Materials for energy and environmentD
Earth abundant and emerging solar energy conversion materials
The global demand for energy is growing inexorably, and thus solar energy production is becoming increasingly important. Photovoltaic and solar fuel research is at the forefront of contemporary materials science; the key challenge is to understand structure-property relationships of emerging materials using state of the art experimental and computational methods.
This symposium will address fundamental and applied aspects of emerging solar absorbers and related materials and will highlight recent developments in both experimental and theoretical/computational approaches. The scope of this symposium is to provide a discussion forum for researchers working on the early stages of development of earth abundant and newly-emerging materials for thin film photovoltaics and solar water splitting. The focus will be on issues that are relevant to development of solar cell and solar fuel technologies outside of those already well developed industrially, ie materials other than Si, CdTe and CIGS. Novel experimental techniques for synthesis and characterization as well as theoretical, computational and modeling methods are of interest. Presentations will focus on relevant materials, interfaces and devices. Layers of interest include solar cell absorbers, water splitting photoelectrodes, transparent conductors, electrocatalysts for oxygen and hydrogen evolution, buffers and other components of importance to thin film photovoltaics and solar fuel devices. Absorber materials will include but will not be limited to SnS, Cu2O, FeS2, Zn3P2, ZnSnN2, ZnSnP2, Cu2S, Cu3N, WSe2, CZTS and related multinary compounds (semiconductor oxides). We are especially interrested in submissions relating to the emerging field of hybrid inorganic-organic halide perovskites, all inorganic halide perovskites, and interfacing semiconductor photoelectrodes with electrocatalysts.
Hot topics to be covered by the symposium:
- Halide perovskites solar cells
- Emerging earth abundant solar absorbers
- Novel p-type transparent conducting oxides
- Computational design for photovoltaics
- Defects analysis of absorber materials
- Interface and surface properties
- Novel solar cell devices
- Integrated solar fuel devices
- Metal oxide photoelectrodes
List of invited speakers:
- Aron Walsh, University of Bath, UK
- Andriy Zakutayev, National Renewable Energy Laboratory, USA
- Tim Veal, University of Liverpool, UK
- Wolfram Jaegerman, University of Darmstadt, Germany
- Hugh Hillhouse, University of Washington, USA
- Sixto Giminez, Universitat Jaimi I, Spain
- Brian Seger, Technical University of Denmark, Denmark
- Feliciano Giustino, University of Oxford, UK
- Jin Suntivich, Cornell University, USA
- Dunwei Wang, Boston College, USA
- Susan Schorr, Frei Universitat Berlin, Germany
- Oki Gunawan, IBM, USA
- Klaus Ellmer, Helmoltz Zentrum Berlin, Germany
Tentative list of scientific committee members:
- Jonathon Scragg, Uppsala University, Sweden
- Alex Cowan, University of Liverpool, UK
- Julien Vidal, ETSF, France
- Keith Butler, University of Bath, UK
- Ken Durose, University of Liverpool, UK
- Xavier Gonze, Université Catholique de Louvain, Belgium
- Stephan Lany, NREL, USA
- Shiyou Chen, Fundan University, China
- Su-Huai Wei, NREL, USA
- S. Guha, IBM, USA
- Robert Kokenyesi, Oregon State University, USA
- Roel van de Krol, Helmholtz Zentrum Berlin, Germany
- Kevin Sivula, EPFL, Switzerland
- Arno Smets, Delft University of Technology, The Netherlands
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Perovskites and emerging absorbers : Davd Scanlon
Authors : Aron Walsh
Affiliations : Centre for Sustainable Chemical Technologies and Department of Chemistry, University of Bath, UK
Resume : There are a large variety of materials being developed for application in thin-film solar cells. The majority is based upon naturally occurring minerals (solar mineralogy). The general procedure has been to take a multi-component system and tune the chemical composition to optimise optical absorption for the terrestrial solar spectrum. Other factors also determine whether a material can be practically employed in a photovoltaic or photoelectrochemical system, for example, the absolute band energies (work functions), defect physics, and chemical stability. I will discuss our recent progress into computing these performance descriptors from materials simulations [1-4], including advances in structure-property relationships in the kesterite (e.g. Cu2ZnSnS4) and perovskite (e.g. CsSnI3 and CH3NH3PbI3) families, in addition to the herzenbergite (SnS) system. New directions in the field, including the development of novel photoferroic materials, will also be addressed.  Atomistic origins of high-performance in hybrid halide perovskite solar cells Nano Letters 14, 2584 (2014).  Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells APL Materials 2, 081506 (2014).  Kesterite Thin-Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4 Advanced Energy Materials 2, 400 (2013).  Band alignment in SnS thin-film solar cells: Origin of the low conversion efficiency Applied Physics Letters 102, 132111 (2013).
Authors : T. Fix1, J.-L. Rehspringer2, G. Ferblantier1, D. Muller1, A. Slaoui1
Affiliations : 1 ICube laboratory (Universit? de Strasbourg and CNRS), 23 rue du Loess BP20 CR, 67037 Strasbourg Cedex 2, France; 2 Institut de Physique et Chimie des Mat?riaux de Strasbourg (IPCMS), UMR 7504 CNRS, University of Strasbourg, 23 rue du Loess, F-67034 Strasbourg Cedex 2, France
Resume : Inorganic thin film photovoltaics (PV) are mainly based on CdTe, amorphous Si or CIGS. At the same time a new route based on hybrid perovskite has emerged with top conversion efficiencies of 20.3 %. This proves that perovskites have a strong potential for photovoltaics, but hybrid perovskites still suffer from stability issues. Another possible route is the use of metal oxides, in particular inorganic perovskites, that are generally stable, non-toxic, abundant and can be synthesized by various methods. The ideal bandgap width of an active photovoltaic layer for the solar spectrum is around 1.3 eV. However conductive oxides with such a low bandgap width are scarce. One of the most studied oxide as active photovoltaic layer is cuprous oxide Cu2O. Its bandgap width of around 2 eV is not ideal for the solar spectrum, and conversion efficiencies do not generally exceed 4%. Novel perovskite oxides that have a lower bandgap width and have promising properties for photovoltaics have been identified, enabling to initiate a new photovoltaic technology based on these materials. One type of these oxides is called Mott insulators, such as LaVO3 that has suitable properties for an active layer, such as high optical absorption in the visible, direct bandgap and p-type properties. Mott insulators can easily become conductive depending on temperature, strain, stoichiometry and defects. We will describe the properties of these new absorbers and show their potential for photovoltaics.
Authors : John Buckeridge, Ben Williamson, David O. Scanlon
Affiliations : University College London, Kathleen Lonsdale Materials Chemistry, Department of Chemistry, 20 Gordon Street, London WC1H 0AJ, United Kingdom
Resume : The search for alternatives to second-generation thin film solar cell absorbers has led to the consideration of combinations of different non-toxic earth-abundant materials. We present a new class of such compounds: MCuP (M = Mg, Ca, Sr, Ba), which are promising as solar cell absorber layers. Using hybrid density functional theory, we demonstrate their thermodynamical stability and study their fundamental electronic and optical properties, finding that the mixing of Cu d and P p states results in highly disperse valence bands and low hole effective masses. Our results are confirmed by experimental synthesis and characterisation made by our collaborators, indicating the favourable p-type conductivity of these materials.
Authors : R. Chierchia 1, F.Pigna 2, M. Valentini 3, C. Malerba 4, E. Salza 1, P. Mangiapane 1, A. Mittiga1
Affiliations : 1 ENEA, UTRINN-FVC, C.R. Casaccia 2 Dipartimento di Ingegneria, Università di Roma, "Sapienza" 3 Dipartimento di Fisica, Università di Roma, "Sapienza" 4 Dipartimento di Ingegneria dei Materiali e Tecnologie Industriali, Università di Trento
Resume : Cu2SnS3 is an earth abundant material suitable for photovoltaic applications. Its low bandgap makes it a good candidate both for single junction solar cells and for IR detectors using heterojunction structures. Unfortunately the material suffers of a low diffusion length of the carriers due to the presence of spurious phases, voids, defects and small grain size. In order to improve the quality of our samples, the influence of the deposition parameters on its structural properties has been studied. The solar cell obtained with the optimized Cu2SnS3 has shown an external quantum yield larger than 80% around 500 nm a conversion efficiency in the order of 3%, a Jsc of 26 mA and a Voc of 240 mV, one of the world best result obtained with a Cu2SnS3 based solar cell. Furthermore the external quantum yield at wavelength larger than 1200 nm is still around 30%. Further improvements could come from the use of a rapid thermal annealing to reduce the formation of spurious phases.
Authors : G. Larramona, S. Bourdais, A. Jacob, C. Choné, B. Delatouche, C. Moisan, G. Dennler, S. Levcenko, and T. Unold
Affiliations : IMRA Europe S.A.S., Sophia Antipolis, France. Helmholtz-Zentrum Berlin für Materialien und Energie(HZB), Berlin, Germany.
Resume : We have developed recently a process allowing to spray coat an ink based on a mixture of water (90%) and ethanol (10%) and containing a Copper Zinc Tin Sulfide (CZTS) colloid prepared with a new, instantaneous, and environmentally friendly method. An appropriate sequential annealing allowed us to form a CZTSSe layer with a ratio Se/(Se+S) of 60%, and devices showing efficiencies about 8.5%. In order to comprehend the main limitations occurring in these solar cells, we have employed temperature dependent admittance spectroscopy and capacitance-voltage profiling. These techniques revealed a large density of charge carriers in the dark (5.10e16 cm-3) and two main acceptor defect levels: One fairly close to the middle of the band-gap and another one much shallower. A controlled tuning of the Sn content in the sprayed ink appears to offer an efficient leverage on the net doping density: By adding only a few percent, the density of charge carriers could be reduced by more than an order of magnitude. A subsequent optimization of the annealing conditions allowed the completion of pinhole free active layers, leading to efficiencies reaching 10% under AM1.5G. In spite of their respectable performances provided the unprecedented environmental friendliness and simplicity of our process, our devices still suffer from a major open circuit voltage deficit. We will discuss the origin of this latter within the context of further experimental analysis and controlled order to disorder ratio.
Water Splitting I : Kevin Sivula
Authors : Brian Seger1,*, Bastian Timo Mei1, Dowon Bae1, Ivano Castelli2, Thomas Pedersen3, Peter Vesborg1, Karsten Jacobsen2, Ole Hansen3, Ib Chorkendorff1
Affiliations : 1: Center for Individual Nanoparticle Functionality (CINF), Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark; 2: Center for Atomic-scale Materials Design (CAMD), Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark; 3: Department of Micro- and Nanotechnology, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
Resume : Modelling has shown that to optimize photoelectrolysis efficiency, a 2 photon tandem device (back to back solar cells) should be used. The underlying principle is that one solar cell should absorb high energy photons while the other absorbs the low energy photons. In theory, either the H2 evolution reaction or the O2 evolution can take place on the large band gap material with the opposite reaction taking place on the small band gap material. I will discuss both possibilities and show how this affects optimal material properties needed for the photocatalysts. Finding an optimal photocatalyst system can be very difficult, thus we break the water splitting process into 3 parts: 1) Photoabsorption, 2) Photoabsorber/Electrolyte interface and 3) H2 or O2 evolution catalysis. In this talk I will discuss our use of Si as a photoabsorber and what we do to maximize photovoltage. To prevent corrosion at the photoabsorber/electrolyte interface we use corrosion protection layers. I will show how TiO2 is a great material for protecting the H2 evolution photocatalyst, whereas NiO is excellent at protecting the O2 evolution catalyst. I will also show our work on improving both the H2 evolution and O2 evolution catalysts.
Authors : M. Rioult(1), H. Magnan(1), D. Stanescu(1), S. Datta(1), R. Belkhou(2), S. Stanescu(2), P. Le Fèvre(2), F. Maccherozzi(3), and A. Barbier(1)
Affiliations : (1): Service de Physique de lEtat Condensé, DSM/IRAMIS/SPEC, CNRS UMR 3680, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France. (2): Synchrotron SOLEIL, LOrme des Merisiers Saint-Aubin, 91192 Gif-sur-Yvette, France. (3): Diamond Light Source, Harwell Campus, Didcot OX11 0DE, Oxforshire, United Kingdom.
Resume : The quest for semiconducting photoelectrodes able to produce hydrogen as a solar fuel by solar water splitting is still to be achieved. Ferroelectric oxide semiconductors are photoactive and exhibit a self-polarization state, acting as a permanent internal electric field. This is of prime interest for tuning the photogenerated charges separation and transport, as it was pointed out for the perovskite photovoltaics breakthrough . This emerging lead brings a new degree of freedom for optimization of photoelectrodes [2,3]. Barium titanate perovskite (BaTiO3) is one of the best candidates considering its high remnant polarization and its stability. However, very little is known about the correlation between the polarization state, the electronic structure (work function, band bending and positions) and the water splitting performances. To tackle these questions, we studied nanometric BaTiO3 epitaxial thin films poled in different remnant polarization states. The electronic structure of the films was investigated by X-ray photoelectron spectroscopy (XPS) and microscopy (X-PEEM), using synchrotron radiation. We also performed photoelectrochemical measurements to correlate the photocurrent to the polarization state. In addition, bilayers of BaTiO3 with a conventional photoanode oxide (Ti:Fe2O3, TiO2) were investigated.  Frost et al., Nano Lett. 2014, 14, 2584-2590  Cao et al., Angew Chem. Int. Ed. 2014, 53, 11027-11031  Ji et al., Appl. Phys. Lett. 2013, 103, 062901
Authors : D.A. Kudryashov, A.S. Gudovskikh
Affiliations : Saint-Petersburg Academic University - Nanotechnology Research and Education Centre RAS 194021 St. Petersburg, Russia Email: email@example.com; Saint-Petersburg Academic University - Nanotechnology Research and Education Centre RAS 194021 St. Petersburg, Russia Saint Petersburg Electrotechnical University "LETI" 197376 St. Petersburg, Russia
Resume : Rapid development of solar cell technology is caused by increasing attention to the problems of environmental protection and hydrocarbon depletion. New types of solar cells are actively developing. Solar cells based on oxide heterojunctions are extremely attractive due to its strong growth potential. ZnO/Cu2O based solar cells are cheap and do not contain toxic materials in its production. Shockley-Queisser efficiency for this structure reaches of 20% . However, the best known samples of ZnO/Cu2O solar cells could achieve an efficiency of 4.1% . To estimate a potentially achievable values of Voc and short-circuit current of ZnO/Cu2O based solar cells a computational model was developed. AZO/ZnO/Cu2O/Cu structure was chosen as a basis. Experimentally measured material parameters were used to calculate bandgap diagram, IV and EQE characteristics. Simulation results demonstrates a good agreement compared to experimental data. Recommendations for efficiency improvement were developed. Possible implementation of ZnO/Cu2O subcell in a multijunction stack will be discussed. The reported study was partially supported by RFBR, research project N15-08-06645 А. 1. Shockley, W., Queisser, H. J. // J. Appl. Phys. 1961, 32, 510-519. 2. Y. Nishi, T. Miyata and T. Minami // J Vac Sci Tech A: Vacuum, Surfaces, and Films, 30(4):04D103-04D103, 2012.
Authors : Saatviki Gupta, Manoj Kumar, Yogita Batra, VR Satsangi, BR Mehta
Affiliations : Dayalbagh Educational Institute: Saatviki Gupta and VR Satsangi Indian Institute of Technology Delhi: Saatviki Gupta, Manoj Kumar, Yogita Batra, BR Mehta
Resume : In this study, CdS-CZTS single nanorod heterojunctions formed using a combination of chemical synthesis and RF sputtering techniques have been studied. Kelvin probe force microscopy (KPFM) has been used to investigate the processes of interface formation, charge generation and separation under dark and light conditions. CdS nanorods were fabricated through a hydrothermal route and process parameters have been optimized to yield nanorods with good crystallinity and well controlled diameter. A thin layer of CZTS was deposited on to these nanorods followed by annealing to facilitate junction formation. The surface potential map of the single nanorod heterojunction in the dark showed distinct regions belonging to CdS and CZTS. On illumination, a clear interfacial region between CdS and CZTS corresponding to the charge generation at the interface is observed. The interface region topographically positioned between the two regions of CdS and CZTS has surface potential values also between those of the two semiconductors. The surface potential scan indicates an overall decrease in surface potential of the sample by ~60mV on illumination which is explained by charge generation and hole accumulation effects. Additionally, the development of photovoltage between the p and n type materials manifests as a relative difference in surface potential between the regions of CdS and CZTS as a consequence of a shift in the Fermi levels. The variation in surface potential has been also been investigated by changing the stoichiometry of the CZTS film and the sulfurization conditions. These studies show that the information obtained from KPFM investigations is extremely useful for studying junction formation in nanoscale devices and can be used to improve the junction quality by optimizing the charge generation and separation processes.
Authors : S. BECHU*, A. GOULLET*, L. CATTIN-GUENADEZ*, D. DUCHE#, J-J. SIMON#, M. GIRTAN$, L. BROHAN*, M. RICHARD-PLOUET*
Affiliations : * IMN-Université de Nantes, Nantes ; # IM2NP, Marseille ;$ LᶲA, Angers
Resume : Third generation solar cells aims at increasing efficiency by improving the solar spectrum capture. According to Marti and Luque , intermediate band cell concept could overtake the 31% theoretical efficiency of simple junction photovoltaic cells established by Shockley and Queisser  in 1961. For several years, the Institut des Matériaux Jean Rouxel of Nantes worked on hybrid photosensitive sols-gels based on titanium oxides . Theses sols-gels are characterised by specific optical and electronic properties. Once illuminated under UV light, an intermediate band appears in the band structure so the absorption spreads over visible to near infrared due to reduction of Ti(IV) in Ti(III). Thanks to this absorption range increase, these sols-gels can be used as active layers in solar cell. In order to optimise light absorption properties of the sol-gel layer, shaping and optical properties studies were carried out. Thin films were obtained with an accurate control over thickness in the range from 150 nm to 10 µm. Ellipsometry and XPS studies were respectively undertaken to determine optimal thickness and electronic structure evolution under UV illumination. This layer with original properties could be implemented as active layers in new hybrid solar cells.  Luque, A., Marti, A., Physical Review Letters, 28, 78, (1997)  Shockley, W., Queisser, H.J., Journal of Applied Physics, 32, 510 (1961)  Cottineau T., Brohan L., Pregelj M., Cevc P., Richard-Plouet M., Arčon D., Advanced Functional Materials, 18, 2602 (2008)
Authors : K. Timmo*, M. Kauk-Kuusik, M. Pilvet, V. Mikli, E. Kärber, T. Raadik, I. Leinemann
Affiliations : Department of Materials Science, Tallinn University of Technology Ehitajate tee 5, 19086 Tallinn, Estonia
Resume : Tin monosulphide (SnS) is a promising candidate for the development of solar cells using earth-abundant-materials. While SnS combines the set of materials characteristics suitable for high-performance photovoltaics, the efficiencies of SnS solar cells has only resently reached 4.6%  and it is far below the theoretical efficiency. This is due to the fact that it is difficult to grow high purity and device quality SnS films. In this study, the results of recrystallization of polycrystalline SnS in the presence of molten CdI2, SnCl2 and KI salts as flux materials to produce unique SnS monograin powder are described. The crystal structure and morphology of the powders were characterised by X-ray diffractometry (XRD), Raman spectroscopy and scanning electron microscopy (SEM). XRD and Raman analysis revealed that single phase SnS powder can be obtained in KI and SnCl2 at 740°C and 500oC, respectively. According to XRD and energy dispersive X-ray spectroscopy, recrystallization of SnS in CdI2 flux resulted in mixture of CdS and Sn2S3 crystals. SEM images showed that morphology of formed crystals can be controlled by the nature of the flux materials: polycrystalline SnS powder forms needles of Sn2S3 together with rounded crystals of CdS in CdI2, smooth flat crystals of SnS in SnCl2 and well-formed crystals of SnS with rounded edges in KI. The temperatures of phase transitions and/or the interactions of SnS and flux materials are determined by differential thermal analysis.  R.G.Gordon et al. 2014
Authors : Rachmat Adhi Wibowo1, Bernhard Klampfl1, Raad Hamid2, Thomas Meier3, Theodoros Dimopoulos1
Affiliations : 1 AIT Austrian Institute of Technology, Energy Department, Photovoltaic Systems, Giefinggasse 2, 1210, Vienna, Austria 2 AIT Austrian Institute of Technology, Mobility Department, Electric Drive Technologies, Giefinggasse 2, 1210, Vienna, Austria 3 AIT Austrian Institute of Technology, Health and Environment Department, Molecular Diagnostics, Donau-City Strasse 1, 1220, Vienna, Austria
Resume : The surface of the Cu/Sn/Zn multilayer, employed as precursor for crystallising kesterite Cu2ZnSnS4 solar absorber, typically demonstrates a rough profile that is far from the required precursor properties. This morphology is mainly caused by the presence of Volmer-Weber-type Sn island deposits on top of the Cu layer. The use of galvanostatic electrodeposition was aimed to improve the precursor surface morphology by a two-stage approach, i.e. the deposition of Cu, followed by the co-deposition of a Sn-Zn layer under galvanostatic/constant current mode. The two-stage deposited precursors exhibit multi-phase of elemental Sn and Cu along with Cu5Zn8/CuZn intermetallic compound and show a macroscopically smooth surface morphology. The origin of as-deposited precursor smooth morphology is attributed to the presence of CuxZny alloy phase, which prevents the formation of Volmer-Weber Sn islands. It is demonstrated that the co-deposition of Sn-Zn layers offers a straightforward technique to improve precursor surface morphology without the use of additional chemical agents as surface modifier or leveler. In this contribution, an insight of Sn Volmer-Weber island formation in precursors is discussed as well. The kesterite Cu2ZnSnS4 films crystallised from the process aforementioned were characterised in order to unveil the impact of the improved precursor morphology on the kesterite properties.
Authors : M. Espindola-Rodriguez1, M. Placidi1, Y. Sánchez1, S. López-Marino1, D. Sylla1, M. Neuschitzer1, V. Izquierdo-Roca1, O. Vigil-Galán2, A. Pérez-Rodríguez1,3 and E. Saucedo1
Affiliations : 1Catalonia Institute for Energy Research- IREC, Jadins de les Dones de Negre 1, 08930 Sant Adrià de Besòs (Barcelona), Spain. 2 Escuela Superior de Física y Matemáticas-Instituto Politécnico Nacional (IPN), México DF, México. 3 IN2UB, Departament dElectrònica, Universitat de Barcelona, C. Martí i Franquès 1, Barcelona, Spain
Resume : In modern architectures human comfort is enhanced by the use of sun and shading, making necessary the use of semitransparent elements. In this context, bifacial solar cells could play an important role allowing the introduction of natural light deep into the interior of constructions by the use of a transparent conducting back contact (TCB) and an appropriate gridding. In this work we report on the use of different absorber thicknesses (0.6-1.3 μm), selenization temperatures (550 500 °C) and different TCBs for CZTSe bifacial solar cells. The absorbers were produced by using a two step synthesis process (sputtering + reactive annealing), onto SnO2:F (FTO), In2O3:SnO2 (ITO) and ZnO:Al (AZO) as TCB. Solar cells were characterized using SEM, XRD and Raman spectroscopy. As a first strategy, solar cells with the Glass/TCB/CZTSe/CdS/i-ZnO/ITO structure were produced. Under single-side illumination efficiencies of 1.7%-front, 0.1%-rear were obtained. The main identified problem was the lack a good ohmic contact at the interface TCB/CZTSe with a high Rs >4 Ω.cm2-front. In a second strategy, a thin layer of sputtered Mo (~25nm) was introduced at the TCB/CZTSe interface. Solar cells with the Glass/TCB/Mo/CZTSe/CdS/i-ZnO/ITO structure were then produced with improved efficiencies of up to 3%-front, 0.3%-rear using FTO and with improved Rs ~3 Ω.cm2-front. Modifications at the TCB/CZTSe interface are currently been performed to increase the efficiency of the CZTSe bifacial solar cells.
Authors : Michaela Meyns (1), Xuelian Yu (1,2), Zhishan Luo (1), Alexey Shavel (1), Xiaoqiang An (3,4), Pablo Guardia (5), Maria Ibañez (1,6), Andreu Cabot (1,7)
Affiliations : (1) Catalonia Energy Research Institute (IREC), 08930 Barcelona, Spain; (2) Institute of Process Engineering, Chinese Academy of Sciences, 100190 Beijing, China; (3) Department of Chemical Engineering, University College London, London WC1E 7JE, United Kingdom; (4) Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, 100085 Beijing, China; (5) Centre Tecnològic de la Química de Catalunya, 43007 Tarragona, Spain; (6) Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich/ Empa - Swiss Federal Laboratories for Materials Science and Technology, Laboratory for thin films and photovoltaics, 8600 Dübendorf, Switzerland (7) Institució Catalana de Recerca i Estudis Avancats (ICREA), 08010 Barcelona, Spain.
Resume : Heterostructured semiconductor-metal nanoparticles are ideal candidates for photocatalytic applications owing to their ability to effectively separate photogenerated charges. Excited electrons generated in the semiconductor component can be quickly transferred to a metallic domain [Mongin et al. ACS Nano 2012, 6, 7034.], while holes are caught by a scavenger. Recently, the formation of heterostructures of well-defined CZTS nanoparticles and metals was reported [Yu et al. J. Am. Chem. Soc. 2014, 136, 92369239.]. The heteronanoparticles of this quarternary material, composed of abundant elements and absorbing light across the spectrum into the infrared region, were successfully applied in water splitting and pollutant degradation reactions. In these and other heterostructures comparatively small changes of reaction parameters and components can lead to significant alterations in the deposition of the metals, affecting the overall colloidal stability and photocatalytic performance of the catalyst. We will show and discuss consequences of changing the metal precursor compound and the driving force for the deposition (irradiation or redox-deposition) in the colloidal seeded-growth synthesis of CZTS-Au and other materials with special emphasis on the photocatalytic properties.
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Authors : Jan Morasch, Christian Lohaus, V. P. Prasadam, Wolfram Jaegermann, Andreas Klein
Affiliations : Technische Universität Darmstadt, Institute of Materials Science, Surface Science Division, Jovanka-Bonschits-Straße 2, 64287 Darmstadt, Germany
Resume : Semiconducting metal oxides tend to show large band gap energies above 2.5 eV. However, for photovoltaic and photocatalytic applications a band gap in the range of 1 to 2.5 eV is required in order to absorb a sufficient part of the solar spectrum. Possible candidates for oxidic absorber materials could be Bi2O3, CuO and Co3O4 with band gaps of approx. 2.5, 1.5 and 1.6 respectively. In this work thin Bi2O3, CuO and Co3O4 films were deposited on glass and glass|ITO substrates by RF-magnetron sputtering. The films were characterized via photoelectron spectroscopy without breaking UHV conditions. To investigate the optical transitions of the materials UV/VIS spectroscopic measurements were carried out. For Bi2O3 and Co3O4 the phase was determined by XRD. Electrical measurements were performed with a four point setup. In addition the band alignments of the absorber layers with the oxidic contact materials ITO and RuO2 were investigated via so called interface experiments. These are stepwise depositions of a material on a freshly prepared substrate followed by XPS measurements after each step. All of the three materials are p-type semiconductors with a high valence band maximum in comparison to other metal oxides. This makes them promising parent materials for ternary metal oxides with a small band gap.
CZTS and its analogues II : Oki Gunawan
Authors : Susan Schorr
Affiliations : Helmholtz Centre Berlin for Materials and Energy
Resume : One of the reasons for Cu(In,Ga)Se2 based thin film solar cells sucess is the remarkable flexibility of its chalcopyrite crystal structure [1,2]. This flexibility is a key point also for the quaternary kesterite type compounds like Cu2ZnSnS4/Se4 (CZTS, CZTSe) because the thin film growth is in fact a non-equilibrium process. Nevertheless the absorber layers exhibits an overall off-stoichiometric composition, thus the existence of intrinsic point defects is strongly correlated with the chemical potential and therefore dependent on the composition of the material. Finally these structural defects influence the electronic properties of the final device, sensitively. Both compounds, CZTS and CZTSe, crystallize in the kesterite type structure (space group I-4) . The structure can be described by a stacking sequence of cation layers Cu/Sn Cu/Zn Sn/Cu Cu/Zn Cu/Sn perpendicular to the crystallographic z-direction. An off-stoichiometric composition, for instance Cu-poor, originates from the prospensity of the structure to stabilze copper vacancies, the charge balancees beeing commonly insured by appropriate substitutions on the cationic sites. If the oxidation states of the cations and anions are retained going from stoichiometric CZTS/Se to off-stoichiometric CZTS/Se, a number of specific substitutions can be envisioned to account for the charge balance in the off-stoichiometric material. These substitutions lead to the formation of point defects in the crystal structur.
Emerging Materials : Andriy Zakutayev
Authors : M. Birkett, A. Welch, T. Whittles, V. R. Dhanak, A. Zakutayev, T. D. Veal
Affiliations : Stephenson Institute for Renewable Energy, University of Liverpool, Liverpool, UK; National Renewable Energy Laboratory, Golden, Colorado, USA
Resume : For solar power to continue to deliver energy at the same or lower price than conventional sources but at TW scale requires new cheap and efficient earth abundant photovoltaic (PV) materials. These will replace the current technologies which rely on Si, an inefficient absorber of photons which is also rather defect and impurity intolerant necessitating relatively expensive refining, and Cu(In,Ga)Se2 and CdTe, two thin film technologies which employ scarce and expensive (In and Te) and/or toxic elements. The leading abundant PV material is quaternary compound CZTS, owing to its investigation for PV for over 25 years and its high power conversion efficiency of up to 12. 6%. Alternative less complex candidates include CuSbS2 and Cu3N. In particular, Cu-based materials have been identified as defect tolerant due to the anti-bonding character of their valence band maxima that result from hybridization between Cu-d and anion-p states . Here, CuSbS2 and Cu3N films grown by sputtering are investigated for PV application. Transmission and reflectance measurements between 4 and 300 K have been used to determine the nature and temperature dependence of the optical transitions of these materials. Additionally, the valence band structure, surface composition and electronic properties have been probed by photoemission spectroscopy.  A. Zakuteyev et al., "Defect Tolerant Semiconductors for Solar Energy Conversion," J. Phys. Chem. Lett. 5, 1117 (2014)
Authors : A. Kuzmin1, A. Anspoks1, A. Kalinko2, J. Timoshenko1, L. Nataf2, F. Baudelet2, T Irifune3
Affiliations : 1)Institute of Solid State Physics, University of Latvia, Latvia; 2)Synchrotron SOLEIL, l'Orme des Merisiers, Saint-Aubin, France; 3)Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
Resume : Copper nitride (Cu3N) has attracted much attention due to its possible use in spintronic devices, optical lithography/metallization layers, resistive random-access memory as well as write-once read-many optical storage media, conductive ink and solar absorber material. At ambient pressure Cu3N crystallizes in the anti-ReO3 type structure (Pm-3m space group) with the open framework built up of NCu6 octahedra. Such structure type together with low decomposition temperature (600-800 K) indicates on the possibility of pressure induced modifications. In fact, the pressure-induced metallization above ~5 GPa has been observed previously in Cu3N by electrical resistivity  and optical absorption  measurements. X-ray [2,3] and neutron  diffraction confirmed the existence of phase transition between 5 and 10 GPa to the high pressure phase I4/mmm  or P4/mmm . At the same time, the local atomic structure evolution under high pressure has been never studied so far. In this work we present the results of the room-temperature pressure-dependent (up to 27 GPa) Cu K-edge x-ray absorption spectroscopy study of Cu3N. Our results confirm the occurrence of transition above 5 GPa, but disagree on the type of high pressure phase.  J. G. Zhao, et al., Phys. Stat. Sol. (b) 243 (2006) 573.  A. Wosylus, et al, Z. Anorg. Allg. Chem. 635 (2009) 1959.  J.G. Zhao, et al, Solid State Commun. 150 (2010) 1521.
Authors : J. Timoshenko, A. Anspoks, A. Kalinko, A. Kuzmin
Affiliations : Institute of Solid State Physics, University of Latvia, Kengaraga street 8, LV-1063 Riga, Latvia
Resume : Copper nitride (Cu3N) has cubic antiperovskite structure that recently attracted the interest due to its potentiality for various technological applications. It is considered as a metastable semiconductor, which loses nitrogen and turns into metal upon temperature increase, and thus is a cheap and promising candidate for write-once memory devices. The structure of Cu3N is compatible with the structure of some organic molecules, including porphyrins that are considered for applications in molecular-based electronics. Cu3N has been recently proposed also as a solar energy absorber material. Deep understanding of the local structure and dynamics of Cu3N is important to fully explain and exploit its unique properties. In this study we investigate the local structure around copper in polycrystalline Cu3N using EXAFS spectroscopy at the Cu K-edge. EXAFS spectra of Cu3N contain very rich information, including contribution not only from the first but also from distant coordination shells; besides, very pronounced is the influence of so called multiple-scattering effects, which encode the information on many-atom distribution functions and correlations of atomic motion. To benefit from this information, the EXAFS data are interpreted by a novel method, based on the use of evolutionary algorithm (EA). The combined EA-EXAFS approach allows us to construct a 3D structure model of Cu3N and to follow the development of its lattice dynamics upon temperature increase from 10 to 300 K.
Water Splitting III : Dunwei Wang
Authors : Sixto Gimenez
Affiliations : Photovoltaics and Optoelectronic Devices Group, Departament de Física, Universitat Jaume I, 12071 Castelló, Spain
Resume : In the present talk, we will focus on different strategies to photoelectrochemically generate hydrogen with earth-abundant materials. Metal oxide semiconductors constitute promising candidates due to their synthetic versatility, low cost and relatively high stability in a wide range of environmental conditions. However, the current conversion efficiencies remain low for further technological exploitation and the basic understanding of carrier dynamics is a key issue to boost the performance of these materials. We will examine the role of surface states on water oxidation with metal oxide semiconductors and the effect of different catalytic and functional layers. Moreover, we will discuss the potential of different strategies exploiting novel optoelectronic effects like quantum confinement, spectral conversion and plasmonics. On the other hand, the potential of organic semiconducting materials for solar fuel production will be highlighted and we will provide the design rules to extract photocurrents for hydrogen reduction in the scale of mA·cm-2. Finally, we show how stability of these organic devices in aqueous solutions can be significantly enhanced by the adequate selection of selective contacts.
Authors : Christian Lohaus, Jan Morasch, Joachim Brötz, Andreas Klein, Wolfram Jaegermann
Affiliations : Technische Universität Darmstadt, Institute of Materials Science, Surface Science Division, Jovanka-Bonschits-Straße 2, 64287 Darmstadt, Germany; Technische Universität Darmstadt, Institute of Materials Science, Surface Science Division, Jovanka-Bonschits-Straße 2, 64287 Darmstadt, Germany; Technische Universität Darmstadt, Institute of Materials Science, Structure Research Division, Alarich-Weiss-Strasse 2, 64287 Darmstadt, Germany; Technische Universität Darmstadt, Institute of Materials Science, Surface Science Division, Jovanka-Bonschits-Straße 2, 64287 Darmstadt, Germany; Technische Universität Darmstadt, Institute of Materials Science, Surface Science Division, Jovanka-Bonschits-Straße 2, 64287 Darmstadt, Germany
Resume : Transition metal oxides (TMOs) are heavily studied as materials for energy applications and catalysis. Their optical and electrical properties mainly result from cation-3d-states which give rise to a variety of different thinkable applications of such oxides. One of the most heavily studied TMOs is the cubic-spinel of Co3O4 with possible applications as (photo-)catalysts or solar cell absorber. In this study, thin films have been prepared by RF-magnetron sputtering. In addition to as deposited films, samples were annealed in vacuum and oxygen-containing atmosphere. The integration of a photoelectron spectrometer into the system allows for the in-situ characterization of the samples by X-Ray Photoelectron Spectroscopy (XPS) using core level binding energies and intensities. The work function dependency on the preparation and sample treatment conditions was obtained by Ultraviolet Photoelectron Spectroscopy (UPS) and reveals a variation of about 1 eV depending on the oxygen content in the gas. Poor photovoltaic behavior is related to Fermi-level pinning which was determined by interface experiments. Here, step-wise depositions of contact materials onto a Co3O4 substrate (or vice versa) are followed by in-situ observation of the Fermi level shifts via XPS. In addition, optical properties were measured using UV/Vis/NIR spectroscopy and a band gap of approximately 0.8 eV was found. Conductivity was measured using a four point measurement setup.
Authors : J. P. Teixeira (1), P. M. P. Salomé (2), R. A. Sousa (1), M. G. Sousa (1), A. F. da Cunha (1), P. A. Fernandes (1,3), S. Sadewasser (2) and J. P. Leitão (1)
Affiliations : (1) Departamento de Física and I3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portuga; (2) INL - International Iberian Nanotechnology Laboratory, Laboratory for Nanostructured Solar Cells (LaNaSC), Avenida Mestre José Veiga, 4715-330 Braga, Portugal; (3) Departamento de Física, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida 431, 4200-072 Porto, Portugal;
Resume : In recent years, Cu(In,Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) have received increasing attention in the thin film technologies as promising photovoltaic materials. Further improvement of the electrical performance of solar cells depends on the knowledge of fundamental physical properties, namely, the electronic levels structure. A few different models have been considered for the assignment of the radiative transitions in CIGS and CZTS, namely: donor acceptor pair recombination (DAP), quasi-DAP recombination (QDAP) and radiative channels involving fluctuating potentials. In this work, we take a special care considering that these materials have a high doping level and strong compensation and thus the radiative and non-radiative recombination channels are strongly influenced by electrostatic fluctuating potentials along the film. As a consequence, bound states for electrons do not occur for single donors but just for large enough clusters of this type of impurities, which is a quite different behavior from the one observed for acceptors. Such discussion is sometimes discarded in the literature but of the utmost importance. Photoluminescence results for CZTS thin films are presented. The observed radiative transitions are discussed in the scope of the previous theoretical analysis. The DAP model is unable to explains the excitation power dependence of the PL. The influence of fluctuating potentials is shown and discussed.
Authors : Joe Briscoe1, Adam Marinovic1, Marta Sevilla2, Steve Dunn1, and Magdalena Titirici1
Affiliations : 1 Queen Mary University of London, UK; 2 Instituto Nacional del Carbón (CSIC), Oviedo, Spain
Resume : We report the use of carbon quantum dots (CQDs) synthesised from the biomass-derived materials chitosan, chitin and glucose as light absorbers in nanostructured solar cells. These materials are synthesised via hydrothermal carbonisation at 200°C and used to sensitise both ZnO nanorods and mesoporous TiO2 for photovoltaic devices, thus avoiding the use of rare, expensive or toxic materials commonly used in absorbers for nanostructured solar cells. It is shown through XPS and FTIR analysis that the key functional groups are retained from the precursor to the final CQDs, and that this chemical differentiation can lead to varying levels of sensitisation and light absorption for the different types of CQDs. The CQD-sensitised ZnO nanorod devices are combined with CuSCN to form a solid-state cell, and mesoporous TiO2 devices are completed with a liquid electrolyte. A maximum power conversion efficiency of 0.15 % is achieved. The efficiency is limited by the low light harvesting efficiency, which is a maximum of ~20 % in the visible region. However, with a high fill factor of 65 %, these materials the devices show good prospects for improvement with increased optical absorption of the CQDs. This can be targeted by further developing the wide range of potential synthesis conditions and biomass-derived precursor materials to develop a new kind of sustainable light-harvesting material.
Authors : Ana Rosa García-Angelmo; R. Romano-Trujillo; O. Gomez Daza; J. Campos; M. T. S. Nair; and P. K. Nair
Affiliations : Instituto de Energías Renovables, Universidad Nacional Autónoma de México Temixco, Morelos-62580, México
Resume : Thin films of SnS deposited by chemical bath method show different crystal structures : zinc blende (SnS-ZB), cubic (SnS-C), or orthorhombic (SnS-Or), depending on the chemical composition and temperature of the deposition bath. In the present work, we used the SnS-ZB thin films with optical band gap (Eg) of 1.6 eV as absorbers in solar cell structures deposited on stainless steel (SS) substrates from a chemical bath containing a soluble Sn(II) complex and thioaetamide. By completing the solar cell structure with sputter-coated ZnO followed by n-ZnO:Al, and evaporated Al electrode, we have observed in SS/SnS-ZB(550 nm)/ZnO(400 nm)/ZnO:Al/Al, an open circuit voltage, Voc, of 259 mV, short circuit current density, Jsc, of 0.64 mA/cm2, and fill factor, FF, of 0.36. However, these parameters are improved in: SS/SnS-ZB/CdS(50 nm)/ZnO/ZnO:Al/Al, with Voc de 489 mV and Jsc of 2.68 mA/cm2 (data normalized under the sun), FF of 0.39 and a conversion efficiency of 0.51 %. We hope to improve these characteristics by optimizing the thicknesses of SnS, CdS and ZnO layers, and by varying the electrode materials.  Ana Rosa García-Angelmo, M. T. S. Nair and P. K. Nair, Solid State Sciences, 30 (2014) 26; doi:10.1016/j.solidstatesciences.2014.02.002
Authors : S. Polivtseva1, I. Oja Acik1, E. Kärber1, A. Mere1, V. Mikli2, M. Krunks1
Affiliations : 1 Tallinn University of Technology, Department of Materials Science, Laboratory of Thin Film Chemical Technologies, Ehitajate tee 5, 19086 Tallinn, Estonia; 2 Tallinn University of Technology, Department of Materials Science, Chair of Semiconductor Materials Technology, Ehitajate tee 5, 19086 Tallinn, Estonia;
Resume : Tin sulfide films were deposited by non-vacuum spray pyrolysis method using aqueous solutions containing SnCl2 and cysteine (HO2CCH(NH2)CH2SH) as a novel source of sulfur instead of commonly used thiourea. The solution that contained SnCl2 (Sn) and cysteine (S) at molar ratios of Sn:S=1:1, 1:2 and 1:4 was sprayed onto glass sheets at temperatures from 200 to 370°C, in air. X-ray diffraction (XRD), Raman and UV-Vis spectroscopies were used to characterize structural and optical properties of the films. According to XRD, spray of 1:1 and 1:2 solutions onto substrates with temperatures up to 370°C results in films that are composed of SnS as a main crystalline phase, but films grown from 1:4 solutions are amorphous when grown below 370°C. Raman spectra confirm presence of SnS phase in all films by spray, the bands characteristic of Sn2S3 and SnS2 phases become more intense when deposited at higher temperatures or using higher amount of cysteine in the spray solution. Increase in optical band gap value from 1.6 eV to 2.3 eV is in correlation with changes in phase composition. Characterisation of the films by energy dispersive X-ray spectroscopy is in progress. In the present study we show how properties of the sprayed SnS films depend on the film growth temperature and amount of cysteine in spray solution.
Authors : Sara Engberg, Yeng Ming Lam, Jørgen Schou
Affiliations : DTU Fotonik, Technical University of Denmark, DK-4000 Roskilde, Denmark; School of Materials Science and Engineering, Nanyang Technological University, Singapore; DTU Fotonik, Technical University of Denmark, DK-4000 Roskilde, Denmark;
Resume : The kesterite material, Cu2ZnSn(SxSe1-x)4 (CZTS), shows great promise as the absorber layer for future thin film solar cells. Solution processing allows for comparatively fast and inexpensive fabrication, and holds the record efficiency in the kesterite family. However, for nanoparticle (NP) solution processing to be a feasible fabrication route, the amount of carbon in the film has to be limited. In our work, we try to limit the organic material in the film by synthesizing larger NPs. Larger particles can be obtained by longer reaction durations, slower reaction rates of the precursors, or slower injection rates of the sulfur/selenium precursors. In our group, we have synthesized NPs larger than 200 nm by controlling the monomer concentration during growth. Transmission electron microscopy (TEM) allows us to image the NPs and determine their individual composition. Size-selective methods can be carried out in order to isolate the desired particle sizes, and films will be deposited through wet-chemical means. Mixing large NPs with small NPs can also improve the film-quality as a result of densification at the optimal packing density. The films are characterized by scanning electron microscopy (SEM) as well as other surface characterization techniques. Our first photovoltaic device consisting of soda lime glass/Mo/CZTS/CdS/ZnO has been built from doctor blading of approx. 20 nm Cu2ZnSnS4 NPs in octanethiol, and annealed in Se-atmosphere. It had an efficiency of 1.4%.
Authors : A.A. Vozny, V.V. Kosyak, A.S. Opanasyuyk
Affiliations : Sumy State University, 2, Rymsky Korsakov Str., 40007 Sumy, Ukraine
Resume : The SnS compound semiconductor with the band gap of 1.2 eV and is considered as a promising material for absorber layers in thin films solar cells. It is known that variation of Sn/S ratio in SnS material lead to the formation of number of the crystal phases such as SnS2 and Sn2S3 with the different band gaps. Also these materials may have p or n- type of conductivity. The aim of this work is to establish the influence of the growth conditions on the formation of the SnS2 and Sn2S3 phases in SnS films obtained by the vacuum closed space sublimation and spray pyrolysis methods. With this propose the in-depth analysis of the samples obtained at different substrate and evaporator temperature were carried out. The structural properties of the samples were studied with the help of SEM, XRD and Raman methods. In order to evaluate the band gap of the material the optical transmission spectra were measured by the UV spectrophotometer. The chemical composition of the films was examined by the EDAX method. As a result the optimal growth conditions of the single phase SnS2 thin films with the high good crystal quality were determined. The analysis of the optical transmission spectra has shown that obtained samples could be used as window layer for p-type base layers in heterojunction solar cells.
Authors : Andrea Cazzaniga, Rebecca Bolt Ettlinger, Andrea Crovetto, Jørgen Schou
Affiliations : DTU Fotonik, Technical University of Denmark, 4000 Roskilde Denmark; DTU Fotonik, Technical University of Denmark, 4000 Roskilde Denmark; DTU Nanotech, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark; DTU Fotonik, Technical University of Denmark, 4000 Roskilde Denmark
Resume : The absorber layer Cu2ZnSnS4 (CZTS) is a promising, earth abundant material for solar cells with a record efficiency of 8.4% obtained with CdS as a buffer layer. The experimental reproducibility of the results is a big issue due to the high volatility of sulfur at high temperatures. Here we investigate the use of an alternative buffer layer of ZnS, such that we can anneal the absorber and the buffer layer at the same time. The decomposition reactions from evaporation are reduced by the buffer layer acting as a cap, and the reproducibility greatly enhanced. This bi-layered structure of CZTS/ZnS is deposited on different substrates to investigate the effect of epitaxy on crystal growth. The substrates compared are amorphous quartz, Soda-Lime Glass (SLG) and Mo-coated SLG. The films are investigated with high resolution X-ray diffraction, Raman spectroscopy and optical measurements. Very clear differences are seen in the diffraction patterns of films deposited on different substrates: secondary phases appear on films grown on amorphous substrates while films grown on Mo/SLG meet state of the art requirements. The films are produced under high vacuum (p < 10-6 mbar) with Pulsed Laser Deposition at a substrate temperature of 300 C and subsequently annealed in a sulfurized atmosphere at 550 C. The thickness of the Cu2ZnSnS4 layer is 400 500 nm, while the ZnS layer is 50 100 nm. The I-V curve of the full solar cell made on Mo-coated SLG is also presented.
Authors : K. Timmo*, M. Altosaar, K. Kaarna, J. Raudoja, V. Mikli, M. Kauk-Kuusik
Affiliations : Department of Materials Science, Tallinn University of Technology Ehitajate tee 5, 19086 Tallinn, Estonia
Resume : Cu2ZnSnSe4 (CZTSe) monograin powders have been used as absorber materials in monograin layer solar cells. In laboratory scale, the practicable sizes of powder grains are in the range of 38-112 µm. Therefore, better knowledge of Cu2ZnSnSe4 monograin powder crystals growth parameters is needed to facilitate the higher production of material yield in this range. The objective of this research was to study the growth mechanism and parameters of Cu2ZnSnSe4 crystals in molten potassium iodide (KI). CZTSe powders were synthesized starting from binary compounds CuSe, ZnSe and SnSe in molten KI at different temperatures (700-760°C) for different time periods in sealed quartz ampoules. Scanning electron microscopy images revealed that longer annealing at higher temperatures caused numerous quantities of sintered grains. Sieving analysis method was used for the determination of particle size distribution and average grain size (dm) of grown powders. The average crystal size (dm) was found from the plots of particle size distribution at constant temperatures and for constant times for every individual powder. The values of dm were used to describe the growth process. The activation energy of linear growth of crystals was found as Ed = 0.59 (±0.13) eV. The determined geometric factor values (n = ~ 4) indicate that two mechanisms - mass diffusion through the liquid phase and sintering of formed grains by material surface diffusion were occuring in the growth process of monograin powder crystals.
Authors : Andrea Crovetto(1), Rebecca B. Ettlinger(2), Jørgen Schou(2), Ole Hansen(1,3)
Affiliations : (1)DTU Nanotech, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark; (2) DTU Fotonik, Technical University of Denmark, DK-4000 Roskilde, Denmark; (3) CINF, Center for Individual Nanoparticle Functionality, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
Resume : The ternary chalcogenide compound Cu2SnS3 (CTS) is of interest for thin film photovoltaic applications. The tetragonal phase of CTS typically exhibits a direct band gap of around 1.35 eV and a high absorption coefficient. Hence it can be potentially employed as a photovoltaic absorber using the same device structure as in Cu(In,Ga)Se2 solar cells. On the other hand, the cubic phase of CTS has a typical bandgap of 0.9 eV and good lattice matching to Cu2ZnSnS4 (CZTS). Therefore it may find an application as a top absorber in a CTS/CZTS tandem cell. In this work, CTS films are grown by pulsed laser deposition (PLD) on fused silica glass and Mo-coated soda lime glass substrates. Cubic and tetragonal phases are obtained by changing deposition parameters (temperature and laser fluence) and post-annealing conditions (annealing temperature and time). Dielectric functions and other optical properties of the resulting CTS films are extracted by spectroscopic ellipsometry. The differences in band gap, absorption coefficient and critical points in the dielectric functions are related to structural, compositional and morphological differences in the CTS films. The validity of the optical models used to derive dielectric functions from ellipsometry is discussed in relation to results from direct measurement methods such as optical transmission, Scanning Electron Microscopy (SEM) stylus profiling and atomic force microscopy (AFM).
Authors : Solange Temgoua, Romain Bodeux, Stephan Borensztajn, Negar Naghavi
Affiliations : IRDEP (Institute of Research and Development on PV Energy 6 quai watier, 78401 Chatou, France
Resume : In this work, we are motivated to study the effect of both CZTS precursor thickness and annealing in a sulfur and selenium containing atmosphere on the efficiency of CZTSSe solar cells. For this purpose Cu, ZnS, and SnS targets are co-sputtered on glass/Mo substrate and then annealed under Se and a mixture of S and Se atmospheres. The composition of precursors was Cu/(Zn+Sn) ≈ 0.8 and Zn/Sn ≈ 1.2, with thicknesses varying from 0.8 to 1.7 µm. The materials characterization included GIXRD, Raman spectroscopy, as well as cross-sectional SEM/EDX analysis. The results obtained show that for samples annealed under Se, Zn(S,Se) secondary phases are distributed in the bulk, whereas they tend to segregate near the back contact of the absorbers when S is added in the annealing atmosphere. Devices with CZTSSe absorbers annealed under Se indicate an increase of the absorption, visible on EQE measurements, with the thickness. However inhomogeneous solar cells performances are revealed by a mapping of the electrical parameters. The S+Se annealing treatment enhances the open circuit voltage, and improves the homogeneity of the electrical properties. Furthermore, the absorption in this case is affected by the S addition, because the short circuit current remains constant whatever the thickness. The fill factor loss is proportional to the thickness and also to the CZTSSe/Mo interface quality. The best devices results 7.1% efficiency were obtained for the thinnest precursors. These observations will be discussed and a correlation between the absorber properties and solar cells will be presented
Authors : Valentini Matteo, Malerba Claudia, Mittiga Alberto
Affiliations : Sapienza University of Rome, Department of Physics, p.le A. Moro 5, 00185 Roma, Italy; University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, 38123 Trento, Italy; ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma, Italy
Resume : CZTS thin films were grown by two-steps processes, which consist in the precursor deposition followed by a thermal treatment. The precursors have been deposited by two different co-sputtering processes: the first one involves three binary sulfides sources (CuS, ZnS, SnS) giving films with a homogeneous elements distribution and nearly stoichiometric sulfur content. Nevertheless binary sulfides targets are expensive, fragile and they dont maintain their composition unchanged on a long time scale, giving rise to reproducibility problems that limit the conversion efficiency improvement of the devices. The alternative process is based on co-sputtering from ZnS, Cu and Sn targets. Even if the employment of metal targets doesnt provide the stoichiometric sulfur content, it still ensures a homogeneous elements distribution in the precursor and a better process reproducibility. A first comparison of these two different growth processes is made looking at the final CZTS films morphology, microstructure and optical properties. The optoelectronic properties of CZTS as absorber layer are also evaluated by analyzing the J-V characteristics and the External Quantum Efficiency of the final PV devices.
Authors : Claudia Malerba (1), Matteo Valentini (2), Marco Fortunato (3), Alberto Mittiga (3)
Affiliations : 1. University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, 38123 Trento, Italy; 2. Sapienza University of Rome, Department of Physics, p.le A. Moro 5, 00185 Roma, Italy; 3. ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma, Italy
Resume : Despite the rapid improvement of CZTS-based solar cells, the device performances remain still far from their theoretical limit of about 30%. The most important limiting factor has been identified with the low value of the open-circuit voltage. Several factors can be responsible for the low Voc of the actual Cu2ZnSnS4 (CZTS) devices, such as a low absorber layer quality and/or a non-optimal behaviour of the front interface and back contact. In this work we try to evaluate the importance of these different factors performing a detailed analysis of the results of several common characterization techniques applied to CZTS solar cells. Our CZTS solar cells have the usual structure: Mo/CZTS/CdS/iZnO/Al:ZnO, where the CZTS absorber is grown by sulphurization of CuS, SnS and ZnS co-sputtered precursors. The devices were characterized by admittance spectroscopy, by measuring the J-V curves (under both AM1.5G illumination and dark conditions) as a function of temperature and by EQE measurements at room temperature. The typical methods used in the literature were applied to analyze these data, but the simple device models used to derive the approximated analytical expressions are unable to explain some important features of our results. Therefore, the fitting procedure was improved using the numerical simulation programs AFORS-het and SCAPS. This allows a more realistic modeling of the influence of interfacial and bulk defects and of high-injection effects.
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Authors : Moreno de Respinis*, Khurram S. Joya**, Huub J. M. De Groot**, Francis DSouza, Wilson A. Smith*, Roel van de Krol,**** Bernard Dam*
Affiliations : * Delft University of Technology, Faculty of Applied Sciences, Department of Chemical Engineering, Materials for Energy Conversion and Storage, 2600GA Delft, The Netherlands; **Leiden Institute of Chemistry, Leiden University Einsteinweg 55, 2300 RA, Leiden, The Netherlands; ***Department of Chemistry, University of North Texas, 1155 Union Circle, Denton, TX 76203, USA; ****Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
Resume : We demonstrate here for the first time the photoelectrochemical properties of a BiVO4 photoanode in conjunction with a molecular catalyst. When the Ru-based molecular catalyst (RuCat) is coupled to a BiVO4 light-absorber the performance of this photoanode improves particularly in the low-bias region (<1.0V vs. RHE). The RuCat-BiVO4 photoanode shows a higher photocurrent than CoPi-BiVO4 under front illumination, and a 0.1 V more cathodic onset potential. The former can be partly explained by the low light absorption of the RuCat (<5% light absorption in the UV-vis-NIR range). For the latter, we propose that the linkers in the RuCat reduce the surface recombination in BiVO4 to a greater extent than CoPi. Finally, we observe that the fill factor of the RuCat-BiVO4 JV characteristic improves after the stability test. The results presented herein not only show the feasibility and potential of the solid state/molecular heterojunctions, they also represent a proof of principle to improve conventional all-solid state systems such as CoPi-BiVO4.
Perovskites I : Jin Suntivich
Authors : Robert G. Palgrave, Hugo Bronstein
Affiliations : University College London
Resume : The field of hybrid perovskite solar cells has rapidly advanced but to date it has proven difficult to alter the composition of the archetype material, CH3NH3PbI3 whilst retaining high efficiency. Only a very narrow range of A and B site cations result in a perovskite structure, meaning the pressing issues of lead toxicity and moisture sensitivity are very difficult to address by modifying the composition of the material. An alternative is to move away from the perovskite structure, which opens up a large composition and structure space for exploration in order to find effective solar absorber materials. However, the loss of the 3D connectivity of the perovskite structure and resultant large diffusion lengths must be mitigated. Here we present a number of approaches to design and synthesis of new hybrid organic-inorganic solar absorber materials, encompassing lead free materials and layered materials.
Emerging Chalcogenides : Talia Gershon
Authors : Wolfram Jaegermann
Affiliations : TU Darmstadt, Surface Science Division, Materials Science Jovanka-Bontschits-Str. 2, D-64287 Darmstadt, Germany
Resume : From a theoretical point of view we propose p-i-n structures of highly absorbing compound semiconductors promising device structures for thin film solar cells. Results obtained so far with single absorber layers show that efficient photoelectrochemical cells for the production of H2 consist of buried tandem structures modified with an electronically and chemically adjusted passivation layer and electronically aligned co-catalysts. As a consequence both research directions follow the same directions and need to find suitable materials with absorber layer bandgaps of about 1.2 and about 2 eV bandgaps. Recently we have started to study investigate novel absorber layers as e. g. SnS and the already previously studied Cu2S for their possible application in p-i-n structures. Our results on the preparation and characterisation of thin films and first simple devices of the materials will be presented and discussed. As the two probably most challenging duties we have identified the control of nucleation and growth also at low substrate temperatures as well as the engineering of interface properties. In very many cases the bulk properties of the deposited materials are not of reasonable quality when the material is deposited at lower sample temperature but the morphology of the films shows the preferential morphology as flat films without detrimental pin-holes. For improving the electronic quality of the film annealing steps art elevated temperatures are applied which often leads to a detrimental restructuring of the crystallites in the film forming larger grains and pin-holes or grain boundary shunts and therefore deteriorates the conversion efficiencies. In addition, in most cases the dissimilar contact materials are not appropriate adjusted electronically which lead to interface defects and only low built-in potentials due to the Fermi level pinning. For the electronic passivation of surface/interface states specifically engineered interfaces are needed which for most contacts will only be possible with the formation of non-abrupt heterointerfaces.
Authors : Ruo Xi Yang, Keith T. Butler, and Aron Walsh
Affiliations : Department of Chemistry, University of Bath, UK
Resume : Antimony sulfide has been studied for decades as an attractive material for photovoltaic devices owning to its narrow band-gap (1.7 eV), high optical absorption coefficient, and environmentally-friendly characteristics. Due to the lone pair of Sb(III), and its asymmetric local coordination environment ranging from 3 to 6, it is able to form various complex structures. Multi-component antinomy sulfides, made with additional metals and/or organic ions, can adopt chain, sheet and three-dimensional network structures. Hybrid organic-inorganic solar cells are currently the subject of intense interest for their attractive properties and high-performance, including the ability to tune both structural and electronic properties. Novel materials, beyond the hybrid perovskite CH3NH3PbI3, can be designed and synthesized to achieve favorable properties. The crystal structure of (CH3NH3)2Sb8S13 is formed of double chains of corner-linked [SbS3]3− pyramids and methylammonium cations located in the ring apertures created within the double chains. The double chains are interlocked by the Sb?S interactions, which form a sheet perpendicularly, hence providing interesting prospect as a pseudo two-dimensional semiconductor. A full description of these crystals has been provided using density functional theory (DFT), including structural, elastic and electronic properties. Based on a thorough understanding of these multi-component antimony sulfides, further screening of other hybrid anti- mony sulfides will be carried out to achieve a full range of optimal properties for application in thin-film solar cells.
Authors : Gustavo Baldissera, Clas Persson
Affiliations : Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden; Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Department of Physics, University of Oslo, P.O. Box 1048 Blindern, NO-0316 Oslo, Norway
Resume : Oxygen doped ZnTe has experimentally been proven experimentally to exhibit beneficial in-gap states for intermediate band solar cells . In this work we further investigate doped ZnTe in order to tune in the in-gap states for optimized solar cell performance, as well to effectuate necessary band filling of the these energy states. Our theoretical analyses are based on the density functional theory (DFT), and we verify the energy gaps and in-gap energy levels and energy-band distribution by means of the GW post-DFT approach . We vary the doping elements as well as the doping concentrations. We find that we can tune the energy levels that correspond to the energy gaps of roughly 0.7, 1.3, and 2.0 eV with partially filled bands for an efficient intermediate band solar cells performance. We analyze and discuss the results in terms on the local density-of-state and the optical absorption coefficient. References:  T. Tanaka, et al., App. Phys. Lett. 100, 011905 (2012);  M. Dou, et al., Int. J. Hydrogen Energy 38, 16727 (2013).
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Perovskites II : Hugh Hillhouse
Authors : Feliciano Giustino
Affiliations : Department of Materials, University of Oxford
Resume : Within less than five years since the initial discovery the power conversion efficiency of solar cells based on hybrid organic-inorganic perovskites increased at an unprecedented rate, up to the current record of 20%. The impressive pace of experimental R&D is gradually reshaping the role and mission of computational modelling for solar energy research, by moving the focus from fundamental studies to materials discovery and design. In this context I will discuss some of our recent work aimed at designing new perovskite absorbers via ab initio computational modelling. Our starting point was the following question: can we design metal-halide perovskites with a given optical gap? In order to answer this question we used a hybrid rational design approach which combines extensive first-principles calculations with a simple mathematical model of the metal-halide network. Using this approach we found that the band gap correlates with the largest metal-halide-metal bond angle. With this information at hand we performed first-principles calculations on a series of hypothetical lead-halide perovskites, and established that the bond angles can be controlled via the size of the organic cations. This study led us to identify several new molecular cations which would allow tuning the gap from the infrared to the visible. One of our proposed compounds was successfully synthesized by our experimental collaborators at Oxford [M. R. Filip et al, Nat. Commun. 5, 5757 (2014)].
Authors : Rongzhen Chen, Clas Persson
Affiliations : Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, SE100 44, Sweden; Department of Physics, University of Oslo, P.Box 1048 Blindern, NO0316 Oslo, Norway
Resume : The chalcopyrite Cu(In,Ga)Se2 alloy is today a rather well-established compound for thin film solar cells. Emerging Cu-based materials are explored to benefit from the energetically high-lying Cu d-state in combination with low effective mass of the minority carriers. In the present study, we explore the details in the optical properties of emerging Cu-based compounds, like for instance Cu2ZnSn(S,Se)4, Cu(Sb,Bi)(S,Se)2, and Cu3(Sb,Bi)(S,Se)3, employing a GW approach beyond the density functional theory. We calculate the electronic structure and we analyze the optical properties in terms of the absorption coefficients. By modeling the quantum efficiency of the compounds, we further discuss the optical response. The results help to understand fundamental physics of the emerging Cu-based materials in order to design and optimize solar cell devices. Refs.: R. Chen and C. Persson, J. Appl. Phys. 112, 103708 (2012); ibid, Thin Solid Films 519, 7503 (2011); M. Kumar and C. Persson, Appl. Phys. Lett. 102, 062109 (2013); ibid Energy Procedia 44, 176 (2014); S.G. Choi, et al., Appl. Phys. Lett. 101, 261903 (2012).
Oxides II : Brian Seger
Authors : Kevin Sivula
Affiliations : Laboratory for Molecular Engineering of Optoelectronic Nanomaterials, Institute of Chemical Science and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
Resume : Delafossite CuFeO2 is a promising material for solar hydrogen production given its favorable light absorption, proven stability, and the availability of its components, but its application is limited by poor photogenerated charge transport and transfer. Here strategies to improve the performance of CuFeO2 electrodes are demonstrated. Optimizing the delafossite layer thickness, using a simple sol-gel deposition technique, and increasing the majority carrier concentration (via the thermal intercalation of oxygen), give insights into the recombination and extraction of photogenerated charges and enables performance improvement. In addition, a porous host scaffold of CuAlO2 is employed to improve charge carrier collection. In O2 saturated electrolyte (sacrificial) photocurrents (1 sun illumination) up to 2 mA/cm2 at +0.35 V vs the reversible hydrogen electrode are observed. Despite the favorable onset potential for photocurrent with the bare delafossite electrodes, water photoreduction is limited by poor hydrogen evolution reaction (HER) catalysis. However, through the use of suitable oxide overlayers and HER catalysts, sustained solar H2 production photocurrents of over 1 mA/cm2 are demonstrated. Finally the application of optimized photocathodes in all-oxide tandem cells for overall solar water splitting is examined along with an outlook for future improvement.  M. S. Prévot, N. Guijarro, K. Sivula, ChemSusChem 2015 doi: 10.1002/cssc.201403146
Authors : Crêpellière Jonathan, Bahlawane Naoufal, Lunca Popa Petru, Siebentritt Susanne, Lenoble Damien
Affiliations : Luxembourg Institut of Science and Technology Material Research and Technology department ; Luxembourg Institut of Science and Technology Material Research and Technology department ; Luxembourg Institut of Science and Technology Material Research and Technology department; laboratory for photovoltaics University of Luxembourg ; Luxembourg Institut of Science and Technology Material Research and Technology department
Resume : N-type transparent conducting oxides are currently used in a number of commercial applications, like transparent electrodes for flat panel displays. The fabrication of electronic devices based on transparent p-n junctions still requires the integration of highly-conductive and tuneable p-type transparent semiconductors. Delafossite materials are thought to hold one of the highest potential. In this study, we report for the first time on highly-conductive CuCrO2 thin-films, grown using an optimized direct-liquid injection Metal-Organic Chemical Vapour Deposition. The fabricated films show a high purity with a carbon contamination below 1%. We particularly highlight the influence of the Cu/Cr ratio in the liquid precursor feedstock, the process temperature and oxygen partial pressure on the thin-film's chemical, morphological structural and electrical properties. It is found that the crystalline structure and the films composition are highly dependent on the Cu/Cr precursors ratio. The room-temperature resistivity is 0.1 Ω.cm. Such a low resistivity has never been reported for intrinsic delafossite CuCrO2 thin-films and their transport mechanisms will be discussed. In the visible range, the optical transmittance is measured at 60% for a 100 nm thin-film with an optical band gap around 3eV. These overall properties re-open the use of delafossite materials for the large-scale fabrication of transparent electronic diodes.
Authors : J. Márquez1*, S. Levcenko2, J. Just2, I. Forbes1 and T. Unold2
Affiliations : 1. Northumbria Photovoltaic Application Centre, Faculty of Engineering and Environment, Northumbria University, Newcastle upon Tyne 2.Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
Resume : Cu2SnS3 (CTS) is starting to gain interest in the PV research community as an alternative earth abundant absorber for thin film photovoltaics. We have synthesised a range of Cu-Sn-S thin films by co-evaporation with Cu/Sn ratios between 1.5 and 2.1 at a substrate temperature of 400 ºC. X-ray diffraction patterns of the films show a very intense peak at 28.2º that can be attributed to Cu2SnS3 phase. No other peaks corresponding to this phase can be observed in the patterns, suggesting that the films are highly orientated, which makes difficult to determine the crystal structure. Raman spectroscopy of the absorber layers show bands at 266, 305, 321 361, and 373 cm-1 which has been attributed in literature to cubic Cu2SnS3. Solar cells were processed out of the series of absorbers. The best device was fabricated with the absorber with Cu/Sn ratio approximately of 2 and had an efficiency of 1.8 %, short circuit current of 28 mA*cm-2, open circuit voltage of 147 mV and a fill factor of 42.9 %. From the quantum efficiency measurement we estimate a band gap of 1.05eV for these devices. Capacitance-voltage (C-V) measurements show charge carrier densities between 2 and 5E16 cm-3. The structure and the composition of the CTS absorbers will be analysed as well as its influence on the optoelectronic properties of the solar cells. Additionally, the potential of CTS absorber layers for photovoltaic applications will be discussed.
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