Thin film chalcogenide photovoltaic materials

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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