constants a and c increased with increasing Li content, x. The band gap energy linearly increased from 2.44 eV for x = 0.0 (CuGaS2) to 2.67 eV for x = 0.35. The ionization energies were measured by photoemission yield spectroscopy (PYS). Energy positions of the valence band maximum (VBM) and conduction band minimum (CBM) were determined from the ionization energies and band bap energies. The VBM level of (Cu1-xLix)GaS2 solid solution slightly decreased with increasing Li content, x and the CBM level slightly rose. [1] T. Maeda, A. Kawabata, and T. Wada, Jpn. J. Appl. Phys. 54, 08KC20 (2015). [2] T. Maeda, C. Zhao, and T. Wada, Thin Solid Films 633, 172 (2017). [3] T. Kusumoto, T. Maeda, and T. Wada, submitted to Jpn. J. Appl. Phys.

Authors : Daniel Siopa, Sara Tombolato, Finn Babe, Max Wolter, Philip Dale, Pedro M. P. Salomé, Kamal Abderrafi, Pedro Anacleto, Sascha Sadewasser
Affiliations : Physics and Materials Science Research Unit, University of Luxembourg, L-4422 Belvaux, Luxembourg; International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga s/n, 4715-330 Braga, Portugal

Resume : Micro concentrator solar cells theoretically have higher power conversion efficiency and use less precious semiconductor than normal large area non-concentrated solar cells. Cu(In,Ga)Se2 is suitable for such cells due to its high efficiency potential, and because it can be fabricated in micron sized areas using a two-step synthesis. We employ electrodeposition, which has a material utilization efficiency of over 90%, to deposit arrays of 200 µm diameter dots of Cu, In, and Ga metal stacks. After selenization, Raman spectra show a peak at 174 cm-1 and room temperature photoluminescence spectra show a radiative transition at 1.02 eV, both consistent with CuInSe2, suggesting that gallium is segregated at the back contact. First devices show a power conversion efficiency of 1.8% with an open-circuit voltage (Voc) of 182 mV and a short-circuit current (Jsc) of 24.2 mA cm-2 under 1 sun. External quantum efficiency measurements reveal a band gap of 1.0 eV consistent with a front surface of CuInSe2. To study the performance under concentrated light, the cells were illuminated with 650 nm light over 3 decades of intensity. From 1 to 33 suns, the efficiency rises from 1.8 to 4.7% due to the increase in Voc, and a maximum diode factor of 2.6 is calculated. These results are comparable to other selective area deposited CuInSe2 state of the art micro solar cell devices.

Authors : J. P. Leitão (1), J. P. Teixeira (1), B. Alves (1), M. Edoff (2), P. M. P. Salomé (3)(4)
Affiliations : (1) Departamento de Física and I3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal (2) Ångström Laboratory, Solid State Electronics, Ångström Solar Center, Uppsala University, SE-751 21 Uppsala, Sweden (3) International IberianNanotechnologyLaboratory,4715-330Braga,Portugal (4) Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal

Resume : The power conversion efficiency of Cu(In,Ga)Se2 (CIGS) based solar cell is improved for slightly Cu-poor thin films. Besides the deviation from stoichiometry the distinct atomic species induce a large diversity of native point defects and compositional fluctuations. Thus, Cu-poor CIGS is typically highly doped and highly compensated, and its optoelectronic properties are governed by the presence of electrostatic and bandgap fluctuating potentials. A set of CIGS samples with a constant value of [Ga]/([Ga]+[In])~0.3 and without any Cu-transitions, were prepared. The [Cu]/([Ga]+[In]) (CGI) ratios studied were 0.53, 0.71, and 0.84. The electrical performance of the samples was studied by J-V measurements, and the experimental evidences of fluctuating potentials influence were discussed from external quantum efficiency (EQE) and photoluminescence (PL) measurements. The samples with the best electrical performance are the ones which present a lower effect of fluctuating potentials relatively to the other two samples. With decreasing electrical performance the effect of fluctuating potentials increases. A clear relation between the electrical performances of the resulting solar cells with the average depth of fluctuating potentials is demonstrated and discussed.

Authors : Natalia Maticiuc, Tim Kodalle, Jakob Lauche, Robert Wenisch, Christian A. Kaufmann and Iver Lauermann
Affiliations : Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany

Resume : Latest record efficiencies of Cu(In,Ga)Se2 (CIGS) solar cells were achieved by means of a rubidium fluoride (RbF) post-deposition treatment (PDT). To understand the effect of the RbF PDT on the surface chemistry of CIGS and its interaction with sodium that is generally present in the CIGS absorber, we performed in-situ X-ray photoelectron spectroscopy (XPS) on RbF-treated CIGSe thin films as-deposited by a three-stage co-evaporation process and after RbF-PDT. The sample transfer from the deposition to the XPS analysis chamber was in part performed via an ultra-high vacuum transfer system. This allows to minimize air exposure, avoiding oxide formation on the CIGSe surface, especially for alkali-treated absorbers. Beside an expected reduction of Cu and Ga at the surface of RbF-treated CIGSe films, we find that Rb penetrates at the grain boundaries of CIGSe and, contrary to the fluorine, it is not completely removed by ammonia etching. The remaining Rb contribution at 110.56 eV may hence belong to an Rb-In-Se phase, since the shift in the Indium Auger transition is similar to the one previously ascribed to K-In-Se. Na is driven towards the surface of CIGSe absorber as a direct result of the RbF PDT. Although both RbF and air exposure represent a driving force for Na, a remaining signal is observed after etching with ammonia. This indicates that air exposure, i.e. oxygen and/or oxides from the CIGSe surface, ensure a complete transfer of sodium from bulk CIGSe to its surface.

Authors : J. P. Teixeira(1), P. A. Fernandes(3)(4), B. P. Falcão(1), M. Edoff(2), P. M. P. Salomé(3),(5), J. P. Leitão(1)
Affiliations : (1) Departamento de Física and I3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal (2) Ångström Laboratory, Solid State Electronics, Ångström Solar Center, Uppsala University, SE-751 21 Uppsala, Sweden (3) International Iberian Nanotechnology Laboratory,4715-330Braga,Portugal (4) I3N, CIETI and Departamento de Física, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugal (5) Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal

Resume : A wide variety of thin film growth methods have been used to fabricate Cu(In,Ga)Se2 (CIGS). The most versatile one is co-evaporation, which allows for a near-perfect control over elemental distribution in time and in depth of the film. An optical study by photoluminescence (PL) of the influence of different Ga profiles on the electronic structure of CIGS is presented. We prepared a set of CIGS samples with different [Ga]/([Ga]+[In]) (GGI) profiles: i) a three-stage sample (A) with a typical Ga notch profile; ii) a one-stage sample (B) with a decreasing linear GGI profile, from 55% at the back to 25% at the front; iii) a flat-gradient sample (C) with flat evaporation profiles with a constant value of GGI~0.3. Despite a blueshift of the luminescence with the increase of the excitation power, striking differences were observed for the three samples. For sample A, two emission bands are observed in the range 0.92-1.16 eV, being asymmetric the one located at a low energy position which progressively dominates the spectrum as the excitation power increases. For sample B, at low excitation power values, a single asymmetric band is observed, whereas with the increase of the excitation power, two new bands are evident for higher values of energy. For sample C, two asymmetric bands are always observed in the excitation power range investigated. The influence of the different Ga profiles on the fluctuating potentials is discussed and related with the J-V and external quantum efficiency results of the resulting solar cells.

Authors : O. Ernst [1], K. Eylers [1], F. Ringleb[1], Th. Teubner[1], B. Heidmann [2] [3], M. Schmid [2] [3], T. Boeck [1]
Affiliations : [1] Leibniz Institute for Crystal Growth; [2] University of Duisburg-Essen and CENIDE Faculty of Physics; [3] Helmholtz-Zentrum Berlin für Materialien und Energie, Department Renewable Energy

Resume : Thin film solar cells made of Cu(In,Ga)Se2 (CIGSe) suffer from high costs, since the rare elements indium and gallium are used for a multitude of technical applications. The concept of micro-concentrator CIGSe solar cells allows substantial material saving. Therefore a reliable bottom up process for the preparation of the absorber islands is required. In addition, this also results in an increase of solar cell efficiency. We show that the indium islands can be grown at predefined locations by novel methods of surface treatment before indium deposition. By this, regular island patterns with a well-defined morphology have been obtained. The controlled deposition of copper, followed by selenization in a PVD system, and selective etching of copper selenides yields polycrystalline CIGSe micro islands with promising material properties. For the samples, we proof ways to place a non-conducting layer in between CIGSe islands to insulate the front and back contact. This layer is obtained by spin-coating and curing a novolak based resist. At first, the photovoltaic active absorber islands is completely covered by the resin. The top of the islands can be stripped by plasma etching for front contacting. A characterization of the obtained devices provided evidence for their functionality.

Authors : Setareh Zahedi-Azad, Roland Clausing, Roland Scheer
Affiliations : Martin-Luther-University Halle-Wittenberg, Fachgruppe Photovoltaik, 06120 Halle, Germany

Resume : It is known that post-deposition treatment (PDT) with heavy alkalis of small bandgap Cu(In1-x,Gax)Se2 (x ~ 0.3) solar cells improves the electrical parameters. In this work we investigate the influence of heavy alkalis (KF, RbF and CsF) on wide bandgap CIGSe samples. It is found that the PDT leads to a reduced VOC-deficit and thus allows the open-circuit-voltage (VOC) to partially overcome its typical limitations for high gallium concentrations. The study of red-light VOC(t) transients, where t is the time, allows to differentiate between dominant bulk and interface recombination. No-PDT samples show a change from bulk (dVOC/dt>0) to interface recombination (dVOC/dt< 0) as the bandgap increases. KF treatment partiallly quenches interface recombination. Comparing CIGSe devices (same Eg) with different alkali treatment, a different slope (dVOC/dt) was measured. Wide bandgap CIGSe samples treated with heavy alkalis (RbF and CsF-PDT) illustrate a smaller slope in comparison to no-PDT samples. Independent from the alkali element, the open circuit voltage increases for cells with a bandgap value of 1.4 eV and reduces sharply for higher bandgap values. In addition, the CIGSe devices treated with heavy alkalis of RbF and CsF-PDT show a higher open circuit voltage compared to the case of KF-PDT or no-PDT.

Authors : Dae-Hyung Cho, Woo-Jung Lee, Hye-Jung Yu, Won Seok Han, Byungha Shin, Yong-Duck Chung
Affiliations : Electronics and Telecommunications Research Institute (ETRI); Electronics and Telecommunications Research Institute (ETRI); Electronics and Telecommunications Research Institute (ETRI); Electronics and Telecommunications Research Institute (ETRI); Korea Advanced Institute of Science and Technology (KAIST); Electronics and Telecommunications Research Institute (ETRI)

Resume : Among the various efforts to improve the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells, the control of energy levels has been a key issue because the depth directional profile of energy levels largely affects the carrier transport. Nowadays, thus, the In-Ga-Se/Cu-Se/In-Ga-Se multi-stage CIGS co-evaporation process has been widely used to make moderate Ga-gradient in CIGS films because the Ga-to-In ratio determines the energy band gap and the conduction band minimum. The world record efficiencies are achieved mainly using this technique. However, conventional multi-stage technique should be modified when using heat-vulnerable plastic substrates such as polyimide (PI) since the elemental inter-diffusivity substantially changes with temperature. In this work, we engineered the process parameters and suggest an optimal elemental profiles in CIGS absorber for highly efficient solar cells at limited thermal budget. The CIGS thin films were fabricated by using independently controlled Cu, In, Ga, and Se effusion cells. The evaporation flux and duration time of each element are controlled at different substrate temperatures under 450 °C. The elemental compositional profiles in depth direction were successfully controlled. The properties of the fabricated films were characterized by a secondary ion mass spectroscopy, an X-ray diffraction, a scanning electron microscopy, and an energy dispersive spectroscopy. To measure the photovoltaic performances, we measured a current density-voltage curve on the solar cell devices with a structure of Al/Ni/ITO/Zn(O,S)/CIGS/Mo/PI. The slope, minimum point position, and height of the compositional Ga-to-In ratio largely influenced on the minority carrier transport and eventually the solar cell efficiency.

Authors : M. Krause1, A. Nikolaeva1, P. Jackson2, D. Hariskos2, W. Witte2, D. Abou-Ras1
Affiliations : 1 Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany; 2 Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstr. 1, 70563 Stuttgart, Germany

Resume : The present study was conducted in order to shed light on the question which are the microscopic material properties in a high-efficiency Cu(In,Ga)Se2 (CIGS) solar cell. A CIGS device with stacking sequence ZnO:Al/(Zn,Mg)O/CdS/CIGS/Mo/glass, of which the CIGS absorber underwent a RbF postdeposition treatment, was investigated. Its conversion efficiency was almost 21% without antireflection coating. Electron backscatter diffraction (EBSD), energy-dispersive X-ray spectrometry (EDX), electron-beam-induced current (EBIC), and cathodoluminescence (CL) measurements were performed on cross-sectional specimens in order to get insight to microstructural, compositional, electrical, and optoelectronic properties. The average grain size was determined to 0.5 µm by EBSD. From the EBIC results, the width of the space charge region, wSCR, and the minority-carrier diffusion length in the quasi-neutral region, LD, were determined. Average recombination velocities at grain boundaries calculated from the CL data were about 5x10^3 cm/s in average. EDX elemental distribution maps did not show any substantial change in composition in the lateral direction; only the Ga/In gradients perpendicular to the substrate were detected, which can be correlated well with the peak shifts visible in the hyperspectral CL map. In contrast, w_SCR and L_D were found to be inhomogeneously distributed along the p-n junction, indicating lateral fluctuations in the net doping and the carrier lifetime. The present contribution discusses these fluctuations as possible origin for the limited open-circuit voltage of CIGS solar cells.

Authors : Tsuyoshi Maeda, Niina Kitagawa, Takahiro Wada
Affiliations : Department of Materials Chemistry, Ryukoku University

Resume : Most of the Cu(In,Ga)Se2 (CIGS) absorbers for high-efficiency solar cells have been formed by physical vapor deposition as typified by the ?3-stage process?. In the first stage of the ?3-stage process?, a (In,Ga)2Se3 precursor layer which was the framework of the CIGS crystal was prepared by the deposition of In, Ga, and Se atoms [1]. The preferred orientation of CIGS films could be controlled by controlling the orientation of the (In,Ga)2Se3 precursor layer [2]. In this study, we investigated the crystal structures and optical properties of (In,Ga)2Se3 system. We prepared the (In1-xGax)2Se3 powders with 0.0 ? x ? 1.0 by mixing the elemental In, Ga, and Se powders and heating at 550? in N2 gas. In2Se3 (x=0.0) has a layered defect wurtzite-type crystal structure [3] and Ga2Se3 (x=1.0) has a defect zinc-blende-type structure. The XRD patterns showed that the (In1-xGax)2Se3 have a layered defect wurtzite-type structure for 0.0 ? x ? 0.6 and ideal wurtzite-type structure for x=0.7. The samples with 0.8 ? x ? 0.9 are a mixture of ideal defect wurtzite-type (In,Ga)2Se3 and defect zinc-blende-type Ga2Se3. The direct band-gap energy of the (In1-xGax)2Se3 increased from 1.95 eV for x=0.0 (In2Se3) to 2.20 eV for x=0.5 and then decreased to 1.85 eV for x=0.6. The samples with 0.7 ? x ? 1.0 have the indirect band gap and their band gap energies increase with Ga content. On the basis of the present results, the deposition of high quality CIGS films with high Ga content is discussed. [1] T. Wada and T. Maeda, Jpn. J. Appl. Phys. 50, 05FA02 (2011). [2] S. Chaisitsak, A. Yamada, and M. Konagai, Jpn. J. Appl. Phys. 41, 507 (2002). [3] A. Pfitzner and H. D. Lutz, J. Solid State Chem. 124, 305 (1996).

Authors : Jonathan Joel, Erik Wallin, Olle Lundberg, Lars Stolt
Affiliations : Solibro Research AB, Vallvägen 5, 75651 Uppsala, Sweden

Resume : This contribution demonstrates how replacing the conventional intrinsic ZnO 2nd buffer in Cu(In,Ga)Se2 (CIGS) solar cells with a tailored Zn1-XMgXO (ZMO) film enables the use of an optically superior ZnO:B (BZO) transparent conductive oxide (TCO) as front electrode, instead of the standard ZnO:Al, while maintaining Voc and FF. The ZMO is deposited in a pulsed DC sputtering process from a compound target. By tuning the Mg content of the sputter target and the ZMO film thickness, the band structure can be designed in such a way that the use of BZO in the device stack is possible, without losing Voc. For our Mo/CIGS/CdS cell stack the ZMO process window is quite broad, with an optimal [Mg]/([Zn]+[Mg]) ratio of 0.05-0.07 and film thickness around 100nm. The BZO layer is produced in an industrial size prototype low-pressure chemical vapor deposition tool, and has excellent optical properties also at lower sheet resistances of about 10 ?/?. IV, EQE, XRD and optical measurements illustrate how the absorption losses in the TCO are reduced, due to the higher carrier mobility, sharper absorption edge in the blue region and advantageous film texture of the BZO. The increased current output corresponds to an absolute efficiency gain of around 0.8% units, compared to the standard window layer (WL). As proof of concept we have produced non-gridded cells (0.11cm2), implementing this improved WL on absorber material from our pilot production line, reaching efficiencies up to 21% (AR-coated).

Authors : Romain Carron, Enrico Avancini, Thomas Feurer, Shiro Nishiwaki, Stefano Pisoni, Fan Fu, Christian Andres, Martina Lingg, Yaroslav E. Romanyuk, Stephan Buecheler, Ayodhya N. Tiwari
Affiliations : Laboratory for Thin films and Photovoltaics, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, 8600 Dübendorf, Switzerland

Resume : The difference of open-circuit voltage (Voc) compared to the Shockley-Queisser limit is an indicator of the recombination losses in a solar cell device. Its computation requires the precise knowledge of the absorber bandgap, especially for absorber alloys such as Cu(In,Ga)Se2, Cd(Te,Se), Cu2ZnSnSe4 or perovskite. However the characterized value of the bandgap depends on the choice of the determination method, and comparison of Voc deficits reported by various sources remains ambiguous. In this contribution we review methods for bandgap determination, such as processing of the quantum efficiency curves (notably inflection point and Tauc plots) and optical measurements of bare absorbers. The bandgap determination methods are compared for different solar cell technologies, and we report systematic differences in the bandgap value related to the choice of determination method. The presented results should facilitate the analysis and interpretation of bandgap and Voc deficits values reported by various sources.

Authors : Shiro Nishiwaki, Romain Carron, Johannes Löckinger, Stephan Buecheler, Ayodhya N. Tiwari
Affiliations : Laboratory for Thin Films and Photovoltaics, Empa, Swiss Federal Laboratories for Materials Science and Technology

Resume : To achieve high efficiency utilizing Cu(In,Ga)Se2 (CIGS) absorber layers alkaline incorporation to the CIGS layers is mandatory. Generally for high efficiency devices, alkaline elements are introduced by diffusion from the soda-lime glass (SLG) substrate and by post deposition treatment (PDT) with alkaline fluoride deposition. In this work, CIGS layers are grown on substrates of polyimide and SLG covered with SiOx alkaline diffusion barrier by the multi stage thermal co-deposition. Co-evaporation of alkaline fluorides was attempted during the growth processes of CIGS layers in addition to the PDT. SEM observation revealed that the microstructure changed with NaF co-evaporation during the 3rd stage, in which In, Ga, and Se are co-evaporated. With increasing the temperature of the NaF effusion cell, faceted morphology at film surface altered into rounded morphology. It was shown that co-evaporation of NaF during 3rd stage improved the efficiency of CIGS solar cells. Compared to samples with only PDT (NaF followed by RbF), alkaline fluoride co-evaporation increases in efficiency from a baseline of about 19% to 20%. A correlation between the efficiency and the composition ratio of [Cu]/([In]+[Ga]) was suggested.

Authors : Jang Mi Lee(1,2), Doh-Kwon Lee(1,2)*
Affiliations : (1) Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Korea; (2) Division of Nano and Information Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Korea

Resume : Building-integrated photovoltaics (BIPV) are receiving increased attention as a feasible way to generate renewable electrical power. For the BIPV window applications, highly stable, efficient and transparent for visible light solar cells are essential. In terms of high stability and power conversion efficiencies (PCEs), CuInSe2 (CISe) thin film solar cells are one of the potential candidates for BIPV applications. But there are some challenges to transparency. General approach to semi-transparent solar cells is using ultrathin absorber layers or to remove some parts of absorber layers on transparent conductive oxide (TCO) as a back electrode. The former is easily limited to provide light transmission in the infra-red region and difficult to achieve high efficiency due to high back surface recombination, while the latter may result in material losses. To tackle this problem, in this study, we demonstrated selectively deposited CISe thin films on patterned Mo back contact using electrochemical deposition enables the material savings without the need of further removal of absorber layer. Furthermore, to reduce the back electrode interface recombination and high shunt conductance caused by the edge of patterned Mo, we introduced a passivation layer in the edge area of patterned Mo. As a consequence, we present a first demonstration of a novel fabrication process for semi-transparent CISe solar cells with upto 30% transparency and over 8% of PCE.

Authors : Karima Bouras1*, Romain Bodeux1,2, Damien Coutancier1,3, Aurélien Duchatelet1,2, and Daniel Lincot1,3
Affiliations : 1-Institut Photovoltaïque d?Ile-de-France (IPVF),  30 RD 128, 91120 Palaiseau, France. 2-EDF, IPVF, 30 RD 128, 91120 Palaiseau, France. 3-CNRS UMR 9006  IPVF, 30 RD128, 91120 Palaiseau, France.

Resume : Wide band gap pure sulfide Cu(In,Ga)S2 (CIGS) solar cells are attracting increased attention since the world record efficiency of 17.6% achieved by Solar Frontier for a band gap of 1.55 eV, using a two-step process. Such material is motivated by implementation in next-generation high-efficiency tandem solar cells as top cells, thanks to their wide band gap. The present paper reports on 1.6-1.8 eV wide gap CIGS absorbers successfully made by a two-step process, implementing the electrodeposition technique instead of sputtering. The sulfurization of the stacked Cu-In-Ga metallic precursors (deposited at room temperature, under atmospheric pressure, up to 15x15 cm2) was performed thermally using H2S gas. We investigate the influence of the composition of the precursors and thermal annealing parameters on the properties of the CIGS absorber as well as on the device performance. Increasing the Ga content allowed us tuning the absorber band gap up to 1.8 eV, which is the widest band gap reported using two-steps process. On the other hand, the morphology and crystalline structure were found to strongly depend on the copper content. An optimization of the processing parameters allowed us to successfully fabricate compact and crystalline wide gap CIGS absorbers without parasitic phases. The resulted devices have shown very promising characteristics, Voc as high as 760 mV and Fill Factor of 60.7 %, for a present efficiency of 5% at 1.6 eV were recorded. In depth characterization of materials at different processing stages and devices (Raman, GXRD, GDOES, PL, SEM, Spectral responses and IV?) will be presented, revealing the great potential of this technology for industrial application.

Authors : Karima Bouras1*, Romain Bodeux1,2,3, Damien Coutancier3, Aurélien Duchatelet1,2,3, and Daniel Lincot1,3
Affiliations : 1-Institut Photovoltaïque d?Ile-de-France (IPVF),  30 Route Départementale 128, 91120 Palaiseau, France. 2-EDF, IPVF, 30 RD 128, 91120 Palaiseau, France. 3-CNRS, UMR 9006,  IPVF, 30 RD128, 91120 Palaiseau, France.

Resume : At the emergence of new concepts based on multijunction solar cells, wide band gap pure sulfide Cu(In,Ga)S2 (CIGS) solar cells have attracted a lot of attention, in particular for implementation in tandem solar cells with Si as top cells. Herein, wide gap CIGS solar cells were fabricated using the same industrial process as Solar Frontier. First, the CIG metallic precursors were deposited in different stack configurations by means of sputtering. The second step consists in sulfurization under H2S-feeled atmosphere. We show that the layer stacking, chemical composition and annealing parameters play a key role on the properties of the CIGS absorber and device performance. Copper-poor absorbers exhibit In2S3 parasitic phases while Cupper-rich ones result in Cu-based secondary phases on both surface and bulk. The chemical composition as well as the deposition sequences were then optimized so as to obtain a compact and crystalline wide gap absorber without parasitic secondary phases. On the other hand, controlling the Ga content and its gradient into the absorber have allowed monitoring the band gap from 1.5 to 1.7 eV. Finally, our best wide gap CIGS cell have shown a 7.3 % efficiency, a Voc of 690 mV, a Jsc of 17.2 and FF of 61%. Thanks to advanced characterization techniques available at IPVF we could identify the limiting factors and evaluate directions for improvements.

Authors : Xiong Zhiyu, Xiao Xudong*
Affiliations : Department of Physics, Chinese University of Hong Kong, Hong Kong, Hong Kong

Resume : Abstract: Spectral conversion has been widely applied in the field of luminescence and is potentially beneficial in photovoltaics. Among different approaches, down conversion shows a theoretical potential to enhance the limit of energy conversion efficiency of a single-junction solar cell from about 32% to 40%. For CIGS solar cells, the main current loss in the short wavelength region (<550 nm) comes from absorption of the CdS buffer layer with a relative small bandgap. As the spectral response of CIGS has an optimal efficiency in the longer wavelength (550-1000 nm) region, spectral down converter that converts photons with wavelength shorter than 550 nm to longer wavelength at a high luminescent efficiency has the potential to reduce the above current loss for CIGS solar cells. In this work, Ce3+ and Yb3+ doped perovskite nanocrystals are successfully synthesized and applied as the down convertor for CIGS solar cells. Their absorption and photoluminescence properties have been investigated. The device conversion efficiencies of CIGS solar cells with and without the down converter have been measured and compared. The results show that Benefiting from the higher inner luminescent quantum yield, the down convertor could enhance the efficiency of CIGS solar cells effectively.

Authors : Idris Bouchama1,2,* and Kamal Djessas3
Affiliations : 1 Département d’Electronique, Faculté de Technologie, Université de Msila, Algeria. 2 Laboratoire des matériaux inorganique, Université de Msila, Algeria. 3Laboratoire Procédés Matériaux et Energie Solaire PROMES-CNRS, Rambla de la Thermodynamique, Technosud, 66100 Perpignan, France.

Resume : CuIn0.7Ga0.3Se2 (CIGS) thin films were deposited by using one-step radio-frequency magnetron sputtering with different substrate temperatures onto glass substrates and an annealing process for improving the crystallinity and the morphology of the prepared films was performed. The annealing step was performed in argon gas atmosphere at 300°C for 1h. The influence of annealing process and the substrate temperatures on the crystalline quality as well as structural, optical and electrical properties of thin layers obtained has been studied. X-ray diffraction showed that the annealed films were highly orientated in the (112) and/or (204)/(220) direction. In2Se3 secondary phase was observed on the samples grown at lower substrate temperatures. The surface morphology of CIGS layers studied by Atomic Force Microscopy (AFM) and Scanning Electronic Microscopy (SEM) has been also discussed. The most surprising and exciting outcome of this study was that the as grown films were of near stoichiometric composition after annealing. Resistivity measurements were carried out using the four point probe method. The optical absorption showed that energy gap values are between 1.13 and 1.18 eV and rather sharp absorption fronts. Thin films resistivities are between 10.7 and 60.9 Ω.cm depending on the experimental growth conditions and annealing. Keywords : Cu(In,Ga)Se2, thin films, rf-magnetron sputtering, Annealing process, Secondary phases.

Authors : Ralf Hunger 1, Stefan Weeke 1, Sebastian Jander 1, Martin Zimmer 1, Ingmar Ratschinski 1, Jovana Djordjevic-Reiss 1, Philipp Kratzert 1, Lars Stolt 2
Affiliations : 1 Solibro-Hitech GmbH, Sonnenallee 32-36, 06766 Bitterfeld/Wolfen, Germany; 2 Solibro Research AB, Vallvägen 5, 756 51 Uppsala, Sweden

Resume : Invest cost for industrial mass production CIGS coating tools contributes substantially to the cost of CIGS PV technology. An increased process speed is an effective leverage to increase the output of a single tool and thus reduce the invest cost for a given nameplate fab capacity. In this contribution, the increase of deposition speed, its effect on CIGS layer and device properties on an industrial scale coevaporator tool was investigated. We found that peak cell efficiencies around the center of deposition are less affected, but lateral non-unformities and non-ideal deposition conditions towards the deposition edges increase with increased deposition speed. Whereas the depth dependent element distribution is actually little affected, the film roughness and electronic quality deteriorate to some degree. Optimization efforts in the process for increased deposition speed focused on Ga depth distribution, deposition temperatures, and alkali content. Finally, an equal module efficiency at 20% increased deposition speed could be achieved. By this acceleration, the total equipment invest for a CIGS turnkey line can be reduced by about 7%

15:45 Coffee Break    
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CIGS: Band Gap Tuning : Byoung Koun Min, Johannes Löckinger
Authors : Marika Edoff, Fredrik Larsson, Nina Shariati Nilsson, Faraz Khavari, Olivier Donzel-Gargand, Tobias Törndahl, Lars Stolt
Affiliations : Division of Solid State Electronics, Angstrom Laboratory, Uppsala University, p.o. box 534, 751 21 Uppsala, Sweden

Resume : Solar cells based on (Ag,Cu)(In,Ga)Se2 (ACIGS) with varying Ag/(Ag+In) ratio from few percent up to 50 % have been studied in this contribution. The substrates were either a high strain point, K-containing glass or a normal soda lime glass. The efficiencies of the solar cells have been in the range of 18 % or above. Interdiffusion of silver and copper in ACIGS was investigated. Ag was added to the co-evaporation of the ACIGS during one of the three stages in the co-evaporation. In contrast to interdiffusion of Ga and In, which has previously been found to be slow in presence of alkali metals and in particular in presence of K, interdiffusion of Ag and Cu is found to be rapid. Even if Ag is added only in the first stage, corresponding to about 25 % of the total ACIGS thickness, the final profile as analyzed by glow discharge optical emission spectroscopy was found to vary less than 10 % in depth. Similar to solar cells based on co-evaporated CIGS layers, cells with ACIGS layers benefit from a post deposition treatment (pdt) with alkali fluorides, such as KF. We have investigated the surface properties of ACIGS with KF pdt after an ammonia etch (a similar treatment as in the beginning of the chemical bath deposition) by X-ray Photoelectron Spectroscopy and Transmission Electron Microscopy. The results point to that the surface reactions for the ACIGS surfaces as compared to the CIGS surfaces are slightly different. In addition to the formation of GaF3, which is washed away together with remaining KF by the ammonia treatment, also Ag-Se phases are formed. This may explain the increased sensitivity for ACIGS to excess KF leading to reduced voltage and fill factor.

Authors : A. Thomere (a,b,c), A. Lafond (a), C. Guillot-Deudon (a), M. T. Caldes (a), R. Bodeux( b,c), N. Barreau (a), S. Jobic (a)
Affiliations : a Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes cedex 3, France; b EDF R&D, 30 Route Départementale 128, 91120, Palaiseau, France; c Institut Photovoltaïque d’Île-de-France (IPVF), 30 Route Départementale 128, 91120, Palaiseau, France

Resume : Up to now, sulfide chalcopyrites lead to less efficient solar cells compared to the selenide counterparts. We have investigated the Cu2S-In2S3-Ga2S3 ternary system through a chemical crystallography approach (X-ray diffraction and chemical analyses) on bulk materials and established the phase diagram of these compounds. The general formulation of the targeted compounds (high temperature ceramic route) is Cu1 z(In1 xGax)1 z/3S2 (0

Authors : Arjan Hovestad1 ,Mirjam Theelen1, Marcel Simor1, Maarten van der Vleuten1, Henk Steijvers1, Karine van der Werf2, Dong Zhang2, Maarten Dörenkämper2, Hans Linden1
Affiliations : 1 TNO - Solliance, Thin Film Technology, High Tech Campus 21, 5656 AE Eindhoven, The Netherlands ; 2 ECN – Solliance, High Tech Campus 21, 5656 AE Eindhoven, The Netherlands

Resume : Reactive thermal annealing of electrodeposited precursors in elemental Se and S is an industrially attractive technology to produce CIGS thin film solar modules, due to the comparably low equipment costs and high metal utilization. However, effort is still required to obtain the double graded band-gap, with a minimum at 1.15 eV, that enables high-efficiency CIGS. We have demonstrated a bandgap optimization to 1.15 eV, giving an open-circuit voltage above 650 mV, by the adaptation of the gallium and sulphur profile in sequentially deposited Cu(In,Ga)(Se,S)2 absorbers in a 30x30 cm2 research line. Electro-optical modelling shows that straightforward selenisation of electrodeposited precursors results in well-behaved CuInSe2/CuGaSe2 bilayer absorbers. By pre-alloying before and extended soaking after selenisation the bandgap was increased from 1.00 to 1.10 eV. The pre-alloying stabilizes the electrodeposited precursors and by soaking the Ga-profile can be tuned. The minimum bandgap was further increased to 1.15 eV by reacting Cu(In,Ga)Se2 with elemental S to form Cu(In,Ga)(Se,S)2 with a few percent S near the junction. This bandgap and the maximum Voc of 665 mV obtained are higher than values earlier reported for semi-industrial routes based on elemental Se and S reactive thermal annealing. A 15.4% maximum active area efficiency (0.5 cm2) and an 13.3% average efficiency over 10x10 cm2 were achieved.

Authors : Valentin Achard1,2, Matteo Balestrieri2, Marie Jubault1,2, Laurent Lombez2,3 Daniel Lincot2,3, Frédérique Donsanti1,2
Affiliations : 1 EDF R&D, IPVF, 30 route départementale 128 91120 Palaiseau 2 IPVF, 30 route départementale 128 91120 Palaiseau 3 CNRS, IPVF,UMR 9006 30 route départementale 128 91120 Palaiseau

Resume : A proper control of Ga concentration profile is mandatory to achieve high efficiency Cu(In,Ga)Se2 (CIGS) solar cells. Deep gradients, detrimental for carriers diffusion, are obtained when CIGS is deposited with a standard three-stage process at low temperature on flexible substrates: an optimization of the process is needed. In this study, we show the impact of the depth of the notch by introducing Ga flux during the second stage from 0 nm/min to 1.1 nm/min. We show a better diffusion of the carrier by EQE measurements performed with bias and obtain a higher VOC compensated by a lower JSC du to a higher band gap energy. We then analyze CIGS growth during the second stage by coupling different characterization techniques: GD-OES, Raman and XRD on various absorbers extracted from the deposition chamber at different moments of the second stage. We were then able to compare the growth mechanisms before and after the optimization. We show a better crystallization and morphology for an absorbers grown with an optimized three-stage process. We also show the presence of less binary compounds at the end of the second stage by Raman spectroscopy. Thus, we manage to explain why less deep Ga grading profiles are obtained when Ga is evaporated during the second stage. To conclude, we discuss the impacts on the growth of CIGS during the last stage. Further investigations of the front grading allowed us to achieve a 17.8% solar efficiency without a KF post-deposition treatment and ARC.

Authors : Thomas Feurer, Stefano Pisoni, Thomas P. Weiss, Enrico Avancini, Romain Carron, Johannes Löckinger, Stephan Buecheler and Ayodhya N. Tiwari
Affiliations : Laboratory for Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstr. 129, 8600 Duebendorf, Switzerland

Resume : With a tunable band gap and high efficiency, Cu(In,Ga)Se2 (CIGS) solar cells have shown great potential for thin film tandem solar cells. For combining them with perovskite top cells, the bottom cell should preferably be based on a 1.0 eV absorber layer, achieving high efficiency and covering a broad spectral range. In this work we present the development of about 18% efficient CIS based bottom cells with an optoelectronic bandgap of 1.0 eV. The cells consist of a low bandgap front for a large part of the absorber as relevant for tandem applications, while a Ga back grading is applied towards the Mo contact to reduce the recombination loss. By Ga back grading and modifying the doping and the front interface, the open circuit voltage of those devices could be increased to above 585 mV while still keeping a high spectral response close to the 1.0 eV band gap. We demonstrate the suitability of these cells for tandem devices and show that using this approach even current matching for monolithic interconnection with perovskite top cells of 1.6eV bandgap is feasible. The tandem devices in 4 terminal configuration display a significant improvement over the single cell efficiencies.

10:00 Coffee Break    
CIGS: Growth and Bulk Properties : Maria Malitchaya, Takashi Koida
Authors : Susanne Siebentritt, Finn Babbe
Affiliations : University of Luxembourg

Resume : From photoluminescence spectroscopy we can determine on a bare absorber several quality indicators of the finished solar cell: the quasi Fermi level splitting, which corresponds to the maximum open circuit voltage, the luminescent efficiency which corresponds to the voltage loss with respect to the band gap of the absorber, and the diode factor of the solar cell. We will discuss the relations between the photoluminescence data and the solar cells properties and how photoluminescence can be used to monitor absorber quality before finishing the solar cell.

Authors : Helena Stange, Roland Mainz
Affiliations : Technische Universität Berlin, Institut für Werkstoffwissenschaften, 10587 Berlin, Germany; Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 14109 Berlin, Germany

Resume : It has been well known for a long time that Cu-Se deposition during the 2nd stage of the famous 3-stage Cu(In,Ga)Se2 (CIGS) co-evaporation process influences the microstructure of the final absorber layer. To gain a comprehensive understanding of the mechanisms and driving forces responsible for the microstructural changes during this stage and its effect on the absorber quality, we have performed a detailed series of in-situ studies, as well as complementing ex-situ studies in the recent past. Our studies provided detailed insights into several correlating changes of the structural and physical properties of the absorber layer during Cu-Se deposition or subsequent annealing, such as defect annihilation, stress relaxation, texture changes, and grain growth. In this contribution, we give a comprehensive overview of these various observations and demonstrate how they can all be explained by a single model of abnormal grain growth, driven by differences of the internal energy of neighboring grains. One of the important consequences is that this abnormal grain growth leads to the reduction of structural defects within the grains. Moreover, from our model we derive limitations and perspectives for future process optimizations of Cu(In,Ga)Se2 deposition by co-evaporation.

Authors : E. Avancini(1), R. Carron(1), S. Nishiwaki(1), D. Keller(2), R. Erni(2), Y. Arroyo(2), A. Priebe(3), T. Feurer(1), R. Menozzi(4), S. Di Napoli(4), G. Sozzi(4), S. Buecheler(1) and A. N. Tiwari(1)
Affiliations : (1)Laboratory for Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstr. 129, 8600 Duebendorf, Switzerland (2)Electron Microscopy Center, Empa-Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstr. 129, 8600 Duebendorf, Switzerland (3)Laboratory for Mechanics Materials and Nanostructures, Empa-Swiss Federal Laboratories for Materials Science and Technology, Feuerwerkerstrasse 39, 3602 Thun (4)Department of Engineering and Architecture, University of Parma, Parco Area delle Scienze 181A, 43124 Parma, Italy

Resume : Among the factors limiting the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells, the presence of unpassivated surfaces near the absorber/buffer interface might play an important role, due to possibly high recombination velocities in a region with large charge carrier densities. In this contribution, we report the presence of voids with diameters of up to 200 nm in the CIGS absorber layer, just below the front surface, as observed by transmission electron microscopy (TEM). CIGS thin films are grown by multi-stage co-evaporation. The deposition of a CdS buffer layer by chemical bath (CBD) leads to the passivation of some of the voids’ internal surface, whereas other voids are completely buried within the CIGS film and cannot be reached by the CBD solution. The density of voids is observed by coupled focused-ion-beam etching and scanning electron microscopy. In multi-stage co-evaporation, the CIGS film is grown non-stoichiometrically in a Cu poor - Cu rich - Cu poor sequence. The elemental distribution is analyzed during the different stages of the CIGS growth by scanning TEM coupled with energy-dispersive x-ray spectroscopy (STEM-EDX). We propose that the voids are formed during the final Cu rich to Cu poor transition, due to the out-migration of Cu and Se from non-homogeneously distributed areas with segregated Cu-Se phases. Possible pathways to eliminate or reduce the density and size of such voids are suggested. Finally, we analyze the impact of passivated and unpassivated void surfaces on photovoltaic properties by 2D and 3D simulations using the Sentaurus TCad Suite software.

Authors : Polyxeni Tsoulka, Nicolas Barreau, Isabelle Braems, Eric Gautron, Sylvie Harel, Ludovic Arzel
Affiliations : Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS-UMR 6502, Nantes

Resume : The limited photovoltaic performance of the CuIn(1-x)GaxSe2 (CIGSe) thin-film solar cells, when x exceeds the threshold value of 0.4, is under investigation for more than 20 years. So far, the best labscale energy conversion efficiency is achieved for Ga content (x) around 0.3, while theoretical predictions propose a better solar cell efficiency for x around 0.75. In our work, by coupling computational and experimental studies, we introduce a new approach to explain the limited performance for large x, which involves the presence of copper-selenide (Cu-Se) detrimental phases at the bulk of the CIGSe layer. These Cu-Se compounds are created during the CIGSe deposition process and they can precipitate at the GBs or/and within the grains, acting as recombination centers. Our XRD, RAMAN and EDS analyses demonstrate that the copper-selenide behavior differs, at low and high x. Thus, it appears that for large x, Cu-rich compounds precipitate at the bulk of the CIGSe layer, while at low x, Cu-Se phases segregate mainly at the surface and so they can be easily removed by wet-chemical etching with potassium cyanide (KCN). Moreover, our study raises questions about the Cu-Se diffusion within an In-rich or Ga-rich environment. Additional TEM analyses were also carried out, in order to further increase our knowledge about the Cu-Se behavior in the CIGSe layers.

Authors : Chen Li (1)*, Ekin Simsek Sanli (1), Helena Strange (2), Marc-Daniel Heinemann (3), Dieter Greiner (3), Norbert Schäfer (3), Roland Mainz (3), Daniel Abou-Ras (3), Peter A. van Aken (1)
Affiliations : (1) Stuttgart Center for Electron Microscopy, Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany; (2) Technische Universität Berlin, Institut für Werkstoffwissenschaften, 10587 Berlin, Germany; (3) Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 14109 Berlin, Germany; *Email:

Resume : The highest Cu(In,Ga)Se2 (CIGS) thin-film solar cell efficiencies are gained by depositing CIGS absorber layers with a three-stage co-evaporation process. During the second stage, Cu-Se is deposited on CIGS and heated, transforming the CIGS layer from Cu-poor to Cu-rich in composition. It has been reported that the Cu-(In,Ga) interdiffusion between CIGS and Cu-Se layers in the second stage is essential for the recrystallization of CIGS. However, little has been known about how the atomic structures evolve during this recrystallization. In this study, Cu-Se is deposited on CuInSe2 (CIS) and the recrystallization is directly monitored during in-situ heating in a scanning electron transmission microscope (STEM), which simultaneously reveals the composition changes via electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX). The results show that Cu favours diffusion along grain boundaries. Planar defects play an important role in grain recrystallization: the grains with high-density planar defects tend to be consumed by the grains without planar defects, and this only occurs in case of excess Cu on CIS. Moreover, Cu-In interdiffusion between CIS and Cu-Se layers transforms some CIS grains to Cu2-xSe grains. The newly formed Cu2-xSe grains show a reconstructed superlattice aligned with the tetragonal lattice of the surrounding CIS grains, suggesting a low-energy transformation mechanism between CIS and Cu2-xSe.

Authors : Torsten Schwarz,1 Guillaume Stechmann,1 Baptiste Gault,1 Oana Cojocaru-Mirédin,1,2 Roland Wuerz,3 Alberto Lomuscio,4 Susanne Siebentritt,4 and Dierk Raabe1
Affiliations : 1. Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany. 2. RWTH Aachen, I. Physikalisches Institut IA, Aachen, Germany. 3. Zentrum für Sonnenenergie‐und Wasserstoff-Forschung Baden‐Württemberg, Stuttgart, Germany. 4. Laboratory for Photovoltaics, University of Luxembourg, Belvaux, Luxembourg.

Resume : The compound semiconductors Cu(In,Ga)(S,Se)2 (CIGSSe) is used as absorber materials in thin-film solar cells. Currently, a record efficiency of 22.9% is achieved for CIGSSe solar cells. When considering the polycrystalline nature of the CIGSSe thin-films, this efficiency is surprisingly high. For further enhancement of the efficiency it is important to understand how the microstructure affects the electrical properties of the absorber and, hence, the device performance. Therefore, one needs to study the relationship between structural and chemical properties of grain boundaries (GB). Compositional changes at GBs in Cu(In,Ga)Se2 thin films have been already reported. However, there are only few studies, which correlate structural and chemical properties of GBs. Here, we investigate the correlation between crystallographic and chemical information of GBs by combining transmission Kikuchi diffraction and atom probe tomography. For the CIGSe system, we detect an elemental redistribution at random GBs but no chemical fluctuations at twin boundaries. We observe Cu depletion and In & Se enrichment at random GBs and, at some random GBs, a slight Ga depletion. This In on Cu scenario is accompanied by co-segregation of Na and K. The amount of impurity segregation varies from one GB to another but also along an individual GB. For the CIS system, we will present our latest results as well and will discuss possible effects of the observed phenomena on the cell performance.

12:15 Lunch    
CIGS: Buffer Layers and Interfaces : Frederik Larsson, Victor van Maris
Authors : N. Terada 1, S. Kawamura 1, Y. Iwamoto 1, K. Tanigawa 1, T. Shimamura 1, T. Kato 2, H. Sugimoto 2, H. Shibata 3, K. Matsubara 3 and S. Niki 3
Affiliations : 1. Kagoshima University, Kagoshima, Kagoshima 890-0065, Japan; 2. Solar Frontier K. K., Atsugi, Kanagawa 243-0206, Japan; 3. Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan

Resume : Electronic structure including lateral profiles of surface potential of Cu(In, Ga)(S, Se)2 (CIGSSe) surface and CdS/CIGSSe interface used in cells with conversion efficiency above 20% is studied by in-situ photoemission, inverse photoemission spectroscopy (PES and IPES) and UHV-Kelvin probe microscopy (KFM). Impact of post deposition treatment using KF (KF-PDT) on these electronic structures is also studied. The measurements have revealed that the surface of the CIGSSe without KF-PDT is characterized with a high S/(S+Se) ratio of about 0.6, an expanded band gap energy of 1.4–1.5 eV and a high conduction band minimum (CBM) of 0.8–0.9 eV. The KFM analysis shows a small built-in potential at grain boundaries and apparent potential fluctuation originated in crystalline faceting. The interface between in-situ evaporated CdS and the CIGSSe has almost “flat” connection of conduction band, where a large difference in CBM of the CdS and the CIGSSe of about -0.4 eV is compensated with a large interface induced band bending (iibb) of 0.44 eV. The KF-PDT results in a lowering of Fermi level of the CIGSSe and suppression of fluctuations in surface potential and photovoltage. The former causes an increase of built-in potential through the interface. iibb of this interface is, however, equivalent to that of the non-treated CIGSSe, which results in a slightly negative conduction band offset. These results have revealed that the current KF-PDT is beneficial mainly to open circuit voltage.

Authors : Mary Blankenship (1), Dirk Hauschild (2,3), James Carter (1), Victor van Maris (2,3), Wolfram Witte (4), Dimitrios Hariskos (4), Wanli Yang (5), Monika Blum (1,5), Michael Powalla (4), Lothar Weinhardt (1,2,3), Clemens Heske (1,2,3)
Affiliations : (1) Department of Chemistry and Biochemistry, University of Nevada, Las Vegas (UNLV), 4505 S. Maryland Parkway, Las Vegas, NV 89154, United States (2) Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Engesserstraße 18/20, 76128 Karlsruhe, Germany (3) Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT), Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany (4) Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstraße 1, 70563 Stuttgart, Germany (5) Advanced Light Source (ALS), Lawrence Berkeley National Lab, 1 Cyclotron Rd, Berkeley, CA 94720, United States

Resume : Cu(In,Ga)Se2-based (CIGSe) thin-film solar cells have continued to increase prominently in efficiency over the last years, reaching over 22% on a laboratory scale. Such high efficiencies are usually achieved with a Ga/(Ga+In) (GGI) ratio of ~0.30. In order to achieve higher open-circuit voltages, larger absorber band gaps are needed, which can be realized, e.g., by increasing the GGI. For such devices with a large GGI, a comprehensive understanding of the electronic and chemical structure of the CIGSe-based absorbers and their interface with the buffer layer is essential. Absorbers with different bulk GGI ratios of ~0.30, ~0.66, and ~0.95 and their interface with a solution-grown CdS buffer layer were studied using a number of different electron and soft x-ray spectroscopy techniques, including lab-based x-ray and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and x-ray-excited Auger electron spectroscopy. Combined with synchrotron-based soft x-ray emission spectroscopy, a full picture of the electronic and chemical structure at and near the surface can be drawn, including the band alignment and intermixing behavior as a function of GGI. In our contribution, initial results of this comprehensive study will be shown and discussed in view of their impact on the performance of corresponding CIGSe solar cells.

Authors : Xiaowei Jin (1), Altaf Pasha (1), Reinhard Schneider (1), Dimitrios Hariskos (2), Wolfram Witte (2), Michael Powalla (2), Dagmar Gerthsen (1)
Affiliations : (1) Laboratorium für Elektronenmikroskopie, Karlsruher Institut für Technologie, Karlsruhe, Germany (2) Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg, Stuttgart, Germany

Resume : Cu(In,Ga)Se2 (CIGS) thin films have been a promising material for solar cells with record conversion efficiencies above 22%. Usually, CdS is used as buffer layer for these devices, but due to the bandgap of 2.4 eV CdS buffers decrease the quantum efficiency of the device in the short wavelength region. Thus, Cd-free buffer materials with wider bandgap are demanded. In this work, three CIGS devices with the same Ga/(Ga+In) ratio (GGI) of about 0.3 deposited by co-evaporation but different solution-grown buffer layers, namely CdS, Zn(O,S), and InxSy, were analyzed. These three devices exhibit conversion efficiencies of 17.0% for CdS buffer, 14.7% for Zn(O,S), and 14.6% for InxSy, respectively. Microstructural investigations by TEM showed that at the bottom of the absorbers pores are present. Moreover, twin lamellas, stacking faults, and other defects exist in the CIGS. The interfacial growth of CdS/CIGS was coherent, while that of Zn(O,S)/CIGS and InxSy/CIGS was incoherent. EDXS results demonstrate that neither the absorbers nor the buffer layers are chemically homogenous on a micrometer scale. In addition to different bandgaps of the buffer layers, it can be supposed that local differences in the GGI and the Cu/(Ga+In) ratio (CGI) of the absorber as well as the buffer/CIGS epitaxial relationship may influence the conversion efficiency of CIGS solar cells, too.

Authors : Elaheh Ghorbani, Karsten Albe
Affiliations : Fachgebiet Materialmodellierung, Institut für Materialwissenschaft, TU Darmstadt, Otto-Berndt-Straße 3, D-64287 Darmstadt, Germany; Fachgebiet Materialmodellierung, Institut für Materialwissenschaft, TU Darmstadt, Otto-Berndt-Straße 3, D-64287 Darmstadt, Germany

Resume : Replacing cadmium sulfide (CdS) with an alternative buffer material in Cu(In,Ga)(S,Se)2 (CIGSSe) based thin film solar cells has been discussed vastly in the literature over the past two decades. One of the prime candidates introduced for this replacement is indium sulfide ( In2S3), which has suitable properties for optoelectronic applications. However, the presence of impurities even at low concentration can strongly influence the optical and electrical properties of In2S3. So-far, several studies documented an intermixed In2S3/CIGS interface, which mainly contains Na, Cu, O and Cl impurities. The existence of Cu and Na in indium sulfide is attributed to the diffusion from the CIGSSe absorber layer into the In2S3 buffer layer. Regarding the anionic impurities, the existence of chlorine and oxygen are due to incomplete sulfurization of the Cl-containing precursor and out-diffusion of oxygens from the ZnO front contact respectively. In spite of the vast experimental confirmation on the existence of impurities, the knowledge of their properties, particularly the role of Cl, in indium sulfide is not well-grounded. In this contribution, we mainly focus on anionic impurities in In2S3 and report on their formation enthalpy, influence on conductivity and optical transition levels. Besides, we present a comparison between the role of cationic (Na and Cu) and anionic (O, Cl) impurities in In2S3. This includes how these impurities influence the absorber/buffer interface, how they compete with each other in seizing available lattice sites, and they contribution in the experimentally observed Photo Luminescences (PL) and persistent electron photoconductivity. All calculations were performed based on Hybrid-Density Functional Theory (Hybrid-DFT) with projector augmented wave pseudopotentials, implemented in Vienna Ab-initio Simulation Package (VASP).

Authors : Takashi Koida, Jiro Nishinaga, Yuko Ueno, Hirohumi Higuchi, Hideki Takahashi, Masayuki Iioka, Hajime Shibata, Shigeru Niki
Affiliations : National Institute of Advanced Industrial Science and Technology

Resume : We investigated the effects of front contact layers composed of transparent conducting oxide (TCO) and transparent oxide semiconductor (TOS) layers on the performances of Cu(In,Ga)Se2 (CIGS) solar cells with a glass/Mo/CIGS/CdS/TOS/TCO in a relaxed and metastable states. After annealing under illumination, the CIGS solar cells with KF- and NaF-post-deposition treatments exhibited metastable increases in open-circuit voltage, fill factor, and the resulting conversion efficiency with a decrease in short-circuit current density. These are partly attributed to an increase in metastable acceptor density, being stable with time-scale of day even at room temperature, with a reduced space charge width. The metastability characteristics varied depending on the TOS/TCO layers, affecting the illumination spectra to the CdS layer during the heat light soaking process and the resulting free-hole densities in the CIGS layers. On the other hand, no large differences in the open-circuit voltage, fill factor, or the acceptor density were observed regardless of the TOS/TCO layers in the relaxed state produced by annealing in dark. The presented results highlight the importance of the design of CdS/TOS/TCO layers in solar cells operated in metastable states.

Authors : Johannes Löckinger, Romain Carron, Shiro Nishiwaki, Yaroslav E. Romanyuk, Stephan Buecheler, Ayodhya N. Tiwari
Affiliations : Laboratory for Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland

Resume : CdS is most commonly applied as buffer layer in photovoltaic devices employing Cu(In,Ga)Se2 (CIGS) as active absorbing material especially in highly efficient devices. The parasitic optical absorption of CdS (Eg = 2.4 eV) results in a reduced photocurrent density. A simple reduction of the CdS layer thickness, however, leads to a reduced device performance due to a loss in open-circuit voltage (Voc) attributed mainly to insufficient coverage on the rough surfaces and possible sputter damage of the CIGS absorber surface during the deposition of the window layer. In this contribution, alternative window layers are investigated replacing the sputtered ZnO. Highly resistive and transparent oxides, such as SnO2, TiO2, Al2O3, ZnO are deposited by atomic layer deposition (ALD) onto the substrate/Mo/CIGS/CdS stack. ALD is chosen for this purpose since for thin and resistive oxide layers a conformal coverage of the rough surface and a precise thickness control are paramount. Solar cells were characterized by (temperature-dependent) current-voltage and external quantum efficiency (EQE) measurements. It is found that high Voc (~0.7 V) can be retained with thinner CdS in combination with ALD-TiO2 and SnO2, and therefore a gain in current density (~1 mA cm-2), owing to a reduced parasitic absorption of CdS, is achieved. The drawback of this approach is a reduced FF of about 73% compared to 76% of the reference structure. The interpretation of the observations is supported by 1D device simulations using SCAPS.

15:30 Coffee Break    
Poster 4: CIGS Solar Cells and Alternative Absorbers : Susanne Siebentritt, Shogo Ishizuka
Authors : Raphael Edem Agbenyeke 1,2, Bo Keun Park 1,2, Gun Hwan Kim 1, Taek-Mo Chung 1,2, Chang Gyoun Kim 1,2, Jeong Hwan Han 1,3
Affiliations : 1. Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-Ro, Yuseong-Gu, Daejeon 34114, Republic of Korea 2. Department of Chemical Convergence Materials, University of Science and Technology (UST), 217, Gajeong-Ro, Yuseong-Gu, Deajeon 34113, Republic of Korea 3. Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea

Resume : The high diffusivity of Cu+ ions in the hexagonal-close-packed structure of Cu2-xS often leads to interesting and unexpected observations. Herein, we present the etching of oxide and sulfide thin film underlayers during the atomic layer deposition of Cu2-xS thin films. The infiltration of the underlayers by Cu+ ions is an essential step that precedes the etching process. However, it is suspected that the eventual etching of the underlayer, and the etch rate strongly depend on the lattice (bond dissociation energy) of the underlayer material. Thin films of ZnS, ZnO, SnS, and SnO were etched to different degrees during the deposition of Cu2-xS while SnO2 exhibited a high resistance to etching. Interestingly, a selective removal of Zn2+ was observed when a ternary Zn1-xSnxO film was used as underlayer. Based on XPS results and findings from other supplementary experiments, a possible reaction mechanism was proposed for the etching process. Finally, the observation was extended to the synthesis of Cu2-xS nanowires that can be used as effective absorbers for photovoltaic cells.

Authors : Ofir Friedman, Alexander Upcher, Tzvi Templeman, Dor Korn, Vladimir Ezersky and Yuval Golan
Affiliations : Department of Materials Engineering, and Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Be’er-Sheva 8410501, Israel

Resume : We present chemical solution deposited cadmium chalcogenide thin films with a well-defined orientation relationship with the monocrystalline substrate,[1,2] which allow to predict and control orientation and morphology of the films. CdS and CdSe films were chemically deposited on (100), (111)A and (111)B oriented GaAs substrates and characterized by analytical transmission electron microscopy, high-resolution scanning electron microscopy and x-ray diffraction. Both CdS and CdSe showed phase change from initial zinc blende at the interface to the stable wurtzite phase when deposited on GaAs(100). Furthermore, the importance of surface polarity was demonstrated for deposition on GaAs(111)A and GaAs(111)B. While in the case of CdS monocrystalline were formed only on GaAs(111)B, CdSe monocrystalline films were obtained also on GaAs(111)A. The monocrystalline films had wurtzite structure, with the c-axis oriented perpendicular to the substrate surface. In addition, the films showed high densities of stacking faults which are attributed to the well-known polytypism in CdS and CdSe. The advanced control over orientation and morphology of thin film enhance the materials potentials in optoelectronics devices. 1. O. Friedman, A. Upcher, T. Templeman, V. Ezersky and Y. Golan, Journal of Materials Chemistry C, 2017, 5, 1660-1667. 2. O. Friedman, D. Korn, V. Ezersky and Y. Golan, CrystEngComm, 2017, 19, 5381-5389.

Authors : Negar Gheshlaghi, Hadi Sedaghat Pisheh
Affiliations : Nano Science & Nanoengineering Program, Istanbul Technical University, Maslak Istanbul, Turkey

Resume : Increasing demand for energy soon will force us to seek environmentally clean alternative energy resources. Recent efforts to design ordered assemblies of semiconductor and metal nanoparticles as well as carbon nanostructures provide innovative strategies for designing next generation energy conversion devices. Recently, synthesis of atomically-flat colloidal nanoplatelets (NPLs) have been also demonstrated by using colloidal approaches. Owing to the extended lateral dimensions of colloidal NPLs larger than the exciton Bohr radius, these NPLs exhibit pure quantum confinement along their vertical thicknesses. NPLs which absorb light in the visible range, can serve as sensitizers as they are able to transfer electrons to large band gap semiconductors such as SnO2. Size quantization often becomes an important factor to drive the energetics to more favorable levels. Since colloidal NPLs has shown unique electronic structure, their optical properties significantly different from the spherical shaped colloidal NCs. With the formation of quantum-well like electronic structure, sharp excitonic transitions are observed from the absorption spectra corresponding to the heavy-hole and light-hole transitions. Here, by using surface modifiers, CdSe NPLs have been assembled onto mesoscopic SnO2 films. Femtosecond transient absorption and emission quenching experiments confirm the injection from the excited state of CdSe NPLs into SnO2 nanoparticles. The injected charge carriers in a CdSe-modified SnO2 film can be collected at a conducting electrode to generate a photocurrent. When the SnO2-CdSe composite employed as a photoanode in a photo electrochemical cell, it exhibits a photon-to-charge carrier generation efficiency of 21%.

Authors : Masafumi Yamaguchi1, Hitoshi Tampo2, Hajime Shibata2, Kan-Hua Lee1, Kenji Araki1, and Nobuaki Kojima1
Affiliations : 1Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, JAPAN E-mail:; 2 National Institute of Advanced Industrial Science and Technology Tsukuba Central 2, Tsukuba 305-8568 JAPAN

Resume : Efficiency potential of future generation solar cells such as wide bandgap CIGSe(CuInGaSe2), CIGS(CuInGaS2), CZTS (Cu2ZnSnS4) and CZTSSe (Cu2ZnSn(S,Se )4) solar cells is discussed based on external radiative efficiency (ERE), open-circuit voltage loss and fill factor loss and non-radiative recombination loss. In summary, 1) CIGSe solar cells have efficiency potential of over 26% such as 26.0 and 26.5% with normalized resistance of 0.05 and by improving in ERE into 5% and 10%, respectively from around 0.5%. 2) CIGS solar cells, efficiencies more than 22% from present 16.9% efficiency will be realized by improvements in ERE into more than 1% from about 0.001%. 3) CZTS(Se) and CIGS solar cells have efficiency potential of more than 20% and 22%, respectively by improvement in ERE from around about 0.001% to 1%. Effects of non-radiative recombination losses upon properties of future generation solar cells are discussed. 4) Regarding wide-gap CIGSe solar cells, lattice mismatching between CIGSe active layer and buffer layer, and increase in non-radiative recombination center density with increase in bandgap energy of CIGSe materials and solar cells are suggested as non-radiative recombination losses in wide-gap CIGSe solar cells. 5) Regarding CZTS(Se) solar cells, existence of unknown recombination loss and resistance loss due to low carrier mobility compared to CIGSe materials are shown in addition to surface and bulk recombination losses.

Authors : Young Kuk Lee
Affiliations : Korea Research Institute of Chemical Technology

Resume : Tin sulfide (SnS)-based thin film solar cells were fabricated using an novel organic precursor, Sn(dmamp)2 (dmamp, di-methyl-amino-methyl-propanolate). SnS thin films were deposited by MOCVD (metal organic chemical vapor deposition) in the temperature range of 150~300 °C. Effect of post annealing temperature and atmosphere on the cell efficiency were investigated. Also, n-type buffer, Zn(O,S) were optimized by estimation of band structure. X-ray diffraction (XRD) showed that formation of the pure SnS phase was observed on the sample grown between 200~250 °C and partial pressure of H2S at 10 Torr. Transmission electron microscope (TEM) images showed that metallic thin precipitates in the films were observed to disappear at moderate annealing condition. Maximum cell efficiency was measured up to ~2.6%.

Authors : Wentao Deng1, Fabiana Lisco2, Jake Bowers2, Laurence Peter1*
Affiliations : 1 Department of Chemistry, University of Bath, Bath, UK BA2 7AY, UK 2 Centre for Renewable Energy Systems Technology, University of Loughborough, Loughborough, LE11 3TU, UK

Resume : Blocking electrolyte contacts offer a convenient non-destructive way of characterizing thin film semiconductor layers such as the CdS buffer layers that are used in CIGS, CZTS and CdTe solar cells. If the layers are deposited on a highly doped substrate such as fluorine-doped tin oxide coated glass (FTO, the doping density and film thickness can be determined by Mott Schottky plots, which show a change in slope when the applied voltage is high enough that the space charge layer extend to the substrate. Whilst modelling the capacitance behaviour of CdS deposited from a chemical bath onto FTO, we observed that the layers evidently are partly porous. Porosity in buffer layers can give rise to shunting in thin film solar cells, to the photovoltaic performance, as it can cause shunting. We have therefore used cyclic voltammetry with a redox electrolyte to investigate the porosity of thin CdS films on FTO. The cyclic voltammetry response shows that the pores in the film behave like an array of ultramicroelectrodes. A new theory is presented that allows estimation of the average pore sizes and pore number density by combining the results of capacitance and cyclic voltammetry measurements. Typical values for a range of CdS films deposited under different conditions are presented and discussed.

Authors : Rokas Kondrotas1,2; Jun Zhang1,2; Liu Nian1,2; Chong Wang1,2; Yuhao Liu1,2; Jiang Tang1,2
Affiliations : 1-Sargent joint research center, Wuhan National Research Center for Optoelectronics and School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, 430074, Hubei, P. R. China. 2-Shenzhen R&D Center of Huazhong University of Science and Technology, Shenzhen, 518000, P. R. China.

Resume : V2VI3 family compounds such as Sb2(S,Se)3, Bi2(S,Se)3, (Sb,Bi)2Te3 have drawn considerable attention as promising earth-abundant, non-toxic materials for thermoelectric and recently for photovoltaic applications. These compounds exhibit lower than 3 crystal structure dimensionality giving rising to strongly anisotropic optical and electrical properties. For instance, a record certified 7.6% power conversion efficiency has been achieved for Sb2Se3 solar cells by carefully optimizing film orientation, which enhanced carrier transport across the absorber. Therefore, the control of thin film growth and consequently orientation is critical for V2VI3 ?based device performance. In this work we analyse the growth process of Sb2Se3 on various substrates from vapour phase. We investigate the influence of deposition parameters as well as the nature of substrate on Sb2Se3 film orientation, grain size, morphology and composition. We found out that as-deposited Sb2Se3 film orientation primarily depends on the substrate temperature and the substrate?s surface energy with respect to Sb2Se3. Because of anisotropic crystal structure of Sb2Se3, the facets with (hk0) indices generally have lower surface energy and therefore are the most dominant under near equilibrium growth conditions (high substrate temperature, Se-rich atmosphere). In this case, despite the chosen substrate, 1D and 2D growth is observed (nanorods, nanoneedles, nanosheets) and films are oriented horizontally (c-axis parallel to the surface). However, if substrate surface energy is high (Sb2Se3 is unlikely to create strong bonds with substrate) and deposition conditions are further away from equilibrium, the growth will become 3D and grains of Sb2Se3 will be oriented non-horizontally (hkl, l?0). The transition from non-horizontally to horizontally aligned films occurs at higher temperature for substrates with high surface energy. According to the findings we propose several strategies to synthesize high crystalline quality films with favourable orientation on various substrates. The results obtained in this study and observed tendencies we believe can be extended not only to V2VI3 family compounds, but also to other e.g. SnS, CuSbS2 anisotropic photovoltaic materials, where carrier transport is of key importance. Obtaining a better understanding of the growth mechanism and how to control film orientation can facilitate the fabrication of high performance devices.

Authors : Ji Hyeon Kim, Ahyeon Ma, Hyunjoon Song,* and Ki Min Nam*
Affiliations : Department of Chemistry, Mokpo National University, 1666 Yeongsan-ro, Cheonggye-myeon, Muan-gun, Jeonnam 58554, Republic of Korea; Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), and Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), 291 Daehak-ro, Yuseong-gu, Daejeon 34341, Republic of Korea

Resume : A well-defined WO3/Bi2S3 heterojunction comprised of single-crystalline Bi2S3 nanowire (Bi2S3NW) layers on top of the WO3 nanoparticles (WO3NP) was synthesized via an in-situ hydrothermal reaction. The single-crystalline Bi2S3 nanowires were uniformly grown and directly anchored on the surface of the WO3 nanoparticle layer. Compared to those of the other Bi2S3 electrodes, the resulting WO3NP/Bi2S3NW heterojunction showed enhanced photoelectrochemical (PEC) activity. The origin of this enhanced PEC activity is mainly attributed to the enhancement of charge separation on the Bi2S3 layer, due to the effective photogenerated electron transfer from the Bi2S3 conduction band to that of WO3. The uniform heterojunction interface with less interfacial defects generated through the in-situ growth method also leads to lower resistance at the interface between the different domains. Furthermore, the single-crystalline longitudinal structure of the Bi2S3 nanowires can provide a direct electrical pathway through a single domain of nanowires.

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Affiliations : -

Resume : -

Authors : Johannes Schoneberg, Jörg Ohland, Patrick Eraerds, Thomas Dalibor, Jürgen Parisi, Michael Richter
Affiliations : Laboratory for Chalcogenide Photovoltaics, Department of Energy and Semiconductor Research, Institute of Physics, University of Oldenburg, D-26111 , Laboratory for Chalcogenide Photovoltaics, Department of Energy and Semiconductor Research, Institute of Physics, University of Oldenburg, D-26111, AVANCIS GmbH, Otto-Hahn-Ring 6, 81739 Munich, Germany, AVANCIS GmbH, Otto-Hahn-Ring 6, 81739 Munich, Germany, Laboratory for Chalcogenide Photovoltaics, Department of Energy and Semiconductor Research, Institute of Physics, University of Oldenburg, D-26111, Laboratory for Chalcogenide Photovoltaics, Department of Energy and Semiconductor Research, Institute of Physics, University of Oldenburg, D-26111,

Resume : Replacing the CdS buffer in a Cu(In,Ga)(Se,S)_2 (CIGSSe) thin film solar cell with alternative materials such as InxSy requires reconsidering the interface properties between buffer and absorber as well as between buffer and intrinsic ZnO. Besides the material properties of the buffer, the interfaces could have a major impact on the junction formation of the solar cell. As the charge and defect properties of an interface are not directly measureable, we used numerical simulation combined with extensive material and device characterization to model the i-ZnO/In_xS_y:Na/CIGSSe interfaces. In this study we present a one-dimensional simulation model which is based on measured electrical characteristics of In_xS_y:Na buffer and i-ZnO layers, as well as capacitance, current-voltage and quantum efficiency measurements. The simulation results show good agreement with the experimental data over a broad temperature range. Derived from this, we postulate a novel band alignment at the junction due to an interface-near region which exhibits an inhomogeneous doping and defect density profile in the CIGSSe as well as donor type interface traps at the window/buffer and buffer/absorber interfaces. We conclude, that the combination of simulation and temperature dependent device characterization enables access to crucial interface parameters and is therefore a productive method to investigate the junction properties of new material combinations and strategies for further solar cell optimization including (alkali-) post deposition treatments.

Authors : Julien Marquardt, Sergiu Levcenco, Alexandra Franz, Thomas Unold, Christiane Stephan, Susan Schorr
Affiliations : Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany, Marquardt; Levcenco; Franz; Unold; Stephan-Scherb; Schorr Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstraße 74-100, 12249 Berlin, Germany, Marquardt; Stephan-Scherb; Schorr Bundesanstalt für Materialforschung und -prüfung (BAM), Unter den Eichen 87, 12205 Berlin, Germany, Stephan-Scherb

Resume : By now, the progress in manufacturing Cu(Ga,In)(S,Se)2 absorbers used in thin film solar cells led to conversion efficiencies of 22.6% [1]. Applying absorber materials with an intermediate band (IB) energy level in between the valence and conduction bands it is proposed that efficiency can be raised up to 63.3% [2]. In literature [3] different transition elements which may cause an IB within the band gap of chalcopyrite type CuGaS2 have been suggested. In our approach we studied solid solutions following the pseudo-binary section of CuGaS2 and MnS as well CuMnS2. Different series of powder samples were synthesized by solid state reaction of pure elements. The samples were analyzed in terms of chemical composition and phase content using WDX spectroscopy as well as in terms of crystal structure by X-ray and neutron diffraction. In addition we performed photoluminescence (PL) measurements to determine the optoelectronic properties. The presentation will summarize structural trends in dependence on the Mn content in CuGaS2 as well as off-stoichiometry. The observed PL spectra will be interpreted with calculations of the band diagram [4] and correlated with the cation distribution obtained from average neutron scattering length method. Moreover we prove that Mn incorporation does not lead to an IB-Material. [1] Jackson et al., Phys. Status Solidi RRL, 10 (2016) [2] Luque et al., Physica B, 382 (2006) [3] Martí et al., J. Appl. Phys., 103 (2008) [4] Zhao et al., Phys. Rev. B, 69 (2004)

Authors : Elif Peksu, Hakan Karaa?aç
Affiliations : Istanbul Technical University, Department of Physics, Maslak, 34469, Istanbul, Turkey

Resume : CuIn0.7Ga0.3Se2 (CIGS) thin films were deposited on both n-type silicon (Si) wafer and Si-nanowire (NW) arrays by one-step thermal evaporation of a stoichiometric powder for the construction of Planer-Si/CIGS and Si-NW/CIGS structured heterojunction solar cells. A core-shell like solar cell was constructed by decoration of electroless-etched silicon (Si) nanowires (NWs) with a 700-thick CIGS thin film. The detailed information about the synthesis of Si NWs have been presented elsewhere [1]. The CIGS thin film with the same thickness was also deposited on an n-type Si wafer (1 cm x 1cm) for the construction of planer counterpart of the core-shell like n-Si-NW/p-CIGS structured solar cell. Following the deposition of CIGS thin films, the whole device structures were annealed at 400 oC under the N2 flow for 30 min. The structural properties of the films deposited on both planer Si and Si-NW arrays were examined using a X-Ray diffractometer in the 2? range of 20o to 70o with a scan rate of 1o/min. Surface morphologies of the deposited films were studied by atomic force microscopy (AFM) operated in tapping mode and scanning electron microscopy (SEM) measurements. Reflectance measurements were performed in the wavelength range of 400-1000 nm to investigate the effect of introducing one-dimensional nanostructures into device structure of solar cells. Results have shown not only that there is a dramatic reduction in the reflection with the incorporation of Si nanowires but also a significant improvement in crystallinity of the deposited CIGS films when compared with those recorded for its planer counterparts. In addition, it was revealed that the post-annealing under the inert gas flow has a significant effect on morphology, electrical resistivity (measured by four point probe) and structural properties of CIGS thin films deposited on Si substrates both in planer and NW configurations. Silver dot contacts (1 mm in diameter) were evaporated by thermal evaporation to form the ohmic top-contacts of the solar cells structure by using a copper-shadow mask. As a back contact of the solar cells, the back side of the n-Si wafer was coated with a 150-nm thick silver thin film layer via thermal evaporation technique. The photovoltaic behaviors of the constructed prototype solar cells were examined under standard test conditions by using a PC-controlled solar simulator. Current ? voltage characteristics of the solar cells under dark and light illumination conditions were recorded and analyzed for the calculation of the solar parameters of the fabricated solar cells including open-circuit voltage (Voc), short-circuit current (Isc), series-/-shunt resistances and power conversion efficiency. [1] H Karaagac, M Parlak, E Yengel, MS Islam (2013) Mater Chem Phys 140: 382.

Authors : H. Shiel, O. Hutter, P. Yates, K. Durose, V. R. Dhanak, T. D. Veal, and J. D. Major
Affiliations : Stephenson Institute for Renewable Energy and Department of Physics, University of Liverpool, Liverpool, UK

Resume : Antimony Selenide (Sb2Se3) is a promising material for thin film PV due to its suitable optoelectronic properties, low cost, low toxicity, and its unusual 1D nanoribbon structure allowing benign grain boundaries. The record efficiency currently stands at 6.5% [1] for a thermally evaporated film. Here, we focus on devices produced by close space sublimation yielding efficiencies of 5.5% [2]. Current champion devices benefit from addition of quantum dots at the device back surface to enhance performance but there is still little fundamental understanding of the surface composition or its effect on cell performance. X-ray Photoemission Spectroscopy (XPS) is powerful for characterising surface chemistry and electronic properties in a material. Here, we present the results of in-depth XPS analysis of Sb2Se3 surfaces following various treatment steps. Commonly for Sb2Se3 films have varying amounts of Sb2O3 and Se at the back contact, yet it is unclear how this affects the performance of the device. We investigate how the surface properties can be controlled via different post-growth annealing processes and chemical etches such as CS2 and (NH4)2S and how this influences cell performance. This offers the opportunity to identify which surface characteristics best benefit device efficiency, as well as develop an understanding of the behaviour of Sb2Se3 following different post-growth treatments. [1] C. Chen et al., ACS Energy Lett. 2, 2125 (2017) [2] L. Phillips et al., submitted, 2018

Authors : Sebastian Siol 1,2; Jan C. Hellmann 2; S. David Tilley 3&; Michael Graetzel 3; Jan Morasch 2; Jonas Deuermeier 2; Wolfram Jaegermann 2; Andreas Klein 2
Affiliations : 1:Empa, Swiss Federal Laboratories for Materials Science and Technology Ueberlandstrasse 129, 8600 Duebendorf, Switzerland 2:Technische Universität Darmstadt, Institute of Materials Science, Surface Science Division, Petersenstrasse 32, 64287 Darmstadt, Germany 3:Ecole Polytechnique Fédérale de Lausanne (EPFL), Institut des Sciences et Ingénierie Chimiques, Laboratory of Photonics and Interfaces, Station 6, CH-1015 Lausanne, Switzerland 4:i3N/CENIMAT, Universidade NOVA de Lisboa and CEMOP/UNINOVA, Department of Materials Science, Faculty of Science and Technology, Campus de Caparica, 2829-516 Caparica, Portugal & Present Address: University of Zurich, Department of Chemistry, Winterthurerstrasse 190, 8057 Zurich, Switzerland

Resume : The search for material combinations with suitable energy band alignments is an important challenge in the development of new PV technologies. In this work it is shown how changes in bulk defect chemistry can dramatically influence the functional properties of an interface, even within the same material system. One striking example for this is the Cu2O/ZnO interface, for which many different values for the band offset have been reported [1]. The origin of this phenomenon is explored in a systematic study conducting multiple in-situ photoelectron spectroscopy interface experiments. A large process dependent variation in energy band alignment is observed, with valence band offsets ranging from ?EVB = 1.45 ? 2.7 eV [2]. This variation can be ascribed to a pinning of the Fermi level in ZnO and Cu2O, which can be traced back to oxygen vacancies in ZnO and to metallic precipitates in Cu2O, respectively. The intrinsic valence band offset for the interface, which is not modified by Fermi level pinning, is derived as ?EVB ? 1.5 eV, much lower than previously reported values in literature. The results from this study explain the variation of ?EVB at the Cu2O/ZnO interfaces reported here and in literature, but more importantly provide a general strategy to control the energy band alignment at semiconductor heterointerfaces. [1] M. Ichimura et al. Japanese Journal of Applied Physics, 50 (5), 5100, 2011 [2] S. Siol et al. ACS Appl. Mater. Interfaces, 8 (33), 21824?21831, 2016

Authors : D. G. Buldu1,2, J. de Wild1,2, T. Kohl1,2, S. Suresh3,5,G. Brammertz1,2, M. Meuris1,2, J. Poortmans1,3,4, B. Vermang1,2
Affiliations : 1 Institute for Material Research (IMO), Hasselt University (partner in Solliance & EnergyVille), Agoralaan gebouw H, Diepenbeek, 3590, Belgium 2 imec division IMOMEC (partner in Solliance & EnergyVille), Wetenschapspark 1, 3590 Diepenbeek, Belgium. 3 imec (partner in Solliance & EnergyVille), Kapeldreef 75, Leuven, 3001, Belgium 4 Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium 5 Photovoltaic Materials and Device, Delft University of Technology, Mekelweg 4, 2628 CD Delft, Netherlands.

Resume : CIGS is one of the most promising thin film solar cell technologies, and reached a record power conversion efficiency of 22.9% [1]. The most commonly used buffer layer is chemical bath deposition (CBD) of CdS, but these solar cells suffer parasitic absorption losses in the short wavelength region. The conventional Cd-Free buffer layer with wider bandgap is Zn(O,S), but this buffer regularly leads to increased recombination at the interface with CIGS, seen as a lower Voc. In this work, we study both buffer layers together with novel alternatives, and new surface treatments prior to passivation and buffer layer deposition. As surface treatment several etching and cleaning steps like NH3 solution, S(NH4)2 and HBr/Br2 solution are tried. CdS and Zn(O,S) buffer layers are grown by CBD, where CdS is the reference for comparison. TiO2 layer is used as a passivation layer to reduce interface recombination. The properties of buffer layer/CIGS interface with and without TiO2 passivation layer are examined by photoluminescence (PL) and time resolve photoluminescence (TR-PL), which are effective to characterize defects and recombination mechanisms. Electrical characterization is also done on finished solar cells. The effect of the surface treatments and passivation layer on CIGS solar cells will be shown in this paper. This work received funding from the European Union?s H2020 research and innovation programme under grant agreement No. 715027. [1] Solar Frontier, Solar Frontier Achieves World Record Thin-Film Solar Cell Efficiency of 22.9%, Press Release 20/12/2012, Available: (Accessed on 17/01/2018)

Authors : S. J. Heise, J. F. López Salas, M. Richter, J. Parisi
Affiliations : Laboratory for Chalcogenide Photovoltaics, Department of Energy and Semiconductor Research, Institute of Physics, University of Oldenburg, D-26111 Oldenburg, Germany

Resume : The analysis of the minority charge carrier lifetime in Cu(In,Ga)Se2 (CIGSe) thin-film solar cells via the contactless and non-destructive method of time-resolved photoluminescence (TRPL) is commonly believed to be most straightforward in low injection conditions. Moreover, to prevent rapid degradation of the blank absorber material, TRPL of CIGSe absorbers is usually measured with a CdS buffer layer. However, our time-resolved device simulations show that the presence of a p-n heterojunction strongly influences the shape of TRPL decays measured under low injection conditions. Simulations also predict that this effect depends on the doping concentration of the absorber layer. By varying the doping concentration via a metastable light-soaking treatment of the samples, we were able to experimentally verify these predictions. The main sources of the observed effects were determined to be charge carrier separation and thermionic emission at the heterojunction. These findings need to be taken into account in the interpretation of TRPL lifetime measurements on the CIGSe/CdS system.

Authors : Anaïs Loubat 1,2, Solène Béchu 1,2, Muriel Bouttemy 1,2, Sofia Gaiaschi 1,3, Valentin Achard 1,4, Mathieu Frégnaux 1,2, Damien Aureau 1,2, Jackie Vigneron 1,2, Frédérique Donsanti 1,4, Marie Jubault 1,4, Neghar Naghavi 1,5, Patrick Chapon 1,3, Daniel Lincot 1,5, Arnaud Etcheberry 1,2
Affiliations : 1. Institut Photovoltaïque d?Ile-de-France (IPVF), 30 RD 128, 91120 Palaiseau, France; 2. Institut Lavoisier de Versailles (ILV), UMR 8180 CNRS-UVSQ, 45 av. des Etats-Unis, Versailles, 78035, France; 3. HORIBA Scientific, avenue de la Vauve, Passage Jobin Yvon, CS 45002, 91120 Palaiseau, France; 4. EDF, IPVF, 30 RD 128, 91120 Palaiseau, France; 5. CNRS UMR 9006 IPVF, 30 RD128, 91120 Palaiseau, France.

Resume : CIGS (Cu(In,Ga)Se2) solar cells present high efficiencies (above 22%) but, to still be competitive in a constant evolving context with new materials and architectures, efficient analyses tools are required. We propose here an innovative coupling of characterization methods, Glow Discharge Optical Emission Spectroscopy (GD-OES) and X-ray Photoelectron Spectroscopy (XPS), to fully determine, at first sight, the chemical composition of CIGS absorber from the surface to the depth and to its further implementation on the final device. By combining these techniques, a high etching rate (GD-OES) and a precise monitoring of elements distribution in depth, enabling an accurate positioning at specific location (buried interfaces) (XPS) directly at the GD-OES bottom crater can be obtained. In a previous study the capability of this coupling has already been established. However, prior to XPS analysis, a preliminary regeneration step after stopping the GD-OES profiling is necessary to eliminate the superficial perturbed layer at the surface of the crater bottom. In this work we study the different strategies that can be employed to proceed to this necessary intermediate step to achieve the regeneration of the initial chemical information: an ex-situ wet chemical engineering (HCl, KCN and Bromine dipping) and a physical curing approach based on the use of Arn cluster sputtering, directly performed inside the XPS analysis chamber.

Authors : S. Siol, L. Baranowski, K. Mokurala, A. Zakutayev
Affiliations : National Renewable Energy Laboratory, Golden, CO, USA

Resume : An important part of the development of thin film chalcogenide solar cell technologies is the improvement of interfacial contact layers. For both established technologies (eg. CdTe, CIGS) as well as new absorber materials tuning the contact materials properties can result in better charge transport across the interface, and higher overall efficiency. Desired properties of contacts include a tunable position of conduction band (CB) position, chemical and structural compatibility to the absorber material, as well as moderately high levels of n-type doping. This presentation will discuss recent progress towards the development of alternative contact materials for thin film chalcogenide solar cells. First, controlling the phase and increasing conductivity of sputtered beta-In2S3 with high CB position, while maintaining low deposition temperature compatible with CIGS and CZTS solar cells, will be discussed[1]. Second, the presentation will discuss tuning conduction band position in wurtzite (Cd,Zn)S layers prepared by atomic layer deposition, and doping these layers with Ga to maintain high conductivity with increasing Zn content[2]. Finally, a new high-throughput technique for the optimization of contact layer thickness using combinatorial chemical bath deposition will be introduced based on the example of CdS[3]. [1] S. Siol, A. Zakutayev et al. ACS Appl. Mater. Interfaces 8, 14004 (2016) [2] L. L. Baranowski, A. Zakutayev et al. Mater. Chem. Front., 1, 1342 (2017) [3] K. Mokurala, A. Zakutayev et al. ACS Comb. Sci. 18, 583, (2016)

Authors : G. Sreedevi 1, M. Vasudeva Reddy 1, K.T. Ramakrishna Reddy 2 and Woo Kyoung Kim1*
Affiliations : 1School of Chemical Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan 38541, Korea. *Corresponding Author: Tel.: +82-53-810-2514, E-mail: 2Department of Physics, Sri Venkateswasra University, Tirupati 517 502, India.

Resume : A great deal of effort has been made to develop alternative earth-abundant, cost-effective and non-toxic semiconducting materials for photovoltaics, batteries, light-emitting diodes, photo catalysis, and other applications. In particular, tin (II) sulfide (SnS) has attracted more interest due to its structural multiformity, appealing catalytic activity and favorable opto-electrical properties. On the other hand, the complete optical analyzation of SnS thin films is essential for more preponderant designing of devices since the optical properties give the information to understand their electronic properties and band structures. Herein, we deposited SnS thin films on glass substrates as a function of concentration of an environmental friendly complexing agent ?tartaric acid (C4H6O6)? by an extremely facile route, chemical bath deposition using analytical grade stannous chloride (SnCl2.2H2O) and thioacetimide (C2H5NS) as source materials for Sn and S ions, respectively. Further, an exhaustive investigation on their optical properties was made using the transmittance and reflectance measurements. Finally electrical parameters such as resistivity, carrier mobility and carrier density were estimated using optical data. A detailed analysis results will be presented and discussed for a clean understanding of electronic characteristics of SnS thin films.

Authors : Marlena Ostrysz1*,Soomin Song1, Jihye Gwak1,2, Ara Cho1,2**
Affiliations : 1 Photovoltaic Laboratory, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea 2 Renewable Energy Engineering, University of Science and Technology (UST), Daejeon, Republic of Korea

Resume : Zn(S,O,OH) has been investigated as an alternative buffer material against CdS because of its direct wide band gap (~3.8 eV) at room temperature and non-toxic character [1]. To produce a buffer layer, chemical bath deposition (CBD) process is typically used but in Zn(S,O,OH) deposition, various factors (such as Zn source, S source, complexing agent, pH of solution, bath temperature, stirring speed and deposition time) are required to control. In this study, we focused on the difference of chemical contents depending on time. In the CBD of Zn(S,O,OH), there are two stages: clear solution stage and turbid stage. However, although chemical composition was not changed in clear stage, it shows different content in turbid stage. Therefore, we controlled the thickness of buffer layer by quartz crystal microbalance (QCM) system with certain temperature. The system follows the equation as below: ?m=(Ar ???)/(2f^2 ) ?F --> ?t=?m/(?c?Ar )) ?m-mass change;Ar-surface area of an electrode ; ?-elastic constant of crystal ;?-density of the crystal ; f- standard frequency . The characters of prepared buffer layer were analyzed by TEM and EDS and more observation will be discussed.

Authors : V. van Maris(1,2), D. Hauschild(1,2), M. Blankenship(3), L. Seitz(2), W. Witte(4), D. Hariskos(4), W. Yang(5), M. Blum(3,5), M. Powalla(4), C. Heske(1,2,3), L. Weinhardt(1,2,3)
Affiliations : (1)Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Engesserstraße 18/20, 76131 Karlsruhe, Germany; (2)Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; (3)Department of Chemistry & Biochemistry, University of Nevada, Las Vegas (UNLV), 4505 South Maryland Parkway, Las Vegas, NW 89154, United States; (4)Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstraße 1, 70563 Stuttgart, Germany; (5)Advanced Light Source (ALS), Lawrence Berkeley National Lab, 1 Cyclotron Road, Berkeley, CA 94720, United States

Resume : High-efficiency Cu(In,Ga)Se2 (CIGSe)-based thin-film solar cells often use a CdS buffer layer grown by chemical bath deposition (CBD). However, alternatives are being developed that are more environmentally friendly and/or more transparent in order to reduce absorption losses due to the buffer layer. Among these alternatives, InxSy is a very promising candidate, reaching CIGSe cell efficiencies very close to, or sometimes even above CIGSe devices with CBD-CdS buffer layers. To deliberately optimize the buffer layer design, an important prerequisite is detailed knowledge of the chemical and electronic structure of CIGSe absorber surfaces and their interfaces with CBD-InxSy buffer layers. To study these properties, we use laboratory-based electron spectroscopy techniques, as well as synchrotron-based soft x-ray emission spectroscopy. With these complementary spectroscopic methods, important insights into the chemical structure, e.g., characteristic species present at the surface of the investigated samples and intermixing processes at the interface, are gained. Furthermore, the electronic structure, i.e., the band edge positions as well as the band alignment at the CBD-InxSy/CIGSe interface, is studied. Initial results of these investigations will be presented and related to the electrical properties of corresponding CIGSe solar cell devices.

Authors : A. Shongalova 1,2, M. R. Correia 2 , B. Vermang 3,4,5, S. Ranjbar 4, S. Garud 4, J.M.V. Cunha 6, J. P. Teixeira 2, J. P. Leitão 2, J. C. González 8, P.M.P. Salomé 6,9, P. A. Fernandes 2,6,7
Affiliations : 1 Satpayev University, Satpayev street, 22a, 050013 Almaty City, Kazakhstan 2 I3N / Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal 3 University of Hasselt ? partner in Solliance, Agoralaan gebouw H, Diepenbeek, 3590, Belgium 4 Imec ? partner in Solliance, Kapeldreef 75, Leuven, 3001, Belgium 5 Imomec ? partner in Solliance, Wetenschapspark 1, Diepenbeek, 3590, Belgium 6 INL - International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga 4715-330 Braga Portugal 7 CIETI / Departamento de Física, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugal 8 Departamento de Física, Universidade Federal de Minas Gerais, 30123-970 Belo Horizonte, Minas Gerais, Brazil 9 Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal

Resume : In this work, we show a method to grow Sb2Se3 thin films which can be used as absorber layer in a solar cell structures. These films were grown on the top of different substrates such as soda-lime glass, Mo coated soda-lime glass and Si. The Sb-Se precursors films were deposited by RF magnetron sputtering and then annealed with an H2Se gas flow. Different annealing temperatures were tested and analyzed. This study also analyses the effects of the use of different substrates on properties of the film. Compositional and morphological analyzes are performed by Energy Dispersive Spectroscopy and Scanning Electron Microscopy, respectively. Two techniques are used to phase identification and structural characterization, namely, X-ray Diffraction and Raman Scattering Spectroscopy. The first technique shows that the main crystalline phase is Sb2Se3 with Pmna orthorhombic structure. Raman scattering analysis show the characteristic peaks of Sb2Se3 located at 150 cm-1 and 188 cm-1. Traces of rhombohedral and amorphous Se secondary phases are also observed. Special attention is taken to Raman scattering characterization conditions in order to avoid misidentification of Sb2Se3 phase. Many authors show results with Sb2O3 Raman modes identified as Sb2Se3. In this work we clearly differentiate the modes from each binary compound and it is shown how prevent the oxidation of Sb. Spectrophotometry measurements allowed to extract a direct bandgap with a value close to 1.06 eV.

Authors : S. Bose1,2, J.M.V. Cunha1, J.P. Teixeira3,4, J.P. Leitão3,4 , J. Jêróme1, J. Gaspar1, P.A. Fernandes1,3,5, M. Edoff1 P.M.P. Salomé1,4,
Affiliations : 1INL ? International Iberian Nanotechnology Laboratory, Avenida Mestre José Veiga, 4715-330 Braga, Portugal 2Ångström Laboratory, Solid State Electronics, Ångström Solar Center, Uppsala University, SE-751 21 Uppsala, Sweden 3I3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal 4Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal 5Departamento de Física, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida 431, 4200-072 Porto, Portugal

Resume : Thin film solar cells based on direct bandgap semiconductor materials like Cu(In,Ga)Se2 (CIGS), have light absorption properties far larger than indirect bandgap materials such as Si. So, absorber layers with thickness of 1-2 ?m are enough to absorb most of incoming light, while c-Si requires two more orders of magnitude thicker absorber layers. Nonetheless, CIGS solar cells with ultrathin absorber layers (0.5 ?m) suffer from incomplete photon absorption and increased charge carrier recombination at the rear electrode. Passivation layers are needed to reduce these detrimental effects. To study the detrimental effects of thinning the CIGS layer, we compare solar cells with and without passivation layers. 25nm Al2O3 layers and point structures have been created on the rear contact using e-beam lithography. The solar cells with ultrathin absorbers layers provide devices with a power conversion efficiency value of 8.2% while with nanostructured passivation layers the efficiency reached 9.1%. SEM and XRD measurements revealed that the CIGS layer does not undergo any significant morphological changes, in agreement with the similar values of carrier density (~8x10E16) and depletion region (~160 nm) extracted using C-V. However, J-V, EQE and photoluminescence results show very different optoelectronic behaviour which is indicative of a successful passivation of the rear contact. A detailed comparison between both types of samples is done and presented.

Authors : *Da Hye Jeong, *Sung Min Youn, **Jin Hyeok Kim, *Chaehwan Jeong
Affiliations : *Korea Institute of Industrial Technology: **Chonnam National University

Resume : CIGS thin films with high absorption coefficient and direct band gap are one of the promising absorbing materials for solar cells. CIGS solar cells on flexible substrate offer the advantage to be lightweight and low cost and can be applied to various fields such as transportation and building. In large-area flexible CIGS solar cells, it?s significant to obtain a higher fill factor. However, there are some limits for high efficiency due to low fill factor. Some reason might be caused by leakage current in pinhole or plasma damage of chemical bath deposited CdS. Therefore, it?s very important to find solutions for increase of fill factor. Both buffer layer with high durability and process condition of i-ZnO thin layer with avoidable plasma damage are needed for large-area flexible solar cells. In this study, Mo back contact layer on stainless steel substrate (100×100 mm) are deposited by a DC sputtering and CIGS absorber layer are deposited by a co-evaporation method. As buffer layers, chemical bath deposited CdS and atomic layer deposited ZnS are prepared with 50 nm thick. These are followed by RF sputtered i-ZnO layer with varying input RF power for avoiding plasma damage. Surface and interfacial properties between buffer layer and i-ZnO layer are investigated using TEM, GD-OES, Microwave PCD and Kelvin probe method. Finally, the photovoltaic properties are analyzed with a solar cell simulator and external quantum efficiency measurement systems.

Authors : J.M.V. Cunha1, P.A. Fernandes1,2,3, B. Vermang4,5,6, J.P. Teixeira3,7, S. Bose1,8, J. Gaspar1, M. Edoff8, J.P. Leitão3,7, P.M.P. Salomé1,7
Affiliations : 1INL ? International Iberian Nanotechnology Laboratory, Avenida Mestre José Veiga, 4715-330 Braga, Portugal 2Departamento de Física, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida 431, 4200-072 Porto, Portugal 3I3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal 4University of Hasselt ? partner in Solliance, Agoralaan gebouw H, Diepenbeek, 3590, Belgium 5Imec ? partner in Solliance, Kapeldreef 75, Leuven, 3001, Belgium 6Imomec ? partner in Solliance, Wetenschapspark 1, Diepenbeek, 3590, Belgium 7Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal 8Ångström Laboratory, Solid State Electronics, Ångström Solar Center, Uppsala University, SE-751 21 Uppsala, Sweden

Resume : Metal-Insulator-Semiconductor (MIS) structures with Al2O3, Si3Nx and SiOx as passivation layers in Cu(In,Ga)Se2 (CIGS) solar cells are studied. The structures consisted of Mo/CIGS/insulator/Al. Several measurements were done to quantify the insulator deposition influence on the CIGS surface, namely: Raman scattering, X-ray diffraction and Photoluminescence. To study the electrical properties of the CIGS-insulator interface, the performed measurements were: i) C-V measurements to estimate the number and polarity of fixed charges (Qf) and ii) G/?-f measurements to estimate the density of interface defects (Dit). In summary, the deposition of the insulators at high temperatures (300 ºC) and the use of sputtering causes surface modification on CIGS which compromises device operation if used as front contact passivation layer. Moreover, by varying the SiOx deposition parameters, it is possible to have opposite charges inside the insulator, +2.8×1011 and -6.1×1010 cm-2, which would allow its use for different architectures. All the insulators have density of interface defects values under 1×1013 eV-1cm-2. This is a range of values that, according to the literature, allows for an effective interface passivation. The material with lower Dit values was Al2O3 deposited by sputtering, 0.8×1012 eV-1cm-2, which is a remarkable result taking into account that its deposition method, sputtering, causes CIGS surface modification.

Authors : Rosemary Jones [1], David J. Lewis [1], Karen L. Syres [3], Aleksander A. Tedstone [2], Zheshen Li [4], Paul O'Brien [1,2], Andrew G. Thomas [1]
Affiliations : [1] School of Materials, University of Manchester, UK [2] School of Chemistry, University of Manchester, UK [3] School of Physical Sciences and Computing, University of Central Lancashire, UK [4] Department of Physics and Astronomy, Aarhus University, Denmark

Resume : Dye, quantum dot and perovskite sensitised metal oxides are a subject of intensive research. An alternative approach to sensitising surfaces is to use small band gap 2-D materials, such as chalcogenides where the band gap can be tuned by varying the number of layers (Brent 2015). In order for such devices to operate the relative positions of valence and conduction bands of the sensitiser and n-type material is important. Here we report on the measurement of band alignment of 2-D SnS deposited on Anatase (101) surface by x-ray photoelectron spectroscopy (XPS). The 2-D SnS was obtained by liquid-phase exfoliation and deposited directly onto an Anatase (101) single crystal surface, which had been cleaned under ultra-high vacuum conditions. To determine the alignment the valence band offset for the heterojunction n-TiO2/p-SnS was measured using soft XPS which gave an overlap of 0.55 eV. Literature values of the band gaps of 2-D SnS (1.6 eV) (Brent 2015) and TiO2 (3.2 eV) (Kaven 1996) were used to determine the conduction band position. Analysis shows that the interface between p-SnS and single crystal Anatase phase n-TiO2 has a type II offset. Under the same conditions Rutile (110) and ZnO (100), with bandgaps of 3.0 eV (Poumellec 1991) and 3.3 eV (Liang 1968) respectively, also demonstrated a type II offset interface with 2-D SnS. Rutile (110) and ZnO (100) showed larger overlaps with 2-D SnS of 0.9 eV and 0.7 eV respectively.

Authors : N. Spalatu, J. Hiie and M. Krunks
Affiliations : Laboratory of Thin Film Chemical Technologies, Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : We report a systematic study of the impact of post deposition thermal treatments (PDT) on properties of SnS thin films and SnS/CdS solar cells prepared by closed-spaced sublimation. Results are analysed for different PDT including annealing in air, N2, vacuum and closed quartz ampoules in the range of 400-500 oC without, and in presence of alkali halide flux. The properties of SnS films were analysed by XRD, SEM, EDX and UV-vis, while the glass/FTO/CdS/SnS/Au solar cells were characterized by I-V and EQE. As deposited SnS films on glass and CdS/FTO/glass substrates had stoichiometric composition, large grains of orthorhombic structure and band gap of 1.4 eV. Hall measurements indicate p-type carrier concentration of 1018 cm-3 and mobility of 0.9 cm2 V-1s-1. Properties of SnS films strongly depend on the PDT conditions. Annealing in evacuated ampoules increases mobility to 2 cm2 V-1s-1 and decreases the density of holes to 5 × 1017 cm?3 which is beneficial for p-n junction formation. Strong activity of alkali halide flux was observed in the PDT, especially highlighted in the air - treated SnS films. The flux generates melted phase and promotes grain growth by mass transport, resulting in recrystallization and sintering processes in films. While as deposited SnS solar cells exhibit conversion efficiency of 0.5 %, then thermal treatment of glass/FTO/CdS/SnS structure in closed ampoules resulted in efficiency of 1.6 % with VOC of 250 mV, JSC of 12.7 mA/cm2 and FF of 50 %.

Authors : Thomas Paul Weiss, Benjamin Bissig, Thomas Feurer, Romain Carron, Stephan Buecheler, Ayodhya N. Tiwari
Affiliations : Laboratory for Thin Films and Photovoltaics, Empa ? Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf

Resume : CuInSe2 (CIS) and Ga back-graded Cu(In,Ga)Se2 (CIGS) devices with efficiencies as high as 16.1 % are characterized by means of time resolved photo luminescence (TRPL). In particular the Ga back graded CIGS device consists of a Ga grading towards the back while the top part is CIS with sufficient thickness for absorption. TRPL measurements with different surface modifications are carried out and the extracted measured decay times are compared to obtain insights into surface recombination velocities, the minority carrier mobility and bulk lifetime. Experimental data is systematically analyzed based on analytical expressions for the CIS and the back graded CIGS absorber and corroborated by Sentaurus TCAD simulations. The presented approach will allow the estimation for upper and lower bounds of the surface recombi-nation velocities based only on the tail decay time of experimentally measured transients. Additionally, it will be shown that the back-graded CIGS device with an optical bandgap of 1.0 eV has a well passiv-ated back contact, which might be the reason for the performance leap to 16.1 % efficiency.

Authors : Daniel Ory 1 2, Nicolas Paul 2 , Vincent Le Guen 2 , Laurent Lombez 1 3
Affiliations : 1 IPVF, 91120 PALAISEAU, France 2 EDF R&D, 91120 PALAISEAU, France 3 CNRS, UMR 9006, 91120 PALAISEAU, France

Resume : We present a numerical model to investigate spectrally resolved photoluminescence images acquired by hyperspectral imager. A common method is to consider the high energy part of the luminance spectrum and extract the temperature and the Quasi-Fermi Level Splitting using a linearized form of the generalized Planck?s law for semiconductors. On the contrary, our method considers the complete luminescence spectrum which is modeled following the work of J. K. Katahara and H. W. Hillhouse. This allows to extract absorption properties as well as thermodynamic properties of the device such as the temperature and the quasi-Fermi level splitting. A three-dimensional deconvolution algorithm has been developed to jointly deal with the spectral and the spatial response of the camera sensor and therefore improve accuracy of the results. The validity of the method is discussed to get rid of any ambiguity that could be inherent to fitting procedures with multi-parameters. We investigate the impact of the noise to signal ratio of the acquired data to properly access the material properties. Spatial correlations between some parameters are inherent to the absorption modelling and may induce wrong interpretation of the results. We apply the method to CIGS polycrystalline thin films solar cells. We show that the Quasi-Fermi Level Splitting is much less spatially varying than shown before and discuss the real link between luminescence mapping and the spatial variations of this material properties.

Authors : Merve Demir 1, Makbule Terlemezoglu 1,2,4, Ozge Bayrakli 1,3,4, Hasan Huseyin Gullu 4,5 Mehmet Parlak1,4
Affiliations : 1- Department of Physics, Middle East Technical University, Ankara, Turkey 2- Department of Physics, Nam?k Kemal University, Tekirdag, Turkey 3- Department of Physics, Ahi Evran University, Kirsehir, Turkey 4- Center for Solar Energy Research and Applications (GUNAM), Middle East Technical University, Ankara, Turkey 5- Department of Electrical and Electronics, Atilim University, 06836, Ankara/Turkey

Resume : Thin ?lms of metal chalcogenides have been point of interest in their potential industrial applications. Among these materials, II?VI semiconductor group of materials with their remarkable optoelectronic properties as ideal direct band gap, high absorption coefficient and photo-sensitive behavior in the visible region of spectrum have attracted the research on the photovoltaic diode applications and investigated an alternative material system in the fabrication of low cost optoelectronic devices. In this study, cadmium selenide (CdSe) thin ?lms were deposited on a soda-lime glass substrates using direct evaporation of stoichiometric CdSe powder with purity 99.99% at room temperature. Following to the deposition, the effect of the annealing temperature on compositional, structural, morphological, optical and electrical properties of the films were discussed. From the analysis of energy dispersive X-ray spectroscopy, it was observed that the stoichiometric transfer of material from source powder to the deposited films was achieved, and additionally under the post-heat treatment this compositional homogeneity was preserved. The X-ray diffraction profile shows the characteristic diffraction peaks in a good agreement with the literature and crystalline quality was improved with increasing annealing temperature. No major peaks corresponding to secondary phases were detected in the diffraction pattern of both as-deposited and annealed films. Moreover, crystal structure of the films was detailed with Raman spectroscopy measurements. The surface imaging and analysis using scanning electron microscopy and detailed surface morphology and roughness analysis using atomic force microscope showed that the deposited CdSe thin ?lms are dense and compact in nature. From the optical transmission studies, films were found in direct optical transition characteristics and the band gap values of the samples were calculated using Tauc plots. The temperature dependent conductivity measurements were carried out to investigate their conduction mechanisms and investigate the change in the electrical conductivity depending on the illumination intensity.

Authors : Torsten Hölscher, Thomas Schneider, Matthias Maiberg, and Roland Scheer
Affiliations : Martin-Luther-University Halle-Wittenberg, Institute of Physics, Von-Danckelmann-Platz 3, 06120 Halle (Saale), Germany

Resume : We present our study about the diminishing impact of air-light exposure (ALE) of bare Cu(In,Ga)Se2 (CIGSe) absorber layers on the electronic properties of the complete solar cells. Former investigations with time-resolved photoluminescence revealed a strong degradation of the minority charge carrier lifetime of uncovered CIGSe layers after illumination in ambient air. The reason could be referred to an increased surface recombination velocity caused by a Na, O, and light induced modification of the CIGSe surface. In this regard for completed solar cells a reduction of open circuit voltage and power conversion efficiency in the range of 10-20% compared to solar cells from non-ALE absorbers is observed. With admittance spectroscopy under an applied DC bias voltage (-1 .. +0.75 V) we detect an additional defect signature in the case of cells with ALE absorbers. The voltage dependence of the attempt to escape frequency of this defect signature is an explicit sign of interface states. Simulations by SCAPS 1d reveal a deep acceptor like defect state Et ? EV ? 350 meV with a broad Gaussian energy distribution ?(Et) ? 100 meV. The high defect density Nt ? 10^10 .. 10^11 cm^-2 causes a reduced type inversion of the absorber at the interface and a drift of the Fermi level to an unfavourable position close to midgap. The critical role of the band alignment or the increased interface recombination velocity on the reduced performance values of ALE absorber solar cells will be discussed.

Authors : Hasan Huseyin Gullu 1,5, Ozge Bayrakli 2,3,5, Makbule Terlemezoglu 2,4,5, Mehmet Parlak 2,5
Affiliations : 1- Department of Electrical and Electronics, Atilim University, 06836, Ankara/Turkey 2- Department of Physics, Middle East Technical University, Ankara, Turkey 3- Department of Physics, Ahi Evran University, Kirsehir, Turkey 4- Department of Physics, Nam?k Kemal University, Tekirdag, Turkey 5- Center for Solar Energy Research and Applications (GUNAM), Middle East Technical University, Ankara, Turkey

Resume : Transparent and conductive molybdenum oxide (MoOx) have become an attractive material in a variety of technological and industrial applications in the field of solar cells. In a variety of techniques, sputtering is a favorable deposition technique to obtain a wide range of electrical and optical properties of thin metal-oxide ?lms. Therefore, in this work, transparent and conductive MoOx thin film samples were deposited on the ultrasonically cleaned soda lime glass substrates by DC sputtering technique. A full understanding of the structural, morphological, optical and electrical properties of the as-deposited films were investigated under the effect of post-annealing treatments with different applied temperatures. The effects of deposition pressure and sputtering power were investigated and optimization process was achieved to obtain amorphous MoOx ?lms with low surface roughness and enhanced conductive behavior with low deposition temperature. From optical transmission measurements, band gap values were calculated in terms of annealing temperature. The optical and electrical characteristics were found to be strongly enhanced by post-deposition annealing. Improvement of these characteristics of the films was achieved as highly transparent in the visible wavelength range and conductive behaviors. An impact on the electrical conductivity was also observed under illuminated current-voltage measurements. The activation energy for electrical conduction were calculated and the photoconductivity behaviors of the films were discussed by applying two-center recombination model

Authors : Y. Vygranenko (1), M. Vieira (1,2), G. Lavareda (1,3), C. Nunes de Carvalho (3,4), P. Brogueira (4,5), A. Amaral (4,5), N. P. Barradas (6)
Affiliations : 1) CTS-UNINOVA, Campus da Caparica, 2829-516 Caparica, Portugal; 2) Departamento de Engenharia Electrónica e Telecomunicações e de Computadores, ISEL, Lisboa, Portugal; 3) Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Portugal; 4) CeFEMA and 5) Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Portugal; 6) Centro de Ciências Tecnológicas e Nucleares, Lisboa, Portugal.

Resume : In this contribution, we report on transparent semiconducting indium sulfur fluoride (ISF) thin-films exhibiting high sensitivity to UV radiation. The films were deposited on fused silica and silicon substrates using a radio-frequency plasma-enhanced reactive thermal evaporation system. The deposition was performed evaporating pure indium in SF6 plasma at a substrate temperature of 423 K. Rutherford backscattering (RBS) measurements were used to determine the chemical composition of the films deposited on silicon substrates. The film characterization includes electrical, optical, and photoconductivity measurements. The synthesized compound is highly-resistive (~700 M?-cm at 300 K) and exhibits an evident semiconducting behaviour. The activation energy of 0.88 eV is deduced from the temperature dependence of electrical resistivity. The indirect band energy gap of 2.78-2.9 eV is determined from transmittance spectra of the ISF films. The photoconductivity band is centred at 342 nm wavelength. The photoconductivity spectrum also shows the Urbach tail with a characteristic energy of 123 meV. ISF is a promising candidate for buffer layers in chalcogenide-based solar cells.

Authors : Marek Maciaszek, Pawel Zabierowski
Affiliations : Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warszawa, Poland

Resume : A narrow p+ layer between the buffer layer and the absorber is often assumed in order to interpret the electrical characteristics of Cu(In,Ga)Se2-based solar cells, in particular I-V and C-V. The model of the p+ layer can explain changes of the fill factor after light soaking and ? at least partially ? so called double-diode behavior. However, it is difficult to obtain the width and the acceptor density of such a layer from standard methods (like C-V or DLCP). Our goal was to analyze this model quantitatively in order to derive equations allowing for extracting parameters of the p+ layer from I-V characteristics. Starting from the electrostatic analysis we derived formulas describing the relationship between parameters of the p+ layer (width and density of acceptors) and the voltage at which the deterioration of the I-V curve begins. It is shown that the potential distribution in the buffer layer has a strong impact on analytical results. Then, we apply obtained equations for the analysis of experimental I-V curves in different metastable states: light-soaked and reverse-biased. Finally, we discuss calculated parameters of the p+ layer in the framework of the existing models, like (VSe-VCu) or InCu. (VSe-VCu) model can be a good explanation of the fill factor changes related to the p+ layer.

Authors : Dumitru Untila1,2, Iuliana Caraman3, Igor Evtodiev1,2, Liliana Dmitroglo1, Nicolae Spalatu4, Mihail Caraman1
Affiliations : 1 Moldova State University, A. Mateevici, 60, MD-2009, Chisinau, Republic of Moldova; 2 Ghitu Institute of Electronic Engineering and Nanotechnologies, Academy of Sciences of Moldova, Academiei, 3/3, MD-2028, Chisinau, Republic of Moldova; 3 Engineering Department, ?Vasile Alecsandri? University of Bacau, Calea Marasesti, 157, RO-600115, Bacau, Romania; 4 Tallinn University of Technology, Department of Materials Science, Ehitajate tee, 5, EE-19086 Tallinn, Estonia

Resume : The structural and photoluminescence (PL) properties are studied for the ZnTe nanolamellar structures obtained by thermal annealing (TA) of the GaTe crystals, in Zn vapours, at 800°C, for 1 h up to 24 h. As a result of the TA, the external surface of the GaTe plates becomes reddish. The surface formations are oriented parallel to the GaTe plates and entirely cover the surface when treatment duration increases up to 24 h. The XRD analysis revealed the presence of the ZnTe crystallites. At the same time, there some phases of the Ga compounds with Te are formed (e.g. Ga2Te3). The Ga2Te3 phase is an intermediate phase in the formation process of the ZnTe compound, while Ga2Te5 and Ga7Te10 phases are obtained as a result of the TA. The presence of the ZnTe layer on the surface of GaTe plates is confirmed by Raman spectroscopy. SEM and EDX analysis showed that Ga atoms, released as a result of ZnTe compound formation, create microislands spread at both structure surface and Van der Waals space between elementary packings (Te-Ga-Ga-Te). The obtained material has PL features characteristic for ZnTe and GaTe compounds. Thus, at 80K, PL spectra contain two types of bands localized near to the fundamental absorption bands of the GaTe and ZnTe compounds. In the PL spectrum, the band of the localized excitons prevails by intensity in the fundamental absorption region of GaTe crystals. The emission band of the donor-acceptor type in ZnTe crystallites, is present in the region h?>Eg(GaTe).

Authors : Hui Li1,2, Fei Qu1, Xinlu Lin1,2, Hongwei Gu1,2, Wenjing Wang1,2
Affiliations : 1Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China, Beijing, China /2University of Chinese Academy of Sciences, Beijing 100049, Beijing, China

Resume : Abstract ? Cu(In,Ga)Se2 (CIGS) polycrystalline thin-film solar cells with an area of 0.25 cm2 were prepared by a three-stage co-evaporation method. By applying ZnMgO secondary buffer layer, we obtained CIGS solar cells with an efficiency of 13.12% with only 20 nm CdS first buffer layer. The thickness of CdS buffer layer was greatly reduced by applying ZnMgO secondary buffer layer. The ZnMgO buffer layer reduced the optical loss and interface recombination, leasing to the enhanced performance of CIGS solar cells.

Authors : Faisal Baig, Yousaf Hameed Khattak, Shafi Ullah, Bernabé María , Abrar Ahmed ,Hanif Ullah
Affiliations : Departament de Física Aplicada, ETS de Enginyeria del Disseny, Universitat Politécnica de Valencia, Camí de Vera València, (Spain) Department of Electrical Engineering, Federal Urdu University of Arts, Science and Technology Islamabad (Pakistan)

Resume : At present various simulation software for numerical modeling are being utilized to estimate the performance of group II-VI based solar cells. In this work, we modelled tin sulfide (SnS) as an absorber layer for solar cell and it belongs to group IV-VI. SnS is an environmental friendly earth abundant material and it is suitable for solar cell application. The structure of solar cell is SnS as absorber layer, Cd1-xZnxS as buffer layer and ZnO as window layer. The performance of this device is simulated numerically by changing the molar concentration of Zn in buffer layer by adjusting conduction band offset. After optimizing the molar concentration of Zn in buffer layer efficiency of solar cell achieved in this work is 26.414%.

Authors : Victoria Elena González-Flores, M. Rohini, Ana Rosa Garcia Angelmo, Rafael Ballinas Morales, Oscar GomezDaza, M.T. Santhamma Nair, P. Karunakaran Nair
Affiliations : Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Temixco, Morelos 62580, MEXICO

Resume : Chemical bath deposition yields SnS thin films of cubic (SnS(CUB)) or orthorhombic (SnS(ORT)) crystal structure with optical band gap of 1.7 eV or 1.1 eV, respectively, and p-type conductivity. In the present work, we have developed solar cells incorporating these films of different crystal structures by sequential deposition on glass substrates coated with SnO2:F (FTO) and CdS. We used a chemical bath with thioacetamide as sulfide source for the cubic films and that with thiosulfate for the orthorhombic thin films. The superstrate solar cell: FTO/CdS/SnS(CUB)/C-Ag showed an open circuit voltage (Voc) of 0.368 V, a short circuit current density (Jsc) of 2.77 mA/cm2, a fill factor of 0.33 and a conversion efficiency (?) of 0.33%. The solar cell parameters became significantly better when an SnS(ORT) thin film is added, which leads to more light generated current through more effective optical absorption in it. Thus, the cell structure FTO/CdS/SnS(CUB)-SnS(ORT)/C-Ag presented Voc of 0.488 V, Jsc of 8.35 mA/cm2, FF of 0.40 and an efficiency of 1.65 %. The paper will also deal with results on our subsequent investigations on modifications in properties of SnS(CUB)-SnS(ORT), heating the cell structure, and application of different electrodes toward improving the performance of these solar cells.

Authors : Fabiola de Bray Sánchez, M. T. Santhamma Nair, P. Karunakaran Nair
Affiliations : Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Temixco, Morelos 62580, MEXICO

Resume : Antimony chalcogenides, Sb2S3, Sb2Se3, and their solid solutions Sb2(S/Se)3, with optical band gaps (Eg) in the interval of 1.88 to 1.1 eV have prospects for application as absorber materials of low toxicity in solar cells. In the present work, we have prepared thin films of Sb2(S/Se)3 by chemical deposition as well as by thermal evaporation of the solid antimony sulfide/selenide, which settled as precipitate during chemical bath deposition of thin film materials. These thin films of Sb2(S/Se)3 have Eg of 1.45 ? 1.71 eV with a coefficient of absorption > 1E5 cm-1 in the visible region. The photoconductivity of the films is typically 1E-6 (ohm cm)-1. The films were integrated into solar cell structures, SnO2:F(TCO)/CdS/Sb2(S/Se)3/C-Ag. Solar cell using the absorber film of Sb2S0.78Se2.22 prepared by thermal evaporation showed a conversion efficiency (?) of 6 % with an open circuit voltage, Voc, of 0.503 V and a short circuit current density, Jsc, of 25 mA/cm2. In solar cells with chemically deposited thin film of lower selenium content Sb2S1.96Se1.04, a lower Jsc of 17.6 mA/cm2 and a conversion efficiency of 4 % were obtained. The paper will deal with these and results of further work on improving the solar cell parameters.

Authors : Lahouaria. Boudaoud1, Nacera. Boudaoud2
Affiliations : Laboratoire Energie, Environnement et Systèmes d?Information faculté des sciences et technologies, Université Ahmed Draya Adrar, Algérie

Resume : In this paper, we present a new approach based on the use of GaSe as buffer thin films for CIGSe solar cells with SCAPS simulator (Solar Cell Capacitance Simulator). In the contrast with the excellent proprieties of the thick CdS layer, the thickness seen to be lowest for a good match between performances and a thickness reduction of CdS was coupled to a dramatic decrease of device performance, due to homogeneity problems. This new approach allows a reduction of the thickness furthermore the parameters lattice are matching, these characteristics will keeping quality of the device and improving the surface coverage of the absorber layer with the buffer. The results of this study indicate that the buffer layer 10 nm thick we obtain value of 30 % efficiency, this is mainly explained by the current density of the solar cells increases and reaches the value of 42.68mA/cm². This opens the possibility to use ultra-thinbuffer layers for high efficiency chalcogenide based solar cells.

Authors : Tvarit Patel, Dr. Emila Panda
Affiliations : Research scholar at IIT Gandhinagar; Associate professor at IIT Gandhinagar

Resume : In order to understand the effect of stoichiometric change of Cu2-xS on the optoelectronic properties, a molecular solution based deposition approach for different composition of Cu2-xS films (x = 1, 0.4, 0.2, 0.04, 0) is proposed. To this end, different amount of Cu and CuS particles were dissolved in the binary thiol-amine alkahest solvent mixture due to its ability of room temperature dissolution and easy recovery at low-temperature solvent evaporation which helped the formation of desired Cu2-xS film compositions. A series of experimental techniques, such as XRD, FE-SEM, EDX, UV-Vis-NIR spectroscopy and Hall Effect measurement were used to understand the microstructure and optoelectronic properties of these films. This provides the systematic understanding of the stoichiometric dependent variation in optoelectronic properties of Cu2-xS films. Furthermore, the scanning tunnelling spectroscopy (STS) analysis provides location of band-edges of individual composition with respect to Fermi energy. This in-depth understanding of composition controlled electronic structure variation offers the guideline for application of identical composition to its respective place.

18:00 High efficiency CIGS solar cell workshop until 19:30    
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Modelling and Characterization : Stephan Heise, Thomas P. Weiss
Authors : Matthias Maiberg*, Torsten Hölscher, and Roland Scheer *Electronic mail:
Affiliations : Institute of Physics, Martin-Luther-University Halle-Wittenberg, 06120 Halle, Germany

Resume : In searching strategies for improving photovoltaic chalcogenides it turns out that the material's properties are not well-known; not least this is because of the material's inhomogeneities. This problem led us to the development of a setup for two-dimensional electro-optical simulation of thin-film solar cells. The novelty of this program is that many simulation parameters are treated as random variables and their values are given by density functions rather than deterministic numbers. We believe this approach to be auspicious, since most simulations of macroscopic experiments lack of an exact mapping of the material's properties and probabilistic statements are the utmost information available. In the first part of this talk we focus on how the random number approach is realized in our simulation setup. Based on a few user requirements, the program calculates completely automatically a grain structure of the absorber, “deposits" conformally additional window layers on top, and distributes defect densities, doping densities and band gaps within the absorber. That way, the program forms the base for establishing a micro-structure model of the material under investigation. In the second part we demonstrate with several examples, how this program can be used to gain deeper insight into the influence of a microstructure on the photovoltaic parameters. We start with the effect of surface roughness, which is mimicked by a polycrystalline geometry with grain height distribution. The calculations reveal increased diode quality factors and saturation current densities, as a result of the enlarged space charge region. With the aid of raytracing it is shown that the optical incoupling in the present case is reduced against all odds. As a second example, we study the effect of grain boundary defects with additional grain size fluctuation. It turns out that the photovoltaic parameters decrease on average, if the grain size distribution is broadened while keeping the mean grain size constant, since the “worst" grain dominates the solar cell. The same applies to our last example of defect densities being distributed non-uniformally to the grains. Again, the photovoltaic parameters are reduced when the distribution is broadened and the mean value is kept constant.

Authors : M. Igalson1, M. Maciaszek1, K. Macielak1, M. Edoff2, N. Barreau3
Affiliations : 1 Faculty of Physics, Warsaw University of Technology; Poland; 2 Angstrom Solar Center, Uppsala University, Sweden; 3. Institut des Matériaux Jean Rouxel, Université de Nantes, France

Resume : The model of defects strongly correlated to the lattice and undergoing configurational changes upon illumination describes well persistent photoconductivity effect (PPC) observed in copper-poor Cu(InGa)Se2 compounds. Such model explains PPC as due to a transition between two configurational states of the defect which requires overcoming the potential barrier. Lany and Zunger relate this effect to one of the intrinsic defects, namely VSe-VCu. In our report we apply this model to evaluate quantitatively concentration of defects responsible for PPC. First we confirm experimentally that in agreement with theory the low-temperature conductivity in the relaxed state corresponds to a frozen non-equilibrium value depending on the cooling rate. Then we propose a method of calculation of the concentration of defect states responsible for PPC using the low-temperature ratio of conductivity in the relaxed and light-soaked state and under assumption that all defects convert from donor to acceptor state due after light soaking. We apply the procedure to a range of copper-poor thin films with different I/III ratio, selenium and sodium content and estimate the concentration of defects to more than 1017 cm-3 in most cases. The results indicate that defect concentration is constant within the range 0.75 - 0.99 of Cu/(In+Ga) ratio. Also sodium content is not the decisive factor while annealing in selenium decreases defect concentration only in case of the sodium-free sample.

Authors : Finn Babbe, Hossam Elanzeery, Max H. Wolter, Korra Santhosh , Susanne Siebentritt
Affiliations : Laboratory for Photovoltaics, Physics and Materials Science Research Unit, University of Luxembourg, L-4422 Belvaux, Luxembourg

Resume : The model for intrinsic defects of the Cu(In,Ga)Se2 chalcopyrite is still under debate. It is commonly agreed that there are at least one shallow donor and two shallow acceptors. A third acceptor A3 was proven to exist by spatially resolved luminescence for CuGaSe2, but is debated in CuInSe2. In the present study we show by low temperature spatially resolved photoluminescence (PL) that the peak at 0.95 eV in CuInSe2, that was previously attributed to the third acceptor, is a phonon replica and not a DA transition. But another pronounced peak at 0.9 eV together with its phonon replica was detected on polycrystalline CuInSe2 samples grown under high selenium excess and moderate copper excess. Intensity and temperature dependent PL reveal that this peak originates from a DA-transition with phonon replicas that have 28meV spacing and a Huang Rhys factor of 0.75. From temperature dependent PL measurements an activation energy of (120±20)meV is determined. It is concluded that there is a third acceptor in CuInSe2. From comparison with recent theoretical results and from growth experiments under various pressure conditions, we propose this might be the indium vacancy. Admittance spectroscopy on Cu-rich and Cu-poor CuInSe2 devices shows a step with an activation energy of 130meV, matching the A3 from PL. With the addition of small amounts of gallium to the composition (GGI up to 0.15) the activation energy determined by both methods increases, moving the defect deeper into the bandgap.

Authors : Konrad Wiśniewski, Paweł Zabierowski, Nicolas Barreau
Affiliations : Warsaw University of Technology, Faculty of Physics, Koszykowa 75, 00 662 Warsaw, Poland; Warsaw University of Technology, Faculty of Physics, Koszykowa 75, 00 662 Warsaw, Poland; Institut des Matériaux Jean Rouxel (IMN)-UMR6502, Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France

Resume : Deep level transient spectroscopy (DLTS) has a limited application for CIGS solar cells since the signals deviate significantly from typical textbook cases. In order to explain the origin of unexpected behaviour of experimental dependencies 4 mutually exclusive models were proposed and the discussion is still ongoing. In this work we simulate capacitance transients by solving a set of differential equations describing the response to a voltage pulse of a solar cell consisting of two junctions and one resistance representing the neutral region. We discuss how this extended two-diode model helps to understand previously unexplained distinctive features of N1 DLTS and RDLTS signals of CIGS solar cells: (i) the sign of signals and their amplitudes, (ii) the difference in the position and widths of RDLTS and DLTS peaks, and (iii) the increase of an amplitude at higher temperatures. We show that the expression for junction capacitance needs to be reformulated since the equivalent circuit and therefore the susceptance of a sample must be calculated differently. The simulation results are compared to the measurements of the samples with a modified surface after KF and NaF treatment. Contrary to literature data, the parameters used to achieve the best fit to experimental data suggest that the secondary barrier explaining the N1 signal is located in the buffer/absorber interface region rather than at the back contact.

09:45 Poster award ceremony    
10:00 Coffee Break    
Characterization and Alternative Absorbers : Malgorzata Igalson, Stefano Pisoni
Authors : Max Hilaire Wolter1, Susanne Siebentritt1, Benjamin Bissig2, Enrico Avancini2, Romain Carron2, Stephan Buecheler2, Philip Jackson3, Dimitrios Hariskos3, Wolfram Witte3
Affiliations : 1 University of Luxembourg, Physics and Materials Science Research Unit, Laboratory for Photovoltaics, 41 rue du Brill, 4422 Belvaux, Luxembourg; 2 Laboratory for Thin Films and Photovoltaics, Empa - Swiss Federal Laboratories for Materials Science and Technology, Überlandstr. 129, 8600 Dübendorf, Switzerland; 3 Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstr. 1, 70563 Stuttgart, Germany

Resume : Photoluminescence (PL) spectroscopy is a powerful tool to investigate defect states, potential and band gap fluctuations, as well as the quasi-Fermi level splitting of semiconductors. It can thus be used to analyze high-quality Cu(In,Ga)Se2 (CIGS) thin-film absorber layers and define properties leading to high power conversion efficiencies. However, CIGS absorbers that exhibit a double band gap grading, a common feature in high-end CIGS devices, are likely to show PL spectra dominated by interference effects. For example, if PL is measured at low temperatures, the interference fringes lead to the appearance of several peaks that could wrongly be attributed to individual defect transitions. Thus the presence of interference effects can lead to erroneous interpretation of data. In particular, interferences influence the low energy part of the PL spectrum, which is essential in the analysis of electrostatic potential fluctuations. Here we introduce a method for removing interference effects in CIGS by depositing an auxiliary surface layer on top of the absorber. This surface treatment consists of the chemical deposition of several monolayers of polystyrene beads. We thus create an additional scattering medium for the emitted photons that consequently lose their coherence. Hence, we can measure nearly interference-free PL spectra down to low temperatures and down to low energies. This approach allows to investigate the extent of electrostatic potential fluctuations in high quality CIGS absorbers. First results indicate that absorbers that underwent an alkali post-deposition treatment (PDT) show reduced potential fluctuations. We conclude that one of the benefits of the PDT seems to be an improved absorber quality by means of reduced non-radiative recombination as a result of smaller potential fluctuations.

Authors : Baptiste Bérenguier, Daniel Ory, Jean-Paul Kleider, Laurent Lombez
Affiliations : Institut Photovoltaïque d'Ile de France, 30 Route Départementale 128, 91120 Palaiseau, France; EDF R&D, 30 Route Départementale 128, 91120 Palaiseau, France / Institut Photovoltaïque d'Ile de France, 30 Route Départementale 128, 91120 Palaiseau, France; GeePs, UMR 8507-CNRS, CentraleSupélec, Univ. Paris-Sud, Université Paris-Saclay, Sorbonne Universités, UPMC Univ Paris 06, 11 rue Joliot-Curie, F-91192 Gif-surYvette Cedex, France / Institut Photovoltaïque d'Ile de France, 30 Route Départementale 128 91120 Palaiseau, France; Institut Photovoltaïque d'Ile de France, 30 Route Départementale 128, 91120 Palaiseau, France;

Resume : Modulated photoluminescence (MPL) as a carrier lifetime measurement method was successfully applied during the past to silicon wafers characterization by several teams, for example at the GeePs laboratory. The goal of this technique is to perform a frequency study of semiconductor materials recombination dynamics by applying an intensity modulated lighting and monitoring the photoluminescence variations. Among other Photoluminescence techniques, Time Resolved Photoluminescence uses for example extremely fast light pulses as an optical excitation when the Quasi Steady-State Photoluminescence uses really slow pulses. The MPL technique is able to go from slow to fast varying excitation by changing the modulation frequency, and to work at a given injection level by adding a light bias, providing additional information on the carrier recombination dynamics. Our team recently built a new modulated photoluminescence setup, covering the [40 kHz, 4 MHz] modulation frequency range. The system was employed to record modulated photoluminescence response of a CdS/CIGS/Mo/Glass Stack. The experiment was performed at several lighting power. The obtained phase vs frequency curves will be analyzed by comparison with numerical simulation, including zero dimensional carrier densities rate equations and 1D drift-diffusion equations. The result will be compared to TRPL measurements and the presence and characteristics of carrier trapping centers discussed.

Authors : C. Hages [1]; J. Marquez [1]; S. Levcenco [1]; M. D. Heinemann [2]; T. Kodalle [2]; C. A. Kaufmann [2]; T. Unold [1]
Affiliations : [1] Dept. of Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin, Germany; [2] PVComB, Helmholtz Zentrum für Materialien und Energie, Berlin, Germany

Resume : Application of a post-deposition treatment (PDT) of alkali metals to Cu(In,Ga)Se2 absorbers during processing has resulted in significant performance enhancements in this material system. Absorbers fabricated with such PDTs using RbF have resulted in world record-device performance for thin-film photovoltaics and enhanced minority carrier lifetimes reported for this material [1,2]. Detailed optoelectronic characterization is needed to understand the role these alkali PDTs have on the enhanced performance reported for this system. Accordingly, in this work we use optical spectroscopy to investigate the role that a RbF-PDT has on the defect properties of CIGSe grown by two-stage coevaporation. Here we find that a RbF-PDT results in a minority carrier lifetime approaching 500 ns, which is the highest value currently reported for this material. Quantitative photoluminescence analysis is used to verify the minority carrier lifetime measured from time-resolved photoluminescence. In contrast to previous reports regarding the benefit of KF-PDT, here we find that the benefits of the RbF-PDT mainly result from reduced interface recombination; the relative impact of interface recombination in samples with and without a PDT is measured using super-continuum excitation time-resolve photoluminescence. Lastly, the RbF-PDT is shown to significantly enhance stability of the Cu(In,Ga)Se2 absorber layer to photodegradation. [1] Jackson et al., RRL Solar, 8, 2016; [2] Jensen et al., JAP, 120, 2016, 063106

Authors : Filipe Martinho (1), Simon López-Marino (2), Sara Engberg (1), Alireza Hajijafarassar (2), Jakob Pedersen (1), Stela Canulescu (1), Eugen Stamate (3), Jørgen Schou (1), Ole Hansen (2)
Affiliations : (1) - Department of Photonics Engineering, Technical University of Denmark, DK-4000 Roskilde, Denmark (2) - Department of Micro- and Nanotechnology, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark (3) Department of Energy Conversion and Storage, Technical University of Denmark, DK-4000 Roskilde, Denmark

Resume : Kesterite solar cells based on pure sulfide CZTS (Cu2ZnSnS4) offer prospects of a cost effective and environmentally benign solution for photovoltaics. While promising on its own, CZTS is now receiving increased interest for applications in monolithic CZTS/Si tandem solar cells due to its high bandgap and low lattice mismatch with Si. However, integrating the Si and CZTS production steps to obtain an efficient monolithic cell is a challenging task. In this work, CZTS absorbers were produced by co- or sequential sputter deposition of the binary precursors SnS and ZnS and of the metallic precursor Cu. The characterization of a baseline single junction CZTS solar cell is presented, and some of the challenges and early results on the CZTS/Si tandem cell integration are reported. CZTS was grown on a Si substrate with a TiN diffusion barrier, to prevent the diffusion of some CZTS elements into Si during annealing – particularly Cu and S – that can degrade the photovoltaic properties of Si. The effects of annealing on the resulting structure were studied as a function of the initial CZTS precursor configuration and composition. The results of this initial integration and their implications for CZTS/Si tandem devices are discussed. The ideal thickness of the CZTS layer, in order to match the Si thickness for complete light absorption, was studied experimentally, in conjunction with optical simulations on complete CZTS/Si tandem structures.

Authors : Shuaicheng Lu, Chao Chen, Jiang Tang*
Affiliations : Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, China

Resume : Antimony selenide (Sb2Se3) is a promising photovoltaic material due to its suitable bandgap, strong light absorption, simple phase, nontoxicity, and earth-abundant constituents. Currently, most efficient Sb2Se3 thin-film solar cells are based on CdS buffer layer. But it is still a big problem that CdS is high-toxic and makes the device unstable due to the Cd diffusion. Hence, a new way using wide-bandgap and non-toxic oxide such as TiO2 and SnO2 as the buffer layer was explored to configure Sb2Se3 thin-film solar cells. For TiO2/Sb2Se3 solar cells, the TiO2 and Sb2Se3 films were fabricated by spray pyrolysis and rapid thermal evaporation (RTE) methods, respectively. Based on the high-throughput combinatorial approach (HCA), key parameters such as TiO2 thickness, post-annealing temperature and Sb2Se3 thickness are systematically optimized in a short experimental period. Finally, in combination with (NH4)2S back surface cleaning, TiO2/Sb2Se3 solar cells with 5.6% efficiency and decent stability were obtained.[1] For SnO2/Sb2Se3 solar cells, spray pyrolysis method was also used to fabricate SnO2 films. By controlling the interfacial defects and optimizing the quality of Sb2Se3 films, SnO2/Sb2Se3 solar cell with 3.05% efficiency was obtained, and the devices showed a superior light-soaking stability compared with CdS/Sb2Se3 solar cells.[2] The related experiments imply that it is possible to configure non-toxic Sb2Se3 thin-film solar cells, and further optimization and more studies are ongoing to improve device performance. Key words?Sb2Se3 thin-film solar cells; non-toxic buffer layer; high-throughput combinatorial approach Reference? [1] C. Chen , Y. Zhao , S. Lu , J. Tang* et. al. Adv. Energy Mater. 2017, 7, 1700866. [2] S. Lu , Y. Zhao , J. Tang* et. al. Adv. Electron. Mater. DOI: 2017, 10.1002/aelm.201700329

Authors : Stefano Rampino, Matteo Bronzoni, Davide Calestani, Edmondo Gilioli, Massimo Mazzer, Michele Sidoli, Giulia Spaggiari, Francesco Pattini
Affiliations : Institute of Materials for Electronics and Magnetism (IMEM) - Italian National Research Council (CNR) Parco Area Delle Scienze 37/A 43124 Parma ITALY

Resume : CuSb(S,Se)2 is emerging as a new absorber for photovoltaic (PV) applications, for its intrinsic p-type conductivity mainly due to Cu vacancies, tunable energy bandgap (1.0÷1.5eV), high absorption coefficient >10^5cm-1 and for the very cheap cost of Sb. Recently, some devices based on CuSbSe2 (CASe) reached a PV efficiency 𝜂 = 5% [1]. We studied structural, compositional and electro-optical properties of CASe films grown by Low-Temperature Pulsed Electron Deposition (LTPED) [2,3] vs. substrate temperature. CASe layers grown in a single stage from ternary targets between RT and 180°C exhibit a chalcostibite phase with Cu-poor composition (Cu/Sb≈0.55), while at higher temperature, a sort of “self-regulated” growth occurs. The Cu deficiency is compensated by the sublimation of Sb2Se3, leading to the formation of stoichiometric CASe phase, as confirmed by XRD/Raman analysis. By moving from solar cells based on Cu-poor CASe absorbers to stoichiometric ones, we found similar Voc (∼280÷320 mV), while the Jsc is boosted from ∼3 mA/cm2 to ∼20 mA/cm2. A FF of ~35÷40 limits the performances in all the cells, mainly due to the CIGS-like architecture and in particular to CdS. A post-growth annealing at 200°C improves all the electrical parameters in stoichiometric CASe cells, leading to a remarkable 𝜂 = 3.8%, very close to the state of the art. [1] A W Welch, et al. Adv Energy Mater 7, 1601935 (2017) [2] S Rampino, et al. APL 101 (13), 132107 (2012) [3] M Mazzer, et al. Sol Mat 166 (2017)

Authors : Laurie J. Phillips, Oliver S. Hutter, Peter J. Yates, Ken Durose, Jonathan D. Major
Affiliations : Stephenson Institute for Renewable Energy, University of Liverpool, Liverpool, UK

Resume : We report on the development of close-space sublimation (CSS) deposited antimony selenide (Sb2Se3) solar cells, and how the choice and modification of partner layers can enhance performance. CSS deposited Sb2Se3 thin films have large grains with preferred alignment along the (112) and (212) directions. However, intermixing between CdS-based window layers and the Sb2Se3 absorber layer has forced the use of more resistive oxides such as titania which has a detrimental effect on the fill factor. Here, we show that by doping the titania with niobium, charge extraction is significantly enhanced. Niobium doped titania window layers are deposited from solutions containing 0 to 10% niobium ethoxide : titanium isopropoxide in ethanol. Films are deposited via spin coating and annealed in air at 500ºC to create compact electron transport layers. The Sb2Se3 absorber layer is deposited on top using a CSS process at 450ºC without post-growth annealing or selenization. The cells are completed with a hole-transporting partner layer and a gold back contact. Performance improves, particularly via the current and fill factor, with cell parameters improving from η = 3.4%, Voc = 0.4 V, Jsc = 22.6 mA/cm2 FF = 37.3%, without Nb, to to η = 4.6%, Voc = 0.4 V, Jsc = 31.6 mA/cm2, = FF 40.8% with 5% Nb doping.

12:15 Closing remarks    

Symposium organizers
Alexander R. UHLUniversity of Washington

Molecular Eng. & Sciences Bldg., Rm. 120 3946 W Stevens Way NE Box 351652, Seattle, WA 98195, USA

+1 206 734 0742
Daniel ABOU-RASHelmholz Zentrum Berlin für Materialien und Energie GmbH

Hahn-Meitner Platz 1 14109 Berlin Germany

+49 30 8062 43218
Negar NAGHAVICentre National de Recherche Scientifique (CNRS)/Institute of Research & Development on Photovoltaic Energy (IRDEP)

IRDEP, EDF R&D 6 quai Watier-78401 Chatou, France

+33 (0)1 30 87 75 12
Stephan BUECHELER (Main Organizer)Empa

Laboratory for Thin Films and Photovoltaics, Überlandstrasse 129, 8600 Dübendorf, Switzerland

+ 41 58 765 6107
Woo Kyoung KIMSchool of Chemical Engineering

Yeungnam University 280 Daehak-ro, Gyeongsan, Gyeongbuk, 38541, Korea

+82 53 810 2514