Earth abundant and emerging solar energy conversion materials

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.

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) [3]. 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.

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 [1]. 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. [1] A. Zakuteyev et al., "Defect Tolerant Semiconductors for Solar Energy Conversion," J. Phys. Chem. Lett. 5, 1117 (2014)

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 [1] and optical absorption [2] measurements. X-ray [2,3] and neutron [2] diffraction confirmed the existence of phase transition between 5 and 10 GPa to the high pressure phase I4/mmm [2] or P4/mmm [3]. 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. [1] J. G. Zhao, et al., Phys. Stat. Sol. (b) 243 (2006) 573. [2] A. Wosylus, et al, Z. Anorg. Allg. Chem. 635 (2009) 1959. [3] J.G. Zhao, et al, Solid State Commun. 150 (2010) 1521.

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.

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.

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.

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.

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.

Resume : Thin films of SnS deposited by chemical bath method show different crystal structures [1]: 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. [1] 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

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.

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%.

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.

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.

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.

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).

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

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 don’t 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 doesn’t 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.

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.

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.

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.

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.

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.

Resume : Oxygen doped ZnTe has experimentally been proven experimentally to exhibit beneficial in-gap states for intermediate band solar cells [1]. 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 [2]. 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: [1] T. Tanaka, et al., App. Phys. Lett. 100, 011905 (2012); [2] M. Dou, et al., Int. J. Hydrogen Energy 38, 16727 (2013).

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)].

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).

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.[1] 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. [1] M. S. Prévot, N. Guijarro, K. Sivula, ChemSusChem 2015 doi: 10.1002/cssc.201403146

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 film’s composition are highly dependent on the Cu/Cr precursor’s 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.

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.