Thin Film Photovoltaic Materials Research Articles

Interface-enhanced germanium selenide solar cells comprising an ultrathin and uniform antimony selenide buffer layer via hydrothermal approach

Germanium selenide (GeSe) is a promising thin film photovoltaic absorber material owing to its excellent optoelectronic properties, high stability, and low toxicity. Interface engineering by introducing an ultrathin antimony selenide (Sb2Se3) buffer layer between the CdS electron transport layer and GeSe absorber layer is an effective technique for enhancing solar cell performance. However, the key to this technique is the fabrication of a uniform and smooth Sb2Se3 buffer layer with minimal thickness. In this study, instead of the conventional closed-space sublimation method, a hydrothermal method was employed to slowly grow an Sb2Se3 buffer layer with a thickness of approximately 8 nm. The Se/Na2SO3 molar ratio in the selenium source during the hydrothermal synthesis was adjusted; a molar ratio of 1:2 led to an uneven Sb2Se3 buffer layer thickness, whereas a molar ratio of 1:10 resulted in the formation of Sb2O3 particles on the buffer layer surface. When the Se/Na2SO3 molar ratio was 1:6, a smooth, uniform, dense, and impurity-free Sb2Se3 buffer layer was obtained, achieving the highest efficiency of 3.33 % in a GeSe solar cell. Moreover, GeSe solar cells with hydrothermally grown Sb2Se3 buffer layers demonstrated superior device interface properties and efficiency comparable with those using Sb2Se3 buffer layers deposited via closed-space sublimation. This technique offers an effective method for steadily improving the performance of GeSe solar cells.

Design and simulation of highly efficient CZTS/CZTSSe based thin-film solar cell

Thin-film solar cells are a substitute for more common crystalline silicon solar cells, which consist of thin semiconductor layers. Thin-film materials comprise direct bandgap and can absorb sunlight more efficiently than silicon. In this article, a double-absorber-based thin-film solar cell comprising CZTS/CZTSSe is designed and optimized through numerical simulation. The proposed solar cell structure consists of a transparent window layer made of aluminum-doped zinc oxide, followed by an intrinsic zinc oxide layer, an n-type cadmium sulfide layer, and a p-type combined absorber layer of copper zinc tin sulfide (Cu2ZnSnS4) (CZTS) and copper zinc tin sulfur-selenium alloy (Cu2ZnSn(S,Se4)) (CZTSSe). The structure is further optimized by introducing two interfacial layers between CZTSSe/CZTS and CZTS/CdS. The highest conversion efficiency is achieved by adjusting the thicknesses of the layers, the doping densities in different layers, and the defect densities in the two absorber layers. The optimized model, with a total thickness of 2.01 μm, demonstrates an open-circuit voltage (Voc) of 0.7669 V, a short-circuit current (Jsc) of 48.57740 mA/cm2, a fill factor (FF) of 70.61%, and efficiency (η) of 26.31%. These results suggest that CZTS is a promising candidate for replacing other thin-film photovoltaic materials, such as CdTe. The proposed double-absorber thin-film solar cell optimizes doping concentration, thickness, and defect density to enhance performance metrics and efficiency while utilizing non-toxic materials to promote cost-effective, environmentally friendly energy solutions.

Compact microstructured Cu2ZnSnS4 thin films with enhanced optoelectronic properties via (NH4)2S tunable hybrid colloidal ink coating and transformation

Kesterite Cu2ZnSnS4 (CZTS) is a quaternary semiconductor material composed of earth-abundant elements that is a promising material for sustainable thin film photovoltaics and other optoelectronic applications. In this work, CZTS thin films are deposited using aqueous colloidal suspensions of binary sulfides (Cu and Zn) and thiostannate complexes formed by in situ reaction of SnS2 with (NH4)2S. In a departure from previous works that used only dissolved thiostannate complexes, we utilize a hybrid state of tin sulfide as both nanoparticles and thiostannate complexes to improve thin film quality. This finding was made by adjusting the (NH4)2S concentration in our aqueous inks, which modified the relative abundance of thiostannate complexes vis-à-vis the amount of SnS2 nanoparticles. In the absence of (NH4)2S, the unstable ink yields nonuniform, partially oxidized thin films. Meanwhile, thiostannate complex formation with the use of (NH4)2S (2–10 vol%) can suppress the presence of secondary phases such as CuxS in the absorber layer and protects against oxidation (avoiding undesirable SnO2 formation). Lowering the aqueous (NH4)2S concentration to 2 vol% yielded the most stable ink formulation, featuring a hybrid state of tin sulfide as both colloidal particles and dissolved thiostannate complexes. The hybrid tin sulfide ink produced compact thin films and reduced the level of Cu-Zn antisite defects in the kesterite crystal structure. We attribute the compact microstructure attained with the hybrid tin sulfide formulation to thiostannate complexes acting as linkers among the three binary sulfides (Cu, Zn, and Sn), enabling their orderly packing into an amorphous, quaternary initial state which is then annealed to grow crystalline kesterite CZTS. UV-Vis absorption characterization shows the Tauc bandgap for 2 vol% hybrid ink coated films to be 1.40 eV with reduced absorption tails based on Urbach energy measurements, while photoluminescence studies showed significantly enhanced emission.

Accelerating the Development of Thin Film Photovoltaic Technologies: An Artificial Intelligence Assisted Methodology Using Spectroscopic and Optoelectronic Techniques.

Thin film photovoltaic (TFPV) materials and devices present a high complexity with multiscale, multilayer, and multielement structures and with complex fabrication procedures. To deal with this complexity, the evaluation of their physicochemical properties is critical for generating a model that proposes strategies for their development and optimization. However, this process is time-consuming and requires high expertise. In this context, the adoption of combinatorial analysis (CA) and artificial intelligence (AI) strategies represents a powerful asset for accelerating the development of these complex materials and devices. This work introduces a methodology to facilitate the adoption of AI and CA for the development of TFPV technologies. The methodology covers all the necessary steps from the synthesis of samples for CA to data acquisition, AI-assisted data analysis, and the extraction of relevant information for research acceleration. Each step provides details on the necessary concepts, requirements, and procedures and are illustrated with examples from the literature. Then, the application of the methodology to a complex set of samples from a TFPV production line highlights its ability to rapidly glean significant insights even in intricate scenarios. The proposed methodology can be applied to other types of materials and devices beyond PV and using different characterization techniques.

Performance evaluation of ZnSnN2 solar cells with Si back surface field using SCAPS-1D: A theoretical study

The earth-abundant semiconductor zinc tin nitride (ZnSnN2) has garnered significant attention as a prospective material in photovoltaic and lighting applications, primarily due to its tunable narrow bandgap and high absorption coefficient. This study focuses on a numerical investigation of ZnSnN2 solar cell structures using the SCAPS 1-D software. The objective is to analyze the influence of various physical and geometrical parameters on solar cell performance. These parameters include the thicknesses of the ZnO window layer, CdS buffer layer, ZnSnN2 absorber layer, and Si back surface field layer (BSF), as well as operating temperature, series and shunt resistances (RS and Rsh), absorber layer defect density, interface defects, and the generation-recombination profile of the n-ZnO:Al/n-CdS/p-ZnSnN2/p-Si/Mo structure. We have evaluated the capabilities of this novel material absorber by investigating its performance across a range of thicknesses. We have started with ultrathin absorber thicknesses and gradually increased them to thicker levels to determine the optimal thickness for achieving high efficiency. Under optimal conditions, a thin solar cell with a thickness (wp) of 1 μm achieved an efficiency (η) of 23.9%. In a practical solar cell operating at room temperature, optimal parameters were observed with a thicker absorber layer (wp = 8 μm) and a BSF width of 0.3 μm. The cell exhibited resistances of Rsh = 106 Ω cm2 and Rs = 1 Ω cm2, along with a low defect density (Nt = 1010 cm−3) in the ZnSnN2 semiconductor. These factors combined to yield an impressive efficiency of 29.5%. Numerous studies on emerging ternary nitride semiconductors (Zn-IV-N2) have highlighted ZnSnN2 as a promising material for thin-film photovoltaics. This compound is appealing due to its abundance, non-toxicity, and cost-effectiveness. Unlike conventional solar cells that depend on rare, toxic, and costly elements, these components are still essential for today's solar cell technology.

Inverse design of omnidirectional coherent absorbers for optical power beaming applications.

An efficient photovoltaic power converter is a critical element in laser power beaming systems for maximizing the end-to-end power transfer efficiency while minimizing beam reflections from the receiver for safety considerations. We designed a multilayer absorber that can efficiently trap monochromatic light from broad incident angles. The proposed design is built on the concept of a one-way coherent absorber with inverse-designed aperiodic multilayer front- and back-reflectors that enable maximal optical absorption in a thin-film photovoltaic material for broad angles. We argue that the broad bandwidth is achieved through an optimization search process that automatically engineers the modal content of the cavity to create multiple overlapping resonant modes at the desired angle or frequency range. A realistic design is provided based on GaAs thin films with inverse-designed multilayer binary AlAs/AlGaAs mirrors. The proposed device can pave the way for efficient optical power beaming systems.

Cubic structured silver antimony sulfide-selenide solid solution thin films for sustainable photodetection and photovoltaic application

Exploration of new materials for thin film photovoltaics has grown considerably to eliminate harmful and expensive material components in the large-scale production. In the present work, we report preparation of AgSbS2−xSex thin films by heating Ag2Se-Sb2S3 precursor layers. X-ray diffraction patterns showed that AgSbS2−xSex thin films with varying stoichiometry x (0.58–0.84) crystallized in face centered cubic structure and a compact surface morphology in scanning electron microscopy analysis. Optical bandgap of the thin films decreased from 1.6 to 1.4 eV as selenium content in the films increased. Photovoltaic structures were fabricated using AgSbS2−xSex films of different x values as absorbers and CdS as window layers. The best device based on AgSbS1.28Se0.72 of thickness ∼400 nm yielded a Voc = 525 mV, Jsc = 15 mA/cm2, FF = 0.53 and a conversion efficiency of 4.2%, the highest value ever reported for this material. In addition, CdS/AgSbS1.28Se0.72 heterojunction was also performed as a self-powered photodiode. Further, simulation results using SCAPS-1D predict that photovoltaic performance of CdS/AgSbS2−xSex can be improved by adjusting the Se concentration and the absorber layer thickness in a single p-n junction configuration.

Hyperspectral mapping of nanoscale photophysics and degradation processes in hybrid perovskite at the single grain level.

With solar cells reaching 26.1% certified efficiency, hybrid perovskites are now the most efficient thin film photovoltaic material. Though substantial effort has focussed on synthesis approaches and device architectures to further improve perovskite-based solar cells, more work is needed to correlate physical properties of the underlying film structure with device performance. Here, using cathodoluminescence microscopy coupled with unsupervised machine learning, we quantify how nanoscale heterogeneity globally builds up within a large morphological grain of hybrid perovskite when exposed to extrinsic stimuli such as charge accumulation from electron beams or milder environmental factors like humidity. The converged electron-beam excitation allows us to map PbI2 and the emergence of other intermediate phases with high spatial and energy resolution. In contrast with recent reports of hybrid perovskite cathodoluminescence, we observe no significant change in the PbI2 signatures, even after high-energy electron beam excitation. In fact, we can exploit the stable PbI2 signatures to quantitatively map how hybrid perovskites degrade. Moreover, we show how our methodology allows disentangling of the photophysics associated with photon recycling and band-edge emission with sub-micron resolution using a fundamental understanding of electron interactions in hybrid perovskites.

A study of bandgap-graded CZTGSe kesterite thin films for solar cell applications

The sustainable and harmless Cu2ZnSn(S,Se)4 kesterite materials for thin-film photovoltaics are still lacking efficiency, which may be solved by bandgap grading, herein obtained by two-step annealing in H2Se gas of metallic precursors.

Innovative back-contact for [formula omitted]-based thin film solar cells

Among the most promising material for thin-film photovoltaics, Sb2Se3 (ASe) plays a pivotal role, being constituted of Earth-abundant elements and owing to its optical and electrical properties. Recently, ASe solar cells exhibited relatively good photovoltaic performances, reaching a considerable efficiency close to 9 %. Despite these remarkable results, much is still to be investigated, particularly regarding the active materials, interfaces, and electrical contacts. In this study we propose an alternative back contact for ASe solar cells. It is based on Fe, S and O elements, and was deposited at room temperature by radiofrequency magnetron sputtering. XRD and Raman measurements revealed that it is actually made of two materials: Fe3O4, in orthorhombic and cubic phases and FeS in troilite phase. This material provided an ohmic contact on top of the ASe film; its contact resistivity, extrapolated from current vs voltage characteristics of complete ASe-based solar cells, was estimated to be 0.8Ω·cm2. Monitoring the photovoltaic parameters, we observed a negligible average change after three months.

Understanding Performance Limitation of CuCdSnS as Photoactive Layer: Physics of Defect States and Recombination Mechanisms

There is a plethora of thin-film photovoltaic materials like copper indium gallium sulfur (CIGS), CIGS-Selenium (CIGSSe), Cu2ZnSnS4 (CZTS), <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {Cu}_{{2}}\text {ZnSn} (\text {S}_{x}, \text {Se}_{{1}-{x}})_{{4}}$ </tex-math></inline-formula> (CZTSSe), and so on that are currently under research and are suitable for commercial use. In this study, a Cu2CdSnS4 (CCdTS)-based thin-film solar cell is simulated using Silvaco technology computer-aided design (TCAD) to extract its optical and electrical properties such as efficiency, electric field (EF), short circuit current, quantum efficiency, and so on. The device is also optimized by variations in physical parameters such as thickness, doping concentration, and material defects of the absorber layer. A remarkable efficiency of 21.2% is reached under the condition of defect absence, whereas the efficiency of 7.5% is obtained for the defect present in the absorber layer. Changes in the behavior of the solar cell with material defects following Gaussian- and tail-type distribution are also analyzed and corresponding conclusions are drawn.

P-type conductivity in Sn-doped Sb2Se3

Antimony selenide (Sb2Se3) is a promising absorber material for thin-film photovoltaics. However, certain areas of fundamental understanding of this material remain incomplete and this presents a barrier to further efficiency gains. In particular, recent studies have highlighted the role of majority carrier type and extrinsic doping in drastically changing the performance of high efficiency devices (Hobson et al 2020 Chem. Mater. 32 2621–30). Herein, Sn-doped Sb2Se3 bulk crystals are shown to exhibit p-type conductivity using Hall effect and hot-probe measurements. The measured conductivities are higher than those achieved through native defects alone, but with a carrier density (up to 7.4 × 1014 cm−3) several orders of magnitude smaller than the quantity of Sn included in the source material. Additionally, a combination of ultraviolet, x-ray and hard x-ray photoemission spectroscopies are employed to obtain a non-destructive depth profile of the valence band maximum, confirming p-type conductivity and indicating a majority carrier type inversion layer at the surface. Finally, these results are supported by density functional theory calculations of the defect formation energies in Sn-doped Sb2Se3, showing a possible limit on the carrier concentration achievable with Sn as a dopant. This study sheds light on the effectiveness of Sn as a p-type dopant in Sb2Se3 and highlights avenues for further optimisation of doped Sb2Se3 for solar energy devices.

Unveiling microscopic carrier loss mechanisms in 12% efficient Cu2ZnSnSe4 solar cells

Understanding carrier loss mechanisms at microscopic regions is imperative for the development of high-performance polycrystalline inorganic thin-film solar cells. Despite the progress achieved for kesterite, a promising environmentally benign and earth-abundant thin-film photovoltaic material, the microscopic carrier loss mechanisms and their impact on device performance remain largely unknown. Herein, we unveil these mechanisms in state-of-the-art Cu2ZnSnSe4 (CZTSe) solar cells using a framework that integrates multiple microscopic and macroscopic characterizations with three-dimensional device simulations. The results indicate the CZTSe films have a relatively long intragrain electron lifetime of 10–30 ns and small recombination losses through bandgap and/or electrostatic potential fluctuations. We identify that the effective minority carrier lifetime of CZTSe is dominated by a large grain boundary recombination velocity (~104 cm s−1), which is the major limiting factor of present device performance. These findings and the framework can greatly advance the research of kesterite and other emerging photovoltaic materials.

Crystal Growth Promotion and Defect Passivation by Hydrothermal and Selenized Deposition for Substrate-Structured Antimony Selenosulfide Solar Cells.

Antimony sulfide-selenide (Sb2(S,Se)3) is a promising light-harvesting material for stable and high-efficiency thin-film photovoltaics (PV) because of its excellent light-harvesting capability, abundant elemental storage, and excellent stability. This study aimed to expand the application of Sb2(S,Se)3 solar cells with a substrate structure as a flexible or tandem device. The use of a hydrothermal method accompanied by a postselenization process for the deposition of Sb2(S,Se)3 film based on the solar cell substrate structure was first demonstrated. The mechanism of postselenization treatment on crystal growth promotion of the Sb2(S,Se)3 film and the defect passivation of the Sb2(S,Se)3 solar cell were revealed through different characterization methods. The crystallinity and the carrier transport property of the Sb2(S,Se)3 film improved, and both the interface defect density of the Sb2(S,Se)3/CdS interface and the bulk defect density of the Sb2(S,Se)3 absorber decreased. Through these above-mentioned processes, the transport and collection of electronics can be improved, and the defect recombination loss can be reduced. By using postselenization treatment to optimize the absorber layer, Sb2(S,Se)3 solar cells with the configuration SLG/Mo/Sb2(S,Se)3/CdS/ITO/Ag achieved an efficiency of 4.05%. This work can provide valuable information for the further development and improvement of Sb2(S,Se)3 solar cells.

11.39% efficiency Cu2ZnSn(S,Se)4 solar cells from scrap brass

Kesterite Cu2ZnSn(S,Se)4 is one of the most promising next-generation thin-film photovoltaic materials. Along with the efficiency advancement of kesterite solar cells, a cost-effective fabrication process with low carbon footprint plays an increasingly important role considering the near-future industrialization of CZTSSe solar cells. In this study, we proposed an environmental friendly process to recover copper and zinc from the scrap brass, which can be directly used as the precursor salts to fabricate efficient CZTSSe thin-film solar cells. The best power conversion efficiency (PCE) of the device using the proposed eco-friendly process is 11.39% (certified at the National PV Industry Measurement and Testing Center) for CZTSSe solar cells.

Optical and Electrical Transport Evaluations of n-Type Iron Pyrite Single Crystals.

Iron pyrite [cubic FeS2 (cFeS2)] is considered as an earth-abundant and low-cost thin-film photovoltaic material. However, the conversion efficiency of cFeS2-based solar cells remains below 3%. To elucidate this limitation, we evaluate the optical and electrical characteristics of cFeS2 single crystals that are grown using the flux method, thus providing us an understanding of the electron transport behavior of cFeS2 single crystals. The oxide layer on the surface of cFeS2, which can possibly have an influence on the electrical characteristics of cFeS2, is removed prior to characterization via optical spectroscopy and electrical transport measurement. The optical property of cFeS2 was found to have both indirect and direct transitions. We also observed the presence of a band tail below the conduction band. The obtained electrical transport behavior indicates that cFeS2 bulk exhibits a high defect density and a disordered phase, thus leading to the hopping conduction mechanism. Our results will pave the way for the development of photovoltaic applications with iron pyrite.

Parametric Open Source Cold-Frame Agrivoltaic Systems

There is an intense need to optimize agrivoltaic systems. This article describes the invention of a new testing system: the parametric open source cold-frame agrivoltaic system (POSCAS). POSCAS is an adapted gardening cold-frame used in cold climates as it acts as a small greenhouse for agricultural production. POSCAS is designed to test partially transparent solar photovoltaic (PV) modules targeting the agrivoltaic market. It can both function as a traditional cold frame, but it can also be automated to function as a full-service greenhouse. The integrated PV module roof can be used to power the controls or it can be attached to a microinverter to produce power. POSCAS can be placed in an experimental array for testing agricultural and power production. It can be easily adapted for any type of partially transparent PV module. An array of POSCAS systems allows for the testing of agrivoltaic impacts from the percent transparency of the modules by varying the thickness of a thin film PV material or the density of silicon-based cells, and various forms of optical enhancement, anti-reflection coatings and solar light spectral shifting materials in the back sheet. All agrivoltaic variables can be customized to identify ideal PV designs for a given agricultural crop.

CZTS nanoparticles as an effective hole-transport layer for Sb2Se3 thin-film solar cells

Sb2Se3 is an emerging and promising thin-film photovoltaic absorber material. However, for commonly CdS/Sb2Se3 heterojunction device structure, low carrier concentration and back contact barrier will prevent the further improvement of device efficiency. Here we have successfully applied Cu2ZnSnS4 (CZTS) nanoparticles (NPs) as a hole-transport layer (HTL) to construct a low-cost n-i-p Sb2Se3 solar cell with FTO/CdS/Sb2Se3/CZTS/Carbon configuration. We found that CZTS HTL can effectively suppress the back interface carrier recombination and improve carrier collection efficiency in the Sb2Se3 solar cells. The average Jsc, FF, and efficiency of Sb2Se3 solar cells with an optimum CZTS HTL are increased by 10.64%, 7.91%, and 17.14%, respectively, compared with Sb2Se3 solar cells without a HTL. Consequently, with CZTS NPs as HTL and carbon as electrode, the n-i-p Sb2Se3 solar cells have achieved a champion efficiency of 6.06%.

Unsupervised discovery of thin-film photovoltaic materials from unlabeled data

Quaternary chalcogenide semiconductors (I2-II-IV-X4) are key materials for thin-film photovoltaics (PVs) to alleviate the energy crisis. Scaling up of PVs requires the discovery of I2-II-IV-X4 with good photoelectric properties; however, the structure search space is significantly large to explore exhaustively. The scarcity of available data impedes even many machine learning (ML) methods. Here, we employ the unsupervised learning (UL) method to discover I2-II-IV-X4 that alleviates the challenge of data scarcity. We screen all the I2-II-IV-X4 from the periodic table as the initial data and finally select eight candidates through UL. As predicted by ab initio calculations, they exhibit good optical conversion efficiency, strong optical responses, and good thermal stabilities at room temperatures. This typical case demonstrates the potential of UL in material discovery, which overcomes the limitation of data scarcity, and shortens the computational screening cycle of I2-II-IV-X4 by ~12.1 years, providing a research avenue for rapid material discovery.

One-Step Synthesis of SnI2·(DMSO)x Adducts for High-Performance Tin Perovskite Solar Cells.

Contemporary thin-film photovoltaic (PV) materials contain elements that are scarce (CIGS) or regulated (CdTe and lead-based perovskites), a fact that may limit the widespread impact of these emerging PV technologies. Tin halide perovskites utilize materials less stringently regulated than the lead (Pb) employed in mainstream perovskite solar cells; however, even today's best tin-halide perovskite thin films suffer from limited carrier diffusion length and poor film morphology. We devised a synthetic route to enable in situ reaction between metallic Sn and I2 in dimethyl sulfoxide (DMSO), a reaction that generates a highly coordinated SnI2·(DMSO)x adduct that is well-dispersed in the precursor solution. The adduct directs out-of-plane crystal orientation and achieves a more homogeneous structure in polycrystalline perovskite thin films. This approach improves the electron diffusion length of tin-halide perovskite to 290 ± 20 nm compared to 210 ± 20 nm in reference films. We fabricate tin-halide perovskite solar cells with a power conversion efficiency of 14.6% as certified in an independent lab. This represents a ∼20% increase compared to the previous best-performing certified tin-halide perovskite solar cells. The cells outperform prior earth-abundant and heavy-metal-free inorganic-active-layer-based thin-film solar cells such as those based on amorphous silicon, Cu2ZnSn(S/Se)4 , and Sb2(S/Se)3.