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Bilateral energy conference


Advanced materials and characterization techniques for solar cells II

The symposium will focus on advances in materials science, processing, and device issues aimed to achieve a cost reduction and an efficiency increase of future solar cells. As a continuation of the last edition, this symposium will provide a large platform for discussion on key aspects of solar cell research and fabrication.



The expected huge increase in the world energy consumption linked to the Earth-scale environmental problems, and the remarkable growth of photovoltaics industry generated a great interest in the scientific community and increasing investments in research and development of materials and concepts useful for future solar cells. Although silicon is the dominant material used in solar cells today, alternative materials are needed to meet long-term clean energy economic goals. Moreover, novel concepts and device ideas can be actually manufactured using present nanotechnology. Advanced analytic tools are also a key factor for achieving efficiency improvement and reducing cost.

The focus of the symposium is on areas of material growth techniques, modeling and advanced characterizations of component materials and individual solar cells. Special emphasis will be placed on the effect of the material properties on the device efficiency with particular interest on cells manufacturing, thin films, nanostructures, phenomena at interfaces, structural defects, bulk and surface properties, carriers transport properties, light trapping, etc. A fruitful mixing of materials science, nanotechnology, device engineering, industry related issues, produced by the presence of excellent invited speakers,  solid selection of contributions, relaxed atmosphere and synergy with other symposia present at the conference, will ensure the success of this symposium. All colleagues interested in the recent progresses and future challenges of photovoltaics are invited to participate and encouraged to submit their contributions for oral and poster presentation.


Hot topics to be covered by the symposium:

  • Novel concepts for photovoltaics applications
  • Advanced synthesis and application of nanostructures for sunlight-energy conversion
  • Innovative multi-junction cells
  • Multiple exciton generation, carrier multiplication
  • Intermediate Band solar cells
  • Up- and down- converters
  • Light trapping and plasmonic effects
  • Photogenerated carriers transport and modeling
  • Si-based and thin film solar cells
  • Surface and interface issues in solar cell design
  • Innovative materials for transparent contacts
  • Advanced glass and flexible substrates
  • Manufacturing issues and In-line characterization of solar cells
  • compound thin film systems (CIGS, CdTe, etc.)
  • organic and dye-sensitized solar cell systems


Tentative list of invited speakers:

  • Harry Atwater (CalTech, USA) Light trapping in printable resonant dielectric nanosphere arrays
  • Christophe Ballif (EPFL, Switzerland) Silicon filaments for next generation Photovoltaics
  • Christiane Becker (HZB, Germany) Nanostructured thin film crystalline silicon for PV applications
  • Kylie Catchpole (Australian Natl Univ) Plasmonics and nanophotonics for photovoltaics
  • Andreas Gerber (IEK5 Forschungscentrum Julich, Germany) - Advanced large area characterization of thin film solar modules
  • David Ginley (National Renewable Energy Lab., USA) Enhanced Efficiency in Organic Solar Cells
  • Thomas Krauss (York University, UK) Engineering gratings for light trapping in photovoltaics
  • Seigo ITO (Univ. Hyogo, Japan) Inorganic printed solar cells
  • Antonio Luque (Univ. Madrid, Spain) Utmost efficiency in photovoltaics
  • Susanne Siebentritt (Luxembourg university) Kesterites: a challenging material for solar cells
  • Marko Topic (University of Ljubljana) Approaches and challenges in optical modeling and optical characterization of thin-film solar cells


Tentative list of scientific committee members:

  • Atilla Aydinly (Bilkent Univ., Turkey)
  • Isodiana Crupi (CNR-IMM, Italy)
  • Guglielmo Fortunato (CNR-IMM, Italy)
  • Blas Garrido (UB, Spain)
  • Karl-Heinz Heinig (Helmholtz Center Dresden-Rossendorf, Germany)
  • Andrej Kuznetsov (Centre for Material Science and Nanotechnology, Oslo, Norway)
  • Arne Nylandsted Larsen (University of Aarhus, Denmark)
  • Antonio Martì (Univ. Madrid, Spain)
  • Ivan Pelant (IP AVCR, Prague, Czechia)
  • Jef Poortmans (IMEC, Belgium)
  • Yosef Shacham (Tel Aviv Univ., Israel)
  • Abdelilah Slaoui (CNRS, France)
  • Baojie Yan (United Solar Ovonic, USA)
  • Margit Zacharias (Freiburg University, Germany)
  • Miro Zeman (Delft Univ., The Netherlands)





Symposium organizers:

Salvo Mirabella                                    
Via Santa Sofia, 64
Phone: +39 095 3785438
Fax: +39 095 3785243

Ivan Gordon
Kapeldreef 75, B-3001
Phone: +32 16 28 82 49
Fax: +32 16 28 15 01

Jan Valenta
Charles University
Ke Karlovu 3, Prague 2
Czech Republic
Phone: +420 2 21911272
Fax: +420 2 21911249

Raşit Turan
Dep. Phys. METU 06531
Phone: +90 312 2105069
Fax: +90 312 2105099

Harry Atwater
Pasadena, CA 91125

Start atSubject View AllNum.Add
Authors : S. Mirabella
Affiliations : CNR-IMM

Resume : introduction

Authors : Ivan Gordon
Affiliations : IMEC


10:00 Break    
Light management I : I. Gordon
Authors : Thomas Krauss
Affiliations : York University, UK

Resume : tba

Authors : M. Aeschlimann 1, T. Brixner 2,3, D. Differt 4, U. Heinzmann 4,5, M. Hensen 4, C. Kramer 2, F. Luekermann 4,5, P. Melchior 1, W. Pfeiffer 4, M. Piecuch 1, C. Schneider 1, H. Stiebig 4,5, C. Strüber 4, P. Thielen 1,6
Affiliations : 1 Fachbereich Physik and Research Center OPTIMAS, Technische Universitaet Kaiserslautern, Erwin-Schroedinger-Str. 46, 67663 Kaiserslautern, Germany; 2 Institut für Physikalische und Theoretische Chemie, Universitaet Wuerzburg, Am Hubland, 97074 Wuerzburg, Germany; 3 Roentgen Research Center for Complex Material Systems (RCCM), Universitaet Wuerzburg, Am Hubland, 97074 Wuerzburg, Germany; 4 Fakultaet für Physik, Universitaet Bielefeld, Universitaetsstr. 25, 33615 Bielefeld, Germany; 5 Institut für Innovationstransfer an der Universitaet Bielefeld, Universitaetsstr. 25, 33615 Bielefeld, Germany 6Graduate School of Excellence Materials Science in Mainz, Gottlieb-Daimler-Str. 47, 67663 Kaiserslautern, Germany

Resume : Light trapping enhances the absorption in thin-film solar cells due to an increased effective light path in the absorber. We investigate the light trapping mechanism in a-Si:H based p-i-n solar cells by means of optical spectral interferometry and coherent two-dimensional (2D) nanoscopy. By probing the electric field of the ultrashort near-infrared laser pulses scattered in the sample, the existence of photonic resonances is revealed. 2D nanoscopy combines coherent 2D spectroscopy with photoemission electron microscopy (PEEM) and enables a high spatial resolution below the optical diffraction limit [1]. In our experiments we observe hot-spot photoemission confirming strongly localized electric field distributions in the a-Si:H layer. Both techniques reveal that these photonic modes feature long coherence lifetimes. Fitting the measured 2D nanospectra with a damped Lorentzian oscillator model we reconstruct the spatially-resolved information about lifetimes, spectral shifts of localized photonic modes, and local absorbed energy density with sub-diffraction resolution. We show that these states account for the enhanced absorption in the long-wavelength cut-off region observed for nanotextured thin-film silicon solar cells. Our observations verify that light localization is a highly efficient absorption enhancement mechanism offering interesting opportunities for the design of future efficient absorber materials. [1] M. Aeschlimann, et al., Science 333, 1723 (2011).

Authors : Lucio Claudio Andreani, Angelo Bozzola, Piotr Kowalczewski, Marco Liscidini
Affiliations : Physics Department, University of Pavia, Italy

Resume : Light trapping is crucial to increase efficiency and to reduce the cost of thin-film solar cells. It is especially important to enhance light absorption at low energy, and to approach the ultimate or Lambertian limit to absorption. In this work we report a theoretical study of thin-film silicon solar cells with various types of photonic structures that are either fully ordered, or randomly rough, or with a combination of order and disorder[1,2]. Ordered photonic lattices like 1D or 2D photonic crystals allow achieving a substantial improvement of the short-circuit current, but Jsc is still far from the Lambertian limit. Photonic lattices with size or position disorder yield a further improvement of Jsc and can still be realized with lithography and etching. Randomly rough surfaces are described by a Gaussian disorder. This model describes very well the scattering properties of actual rough substrates, and optimization of the disorder parameters allows to reach the Lambertian limit for absorption. However, this requires values of the roughness that are too high for growth of good-quality material. Thus we introduce hybrid structures that combine moderate Gaussian disorder with a photonic crystal: such structures have still a Jsc close to the Lambertian limit. [1] A. Bozzola et al, Opt. Express 20, A224 (2012); Progr. Photov. Res. Appl. (2013), DOI: 10.1002/pip.2385 [2] P. Kowalczewski et al, Opt. Lett. 37, 4868 (2012); Opt. Express 21, A808 (2013).

Authors : A. Hoffmann, U.W. Paetzold, C. Zhang, K. Bittkau, U.Rau
Affiliations : IEK 5 - Photovoltaik, Forschungszentrum Jülich

Resume : Tandem thin-film silicon solar cells consist of an amorphous silicon top cell and a microcrystalline silicon bottom cell which are stacked and connected in series. To match the photocurrents of the top cell and the bottom cell, a proper light management is essential. To this end, intermediate reflectors are applied between the top and the bottom solar cells. State-of-the-art single-layer intermediate reflectors are made of low refractive index materials but show poor spectral selectivity and cause parasitic reflection losses in the external quantum efficiency of the bottom cell. We report on the design of a multilayer intermediate reflector based on aluminum doped zinc oxide and microcrystalline silicon oxide with a spectrally selective reflectance. Rigorous optical simulations are used to examine the intermediate reflector even in textured thin-film solar cells. In a subsequent step, this intermediate reflector was successfully integrated into state-of-the art tandem solar cells deposited on a rough front contact. In agreement to simulation, an improved spectral selective reflectance of incident light is realized which increases the total charge carrier generation of the tandem solar cell by 0.7 mA/cm² in comparison to the state-of-the-art single-layer intermediate reflector.

Authors : V. Strano(1), R.G.Urso (2), I. Crupi (1), F.Simone (1), E.Ciliberto (2), S.Mirabella (1)
Affiliations : (1) MATIS CNR-IMM and Dipartimento di Fisica e Astronomia Università di Catania; (2) Dipartimento di Chimica Università di Catania

Resume : Light management has become more and more important to improve energy conversion efficiency in solar cells. Efficient light management can be pursued by exploiting plasmonic nanostructures, gratings, photonic crystals, while random structures, as disordered clusters or randomly textured surface, can give advantages due to their broadband and wide-angle properties. A light diffuser top layer is widely used in thin film technology solar cells for extending the path length of incident light into the absorbing layer. Typically used front window in photovoltaic devices shows Haze in transmission (ratio of diffused over total transmitted light) below 50% at 400nm. We report on optical properties of arrays of ZnO nanorods (NRs) synthesized on transparent substrates by low temperature chemical bath deposition using a solution containing zinc nitrate hexahydrate and hexamethylenetetramine. The aspect ratio and the coverage of ZnO NRs have been modulated by changing experimental conditions including reaction temperature, molar concentration of the reagents and growth time. In order to investigate the influence of NRs morphology on the radiation diffusion, optical spectra were acquired. ZnO nanorods (90nm in diameter and 450 nm in length) show high transmittance (90%) and ultra-high optical Haze (>80%) in wavelength range from 400 to 600 nm. Our results suggest that zinc oxide NRs may represent a promising alternative for very efficient light scattering layers in solar cells.

Authors : Paola Lova 1,2, Valentina Robbiano 3, Franco Cacialli 3, Davide Comoretto 4, and Cesare Soci 2.
Affiliations : 1 Energy Institute at NTU (ERI@N), Interdisciplinary Graduate School, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore; 2 School of Physical and Mathematical Sciences, Division of Physics and Applied Physics, Nanyang technological University, 21 Nanyang Link, 637371, Singapore; 3 Department of Physics and Astronomy and London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, United Kingdom; 4 Dipartimento di Chimica e Chimica Industriale, Università degli studi di Genova, via Dodecaneso 31, 16146 Genoa, Italy

Resume : The growing demand for photovoltaic energy generation technologies has fostered research on highly absorbing, low-bandgap semiconductors to allow thinner photoactive layers with reduced cost, and on light management strategies to increase solar cell power conversion efficiency. Here we report a simple and scalable process for the production of “black GaAs”, where remarkable antireflective properties are obtained roughening the GaAs wafer surface by Metal Assisted Chemical Etching (MACE). In this process, anisotropic etching of the wafer surface along the <111> and <311> planes is mediated by metal nanoparticles. Similar to “black silicon”, where high aspect ratio nanowires arrays are used to enhance light harvesting in the photoactive layer, microstructured GaAs shows surface reflectance as low as 10% throughout the visible to near-infrared spectral range. MACE could be used in conjunction with high-efficiency, thin film GaAs solar cells to increase both photon absorption and emission1, as well as to produce high aspect ratio GaAs structures to be used as additives in hybrid organic-inorganic photovoltaic devices. 1 O. D. Miller, E. Yablonovitch and S. R. Kurtz, IEEE Journal of Photovoltaics 2 (3), 303-311 (2012).

12:30 Lunch    
Light management II : T. Krauss
Authors : T. Gregorkiewicz
Affiliations : Van der Waals - Zeeman Institute, University of Amsterdam

Resume : The most important limitation for efficiency of photovoltaic energy conversion appears due to the enormous mismatch between the broad-band character of the solar radiation and the discrete operation mode of solar devices, as determined by the specific bandgap energy of the active medium. One remedy for that would be the spectral transformation of the solar radiation before it enters a photovoltaic device. Specifically, such a “solar shaper” should split the large energy photons into smaller ones, whose conversion efficiency is optimal. In my presentation, I will discuss how shaping of the solar spectrum can be achieved using layers of Si nanocrystals with and without Er doping. In such systems efficient photon transformation can be realized by quantum cutting [1] and/or emission from Er3+ ions sensitized by Si nanocrystals [2]. I will present evaluation of external quantum efficiency of emission from thin layers of Si nanocrystals in SiO2 and its evolution upon introduction of Er3+ ions. Using differently prepared materials I will evaluate how this can be maximized. I will also discuss energy efficiency of solar shapers based on Si nanocrystals and Er3+ ions. In a separate part, I will discuss also strategies how Si nanocrystals can be employed for quantum “pasting” by which low-energy photons could be used in photovoltaic conversion by a standard Si solar cell. [1] M.T. Trinh et al., Nature Photonics 6, 316-321 (2012). [2] S. Saeed et al., under consideration

Authors : E. M. L. D. de Jong and T. Gregorkiewicz
Affiliations : Van der Waals-Zeeman Institute, University of Amsterdam, Netherlands

Resume : Si-based solar cells feature very good efficiency in the visible regime, but significant losses occur in the infrared. These low-energy photons could be used through up-conversion, taking place before they enter Si cell. In this study, we investigate how this can be achieved with Si nanocrystals (SiNCs), which we have shown to be attractive for photovoltaic applications1,2, among others. We explore how hot carriers can be generated in SiNCs by low-energy excitation making use of multi-photon absorption, Auger recombination of multiple excitons, and free carrier absorption. We use Er3+ ions to detect the hot carriers, and in that way reveal the up-conversion process. In the investigated material, SiO2 doped with SiNCs and Er3+ ions, hot carriers enable very characteristic ultrafast excitation into the 4I13/2 state responsible for the 0.8eV photoluminescence. We monitor dynamics of this emission band and from these data we extract information on hot carrier generation. The results obtained under various excitation modes corresponding to the above mentioned scenarios for up-conversion allow us to investigate their feasibility and efficiency. The investigated mechanisms will be relevant for future photovoltaics, where a more efficient use of low-energy photons will be possible. 1 M.T. Trinh et al., Nat. Photon. 6, (2012) 2 F. Priolo et al., Nat. Nanotechnol. 9, (2014)

Authors : N.X. Chung, R. Limpens, B. Bruhn, T. Gregorkiewicz
Affiliations : Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098XH Amsterdam, the Netherlands

Resume : Photon down-conversion and also efficient multiple exciton generation (MEG) can be achieved with solid-state dispersions of Si nanocrystals (SiNCs) in an SiO2 matrix. Past research has shown that in order to maximize efficiency of the photovoltaic conversion, the photon conversion energy should be around 0.8 eV. This cannot be achieved with SiNC whose bandgap always exceeds that of bulk Si. Here we report on investigations how the photon conversion at the optimal energy can be achieved with SiNCs co-doped with boron and phosphor. In this case (radiative) recombination takes place between impurity levels in the bandgap, and therefore lower energy photons are generated. We investigate the absolute external quantum yield (AQY) of this emission as function of excitation energy. Materials with different doping characteristics are used and the results are compared with those for undoped NCs of the similar size. MEGs visualized by a gradual QY increase towards higher energies is concluded, with the threshold energy marking the onset of the multiple exciton generation. We demonstrate that the threshold energy downshifts upon co-doping. Finally. we discuss the application potential of doped Si NCs for solar shaping in photovoltaics.

Authors : T.Deschamps1,2, E.Drouard1,2, A.Pereira3, R.Peretti1,2, L.Lalouat1,2, A.Guille3, M.Le Coz1,4, E.Fourmond1,4, A.Fave1,4, R. Orobtchouk1,4, B.Moine3, C.Seassal1,2,4
Affiliations : 1 Université de Lyon, INL UMR5270 CNRS-INSA-ECL-UCBL 2 Ecole Centrale de Lyon, 36 avenue Guy de Collongue, 69134 Ecully Cedex, France 3 Université de Lyon, ILM UMR5306 CNRS-UCBL, 69622 Villeurbanne, France 4 INSA de Lyon, 7 avenue jean Capelle, 69621 Villeurbanne, France

Resume : Two of the major sources of optical losses in solar cells are due to charges thermalization and to weak external quantum efficiency of solar cells at key wavelength ranges. These limitations can be theoretically circumvented by spectral shaping, meaning the adaptation of the solar spectrum to the spectral sensitivity of the cells. A frequency converter material is necessary for such application. However, to limit material consumption, thin layers have to be used, which limit the conversion yield. For this reason, we have developed ultra-compact structures combining rare-earth doped thin layer with a 2D planar photonic crystal (PhC) in order to trap photons and control their frequency conversion. Two types of conversion layers have been studied respectively for down-shifting and down-conversion: Eu3+ doped Y2O3 and Pr3+,Yb3+ doped CaYAlO4. The first one allows a conversion of photons from 400 nm to 620nm, and the second one from 450 nm to 980 nm with a quantum efficiency higher than 1. Above these converter layers, a Si3N4 thin film have been deposited and nanopatterned as a 2D PhC with specific topological parameters, in order to make the structure resonant at the absorption wavelengths of the rare-earth ions. We will introduce the design and elaboration of such PhC-assisted frequency converter structure. Results of simulations, topological characterizations, and optical measurements, which demonstrate a drastic enhancement of the conversion yield, will be discussed.

Authors : S. Saeed, K. Dohnalova, E. M. L. D. de Jong, T. Gregorkiewicz
Affiliations : Van der Waals-Zeeman Institute, University of Amsterdam, The Netherlands

Resume : Conversion of light into free electron-hole pairs is the major physical process in the fields of photodetection and photovoltaics. For high-energy photons this process leads to generation of “hot” carriers, with large excess energy. The loss of the excess energy to heat is the most important limiting factor in the way of increasing efficiencies of photo-detectors and solar cells. Extraction of this energy to the outside, is extremely challenging, in view of rapid thermalisation of hot carriers. In this contribution a new way of optical extraction of this excess energy using Er-doped SiO2 sensitized with silicon nanocrystal material is propsed, where excess energy of hot carriers created in silicon nanocrystals can be streamlined into IR photons with the help of Er3+ ions. We develop methodology and present for the first time the results on external quantum yield of Er-related photoluminescence. The efficiency of this process depends on material characteristics – concentration of Er3+ ions and Si nanocrystals, and their relative coupling. We discuss possible utilization of these findings for development of a dedicated solar spectral shaper for photovoltaics.

Authors : K. Bouras1*, J.- L. Rehspringer2, G. Schmerber2, G. Ferblantier1, S. Colis2, A. Dinia2 and A. Slaoui1
Affiliations : 1ICube, CNRS-Université de Strasbourg, 23 rue du Loess, BP 20 CR, 67037 Strasbourg Cedex 2, France 2IPCMS, CNRS-Université de Strasbourg, UMR7504, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France

Resume : Tin dioxide is one of the most attractive materials studied in the last decade due to its several applications such as optoelectronic devices, gas sensors, and solar cells. It is n- type semiconductor with a wide band gap (around 3.6 eV for bulk material at 300 K). Doping SnO2 films with rare earth (RE) elements (such as Yb, Pr, Tb, Nd) can enhance their photoluminescence properties thanks to the optical transitions involving the 4f shell of the dopant. However, the transparency and structural properties of the RE doped SnO2 films should not be affected. In this work, we have synthesized undoped SnO2 and Nd doped SnO2 (Nd:SnO2) powders and thin films by sol gel method in order to understand the insertion process of Nd in SnO2 matrix and to produce luminescent materials for photon shifting purpose. The crystallinity, structure and particles size for powder and thin films were determined by XRD measurements. We show that the tetragonal crystalline phase is achieved for the Nd:SnO2 structure at an annealing temperature of 700°C. Furthermore, a slight diffraction angle shift toward lower angles is observed with increasing Nd content, indicating Nd incorporation. No secondary phases like Nd oxide are detected. A change of optical band gap is also observed from UV-Visible spectroscopy confirming XRD results. The intensity of Raman peaks decreases gradually as the Nd content increases. SEM micrographs show that the grain size is homogenous for Nd:SnO2 powders and the surface of spin-coated films is uniform. Finally, photoluminescence measurements are carried out to check if an electronic transfer from SnO2 to Nd+3 ions is achieved

Authors : P. Morvillo, A. De Girolamo Del Mauro, R. Ricciardi, G. Nenna, R. Diana, C. Minarini
Affiliations : ENEA, P.le E. Fermi, 1, 80055 Portici (NA), Italy

Resume : In this work we improved the performance of ITO-free polymer solar cells (PSCs) by incorporating silver nanoparticles (AgNPs) in the highly conductive PEDOT:PSS anode. The AgNPs were synthetized in-situ in the PEDOT:PSS water dispersion. This anode was used to realize PSCs with the following geometry: glass/HC-PEDOT:PSS)/PBDTTT-C:[70]PCBM/Ca/Al. All the devices were characterized by UV-VIS spectroscopy, impedance spectroscopy, IV light, IV dark and quantum efficiency measurements. We made a comparative study of the electrical behavior of different PSCs in order to investigate the influence of AgNPs concentration and size. The best device reached a power conversion efficiency greater than 6%. The presence of AgNPs in the HC-PEDOT:PSS anode contributes to improve the absorption of the photoactive layer (plasmonic effect) and to lower the resistivity of the anode. The impedance spectroscopy was used to derive the circuit elements of the device in order to explain the change in the photovoltaic performances of the realized PSCs.

16:00 Break    
16:15 Poster session I : A. Terrasi and R. Turan    
Authors : S.F.U. Farhad*1, 2 , David Cherns1
Affiliations : 1School of Physics, University of Bristol, HH Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, United Kingdom. 2Industrial Physics Division, BCSIR Laboratories, Dhaka 1250; Bangladesh Council of Scientific and Industrial Research, Bangladesh.

Resume : Phase pure Cu2O is desirable as an absorber material for solar cells using ZnO electrodes because of its reported bandgap (~2.1 eV), a suitable band alignment with ZnO, and the ability to dope both n- and p-type. Pulsed laser deposition (PLD) has been used to grow Cu2O on ZnO and other substrates at low substrate temperatures and in oxygen ambient. Transmission electron microscopy and X-ray diffraction analyses showed a single phase Cu2O with (111) and (200) textures while growing at 200 C and 25 C substrate temperatures respectively. Electrical measurements, including conductivity and electrochemical Mott-Schottky measurements, showed n-type behaviour with resistivity as low as ~6x10^-3 Ω.cm, much lower than previously reported results at ~25 C without any oxygen injection in the PLD chamber. The as grown Cu2O thin films also showed a transition from highly conductive n-type to semi-insulating (~2x10^4 Ω.cm) p-type behaviour as the oxygen concentration was optimum. UV-Vis diffuse reflectance spectroscopy was used to estimate the optical bandgap of the polycrystalline thin films to be in the range 1.96 -2.08 eV. The paper will describe the growth, structural and electrical properties of the Cu2O films, and the properties of Cu2O solar cells grown on ZnO electrodes.

Authors : Sakellis I.1, Moschos I.1, Giamini S.1, Chandrinou C.1, Travlos A.1, Kim C.-Y.2, Lee J.-H.2, Kim J.-G.2 and Boukos N.1
Affiliations : 1 National Center for Scientific Research ‘Demokritos’, IAMPPNM, GR 15310 Agia Paraskevi Attikis, Athens, Greece; 2 Korea Basic Science Institute, Daejeon 305-806, Korea

Resume : A novel two step electrochemical method -promising for the synthesis of heterojunction solar cell- is reported for the fabrication of Cu2O/ZnO heterojunctions. The proposed method is of low capital cost and energy consumption and can be applied to large scale production. After ZnO nanorods were grown on seeded oxidized silicon substrates, they were employed as working electrodes for the electrodeposition of Cu2O in an aqueous formamide solution. Scanning and transmission electron microscopy as well as X-ray diffraction were employed for the study of the resulting nanostructures. It was evidenced that single crystalline Cu2O particles of micrometer size were grown on top of the -perpendicular to the substrate- ZnO nanorods. Varying the growth parameters different Cu2O morphologies can be obtained, ranging from isolated particles to a continuous layer. Finally electrical characterization of the derived films showed the formation of Cu2O/ZnO heterojunctions.

Authors : P. M. Sberna1,2, I. Crupi2, V. Privitera2, F. Simone1, M. Miritello2
Affiliations : 1 Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, Italy 2 MATIS CNR-IMM, via S. Sofia 64, 95123 Catania, Italy

Resume : Cu2O exhibits numerous features suitable for the development of novel technology for all oxide hetero-junction solar cells. It has advantages such as direct optical energy gap, high minority carrier diffusion length, high absorption coefficient in the visible region, non-toxicity and low cost production. The physical properties of this spontaneously non-stoichiometric p-type semiconductor can be properly tuned. It has been demonstrated, for instance, that opportune doping induces conductivity enhancement and energy band gap modulation. In the present work we report our results concerning the effect of nitrogen ion implantation at two different doses, 1.2 N% and 2.5 N%, into Cu2O thin films deposited by radio frequency magnetron sputtering. Post growth annealing was optimized at the temperature of 200°C under oxidizing atmosphere. X-ray diffraction analysis and spectrophotometric characterizations revealed a decrease of the lattice inter-plane distances and a narrowing of optical band gap in respect to the undoped samples. It suggests that nitrogen doping induces the introduction of oxygen vacancies. Moreover nitrogen causes one order of magnitude decrease of the resistivity with respect to that one measured in the undoped samples; this result reveals that the introduction of nitrogen is efficient to create charge carriers as donors.

Authors : M.A. Jafarov, E.F. Nasirov
Affiliations : Baku State University, Baku 1045, Z.Khalilov st.23, Azerbaijan

Resume : The intensive development of solid-state electronics stimulates the complex physical and technological investigations of the semiconductive films with a complex chemical composition. A simple and a low-cast technology high reproducibility and modification of their physical properties characterize the films obtained by method of chemical deposition from the aqueous solution. The investigation of Cd1-xZnxS films deposited from the solution showed that their high sensitivity could be realized on a wide spectral range (0.3-1.2m). The Cd1-xZnxS (0x0.6) films of 8-10m thickness were deposited on glass ceramic substrates from aqueous solution by the method described in3. We have created and investigated some properties of the thin-film solar cells with heterojunctions CuInGaSe2-Cd1-xZnxS(Se). In this case the studied heterojunctions (HJ) obtained by precipitation from solution in a single process cycle. CuInGaSe2 film deposited from an aqueous solution of CuCl2 InCl3 GaCl3 HCl. The Cd1-xZnxS films composition (0x1) was changed by partial substitution of the thiourea and was controlled by chemical, spectral and X-ray phase analyses. A part of films was subjected to heat-treatment in the air at temperatures of 400-5000C for 0-30 min. In recent years, resumed intensive studies of heterojunctions, in connection with the possibility of using them on the most economical and relatively efficient photodetectors. In this economy is determined by the structure of production technology and the value of the source material. Thus there is a substitution reaction on the surface of the films Cd1-xZnxS formed a second layer of CuInGaSe2 whose thickness is determined by the time of deposition. When light 1,45104 lux photocells studied CuInGaSe2-Cd1-xZnxS(Se)generated voltage 0,580,65V and in 0,50-0,58V, current Jsc 18  22 mA/sm2 and 15-18mA/cm2 and efficiency were = 11%. and 8%. respectively. It is established that with increasing content of Zn in the base material-circuit voltage Uos photocell increases and short-circuit current decreases. Using as a base material of solid solutions of Cd1-xZnxS and Cd1-xZnxSe causes an increase in the potential barrier at the contact. On the other hand, the discrepancy decreases the lattice constants of contacting materials, which leads to a decrease in the density of states at the interface of HJ, as well as the rate of degradation. The prospects of practical application of non-ideal heterojunction is related primarily to the more cost-effective technologies to create polycrystalline heterostructures in comparison with single crystal. One of the directions in the study of nonideal heterojunctions is the possibility of applying the criteria developed in the classical photographic sensitometry, the optical image converters into an electrical signal based on heterojunction CuInGaSe2-Cd1-xZnxS(Se). Consider the possibility of such a system in relation to the registration of optical images of different spectral composition. The maximum effect is achieved at 445 nm (absorption edge of x=0,6). More shortwave light is strongly absorbed in the base layer of cadmium sulfide, so the concentration of photoexcited holes in the vicinity of space-charge region determined by the thickness of the layer of cadmium sulfide and the diffusion length of holes in this material. AES not reach all the photogenerated holes, which leads to a decrease in short-stimulation. The sharp drop in sensitivity of the sample in the short-range due to the fact that the generated charge carriers recombine in the volume of the layer of Cd1-xZnxS, without having to reach the space charge region, absorption occurs in the surface layer of Cd1-xZnxS. Decline of sensitivity in the wavelength region shows the decrease of the coefficient of HJ Cd1-xZnxS- CuInGaSe2 the presence of the impurity centers in Cd1-xZnxS, involved in the processes of generation of carriers. To increase the sensitivity should either reduce the thickness of the base layer, or create an optical image from a thin layer of CuInGaSe2 layer. Thickness reduction of the CuInGaSe2absorber layers would not only be useful to reduce the material cost in the production process but could also lead to better solar cell properties by reducing recombination losses in the bulk. However, control of the film growth and recrystalsation due to post-deposition treatment is necessary to obtain thin films which are compact and free of pinholes. Employing the CSS technique; this is the best technique of deposition the CuInGaSe2 layer as it provides high rates and good film quality while the material yield is very high. CSS- CuInGaSe2 films can consist of large grains being useful for the electronic transport properties. However, the actual film thickness needed may be larger than the absorption depth due to the possibility of increased shunting in thinner films. Thus, considering all these consequences the CuInGaSe2 layer thickness was selected at 1 μm for this investigation. However there are possibilities of increasing SR and cell conversion efficiency further more if the ZnxCd1-xS window layer thickness (100 nm) can be reduced with proper buffer layer insertion in between front contact (ITO) and ZnxCd1-xS layer. When the ZnxCd1-xS layer thickness was reduced, the absorption loss in the blue region due to thin ZnxCd1-xS layer also reduces, which improves mainly Jsc and consequently the cell conversion efficiency. As a result, cell conversion efficiency was increased with reduced ZnxCd1-xS layer. The SR of ZnxCd1-xS layer thickness variation from 20 nm to 0.2 μm was investigated through AMPS simulation and it was noticed when the wavelength was in between 400-540 nm the quantum efficiency (QE) is much affected with the increasing ZnxCd1-xS layer thickness which influences the cell conversion efficiency. However, in considering fabrication challenges and to fabricate good quality cells we have selected the ZnxCd1-xS film thickness of 60 nm with conversion efficiency of 12%. The improvement in conversion efficiency was achieved due to the improvement of Jsc with reduced ZnxCd1-xS layer. However, this reduced ZnxCd1-xS layer might allow forward leakage current to front contact through possible pinholes of ultra thin ZnxCd1-xS layer although the pinholes is greatly reduced using ZnxCd1-xS window layer instead of CdS in our proposed solar cell. In order to prevent this unwanted forward leakage current a high resistive buffer layer of suitable material must be inserted in between front contact and ZnxCd1-xS layer. Further numerical analysis was done aiming to improve the conversion efficiency of the ultra thin CuInGaSe2-Cd1-xZnxS(Se) cell by inserting a suitable buffer layer. It is possible to take advantage of the different properties of two TCOs by forming a buffer layer. High-efficiency CIGS devices are generally fabricated with such buffer layer structures. The modified proposed cell structure consists of a highly conducting layer (ITO) for low-resistance due to contact and lateral current collection and a much thinner high-resistivity layer (called buffer layer) of a suitable material. By incorporating a very thin resistive buffer layer, the ZnxCd1-xS layer thickness can be reduced to 60 nm, which significantly improves the blue response, ZnxCd1-xS film morphology and conversion efficiency of the CuInGaSe2- Cd1-xZnxS(Se) devices.

Authors : Vanessa L. Pool, M. Imteyaz Ahmad, C. Jackson Stolle, Taylor B. Harvey, Brian A. Korgel, Michael F. Toney
Affiliations : 1 The University of Texas at Austin, McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science, Austin, TX 2SSRL, Materials Science Division, SLAC National Accelerator Laboratory, Menlo Park, CA

Resume : Cu(In,Ga)(S,Se)2 (CIGS) is a promising absorber material for photovoltaic devices. Vacuum based processing of CIGS films has yielded solar cells having efficiencies greater than 20%, however, the vapor deposition technique is not a cost or energy effective option. High throughput solution based processing has been shown to be cost effective and can deposit films of uniform thicknesses over relatively large areas. This solution-based processing involves deposition of films using a suspension of CIGS nanoparticles. In order to achieve greater efficiency, the nanoparticles must be sintered to micron-sized grains without leaving any inter-granular impurities. Efficiencies up to 12% have been achieved by sintering at 500 ⁰C in a selenium rich atmosphere, and the processing plays an important role in achieving higher-efficiency devices. However, the effect of these processing parameters on the phase evolution and grain growth kinetics is scarcely understood, limiting progress in achieving higher efficiencies. In-situ characterization of CIGS film formation using x-ray diffraction and florescence with a time resolution up to 100 ms allows for a deeper insight into the reaction pathways during the thermal processing. Our in-situ measurements of the grain growth and phase evolution of CIGS nanoparticle films, deposited on molybdenum coated glass substrates, will be presented.

Authors : Tetsuhito Okamoto, Hironori Komaki, Junji Sasano, Shigeru Niki, Masanobu Izaki
Affiliations : Toyohashi University of Technology, National Institute of Advanced Industrial Science and Technology (AIST)

Resume : Cu(In,Ga)Se2 (CIGS) solar cell is a thin film solar cell with high efficiency and low cost. The buffer layer in the CIGS cell plays a role to suppress the recombination loss, and the Zn(O,S) has been used as the buffer layer alternative to the CdS because of the environmental aspect and wide bandgap energy. In this study, the chemical bath deposition (CBD) method used for fabricating the Zn(O,S) buffer layer was optimized based on the structural characteristics. Zn(O,S) buffer layers were formed on CIGS/Mo/SLG substrate by CBD process using the aqueous solution containing ammonia water, zinc nitrate, and thiourea, and then the layer was immersed in ammonia water. The CIGS solar cells were constructed by stacking the ZnO and Al layers, and the photovoltaic performance was estimated under AM1.5 G illumination. The Zn(O,S) layer was constructed by the upper zinc hydroxide and lower crystalline Zn(O,S) layers, and the zinc hydroxide was removed by post ammonia water immersion, resulting in the improvement of the conversion efficiency from 6.8 to 13.7%. The optimization of the deposition time gave the further improvement of the efficiency from 13.7 to 15.5%. This is considered to be due to the improvement of homogeneity and film thickness of the buffer layers. This result shows the importance of the structural characteristics and open new door to improve the efficiency of CIGS solar cell.

Authors : M. Theelen, H. Steijvers, Z. Vroon, N. Barreau, M. Zeman
Affiliations : M. Theelen1,2,3; H. Steijvers1; Z. Vroon1; N. Barreau4; M. Zeman2 1 TNO, dept. Thin Film Technology - De Rondom 1, 5612 AP, Eindhoven, The Netherlands 2 Delft University of Technology, Photovoltaic Materials and Devices, Mekelweg 4, 2628 CD Delft, The Netherlands 3 Materials innovation institute (M2i), Mekelweg 2 2628 CD DELFT, The Netherlands 4 Institut des Matériaux Jean Rouxel (IMN)—UMR 6502, Université de Nantes, CNRS, 2 rue de la Houssinière B.P. 32229, 44322 Nantes Cedex 3 France

Resume : The reliability of thin film Copper Indium Gallium Selenide (CIGS) solar cells is currently tested according to IEC standards, which do not give a complete picture of the degradation mechanisms. We have designed and built an ‘hybrid’ degradation setup, in which humidity, temperature and illumination are all used as loads in order to accelerate degradation of CIGS. The setup consists of a climate chamber, which can vary the temperature and humidity. Furthermore, an area of 80x80 cm2 is illuminated, which allows both the study of light induced degradation and the in-situ measurement of the IV curve of the cell during the test, so the degradation behaviour can be observed in time. The IV output is automatically logged and can be converted to current, voltage and resistance characteristics. The 40x40 cm2 in the center of the illuminated area was calibrated BAA according to IEC norm 60904-9. Twelve cells or minimodules can be degraded and measured in-situ at the same time. Extra external loads can be applied to make sure that the cells operate at the maximal powerpoint to optimally simulate field operation. The continuous in-situ IV measurements allowed us, for example, to observe reversible degradation of the Voc and the Rsh, which would not be visible in regular damp heat tests. It also allowed us to very accurately determine the temperature dependency and the temperature hysteresis of our cells by slow heating and cooling.

Authors : David A Keller, Klimentiy Shimanovich, Hannah-Noa Barad, Assaf Y Anderson, Yaniv Bouhadana, Adam Ginsburg, Eli Rosh-Hodesh, Koushik Majhi, David Sriker, Sven Rühle, Arie Zaban
Affiliations : Bar-Ilan University

Resume : All-oxide photovoltaic devices are a new and emerging type of solar cells. The all-oxide solar cells are based solely on semiconducting metal oxides that act as the absorber, the electron conductor, and as selective back contacts. Fe2O3 has been extensively investigated for its photocatalytic properties as it is highly abundant and has a high thermodynamic stability. Most of the research on Fe2O3 has focused on doping Fe2O3 nano-particles and thin films with various dopants, since Fe2O3 has poor electrical properties, such as very high resistivity and very short charge carrier diffusion lengths. These properties have prevented wide exploration of Fe2O3 as a light-absorber for photovoltaic applications. In order to overcome these obstacles, a combinatorial material science approach was selected. A thin film, between 100-300 nm thick, with continuous compositional spread of Fe-Nb-O was created using Fe2O3 and Nb2O5 as starting materials, and deposited by pulsed laser deposition (PLD). Electrical, optical, and structural high-throughput measurements were performed in order to characterize the different mixing ratios between Fe, Nb, and O. These measurements were conducted using home built scanners such as sheet resistance, optical spectroscopy, etc. Specific mixing ratios between Fe, Nb, and O showed much lower resistivity than the pure Fe2O3 or Nb2O5. Further improvement in Fe2O3 properties will allow its future use as the main light absorber in all-oxide PV devices.

Authors : Kuo-Chin Wang, Po-Shen, Shena, Peter Chen
Affiliations : Department of Photonics, National Cheng Kung University, Tainan, Taiwan 701

Resume : We present a new paradigm for organometallic hybrid perovskite solar cell using NiO inorganic metal oxide nanocrystalline as electrode material and realized the first mesoscopic NiO/perovskite/PCBM heterojunction photovoltaic device. The photo-induced absorption spectroscopy results verified that the architecture is an effective sensitized junction which is the first inorganic p-type metal oxide contact material for perovskite-based solar cell. Power conversion efficiency of 9.51 % was achieved under AM 1.5 G illumination, which significantly surpassed the reported conventional p-type sensitization dye solar cells. The replacement of organic hole transport materials by metal oxide is advantageous for providing robust device architecture and the development of fully inorganic perovskite-based thin film solar cells. We believe this significant advance will stimulate further contributions for novel configurations that could facilitate the future design of low-cost fully inorganic thin film and tandem photovoltaics.

Authors : Artem A. Bakulin, Huib J. Bakker, Zhenhua Sun, Zhuoying Chen
Affiliations : FOM institute AMOLF, Science Park 104, Amsterdam 1098 XG, The Netherlands; Laboratoire de Physique et d'Etude des Matériaux, ESPCI/CNRS/UPMC UMR 8213, 10 rue Vauquelin, 75005 Paris, France;

Resume : Solution-processed organic-inorganic hybrid materials like colloidal quantum dots (QD) or perovskites hold promise for cost-effective thin-film solar cells. The carrier dynamics in these materials is determined by the conduction and valence band structure, distribution of trapping states and the pathways for carrier relaxation. In current contribution, we apply a set of novel ultrafast electro-optical techniques, including Vis-pump – IR-probe, pump-push photocurrent, and 3-pulse transient absorption spectroscopies to elucidate the carrier relaxation and trapping dynamics in PbS QD and methylammonium lead halide perovskite films. The material properties were controlled by the QD ligand-exchange and perovskite surface passivation. We show that the carrier relaxation in the studied systems can occur on a variety of timescales form 200 fs to longer than 10 ps. The particular charge relaxation and recombination rates strongly depend on the presence of defect states and the strength of electron-hole coupling. In QD photovoltaic device we observed that some photogenerated electron-hole pairs are intrinsically weakly bound. At the same time, charge diffusion in the QD films leads to substantial charge trapping on the ~ns timescale. Our study provides important information for the development and implementation of hot-electron extraction and carrier multiplication in third-generation photovoltaic devices.

Authors : W. El Huni [1], A. Migan [1.2], Z. Djebbour [1.3], S. Sundaram [4], K. Pantzas [4.5], J-P. Salvestrini [6.7], P.L. Voss [4.5], A. Ougazzaden [4.5]
Affiliations : [1]LGEP, UMR8507, CNRS, Supélec, U. Paris-sud 11, UPMC, 11 Rue Joliot-Curie, 91192 Gif-sur-Yvette cedex, France; [2]Université Pierre et Marie Curie, 4 Place Jussieu, 75005 Paris, France; [3]Département des Sciences Physiques, UVSQ, 45 Avenue des États-Unis, 78035 Versailles, France; [4]CNRS, UMI2958 Georgiatech-CNRS, Metz, France; [5]Georgia Institute of Technology, 2–3 Rue Marconi, 57070 Metz, France; [6]Université de Lorraine, LMOPS, EA4423, 2 Rue E. Belin, 57070 Metz, France; [7]Supélec, LMOPS, EA4423, 2 Rue E. Belin, 57070 Metz, France

Resume : InxGa1−xN alloys have recently emerged as promising solar cell materials due to their direct band gaps which span almost the whole solar spectrum while exhibiting stong optical absorption of approximately 10e5 cm−1 near the band edge [1–3]. Several challenges still limit the performance of these materials. One of them is the ability to systematically grow thick single-phase epitaxial InGaN layers [4]. Indeed, in order to absorb most of incident light, a relatively thick InGaN layer of approximately 200nm is required. One proposed solution is the semibulk structure, which is obtained by adding periodically ultra-thin layers of GaN [4]. Using Silvaco-Atlas software, we have modeled InGaN-based solar cells with graded junctions (GaN-p/InGaN-i/GaN-n) and (GaN-n/InGaN-i/GaN-p) by changing the nature of the intrinsic layer (i.e. bulk or semibulk InGaN) and taking into account polarization effects at heterointerfaces and in graded layers. We have shown that solar cell performance is not globally affected by semibulk structure for designs for high indium content. This shows that the experimentally demonstrated high quality of the semibulk InGaN can be compatible with good cell performance. [1] R. Dahal et al., Appl. Phys. Lett. 94 (2009) 063505-1. [2] C.J. Neufeld et al., Appl. Phys. Lett. 93 (2008) 143502-1. [3] M.J. Jeng et al., J. Phys. D Appl. Phys. 42 (2009) 105101. [4] K. Pantzas et al., Journal of Crystal Growth 370 (2013) 57–62

Authors : Anna Mukhtarova, Sirona Valdueza-Felip, Louis Grenet, Catherine Bougerol, Christophe Durand, Eva Monroy, Joel Eymery
Affiliations : CEA-Grenoble, INAC/SP2M/NPSC, 17 rue des Martyrs, 38054 Grenoble, France CEA, LITEN, 17 rue des Martyrs, 38054 Grenoble, France CNRS-Institut Néel, 25 rue des Martyrs, 38042 Grenoble, France

Resume : We report on the structural and photovoltaic characteristics of solar cells based on 15 and 30 InxGa1-xN/GaN (x = 0.10 and 0.19) multi-quantum wells (MQWs) grown by metalorganic vapor phase epitaxy on sapphire substrates. In-content and strain state were calculated by the combination of x-ray diffraction and transmission electron microscopy. The QWs are coherently strained on the GaN buffer layer for all samples. Doubling the number of MQWs enhances the peak external quantum efficiency by a factor of 2 for both In contents. Higher In contents leads to a 20-fold increase of the overall conversion efficiency. Best results are obtained for solar cells with 19% of In, which exhibit an open circuit voltage of 1.7 V, a short circuit current density of 3.00 mA/cm2, and a fill factor of 39.3% under 1 sun of AM1.5G illumination, leading to a conversion efficiency of 2.00%. These results are obtained for devices without antireflection coating or surface/backside treatment, for the spectral cutoff wavelength at 465 nm, making them promising for hybrid integration with non-III-nitride photovoltaic devices.

Authors : Hasan Huseyin Gullu 1-3, Emre Coskun 1-2-3, Ozge Bayrakli 1-3, Idris Candan 1-3, Mehmet Parlak 1-3
Affiliations : 1-Department of Physics, Middle East Technical University (METU), Ankara 06800, Turkey 2-Department of Physics, Çanakkale Onsekiz Mart University, Çanakkale 17100 Turkey 3-Center for Solar Energy Research and Applications (GÜNAM), METU, Ankara 06800, Turkey

Resume : In this study, the CZST thin films were prepared by sequential-sputtering from compound targets of Cu, SnTe and ZnTe in an argon atmosphere by using three magnetron DC/RF sputtering system. During the deposition process, the substrate temperature was kept at about 200°C. The composition of the films was determined by EDXA and the percentage of atomic ratios were found as about Cu:Zn:Sn:Te = 13:10:21:56 %. XRD measurements of the thin films showed the main orientation along (112) plane direction. All of the as-grown films were in polycrystalline structure, with the mixed phases of CZST, ZnTe and SnTe. The thicknesses of the films were found as about 210 nm with ±1 nm uncertainty. Optical measurements showed that the thin films have direct energy gap of about 1.5 eV. The measurement of the electrical properties by the four-point probe method at room temperature showed that it had a sheet resistance of about 106 Ω/sq. Moreover, all films were determined to be p-type by Hall-effect measurements with ~2.3 x 1018 cm-3 carrier concentration and ~18.5 cm2/V.s mobility. A detailed device characterization of the Ag/n-Si/p-CZST/In sandwich structure was performed as a function of sample temperature. The temperature dependent current-voltage (I-V) characteristics of the sample were studied to determine the possible conduction mechanisms and device parameters. They showed a good diode behavior with about 2 order rectification factor. The ideality factor n and the barrier height values were determined. The spectral photo-response measurements were also performed in order to see the effects and contributions of n-Si and p-CZST in the device structure.

Authors : Yu Bi , Arjan J. Houtepen, Gilles Dennler, Tom J. Savenije.
Affiliations : Yu Bi, Arjan J. Houtepen, and Tom J. Savenije Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, the Netherlands; Gilles Dennler, IMRA Europe, 220 rue Albert Caquot BP 213, 06904 Sophia Antipolis, France.

Resume : Iron pyrite is a promising solar cell material due to its suitable bandgap (0.95eV) and high absorption coefficient (105 cm-1). In addition, it is nontoxic, cheap and abundant1. However, the highest efficiency (2.8%) reported is based on the photoelectrochemical cell in the 1980s2. The limiting factor is the high dark current, which results in small open circuit voltages of less than 0.2V. This low open voltage is attributed to trace impurities of marcasite or iron monosulfide phase, or to a large density of surface states. Previously we reported on the surfactant (TOPO) assisted hot injection method to fabricate phase pure, air stable FeS2 nanocubes of 60 to 200 nm3. Here, drop-casted films of these capped particles are investigated by the time resolved microwave conductivity technique (TRMC) for the first time to explore the charge carrier dynamics. Surprisingly, long-lived photo-excited charge carriers are observed, however with low mobilities, indicating trapping is involved. Additional femtosecond transient absorption measurements are presently carried out to confirm the fast trapping events. 1. Wadia, C.; et al. Environ Sci Technol 2009, 43 (6), 2072-2077. 2. Ennaoui, A.; et al. Sol Energ Mat Sol C 1993, 29 (4), 289-370. 3. Bi, Y.; et al Nano Lett 2011, 11 (11), 4953-4957.

Authors : Kuang-I Lin,1,* Kuo-Lung Lin,2 Bo-Wei Wang,3 and Hao-Hsiung Lin3
Affiliations : 1Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan; 2Department of Electrical Engineering, National Chung Hsing University, Taichung, Taiwan; 3Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan; Funding: NSC102-2112-M-006-018 and NSC102-2221-E-002-191-MY3

Resume : The performance of a 1 eV GaAsSbN-based photovoltaic cell has been reported [1]. Recently, an improvement in the photocurrent production and open-circuit voltage has been observed in GaAsSbN intermediate-band solar cells [2]. A double-band anticrossing (DBAC) model is introduced to describe the influence of N and Sb incorporation on the band structure of GaAsSbN alloys. However, the values of the EN level and the interaction potential are still disputable matters. Recently, the suitability of photomodulated reflectance (PR) as a powerful diagnostic tool for advanced photovoltaic structures has been demonstrated [3]. In this work, PR is utilized to study as-grown GaAsSbN alloys from 0.75 to 2.4 eV. In addition to the band gap and the spin-orbit splitting, the transition of the VB maximum to the N-induced upward CB, for the first time, is demonstrated and analyzed using the DBAC model. The EN level with respect to the GaAs VB maximum and the interaction potential are precisely determined as 1.540 and 2.839 eV, respectively, for the as-grown GaAsSbN alloys. The result of the DBAC model is helpful information for the determination of the band offset between GaAsSbN and other semiconductors and on the design of GaAsSbN-based devices, e.g., intermediate-band solar cells. 1. K. H. Tan et al., J. Cryst. Growth 335 (2011) 66; 2. N. Ahsan et al., IEEE J. Photovolt. 3 (2013) 730; 3. D. Fuertes Marrón et al., Mater. Sci. Eng. B 178 (2013) 599.

Authors : M. Emziane
Affiliations : Solar Energy Materials and Devices Laboratory Masdar Institute of Science and Technology Masdar City, PO Box 54224, Abu Dhabi, UAE.

Resume : We report on new tandem device designs based on different semiconductor materials suitable for the top and bottom sub-cells. In addition to the extended spectral coverage leading to more photons being converted, three and four-terminal device configurations were considered in order to avoid the current matching and the associated tunnel junctions between the two sub-cells. A comprehensive modeling analysis is presented where device structures were designed and optimized, and the behavior of the sub-cells studied. Optimal cell characteristics were obtained with the quantum efficiency. The applications of these devices were assessed for PV, TPV and CPV applications and the output parameters were predicted as a function of the device simulated operating conditions. Keywords: Tandem solar cells, device design, three terminals, four terminals, CPV, TPV.

Authors : E. Dumiszewska (1), P. Knyps (1), M. Wesolowski (1), A. Krajewska (1), W. Strupinski (1), J. Kalbarczyk (1)
Affiliations : (1) Institue of Electronic Materials Technology, Warsaw, Poland

Resume : Epitaxial lift-off (ELO) is a process which enables the removal of solar cell structures (one junction GaAs, two junction GaAs/InGaP or three junction GaAs/InGaAs/InGaP) from the substrate on which they are grown and their transfer onto lightweight carriers such as metal or polymeric insulator films. Such solar cells exhibit superior power conversion efficiency compared with alternative single-junction photovoltaic cell designs such as those based on crystalline Si, copper indium gallium sulfide (CIGS) or CdTe. The major advantage of ELO solar cells is the potential for wafer reuse, which can enable significant manufacturing cost reduction by minimizing the consumption of expensive wafers. Here in this work we have grown one junction GaAs solar cells on GaAs (100) substrates. A 10 nm thick AlAs layer has been used as a release layer, which has been selectively etched in HF solution. We have investigated different methods of transferring thin films onto polymer and copper foils, including the usage of temporary mounting adhesives and electro-conductive pastes. We have also analyzed techniques for reusing GaAs substrates and their repolishing aimed at obtaining the epi-ready quality wafers to be used in next epitaxial growths. We have achieved a thin film solar cell without any defects and cracks on 3-inch substrates. Lift-off has been demonstrated to be a very promising technique for producing affordable solar cells with a very high efficiency of up to 30%.

Authors : André Luis F. Cauduro, Michal Radziwon, Horst-Günter Rubahn, and Morten Madsen.
Affiliations : NanoSYD, Mads Clausen Institute, University of Southern Denmark, Alsion 2, 6400-Sønderborg, Denmark.

Resume : Modern organic photovoltaic (OPV) devices have attracted great interest over the past two decades due to their appealing features such as mechanical flexibility and light-weight, combined with a low manufacturing cost. In the strive for achieving higher power conversion efficiencies, one utterly important task is to control the morphology of the active layer, in order to ensure both efficient exciton dissociation and carrier transport in the devices. In this work, the effect from different morphologies of alpha-sexithiophene (a-6T) thin-films in a-6T/C60 bilayer solar cells is investigated and correlated to the device performances. The a-6T/C60 solar cells are fabricated from organic molecular beam deposition, using Molybdenum Oxide (MoOx) as a hole transport layer in the devices. The role of the MoOx layer in the solar cells is first investigated and optimized in terms of transport properties by fabrication of hole-only devices, with the MoOx layers being deposited by both evaporation under ultra-high vacuum and RF sputtering. By control of the substrate temperature during growth of the a-6T donor layer, a variety of a-6T nanostructures are obtained, which are subsequently implemented in the bilayer devices. The morphology of the a-6T layer is correlated to the performances of the developed devices in order to optimize their power conversion efficiencies.

Authors : Yoon-Jung Na, Oh Young Kim, Seok-Ho Hwang*
Affiliations : Department of Polymer Science & Engineering, Dankook University

Resume : It is well established that polymer absorbers containing a long, planar, π-conjugated backbone can offer broad overlap of their absorption with the solar spectrum and often demonstrate high charge-carrier mobility due to π-π stacking interactions. In this presentation, we designed a series of semi-random conjugated copolymers composed of a benzothiadiazole and 2,6-dibromo-4,8-bis(2-ethylhexyloxy) benzo[1,2-b:4,5-b']dithiophene. The carbazole moiety in this study is used as bridge molecule to combine two different donor and acceptor units. First of all, 3-bromo-9H-carbazole is coupled with 4,7-dibromobenzo[c][1,2,5]thiadiazole via Ullmann reaction in the presence of copper iodide and K2CO3 to form the 3-bromo-9-(4-(3-bromo-9H-carbazol-9-yl)benzo[c][1,2,5]thiadiazol-7-yl)-9H-carbazole (A). And then boronic ester were introduced to the A by Miyaura borylation reaction with bis(pinacolate)diboron to obtain 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-(4-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazol-9-yl)benzo[c][1,2,5]thiadiazol-7-yl)-9H-carbazole. The copolymerization of two different monomers was conducted by Suzuki-Miyaura cross-coupling reaction in the presence of Pd(PPh3)4 as a catalyst. Solar cells were produced from blends of synthesized copolymer with the fullerene acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). In the following, the synthesis and optical, electrochemical and morphological properties are presented and the performance of organic solar cell devices under an illumination of AM1.5G, 100 mW cm−2 will be discussed.

Authors : Amy L. Pitman1, I.S. Zhidkov2,3, A.I. Kukharenko2,3, A. Kucherov2,3, E.Z. Kurmaev3, A. Moewes1, S. Achilleas4, S.A. Choulis4 and S.O. Cholakh3
Affiliations : 1Department of Physics and Engineering Physics, University of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan, Canada, S7N 5E2; 2Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620990 Yekaterinburg, Russia; 3Ural Federal University, 19 Mira Str., 620002 Yekaterinburg, Russia; 4Department of Mechanical Engineering and Material Science and Engineering, Cyprus University of Technology, 3603 Limassol, Cyprus

Resume : The results of measurements of X-ray emission (XES), X-ray absorption (XAS) and X-ray photoelectron spectra (XPS) of donor (P3HT), acceptor (C[60]BM) compounds and P3HT/C[60]BM interfaces are presented. The XES spectra are measured at Beamline 8.0.1 at the Advanced Light Source, while the XAS spectra are measured at the SGM beamline at the Canadian Light Source. XPS measurements are studied with help of PHI XPS Versaprobe 5000 spectrometer (ULVAC-Physical Electronics) equipped by C60+ cluster source ion gun specially designed for surface cleaning of organic compounds. Basing on XPS and XES measurements of valence bands and carbon C 1s XAS which probe the occupied and vacant electronic states, respectively, the HOMO-LUMO gaps are determined for donor (P3HT) and acceptor (C[60]BM) compounds and P3HT/C[60]BM interfaces. It is found that HOMO-LUMO gap of isolated P3HT is increased by linking with PCBM in P3HT/PBCM interfaces in accordance with electronic structure calculations [1-2]. This work is supported by Russian Foundation for Basic Research (Project 14-08-31088). 1. David P. McMahon, David L. Cheung, and Alessandro Troisi, J. Phys. Chem. Lett. 2 (2011) 2737. 2. Xiaoyin Xie, Heongkyu Ju and Eun-Cheol Lee, Journal of the Korean Physical Society, 57 (2010), 144.

Authors : Goszczak A. J., Rubahn H.G., Madsen M.
Affiliations : Mads Clausen Institute, University of Southern Denmark, Alsion 2, S?nderborg, Denmark

Resume : Organic solar cells (OSC?s) have attracted much attention in the past years due to their potential low-cost, light-weight and mechanical flexibility. A method for improving the power conversion efficiencies of the devices is by incorporating structured electrodes in the solar cell architecture, as they can improve light absorption in the active layers of the devices. In this direction, a cheap and large-scale compatible method for structuring the electrodes in OSC?s is by the use of Anodic Alumina Oxide (AAO) membranes. In this work, high purity Al films are formed via sputter deposition and anodized to form nano-scale pores of controlled sizes. The Al deposition conditions are regulated in order to control the roughness and the grain size of the Al layer, as it critically affects the subsequent pore formation during the anodization process. The anodization of the prepared samples occurs in an electrochemical cell in H2SO4, H2C2O4 and H3PO4 solutions, in order to tune the AAO pore diameter and interpore distance. Subsequently, the fabricated AAO is selectively etched in H2CrO4/H3PO4 mixtures, in order to reveal the underlying Al nanoscale dimples, which are present at the bottom of the pores. These dimples are then employed to fabricate nanostructured electrodes in P3HT/PCBM organic solar cells. The impact from different dimple dimensions on the light absorption in the active layer is investigated to optimize the efficiency of the solar cells.

Authors : Marco Seeland1, Christian Kaestner1, Daniel A. M. Egbe2, Harald Hoppe1
Affiliations : 1: Institute of Physics, Technische Universität Ilmenau, Langewiesener Str. 22, D-98693 Ilmenau, Germany; 2: Linz Institute for Organic Solar Cells, Johannes Kepler University Linz, A-4040 Linz, Austria

Resume : Luminescence imaging has evolved to a versatile characterization method for studying the laterally resolved behavior of polymer solar cells. Especially in degradation studies the use of luminescence imaging is beneficial as it is non-invasive and offers short measurement times. However, except for the correction of the active area in degradation studies the data analysis so far is mainly qualitative, i.e. interpretation of the measured luminescence image by comparison with other techniques. In a first step we introduced a quantitative analysis for calculating the current-voltage characteristics of the active layer as well as for determination of the involved resistances in an equivalent circuit network model. However, this analysis was restricted to laterally homogeneous solar cells. In this work we present a quantitative analysis of electroluminescence images of laterally inhomogeneous polymer solar cells. By decoupling the local equivalent circuit parameters within an iteration procedure this analysis allows calculation of the local current flow through and the local voltage applied to the active layer. Furthermore quantitative images of the local series resistance and the saturation current-density are achieved. Applied to polymer solar cells based on an anthracene containing PPE-PPV alternating copolymer blended with PCBM the local saturation current-density contrast was found to correlate perfectly with the strong lateral phase separation occurring in these devices. Further analysis of the lateral difference in the saturation current-densities delivers information on the thermal activation of charge carriers at the donor/acceptor-interface and in the phase separated bulk.

Authors : G. Salvinelli1, G. Drera1, A. Braga23, C. Baratto3, L. Sangaletti1
Affiliations : 1 I-LAMP and Dipartimento di Matematica e Fisica, Università Cattolica del Sacro Cuore, Via dei Musei 41, 25121, Brescia, Italy; 2 Nanochemistry Department, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy; 3 CNR-IDASC SENSOR Lab and Department of Information Engineering, Brescia University, Via Valotti 9, 25131 Brescia, Italy;

Resume : This work is focused on an Angle-Resolved X-ray Photoelectron Spectroscopy (AR-XPS) analysis of the interface between a novel transparent conductive oxide (TCO) – i.e. an amorphous non-stoichiometric cadmium-tin oxide (CSO) [1] – and an ultra-thin (1 to 4.5 nm) titanium dioxide (TO) layer. These heterojunctions constitute the front contact of dye-sensitized solar cells (DSC). In particular, TO acts as a blocking layer (BL) which is generally accepted to increase the overall efficiency of DSCs by preventing the electron recombination at the electrolyte/TCO interface. Nevertheless, the BL role is still matter of debate as the best chemical composition, crystal phase, thickness and deposition method have not yet been univocally assessed. This investigation reveals a stoichiometry gradient and a cation interdiffusion at the CSO/TO interface. The valence/conduction band offsets are measured and the band diagram at the interfaces is evaluated in order to interpret the overall DSC efficiency gains in relation with the increasing BL thickness [2]. [1] C. Baratto, et al., Thin Solid Films, 520 (2012) 2739. [2] A. Braga, et al., Phys. Chem. Chem. Phys., 15 (2013) 16812.

Authors : G. Sarau1, B. Hoffmann1, P. Klement2, S. Geissendoerfer2, S. Christiansen1
Affiliations : 1. Max Planck Institute for the Science of Light, Günther-Scharowsky-Str. 1, 91058 Erlangen, Germany; 2. NEXT ENERGY, EWE Research Centre for Energy Technology, Carl-von-Ossietzky-Str. 15, 26129 Oldenburg, Germany

Resume : The main challenge to a detailed in-depth characterization of thin films for solar cells is their intrinsic small thickness up to a few micrometers. For this purpose, two methods are used to virtually increase the thickness and thus the number of data points: mechanical polishing or chemical etching to form wedges along the entire film depth or successive surface removal and probing of the remaining planar film. Here, we employed the focused gallium ion beam (FIB) technique in a SEM to prepare wedges in a-Si:H/µc-Si:H by PECVD and mc-Si by laser crystallization and solid phase epitaxy (LC-SPC) thin film solar cells at selected positions without damaging and contaminating the entire sample (not provided by the other approaches). The implantation of Ga atoms and the local heating that may lead to in-depth mechanical damage and amorphization of the thin films was found negligible when using low ion energy and current values of 10 keV and 100 pA, respectively. This was confirmed on 4o wedges with widths between 10–140 µm and stretching factors up to 14.3 by the micro-Raman mapping of the crystallinity in the a-Si:H/µc-Si:H cells and by the EBSD mapping of the grain structure in LC-SPC cells. Furthermore, we were able to study the interface between the layers themselves and the substrate. Our work opens the way towards advanced depth-resolved spectroscopy and microscopy investigations of bulk and interface properties of thin film solar materials at pre-characterized positions.

Authors : Teimuraz Mchedlidze,1 Christian Möller,2 Kevin Lauer,2 and Jörg Weber1
Affiliations : 1Technische Universität Dresden, 01062 Dresden, Germany; 2CiS Forschungsinstitut für Mikrosensorik und Photovoltaik GmbH, Konrad-Zuse-Str. 14, 99099 Erfurt, Germany

Resume : Utilization of low quality feedstock for Si crystal growth was acknowledged as a valuable strategy for further price reduction of solar cell production. However, this strategy requires correct determination of the acceptable limits for the feedstock cleanness and relevant tailoring of the solar cell fabrication process. Recently, we reported about an influence of the fabrication steps on the bulk lifetime in the wafers produced from the various feedstock materials [1]. Deep traps of majority carriers were detected by deep level transient spectroscopy (DLTS) in the similar samples in near to the junction volume (NJV) using mesa-structured n+p-junctions prepared from the processed Si solar cells [2]. For proper detection of total iron content in the samples, a high temperature annealing sequence followed by fast quenching to 273 K (HTAQ) was used to transform iron silicide precipitates, formed after the crystal growth, to iron-containing defects detectable by DLTS [3]. In this report we present comparison of the lifetime measurement and the DLTS results for the crystals grown from various feedstock materials at various stages of solar cell fabrication process. Apparently, the NJV traps differ from those formed in the bulk of the cells and respond differently to the various process steps. However, in the both cases the trap densities correlate with the total content of iron in the crystals determined for the HTAQ subjected samples. Our results suggest changes to the solar cell fabrication processes that would minimize the influence of iron contamination in the as-grown Si crystal. [1] K. Layer, et al., Energy Proc. 38 589 (2013) [2] T. Mchedlidze, et al., Appl. Phys. Lett., 103, 013901 (2013) [3] T. Mchedlidze and J. Weber, Phys. Status Solidi RRL (2014) / DOI: 10.1002/pssr.201308327

Authors : M. Ledinský (1,2), K. Ganzerová (1), A. Vetushka (1), A. Fejfar (1), G. Bugnon (2), F. Meillaud (2) and C. Ballif (2)
Affiliations : (1) Institute of Physics, Academy of Sciences of the Czech Republic, v. v. i., Cukrovarnická 10, 162 00 Prague, Czech Republic (2) Photovoltaics and Thin Film Electronics Laboratory, École Polytechnique Fédérale de Lausanne (EPFL), Rue de la Maladière 71, CH-2000 Neuchâtel, Switzerland

Resume : Light management is among the most studied approach for improving the conversion efficiency of thin-film silicon solar cells. In order to enhance insufficient absorption of thin-film silicon solar cells in the red and near infrared (NIR) part of the sun spectra, different optical light-trapping structures are used. The main goal is to enhance the light path in the absorber layer by light scattering. Light-trapping properties of randomly rough ZnO front electrodes with different roughness will be evaluated in this paper by a newly developed method based on Raman spectroscopy. As the light-trapping prolongs the photons path in the solar cell, the probability of Raman scattering effect increases. Therefore Raman spectra measured with (in silicon weakly absorbed) 785 nm laser reflect the layer optical thickness which is directly related to light-trapping in the silicon thin film. Raman spectroscopy is a non-contact method, unaffected by electrical quality of the film. It may be used for samples even without contacts or/and doped layers. We show that, in the case of high quality solar cells, absolute Raman intensities and EQE follow the same trend. However, we find that, when the EQE value decreases without corresponding decrease of Raman signal, the reason has usually to be found in electrical quality of the cell and in carrier collection problems. In summary, Raman spectroscopy is found a suitable and simple characterization tool for light-trapping properties evaluation.

Authors : Orman Gref1, Jon Sandström1, Moshe Weizman2, Holger Rhein3, Stefan Gall3, Rutger Schlatmann2, Christian Boit1, Felice Friedrich1
Affiliations : 1 Technische Universität Berlin, Semiconductor Devices Division / PVcomB, Sekr. E4, Einsteinufer 19, 10587 Berlin, Germany; 2 University of Applied Sciences (HTW) Berlin / PVcomB, Schwarzschildstr. 3, 12489 Berlin, Germany; 3 Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), Silicon Photovoltaics / PVcomB, Kekulestr. 5, 12489 Berlin, Germany

Resume : Rear side contacting is a promising route for wafer as well as thin-film silicon solar cells. Recently, it was shown that for crystalline silicon on glass solar cells, featuring a rear side point contacting scheme, an initial solar cell record efficiency of 11.7 % could be achieved. These solar cells, however, suffered from an efficiency degradation which was shown to be related to the aluminum-silicon absorber contact. A few months ago, it was reported that a subsequent laser firing of these contacts could improve the series resistance and helped to stabilize the cell efficiency. In this current study, such laser-fired contacts were investigated by conductive probe atomic force microscope (c-AFM) in order to detect, with an adequate spatial resolution, the local improvement in conductivity. By using the advantage of simultaneously mapping the topography and current distribution, a drastic increase in the local conductivity within the laser-fired region over a spot of about 30 µm in diameter was observed. Current profiling of the laser treated contact with the c-AFM technique thus provides the means to systematically optimize the laser focusing and energy fluence.

Authors : B. Soleymanzadeh 1, W. Beyer 2,3, F. Luekermann 1, W. Pfeiffer 1, H. Stiebig 1,4
Affiliations : 1. Molecular and Surface Physics, University of Bielefeld, D-33615 Bielefeld, Germany; 2. Institut fuer Silizium-Photovoltaik, HZB, Kekulestrasse 5, D-12489 Berlin, Germany; 3. IEK5-Photovoltaik, Forschungszentrum Juelich GmbH, D-52425 J?lich, Germany; 4. Institut fuer Innovationstransfer an der Universitaet Bielefeld, Universitaetsstr. 25, D-33615 Bielefeld, Germany

Resume : Laser processing of a-Si:H enables the realization of polycrystalline-Si devices. We have studied the material properties of a-Si:H deposited with different hydrogen (H) content before and after femtosecond (fs) laser processing. Single 30 fs-pulses of an amplified Ti:Sapphs laser (λ=790nm) with peak fluences between 30 mJ/cm2 and 120 mJ/cm2 were applied. Material characterization is performed by optical microscopy, imaging ellipsometry, Raman mirco-spectroscopy (473 and 633nm) and SEM. The investigation focuses on fluences below the ablation threshold observed at 46, 60, and 90mJ/cm2 for 30%, 13%, and <1% H-content, respectively. Raman spectroscopy reveals critical fluences for Si-H dissociation and the onset of recrystallization. The different penetration depth of blue and red Raman probing lasers allows determining the thickness of the recrystallized layer. At medium energies, only a thin surface layer crystallizes. This indicates a strong absorption of fs pulses at 790nm, i.e. at a wavelength that is only weakly absorbed. This efficient absorption is explained using a two-photon absorption (TPA) coefficient of 1nm/W showing that fs-laser induced recrystallization requires a locally deposited energy density of 3 kJ/cm3. At certain conditions, recrystallization increases the surface roughness and enhances light absorption similar to absorption in black silicon. Based on the TPA model the prospects of flexible fs-laser materials processing of a-Si:H layers will be discussed.

Authors : H. Predatsch 1, U. Heinzmann 1, H. Stiebig 1,2
Affiliations : 1. Molecular and Surface Physics, University of Bielefeld, D-33615 Bielefeld, Germany; 2. Institut für Innovationstransfer an der Universitaet Bielefeld, Universitaetsstr. 25, D-33615 Bielefeld, Germany

Resume : Large area inspection tools such as electroluminescence (EL) become increasingly important for thin-film silicon module analysis. Small area modules of a-Si:H, µc-Si:H and a-Si:H/µc-Si:H tandem cell structures deposited by large area equipment (>1m2) were investigated. Generally, it is assumed that the EL-signal is proportional to the local current density (J). However, we will discuss circumstances when this assumption is not valid. One kind of local defects in a-Si:H based devices shows a strong spectral dependence. A bright local EL-signal (observed with a c-Si CCD) disappears, when a cut of filter of 1.42 eV is used. Performing spectral EL-imaging these hot spots show increased emission at 1.0-1.1 eV, which can be attributed to radiative tail-tail recombination. SEM images point out that these local defects originate from undesirable inclusions within the device. These defects has led to a local disturbance of the a-Si:H network, which results in broader tail states. The enhanced EL-signal can be attributed to an increased tail-tail recombination rather than an increased J. Another effect can be observed at a-Si:H based solar modules. Often a local decrease of EL-Signal occurs close to the plus-terminal within every cell of a series connected module. If EL-signal is proportional to J, J should decrease as well. However, an induced, locally enhanced deep defect density leads to an increased non-radiative recombination and a reduced radiative recombination detected by EL.

Authors : J. Holovský (1), N. Neykova (1, 2), M. Vaněček (1), S. De Wolf (3), C. Ballif (3)
Affiliations : (1) Institute of Physics, ASCR v. v. i. , Cukrovarnická 10, 16200 Prague, Czech Republic ; (2) Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Trojanova 13, 120 00 Prague, Czech Republic; (3) EPFL-IMT-PVLab, Rue de la Maladière 71b, CP 526 CH-2002 Neuchâtel, Switzerland

Resume : The absorption spectroscopy of defect states or of hydrogen-related bonds in bulk silicon-based materials has contributed greatly to the development of photovoltaics in past years. From the bulk properties, however, the interest shifts to surfaces, interfaces and ultrathin layers. We discuss spectroscopic methods tailored towards the layers going down to zero thickness. We developed a special evaluation procedure based on the thin-film limit and demonstrate it on a graphene monolayer measured by photothermal deflection spectroscopy (PDS) [1]. This simple and fast approach allowed investigation of time and light-soaking stability of surface states on 300nm thick amorphous silicon films [2]. Results correlated with measurements on 10nm thick layers of amorphous silicon on fused silica. The attenuated total reflectance infrared spectroscopy (ATR-FTIR) was developed and used for microstructure characterization of such ultrathin layers during light soaking and annealing experiments [3]. Interestingly, these layers differ diametrically from their thick counterparts. [1] J. Holovský, et al., Effect of thin film limit on measurable properties of graphene, Submitted [2] J. Holovský, et al., Time evolution of surface defect states in hydrogenated amorphous silicon studied by photothermal and photocurrent spectroscopy and optical simulation, J. Non-Cryst. Solids. 358 (2012) 2035. [3] E. El Mhamdi, et al., Is light-induced degradation of a-Si:H/c-S nterfaces reversible?, Submitted

Authors : M. Bär,1,2,3 D. Gerlach,1,4 M. Wimmer,1 R.G. Wilks,1 L. Weinhardt,3,5,6 R. Félix,1 F. Ruske,1 K. Lips,1 J. Hüpkes,7 M. Blum,3,8 C. Lupulescu,1,9 F. Kronast,1 M. Gorgoi,1 W. Yang,8 C. Heske,3,5,6,10 W. Eberhardt,1,9 and B. Rech1
Affiliations : 1Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 14109 Berlin, Germany; 2Institut für Physik und Chemie, Brandenburgische Technische Universität Cottbus-Senftenberg, 03046 Cottbus, Germany; 3Department of Chemistry, University of Nevada, Las Vegas, Las Vegas, NV 89154-4003; 4National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan; 5ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany; 6Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany; 7IEK5 – Photovoltaics, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany; 8Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720; 9Institute for Optics and Atomic Physics, Technische Universität Berlin, 10623 Berlin, Germany; 10Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany

Resume : One route to produce high-quality and low-cost thin-film silicon photovoltaic devices involves the deposition, subsequent solid-phase crystallization (SPC), and rapid thermal processing (RTP) of hydrogenated amorphous silicon to form poly-crystalline silicon (poly-Si) thin films. A simple electric contact can be implemented by a transparent conductive oxide (e.g., ZnO:Al) deposited on the glass substrate. The formation and properties of the poly-Si p/n junction and the poly-Si(n+)/ZnO:Al heterocontact are of inherent importance to the characteristics of the device. In our contribution, we will use a combination of different synchrotron-based techniques (including soft x-ray emission spectroscopy, photoemission electron microscopy, and hard x-ray photoelectron spectroscopy) to study the chemical and electronic structure of silicon-based p/n junctions and the buried silicon/zinc oxide heterointerface with a focus on the modifications upon SPC and RTP. We find evidence for a SPC- and RTP-induced activation and diffusion of the dopants and the formation of Si-O bonds at the Si/ZnO:Al interface. The data also suggests an accumulation and/or diffusion of zinc and aluminum at or across the Si/ZnO:Al interface. This elemental redistribution is more pronounced for aluminum than for zinc. A chemical pathway for the observed interaction at the silicon/zinc oxide interface will be presented and the relevance of our findings for the overall device performance discussed.

Authors : Jan Valenta
Affiliations : Charles University in Prague, Department of Chemical Physics & Optics, Faculty of Mathematics & Physics, Prague, Czechia

Resume : Luminescence spectroscopy is an important method for characterization of solar cells (SC) as it reveals radiative recombination of photo-generated carriers – the process concurrent to generation of electrical current but unavoidably present in highly-efficient SC devices. Especially electroluminescence is applied to characterization of SCs in both the development and production. For technical difficulties, most of luminescence measurements are done as relative experiments, it means without absolute (energetic or quantum) scale of luminescence intensity. However, the absolute calibration should enable to get much more insight into the processes taking place in a studied SC. Here we describe our imaging-micro-spectroscopy set-up with two parallel detection channels for visible and near-infrared regions. This apparatus is absolutely calibrated using special thin-layers of luminescing Si nanocrystals with the luminescence quantum yield determined in a separate apparatus. The capabilities of our methods are demonstrated by studies of several model materials.

Authors : C. Major1, G. Juhasz1, P. Petrik1, Z. G. Horvath2, M. Fried1,3
Affiliations : 1. Institute for Technical Physics and Materials Science, Research Centre for Natural Sciences (MTA TTK MFA), H-1525 Budapest, POB 49, Hungary 2. Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences 3. Doctoral School of Molecular – and Nanotechnologies, Faculty of Information Technology, University of Pannonia, Egyetem u.10, Veszprém, H-8200, Hungary

Resume : A macro imaging spectroscopic ellipsometer has been developed for high speed mapping of large area multilayer coated substrates. Non-contact or touchless characterization techniques based on spectroscopic ellipsometry (SE) are widely used by the photovoltaic industry for process or quality control in production. The commercialization of thin film photovoltaic (PV) technologies and the related increasing surfaces lead to many key problems such as reduced efficiency caused by multiple non-uniformities of the layers properties over the entire panel resulting from the technological steps of individual layer components. Scanning methods, based on the conventional narrow beam spectroscopic ellipsometry measurements provides high accuracy but suffer from long mapping times as the polarization state of the reflected beam must be detected. Our new instrument provides a line image of spectroscopic ellipsometry (wl=350-1000 nm) data with a resolution of 10 mm, thus SE information of 900 points can be collected in 210 sec over a 300 x300 mm PV material and it could be several 10 times faster than a conventional scanning method. In this paper calibration on 300 mm Si-c/SiO2 and test measurements on 300x300 mm ZnO/Mo and ZnO/MoO3/Mo structures are presented.

Authors : N. Segercrantz, D. Vial, F. Tuomisto, J. Slotte, J. Puustinen and M. Guina
Affiliations : Department of Applied Physics, Aalto University, P.O. Box 14100, FI-00076 Aalto, Finland; Department of Applied Physics, Aalto University, P.O. Box 14100, FI-00076 Aalto, Finland; Department of Applied Physics, Aalto University, P.O. Box 14100, FI-00076 Aalto, Finland; Department of Applied Physics, Aalto University, P.O. Box 14100, FI-00076 Aalto, Finland; Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, FI-33101, Tampere, Finland; Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, FI-33101, Tampere, Finland

Resume : Due to its many possible technical applications, GaAsBi has attracted increasing interest during the last decade. Properties such as large spin-orbital splitting, large band gap reduction and relatively weak temperature dependence makes GaAsBi a promising candidate for both long wavelength optoelectronic as well as spintronic related devices. Using positron annihilation spectroscopy (PAS) in Doppler broadening mode, we have studied MBE-grown epitaxial layers of GaAsBi on SI-GaAs. The samples had a constant layer thickness of 250 nm and Bi concentration of 1.5 or 1.6 % Bi. The growth temperature was 220 °C or 315 °C and the As/Ga flux ratio 1.5 or 1.6, respectively. As-grown samples as well as samples treated in a rapid thermal annealing (RTA) furnace at temperatures 500-700 °C for 60 s were studied. From the results, a clear difference between the as-grown samples and the RTA-threated samples can be seen. For the samples with 1.6 % Bi in the epitaxial layers, annealing at 500 °C led to a decrease in the S parameter and a lowered monovacancy concentration compared to the as-grown sample as well as the sample annealed at 600 °C. The W(S)-curve indicates that larger open volume defects are generated when the samples are annealed at 700 °C. Comparatively, the S parameter decreased compared to the as-grown sample when the sample with 1.5 % Bi was annealed at 600 °C.

Authors : Y. Ohshita1), T. Nishi1), D. Kodera1), K. Ikeda1), K. Shimomura1), H. Suzuki2), T. Sasaki3), I. Kamiya1) and M. Takahasi3)
Affiliations : (1)Toyota Technological Institute (2)University of Miyazaki (3)Japan Atomic Energy Agency

Resume : Dislocation-mediated strain relaxation during lattice-mismatched InGaAs/GaAs(001) heteroepitaxy was studied through in-situ x-ray reciprocal space mapping (in-situ RSM). Tandem type solar cell composed of lattice-mismatched III-V compound semiconductors is considered one of the candidates for realizing the super high conversion efficiency. However, the high density of threading dislocations in active layers grown on buffer layers causes the short minority carrier lifetime, which deteriorates the solar cell performance. Therefore, understanding the strain relaxation processes during the crystal growth is important to realize the expected high efficiency. The in-situ observation experiments were performed at the synchrotron radiation beamline 11XU at SPring-8, Japan, where a molecular beam epitaxy (MBE) – X-ray diffraction (XRD) system was used. RSM of 022 reflections were obtained from the InxGa1-xAs growing layer on a GaAs(001) substrate. To understand the strain relaxation kinetics, the effect of growth interruptions on the strain relaxation behavior was studied. During the growth interruption, the relaxation ratio quickly increased and reached a maximum. The relaxation rate at the initial stage after the interruption increased as the growth temperature increased. On the other hand, the maximum values were constant independent of the temperature. The relaxation during the interruptions along the [110] reached saturation more readily than along [1-10] direction. This result suggested that the blocking process with orthogonally aligned α dislocations limited β dislocation motions.

Authors : David Jacques
Affiliations : Doctoral Training Centre in Low Carbon Technologies, Energy Building, Faculty of Engineering, University of Leeds, Leeds LS2 9JT, U.K.

Resume : Current silicon cells require a thick layer of doped silicon which requires expensive processing. Silicon is also highly susceptible to global trade issues and therefore a long-term low price cannot be guaranteed. On top of the issues of price and efficiency, material resource use is a matter that also requires careful consideration. Any solar cell made for wide-scale future deployment must not bring a risk of over-consumption and material depletion. A potential solution to these issues is the fabrication of a novel nanocrystal activated Schottky barrier solar cell. Thin film titanium oxide, silver nanowires and copper oxide nanocrystals are synthesised and for each synthesis procedure, variables are adjusted to optimise the processes. The produced materials are then be characterised through utilising techniques such as XRD, XPS, SEM (including EDX) and UV-Vis spectroscopy. These materials will then be used to fabricate the nanocrystal activated Schottky barrier devices. The devices fabricated will then be characterised to assess the reasons behind their relative effectiveness. Cell characterisation will be performed through IV curves at white and monochromatic light, Voc and ICC measurements and also through probe measurements. This paper aims to introduce this new cell architecture and discuss its potential to offer a novel, cheap, robust solar cell that will utilise a variety of materials and also has its potential to overcome current theoretical efficiency limits

Authors : Fang Liu , S.J. Xu
Affiliations : Fang Liu,Department of physics, The University of Hong Kong; S.J. Xu, Department of Physics, The University of Hong Kong

Resume : The efficiency of multi-junction (MJ) solar cells can be significantly enhanced with the wider coverage of the solar spectrum and higher conversion. Time resolved optical spectroscopy is a powerful tool for optical study in semiconductor industry and for the measurement of time constants of physical processes such as absorption or emission of a given material. In this work, we report on the measurement of the time-resolved spectroscopy of the GaxIn1-xP-GaAs double-junction photovoltaic structures under different conditions of temperature and reverse bias voltage by using laser pulses as the excitation source and attempt to obtain some important time constants of carriers, such as, transit time of carriers in this kind of complex double-junction solar cells. The measurements of these time-resolved events can provide important information about the performance of the GaxIn1-xP-GaAs double-junction photovoltaic structures and thus let us know how to improve the efficiency of the tandem solar cells. The device system for the study of time resolved photocurrent was the GaxIn1-xP-GaAs double-junction photovoltaic structure grown with metalorganic chemical vapour deposition technique. The excitation laser pulses were provided by a Nd:YAG pulse laser with the wavelength of 532 nm and pulse width of ~10 ns. Time-resolved photocurrent signal of the GaxIn1-xP-GaAs double-junction photovoltaic structure was measured and recorded by a Boxcar (SRS250). In order to obtain the efficiency and accuracy in luminescence measurement, the Boxcar is interfaced to a computer and a developed software that allows synchronous scan control of the gate delay time in the boxcar integrator is provided. The effects of temperature and reverse bias voltages changing were evaluated by photocurrent, carried out using a pulsed laser with 532 nm as an excitation source. Compared to the photocurrent measured under 10K, significant reduction of the rising time was observed in the photocurrent spectra. The rising time is cut down remarkably with the growth of the reverse bias voltage applied. The spectra shift can be explained in terms of conductivity in the solar cell structure: as the temperature and the bias voltage increases, the relaxation time of electrons is reduced.

Authors : Fang Huang, Xiangxin Liu, Biao Yang, Hui Li Junfeng Han, Marie-paule Besland
Affiliations : The Key Laboratory of Solar Thermal Energy and Photovoltaic System, Institute of Electrical Engineering, Chiness Academy of Sciences; Institut des Mat?riaux Jean Rouxel (IMN), Universit? de Nantes, UMR CNRS;

Resume : Unlike conventional PN junctional photovoltaic devices, nanodipole solar cells require only one layer for both light-harvesting and carrier-separating, which are expected to reach high conversion efficiency by forming strong polarized nanodipole electric field of ~10^5 V/cm. A CdS nanodipole thin film solar cell is realized by phase segregation of a CdS-CdTe pseudobinary system. In this paper, we are going to report the preparation processes of the CdS nanodipole films, and the optimized deposition temperature, deposition pressure and heat-treatment condition which are 291 ℃, 2Pa and 400℃ in N2 atmosphere, respectively. By applying these optimization preparation conditions, our present best nanodipole devices with a structure of "ITO/ CdS nanodipole-CdTe mixed layer /Cu/Au" has reached device efficiency of 8.81%. Through transmission electron microscopy, we also observed uniformly distributed nano-particles with size of 5 nm in the photovoltaic layer, which confirms the phase segregation as a feasible fabrication mechanism of nanodiopole solar cells.

Authors : R. Pietruszka1, G. Luka1, B. S. Witkowski1, L. Wachnicki1, S. Gieraltowska1, E. Zielony2, P. Bieganski2, E. Płaczek-Popko2, M. Godlewski1,3
Affiliations : 1Institute of Physics, Polish Academy of Sciences, Warsaw, Poland 2Institute of Physics, Wroclaw University of Technology, Wroclaw, Poland; 3 Department of Mathematics and Natural Sciences College of Science, Cardinal Stefan Wyszynski University, Warsaw, Poland

Resume : ZnO, a wide band gap semiconductor with 3.3 eV band gap at room temperature, is intensively studied for photovoltaic (PV) applications mostly as a transparent conductive oxide (TCO) and n type partner for p-type substrates. In this work we study PV structures based on n type ZnO grown at low temperature either by atomic layer deposition (ALD) or hydrothermal method (nanorods). In the first approach, we deposited thin films of n-type ZnO on p-type silicon substrates. Such obtained PV structures of the II generation (ZnO:Al/ZnO/Si/Al) show efficiency of about 6%. In the second approach, we grown ZnO nanorods (ZnONR) on p-type Si. The growth process was initialed by 15 cycles of ALD process forming ZnO nanoseeds on a Si substrate. Then, the so-deposited ZnO nanoseeds nucleate growth of ZnO nanorods in a hydrothermal process. So obtained ZnONR were covered with n-type ZnO and ZnO:Al films by ALD process. The best PV efficiency for such IV generation structures (ZnO:Al/ZnO/ZnONR/Si/Al) was equal to 12.5%. This work was partially supported by the Innovative Economy grant (POIG.01.01.02-00-108/09, the National Centre for Research and Development grant (PBS1/A5/27/2012), and (E. Zielony, P. Biegański and E. Popko) by the National Laboratory of Quantum Technologies (POIG. 02.02.00-00-003/08-00).

Authors : Fang Liu, Z. Deng, J. Q. Ning, S. J. Xu
Affiliations : Department of Physics and HKU-Shenzhen Institute of Research and Innovation (HKU-SIRI), The University of Hong Kong, Pokfulam Road, Hong Kong, China

Resume : Multi-junction (MJ) solar cells with high efficiency have been a hot topic due to strong demand for clean energy and energy saving over the world. The efficiency of MJ solar cells is thus a central issue in this field. In order to improve the efficiency of MJ solar cells, a deeper understanding of carriers’ transport and loss mechanisms is necessarily vital. Time resolved optical spectroscopy is a powerful tool for optical study in semiconductor industry and for the measurements of time constants of physical processes such as transport and annihilation of carriers in materials and devices. In this work, we report on the detailed measurements of the time-resolved photocurrent spectra of GaxIn1-xP-GaAs double-junction photovoltaic structures under different conditions such as different temperatures and reverse bias voltages by using ns laser pulses as the excitation source. We attempt to obtain some important time constants of carriers, such as, transit time of carriers in this kind of complex double-junction solar cells. The measurements of these time-resolved events can provide important information about the internal physical processes in the GaxIn1-xP-GaAs double-junction photovoltaic structures and thus let us know how to improve the efficiency of the tandem solar cells. The device system for the study of time resolved photocurrents was the GaxIn1-xP-GaAs double- junction photovoltaic structures grown with metalorganic chemical vapour deposition technique. The excitation laser pulses were provided by a mode-locked Nd:YAG laser. The wavelength of the pulses was 532 nm and the pulse width was ~10 ns. Time-resolved photocurrent signal of the samples was measured and recorded with a Boxcar (SRS250) which is interfaced to a computer. The labview based control software was coded to allow a synchronous scan control of the gate delay time in the boxcar integrator. The effects of temperature and reverse bias voltages on the time-resolved photocurrents were evaluated in depth. Compared to the photocurrents measured at 10K, significant reduction of the rising time was observed in the time-resolved photocurrent spectra. It is also found that the rising time drops remarkably with increasing the reverse bias voltage. These interesting findings and analysis will be presented in the report. This work was financially supported by NSFC Grants (Grant No. 11374247).

Authors : S. Polivtseva1, I. Oja Acik1, A. Mere1, V. Mikli2, M. Krunks1
Affiliations : 1 Tallinn University of Technology, Department of Materials Science, Laboratory of Thin Film Chemical Technologies, 19086 Tallinn, Estonia; 2 Tallinn University of Technology, Department of Materials Science, Chair of Semiconductor Materials Technology, 19086 Tallinn, Estonia

Resume : SnxSy films as were grown by the chemical spray pyrolysis method. Aqueous solutions consisted of tin chloride (SnCl2) and thiourea (SC(NH2)2) at molar ratio of Sn:S=1:1, 1:2 and 1:4 were deposited onto preheated glass substrates at temperatures Ts=200-400°C in air. The films were characterized by X-ray diffraction (XRD), UV-VIS spectroscopy and scanning electron microscopy (SEM). According to XRD, the films deposited at Ts=200-320°C were composed of polycrystalline SnS as main crystalline phase independent of the Sn:S ratio in the spray solution. By increasing the Ts from 200 to 320°C, the film thickness decreases and the mean crystallite size of SnS decreases from 29 to 14 nm and from 17 to 7 nm while spraying 1:4 and 1:1 solutions, respectively. At similar growth temperatures, the thickness of films from 1:4 solutions was approximately twice higher than from 1:1 solutions. The bandgap of the films grown at Ts270°C from 1:4, 1:2 and 1:1 solutions was 2.1, 2.2 and 2.6 eV, respectively. According to XRD, films obtained at Ts370C from 1:4 solutions still contained of SnS as the main phase with addition of SnS2 as a secondary phase, while films from 1:1 solutions were composed of SnS2 and SnO2 phases. Characterisation of the films by XPS and EDX techniques is in progress. In this study we showed that the precursors’ molar ratio in the solution is an important parameter controlling the properties of sprayed Sn-S films.

Authors : M.V. Yakushev1,2; P. Maiello3; T. Raadik4; M.J Shaw1; P.R. Edwards1; J. Krustok4; A.V. Mudryi1,4; I. Forbes3; and R.W. Martin1
Affiliations : 1Department of Physics, SUPA, University of Strathclyde, Glasgow, G4 0NG, UK; 2Academy of Science of Russia and URFU, Ekaterinburg, Russia; 3Northumbria Photovoltaics Applications Centre, Northumbria University, Newcastle upon Tyne, UK; 4Tallinn University of Technology, Tallinn, Estonia; 5Scientific-Practical Material Research Centre of the National Academy of Science of Belarus, Minsk, Belarus

Resume : Cu3BiS3 is a semiconductor with high potential for the absorber layer in sustainable thin film solar cells. Thin films of p-type material were synthesised by heating metallic Cu-Bi precursors with a layer of thermo-evaporated S in a tube furnace. The metal multilayer precursors were magnetron sputtered onto Mo/glass substrates. The elemental composition, structural and electronic properties have been studied. The Raman spectrum shows four modes with the dominant peak at 292 cm-1. Photoreflectance spectra demonstrate two band gaps EgX and EgY and their evolution with temperature. These bands are associated with the X and Y valence sub-bands split due to the influence of the crystal field and spin orbital coupling. Analysis of the temperature dependencies of the band-gaps gives values of 1.24 and 1.53 eV for the X and Y band gaps at 0 K as well as the average phonon energy. Low temperature photoluminescence spectra reveal two broad emission bands at 0.84 and 0.99 eV, which simultaneously quench with anactivation energy of 40 meV. The photocurrent excitation measurements demonstrate a photoresponse suggesting a direct allowed nature of the X band gap.

Authors : A.I. Baranov, A.S. Gudovskikh, K.S. Zelentsov, E.V. Nikitina, A.Yu. Egorov
Affiliations : St Petersburg Academic University-Nanotechnology Research and Education Centre of Russian Academy of Sciences, Hlopina str. 8/3, 194021, St.-Petersburg, Russia

Resume : GaPNAs with concentration of nitrogen above 0.5% is a direct band material, which can be lattice-matched with silicon wafers in range of bandgaps (1.5-2.0 eV). According to theoretical estimations lattice-matched GaPNAs/Si triple-junction solar cell efficiency can reach 51.2%. However GaPNAs is relatively new material and its electrical properties are not studied enough. In this paper we present the defect study of GaPNAs layers grown on GaP and Si wafers. Silicon doped n-GaPNAs and undoped GaPNAs layers were studied by admittance spectroscopy in p-n and p-i-n solar cell heterostructures, respectively. For doped n-GaP layer on GaP substrate point defect with activation energy of Ea=0.22 eV(T1) was found. It is attributed with SiGa+VP system similar to GaP:N material. For undoped GaPNAs on GaP substrate two types of defects with Ea=0.18 eV(B) and Ea=0.24 eV(B2) were obtained. Defect B2 disappeared after annealing at 500 °C, defect with Ea=0.18 eV(B) was not changed, while another defect with Ea=0.31 eV(C1) was detected. Also we studied undoped GaPNAs layers in double-junction solar cells on Si wafer. In result, only defects with Ea=0.24 eV were detected, but after annealing at 500 °C no defect was detected with admittance spectroscopy. Further defect study is expected using DLTS technique.

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Light management III : T. Gregorkiewicz
Authors : Kylie Catchpole
Affiliations : Australian National University

Resume : Light trapping is of fundamental importance in many types of solar cells to allow maximum efficiencies, and hence lowest costs, to be reached. We show that that light trapping can lead to substantial efficiency increases using rear surface scattering, near-field enhancement and for tandem solar cells. A doubling of the photocurrent due to light trapping is demonstrated by the combination of silver nanoparticles with a highly reflective back scatterer on the rear of a silicon thin film solar cell. We also propose a planar ultra-thin absorber concept exploiting plasmonic resonance absorption enhancement. We calculate a maximum absorption of 90% for TM polarized normally incident light in a 5 nm thin-film absorber with a single-pass absorption of only 1.7%. Broadband and wide-angle absorption is demonstrated. Tandem solar cells based on crystalline silicon present a practical route toward low-cost cells with efficiencies above 30%. We evaluate inorganic thin-film top cells in a tandem stack with a high-efficiency c-Si bottom cell. We show when light trapping is incorporated, even relatively low quality earth-abundant semiconductor materials with luminescence efficiencies of 10-5 and diffusion lengths below 100nm are compatible with tandem cell efficiencies above 30%

Authors : Seweryn Morawiec1,2, Manuel J. Mendes1, Sergej A. Filonovich3, Tiago Mateus3, Salvatore Mirabella1, Hugo Águas3, Isabel Ferreira3, Francesca Simone2, Elvira Fortunato3, Rodrigo Martins3, Francesco Priolo1,2,4, and Isodiana Crupi1
Affiliations : 1 MATIS IMM-CNR, via S. Sofia 64, I-95123 Catania, Italy; 2 Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, I-95123 Catania, Italy; 3 CENIMAT/I3N, Departamento de Ciência dos Materiais, and CEMOP/UNINOVA, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal; 4 Scuola Superiore di Catania, Università di Catania, Via Valdisavoia 9, 95123 Catania, Italy

Resume : Plasmonic light trapping is nowadays considered a promising solution for retaining high efficiency in thin film silicon solar cells while reducing their volume of semiconductor material. However, to produce significant efficiency enhancement, it is essential to achieve broadband light scattering from subwavelength metallic nanoparticles (NPs) together with suppression of the parasitic absorption in the particles. Here we study the performance of a-Si:H solar cells, with a substrate n-i-p configuration, using self-assembled silver NPs incorporated in the cells’ rear contact, forming a plasmonic back reflector (PBR). The optical properties of the PBRs are optimized according to the morphology of the nanostructures, which can be tuned by the parameters of the solid state dewetting process [1]. A broadband photocurrent enhancement of 22.3% is achieved and attributed to both the plasmon-assisted light scattering from the NPs and the front surface texture originated from the conformal growth of the cell material over the particles. Based on the analysis of more than 40 cells built on distinct PBRs we demonstrate a linear relation between the PBRs’ diffuse reflection and the photocurrent enhancement in the a-Si:H light trapping window (600–800 nm), which reaches values as high as 60%. Additionally, remarkably high values of Jsc and Voc are achieved, relative to those reported in the literature for the same type of devices. [1] S. Morawiec et. al., Nanotechnology, 24 (2013) 26560

Authors : J. Ortiz Gonzalez, T.S. Frenkel, R. Santbergen, T.V. Pfeiffer, H. Tan, A. Schmidt-Ott, M. Zeman, A.H.M. Smets
Affiliations : Photovoltaic Materials and Devices, Delft University of Technology, the Netherlands; Materials for Energy Conversion and Storage, Delft University of Technology, the Netherlands

Resume : Plasmonic silver nanoparticles can provide excellent light-trapping in thin-film solar cells. It is however crucial that non-absorbing, strongly scattering nanoparticles with a diameter on the order of 100 nm are used. Our objective is to fabricate a film of such metal nanoparticles over a large area at low cost. We have previously achieved this goal using the metal island film method at a 400°C anneal temperature, resulting in state-of-the-art solar cell performance [1]. Here we present two alternative methods that do not require substrate heating. This makes it possible to deposit the particles on already deposited solar cell layers without inducing cell damage. In the first method we generate an aerosol of spherical silver particles in an inert gas. A differential mobility analyser is used to size-select the particles before deposition. We show that the obtained surface coverage is proportional to the deposition time. The optical properties of nanoparticle films with different combinations of particle size and surface coverage are analysed in detail and compared to theoretical results. The nanoparticle films that exhibit strongest light scattering are integrated in single and multi-junction solar cells. We find that photocurrent density is enhanced compared to a flat reference device. A second method, based on porous anodized aluminium oxide used as metal deposition mask, will be presented as well. [1] H. Tan et al., Nano Lett. 12 (2012) 4070.

Authors : C. S Schuster (1), S. Morawiec (2), M. J Mendes (2), P. Kowalczewski (3), M. Patrini (3), E. R Martins (4), L. Lewis (5), I. Crupi (2), F. Priolo (2,6), L. Andreani (3), T. F Krauss (1)
Affiliations : (1) Department of Physics, University of York, York, YO10 5DD, UK (2) MATIS CNR-IMM and Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, Italy (3) Department of Physics, University of Pavia, Via Bassi 6, 27100 Pavia, Italy (4) School of Physics and Astronomy, SUPA, University of St Andrews, St. Andrews, KY16 9SS, UK (5) Photonics Device Dynamics Group, Tyndall National Institute, Lee Maltings, Cork, Ireland (6) Scuola Superiore di Catania, Università di Catania, via Valdisavoia 9, 95123 Catania, Italy

Resume : Thin-film photovoltaics strongly benefits from novel light trapping approaches. Both plasmonic and dielectric scattering structures have been shown to enhance light trapping, a direct comparison has not been carried out yet. Here, we quantify the difference between plasmonic and dielectric light trapping experimentally and over a broad wavelength range. All samples use the same substrate and the same hydrogenated amorphous silicon film of 300 nm nominal thickness. We carefully characterised all the material properties to allow detailed comparison with simulations. Our study compares the performance of an optimized plasmonic design, consisting of silver nanoparticles on a spacer layer created by thermal annealing (100 nm diameter), with a dielectric scatterer based on our diffractive supercell grating design (1.8 um period, 80 nm etching depth). The comparison is made in a superstrate configuration, i.e. the light is incident from the substrate side so the scattering structure is at the back of the absorbing layer. This minimises the intrinsic losses of the plasmonic nanoparticles and avoids a distortion of the results due to antireflection effects. Hence, we can separate the scattering properties of the respective structures as well as experimentally possible. We find that both approaches are comparable up to 750 nm wavelength, but that the plasmonic design appears to have higher parasitics at the longer wavelenghts where the material absorption starts to drop.

Authors : Hisham Nasser, Firat Es, Alpan Bek, Mehmet Can Gunendi, Oguz Gulseren, Rasit Turan
Affiliations : Hisham Nasser: Micro and Nanotechnology Graduate Program and The Center for Solar Energy Research and Applications (G?NAM), Middle East Technical University; Firat Es: Micro and Nanotechnology Graduate Program and The Center for Solar Energy Research and Applications (G?NAM), Middle East Technical University; Alpan Bek: Department of Physics and The Center for Solar Energy Research and Applications (G?NAM), Middle East Technical University; Mehmet Can Gunendi: Department of Physics, Bilkent University; Oguz Gulseren: Department of Physics, Bilkent University; Rasit Turan: Department of Physics and The Center for Solar Energy Research and Applications (G?NAM), Middle East Technical University

Resume : Metal nanoparticles possess localized surface plasmons (LSP) upon interacting with incident light that offer the possibility of improved light absorption in solar cells. Proper engineering and manipulations are essential to enhance the scattering efficiency of the light into the absorber layer of the solar cell and to reduce various losses. However, the deposition of metal nanoparticles in direct contact with the active absorber layer reduces the corresponding photon conversion efficiency of the solar cell due to possible contaminations and thus carrier recombination induced by the deposited metal islands. Furthermore, to attain higher efficiency, the dangling bond passivation using dielectric material such as SiO2 is necessary. Therefore, to integrate metal nanoparticles in the fabrication of solar cells, it is crucial to fabricate them on a dielectric spacer layer either on the front or back surface of the solar cell. The use of a spacer layer between the absorbing layer of the cell and the nanoparticles is of great interest both from the fabrication and the photon management points of view. In this study, we investigate the influence of SiO2 spacer layer thickness on the average metal nanoparticles size and shape, nanoparticle size distribution, and thus on the excitation of localized surface plasmon properties as the optical response of metal nanoparticle can be tuned by varying their size or shape, size distribution, or by modifying the local dielectric environment. The thickness of the dielectric spacer plays an important role in the plasmonic coupling of the incoming photon field into the underlying active device. By carefully studying the thickness of the spacer layer, we have identified the critical thickness that defines the border between coupled and uncoupled regimes. In order to explain the experimental data, we have carried out a series of theoretical calculations based on the time evolution of Maxwell equations in a discretized space (FDTD model). We have successfully regenerated the experimental observation and found that plasmonic behavior of the nanoparticles is sensitive to the spacer layer thickness and its size distribution.

10:00 Best Poster Award (for poster session I)    
10:10 Break    
Advanced concepts I : K. Catchpole
Authors : Tonio Buonassisi
Affiliations : Massachusetts Institute of Technology

Resume : Earth-abundant solar-cell materials have been of interest for many decades, but only two non-silicon material systems, Cu2S and CZTS, demonstrate efficiencies above 10%. This invites the question: are these “alternative” solar-cell materials fundamentally limited, or can they be engineered into high-performance devices? Herein, we consider this question in context of two candidate Earth-abundant absorber materials — thin-film cuprous oxide (Cu2O) and tin monosulfide (SnS) — and detail their recent efficiency evolution to >4%. We detail our efforts to improve bulk minority-carrier collection length, minimize contact resistance, and tune the composition of buffer layers to minimize interface recombination between the absorber and transparent-conducting oxide. We also present a summary of our simulation and modeling efforts, aimed at identifying the governing efficiency loss mechanism(s). A key finding is that these particular materials systems do not (yet) appear fundamentally limited, but lack appropriate pairing with neighboring materials in the device stack. The development of new absorbers often requires novel buffer layers, contact materials, and interface engineering. From realized device-efficiencies and simulated further improvements, a picture emerges of a rational path to improve Earth-abundant solar-cell device performance, the lessons of which may accelerate the development of other candidate materials.

Authors : Y. Ievskaya1, R. Hoye1, K. Musselman2 and J.L. MacManus-Driscoll1
Affiliations : 1, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, United Kingdom; 2, Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom.

Resume : Cuprous oxide is a non-stoichiometric p-type semiconductor with a direct band gap of 2.0 eV and theoretical conversion efficiency of approximately 20%. However, the best Cu2O-based heterojunction solar cells to date do not exceed 5.4% [1]. One of the reasons for the impaired performance is poor quality of the heterojunction interface due to thermodynamic instability of cuprous oxide at ambient conditions. Heterojunction ZnO/Cu2O solar cells were fabricated with high quality thermally oxidised crystalline Cu2O sheets ensuring low resistivity and good carrier mobility of the absorber. In this work, ZnO films were deposited by a scalable low damage technique - atmospheric atomic layer deposition (aALD). Optimisation of aALD conditions, doping of ZnO, application of buffer layers and other modifications to avoid ambient exposure were employed in order to preserve the pristine surface of Cu2O substrates and minimize the formation of phases detrimental to heterojunction interface quality. This has tripled the open circuit voltage and doubled the short circuit current, as well as improved the fill factor, resulting in a manifold increase in conversion efficiency of the cells. The performance of the cells was characterized by J-V measurements, external quantum efficiency, transient photocurrent and photovoltage and light intensity dependent measurements. [1] T. Minami et al., Thin Solid Films (2013)

Authors : Riley E. Brandt, Yun S. Lee, Niall Mangan, Jian V. Li, Matthew Young, Tonio Buonassisi
Affiliations : Massachusetts Institute of Technology, Cambridge, MA (USA); National Renewable Energy Laboratory, Golden, CO (USA)

Resume : The Earth-abundant absorber cuprous oxide (Cu2O) is promising as a scalable PV material. We have reported thin-film Cu2O efficiencies of 2.65%, and have recently exceeded 4.4% [unpublished]. The p-type Cu2O is paired with a high work-function metal back contact, and an n-type buffer layer, transparent conductor, and metal finger electrode front contact. At present, a dominant deficiency in Cu2O cells is the low open-circuit voltage (VOC), well below the detailed-balance limit for its 2 eV bandgap. This is suggestive of an insufficient energetic barrier to recombination produced by the heterojunction interface. Here, we show that an unfavourable conduction-band offset between Cu2O and the ¬¬n-type material is the primary cause of the low built-in potential, and therefore low VOC; 1-D device simulations support this explanation in addition to low bulk minority carrier lifetime. Temperature-dependent J-V measurements are used to measure the energetic barrier to carrier recombination by extrapolating VOC to 0 K. Heterojunction band offsets and the oxidation state of species at the interface are both determined through x-ray photoelectron spectroscopy (XPS) measurements. The VOC is shown to be correlated with heterojunction band offsets, as well as interface chemical composition set by specific fabrication conditions. These trends are compared to simulated devices to explain the VOC values (ranging from 0.3 – 1 V) observed in devices fabricated with different buffer layers.

Authors : Kieren Bradley, David Cherns, David Fermin, Martin Cryan
Affiliations : University of Bristol, University of Bristol, University of Bristol, University of Bristol

Resume : Extremely thin absorber (ETA) solar cells are next generation photovoltaics with the advantages of dye sensitised solar cells but as they can be entirely inorganic they have the potential for longer lifetimes and greater stability. The base material for our ETA is zinc oxide in the form of nanorods; they are low cost, easily grown, transparent to visible light and are n-type semiconductors as grown. Added advantages may occur due to their disordered semi-periodic structures inducing photonic behaviour, antireflection properties, scattering of the light for multiple absorption chances and light concentrating abilities acting like waveguides. To study the electrical and optical properties of zinc oxide nanorods we are using photoelectrochemistry to probe the charge transport within the nanostructures. Photocurrent transients show differences between the as-grown nanorods and nanorods that have undergone a 5h annealing process; annealed rods show a greater photon to current conversion efficiency, with the as-grown rods having a rise time that depends on potential. The measured transients were modelled as a single electron trap system; fitting the measured data to the model allowed for estimation of trapping rates, defect densities and the potential at which the defect states occur. The results suggest that the defect states are ~0.8V below the conduction band; although these may correspond with bulk defect states often seen in photoluminescence, due to the surface dependence of the photocurrent technique they may actually be an indication of surface defects. A method of calculating the energy level of the defect states will likely enable the identification of the state and the annealing process can be improved. The current semi-empirical model requires an input from the measurements to provide a steady-state current from which the transient is then calculated. Drift diffusion modelling of steady-state photocurrents with carrier generation rates derived from optical modelling may provide a description of the entire optoelectronic process.

Authors : Assaf Y. Anderson, Hannah-Noa Barad, Adam Ginsburg, David A Keller, Klimentiy Shimanovich, David Sriker, Koushik Majhi, Yaniv Bouhadana and Arie Zaban
Affiliations : Department of Chemistry, Center for Nanotechnology & Advanced Materials, Bar Ilan University, 52900 Ramat Gan, Israel

Resume : All-oxide-based photovoltaics (PVs) encompass the potential for extremely low cost solar cells, provided they can obtain an order of magnitude improvement in their power conversion efficiencies. To achieve this goal, we perform a combinatorial materials study of metal oxide based light absorbers, charge transporters, junctions between them, and PV devices. Using the combinatorial approach we discover preferred properties such as enhanced absorption, bandgap tuning, enhanced charge carriers lifetime, and photoconductivity for several binary and ternary metal oxides. We then reveal the photovoltaic activity once these materials are assembled in a heterojunction configuration. We present here the concepts of the all oxide PVs and the combinatorial approach. We show the progress, the trends in power conversion efficiencies, newly discovered materials, our current understandings and future challenges. doi/abs/10.1021/co3001583 doi/abs/10.1021/jz3017039

Authors : .J.N. Hart, M. Cutini, N. L. Allan
Affiliations : School of Materials Science and Engineering, UNSW Australia, UNSW, NSW, Australia; School of Chemistry, University of Bristol, Bristol, United Kingdom; School of Chemistry, University of Bristol, Bristol, United Kingdom

Resume : Many potential semiconductors for photocatalysis of water splitting have band gaps that are too large for absorption of visible light and hence give low efficiencies under sunlight. Two such examples are ZnO and ZnS. Solid solution formation is one approach to reducing band gaps and enhancing visible-light absorption. For example, it has been shown that GaN-ZnO solid solutions are promising visible-light photocatalysts [1]. In this work, we show based on density functional theory calculations that both ZnO-AlN and ZnS-GaP solid solutions are promising photocatalysts. The solid solutions have small energies of mixing from the two constituents (lower than ZnO-GaN in the case of ZnO-AlN). The solid solutions have tunable band gaps that depend on both composition and atomic ordering. Addition of only a small amount of AlN to ZnO and GaP to ZnS reduces the band gap into the correct energy range for absorption and emission of visible light and close to the optimum for photocatalysis of water splitting under sunlight [2]. Significantly, the band gaps of the solid solutions are smaller than those of either constituent on their own. Furthermore, addition of a small amount of ZnS to GaP produces a direct band gap semiconductor, in contrast to pure GaP which has an indirect band gap, and this should therefore increase the efficiency of light absorption. [1] K. Maeda, et al., Nature, 440 (2006) 295. [2] J. N. Hart, N. L. Allan, Advanced Materials, 25 (2013) 2989.

Authors : Robert Karsthof, Holger von Wenckstern, Marius Grundmann
Affiliations : Universit?t Leipzig, Institut f?r Experimentelle Physik II, Linn?str. 5, 04103 Leipzig, Germany

Resume : Transparent pn-heterojunctions of zinc oxide and nickel oxide are promising for the realization of photovoltaic devices that transmit visible light and at the same time convert photons from the UV part of the solar spectrum for power generation. Such solar cells could be employed on large areas like windows or glazed roofs without impairing the function of these structures. P-type nickel oxide was grown by reactive sputter deposition of metallic Ni on top of n-type zinc oxide grown by pulsed laser deposition. The resulting pn-heterojunctions were semi-transparent with an average transmission of approx. 64% in the visible spectral range, mostly due to absorption losses in the NiO which has amorphous structure and a high density of defect states. Under illumination with an AM1.5G spectrum the devices showed photovoltaic activity with photovoltages of more than 500 mV. The devices were characterized by means of temperature dependent current-voltage measurements. Rectification ratios of up to 8 orders of magnitude were detected. The influence of NiO film properties, such as film thickness and stoichiometry, on the temperature behavior of the IV characteristics was investigated.

12:30 Lunch    
Advanced concepts II : T. Buonassisi
Authors : Susanne Siebentritt
Affiliations : University of Luxembourg

Resume : Kesterites (Cu2ZnSn(S,Se)4) are compound semiconductors which do not make use of any rare metals, have band gaps suitable for the use in solar cells and a crystal structure similar to the successful Cu(In,Ga)Se2 compounds. Therefore theses materials are worldwide intensely studied as solar cell absorbers. Efficiencies up to 12% have been reached by IBM, and around 10% by a number of labs worldwide. However, to be competitive the efficiencies have to improved to a level comparable to CIGS and CdTe. The presentation will summarise the current state of the art of kesterite solar cells and the challenges on the path to higher efficiencies.

Authors : Seigo ITO
Affiliations : University of Hyogo

Resume : The hybrid organic-inorganic methylammonium lead halide perovskites (CH3NH3PbX3, X = Cl-, Br-, I-) pioneered for use in thin film transistors by Mitzi, and introduced as a light harvester in dye sensitized solar cell configurations by Miyazaka have attracted intensive attention for thin-film photovoltaics, due to their large absorption coefficient, high charge carrier mobility and diffusion length. Power conversion efficiencies (PCEs) of over 15% were obtained with both mesoporous metal oxide scaffold and in planar heterojunction architectures. Despite the rapid increase in efficiency associated with the evolution of different types of perovskites and device fabrication techniques, the hole transporting material (HTM) used were mainly limited to organic compounds, the start-of-the-art 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD), conducting polymers, and small molecules. Their low hole mobilities in comparison to the n-type semiconductors (TiO2, ZnO) and perovskite itself was one of the main limitation of these materials. Compared to organic HTMs, inorganic p-type semiconductors appear to be an ideal choice from the point of view of high mobility, stability ease of synthesis and low cost. Here, we report that combining the perovskite CH3NH3PbI3 with CuSCN as p-type HTM lead to solar cells with very high power conversion efficiency (12.4%) under full sun illumination.

Authors : Antonio Abate, Henry J. Snaith
Affiliations : Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom

Resume : Organometal trihalide perovskites, such as CH3NH3 Pb X3 (X = I-, Br-, Cl-), are attracting growing interest to prepare low cost solar cells that are capable of converting sun light to electricity at the highest efficiencies. Despite negligible effort on enhancing materials purity or passivation of surfaces, high efficiencies have already been achieved. However, we show that trap states at the perovskite surface generate charge accumulation and consequent recombination losses in working solar cells. We identify that under-coordinated iodine ions within the perovskite structure are responsible and establish a supramolecular strategy to successfully passivate these sites. Following this strategy we demonstrate solar cells with stabilized power conversion efficiency over 15% under simulated full sun light.

Authors : K. Ben Messaoud, M. Buffiere, G. Brammertz, H. ElAnzeery, S. oueslati, M. Meuris, M. Amlouk, J. Poortmans
Affiliations : KACST-Intel Consortium Center of Excellence in Nano-manufacturing Applications (CENA), Riyadh, KSA ; imec division IMOMEC – partner in Solliance, Wetenschapspark 1, 3590 Diepenbeek, Belgium ; Institute for Material Research (IMO) Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium ; Unité de Physique des Dispositifs à Semiconducteurs Department of Physics, Faculty of Sciences of Tunis, El Manar, Tunisia ; Department of Physics, Faculty of Sciences of Bizerte, Tunisia ; Department of Electrical Engineering, KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium ; imec – partner in Solliance, Kapeldreef 75, 3001 Leuven, Belgium ; Microelectronics System Design department, Nile University, Cairo, Egypt

Resume : The present contribution aims at determining the impact of modifying the properties of the CZTSe/CdS interface on the electrical performance of Cu2ZnSnSe4 (CZTSe) thin film solar cells by using a Cd treatment of the absorber before the buffer layer deposition. In this work, solar cells with and without Cd treatment were compared to their respective CIGSe references. CZTSe absorbers were prepared by DC sputtering of CuSn, Zn and Cu layers on glass/Mo substrates, followed by subsequent annealing under H2Se. The absorbers were immersed in KCN solution (5%wt, 2min) in order to avoid the presence of CuxSe secondary phase. The reference CIGSe absorbers were synthetized by co-evaporation using a CuPRO process. The Cd treatment was performed on some of these samples in a chemical bath for 7 min at 70 C using a basic solution of cadmium acetate. The solar cells were completed by CdS buffer layers deposited by chemical bath deposition and a i-ZnO/ZnO:Al bilayer deposited by RF sputtering. XPS measurements were performed to evaluate the diffusion profile of Cd into the CZTSe absorbers as well as the possible defect passivation. The solar cells were characterized using JV, EQE and CV measurements. We have observed that the fill factor (FF) increased from 57.7% to 64.0% leading to the improvement of the efficiency () from 8.33% to 9.01% for the CZTSe samples with a Cd treatment, as for the CIGSe solar cells reference ( FF from 58.3% to 63.2% and  increasing from 9.56% to 10.2%). This effect comes from a considerable reduction of the series resistance (Rs) of dark and light JV, determined using the one diode model. The crossover between dark and light JV seems also affected by Cd treatment. Finally, a model explaining the improvement of the efficiency of the devices when using Cd treatment will be discussed.

Authors : N. von Morzé, S. Fengler, T. Dittrich, S. Wiesner, T. Münchenberg, C. A. Kaufmann, M. Rusu, M. Ch. Lux-Steiner
Affiliations : Bereich Solarenergieforschung, Helmholtz-Zentrum Berlin für Materialien und Energie, Lise-Meitner Campus, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

Resume : Charge separation processes at hybrid interfaces between single as well as blend layers of various small organic molecules (C60, C70, ZnPc, MgPc, SubPc and ZnPc:C60) and inorganic Na-free and Na-treated CuInSe2 (CISe) thin films were investigated. The organic materials were deposited by organic vapor phase deposition on CISe/glass as well as on ITO/glass substrates for reference measurements. The CISe layers were deposited by physical vapor deposition on Mo coated soda lime glass substrates with and without Na diffusion barrier at the Mo/glass interface. Surface photovoltage (SPV) measurements on Na-free CISe layers show opposite signals compared to those from Na-treated CISe layers indicating different surface conductivity types. The temperature and wavelength dependent transient measurements show that the latter effect originates from at least two opposite charge transfer processes. One process is strongly affected by surface defect states; the presence of Na helps to reduce the concentration of such defects. The donor materials MgPc and SubPc show strongest SPV-signals on Na-free CISe whereas the acceptor C60 and the blend C60:ZnPc show highest signals on Na-treated CISe. For C60, C70, ZnPc:C60, and SubPc on Na-free CISe an intensity and wavelength dependent polarity of charge separation was observed. Our results show that the combination of Na-treated CISe and C60 as well as C60-containing donor-acceptor blends is promising for the preparation of hybrid solar cells.

Authors : M. Buffiere1,2,3*, A.E. Zaghi2,3,4, N. Lenaers2,3,4, M. Batuk5, S. Khelifi6, J. Drijkoningen7,8, V. Afanasiev9, J. Kepa9, A. Stesmans9, J. Hadermann5, J. D’Haen7,8, J. Manca7,8, J. Vleugels9, M. Meuris7,8, J. Poortmans1,2
Affiliations : 1 Department of Electrical Engineering (ESAT), KU Leuven, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium 2 imec- partner in Solliance, Kapeldreef 75, 3001 Leuven, Belgium 3 SIM vzw, Technologiepark 935, 9052 Zwijnaarde, Belgium 4 Department of Metallurgy and Materials Engineering (MTM), KU Leuven, Kasteelpark 44, 3001 Heverlee, Belgium 5 Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium 6 Electronics and Information Systems department (ELIS), University of Gent, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium 7 imec division IMOMEC - partner in Solliance, Wetenschapspark 1, 3590 Diepenbeek, Belgium 8 Institute for Material Research (IMO) Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium 9 Department of Physics and Astronomy (FYS), KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Resume : Printed chalcopyrite thin films have attracted considerable attention in recent years due to their potential in the high-throughput production of photovoltaic devices. To improve the homogeneity of printed CuInSe2 (CISe) layers, chemical additives can be added to the precursor ink. In order to get crack-free precursor thin films, organic binders are in general used to maintain the particles forming the precursor joined once the solvents evaporates. However, using additives might also have drawbacks on the solar cell performances when resulting in the contamination of the absorber. In this contribution, we investigate the influence of dicyandiamide (DCDA), used as a binder in the precursor ink, on the physical and electrical properties of printed CISe solar cells. The precursor was prepared using mechanically alloyed CuInSe nanopowder dispersed in pentanediol, while three different amounts of DCDA (0 wt%, 10 wt%, 20 wt%) were added to the ink. The resulting inks were printed using a Doctor Blade coating tool and annealed under selenium vapor using a two-step process (250 oC, 10 min; 500 oC, 20 min). The solar cells were completed at the same time, using a standard process flow for the deposition of CdS/ZnO/ZnO:Al buffer/window layers. A complete analysis, consisting in both physical (XPS, SEM, TEM, ESR) and electrical (I-V, C-V, DLCP, C-AFM, KPFM) measurements, was carried out on the devices. It is found that the use of this binder does not affect only the morphology of the absorber layer, but also the efficiency of the resulting device, the doping profile of the absorber as well as the passivation of the grain boundaries in the polycrystalline CISe thin film. A model based on the modification of the conversion and the sintering mechanisms of the precursor when using a binder is proposed to explain these observations. The Flemish ‘Strategisch Initiatief Materialen’ (SIM) SoPPoM program is acknowledged for its support.

16:00 Break    
16:15 Poster session II : S. Christiansen and J. Valenta    
Authors : M. Balestrieri (1), G. Ferblantier (2), S. Colis (1), G. Schmerber (1), M. Ziegler (1), M. Gallart (1), D. Muller (2), P. Gilliot (1), A. Slaoui (2) and A. Dinia (1)
Affiliations : (1) Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS UMR 7504, 23 rue du Lœss, B.P. 43, F-67034 Strasbourg Cedex 2, France; (2) ICube, Université de Strasbourg, CNRS UMR 7357, 23 rue du Lœss, B.P. 20, F-67037 Strasbourg Cedex 2, France

Resume : Doping wide band gap semiconductors, such as ZnO, with trivalent rare earth (RE) ions is well known to enhance their optical activity. In fact, RE are well known for their optical transitions involving the 4f shell. The main purpose of this work is to study the electronic transfer between ZnO and the rare earth (RE) for photon shifting and possible applications for silicon-based solar cells. Trivalent Neodymium ions Nd+3 exhibit intense luminescence at 900 nm, just above the band gap of silicon. The effect of deposition temperature and annealing on the photoluminescence (PL) of ZnO:Nd films is reported. The structural and optical properties of the ZnO:Nd films were characterized. XRD structural measurements of the as-grown Nd-doped ZnO films show that high-quality strongly-oriented ZnO:Nd films can be obtained by magnetron reactive sputtering, even when the substrate temperature during deposition is as low as 15°C. Rutherford back scattering (RBS) measurements show that Nd is uniformly distributed inside the ZnO matrix. Photoluminescence measurements indicated that an efficient electronic transfer from ZnO to Nd+3 ions is achieved. In particular, excitation-dependent PL (PLE) allows deeper insight into the Nd electronic level structure. This conversion layer will be inserted in a complete solar cell in order to test its effect on the efficiency of the cell.

Authors : Marta Lluscà, Julià Lopez-Vidrier, Isabel Sánchez, Aldrin Antony, Sara Lauzurica, Carlos Molpeceres, Sergi Hernández, Blas Garrido, Joan Bertomeu
Affiliations : Universitat de Barcelona; Indian Institute of Technology Bombay, Universidad Politécnica de Madrid.

Resume : Er and Yb co-doped systems have been extensively studied because of the ability of both ionic species to cooperate together to convert infrared light into visible light, thanks to the energetic alignment of their energy levels for λ = 980 nm. In particular, ZnO is a transparent and conducting material that can be used as their host matrix, propitiating the up-conversion process and, thus, the creation of photons in the visible range. Therefore, ZnO:Er:Yb becomes a potential candidate to be employed as top-cell electrodes with infrared up-conversion properties in conventional photovoltaic cells. In this study, a ZnO:Er:Yb film was deposited by means of sputtering onto a glass substrate. In order to act as an optically active center and thus to allow radiative 4f-4f transitions, Er needs to be surrounded by oxygen atoms (ErO6) forming a pseudo-octahedron structure with a C4v symmetry. This means that Er replacing Zn in the ZnO matrix does not act as an optically active center, and an annealing treatment is usually needed to change the Er local structure. With the aim of activating Er ions, while preserving the transparency and conductivity of the films, three different annealing methods were carried out and compared: 1) at 800 ºC in air atmosphere, 2) at 800 ºC in vacuum, and 3) laser annealing. The composition, structure, electrical and optical properties of the films, as well as their up-conversion emission, were studied before and after the different annealing treatments.

Authors : L. Dumont*, P. Benzo*, J. Cardin*, C. Labbé*, I-S YuϮ, F. Gourbilleau*
Affiliations : * CIMAP CNRS/CEA/ENSICAEN/UCBN, 6 Boulevard Maréchal Juin, 14050 Caen Cedex 4, France Ϯ Department of Materials Science and Engineering, National Dong Hwa University, Da Hsueh Rd, Shoufeng, Hualien 97401,Taiwan

Resume : Management of the solar spectrum is one of the key issues to improve the solar cell efficiency. Among the existing solutions, one consists in a layer absorbing energetic UV photons and converting them into two IR photons that match the absorption band of classical Si solar cells. This mechanism called frequency- or down- conversion process may increase the cell efficiency by almost doubling the number of IR photons absorbed as well as reducing the thermalisation losses. In this paper, we propose an innovative approach using a co-doped SiN matrix that presents the advantages of being compatible with Si solar cells fabrication process. Layers were deposited on Si substrates by reactive magnetron co-sputtering of Si, Tb, and Yb targets under a nitrogen-rich plasma. The microstructure of the grown layers were investigated using TEM, EDX and Glow Discharge Mass Spectroscopy while their optical properties and excitation mechanisms were analysed by means of ellipsometry, photoluminescence, and photoluminescence excitation spectroscopies. The quantum efficiency of the system has been studied through time-resolved photoluminescence experiments. Finally, optimized layers have been deposited on a solar cell to measure the carrier lifetime as well as the external quantum efficiency under AMG 1.5 illumination.

Authors : Umme Aiman Mahmood, Keivan Sedhigi, Benjamin Riedmueller, Ulrich Herr
Affiliations : Institute for Micro- and Nanomaterials, Ulm University, D-89081 Ulm, Germany

Resume : ZnO is a promising material for photovoltaic and photocatalytic applications since its band gap is in the near UV. It is already applied in large quantities for UV protection. We report about a study of nanocrystalline commercial ZnO which has been subjected to an annealing treatment in reducing atmosphere. Intensive blue-green luminescence is found after that treatment, which is attributed to defects. The material has been characterized in detail using photoluminescence spectroscopy, and subsequently been combined with a commercial Si solar cell. A quantitative model has been developed to compare the experimental results with theoretical predictions. For this purpose, we have determined the photoluminescence quantum efficiency of the powders. In addition, we have determined the transmission spectrum of glass slides coated with PDMS films containing varying amounts of the luminescent powder. By combining the measured data with spectrally resolved photocurrent measurements of plain and ZnO-covered Si cells, we are able to quantitatively model the behavior of the system. I t is found that under the present conditions the strong back-scattering from the ZnO layer leads to a reduction of the overall efficiency of the system. However, in the UV region we find an enhancement of the photocurrent. The model allows us to predict the conditions for improving cell efficiency by this method and amount of overall increase of the cell efficiency which can be reached.

Authors : M. Drev1,2, U. Opara Kra?ovec2, Andrej Čampa2, M. Topič2
Affiliations : 1CBS Institute, Prijateljeva cesta 12, Trebnje SI8210, Slovenia. 2Faculty of Electrical Engineering, Tr?a?ka 25, SI1000 Ljubljana.

Resume : The challenge of more cost effective solar cells still persists. The main theoretical limitation of the solar cell efficiency is the spectral mismatch of the solar spectrum and the absorber layer in the solar cell. One way to overcome this limitation is by using down-conversion (DC) process where a high energy photon could be split into two photons with lower energies, thereby increasing the photocurrent. The DC-layer placed on the front of the solar cell consists of a host material and a luminescent species where the host material can mitigate the reflection losses on the air/glass interface and act as antireflective coatings (ARC) in the system. Sol gel films in the TiO2 ? SiO2 binary system has already been widely investigated for various optical applications. With the control of the TiO2 content we tailor the film refractive index (determined from R&T measurements using NIKA software) from 1.48 at wavelength 550 nm for silica films to 2.10 for titania films. In the experimental part we focus on the inclusion of the DC/AR concept into the dye sensitized solar cells (DSSCs). Films with different refractive indexes, i.e. pure SiO2, TiO2/SiO2 = 1/1, TiO2/SiO2 = 1/20 and pure TiO2 were prepared by sol-gel synthesis. The concentrations of La3 (La = Sm, Eu) ions vary from 0 to 2 mol %. The comparison of the DC/AR effect of different layers in DSSCs will be presented. In addition, the spectral response of the DSSCs together with I/V measurements will be reported.

Authors : Gianmarco Griffini, Luigi Brambilla, Marinella Levi, Chiara Castiglioni, Mirella Del Zoppo, Stefano Turri
Affiliations : Department of Chemistry, Materials and Chemical Engineering "Giulio Natta" of Politecnico di Milano (Italy)

Resume : In the field of sunlight conversion and management, luminescent solar concentrators (LSCs) represent a promising technology to reduce manufacturing and installation costs of traditional photovoltaic (PV) systems. In the best performing LSC devices, organic dyes are commonly employed as the luminescent species, with perylene-based dyes being the most widely utilized. One limitation of these dyes is their need to be highly soluble in the carrier matrix material (typically polymers) in order to obtain high device efficiencies. In fact, poor dye solubility may lead to the formation of non-luminescent dimers and aggregates that causes drop of fluorescence quantum yield and device performance. In the attempt to overcome this limitation, we report on the preparation of fluorescent films based on PMMA doped with anthracene (Ac)-tetracene (Tc) host-guest cocrystals and their use as down-converting systems in LSCs. Since in these cocrystals the guest molecule is the main responsible for the fluorescence emission and is present at very low concentrations compared to the host molecule, formation of non-luminescent aggregates is not expected. Different process parameters were varied and their effect on LSC device performance was evaluated, leading to optical efficiencies approaching 24%. This first demonstration of the use of molecular cocrystals as novel fluorescent species in LSC devices opens up new strategies for the preparation of efficient fluorophores for LSC applications.

Authors : Gianmarco Griffini, Marinella Levi, Stefano Turri
Affiliations : Department of Chemistry, Materials and Chemical Engineering "Giulio Natta" of Politecnico di Milano (Italy)

Resume : Fluorescent polymeric coatings may be successfully employed in the field of solar energy management as thin-film luminescent solar concentrators (LSCs). Although polymers such as polycarbonate or poly(methyl-methacrylate) (PMMA) are commonly used as dye-doped host matrix materials in LSCs, devices based on these systems still present limited lifetimes resulting from the relatively poor photo-stability of the polymeric carrier. To overcome this durability limitation, new crosslinked fluoropolymeric systems are prepared and characterized with the aim of proposing potential alternative host matrix systems for LSC applications. After exposing these systems to long-term UV–vis light exposure (over 1000 h of continuous accelerated weathering), the chemical, physical and morphological modifications occurring to the crosslinked fluorinated coatings were correlated with the photovoltaic response over long-term operation of LSC devices. Better operational stability compared to reference PMMA-based devices was found in LSC systems based on the new crosslinked fluorinated coatings. Furthermore, the addition of commercial stabilizers (radical scavengers) on the fluoropolymer fluorescent coatings was found to further improve LSC device lifetime in the long-term light exposure regime (>600 h). The results of this study provide useful guidelines for the design of high durability fluorescent coatings for light management and photovoltaics.

Authors : Qingfeng Lin,1 Kwong-Hoi Tsui,1 Hungtao Chou,2 Qianpeng Zhang,1 Huiying Fu,1 Pengfei Qi,2 and Zhiyong Fan1*
Affiliations : 1Department of Electronic and Computer Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China SAR; 2 Clean Energy International, 46535 Fremont Blvd, Fremont, CA 94538, USA.

Resume : A photovoltaic device is essentially a solar energy harvesting device converting the incoming photons to charge carriers. Therefore, the capability of capturing the incident photons is one of the key characteristics of a solar cell device. Since the reflectance loss of light leads to inefficient utilization of the incident photon, various anti-reflection (AR) schemes have been developed to achieve high efficiency solar cell devices. Conventionally, quarter-wavelength (λ/4) AR coatings have been widely used on the front surface of photovoltaic devices/modules. However, its effectiveness typically has wavelength and incident angle dependence, and high quality AR coating relies on chemical or physical deposition processes which increase the production cost. Meanwhile, nano/microstructures have been discovered with broadband light trapping capability which can significantly suppress device front surface reflection. Hence, a variety of nano/microstructures, such as nano/micro-pyramids, nanowires, nanopillars, nanocones, nanodomes, nanospheres, and so forth, have been extensively studied with different photonic materials, such as Si, Ge, CdTe, and Cu(In, Ga)Se (CIGS), etc. Although these structures have demonstrated appealing performance on photon management, many of them have been fabricated with costly and/or destructive methods, such as lithographic and wet/dry etching approach. And these approaches may not be necessarily applicable for thin film photovoltaics, especially for flexible applications. On the other hand, fabricating active photovoltaic materials into nano/microstructures introduces defects and the increased surface recombination, hence it needs to be carefully designed and performed. In this work, we have utilized a facile molding process to fabricate flexible plastic AR films with three-dimensional nanocone arrays on the front surface. The geometry of the nanocones, i. e. pitch and height, can be precisely controlled by tuning the structure of the inverse nanocone mold fabricated with anodization in conjunction with nanoimprint. The AR films can be readily attached to flat substrates, such as glass and Si, without adhesive glue. Therefore, their effectiveness has been examined on high efficiency CdTe thin film solar cells fabricated on glass substrates. The optical reflectance measurements and simulations have shown that the nanocone structure can significantly reduce the reflectance of the glass window above the CdTe light absorbing layer, resulting in appreciable device performance improvement confirmed by both current-voltage characteristics and quantum efficiency measurements. Furthermore, it was found that the improved AR effect can be observed with oblique light incident angle, which is highly beneficial for practical deployment of photovoltaic panels. Particularly for the studied high performance CdTe solar cell devices, it was found that over 1 kWh/m2 daily electrical energy output can be achieved with the nanocone AR film, indicating ~7% overall enhancement over the same device without the nanocone AR film. Besides intriguing AR property, it was also discovered that the nanocone structures are superhydrophobic with high water contact angle. This effect suggests the plastic AR film can possess self-cleaning function which is favorable for solar cells/modules deployed in dusty environment.

Authors : Yunae Cho (1), Minji Gwon (1), Dong-Wook Kim *(1), Joondong Kim (2)
Affiliations : (1) Department. of Physics, Ewha Womans University, Seoul, 120-750, Korea; (2) Department of Electrical Engineering, Incheon National University, Incheon 406-772, Korea

Resume : We have fabricated and investigated photovoltaic characteristics of crystalline silicon (Si) solar cells with nanostructures on the surface. The periodic nano-patterns were fabricated by nano-imprint lithography. Overall photovoltaic performance of the nanostructured cell surpassed that of the planar counterpart. In particular, increase of the short-circuit current was noticeable. We investigated influences of the geometric parameters (including height and period) on light absorption enhancement using FDTD (finite difference time domain) simulations. The numerical results clearly showed that the patterned nanostructures enabled surface concentrated field distribution. Our comparative studies, including both experiments and simulation, could suggest strategies to realize ultrathin crystalline Si solar cells.

Authors : Sangho Kim, Vinh Ai Dao, Chonghoon Shin and Junsin Yi*
Affiliations : Department of Energy Science, Sungkyunkwan University, Suwon, 440-746, Korea

Resume : Anisotropic etching of single-crystalline solar cells is used to increase the light absorption and surface area for high conversion efficiency. Because the conventional anisotropic etching process is limited for increasing surface area, unique surface structures are necessary. In this paper, we use the etching variety of techniques for the angle is increased, making the structure a large aspect ratio. The etching technique used is a metal assisted chemical etching and inductively coupled plasma reactive ion etching (ICP-RIE). In order to improve the uniformity and reproducibility, the surface of the substrate, forming a mask using photolithography. Further, in case there is damage to the surface by the process, were damage remove etching due to defect of the surface. Using this process, we achieved pillar-type surface structure with 1:1.5 aspect ratios in reactive ion etching (RIE), and the aspect ratio was increased further to 1:2.3 with various etching conditions. The angle of pillar structure was increased 68.2° to 82.9°. This new technique can be used to increase the aspect ratio and surface area for high efficiency c-Si solar cells. We Were analyzed by measuring Field Emission Scanning Electron Microscope(FE-SEM), reflection, and lifetime for the surface analysis.

Authors : Di Zhou1, Y. Pennec1, B.Djafari-Rouhani1, O. Cristini2, T. Xu3, Y. Lambert1 and D. Stiévenard1
Affiliations : 1- Institut d’Electronique et de Microélectronique et de Nanotechnologies, IEMN, (CNRS, UMR 8520), Groupe de Physique, Cité scientifique, avenue Poincaré, 59652 Villeneuve d’Ascq, France 2 - PHLAM, UMR8523, Université de Lille 1, 59652 Villeneuve d’Asq Cédex, France 3- Key Laboratory of Advanced Display and System Application, Shanghai University, 149 Yanchang Road, Shanghai 200072, People's Republic of China

Resume : Surface nanostructuration is an important challenge for the optimization of light trapping design in solar cell. We present simulations of the optical properties and efficiency of nanostructured (micropilars-MPs- or nanocones-NCs-) silicon based solar cells as well as measurements of their optical absorption. We address the simulation using the Finite Difference Time Domain (FDTD) method, well-adapted to deal with a periodic set of nanostructures. We study the effect of the period, the bottom diameter, the top diameter and the height of the MPs or NCs on the efficiency of the associated solar cell, assuming that one absorbed photon induces one exciton.. We give general trends taking into account diameter, length, period at the same time. Moreover, nanostructured structures have been processed and allow to compare experimental results with simulations. In every case, a good agreement is found.

Authors : Angelo Bozzola , Piotr Kowalczewski, and Lucio Claudio Andreani
Affiliations : Department of Physics, University of Pavia, Via Bassi 6, I-27100 Pavia, Italy

Resume : Thin-film silicon solar cells with light trapping emerged as an alternative to crystalline silicon (c-Si) devices based on bulk wafers. Although material costs can be reduced adopting a thin-film approach, conversion efficiency is typically lower than in bulk c-Si counterparts [1]. This is mainly due to optical losses and recombination induced by defects in the bulk of the active material and at its interfaces. The aim of this theoretical work is to quantify the impact of transport losses in devices incorporating light trapping. We illustrate a new electro-optical model [2] which is based on analytic solutions of drift-diffusion equations in p-n junctions c-Si solar cells, and it is validated against numerical calculations obtained with the Silvaco ATLAS software. By calculating efficiency with different bulk and surface qualities, we find that devices with thickness in the range 10-80 μm and light trapping at the Lambertian limit can outperform bulk cells with equivalent material qualities. By calculating the constrains on bulk and surface recombinations that ensure these results, we find that the main ingredients for highly efficient thin-film c-Si solar cells are already within present-day technological capabilities, although they still have to be implemented jointly into a single device. [1] M. A. Green et al., Progr. Photovolt: Res. Appl. 21, 1 (2013). [2] A. Bozzola et al., J. Appl. Phys., submitted. [3] M. A. Green, Progr. Photovolt: Res. Appl. 10, 235 (2013).

Authors : Piotr Kowalczewski, Angelo Bozzola, Marco Liscidini, and Lucio Claudio Andreani
Affiliations : Department of Physics, University of Pavia, Via Bassi 6, I-27100 Pavia, Italy

Resume : In our previous work, we have demonstrated that a simple model of Gaussian roughness is able to describe the optical properties of state-of-the-art rough substrates (e.g., Neuchatel and Asahi-U) [1], and that the optimized rough texture allows to reach the absorption corresponding to a perfectly random (Lambertian) scatterer, often taken as a theoretical limit. In this contribution, we use rigorous electro-optical modelling to perform a complete analysis of randomly rough thin-film silicon solar cells approaching the Lambertian Limit. Within the presented framework, we determine the efficiency dependence on the absorber thickness, showing that thin-film c-Si solar cells can reach 20% efficiency with a bulk material quality typical for photovoltaic modules. We also discuss the role of surface recombination, demonstrating that the efficiency of the cell is limited by the recombination at the rear interface, and thus it is possible to engineer the front surface to a large extent without compromising on efficiency. This general approach can be also applied to different light-trapping structures, such as photonic crystals, and provides a route to design high efficiency solar cells with a micron-scale absorbing layer. [1] P. Kowalczewski, M. Liscidini, and L.C. Andreani, Opt. Lett. 37, 4868–4870 (2012). [2] P. Kowalczewski, M. Liscidini, and L.C. Andreani, Opt. Express 21, A808–A820 (2013).

Authors : Manuel J. Mendes(1), Estela Hernández(2), Esther López(2), Pablo García-Linares(2), Iñigo Ramiro(2), Irene Artacho(2), Elisa Antolín(2), Ignacio Tobías(2), Isodiana Crupi(1), Antonio Martí(2), Antonio Luque(2)
Affiliations : 1) MATIS CNR-IMM, via S. Sofia 64, 95123 Catania, Italy 2) Instituto de Energía Solar, E.T.S.I. Telecomunicación, Universidad Politécnica de Madrid, Avda. Complutense 30, 28040 Madrid, Spain

Resume : A major loss mechanism in conventional solar cells is their inability to exploit below-bandgap photons. Quantum dot intermediate band solar cells (QD-IBSCs) are a promising route to overcome this limitation. These cells contain an array of QDs whose confined ground-states can generate current from photons with energy below their host semiconductor bandgap, enabling higher cell efficiency. This has been verified in prototype devices, but the impact of the dots on the cell performance is still marginal mainly due to their weak light absorption at the confined levels. A novel approach is presented here to increase the QDs absorption by coupling them with metal nanoparticles (MNPs) sustaining surface plasmons (SPs). A wet-coating method was developed to self-assemble uniform arrays of colloidal quantum dots (CQDs) and MNPs using scalable and inexpensive procedures [1]. The CQDs are patterned side-by-side with the MNPs in the array, with controllable inter-particle distances. The MNPs act as dipolar antennas at their SP resonance, bringing the incident light from the surroundings and focusing it in their intense scattered near-field where the CQDs are located. This can pronouncedly enhance the light absorption in the QDs by up to two orders of magnitude. These developments open the way for new plasmon-enhanced IBSC designs, with the intermediate band formed by the confined ground-states of colloidal QDs coupled with MNPs. [1] M.J. Mendes et al. Nanotechnology 24 (2013) 345402

Authors : I. Crupi 1, S. Morawiec 1, M. Müller 2, K. Ganzerová 2, J. Holovský 2, A. Vetushka 2, M. Ledinský 2, M. J. Mendes 1, S. Mirabella 1, F. Priolo 1,3, A. Fejfar 2
Affiliations : 1) MATIS CNR-IMM and Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, Italy 2) Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague, Czech Republic 3) Scuola Superiore di Catania, Università di Catania, via Valdisavoia 9, 95123 Catania, Italy

Resume : Metallic nanoparticles (MNPs) are gaining importance for application in silicon solar cells as an efficient method to enhance the light absorption in the active layer due to their pronounced scattering properties at the surface plasmon resonance. It has been recently demonstrated that the preferential location for the MNPs in solar cells is in the rear contact, embedded in the transparent conductive oxide (TCO) layer which separates the back mirror from the absorber, forming what is known as a plasmonic back reflector (PBR) [1]. Here we study how light trapping in silicon thin films is influenced by the distinct layers composing the PBR structures. The PBRs were fabricated by sequential depositions of silver (Ag) and aluminium doped zinc oxide (AZO) and the MNPs were formed in a self-assembly manner by solid-state dewetting of thin Ag films. A 1 m thick hydrogenated nanocrystalline silicon (nc-Si:H) layer was deposited on top of the PBR structures at different stages of completion. The obtained surface topographies were investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The light trapping induced by the MNPs is evidenced by the enhancement of the Raman signal (acquired with weakly absorbed excitation laser excitation at 785 nm) in the nc-Si:H film and spectrally analyzed by diffused and total reflectance in the UV-Vis-NIR wavelength range. [1] S. Morawiec et. al., Nanotechnology, 24 (2013) 265601

Authors : Rachid Ouertani, Chohdi Amri, Abderrahmen Hamdi, Wissem Dimessi, Ezzaouia Hatem.
Affiliations : Laboratoire de Photovoltaique, Centre de Recherches et des Technologies de l'Energie, Technopole de Borj-Cedria, BP95, 2050 Hammam-if.Tunisie.

Resume : in this paper, We use a novel method (MAC-VEP) to perform a wild grooving of p type (100) oriented silicon. The MAC-VPE method is a combination of the metal assisted chemical wet etching and vapor phase etching methods. Silver loaded samples were exposed to the vapor of HF- H2O2. While all processing parameters are maintained constant, etch time was varied from 20 to 120 min. We explain the role of the reactants before and after vapor etching. We focus on the double role of silver. First the noble metal catalyses the reductions of hydrogen peroxide molecules and hydrogen formation. Second, when dissolution or mass transport of by product transport slows down, Silver is redistributed to decorate wall macropores and catalyses a local anisotropic chemical new etching. Finally residual silver trapped inside the pores affects the reflectivity of the nanostructured samples. We investigate the effect of an iterative removal of residual silver on reflectivity of the nanostructured silicon. SEM images show a complex nanostructure composed of forests of nonobelts and some sporadic nanowires. The antireflective character of vapor etched silicon and the residual silver has been evaluated by reflectivity measurement.

Authors : R. Mailhes, T. Nychyporuk, M. Lemiti and V. Lysenko
Affiliations : Institut des Nanotechnologies de Lyon (INL)

Resume : With the aim of reducing the cost of photovoltaic, several strategies are investigated. Reducing the amount of raw material by reducing the thickness of the absorbing layer can lower the manufacturing costs. Unfortunately absorption of a thin layer is lowered compared to its thick counterpart. One of the innovative way to improve the light trapping within a thin layer, thus enhancing the absorption and the cell performance, is to take advantage of the plasmonic effects, which have seen a growing interest these last years. While the major tendancy is to use metallic nanoparticles on the surface of solar cells, we investigated a novel design where an array of vertical silver nanopillars is embedded within the silicon layer. This particular design has been found thanks to numerical simulations, and we report here on the results showing an increase of the absorption in the near infrared spectral range. Preliminary results in engineering such modelled plasmonic substrates are also presented in this communication.

Authors : Dongsheng Li, Meng Yuan, and Deren Yang
Affiliations : State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University

Resume : Changing the size and morphology of silver nanoparticles leads to kinds of distinctive scattering properties and localized surface plasmon resonant effect, which has demonstrated a huge potential in photovoltaic applications. We have studied lumpy silver particles as rear located strengthen materials for silicon thin-film solar cell. Though theoretical simulations, we find that the large-size lumpy silver particle has a more advantageous property of scattering incident light back than the spherical particle in a broad wavelength range. This kind of large-size silver particles can be used as rear-position strengthening materials for silicon thin-film solar cells. We demonstrate that when the Ag particles' coverage density is 10%, the light absorption enhancement is optimal.

Authors : A. Čiegis, Š. Meškinis, A. Vasiliauskas, K. Šlapikas, R. Gudaitis, S. Tamulevičius
Affiliations : Institute of Materials Science of Kaunas University of Technology, Savanorių 271, 50131 Kaunas, Lithuania

Resume : In last few years group I metal (Ag, Au, Cu) nanoparticle based plasmonic layers received considerable interest due to the increased light trapping in the underlying semiconducting absorber layer. Environmental stability of these nanoparticles can be increased by coating them with ultrathin protective dielectric (or semiconductor) layer. In addition such layer can be used for additional control of the position of the surface plasmon resonance by changing refractive index of the media surrounding nanoparticle. Along with the coated core shell nanoparticles, metal and dielectric nanocomposites can be used for such a purpose. Diamond like carbon is prospective matrix material of such a nanocomposite due to the high optical transmittance, high hardness, corrosion resistance as well as possibility control refractive index and electrical properties in wide range. In present study diamond like carbon films containing Ag nanoclusters (DLC:Ag) were deposited by reactive unbalanced magnetron sputtering. Optical properties of synthesized nanocomposite films were investigated in 180-1100 nm range. Structure of the films was studied by Raman spectroscopy, x-ray diffractometry and atomic force microscopy. Effects of the annealing on optical and electrical properties as well as structure of DLC:Ag films were studied. Significant changes of the position, shape and width of the plasmon resonance peak were observed.

Authors : P. Basa, L. Makai, A. A. Khosroabadi, P. Gangopadhyay, R. A. Norwood
Affiliations : P. Basa; L. Makai: Semilab Semiconductor Physics Laboratory Co. Ltd., Budapest XI., Prielle K. u. 2. 1117, Hungary A. A. Khosroabadi; P. Gangopadhyay; R. A. Norwood: College of Optical Sciences, The University of Arizona, Tucson, Arizona 85721, USA

Resume : Plasmonic solar cells are devices where photovoltaic energy conversion efficiency is enhanced by optical plasmons. In these structures the surface plasmon resonance (SPR) is generated by metal or metal-coated nanoparticles which results in increased optical absorption in the active layer. Application of such advanced structures are intensively studied nowadays both for inorganic and organic thin film solar cell devices. The optical absorption around plasmon resonance frequencies is strongly dependent on the size, shape, and structural arrangement of the nanoparticle layer, therefore it is essential to study the dependence of their optical response on the actual sample configuration. Spectroscopic ellipsometry (SE) is proven to be a powerful tool for characterizing various thin films as well as 3D nanostructures [1]. It provides the optical thickness, refractive index and absorption spectra based on optical modeling of the sample. In this work, samples with ITO/Ag hybrid nanorod layers were studied along with their planar reference pairs by SE to study the surface plasmon enhancement of the 3D structure. The results show clear SPR frequency dependence on the nanostructure composition, as well as on the refractive index of the embedding medium. [1]. A. A. Khosroabadi et. al, Opt. Lett. 38,3969 (2013).

Authors : Shuhei Miura, Kazutoshi Suzuki, Shinichi Noda, Shuichi Nonomura
Affiliations : Environmental and Renewal Energy Systems Division Graduate School of Engineering, Gifu University

Resume : The surface textured morphology of transparent conductive oxide (TCO) substrates enhances light absorption in Si thin-films by trapping light, results in an increase in the current density of Si thin-film solar cells. However, the highly textured morphology often forms structural defects within the Si active layer that decrease the photovoltaic performance parameters of solar cells, such as the VOC and fill factor FF. To overcome this trade-off, we fabricated a flattened light-scattering TCO substrate based on a strongly light-scattering metal-oxide nanoparticle layer. In this work, we used two types nanoparticle layer of ZnO and TiO2. The metal-oxide nanoparticle layer was formed between glass substrate and low-resistivity AZO film. The nanoparticle layer scatters light strongly without forming textured morphology on its surface. Moreover, haze value of nanoparticle layers significantly increased with the constitution particle size. After AZO film deposition, nanoparticle TCO substrates showed very low root-mean surface (RMS) roughness of less than 10 nm, and good sheet resistance of approximately 10 Ω/□. Furthermore, haze value of nanoparticle TCO substrates showed high relativity with based nanoparticle layer. These results suggest that metal-oxide nanoparticle layer is effective for improvement of light-scattering property of TCO substrate without increasing its surface roughness.

Authors : A. Gentile, G. Cacciato, F. Ruffino, R. Reitano, G. Scapellato, E. Bruno, M. Zimbone, S. Lombardo, A. Battaglia, M. G. Grimaldi
Affiliations : A. Gentile, G. Cacciato, F. Ruffino, G. Scapellato, E. Bruno, M. Zimbone, M. G. Grimaldi Department of Physics and Astronomy and MATIS- CNR-IMM - University of Catania, via S. Sofia 64 95123 Catania, Italy R. Reitano Department of Physics and Astronomy - University of Catania, via S. Sofia 64 S. Lombardo CNR-IMM, Stradale Primosole 50, I-95121 Catania, Italy A. Battaglia 3SUN S.r.l. Contrada Blocco torrazze sn - Zona Industriale 95121 - Catania, Italy

Resume : Experimental and numerical investigations have shown that metallic nanoparticles (MNPs) exhibit localized surface plasmon resonances leading to selective photon absorption and enhancement of local electromagnetic field near the MNPs by order of magnitude. As a consequence, MNPs located near to a solar cell active layer can induce light trapping improvement and so increase the cell efficiency. However, to obtain high amplification factors of the incident field, the control over shape, size and spatial order of MNPs is crucial. Starting from these ideas, in this work we propose a laser-based methodology to form MNPs on transparent conductive oxide (TCO) surfaces by irradiations of deposited Au and Ag films. In particular, the breakup of metal films into MNPs is observed as a consequence of melting and solidification processes induced by the irradiations. The mean MNPs size and surface density evolution is also analysed as a function of the laser fluence to find correlations allowing the MNPs structural characteristics tuning. Optical characterizations of the systems highlight, in the absorption spectra, plasmonic peaks whose shapes and positions can be tuned by size and morphology of MNPs. Furthermore, electrical characterizations evidence a reduction in the sheet resistance of the TCO due to the MNPs presence. We compare these results with those obtained for the same systems when standard furnace annealing processes are used to form the MNPs instead of the laser ones.

Authors : Mona Zolfaghari Borra (a-b), Seda Kayra Güllü (b-c-d), Raşit Turan (a-b-c), Alpan Bek (a-b-c)
Affiliations : a- Micro and Nanotechnology Program of Graduate School of Natural and Applied Sciences, Middle East Technical University, Ankara 06800, Turkey b- Center for Solar Energy Research and Applications, Middle East Technical University, Ankara 06800, Turkey c- Department of Physics, Middle East Technical University, Ankara 06800, Turkey d- Electrical and Electronics Engineering Department, Physics Unit, Atılım University, Ankara 06836, Turkey

Resume : One of the plasmonic enhancement mechanisms in photovoltaic solar cells (SCs) relies on increased light absorption due to increased optical path length of incident photons in the active region. Plasmonic enhancement interfaces for SCs are thin but strong light scattering layers of a few nanometers that redirect incident photons in to the plane of p-n junction. In this study, the decoration of metal nanoparticles (MNPs) by the self-organized mechanism of dewetting is utilized as a suitable method for plasmonic interface integration to large area full-scale crystalline silicon (Si) SC devices. Reflection measurements are performed on both flat and textured Si SCs in order to investigate the local plasmonic resonances of the MNPs. The effects of particle size and thickness of silicon nitride (Si3N4) anti-reflection coating layer are investigated by reflection measurements and the shift of plasmon resonance peak position. It is found that surface roughness, annealing time, annealing temperature, and varying Si3N4 thicknesses can be used as mechanisms to control the size distribution, shape of the resultant nano-islands, and SC efficiency. The findings on the most suitable nanoparticle system production parameters by this method, depends on the applied substrate properties which are expected to guide further applications of plasmonic interfaces and also to the other kinds of device structures in the ultimate quest for attaining affordable high efficiency SCs.

Authors : P. Dubcek1, B. Pivac1, N. Radic1, S. Bernstorff2
Affiliations : 1 R. Boskovic Institute, P.O. Box 180, Zagreb, Croatia; 2 Elettra-Sincrotrone Trieste, SS 14, km 163.5, Basovizza (TS), Italy

Resume : It is known that Al nanoparticles embedded in the photoactive layer of a bulk heterojunction enhances the efficiency of photovoltaic devices. Doping with plasmonic nanoparticles leads to a power conversion efficiency improvement that can reach 20%. A good control of the size and space distribution is crucial for future device aplications. Here we report the investigation of post annealed magnetron sputtered aluminium thin film on monocrystalline silicon. The AFM and GISAXS techniques were employed in the size and size distribution investigation. The nominal film thickness was varied from 5 to 40 nm, while the annealing temperature was up to 600 C. Optimal sizes and narrow size distributions were obtained for higher temperatures, which are however prohibitive when a silicon heterojunction is present. Some compromise will be necessary in functional device production.

Authors : Kuk-Hyun Cho, Hyo Sik Chang
Affiliations : Graduate school of Energy Science Technology, Chungnam National University

Resume : The surface passivation is very important to develop high efficiency crystalline silicon solar cells. Al2O3 film provides outstanding passivation quality for PERC solar cell. Al2O3 layer as the passivation layer is usually stacked with thicker capping layers, such as SiO2, SiNx and SiON films. These capping layers protect the thin Al2O3 layer at high thermal process. The thermal stability was compared to investigate Al2O3-SiON stacks and Al2O3-SiNx stacks after annealing process at 450o and 850oC. Subsequently, the surface morphology and element are observed by SEM-EDS. The minority carrier lifetime is measured by the quasi steady state photoconductance decay (QSSPC). After annealing, the hillock occurred on Al2O3-SiNx Stacks surface by blistering phenomena but Al2O3-SiON Stacks surface were clean. Lifetime of Al2O3-SiON increased when it was annealed at 450℃ in forming gas ambient, whereas lifetime of Al2O3-SiNx stacks decreased. Although lifetimes for Al2O3-SiON stacks reduced after annealing at 850℃, the lifetime for SiON capping layer was higher than that of Al2O3-SiNx stacks layer. The results indicate that Al2O3-SiON Stacks have excellent thermal stability of surface passivation layer than Al2O3-SiNx stacks.

Authors : M. Ben Rabha1,2, M. Hajjaji2, M. Gaidi1,3, B. Bessais2
Affiliations : 1Riyadh College of Technology, Technical and Vocational Training Corporation 2Laboratoire de Photovoltaïque, Centre de Recherches et des Technologies de l’Energie, Technopole de Borj-Cédria, BP 95, 2050 Hammam-Lif, Tunisia 3Emirates college of Technology

Resume : Metal-nanoparticle-assisted chemical etching is an excellent developed wet etching method of producing uniform mono and multicrystalline silicon nanostructure from patterned metallic film on crystalline silicon surface. Creation of different silicon nanostructure morphologies by changing the etching time and its effects on optical and optoelectronic properties was investigated. The investigation of average nanorod lengths from 0 nm to 1.4 μm reveals that the Si wafer decorated with 0 nm thick nanorods has optical reflection of 35% but the one with 1 μm thick nanorods has optical reflection of 2% in 300- 1200 nm wavelength range. Keywords: semiconductor nanostructure; chemical etching; optical property.

Authors : R. Ouertani, C. Amri, M. Ben Rabha
Affiliations : Laboratoire de Photovoltaïque, Centre de Recherches et des Technologies de l’Energie, Technopole de Borj-Cédria, BP 95, 2050 Hammam-Lif, Tunisia

Resume : In this work, preparation and surface modification of porous silicon (PS) and silicon nanowires (SiNWs) grown by chemical etching method had been investigated. The detail study of optical and optoelectronic properties of PS and SiNWs were demonstrated. The morphological, optical and optoelectronic properties were studied using scanning electron microscope (SEM), LAMBDA 950 UV/Vis/NIR Spectrophotometer equipped with an integrating sphere and Laser-beam-induced current (LBIC) technique. Results show that chemical etching of the silicon surface drops the total reflectivity to about 1 % in the 400-1100 nm wavelength range of SiNWs and the minority carrier diffusion length enhances to about 180 µs of the PS. Keywords: Porous silicon; silicon nanowires; LBIC; reflectivity.

Authors : A. HAJJAJI1, 2, I. Ka1, M. Ben Rabha2, M. GAIDI2, B. BESSAIS2, and M. A. El KHAKANI1
Affiliations : 1Institut National de la Recherche Scientifique, INRS-Énergie, Matériaux et Télécommunications, 1650, Blvd. Lionel-Boulet, Varennes, QC, Canada J3X-1S2 2Laboratoire de Photovoltaïque, Centre de Recherches et des Technologies de l’Energie, Technopole de Borj-Cédria, BP 95, 2050 Hammam-Lif, Tunisia

Resume : The surface passivation of the silicon nanowires (SiNWs) by Al2O3 and its effects on optical and optoelectronic properties was investigated in this work. An Al2O3 film with a thickness of 80 nm is deposited by pulsed laser deposition (PLD). The level of surface passivation is determined by technique based on photoconductance. As a result, the effective minority carrier lifetime increase from 1µs to 100 µs and the reflectivity reduce from 37% to about 1% in 300-1100 nm wavelength range after SiNWs /Al2O3 passivation. Keywords: SiNWs; surface passivation; reflectivity; Al2O3; PLD.

Authors : André Luis F. Cauduro, Stefan N. Johansen, Horst-Günter Rubahn, and Morten Madsen
Affiliations : NanoSYD, Mads Clausen Institute, University of Southern Denmark, Alsion 2, 6400-Sønderborg, Denmark.

Resume : A general way to enhance the device performance of either organic or inorganic solar cell devices is to increase the light absorption in the active layer by employing nano- or micro-structured features that can trap light at specific wavelengths. One simple and effective version of this can be by integration of grating structures with tuned pitch distances on the back electrodes of solar cells [1]. In that direction, structuring the surface of polydimethylsiloxane (PDMS) opens up as an interesting cost-effective method, due to its ease of processing, large-scale compatibility and flexible nature, which has made it highly applicable in soft-lithography processes. In this work, we have used wrinkled surfaces produced in a fast, cheap and lithography-free way on PDMS substrates to increase the optical path length of light in P3HT:PCBM solar cells and thus enhance both the light absorption and the efficiency of the devices. The PDMS substrates were fabricated by controlled stretching of the samples during exposure to oxygen plasma, which led to formation of groove-like structures with controlled pitch distances in the range of 200- 800 nm. The effect of structure dimensions on the absorption of light in the cells is studied to optimize the efficiency of the cells. [1] R.M. de Oliveira Hansen, Y. Liu, M. Madsen, and H.-G. Rubahn, Nanotechnology, 24, 145301, (2013).

Authors : Abdennacer Benali(a), Jérôme Michallon(c), Philippe Regreny(a), Emmanuel Drouard(a), Pedro Rojo(a), Alain Fave(b), Anne Kaminski-Cachopo(c), Michel Gendry(a)
Affiliations : a Institut de Nanotechnologies de Lyon, UMR 5270 – Ecole Centrale de Lyon, 36 avenue Guy de Collongue, 69134 Ecully, France b Institut de Nanotechnologies de Lyon, UMR 5270 – INSA de Lyon, Bâtiment Blaise Pascal, 20 avenue Albert Einstein, 69100 Villeurbanne, France c Institut de Micro Electronique et de Photonique – Laboratoire d’Hyperfréquence et de Caractérisation, UMR 5130, 3, rue Parvis Louis Néel, BP 257, 38016 Grenoble, France

Resume : Arrays of III-V direct-bandgap semiconductor nanowires are promising candidates for future photovoltaic devices due to their high optical absorption and their ability to be grown on low cost semiconductor substrates like silicon. The core-shell structure is particularly interesting as the electron-hole pair separation occurs in the radial direction and the photogenerated minority carriers have to travel short distances (typically the radius of the nanowires) improving the collection probability in case of well passivated surfaces and interfaces. The aim of this study is to find the optimal geometry (length, height and diameter) of GaAs nanowire array grown on a silicon substrate in order to have the best absorption of the incident photons. For this purpose, we perform electromagnetic simulations by using homemade RCWA (Rigourous Coupled Wave Analysis) software. Our simulations take into account the core-shell structure, the passivation layer (GaAlAs) and the anti-reflection coating, but also the necessity to achieve current matching between the GaAs nanowire based solar cell and the silicon substrate solar cell. In fact, in the final multijunction solar cell structure we aim to realize the silicon (substrate) and GaAs (nanowire array) solar cells connected in series.

Authors : Kudryashov Dmitry, Gudovskikh Alexander, Morozov Ivan
Affiliations : SPbAU RAS - St. Petersburg Academic University

Resume : There are currently many investigations in the field of photovoltaics making current technology solar cells more efficient and cheaper. Multijunction solar cells based on GaPNAs/Si lattice matched heterostructures are very promising. However GaPNAs grown to-date appears to have very short diffusion lengths. Thus it is very hard to provide current matching of GaPNAs- and Si-subcells. This paper presents a 2D-simulation of new design two junction nanowire (NW) solar cells based on GaPNAs/Si lattice matched heterostructures. The effects of NW's height and distance on the electrical characteristics were investigated. The efficiency of NW solar cells increases with increasing of NW height due to collecting more carriers and reaches a plateau at 4 microns. Decreasing the distance between NWs leads to increase of solar cells efficiency due to increase of current spreading. Also it was shown that NW solar cells with lower thickness of GaPNAs layer are able to reach higher efficiency compared to its flat design.

Authors : Chien-Ting Liu, Subramani Thiyagu, Chen-Chih Hsueh, Hong-Jhang Syu, Sung-Ting Yang, Pin-Chun Shen, Yu-Wen Cheng, Hao-Yu Wu and Ching-Fuh Lin
Affiliations : Graduate Institute of Photonics and Optoelectronics, National Taiwan University.

Resume : Novel silicon nanostructures featuring excellent light trapping effect, high mobility of Si and manufacturing compatibility, hence providing a suitable candidate for solar cell application. Here, we demonstrate a promising structure and key factors to achieve high-efficiency hybrid heterojunction solar cells, which can facilitate development for potential industrial production. Compared with traditional solar cells textured with only pyramid structures, the multi-textured pyramid/ silicon nanowire (SiNW) structure formed here exhibits much lower reflection and gives larger junction areas, hence increases device current density. Besides, by designing appropriate hole transporting layer (HTL) for matching effective refractive index with silicon, we can obtain extra anti-reflection effect and then further increase absorption of silicon. The total reflectance of multi-textured pyramid/SiNW with HTL is reduced to very low, only 2 to 4% for spectral range from 400 nm to 1000 nm in wavelength. Moreover, we manage to obtain higher fill factor and current density by further modification of HTL conductivity. Accordingly, by adopting the above promising tactic, we achieve high-efficiency hybrid heterojunction solar cells with power conversion efficiency of 11.50%, Voc of 0.49 V, Jsc of 39.17 mA/cm2 and FF of 59.35%. Hence it is promising for future commercial practice and application.

Authors : A.-L. Joudrier (1), F. Proise (1,2), R. Grapin (1), J.-L. Pelouard (2), and J.-F. Guillemoles (1)
Affiliations : (1) Institute of Research and Development on Photovoltaic Energy, EDF-CNRS-ENSCP, UMR 7174, 6 Quai Watier, 78401 Chatou cedex, France ; (2) Laboratory of Photonic and Nanostructures – CNRS, route de Nozay, 91460 Marcoussis, France

Resume : The decrease of photovoltaic devices cost is one of the main goals of the photovoltaic community. The main ways investigated are the increase of power conversion efficiency or the reduction of expensive material usage. Sun light concentration is a way to address this issue. An original method to concentrate light is the use of luminescent sheet concentrators (LSC), which acts as waveguide to concentrate light towards the photovoltaic (PV) cells. Its main advantage over imaging concentrator systems is that both direct and diffuse sun light components could be concentrated. Theoretically, concentration factors above 1000 are possible, but practically performances fall far short of these expectations. Computational simulations were realized for different systems: idealistic LSC, non-idealistic and with the addition of a photonic band stop above the luminescent sheet concentrator. They enabled to distinguish loss mechanisms, to determine their respective weight and to correlate them to physical parameters, such as the coverage fraction. Moreover, the high interdependency of loss mechanisms has been studied in some cases. In other hand, photonic band stop with an opal structure have been realized. Good monodisperse nanoparticles were obtained and the optical characterizations are in accordance with the dye used. The fabrication of the complete devices and their characterizations will be presented.

Authors : Aijaz Ahmad, Akshay Balgarkashi, S. Sengupta and S.Chakrabarti*
Affiliations : Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai-400076, Maharashtra, India

Resume : A comprehensive study on the impact of GaAs and InGaAs capping layers on the carrier transport in quantum dots (QDs) for infrared detectors and solar cell application is carried out. The effect of variation in dot monolayer coverage has been studied. For GaAs capped QDs, the activation energies obtained from temperature-dependent I-V measurements is found to increase with monolayer(ML) coverage upto 3.0 ML. This can be attributed to increase in width of the potential well and corresponding increase in bound-to-continuum energy gap. The spectral peaks were observed to be decreasing from 8.37μm to 7.38μm which affirms the above trend. For InGaAs capped QDs, the corresponding activation energies with increase in ML coverage were found to be much higher than those for the GaAs capped structures. Due to higher lattice mismatch in GaAs capped QDs, strain enhancement at apex of dots leads to significant In adatom migration reducing the height of QDs. InGaAs capping having smaller lattice mismatch with InAs QDs, the In atom migration is hindered and the QD size remains significantly unchanged which justifies the higher activation energies in these structures. The spectral peaks obtained here at lower wavelengths complements the above results. A decreasing trend in activation energy is observed for InGaAs capped structures. This can be ascribed to the increase in defect states arising out of higher strain induced due to higher ML coverage. Riber, France and DST, India is acknowledged.

Authors : Akshay Balgarkashi, Aijaz Ahmad, S. Sengupta and S.Chakrabarti*
Affiliations : Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai-400076, Maharashtra, India

Resume : We have studied the effect of post-growth rapid thermal annealing (RTA) on the stability of InGaAs capped quantum dots (QDs) for infrared photodetectors and solar cell applications. For comparison, GaAs capped quantum dots heterostructures have also been investigated. QDs with different monolayer (ML) coverage were grown using molecular beam epitaxy (MBE). It is observed that with increasing monolayer coverage the shift in the photoluminescence (PL) peak with RTA is suppressed. This is attributed to higher stability attained due to larger size of dots. Higher ML coverage also prevents the interdiffusion of In and Ga, which results in a reduced blueshift even at temperatures upto 7500C. A temperature of 8000C is, however, sufficient to elevate the interdiffusion enhancing the blueshift. The activation energies calculated from temperature-dependent PL measurements show minor variation with annealing upto 7500C for higher monolayer dots which is in excellent agreement with the above discussion. With the InGaAs capping, the PL peak shift is further reduced in contrast to the GaAs capped dots. The capping layer prevents In interdiffusion to a higher extent consequently reducing shift in the PL peaks. The restrained shift in luminescence even at higher annealing temperatures indicates good thermal stability which is preferable for QD device applications. Riber, France and DST, India are acknowledged.

Authors : M. Perani (1), N. Brinkmann (2), D. Cavalcoli (1), B. Terheiden (2)
Affiliations : (1) Department of Physics and Astronomy, University of Bologna, Italy; (2) Department of Physics, University of Konstanz, Germany

Resume : Silicon heterojunction solar cells (SHJ) attracted a lot of attention in recent years due to their high efficiency potential and low temperature production process. Nano-crystalline Silicon Oxy-Nitrides layers (nc-SiOxNy) are presently studied in view of their application as emitters in SHJ. SiOxNy films were deposited by Plasma Enhanced Chemical Vapor Deposition with different parameters and doping levels. A subsequent annealing treatment has allowed us to obtain the formation of nano-crystals. The nano-crystalline films have showed very high conductivity (1-40 S/cm) and low parasitic optical absorption. In order to better optimize these layers careful and extensive morphological, structural, electrical and optical characterizations were carried out. Structural and compositional analyses were performed by Raman and Fourier-Transform Infra-Red (FTIR) spectroscopy, which show an evolution of the crystalline fraction and film composition with deposition conditions. IV measurements were carried out in order to investigate the electrical properties of the layers. The films were optically characterized by spectral transmission analyses. Scanning and Transmission Electron Microscopy and Atomic Force Microscopy were carried out in order to understand at a microscopic level the physical properties of the films. The combination of macroscopical and microscopical techniques gives a fundamental insight on the properties of the material.

Authors : Aude Berbezier, Urs Aeberhard
Affiliations : IEK-5: Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany

Resume : Quantum dot (QD) based solar cells are promising concepts aiming to increase photovoltaic solar energy conversion efficiency. However, in these devices, to explain the physical mechanisms of carrier generation and recombination coupled to quantum transport at the nanoscale, one needs to resort to advanced quantum kinetic theories such as the nonequilibrium Green's function formalism (NEGF). We study the influence of the strength of interdot and dot-contact couplings on transport and radiative rates and consequently on the ultimate performance of photovoltaic devices with QD arrays as active medium. Using an effective model based on NEGF, we investigate the photovoltaic response of QD arrays in a wide band gap host matrix under non-polarized illumination. The dot-contact coupling strength is obtained via hybridization of electrode and adjacent quantum dot states, while interdot coupling parameters are derived using a localized QD orbital basis. Photogeneration and radiative recombination processes are described via a self-energy for electron-photon interaction in self-consistent Born approximation and for coupling to the classical electromagnetic field, which is obtained from rigorous solution of Maxwell's equations with optical material parameters computed from the same electronic structure as used for the transport simulation. We conclude that interdot and dot-contact couplings have a strong influence on the photovoltaic energy conversion process in QD based solar cells.

Authors : B. Dridi Rezgui1, R. Jemai2, K. Khirouni2, and B. Bessais2
Affiliations : 1Laboratoire de Photovoltaique, Centre de Recherches et des Technologies de l'Energie, Technopole de Borj-Cedria, BP 95, 2050 Hammam-Lif, Tunisia 2Laboratoire de Physique des Materiaux et des Nanomateriaux appliquee a l'Environnement, Faculte des Sciences de Gabes cite Erriadh, 6079 Gabes, University of Gabes, Tunisia

Resume : The concept of third generation photovoltaics which aims to significantly increase device efficiencies has been widely used in recent years. As a result, different semiconductor materials and nanostructures have gained researchers? interests for the development of various solar cells approaches. To this end, solar cells based on silicon quantum dots (Si-QDs) offer the best prospects to overcome the shortcomings of 2nd generation photovoltaics as silicon is abundant on the earth and allows for well-established and cost-effective large-scale fabrication techniques. This article describes the results of a systematic investigation of the influence of process parameters in HWCVD of Si-rich SixC1-x on several properties of the resulting films. The samples are characterized by Raman spectroscopy and Fourier transformed infrared spectroscopy (FTIR) to study the changes in the film structures with the variation of the deposition parameters and after an annealing treatment. The optical characterization of the nanostructured films by spectroscopic techniques such as photoluminescence and optical absorption measurements are performed in order to obtain a better knowledge of their physical characteristics related to the photovoltaic (PV) application.

Authors : Alessandro Mattoni[1], Giorgia Fugallo[2], Luigi Bagolini[1,3], Mark T. Lusk[3]
Affiliations : [1]Consiglio Nazionale delle Ricerche, Istituto Officina dei Materiali (CNR-IOM Cagliari, I-09042, Monserrato (Ca), Italy [2] European Theoretical Spectroscopy Facility (ETSF) / Laboratoire des Solides Irradiés (LSI), École Polytechnique, 91128 Palaiseau cedex, France. [3] Department of Physics, Colorado School of Mines, Golden, CO 80401, USA

Resume : Hydrogenated nanocrystalline silicon (nc-Si:H) is an emerging thin-film photovoltaic material that combines advantages of silicon (c-Si), like high carrier mobility, with less expensive production methods of amorphous silicon (a-Si). Among several processing issues, hydrogenation is critical in affecting the structural and electronic properties of nc-Si[1]. Here, we report molecular dynamics theoretical results on the effect of dissolved hydrogen on the thermally induced recrystallization[2] of nanocrystalline silicon. The recrystallization rate decreases exponentially with hydrogenation with a tendency of H atoms to out-diffuse to the crystal phase at low concentration and forming immobile SimHn hydrides at higher concentration[3]. The tendency of H to segregate in the amorphous enables quantum confinement phenomena with the holes localized within the crystal grains. The possibility to tune the electronic gap of the material by the grains size is showed by semi-empirical and ab initio electronic structure calculations on large scale atomistic models[4]. [1] L. Bagolini et al. PRL 104, 176803 (2010); [2] A. Mattoni et al., PRL 99, 205501 (2007); 78 075408 (2008); [3] G. Fugallo and A. Mattoni, PRB 89, 045301 (2014) (2013); [4] A. Mattoni et al., 79, 245302 (2009);

Authors : B. Liedke,1 D. Friedrich,1 B. Schmidt,1 K. H. Heinig,1 A. Mücklich,1 R. Hübner,1 D. Wolf,2 S. Kölling 3
Affiliations : 1 Helmholtz-Zentrum Dresden – Rossendorf, Bautzner Landstr. 400, 01328 Dresden, Germany 2 Triebenberg Laboratory, Institute of Structure Physics, Technische Universität Dresden, 01062 Dresden, Germany 3 Fraunhofer Center Nanoelectronic Technologies, Königsbrücker Str. 180, 01099 Dresden, Germany

Resume : To increase the market share of Si-based thin film PV cells their efficiency has to be improved without increasing of the module costs. Sponge-like Si-SiO2 nanocomposite has a potential to be a low cost and efficient absorber for next generation PV. It consists of Si embedded in SiO2 fabricated by spinodal decomposition of sputter-deposited silicon-rich oxide SiOx≈1. Thermal treatment using rapid thermal processing and furnace annealing requires annealing times of few tens of sec. up to few tens of min. However, in a thin film technology the phase separation of SiOx at high temperatures requires a very rapid thermal processing of few tens of ms in order to avoid substrate damage. Here, the structure of the Si-SiO2 nanocomposite was investigated by energy-filtered transmission electron microscopy (EFTEM), EFTEM tomography and atom probe tomography which revealed a percolated Si morphology [1]. This is in excellent agreement with atomistic simulations using kinetic Monte-Carlo method [2]. Depending on the annealing time and temperature a feature size in the range of 2..5 nm was found, which is small enough for band gap widening due to quantum confinement [3]. We show that the favorable properties of Si-SiO2 nanocomposite, e.g. quantum size effect and percolated morphology, make it a suitable material for PV absorber. [1] Friedrich, et al. Appl. Phys. Lett. 103,133106(2013) [2] Liedke, et al. Appl. Phys. Lett. 103,131911(2013) [3] Keles, et al. Appl. Phys. Lett. 103,203103(2013)

Authors : A. Haj Salem, M. Carrada, R. Carles, G. Ben Assayag
Affiliations : CEMES-CNRS, Université de Toulouse, 29 rue J. Marvig, 31055 Toulouse, France

Resume : Hybrid systems composed of silicon and silver nanocrystals (Si-NCs and Ag-NPs respectively) are very interesting for their applications in photon conversion solar cells. The optimization of the coupling in these systems depends on several factors such as the distance between the two types of particles, Ag-NPs size, shape and spatial distribution. We have developed an original method based on dual Ultra Low Energy Ion Beam Synthesis (ULE-IBS) for the controlled synthesis of Si-NCs and Ag- NPs in the same matrix. First, Si-NCs are synthesized by Si ion implantation followed by high temperature thermal annealing. Then, Ag ion implantation is performed. In this work, we have investigated the role of an additional thermal annealing (400 ° C to 600 ° C) after Ag ion implantation. Indeed, even if the formation of Ag-NPs occurs during the Ag ion implantation and would not require any annealing step, this one is essential to the optimization of the NCs-Si/NPs-Ag coupling. This annealing allows a passivation of the Si-NCs in order to increase the PL emission, to recover the defects in the matrix to prevent aging and oxidation of Ag-NPS. This step adds a degree of freedom allowing a fine control of the characteristics of Ag-NPs (position and size). The structural and optical properties of the synthesized systems have been studied by transmission electron microscopy (HREM, EFTEM), Raman spectroscopy and PL spectroscopy.

Authors : Ansoon Kim, Songwoung Hong, Jong Sik Jang, Hyun-Jeong Baek, Taewoon Kim, Kyung Joong Kim
Affiliations : Korea Research Institute of Standards and Science (KRISS), Daejeon, Korea

Resume : Nanostructured solar cells have been intensively studied to overcome the limited efficiency of crystalline Si solar cells by band gap engineering of the nanostructures. Self-assembled silicon quantum dot (Si QD) solar cell has one major advantage which is easily to engineer the effective band gap of the Si QDs by varying the size of Si QDs. Despite of the advantage, the Si QDs in a SiO2 matrix exhibits the limitations of low short-circuit currents and high resistivities. In order to overcome the limitations, polycrystalline Si interlayers were introduced between the Si QD layers. Here, we will present the effect of doping concentrations as well as thickness of the polycrystalline Si interlayers on the photovoltaic properties of Si QD cells.

Authors : J. López-Vidrier1, Y. Berencén1, S. Hernández1, O. Blázquez1, S. Gutsch2, J. Laube2, D. Hiller2, P. Löper3, M. Schnabel3, S. Janz3, M. Zacharias2, and B. Garrido1
Affiliations : 1MIND-IN2UB, Departament d’Electrònica, Universitat de Barcelona, Martí i Franquès 1, E-08028, Barcelona, Spain; 2IMTEK, Faculty of Engineering, Albert-Ludwigs-University Freiburg, Georges-Köhler-Allee 103, D-79110, Freiburg, Germany; 3ISE, Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, D-79110, Freiburg, Germany.

Resume : The size-dependent electronic and optical properties of silicon nanocrystals (Si-NCs) embedded in SiO2 matrix have been extensively studied during the last years, as they are fundamental to exploit this system for light-emission or photovoltaic applications. Besides, to guarantee a good control of the NC size, the superlattice (SL) approach has demonstrated to be an excellent method for obtaining Si-NCs with controlled size. Nevertheless, some aspects of their transport and electro-optical properties have not been fully studied. SiOxNy/SiO2 SLs have been deposited on p-type c-Si substrate by means of plasma-enhanced chemical-vapor deposition, varying the thickness of the Si-rich layers as well as their Si excess. A post-deposition annealing treatment was carried out at 1150 °C for 1 h in N2 ambient, to precipitate and crystallize the Si excess in the form of NCs. A MOS device structure was fabricated by sputtering ITO on top and Al on the bottom. The dependence of the electrical properties on voltage and temperature confirmed Poole-Frenkel (PF) as the main transport mechanism in our SL system. In addition, the electroluminescence (EL) study showed a clear emission peak in all devices, coming from the radiative recombination of electron-hole pairs within NCs. A blue-shifted emission is observed in devices containing SiOxNy layers with thinner thickness and lower Si excess, attributed to the quantum confinement effect. Our experimental observations support an EL excitation mechanism consisting of electron impact ionization on the Si NCs, that can be correlated to the PF transport through NCs.

Authors : N.L. Dmitruk, O.Yu. Borkovskaya, S.V. Mamykin, I.B. Mamontova, N.V. Kotova
Affiliations : V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kyiv 03028, Ukraine

Resume : Surface microrelief of polar semiconductor GaAs and InP formed in the process of special chemical anisotropic etching have been used for deposition of gold nanoparticles on the ridges of relief with formation of metal nanowires. Morphology and statistical charachteristics of nanowires array formed have been characterized by atomic force microscopy and scanning electron microscopy with X-ray energy dispersive element analysis. Modeling of optical properties of system has been fulfilled by the anisotropic effective medium approximation method. The model solar cells were created by following preparation of gold contact grid perpendicularly to array of nanowires. Comprehensive experimental investigations of optical and photoelectric properties of these structures allowed to find out essential enhancement of both the short-circuit photocurrent and efficiency. The positive influence of gold nanowires array on photoelectric parameters is caused by several effects: i) reduction of optical losses due to randomization of light propagation through microrelief: ii) excitation of surface plasmons and surface plasmon polariton in nanowires and their array, correspondingly; iii) improvement of non-equilibrium charge carriers collection outside the barrier Au-GaAs, Au-InP contacts.

Authors : C.Prastani,C.Saguy, M. Nanu, D.E. Nanu, R.E.I. Schropp, J.K. Rath
Affiliations : 1Utrecht University, Faculty of Science, Debye Institute for Nanomaterials Science-Physics of Devices, High Tech Campus 5, 5656 AE Eindhoven, The Netherlands; Solid State Institute, Technion City 32000 Haifa, Israel; Thin Film Factory, Foeke Sjoerdwei 3, 8914 BH Leeuwardeen, The Netherlands; Thin Film Factory, Foeke Sjoerdwei 3, 8914 BH Leeuwardeen, The Netherlands; Energy research Center of the Netherlands (ECN), Solar Energy, High Tech Campus Building 5 (WAY); p 057, 5656 AE Eindhoven and Eindhoven University of Technology (TU/e), Department of Applied Physics, Plasma & Materials Processing, P.O. Box 513, 5600 MB Eindhoven; Utrecht University, Faculty of Science, Debye Institute for Nanomaterials Science-Physics of Devices, High Tech Campus 5, 5656 AE Eindhoven, The Netherlands

Resume : The possibility to dope semiconductor nanoparticles and quantum dots plays a crucial role for application in electronic devices. Hence, in the last years, the growth of doped nanoparticles has gained great interest. However, the doping process of nanoparticles is still unclear and the mechanism that controls this phenomenon is still under investigation, both experimentally and theoretically. To that end, we investigated doping effects in quantum dot sized SnS nanoparticles by various techniques. The presence of dopants is indicated by the measured optical absorption spectrum. The presence of a paramagnetic center with an unpaired electron has been confirmed by electron spin resonance (ESR) measurements. These electrons are donated, most likely, by sodium atoms, as sodium sulphide was used as a precursor for the synthesis of these SnS nanoparticles. Moreover, the nanoparticles have been investigated by Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) in ultra-high-vacuum in order to obtain electrical behavior information. STS measurements showed different results for nanoparticles with different sizes. While SnS nanoparticles with size around 2 nm behave as quantum dots, exhibiting a band gap and quantized energy levels, SnS nanoparticles with size of 4 nm show a high conductivity and no band gap could be detected. We infer that the doping of SnS nanoparticles critically depend on particle size. We propose two possible causes for this behavior; (i) the 2 nm nanoparticles expel the dopant atoms by self-purification, or (ii) the Lowest Unoccupied Quantum-Confined Orbital for 2 nm SnS quantum dots lies above the reduction potential of sodium to accept the electrons. This study shows the possibility of doping SnS nanoparticles, paving the way for their use in optoelectronic applications, such as solar cells.

Authors : Basma EL Zein1, Elhadj Dogheche2, Enrico Traversa1
Affiliations : 1 King Abdullah University of Sciences and Technology (KAUST) –- Thuwal, Saudi Arabia. ; 2 Institute of Electronics, Microelectronics and Nanotechnology- IEMN UMR 8520 CNRS, University Valenciennes- Le Mont Houy, 59309 Valenciennes, Cedex, France

Resume : Semiconductor nanostructures with low dimensions such as quantum dots and nanowires are considered as important building blocks for the third generation solar cells, due to their useful optical and electrical properties. Quantum dots and nanowires hold promising potency to improve the performance of solar cells by enhancing light trapping and light absorption, exciton generation and photo-excited carrier transferring and collection. Quantum dot sensitized solar cells consist of three components: semiconductor quantum dots, one dimensional (1D) metal oxide n-type and hole-transport layer materials. Quantum dots are characterized by a tunable absorption, and multiple exciton generation. Nanowires are of great importance because of their large surface area, direct light absorption and light trapping between boundaries of the nanowire arrays. In this work, the synthesis and the characterization of zinc oxide (ZnO) nanowires are presented. We have also investigated the in-situ growth of lead sulfide (PbS) quantum dots on the nanowires, highlighting the importance of electron transfer process from quantum dots to nanowires for photovoltaic applications.

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Advanced concepts III : S. Ito
Authors : Gavin Conibeer, Santosh Shrestha, Shujuan Huang, Robert Patterson, Hongze Xia, Yu Feng, Pengfei Zhang, Neeti Gupta, Suntrana Smyth, Yuanxun Liao, Shu Lin, Pei Wang, Xi Dai, Simon Chung
Affiliations : University of New South Wales, Sydney 2052, Australia

Resume : The key property for a hot carrier absorber is to slow the rate of carrier cooling from the picosecond timescale to at least 100s of ps, but preferably ns to be similar to the rate of radiative recombination. Hot carriers cool primarily by emission of LO phonons. The general properties of phonons and carriers required of a hot carrier absorber have been defined. Materials and structures that exhibit some of these properties fall into three categories: (a) Bulk materials with large difference in mass between constituent atoms such that there is a large difference between optical and acoustic phonon energies. These suppress the main carrier cooling mechanism of decay of optical phonons. Materials include III-nitrides; transition metal nitrides and group IV compounds. (b) Low dimensional multiple quantum well (MQW) systems have also been shown to have lower carrier cooling rates. This has been seen in various lattice matched MQW systems and more recently in strain balanced MQWs in which barrier height and well width affect the temperature. (c) Nanoparticles have also exhibited long hot carrier lifetimes. This is due to confinement of optical phonons and folding of acoustic phonon modes leading to restriction in the allowed decay paths for hot optical phonon populations. These result in longer hot carrier lifetimes. Data on carrier cooling rates and phonon energies for materials from each of these groups will be presented and mechanisms for slowed carrier cooling discussed.

Authors : O. Durand1, S. Almosni1, P. Rale2,3,4, J. Rodi?re2,3,4, Y. PingWang1, A. Letoublon1, H. Folliot1, A. Le Corre1, C. Cornet1, J. Even1, A. Ponchet5, L. Lombez2,3,4, J.-F. Guillemoles2,3,4
Affiliations : 1Universite Europeenne de Bretagne, INSA, FOTON-OHM, UMR 6082, 35708 Rennes, France. 2EDF R&D, Institute of Research and Development on Photovoltaic Energy (IRDEP), Chatou, France 3CNRS, IRDEP, UMR 7174, 78401 Chatou, France 4Chimie ParisTech, IRDEP, 75005 Paris, France 5CEMES-CNRS, Universit? de Toulouse, 29 rue Jeanne Marvig, BP94347 Toulouse cedex 04, France

Resume : By their potential to overcome the Shockley?Queisser limit of a single junction solar cell, the third generation solar cells would enable a lower material usage and/or installation cost share. To this end, a solution consists in the MBE-epitaxial growth of III-V heterostructures towards the elaboration of innovative concepts of solar cells. First, this paper describes our approach to obtain high efficiencies tandem solar cells on Si substrates. GaAsPN diluted nitride alloy is studied as the top junction material due to its perfect lattice matching with the Si substrate and due to its ideal bandgap energy for current matching with the Si bottom cell. Through structural analyses (XRD and TEM), we have developed a growth strategy which dramatically reduces the III-V/Si defects interface. An optimisation of the GaAsPN absorber using a post-growth annealing treatment and first results on p-i-n junctions grown onto GaP substrates are also reported. A second approach consists in the study of InGaAsP/InP MQWs, towards the elaboration of Hot-Carrier Solar Cells, which aim to reduce both the non-absorption of the incoming light (small bandgap) and the thermalization process (electron-phonons interactions). Using different PL techniques, the quasi Fermi level splitting Δμ is estimated as a function of the excitation power. High value of Δμ is found as well as high carrier temperature. These results are compared to electrical measurements. Support: ANR project MENHIRS 2011-PRGE-007-01.

Authors : T. Molière1,2, C. Renard1, A. Jaffré2, L. Vincent1, R, D. Bouchier1, J. Alvarez2, J.P. Kleider2, D. Mencaraglia2, N. Cherkashin3, A. Michel3, A. Claverie3, J. P. Connolly4
Affiliations : 1 IEF, CNRS-UMR 8622, Bat 220, Univ Paris-Sud, 91405 Orsay, France 2 LGEP, UMR 8507 CNRS, Supélec, Universités Paris VI et XI, 11 rue Joliot Curie, 91192 Gif-sur-Yvette 3 CEMES-CNRS, Université de Toulouse, 29 rue J. Marvig, Toulouse, 31055, France 4 Universidad Politecnica de Valencia, Spain

Resume : Up to now, high efficiency multijunction solar cells have been only developed on crystalline Ge substrates for space applications. This is due to the fact that Ge is lattice matched with the specific III-V compounds useful for the implementation of the multispectral cell. Despite rapid progress allowing this technology to go beyond the 31% single junction conversion efficiency limit and its development for space applications, its major drawback remains the high fabrication cost hampering its use for terrestrial PV applications. In order to drastically reduce the cost, we propose an interesting concept that permits the heteroepitaxy of mismatched III-V compounds on Si substrate without any substantial mechanical stress or major defect detrimental to PV applications. The aim is to obtain a GaAs/Si multispectral solar cell demonstrator achieving a very high efficiency (close to 30%) under the global AM1.5G spectrum without concentration. This goal is achievable with epitaxial lateral overgrowth on ultra-thin Si oxide by CBE. This technique allows us to obtain perfect integration of heterogeneous GaAs islands of microscopic size through a Si substrate. X-ray diffraction and transmission electron microscopy analysis indicate that GaAs islands are perfectly relaxed and in epitaxial relation with the Si substrate. Additional measurements by confocal Raman microscopy, µ-PL, and EBIC were also performed. These different results will be presented and discussed during the communication.

Authors : Adam Ginsburg*, David Keller, Hannah-Noa Barad, Yaniv Buhadana, Klimenty Shimanovich, Koushik Majhi, Assaf Anderson, Arie Zaban
Affiliations : Department of Chemistry and the Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel

Resume : All-oxide photovoltaics is an emerging discipline in photovoltaics research. Metal oxides are mostly low-cost, abundant, easy to fabricate and non-toxic materials. These properties make metal-oxides well suited materials to investigate for photovoltaic devices based on p-n heterojunction. However, metal oxides have disadvantages that prevent their use in photovoltaics devices, i.e.short diffusion length and exciton lifetime, high resistance and poor charge separation. Metallic Manganese has a range of stable oxidation states which results in four different materials MnO, Mn2O3, Mn3O4 and MnO2, all having their unique properties. DiManganese trioxide (Mn2O3) is a p-type material with a reported band gap of 1.2eV, which allows sufficient light-harvesting from the sun spectrum. In addition,it has relatively good conductivity and thermodynamic stability. These features allow manganese oxide to be a potentially good candidate for all-oxide photovoltaic devices. The use of high temperature for Mn2O3 synthesis that has been reported so far prohibited the use of glass substrate. In this work, we report for the first time a one-step synthesis of pure, single phase Mn2O3 via spray pyrolysis in atmospheric ambiance. Furthermore, we present high throughput characterization,and in particular the Mn2O3 photovoltaics properties, i.e. optical absorptance, band gap and resistivity. In addition, we demonstrate I-V characteristics for Mn2O3 and TiO2 in a p-n heterojunction structure solar cell.

Authors : Francesco Pastorelli1,2, Sebastien Bidault3, Pablo Romero Gomez2, Rafael Betancur4, Alberto Martinez-Otero2, Jordi Martorell2,5, Nicolas Bonod1
Affiliations : 1 Institut Fresnel, Aix-Marseille Université, CNRS, Domaine Universitaire de St Jérôme, Marseille 13397, France; 2 ICFO-Institut de Ciències Fotòniques, Mediterranean Technology Park, Castelldefels (Barcelona) 08860, Spain; 3 Institut Langevin, CNRS, ESPCI Paris Tech, 1 rue Jussieu, Paris 75005, France; 4 Centro de investigación, innovación y desarrollo de materiales - CIDEMAT Universidad de Antioquia, Medellín Colombia; 5 Departament de Fisica i Enginyeria Nuclear, Universitat Politecnica de Catalunya, Terrassa 08222, Spain;

Resume : The goal of our work is to increase light-matter interactions in organic photovoltaics. Our first design involves self-assembled gap antennas with interparticle distances of a few nanometers controlled by a molecular cross-linker (dithiothreitol). The spacing molecules ensure a minimum distance that plays a fundamental role in the formation of intensity hot spots in the nanogap as well as large and red-shifted scattering peaks. This device exhibited an efficiency 14% higher than the reference one showing a relevant enhancement in the red part of the EQE measurements. As second design we build up a photonic crystal and a metal cavity around a transparent organic solar cell. We enclosed the active material in between two metal electrodes forming an optical cavity designed to optimize photon trapping inside the cell. To increase near IR light trapping, while maintaining transparency in the visible, an anti-reflection coating was deposited on top of the front metal contact while a non-periodic multi-layer was inserted in between the back metal contact and the substrate. The cavity configuration was designed specifically for the cell architecture used and we achieved semi-transparent cells with 5.3% PCE, corresponding to 90% the PCE of the opaque cell.

10:00 Best Poster Award (for poster session II)    
10:10 Break    
Advanced characterization : M. Topic
Authors : Andreas Gerber
Affiliations : IEK5 Forschungscentrum Julich, Germany

Resume : tba

Authors : Jean-Paul Kleider, José Alvarez, Aurore Brézard-Oudot, Marie-Estelle Gueunier-Farret, Olga Maslova
Affiliations : LGEP; CNRS UMR8507; SUPELEC ; Univ Paris-Sud ; UPMC Univ Paris 06 ; 11 rue Joliot-Curie, Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France

Resume : Capacitance techniques have been widely applied to crystalline semiconductors homojunctions in order to study defect levels or doping profiles. The theory was also extended to crystalline semiconductors heterojunctions, where for instance it was proposed how to use the bias dependence to determine band offsets. In amorphous semiconductors, the theory was modified in order to take into account the predominant role of the continuum of defects in the band gap. In this presentation, we will briefly review the basic concepts of capacitance techniques and the peculiarities related to amorphous semiconductors, paying tribute to J. D. Cohen and to his pioneering work. Then, we will extend the discussion to very high efficiency silicon heterojunction (SiHET) solar cells that gather the ingredients of both amorphous semiconductors and heterojunctions. By presenting both modeling and experimental results, we demonstrate that the conventional theory of heterojunction capacitance based on the depletion approximation, cannot reproduce the measured temperature and bias dependence, and leads to strong errors if applied to the determination of band offsets. We show that this is not related to the amorphous nature of a-Si:H, but to the existence of a strongly inverted c-Si surface layer. SiHET solar cells thus offer a unique opportunity to reconsider the theory of junction capacitance and to explain how, in this case, capacitance measurements can bring insight into the device physics.

Authors : Iain D Baikie, Angela C Grain, James Sutherland, Jamie Law
Affiliations : KP Technology Ltd, Burn Street, Wick, Caithness, KW1 5EH, UK

Resume : We demonstrate a dual-mode Kelvin probe featuring two novel detection modes comprising Air Photoemission Spectroscopy (APS), which yields information on the absolute work function (Φ) of a surface/thin film, and Surface Photovoltage Spectroscopy (SPS) which produces information relevant to spectroscopic characterization of solar cells. These measurement modes allow a full characterization of the electronic energy band diagram including valance band energy, fermi-energy and surface potential (band-bending) under standard conditions. The traditional Kelvin probe measures small changes (1-3 meV) in a non-contact fashion, using a vibrating tip. This system is extremely versatile, capable of automatic monitoring of changes in Φ or sample fermi-level under ambient, controlled atmosphere and UHV environments. Using a combination of Visible/IR and deep UV illumination (1.2 - 7.3 eV) we have characterized the work function of metallic and ionization potential of semiconducting thin films and TCO’s utilised in device fabrication such as Au, Ag, Al, Cu, cSi, mcSi, aSi, ITO, CuO, ZnO, TiO2, Pedot, GaP, Graphene. Other examples include the near fermi-level Density of States (DOS) in Cobalt-Phylocyanine (NiPc). The resulting tool is extremely useful for surface characterization of organic semiconductors and solar cells, allowing clarification of the energy band diagram. Further we show how Pulsed Light SPV Transients can be used to characterize interface trapping and surface recombination.

Authors : Mohit Raghuwanshi, Emmanuel Cadel, Sébastien Duguay, Philippe Pareige, Nicolas Barreau
Affiliations : Groupe de Physique des Matériaux (GPM), University of Rouen; IMN, University of Nantes

Resume : Photovoltaics is currently dominated by Silicon solar cells, however there is a need to develop and improve other alternative materials to synthesize cost-effective high efficient solar cells. We utilize Atom Probe Tomography (APT) technique to understand and enhance efficiency of Copper Indium Gallium Selenide (CIGS) solar cells. CIGS is currently the most efficient solar cell (efficiency > 20%) under thin film category. This high efficiency is obtained for polycrystalline CIGS due to Na atoms (diffused from the glass substrate) segregation along Grain Boundaries (GBs). This increased efficiency due to presence of GBs is both surprising and interesting; to understand the role of GBs in changing the electrical properties of CIGS advanced characterization techniques like APT must be used. APT can resolve materials and provide 3D atomic information at sub-nanometer level. Herein we combine EBSD (Electron Back Scattered Diffraction) to locate GBs and APT to explore nano-chemistry of GBs in CIGS for different Ga concentrated samples. Results show that: composition profile at GBs strongly depends on Ga ratio (Ga/In+Ga). Depletion and enrichment in Cu conc. is observed at GB for Ga poor and Ga rich samples respectively, suggesting different phase compositions at GBs for different Ga ratios. Efficiency variation of CIGS for different Ga concentration is linked for the first time with its GB composition profile will be discussed.

Authors : Hariharsudan Sivaramakrishnan Radhakrishnan, Ivan Gordon, Robert Mertens, Jef Poortmans
Affiliations : KU Leuven, imec; imec; KU Leuven, imec; KU Leuven, Universiteit Hasselt, imec

Resume : In wafer-equivalent silicon solar cells, measuring the quality of the p-type epitaxial layers (“epilayers”) grown on a heavily-doped p+ substrates is important and challenging. Lifetime measurements on such p/p+ structures are strongly influenced by the substrate. One technique used for this is simulation-assisted photoluminescence (sim-PL), where a ratio of PL intensities from epitaxial layers of two different thicknesses are measured and related to bulk lifetime using numerical modeling of the excess carrier densities in the epilayer. It is shown that the substrate influences the measured signal significantly due to a non-negligible substrate excess carrier densities. Neglecting this can lead to a significant bulk lifetime underestimation. For a thin 20 µm-thick layer grown on a p+ substrate with a B doping concentration of 1019 cm-3, substrate PL contributes ~40% to the measured signal. A correction procedure is presented based on the analytical modeling of the excess carrier densities in the substrate, which can be dominated by carrier injection from the epilayer over the p/p+ barrier or direct substrate photo-generation. In the first case, an elegant formula is derived for correcting the substrate PL, which depends only on the substrate doping concentration. Significantly, no prior knowledge about the epilayer parameters are needed. In the second case, a PL measurement on the substrate can be used for the correction after accounting for the absorption in the epilayer.

Authors : P. Rale [1], L. Lombez [1], A. Delamarre [1], G. El Hajje [1], K. Watanabe[2], R. Tamaki [2], Y. Shoji [2], Y. Okada [2], M. Sugiyama [3], J-F. Guillemoles [1]
Affiliations : [1] Institute of Research and Development on Photovoltaic Energy, 6 quai Watier,78401 Chatou, France; [2] Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Tokyo, 153-8904, Japan; [3] School of Engineering, The University of Tokyo, Tokyo 1138656, Japan

Resume : One of the solutions to overpass the Schockley-Queisser limit is to use intermediate band solar cells (IBSC). This kind of cells has a potential efficiency limit of 63% under concentration, thanks to its three optical transitions allowed in the material. Nevertheless the intermediate band behavior has never been quantitatively demonstrated yet. One key step would be the access of the three quasi-Fermi levels (QFL) splitting corresponding to the three transitions, the quasi-Fermi level separation of the largest transition being the sum of the two others. In order to address this issue, we have used a calibrated hyperspectral imager that can record spatially and spectrally resolved luminescence fluxes. Therefore, we are able to map QFL splitting of transitions from the visible to the NIR range via photoluminescence (PL) measurements and using the generalized Planck’s law. This method, previously validated on GaAs device, has been used in the present work, on state of the art quantum confined heterostructures (quantum wells and quantum dots devices). Two QFL splitting are optically measured and compared to electrical measurement. Collection efficiency is also investigated by photoluminescence-excitation (PLE) and luminescence measurements at different electric bias.

12:30 Lunch    
Thin film solar cells I : A. Gerber
Authors : Marko Topič
Affiliations : University of Ljubljana, Slovenia

Resume : Performance of thin film solar cells needs accelerated improvements. Where is the unused potential? Apart from inevitable energy conversion losses, both electrical and optical losses need to be defragmented per thin-film cell structure to path a way to further improvements. In case of TF-Si photon management is crucial for further improvement of conversion efficiency. By means of optical modeling and simulations losses will be discussed and potential improvements in quality of materials discussed. Optical simulators developed at University of Ljubljana SunShine, FEMOS and CROWM will be briefly reviewed and combined with rigorous 3D simulations based on COMSOL software. Finally, the record thin film solar cell will be compared to the record crystalline solar cells and unused potential in performance of thin film solar cells will be pointed out.

Authors : Thu Nhi Tran Thi1, Sébastien Dubois2, José Baruchel1, Nicolas Enjalbert2, Bruno Fernandez3, Tobias Schülli1, Rafael Kluender4 and Tamzin Lafford1
Affiliations : 1: European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, 38043 Grenoble cedex 9, France; 2: CEA-INES, Savoie Technolac, 50 avenue du Lac Léman, 73375 Bourget du Lac, France; 3: Institut Néel CNRS/UJF UPR2940, 25 rue des Martyrs, BP 166, 38042 Grenoble cedex 9, France; 4: CEA-LETI, 17 rue des Martyrs, 38000 Grenoble, France

Resume : Today's best photovoltaic (PV) conversion efficiencies industrially are up to 19% for p-type Si solar cells and 23% with Czochralski (Cz)-Si n-type technology. We study how the architecture of the rear side of the cell influences these efficiencies by modifying the structural properties. Synchrotron X-ray diffraction imaging in section and nano-XRD give quantitative, spatially-resolved information on deformations induced by the back contacts. For conventional p-type Si solar cells with screen-printed Al back-planes, back contacts are created with a p doped layer/eutectic Al-Si layer structure at the Si surface. Inhomogeneity in the eutectic layer causes distortion and strain, which affect PV conversion efficiency. Cross-section maps of diffraction peak FWHM of full p-type Si solar cell structures show significant variations from the back surface to the bulk. Such distortions could have a noticeable influence on carrier mobility and lifetime, and on the properties of junctions and dopant-rich layers. We associate the inhomogeneity of the eutectic layer with the size variations of the Al particles in the pastes. Through measurements of Jcc, Voc and IQE we correlate the overall stress and local strain in the vicinity of the p layer with the PV conversion efficiency: the lower the stress and strain, the higher the PV performance. For n-type cells with back-planes of SiO2 and Si-nitride, distortions in the Si are much lower, which could contribute to their higher PV efficiency.

Authors : W. Soppe, M. D?renk?mper, D. Zhang, C. van der Werf and R. Schropp
Affiliations : ECN-Solliance, High Tech Campus 5, 5656 AE Eindhoven, the Netherlands

Resume : Micro-crystalline silicon (uc-Si) has already been discovered in 1968 by Veprek and Maracek, but it is still an interesting and promising material for solar cells, because of the low processing temperatures and the abundance of the feedstock material. uc-Si, however, also has some drawbacks, and one of them is that the material consists of small grains, requiring excellent passivation of the grain boundaries. The latter is accomplished by tuning the conditions of the PECVD process such that the material contains a certain fraction of hydrogenated amorphous silicon (a-Si:H). a-Si:H is capable of passivating the grain boundaries of uc-Si very well, but in practice an over-proportional fraction of around 30 % a-Si:H in the layers is required to obtain reasonable open-circuit voltages (Voc). This high fraction of a-Si:H not only leads to a non-negligible light induced degradation of the solar cells, but also to a certain loss in short-circuit current (Jsc): charge carriers generated in the a-Si:H tissue have little chance to contribute to the current of the solar cell. In order to reduce the a-Si:H fraction in the uc-Si layers, but maintaining the grain boundary passivation, we have conducted post-deposition hydrogenation experiments. We made uc-Si layers with a relatively high crystalline fraction and subsequently hydrogenated these layers by two different methods: microwave (MW)-PECVD and hot wire(HW)-CVD. Both methods resulted in a significant improvement of the ratio of the photo to dark conductivity. Before hydrogenation this ratio was about 50 and after hydrogenation this ratio improved to a value of 400 for HW-CVD and to 775 for MW-PECVD. Cell fabrication is ongoing and the results of these experiments will be reported at the conference.

Authors : Selcuk Yerci, Matthew Branham, Gang Chen
Affiliations : Middle East Technical University Massachusetts Institute of Technology

Resume : Solar industry has been dominated by crystalline silicon (c-Si) despite its relatively poor light absorption coefficient. However, the price of silicon is responsible for ~40% of the total module cost. Therefore, thin (~10-20 m) c-Si solar cells with advance surface texturing that can absorb light with a similar efficiencies of thick cell and collect carriers potentially with a higher efficiency are desired. Recently, various light trapping structures operating in wave optics regime have been introduced to boost the absorption in thin silicon films. However, it is not known what structure will be optimum for both optical absorption and also carrier collection perspectives. Here, we performed a multiphysics simulation which includes wave optics and drift-diffusion models. Our simulation is capable of quantifying the losses due to the Schottky-Read-Hall, Auger bulk recombinations, surface recombinations, and contact losses as well as the mobility quenching due to doping. We found that efficiencies above 20% can be achieved using thin film silicon solar cells. Furthermore, we calculated the optimum contact separation in case of interdigitated back contact scheme for various minority carrier lifetimes for thin c-Si solar cells.

Authors : Hariharsudan Sivaramakrishnan Radhakrishnan, Roberto Martini, Valerie Depauw, Kris Van Nieuwenhuysen, Ivan Gordon, Robert Mertens, Jef Poortmans
Affiliations : KU Leuven, imec; KU Leuven, imec; imec; imec; imec; KU Leuven, imec; KU Leuven, Universiteit Hasselt, imec

Resume : Kerf-less layer transfer of epitaxial thin (<50 µm) silicon foils (“epifoils”) using porous silicon (PS) as the detachment layer reduces the silicon used per m^2 and hence the cost of silicon solar cells. Two critical aspects of this technology are addressed: Firstly, a yield for the PS-based detachment process of near 100% must be attained. Secondly, the PS must be optimised to allow high quality epitaxy on its surface. The PS stack is electrochemically etched to consist of a high porosity detachment layer (HPL) underneath a low porosity template layer (LPL). During subsequent high temperature sintering, strong inter-layer interaction driven by vacancy diffusion determines the PS morphology of both the layers. The yield of detachment depends on the density and width of the interconnections in the HPL. By increasing the vacancy supply from the LPL, the porosity of the HPL was increased, improving the detachment yield from ~60-70% to nearly 100%. However, this makes the LPL a poorer template for epitaxy, increasing the epifoil defect density ~1.5 times. Therefore, we introduce novel multi-layer PS stacks, which rely on controlling the surface porosity and vacancy concentration gradients to achieve a surface zone or an entire layer of 100-250 nm that is almost void-free. With such stacks, diffusion lengths of ~700 µm have been achieved in a 40 µm thick epifoil. Further research is ongoing to attain epifoils of even higher quality with 100% detachment yield.

Authors : F. E. Rougieux, N. E. Grant, P. Zheng and D. Macdonald
Affiliations : The Australian National University

Resume : Common metallic-related impurities such as iron, and the boron-oxygen defect, which are critical in determining the performance of lower quality p-type solar cells, are of much less concern in high purity n-type monocrystalline silicon used for very high efficiency solar cells. However, in this material, the electronic quality may still be limited by the presence of grown-in defects, involving, for example, silicon vacancies or self-interstitials, and complexes formed with dopant atoms or light elements such as oxygen and carbon. In this study, we uncover a recombination-active grown-in defect reducing the minority carrier lifetime of Czochralski grown n-type silicon from 5ms to below 2ms. We also demonstrate that the defect can be de-activated by annealing between 300°C and 360°C. As the defect is likely to be in too small concentrations to be detectable by DLTS, we characterize the recombination activity of the defect using minority carrier lifetime measurements. Our experimental findings suggest that vacancy-related pairs incorporated during ingot growth may be responsible for the decreased minority carrier lifetime.

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Thin film solar cells II : G. Conibeer
Authors : C. Becker (1), V. Preidel (1), J. Xavier (1), P. Wyss (1), D. Eisenhauer (1), J. Probst (1), S. Burger (2), F. Schmidt (2), F. Back (3), E. Rudigier-Voigt (3), D. Amkreutz (1), J. Haschke (1), B. Rech (1)
Affiliations : (1) Helmholtz-Zentrum Berlin für Materialien und Energie, Kekuléstr. 5, 12489 Berlin; (2) Zuse-Institute Berlin, Takustr. 7 14195 Berlin; (3) SCHOTT AG, Hattenbergstr. 10, 55122 Mainz, Germany

Resume : Crystalline silicon (c-Si) thin-film solar cell technology recently underwent rapid advances in high-quality absorber fabrication by liquid phase crystallization (LPC) techniques and vivid progress in nanophotonic light trapping systems. We present a solar cell concept combining LPC of sub-10µm-thick silicon films with nanoimprint-textured glass substrates for systematic light management. This technology enables an excellent material quality with open circuit voltages up to 600 mV, it has the inherent advantages of thin-film technology (low material usage, large-area fabrication, monolithic devices) and permits manifold degree of freedom to systematically control size and shape of the light trapping features. A periodically double-sided textured c-Si thin-film solar cell device was designed with an integrated absorption corresponding to a short circuit current density of 38 mA/cm^2 and efficiencies above 8%. Three main challenges are addressed: Passivation of the c-Si absorber interfaces which are enlarged by nanostructuring; obtaining an excellent electronic material quality comparable to the bulk reference; and, in particular, optimization of nanophotonic structures enabling wide-angle and broadband spectral absorption. For defining structure designs with improved optical properties we apply 3D finite-element modeling. Periodic, quasiperiodic and polysymmetric c-Si nanostructures are experimentally realized and analyzed with respect to their optical performance.

Authors : I. Cosme1, R. Cariou1, M. Foldyna1, P. Roca i Cabarrocas1, K.D. Lee2, C. Trompoukis3-4, V. Depauw3.
Affiliations : 1. LPICM-CNRS, Ecole Polytechnique, 91128 Palaiseau, France; 2. Obducat Technologies AB, Scheelevägen 2, 223 63 Lund, Sweden; 3. IMEC, Kapeldreef 75, B-3001 Belgium; 4. KUL, Departement Elektrotechniek – ESAT, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium

Resume : In the field of c-Si PV, strong efforts focus on reducing wafer thickness, however reducing absorber layer down to 40-50 µm or 10-20 µm has resulted in efficiencies above 20 % and 15%, respectively, and this limitation is even stronger in the range of few microns. To compensate, nanophotonic concepts have been developed enabling the manipulation of light on sub-wavelength scale to enhance the absorption and to reduce the total reflectance. However, these approaches result in higher surface areas and as a consequence in higher surface recombination velocities. In this study, we defined nanostructures, with different surface area enhancement, by nanoimprint lithography and etching (dry or wet). We focus on lifetime characterization of nanostructured c-Si wafers (280 µm), thin c-Si wafers (25 µm) and ultrathin (~1-5 µm) epitaxial and epi-free layers. The net effect of the nanostructured patterning on the defect creation is investigated by means of photoconductance (PC) and time-resolved microwave conductivity (TRMC) measurements. The c-Si wafers were passivated by a-Si:H films: a flat wafer, a dry etched wafer, and a TMAH wet etched wafer. The τeff were 2.2 ms (ref), 484 μs (dry) and 709 μs (wet). It is noteworthy that despite the plasma damage associated to the dry etching process, the lifetime after passivation remains high enough for high efficiency ultrathin solar cells. This approach is therefore being extended to ultrathin wafers, epi-free and thin PECVD epitaxial layers.

Authors : J. A. Töfflinger (1), A. Laades (2), C. Leendertz (1), L. M. Montañez (1), L. Korte (1), U. Stürzebecher (2), H.-P. Sperlich (3), B. Rech (1)
Affiliations : 1: Institute for Silicon-Photovoltaics, Helmholtz-Zentrum Berlin, Kekuléstraße 5, 12489 Berlin, Germany; 2: CiS Forschungsinstitut für Mikrosensorik und Photovoltaik GmbH, Konrad-Zuse-Straße 14, 99099 Erfurt, Germany; 3: Roth & Rau AG, An der Baumschule 6-8, 09337 Hohenstein-Ernstthal, Germany

Resume : Aluminum oxide (AlOx) capped by silicon nitride (SiNx) is newly applied for excellent surface passivation of high-efficiency crystalline silicon (c-Si) solar cells [1] reaching efficiencies >20% [2]. Our study aims at a better understanding of the charging mechanisms including trapping-detrapping phenomena in the PECVD-AlOx/SiNx-system and of the c-Si/AlOx interface defects which have great impact on the field-effect and chemical passivation, respectively. To achieve this we apply constant voltage stress (Vstress) combined high frequency (1MHz) capacitance-voltage and capacitance-time measurements. For instance, the initially high negative charge state of the system can be manipulated, by first charging traps positively and then negatively again through charge injection from the c-Si by applying a negative and a positive Vstress, respectively. In addition, the defect state density at the c-Si/AlOx interface over the entire Si band gap is monitored. A large Vstress induces a degradation of chemical passivation due to the generation of additional Si dangling bond defects. These results are of interest for understanding the charge trapping mechanisms and interface properties of c-Si/AlOx/SiNx and also in regard to their implementation in c-Si solar cells where Vstress induced degradation of passivation will influence their performance. [1] G. Dingemans and W. M. M. Kessels, J. Vac. Sci. Technol. A 30, 040802 (2012). [2] J. Schmidt et al., Energy Procedia 15, 30 (2012).

Authors : M. Pawlik 1, J. P. Vilcot 1, M. Halbwax 1, D. Aureau 2, A. Etcheberry 2, A. Slaoui 3, T. Schutz-Kuchly 3, R. Cabal 4
Affiliations : 1 Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN) UMR 8520, Université Lille1 Sciences et Technologies, CS 60069, 59652 Villeneuve d’Ascq, France 2 Institut Lavoisier de Versailles UMR 8180, Université de Versailles-St-Quentin en Yvelines, 45 avenue des Etats Unis, 78000 Versailles, France 3 Laboratoire des sciences de l’Ingénieur, de l’Informatique et de l’Imagerie (ICube), UMR 7357, UdS/CNRS, 23 rue du Loess, BP 20 CR, 67037 Strasbourg Cedex2, France 4 CEA-INES 50 avenue du Lac Léman, 73375 Le Bourget du Lac, FRANCE

Resume : Passivation process is a key feature to improve the efficiency of silicon solar cells. So far, a 20nm-thick layer of Al2O3 grown by Plasma Enhanced Atomic Layer Deposition (PE-ALD) followed by a 450°C anneal during 15 to 30 min gives the best results in surface carrier recombination velocities on p-type silicon. In this study we used chemical (XPS profiling, SIMS), and electrical (C-V, QSS-PCD) characterisation to obtain a better understanding of the passivation activation process. A 20nm-thick Al2O3 film is deposited on 200µm Cz p-type c-Si (100) wafers with a resistivity of 5Ω.cm by PE-ALD. Starting from as-deposited state to anneal time up to 1 hour, the evolution of chemicals components at the interface Al2O3/Si is recorded in conjunction with macroscopic parameters (Density of effectives charges (Qeff), Density of interface defects (Dit) and carrier lifetime). First XPS analyses show that the SiO2 interface layer is not the driving parameter of a good passivation since it is not affected by annealing. However chemical bonds, such as Al-O-Si appear as the sample is annealed. At the same time, SIMS measurements confirm an outgassing of hydrogen, contained in the Al2O3 layer, toward the silicon during the thermal treatment with a maximum penetration depth into the silicon at 30 min of annealing. This evolution is in good agreement with the electrical values, deduced from the C(V) measurements, where the Dit at 30min is low (109 cm-3) and the Qeff is high (-1.1013 cm-3).

Authors : A. Slaoui1, S. Roques1, O. Lunder2, A. Ulyashin3, O. Mahboub4, Z. Sekkat4,
Affiliations : 1 ICube, University of Strasbourg-CNRS, 23 rue du Loess, B.P.20, F-67037 Strasbourg, France 2 SINTEF, Material and Chemistry, Høgskoleringen 5, NO- 034 ,Trondheim, Norway 3 SINTEF, Material and Chemistry, Forskningsveien 1, NO-0314, Oslo, Norway 4 MAScIR, MAScIR, Rue Mohamed Al Jazouli – Madinat Al Irfane Rabat 10 100 – Morocco

Resume : Thin film silicon solar cells on low cost foreign substrates could be attractive for highly efficient and low cost production of photovoltaic electricity. This work aims at the synthesis of high-quality continuous polycrystalline silicon (pc-Si) layers on flexible aluminium based substrates using direct crystallization (DC) or through the aluminium induced crystallization (AIC) procès of amorphous silicon. Pure aluminium (Al) or anodic alumina (Al2O3/Al structures) were used as substrates. Amorphous silicon films with thicknesses ranging from 200 to 1000 nm were deposited by ECR-PECVD and PVD on the substrates The direct crystallization using a conventional furnace was carried out at temperatures ranging from 450 to 550°C and durations from 1h to 5h. Rapid thermal annealing using halogen lamps at températures of 700-800°C and durations lower than 15min were attempted. For the AIC process, a 200nm aluminium layer was first evaporated on the substrates prior to depositing amorphous silicon. Similar thermal treatments than above were performed. The resulting crystallized layers were characterized by Raman, UV reflectance spectroscopy as well as by XRD analysis. It was found that the complete crystallisation of the a-Si layers is strongly dependent on the initial layer thickness and the thermal budget. On the other hand, the as-grown AIC pc-Si films were found to be continuous and densely packed without amorphous phase. The migration of impurities from the substrate to the pc- Si films were studied systematically in terms of chemical and stress level analysis, which are the important aspects to be considered when metallic foils are used as substrates. Complementary experiments and analysis are currently carried out to improve the quality of DC and AIC pc-Si films.

10:00 Break    
Nanostructures I : H. Stiebig
Authors : A. Fejfar, M. Hývl, A. Vetushka, M. Ledinský, S. Misra, M. Foldyna, Linwei Yu, P. Roca i Cabarrocas
Affiliations : a) Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, 162 00 Prague 6, Czech Republic b) Laboratoire de Physique des Interfaces et des Couches Minces (LPICM), Ecole Polytechnique, CNRS, F-91128 Palaiseau, France

Resume : Radial junctions based on silicon nanowires (SiNWs) are an example of nanostructured design of solar cells with excellent light trapping and efficient photogenerated charge collection. A single pump-down process used to prepare a randomly grown matrix of SiNWs and conformal p-i-n radial junctions led to cells with efficiencies over 8% [1]. Considerable influence of irregularities in SiNWs lengths, orientations, shapes and mutual interactions on the photovoltaic action can be expected. Direct measurement of these effects requires microscopic measurements of photoresponse. This is possible using atomic force microscopy (AFM) with conductive cantilever which serves as a contact to individual radial junctions [2]. At the same time the cantilever can measure the local nanomechanical properties, including local stiffness of the wires, which can only sustain contact forces up to ~1 nN. Resulting conductivity maps show substantial variation of the local electronic properties. The AFM tip cannot reach deeper into the SiNWs matrix and correlation with scanning electron microscopy of the identical nanowires was sought in order to identify the reason for conductivity variations. The results are discussed in terms of random photodiode arrays connected in parallel with overall performance limited by weak diodes. [1] S. Misra et al., Sol. Energy Mat. Sol Cells. 118 (2013) 90–95. [2] A. Fejfar et al., Sol. Energy Mat. Sol. Cells. (2013) 228–234.

Authors : S. Christiansen(1,2), S. Schmitt(2), S. Jäckle(2), Ch. Tessarek(2), G. Sarau(2), M. Heilmann(2), M. Latzel(2), M. Göbelt(2), G. Shalev(2), M. Bashouti(2), A. Mahmoud(2), K. Höflich(1)
Affiliations : (1) Helmholtz-Zentrum für Materialien und Energie, Kekulestr. 7, 12489 Berlin, Germany; (2)Max Planck Institute for the Science of Light, Günther-Scharowsky-Str. 1, 91058 Erlangen, Germany

Resume : Aligned silicon nanowire (SiNW) arrays for the efficiencies >15% - era are fabricated on multi-crystalline Si layers on glass using reactive ion etching with lithographic large area nano-patterning using densely packed polystyrene (PS) spheres. Diameter, length, density and shape of SiNWs can be tuned for highest absorptions (close to 90%) and as small as possible surface areas, since surfaces are prone to carrier recombination. Various SiNW cell concepts are: (i) a hybrid organic/inorganic cell with SiNWs absorber and a hole conducting polymer (PEDOT:PSS - encapsulation procedures for long term stability suggested); (ii) a semiconductor-insulator-semiconductor (SIS) cell with SiNW absorber, oxide (few Å Al2O3 by atomic layer deposition-ALD) ) tunneling barriers for charge carrier separation and a transparent conductive oxide (TCO – here: Al:ZnO, by ALD). Initial thin film solar cell prototypes reached open-circuit voltages of > 630 mV, short-circuit current densities of even ~ 30 mA/cm2 and efficiencies >13%. Analytics to improve materials / cells are: (i) electron beam induced current (EBIC) - charge carrier distributions; (ii) electron backscatter diffraction (EBSD) - structural quality of the multi-crystalline Si layer; (iii) integrating sphere measurements, external quantum efficiency - optical properties and (iv) 4-point nano-probing of individual NWs - electrical properties. Novel electrodes (e.g. graphene, silver nanowire webs) to further improve the cells are shown.

Authors : Minji Gwon, Yunae Cho, and Dong-Wook Kim
Affiliations : Department of Physics, Ewha Womans University, Seoul, 120-750, Korea

Resume : Si nanowire (NW) arrays prepared on Si wafers could significantly enhance the optical absorption of the cells, with the aid of graded refractive index, resonant guided modes, scattering, and diffraction. We have carried out finite-difference time-domain simulation studies to design optimal Si nanowire array for solar cell applications. Optical reflectance, transmission, and absorption can be calculated for nanowire arrays with various diameter, length, and period. The NW height (500 nm) and the wafer thickness (10 m) fixed. The absorption in the planar wafer has the peak position, distinct from the total absorption by both the NWs and the wafer. This result provides a design guideline for selecting optimal antireflective NW arrays, maximizing the optical absorption in the thin absorber and minimizing the surface recombination loss in the NWs.

Authors : M. Daanoune(1), D. Kohen(2), A. Kaminski-Cachopo(1), C. Morin(2), P. Faucherand(2), S. Perraud(2), D. Blanc-Pélissier(3)
Affiliations : (1)IMEP-LAHC, Grenoble INP, Grenoble, France;(2)CEA, LITEN, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France;(3)INL, INSA, 7 avenue Jean Capelle, 69621 Villeurbanne Cedex

Resume : Nanowire-based solar cells are interesting structures for photovoltaic applications as they enhance properties such as light absorption, trapping efficiency and carrier collection. Consequently, the potential to decrease the cost of photovoltaic energy thanks to these structures is not negligible. However, up to now, their efficiency has been limited mainly because of the recombination at the interfaces and in the volume. The effective minority carrier lifetime is a key parameter which is strongly connected to volume, interface and surface recombination properties. In this work, we have used a purely electrical approach called reverse recovery transient (RRT) to perform measurements of minority carrier lifetime in core-shell nanowire-based solar cells under dark conditions. The structures are based on crystalline silicon nanowires grown on silicon wafers and embedded in a radial amorphous silicon shell. The electrical contacts for this heterojunction structure are transparent conductive oxide for the front surface and aluminum for the backside. A planar solar cell has also been fabricated to be used as a reference. By comparing RRT measurement on the nanowire-based solar cell and on the planar reference solar cell with simulations, we extract the lifetime of the nanowires.

Authors : Yingchao Cui, Jia Wang, Sebastien R. Plissard, Alessandro Cavalli, Thuy T. T. Vu, Rene P. J. van Veldhoven, Lu Gao, Mike Trainor, Marcel A. Verheijen, Jos E. M. Haverkort, Erik P. A. M. Bakkers
Affiliations : COBRA Research Institute, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands; Philips Innovation Services, High Tech Campus 11, 5656 AE, Eindhoven, The Netherlands; Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA, Delft, The Netherlands

Resume : Semiconductor nanowires (NWs) are very promising for multiple junction solar cells due to their small footprint which makes it' s possible to stack lattice mismatched materials on top of each other without introducing lattice misfit dislocations. In addition, nanowire arrays show increased light absorption and low reflection loss, even at an appreciable nanowire spacing. We demonstrate an efficiency enhancement of an InP nanowire (NW) axial p−n junction solar cell by cleaning the NW surface. NW arrays were grown with in situ HCl etching on an InP substrate patterned by nanoimprint lithography, and the NWs surfaces were cleaned after growth by piranha etching. We find that the postgrowth piranha etching is critical for obtaining a good solar cell performance. With this procedure, a high diode rectification factor of 10E7 is obtained. With only 75nm in nanowire diamter, the resulting NW solar cell exhibits an open-circuit voltage of 0.73 V, a short-circuit current density of 21 mA/cm^2, and a fill factor (FF) of 0.73 at 1 sun. This yields a power conversion efficiency of up to 11.1% at 1 sun and 10.3% at 12 suns.

Authors : S. Gaiaschi, E.V. Johnson, M-E. Gueunier-Farret, C. Longeaud, P. Chapon, J-P. Kleider
Affiliations : LGEP–CNRS/SUPELEC, 11 rue Joliot Curie - Plateau de Moulon, 91192 Gif sur Yvette, FRANCE LPICM-CNRS, Ecole Polytechnique, 91128 Palaiseau, FRANCE; LPICM-CNRS, Ecole Polytechnique, 91128 Palaiseau, FRANCE; LGEP–CNRS/SUPELEC, 11 rue Joliot Curie - Plateau de Moulon, 91192 Gif sur Yvette, FRANCE; LGEP–CNRS/SUPELEC, 11 rue Joliot Curie - Plateau de Moulon, 91192 Gif sur Yvette, FRANCE; HORIBA Jobin Yvon, 16-18, rue du Canal, 91165 Longjumeau CEDEX, FRANCE; LGEP–CNRS/SUPELEC, 11 rue Joliot Curie - Plateau de Moulon, 91192 Gif sur Yvette, FRANCE

Resume : Hydrogenated microcrystalline silicon-carbon alloys (µc-Si1-xCx:H) can be expected to be an alternative to the unstable hydrogenated amorphous silicon (a-Si:H) in tandem solar cells. These thin films can be obtained by standard Radio Frequency Plasma Enhanced Chemical Vapour Deposition (RF-PECVD). Their energy gap depends on the carbon incorporation and values close to the band gap value of a-Si:H can be obtained. Actually, according to the binary Si-C phase diagram, silicon carbide is the only stable compound and alloys with small carbon content are thermodynamically metastable. They are characterized by silicon crystallites embedded in an a-Si1-xCx:H matrix. We studied µc-Si1-xCx:H samples deposited by RF-PECVD at 175 °C from a silane and methane gas mixture highly diluted in hydrogen. The material quality was assessed through Raman spectroscopy, X-ray diffraction and transport property measurements, as a function of the carbon content determined from radio frequency glow discharge optical emission spectroscopy measurements. P-I-N junction photodiodes were made using these alloys as the absorber layer. The influences of RF power and carbon content on the diode characteristics under dark and on the solar cell performance were studied, finding a good agreement between material and device properties.

12:30 Lunch    
Nanostructures II : A. Fejfar
Authors : S. Cosentino1, S. Mirabella1, M. Miritello1, I. Crupi1, P. Liu2, A. Zaslavsky2, D. Pacifici2, A. Terrasi1
Affiliations : 1 MATIS IMM-CNR and Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, ITALY; 2 School of Engineering, Brown University, Providence, RI 02912, USA

Resume : Quantum confinement effect (QCE) can tune many properties relevant to light harvesting, such as optical bandgap, efficiency of luminescence and oscillator strength of the optical transition. Since Ge has a quite large Bohr exciton radius (24 nm), the quantum confinement regime in Ge nanostructures (NS) is relatively easy to get. In addition, Ge shows an absorption coefficient more than 10 times higher with respect to Si in the solar energy range. Thus, Ge NS are really attractive as active absorber for efficient light harvester, solar cells and novel optoelectronic devices. This work aims to show how and to what extent the QCE modifies the optical behavior of a variety of Ge NS confined in SiO2. Materials and devices, prepared by sputtering deposition of Ge-rich SiO2 thin films in single or multiple layer configuration, comprise: Ge quantum dots (QD, 2-10 nm in size) with QD-QD distance accurately fixed between 3 and 20 nm, and single Ge quantum well (2-30 nm in thickness). Light absorption spectroscopy with effective mass simulation evidenced the relationships among synthesis parameters and response to light absorption in prototypal devices, getting light detection with photoconductive gain up to 1500%. Our data fix the NS size as a critical parameters for QCE, but also demonstrate that proximity and interaction among NS play key roles in the efficiency of photon absorption process. Ref. S. Cosentino et al. Nanoscale Res. Lett. 8, 128 (2013) S. Mirabella et al. Appl. Phys. Lett. 102, 193105 (2013) S. Cosentino et al. Appl. Phys. Lett. 98, 221107 (2011)

Authors : Salvatore Cosentino1, Emel Sungur Ozen2, Rosario Raciti1, Antonio M. Mio3, Giuseppe Nicotra3, Francesca Simone1, Maria Miritello1, Isodiana Crupi1, Rasit Turan4, Antonio Terrasi1, Atilla Aydinli2, Salvo Mirabella1
Affiliations : 1 MATIS IMM-CNR and Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, ITALY 2 Department of Physics, Bilkent University, 06800, Ankara, Turkey 3 IMM-CNR, VII strada 5, 95121 Catania, ITALY 4 Department of Physics, Middle East Technical University, 06531 Ankara, Turkey

Resume : Ge quantum dots (QDs) are gaining a renewed interest because of their lower synthesis temperature, larger optical absorption and stronger quantum confinement effect (QCE) compared to Si QDs. Though size dependent bandgap is typically advocated for tuning the light absorption, QCE does not always explain experimental data. In this regard, it is essential to disentangle the role of size from the other effects related to the hosting matrix or preparation techniques. For this reason, we report on the structural and optical properties of Ge QDs prepared by annealing of Ge rich SiO2 or Si3N4 thin films produced by magnetron sputtering, plasma enhanced chemical vapor deposition or Ge implantation in stoichiometric matrices. By varying the Ge content, the QD size can be tuned in the 3-9 nm range in SiO2 matrix, or in the 1-2 nm range in Si3N4 matrix, as revealed by transmission electron microscopy. Si3N4 matrix hosts QDs at higher density than SiO2, while Raman analysis reveals a higher threshold for amorphous-to-crystalline transition for Ge QDs in Si3N4. Light absorption by Ge QDs strongly depends on the type and quality of the matrix, revealing a marked size-dependent shift of the optical bandgap and being more effective in Si3N4, with a strong role of the abundance of defect states. These data will be presented and discussed, opening new routes for application of Ge QDs in light harvesting devices. [1] Mirabella et al. APL 101, 011911 (2012) [2] Cosentino et al. accepted by JAP

Authors : E. G. Barbagiovanni (a), S. Cosentino (a), A. Terrasi (a), S. Mirabella (a), D. J. Lockwood (b), R. N. Costa Filho (c)
Affiliations : (a) MATIS IMM-CNR and Dipartimento di Fisica e Astronomia, Universita di Catania, Catania 95123, Italy (b) Measurement Science and Standards, National Research Council, Ottawa, Ontario K1A 0R6, Canada (c) Departamento de Fisica, Universidade Federal do Ceara, Caixa Postal 6030, Campus do Pici, 60455-760 Fortaleza, Ceara, Brazil

Resume : There is renewed interest in Ge nanostructures (NSs) for photovoltaic and optoelectronic applications due to recent fabrication advances. Here, we investigate the structural and optical properties of Ge quantum dots (QDs) formed in different matrices. High quality Ge QDs were prepared by sputtering, plasma enhanced chemical vapour deposition (PECVD) and ion implantation embedded in either a SiO2 or Si3N4 matrix [1]. Optical absorption measurements indicate that Ge QDs in a Si3N4 matrix have a higher absorption efficiency compared to a SiO2 matrix, due to a higher areal density. On the other hand, Ge QDs embedded in the SiO2 matrix exhibit a larger increase in the optical energy gap (EG) with decreasing QD diameter (D) compared to the Si3N4 matrix. This result is theoretically modelled using a finite confinement potential as a function of the lowered inter-facial EG between Ge/Si3N4 compared to Ge/SiO2 [2]. Regardless of the preparation method, Ge QDs exhibit a larger confinement effect compared to their Si counterpart. To explain this observation, we have developed a new theoretical model under the effective mass approximation (EMA) and a spatially dependent effective mass (SPDEM) [2]. Within this model, we calculated a reduction in the NS effective mass (EM) from the bulk value, which increases EG. Further differences in the confinement energy due to the matrix arise as a function of the interface defect states. Furthermore, our results indicate that Ge NSs experience an additional confinement mechanism due to the dimensionally dependent EM, which modifies the dispersion relation and can be exploited for device fabrication. [1] S. Cosentino et al., submitted to J. Appl. Phys. (2014) [2] E. G. Barbagiovanni et al., accepted J. Appl. Phys. (2014)

Authors : Julie Goffard1-2, Patrice Miska*2, Davy Gérard1, Michel Vergnat2 and Jérôme Plain1
Affiliations : 1) Université de Technologie de Troyes-12 rue Marie Curie 10000 Troyes (France) 2) Institut Jean Lamour UMR CNRS 7198 – Nancy Université – UPV Metz , Boulevard des Aiguillettes, 54500 Vandoeuvre-les-Nancy France

Resume : The use of silicon nanocrystals (SiNC) in optoelectronic devices has risen from a decade thanks to the discovery of photoluminescence in porous silicon in 1991 [1]. However the SiNCs exhibit a low quantum yield, which prevent from currently using them in optoelectronic devices. This problem can be bypassed by using localized surface plasmons (LSP). Indeed LSPs can modify emitters’ photoluminescence by changing the optical local density of states and/or increasing the local excitation field. Numerous studies have been performed with different emitters, but the analyze of LSP coupled to SiNC is scarce. Seminal studies by Biteen and coworkers [2,3] have paved the way for increasing SiNCs’ performances with LSPs. In these preliminary works only the SiNCs’ emission wavelength was coupled to LSPs. In this work, we first present a study where LSPs resonances of gold nanodisks (GNDs) are coupled to SiNCs [4]. Then, we show results concerning the coupling of both the absorption and emission wavelengths of SiNCs to gold nanorods. In both cases, the use of an original fabrication method gives us the control all the geometrical parameters that modify the SiNCs – LSPs coupling. In the first study, we use GNDs with different diameters and located at different distances to the SiNCs. We then evaluate the maximum quantum yield enhancement for the sample with the optimized geometry. The use of nanorods with different shapes and metallic materials allows us to obtain in the same structure two surface plasmon modes exhibiting different polarizations. To understand them, we studied the coupling of LSP to the SiNC’s photoluminescence intensity, polarization and spatial redirection. [1] A. G. Cullis, L. T. Canham, Nature 1991, 353, 335–338. [2] J. S. Biteen, D. Pacifici, N. S. Lewis, H. A. Atwater, Nano Lett. 2005, 5, 1768–1773. [3] J. S. Biteen, N. S. Lewis, H. A. Atwater, H. Mertens, A. Polman, Appl. Phys. Lett. 2006, 88, 131109. [4] J. Goffard, D. Gérard, P. Miska, A.-L. Baudrion, R. Deturche, and J. Plain, Scientific Reports 3, 2672 (2013).

Authors : M. Perani (1), D. Cavalcoli (1), M. Canino (2), M. Allegrezza (2), M. Bellettato (2), C. Summonte (2)
Affiliations : (1) Physics and Astronomy Dept University of Bologna, viale B. Pichat 6/2, 40127 Bologna, Italy; (2) CNR-IMM, via Gobetti 101, 40129 Bologna, Italy

Resume : Silicon nano-crystals (Si NCs) embedded in a dielectric matrix are presently studied in view of their application within the framework of the third generation photovoltaic. This system provides a tunable band gap layer that can be used as a top absorber in all-silicon multi-junction photovoltaic cells. Although high conductivity and high mobility are requirements for the matrix in order to guarantee an efficient carrier collection, the ideal properties of the NCs are more related to the ability of absorbing photons beyond a given energy, and emitting photogenerated carriers into the surrounding matrix. SiC/SRC (Silicon Rich Carbide) multilayers produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) have been annealed to obtain NCs formation. The properties of the multilayers have been studied as a function of different experimental conditions, such as NCs size. AFM maps are obtained in tapping mode in order to investigate the topography of the layers and the energy dissipated between the tip and the sample. Conductive-AFM is performed to identify conductive paths at the nano-scale, showing the presence of conductive clusters. Macroscopical conductivity and local IV (with AFM in contact mode) measurements are performed and show a similar increasing trend with respect to SRC layer thickness. The results are correlated with crystalline fraction and structural properties of the layers as obtained by optical analyses and transmission electron microscopy measurements.

Authors : C. Weiss, M. Schnabel, P. Löper, S. Janz
Affiliations : Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany; Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany; École Polytechnique Fédérale de Lausanne, Rue de la Maladière 71b, CP 526, CH-2002 Neuchâtel 2, Switzerland; Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany

Resume : Please delete this abstract!

Authors : K. Kusova, P. Hapala, P. Jelinek, J. Valenta, L. Ondic, O. Cibulka and I. Pelant
Affiliations : K. Kusova; P. Hapala; P. Jelinek; L. Ondic; O. Cibulka and I. Pelant Institute of Physics ASCR, Prague, Czech Republic J. Valenta Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic

Resume : Bulk silicon is notoriously known as an inefficient light emitter due to its indirect bandgap, which is usually perceived as an obstacle towards the utilization of this material in optoelectronics and bioimaging, despite the evident advantages of this approach. Some alleviation of the poor light emission performance of bulk silicon can be achieved by switching to silicon nanocrystals, whose light-emission efficiency can be reasonably high, but whose photon emission rates are still usually low due to the remaining character of the indirect badstructure. In this contribution, we show that the bandstructure of silicon nanocrystals can be strain-engineered to achieve fundamental direct bandgap, leading to a 10 000x increase in the photon emission rate. We will first briefly discuss the validity of the bandstructure concept on the nanoscale [1] and then present both theoretical and experimental evidence that the joint action of quantum confinement and tensile strain induced by surface capping lead to indirect-to-direct bandgap cross-over [2]. [1] P. Hapala et al.: `Theoretical analysis of electronic bandstructure of 2-to-3-nm Si nanocrystals,' Phys. Rev. B 2013, 87(19) 195420. [2] K. Kusova et al.: `Direct Bandgap Silicon: Tensile-Strained Silicon Nanocrystals,' Adv. Mater. Int. 2014, in press, DOI: 10.1002/admi.201300042.

Poster session III : T. Lafford and R. Turan
Authors : Osama Tobail
Affiliations : Egypt Nanotechnology Center

Resume : This paper reports on our progress in developing and optimizing low band gap bottom solar cells for tandem structures. We focus on three types of material grown by Plasma Enhanced Chemical Vapour Deposition (PECVD): (i) Hydrogenated microcrystalline silicon (µc-Si:H). (ii) Hydrogenated amorphous silicon germanium a-SiGe:H alloy. (iii) Hydrogenated microcrystalline silicon germanium (µc-SiGe:H) alloy. The material crystallinity was determined by micro-Raman spectroscopy and optimized for the best cell performance. The µc-Si:H solar cell fill factor was below 32% due to the oxygen contamination, which is in the range of 7x1019cm-3 as determined by SIMS. Oxygen concentration was reduced by one order of magnitude by increasing the pumping duration and purging the chamber before the process. Another disadvantage of µc-Si:H is its low growth rate of 2 to 7A/s. To overcome those problems, we developed a process for a-SiGe:H and µc-SiGe:H. This paper shows the advantages of those materials over µc-Si:H as the band gap can be tailored, and higher growth rate is achieved. The AMPS-1D simulation of the experimentally fabricated cells show that the fill factor limitation of the low band gap cells is due to a 0.5eV barrier due to the p/i interface. This barrier can be reduced to 0.1eV by adding Ge to the p-type window layer. The resulted cells have an increase of the fill factor from 53.2 to 58.8% due to a reduction of cell series resistance from 8.3 to 4.8Ohm/cm2.

Authors : Wen Ding1, Goshi Morioka1, Akio Suzuki1, Hidetoshi Suzuki2, Atsuhiko Fukuyama1, Masafumi Yamaguchi3, and Tetsuo Ikari1
Affiliations : 1Faculty of Engineering, University of Miyazaki, 1-1 Gakuen Kibanadai-nishi, Miyazaki 889-2192, Japan; 2Interdisciplinary Research Organization, University of Miyazaki, Miyazaki, Japan; 3Toyota Technological Institute 2-12-1 Hisakata, Tempaku-ku, Nagoya, Aichi 468-8511, Japan

Resume : Dilute nitride GaAsN has been expected as an absorbing layer material for ultrahigh-efficiency tandem solar cells and its unique property of the band gap energy has been understood in terms of a band anti-crossing (BAC) model. However, it is not clear whether the localized nitrogen state (E_N) is determined by an averaged interatomic distance between N atoms or formation of N related clusters. Since the distribution and environment of N atoms may be changed by the growth method, we investigate the effect of crystal growth method on E_N. The photoreflectace (PR) measurements were used for estimating E_N. Chemical beam epitaxy (CBE) and flow-rate modulated chemical beam epitaxy (FM-CBE) were employed for growing the samples with N contents of 0.3-1.8% and 0.6-1.4%, respectively. We found the temperature coefficient of E_N (-dE_N/dT) decreased by N composition. Both the value and the reduction rate of -dE_N/dT for FM-CBE are larger than those for CBE. Since -dE_N/dT is strongly related to the distance between the adjacent N atoms, the effect of interatomic distance on their localized electronic levels could be understood. The nature of the electronic level depends on the N-supplying sequence during the crystal growth. It is concluded that a pair of adjacent N atoms is included in the microscopic structure and this pair determines E_N. The N atoms were distributed uniformly for the FM-CBE sample and this resulted in the knowing of the detailed electronic state of N in dilute GaAsN.

Authors : S. Prucnal and W. Skorupa
Affiliations : Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, P.O. Box 510119, 01314 Dresden, Germany

Resume : The incredible growth rates of the PV solar industry have allowed manufacturing efficiencies that are unheard of in other industries. Nowadays in the solar cells industry the main effort is directed to the cost reduction of the solar panels fabrication which will decrease the average price per kWh from presence 0.20 €/kWh down to 0.07 €/kWh in 2020. Generally it is realized by using much cheaper polycrystalline wafers, reduction of the overall wafer thickness and/or simplification of the production complexity. We propose the simplification of the production process of silicon solar cells using only one step millisecond annealing for the whole solar cell processing and replacing standard phosphorous thermal diffusion by plasma immersion ion implantation. Our technology can be directly transferred to an in-line production process leading to significant cost reduction and decreasing the amount of chemicals used during solar cell manufacturing. Due to one step millisecond range flash lamp annealing (FLA) the overall thermal budget needed for the solar cell fabrication is significantly reduced. Moreover the emitter formed by ion implantation and FLA is clean and allows the precise control of the dopant concentration and width of the p-n junction.

Authors : K. Kacha1, F. Djeffal1, T. Bentrcia2 and I. Berbezier3
Affiliations : 1) LEA, Department of Electronics, University of Batna, Batna 05000, Algeria. 2) Department of Physics, University of Batna,Batna 05000, Algeria. 3) IM2NP Aix-Marseille Universités, UMR CNRS n°7334, Faculté des Sciences St-Jérôme - Case 142, 13397 Marseille Cedex 20 France. E-mail:,, Tel/Fax: 0021333805494

Resume : Nowadays, solar cell technology attracts much attention in reliable and high efficiency photovoltaic applications. Thin-film SiGe solar cells have important advantages such as high photocurrent and their compatibility with the process developed for pure Si cells. In order to improve the electrical efficiency performances of the conventional SiGe solar cell, we have introduced a new multi-trench technique. In the proposed method, the multi-trench is created in the silicon layer and filled with n-type doped SiGe. The p-type trenches under the SiGe layer improve the electrical performance of the proposed design. By 2-D numerical simulation, we have investigated the electrical performances of the proposed design and compared with it a conventional thin-film SiGe solar cell. The proposed accurate numerical models have been used as objective function to optimize the electrical performance of the SiGe-based solar cell. The obtained results show that the proposed design can be considered as a potential candidate for high performance photovoltaic applications.

Authors : Hui-Min Chuang,†, Chun-Ting Li,†, Min-Hsin Yeh,† Chuan-Pei Lee,† R.Vittal†, and Kuo-Chuan Ho†,‡,*
Affiliations : †Department of Chemical Engineering and ‡Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan Corresponding Author: *E-mail:

Resume : A coral-like film of nickel@nickel sulfide (Ni@NiS) was obtained on a conducting glass through an electrochemical method, in which the Ni film functioned as a template. Three types of Ni thin films were electrodeposited on fluorine-doped tin oxide (FTO) substrates by the pulse current technique at the passed charge densities of 100, 200, and 300 mC/cm2, which rendered custard apple-like, coral-like, and cracked nanostructures, respectively. Subsequently, NiS films were coated on these Ni films by using the pulse potential technique. Due to the template effect of the Ni films, the composite films of Ni@NiS also assumed the same structures as those of their nickel templates. In each case of the films the particle of the film assumed a core-shell structure. The Ni@NiS coated FTO glasses were used as the counter electrodes for dye-sensitized solar cells (DSSCs). The DSSC with the coral-like Ni@NiS film on its counter electrode exhibits the highest power conversion efficiency (η) of 7.84%, while the DSSC with a platinum film on its counter electrode shows an η of 8.11%. The coral-like Ni@NiS film exhibits multiple functions, i.e., large surface area, high conductivity, and great electrocatalytic ability for iodine/triiodine (I /I3-) redox reaction. X-ray photoelectron spectroscopy (XPS), X-ray diffraction pattern (XRD), scanning electron microscopy (SEM), and four-point probe technique were used to characterize the films. The photovoltaic parameters are substantiated using incident photon–to–current conversion efficiency (IPCE) curves, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization plots. The IPCE curves were further used to calculate the theoretical short-current densities of the cells.

Authors : Jeeyoung Lee, Jisuk Park, Junyeon Heo, Hyungseok Min, Myeongkyu Lee
Affiliations : Department of Materials Science and Engineering, Yonsei University, Seoul, Korea

Resume : The light harvest in a solar cell can be greatly improved if the sunlight is made obliquely incident into the absorption layer. Not only enhancing light absorption by increasing the optical path length, this configuration is also favorable for the extraction of charges as they are generated closer to the collecting electrode. Despite many advantages, the deflection of incident light has not been employed for dye-sensitized solar cell (DSSC) because of the inherent difficulty of incorporating a relevant structure into DSSC. Here we report that a refractive-index grating can be embedded into the TiO2 electrode and this architecture highly increases light absorption in DSSC by diffracting the incident light. A thin TiO2 film spin-coated on conducting glass was molded by imprinting and treated with a higher TiCl4 concentration than the later-coated thicker TiO2 absorption layer. Due to the refractive index difference between two layers, the incident light was widely diffracted, giving diffraction efficiencies over 70% in the visible range. As a result, the IPCE and current density of the cells were much improved. This facile approach can be effectively utilized to increase the light harvest of DSSC and its energy conversion efficiency.

Authors : J. Montero, C. Guillen, C. G. Granqvist, J. Herrero, G. A. Niklasson
Affiliations : Department of Engineering Sciences, The Ångstrom Laboratory, Uppsala University (J. Montero, C. G, Granqvist and G. A. Niklasson) Department of Energy, Ciemat (C. Guillen and J. Herrero)

Resume : Antimony doped tin oxide (ATO) is a promising transparent conductor oxide which has wide applications as electrode in semiconductor devices such as solar cells and organic light emitting diodes. The successful application of polycrystalline ATO films as transparent electrodes in such devices depends on the band alignment of the conductor oxide to the semiconductor, which in turn depends on the ATO electronic band structure (Fermi level, gap energy, etc) that is related to the surface termination given by its crystallographic orientation. In the present work, ATO films were deposited on glass substrates without intentional heating by reactive sputtering from a metallic target of Sn:Sb (5wt.%) in several O2/Ar plasma mixtures. The samples displayed a nanocrystalline rutile structure with preferential orientation changing from (101) to (211) or (110) as the deposition conditions were altered. These structural changes are consistent with variations in the optical and electrical characteristics. A detailed analysis of the various samples shows how the surface orientation of the sputtered ATO films can be controlled by different preparation parameters such as the O2/Ar ratio and the discharge power density. These insights serve as guidelines for tailoring ATO films for specific device applications.

Authors : F. L. Bregolin1, P. Lindberg2, K. Wiesenhuetter1, L. Vines2, S. Prucnal1, B. G. Svensson2, W. Skorupa1
Affiliations : 1) Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, P.O. Box 510119, 01314 Dresden, Germany; 2) Department of Physics/Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway

Resume : Currently, indium tin oxide (ITO) is the most widely used transparent conductive oxide due to its outstanding properties. However, because of its high cost, several alternatives are being sought to replace it. Among them, the aluminum-doped zinc oxide (AZO) films are one of the most promising candidates for PV applications due their low resistivity, high transparency and most of all, their relative low cost of fabrication. In this work, AZO films were deposited over Si wafers via r.f. magnetron sputtering and subsequently treated by millisecond-range flash lamp annealing (FLA). The fabricated layers were then characterized by sheet resistance, photoluminescence spectroscopy, X-ray diffraction and Hall effect measurements. The influence of the deposition temperature and FLA parameters on the microstructure and optoelectronic response of the AZO layers was studied in detail. It was demonstrated that the FLA technique significantly improves the electrical conductivity of the as-deposited AZO layers due to the Al activation, the increase in crystallinity as well as the passivation of defects and grain boundaries. In particular, the room temperature sputtered AZO films subsequently treated by FLA have shown performance characteristics similar to those sputtered at 400 ?C, opening the possibility for further cost reductions in the fabrication process. The FLA technique is a cost-effective and high-throughput alternative for the processing of Si-based heterojunction solar cells.

Authors : Rosa Chierchia, Alberto Mittiga, Enrico Salza, Luca Serenelli, Mario Tucci
Affiliations : Enea Italian National Agency for New Technologies, Energy and Sustainable Economic Development

Resume : The Al doped Zinc Oxide (ZnO:Al) is a transparent and conductive oxide used in alternative to the Indium Tin Oxide (ITO) as contact and antireflection layer in amorphous silicon heterojunction and chalcogenide based solar cell. Generally good film quality can be obtained by low cost magnetron sputtering, even though it is difficult to obtain a resistivity below 10-3 ohm cm when grown at the temperature below 200°C commonly used in solar cells manufacturing to avoid thermal stress. Following the recent theoretical and experimental confirmations of the Hydrogen doping property in Al:ZnO films, in this work the Hydrogen effect on the Al:ZnO film as optical, electrical and structural properties is investigated. Two different growth techniques have been considered: DC and Pulsed DC magnetron sputtering. A clear improvement in the electrical properties of the layer has been noted in particular when films have been grown by DC pulsed magnetron sputtering. Resistivity of 6.7x10-4 ohm cm, carrier concentration of 5.4x1020 cm-3 and electrical mobility of 17 cm2/Vs have been obtained even on film thinner than 100nm. While the low mobility value suggests that the film grain size still needs to be increased, XRD measurements show better quality material. Finally the stability of the hydrogen doped ZnO:Al film properties is evaluated when the film is subjected to thermal stress as in solar cell manufacturing applications.

Authors : David Dodoo-Arhin(ab), Richard Howe(a), Guohua Hu(a), Na Jiao(a), Pritesh Hiralal(c), Gehan Amaratunga(c), Tawfique Hasan(a)
Affiliations : (a) Cambridge Graphene Centre, Cambridge University, UK; (b) Department of Material Science and Engineering, University of Ghana, Ghana; (c) Engineering Department, Cambridge University, UK.

Resume : Dye-sensitized solar cells (DSSCs) have been intensively studied as prospective alternative to conventional silicon-based solar cells, largely due to its simple fabrication process, relatively high energy conversion efficiency, and low-cost materials. However, two components of the DSSC, namely the counter electrode and the dye, still have significant potential for cost reduction. Graphene is a promising candidate for platinum substitution in DSSCs due to its high exchange current density, low charge-transfer resistance high specific surface area, abundance and low cost. In this work, we present studies on DSSCs based on graphene ink counter electrodes produced via liquid phase exfoliation of graphite. We use natural tropical (Ghanaian) dye extracts from Pennisetum glaucum, Hibiscus sabdariffa and Caesalpinia pulcherrima as photosensitizers. Properties of the various dyes have been investigated via UV-Vis, FTIR and Photoluminescence. Typical, graphene ink based DSSCs sensitized with natural dye extracts show Jsc values from 3.36 mA to 1.67 mA, the Voc from 0.589 V to 0.498 V, fill factor from 0.712 to 0.663, and conversion efficiency from 1.41% to 0.55%. on the other hand, graphene ink counter electrode DSSCs assembled with N719 dye gave Jsc of 7.436 mA, Voc of 0.644V, FF of 0.620 and a conversion efficiency of 2.97%. The effects of the graphene ink and the natural dyes are individually discussed in comparison with their traditional counterparts.

Authors : L. Mazzarella1, S. Kirner1, B. Stannowski1, L. Korte2, R. Schlatmann1, B. Rech2
Affiliations : 1 PVcomB, Helmholtz-Zentrum Berlin für Materialien und Energie, Schwarzschildstr. 3, 12489 Berlin, Germany; 2 Institute for Silicon Photovoltaics, Helmholtz-Zentrum Berlin für Materialien und Energie, Kekuléstraße 5, 12489 Berlin, Germany

Resume : In heterojunction (HJ) silicon solar cells, wide optical band gap (Eg) emitter layers are intensively investigated to obtain high performance devices by reducing parasitic absorption and at the same time have low ohmic contact resistance to the TCO. Eg is strongly influenced by PECVD deposition parameters, such as the choice of precursor gases. In p-doped a-Si:H layer deposition, Diborane (B2H6) or Trimethylboron (TMB) are commonly used as dopant gases. In this study, we systematically compared the optoelectronic properties of p-doped a-Si:H single layers using B2H6 and TMB and varying gas concentration and deposition temperature over a wide range. Film properties were investigated using spectrophotometry, FTIR-spectroscopy, as well as electrical characterizations. We furthermore compared HJ solar cells fabricated with both TMB- and B2H6 based emitter p-layers on textured and smooth n-type Si wafers. Both, solar cell and single layer results as well as literature data suggest that TMB doped emitters give systematically lower fill factors that we attribute to the lower conductivity of these layers. On the other hand, Jsc is significantly increased up to values close to 40 mA/cm². We ascribe this to CHx complexes built into the layer, which at the same time enhance the optical gap and reduce doping efficiency. The best solar cell produced had an efficiency of 20.6%.

Authors : Olesia Synooka,¬ Kai–Rudi Eberhardt, Chetan Raj Singh, Gernot Ecke, Bernhard Ecker, Elizabeth von Hauff, Gerhard Gobsch and Harald Hoppe
Affiliations : Olesia Synooka, Kai-Rudi Eberhardt, Chetan Raj Singh, Felix Hermann, Prof. Gerhard Gobsch, Dr. Harald Hoppe Department of Experimental Physics 1, Institut für Physik and Institut für Mikro- und Nanotechnologien, TU Ilmenau, PF 100565, 98693 Ilmenau, Germany E-mail: or Gernot Ecke Department of Nanotechnology, Institut für Mikro- und Nanotechnologien, TU Ilmenau, PF 100565, 98693 Ilmenau Dr. Bernhard Ecker, Prof. Elizabeth von Hauff Organic Photovoltaics & Electronics Group, Institute of Physics, University of Freiburg, 79104 Freiburg, Germany Fraunhofer ISE, Heidenhofstr. 2, 79110 Freiburg, Germany

Resume : As reported earlier, the photovoltaic performance of PCDTBT:PCBM polymer solar cells drastically decreases upon thermal annealing. It was demonstrated in the literature, that thermal annealing leads to increased trap formation and as a consequence disturb solar cell performance, especially via a reduced fill factor. This has been demonstrated by space-charge-limited-current analysis and ellipsometry, as well as, structural changes analyse of PCDTBT upon annealing. However, we decided in addition to investigate morphological changes occurring within PCDTBT:PCBM photoactive blend layers upon thermal annealing, as these must have an impact on charge transport. By application of several characterizations techniques, and especially supported by results of Impedance Spectroscopy and Auger Electron Spectroscopy (AES), indeed the existence of an unfavourable compositional gradient within the photoactive layer could be revealed. This compositional gradient may be in part accounted for harming the transport of electrons and holes in either direction.

Authors : Li-Wei Chou, Albert T. Wu
Affiliations : Ph.D. Candidate; Professor

Resume : Highly texture surface tin oxide transparent conductive oxide (TCO) thin films have been directly deposited via atmospheric pressure chemical vapor deposition (APCVD) technique on nanoparticle-coated glass substrates. A simple nozzle spraying process was developed in tin oxide nanoparticle coating process for texture tin oxide thin films. It was found that with applying nozzle spraying process, the surface morphology of tin oxide films changed from pyramidal shape to flower-like double texture. The optimum nanoparticle-coated tin oxide thin films has a haze value of 35.0±4.0% and an average visible optical transmittance of 80.6±2.2% in wavelength range of 400-900 nm. The carrier concentration and the mobility of the film were 1.3±0.3 × 10^20 cm-3 and 4.0±0.6 cm^2/V*s, respectively. The morphological evolution of tin oxide thin films was shown on the hetero-surface of amorphous glass and multicrystalline tin oxide nanoparticle. This result reveals that the crystalline tin oxide nanoparticle played an important role for the fabrication of flower-like double texture, and the texture tin oxide thin films are promising TCO materials for thin film solar cells.

Authors : K. Lovchinov, M. Petrov, O. Angelov, D.Karashanova, D. Dimova-Malinovska
Affiliations : Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences; Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl.109, 1113 Sofia, Bulgaria

Resume : The systems comprised of layered structures transparent conductive oxide (TCO)/noble metal (Cu, Au, Ag) are recently intensively studied because of the possibility to be applied in thin film solar cells. The difference in the work functions between the TCO and the noble metals give an opportunity to increase the conductivity of the TCO as a rear electrode. The main tasks in development and fabrication of thin film solar cells are increasing of the absorption of light, minimization of the reflection losses and increasing of the reflection of the transmitted light through the solar cell. Multilayer stack structures ZnO:Al/Ag/ZnO:Al and ZrO2/Ag/ZrO2 are prepared by r.f. magnetron sputtering. The ZnO:Al, ZrO2 and Ag layers are deposited on glass substrates without heating. Optical, structural and electrical properties of the as-deposited and annealed three-layer stacks are studied. The TEM, SEM and AFM analysis demonstrate changes of the stack structure with Ag film after annealing. The transmittance and reflectance spectra demonstrate bands of Ag electrons plasma oscillations and inter-band d-shell Ag electrons which are red shifted after annealing. The optical spectra of the as-deposited and annealed ZrO2/Ag/ZrO2 multilayer stacks do not change and they have a higher values of diffuse reflectance than the ZnO:Al/Ag/ZnO:Al ones. The resistivity of the as-deposited ZnO:Al/Ag/ZnO:Al films is 4.0x10-5 Ω.cm and after annealing it increases to 6.0x10-5 Ω.cm. The resistivity of the as-deposited ZrO2/Ag/ZrO2 films is 1.5x10-5 Ω.cm and after annealing it decreases slightly to 1.3x10-5 Ω.cm. The better resistivity of the ZrO2/Ag/ZrO2 multilayer stacks is due to the better structure of the Ag film leading to transformation of the conductivity from thermally activated hopping mechanism to metallic type. The obtained structures reveal a potential for application at the rear side of thin film Si solar cells for improvement of their light trapping.

Authors : M. Petrov, K. Lovchinov, O. Angelov, H. Nichev, D.Karashanova, D. Dimova-Malinovska
Affiliations : Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, bulv. Tzarigradsko chaussee, 72, 1784 Sofia, Bulgaria; Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl.109, 1113 Sofia, Bulgaria

Resume : ZnO thin films and coatings have attracted a great interest due to their suitable properties for application in optoelectronic devices, sensors, displays and different thin film solar cells. Recently, development of cost-effective methods for fabrication of one dimensional ZnO nanostructures, such as nanowires, nanorods and nanotubes have attracted extensive attention because of their potential applications for the advanced solar cell structures. The nanometer size ZnO based nanostructures have very large surface areas per unit volume, which offers a possibility to improve light harvesting properties of solar cell structures in case if such nanostructures are used as antireflection coatings. In this paper we report results of fabrication of ordered ZnO nanorods (NRs) or nanowalls (NWs) electrochemically deposited on different highly conductive substrates. The following types of conductive substrates have been used: (i) glass/ITO, (ii) glass/SnO2, (iii) glass covered by 2 types multilayer stack structures ZnO:Al/Ag/ZnO:Al and (iv) ZrO2/Ag/ZrO2, v) highly doped p-type multi-crystalline Si and stainless steel thin plates. ZnO nanostructured films are deposited by an electrochemical process using a three-electrode potentiostatic system with saturated calomel electrodes as reference electrodes, from aqueous solution containing ZnCl2, KCl, pH = 4.00. An analysis of the surface morphology of such layers is performed by means of Scanning Electron Microscope (SEM). Measurements of the diffused reflection spectra of ZnO based nanostructures deposited substrates have been performed as well. It is observed that morphology of the deposited ZnO structures depends strongly on the type of the substrate used. It is found that ZnO structures with different shapes, such as nanorods, nanotubes or nanowalls can be grown on top of highly conductive substrates used in this work. It is observed that optical reflection of the deposited layers depends on the substrate used. Possible applications of ZnO NRs or NWs based structures for the processing of advanced Si based solar cells are discussed.

Authors : L. V. Mercaldo, E. M. Esposito, I. Usatii, P. Delli Veneri
Affiliations : ENEA, Portici Research Center, P.le E. Fermi 1, 80055 Portici (Napoli), Italy

Resume : Thin film silicon solar cell technology, with its potential to reduce material and fabrication costs, is considered as a promising approach for a large-scale deployment of photovoltaics. The reduction of parasitic absorption in inactive layers is one important aspect to enhance the absorption of the sunlight in the intrinsic absorber layers and then the use of poorly absorbing doped layers is desired. Lower absorption, lower refractive index, and tunable resistance are three advantages of mixed phase p-type SiOx compared to microcrystalline silicon , when used as a p-type layer in solar cells. In the present study we report on the development of p-type material with lower optical constants with respect to Si for use as advanced window layer in superstrate devices. Boron-doped SiOx films have been deposited on glass by VHF-PECVD (40 MHz) at 150°C from a mixture of CO2, SiH4, H2, and B(CH3)3 (TMB). The H2/SiH4 dilution ratio and the TMB/SiH4 doping ratio were held constant, while the CO2/SiH4 gas flow rate ratio was varied to obtain films with different stoichiometry for different plasma power densities. The series of films have been characterized by micro-Raman, spectroscopic ellipsometry and lateral conductivity measurements. The beneficial effect in thin film μc-Si:H solar cells has been demonstrated with a significant increase of the quantum efficiency in the short wavelength range (Jsc gain of 1 mA/cm2), thanks to reduced absorption and reflection losses.

Authors : S. Boscarino1,2, G. Torrisi3, I. Crupi2, A. Alberti4, S. Mirabella2, F. Ruffino1,2, F. Simone1, A. Terrasi1,2
Affiliations : 1Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, Italy; 2MATIS IMM-CNR, via S. Sofia 64, 95123 Catania, Italy; 3Distretto Tecnologico Sicilia Micro e Nanosistemi, via Strada VIII 5, 95121 Catania, Italy; 4CNR-IMM, via Strada VIII 5, 95121 Catania, Italy

Resume : The combination of low electrical resistivity with high optical transparency make Al-doped ZnO (AZO) films widely used as transparent electrodes in solar cells. In order to improve conductivity and transmittance of these films, annealing treatments are normally employed during or after the growth process. We report on the increase of the conductivity of ultra-thin AZO films upon irradiation with high energy O2+ ions. This improvement is linked to the modifications of both optical and structural properties of the material. The AZO thin films (60 nm) have been grown by RF-magnetron sputtering at room temperature, then irradiated with 350 MeV passing O2+ ions at fluencies of 3x1015, 1x1016 and 3x1016 ions/cm2. By X-ray diffraction analysis, all films have shown hexagonal wurtzite structure, improving the crystalline quality as a function of the ions fluencies. AFM analysis for the surface morphology was also performed. In particular, we observed an enhancement of the electrical conductivity of more than two orders of magnitude, along with grain size increasing and in-plane strain change from compressive to tensile values. The optical bandgap increases with the ion doses, partly due to the Burstein-Moss effect connected with an improved crystalline quality. Annealing effects, up to 400°C, are also reported and compared with the irradiation ones. Our results indicate that the structural quality and conductivity of the AZO thin films can be improved by specific ion implantation processes.

Authors : L. Serenelli 1, R. Chierchia 1, M. Izzi 1, M. Tucci 1, L. Martini 2 D. Caputo 2, R. Asquini 2, G. de Cesare 2
Affiliations : 1 ENEA, Research Centre Casaccia, via Anguillarese, 301– Rome, Italy 00123; 2 Dept. of Information, Electronic and Telecommunication Engineering, Sapienza Univ. of Rome, via Eudossiana, 18 – Rome, Italy 00184;

Resume : The most attracting way to fabricate high efficiency solar cells is the amorphous/crystalline silicon technology, because of the high Voc obtainable as a consequence of excellent c-Si surface passivation offered by a-Si:H films. This passivation is obtained by saturation of silicon dangling bonds at c-Si/a-Si interface and can be influenced by the hydrogen inclusion within the a-Si:H layer. In this work we propose the use of hydrogen plasma treatments subsequently performed on the thin amorphous silicon layer deposited to passivate the c-Si surface. We compare the hydrogen effect on the interface with that of thermal annealing of the interface to identify how the hydrogen evolves inside the a-Si:H network modifying the defect density and the a-Si:H/c-Si interface. To monitor the hydrogen effect on the heterointerface we propose the use of surface photovoltage technique as a contact-less tool for the evaluation of the energetic distribution of the state density at amorphous/crystalline silicon interface. This technique results to be very sensitive to the different experimental treatments, and therefore it can be considered a precious tool to monitor and improve the interface electronic quality. These characteristics are compared with FTIR spectra measurements performed on both hydrogen plasma post-treated and thermal annealed a-Si:H/c-Si samples. We find out that hydrogen plasma post treatment is effective to stably reduce interface state density.

Authors : Leo Farrell, Elisabetta Arca, David, Caffrey, Daragh Mullarkey, Igor Shvets
Affiliations : Trinity College Dublin, College Green, Dublin 2, Ireland.

Resume : Transparent conducting oxides (TCO) are used in flat screen displays (TFTs), organic light emitting diodes and thin film solar cells. p-type TCOs used as the hole injection layer are an integral component of these devices. However, p-type TCOs have always exhibited poorer performances than their n-type counterparts. As a result, new p-type materials remain an important area of research. Cr2O3 doped with Mg is a candidate p-type TCO material. In this study we improved on the electrical and optical properties of Cr2O3:Mg. The samples were epitaxially grown by PLD and MBE where the stoichiometry was finely tuned in order to investigate the effect on the structural, electrical and optical properties. The influence of the Mg dopants and the oxygen partial pressure were also investigated by Seebeck and resistivity measurements. Carrier transport properties are examined. The negative effect of compensating defects such as oxygen vacancies are known for p-type materials and post-annealing methods are demonstrated. The origin of the p-type conductivity is correlated to the expected doping mechanism. The role of polaronic reduction in hole mobility for this material is also discussed. Investigating the fundamental properties in epitaxial material will allow us to add to our understanding of the role of defects in p-type TCOs, helping to improve material grown by other more industrial relevant methods.

Authors : Y. Vygranenko, M. Fernandes, M. Vieira, A. Khosropour, A. Sazonov
Affiliations : Electronics, Telecommunications and Computer Engineering Department, ISEL, Lisbon, Portugal; CTS-UNINOVA, 2829-516 Caparica, Portugal; Electrical and Computer Engineering Department, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

Resume : Solar cells on lightweight and flexible substrates have advantages over the glass- or wafer-based photovoltaic devices in both terrestrial and space applications. Here, we report on amorphous silicon solar cells on plastic foils in the substrate configuration having a front metal grid. A two-dimensional distributed circuit model of the photovoltaic cell has been developed for performance analysis and device design optimization. The circuit simulator SPICE is used to calculate current and potential distributions in a network of sub-cell circuits. The equivalent circuit of each sub-cell includes a diode, a current source representing current generation in the n-i-p structure, and resistors representing the p-layer, transparent-conducting oxide (TCO) electrode, top metallization and shunting resistances. The input parameters also include performance characteristics of the n-i-p structure, which can be determined experimentally. This approach enables a realistic device model that predicts output current-voltage characteristics and maps Joule losses in the TCO electrode and the metal grid. As an example of usage the optimization of contact grid geometry at various TCO sheet resistances has been performed. These results are used to determine the optimal thicknesses for the ZnO:Al electrode and top metallization in the developed photovoltaic module thus boosting its power conversion efficiency.

Authors : Yow-An Leu1,2, Hui-Min Chuang2, Min-Hsin Yeh2, Lu-Yin Lin2, Ta-Jen Li2, Ling-Yu Chang1, Wei-Hung Chiang3,4, Jiang-Jen Lin1, and Kuo-Chuan Ho1,2*
Affiliations : 1Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan 2 Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan 3Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan 4Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu, 30011, Taiwan * Corresponding author (

Resume : Investigating alternative Pt–free catalytic films is crtitical to reduce the fabricating cost for the counter electrode (CE) of dye–sensitized solar cells (DSSCs). Multi–walled carbon nanotube (MWCNT) is one of the potential substitutions due to its excellent catalytic ability and high conductivity. A dispersant is necessary for preparing the MWCNT slurry before coating on the conductive substrates as well as improving the uniformity of the MWCNT film. A polymeric surfactant, poly(oxyethylene)-segmented imide (POEM), was synthesized and applied as the dispersant for preparing a well dispersed MWCNT slurry. However, the nonconductive POEM should be removed from the resulting film by annealing to avoid reducing the conductivity. In this study, boron–doped MWCNT prepared by a substitution method with NH3 and B2O3 as the etching gas and the boron source, respectively, was proposed as the catalyst on the CE of DSSCs. During the film fabricating process, the boron–doped MWCNT shows higher thermal stability to sustain higher temperature (500 oC) than MWCNT (400 oC). As a result, the nonconductive POEM was removed more completely for the film of boron–doped MWCNT. Higher cell efficiency (η) of 8.11% was obtained for the pertinent DSSC than that of the cell with a pristine MWCNT film (η = 6.16%). The better performance for the boron–doped MWCNT based DSSC is attributed to the higher conductivity and better catalytic ability resulting from the less POEM residues in the catalytic film and the more defects produced by boron doping, respectively. A DSSC with Pt–sputtered CE was also fabricated as a reference with an η of 8.03%.

Authors : M. Emziane, S. Alshkeili
Affiliations : Solar Energy Materials and Devices Laboratory Masdar Institute of Science and Technology Masdar City, PO Box 54224, Abu Dhabi, UAE.

Resume : Photovoltaic (PV) devices are rated according to standard testing conditions (STC) of 25 °C, AM1.5G under 1 sun which allows for an easy comparison between devices reported in other parts of the world. However, PV devices are expected to perform under various temperatures and spectra in the field. In this paper, PV devices performance was investigated under simulated Abu Dhabi conditions of temperature and spectra. Actual field data of Abu Dhabi were used to benchmark our simulated conditions. Computer based modeling was conducted to rate the performance of the PV devices under simulated Abu Dhabi conditions. Each condition effect was studied separately and then the combined effect on PV devices was reported. It was found that the effect of temperature on the performance of PV devices was greater than that of the spectra. Si/Ge, AlGaAs/CIGS and CdTe/InGaAs tandem PV devices under simulated Abu Dhabi conditions were found to have losses in efficiency ranging between 5.2% and 6.7% absolute compared to their expected performance under STC. The performance of the tandem PV devices under Abu Dhabi simulated conditions is lower than that of STC due to lower solar irradiance and higher expected operating cell temperatures. Key words: Solar cells, Tandem devices, Modeling, Temperature, Spectrum

Authors : Chi-Ta Lee, Jia-De Peng, and Kuo-Chuan Ho
Affiliations : 1. Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan 2. Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan * Corresponding author:

Resume : Dye-sensitized solar cell (DSSCs) have been widely researched in recent years for their possible high power conversion efficiency, low-cost production, and easy fabrication process. Platinum is an excellent catalytic material for counter electrode; however, it is considerably expensive. Consequently, numerous researches have been done for exploring competent substitutes for platinum to reduce the cost and enhance the power conversion efficiency. Recently, many materials such as sulfides, nitrides, oxides and selenides were applied in DSSCs since their remarkable catalytic ability for the redox reaction of I-/I3- couple in the electrolyte.[1] In this study, we synthesized different morphologies of nickel selenide by using methanol, ethanol, and propan-1-ol as the solvents. The results show that the morphologies of nickel selenide vary significantly by changing the solvent. The SEM images show that the nickel selenide synthesized by methanol had rougher surface, which implies that it would have a larger surface area. As the carbon chain length of solvent increases, the surface of nickel selenide becomes smoother. A well-dispersed nickel selenide solution was used along with drop-coating method to prepare the counter electrode. Finally, the performance of DSSCs with nickel selenide CE synthesized in methanol reached an cell efficiency of 8.48%, which is higher than that of the DSSC using platinum CE (8.05%). The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and incident photon–to–current conversion efficiency (IPCE) are also employed in this study to verify the results. Key reference: [1] F. Gong , H. Wang, X. Xu, G. Zhou, and Z. S. Wang. J. Am. Chem. Soc. 2012, 134, 10953–10958.

Authors : Olfat Hamdan, Ali Hajjiah, Moustafa Y. Ghannam
Affiliations : Electrical Engineering Dept., College of Engineering and Petroleum, Kuwait University, P.O. Box 5969, 13060 Safat, Kuwait

Resume : Saw damage removal (SRD) in 30% KOH alkaline solution is an early step carried out in silicon processing and in solar cell manufacturing with the purpose of removing a few microns from the damaged (100) oriented silicon surface. Such a step increases the roughness of the polished surface which should have an impact on subsequent surface texturing carried out in an alkaline solution with a low KOH concentration. Therefore, in the present work, the surface topography of SDR-treated samples is compared to that of non-treated samples which received only RCA cleaning. The investigations are carried out by surface examination using Atomic Force Microscopy (AFM) and Scanning electron Microscopy (SEM), and by spectral reflectance measurements. The texturing is carried out in a 1-1.5%, 5% and 10% KOH solution with 6% IPA in the temperature range 70-85oC for variable times. It is found that texturing in the 1% KOH solution at 70oC to 80oC is incomplete and results in non-acceptable reflectance for SDR-treated as well as non-treated samples. On the other hand, SDR-treated samples systematically exhibit a smaller reflectance than non-treated samples when textured in 5% KOH solution. A minimum reflectance of 7.8% with no ARC and 0.8% with 115nm thick SiO2 could be obtained for SDR-treated samples after 40 minutes texturing at 70oC and 80oC, and after only 10 minutes texturing at 85oC. For SDR-treated samples the pyramids are smaller and denser than for non-SDR treated samples. Longer etching time results in a larger reflectance due to coalescence of neighboring small pyramids to form larger structures separated by voids. Finally, texturing in a 10% KOH solution results in large sparsely distributed pyramids leading to a surface reflectance of at least 20% even after 40 min etching.

Authors : A. Morales-Vilches*, C. Voz, M. Colina, I. Martín, P. Ortega, G. López, R. Alcubilla
Affiliations : Polytechnic University of Catalonia (UPC), Barcelona, Spain

Resume : In crystalline silicon (c-Si) solar cell technology, low temperature processes are a matter of necessity in order to reduce manufacturing costs. Concerning the back contact for p-type c-Si substrates, different alternatives to the traditional aluminum back-surface-field contact are under investigation. One of the most promising is the laser-firing technique that can be used to diffuse aluminum point contacts through different passivating layers. This method has already allowed high efficiency devices, but it necessarily causes some degradation of the back surface passivation quality along with the complexity of using pulsed lasers and beam positioning systems. In an attempt to solve this problem, we investigate an alternative based on hole tunneling through an stack of an ultrathin (2-3 nm) alumina (Al2O3) layer and a 40-80 nm thick ZnO:Al layer. In this structure the Al2O3 layer is a very efficient passivating buffer while the ZnO:Al layer acts as an electron donor. Thus, holes at the Si surface could be collected by recombining with electrons tunneling through the alumina layer. In this work, we study the optimization of the Al2O3/ZnO:Al stack to combine a good surface passivation (Seff < 25 cm/s) with a low enough full-area finished device resistance (< 5 ohm·cm2). Finally, these structures are incorporated as the back contact of silicon heterojunction solar cells to analyze their potential in finished devices.

Authors : S.Tata1, N. Khelifati 2, A.fedala 1 and A.Rahal1
Affiliations : 1Laboratoire de Physique des Matériaux: Couches Minces et Semi-conducteurs, Faculté de Physique, USTHB, BP 32 El Alia, 16111 Bab-Ezzouar, Alger, Algerie 2L'Unité de Développement de la Technologie du Silicium (UDTS), UDTS - 2, bd. dr. Frantz FANON - B.P.140 Alger Sept Merveilles, 16027 Alger – ALGÉRIE

Resume : The measurements of SPV (surface photovoltage) present a very interesting way to characterize the effect of light on semiconductors. We can extract important parameters from this technique such as diffusion length. The principle of SPV requires a Schottky junction device (Metal / SC). It also requires a photoconductive material. At first we must take the spectral response of the photo-device by measuring the open circuit voltage as a function of the wavelength (Voc(λ)) when the junction is illuminated with a monochromatic light. In the second time we adjust the luminous flow to maintain constant the open circuit voltage (Voc) according to the wavelength. This technique requires an analysis of theoretical development and its assumptions. We must then achieve these conditions for an experimental point of view. In this work we estimated the diffusion length of our material (hydrogenated amorphous silicon a-Si: H) by adapting the technique SPV to our frame work. Samples are prepared in our laboratory by DC magnetron sputtering technique. The results obtained show a measured diffusion length around 0.3 µm in the same order as those found in the literature [1]. [1] L. Kronik, Y. Shapira / Surface photovoltage phenomena: theory, experiment, and applications Surface Science Reports 37 (1999) 1-206

Authors : Gurpreet Singh Selopal1 2, Nafiseh Memarian 3, Riccardo Milan 1 2, Isabella Concina 1 2, Giorgio Sberveglieri 1 2, Alberto Vomiero 1 2
Affiliations : 1 SENSOR Lab, Department of Information Engineering, University of Brescia, Via Valotti 9, 25133 Brescia, Italy; 2 CNR-INO SENSOR Lab, Via Branze 45, 25123 Brescia, Italy; 3 Faculty of Physics, Semnan University, 35195-363, Semnan, Iran;

Resume : Dye sensitized solar cells (DSSCs) are one of the most promising architectures in the panorama of third generation solar cells. Application of a compact blocking layer (BL) between the conductive glass and the photoanode[1,2] is an effective strategy to reduce electron recombination, which is among the most detrimental processes in these third generation photovoltaic devices. However, in DSSCs based on TiO2 such strategy has limited impact due to high series resistance and consequent fill factor reduction. Herein, we present a systematic investigation of the effect of a ZnO compact BL in DSSCs based on ZnO photoanodes. BLs of different thicknesses are generated through spray deposition onto conducting glass before the deposition of a ZnO active layer. Functional properties of DSSCs are investigated as a function of the thickness of the BL for two different kinds of ZnO active layer (hierarchically self-assembled nanoparticles [3] and micro-cubes composed of closely packed ZnO nano-sheets). Presence of BL leads to dramatic improvement of photoconversion efficiency (PCE=7.5%), by physically insulating the electrolyte and the glass, resulting in noteworthy increase in photocurrent (higher than 15 mA cm-2) and unprecedented photoconversion efficiency (7.5%) for a ZnO-DSSC. Mechanisms boosting the functional features of the devices are discussed in detail. (1) O’Regan, B. et al. Nature 1991, 353, 737. (2) Cameron, P. et al. J. Phys. Chem. B 2003, 107, 14394 (3) Memarian, N. et al. Angew. Chem. 2011, 50, 12321.

Authors : B. Grew, J. W. Bowers, F. Lisco, N. Arnou, J. M. Walls, H. M. Upadhyaya
Affiliations : Heriot-Watt University; Loughborough University

Resume : Titanium-doped indium oxide (ITiO) is a high mobility TCO that has lower carrier absorption losses in the near IR due to its high mobility and may be used in tandem solar cells. Research has mostly been concerned with RF sputtering of ITiO. Pulsed DC magnetron sputtering (PDMS) is a process that has industrial advantages over RF sputtering. It removes the need for costly impedance matching networking equipment and has the advantage of a higher rate of deposition. This leads to higher throughput and reduces production costs when scaled up. Despite these advantages, the use of a PDMS power supply may damage the film, leading to a reduced mobility, potentially leading to electrical performance losses when used in a tandem solar cell. We compare ITiO films deposited by both RF and PDMS onto soda-lime glass. Each film was deposited to a 200 nm thickness and exhibited excellent transmission properties of 75-90% between 400-800nm and >80% to 1800nm. XRD data of films deposited by RF reveal a single intense peak at 2θ=35°, matched to the (400) peak of In2O3, whilst PDMS films gave several diffraction peaks that were indexed to pure In2O3. This implies a more textured film is produced from the PDMS supply. Hall measurements confirm a reduced mobility in the PDMS film. Despite a reduced mobility, the PDMS films have desirable properties for use in tandem solar cells. Studies are underway to understand the role of process parameters to deliver further improvements in film quality.

Authors : P J M Isherwood, J W Bowers, J M Walls
Affiliations : CREST, Holywell Park, School of Electrical, Electronic and Systems Engineering, Loughborough University, LE11 3TU, UK

Resume : Metal oxide semiconductors are widely used as transparent contact materials for solar cells. However the vast majority are n-type. This is because of the typical band structure of metal oxides and the high electronegativity of oxygen. The development of transparent conducting p-type oxides could be of use in a range of PV applications such as bi-facial and multijunction cells, as well as possible high work-function back contact materials for organic or CdTe cells. Cupric oxide (CuO) is a p-type material with an indirect band gap of 1.2 eV. This study examines the effects of doping on the resistivity of sputtered CuO, and aims to increase the material band gap by co-sputtering with SnO2. It was found that a target doped with 2% Na produced films with resistivities of four orders of magnitude lower than equivalent undoped films. Addition of oxygen to the films was found to reduce the resistivity further. The best films were found to have resistivities of 4.3x10-2Ω.cm, which is an order of magnitude better than has been reported for other p-type oxides. Addition of SnO2 was found to increase the band gap significantly, although it also caused an increase in the resistivity. Despite not being as conductive as n-type oxides such as ITO or AZO, this study demonstrates that cupric oxide-based materials have potential as effective p-type transparent conductors.

Authors : 1-Arezoo Hosseini(1-4) 2-kerem çağatay içli (2-3-4) 3- Macit Özenbaş(3-4) 4-Ayşe Çiğdem Erçelebi) (1-4)
Affiliations : 1Middle East Technical University, Department of physics, Ankara, Turkey 2 Middle East Technical University, Micro and Nanotechnology Graduate Program, Ankara, Turkey 3Middle East Technical University, Department of Metallurgical and Materials Engineering, Ankara, Turkey 4 The Center for Solar Energy Research and Applications (GÜNAM), METU, Turkey

Resume : Titanium dioxide is a high band gap semiconductor, which, due to its photo-conversion properties in the UV spectrum range, shows various useful applications. Different deposition techniques can be used for TiO2 thin films production like e-beam PVD, spray pyrolysis, electrophoretic deposition, Successive Ionic Layer, sputtering and spin-coating methods. In this study, highly porous TiO2 films were deposited on glass and ITO coated glass substrates using spin coating method for 2000 rpm, 5000 rpm, 2000 rpm followed by successive coating run of 5000 rpm (2000-5000) spinning rates. XRD analysis of TiO2 films coated on glass substrates for 2000 and 5000 rpm spinning rates showed that these films were amorphous which was reported elsewhere. Whereas, 2000-8000 rpm spin-coated films showed crystalline phases with preferred orientation along (1 1 1) plane and diffraction angle 2ϴ~26° with crystallite size of 6.5 nm calculated using Scherrer’s formula. The XRD patterns of TiO2 films deposited on ITO coated glass substrates all show crystalline structure. It is observed that both substrate nature and thickness are important factors to determine the crystalline or amorphous structure of the films. Using high-resolution SEM images of spin-coated thin films, the thickness and particle size of films were determined which are consistent with the results obtained from XRD and BET measurements. The roughness values of the coated films measured from the high-resolution AFM images showed that these values are independent of substrate nature. Using the room temperature transmission measurements, the characterization of the films deposited on both substrates at different spin rates indicated direct band-gaps with values of Eg=3.50- 3.6 eV for spin coated films at 2000 and 5000 rpm respectively. The band gap value for the film coated with two successive spin rates with 2000-5000 rpm, was obtained as 3.4 eV. The absorption coefficient was found to be of 105 for 2000 and 5000 rpm spin coated films which was decreased to 104 for 2000-5000 rpm spin coated films.

Authors : Daniel Bryant1, Peter Greenwood1, Joel Troughton1, Matthew Carnie1, Matthew Davies1,Trystan Watson1, Maarten Wijdekop2, Dave Worsley1
Affiliations : 1 SPECIFIC, Baglan Bay Innovation Centre, Baglan Energy Park, Baglan, Port Talbot, SA12 7AX, United Kingdom ; 2 TATA Steel Europe, PV Accelerator, Shotton Works, Deeside, Flintshire, CH5 2NH, United Kingdom

Resume : Here we show for the first time the use of a novel combination of a lamination of a conductive grid structure using a newly developed transparent conductive adhesive (TCA) formulation to make both mechanical and electrical contact and applicable to a wide range of photovoltaic and other optoelectronic devices. The method uses lamination under ambient conditions so as not to damage the device architecture and has enabled devices to be manufactured using the TCA-laminate electrode with only a 0.5% drop in efficiency compared to ones with traditional evaporated metallic contacts. This has for the first time allowed ITO/FTO free, atmospherically produced, fully printed, roll-to-roll compatible Solid-State Dye sensitised solar cells (SDSC) and Organolead Halid Perovskite to be fabricated on cheap metallic substrates. One of the major steps in being able to manufacture hybrid organic photovoltaics (such as SDSC, Organolead Halid Perovskite and Organic Photovoltaics (OPV)) is the production of a flexible top electrode that can easily be applied in a continuous manufacturing process. Here we present a simple solution to the fabrication of a flexible laminate with a conducting adhesive and we demonstrate its use in a solid state dye sensitised solar cell at large area. This is really significant since the recent publication of new HOPV device efficiencies of over 15% for the Organolead Halid Perovskite technology and with established technologies for DSC and OPV giving 12.3% and 12% respectively. At the laboratory research stage of these third generation PV technologies, the counter electrode is typically applied by sputtering or evaporating of an opaque metallic contact onto the cell. This limits their potential for scale up as the metals used are inherently expensive Gold or Silver, and vacuum processing makes full R2R production rather expensive. Furthermore this standard architecture dictates that the working electrode of the cell must be transparent. Both FTO and ITO glass working electrodes are used with the transparent conducting oxide the basis for attaching the active layer (with our without an additional blocking layer). Whilst these are dimensionally stable and can withstand sintering they have certain disadvantages particularly for flexible metal construction products. For metal building products (both steel and aluminium) the PV applied ideally needs to be flexible. For this reason, a cost-effective metal foil such as carbon steel is ideal as a fully flexible working electrode. For such fully flexible PV cells to be produced using a roll-to-roll process on a metal substrate a new approach is required for the counter electrode. Whilst ITO coated transparent polymers such as PET can be used they are both extremely expensive and somewhat easily damaged. The TCA-Laminate is formulated using a transparent adhesive and the transparent conductor PEDOT:PSS. The various properties of the TCA such as conductivity, tack and transparency have been found to vary with not only the concentration of PEDOT:PSS but also the coating’s thickness. Experimentally relationships have been found between the properties of the TCA and more importantly how these affect photovoltaic efficiencies when used in devices. The critical advantage of the new room temperature lamination method shown in this work is that it is well suited to scale up and mass production and means that metal substrates can be successfully employed. Further work is in progress to drive up the conductivity of the adhesive laminate and also critically to understand its stability in longer term weathering exposure however this represents a major breakthrough in this technology and the route to scale up of this technology can now be clearly defined.

Authors : L. Bergerot, C. Jimenez, J.L. Deschanvres, O. Chaix
Affiliations : Laboratoire des Matériaux et du Génie Physique CNRS - Grenoble INP, 3 parvis Louis Néel CS 50257, 38016 Grenoble, France.

Resume : Transparent conducting oxides are used in a wide variety of applications. Currently, those with the best characteristics are all n-type, when those that are p-type have conductivities that are one order of magnitude inferior. Cuprous oxide Cu2O appears as a promising material for use as a p-type transparent conducting oxide. According to theoretical simulation by M. Nolan et al, it is possible to tune its properties by doping it with strontium. This work aims at growing and characterizing thin film of cuprous oxide doped with strontium by metal-organic chemical vapour deposition method. The influence on optical and electronic properties of deposition parameters such as substrate temperature and oxygen partial pressure in the reactor, as well as the influence of dopant concentration, are explored. In addition, the effects of thermal annealing on deposited thin films are studied. The combination of strontium doping and post deposition annealing have led to the best film properties. Such films show promising results, with transparencies of approximately 70% and sheet resistance as low as 10^4 Ω/□. Effects of deposition parameters, doping with strontium and annealing on the structural, optical and electronic properties of the thin films, as well as optimization of said parameters, will be presented. Reference: Michael Nolan, Simon D. Elliott. Thin Solid Films 516 (2008) 1468–1472

Authors : Giovanni Carapezza, Giuseppe Fisicaro, Cosimo Gerardi, Giuseppe Nicosia, Antonino La Magna, Salvatore Lombardo, Giovanni Mannino, Andrea Patanè, Vittorio Romano, Andrea Santoro
Affiliations : CNR-IMM VIII Strada 5, Catania (Italy); Dipartimento di Matematica Università di Catania, Via Santa Sofia 64, Catania (Italy)

Resume : We present a composite design methodology for the simulation and optimization of the solar cell performances. Our method is based on the synergic use of different simulation techniques and it is especially designed for the thin-film cell technology. In particular, we aim to efficiently simulate light trapping and plasmonic effects to enhance the light harvesting of the cell. Our method is based on the sequential application of a hierarchy of approaches: a) full Maxwell simulations are applied to study of the light scattering in systems presenting textured interfaces; b) calibrated Photonic Monte Carlo is used in conjunction with the scattering matrixes method to evaluate coherent and scattered phonon absorption in the full cell architectures; c) the results of these advanced optical simulations are used as the pair generation terms in model implemented in a conventional TCAD tool for the derivation of the cell performances; d) genetic-type algorithms are applied to the automatic optimization procedure and the cell design. The material calibration includes optical and electrical properties of common substances used in thin film solar cells, while both full Maxwell simulations and experimental measurements are used to calibrate the angular scattering probability used in the Photonic Monte Carlo method. We discuss the application of our methodology to the study and the optimization of different typologies of solar cell.

Authors : Hariharsudan Sivaramakrishnan Radhakrishnan, Ferenc Korsos, Nick Cowern, Maarten Debucquoy, Ivan Gordon, Robert Mertens, Jef Poortmans
Affiliations : KU Leuven, imec; Semilab; Newcastle University; imec; imec; KU Leuven, imec; KU Leuven, Universiteit Hasselt, imec

Resume : In wafer-equivalent epitaxial silicon solar cells, a 20-30 µm thick epitaxial layer (epilayer) is grown on low-cost and often low-purity substrates, with the perceived advantage of making the cell concept cheaper by using less high quality silicon per m2. A porous silicon (PS) layer embedded at the interface between the epilayer and the low-purity substrate is crucial since it getters metal impurities and “cleans” up the epilayer in its proximity. Intentional metal contamination and gettering experiments were performed on epitaxial p/p+ structures. For Fe-contaminated samples, Fe-B pair dissociation combined with lifetime measurements were used to calculate the [Fe] in both gettered and non-gettered epilayers, which showed significantly reduced [Fe] in PS-gettered epilayers and a large gettering ratio of >100. In addition, PS void size reduction was used to enhance the gettering efficiency for Fe by 3 times. Similar experiments on Ni and Cu also prove the efficacy of proximal PS gettering. Based on this, a methodology is presented to estimate the specifications for the maximum allowable metal concentration in the substrate and the Si feedstock used to crystallise the substrate, which accounts for impurity segregation in the melt (via Scheil equation), out-diffusion from substrate to epilayer as well as PS gettering. It is shown that with PS gettering, an epilayer diffusion length of 250 µm (~10 times the epilayer thickness) can be attained with UMG substrates.

Authors : 1 Arezoo hosseini (1-3) 2 Hasan Hüseyin Güllü (1-3) 3 emre coskun (1-2) 4 Raşit Turan (1-3) 5 Ayşe Çiğdem Erçelebi (1-3)
Affiliations : 1 Department of Physics, Middle East Technical University (METU), Ankara 06800, Turkey 2 Department of Physics, Çanakkale Onsekiz Mart University, Çanakkale 17100 Turkey 3 Center for Solar Energy Research and Applications (GÜNAM), METU, Ankara 06800, Turkey

Resume : Titanium dioxide is a high band gap semiconductor that shows various useful applications due to its photo-conversion properties in the UV spectrum range. It has three crystalline phases: anatase (tetragonal), rutile (tetragonal), and brookite. Both anatase and rutile are more common phases than the brookite and have wide commercial applications. In this study, n-type TiO2 thin films were deposited on to p-type Si (111) wafers by sputtering of single TiO2 target. During the deposition process, the substrate temperature was kept at about 200°C. The room temperature transmission measurements were performed for the optical characterization of the films. The film’s maximum transmittance was about 90% at the wavelength interval of around 320-1100 nm. The optical measurements of the films exhibited direct band-gap energy of around 3.6 eV. A detailed device characterization of the Al/p-Si/n-TiO2/Al sandwich structure was performed at room temperature. The current-voltage (I-V) and frequency dependent capacitance-voltage (C-V) characteristics of the heterojunction were studied to determine the possible conduction mechanisms and material/device parameters respectively. The fabricated p-n hetero-junctions showed diode behaviors with rectification ratıo of about one order of magnitude. The diode series and shunt resistance values were obtained as 3.5 ×104 Ω and 3.02 ×105 Ω respectively. . The ideality factor n and the barrier height values of the diode were determined by performing different I-V plots.

Authors : A. Sytchkova*, A. Ulyashin**
Affiliations : *ENEA, Rome, Italy **SINTEF Materials and Chemistry, Oslo, Norway

Resume : Si based heterojunction solar cell structures with ITO and a-Si:H layers have been analysed by spectroscopic variable angle ellipsometry in order to trace dependence of free carriers’ distribution along the film depth as a function of film thickness as well as its change upon annealing. ITO layers were deposited at RT or 200°C preheated silicon substrates. Properties of HJ based solar cell structures have been compared with the ITO film optical constants’ inhomogeneity, i.e. material properties along the film thickness. The obtained results show that optical as well as electrical properties of thin ITO films prepared by pulsed DC sputtering are depth dependent. In particular, the deposition conditions used ensure a well-determined reproducible non-uniform distribution of free carriers within the film thickness. It has been found that distribution of free carriers in annealed around 300 °C samples is qualitatively different from that of as-deposited layers. It is shown that such annealing leads to an improvement of the HJ solar cell efficiencies. It is concluded that in-depth variations of the optical function of ITO layers responsible for the improvement of Si-based heterojunction ITO/a-Si:H/Si structures upon anneals.

Authors : Hanane Lachachi, Abdellatif Zerga
Affiliations : Materials and Renewable Energies Research Unit (URMER), Faculty of Sciences, University of Tlemcen, PO Box 119, 13000 Tlemcen, Algeria

Resume : The formation of the emitter is considered as a crucial step during the manufacture of solar cells. Several techniques are used in the photovoltaic industry for the formation of the emitter and the most widespread one is based on the BCl3 diffusion in cylindrical quartz tube. This distribution is often difficult to control and requires an adequate understanding of physical phenomena in order to optimize the best profile of dopant diffusion. However, this work is focused on the optimization of boron diffusion profile by investigating the effects of temperature, the diffusion time and the surface concentration on the performance of solar cells based on n-type silicon. A comparison with the experimental profile measured by SIMS (Spectroscopy Secondary Ion Mass) was performed. The simulation code was developed via Athena and Atlas modulus of SILVACO software package. The design and technological parameters as doping profile, junction depth and surface recombination velocity are investigated our obtained results on N-type silicon solar cells showed a maximum efficiency improvement of about 1.20%. These results are confirmed also by calculating the internal quantum efficiency.

Authors : B.Benabadji, A.Zerga, Lachachi
Affiliations : Materials and Renewable Energies Research Unit (URMER), Faculty of Sciences, University of Tlemcen, PO Box 119, 13000 Tlemcen, Algeria

Resume : Actually, more than 80% of the industrial solar cells are fabricated based on p-type silicon material. The n-type solar cell is also interesting because it has a lower recombination velocity and a higher carrier lifetime. Comparative simulation of EWT (Emitter Wrap Through) silicon solar cell structure using Silvaco software package has been carried out. EWT silicon solar cells are reported by researchers to have high efficiencies (21.4% by Fraunhofer ISE) because there is no shadow rate and the p-n junction is increased. This structure has an emitter on the front side of the cell, but all the contacts are on the rear side. Holes, usually buried with a laser, act as interconnection between the front and the rear side of the cell. The study is based on device (n+np+ and p+pn+) simulation and some physical and geometrical parameters of base and emitter are set for best performance solar cells. Conversion efficiency, the internal and external quantum efficiencies are computed. A solar cell of efficiency close to 20% is demonstrated by simulation.

Authors : P. Prepelita, V. Craciun
Affiliations : National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor Street, PO Box MG-36, Magurele 077125, Ilfov, Romania

Resume : Transparent contact electrodes are fascinating oxides with interesting physical properties. AZO, ITO and SnO2 were tested for the development of photovoltaic devices, such as solar cells. They have good electrical properties and are usually used as transparent contact electrodes in such devices. In the present paper we discuss the application of RF magnetron deposited AZO, ITO and SnO2 thin films as transparent contact electrodes in high quality chalchogenide solar cells. Therefore, a reliable assessment of their crystallographic and optical properties is essential. The influence of growth parameters (i.e. oxygen pressure, flow rates, substrate type) on the morpho-structural characteristics of the thin films were also investigated. AZO, ITO and SnO2 films with a thickness of ~ 500nm were deposited by RF magnetron sputtering onto glass substrate, using a constant deposition rate of 6 Å/s. Morphological features were investigated by Atomic Force Microscopy and Scanning Electron Microscopy, which showed profiles that are densely packed and well adherent to the surface of the substrate. The X-ray diffraction analyzes revealed that all films are polycrystalline and show a strong orientation after the main planes perpendicular to the substrate. It was highlighted that ITO and SnO2 have good conductivity and are transparent (90-94%) materials, whereas AZO depends on the Al (3%) doping to exhibit improved properties.

Authors : P. Prathap*, R. Nautiyal, M. Srivastava, Vandana, S.K. Srivastava, R.K. Singh, C.M.S Rauthan, P.K. Singh
Affiliations : Silicon Solar Cell Group, CSIR-Network of Institutes for Solar Energy, CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110012, India

Resume : Nitric acid oxidation of silicon (NAOSi) is an attractive method to oxidize silicon to form ultra-thin SiO2 layers with good interface quality at lower temperatures. In the present investigation, (100) n-type Si wafers were subjected to two-step nitric acid oxidation, involving a treatment in 40 % HNO3 for 10 min followed by 68 % HNO3 treatment at 121 oC for several time periods. The grown samples were annealed in different ambients. SiO2 thickness practically stopped after attaining a critical thickness. Quasi-steady state photoconductivity measurements on the oxidized samples indicated improvement in minority carrier lifetime after annealing at 400 oC for 30 min in forming gas. The passivation observed in n-type silicon surface was attributed to chemical passivation. Capacitance – Voltage characteristics revealed that SiO2 layers formed using NAOSi route has positive fixed charges. A short high-temperature annealing affected the local bonding configuration and enhanced the positive charge density of SiO2 layers after hydrogen annealing. The results indicate that NAOSi grown oxide layers may find application in silicon solar cells as passivating layers alone or as buffer layer in conjunction with other surface passivation layers such as Al2O3 or Si3N4.

Authors : P Prathap1, D Praveen Kumar2, G. Sumana1, M V Shankar2
Affiliations : 1CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110012, India 2Nanocatalysis Research Lab, Department of Materials Science & Nanotechnology, Yogi Vemana University, Kadapa ? 516 003, India

Resume : Nitric acid oxidation of silicon (NAOSi) is an attractive method to oxidize silicon to form ultra-thin SiO2 layers with good interface quality at lower temperatures. In the present investigation, (100) n-type Si wafers were subjected to two-step nitric acid oxidation, involving a treatment in 40 % HNO3 for 10 min followed by 68 % HNO3 treatment at 121 oC for several time periods. The grown samples were annealed in different ambients. SiO2 thickness practically stopped after attaining a critical thickness. Quasi-steady state photoconductivity measurements on the oxidized samples indicated improvement in minority carrier lifetime after annealing at 400 oC for 30 min in forming gas. The passivation observed in n-type silicon surface was attributed to chemical passivation. Capacitance ? Voltage characteristics revealed that SiO2 layers formed using NAOSi route has positive fixed charges. A short high-temperature annealing affected the local bonding configuration and enhanced the positive charge density of SiO2 layers after hydrogen annealing. The results indicate that NAOSi grown oxide layers may find application in silicon solar cells as passivating layers alone or as buffer layer in conjunction with other surface passivation layers such as Al2O3 or Si3N4.

Authors : Kwang-Won Park+, Sungwoo Ahn+, Jiyoon Kim, Kang Min Ok, Kyungwon Kwak, Jongin Hong* + Equal contribution to this presentation * Corresponding author
Affiliations : Department of Chemistry, Chung-Ang University, Seoul 156-756, South Korea

Resume : We present the facile synthesis of metal free dyes SC-01 and SC-02, their solution-based optical and redox properties and their use as sensitizers in dye-sensitized solar cells (DSSCs). Our studies indicate that the addition of the second thiophene unit in SC-02 decreases the oxidation and reduction potential and consequently the bandgap of the molecule compared to SC-01. Furthermore, increasing the length of the conjugated spacer also affects the properties of the DSSCs, with SC-02 providing a higer power conversion efficiency compared SC-01 (PCE = 4.49 % versus 3.23%).

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Thin film solar cells III : C. Becker
Authors : Wade BRAUNECKER
Affiliations : National Renewable Energy Lab., USA

Resume : tba

Authors : Andreas Gerl*, Moses Richter**, Gebhard Matt**, Heiko B. Weber* and Christoph J. Brabec**
Affiliations : ** Department of Materials for Electronics and Energy Technology, Friedrich-Alexander University Erlangen-Nürnberg, Martensstraße 7, 91058 Erlangen; * Department for Applied Physics, Friedrich-Alexander University Erlangen-Nürnberg, Staudtstr. 7, 91058 Erlangen;

Resume : In this work we present a model which describes the current-voltage characteristics of an organic solar cell under illumination with 0.25 – 2 sun equivalents. The IV characteristics of an ideal solar cell, under illumination, are described by the sum of the dark current (J_dark) and photo-generated current (J_photo). In a practical solar cell it is also necessary to consider the recombination rate (R) of the photo-generated charge carriers, which in fact reduces the power output of the solar cell (i.e. decreased fill factor). Practically it is a challenging task to determine the recombination rate. Here we present a novel approach based on charge extraction (CE) and transient photo-voltage measurements (TPV) to calculate R. For the calculation of R, the voltage dependent carrier density n(V) must be known as well as the recombination order, often described by the recombination coefficient α. Latter is derived from the charge carrier lifetime dependence on the charge density. Both parameters are accessed by TPV and CE measurements. Based on this, the reconstructed IV characteristic is in excellent agreement with the measured data. It is noted, that the herein introduced model does not need any free fit parameters and allows a consistent recreation of the IV characteristics of an organic solar cell.

Authors : T. Sameshima, S. Yoshidomi, and M. Hasumi
Affiliations : Tokyo University of Agriculture and Technology

Resume : We report the mechanical stacking technology used for fabricating large size multi-junction solar cells. 1) We developed transparent conductive adhesive and stacking technology. 20 micrometer diameter ITO conductive particles were dispersed in epoxy-type transparent adhesive jell named Cemedine. We also developed a gas press method for stacking semiconductor substrates up to 8 inch size. Two low-resistivity silicon substrates were pasted together by the Cemedine adhesive including 6 wt% ITO particles as an intermediate layer. Then, the sample was placed in a pressure proof chamber. Air gas was introduced in the camber at 0.7 MPa for 2 h to hardening the intermediate Cemedine adhesive at room temperature. Current voltage measurement of the sample with the structure Si/adhesive/Si revealed a good ohmic characteristic with a connecting resistivity of about 1 ohm cm2, which was enough low for fabricating multi-junction solar cells. 2) We experimentally demonstrate multi-junction solar cells using our mechanical stacking technique as described above. Commercial GaInP/GaAs/Ge three junction solar cells with a size of 8x4 cm2 were prepared. The initial solar cell characteristics were first measured under AM1.5 light at 100 mW/cm2. The open circuit voltage (Voc), fill factor (FF), and efficiency were 2.51V, 0.85, 31.6%, respectively. Then, the rear metal electrodes were removed and Ge rear surfaces were pasted to a low resistivity silicon substrates using the Cemedine adhesive including 6 wt% ITO particles. Measurement of solar cell characteristics under AM1.5 light at 100 mW/cm2 resulted in Voc, FF, and efficiency as 2.53V, 0.84, 31.4%. The solar cell performances were almost the same as the initial ones. Those results show that our transparent conductive adhesive has a capability of fabrication of large size multi-junction solar cells. In the conference, we will also report solar cells fabricated by stacking GaInP/GaAs epitaxial thin solar cells on Ge cells and stacking amorphous silicon thin film solar cells on crystalline silicon solar cells using our transparent conductive adhesive.

Authors : Ivan G. Samatov*, Sanjay K. Ram*, Sabrina R. Johannsen*, Pekka T. Neuvonen**, John Lundsgaard Hansen *, Jacques Chevallier*, A. Nylandsted Larsen*
Affiliations : * Department of Physics and Astronomy - iNANO, Aarhus University, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark; **University of Oslo, Norway, 0316 Oslo

Resume : High mobility transparent conducting oxides (TCO) have emerged as a potential alternative to the dominant conventional TCOs (SnO2:F, In2O3:Sn and ZnO:Al) due to their better optical response in the NIR region. In this respect, Mo and W-doped indium oxides (IMO and IWO) are promising materials as there is a valence difference of 3 between the dopant and substituted ion, which gives a great advantage to obtain a TCO film having both high conductivity and high transparency. Since these materials are in their infancy, therefore, great emphasis is on to obtain device quality IMO and IWO film without looking much into the aspect of substrate temperature. Nevertheless, success of TCO material lies in their applicability within flexible devices. Therefore, in this work, we studied the opto-electronic properties of ~70 nm IMO and IWO films deposited by RF magnetron sputtering at room temperature in the framework of varying power and pressure. XRD study of the IMO films confirms that the films, sputtered at low pressure are crystalline (less transparent, 50% and resistive, 300 S/cm) while those grown at higher pressure are mostly amorphous (highly transparent, 77% and conducting, 1000 S/cm). High optical quality of these films was confirmed by low values of absorptance (A=100%-R-T), which doesn’t exceed 3% in the spectral range of Si solar cell’s performance. The minimum bulk resistivity of 0.0006 Ohm*cm was achieved in the IWO samples, which makes it comparable to the ITO films.

Authors : D. P. Langley1,2, M. Lagrange1, D. Muñoz-Rojas1, C. Jiménez1, Y. Pellegrin3, N. D. Nguyen4, Y. Bréchet5, D. Bellet1
Affiliations : 1 Laboratoire des Matériaux et du Génie Physique CNRS - Grenoble INP, 3 parvis Louis Néel CS 50257, 38016 Grenoble, France. 2 Laboratoire de Physique des Solides, Interfaces et Nanostructures Département de Physique, Université de Liège Allée du 6 Août 17, B-4000 Liège, Belgique. 3 Laboratoire Chimie Interdisciplinarité, synthèse, analyse, modélisation (CEISAM) 2, rue de la Houssinière, Université de Nantes, BP 92208 ; 44 322 Nantes cedex 03, France. 4 Laboratoire de Science et Ingénierie des Matériaux et des Procédés CNRS - Grenoble INP, 1130 rue de la piscine 38042 Saint-Martin d’Hères, France.

Resume : Research is increasingly being dedicated towards replacing rare earth elements in Transparent Conductive Materials (TCMs). In this work we present the investigation of silver nanowire (AgNW) networks as transparent electrodes for solar cell applications. Metallic nanowire networks can be deposited via low cost deposition techniques and exhibit very interesting electrical, optical and mechanical properties. Experimental and simulation approaches aim at improving their physical properties. Indeed, we show that a thermal annealing can drastically improve transport properties of the nearly transparent networks. We also explore the optimization of the network density. These percolating networks exhibit excellent properties (i.e. sheet resistances (Rs) of about 10 Ω/ and optical transparency of approximately 90%) compatible with solar applications requirements. This makes them very appropriate for future uses in low cost, large area and flexible solar and display technologies. A comprehensive understanding of the main physical properties of this promising nanostructured network and its integration with solar cells will then be presented. References: D.P. Langley, G. Giusti, M. Lagrange, R. Collins, C. Jiménez, Y. Bréchet, D. Bellet Solar Energy Materials and Solar Cells (2013) D.P. Langley, G. Giusti, C. Mayousse, C. Celle, D. Bellet, J.P. Simonato. Nanotechnology 24 (2013) 452001.

10:00 Best Poster Award (for poster session III)    
10:10 Break    
Dye-sensitized solar cell : W. Braunecker
Authors : G.G. Selopal (1,2), R. Milan (1,2), K.T. Dembele (3), L. Ortolani (4), R. Rizzoli (4), V. Morandi (4), I. Concina (1,2), G. Sberveglieri (1,2), F. Rosei (3,5), A. Vomiero (1,2)
Affiliations : 1. Department of Information Engineering, University of Brescia, Brescia, Italy; 2. CNR-INO SENSOR Lab Brescia, Italy; 3. Institut National de la Recherche Scientifique,Énergie, Matériaux et Télécommunications, Varennes, QC, Canada; 4. CNR-IMM Section of Bologna, Bologna, Italy; 5. Center for Self-Assembled Chemical Structures, McGill University, Montréal, QC, Canada.

Resume : The outstanding properties of graphene (especially its high electrical conductivity and high transparency) allow its exploitation in two critical parts of dye-sensitized solar cells: (i) graphene addition to TiO2 mesoporous film creates a preferential path for transport and collection of photogenerated charges [1,2]; (ii) a homogeneous single layer of graphene can be applied as front contact, as viable alternative to the usual transparent conducting oxide. Herein we present a combined investigation on the effect of graphene sheets into TiO2 photoanodes and the application of graphene thin film as front contact in DSSCs. Graphene addition in TiO2 can boost photoconversion efficiency up to 8.8 % (compared to 6.3% of pure TiO2), mainly increasing the photogenerated current and the collected charges, due to fast electron transport and reduced charge recombination. We also demonstrated the possibility of replacing indium tin oxide (ITO) and fluorine tin oxide (FTO) by the graphene thin film front contact of DSSCs obtaining very promising photoconversion efficiency (above 2%, which is the highest recorded for a graphene front contact in DSSCs). These results indicate the potential benefits offered by graphene towards one of the most promising technologies and architectures of third generation photovoltaics, with the possibility of a significant advance in the field. [1] K.T. Dembele et al. J. Phys. Chem. C 117 (2013) 14510. [2] K.T. Dembele et al. J. Power Sources 233 (2013) 93.

Authors : Giovanna Pellegrino1, Guglielmo Guido Condorelli2, Francesca De Rossi3, Thomas M. Brown3, Francesco Giovenale4 , Corrado Bongiorno1 and Alessandra Alberti1*.
Affiliations : 1 IMM-CNR, Zona Industriale, VIII Strada 5, Catania; 2 Università degli Studi di Catania and INSTM UdR Catania; Viale Andrea Doria 6, Catania; 3 CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of Rome—Tor Vergata, via del Politecnico 1, I-00133, Rome, Italy; 4 Hamamatsu Photonics Europe, Arzbergerstr. 10, D-82211 Herrsching am Ammersee, Germany

Resume : Nano-sized TiO2 films have been grown by DC-Reactive Sputtering at ~150 °C on [0001] ZnO:Al substrates to be applied in Dye Sensitized Solar Cells. The effects of post-deposition thermal treatment at 500°C on the optical and structural (density, grain size) properties of the TiO2/ZnO:Al bi-layers have been investigated in details. The effects of such modifications on the surface properties, have been evaluated by dye-sensitization with 5,10,15,20 Tetrakis(4-carboxyphenyl) porphyrin (TCPP). Different interactions of the probe-molecules with the as grown and annealed TiO2 surfaces have been observed. In particular, Quantum Yield (QY) analysis of TCPP-sensitized films showed that the emitted/absorbed photons ratio in the as grown layers is a factor of ~3 lower than in the annealed layers. Based on simplified test solar cells, the electron injection is thought to be the most appropriate mechanism to explain the QY results. Our approach gives particular emphasis to the role of the surfaces and the interfaces involved in the TCPP/ TiO2/AZO multilayer structures with the intent of proposing the as sputtered TiO2 layer as blocking layer in complete architecture for low thermal budget solutions (<200°C).

Affiliations : Universitaet Münster, Institut fuer Materialphysik, 48149 Muenster

Resume : The paper highlights some remarkable results obtained by applying the radiotracer diffusion (RTD) technique to the study of iodine transport in polymer electrolytes for dye-sensitized solar cells (DSSY). In this area, RTD using the isotope I-125 fills a gap, as field-gradient NMR techniques are not suitable to monitor iodine diffusion. The RTD technique is based on the determination of a radiotracer depth profile by serial sectioning following isothermal diffusion annealing [1]. The method has been applied to Graetzel-cell electrolytes based on poly(ethylene oxide) (PEO) and various salts, including NaI and the ionic liquids 1-ethyl-3-methyl-imidazolium iodide (EMImI) and 1-propyl-3-methyl-imidazolium iodide (PMImI) [2,3]. Combined with DC conductivity data and cation diffusion measurements, RTD provides a virtually complete picture of mass and charge transport in these DSSY polymer electrolytes. Evaluating all data simultaneously in a comprehensive ion transport model, we find a strong role of cation-anion pair formation, which leads to a substantial decrease of the charge transport capacity. A remarkable effect is the observed enhancement of iodine and charge transport with increasing additions of pure iodine to the electrolytes. We will show that this enhancement can be consistently explained by the formation of the triiodide species and the concomitant reduced tendency to ion association. [1] N.A. Stolwijk, M. Wiencierz, J. Foegeling, J. Bastek, Sh. Obeidi, Z. Phys. Chem. 234 (2010) 1707-1733 [2] F. Call, N.A. Stolwijk, J. Phys. Chem. Lett. 1 (2010) 2088-2093; Correction: J. Phys. Chem. Lett. 1, (2010) 3213–3213 [3] T. Eschen, J. Koesters, M. Schoenhoff, N.A. Stolwijk, J. Phys. Chem. B 116 (2012) 8290-8298

Authors : Jia-De Peng1, Chi-Ta Lee1, Chuan-Ming Tseng2, and Kuo-Chuan Ho*1,3
Affiliations : 1 Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan; 2 Institute of Physics, Academia Sinica, Taipei 11529, Taiwan; 3 Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

Resume : In this study, mono-dispersed microspheres of highly exposed (001)-facets anatase TiO2 were synthesized by a facile hydrothermal route. Each of these microspheres consists of nanosheets with side length of 30–50 nm and thickness of 3–5 nm, a pore size of 30 nm and a surface area of 132 m2 g-1. The TiO2 film served as the semiconductor layer for a dye-sensitized solar cell (DSSC), exhibiting a power conversion efficiency (η) of 9.68%. This represents a 45.1% improvement over that obtained for a DSSC using the common P25-TiO2 (η = 6.67%), owing to higher light scattering ability of the (001)-facets TiO2 film (diffuse reflection spectra), its higher dye loading, compared to those of the P25-TiO2 film. Moreover, a higher diffusion coefficient (Deff) is shown as the reason for the higher Jsc in favor of the cell with (001)-facets TiO2. Higher band gap energy of the (001)-facets TiO2 compared to that of P25-TiO2 (differential reflectance spectroscopy and Kubelka–Munkand plots), higher steady-state electron density in the TiO2 conduction band (ns), higher electron lifetime (τeff) are shown as the causes for the higher Voc in favor of the cell with (001)-facets TiO2.[1] Here we have proposed hierarchically assembled microspheres of nanosheets TiO2 structure for DSSC application, and the highest power conversion efficiency was obtained for a DSSC using (001)-facets TiO2 film as the semiconductor layer. Key reference: [1] M. Adachi, M. Sakamoto, J. T. Jiu, Y. Ogata, S. Isoda, “Determination of parameters of electron transport in dye-sensitized solar cells using electrochemical impedance spectroscopy,” J. Phys. Chem. B 2006, 110, 13872-13880.

Authors : Hsin–Wei Chen1, Chen-Yu Hong2, Chia-Yuan Chen3, Chung-Wei Kung1, Chun-Guey Wu3, Chung-Yuan Mou2 and Kuo-Chuan Ho1,4,*
Affiliations : 1 Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan 2 Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan 3 Department of Chemistry, National Central University, Jhong-Li, 32001 Taiwan 4 Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

Resume : Plasmonic effect is a promising new approach to enhance the light trapping properties of thin film solar cells. Metal nanoparticles such as Au or Ag would induce the surface plasmons, which are used to couple light into the underlying optical modes of the active layer. Tuning the surface plasmon resonance can be used to enhance the light absorption in the required wavelength region. Excitation of surface plasmons in the mesoporous TiO2 will induce a strong scattering effect and enhance the electric field around the vicinity of the metal nanoparticle. Photocurrent enhancements have been reported from both inorganic and organic solar cells due to the surface plasmonic effect. In this study, a plasmon enhanced Au loaded mesoporous titania nanoparticles (Au@MTNs) with different concentrations of Au are synthesized by one-step method. When Au@MTNs are used as the scattering layer for the plastic-based dye-sensitized solar cells (DSSCs), a power conversion efficiency of 6.47% is achieved under illumination of 100 mW cm-2, which exhibits a 18% increase compared to the DSSCs fabricated with P25 TiO2 photoanodes because of the plasmonic scattering enhancement of the Au nanoparticles (NPs).1 Key reference: 1) S. Pillai and M. A. Green, Sol. Energy Mater. Sol. Cells, 94, 1481-1486, 2010.

12:00 Lunch