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Thin film chalcogenide photovoltaic materials

The Thin Film Chalcogenide Photovoltaic Materials symposium 2020 will closely follow the research in the field of chalcogenide materials for PV applications based on derivates of CdTe, Cu(In,Ga)(S,Se)2, Cu2ZnSn(S,Se)4 and their related alloys. It aims to provide a platform for presenting recent and on-going researches for further efficiency improvements and cost reduction of these systems.


Chalcogenides are highly interesting for their use as light absorber layers in solar cells due to their uniquely high absorbance, bandgap tenability and defect chemistry. Stability and reliability, monolithic deposition, aesthetical appearance, applicability onto flexible substrates and in roll-to-roll production, superior temperature coefficient, potential in tandem applications are other advantages that chalcogenide technology offers. These materials include CdTe and its alloys, CuInSe2 – CuGaSe2 – CuInS2 – AgInSe2 -CuAlSe2 system (commonly called CIGS) and their alloys and Cu2ZnSn(S,Se)4.

Presently, record efficiencies of 23.3% for Cu(In,Ga)Se2 and 22.1% for CdTe were obtained, which outperform 22.3% efficiency of multicrystalline silicon. Moreover, industrial activities continue to expand, especially in the field of CdTe and Cu(In,Ga)Se2 which are now produced at the gigawatts peak (GWp) scale.

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. New findings will lead to a better understanding of the fundamental physical properties of the semiconductors, which ultimately will lead to increased efficiencies of the solar cell devices and thereby improved cost structures of the photovoltaic systems.

This symposium aims to bring together different actors on these technologies and to provide a platform for presenting recent and on-going researches for further efficiency improvements and cost reduction
employing new concepts of solar cell architectures such as passivation and light management, tandem multi-junction architectures, bulk and interfacial optimization, novel related absorber materials as well as deep characterization and modelling.

The symposium has a long tradition on attracting the most successful researchers in the world and is becoming over the years one of the largest symposia at E-MRS Spring Meetings. 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 2020.

Hot topics to be covered by the symposium:

  • Chalcogenide PV materials, theory and modeling
  • Novel/ alloyed chalcogenide materials
  • Processes for film synthesis
  • Thin film growth, theory and experimental aspects
  • Material combinations and heterostructures
  • 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:

  • Roland Scheer (Martin-Luther-University Halle-Wittenberg, Germany): heavy alkali effects on wide bandgap CIGS based solar cells
  • Nicolas Barreau, (University of Nantes, France): Solar cell based on co-evaporated Cu(In,Ga)S2: process tricks and material issues
  • Daniel Abou-Ras (HZB, Germany): Limiting factors for CIGS solar cells from microscopic analyses
  • Bart Vermong (IMEC, Belgium): Perovskite, marriage material for chalcogenide?
  • Aron Walsh (Imperial college, UK) :Microscopic origins of voltage loss in chalcogenide solar cells
  • M. A. Scarpulla (University of Utah, US) :Composition inhomogeneity and device implications of grain boundaries in CdTe
  • Chantana Jakapan (Ritsumeikan University, Japan):Enhancement of conversion efficiency up to 22% for Cd-free Cu(In,Ga)(S,Se)2 solar cell with sputtered (Zn,Mg)O buffer layer
  • Jonathan Scragg (Uppsala University, Sweden)
  • Maarja Grossberg (Tallinn University of Technology, Estonia)
  • Susanne Siebentritt (University of Luxembourg, Luxembourg) 

Tentative list of scientific committee members:

  • D. Lincot (IPVF), France
  • T. Wada, (Ryokoku Univ.), Japan
  • A.N. Tiwari (Empa), Switzerland
  • J.F. Guillemoles (IPVF), France
  • 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
  • D. Abou-Ras (HZB), Germany


Selected papers will be published in a topical issue of Thin Solid Films.

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09:00 Welcome    
CIGS: Materials & devices : Susanne Siebentritt/Charlotte Platzer Björkman
Authors : N. Barreau
Affiliations : Université de Nantes, CNRS, Institut des Matériaux Jean Rouxel, IMN, F-44000 Nantes, France

Resume : CuIn0.7Ga0.3S2 semi-conductor has a bandgap energy of 1.7 eV, which is well adapted to applications as top cell in tandem structures with c-silicon bottom cell. Recently, we have shown that solar cells with standard structure, namely Glass/Mo/CuIn0.7Ga0.3S2/CdS/ZnO/AZO, can reach efficiencies above 14 %; the CuIn0.7Ga0.3S2 layer being grown by co-evaporation following a 3-stage process. Determination of compositional depth profile of these absorbers revealed that the distribution of group III elements (i.e. In and Ga) is not homogeneous throughout the layers, as expected for films grown by means of sequential process. However, in contrast to V-shaped GGI evolution commonly observed in selenide absorbers, these sulfide films appear composed of two distinct layers, a first with GGI  0.7/0.8 near the back contact and a second with GGI  0.2/0.3 close to the surface; note that the abrupt interface between these layers is located at about half of total film thickness. A large part is the present contribution will focus on the origins for such bilayered compositional structure, which will be discussed accounting relative phase stability while copper content is increased during the second stage. By taking the advantage of this latter phase separation, we could prepare devices from absorbers having diverse GGI profiles, showing the high Ga containing chalcopyrite near the back likely acts as an electron reflector.

Authors : M. Stoelzel, A. Weber, H. Elanzeery, A. Zelenina, M. Fuerfanger, S. Gruensteidl, M. Algasinger, M. Hala, J. Roeder, P. Borowski, P. Eraerds, R. Lechner, T. Dalibor, J. Palm
Affiliations : AVANCIS GmbH, Otto-Hahn-Ring 6, 81739 Munich, Germany

Resume : The CIGSSe thin film technology continues to demonstrate highest efficiency records - one important prerequisite for the GWp industrial production scale. However, the complex interplay between material parameters and optoelectronic device properties needs to be controlled for high module efficiency not only at standard test conditions, but also at elevated temperatures and under low light illumination. At AVANCIS, we apply the sequential absorber formation via rapid thermal processing of stacked elemental layer precursors (SEL-RTP). To form the absorber-buffer heterojunction and to complete the device, sputtered Zn(O,S) buffer layers and n-type ZnO:Al front contact layers are deposited. We will present results from process development on 30×30 cm² CIGSSe modules with systematic variations of the individual device stack components. We have increased the minimum absorber band gap by introduction of a Ga rich precursor and applied Na post deposition treatment to such absorbers. We will show how these changes improve both the resulting efficiency and the performance ratio parameters. Temperature coefficients and lowlight behavior are compared for data taken from lab and outdoor measurements. Apparently, measures to reduce the Voc deficit as evaluated from STC-IV data, also seem to reduce the Voc temperature coefficient. (Accepted contribution for the 2020 EMRS Spring meeting, updated for re-submission for the EMRS 2021 Spring Meeting by the deadline of 28th January 2021).

Authors : Faraz Khavari, Jan Keller, Jes K. Larsen, Kostiantyn V. Sopiha, Tobias Törndahl, Marika Edoff
Affiliations : Department of Materials Science and Engineering, Solar Cell Technology, Ångström laboratory, Uppsala University, Sweden

Resume : In this study, sulfur incorporation of CuInSe2 (CISe), and CuGaSe2 (CGSe) absorber layers is investigated in order to better understand the sulfurization process of Cu(In,Ga)Se2 (CIGSe). We found that for all investigated annealing temperatures from 405 to 530°C, sulfur is always incorporated into the CISe absorber, forming a thin CuInS2 (CIS2) layer at the surface. By increasing the annealing temperature, this surface layer becomes thicker and acts as an electron barrier, which deteriorates the device performance by decreasing fill factor (FF), and current density (JSc). In contrast, for the off-stoichiometric CGSe samples (Cu-poor), S could only be introduced at a temperature of 530°C or above, mainly forming a mixed crystal of CuGa(S,Se)2 (CGSSe). In Cu-rich samples, however, selenium (Se) could substitute with S independent of the annealing temperature, which could be explained by a sequential phase transformation of Cu2-xS to Cu2-xSe to CuGaS2. Finally, the solar cell parameters of the CISe solar cells are improved by sulfurization with respect to their references, while the electrical characteristics deteriorated for the CGSe cells. Keywords: Sulfurization, CISe, CGSe, phase transformation, Cu2-xS, Cu2-xSe, and OVC.

Authors : Shih-Chi Yang, Jordi Sastre, Maximilian Krause, Xiaoxiao Sun, Ramis Hertwig, Mario Ochoa, Ayodhya Nath Tiwari and Romain Carron
Affiliations : Laboratory for Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology

Resume : Achieving high efficiency for very low temperature (≤450°C) grown Cu(In,Ga)Se2 (CIGS) solar cells is challenging because of insufficient thermal energy for grain growth and defect annihilation, which result in poor crystallinity, presence of defects and degraded device performance. In this work, we begin with temperature (about 450°C) compatible with polyimide substrates and explore the possibilities for high performing solar cells grown at even lower temperatures. By alloying CIGS with the Ag precursor layer method, ACIGS solar cells grown at 450°C reaches an efficiency of 20.1%. Only 0.5% and 1.6% reduction in efficiency are observed for 390°C and 340°C growth. The high efficiency for such low temperatures grown ACIGS is mainly driven by less VOC degradation than that in CIGS. The root cause of the difference in VOC between ACIGS and CIGS solar cells is further investigated by SEM, Urbach tails and TRPL. As compared to CIGS, ACIGS shows improved morphology, reduced tail states and higher doping. The proposed approach offers a range of benefits in view of CIGS deposition on temperature-sensitive substrates. Silver also enlarges the parameter window of substrate temperature. Increased Cu diffusion promoted by silver enables the use of end-point detection in the three-stage process at low temperatures below 300°C. This method requires minimal modification for implementation into existing processes and deposition equipment, and shows potential for the development of flexible applications.

Authors : Ha Kyung Park (1), Yunae Cho (1), Kihwan Kim (2), Jae Ho Yun (2), Jihye Gwak (2), William Jo (1)*
Affiliations : 1 Department of Physics and New and Renewable Energy Research Center (NREC); Ewha Womans University, 2 Photovoltaic Laboratory; Korea Institute of Energy Research (KIER)

Resume : Cu(In,Ga)Se2 (CIGS) thin film solar cell synthesis have been widely studied to maximized its performance. Incorporation of Ag in CIGS thin film develop the synthesis of CIGS by facilitating CIGS recrystallization at lower temperature and enhance the cell performance. In this work, CIGS on soda-lime glass with and without Ag were prepared to examine the effect of Ag on CIGS surface electrical properties. To investigate the local electrical properties, Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (c-AFM) were utilized. Grain size increased in Ag incorporated CIGS indicating the grain growth of CIGS was enhanced with incorporation of Ag. As a result of KPFM, surface potential distribution was proved. Both of CIGS samples with and without Ag incorporation showed upward potential bending at grain boundaries (GBs), indicating the collection of electrons at GBs. Also, surface current map was obtained via c-AFM external bias voltage of 0.5 V. In Ag incorporated CIGS, surface current was formed along the GBs while formation of surface current under the same bias voltage was not observed in the sample without Ag. Therefore, flow of collected electrons along GBs was affected by incorporation of Ag. For the further understanding of implication of carrier transport, formation and recombination of photogenerated carriers will be studied as future work.

10:40 Discussion/Break    
CIGS absorbers for tandem solar cells : Nicolas Barreau/Romain Carron
Authors : Bart Vermang, Sarallah Hamtaei, Jessica de Wild, Merve Tutundzic, Yinghuan Kuang, Tom Aernouts, Jonas Hanisch, Erik Ahlswede, Ihteaz Hossain, Tobias Abzieher, Fabrizio Gota, Ulrich Paetzold, Naresh Kotadiya, Romain Carron, Maximilian Krause, Fan Fu, Ayodhya Tiwari, Pieter Bolt, Marcel Simor, Hans Linden, Philip Schulz, Vincent Dufoulon, Cecilia Tel, Daniel Lincot, Neethi Rajagopalan, Steven Claes, Carolin Spirinckx, Sebastien Lizin, Alessandro Martulli, Jacopo Sala, Michaël Daenen, Hans-Gerd Boyen, Derese Desta, Stéphanie Narbey, Toby Meyer, César Omar Ramírez Quiroz, Bernhard Dimmler, Heping Shen, Ingrid Repins
Affiliations : imec; ZSW; KIT; Empa; TNO; CNRS-IPVF; VITO; UHasselt; Solaronix; NICE Solar Energy; ANU; NREL

Resume : A promising approach to increase the efficiency of photovoltaic devices above the Shockley-Queisser single-junction limit is the realization of tandem devices. In the first part, the presentation will start with the history of and rationale for all thin-film perovskite on chalcogenide tandem devices. This tandem arrangement (i) allows a more ideal band gap combination (i.e., 1.6-1.7 eV top solar cell and 1.0 eV bottom solar cell) as compared to perovskite on silicon tandem structures, and (ii) outperforms not only the stand-alone perovskite and chalcogenide devices, but also best single-junction silicon devices. In the second part, the presentation will provide an overview on the approach and latest results of the PERCISTAND project. The project focuses on 4-terminal tandem solar cell and module prototype demonstration on glass substrates, but also current- and voltage-matched 2-terminal proof-of-concept device structures are envisaged. Key research activities are the development and optimisation of top wide band gap perovskite and bottom low band gap CuInSe2 devices, suitable transparent conductive oxides, and integration into tandem configurations. The emphasis is on obtaining high efficiency, stability, and large-area manufacturability, at low production cost and environmental footprint. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 850937.

Authors : Nicolas Gaillard, Wilman Septina, Joshua Crunk, Kai Outlaw-Spruell and Thomas West
Affiliations : Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, HI 96822, USA

Resume : Despite years of research on multijunction (MJ) solar devices, single junction solar cells still dominate the photovoltaic (PV) market. On one hand, the cost of highly efficient III-V MJ devices is still too prohibitive to be economically sound, and on the other the efficiency of cheaper alternatives, such as a-Si/a-Si or perovskite/c-Si MJ devices, barely exceed that of their single junction counterparts. The chalcopyrite class falls further behind, with MJ devices exhibiting efficiencies in the range of 2.7-8.0%. However, this low performance range is surprising given that chalcopyrites themselves possess highly desirable characteristics for MJ devices, the most salient of them being bandgap tunability. We believe that one reason for the poor performance of chalcopyrite MJ devices is in part due to research efforts focusing predominantly on only one wide bandgap candidate (1.68 eV CuGaSe2). Therefore in this communication, we report on our recent efforts to develop alternative wide bandgap chalcopyrites. We first present theoretical modeling of selected materials candidates, including Cu(In,Al)Se2, Cu(In,B)Se2 and Cu-poor ordered vacancy compounds (e.g., CuGa3Se5) and report on their bulk energetics and conduction band offsets with common n-type buffer materials (CdS, In2S3). Then, we present some of the strategies developed by our group to fabricate these candidates (e.g., co-evaporation, molecular ink approach) and report on their solid-state properties. As an example, we show that pairing co-evaporated 1.8 eV CuGa3Se5 with MgxZn1-xO buffer can lead to single junction cells with open circuit voltage (VOC) exceeding 925 mV. Another major reason for low performance of chalcopyrite MJ devices is due to the reliance on conventional MJ device approaches such as mechanical and monolithic stacking. To address this, we present a new strategy to integrate wide bandgap chalcopyrites into MJ devices. In this approach, we process each individual cell independently, then exfoliate and bond the top cell onto the bottom cell using a transparent conductive binder (TCB). This way, the bottom cell is not subjected to unnecessary thermal budgets, which has been identified as the main culprit for poor efficiency in monolithic chalcopyrite-based MJ devices. Being an essential component of the semi-monolithic device, we present our development of a TCB that exhibits ~90% transmittance from 300-2000 nm as well as an out-of-plane series resistance on the order of 10-1 Ω-cm2 (comparable to that of molybdenum in Cu(In,Ga)Se2 solar cells). We then present new exfoliation methods that have been investigated by our team to lift off wide-bandgap chalcopyrite devices as large as 25mm x 25mm and the challenges and merits associated with each. We will conclude the presentation with data measured on exfoliated chalcopyrite single junction and fully integrated semi-monolithic MJ devices. We show for instance that over 80% of the baseline short-circuit photocurrent density can be preserved after the exfoliation process, while the VOC of a fully integrated semi-monolithic CuGaSe2/Si tandem exceeds 80% of the sum of the sub-cell’s VOC.

Authors : Ihteaz M. Hossain, Thomas Feeney, Saba Gharibzadeh, Tobias Abzieher, Paul Fassl, Stefan Paetel, Jan-Philipp Becker, Erik Ahlswede, Jonas Hanisch, Ulrich W. Paetzold
Affiliations : Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von -Helmholtz- Platz 1, 76344 Eggenstein-Leopoldshafen, Germany; Light Technology Institute, Karlsruhe Institute of Technology, Engesserstr. 13, 76131 Karlsruhe, Germany; Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstr. 1, 70563 Stuttgart, Germany

Resume : Organometal halide perovskite solar cells have demonstrated breath-taking progress over the last decade, with monolithic two-terminal perovskite/silicon tandem solar cells approaching power conversion efficiencies of 30%. Similar advances were demonstrated for perovskite/CIGS tandem solar cells, but their maximum power conversion efficiencies lag behind for both two- (24.2%) and four-terminal (25.9%) architectures. In this contribution, we demonstrate that by systematic optimization of the semi-transparent perovskite top solar cell, power conversion efficiencies up to 27.3% in four-terminal perovskite/CIGS tandem solar cells are obtained. We report on improvements in light management and material composition of the semi-transparent perovskite solar cell, addressing transparent conductive oxide electrodes, anti-reflection coatings, compositional engineering, and passivation strategies of the perovskite top solar cell. Our study shows that by employing an optimized anti-reflection coating, reflection losses are reduced significantly, resulting in an enhanced transmission of the semi-transparent perovskites solar cells, with values around 90% in the near infrared wavelength regimes. Furthermore, we show that utilising alternative transparent conductive oxides with low parasitic absorption and a high refractive index is critical in improving the performance of both the top and bottom solar cell in four-terminal perovskite/CICG tandem solar cells.

Authors : Damilola Adeleye, Mohit Sood, Sudhanshu Shukla, Michele Melchiorre, Susanne Siebentritt
Affiliations : University of Luxembourg

Resume : Copper indium gallium disulfide, Cu(In,Ga)S2, in addition to being well-suited for use as a single-junction solar cell, is an excellent candidate for the top cell of a tandem device. However, this alloy still suffers from a high open circuit voltage (Voc) deficit. The highest recorded conversion efficiency for Cu(In,Ga)S2 solar cell is 15.5 % with a Voc-deficit of ~400 mV with respect to the Shockley-Queisser Voc, quite high compared to Cu(In,Ga)(S,Se)2 with a Voc-deficit of ~100 mV. The deficit is attributed to high non-radiative recombination in the bulk, as well as at absorber/buffer interfaces, which reduces quasi-Fermi level splitting (QFLS) and Voc. In this work, we investigate the bulk defects in the Cu(In,Ga)S2 absorbers and causes of the non-radiative losses at the absorber/buffer interface. Cu(In,Ga)S2 absorbers with atomic ratio of Cu/(Ga + In) (CGI) ranging from 1.29 to 0.93, but having similar (Ga)/(Ga+In) (GGI) are investigated. The optoelectronic properties and deep defects are studied by calibrated photoluminescence (PL) and low temperature PL at 10 K, respectively. We show that, Ga-containing Cu-poor Cu(In,Ga)S2 absorbers present high quality devices. The Cu-poor absorbers exhibit decreased QFLS-loss and show increased lifetime compared to Cu-rich absorbers. The increase in QFLS is enhanced due to the suppression of deep bulk defects, which are sources of high bulk recombination. This appears to contradict our previous results on CuInS2 (no Ga), where we found higher QFLS with higher Cu-excess. But it should be noted, that this improvement is mainly due to an increase in doping level and that we were not able to grow Cu-poor CuInS2, in agreement with the published phase diagram. Devices made on the Cu-poor Cu(In,Ga)S2 absorbers showed, that interface recombination is eliminated when using a suitable buffer layer. The highest efficiency is 15.2% with anti-reflective coating. This sample shows QFLS of 972 meV and a Voc-deficit of 388 mV. [1] By optimizing the Ga gradient of the Cu-poor absorber we can increase the QFLS of 1004 meV, resulting in Voc of 921 mV (Voc-deficit ~ 370 mV). This result demonstrates that, higher conversion efficiency is attainable from further optimization. [1] S. Shukla, M. Sood, D. Adeleye, S. Peedle, G. Kusch, D. Dahliah, M. Melchiorre, G. Hautier, R. Oliver, S. Siebentritt, Over 15 % Efficient Wide-Bandgap Cu(In,Ga)S2 Solar Cell: Suppressing Bulk and Interface Recombination through Composition Engineering, under review.

12:15 Discussion    
12:30 Lunch    
CIGS: Materials & devices II : Ayodhya Tiwari/Roland Mainz
Authors : R. Scheer1, S. Zahedi-Azad1, M. Maiberg1, H. Tangara2, T. Sakurai2, W. Witte3, D. Hariskos3
Affiliations : 1Martin-Luther-Universität Halle, Institute of Physics, Von-Danckelmann-Platz 3, 06120 Halle/Saale 2Institute of Applied Physics, University of Tsukuba, Ibaraki 305-8573, Japan 3Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstr. 1, 70563 Stuttgart, Germany

Resume : Heavy alkali (HA) treatment is the latest milestone in improving Cu(In,Ga)Se2 device efficiency [1]. It modifies the Cu(In,Ga)Se2 surface allowing for thinner buffer layers without risk of interface recombination. In addition, the HA treatment reduces bulk recombination [2]. All this is valid for low bandgap Cu(In,Ga)Se2 devices which are limited by bulk recombination. However, is the HA treatment also effective for high Gallium, wide bandgap solar cells where the open circuit voltage (Voc) is limited by interface recombination (IFR)? In this contribution, it is shown that in the case of high Gallium Cu(In,Ga)Se2 the HA treatment strongly increases Voc, albeit the cells remain in the IFR limit [3]. Based on an observed current barrier it is postulated that by employing the residual Indium amount, a HAInSe2 surface layer is formed which reduces IFR [3] to an extend that bulk properties become important. In fact, HA alkali treatment reduces the Urbach energy similar as for low Gallium samples. Moreover, the treatment is compatible with high temperature Cu(In,Ga)Se2 growth where the crystal structure is largely improved and by which the Voc deficit can be reduced to 0,64 V. 1. A. Chirilă et al., Nature Materials, 2013, 12, p. 1107. 2. S. Siebentritt et al., Advanced Energy Materials, 2020, 10(8), p. 1903752. 3. S. Zahedi-Azad et al., Progress in Photovoltaics: Research and Applications, 2020, 28(11), p. 1146.

Authors : Kosuke Beppu, Hiroki Kishino, Takahiro Wada
Affiliations : Department of Materials Chemistry, Ryukoku University, Seta Otsu 520-2194, Japan

Resume : Cu(In,Ga)Se2 (CIGS) has been paid much attention as a thin-film photovoltaic device material. Recently, (Cu,Ag)(In,Ga)Se2 (CAIGS) solar cells achieved over 20% efficiency. The substitution of Ag for Cu in CIGS film induced the enlargement of the grain size and the homogeneously distribution of Ag element in the grains.[1-3] We studied CIGS and Cu(In,Ga)(S,Se)2 films by depth-resolved XAFS.[4,5] We have also investigated the thermal properties of chemical bonds in the CIGS using low-temperature XAFS.[6] Einstein temperatures of the chemical bonds, Cu-Se, In-Se, and Ga-Se in CIGS, were parameters for the diffusion of Cu, In, and Ga atoms in the CIGS. In this work, in order to evaluate the substitution effect of Ag for Cu in the CIGS, the thermal properties of the chemical bonds in AIGS were studied by low-temperature XAFS. The AIGS samples were prepared by a mechanochemical process and subsequent heating. The chemical bonds such as Ag-Se, In-Se, and Ga-Se in AIGS were characterized by Ag, In, and Ga K-edge low-temperature XAFS. In the chalcopyrite-type AIGS, Ag, In, and Ga atoms are tetrahedrally coordinated by four Se atoms. The chemical bonds were analyzed on the basis of Einstein model (each bond is a harmonic oscillator). Einstein temperatures were calculated by the temperature dependence of the Debye-Waller factors in Ag-Se, In-Se, and Ga-Se bonds, which were estimated by EXAFS curve fittings for these bonds. Although the Einstein temperatures of the bonds in AIGS had almost no dependence of Ga/(In+Ga) ratio, the evaluated Einstein temperatures were as follows: Ag-Se (190 K) < In-Se (290 K) < Ga-Se (330 K). The result indicated that the oscillator of the Ag-Se bond is the easiest to be excited and that of the Ga-Se bond is the most difficult to be excited. Then, the Einstein temperatures of chemical bonds in AIGS were compared with those in CIGS. Einstein temperatures of In-Se and Ga-Se bonds in AIGS almost corresponded with those in CIGS. Einstein temperature of Ag-Se (190K) in AIGS was clearly lower than that of Cu-Se (230 K) in CIGS. The result indicates that activation energy of Ag diffusion in AIGS is lower than that of Cu diffusion in CIGS. These findings are consistent with the reported experimental results. [1] L. Chen et. al., IEEE J. Photovolt. 4, 447 (2014). [2] M. Edoff et. al., IEEE J. Photovolt. 7, 1789 (2017). [3] N. Valdes et. al., Sol. Energy Mater Sol. Cells 195, 155 (2019). [4] K. Beppu et. al., Jpn. J. App. Phys. 58, 105502 (2019). [5] K. Beppu et. al., Appl. Phys. Lett., 117, 043901 (2020). [6] K. Beppu et. al., 23th Japan XAFS conference proceedings, O3-1 (2020).

Authors : R. Fonoll-Rubio, S. Paetel, S. Estradé, F. Peiró, I. Becerril-Romero, A. Pérez-Rodríguez, M. Guc, V. Izquierdo-Roca
Affiliations : Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs-Barcelona, Spain; Zentrum für Sonnenenergie- und Wasserstoff-Forschung, Baden-Württemberg (ZSW), Meitnerstr. 1, 70563 Stuttgart, Germany; LENS-MIND, Departament d’Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, 08028, Barcelona, Spain; LENS-MIND, Departament d’Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, 08028, Barcelona, Spain; Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs-Barcelona, Spain; Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs-Barcelona, Spain; Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs-Barcelona, Spain; Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs-Barcelona, Spain

Resume : Post deposition treatments (PDTs) based on heavy alkali elements such as K, Rb and Cs have allowed to achieve record energy conversion efficiencies in thin film photovoltaic technologies based on Cu(In,Ga)Se2 (CIGSe), which offers versatile alternative to silicon allowing to expand the possible applications of the solar cells. While there are no doubts about the benefits on the CIGSe performance of applying a heavy alkali-based PDT, the reasons for such a positive impact of this treatment on the devices remains under discussion. In this work, we explore the formation mechanisms associated to the RbF-PDT in CIGSe-based solar cells and how such treatments can be applied to modify the CIGSe absorber surface for the Voc optimization. To achieved this, two set of samples were characterized by Raman spectroscopy, and suported by TEM, which allowed to study the compositional and structural properties of the first tens of nanometres of the absorber, and by photoluminescence (PL) spectroscopy, which allowed to assess the irradiative recombination at the absorber surface. The first set consisted in CIGSe-based devices treated by RbF-PDT using four different source temperatures, while keeping constant [Cu]/([Ga]+[In]) (CGI) ratio. The second set consisted in CIGSe-based devices treated by RbF-PDT using a constant source temperature and varying the CGI ratio. All samples were prepared at a pre-industrial pilot line that allowed to produce and analyse tens of individual solar cells assuring the high statistics of the obtained data. Characterized solar cells achieved efficiencies up to 18.9 % (without ARC), thus assuring the relevance of the obtained results to the scientific community, while preparation of the samples at the pre-industrial pilot line also makes the conclusions of present study valuable for the thin film industry. Performed analysis showed that RbF-PDT redistributes VCu defects between the OVC and CIGS phases at the absorber surface. This redistribution can be controlled by the source temperature of the PDT. Furthermore, a direct correlation is found between the Voc of the devices and relative concentration of the OVC phase in both set of samples, thus revealing that the positive impact of the RbF-PDT arises from the modification of the OVC concentration at the CIGSe surface. This was also correlated with quantum yield of the PL signal, that was increasing with decrease of the OVC phase content, denoting the reduce of the non-radiative recombination at the absorber surface. Combination of Raman and PL spectroscopy analysis allowed to propose fast, non-destructive and industrial compatible methodology to predict the Voc of CIGSe-based solar cells treated by RbF-PDT prior to the complete device fabrication. Developed methodology was tested on the pre-industrial samples with size 30 × 30 cm2.

Authors : Stephanie Essig, Stefan Paetel, Theresa Magorian Friedlmeier, Michael Powalla
Affiliations : Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstraße 1, 70563 Stuttgart, Germany

Resume : Ag-alloying of Cu(In,Ga)Se2 absorber layers offers the advantages of larger grains and lower deposition temperatures and is thus considered to be an important key towards high-efficiency thin-film solar cells. To further explore its potential, we have investigated in-depth the deposition of (Ag,Cu)(In,Ga)Se2 absorber films (ACIGS) by 3-stage co-evaporation in a manufacturing-like inline deposition system. In our study, up to 20% of the Cu atoms were replaced by Ag and the ACIGS films were analysed by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy and glow discharge optical emission spectroscopy (GDOES). Depending on the Ag content and group-I richness during the ACIGS evaporation, crevices and cavities were observed in the absorber layers which can reduce the shunt resistance of the solar cell. The root cause was found in the segregation of Ag-Se phases and a compared to Cu slow diffusion of Ag atoms during the high-temperature stage of the growth process. Furthermore, the bandgap grading towards the rear side of the chalcopyrite solar cell, i.e. slope of Ga-to-In ratio, is reduced when adding Ag during the Cu evaporation. EDX analysis also revealed an anti-correlation of the Ga and Ag content on a microscopic scale. Despite these challenges, optimization of the deposition conditions enabled the fabrication of an ACIGS solar cell with 20.5% cell efficiency without alkali post-deposition treatment.

Authors : Hisham Aboulfadl*, Kostiantyn V. Sopiha**, Jes K. Larsen**, Jan Keller**, Jonathan J. S. Scragg**, Mattias Thuvander*, Marika Edoff*
Affiliations : * Division of Microstructure Physics, Chalmers University of Technology, 41296 Göteborg, Sweden ** Division of Solar Cell Technology, Department of Materials Science and Engineering, Uppsala University, 75236 Uppsala, Sweden

Resume : Ag incorporation in Cu(In,Ga)Se2 (CIGSe) absorbers has been established as an effective route to enhance the open-circuit voltage and increase the carrier collection length [1]. Combined with optimum levels of [Ga]/([Ga]+[In]) (GGI) in CIGSe, Ag alloying of CIGSe has shown to produce wide-gap absorbers suitable as top cells in tandem devices [2,3]. However, the caveat is that (Ag,Cu)(In,Ga)Se2 (ACIGSe) with high GGI values (~0.85 and above) was predicted to have an extended miscibility gap, which can induce phase separation during solar cell fabrication and processing [4]. Formation of secondary phases is undesirable as it can severely degrade the device by creating recombination centers and/or forming barriers at the interfaces. In this work, we examine Ga-rich ACIGSe absorbers on account of local compositional variations at the nanoscale using atom probe tomography (APT). Energy-dispersive X-ray spectroscopy (EDS) in scanning transmission electron microscopy (STEM) is also performed as a complementary technique to study the microstructure of the absorbers. The microstructure of as-deposited and post-annealed absorber films is characterized and compared. We find clear evidence of the predicted phase separation in form of co-existing ACIGSe regions with different [Ag]/([Ag]+[Cu]) ratios. Interestingly, it was possible to detect early stages of phase decomposition where compositional fluctuation at the scale of 1-5 nm were observed. In addition, the compositional irregularities were found to interfere with the dispersion of alkali elements in ACIGSe. All these composition inhomogeneities might cause undesired spatial variations in electronic properties, which should be avoided in order to realize high-efficiency top-cell devices. [1] W. Shafarman, C. Thompson, J. Boyle, G. Hanket, P. Erslev, J.D. Cohen, 2010 35th IEEE Photovolt. Spec. Conf. Honolulu, HI (2010) 325–329. [2] J. Keller, L. Stolt, K. V. Sopiha, J.K. Larsen, L. Riekehr, M. Edoff, Sol. RRL 4 (2020) 2000508. [3] K. Kim, S.K. Ahn, J.H. Choi, J. Yoo, Y.J. Eo, J.S. Cho, A. Cho, J. Gwak, S. Song, D.H. Cho, Y.D. Chung, J.H. Yun, Nano Energy 48 (2018) 345–352. [4] K. V. Sopiha, J.K. Larsen, O. Donzel-Gargand, F. Khavari, J. Keller, M. Edoff, C. Platzer-Björkman, C. Persson, J.J.S. Scragg, J. Mater. Chem. A 8 (2020) 8740–8751.

15:30 Discussion    
15:45 Break    
CIGS & Sb-based materials & solar cells : TBD
Authors : Lung-Hsin Tu , Ching-Yang Chu , Chih-Huang Lai
Affiliations : Department of Materials Science and Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan ROC

Resume : The non-uniformity issue is a critical issue for the mass production of the CIGS solar panels by sequential process which involves the precursor deposition followed by selenization and (or) sulfurization. The commonly used precursor structure is CuGa and In. As already known that the CuGa is a smooth surface from several group reported. However, the deposition of smooth Indium surface is quiet challenging so far due to the low melting point and high surface tension. The purpose of this work focuses on the reduction of In accumulation on the surface of metallic precursor. We fabricate the CuGa/In metallic precursor by using 5 at% Se-doped In target. The effect of Se-doped In target on surface roughness of precursor is investigated. Compared with the commonly used CuGa/In precursor with single In layer, the precursor with Se doped In reveal the smoother surface and the roughness successfully reduce from 409 nm to 301 nm. By optimizing the precursor roughness, we can solve the problem of rough CIGS surface and lateral non-uniformity of composition. The photoluminescence spectrum and images analyze the bandgap change, defect characteristic, and band tailing of CIGS device and show that the improve of uniformity of the CIGS surface.

Authors : Donald Valenta *(1), Hasan Arif Yetkin (2), Tim Kodalle (2), Jakob Bombsch (1), Raul Garcia-Diez (1), Claudia Hartmann (1), Shigenori Ueda (4), Johannes Frisch (1,3), Regan G. Wilks (1,3), Christian A. Kaufmann (2), and Marcus Bär (1,3,5,6)
Affiliations : *(1) Interface Design, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany; (2) PVcomB, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany; (3) Energy Materials In-Situ Laboratory Berlin (EMIL), Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany; (4) NIMS Beamline Station at SPring‐8, National Institute for Materials Science (NIMS), Kouto, Sayo, Hyogo, 679‐5148 Japan; (5) Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy, Berlin, Germany; (6) Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany;

Resume : Post-deposition annealing treatments (PDAT) can improve the efficiency of CdS/Cu(In,Ga)Se2 (CIGSe) based solar cells. However, annealing above a critical temperature leads to deterioration. PDAT enhances Na diffusion into the absorber. While the presence of Na in the absorber is known to be beneficial, its accumulation in the absorber surface/buffer region and diffusion into the emitter seems to be detrimental. Furthermore, strong (annealing-induced) elemental intermixing at the CdS/CIGSe interface also leads to a deteriorated performance. Incorporation of GaOx, a promising candidate for field-effect passivation, at the buffer/absorber interface might be a viable route to avoid detrimental diffusion/intermixing effects upon PDAT. Photoelectron spectroscopy provides detailed insight into the chemical and electronic structure of CdS and GaOx/CIGSe interfaces and their modifications by PDAT. While there is no significant PDAT-enhanced diffusion of CIGSe elements, a clear thermally induced Na diffusion is observed in both CdS/ and GaOx/CIGSe samples sets, with significantly lower Na amounts in GaOx samples. Additionally, we find an almost aligned conduction band (CB) at the CdS/CIGSe interface and large offsets in the CB and valence band (VB) at the GaOx/CIGSe interface, i.e. the ideal situation for a passivation layer. However, subgap states are observed above the VB maximum of GaOx in some cases, which might affect its passivation capacity.

Authors : Ana-Maria PANAITESCU (a), Ovidiu TOMA (a), Sorina IFTIMIE (a), Adrian RADU (a), Ana-Maria RĂDUȚĂ (a), Dumitru MANICA (a), Luminița DAN (a), Lucian ION (a), Ștefan ANTOHE (a,b), and Vlad-Andrei ANTOHE (a,c,*)
Affiliations : (a) University of Bucharest, Faculty of Physics, 077125 Măgurele, Ilfov, Romania; (b) Academy of Romanian Scientists, 030167 Bucharest, Romania; (c) Université Catholique de Louvain (UCLouvain), Institute of Condensed Matter and Nanosciences (IMCN), B-1348 Louvain-la-Neuve, Belgium; *Corresponding author: Assoc. Prof. Ph.D. Eng. Vlad-Andrei ANTOHE, e-mail:

Resume : Zinc selenide (ZnSe) thin films were deposited by RF magnetron sputtering onto optical glass substrates. Films of different thicknesses were prepared by varying the RF plasma power between 60 W and 120 W, while keeping constant the target-substrate distance, the Argon pressure and the substrate temperature. The structural properties were studied by X-ray diffraction (XRD), showing that ZnSe films are polycrystalline with a marked (111) texture. Crystalline structure parameters were determined by analyzing the samples in Bragg-Brentano theta-theta geometry. Morphological investigations were made by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Absorption and transmission measurements were performed in a spectral range between 200 nm and 1200 nm, at room temperature. Thicknesses and band gap energies of the ZnSe thin films were determined. Values between 37.6 nm and 340.5 nm were obtained for the thickness of the ZnSe films, and values between 2.39 eV and 2.66 eV were obtained for their band gap energies. These values were compared with the ones resulting from the spectroscopic ellipsometry (SE). Also the optical constants (refractive indices and extinction coefficients) of the RF-sputtered ZnSe thin films were investigated. Finally, electric measurements were performed on the sputtered ZnSe thin films. Keywords: ZnSe thin films; RF magnetron sputtering; XRD; SEM; spectroscopic ellipsometry. Acknowledgements: Support from the "Executive Unit for Financing Higher Education, Research, Development and Innovation" (UEFISCDI, Romania) through the grants: PN-III-P1-1.1-TE-2019-0868 (TE 115/2020), PN-III-P1-1.1-TE-2019-0846 (TE 25/2020) and 18PCCDI/2018, as well as from the University of Bucharest (UB) grant: 11844/2012, is acknowledged.

Authors : M. Rusu1, T. Kodalle2, L. Choubrac1, N. Barreau3, C. A. Kaufmann2, R. Schlatmann2, T. Unold1
Affiliations : 1Struktur und Dynamik von Energiematerialien, Helmholtz-Zentrum Berlin für Materialien und Energie, Lise-Meitner Campus, Hahn-Meitner-Platz 1, 14109 Berlin, Germany 2PVcomB, Helmholtz-Zentrum Berlin für Materialien und Energie, Schwarzschildstr. 3, 12489 Berlin, Germany 3Université de Nantes, CNRS, Institut des Matériaux Jean Rouxel, IMN, F-44000 France

Resume : Ambient pressure Kelvin probe and photoelectron yield spectroscopy methods were employed to investigate the impact of the KF and RbF post-deposition treatments (KF-PDT, RbF-PDT) on the electronic properties of Cu(In,Ga)Se2 (CIGSe) thin film surfaces and the CdS/CIGSe interface in a CdS thickness series, that has been sequentially prepared during the chemical bath deposition (CBD) process in dependence of the deposition time.[1] We observe distinct features correlated to the CBD-CdS growth stages. In particular, we find that after an initial CBD etching stage, the valence band maximum (VBM) of the CIGSe surface is significantly shifted (by 180-620 mV) towards the Fermi level. However, VBM positions at the surface of the CIGSe are still far below the VBM of the CIGSe bulk. The CIGSe surface band gap is found to depend on the type of post deposition treatment, showing values between 1.46 eV–1.58 eV, characteristic for a copper-poor CIGSe surface composition. At the CdS/CIGSe interface, the lowest VBM discontinuity is observed for the RbF-PDT sample. At this interface, a thin intermediate-layer with a graded band gap is found. We also find that K and Rb act as compensating acceptor defects in the CdS layer. Detailed energy band diagrams of the CdS/CIGSe heterostructures are proposed. [1] 1M. Rusu1, T. Kodalle2, L. Choubrac1, N. Barreau3, C. A. Kaufmann2, R. Schlatmann2, T. Unold1. Electronic Structure of the CdS/Cu(In,Ga)Se2 Interface of KF and RbF-Treated Samples by Kelvin Probe and Photoelectron Yield Spectroscopy. ACS Appl. Mater. Interfaces 2021.

Authors : Juneja, N. *(1) Kriisa, M. (1), Kärber, E. (1), Dedova, T. (1), Daskeviciene, M. (2), Vaitukaityte, D. (2), Getautis, V. (2), Oja Acik, I. (1) & Krunks, M. (1)
Affiliations : (1) Laboratory of Thin Film Chemical Technologies, Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia (2) Department of Organic Chemistry, Kaunas University of Technology, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania

Resume : A constant increase in the greenhouse gas emission levels and the global climate change urges the mankind for minimising the ecological footprint of our generation. Harvesting and utilising solar energy as one of nature’s renewable energy resources in zero-emission buildings with integrated photovoltaics is one possibility out of many. Using solar cells in buildings as windows requires the solar windows to be light-weight, efficient and semi-transparent. However, in a variety of solar cells the current limiting factor for manufacturing a semi-transparent solar cell is the use of non-transparent poly(3-hexylthiophene-2,5-diyl) (P3HT) as a hole transport material (HTM). Here, a simple and semi-transparent fluorene-based styrene, either HTM V-1187 or V-1145, is integrated into ultrasonic spray pyrolysis produced ITO/TiO2/Sb2S3/HTM/Au solar cell structure. These small fluorene-based HTM-s were synthesized via an extremely simple route (two steps, no sublimation purification) from commercially available and relatively inexpensive starting reagents, resulting in more than one order of magnitude lower cost of the final product compared to the commercial P3HT. Corresponding solar cell results will be presented and discussed.

Authors : Robert Fonoll-Rubio1,Tim Kodalle2, Ignacio Becerril-Romero1, Alejandro Pérez-Rodríguez1,3, Christian A. Kaufmann2, Victor Izquierdo-Roca1, Maxim Guc1
Affiliations : 1-Catalonia Institute for Energy Research (IREC), Sant Adrià del Besòs-Barcelona, Jardins de les Dones de Negre 1 2pl., 08930, Spain; 2-Helmholtz-Zentrum Berlin für Materialien und Energie, Schwarzschildstr. 3, 12489 Berlin, Germany; 3-IN2UB, Departament d’Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain

Resume : In the recent years, alkali-based post-deposition treatments (PDTs) have been one of the driving forces boosting the performance of the Cu(In,Ga)Se2 (CIGS) solar cell technology. One of the alkali metals that have led to a continuous approach of device efficiency towards the Shockley-Queisser limit is Rb. However, the origin of its beneficial effects is still not completely understood. The present work aims at providing direct insight into this topic by studying the formation of RbInSe2 on the surface of CIGSe absorbers and, as such, the effect of Rb-PDTs on the performance of resulting devices. RbInSe2 layers with nominal thicknesses of 5–30 nm were directly deposited onto the surface of CIGSe absorbers and were analyzed by resonant Raman scattering spectroscopy. The analysis was completed by a comparison with CIGSe absorbers exposed to a more conventional PDT process using the RbF surface treatment. Although Raman scattering has already shown a high potential for the assessment of nanometric layers of different compounds and is highly sensitive to surface modifications, only the thickest RbInSe2 layers were directly detected both on bare glass/Mo and glass/Mo/CIGSe substrates, and no formation of this phase could be confirmed on the samples with RbF treatment. However, a gradual increase of the intensity of the peaks related to the ordered vacancy compound (OVC) phase was detected with increasing thickness of the RbInSe2 layers, which was found to be in contrast to the effect observed in the samples subjected to the RbF-PDT process. The combination of the results obtained in the samples treated by direct RbInSe2 deposition and by RbF allow to propose a comprehensive model of the effect of Rb-PDT on the surface of the CIGSe layer. This suggested that, in both cases, Rb interacts mainly with the OVC phase, impacting the redistribution of the point defects in the CIGSe and OVC phases, and eventually generating or maintaining an (additional) RbInSe2 layer at the absorber/buffer interface. Possible consequences of these observations on the electrical properties of the respective solar cell devices will be discussed.

Authors : Sarallah Hamtaei, Guy Brammertz, Thierry Kohl, Dilara Gokcen Buldu, Gizem Birant, Jessica de Wild, Marc Meuris, Jef Poortmans, Bart Vermang
Affiliations : Institute for Material Research (IMO), Hasselt University (partner in Solliance), Diepenbeek, Belgium;Institute for Material Research (IMO), Hasselt University (partner in Solliance), Diepenbeek, Belgium;Institute for Material Research (IMO), Hasselt University (partner in Solliance), Diepenbeek, Belgium;Institute for Material Research (IMO), Hasselt University (partner in Solliance), Diepenbeek, Belgium;Institute for Material Research (IMO), Hasselt University (partner in Solliance), Diepenbeek, Belgium;Institute for Material Research (IMO), Hasselt University (partner in Solliance), Diepenbeek, Belgium;Institute for Material Research (IMO), Hasselt University (partner in Solliance), Diepenbeek, Belgium;Institute for Material Research (IMO), Hasselt University (partner in Solliance), Diepenbeek, Belgium;Institute for Material Research (IMO), Hasselt University (partner in Solliance), Diepenbeek, Belgium

Resume : This research addresses the missing or often undisclosed empirical data on processing criteria of CI(G)S absorbers by the sequential two-step technique. This route is the more industrially viable approach, compared to the well-researched co-evaporation technique. Therefore, a systematic analysis thereof can benefit both the research and industrial photovoltaic communities. To this end, we employed a design of experiment approach. Here a In/ CuGa/ Mo/ Soda lime glass precursor is selenized and then sulfurized by use of hydrogen selenide and hydrogen sulfide gases, respectively. In three annealing stages, we designed a generic sulfurization after selenization (SAS) route capturing up to 16 main factors. We empirically screened their individual effects in different metrics (visible and μ-structural resemblance, surface roughness and (time-resolved) photoluminescence- optical bandgap, PL intensity and decay time. Subsequently, we identified the active factors by variance analysis and Lenth’s method in quantitative metrics (e.g., roughness), and via heuristics in qualitative metrics (i.e., resemblance). Upon outcomes, we could identify a handful of significant factors common between most of the metrics: First annealing temperature, selenium feed flow during ramping to second anneal, sulfur introduction temperature and its feed flow, final cooling rate, the selenium feed flow at the end, and to a less extent, the ramp rate to final anneal. Other factors not shortlisted can be set at values which favor other requirements, e.g., resources. Subsequently, running a fractional factorial design experiment, we identified further considerable two-factor interactions between certain pairs. Results of this study narrows down the experimental space within which one can design, and consequently optimize, fabrication route of CI(G)S thin film solar cells through the sequential two-step approach. In the meantime, the study is being complemented with solar cells made up of the optimized absorbers and the results thereof will be communicated accordingly. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 850937.

Authors : Oluwabi Abayomi*, Atanas Katerski, Nikolae Spalatu, Arvo Mere, Malle Krunks, Ilona Oja
Affiliations : Laboratory of Inorganic Thin film Chemical Technology, Tallinn University of Technology, Estonia

Resume : Recently, nickel oxide (NiOx) has been an excellent choice of material for the hole transport layer (HTL) in solar cell owing to its p-type conductivity, wide bandgap, and high work function. However, to fabricate NiOx thin film with desired properties, the deposition method and condition must be carefully controlled. In this perspective, the deposition of NiOx thin film from nickel acetylacetonate precursor source was investigated, and chemical spray pyrolysis method; which is a cost-effective method was employed. The deposition condition was optimized by varying the deposition temperature (from 280 to 420 oC in steps of 50 oC) and the precursor concentration (from 10 mM to 40 mM). The film’s morphology obtained through scanning electron microscope (SEM) images revealed that the films were uniform and compact with columnar structure growth. The X-ray diffraction (XRD) results revealed that the deposited NiOx thin films were polycrystalline with cubic structure. The optical transmittance results revealed that the films were transparent and the optical bandgap reduces from 3.95 eV to 3.67 eV for films deposited from 330 oC to 450 oC, respectively. At optimized deposition condition (400 oC), lithium doped NiOx (NiOx-Li) thin film was deposited, and the influence of varying lithium impurities (from 0 to 60 mol %) in the precursor solution on the film’s physical properties was studied. The optical transmittance increases with increase in the Li amount in the spray solution. The optical bandgap also increases from 3.64 eV to 3.72 eV by increasing the Li amount in solution from 0 mol % to 60 mol % respectively. The shift in the XRD peak from 43.48⁰ (0 mol%) to 43.56⁰ (60 mol%) revealed that Li atom was incorporated into the NiOx lattice. Furthermore, the films show p-type conductivity behavior and the resistivity was reduced from 104 Ωcm (0 mol %) to 102 Ωcm (60 mol %). Based on this result, spray deposited NiOx-Li thin film has the potential to be integrated at the hole transport layer in solar cell.

Authors : S. Iftimie1,*, C. Locovei1,2, A.G. Sovaiala1, O.D. Baban1, A.M. Raduta1, A.M. Panaitescu1, A. Radu1, V.A. Antohe1,3, A. Dumitru1, L. Ion1, and S. Antohe1,4
Affiliations : (1)University of Bucharest, Faculty of Physics, Bucharest, Romania; (2)National Institute of Materials Physics, Magurele-Bucharest, Romania; (3)Catholic University of Louvain, Institute of Condensed Matter and Nanoscience, Louvain-la-Neuve, Belgium; (4)Academy of Romanian Scientists, Bucharest, Romania

Resume : We discuss the electrical and photo-electrical performances of both substrate and superstrate configurations based on CdTe/CdS heterojunction, together with the optical, structural, and morphological properties of their components, on both silicon (Si, (100)) and optical glass substrates. The shunt and series resistances and the quality diode factor of ITO/CdS/CdTe/Cu:Au structure will be analyzed in the frame of Si(100)/CdTe/CdS/ITO device, together with their behavior under illumination in AM 1.5 standard conditions. The parameters characterizing a photovoltaic cell, i.e. fill factor, short-circuit current, open circuit voltage, maximum power, and the power conversion efficiency, will be evaluated and analyzed. The growth of the transparent electrode, the active layer, and the window layer was performed by radio-frequency magnetron sputtering, while the metallic electrode was fabricated by thermal evaporation. The optical measurements included absorption spectroscopy, and the structural and morphological features were considered by X-ray diffraction and atomic force microscopy. Keywords: cadmium telluride, cadmium sulfide, photovoltaic structures, substrate configuration, superstrate configuration Acknowledgments: This work was financially supported by The Romanian National Authority for Scientific Research, UEFISCDI, under the project 25TE/2020.

Authors : A. Eslam (1,2), D. Hauschild (2,3,4), W. Hempel (1), L. Weinhardt (2,3,4), R. Wuerz (1), C. Heske (2,3,4), M. Powalla(1,5)
Affiliations : 1) Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), Meitnerstraße 1, 70563 Stuttgart, Germany 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-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 4) Department of Chemistry and Biochemistry, University of Nevada, Las Vegas (UNLV), 4505 Maryland Pkwy, Las Vegas, NV 89154-4003, USA 5) Light Technology Institute (LTI), Karlsruhe Institute of Technology (KIT), Engesserstraße 12, 76131 Karlsruhe, Germany

Resume : In the last decade, Cu(In,Ga)Se2 (CIGSe)-based thin film solar cells have experienced a significant efficiency boost, which can mainly be attributed to alkali post-deposition treatments (PDTs) of the CIGSe absorber layer. PDT-treated CIGSe layers often show an increased carrier lifetime, a reduced defect density, and a copper-depleted CIGSe surface. Typically, these PDTs are used on alkali-containing absorbers, e.g., with sodium diffusion from the glass substrate, and are optimized empirically for this particular alkali distribution. To gain a detailed understanding of the PDT’s impact, it is thus important to separate the effects of these two alkali sources. For this purpose, we have studied the effects of NaF and KF PDTs on alkali-free CIGSe solar cell absorbers with varying Cu content by means of electric cell analysis (IV, EQE, CV), time-of-flight secondary-ion mass spectrometry (ToF-SIMS), as well as x-ray and ultraviolet photoelectron spectroscopy (XPS, UPS). We find a significant cell efficiency improvement with NaF or KF PDT. However, compared to Na, a larger amount of K is needed to achieve the same cell efficiency increase. At the same PDT temperature, we find more K than Na in the CIGSe layer and observe different alkali depth profiles throughout the layer. In contrast to NaF PDT, low-temperature (105°C) KF PDT massively decreases the cell efficiency, which is accompanied by a Cu-enriched CIGSe surface. The results will be discussed in relation to the respective CIGSe solar cell efficiencies.

Authors : Oana Cojocaru-Mirédin
Affiliations : RWTH Aachen University, I. Institute of Physics, Aachen, Germany

Resume : Thin-film solar cells with polycrystalline Cu(In,Ga)Se2 absorber layers exhibit record power-conversion efficiencies of currently 22.6%. Such performance is impressive in view of the rather small average grain sizes of 0.5-1.5 µm with Cu(In,Ga)Se2 layer thicknesses of 2-3 µm. The present work gives insight to the chemistry at grain boundaries in Cu(In,Ga)Se2 absorber with and without Na doping by using correlative microscopy approach. Surprisingly, the planar defects in the Na-free absorber show strong In, Se-enrichment and Cu-depletion. This is contrary to what have been observed for the grain boundaries in Na-containing absorber, i.e. very weak In-enrichment and Cu-depletion accompanied by Na segregation. This work clearly proves that Na does not only act as a dopant, but also as a passivator inside the Cu(In,Ga)Se2 absorber layer, explaining in this way his undoubtful beneficial effect on the cell performance.

Authors : Taowen Wang1*, Florian Ehre1, Boris Veith-Wolf2, Jan Schmidt2,3, Valeriya Titova2, Nathalie Valle4, Michele Melchiorre1 and Susanne Siebentritt1
Affiliations : 1Laboratory for Photovoltaics, Department of Physics and Materials Science, University of Luxembourg, 41 rue du Brill, L-4422, Belvaux, Luxembourg Luxembourg. 2Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, D-31860 Emmerthal, Germany. 3Institute of Solid-State Physics, Leibniz University Hannover, Appelstr. 2, 30167 Hannover, Germany 4Luxembourg Institute of Science and Technology—Materials Research and Technology Department, 41 rue du Brill, L-4422, Belvaux, Luxembourg.

Resume : Abstract Photoluminescence measurements are used to determine quasi-Fermi level splitting (EF), i.e. the open circuit voltage of the absorber and the power law exponent, i.e. the “optical diode factor” (ODF). We investigate the influence of back surface recombination on EF and ODF in CuInSe2 (CISe) solar cells, with the aim of using them as bottom cells in tandem devices. CISe solar cells with flat bands suffer from serious back surface recombination which results in significant EF losses. This problem can be mitigated by incorporation of gallium to form a conduction band grading or by inserting a dielectric layer at the back contact. The passivation effects are studied by intensity calibrated photoluminescence, time resolved photoluminescence and SCAPS simulations. The results demonstrate that both efforts lead to the same passivation effect, reducing the effective recombination velocity by almost 3 orders of magnitude and increasing EF by ~50 meV, compared to reference samples without passivation. Furthermore, the back surface recombination decreases the ODF, indicating that a low ODF alone is not a sufficient indicator of good absorber quality. The study strongly indicates the serious back surface recombination, leading to a significant Voc loss, which must be avoided to make use of these low bandgap solar cells in tandem devices. *Author for correspondence:

Authors : Stephan J. Heise [1], Janet Neerken [1], Raymund Schäffler [2]
Affiliations : 1: Ultrafast Nanoscale Dynamics, Institute of Physics, University of Oldenburg, D-26111 Oldenburg, Germany; 2: NICE Solar Energy GmbH, Alfred-Leikam-Str. 25, D-74523 Schwäbisch Hall, Germany

Resume : One aspect of a solar module’s quality is its resilience towards partial shading, which is especially relevant for building-integrated photovoltaics. Because of the series connection of cells in a module, shaded cells are at negative voltages. Therefore, it is important to gain a firm knowledge and understanding of the material’s reverse current-voltage (IV) characteristics. In this study we investigate how the reverse IV characteristics under illumination are affected by a heavy-alkali post-deposition treatment (PDT), which has become a popular method to enhance cell performance over the last few years. Cells with a PDT step after the (co-evaporated) deposition of the absorber layer are compared to cells without a PDT. Since alkali-treated absorbers allow for thinner CdS buffer layers, we examine cells with different buffer thicknesses in order to distinguish between effects resulting from the PDT and from the thinning of the buffer. For a further analysis of the role of the buffer layer, measurements are also performed with red and white illumination. The results show a clear difference of the reverse IV characteristics between cells with and without PDT: The (absolute) voltage at which the blocking behaviour of the diode starts to break down is higher for cells with PDT compared to cells without PDT. We discuss these results with respect to the mechanisms that are responsible for the reverse IV characteristics, taking into consideration the role of the buffer layer.

Authors : Wooseok Yang, S. David Tilley
Affiliations : Department of Chemistry, University of Zurich

Resume : Antimony triselenide (Sb2Se3) has recently emerged as a promising semiconductor material for photovoltaics and photoelectrochemical (PEC) water splitting, due to its low-cost and small band-gap (1.1~1.2 eV), high absorption coefficient, and good optoelectrical properties.1-4 In order to fabricate an efficient and stable photoelectrode for PEC water splitting, a multilayer structure, consisting of a protection layer (e.g., TiO2) and a catalyst layer (e.g., Pt), is generally required.5 However, despite the importance of understanding the photo-physics underlying these multilayer photoelectrodes, the detailed analysis of them under operando conditions is challenging due to the complexity of the device structures. In this talk, we will present the characterization results of Sb2Se3 photocathodes by using electrochemical impedance spectroscopy (EIS). As a successive study of our previous work on EIS analysis of p-Si and Cu2O photocathodes,6 the current study further demonstrates the versatility of the EIS technique as a useful and easily-accessible technique for identifying the limiting factors in the multilayer photocathodes. By comparing the similarities and differences among the EIS results obtained by the different light absorbers, the correlation between the material’s properties and the EIS results will be also discussed. References 1. Wooseok Yang et al., J. Mater. Chem. A., 2019, 7, 20467 2. Wooseok Yang et al., Adv. Energy Mater., 2018, 8, 1702888 3. Wooseok Yang et al., ACS Nano, 2018, 12, 11088 4. Rajiv R. Prabhakar et al., J. Mater. Chem. A.,2017, 5, 23139 5. Wooseok Yang et al., Chem. Soc. Rev., 2019, 48, 4979 6. Thomas Moehl et al., Sus. Energy & Fuels, 2019, 3, 2067

Authors : Patrick Pearson, Volodymyr Kosyak, Jan Keller, Jes Larsen, Charlotte Platzer Björkman
Affiliations : Uppsala University; Midsummer AB ;Uppsala University; Uppsala University; Uppsala University

Resume : The flexibility and low weight of thin film solar cells compared to Si or GaAs devices makes them strong candidates for use in space. Cu(In,Ga)Se2 (CIGS) is a particularly promising thin film technology with competitive performance values and has been the subject of many radiation studies [1,2]. Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) technologies have, however, had almost no radiation studies published [3-5]. In November 2017 CIGS, sulphide CZTS and mixed CZTSSe thin film solar cells were irradiated by 3 MeV protons with fluence in the range 10^10-10^13 cm^(-2) with the aim of evaluating the devices’ radiation hardness and any self-healing exhibited, using JV and EQE measurements [3]. CIGS was seen to have a radiation hardness of approximately one order of magnitude greater than Si or GaAs, with CZTS(Se) being almost one order of magnitude more radiation hard than CIGS. After one month of recovery time the performance of the CIGS device had recovered to ~70% of its pre-irradiated state and the CZTS and CZTSSe devices to ~95%. The main performance loss was in V_OC. It is speculated that different mechanisms are involved in the irradiation-induced degradation and subsequent self-healing of the chalcogenides and kesterites. Indicators of this difference are the significantly different healing rates and overall recovery, as well as different photoluminescence spectra responses to irradiation [2,4]. A thesis work in 2018 used 0.25MeV protons with a fluence of 10^12 cm^(-2) to irradiate CIGS and CZTS cells. The effect of annealing on recovery was also investigated for the CZTS cells. Both devices were investigated using JV and EQE measurements, whilst the CZTS cells were also examined using capacitance-based measurements. Again it was found that CZTS is significantly more radiation hard than CIGS, with enhanced recovery. Annealing was seen to recover V_OC almost to the pre-irradiated value and J_SC somewhat less, with temporary improvements in FF and η. Since the initial studies, the devices have been kept in dark conditions and we now seek to evaluate any further healing that has taken place in the time interval. Furthermore, we will attempt to understand the mechanisms leading to the self-healing observed in the devices and why there is such a difference between the kesterite and chalcogenide radiation responses. The devices shall be studied using JV and EQE measurements, in addition to capacitance techniques with the possibility to conduct measurements in the temperature range of 100 – 300K. Annealing stages will also be included to further investigate the recovery process. Poster Preferred. [1] Jasenek A, Rau U. Journal of Applied Physics 90 (2001), 650. [2] Hirose Y, et al. Japanese Journal of Applied Physics 51 (2012), 111802. [3] Suvanam S.S, et al. Energy Materials and Solar Cells 185 (2018), 16-20. [4] Sugiyama M, et al. Thin Solid Films 642 (2017), 311-315. [5] Sulimov M.A, et al. Materials Science in Semiconductor Processing 121 (2021), 105301.

Authors : Thomas Paul Weiss, Florian Ehre, Valentina Serrano-Escalante, Taowen Wang, Susanne Siebentritt
Affiliations : University of Luxembourg, Laboratory for Photovoltaics, Department of Physics and Materials Science, 41, rue due Brill, L-4422 Belvaux, Luxembourg

Resume : Thin-film Cu(In,Ga)Se2 solar cells reach power conversion efficiencies exceeding 23 % and non-radiative recombination in the bulk is reported to limit device performance. The diode factor has not received much attention, yet, although it limits the fill factor, and hence the efficiency of state-of-the-art solar cells. Here, we compare the diode factor of Cu(In,Ga)Se2 absorbers, measured by photoluminescence spectroscopy, and of solar cells, measured by current-voltage and capacitance-voltage characteristics, supported by simulations using rate equations of generation and recombination. It is found that the diode factor is already increased in the neutral zone of the absorber, even at low injection rates, due to metastable defects, such as the VSe-VCu defect found in Cu(In,Ga)Se2. The reason is the increased net acceptor density upon minority-carrier injection, a phenomenon generally found in Cu(In,Ga)Se2 solar cells and absorbers from various institutes. In particular, we show that the increased net acceptor density after light soaking can directly be linked to a diode factor of the absorber above the ideal value of 1 and thus to a reduced fill factor of the solar cell. In addition, we present a novel technique based on time-resolved IV measurements, which directly demonstrates the influence of these metastable defects on the fill factor. SCAPS simulations are used to support these findings. It is demonstrated that efficiencies exceeding 24 % are expected for state-of-the-art Cu(In,Ga)Se2 solar cells if metastable defects can be suppressed and hence diode factors of 1 can be obtained.

Authors : Mohit Sood* 1, Aleksander Urbaniak 2, Christian Kameni Boumenou 1, Hossam Elanzeery 1, Florian Werner 1, Michele Melchiorre 1, Susanne Siebentritt 1
Affiliations : 1Department of Physics and Materials Science, University of Luxembourg, Belvaux, L-4422, Luxembourg; 2Faculty of Physics, Warsaw University of Technology, Koszykowa 79, Warszawa 00-662, Poland

Resume : The presence of interface recombination in a complex multilayered thin-film solar cell structure causes a disparity between the measured internal open-circuit voltage (Voc,in) and the external open-circuit voltage (Voc,ex) i.e. an interface Voc deficit. The Voc,in, calculated from the ratio of total radiative recombination in the device to the total number of injected photons in a one sun calibrated PL measurement is the energetic difference between the hole quasi Fermi level (Fh) and electron quasi-Fermi level (Fe) in the bulk. While the Voc,ex measured in a current-voltage measurement under one sun illumination is the energetic difference between the Fh at the hole contact and the Fe at the electron contact. The mismatch of the energy bands at the interface between absorber and charge transport layer, and Fermi level pinning are the two commonly evoked models to explain why and in which case interface recombination dominates. Here, we introduce sub-surface defect model as an alternate plausible reason for the interface Voc deficit in solar cells. We build a model by taking Cu-rich CuInSe2 solar cells as an exemplary case study. These solar cells are limited by interface recombination and show an interface Voc deficit. However, the band alignment with the buffer cannot explain this, neither is there any experimental evidence for Fermi level pinning. To build a reliable model the characteristics of these sub-surface defects is probed in depth using admittance spectroscopy and deep-level transient spectroscopy. Admittance spectroscopy yields the sub-surface defect nature and defect density, while deep-level transient spectroscopy confirms the acceptor nature of these defects. Building on these observations the sub-surface defect model is realized for CuInSe2 and numerical modelling using SCAPS-1D is performed. The results of numerical modelling are reviewed together with optical and electrical experimental observations. The study shows deep defects limit the Voc,ex of the solar cell, as sub-surface defects lead to a strong decrease in Voc,in near the absorber/buffer interface and thus leads to Voc deficit in the device. Moreover, the model simulates an activation energy of recombination channel lower than bandgap of absorber, which is usually attributed to conduction band ‘cliff’ or Fermi level pinning at the absorber buffer interface in the devices. The results show that sub-surface defects cause similar limitations as a dominant interface recombination. In addition, experimentally we also probe the origin for presence of sub-surface defects in CuInSe2 absorbers grown under Cu-excess conditions. The two different etching processes: KCN and aqueous bromine unravel the exact origin of this defect. Findings of admittance and low temperature current-voltage analysis reveal the presence of the deep defect independent of etchant, establishing the removal of Cu2-xSe phase as the root cause of the deep defect formation. Our work demonstrates that subsurface defects lead to IV characteristics that show all signatures of interface recombination. Most importantly: subsurface defects lead to an additional loss in Voc.

Authors : Yukiko Kamikawa1, Marco Nardone2, Hajime Shibata1, Shogo Ishizuka1
Affiliations : 1 National Institute of Advanced Industrial Science and Technology (AIST) 2 Bowling Green State University

Resume : Suppression of interfacial recombination is indispensable for improving the characteristics of semiconductor devices. Research on the reduction of interfacial recombination has been vigorously conducted in Si semiconductors since the 1950s. As a typical example, the method using a thermal oxide film (SiO2) is the oldest [1]. In solar cells, the effectiveness of SiO2, SiNx, Al2O3, etc. is known. Regarding CIGS solar cells, in 2013, a group at Uppsala University (Sweden) introduced an oxide film (Al2O3) into thin Cu (In, Ga) Se2 (CIGS) solar cells by applying the technology of Si solar cells. Reported improved characteristics [2]. The Coulomb repulsion region by a negative fixed charge in Al2O3 is one reported mechanism, analogous to the reported in Si solar cells. In CIGS solar cells, alkali metal movable ions (Na +. K +) are present in CIGS. The influence of the movable ions to the interface properties have not been fully understood. In this study, we introduced an A2O3 passivation film into a thin CIGS solar cell, discussed its effect on its characteristics, and investigated its interfacial characteristics. An Al2O3 film with a film thickness of 1 to 30 nm was formed by the atomic layer deposition (ALD) method. In the solar cell structure, an Al2O3 film was introduced between the back electrode (Mo) and the CIGS absorption layer, and contact holes were formed by using photo lithography. MOS structures were also prepared for interface analysis. The CIGS surface was etched off with a Br-based solution of about 1 µm to remove strong Cu deficient region [3]. The gate voltage dependence of the electrical capacity of the MOS structure showed hysteresis phenomena. In the initial state, the charged state of the interfaces was considered to be close to neutral considering flat band voltage position. On the other hand, once a negative voltage is applied, the flat band voltage is shifted to the negative voltage side, suggesting that the charge state at the interface is varied. In the solar cell structure, in some cases, the VOC is improved by introducing the Al2O3 layer. The passivation effect differs depending on condition of alkali metal supply. It is possible that the alkali metal plays important roles on the interface characteristics. References [1] C. J. Frosch et. al., J. Electrochemical Society, 104 (1957) 547. [2] B. Vermang et. al., IEEE J. Photovoltaics, 4 (2014) 48 [3] Y. Kamikawa et. al., ACS Applied Materials and Interfaces 12 (2020)45485. This work was supported partly by JSPS KAKENHI Grant Number JP 20K0534. This work was also partly supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).

Authors : Vikash Kumar, Elisa Artegiani, Prabeesh Punathil, Alessandro Romeo
Affiliations : Laboratory of Photovoltaics and Solid State Physics, Department of Computer Science, University of Verona, Verona, Italy.

Resume : In the recent years, Antimony Selenide (Sb2Se3) has become a promising absorber material for thin film solar cell application, with high potentials to replace CdTe because of its abundance and better environmental perception. A high optical absorption coefficient along with an optimal band gap and with availability at low cost are the strong advantages of this material. In the present study, Antimony Selenide (Sb2Se3) thin films solar cell in superstrate (Glass/ITO/ZnO/CdS/Sb2Se3) configuration are fabricated with a low temperature thermal evaporation method. Our best solar cell showed a conversion efficiency of 3.5%. However in this configuration the material is selenium poor. In this work co-evaporation of Se and Sb2Se3 at low temperature will be analyzed and discussed. The material has shown to be very sensitive to the Se flux, and for this reason different evaporation rates to adjust the stoichiometry will be presented and discussed. The structural, optical, morphological and electrical properties of the obtained films have been studied and the results will be compared and discussed in detail.

Authors : Aleksei Penezko, Maarja Grossberg, Olga Volobujeva, Marit Kauk-Kuusik
Affiliations : Department of Material and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : Cu-Sb-Se (CASe) has a great potential as effective and low cost absorber material for thin film solar cells. In this study, the influence of the deposition process conditions (substrate temperature, sputtering speed etc.) on the structural and optoelectronic properties of the CASe thin films by magnetron co-sputtering and corresponding thin film solar cells was studied. To improve the properties of as-deposited absorber material, various post-deposition treatments including annealing in selenium vapour in a two temperature-zone furnace at different temperatures were used. Selenization was done in a degassed and sealed quartz ampoule. The annealing temperature and annealing time in selenium vapour were varied in the range from 380°C to 420°C for 10 min to 60 min, respectively. The selenium pressure of 1 Torr was kept constant. The properties of the synthesized and post-treated CASe thin films were investigated using Raman scattering, X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy Dispersive Spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). SEM images showed the effect of selenization on the morphology and grain structure of the film, which becomes double layered after the selenization. The thickness of the top layer increased from 300 nm to 500 nm when increasing the treatment time from 10 minutes to 60 minutes at 380°C, and treatment temperature from 380°C to 420°C for 60 minutes, respectively. XRD and Raman analysis showed that as-deposited films contain mainly CuSbSe2 and minor part of CuxSe and Sb2Se3 phases. After annealing in selenium atmosphere, the films showed double layer structure with Cu3SbSe4 (top layer) and Sb2Se3 (below). The annealed Cu-Sb-Se films were etched with 10% KCN alkaline solution for 60 seconds before making solar cells. The I-V characteristics and External Quantum Efficiency (EQE) measurements of the devices were analysed. The thin film solar cells based on the post-annealed and etched absorbers showed power conversion efficiencies of 3% with open circuit voltage of 373 mV, fill factor 52.1% and short circuit current density 16.5 mA/cm2. The EQE spectra indicated to the strong recombination of the photogenerated charge carriers in the absorber material. EQE curves were also used to estimate the bandgap energy value of absorber layer.

Authors : Yao Gao, Yong Li, Jan Lucaßen, Martina Schmid
Affiliations : University of Duisburg-Essen, Faculty of Physics and CENIDE, Lotharstr. 1, 47057 Duisburg, Germany

Resume : Copper indium gallium diselenide-based (CIGSe) solar cells are one of the most promising thin film technologies, for which a champion record efficiency of 23.35% has been achieved with a 2-3 µm thick absorber layer. To reduce the usage of scarce elements (In and Ga) and hence costs, thinner CIGSe absorber layers are desirable. However, when the absorber layer thickness drops below 1 µm (submicron), the efficiency of the device degrades severely. The reasons are a reduced short circuit current density (jsc)[1] as well as the risk of increased shunt paths.[2] Another attempt for low-cost processes is based on absorber formation from solution followed by selenization in a low-vacuum environment. However, there is still a large room to improve the device performance by reducing shunt paths. Alkali metal post-deposition treatment has been proven to passivate the shunt paths.[3] Our work focusses on submicron (here 740 nm absorber layer thickness) solution-processed CuIn(S,Se)2 (CISSe) solar cells and aims at performance enhancement by Na incorporation. Therefore, the as-prepared Cu-In-S precursor films were soaked in a concentration series (0.2-1 M with a step of 0.2 M) of aqueous NaCl solution before selenization.[4] Compared to CISSe devices without NaCl treatment, open circuit voltage (Voc), fill factor (FF) and efficiency (η) of NaCl-treated CISSe devices improved significantly. With the optimum NaCl concentration of 1 M, Voc, jsc, FF and η increased by more than 21%, 7%, 11% and 45% relative, respectively. For in-depth investigation of the improvement, the devices were characterized by XRD, XRF, GDOES, SEM, and CV measurement. In the sum, aqueous NaCl solution soaking process was demonstrated a promising method for improving submicron CISSe device performance. [1] van Lare C, Yin G, Polman A, et al. Light coupling and trapping in ultrathin Cu(In,Ga)Se2 solar cells using dielectric scattering patterns[J]. ACS nano, 2015, 9(10): 9603-9613. [2] Saifullah, Muhammad, et al. Development of semitransparent CIGS thin-film solar cells modified with a sulfurized-AgGa layer for building applications[J]. Journal of Materials Chemistry A, (2016), 4(27): 10542-10551. [3] Kong Y, Huang L, Chi Z, et al. Failure and Recovery Modes of Submicron Cu(In,Ga)Se2 Solar Cells with High Cu Content[J]. ACS Applied Materials & Interfaces, 2020, 12(47): 52857-52863. [4] Guo Q, Ford G M, Agrawal R, et al. Ink formulation and low‐temperature incorporation of sodium to yield 12% efficient Cu(In,Ga)(S,Se)2 solar cells from sulfide nanocrystal inks[J]. Progress in Photovoltaics: Research and Applications, 2013, 21(1): 64-71.

Authors : Roland Würz (1), Alexander Eslam (1), Wolfram Hempel (1)
Affiliations : (1) Zentrum für Sonnenenergie und Wasserstoff-Forschung Baden Württemberg (ZSW), Meitnerstrasse 1, 70563 Stuttgart, Germany

Resume : The potassium fluoride (KF) post-deposition treatment (PDT) of Na-doped Cu(In,Ga)Se2 (CIGS) thin films led to a strong boost in cell efficiency. An ion-exchange mechanism between Na and K was found which causes a pronounced accumulation of K at grain boundaries. So far, only preliminary results for diffusion coefficients of K are available, but detailed diffusion studies of K in CIGS thin films are missing. Hence, we investigated the diffusion of K into alkali-free CIGS thin films by time-of-flight secondary ion mass spectrometry measurements (ToF-SIMS) in the temperature range of 100-300°C. By comparing K diffusion profiles in coarse- and fine-grained CIGS thin films, the diffusion of K along the grain boundaries (GB) could be distinguished from diffusion into the grain interior (GI). We found that the diffusion coefficient of K along GBs and into the grain interior is smaller than the corresponding diffusion coefficient of Na. Nevertheless, the K concentration in CIGS layers after a KF PDT is much higher than for layers after a similar PDT with NaF PDT. Possible reasons for this discrepancy will be discussed. Furthermore the diffusion of K in Na-containing CIGS layers is compared to the diffusion of K in alkali-free CIGS layers in order to investigate the ion-exchange mechanism. We found that diffusion of K is enhanced if Na is already present in the CIGS layer. Finally, the activation energies for GB and bulk diffusion of K in alkali-free CIGS layers are compared to the corresponding activation energies for diffusion of Na and Rb.

Authors : Panagiotis S. Ioannou1, Evripides Kyriakides1, Christiana Nicolaou1, Vasiliki Paraskeva2, Maria Hadjipanayi2, George E. Georghiou2, Paris Papagiorgis3, Grigorios Itskos3 & John Giapintzakis1
Affiliations : 1. Department of Mechanical and Manufacturing Engineering, University of Cyprus, 75 Kallipoleos Avenue, P.O. Box 20537, 1678, Nicosia, Cyprus; 2. PV Technology Laboratory, FOSS Research Centre for Sustainable Energy, Department of Electrical and Computer Engineering, University of Cyprus, 75 Kallipoleos St., Nicosia, 1678, Cyprus; 3. Department of Physics, Experimental Condensed Matter Physics Laboratory, University of Cyprus, Nicosia 1678, Cyprus

Resume : Thin film chalcogenide solar cells based on Cu(In, Ga)Se2 (CIGS) have been extensively studied both by the research community and the industry due to their high conversion efficiencies, stability and radiation resistance compared to crystalline silicon or other thin film technologies. Recent improvements in CIGS-based cells have led to record performances of up to 23.3%, outperforming multicrystalline silicon cell efficiencies, paving the way for highly promising stand-alone or tandem solar applications. Commonly, for the fabrication of CIGS-based solar cells with high efficiency, an extended range of vacuum and solution-based deposition techniques are utilized. Specifically, multi-stage co-evaporation is utilized for the deposition of CIGS p-type absorber on metal-coated substrates, while the CdS n-type buffer layer is formed through chemical bath deposition. In addition, reactive sputtering is used for the deposition of the i-ZnO/ZnO:Al window layers. In this work, we investigate the potential of Pulsed Laser Deposition (PLD) as a single technique for the complete growth of the abovementioned constituent layers in a sequential procedure. Growth conditions optimization for the multi-layer structure, derived from detailed structural, compositional, morphological, electrical and optical characterization of constituent layers is discussed. Additionally, plume control strategies to enhance interface quality towards the improvement of PLD-derived cell performance are examined. This work was co-funded by the European Regional Development Fund and the Republic of Cyprus through the Research and Innovation Foundation (Project: EXCELLENCE/1216/0232)

Authors : Krause, Maximilian1; Nishiwaki, Shiro1; Ayodhya N. Tiwari1; Carron, Romain1
Affiliations : 1 Laboratory for Thin Films and Photovoltaics, Empa ‐ Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland

Resume : Tandem solar cells are of high interest for low-cost photovoltaic applications to improve power con-version efficiencies beyond the Shockley-Queisser limit. Promising candidates for the tandem architecture are the widely tunable halide perovskites as top cell, and the low band gap CuInSe2 as bottom cell. One challenge in 2-terminal solar cell engineering is that the bottom cell have to be a suitable substrate for the top cell. The present work reports about potential optimization concepts for device performance improvements. We also report on advanced CuInSe2 absorber surface treatments aiming at reducing the surface roughness, of high relevance for monolithic 2-terminal tandem applications. We consider a 3-stage growth process with the multilayer stack ZnO:Al/i-ZnO/CdS/CIGSe/Mo/glass, of which we propose to flatten the device surface by various approaches, notably the deposition of nanoparticles, or the use of an additional silver surface treatment besides the alkali postdeposition treatment. Microscopic surface studies were performed, notably by atomic-force microscope and 3D-microscope analyses from which surface roughness values are acquired and correlated to the impact on the device performance.

Authors : T.S.Lopes1,2,3,4,,3,4, C.Rocha1, A.Violas1, J.M.V.Cunha1,5,6, J.P.Teixeira1, M.A.Curado1,7, A.J.N.Oliveira1,5, K.Oliveira1, J.R.S.Barbosa1, M.Monteiro1, J.Borme1, G.Birant2,3,4, G.Brammertz2,3,4, P.A.Fernandes1,8, B.Vermang2,3,4, P.M.P.Salomé1,5
Affiliations : 1-INL – International Iberian Nanotechnology Laboratory, Avenida Mestre José Veiga, 4715-330 Braga, Portugal; 2-Institute for Material Research (IMO), Hasselt University (partner in Solliance), Agoralaangebouw H, Diepenbeek, 3590, Belgium; 3-Imec division IMOMEC (partner in Solliance), Wetenschapspark 1, 3590 Diepenbeek, Belgium; 4-EnergyVille, ThorPark, Poort Genk, 8310 & 8320, 3600 Genk, Belgium; 5-Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal; 6-i3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal; 7-University of Coimbra, CFisUC, Department of Physics, R. Larga, P-3004-516 Coimbra, Portugal; 8-CIETI, Departamento de Física, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, 4200-072, Porto Portugal

Resume : Economically and sustainably driven goals have been steering the research in CIGS solar cells from 2000 nm CIGS solar cells to thinner absorbers, to what is called the ultrathin range (i.e. around 500 nm). Nevertheless, ultrathin CIGS devices have their performance limited by recombination losses, namely at the interfaces. Rear interface recombination has been addressed by implementing a dielectric layer between the Mo rear contact and the CIGS layer. However, there are a few reports where the ultrathin rear passivated devices' electrical performance is lower when compared with conventional non-passivated devices. To fully understand the electrical benefits provided by this rear interface passivation strategy, more fundamental studies are needed. In this work, to better understand the passivation effect on ultrathin CIGS devices, a nanofabricated point contact structure based on an Al2O3 dielectric layer was deployed together with potassium fluoride (KF) post-deposition treatment (PDT), which is used to improve the CIGS bulk and front interface. E-beam lithography was used to pattern the Al2O3 layer to establish an electrical contact at the rear interface, whereas a KF-PDT treatment was employed by spin coating followed by annealing in an N2 atmosphere. Current density vs Voltage measurements shows that applying both passivation strategies leads to the highest open-circuit voltage (VOC) obtained in this work: 604 mV. The application of both values of leads to an increase of 85 mV (abs) over the device with only KF-PDT (604 mV vs 519 mV, respectively) and 178 mV (abs) over the device with only rear interface passivation strategy (604 vs 426 mV, respectively). Temperature vs VOC and Time-Resolve Photoluminescence results are compatible with a dominant bulk recombination mechanism for the device with both strategies. Whereas for devices with only one treatment interface recombination is dominant. These results may indicate that rear passivation is only effective when rear interface recombination is the dominant loss factor. Hence, both strategies must be used in tandem to maximize the electrical performance of ultrathin CIGS solar cells. Furthermore, Capacitance vs Voltage (C-V-f), AC equivalent circuit fitting of the capacitance-conductance-frequency (C-G-f) measurements, and SCAPS 1-Dimensional solar cell device electrical simulations will be shown to support our experimental results.

Authors : R. Krautmann*, N. Spalatu*, R. Gunder**, D. Abou-Ras**, T. Unold**, S. Schorr**, I. Oja Acik*
Affiliations : * Laboratory of Thin Film Chemical Technologies, Department of Materials and Environmental Technology, Tallinn University of Technology; ** Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin für Materialien und Energie

Resume : Sb2Se3 has emerged as a potential future photovoltaic absorber material exhibiting suitable optical and electronic properties. Despite rapid increase in device efficiency, Sb2Se3 solar cells still perform far below their theoretical efficiency. Undesired grain growth and deep defects are some of the most discussed factors that limit the device performance. While substrate solar cells seem to benefit from structures that comprise vertical Sb2Se3 ribbons, superstrate solar cells have so far performed better with dense structures of large grains that are not preferentially oriented. Thus, the role of preferred orientation on the device performance remains unclear. Here we present a superstrate TiO2/Sb2Se3 solar cell, which was fabricated by close-spaced sublimation technique (CSS) and which included the deployment of seed layer to influence Sb2Se3 absorber growth. This study aimed to inspect the ordering of Sb2Se3 grains more closely to evaluate its effect on the device performance. Pole figure measurement and electron backscatter diffraction were performed on the Sb2Se3 that revealed the presence of desirable (211), (221) crystal phases but also showed that growth control needs significant improvements to achieve preferred orientation. As for electrical properties of Sb2Se3 absorber films, theoretical studies have predicted numerous types of defects that potentially lower the performance of Sb2Se3 solar cells. Capacitance-voltage profiling and temperature-dependent admittance spectroscopy were performed on the 3.9%-efficient superstrate TiO2/Sb2Se3 solar cells to characterize possible performance limiting factors. Admittance spectroscopy study revealed a deep defect at around 0.45 eV, which could be assigned to a selenium vacancy.

Authors : Mohmed Ould Salem (1), Angelica Thomere (1), Robert Fonoll-Rubio (1), Yudania Sanchez (1), Marcel Placidi (1,2), Victor Izquierdo-Roca (1), Alejandro Pérez-Rodríguez (1,3), Zacharie Jehl Li-Kao (2), and Edgardo Saucedo (2)
Affiliations : 1. Institut de Recerca en Energia de Catalunya (IREC), Jardins de les Dones de Negre 1 2pl, 08930 Sant Adrià de Besòs, Barcelona, Spain. 2. Departament d’Enginyeria Electrònica, Universitat Politècnica de Catalunya, Eduard Maristany 16, Campus Besòs, 08930 Sant Adrià del Besòs, Barcelona, Spain. 3. Departament d’Enginyeria Electrònica i Biomèdica IN2UB, Universitat de Barcelona, Martí i Franqués 1, 08028 Barcelona, Spain

Resume : Among wide band gap thin film chalcogenide (WBG) materials, Ga-rich Cu(In-Ga)Se2 (CIGSe) is considered one of the best candidate as top cell in tandem devices applications, due to their good conversion efficiency, very good stability, low cost, and relatively easy fabrication processes. However, development of WBG CIGSe thin film solar cells onto transparent substrates is considered one of the main challenges for the manufacturing of highly efficient tandem solar cells with this technology. Recently, we fabricated WBG CIGSe solar cells (70% of Ga) on transparent substrate using sodium pre-deposition treatment technique with efficiency over 10 %. On the other hand, the alkali Post-deposition treatment (PDT) using KF, RbF, and CsF applied to CIGSe absorber layers, is considered one of the most important strategies for fabricating high-efficiency solar cell devices with this technology. Among the beneficial effects of heavy alkaline treatment one can mention the increase on the carrier concentration in the absorber, the improvement of the p-n junction quality, with the concomitant reduction of the recombination at the CdS / CIGSe interface, and a change in the grain boundaries composition, which all together leads to a general increases of the Voc, FF, and Jsc of the solar cell devices, and then to largely enhance the conversion efficiency. In this work, Glass/FTO is used as transparent substrate for the fabrication of Ga-rich CIGS films with Ga/(Ga+In) ratio equal to 0.7, investigating the effect of post deposition treatment (PDT) with heavy alkalinen elments. A thin layer of KF, RbF or CsF was evaporated on top of the CIGSe absorber, and then re-annealed using a short selenization of 20min at 350°C. Our results show a large improvement of the conversion efficiency from 9.7% for the non-treated absorber, up to 9.8% using RbF, 10.3% uing KF and as high as 11 % using CsF during the PDTs. A complete morphological, structural and optoelectronic characterization of the absorbers treated with the different heavy alkaline elements will be presented. In summary, a record efficiency of 11% for a WBG CIGSe onto transparent substrate was achieved, without any anti-reflection coating, close to the state of the art for this class of absorbers.

Authors : A. Czudek1, A. Urbaniak1, P. Zabierowski1, A. Eslam2, R. Wuerz2, M. Igalson1
Affiliations : 1 Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland 2 Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstr. 1, 70563 Stuttgart, Germany

Resume : Recently we observed that the sodium concentration has a large impact on the magnitude of persistent photoconductivity (PPC) in CIGS layers and solar cells [1]. We proposed the effect to be either caused by the presence of sodium atoms in the metastable defect complexes, and their direct influence on them, or the impact of sodium atoms on the passivation of grain boundaries (GB). To narrow down the possibilities, investigations of cells and layers after a KF post deposition treatment (PDT), as well as a further study of Na-containing devices prepared on a different substrate were performed. In this work, we present the dependence of the magnitude of the PPC on alkali concentration and show that qualitatively the effect of K is the same as for Na: more potassium enhanced both the observed PPC effect and the relaxed conductivity. As the spatial distribution of sodium and potassium atoms differ from each other, with potassium rather occupying grain boundaries than grain interior, our present results point to the influence of K and Na on the transport barriers at grain boundaries and their alkali-induced passivation in both cases. Additional arguments supporting GB explanation are investigations of PPC after illumination with different light intensity, and the increase in calculated hole mobility upon addition of alkali metals. We observed that the metastable increase in hole concentration is closer to the prediction of the Lany-Zunger model for higher alkali content. Thus we conclude that the main effect of alkali elements are diminished potential barriers at grain boundaries, which leads to the removal of distortions in electrical characteristics. [1] A. Czudek, A. Urbaniak, A. Eslam, R. Wuerz and M. Igalson, "Dependence of the Magnitude of Persistent Photoconductivity on Sodium Content in Cu(In,Ga)Se2 Solar Cells and Thin Films," IEEE Journal of Photovoltaics, vol. 10, no. 6, pp. 1926-1930, 2020

Authors : Ramis Hertwig, Shiro Nishiwaki, Ayodhya N. Tiwari, Romain Carron
Affiliations : Laboratory for Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland (

Resume : Thin film solar cells based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorbers benefit from the presence of a buffer layer between the absorber and the front contact. CdS is the most com-mon buffer material, but its low bandgap of 2.4 eV causes parasitic optical absorption and lim-its the short circuit current (Jsc), while the solution based chemical bath process has a low ma-terial yield and require a complex management of process waste. ZnMgO is a promising alter-native to CdS thanks a bandgap of 3.2 eV and higher. This work explores the impact of treat-ments of the absorber surface combined with different buffer deposition methods on solar cell performance. Prior to buffer deposition the CIGS absorber surface is exposed to different chemicals, among them potassium cyanide, formamidine sulfinic acid, ammonium hydoxide, thiourea, to investigate the effect of surface species on the absorber/buffer interface. The ZnMgO buffer layer is deposited on CIGS absorbers using either ALD, single target sputter-ing or co-sputtering. Full solar cell devices are fabricated and characterized by (temperature-dependent) current voltage (JV) and external quantum efficiency (EQE) measurements.

Authors : Aninat, R.* (1), Bakker, K. (1, 2), Jouard, L. (1), Campillay Ott Cruz, M. (1), Gomez, G. (1), Tsikas, C. (1) & Theelen, M. (1)
Affiliations : (1) Nederlandse Organisatie voor toegepast-natuurwetenschappelijk (TNO), The Netherlands (2) TU Delft, The Netherlands * lead presenter

Resume : With around 40% of the thin-film production and 1.9GWp produced in 2017, CIGS is a steadily growing thin film PV technology. This increasing installation capacity calls for a better understanding and control of the reliability of CIGS panels. To work out the root cause of module failure, comprehensive lab-scale analyses must be applied to commercial modules. We achieve this by coring the areas of interest in the modules and removing unactive layers (“unpackaging”) for further analyses. As an application, a commercial Cu(In,Ga)(S,Se)2 module underwent a partial shading experiment: under AM1.5 illumination, a few cells were shaded with a mask, causing locally “wormlike” defects to form. Samples were then extracted from both defected and worm-free areas of the module by coring, and unpackaged. The I-V results on a worm-free reference show that the PV stack was intact after the upper layers removal. On the defected sample, however, PL and ILIT show a strong shunting behaviour where worms are present. Chemical analysis indicate an increased sulphur content near the edge of the worms. Specific defects, likely created during manufacturing, are also shown to affect the propagation path of the wormlike defects. Finally, we highlight our most recent results regarding coring of different types of modules with various histories. Our results show how coring can close in the gap between lab-scale and field scale failure analysis and bring new insights into the root cause of module failure.

Authors : Jako Siim Eensalu, Atanas Katerski, Ilona Oja Acik, Malle Krunks
Affiliations : Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : Sb2S3, a semiconductor with anisotropic structure, Eg 1.7 eV, and α >2×105 cm-1 at 450 nm, has been used as a solar cell absorber to attain 7.5% power conversion efficiency [1]. Previously, we made uniform Sb2S3 thin films, grown from SbCl3 and SC(NH2)2 by ultrasonic spray in air at 210°C, then annealed in vacuum to achieve a solar cell with efficiency of 5.5% [2]. As low-cost processing is vital for emerging PV, a lower deposition temperature is preferable. Antimony ethyl xanthate (SbEX) decomposes into Sb2S3 at 160°C. Thus, solar cells with efficiency of 2.85% have been made [3]. In this study, the thermal decomposition of SbEX is studied by thermal analysis techniques (TG/DTA+EGA) to establish the basis for depositing compact uniform Sb2S3 thin films by ultrasonic spray pyrolysis in air from SbEX. Furthermore, the effect of fabrication conditions and additives on the structure, morphology, chemical composition, and optoelectronic properties of Sb2S3 films sprayed from SbEX is studied by Raman, XRD, SEM, EDX, and UV-Vis methods for future application in Sb2S3 based thin film solar cells. For the first time, thin films of crystalline Sb2S3 (Eg 1.7-1.8 eV), free of organic residues, were prepared from SbEX in this study via spray pyrolysis by employing thioamide additives, and post-annealing in vacuum at 225°C. [1] Choi et al. Adv. Funct. Mater., 2014, 24, 3587-3592 [2] Eensalu et al. Beilstein J. Nanotechnol. 2019, 10, 2396-2409 [3] Li et al. Energy Technol. 2019, 1900841

Authors : Thomas Schneider, Torsten Hölscher, Christiane Dethloff, Heiko Kempa, Roland Scheer
Affiliations : Institute of Physics, Martin-Luther-University Halle-Wittenberg, Germany

Resume : The so-called N1 signal in CIGS solar cells manifests as a characteristic step in frequency-dependent admittance measurements with a temperature-dependent position. Despite long-lasting investigations, no consensus was achieved on the physical origin of this signal. Among others, a defect at the absorber/buffer interface and a hole extraction barrier at the absorber/back contact interface have been put forward as explanations. We present results of a comparative study of CIGS solar cells with conventional Mo back contacts and alternative ITO contacts, and with a variation of the absorber layer thickness. Most notably, we find that the N1 step in admittance does not occur for samples with ITO back contact. For samples with Mo back contact, the N1 step disappears below a certain absorber layer thickness. When recording the temperature dependence of the open-circuit voltage, we observe a linear behaviour with a constant slope of the samples with ITO back contact, while the samples with Mo back contact exhibit different slopes in low and high temperature ranges, respectively. The difference in activation energies extracted from the two regimes in case of the Mo back contact samples, however, corresponds to the activation energy related to the N1 step in admittance. We interpret the experimental findings as strong evidence for a back contact barrier as origin of the N1 signal. This is supported by simulation results reproducing the disappearance of the admittance step at low absorber thickness due to an overlap of the diodes at the front and at the back interface.

Authors : Setareh Zahedi-Azad, Owen C. Ernst, Simon Fleischmann, Thomas Teubner, Torsten Boeck
Affiliations : Leibniz-Institut für Kristallzüchtung (IKZ), Berlin, 12489, Germany

Resume : The micro-concentrator design for Cu(In,Ga)Se2 solar cells is a promising approach to reduce the quantity of indium and gallium being consumed. In order to save these rare and thus expensive materials, other methods like thinning the absorber material has been suggested [1]. However, the high recombination velocity at the back contact leads to a low overall solar cell efficiency [2]. Therefore, optical light management is required to obtain highly efficient ultra-thin film CIGSe solar cells [2]. With the micro-concentrator setups, due to the concentrated light, a higher efficiency can be expected compared to a classic design. In this work, the local nucleation of Cu(In,Ga)Se2-absorbers at a micrometre scale is being investigated. Our results show that the selective formation of CIGSe islands on Mo-coated soda-lime glass-substrates is possible using localized laser heating by a cw laser and a diffractive optical element. Using trimethylindium and trimethylgallium precursors, the nucleation of (In,Ga)-islands can be locally controlled with this sophisticated heating setup. Subsequent evaporation of copper in a PVD-chamber followed by selenization leads to the formation of Cu(In,Ga)Se2-islands that can serve as micro-absorbers. Since the size and pattern of the formed Cu(In,Ga)Se2-islands can be easily controlled using this laser-heating/DOE-setup, this novel method can also be applied to large-scale production. Compared to a conventional layer, this technology makes it possible to save about 99% of indium and gallium on a substrate. [1] Naghavi, N., et al. "Ultrathin Cu (In, Ga) Se2 based solar cells." Thin Solid Films 633 (2017): 55-60. [2] Sadewasser, Sascha. "Geometry and materials considerations for thin film micro-concentrator solar cells." Solar Energy 158 (2017): 186-191.

Authors : Alok Kumar Jain, M.G.Srinivasan, P.Malar
Affiliations : Department of Physics and Nanotechnology, SRM Institute of Science and Technology,Kattankulathur , Chennai 603203, India

Resume : Orthorhombic Sb2Se3 has emerged as a potential candidate for thin-film absorber material for solar cell application. In the present work, Sb2Se3 was synthesized by mechanical alloying. Sb2Se3 thin film was grown on plain Mo using closed space evaporation. Growth of thin films was done at a substrate temperature of 300 and 3200C. The as-grown film will be characterized by XRD, Raman spectra, transmission spectroscopy and scanning electron microscopy. The device with substrate configuration i.e Mo/Sb2Se3/CdS/ZnO/ITO and will be fabricated and their J-V characteristics will be studied in details.

Authors : Rohini Anandan1,2, Srinivasan M G1,2 and Malar Piraviperumal1,2,*
Affiliations : 1 Department of Physics and Nanotechnology, SRM Institute of Science and Nanotechnology, Kattankulathur, India- 603 203 2 Thinfilm Photovoltaic Laboratory, SRM Research Institute, SRM Institute of Science & Technology, Kattankulathur, India-603 203

Resume : Chalcostibite CuSbSe2 has gained attention as a potential absorber material for thin-film solar cells, since it exhibits a high absorption coefficient and is also a low-cost absorber material. In this work, we report the formation of the copper antimony selenide (CuSbSe2) thin films by thermally assisted interlayer diffusion reaction of copper onto Sb2Se3 thin films. Sb2Se3 thin films were deposited on glass substrates from a pre-synthesized ball milled Sb2Se3 source through the close space evaporation method at a substrate temperature 320oC. Subsequently, a thin copper film was deposited on the surface Sb2Se3 film by thermal evaporation. The Sb2Se3/Cu layers on glass substrates were subjected to vacuum annealing at different temperatures to facilitate the copper diffusion into the Sb2Se3 layer. Further, selenization was done to meet the stoichiometry of CuSbSe2. The structure, Phase, morphology, elemental composition, optical and electrical properties of the obtained CuSbSe2 thin film will be analyzed using different characterization techniques such as XRD, RAMAN, SEM, EDS, UV-Vis-NIR and Hall measurement.

Authors : Srinivasan. M. G1,2 and Malar Piraviperumal1,2 *
Affiliations : 1 Department of Physics & Nanotechnology, SRM institute of Science and Technology, Kattankulathur, Tamil Nadu, India, 603 203. 2 Thinfilm Photovoltaics laboratory, SRM Research Institute, SRM institute of Science and Technology, Kattankulathur, Tamil Nadu, India, 603 203

Resume : Antimony selenide (Sb2Se3) is one the of most promising absorber materials in thin film solar cells due to its high absorption coefficient and earth abundant constituents. In this work, Sb2Se3 thin films were grown by close space evaporation method using pre synthesized Sb2Se3 source material on glass substrate at 320oC substrate temperature. Two different substrate to source distance were employed such as 3 and 5 cm. The phase formation of Sb2Se3 thin films were studied in detail using X-ray diffraction and Raman spectroscopy analysis. Scanning electron microscopy and Energy dispersive spectroscopy were performed to analyse the morphology and stoichiometry respectively. Optical and electrical performances of the thin films were evaluated by UV-Vis-NIR Spectroscopy and Hall measurements. Photo response behaviors were studied under dark and light illumination.

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09:00 Welcome and brief introduction    
Tutorial on Advanced Characterization Techniques for Thin-Film Solar Cells : Daniel Abou-Ras
Authors : Jan Keller
Affiliations : Department of Material Science and Engineering, Division of Solar Cell Technology Uppsala University, PO Box 534, SE‐75121 Uppsala, Sweden; phone: +46 18 4716378

Resume : How to accurately characterize solar cells by current-voltage (IV) and quantum efficiency (QE) measurements; variation in temperature and/or illumination power (for IV) or the application of a voltage and/or light bias (for QE); methods to extract important solar cell parameters, like the diode ideality factor and parasitic resistances, as well as absorber properties, like the diffusion length and Urbach energy; reliability of the obtained values; conclusions about internal loss mechanisms.

10:45 Coffee break    
Authors : Evandro Martin Lanzoni
Affiliations : University of Luxembourg, Faculté des Sciences, des Technologies et de Médecine 162A, Avenue de la Faiencerie, 1511 Luxembourg, phone: +352 621 749 761

Resume : Basic principles of the different SPM techniques and main advantages, limitations, as well as artifacts from the tip-sample interaction; mechanism to acquire surface topography by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in various operation modes, as well as the electrical characterization techniques such as electrostatic force microscopy (EFM), Kelvin-probe force microscopy (KPFM), conductive AFM (c-AFM), scanning tunneling spectroscopy (STS) and piezo response; overview of novel SPM developments.

12:30 Closing / Lunch break    
Characterization I : Stephane Collin/Claudia Schnohr
Authors : Daniel Abou-Ras
Affiliations : Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

Resume : In spite of current conversion-efficiency levels of up to more than 23%, Cu(In,Ga)Se2 (CIGS) solar cells still exhibit substantial limitations of their performance. One aspect is losses in open-circuit voltage (Voc), which are linked closely with recombination of excited charge carriers. The present contribution gives an overview on various origins of Voc losses that can be detected by means of microscopic analyses. Recombination at grain boundaries can be quantified by determining the corresponding recombination velocities. It will be shown that even for high-efficiency solar cells, the recombination velocities are on the order of few 100 cm/s, resulting in Voc losses of 20-30 mV. In contrast, dislocations in CIGS thin films were found to be optoelectronically inactive (or similar to the CIGS bulk). I will discuss structural and compositional differences of planar and line defects in CIGS absorber layers, potentially leading to different recombination behaviors. Moreover, electrostatic potential and band-gap fluctuations are unavoidable features of CIGS solar cells. I will provide a summary of the current understanding of these fluctuations and their impacts on the device performance.

Authors : Jiro Nishinaga, Shogo Ishizuka
Affiliations : National Institute of Advanced Industrial Science and Technology (AIST)

Resume : CIGS solar cells with conversion efficiencies over 20% have been reported from several groups. Most studies demonstrating such high efficiencies have employed alkali-metal fluoride postdeposition treatment (PDTs). Thus, the post-deposition treatments using KF, RbF, or CsF are already recognized as an essential technique for high conversion efficiencies. In contrast to the success of these alkali-halide PDTs, the detailed mechanisms by alkali-metal incorporations into CIGS layers are still open to discussion. In this study, we investigated effects of alkali-halide doping on epitaxial CIGS solar cells. Epitaxial CIGS layers on GaAs substrates were fabricated by molecular beam epitaxy methods at 520-degree C. The presence of CIGS layer without grain boundaries was confirmed by scanning transmission electron microscopy. Sodium and potassium ion incorporations were achieved by doping during growth and post-deposition treatments. The best conversion efficiency of 21% was achieved using Cu poor CIGS layers with NaF doping and KF-PDT. The time-resolved photoluminescence (TRPL) was used to compare the minority carrier lifetimes of CdS capped CIGS layers with and without alkali-halide doping, and NaF doping and KF-PDT were confirmed to have positive effects on minority carrier lifetime in the same manner as poly-crystalline CIGS solar cells.

Authors : Roland Mainz, Stefan Schäfer, Jan-Peter Bäcker, Jose Marquez, Helena Stange, Sebastian Schmidt, Reiner Klenk
Affiliations : Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

Resume : It is well known that the Ga distribution has a strong influence on the performance of Cu(In,Ga)Se2 solar cells since its bandgap varies with the [Ga]/[In] ratio. However, in the sequential process, where a metallic precursor film is annealed in an Se-containing atmosphere, the distribution of Ga is difficult to control. Typically, in this process type a strong Ga gradient forms with an accumulation of Ga near the back contact and with a bandgap in the upper part of the film below its optimum below the optimum for a single junction cell. Here, we summarize our current understanding of the formation of Ga gradients during the sequential process, the limitations of reducing this gradient by annealing, as well as the prevention of Ga gradient formation during the selenization process. We gain this understanding from a combination of real-time film growth studies by X-ray diffraction and fluorescence analysis, ex-situ depth profile analysis as well as numerical modelling of the In-Ga interdiffusion process and simulation of the measured XRD and XRF signals. The diffusion modelling shows that a possible explanation for the persistence of the Ga gradient is the formation of elastic stress that counteracts In-Ga interdiffusion. Hence, it is desirable to prevent or reduce Ga gradient formation already during film growth, which can be achieved by finetuning the Se partial pressure during the selenization process.

Authors : Evandro Martin Lanzoni, Omar Ramirez, Amala Elizabeth, Harry Mönig, Susanne Siebentritt, Alex Redinger
Affiliations : Evandro Martin Lanzoni, Omar Ramirez, Susanne Siebentritt, Alex Redinger Department of Physics and Materials Science, University of Luxembourg, Luxembourg; Amala Elizabeth, Harry Mönig Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, Münster 48149, Germany; Amala Elizabeth, Harry Mönig Center for Nanotechnology (CeNTech), Heisenbergstrasse 11, Münster 48149, Germany

Resume : Alkali Post-deposition treatments (PDT) of Cu(In,Ga)Se2 (CIGSe) absorbers are known to improve the power conversion efficiency of the solar cell devices. The majority of the PDT treatments are based on thermal evaporation of alkali metal fluorides under selenium base pressure atmosphere onto a heated polycrystalline CIGSe. Enhanced concentrations of K were detected at the grain boundaries compared to the bulk [1] and a KInSe2 secondary phase was reported to form at the top of the absorber layer [2]. Furthermore, an ion-exchange mechanism with Na was reported [3]. The combination of all these effects leads to an improvement in efficiency, but it is hard to disentangle the individual contributions of the grain boundaries, the surface, and the absorber bulk. In this work, an alkali metal dispenser was used to evaporate metallic potassium onto epitaxial CIGSe absorbers. The experiments were designed such that very small amounts (in the order of a few monolayers) were first evaporated at room temperature onto epitaxial CIGSe absorbers. Subsequently, the absorber layers were heated in-situ to monitor possible chemical reactions and diffusion into the bulk. Due to the absence of grain boundaries, only bulk diffusion was possible. Changes in the optoelectronic properties were monitored by means of quantitative photoluminescence spectroscopy after the surface treatments. In-situ measurements in UHV include Kelvin probe force microscopy (KPFM) measurements to analyze changes in workfunction and X-ray photoelectron spectroscopy (XPS) before and after heating the samples to 200°C in UHV to analyze the compositional changes. We found a reduction of the surface workfunction in the order of 1 eV after deposition of metallic K. Upon annealing to 200°C we observed a recovery of the work function to its original value, which suggested diffusion of K into the bulk of the absorber. This trend was corroborated with XPS measurements on Cu-poor material. The K-concentration was reduced by a factor of two, within the information depth of the XPS for the Cu-poor films whereas it stayed almost constant for the Cu-rich material. Furthermore, we found changes in band bending after K evaporation and annealing, compared to the as-grown films. In the presentation, we will show a detailed model to explain our experimental observations. The use of epitaxial material combined with K evaporation allows understanding how K influences the absorber bulk and surface properties. References: [1] Schöppe, P. et al., Nano Energy 42, 307–313 (2017). [2] Handick, E. et al., ACS Appl. Mater. Interfaces 2, acsami.6b11892 (2017). [3] Chirilă, A. et al., Nat. Mater. 12, 1107–1111 (2013).

Authors : Ashwin Hariharan, Stephan Heise
Affiliations : Ultrafast nanoscale dynamics, Institute of Physics, University of Oldenburg, D-26111 Oldenburg, Germany

Resume : Minority carrier lifetime plays an important role in assessing the quality of a material to develop into a device. For photovoltaic applications, theoretically, a direct correlation should exist between carrier lifetime and device efficiency. The definition and extraction of the lifetime using time-resolved photoluminescence (TRPL) has been recently studied in considerable detail for a CIGSe absorber layer using a two-defect level model. However, freshly fabricated absorbers suffer from surface degradation problems which necessitate the deposition of the Cadmium-Sulfide buffer layer. But, this leads to the formation of a space charge region (SCR) which leads to further complicating the interpretations of the decay. Mainly, inside an SCR a defect’s behavior is a rapid function of space and time thus creating highly nonlinear effects that are nearly impossible to study analytically. In order to underpin the understanding of such a system, the two-defect model for a bulk system has been extended to a CdS/CIGSe heterojunction where simulations (“Synopsys TCAD”) has been carried out by varying key defect parameters such as energy level, capture rate ratios, and the absolute value electron’s capture rate. Simulation results are supported by experimental measurements using different excitation wavelengths, thereby producing different generation depth profiles. Qualitative results have been obtained regarding the behavior of defects inside the SCR, and specific cases have been highlighted where minority carrier lifetime can be extracted with reasonable confidence in a junction system.

15:30 Discussion    
15:45 Break    
Characterization II : Thomas Unold/Alex Redinger
Authors : Susanne Siebentritt, Thomas Paul Weiss, Mohit Sood, Taowen Wang
Affiliations : Laboratory for Photovoltaics, Department of Physics and Materials Science, University of Luxembourg

Resume : The efficiency of solar cells depends on the photocurrent, on the open circuit voltage, which is in the best case given by the quasi-Fermi level splitting (qFls) of the absorber, and on the fill factor, which depends in the best case only on the diode factor. The latter two, qFls and (optical) diode factor (ODF), can be determined by photoluminescence (PL). We use PL measurements and SCAPS simulations to study the role of various recombination channels (bulk, front and back contact recombination) and of metastable defects on qFls and ODF. We will show how much the qFls is reduced by backsurface recombination and how well this loss is mitigated by Ga gradients or dielectric layers at the back contact. We will discuss how defects near the interface reduce the open circuit voltage of the device. Furthermore, we will show how the PL intensity is affected by the presence of metastable defects, which are responsible for an increased net acceptor density after light soaking. These defects increase the diode factor and reduce the fill factor of the solar cell.

Authors : Kostiantyn Sopiha, Jes Larsen, Jan Keller, Charlotte Platzer-Björkman, Jonathan Scragg
Affiliations : Division of Solar Cell Technology, Department of Materials Science and Engineering, Uppsala University, Sweden

Resume : While the most efficient Cu(In,Ga)Se2 (CIGSe) absorbers nowadays are made of Cu-poor material [1], the atomistic origin of such functional off-stoichiometry rarely receives due attention. Here, we present a revised model for off-stoichiometric chalcopyrites that, apart from providing more accurate structures for Cu-poor absorbers and their ordered vacancy compounds (OVCs), and being fully consistent with experimental findings, also enables new insights into the formation and roles of extended defects. A common way of treating off-stoichiometry in CIGSe is to assume a large concentration of Cu vacancies, which must be compensated with In-on-Cu defects in order to explain the relatively low concentration of holes seen in experiments. However, a typical [Cu]/[III]~0.8-0.9 ratio in real absorbers makes the formation of isolated complexes impossible – there is simply insufficient volume to avoid defect interactions. Early first-principles calculations indeed found a tendency for clustering of the defect complexes, which qualitatively explained the formation of OVCs [2]. Still, neither those nor the subsequent calculations were able to identify a realistic structure for near-stoichiometric CIGSe. In these circumstances, the model of isolated defect complexes remained the best explanation at our disposal. In this study, we screened through ~105 different Cu-poor structures in the pseudo-binary In2Se3-CuInSe2 system. The focus was on the ground state stability, which was assessed with a combination of first-principles calculations within density functional theory (DFT) and cluster expansion energy evaluations. We found a series of stable ordered defect phases forming a compositional continuum, with the structures differing by the separation and orientation of Cu-free regions. Among them, the first realistic structure of near-stochiometric CIGSe was uncovered. Some features inherent to it can easily be recognized in standard Cu-poor absorbers (e.g. band gap and hole mobility), while others are nearly identical to those of the ideal chalcopyrite phase (e.g. XRD pattern), explaining how these defect phases could have remained unidentified for so long. Structures of the well-known Cu(In,Ga)3Se5 and Cu(In,Ga)5Se8 OVCs were also refined by the calculations, yielding considerably more stable analogues for both compositions. The results were then extrapolated to other I-III-VI systems (I=Ag,Cu; III=In,Ga; VI=Se,S) and compared. This presentation will discuss the computed properties and their impact on the synthesis and performance of chalcopyrite absorbers. References: [1] Siebentritt, S., et al. Solar Energy Materials and Solar Cells 119 (2013): 18-25. [2] Zhang, S. B., et al. Physical Review Letters 78.21 (1997): 4059.

Authors : Sinju Thomas1, Tobias Bertram2, Christian Kaufmann2, Tim Kodalle2, José A. Márquez Prieto1, Hannes Hempel1, Thomas Unold1, Wolfram Witte3, Dimitrios Hariskos3, Jiro Nishinaga4,5, Takeyoshi Sugaya4,5, Daniel Abou-Ras1
Affiliations : 1. Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany 2. Competence Centre Photovoltaics Berlin (PVcomB) / Helmholtz Zentrum Berlin für Materialien und Energie (HZB), Schwarzschildstr. 3, 12489 Berlin, Germany 3. Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstr. 1, 70563 Stuttgart, Germany 4. Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, /baraki 305-8568, Japan 5. Renewable Energy Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Koriyama, Fukushima 903-0298, Japan

Resume : Band-gap fluctuations in Cu(In,Ga)Se2 (CIGSe) thin films have been discussed since several years as possible origin for losses in open-circuit voltage Voc of the corresponding solar cells [1]. Indeed, even high-efficiency devices exhibit amplitudes of the band-gap fluctuations, 𝜎g, of around 40 meV, which lead to Voc of losses of around 30 mV [2]. In an effort to obtain a better understanding of the origins of the band-gap fluctuations and to find ways to reduce them, we quantified these fluctuations for numerous CIGSe solar cells evaluating the absorption edge in the external quantum efficiency (EQE) spectra according to an approach described in Ref. [3]. The 𝜎g values for all these CIGSe solar cells ranged from 18 to more than 100 meV. As a next step, 6 solar cells were selected, for which the CIGSe layers were grown at different substrate temperatures and also featured different minimum band-gap energies due to different integral [Ga]/([In]+[Ga]) and [Cu]/([In]+[Ga]) ratios. By means of glow-discharge optical emission spectrometry and energy-dispersive X-ray spectroscopy, the Ga gradients perpendicular to the substrate were measured. From these gradients, the distribution of the band-gap energy as well as the curvature of the Ga gradient around the notch point was quantified. It was found that the larger the curvature value, the larger is the amplitude 𝜎g. By means of electron backscatter-diffraction maps, it was confirmed that larger curvature induces lattice strain, indicated by smaller average grain size in the CIGSe absorber. We were able to rule out influences by the minority-carrier diffusion length on the shape of the absorption edge by performing EQE at reverse bias. Also, we conducted one-dimensional device simulations using the software SCAPS to elucidate the effect of only the Ga gradients (without band-gap fluctuations) on the shape of the absorption edge, which was negligible, irrespective of the curvature. In conclusion, we found that 𝜎g values below 20 meV and thus Voc losses below 10 mV can be achieved by a flat Ga gradient in the top part of the CIGSe absorber. This result provides an important hint for the design of CIGSe absorbers not only for enhanced absorption and thus increased photo-current, but also for increased Voc of the corresponding solar cells. [1] Mattheis, J., Rau, U. & Werner, J. H. Light absorption and emission in semiconductors with band gap fluctuations-A study on Cu (In,Ga) Se2 thin films. J. Appl. Phys. 101, (2007). [2] Krause, M. et al. Microscopic origins of performance losses in highly efficient Cu(In,Ga)Se2 thin-film solar cells. Nat. Commun. 11, (2020). [3] Rau, U., Blank, B., Müller, T. C. M. & Kirchartz, T. Efficiency Potential of Photovoltaic Materials and Devices Unveiled by Detailed-Balance Analysis. Phys. Rev. Appl. 7, (2017).

Authors : Khalil El hajraoui1 Mario Navas1, Diego Colombara1,2, Jose Virtuoso1,2, Pedro M. F.J. Costa4, Francis Leonard Deepak1, Sascha Sadewasser1
Affiliations : 1 International Iberian Nanotechnology Laboratory, Av. Mestre José Veiga s/n, Braga, Portugal. 2 Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Genova, Italy. 3 Universidade Politécnica de Madrid, Avda. Complutense 30, 28040 Madrid, Spain. 4 King Abdullah University of Science and Technology, 23955-6900 Thuwal, Saudi Arabia.

Resume : Cu(In1-xGax)Se2 (CIGS) is a direct band gap semiconductor widely used in photovoltaic (PV) energy conversion devices due to its high sunlight absorbance and high temperature stability. A conventional CIGS solar cell is presented as a stack of glass/Mo/CIGS/CdS/i-ZnO/ZnO:Al. The highest efficiencies are typically obtained with a CdS buffer layer deposited by chemical bath deposition (CBD). However, CdS CBD at industrial scale is material inefficient and environmentally unfriendly, as it generates large amounts of toxic/carcinogenic waste. Therefore, a transition to Cd-free buffer layers deposited by a dry vacuum process is mandatory for sustainable CIGS PV in-line production. ZnO1-xSx (ZnO0.75S0.25) via sputtering is an alternative to the CdS CBD in CIGS solar cells, providing a negative conduction band offset [1]. Recently, a significant efficiency enhancement was reported after an annealing treatment of the complete solar cell stack with the sputtered buffer ZnO0.75S0.25 [2]. Among the possible causes of this enhancement inter-diffusion occurring at the absorber/buffer layer interface could play a major role. In the following, we investigate the interface of a similar CIGS solar cell before and after 200°C annealing using advanced scanning transmission electron microscopy (STEM) techniques. Our first results, obtained by high resolution STEM (HR-STEM) and energy dispersive X-ray spectroscopy (EDX), demonstrate the absence of any inter-diffusion or intermixing layer at the absorber/buffer layer interface. Interestingly, we systematically observe the presence of stacking faults in CIGS in close proximity to the absorber/buffer layer interface, independently from the annealing process. HR-STEM imaging reveals an order occurring between ZnO0.75S0.25 crystals and the CIGS crystals, where an epitaxial relationship arises subsequent to the 200°C annealing. This change at the CIGS/buffer interface could result in a lower density of interface defects, which in turn would explain the efficiency enhancement observed in the annealed solar cell stack. [1] C. Platzer-Björkman et al, Journal of Applied Physics, 100 (2006) 044506-1 – 044506-9. [2] M. Zutter et al, Phys. Status Solidi RRL, 13 (2019) 1900145-1 - 1900145-8.

17:15 Discussion    
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Passivation, interfaces and contacts : Negar Naghavi/Jiro Nishinaga
Authors : Jakapan Chantana, Takahito Nishimura, Yu Kawano, Takashi Minemoto
Affiliations : Department of Electrical and Electronic Engineering, Ritsumeikan University, Japan

Resume : Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cells have been reported with conversion efficiencies (η) of over 20%. CdS film is generally used as buffer layer of the CIGSSe solar cells with high η. Nevertheless, the CdS film consisting of toxic Cd absorbs light at the wavelength of about 520 nm owing to its band-gap energy (Eg) of 2.40 eV. Sputtered Zn1-xMgxO film, which is deposited by sputtering method, is thus utilized as alternative buffer to reduce optical loss to enhance short-circuit current density (JSC) for the Cd-free CIGSSe solar cell. In this work, the CIGSSe thin films prepared by the selenization and sulferization (SAS) process have relatively high contents of In and S near their surface [1]. It is therefore considered that the oxidized surface in near-surface In- and S-rich CIGSSe thin films should result in self-forming Inx(O,S)y and/or modified buffer near CIGSSe surface, thus increasing η. The Cd-free CIGSSe solar cells with Zn1-xMgxO buffers were therefore prepared by the all-dry process, where the aged CIGSSe absorbers with different aging times up to 10 months for the oxidized surface were applied. The influence of the aging time on their photovoltaic performances is investigated. In addition, the impact of Eg values of Zn1-xMgxO buffer on cell performances is examined. It is found that the UV-TRPL carrier lifetimes near the Zn1-xMgxO buffer/CIGSSe interface in the Cd-free solar cells is increased with enhancing the Eg of Buffer from 3.30 to 3.94 eV, thus increasing open-circuit voltage and fill factor. Additionally, the JSC is enhanced owing to high transparent Zn1-xMgxO buffer. Ultimately, 22.6%-efficient Cd-free solar cell with the Eg of 3.7 eV of Zn1-xMgxO buffer, prepared by all dry process, is obtained, where the aged CIGSSe absorber with aging time of 10 months is used. The detail will be discussed. [1] J. Chantana, T. Nishimura, Y. Kawano, N. Suyama, A. Yamada, Y. Kimoto, T. Kato, H. Sugimoto, T. Minemoto, Aging effect of a Cu(In,Ga)(S,Se)2 absorber on the photovoltaic performance of its Cd-free solar cell fabricated by an all-dry process: Its carrier recombination analysis, Adv. Energy Mater. 9 (2019) 1902869. Acknowledgements This work is partly supported by NEDO (the New Energy and Industrial Technology Development Organization) in Japan. We would like to thank Dr. Takuya Kato, Dr. Hiroki Sugimoto, and Dr. Yoshinori Kimoto from Atsugi Research Center, Advanced Technology Research Laboratories, Idemitsu Kosan Co.,Ltd. for useful discussion and help of some experiments.

Authors : Dimitrios Hariskos, Wolfram Hempel, Stefan Paetel, Richard Menner, and Wolfram Witte
Affiliations : Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), 70563 Stuttgart, Germany

Resume : Thermal evaporated indium sulfide (InxSy) is already commercially used as a buffer layer in chalcopyrite-based thin-film solar modules. In general, sputtering of InxSy is a much faster deposition method compared to other common techniques such as evaporation, chemical bath deposition (CBD) or atomic layer deposition. It can be easily implemented in an industrial environment to grow the buffer layer on top of the Cu(In,Ga)Se2 (CIGS) absorber, even in a roll-to-roll coater for flexible substrates. From the absorber side, the use of a post-deposition treatment (PDT) with alkali compounds has been established as a key feature during recent years, to fabricate high efficiency CIGS solar cells. In this context the question directly arises, if an all dry process line with CIGS and alkali PDT in combination with InxSy sputtering is possible or if an additional rinsing step of the CIGS surface is mandatory to reach decent efficiencies. In this work, CIGS with a Ga/(Ga In) ratio of 0.3 was deposited in a multi-stage co-evaporation process with or without a rubidium fluoride (RbF) PDT on molybdenum-coated soda-lime glass substrates. The CIGS process was carried out in an industry-relevant in-line machine with 30 x 30 cm2 carriers. In the next step, various rinsing procedures were applied to the CIGS surface and subsequently InxSy buffers were prepared by rf-magnetron sputtering from an In2S3 target at different substrate temperatures. This temperature significantly influences the performance of CIGS solar cells and the best efficiencies were obtained at around 200 C. We observed a sodium accumulation accompanied by a copper depletion at the CIGS/buffer interface from time-of-flight secondary ion mass spectrometry (ToF-SIMS) for higher temperatures during InxSy sputtering on CIGS without PDT. Our experiments indicate that wet chemical treatments like water or ammonia (NH3) solution rinsing after CIGS deposition with RbF-PDT are not needed to reach good cell efficiencies. As a result, the fast InxSy sputtering can be directly applied to the RbF-treated CIGS absorber establishing a fast and all-dry process line. We present efficiency results on the different InxSy-buffered cells and compare them to reference cells with solution-grown CdS buffers. In case of CIGS with RbF-PDT, ToF-SIMS in combination with inductively coupled plasma mass spectrometry revealed that fluorine is completely removed by the NH3 solution rinsing, simulating the conditions during CBD, and the rubidium amount decreases significantly with NH3 solution rinsing compared to the non-rinsed absorbers. Our best cell with sputtered InxSy buffer on a CIGS absorber with RbF-PDT exhibits an efficiency of 17.8% (in-house, with anti-reflective coating), a high open-circuit voltage of 703 mV, and a slightly increased short-circuit current density with delta jSC = 1.7 mA/cm2 compared to the CdS-buffered reference cell with 19.3% efficiency. The efficiency gap is mainly a result of the reduced fill factor for the device with InxSy.

Authors : Alice Debot , Van Ben Chu , Damilola Adeleye , Jérôme Guillot , Régis Vaudemont , Michele Melchiorre , Phillip Dale
Affiliations : Physics and Materials Science Research Unit, University of Luxembourg, 41, rue du Brill, L-4422 Belvaux, Luxembourg ; Physics and Materials Science Research Unit, University of Luxembourg, 41, rue du Brill, L-4422 Belvaux, Luxembourg;Physics and Materials Science Research Unit, University of Luxembourg, 41, rue du Brill, L-4422 Belvaux, Luxembourg; Luxembourg Institute of Science and Technology, Materials Research and Technology Department, 41, rue du Brill, L-4422 Belvaux, Luxembourg;Luxembourg Institute of Science and Technology, Materials Research and Technology Department, 41, rue du Brill, L-4422 Belvaux, Luxembourg;Physics and Materials Science Research Unit, University of Luxembourg, 41, rue du Brill, L-4422 Belvaux, Luxembourg;Physics and Materials Science Research Unit, University of Luxembourg, 41, rue du Brill, L-4422 Belvaux, Luxembourg

Resume : Today, most Cu(In,Ga)(S,Se)2 (CIGS) absorber layers are still covered with a CdS buffer layer to make working solar cells. CdS thin films, which are highly toxic, are usually deposited from chemical bath, creating waste, typically 500 mL per deposition in the research lab. In order to avoid damaging the environment, it is thus urgent to find a less toxic buffer and to reduce waste. Here, indium sulfide buffer layers were inkjet printed on 1 inch square CIGS using only 0.5 microliters of ink. The ink consists of an indium-thiourea complex dissolved in water. The CIGS was UV-ozone treated for 120s to increase the wettability of its surface. And then, a single layer of the ink was printed on the absorber with a drop spacing of 35 micromiters. The incomplete solar cells were then annealed on a hot plate at 250°C for 5 min in air or in nitrogen. A thermogravimetric analysis of the bulk powder of the metal complex showed that it started to decompose at 220°C with residual impurities. X-ray photoelectron spectroscopy measurements of thin films of the complex showed less than 10 at% of impurities, of which less than 1 at% were oxygen and carbon. The nitrogen and chlorine content in the nitrogen annealed samples were found to be slightly higher than in the air annealed sample. GIXRD confirmed the formation of a beta-In2S3 structure which had a tetragonal crystal system with a unit cell volume of 1874 Å3 in excellent agreement with the literature. Spectrophotometry and photoluminescence were used to try to determine the band gaps of the air and nitrogen annealed samples. A Tauc plot of absorption data revealed direct band gaps of 2.5 eV and 2.3 eV for air and nitrogen annealed samples respectively. Photoluminescence measurements pointed to a common band gap of 2.3 eV for both samples. The difference between the results appears to be due to rather broad absorption edge making exact determination of the band gap difficult from the Tauc plot. The best In2S3 air annealed device showed an efficiency of 11.9%, which is twice the efficiency of the nitrogen sample, while reference CdS device showed an efficiency of 11.6%. The short circuit densities were systematically decreased for the In2S3 buffer layers compared to both reference CdS and buffer free samples. In contrast, the open circuit voltages were similar for both In2S3 samples and higher than the other samples. The external quantum efficiency measurements showed that the air annealed In2S3 devices had lower collection at all wavelengths compared to the CdS device indicating the presence of a barrier. Thus, this work showed that no waste air annealed inkjet printed indium sulfide buffer layers can be a good alternative to the chemical bath deposited toxic CdS to achieve efficient CIGS devices.

Authors : Gizem Birant (1,2,3), Jessica de Wild (1,2,3), Thierry Kohl (1,2,3), Dilara Gokcen Buldu (1,2,3), Guy Brammertz (1,2,3), Marc Meuris (1,2,3), Jef Poortmans (1,3,4,5), Bart Vermang (1,2,3)
Affiliations : 1 Institute for Material Research (IMO), Hasselt University (partner in Solliance), Wetenschapspark 1, 3590 Diepenbeek, Belgium; 2 Imec division IMOMEC (partner in Solliance), Wetenschapspark 1, 3590 Diepenbeek, Belgium; 3 EnergyVille, Thorpark, Poort Genk 8310 & 8320, 3600, Belgium; 4 imec (partner in Solliance), Kapeldreef 75, Leuven, 3001, Belgium; 5 Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium

Resume : Ultra-thin CIGS solar cells gain emerging attention due to lowered usage of raw elements, i.e., industrial feasibility,with an environmentally friendly approach.However,benefitting from the sub-micron thick absorber layer brings its own problems,such as insufficient light absorption and interface recombination.One way to overcome this problem is passivating the rear surface with a dielectric material [1].While depositing the dielectric layer,i.e.,the current blocking layer, one should find a way to facilitate current flow between the rear contact and absorber layer.The most commonly used tool is lithography to create contact openings inside the dielectric layer.However,it is an expensive method to use.In this work we present an alternative approach. During the atomic layer deposition (ALD) of the dielectric layer,methyl-based gasses trap inside the layer and create visible bubbles on the surface [2].To create the contact openings, we are cracking those bubbles.We proved that this approach can be used for AlOx and HfOx dielectric layers with remarkable passivation effects, for dielectric layer thicknesses up to 30nm [3], [4].However,for cracking them, several conditions are needed to be satisfied at the same time: the presence of an alkali salt either on top of the passivation layer or coming from the substrate, a certain environmental temperature which depends on the alkali element, trapped methyl-based gasses inside the dielectric layer, and the presence of selenium gas.We also tested different conditions like ALD temperature, alkali salt-dielectric layer matching, the alkali salt deposition method and the effect of the dielectric layer's thickness on distribution of cracked bubbles.After validating all conditions, we develop an idea about this process's chemistry.According to the patented study, to create a dimethyl diselenide (DMDSe) compound, there should be alkali, selenium, and methyl in the high-temperature environment [5].With the patented study's help and our verified conditions, we realized that we unintentionally created the DMDSe compound.As a result, the cracked bubbles, i.e., contact openings, were created with the shape of the alkali salt's traces, by escaped methyl gases through the dielectric layer to create DMDSe with selenium.We did not find any detrimental impact on our solar cells caused by the DMDSe compound until now.However, this approach is still open for improvements to create a more controllable contact openings population and distribution.In the end, this is still one of the easiest and cost-effective approaches to creating the contact openings inside the dielectric-based rear surface passivation layer. This work received funding from the European Union’s H2020 research and innovation program under grant agreement No. 715027. [1]G.Birant, et al,10.3390/app9040677 [2]B.Vermang et al.,10.1109/PVSC.2011.6185916 [3]G.Birant et al.,10.1016/j.solener.2020.07.038 [4]G.Birant et al.,10.1051/epjpv/2020007 [5]舒绪刚张小梅李大光许祥, CN101735130A

11:00 Discussion    
11:15 Break    
Passivation, interfaces and contacts : Wolfram Witte/Bart Vermang
Authors : E. Pyatenko (1,2), D. Hauschild (2,3,4), V. Mikhnych (2), M. Blankenship (4), L. Both (2,3), D. Hariskos (5), W. Witte (5), M. Powalla (5), C. Heske (2,3,4), and L. Weinhardt (2,3,4)
Affiliations : (1) Laboratory for Applications of Synchrotron Radiation (LAS), Karlsruhe Institute of Technology (KIT), Kaiserstraße 12, 76131 Karlsruhe, Germany (2) Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT), Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany (3) Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Engesserstraße 18/20, 76128 Karlsruhe, Germany (4) Department of Chemistry and Biochemistry, University of Nevada, Las Vegas (UNLV), 4505 Maryland Parkway, Las Vegas, NV 89154-4003, United States (5) Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstraße 1, 70563 Stuttgart, Germany

Resume : High-efficiency Cu(In,Ga)Se2 (CIGSe)-based solar cells are traditionally processed with a chemical-bath deposited (CBD) CdS buffer layer. In the last years, significant steps were made to replace the CBD-CdS by alternative buffer layers, e.g., InxSy, Zn(O,S), or oxides like (Zn,Mg)O, HfOx, and GaOx. Such buffer layers can be deposited in a dry process (such as sputtering), which is more compatible for in-line processing and the application in a roll-to-roll coater. Additionally, alkali-fluoride post-deposition treatments (PDT) lead to a significant performance increase for CIGSe-based solar cells. However, the PDTs typically involve a rinsing step (e.g., with H2O or NH3) when combined with a vacuum-processed subsequent layer, again interrupting a complete dry in-line production process. To avoid this rinsing step, a detailed understanding of its impact on the absorber surface and the buffer/absorber interface properties is necessary. In this work, we have investigated the interface formation between a CIGSe absorber prepared with an in-line co-evaporation process and a sputter-deposited GaOx buffer layer; the CIGSe absorber underwent RbF PDT, but no rinsing step. The chemical and electronic structure of this interface was studied using x-ray photoelectron spectroscopy (XPS), x-ray-excited Auger photoelectron spectroscopy (XAES), hard x-ray XPS (HAXPES), UV photoelectron spectroscopy (UPS), and inverse photoemission spectroscopy (IPES). While no diffusion into the GaOx buffer layer is observed for the absorber-related elements, we find a clear diffusion/segregation of Rb, F, and Na. Furthermore, a significant additional band bending in the absorber is caused by the interface formation. With the combination of our techniques, a detailed picture of the chemical and electronic structure of the non-rinsed RbF-PDT CIGSe absorber and its interface with GaOx can be painted. The results are discussed with particular focus on comparing with the interface properties when a rinsing step prior to GaOx sputter deposition is included.

Authors : Yong Li, Guanchao Yin, Martina Schmid
Affiliations : Yong Li, Martina Schmid-University of Duisburg-Essen, Faculty of Physics and CENIDE, Lottharstr. 1, 47051 Duisburg, Germany Guanchao Yin-Wuhan University of Technology, College of Materials Science and Engineering, Luosi Road 122, 43407 Wuhan, China

Resume : On Mo back contact, doping with alkali elements has been proven beneficial for Cu(In, Ga)Se2 (CIGSe) solar cells, both by diffusion from soda-lime glass substrates as well as by post-deposition-treatment (PDT). On the degenerated semiconductor In2O3: Sn (ITO) however, the ITO back contact was reported to impede the Na diffusion from the glass substrate when utilized as replacement for Mo. Yet, a transparent back contact is of interest for semi-transparent, bifacial, and tandem solar cell applications, as is an ultra-thin absorber, which additionally offers reduction of raw material consumption. Therefore, finding an adequate Na doping strategy for ultra-thin CIGSe on ITO back contacts is essential. In this contribution, firstly, Na doping by diffusion from the glass substrate is compared for soda-lime glass and for alkali-free barium boron-silicate glass (PGO glass). Then, on the PGO glass, NaF precursor doping is compared to PDT doping, followed by a combination of precursor doping with PDT as a novel Na doping method. Current-voltage characteristics under AM1.5 solar simulator were measured to derive the photovoltaic parameters of the solar cells, along with capacitance-voltage characteristics to derive the doping density NA in the CIGSe absorber. GD-OES (Glow Discharge Optical Emission Spectroscopy) characterization revealed the Na, CGI (Cu/(Ga+In)) and GGI (Ga/(Ga+In)) depth distribution in the device. It was discovered that under full Na doping optimization, PDT is the optimum method for high performing ultra-thin CIGSe on ITO back contact.

Authors : K. Oliveira1, J. M. V. Cunha1,2,3, A. J. N. Oliveira1,2, J. Lontchi4, D. Flandre4, T. S. Lopes1,5,6,7, M. A. Curado1,8, W. C. Chen9, J. R. S. Barbosa1, A. Violas1, M. Monteiro1, M. Edoff9, J. P. Teixeira1, P. A. Fernandes1,3,10 and P. M. P. Salomé1,2
Affiliations : (1) INL – International Iberian Nanotechnology Laboratory, Avenida Mestre José Veiga, 4715-330, Braga, Portugal (2) Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193, Aveiro, Portuga (3) I3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193, Aveiro, Portugal (4) Université catholique de Louvain, ICTEAM institute, Place du Levant 3, 1348 Louvain-la-Neuve, Belgium (5) Institute for Material Research (IMO), Hasselt University (partner in Solliance), Agoralaangebouw H, Diepenbeek, 3590, Belgium (6) Imec division IMOMEC (partner in Solliance), Wetenschapspark 1, 3590 Diepenbeek, Belgium. (7) EnergyVille, Thorpark, Poort Genk 8310 & 8320, 3600, Belgium (8) CFisUC, Departamento de Física, Universidade de Coimbra, 3004-516 Coimbra, Portugal (9) Ångström Laboratory, Solid State Electronics, Ångström Solar Center, Uppsala University, Uppsala SE-751 21, Sweden (10) CIETI, Departamento de Física, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Porto 4200-072, Portugal

Resume : Cu(In,Ga)Se2 (CIGS) has already shown to be on the vanguard of thin film solar cells applications, by achieving a light to power conversion efficiency value of 23.35%. However, an absorber thickness of about 500 nm is required to pursue a leading role in the market. Albeit, entering the ultrathin regime raises new optoelectronic concerns for CIGS devices. In order to overcome such concerns and achieve high performance substrates (HPS), different ways of incorporating SiOx as a rear passivation layer have been studied. To be able to improve the performance of the HPS, the SiOx layer had to be patterned to provide electrical contact between the rear contact and the absorber layer. The patterning of such layers was carried by lithographic processes using e-beam lithography and photolithography, to produce a hole and line pattern respectively. After a careful analysis of the figures of merit of those cells, we were able to observe that the cell fabricated using the photolithography process was able to produce the most significant improvement compared to a conventional ultrathin CIGS solar cell without passivation layer. Following this study, the lithographic method that produced better results was photolithography, and it was used to fabricate new substrates with line patterned SiOx layers of different thicknesses. Extensive characterizations and complementary optical and electrical simulations of those architectures were carried to better understand the impact of the SiOx layers in the figures of merit of CIGS devices. Introducing a line patterned layer of SiOx, with a thickness of 8nm, between the rear contact and the CIGS absorber layer provided an increase of 1.3% of the light to power conversion efficiency value when compared to a reference device. This novel SiOx based substrate was able to attain efficiency values up to 13.23%. This work demonstrates the potential of SiOx as a passivation layer and paves the way to extend SiOx passivation layers to the industry, as its deposition process (Chemical Vapor Deposition) and the contact patterning (photolithography) have the potential to be up scalable to the industry.

12:15 Discussion    
12:30 Lunch    
CIGS:Interfaces and alloys : Pedro Salome/Roland Scheer
Authors : Claudia S. Schnohr
Affiliations : Felix Bloch Institute for Solid State Physics, University of Leipzig, Germany

Resume : The material properties of Cu(In,Ga)Se2 (CIGS) can be specifically tailored by tuning its composition and stoichiometry. However, similar to other complex semiconductors, the local arrangements of the atoms typically deviate from the long-range crystallographic structure and affect the electronic properties of the material. The element-specific atomic scale structure of the entire Cu-(In,Ga)-Se system has therefore been investigated by synchrotron-based X-ray absorption spectroscopy. For Cu(In,Ga)Se2, the element-specific bond lengths differ significantly from each other and correspond to severe anion displacements, which contribute to the nonlinear change of the band gap energy with In/(In+Ga) ratio [1]. The bond length variation, and hence the bulk disorder on the subnanometer scale, does not depend on the In/(In+Ga) ratio, the Cu history or the alkali treatment of co-evaporated thin films, but it shows a clear and reversible correlation with the final Cu content [2]. A strong increase of local disorder is further observed for the Cu-poor phases Cu(In,Ga)3Se5 and Cu(In,Ga)5Se8 [3]. Still, their element-specific bond lengths show the same dependence on the In/(In+Ga) ratio as observed for Cu(In,Ga)Se2, however, they slightly increase with decreasing Cu content although the lattice constants shrink. These results clearly highlight the remarkable differences between atomic scale structure and crystallographic structure in CIGS and its alloys. [1] Appl. Phys. Rev. 2, 031304 (2015). [2] Acta Mater. 153, 8 (2018). [3] J. Alloys. Comp. 774, 803 (2019).

Authors : Tzu-Ying Lin 1,*, Takahiko Yashiro 2, Ishwor Khatri 3, Ayaka Kanai 2, Mutsumi Sugiyama 2,3
Affiliations : Department of Materials Science and Engineering, National Tsing Hua University, Taiwan1 Faculty of Science and Technology, Tokyo University of Science, Japan2 Research Institute for Science and Technology, Tokyo University of Science, Japan3

Resume : Cu(In, Ga)Se2 (CIGS) solar cells. The strong chemical affinity of alkali metal to In and Se during post-deposition treatment (PDT) results in alkali-InSe2 phase on the CIGS surface. In this work, we found that the Cs-contain compound with the CsInSe2 phase was formed after CsF-PDT even without an extra assistant (In or Se) species supply. The Cs-compound doesn’t perform the nanopattern structure on the surface, but tends to segregate at grain boundaries (GBs); also, a pronounced Cu-deficient layer at grain surface was observed. By modifying the PDT level, the size of the Cs-compound and Cu-deficient layer can be controlled and significantly impact the device performance. In addition, this surface modification structure also brings new phenomena on proton irradiation properties. From the photoluminescence (PL) spectra, the Cs-treated sample behaves stronger radiative recombination than the standard sample after high radiation fluence. Cell efficiencies of Cs-treated sample also easily recover after the heat-light soaking.

Authors : Elizabeth Palmiotti (1), Angus Rockett (1), Benjamin Belfore (2), Deewakar Poudel (2), Sylvain Marsillac (2)
Affiliations : (1) Colorado School of Mines; (2) Old Dominion University

Resume : Thin-film photovoltaic technologies are a prevalent alternative to silicon-based systems due to their greater device efficiencies, high-throughput deposition methods, and lower material requirements. Developments in the commercial manufacturing of thin-film photovoltaic modules has been directed to optimizing a low-cost, high-rate deposition process. Notably, the success of CdTe manufacturing has relied significantly on the high-rate, low-temperature deposition of CdTe films followed by a post-deposition CdCl2 vapor treatment. In this treatment, CdTe films are recrystallized by a short anneal in a CdCl2 environment with some oxygen. Such treatments have resulted in the low-cost production of uniform, high performance devices with minimal chlorine contamination. The thin-film chalcogenide CuInxGa(1-x)Se2 (CIGS) is a high-efficiency photovoltaic absorber material typically deposited by an expensive and time-intensive three-stage co-evaporation process at elevated temperatures. To be more competitive amongst thin-film technologies, CIGS must develop a similar post-deposition recrystallization procedure to that of the CdCl2 vapor treatment of CdTe. Our previous work has investigated post-deposition heat treatment conditions necessary to recrystallize low-temperature co-evaporated CIGS films. In this work, CIGS films were co-evaporated by collaborators onto molybdenum-coated soda-lime glass at 350°C and 400°C and annealed at various times and temperatures with chloride and bromide vapors. The treatments resulted in increased grain size and improved crystallinity. Treatments resulted in varying changes to composition; it is notable that for all treatments the amount of gallium in the films decreased significantly. Additionally, varying amounts of chlorine remained in the for those treated ith a chloride compound. A better understanding of the recrystallization kinetics would provide insights to better control composition changes and optimize treatments. High-temperature x-ray diffraction (HT-XRD) is an in-situ characterization method used to study the kinetic and structural evolution of a material at different temperatures. An Anton Paar DHS 1100 heater stage was installed on a Malvern Panalytical Empyrean X-Ray Diffractometer. The heater stage was contained by a polyether ether ketone (PEEK) dome with flowing N2 gas. The stage was annealed to 150°C and the first XRD scan collected. The stage temperature was increased by ten degree increments and a scan collected at each up to 450°C. In this study, 1μm thick amorphous CuInSe2 (CIS) films were deposited onto silicon and soda-lime glass substrates by rf-magnetron sputtering from a single target at room temperature. Metal halide treatments were conducted using a solution treatment method. Two drops of a 0.1M solution of InCl3 in methanol were put onto the surface of the film then annealed at 55°C for ten minutes in flowing argon to evaporate the methanol. The heating profile was used to study a CIS film as-deposited onto silicon (Sample 1), CIS as-deposited onto soda-lime glass (Sample 2), CIS on silicon with InCl3 (Sample 3), and CIS on soda-lime glass with InCl3 (Sample 4). Peak parameters for each XRD pattern were determined using HighScore Plus and phases identified using the International Center for Diffraction Data (ICDD) database. Micrographs were collected using scanning electron microscopy (SEM) in a FEI Helios Nanolab 600i FIB. XRD patterns collected at 150°C for all samples showed an amorphous CIS starting material, noted by broad, low-intensity peaks corresponding to the (112), (220/204), and (316) orientations. As the temperature of the heater stage was increased, these characteristic peaks began to sharpen and increase in intensity showing crystallization of the CIS. The rate of peak intensity increase slowed to a constant at approximately 380°C, suggesting CIS crystallization is fully achieved at this temperature. This slowing of crystallization also suggest a diffusion-controlled kinetic process. For Samples 1 and 2 an intermediate CuSe phase crystallized rapidly starting at 200°C, producing very obvious platelets extending above the surface of the film. These decreased in peak intensity, and were no longer present above 350°C. CIS/Si and CIS/SLG samples were annealed in an Ar-purged tube furnace at 250°C for five minutes. Using SEM and EDS, platelets were seen across the CIS surface and confirmed to be CuSe. The InCl3 in Samples 3 and 4 resulted in the formation of InOCl and Cu(2-x)Sex intermediate phase formation. This is due to the hygroscopic nature of InCl3; when samples were moved from the furnace to the XRD, oxygen was introduced to the InCl3 and incorporated into the film during the heating profile. To study the crystallization kinetics, the intensity in counts of the peak corresponding to the (112) orientation was plotted for each temperature of each sample. The peak intensity values and rate of peak intensity growth with temperature was greatest for both substrates in the presence of InCl3. In both cases the samples on SLG crystallized faster than those on silicon, showing the influence of the substrate chemistry (presumably as a sodium source). The data was fitted to an Arrhenius relationship and an activation energy extracted for each. The activation energy for Sample 1 was 0.188 eV/atom; Sample 2 0.280 eV/atom; Sample 3 0.392 eV/atom; and Sample 4 0.550 eV/atom. Based on these results, it is apparent that sodium from the soda-lime glass and InCl3 increased the crystallization of CIS. Experiments are underway to repeat this work by depositing the InCl3 in-situ to avoid oxidation and isolate the effects of the metal halide on recrystallization. A heater coil is being installed into the sputtering system to allow the evaporation of a layer of InCl3 during CIS deposition. This technique will be used to continue studying InCl3 and study other metal halides of interest, such as AgBr. HT-XRD will continue to be used to study these samples over similar temperature ranges.

Authors : Deborah L. McGott, Christopher P. Muzzillo, Craig L. Perkins, Joseph J. Berry, Kai Zhu, Joel N. Duenow, Eric Colegrove, Colin A. Wolden, Matthew O. Reese
Affiliations : National Renewable Energy Lab, Colorado School of Mines

Resume : Three leading thin-film photovoltaic (PV) technologies – cadmium telluride (CdTe), CuIn1-xGaxSe2 (CIGS), and perovskite solar cells (PSCs) – are all polycrystalline, but otherwise appear to have little in common. A comprehensive examination of these technologies, however, reveals a common theme: the formation of two-dimensional (2D) van der Waals materials at three-dimensional (3D) absorber interfaces and grain boundaries. In CdTe, the 2D compound is CdCl2; in CIGS, it is XInSe2 (X= K, Rb, Cs) with X depending on the heavy-alkali post-deposition treatment used; and in lead halide PSCs, PbI2 forms naturally, but many new, more stable, 2D perovskites have also been incorporated. Generally, these 2D interfacial materials are present not by design, but instead have evolved from their 3D counterparts during standard device processing. Here, new data, together with evidence compiled from the literature, are presented to illustrate both the existence of 3D/2D interfaces in CdTe, CIGS, and PSCs, and their correlation with improved passivation and device performance. This suggests that 3D/2D passivation may be a heretofore unappreciated key to successful polycrystalline thin-film PV. Finally, the desired attributes of successful low-dimensional layers are presented with rational design strategies for next generation polycrystalline solar cells.

Authors : Mary Blankenship(1), Dirk Hauschild(1)(2)(3), Elizaveta Pyatenko(2)(3), Wolfram Witte(4), Dimitrios Hariskos(4), Stefan Paetel(4), 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 Maryland Parkway, Las Vegas, NV 89154-4003, 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

Resume : The introduction of alkali post-deposition treatments (PDTs) for chalcopyrite-based thin-film solar cells has boosted conversion efficiencies to above 23% on a laboratory scale. PDTs can increase the open-circuit voltage of a device, similar to increasing the Ga/(Ga+In) (GGI) ratio above the customary value of ~0.3 to create a larger absorber band gap. Thus, combining a PDT with a high absorber GGI ratio is a viable pathway, but a comprehensive understanding of the electronic and chemical properties of absorbers and their buffer interfaces is crucial to further optimize such structures. In this work, we have investigated in-line deposited Cu(In,Ga)Se2 (CIGSe) absorbers with a bulk GGI ratio of ~0.9, with and without RbF-PDT, and have studied their interfaces with a CdS buffer layer prepared by chemical bath deposition (CBD). We discuss results from a number of different electron and soft x-ray spectroscopies, including laboratory-based x-ray and UV photoelectron spectroscopy (XPS and UPS), inverse photoemission (IPES), and x-ray-excited Auger electron spectroscopy (XAES). From this combination, a complete picture of the electronic and chemical structure at the absorber surface and the buffer/absorber interface for high GGI absorbers with and without RbF-PDT can be assembled, including band alignment and intermixing behavior. These results will be discussed in view of their impact on the performance of corresponding CIGSe solar cells.

15:30 Discussion    
15:45 Break    
Kesterite and alternative materials : Jose Prieto Marquez/Victor Izquierdo
Authors : Vanira Trifiletti1,2, Giorgio Tseberlidis2, Marco Colombo2, Alberto Spinardi2, Mati Danilson3, Maarja Grossberg3, Sally Luong1, Oliver Fenwick1, Simona Binetti2
Affiliations : 1 School of Engineering and Materials Science, Queen Mary University of London, 327 Mile End Road, London E1 4NS, UK; 2 Department of Materials Science and Solar Energy Research Center (MIB-SOLAR), University of Milano-Bicocca, Via Cozzi 55, I-20125 Milano, Italy; 3 Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate Tee 5, 19086 Tallinn, Estonia.

Resume : Photovoltaics is a promising technology to produce sustainable energy, thanks to the high potential provided by the amount of energy transmitted by the sun. A way for having solar cells with low production costs is to apply the thin-film technology and to employ earth-abundant raw materials. A keen interest is arising in kesterite compounds, which is one of the chalcogenides that use abundant and non-toxic materials. They have already achieved excellent performance at the laboratory level. Here, we report about the synthesis and characterisation of mixed chalcogenides based on Copper, Zinc, Iron, and Tin. Solutions have been studied with different Zinc and Iron ratio. The distortion of the elementary cell of kesterite increases with the iron addition until a phase transition to stannite occurs. The process of synthesis and deposition herein proposed is cheap and straightforward, based on the sol-gel technique. The produced thin-films proved to be particularly attractive for use in cheap and easy processable solar cells. The synthesised layers have been characterised by x-ray diffraction, Uv-Vis, Raman, x-ray photoelectron spectroscopy measurements.

Authors : G. Gurieva1, A. Franz1, S. Levcenko1, K. Muska2, K. Ernits2, S. Schorr1,3
Affiliations : 1 Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany 2 crystalsol OÜ, Akadeemia tee 15a, 12618 Tallin, Estonia 3 Freie Universität Berlin, Institute of Geological Sciences, Malteserstr. 74-100, 12249 Berlin, Germany

Resume : Kesterite-type based thin film solar cell technologies are mainly based on polycrystalline absorber layers. A promising low cost alternative technology uses Cu2ZnSn(S,Se)4 (CZTSSe) monograins (single crystals of 50-100 μm size) which are fixed in a polymer matrix to form a flexible solar cell [1]. It is agreed in literature that large band tailing observed in Cu-based kesterite-type semiconductors causes voltage losses limiting the efficiency of kesterite-based devices. The Cu/Zn disorder (CuZn and ZnCu anti-sites in Cu-Zn planes at z=¼ and ¾), which is always present in these compounds [2], is discussed as a possible reason for band tailing. The experimental determination of the order parameter Q which is a quantitative measure of the degree of Cu/Zn disorder [3] requires a differentiation between the isoelectronic cations Cu and Zn2 . An in depth analysis of neutron diffraction data provides information on the cation distribution in the crystal structure allowing the determination of type and concentration of intrinsic point defects including a distinction between Cu and Zn [2]. On the other hand neutron diffraction requires large sample volumes, thus kesterite monograins offer the unique possibility to correlate structural disorder in kesterite-type absorbers with device performance parameters. We will present a detailed structural investigation of the effect of the disordering procedure and the long low temperature – “ordering” annealing on the Cu/Zn disorder and optical properties of the CZTSSe monograins will be presented. [1] [2] Gurieva et al., J. Appl. Phys. 123 (2018) 161519 [3] Toebbens et al. Phys. Stat. Sol. B 253 (2016) 1890

Authors : Konrad Ritter(1)*, Galina Gurieva (2), Maurizio Ritzer(3), Cora Preiß(3), Charles Hages(4), Rakesh Agrawal(5), Susan Schorr(2,6), Claudia S. Schnohr(1)
Affiliations : (1)Felix-Bloch-Institut für Festkörperphysik, Universität Leipzig, Linnéstraße 5, 04103 Leipzig, Germany; (2)Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany (3)Institut für Festkörperphysik, Friedrich-Schiller-Universität, Max-Wien-Platz 1, 07743 Jena, Germany; (4)Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA; (5)Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA; (6)Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany

Resume : Alloying of kesterite type materials allows material properties like the band gap energy to be tuned directly by the ratio of certain elements. Furthermore, Ge shows great potential in reducing problems associated with the Sn in Cu2ZnSn(S,Se)4 and Ag is discussed as a partial replacement of Cu in the context of Cu-Zn disorder. As shown for many semiconductor materials, alloying changes the local atomic scale structure and thereby directly influences the band gap energy [1]. To probe the local atomic structure, low temperature Extended X-ray Fine Structure Spectroscopy (EXAFS) measurements were performed and compared to lattice parameters from grazing incidence X-ray diffraction (GIXRD) measurements. Analysis of this kind of data has successfully allowed to connect experimental bond lengths and anion positions with band gap energy for Cu2Zn(Ge,Sn)Se4 powder samples [2] and is now applied to technologically relevant (Ag,Cu)2ZnSn(Se,S)4 and Cu2Zn(Ge,Sn)(Se,S)4 thin films. Ab initio theoretical calculations yield the band gap change with the experimentally acquired structural parameters. This allows an estimation of the part of the band gap bowing, that results from a change of the structural parameters. [1] C. S. Schnohr, Appl. Phys. Rev. 2, 031304 (2015). [2] K. Ritter et al., JPhys. Energy 2, 035004 (2020).

Authors : J. El Hamdaoui(a), M.Farkous(a), E. Feddi(a), L.M. Pérez(b), D. Laroze(b),
Affiliations : a) Group of Optoelectronic of Semiconductors and Nanomaterials, ENSET de Rabat, Mohammed V University in Rabat, Morocco. b) Instituto de Alta Investigación, CEDENNA, Universidad de Tarapacá, Casilla 7 D, Arica, Chile.

Resume : In this work, we investigate the structural, electronic and optical properties of the new chalcogenide Cu2NiGeS. Our calculations are done using the first principle approach. We have employed the full-potential linearized augmented plan-wave (FP-LAPW) method implemented in wien2k code. To treat the exchange and correlation effect our numerical calculations we use the Tran-Blaha modified Becke-Johnson potential combined with the Hubbard potential U (mBJ+U). Our numerical calculations show that CNGS is a promising material for photovoltaic application with high absorption and direct bandgap. We also determine the real and imaginary parts of the dielectric function, the reflectivity, and the refraction index.

Authors : Neringa Petrasauskiene, Rasa Alaburdait?, Edita Paluckien?
Affiliations : Department of Physical and Inorganic Chemistry, Kaunas University of Technology, Lithuania

Resume : Copper sulfide (CuxS) is an important p-type metal chalcogenide semiconductor due to its potential uses in many photovoltaic and photothermal applications such as selective solar control coatings, transparent and conductive coatings, and thin film Cu sensor electrodes. Consequently, polymers modified by CuxS are used: as the conductive substrates for deposition of metal and semiconductors; as gas sensors; as polarizer of infrared radiation; and as active absorbents of radio waves. Polyamide (PA) as semi-hydrophilic, flexible polymer is capable to absorb ions or molecules of various electrolytes from aqueous and no aqueous solutions. CuxS thin films were deposited on PA (PA6, Tecamid 6, density 1.13 g?cm?3, 500 ?m thick) using CuCl2 and Na2S2O3 mixture for 16 h. By repeating such deposition cycles for 3 times, we have obtained CuxS thin films in the surface of PA. The main aim of the present work was to evaluate how composition, structure, and morphology of CuxS thin films, formed on the surface of PA by the chemical bath deposition method, depends on the conditions, which are applied for the formation of thin films. The results of X-ray structural analysis confirmed the formation of copper sulfide (CuS - covellite) in the surface of PA. The room temperature electrical resistivity was about 100-200 ?/?. The optical bandgaps for the CuS with different morphologies were measured to be in the range of 1.9-2.5 eV. The obtained layers are promising for photovoltaic applications. The films were found to have average transmittance of about 40 % in the UV-VIS regions while exhibiting average reflectance of about 20 % in the same regions. These characteristics are relevant in solar radiation control applications.

Authors : Leo Choubrac(1), José Márquez Prieto(1), Hannes Hempel(1), Justus Just(2), Taiseer Jokadar(1), Thomas Unold(1)
Affiliations : (1) Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH (2) MAX IV Laboratory, Lund University, P.O. Box 118, SE‐221 00 Lund, Sweden

Resume : Kesterite solar cell conversion efficiency stagnates for a few years due to a low VOC compared to the Schockley-Queisser limit. The losses can be classified into 3 groups: Non-radiative (“NR”) – related to short minority carriers lifetime, Radiative losses (“Rad”-affected by the density of tail states) and device losses (“DL” – imperfect resistances, interface losses…). NR and Rad losses are the most critical as those arises from absorbers intrinsic properties, and a bright future for kesterite will not come without a significant reduction in (NIA+rad) losses. To overcome this, numerous strategies, such as doping, alloying, ordering, Sn supply during selenisation have been developed, with limited success. Implementing a new processing strategy has several impacts at the same time, hence requiring additional work of composition and device re-optimization to demonstrate or rebut their beneficial effect. We produced kesterite layers with lateral compositional gradients and used them to explore the effect of various synthesis parameters. Then, high throughput characterisation (XRF, Raman, absolute photoluminescence and UV-vis hyperspectral imaging) were performed to extract the NR and radiative VOC losses for a very broad range of composition. As a result, we can conclude on the effects of those parameters on kesterite intrinsic properties with high statistics, for a broad range of composition. Moreover, it minimizes the risk that an improvement of intrinsic properties is masked by a third party phenomenon (drift of optimum composition, reproducibility issue, device-related losses) that could be resolved elsewhere. In particular, we discuss the effect of selenisation conditions and the role of Na and structural ordering on the different losses for a large composition range within the ternary phase diagram.

Authors : Ha Kyung Park (1), Yunae Cho (1), Juran Kim (1), Gee Yeong Kim (2), Woo-Lim Jeong (3), Kyung-Pil Kim (3), Dong-Seon Lee (3), William Jo (1)*
Affiliations : 1 Department of Physics and New and Renewable Energy Research Center (NREC); Ewha Womans University, 2 Advanced Photovoltaics Research Center; National Agenda Research Division; Korea Institute of Science and Technology (KIST), 3 School of Electrical Engineering and Computer Science; Gwangju Institute of Science and Technology (GIST)

Resume : Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) materials have been widely studied to fabricate highly efficient thin film solar cells. One of the issues to enhance the kesterite cell performance is defect passivation by incorporation of Na in CZTSSe. In this study, four CZTSSe thin film with various Na content were prepared to examine the optoelectronic properties. Na content was proved by X-ray photoelectron spectroscopy (XPS) and remaining Na atoms on CZTSSe surface was obtained. Na content affect to cell parameter strongly and open-circuit voltage deficit which is proportional to the concentration of recombination center decrease in Na-sufficient samples. Photogenerated electrical properties was investigated through photo-assisted Kelvin probe force microscopy (KPFM) using laser assistant (405, 532, and 640 nm wavelength). Na-sufficient sample showed the larger potential bending under illuminated condition compared to dark state while Na-deficient sample showed less significant difference. Therefore, photogenerated carrier separation under illumination is enhanced in Na-sufficient sample. In Na-deficient sample, the formation of SPV was limited due to recombination of photogenerated carriers at defect sites. In contrast, noticeable positive SPV with maximum 101 mV was measured in Na-sufficient sample. The maximum SPV was formed under 405 nm wavelength illumination due to the defect compensation of blue photon suppressing the recombination. In conclusion, we demonstrated defect passivation effect by sufficient Na content preventing the recombination of photogenerated carrier through a change of surface voltage.

Authors : Daniel Fritsch (1), Susan Schorr (1,2)
Affiliations : (1) Department Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany; (2) Department of Geosciences, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany

Resume : Cu2ZnSnSe4 and Ag2ZnSnSe4 are both crystallising in the kesterite structure with the structurally similar stannite structure being energetically slightly less favourable. In the solid solution, however, there is experimental evidence that for some intermediate concentrations the stannite structure is energetically favoured. The reason for that structural change is so far not completely understood. Here, we're using first-principles calculations based on density functional theory to shed some light into the structure-property relations in the (Cu,Ag)2ZnSnSe4 solid solution. In order to simulate the different concentrations within the solid solution, we're employing the supercell approach based on the respective end members in the kesterite and stannite structure [1]. The Ag and Cu cations are distributed randomly within the supercell, thereby creating several structure models for the solid solutions for further analysis. All random structure models are geometry optimised employing different exchange and correlation functionals, namely the PBEsol and the recently developed SCAN functionals. In order to obtain more reliable electronic and optical properties, selected optimised structures are subjected to one-shot calculations employing the more accurate hybrid functional HSE06. This work made use of computational resources provided by the North-German Supercomputing Alliance (HLRN), and the Curta and Dirac HPC facilities of the FU Berlin and the Helmholtz-Zentrum Berlin, respectively. [1] D. Fritsch and S. Schorr, J. Phys. Energy 3, 015002 (2021).

Authors : R.A. Redko a,b, M.O. Semenenko a, O.V. Hladkovska с, S.M. Redko a
Affiliations : a V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 03680, Kyiv, Ukraine; b State University of Telecommunications, 7, Solomenska str., 03680 Kyiv, Ukraine; c Plasma Physics and Plasma Technology Department at Institute for Nuclear Research of NAS of Ukraine, 47 pr. Nauki, Kyiv, 03028, Ukraine

Resume : Kesterite Cu2ZnSnS4 is an attractive earth-abundant material for fabrication of thin film solar cells. Thin films of this semiconductor were deposited using direct current magnetron sputtering and sulfurized at argon atmosphere pressures of 950, 460 and 50 mbar (sample A, B and C, respectively) are studied in a view of solar cell absorber fabrication. Steady state photoluminescence (PL) spectra of studied thin films were measured at 500 nm excitation at room temperature. It was observed broad bands with peaks position ~1.38 eV, which are close to observed ones in other works. PL spectra could be quality criteria to separate best samples for solar cells. To improve luminescence properties H-plasma treatment (І=2 А, P=150 W, f=13,56 MHz, p=6•10-2 Torr, Н≈1,2•104 А/м, Uback=- 60 В, t=10 min) was used. For samples A and C we have decrease of the intensities (~50 and ~30%, respectively) while for sample C we have an increase one (~45 %). These features could testifies about broken bounds passivation due to H-plasma treatment for samples C, that results in decrease of non-radiative surface centers and increase in the PL intensity. While in samples A and C we have the vice versa – H-plasma treatment results in the addition of surface non-radiative centers and decrease PL intensity. Such different experimental results, obliviously, related to the different defect states of the surfaces of the studied samples. But different surfaces are appeared due to different pressure at growing process. Possible mechanism of observed features are discussed.

Authors : Jae Yu Cho, Jaeyeong Heo
Affiliations : Chonnam National University

Resume : Orthorhombic tin sulfide (SnS) has recently been emerged as a very promising absorber material for thin-film solar cells (TFSCs). It has an ideal optical band gap (~1.3 eV) and it comprises of relatively earth abundant constituents and non-toxicity. But till date, the highest efficiency obtained from the SnS-based solar cells is 4.36%, which is fairly low compared to its theoretical limit of ~32%. SnS is a non-cubic material unlike CIGS or CdTe, crystallizing in an orthorhombic structure (JCPDS No. 39-0354, a = 4.3291 Å, b = 11.1923 Å, c = 3.9838 Å). It easily leads to the formation of layered features. Therefore, controlling the morphology of the SnS absorber with dense and pinhole-free grains is crucial. In this study, the influence of vapor transport deposition (VTD) conditions of tin sulfide (SnS), i.e., growth temperature and duration, on the formation of secondary phases, preferred orientation, and solar cell performance, was investigated. In the growth temperature effect experiment, the film grew as a plate form with increasing temperature and the secondary phase was found at low temperature. Also, it was confirmed that the film thickness was increased linearly with duration. When the growth duration increases to 10 min, a dramatic improvement in the device performance is noted. Finally, fabricated SnS/CdS solar cell achieved 3.93% efficiency (VOC = 0.342 V, JSC = 19.8 mA cm-2, FF = 58.0%) at the growth temperature of 600 oC and the duration of 10 min.

Authors : Neringa Petrasauskiene, Rasa Alaburdaite, Edita Paluckiene
Affiliations : Department of Physical and Inorganic Chemistry, Kaunas University of Technology, Lithuania

Resume : Copper sulfide (CuxS) is an important p-type metal chalcogenide semiconductor due to its potential uses in many photovoltaic and photothermal applications such as selective solar control coatings, transparent and conductive coatings, and thin film Cu sensor electrodes. Consequently, polymers modified by CuxS are used: as the conductive substrates for deposition of metal and semiconductors; as gas sensors; as polarizer of infrared radiation; and as active absorbents of radio waves. Polyamide (PA) as semi-hydrophilic, flexible polymer is capable to absorb ions or molecules of various electrolytes from aqueous and no aqueous solutions. CuxS thin films were deposited on PA (PA6, Tecamid 6, density 1.13 g∙cm–3, 500 μm thick) using CuCl2 and Na2S2O3 mixture for 16 h. By repeating such deposition cycles for 3 times, we have obtained CuxS thin films in the surface of PA. The main aim of the present work was to evaluate how composition, structure, and morphology of CuxS thin films, formed on the surface of PA by the chemical bath deposition method, depends on the conditions, which are applied for the formation of thin films. The results of X-ray structural analysis confirmed the formation of copper sulfide (CuS - covellite) in the surface of PA. The room temperature electrical resistivity was about 100-200 Ω/􀀀. The optical bandgaps for the CuS with different morphologies were measured to be in the range of 1.9-2.5 eV. The obtained layers are promising for photovoltaic applications. The films were found to have average transmittance of about 40 % in the UV-VIS regions while exhibiting average reflectance of about 20 % in the same regions. These characteristics are relevant in solar radiation control applications.

Authors : X. Li*, K. Timmo, M. Pilvet, M. Grossberg, V. Mikli, M. Danilson, M. Kauk-Kuusik
Affiliations : Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : The copper-chalcogenide materials reveal useful properties and have been proposed as potential candidates for absorber materials in photovoltaic cells. In the last decade, although copper-chalcogenide photovoltaic materials, Cu2ZnSn(S1-xSex)4 (CZTSSe), is explored as the promising alternate absorber material to Cu(InxGa1-x)Se2 (CIGS), the solar cells based on the CZTSSe absorber material have shown a slight increase in power conversion efficiency, from about 10% up to 13% [1-2]. Thus, the search for alternative materials with promising properties in the chalcogenide systems is needed. In our previous study [3], Cu2CdGe(SxSe1-x)4 (CCGSSe) compounds has been synthesized, and the solar cell record efficiency 6.4% based on the CCGSSe with 80 mole% selenium to 20 mole% sulfur has been achieved. In this work, as a Cd-free and high quality ternary Cu2Ge(SxSe1-x)3 (CGSSe) monograin powders were synthesized in closed quartz ampoules using elemental Cu, Ge, Se and S as precursor materials and LiI as flux material. The Cu2GeSe3 compound is found to crystallize only in orthorhombic structure with bandgap ~0.78-0.81 eV [4]. The pure Cu2GeS3 crystals exist in different crystal structure, such as tetragonal, orthorhombic, cubic and monoclinic structure. The crystal structure of ternary Cu2GeS3 transfer from the cubic (~ 1.3eV) to the monoclinic (~ 1.58 eV) crystalline structure is observed with the increase of the temperature to 500oC [5]. Theoretical calculations suggest that the optimal bandgap for absorber materials is around 1.4 eV [6]. To obtain the ideal bandgap for CGSSe-based single junction solar cells, the effect of S/(S+Se) precursor composition ratio on the properties of Cu2Ge(SxSe1-x)3 has been investigated. The structural properties of Cu2Ge(SxSe1-x)3 (x= 0-1) monograin powders were investigated by X-ray powder diffraction and Raman, morphology and composition by high-resolution scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy, and optical properties by Photoluminescence spectroscopy. Regardless of the composition of the CGSSe crystals, all materials showed p-type conductivity. The band gap values of Cu2Ge(SxSe1-x)3 powder crystals determined from external quantum efficiency measurements were between 0.8-1.58 eV depending on the S/Se ratio. All powders were used as absorber materials in single junction solar cells and results will be presented in detail in the conference. References (1) W. Wang, et al. Adv. Energy Mater. 2014, 4 (7), 1301465. (2) Todorov T K, et al. Advanced materials, 2010, 22(20): E156-E159. (3) X. Li, et al. Solar Energy 209 (2020): 646-652. (4) Marcano, G., et al. Solid state communications 146.1-2 (2008): 65-68. (5) Robert E V C, et al. International Society for Optics and Photonics, 2016, 9936: 993607. (6) M. Pilvet, et al. Thin Solid Films, 582, 180-183.

Authors : Maayan Perez, Michael Shandalov, Yuval Golan, Tzvi Templeman, Vladimir Ezersky, and Eyal Yahel
Affiliations : Department of Materials Engineering, Ben-Gurion University of the Negev, Beer Sheva 8410500, Israel. Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer Sheva 8410500, Israel Department of Physics, Nuclear Research Center Negev, P.O. Box 9001, Beer Sheva, Israel

Resume : Chemical solution deposition (CD) is a widely studied method for the deposition of high quality thin films, in particular lead chaclcogenides.[1–5] While earlier reports described chemical deposition from acidic baths, the vast majority of the studies were carried out in alkaline baths.[3,6] In some cases, chemically deposited films may contain a significant amount of metal hydroxides and other oxygen-containing compounds due to precipitation of oxides/ hydroxides in the alkaline aqueous environment, these impurities are likely to affect the electrical properties of the films.[7–10] Switching from an alkaline bath to an acidic bath can reduce these impurities in the film while maintaining the simplicity and cost effectiveness of CD. The mechanisms of the precipitation of lead sulfide from solutions containing thioacetamide were previously studied.[3,11,12] It was found that at pH=3 there is a minimum point vertex, due to the change in deposition mechanism, where the rate of the deposition is the slowest.[13,14] In this work we have obtained, for the first time, high-quality epitaxial monocrystalline PbS thin films on GaAs obtained from an acidic bath. The effect of pH on the quality of the PbS film was studied, and a sharp transition in the active deposition mechanism was obtained at pH=3. The optical properties of the films were studied and characterized using the Urbach theory on the tail of the absorption edge. The results showed a transition point at pH=3, signifying that samples grown at the transition point between deposition mechanisms include less defects and show better film quality.[15] This result is in agreement with the rocking curve results which showed that the sample grown at pH=3 is less defective, which is reasonable due to the slower reaction rate, giving rise to better controlled deposition at this pH. Deposition from acidic bath proved to be highly beneficial for doping semiconducting thin films without introducing high levels of impurities. Additionally, the acidic bath developed here paves the road to depositing Th alloyed PbS thin films with very low oxygen content, which are required for radiation damage studies of non-radioactive thin films. 1 T. Templeman, M. Biton, T. Safrani, M. Shandalov, E. Yahel and Y. Golan, CrystEngComm, 2014, 16, 10553–10559. 2 S. Sengupta, M. Perez, A. Rabkin and Y. Golan, CrystEngComm, 2016, 18, 149–156. 3 G. Hodes, Chemical Solution Deposition of Semiconductor Films, Marcel Dekker, Inc., New York-Basel, Marcel Dek., 2002. 4 M. Perez, T. Templeman, M. Shandalov, V. Ezersky, E. Yahel and Y. Golan, CrystEngComm, 2019, 21, 1818–1825. 5 M. Shandalov and Y.Golan, Eur. Phys. J. Appl. Phys., 2004, 28, 265–291. 6 C. D. L. R.S. Mane, Mater. Chem. Phys., 65, 1–31. 7 T. Templeman, M. Shandalov, E. Yahel, V. Ezersky, G. Sarusi and Y. Golan, RSC Adv., 2016, 6, 88077–88084. 8 T. Templeman, M. Shandalov, M. Schmidt, A. Tal, G. Sarusi, E. Yahel, I. Kelson and Y. Golan, Sci. Rep., 2017, 7, 2780. 9 I. Pop, C. Nascu, V. Popescu, E. Indrea and I. Bratu, Thin Solid Films, 1997, 307, 240–244. 10 M. Weber, J. Krauser, A. Weidinger, J. Bruns, C. ‐H. Fischer, W. Bohne, J. Rohrich and R. Scheer, J. Electrochem. Soc., 1999, 146, 2131–2138. 11 C. D. Lokhande, Mater. Chem. Phys., 1991, 27, 1. 12 E. H. Swift and E. A. Butler, Anal. Chem., 1956, 28, 146–153. 13 Oswald M. Peeters and Carniel J. de Ranter, J. Chem. Soc. Perkin Trans. 2, 1974, 4, 1832–1835. 14 E. A. Butler, D. G. Peters and E. H. Swift, Anal. Chem., 1958, 30, 1379–1383. 15 A. Gordijn, L. Hodakova, J. K. Rath and R. E. I. Schropp, J. Non. Cryst. Solids, 2006, 352, 1868–1871.

Authors : Fairouz Ghisani*, Kristi Timmo, Mare Altosaar, Valdek Mikli, Maarja Grossberg and Marit Kauk-Kuusik
Affiliations : Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia.

Resume : Cu10(Zn,Cd)2Sb4(S,Se)13 compounds have optical band gaps, high absorption coefficients and p-type electrical conductivity suitable for use in energy related applications [1]. These properties have encouraged significant scientific effort to synthesize tetrahedrite based semiconducting compounds. In this work, Cd-substituted tetrahedrite Cu10Cd2Sb4S13 (TH-Cd) monograin powders (MGPs) were synthesized from Cu2S, CdS and Sb2S3 in two different molten salt (CdI2 and LiI) environments. The syntheses were performed in sealed vacuum quartz ampoules by heating at 495 oC for two weeks. The influence of the nature of the molten salt used as flux material on the elemental and phase composition, powder particle size distribution, morphology of crystals as well as the rate of agglomeration was studied. X-ray diffraction of the materials indicated mainly single phase of tetrahedrite Cu10Cd2Sb4S13 compound. XRD patterns of TH-Cd crystals grown in LiI revealed a considerable shift of the main reflexions, lower CdS content and a smaller lattice parameter in comparison with those formed in CdI2 (LiI: a=b=c=10.509 Å, CdI2: a=b=c= 10.512 Å). From these findings it was concluded that Li from the molten flux incorporated into the crystal structure of TH-Cd partly replace Cu. Raman analysis of both materials gained from CdI2 and LiI salts showed characteristic modes of the Cd substituted tetrahedrite at 95, 111, 136, 171, 247, 295, 335, 355 and 363 cm−1 [2]. Energy dispersive X-ray spectroscopy revealed a composition close to the stoichiometry of Cu10Cd2Sb4S13 of tetrahedrite grown in CdI2 and Cu-poor grown in LiI. Scanning electron microscope images showed that the morphology of TH-Cd crystals can be controlled by the nature of the used flux materials: flat surfaces and sharp edges with higher rate of agglomeration in CdI2, and well-formed individual crystals with rounded edges and smooth facets in LiI had been formed. The photoluminescence study of Cu10Cd2Sb4S13 microcrystals at T = 10 K showed a single broad, asymmetric PL bands with the maxima detected at around 1.08 and 1.16 eV, respectively for materials grown in CdI2 and LiI. The formation of TH-Cd materials and/or the interactions of used precursors with flux materials in different flux salts were studied by differential thermal analysis method. References [1] D.S.P. Kumar, M. Ren, T. Osipowicz, R.C. Mallik, P. Malar, 2018, Solar Energy, 174, 422430 [2] F. Ghisani, K. Timmo, M. Altosaar, J. Raudoja, V. Mikli, M. Pilvet, M. Kauk-kuusik, M. Grossberg, 2020, Mater. Sci. Semicond. Process., 110, 104973

Authors : Rosa Estefanía Almache, Benjamin Andres Pusay, Kunal Tiwari, Eloi Ros, Gerard Masmitja, Cristóbal Voz, Joaquim Puidollers, Edgardo Saucedo, Pablo Ortega
Affiliations : Universitat Politecnica de Catalunya (UPC) - Departament d'Enginyeria Electronica; Universitat Politecnica de Catalunya (UPC) - Departament d'Enginyeria Electronica; Catalonia Institute for Energy Research (IREC); Universitat Politecnica de Catalunya (UPC) - Departament d'Enginyeria Electronica; Universitat Politecnica de Catalunya (UPC) - Departament d'Enginyeria Electronica; Universitat Politecnica de Catalunya (UPC) - Departament d'Enginyeria Electronica; Universitat Politecnica de Catalunya (UPC) - Departament d'Enginyeria Electronica; Universitat Politecnica de Catalunya (UPC) - Departament d'Enginyeria Electronica; Universitat Politecnica de Catalunya (UPC) - Departament d'Enginyeria Electronica.

Resume : Kesterite (CZTSSe) is an attractive thin-film material for photovoltaic applications due to its excellent optical properties and the use of abundant elements. CZTSe solar cells routinely include opaque Mo as Hole Transport Layer on the back surface. However, advanced applications as bifacial and Tandem solar cells require transparent or semitransparent top solar cells. In this way, it can be interesting to replace the Mo-based contacts by means of selective collection electrodes based on transition metal oxides (TMOs). Dopant-free materials based on TMOs such as MoOx and V2Ox, or alternatively TiO2 films, are good candidates to form hole or electron transport layers respectively in silicon PV technology. With this idea in mind, we have applied V2Ox deposited by Atomic Layer Deposition (ALD) as hole transport layer (HTL). In all cases, the HTL scheme was deposited onto FTO/glass substrates resulting in a fully transparent rear contact. Specific contact resistivity was extracted using Transfer Length Method (TLM) measurements, reaching remarkable values below 10 mΩcm2. Fabricated solar cells incorporating the aforementioned transparent rear contact exhibit efficiencies up to 3.4%, achieving open circuit voltages of 325 mV with reasonably short-circuit current densities of 25.4 mA/cm2 even in this case of cell architecture without a back reflector scheme.

Authors : Kunal J. Tiwari1, Robert Fonoll Rubio1, Sergio Giraldo1, Lorenzo Calvo-Barrio2,3, Victor Izquierdo-Roca1, Marcel Placidi4, Yudania Sanchez1, Alejandro Pérez-Rodríguez1,3, Edgardo Saucedo4, and Zacharie Jehl Li-Kao4
Affiliations : 1. Institut de Recerca en Energia de Catalunya (IREC), 1,2apl. Jardins de les Dones de Negre, 08930 Sant Adria de Besos, Barcelona, Spain. 2. Centres Científics i Tecnològics (CCiTUB), Universitat de Barcelona, C/ Lluis Solé i Sabaris 1-3, 08028 Barcelona, Spain. 3. IN2UB, Departament d′Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, C/ Martí i Franquès, 1, 08028 Barcelona, Spain. 4. Electronic Engineering Department, Polytechnic University of Catalonia (UPC), c/ Jordi Girona 1, 08034 Barcelona, Spain.

Resume : This work presents a method based on chemical etching for a depth analysis of Kesterite absorbers and the profiling of the main limitations of this material. The method proposed here is to some extent material agnostic and could possibly be applied for the evaluation of various other emerging photovoltaic absorbers. The prevalence of deep defects and secondary phases has been ascribed to the limited efficiency of Kesterite solar cells, and specifically to their problematic voltage deficit. Additionally, a poor material quality at the vicinity of the back interface is often reported, further impairing the performance of Kesterite-based photovoltaic devices. Investigating these limitations is not straightforward, especially regarding the defect distribution profile which can usually be indirectly inferred from Admittance Spectroscopy and Deep-Level Transient Spectroscopy. Designing a method for a direct observation of those defects, and their profiling throughout the absorber would be valuable in that context. The chemical etching of PV absorbers by Bromine-based solutions is not new, and has been successively investigated both for CIGSe and to a more limited extent for Kesterite. In this work, we propose to build on this knowledge to design an experimental process combining a Bromine-Methanol chemical etching of a standard Kesterite absorber with the surface analysis of the resulting well-defined Kesterite slabs. The observation of successively etched samples by 3D Optical Profilometry reveals not only the presence of large voids toward the back interface of the films, but allows for the first time to quantify the surface area of the absorber disconnected from the Mo back contact. The analysis of the differently etched sample by X-ray Photoelectron Spectroscopy yields an accurate composition profile of the entire absorber, showing finer grading features with a higher resolution than obtained using a physical-based etching method (sputtering). The main result of this work is obtained from the Raman analysis of the etched samples, which allows to accurately reconstruct the depth defect profile, along with the secondary phase depth profile. An increasing prevalence of the ZnSn point defect is observed, while the VCu point defect has a more homogeneous distribution throughout the film’s thickness. The SnSe2 secondary phase is also found prevalent toward the back interface, which aligns well with the poor film quality often observed in that region. This characterization is to the best of our knowledge the first direct observation of critical limitation of standard Kesterite absorbers. To conclude this study, an application of bromine etching to wide bandgap germanium-based kesterite absorbers in complete solar cell is presented. Short Br2 etchings (less than 30nm) are found to markedly improve the PV performance of the devices, which will be discussed in the context of XPS and admittance spectroscopy analysis. Reference: K. J. Tiwari et al., Applied Surface Science, vol. 540, p. 148342, Feb. 2021, doi: 10.1016/j.apsusc.2020.148342.

Authors : H. HammamI M. Ben Rabeh M. Kanzari
Affiliations : University of Tunis El Manar, National Engineering School of Tunis, Photovoltaic and Semiconductor Materials Laboratory, 1002, Tunis, Tunisia University of Tunis, Preparatory Institute for Engineering Studies of Tunis, Photovoltaic and Semiconductor Materials Laboratory, 1002, Tunis, Tunisia

Resume : In the current study, Cu2CoSnS4 (CCTS) thin films were grown by evaporation of powdered CCTS on various glass substrate temperature. As-grown thin films were annealed in sulfur atmosphere at 400°C for 2 hours. We have systematically studied the structural and optical properties of post-sulfurized CCTS thin films by means of X-ray diffraction, Raman Scattering and UV-vis spectroscopy. CCTS films exhibit polycrystalline structure with stannite phase and (112) preferred orientation. Profound analysis has been thoroughly made to investigate the variation of the optical and dielectric properties and other related parameters as a function of substrate temperature. Optical constants of post-sulfurized CCTS thin films such as absorption coefficient, Urbach energy, extinction coefficient and refractive index were investigated by optical measurement from transmittance and reflectance spectra in the UV-VIS-NIR range. We have discussed the dispersion of the refractive index in terms of the Wemple–DiDomenico single oscillator model from which optical dispersion parameters such as the single oscillator energy (Eo) and dispersion energy (Ed) were calculated in the investigated wavelength range. We have calculated the optical conductivity and the dissipation factor of the studied films. The electric free carrier susceptibility and the carrier concentration on the effective mass ratio were determined by using the model of Spitzer and Fan.

Authors : H. Oueslati1, M. Marzougui1, H. Hammami1, M. Ben Rabeh*,1 and M. Kanzari2
Affiliations : 1University of Tunis El Manar, National Engineering School of Tunis, Photovoltaic and Semiconductor Materials Laboratory, 1002, Tunis, Tunisia 2University of Tunis, Preparatory Institute for Engineering Studies of Tunis, Photovoltaic and Semiconductor Materials Laboratory, 1002, Tunis, Tunisia

Resume : We have successfully produced Cu2CoSnS4 (CCTS), Cu2NiSnS4 (CNTS), Cu2ZnSnS4 (CZTS) and Cu2FeSnS4 (CFTS) materials by direct melting. Constituent elements taken in stoichiometric compositions were used as starting points. After that, Cu2XSnS4 (M=Co, Ni, Zn and Fe) thin films were growth using a single source vacuum thermal evaporation method. The resulting Cu2XSnS4 films were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis and optical spectrophotometry to investigate the crystal structure, composition, morphology, absorption coefficient and optical band gap. Hot probe method shows that Cu2XSnS4 (M=Co, Ni, Zn and Fe) ingots and fabricated samples exhibit p-type semiconductor. This work demonstrates that Cu2XSnS4 (M=Co, Ni, Zn and Fe) thin films are promising absorber materials for energy-conversion applications. Keywords: Cu2XSnS4; Thin films, Thermal evaporation; Structural properties; Optical Properties.

Authors : M. Marzougui *, 1, M.Ben Rabeh 1 and M. Kanzari 1,2
Affiliations : 1University of Tunis El Manar, National Engineering School of Tunis, Photovoltaic and Semiconductor Materials Laboratory, 1002, Tunis, Tunisia 2University of Tunis, Preparatory Institute for Engineering Studies of Tunis, Photovoltaic and Semiconductor Materials Laboratory, 1002, Tunis, Tunisia

Resume : Quaternary chalcogenide semiconductors’ family, particularly Cu2ZnSnS4(CZTS) material acquired an enormous research interest thanks to its low-cost, non-toxic and abundant components in the earth’s crust. Furthermore, its direct band gap around 1.5 eV, a high absorption coefficient (105cm-1 ) a reasonable efficiency rate 12.6% and a p-type conductivity make him a suitable candidate as absorber layer for solar cells application. Alkali metal doping represents one of the main ways to achieve high-efficiency CZTS solar cells. Na was incorporated into CZTS bulk crystals compound from elemental Cu, Zn, Sn, S using the direct melting method. Crushed powders for 10 min of these ingots were used as raw materials for the thermal evaporation. Thin films of undoped and Na-doped CZTS were deposited onto glass substrates at 100°C temperature by thermal evaporation technique. After the deposition, all CZTS thin films were annealed in a furnace in sulfur atmosphere at a temperature of 400 °C during 30 min. In this paper, the samples were analyzed for their microstructural, electrical and optical properties before and after sulfurization. Na ions incorporation in CZTS thin films could improve the crystallinity. X-ray analysis indicates that all sulfurized CZTS films present a polycrystalline nature and exhibit a preferential orientation along (112) plane. Cu2ZnSnS4 crystallize in kesterite structure with I-4 ̅ space group. Raman analysis confirm the presence of Cu2ZnSnS4 phase. Transmittance and reflectance spectra revealed that the films are homogenous. The direct band gap energies of sulfurized CZTS and Na-doped CZTS films were in the range 1.64–1.52eV. After sulfurization and doping with sodium were found to decrease the band gap energy and the resistivity of the samples. All films exhibited p type electrical conductivity. Keywords: Cu2ZnSnS4; Na-doped; Thermal vacuum evaporation; Thin films; Absorber layer; Photovoltaic

Authors : Pravin S. Pawar, Jaeyeong Heo
Affiliations : Chonnam National University

Resume : Among the p-type chalcogenides family, orthorhombic tin sulfide (α–SnS) has recently emerged as a promising absorber layer for a photovoltaic device application due to its suitable optical bandgap of ~1.32 eV and high optical absorption coefficient of ˃ 105 cm-1. Despite of having great absorber properties, the highest efficiency obtained from the SnS-based TFSCs is 4.8%, which is fairly low compared to its theoretical limit of ~32%. SnS is a non-cubic material unlike CIGS or CdTe, crystallizing in an orthorhombic structure. It easily leads to the formation of layered features. Therefore, controlling the morphology of the SnS absorber with dense and pinhole-free grains is crucial. In this study, SnS thin films were deposited using a simple and low-cost spin coating method for solar cell applications. The present study demonstrated the deposition of SnS thin films on soda-lime glass (SLG) and SLG/Mo substrates. The developed films were subsequently applied for the fabrication of a thin-film solar cell. The effect of the annealing temperature on the structural, optical, and morphological properties of the deposited SnS films was analyzed. The precursor concentrations and the annealing temperature played a critical role in determining the phase composition and morphological characteristics of the SnS thin films. The results obtained from X-ray diffraction revealed that the films annealed above 300 °C for one hour exhibit a pure SnS phase with the single (040) plane. TFSC with SLG/Mo/SnS/CdS/i-ZnO/AZO/Al configuration was fabricated using the optimal precursor ratio, i.e., Sn:S = 1:1.2, and this device showed a photoconversion efficiency of 0.076%.

Authors : Yvonne Tomm, Susan Schorr
Affiliations : Helmholtz-Zentrum Berlin für Materialien und Energie

Resume : “Adamantine”-type compounds, including kesterites, are currently the most promising material for fully inorganic thin film photovoltaic technology that is free of critical raw‐materials and thus provides sustainable solutions. Adamantines are compounds crystallizing in a structure, in which every atom is tetrahedrally bonded to four nearest neighbours and if they contain vacancies in the crystal structure they are called “defect adamantines” [1]. To find new compounds for absorber layers as well as window materials in thin film solar cells, the I-III-VI2 chalcopyrite compound family is extended by chemical substitution to form quaternary “defect adamantines” such as I-•-III-IV-VI4 compounds. Coming from the chalcopyrite structure (space group I-42d), one I+1 and one III+3 cation are replaced by one IV+4 cation. This substitution will halve the occupation of lattice sites 4a and 4b. Accordingly, the structure is compensated by vacancies. A number of studies on defect adamantine selenides have been published [2,3], but less information is available on sulphides [4]. First we started on I-•-Ga-IV-S4 with A = Ag, Cu and C = Ge, Sn. Besides the question of the crystal structure of these compounds, we mainly investigated the possibilities of preparing single-phase materials. Starting with the ternary compound CuGaS2, the conditions for growing single crystals of I-•-Ga-IV-S4 were studied. Here we report on the growth of single crystals and their structural as well as optoelectronic properties of the quaternary compounds I-•-Ga-IV-S4 with A = Ag, Cu and C = Ge, Sn such as CuGaGeS4, CuGaSnS4, and AgGaSnS4. The single crystals were grown by chemical vapor transport using iodine as transport agent. The evolved material and the crystals grown were characterized by X-ray diffraction (XRD) at 298K and LeBail analysis of the diffraction pattern. For CuGaGeS4 we determined the crystal structure to I-42d. The band gap energy of the material was revealed by solid-state UV/Vis reflectance spectroscopy and tauc-plot analysis. All in all we show first results on growth and characterization of the novel compound semiconductor CuGaGeS4. This indicates a potential for future applications. [References] [1] B.R. Pamplin, Prog. Crystal Growth Charact. vol.3, pp. 179-192 (1981) [2] H. Matsushita, A. Katsui; J. Phys. Chem. Solids 66 (2005) 1933-1936 [3] T. Maeda, H. Matsushita, A. Katsui; Jpn. J. Appl. Phys. 39 (2000) Suppl. 39-1, 41-43 [4] L. Garbato, A. Geddo-Lehmann, F. Ledda; J. Crystal Growth 114 (1991) 299-306

Authors : Veaceslav Sprincean(1), Liviu Leontie(2), Igor Evtodiev(1), Iuliana Caraman(3), Dumitru Untila(4), Mihaela Girtan*(5), Silviu Gurlui(2), Petru Lisnic(2), Mihail Caraman(3)
Affiliations : (1) Applied Physics and Informatics Department, Faculty of Physics and Engineering, Moldova State University, A. Mateevici, 60, MD-2009, Chisinau, Republic of Moldova; (2) Faculty of Physics, Alexandru Ioan Cuza University of Iasi, Bulevardul Carol I, Nr. 11, RO-700506 Iasi, Romania; (3) Laboratory of Scientific Research „Physics of Semiconductor Devices”, Faculty of Physics and Engineering, Moldova State University, A. Mateevici, 60, MD-2009, Chisinau, Republic of Moldova; (4) Ghitu Institute of Electronic Engineering and Nanotechnologies, Academy of Sciences of Moldova, Academiei, 3/3, MD-2028, Chisinau, Republic of Moldova; (5) Photonics Laboratory, (LPhiA) E.A. 4464, SFR Matrix, Faculty of Sciences, Angers University, 2 Bd Lavoisier, 49045 Angers, France.

Resume : Indium selenide (InSe) is a typical representative of group III-VI layered materials, with a direct band gap of about 1.3 eV and indirect one of 1.26 eV at room temperature. By splitting bulk InSe single crystals perpendicularly to the C6 axis, plan-parallel lamellae with atomically flat surfaces and thickness down to nanometer range can be obtained. InSe lamellae are comprised of elementary Se-In-In-Se packings bound by van der Waals forces, with closed valence bonds at surface. Due to mentioned characteristics, together with nature of optical transitions in the center of Brillouin zone and low surface-state density, InSe is considered among the first 5 most promising materials for photovoltaic, water splitting and transistor technology applications. In this work, optical, photoluminescence and photosensitivity characteristics of micrometer-sized flexible (n- and p-InSe)/In2O3 heterojunctions, obtained by heat treatment in water vapour and oxygen enriched atmosphere of single crystalline Cd- and Sn-doped (0.1-3.0 at. %) InSe lamellae, have beeen investigated. Thin films of In-Cd alloy were used as electrodes. The absorption edge of In2O3:Sn and In2O3:Cd structures on InSe substrate was studied by means of ultraviolet reflectance spectroscopy. The forbidden bandwidth of native oxide, determined from its reflection band edge, is equal to 3.45 eV and ~ 3.52 eV for In2O3 layer on InSe:Cd and InSe:Sn substrates, respectively, for doping concentrations in range of 0.1 0.5 at. %. From the analysis of the photoluminescence spectra, excited by radiation corresponding to the fundamental absorption band of the native oxide layer, the energy band diagram of Sn and Cd impurity levels formed within the In2O3 band gap was determined. The energies of the levels induced by dopants and oxygen atoms diffused in the InSe layer from the native oxide/semiconductor interface were determined from the analysis of photoluminescence spectra of the InSe:Sn and InSe:Cd layers from In2O3/InSe interface. The (n- and p-InSe)/In2¬O¬3 structures display significant photosensitivity over a wide wavelength range, from ultraviolet to near infrared. The influence of Cd and Sn doping concentrations on the photosensitivity, mean free path and recombination rate of nonequilibrium charge carriers in n- and p-InSe layers from the InSe/native oxide interface was also studied.

Authors : A. Ruiz-Perona1, Y. Sánchez2, M. Guc2, T. Kodalle3, M. Placidi2, J.M. Merino1, M. León1, R. Caballero1
Affiliations : 1Universidad Autónoma de Madrid, Applied Physics Department, C/ Francisco Tomás y Valiente 7, 28049 Madrid, Spain. 2Catalonia Institute for Energy Research, C/ Jardins de les Dones de Negre 1,Sant Adriá del Besòs, 08930 Barcelona, Spain 3PVComB-Helmholtz Zentrum Berlin für Materialien und Energie, Schwarzschildstrasse 3, 12489 Berlin, Germany

Resume : Semi-transparent solar cells extend the range of applications of photovoltaics in our daily life. In the last years, kesterite-type material has attracted a special attention to be used as absorber in thin-film solar cells because of its low toxicity and earth abundant constituents. The substitution of Sn with Ge in this type of absorbers increases their band gap energy Eg paving the way towards its use in semi-transparent photovoltaic devices. The potential of the substitution of Sn with Ge has been demonstrated by achieving an efficiency of 8.5 % and Voc values of 65 % of the theoretical limit for Cu2ZnGeSe4 (CZGSe)-based solar cells [1]. In this work, CZGSe thin films are grown onto Mo/V2O5/FTO/glass stacks by co-evaporation and subsequent annealing at a maximum temperature of 480 °C to 525 ºC. The influence of the annealing temperature on the composition, formation of secondary phases, distribution of elements, vibrational properties and transparency of CZGSe layers is investigated by EDX, GIXRD, GD-OES, Raman spectroscopy and transmittance measurements. The optoelectronic properties of the complete solar cell devices are studied by J-V and QE measurements. The increase of the annealing temperature from 480°C to 525 ºC leads to a more uniform distribution of Cu, Zn, Ge and Se through the absorber layer, a reduction of the presence of the GeSe2 secondary phase, and the formation of a thicker MoSe2 layer at the CZGSe/back contact interface. The strategy of increasing the annealing temperature allows to reduce the cross-over between the illuminated and dark J-V curves and higher EQE resulting in an improvement on the device performance from 4.2 to 5.3 % (active area: 5.8 %) when annealing at 480 and 525 ºC, respectively. Both absorber layers present an optical band gap energy of 1.47 eV. However, the transmittance in the near-infrared spectral range decreases with temperature from 38 % (480 ºC) to 33 % (525 ºC) approximately [2]. In order to increase the transparency, kesterite thin films with higher Eg are fabricated by sulfurization of co-evaporated CZGSe layers – resulting in Cu2ZnGeSe4 (CZGSSe) - on transparent substrates. CZGSSe-based solar cells with efficiencies of 3.1, 2.7 and 1.5 % with Eg = 1.73, 1.85 and 1.91 eV respectively, are achieved. Further optimization of the sulfurization process and the transparent back contact is in progress. This work shows first promising semi-transparent CZG(S)Se-based solar cells with a great potential for, among others, Building Integrated Photovoltaic applications. [1] L. Choubrac et al., Appl. Energy Mater. 3 (2020) 5830. [2] A. Ruiz-Perona et al., submitted to J. Phys.: Mater.

Authors : M. Marzougui,1, H. Hammami1, H. Oueslati1, Rosine coq germanicus 2, M. Ben Rabeh 1 and M. Kanzari 3
Affiliations : 1: University of Tunis El Manar, National Engineering School of Tunis, Photovoltaic and Semiconductor Materials Laboratory, 1002, Tunis, Tunisia 2: Normandie Université, ENSICAEN, UNICAEN, CNRS, CRISMAT, 14000, Caen, France 3: University of Tunis, Preparatory Institute for Engineering Studies of Tunis, Photovoltaic and Semiconductor Materials Laboratory, 1002, Tunis, Tunisia

Resume : This paper aims to investigate the effect of Na doping on the physical properties of Cu2ZnSnS4 (CZTS: Na) ingots synthesized by direct fusion method from high purity (99.999%): copper (Cu), zinc (Zn), tin (Sn), sulfur (S) and sodium (Na) elements. Crystal structure and phase of CZTS were analyzed by X-ray diffraction (XRD) technique and Raman spectroscopy. The surface morphology of CZTS was observed by Scanning electron microscopy (SEM). Further, the optical parameters, like optical band gap were extracted by PL measurement and UV-Visible spectroscopy. XRD analysis indicate the growth of kesterite phase of CZTS with polycrystalline nature and a preferential orientation along (112) plane. SEM images show large grains with increasing the doping concentration. Optical measurement analysis reveals that the band gap of CZTS decrease from 1.66 to 1.53 eV with increasing Na-doping, approaching the optimum value, which desirable for solar cell applications. Electrical measurement indicates that all ingots exhibit p-type conductivity. Keywords: CZTS; Direct fusion method; Ingots; Semiconductor; Na-doping; Caracterization

Authors : Ahmad Ibrahim,Asim Guchhait,Shreyash Hadke,Hwee Leng Seng, Lydia Helena Wong
Affiliations : School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798;Department of Physics, Prabhat Kumar College, Contai, West Bengal 721401, India;School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798;Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), Singapore 138634;School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798

Resume : Open-circuit voltage (VOC) remains the primary limiting factor for Cu2ZnSnS4 (CZTS) solar cells to date. Many strategies have been proposed to alleviate this problem. One particular strategy, similar to its predecessor Cu(In,Ga)Se2 (CIGS), is doping and alloying through cation substitution especially with alkali metals. On CIGS, this group of elements was instrumental in boosting the efficiency to over 20%. Meanwhile, the same huge impact has not been observed on CZTS. Nevertheless, the alkali metals have been utilized for some of CZTS studies. For instance, potassium (K) was found to enhance the grain growth and facilitate small Cd inter-diffusion towards the absorber, reducing CuZn defect formation near the absorber surface. Another element that is touted to help in reducing VOC deficit of CZTS is silver (Ag). The large ionic radii difference with Zn could be beneficial in avoiding the formation of CuZn defect. It was also observed that the placement of Ag might give different advantages towards the interfaces, such as reducing voids formation near the absorber/Mo interface, or Cu-depletion near the absorber/CdS interface. These mechanisms could help reduce the VOC deficit of CZTS. In this work, the codoping of Ag and K utilized both improvement mechanisms on CZTS. The Ag-doped and K-doped CZTS precursor solutions were deposited at the bottom and top layers, respectively. By using this method, the avoidance of voids formation and improved carrier collection at the bulk and near the junction contributed to the performance improvement from 6.82% to 8.24% (active area). This result shows that elements codoping at different absorber layers could be benign for CZTS performance improvement.

Authors : Prabeesh Punathil, Solidea Zanetti, Elisa Artegiani, Vikash Kumar, Alessandro Romeo
Affiliations : Laboratory of Photovoltaics and Solid State Physics, Department of Computer Science, University of Verona, Italy

Resume : Copper zinc tin sulphide (Cu2ZnSnS4) is still considered a promising material for absorber layers in thin-film solar cells, despite the limitation experienced by the research community in overcoming the 12% efficiency limit. Among the many different growth techniques, the solution-based ones are interesting for their potential low cost compared to vacuum-based techniques. In our lab, Cu2(Zn,Sn)(S,Se)4 thin-film absorber layers are prepared by spin coating (on Mo coated glass) solution-based precursors. The stack is subsequently annealed in selenium ambient in a temperature range from 450 °C to 550 °C for 30 min. Solar cells with efficiencies of around 4.5% are routinely obtained. In this work we investigate the opportunity of doping the CZTS by inserting dopant (Cd, Ge) in the solution and/or surface treatment of the precursor films with dopant solution. Moreover, the insertion of passivating alkali elements with similar procedure will also be demonstrated and investigated. The influence of these process on the material properties of the absorber and on the finished devices will be analysed by means of different spectroscopies such as X-ray diffraction, Raman, X-ray photoelectron spectroscopy, and microscopy techniques such as atomic force microscopy and tunnelling electron microscopy. The performance of the finished cells will be discussed in sight of the information collected.

Authors : Thomas Ratz, Jean-Yves Raty, Guy Brammertz, Bart Vermang, Ngoc Duy Nguyen
Affiliations : CESAM | Q-MAT | Solid State Physics, Interfaces and Nanostructures, Physics Institute B5a, Allée du Six Août 19, B-4000 Liège, Belgium and Institute for Material Research (IMO), Hasselt University, Agoralaan gebouw H, B-3590 Diepenbeek, Belgium ; CESAM | Q-MAT | Solid State Physics, Interfaces and Nanostructures, Physics Institute B5a, Allée du Six Août 19, B-4000 Liège, Belgium and University of Grenoble Alpes | CEA-LETI | MINATEC Campus, Rue des Martyrs 17, F-38054 Cedex 9 Grenobles, France ; IMEC division IMOMEC, partner in Solliance, Wetenschapspark 1, B-3590 Diepenbeek, Belgium ; Institute for Material Research (IMO), Hasselt University, Agoralaan gebouw H, B-3590 Diepenbeek, Belgium and IMEC division IMOMEC, partner in Solliance, Wetenschapspark 1, B-3590 Diepenbeek, Belgium and Energyville, Thor Park 8320, B-3600 Genk, Belgium ; CESAM | Q-MAT | Solid State Physics, Interfaces and Nanostructures, Physics Institute B5a, Allée du Six Août 19, B-4000 Liège

Resume : In the past twenty years, the main endeavour concerning Cu2ZnSn(S,Se)4 thin films for solar cell applications has been focused on increasing the cell efficiency. And, as the efficiency improved, kesterite inorganic semiconductors using earth-abundant elements gained in interest as alternatives for the well-known chalcogenide Cu2InGa(S,Se)4 alloys. Nowadays, researchers deal with challenges concerning further efficiency improvement resulting of large open circuit voltage (VOC) deficits. Several factors have been pointed out as possible culprits for this VOC limitation, including interface recombination, secondary phases formation and high intrinsic point defects concentration. To overcome these shortcomings, elemental substitution has been proposed as a possible solution. In this work, we conduct first-principles calculations of Cu2ZnXS4 (X=Sn, Ge, Si) materials to highlight the impact of the cationic substitution on the structural, electronic and optical materials properties. Ultimately, using an improved version of the Shockley-Queisser model, we predict the solar cell efficiency based on the previously computed properties, putting into perspective the ab initio results with the solar cell characteristics. First, we report direct bandgaps values of 1.32, 1.89 and 3.06 eV respectively for Sn-, Ge- and Si-containing kesterites and, in addition, we highlight the applicability of these materials for solar cell application as absorption coefficient values of the order of 1E4 1/cm are obtained. Secondly, we report a slight increase of the effective masses as well as a decrease of the high frequency dielectric constant following the sequential substitution. Then, using predicted optimal absorber layer thicknesses, we study the variation of the solar cell maximal efficiency as a function of the non-radiative recombination rate. First, for vanishing non-radiative recombination rate, maximal efficiencies of 25.88, 19.94 and 3.11 % are reported respectively for Cu2ZnSnS4, Cu2ZnGeS4 and Cu2ZnSiS4. Subsequently, using non-radiative recombination rate values providing experimentally comparable VOC, we report efficiencies of 15.88, 14.98 and 2.66 %. As a result, we point out that possible efficiency improvements of 10 and 4.96 % respectively for Sn- and Ge-containing materials could be enabled via the reduction of the non-radiative recombination. We thus strengthen the interest of Cu2ZnSnS4 for single-junction solar cells while indicating Cu2ZnGeS4 as a possible candidate as top layer for tandem cells.

Authors : Fairouz Ghisani*, Kristi Timmo, Mare Altosaar, Maarja Grossberg and Marit Kauk-Kuusik
Affiliations : Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia.

Resume : Cu12Sb4S13 with the tetrahedrite structure is a highly conductive p++ type compound and it is known as thermoelectric material. Theoretical calculations and experimental results [1] have shown that the electrical and optical properties of Cu12Sb4S13 can be modified by substitution of two-valent copper by group II metals (M = Cu, Cd, Mn, Zn). Therefore, recently research has been focused on substituted tetrahedrites Cu12−xMxSb4S13 where a part of copper is replaced by other elements that help to lower the p-type conductivity of tetrahedrite materials [2][3]. However, the use of these compounds for photovoltaic applications is not been widely explored yet. In this work, Cd(Zn)-substituted tetrahedrite Cu12-xMxSb4S13 monograin powders (MGPs) were synthesized from Cu2S, Cd(Zn)S and Sb2S3 in molten salt environment. The syntheses were performed in sealed vacuum quartz ampoules by heating at 495 oC for two weeks. The effect of substitution and compositional modification on the photoelectrical and optical properties of Cu12-xMxSb4S13 (M = Cd, Zn) was investigated. Raman spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, photoluminescence analysis and photoelectrochemical measurements were used to characterize the formed powder crystals. For the first time, Mott–Schottky method was used to calculate and conclude the band positions of Cu12-xMxSb4S13 MGPs. Monograin layer (MGL) solar cells with different chemically deposited buffer materials were produced and characterized by I-V measurements. The influence of different compositions of Cu12-xMxSb4S13 monograin powder materials and different buffer layers to the MGL performance was studied. References [1] J. Heo, R. Ravichandran, C. F. Reidy, J. Tate, J. F. Wager, and D. A. Keszler, “Design Meets Nature: Tetrahedrite Solar Absorbers,” Adv. Energy Mater., vol. 5, no. 7, p. 1401506, Apr. 2015, doi: 10.1002/aenm.201401506. [2] D. S. Prem Kumar, S. Tippireddy, A. Ramakrishnan, K. H. Chen, P. Malar, and R. C. Mallik, “Thermoelectric and electronic properties of chromium substituted tetrahedrite,” Semicond. Sci. Technol., vol. 34, no. 3, 2019, doi: 10.1088/1361-6641/aafa31. [3] S. Bera, A. Dutta, S. Mutyala, D. Ghosh, and N. Pradhan, “Predominated Thermodynamically Controlled Reactions for Suppressing Cross Nucleations in Formation of Multinary Substituted Tetrahedrite Nanocrystals,” J. Phys. Chem. Lett., vol. 9, no. 8, pp. 1907–1912, Apr. 2018, doi: 10.1021/acs.jpclett.8b00680.

Authors : Alex Jimenez-Arguijo1,2, Alejandro Navarro-Güell1, Yudania Sánchez2, Víctor Izquierdo-Roca2, Zacharie Jehl1, Joaquim Puigdollers1, Sergio Giraldo2, and Edgardo Saucedo1.
Affiliations : 1. Electronic Engineering Department, Polytechnic University of Catalonia (UPC), Barcelona, Spain 2. Catalonia Institute for Energy Research (IREC), Sant Adrià del Besòs, Barcelona, Spain

Resume : Cu2ZnSnSe (CZTSe) is currently one of the most promising emerging fully inorganic thin film photovoltaic technologies based exclusively on earth abundant elements. However, the efficiency of CZTSe based solar cells has plateaued around 12-13% for as much as 7 years. In the frame of the European project CUSTOM-ART ( our group works on the improvement of kesterite absorbers in order to achieve the required efficiency threshold to become commercially viable. The most relevant and discussed problem among the kesterite community is the open circuit voltage (Voc) deficit. The origin of this issue still is a matter of intensive research. Recently, it has been suggested that Sn-based kesterites suffer from severe non-radiative recombination due to its native point defects, most probably SnZn. In order to mitigate the effect of the deep defect SnZn several strategies have been suggested. These strategies focus on reducing the concentration of these recombination centres either by extrinsic doping or Ag-alloying, Ge-alloying and fine Sn-concentration tuning. Regarding extrinsic doping, the most promising approach consists in hydrogen doping. Hi defects have a low formation energy in the CZTSe matrix. These defects are donors, i.e. they increase the Fermi Level, which, as a consequence, increases the formation energy of SnZn. After the synthesis of the absorber, due to the small atomic radius of H, it is expected to easily diffuse towards the grain boundaries or outgas even at room temperature. However, hydrogen doping has not been proved successful due to the high stability of the H2 molecule and the unavoidable formation of H2Se at high temperatures. The use of hydrides as sources of hydrogen circumvent the before mentioned issues since the formation of Hi defects is expected to be increased due to the higher reactivity of hydrogen anion(H-). Additionally, once ionized twice, the H- ion will increase the Fermi Energy even more than a Hi formed from a single H atom. This work will focus on the study of the effects of the thermal decomposition of two different hydrides (NaBH4, LiAlH4) on the optoelectronic properties of CZTSe solar cells. First experiments showed an increased reactivity of Se, deteriorating the Mo-CZTSe back interface, suggesting that a fine tuning of the XBH4/Se availability is required. We will show how the optimization of this ratio will solve this issue, affecting the electrical and morphological properties of the absorber. We will present in addition a phenomenological model on how hydrogen can be introduced using XBH4 precursor, opening interesting perspectives for the doping of kesterite.

Authors : Fabien Atlan1, Sergio Giraldo1, Ignacio Becerril-Romero1, Galina Gurieva2, Susan Schorr2,3, Edgardo Saucedo1,4, Alejandro Pérez-Rodríguez1,5, Maxim Guc1, Victor Izquierdo-Roca1
Affiliations : 1Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 2ª pl., 08930 Sant Adrià del Besòs Barcelona, Spain 2Department Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin fur Materialien und Energie, Helmholtz-Zentrum Berlin für Materialien und Energie Hahn-Meitner-Platz 1, 14109 Berlin Germany 3Institute of Geological Sciences, Freie Universitat Berlin, Malteserstraße 74-100, 12249 Berlin, Germany 4Photovoltaic Group, Electronic Engineering Department, Universitat Polytècnica de Catalunya (UPC), 08034, Barcelona, Spain 5UB, Departament d’Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain

Resume : Recently performed advanced studies of kesterite-based devices and bare kesterite materials have deepened into the limitations of this promising earth-abundant photovoltaic (PV) technology [1]. However, the high complexity of thin-film PV devices and high amount of processes involved in their production requires further investigation in order to identify critical fabrication parameters that may be influencing device performance. One of these parameters is the temperature of the different fabrication steps. The highest temperatures (500-600°C) are usually employed in the synthesis stage (i.e. annealing of absorbers) so are expected to only influence the initial layers of the device. On the other hand, softer temperatures (100-200°C) are applied at the final production or even at the post-production stages. As such, they can influence the whole device. In this regard, several groups have reported beneficial effects of low-temperature post-deposition annealings in kesterite solar cells [2,3]. However, the available information about the mechanisms behind the observed effects is quite limited. In the present study, we present an investigation on the effect of a controllable soft thermal treatment (~200 °C) on the properties of a 50×50 mm2 Cu2ZnSnSe4 (CZTSe) sample consisting of 225 individual 3×3 mm2 devices which were fabricated following the standard IREC technology [1,2]. A complex study of the solar cells was made by XRF, Raman, photoluminescence (PL), and I-V characterization comparing the properties of the devices before and after the annealing. The high amount of devices allowed working with high statistics and detecting small variations in their different layers. Multiwavelength Raman and PL spectroscopies revealed small changes in the absorber surface, associated mainly with Cu/Zn disordering (the sensitivity of Raman spectroscopy to Cu/Zn disordering was proven in high-quality CZTSe powder samples on which the ordering parameter was calculated based on the anomalous XRD [4]). In addition, changes in the CdS buffer layer were also detected by resonant Raman spectroscopy. These were related mainly to an improvement of its crystalline quality. Additional experiments allowed concluding that the main beneficial effect of the soft thermal treatment is related to the improvement in the CdS layer while the changes in the absorber surface are less relevant, or even harmful, for device efficiency. The results of this work give proof of the importance of understanding the effect of low-temperature production stages in device performance and represent one step forward for developing fabrication strategies that allow the kesterite PV technology to reach higher efficiency levels. [1] R. Fonoll-Rubio, Energy Environ. Sci. (2021) DOI 10.1039/D0EE02004D [2] M. Dimitrievska, Sol. Energy Mater. Sol. Cell 157 (2016) 462 [3] P. D. Antunez, Nat. Energy 2 (2017) 884 [4] D. M. Többens et. al., Phys. Stat. Solidi B 253 (2016) 1890

Authors : Katriin Kristmann1, Mare Altosaar1 , Jaan Raudoja1, Jüri Krustok1,2 , Taavi Raadik1*, Mati Danilson1, Maarja Grossberg1,
Affiliations : 1 Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate Tee 5, 19086, Tallinn, Estonia 2 Division of Physics, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : Pyrite - iron disulphide FeS2, also known as “fools gold” has all the necessary parameters to be a good absorber material for solar cell, it has p-type conductivity, high absorption coefficient 10^5 cm-1 and direct optical bandgap at 0.95 eV. Pyrite is abundant material in earth crust and it is listed as one of the most economically attractive inorganic PV material, making it perspective solution to reach multi-TW scale usage of solar energy. Nevertheless, the maximum achieved power conversion efficiency is as low as 3%, but theoretical maximum is exceeding 30%. Main problem with pyrite solar cell is low open circuit voltage, which is attributed to the sulphur vacancies and surface defects. When in the past main investigation with pyrite has been done with thin film solar cells, then in our work we will report about the progress of monograin layer solar cell (MGL) that uses pyrite microcrystals as a absorber layer. The absorber is a monolayer of nearly unisize, with a typical diameter of 50 μm, semiconductor powder crystals embedded into a layer of epoxy. MGL technology is completely different from traditional crystalline or thin film solar cell technologies. MGL lightweight solar panel technology combines the advantages of high-efficient single-crystalline material and low-cost roll-to-roll panel production, enabling to manufacture flexible, lightweight, and cost-efficient solar panels from powders of crystalline semiconductor absorber material. In this work the conditions for synthesis and growth of FeS2 microcrystalline powders were found. FeS2 synthesized in sulphur and recrystallized in molten KI as flux, had cubic structure of pyrite phase with stoichiometric composition, confirmed by XRD, Raman and EDX analyses, respectively. The sieved fraction of the crystals with diameter around 50 μm was used for making absorber membranes for monograin membrane solar cells. Schottkey diodes were formed to evaluate the material’s properties. In this paper we will report the conditions of synthesis-growth of phase pure FeS2 microcrystals and their electrical and optical properties.

Authors : Taavi Raadik1, Nikolae Spalatu1, Jüri Krustok1,2, Raavo Josepson2, Maarja Grossberg1
Affiliations : 1 Department of Materials and Environmental Technology, Tallinn University of Technology, 2 Ehitajate Tee 5, 19086, Tallinn, Estonia Division of Physics, Tallinn University of Technology, Ehitajate Tee 5, 19086, Tallinn, Estonia

Resume : Orthorhombic SnS has all the necessary parameters to be a good absorber material for solar cell, it has p-type conductivity, high absorption coefficient 10^4-10^5 cm^-1 and direct optical bandgap. Unfortunately, the maximum efficiency obtained with SnS solar cell is only 4.36%. However, according to the theoretical calculations the maximum efficiency can exceed 32%, meaning there is still a long way to go. The poor performance of SnS solar cells has been attributed to the weak material quality, band alignment problems and rapid carrier recombination at trap states near the interface between SnS and the n-type buffer layer. In current study the optical and electrical properties of high quality SnS solar cell, manufactured by close space sublimation technique, was studied by temperature dependent electroreflecance spectroscopy, - quantum efficiency and - I-V curves in the range of 20-300K. The optical properties of polycrystalline SnS thin films are quite similar to the monocrystalline SnS. However, from EQE of the studied SnS solar cell, two optical transitions were detected, indicating to the valence band splitting, that has not been observed in SnS monocrystals. Temperature dependent effective bandgap energy found from EQE follows the trend of bandgap energy found by electroreflectance, but the values are slightly higher. Although, CdS and SnS both were fabricated with CSS resulting in high quality p-n junctions, the temperature dependent Voc measurements indicate that interface recombination is still dominant in the device structure. Results of the electrical and optical characterization of CSS fabricated SnS solar cell are presented and discussed.

Authors : S. Marchionna(1), L. Frioni (1-2), and S. Binetti (2)
Affiliations : (1) RSE Spa, Ricerca sul Sistema Energetico, Via Rubattino 54, Milano, Italy, (2) University of Milano-Bicocca, Department of Materials Science and MIBSOLAR center, via Cozzi 55, Milano, Italy

Resume : A way to push photovoltaic (PV) on the terawatt scale in an eco-sustainable way is to investigate and validate possible alternatives to Cu2ZnSnS4 among I2-II-IV-S4 Earth-abundant chalcogenides. A further alternative belonging to this class of materials is Cu2FeSnS4 (CFTS), which consists of abundant and non-toxic elements and shows high absorption coefficient and direct band gap suitable for PV applications. In this work, CZTS and CFTS thin films were grown on Mo-coated soda lime glasses by a two-step process: sputtering, that combine high-throughput with an elevated reproducibility has been used to deposit stacked metallic precursors, successively annealed in sulfur vapors. A comparative study has been carried out in order to identify the main similarities and differences among the deposition parameters for pure thin films of this two chalcogenide. The homogeneity, crystallinity, and chemical composition of the thin films have been used like references for our comparative analysis. The PV parameters of some proof of concept solar devices will be also compared and exploited to validate CFTS like a potential alternative to CZTS for PV application. The target of this work is to study a correlation between the PV parameters and the ratios of the metallic elements in CFTS thin films in order to identify on the phase diagram the stoichiometric tolerance range (vs rich/poor metals conditions) for this chalcogenide.

Authors : Souhaib Oueslati, Maris Pilvet, Dieter Meissner, Marit Kauk-Kuusik, Jüri Krustok and Maarja Grossberg
Affiliations : Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : Photovoltaic (PV) cells and Photoelectrochemical (PEC) cells as a desirable system to produce electricity and to split water for practical and commercial applications require compact and stable device designs. Monograin membranes exhibit one of the most promising structures for solar cells and solar water splitting devices. Here semiconductor single crystals provide the reaction center-like means for light-induced charge carrier generation and separation. This work aims at combining the fabrication and characterization of Kesterite monograins (CZTSSe) p-type semiconductors for both PV and PEC application. The kesterite materials are stable, made out of abundant and non-toxic component, achieved relatively high performance on PV application, and considered as potential material for PEC application. The single crystals are synthesized from Cu, Zn, Sn, Se, and S (5N) precursors in potassium iodide as a flux material at a temperature of about 740oC. After synthesis, the monograins are washed with water to remove the salt and sieved to narrow fractions. A heat treatment step at 740°C for 35 minutes is needed to heal the crystal surface. The monograins membrane consists of single crystals semiconductor partially embedded in a thin polymer layer. The thickness of the epoxy layer applied by the doctor blade on a cleaned plastic foil is optimized so that approximately 50% of the grains remained uncovered. The uncovered part of the grains is exposed as a front, photo-active side of the membrane, whereas the other side is covered with the epoxy layer. The latter is removed by mechanical polishing, and a graphite paste is applied as a back contact. The membranes are finished as electrodes by fixing an electric wire on the graphite contact, and the whole backside area is encapsulated with epoxy. Whereas, the solar cells are completed by depositing a CdS buffer layer and an i-ZnO/ZnO:Al front contact on the active surface of the above-described electrode structure. The photoelectrochemical measurements are performed using a three-electrode system consisting of kesterite monograins working electrode, a Pt counter electrode, and a saturated calomel as a reference electrode. These electrodes are immersed in aqueous electrolytes for the electrolysis of water. We will present the first results trying to cover different aspects to develop Kesterite wide-bandgap absorbers as photocathode electrode for water splitting and to identify inherent bottlenecks that may limit the electrode performance, and present different optimization approaches. We studied the electronic properties of the Kesterite monograins electrodes using the electrochemical impedance spectroscopy measurement at room temperature. Using the Mott-Schottky plot, the flat band potential and the carrier concentration of Kesterite semiconductors were investigated. A comparison study of the performance of the electrodes and the solar cells fabricated using the same materials will be presented as well.

Authors : D. Spoială(1), Ig. Narolschi(1,2), O. Shapoval(3), A. Belenciuc(3), C. Rotaru(1,4), C. Antoniuc(1), S. Vatavu(1,2,4)
Affiliations : (1) Physics of Semiconductors and Devices Lab, Research and Innovation Institute, Moldova State University, 60 A. Mateevici str., MD 2009, Chisinau, Moldova; (2) Faculty of Physics and Engineering, Moldova State University, 60 A. Mateevici str., MD 2009, Chisinau, Moldova; (3) Solid State Structures Lab, Institute of Electronic Engineering and Nanotechnologies, 3/3 Academiei str., MD 2028, Chisinau, Moldova (4) CaRISMA Research Center, Research and Innovation Institute, Moldova State University, 60 A. Mateevici str., MD 2009, Chisinau, Moldova

Resume : Ga2S3 thin films are highly perspective for using in optoelectronic devices, targeting applications both in energy conversion and energy detection. Ga2S3 thin films have been prepared by using of Close-Spaced Sublimation (CSS) technique onto quartz plates and SnO2/Glass, having as-synthesized Ga2S3 as source compound. The substrate temperature up to 500°C has been used, while the source temperature was 800°C. Raman spectroscopy measurements were collected in the backscattering configuration by using 532 nm or 633 nm lasers for excitation and unpolarized light detection. The laser intensity was reduced down to 0.1-5% in order to avoid local heating. The well-defined Raman spectrum is attributed to Ga2S3 and Ga2O3 (depending on substrate). The morphology of the samples has been investigated both by SEM and AFM revealing crystallites conglomerates having linear dimensions in the range of 200 nm. GI-XRD (CuKa) have been used for structural characterizations. The investigations summarize in a direct connection of the physical properties to the deposition technology.

Authors : Ayaka Kana, Mutsumi Sugiyama
Affiliations : Faculty of Science & Technology, Tokyo University of Science, Research Fellow of Japan Society for the Promotion of Science; Faculty of Science & Technology, Tokyo University of Science, Research Institute for Science and Technology, Tokyo University of Science

Resume : The ternary Cu2SnS3 (CTS) compound consists of earth-abundant and non-toxic elements. In addition, the CTS compound has a direct bandgap and high optical absorption coefficient. Therefore, it has the potential for being utilized as a solar cell material. However, the CTS thin-film solar cell conversion efficiency is still low for various reasons, including unclarified carrier/defect properties of CTS thin films. To improve efficiency, other chalcogenide solar cells, such as CIGS and CZTS, are controlled carriers through intrinsic and/or extrinsic defects. Recently, a relatively high efficiency exceeding 5% in solar cells using CTS was reported by the use of extrinsic Na infusion. However, the significance of Na on CTS defects has not yet been completely investigated. Therefore, it is necessary to understand and control the carrier of CTS thin films to improve solar cell performance. In this presentation, to understand the origin of the efficiency improvement of Na-infused CTS thin film solar cells, various changes to their characteristics will be investigated by X-ray diffraction, impedance spectroscopy, and photoluminescence. A Cu/SnS2 stacked precursor was deposited by RF sputtering on Mo/alkali-free glass (AFG) substrates. A NaF layer was then deposited by thermal evaporation. The deposited NaF layer thickness was varied from 0 to 120 nm. The precursors were then sulfurized using sulfur vapor at 560 ℃ for 1.5 h. Subsequently, CTS solar cells were fabricated with an Al-Ni/ZnO:Al/ZnO/CdS/CTS/Mo/AFG structure using the generated CTS thin films.

Authors : Tim Freund, Peter Wellmann
Affiliations : Friedrich-Alexander Universität Erlangen-Nürnberg, Institute of Materials for Electronics and Energy Technology; Friedrich-Alexander Universität Erlangen-Nürnberg, Institute of Materials for Electronics and Energy Technology

Resume : Chalcogenide Perovskites are a novel class of materials. They possess Pervoskite-type crystal structure with general formula ABX3, where A typically is either Ca, Ba or Sr, B is either Zr, Ti or Hf and X is either S or Se. [1] Although they were reportedly synthesized for the first time in 1957 by Hahn and Mutschke, [2] they only gained larger attention recently due to an search for alternative solar cell materials. This search was focused to perovskite-type materials due to the large and quick success of the halide perovskites since their introduction to photovoltaics in 2009, with the current certified record efficiency for a solar cell of 25.5%. [3] However, this group of materials face difficulties for the application regarding long-term stability and toxicity. [4, 5] To solve these issues, several Chalcogenide Perovskites were identified as potential solar cell material potentially reaching high efficiencies competing with existing materials. [1, 5, 6] Until recently, no reports of successful thin film synthesis existed. Previous attempts obtained powdered products by solid state reaction of the binary precursors at elevated temperatures, often modified to reduce the reaction temperature and time by the addition of Halides or salt fluxes. [2, 4, 7-10] The majority of the reported thin film synthesis follow an approach where the respective Oxide Perovskite is sulfurized using either H2S or CS2 at elevated temperatures (1050°C). [11-13] Others used co-sputtering followed by rapid thermal processing [14] or spin coated colloidally stabilized powders. [15] Here, we want to report on our efforts to synthesize thin films of BaZrS3 and similar materials utilizing an approach based on the Stacked Elemental Layer (SEL) technique commonly used to synthesize Cu(In,Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS). Therefore, precursor layers of BaS and Zr were previously deposited on various substrates and annealed at temperatures up to 1200°C. To enable the reaction to form BaZrS3, elemental sulphur was added to the annealing crucible. These experiments however did not yield the desired product so far, but indicate several parameters that seem to be key to enable this approach. Namely, for future experiments we want to increase the annealing temperature and improve the purity of our precursor layers.   1. Sun, Y.Y., et al., Chalcogenide perovskites for photovoltaics. Nano Lett, 2015. 15(1): p. 581-5. 2. Hahn, H. and U. Mutschke, Untersuchungen über ternäre Chalkogenide. XI. Versuche zur Darstellung von Thioperowskiten. Zeitschrift für anorganische und allgemeine Chemie, 1957. 288(5‐6): p. 269-278. 3. National Renewable Energy Laboratory. Best Research-Cell Efficiency Chart. 2020 last accessed on January 27, 2020; Available from: 4. Niu, S., et al., Bandgap Control via Structural and Chemical Tuning of Transition Metal Perovskite Chalcogenides. Adv Mater, 2017. 29(9). 5. Nie, R., et al., Lead-free perovskite solar cells enabled by hetero-valent substitutes. Energy & Environmental Science, 2020. 13(8): p. 2363-2385. 6. Swarnkar, A., et al., Are Chalcogenide Perovskites an Emerging Class of Semiconductors for Optoelectronic Properties and Solar Cell? Chemistry of Materials, 2019. 31(3): p. 565-575. 7. Nitta, T., K. Nagase, and S. Hayakawa, Formation, Microstructure, and Properties of Barium Zirconium Sulfide Ceramics. Journal of the American Ceramic Society, 1970. 53(11): p. 601-604. 8. Wang, Y., et al., Synthesis of BaZrS3 in the presence of excess sulfur. Journal of Alloys and Compounds, 2000. 311(2): p. 214-223. 9. Wang, Y., N. Sato, and T. Fujino, Synthesis of BaZrS3 by short time reaction at lower temperatures. Journal of Alloys and Compounds, 2001. 327(1-2): p. 104-112. 10. Meng, W., et al., Alloying and Defect Control within Chalcogenide Perovskites for Optimized Photovoltaic Application. Chemistry of Materials, 2016. 28(3): p. 821-829. 11. Shaili, H., et al., Synthesis of the Sn-based CaSnS3 chalcogenide perovskite thin film as a highly stable photoabsorber for optoelectronic applications. Journal of Alloys and Compounds, 2021. 851. 12. Gupta, T., et al., An Environmentally Stable and Lead-Free Chalcogenide Perovskite. Advanced Functional Materials, 2020. 30(23): p. 2001387. 13. Pandey, J., et al., Local ferroelectric polarization in antiferroelectric chalcogenide perovskite BaZrS3 thin films. Physical Review B, 2020. 102(20). 14. Comparotto, C., et al., Chalcogenide Perovskite BaZrS3: Thin Film Growth by Sputtering and Rapid Thermal Processing. ACS Applied Energy Materials, 2020. 3(3): p. 2762-2770. 15. Ravi, V.K., et al., Colloidal BaZrS3 Chalcogenide Perovskite Nanocrystals for Thin Film Device Fabrication. Nanoscale, 2021.

Authors : Rokas Kondrotas1, Saulius Tumenas1, Bronislovas Cechavicius1, Xiaofeng Li2, Maarja Grossberg2, Marit Kauk-Kuusik2
Affiliations : 1. Center for Physical Sciences and Technology, Savanorių Ave. 231, Vilnius, 02300, Lithuania 2. Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : Currently, low-cost multijunction solar cells are gaining more and more attention thanks to the unprecedented development of perovskite thin film technology. Namely, Si and perovskite tandem device have demonstrated nearly 30% power conversion efficiency (PCE). However, the infrared part of solar radiation, i.e., below 1.1 eV of such tandem device is not utilized and has potential to be increased by 5% in absolute efficiency. Considering triple multijunction solar cell with c-Si as the middle sub-cell, the optimum band gap in terms of PCE for bottom sub-cell is 0.7 eV. Conventional materials operating in this region include Ge, GaSb, alloys of III-V and II-VI, but are very expensive to synthesize (molecular beam epitaxy, metal organic chemical vapour deposition). Employing these materials would augment the overall cost of multijunction device beyond affordable. Herein we seek to explore new earth-abundant materials with band gap in the optimum region and compatible with low-cost synthesis methods. Inorganic chalcogenide-perovskites are underexplored class of materials with potential to be applied in infrared photovoltaics. Theoretical calculation has shown that depending on the adopted crystal structure, the band gap of AIIBIVX3 compounds (X=S, Se) is in 0.0 – 2.3 eV range. However, there is a lack of experimental work reporting optical properties of these compounds. In this work, we selected some of AIIBIVX3 compounds which are anticipated to have low band gap and synthesized via solid state reaction. Many of compounds, although theoretically predicted, do not exists in ternary phase and decomposes into two binary compounds, namely AX and BX2. Other compounds in contrast to predicted perovskite or distorted perovskite phases adopt lower symmetry crystal structures as their equilibrium phase. We found that one of potential compound, SrTiS3, crystallizes into hexagonal crystal structure (prototype structure BaNiO3) and exhibited nearly optimum band gap – 0.65 eV. Synthesis conditions were further adjusted to facilitate and stabilize some other chalcogenide-perovskite phases. XRD, EDS and SEM measurements were carried out to identify compound composition, structure and morphology.

Authors : Filipe Martinho1, Adèle Debono1, Sara Engberg1, Ignacio Minguez Bacho2, Alireza Hajijafarassar3, Moises Espíndola-Rodríguez4, Eugen Stamate3, Jørgen Schou1, Ole Hansen3, Julien Bachmann2, Stela Canulescu1
Affiliations : 1) Department of Photonics Engineering, Technical University of Denmark, DK-4000 Roskilde, Denmark. 2) Interdisciplinary Center for Nanostructured Films, University of Erlangen-Nuremberg, 91058 Erlangen, Germany 3) DTU Nanolab, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark 4) DTU Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark.

Resume : Inorganic chalcogenide semiconductors such as Cu2ZnSnS4 (CZTS) are promising for applications such as multijunction photovoltaics or catalytic electrochemical processes such as water splitting, where a cheap, stable and high-bandgap semiconductor is required. As with all kesterites, the performance of CZTS is currently limited by a large open-circuit voltage (Voc) deficit. In general, the Voc deficit results from the sum of all recombination components, notably interface recombination and bulk recombination. Contrary to its other kesterite counterparts, the interface recombination component of CZTS remains notably high, as evidenced for example by a large discrepancy (>0.3 eV) between the 0 K intercept and the absorber bandgap in temperature dependent current-voltage measurements (IV-T), even in world record devices >9% efficiency. Therefore, suitable interface passivation strategies are required to advance CZTS devices. In this work, we present a comparative study of different passivation strategies for CZTS using ultrathin layers deposited by atomic layer deposition (ALD). We focus on surface charge passivation with Al2O3 and lattice-matching passivation using ZnS, in combination with different surface treatments such as chemical etchings and plasma ashing. By using nanopatterned contact openings or self-organized patterns at the back interface (Mo/CZTS), we show that besides the reduction in recombination, there is the additional benefit of increasing the compatibility with different types of Mo. At the front interface (CZTS/CdS), we find that both Al2O3 and ZnS are effective passivation layers, although the passivation mechanism differs significantly. In particular, we show for the first time that the passivation quality is dependent on the surface etching and post-deposition heat treatments.

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New absorber materials I : Susan Schorr/Phillip Dale
Authors : J. D. Major
Affiliations : University of Liverpool

Resume : Antimony selenide solar cells are rapidly approaching 10% efficiency however a number of key materials properties are still not fully understood. In particular the mechanism by which doping is controlled in the material has not been established and there is a high degree of variability in the literature on the level of doping attainable and even carrier type. This talk will discuss the identification of potential sources of doping in Sb2Se3, the influence this has on carrier concentration and the impact for device performance. Through the use of Sb2Se3 bulk crystals we will further demonstrate routes to achieve p, i and n doping and methods to transfer this to thin film devices.

Authors : P. Vidal-Fuentes, J. Blanco, I. Becerril-Romero, M. Guc, F. Peiró, E. Saucedo, A. Pérez-Rodríguez, S. Estradé, V. Izquierdo-Roca
Affiliations : Catalonia Institute for Energy Research (IREC), Sant Adrià del Besòs-Barcelona, Spain: P. Vidal-Fuentes; I. Becerril-Romero; M. Guc; A. Pérez-Rodríguez; V. Izquierdo-Roca. LENS-MIND, departament d'Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, 08028, Barcelona, Spain: J. Blanco; F. Peiró ;S. Estradé Electronic Engineering Department, Polytechnic University of Catalonia (UPC), Barcelona, Spain: E. Saucedo IN2UB, Departament d’Electrònica, Universitat de Barcelona, Barcelona, Spain: A. Pérez-Rodríguez

Resume : Antimony selenide thin film solar cells have experienced an impressive development in less than five years of dedicated research. Three main approaches have been employed in the development of Sb2Se3. Firstly, a superstrate configuration based on that of CdTe devices and usually grown by CSS and VTD with a record PCE of 7.6%. Then, a substrate configuration, which can be subdivided in planar and non-planar architectures. The non-planar architecture, consisting in nanoribbons oriented perpendicular to the substrate, holds the record PCE of 9.2%. Finally, the planar architecture holds a 6.7% record efficiency using a standard thin film technology structure similar to that of CIGS but with a (Cd,Zn)S buffer layer. In this work, we investigated thoroughly the limitations found in the standard CIGS-like structure (SLG/Mo/MoSe2/Sb2Se3/CdS/i-ZnO/ITO) that may hamper the PCE evolution of the system. In this regard, a device with PCE close to 6% (potentially 6.5% after applying antireflective coating) was fabricated. The synthesis process consisted of a sequential reactive annealing under selenium atmosphere at low temperature (320 ºC) of an elemental Sb layer deposited by e-beam evaporation. The structural quality of the Sb2Se3 grown film was explored by a detailed analysis of the devices using a combination of advanced electron microscopy techniques (TEM/HAADF and X-EDS) together with Raman spectroscopy, XRF and XRD to obtain a full picture of the different issues present at the interfaces of the device and in the bulk of the absorber. The characterization was completed by J-V-T, spectral response and C-V optoelectronic studies. The simultaneous investigation of the bulk and the interfaces allowed the assessment of the impact of the found issues on device performance leading to the design of optimal mitigation strategies. In this regard, a good crystalline quality of the absorber material and a homogeneous composition in the bulk were found, reporting direct evidence of a continuous ribbon orientation inside the grains of Sb2Se3. However, the observations showed significant compositional instability at the front interface of the device, in particular regarding the inter-diffusion of elements between Sb2Se3 and CdS n-type buffer layer. This was proved by Raman scattering spectroscopy that showed a temperature dependent selenium migration towards the CdS buffer layer and the formation of a Cd(S,Se) solid solution that completely degrades the devices annealed at 200ºC. Furthermore, this inter-diffusion of elements between the absorber and the buffer was found to start already at 100ºC, usual temperature of device fabrication. This may limit the development of this technology and must be taken into account in order to achieve efficiencies higher than 10%. As such, the results reported in this work represent a step forward in the comprehension of the limitations of the Sb2Se3 PV necessary to overcome the 10% PCE barrier.

Authors : Nicole Fleck, Oliver S. Hutter, Laurie J. Phillips, Huw Shiel, Theodore D. C. Hobson, Vin R. Dhanak, Tim D. Veal, Frank Jaeckel, Ken Durose, Jonathan D. Major
Affiliations : Oliver S. Hutter: Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle upon Tyne NE1 8ST, UK. Remaining: Stephenson Institute of Renewable Energy and Department of Physics, University of Liverpool, Liverpool, L69 7ZF, UK.

Resume : Sb2Se3 has recently attracted great interest as an absorber layer for photovoltaic devices. It has a suitable band gap, high absorption coefficient and good stability. Efficiencies using Sb2Se3 in PV devices have increased rapidly, now reaching 10%. As with most solar cell fabrication procedures, the cells are exposed to air at intervals during fabrication. While Sb2Se3 solar cell and material papers often comment on oxides present in the sample, little work has been undertaken on the analysis of the effects of this oxide. Here we show that oxidation of the surface plays a significant role in device performance and back interface passivation in superstrate cells [1]. We discuss the effect of the formation of a surface oxide on solar cell performance which, contrary to expectation, we have observed to improve over time. Oxidation takes place within the first 48hrs after deposition of Sb2Se3, a timeframe which is not generally studied in relatively stable solar absorber materials. A combined device characterisation and surface science (x-ray photoelectron spectroscopy) study is presented over a timescale of several days/weeks demonstrating that exposure to oxygen forms a thin, passivating and recombination-reducing layer on the order of nanometers. In particular, oxidation of the Sb in the Sb2Se3 film leaves a more Se-rich back surface which is also shown to improve device performance by ~10%. Replicating these changes via coating the back-surface with thin films of Se, Sb2O3, or both produces a similar reduction in recombination at the Sb2Se3-Au interface achieving peak solar cell efficiencies of over 5.8%. We conclude that that inclusion of an intentional oxide layer at the back contact, either by deposition of an oxide or controlled oxidation represents a method to increase the power conversion efficiency of Sb2Se3 solar cells and may also be relevant to device production of other chalcogenide-based photovoltaics. [1] Fleck et al. ACS Appl. Mater. Interfaces, Nov. 2020, 12, 47, 52595–52602.

Authors : A. Shongalova,B. Vermange, J.M.V. Cunha, P.M.P. Salomé, P.A. Fernandes, M.R. Correia
Affiliations : A. Shongalova (a,b); B. Vermange (c); J.M.V. Cunha (d); P.M.P. Salomé (b,d); P.A. Fernandes (b,d,e); M.R. Correia (b,*) (a) Satbayev University, Satbayev street, 22a, 050013, Almaty City, Kazakhstan. (b) i3N / Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal (c) University of Hasselt – Partner in Solliance, Agoralaan gebouw H, Diepenbeek 3590, Belgium; Imec – Partner in Solliance, Kapeldreef 75, Leuven 3001, Belgium; Imomec – Partner in Solliance, Wetenschapspark 1, Diepenbeek 3590, Belgium. (d) International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal. (e) CIETI, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugal.

Resume : Sb2Se3 thin films have attracted the attention of numerous research groups for their optical and electronic properties. Photovoltaic technology has been pointed out among others as a possible application for this semiconductor material. Therefore, Sb2Se3 has been studied using several characterization techniques and Raman scattering spectroscopy is one of the most widely used techniques for crystalline phase identification and structural analysis. Nevertheless, the local modifications that might be induced by laser in-situ probing during the measurement should be better understood and controlled. This effect is particularly relevant and cannot be neglected. We used µ-Raman technique to identify Sb2Se3 Raman modes and identify possible other crystalline phases that may coexist due to the thin film growth process and promoted also by in-situ chemical reaction during the Raman analysis. The results showed strong evidences that, under certain conditions, this binary compound can be structurally modified, and the oxidation of the films occur. The emergence of a 250 cm-1 Raman mode, not belonging to Sb2Se3, is a proof of in-situ oxidation process. The evolution of the Raman spectrum for different incident laser power densities clearly show Sb2O3 formation. This study was complemented by X-ray diffraction measurements during an in-situ annealing.

11:00 Discussion/Break    
New absorber materials II : Edgardo Saucedo/Jon Major
Authors : Corrado Comparotto(1), José Márquez Prieto(2), Kostiantyn Sopiha(1), Jonathan Scragg(1)
Affiliations : (1) Uppsala University, Sweden; (2) Helmholtz Zentrum Berlin, Germany

Resume : The excellent intrinsic material properties of lead-halide perovskites have opened a new chapter in photovoltaics research, full of new possibilities but also scientific challenges. A particular issue for hybrid lead halide perovskites, already on the edge of commercialisation, will be reaching comparable robustness to existing silicon or chalcopyrite-based technology. This is a high and rising benchmark, and the well-documented lower chemical stability could hold perovskites back. Chalcogenide perovskites, in which halide anions are replaced by the chalcogens S or Se, might offer a new path forward. They are all-inorganic, lead-free and environmentally stable. The solar cell-relevant properties of these materials are projected to be comparable to halide perovskites, and their chemistry provides them with excellent inherent stability under conditions where halide perovskites would rapidly degrade, e.g. in water, humid air or at high temperatures. While this sounds highly promising, the hype is also dampened by some contradictory results and unexpected challenges for material synthesis. In this talk, we review the most important findings from the literature to date concerning chalcogenide perovskites, to give a balanced appraisal of their possible impact on the field. We summarise the chalcogenides that are known or expected to crystallise in perovskite structures, and which are of most relevance for PV applications. We discuss the knowns and unknowns of their defect chemistry and assess their photovoltaic potential, including highlighting some unusual absorption and emission characteristics that are not yet fully explained. We look shortly at the formation chemistry and shed light on some of the synthesis challenges, while also discussing the progress made in our group toward growth of high quality BaZrS3 films and solar cells.

Authors : Ignasi Burgués-Ceballos (1), Yongjie Wang (1), Gerasimos Konstantatos (1,2)
Affiliations : (1) ICFO - The Institute of Photonic Sciences, The Barcelona Institute of Science and Technology, Barcelona (Spain) (2) ICREA - Institució Catalana de Recerca i Estudis Avançats, Barcelona (Spain)

Resume : Colloidal AgBiS2 nanocrystals are gaining attention in the last years amongst heavy-metal-free ternary chalcogenides considered for photovoltaic applications. They are easy to produce from inexpensive, earth-abundant and non-toxic materials and exhibit a high absorption coefficient as well as a suitable bandgap for light harvesting. In addition, the cubic rocksalt AgBiS2 structure shows intrinsic stoichiometric stability. In this talk we show our recent findings on this promising material as well as our latest developments in AgBiS2 based solution-processed solar cells. Firstly, we modify the synthetic conditions to obtain ternary AgBiS2 nanocrystals with a significant increase in the average diameter and a concomitant higher mobility, a reduced trap-assisted recombination and a lower midgap trap state density. In parallel, we manage to tune the energy levels of the absorber by introducing stoichiometric variations to the system. Then, we combine the customised nanocrystals in advanced device configurations to push, for the first time, the power conversion efficiency beyond 7%. We also discuss strategies to further exploit the potential of this material.

Authors : José A. Márquez1, Marin Rusu1, Hannes Hempel1, Ibbi Y. Ahmet2, Moritz Kölbach2, Ibrahim Simsek1, Leo Choubrac1, Rene Gunder1, Galina Gurieva1, Susan Schorr1 and Thomas Unold1
Affiliations : 1 Department of Structure and Dynamics of Energy Materials. Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany 2 Institute of Solar Fuels. Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

Resume : The earth-abundant ternary compound BaZrS3, which crystallizes in a perovskite-type structure, has been discussed as a promising candidate for photovoltaic applications. We present here the synthesis of polycrystalline perovskite-type BaZrS3 thin-films from oxide precursors by annealing in H2S at different temperatures, leading to thin films with variable S and O content. XRD indicates that the thin-films crystallize with domains of either cubic-BaZrO3 or orthorhombic-BaZrS3 both in a perovskite-type structure. No crystalline BaZr(S,O)3 solid solutions were detected by X-ray diffraction studies. The absorption onset of the films undergoes a red-shift as the sulfur content increases and results in an increase in the electron and hole mobilities in the films, as measured by optical-pump terahertz-probe spectroscopy. The charge carrier mobility of the films synthesized at temperatures above 1000 °C reach values of ~2 cm2 /(V s), and large absorption strength above the bandgap, surpassing the ones measured in other compound semiconductors used in thin film photovoltaics including halide perovskites. We also analyze the radiative recombination properties of the BaZrS3 films by temperature-dependent photoluminescence, which reveals the presence of deep defect states. These defects may currently dominate non-radiative recombination in these thin films and strongly quench the luminescence efficiency.

12:15 Discussion    
12:30 Break    
Kesterite absorbers & devices : Jonathan Scragg
Authors : Maarja Grossberg, Marit Kauk-Kuusik, Kristi Timmo, Jüri Krustok, Mare Altosaar, Katri Muska, Valdek Mikli, Mati Danilson, Taavi Raadik, Maris Pilvet, Jelena Maricheva, Souhaib Oueslati
Affiliations : Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

Resume : Kesterite absorber materials Cu2ZnSn(S,Se)4 are attractive candidates for abundant, non-toxic and high efficiency solar cells. Kesterite based unique monograin layer solar cell technology (MGL SC) was developed by the Tallinn University of Technology researchers more than a decade ago targeting building integrated applications as light-weight, semitransparent and flexible photovoltaic modules. The core innovations in the MGL SC technology are: a) the light absorbing layer made of high quality micro-crystalline semiconductor powder, and b) the low cost (no high vacuum methods) and easily up-scalable roll-to-roll PV module production process. The whole kesterite community is looking forward to a breakthrough in the kesterite solar cell device efficiency that has reached close to 13%, which has turned out to be difficult to outperform for many years already. Hereby we present recent advancements in the development of kesterite MGL solar cells, including modifications of the synthesis process in order to control the structural polymorphism and multivalency of Sn, novel approaches for p-n junction engineering, and promising alloying strategies. The developed strategies have enabled us to reduce interface as well as bulk recombination in the CZTSSe MGL solar cells leading to improved device performance. The solar cell device performance will be discussed and correlated with the structural, compositional, optical and electrical properties of the kesterite absorber studied by various methods including scanning electron microscopy tools, photoluminescence spectroscopy, Raman scattering, X-ray photoelectron spectroscopy, admittance spectroscopy etc.

Authors : H. Hempel1, José A. Márquez1, Alex Redinger2, T. Unold1
Affiliations : 1 Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin für Materialien und Energie, Lise-Meitner Campus, Hahn-Meitner-Platz 1, 14109 Berlin, Germany 2 Scanning Probe Microscopy Laboratory, Department of Physics and Materials Science, University of Luxembourg

Resume : The performance of kesterite-based solar cells is strongly limited by low values of the open-circuit voltage (VOC) commonly attributed to large non-radiative recombination and high density of bandtail states. With detailed balance arguments, we decouple the contribution of bandtails and non-radiative recombination to the total VOC loss by using absolute photoluminescence at equivalent one sun conditions. For kesterites, we estimate that bandtails or non-ideal absorption contribute to less than 20% of the total VOC loss and the main culprit of the poor device performance of these materials is large non-radiative recombination. We find that the substitution of Cu by Ag can help to reduce the VOC loss due to non-radiative recombination. We additionally find that the losses due to bandtails can be further reduced by the substitution of Zn by either Cd or Ba and thus is a promising route to increase the radiative limit of the VOC towards the Shockley-Queisser limit. A simple methodology to estimate and decouple bandtail and non-radiative losses is presented and applied to assess highly efficient kesterites, CIGSe and halide perovskites.

Authors : Sara Engberg, Kristoffer Bentsen, Io Mizushima, Filipe Martinho, Denys Miakota, Peter Poulsen, Eugen Stamate, Ole Hansen, Torben Tang, Jørgen Schou, Stela Canulescu
Affiliations : DTU Fotonik; DTU Fotonik; IPU, Kgs. Lyngby, Denmark; DTU Fotonik; DTU Fotonik; DTU Fotonik; DTU Nanolab; DTU Nanolab; IPU, Kgs. Lyngby, Denmark; DTU Fotonik; DTU Fotonik, Technical University of Denmark, Denmark

Resume : Non-vacuum techniques can be used for large-scale, fast, and inexpensive synthesis of thin-film materials. Compared to vacuum approaches, solution-processing is interesting from an economic perspective, as it has the potential to provide a lower capital expenditure (CAPEX), which are the funds required to establish and maintain equipment. Additionally, solution-based methods can provide high material utilization, and an overall lower environmental impact can be achieved. Since 2010, the record Cu2ZnSn(S,Se)4 (CZTSSe) device has been made by solution-processing. Another advantage of this approach is the ease of screening new elements for concurrent research topics, such as doping and alloying. New solvents, salts, and additives can conveniently be incorporated in typical lab-settings, offering a vast parameter-space within reach. In this work, we summarize our results achieved from preparing Cu2ZnSnS4 (CZTS) solar cells through three of the main solution-based methods, including nanoparticles [1], molecular inks [2], and electrodeposition. We have demonstrated efficiencies of 2% and 5.85% for the nanoparticle and molecular ink approaches, respectively, when no doping or alloying was used. For the nanoparticle approach, we have used both CZTS and oxide particles, and for both types, we detect the infamous fine-grain layer. For the CZTS nanoparticles, we show that upon the addition of alkali elements, the fine-grain layer can be avoided [3]. For the oxide particles, we find a linear relationship between the film thickness within a few microns range and the device efficiency. Based on this finding and other results, we propose a formation mechanism for converting these oxide particles to CZTS. The molecular ink approach results in our highest efficiencies as of yet. Our study on the heat treatment between 250C and 450C during the spin-coating procedure suggests that a less crystalline precursor is advantageous for the final device performance, as this treatment results in a more uniform large-grain morphology from a higher sulfur-incorporation during the annealing. Finally, we will discuss our recent results on the synthesis of CZTS thin films by electrodeposition of metallic stacks. We review the synthesis approaches in terms of reproducibility, scope, CAPEX, and the best results obtained by us and reported in the literature. Samples will be characterized with SEM, Raman spectroscopy, XRD, EDX and/or XRF, PL, EQE, and J-V characteristics. [1] N. Mirbagheri et al. Nanotechnology 27, 185603 (2016) [2] S. Engberg et al. Sci Rep 10, 20749 (2020) [3] S. Engberg et al. RSC Adv. 13 (2018)

Authors : Susan Schorr, Galina Gurieva, Sara Niedenzu, Alexandra Franz
Affiliations : Helmholtz-Zentrum Berlin for Materials and Energy, Dep. Structure and Dynamics of Energy Materials, H-Meitner-Platz1, 14109 Berlin, Germany

Resume : Alloying of cations or anions is an established method for tailoring the band gap energy. In this work we discuss the solid solution series Cu2(Zn,Fe)S4, Cu2(Zn,Cd)S4 and Cu2(Zn,Mn)SnSe4 in which divalent Zn is substituted systematically by Fe, Cd and Mn. The end members Cu2ZnSnS4 and Cu2ZnSnSe4 crystallize in the kesterite structure (s. g. I4 ̅ ) [1], whereas Cu2FeSnS4 [1], Cu2CdSnS4 [2] and Cu2MnSnSe4 crystallize in the stannite structure (s. g. I4 ̅2m) [2]. Both the kesterite and the stannite structure are based on a tetrahedral coordination but different cation arrangements: perpendicular to the crystallographic c-axis in the stannite structure Zn-Sn planes change with Cu-Cu planes, whereas in the kesterite structure Cu-Sn planes and Cu-Zn planes, showing Cu/Zn disorder, are present. We studied the kesterite - stannite structural transition in these three solid solution series as well as the band gap energy variation with chemical composition. The detailed structural investigations are based on neutron powder diffraction experiments which provide the cation distribution in the crystal structure allowing a distinction between the isoelectronic cations Cu and Zn2 as well as the electronic similar cations Mn2 and Fe2 . A remarkable structural feature is the abrupt change of the tetragonal deformation c/2a (a and c are lattice parameter). In Cu2Zn1-xFexS4 and Cu2Zn1-xCdxS4 this change occurs at the same x value (x~0.3). In Cu2ZnSnS4 is c/2a~1, where as in Cu2ZnSnSe4 is c/2a<1. Thus in the Cu2(Zn,Mn)SnSe4 series c/2a increases by substituting Zn by Mn until a value of c/2a~1 is reached. The structural transition is characterized by a cation re-distribution process which will be discussed in detail. [1] Schorr et al., Europ. J. Mineral. 19 (2007) 65 [2] Schäfer et al., Z. Krist. 145 (1977) 356

Authors : Enric Grau-Luque, Ignacio Becerril-Romero, Nada Benhaddou, Ikram Anefnaf, Safae Aazou, Zouheir Sekkat, Maxim Guc, Victor Izquierdo-Roca
Affiliations : Catalonia Institute for Energy Research - IREC, Sant Adria de Besos, Barcelona, Spain; Catalonia Institute for Energy Research - IREC, Sant Adria de Besos, Barcelona, Spain; Department of Chemistry, Faculty of Sciences, Mohammed V University in Rabat, Morocco. Optics & Photonics Center, Moroccan Foundation for Advanced Science, Innovation and Research – MAScIR, Rabat, Morocco; Department of Chemistry, Faculty of Sciences, Mohammed V University in Rabat, Morocco. Optics & Photonics Center, Moroccan Foundation for Advanced Science, Innovation and Research – MAScIR, Rabat, Morocco; Department of Chemistry, Faculty of Sciences, Mohammed V University in Rabat, Morocco. Optics & Photonics Center, Moroccan Foundation for Advanced Science, Innovation and Research – MAScIR, Rabat, Morocco; Department of Chemistry, Faculty of Sciences, Mohammed V University in Rabat, Morocco. Optics & Photonics Center, Moroccan Foundation for Advanced Science, Innovation and Research – MAScIR, Rabat, Morocco. Department of Applied Physics, Osaka University, 2-1 Yamadaoka, Suita, Osaka, Japan; Catalonia Institute for Energy Research - IREC, Sant Adria de Besos, Barcelona, Spain; Catalonia Institute for Energy Research - IREC, Sant Adria de Besos, Barcelona, Spain

Resume : Thin film optoelectronic devices are multi-component and multi-layer complex structures composed of an elevated number of micro- and nano- layers of different materials and of the resulting heterogeneous interfaces. As such, their performance is controlled by an extremely large number of complexly intertwined variables associated to the characteristics of each layer and interface. Although it is possible to achieve partial understanding through conventional investigation approaches, it is necessary to take a step forward and develop advanced characterization strategies based on combinatorial analysis (CA) and high statistics if one truly wants to deepen into comprehending such complex devices and find the most critical parameters that control their performance. One of the main issues of CA is dealing with a very large amount of data from different characterization techniques and finding relevant correlations. In this regard, combining CA with machine learning (ML) data analysis algorithms presents a huge potential. Unfortunately, these practices are not extended among the materials research community. In this work, we provide a practical example of the potential of CA and ML applied to the investigation of defect formation and its impact on device performance in Cu2ZnGeSe4 (CZGSe) based solar cells, which are considered a promising wide bandgap evolution of kesterites that avoids Sn-related issues (volatile phases, oxidation state instability, etc.). In order to carry out the study, a combinatorial sample comprised by 196 individual solar cells (9 mm2) with different [Zn]/[Ge] ratios was synthesized. The chemical composition, optoelectronic and structural properties of each individual cell were obtained by means of XRF, I-V and multiwavelength Raman spectroscopy. Firstly, an analytical approach allowed defining the off-stoichiometric limits of formation of the CZGSe kesterite phase and the compositional range to obtain the highest efficiencies (~6%) in terms of the [Zn]/[Ge] ratio: 0.7-1.2 and 1.05-1.15, respectively. Solar cell performance was found to depend mainly on the concentration of the VCu, ZnCu and GeCu point defects estimated from the XRF and Raman scattering analysis of the devices. Then, employing an LDA-based ML algorithm, the different cells were classified according to [Zn]/[Ge], Voc and efficiency by reducing the multi-dimensional Raman data, consisting on the combination of all the multiwavelength Raman spectra for each cell, to a 2D discriminant space. The ML methodology proves to enable effective prediction of cell efficiency based only on Raman spectra, while the resulting ML discriminants show a linear correlation with VCu concentration and device efficiency, which proves the strong interconnection between VCu, Raman spectra, [Zn]/[Ge] ratio and cell performance. These results represent a powerful example of how CA can be used to unravel the critical parameters that govern the performance of complex optoelectronic devices.

15:30 Discussion    
15:30 Break    
CdTe based solar cells : Amit Munshi/Alessandro Romeo
Authors : Mike Scarpulla
Affiliations : MSE & ECE Departments, University of Utah

Resume : With the exception of amorphous materials, thin film solar cells utilize polycrystalline material layers. Thus the key defining feature of the microstructure is the presence of grain boundaries (GBs). In the bulk of CdTe-based solar cell absorber layers, GBs frequently exhibit Fermi-level pinning, associated potentials, and minority carrier recombination. The chlorine-based annealing treatment is a unique and complex step in CdTe-based solar cell processing which passivates the responsible defect states to some degree, as well as facilitating grain growth processes in some cases. I will discuss our research into the mechanisms of microstructural transformations of CdTe absorber layers and prospects for future investigations. A second processing step used commonly in research labs is etching to prepare the back surface of the absorber layer for contact formation. I will discuss our TEM-based investigations that revealed that dramatic Cd-selective etching occurs at the GBs intersecting the back contact. Further, the etching attacks CSL GBs more aggressively than random grain boundaries, and results in more enhanced recombination near the back contact. Thus additive contacting processes should be superior to those involving etching. Throughout the presentation, universalities and analogous behavior will be discussed especially with CIGSe-based solar cells.

Authors : Elisa Artegiani, Prabeesh Punathil, Vikash Kumar, Alessandro Romeo
Affiliations : Laboratory for Photovoltaics and Solid State Physics, Department of Computer Science, University of Verona, Verona, Italy.

Resume : CdTe thin film solar cells are the most successful thin film photovoltaic devices in terms of production yield. In the last few years, by changing the original solar cell structure, the efficiency was increased from around 17 % to 22 %. A CdSexTe1-x layer and a high band gap buffer layer (i.e. MgxZn1-xO) have been introduced. These allow to narrow the band gap of the absorber near the junction and to improve the absorption in the long wavelength region, respectively. CdSexTe1-x is typically formed either by the deposition of a CdSe layer at the junction subsequently mixed with CdTe or by depositing directly a CdSexTe1-x compound. We have introduced a different method where CdSexTe1-x is formed by treating a thin CdTe layer at high temperature in selenium atmosphere. The finished device is fabricated by substituting CdS with SnO2 and efficiencies exceeding 14 % with current densities exceeding 26 mA/cm2 are obtained with a low temperature substrate deposition. In this work we analyze and compare the standard ITO/ZnO/CdS/CdTe devices with the novel FTO/SnO2/CdSexTe1-x/CdTe by capacitance voltage, drive level capacitance profiling, and admittance spectroscopy made at different temperatures. The results will highlight the effect of the selenization process in terms of carrier concentration, carrier profile, comparison between deep and shallow defects, passivation. An analysis of the selenized layer properties will be presented, together with the overall performance of the completed devices.

Authors : Bérengère Frouin, Thomas Bidaud, John Moseley, Andrea Cattoni, Mahisha Amarasinghe, Mowafak Al-Jassim, Wyatt K. Metzger, Stéphane Collin
Affiliations : Centre de Nanosciences et Nanotechnologies (C2N) CNRS Université Paris-Saclay Palaiseau France;C2N-CNRS; National Renewable Energy Laboratory (NREL) Boulder USA;C2N-CNRS;NREL;NREL;NREL;C2N-CNRS

Resume : Polycrystalline Cadmium Telluride (CdTe) thin-film solar cells have a very low levelized cost of energy. However, their efficiency is still hindered by the presence of defects in grain interiors and at grain boundaries. Interestingly, it has been shown recently that the addition of Se in CdTe significantly improves the short-circuit current by bandgap lowering, but without open-circuit voltage (Voc) losses. This positive impact of Se is attributed to a passivation effect on grain boundaries and grain interiors. In this work, we investigate a series of CdSexTe1-x samples with different concentrations of Se from x=0 to x=0.4, where bevels have been prepared to perform in-depth analysis. We present high-resolution cathodoluminescence mapping performed at both room- and low-temperature on the same area. The Se concentration gradient in each layer is monitored with the luminescence peak energy, and correlated to the CL intensity. We show that the radiative efficiency depends on the Se concentration and increases by one order of magnitude from CdTe to CdSe0.4Te0.6, thus reducing the Voc deficit of CdSeTe solar cells. Low-temperature measurements at the same location provide additional insights in the defect energy and density. We will also present preliminary assessment of the internal radiative efficiency based on the calibrated CL flux and temperature-dependent measurements.

Authors : S. Vatavu(1,2), C. Rotaru(1), L. Choubrac(3), C. Antoniuc(1), Ig. Narolschi(1,2), T. Unold(3), M. Rusu(2,3)
Affiliations : (1) Physics of Semiconductors and Devices Lab, Research and Innovation Institute, Moldova State University, 60 A. Mateevici str., MD 2009, Chisinau, Moldova; (2) Faculty of Physics and Engineering, Moldova State University, 60 A. Mateevici str., MD 2009, Chisinau, Moldova; (3) Struktur und Dynamik von Energiematerialien, Helmholtz-Zentrum Berlin für Materialien und Energie, Lise-Meitner Campus, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

Resume : One of the factors determining the formation of the CdS/CdTe interface and thus affecting the performance of the solar cells is the CdS deposition technology. Electronic properties of the CdS thin films deposited by Close-Spaced Sublimation (CSS) have been investigated by Kelvin Probe (KP) and Photoelectron Yield Spectroscopy (PYS). The CdS thin films have been deposited by CSS onto TCO (AZO, SnO2)/glass substrates, varying the substrate temperature between 280-460°C and keeping the evaporation temperature at 630°C. The substrate temperature increase reveals a saturation trend of the ionization energy for CdS/AZO heterojunctions while the same parameter for the CdS/SnO2 heterojunction depending linearly. The obtained results demonstrate an influence of the substrate temperature for CdS deposition on the Fermi level position and the charge carrier concentration, respectively. The Fermi level position changes between 180 meV and 320 meV below the conduction band edge (CBM) for the CdS/AZO heterojunction while the CdS thin films on SnO2 substrates show degeneration. The KP and PYS results are correlated with the thin-film composition, structure and optical properties.

17:15 Discussion    

Symposium organizers
Alex REDINGERUniversity of Luxembourg

Scanning Probe Microscopy Laboratory - 162a, avenue de la Faïencerie - L-1511 Luxembourg

+352 4666 44 6358
Amit MUNSHIColorado State University

Department of Mechanical Engineering, 1374 Campus Delivery, Fort Collins, CO 80523, USA

+1 970 491 8861
Jiro NISHINAGANational Institute of Advanced Industrial Science and Technology (AIST)

Central 2, 1-1-1, Umezono, Tsukuba, Ibaraki, 305-8568, Japan

+81 29 861 5042
Negar NAGHAVI (Main)Centre National de Recherche Scientifique (CNRS) / Eco-Efficient Products and Process Laboratory (E2P2L)

3 rue Michel Ange, 75016 Paris, France

+33 6 79 50 90 69
Romain CARRONEmpa

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

+41 58 765 4791