Active Passivation of Anion Vacancies in Antimony Selenide Film for Efficient Solar Cells.

Binary antimony selenide (Sb2Se3) is a promising inorganic light-harvesting material with high stability, non-toxicity and wide light harvesting capability. In this photovoltaic material, it has been recognized that deep energy level defects with large carrier capture cross section, such as VSe (selenium vacancy), lead to serious open-circuit voltage (VOC) deficit and in turn limit the achievable power conversion efficiency (PCE) of Sb2Se3 solar cells. Understanding the nature of deep-level defects and establishing effective method to eliminate the defects are vital to improving VOC. In this study, we propose a novel directed defect passivation strategy to suppress the formation of VSe and maintaining the composition and morphology of Sb2Se3 film. In particularly, through systematic study on the evolution of defect properties, we reveal the pathway of defect passivation reaction. Owing to the inhibition of defect-assisted recombination, the VOC increases, resulting in an improvement of PCE from 7.69% to 8.90%, where is the highest efficiency of Sb2Se3 solar cells prepared by thermal evaporation method with superstrate device configuration. This study proposes a new understanding of the nature of deep-level defects and enlightens the fabrication of high quality Sb2Se3 thin film for solar cell applications. This article is protected by copyright. All rights reserved.

Effect of Energy-driven Molecular Precursor Decomposition on the Crystal Orientation of Antimony Selenide Film and Solar Cell Efficiency.

Antimony selenide (Sb2Se3) consists of 1D (Sb4Se6)n ribbons, along which the carriers exhibit high transport efficiency. By adjusting the deposition parameters of vacuum-deposited methods, such as evaporation temperature, chamber pressure, and vapor concentration, it is possible to grow the (Sb4Se6)n ribbons vertically or highly inclined towards the substrate, resulting in films with [hk1] orientation. However, the specific mechanisms by which these deposition parameters affect the orientation of thin films require a deeper understanding. Herein, a molecular beam epitaxy technique is developed for the preparation of highly [hk1]-oriented Sb2Se3 films, and the effect of evaporation parameters on the film orientation is investigated. It is found that the evaporation temperature can affect the decomposition degree of Sb2Se3, which in turn determines the vapor composition and film orientation. Additionally, the decomposition of Sb2Se3 related to evaporation temperature leads to significant changes in the elemental composition of the film, thereby passivating deep-level defects under Se-rich conditions. Consequently, the Sb2Se3 films with highly [hk1] orientation achieve a power conversion efficiency of 8.42% for the solar cells. This study provides new insights into the control of orientation in antimony-based chalcogenide films and points out new directions for improving the photovoltaic performance of solar cells.

Critical Review on Crystal Orientation Engineering of Antimony Chalcogenide Thin Film for Solar Cell Applications.

The emerging antimony chalcogenide (Sb2 (Sx Se1-x )3 , 0 ≤ x ≤ 1) semiconductors are featured as quasi-1D structures comprising (Sb4 S(e)6 )n ribbons, this structural characteristic generates facet-dependent properties such as directional charge transfer and trap states. In terms of carrier transport, proper control over the crystal nucleation and growth conditions can promote preferentially oriented growth of favorable crystal planes, thus enabling efficient electron transport along (Sb4 S(e)6 )n ribbons. Furthermore, an in-depth understanding of the origin and impact of the crystal orientation of Sb2 (Sx Se1-x )3 films on the performance of corresponding photovoltaic devices is expected to lead to a breakthrough in power conversion efficiency. In fact, there are many studies on the orientation control of Sb2 (Sx Se1-x )3 colloidal nanomaterials. However, the synthesis of Sb2 (Sx Se1-x )3 thin films with controlled facets has recently been a focus in optoelectronic device applications. This work summarizes methodologies that are applied in the fabrication of preferentially oriented Sb2 (Sx Se1-x )3 films, including treatment strategies developed for crystal orientation engineering in each process. The mechanisms in the orientation control are thoroughly analyzed. An outlook on perspectives for the future development of Sb2 (Sx Se1-x )3 solar cells based on recent research and issues on orientation control is finally provided.

KCl Treatment of CdS Electron-Transporting Layer for Improved Performance of Sb2(S,Se)3 Solar Cells.

Antimony sulfoselenide (Sb2(S,Se)3) is a promising light absorption material because of its high photoabsorption coefficient, appropriate band gap, superior stability, and abundant elemental storage. As an emerging solar material, hydrothermal deposition of Sb2(S,Se)3 solar cells has enabled a 10% efficiency threshold, where cadmium sulfide (CdS) is applied as an electron transport layer (ETL). The high-efficiency Sb2(S,Se)3 solar cells largely employ CdS as the ETL. In terms of efficiency improvement, there are two questions regarding the CdS substrate: (1) the high roughness of CdS grown on F-doped tin oxide glass which increases the roughness of the absorber layer and (2) the low conductivity of CdS films because of low purity of CdS film grown by chemical bath deposition. In this study, we demonstrate an effective potassium chloride (KCl) post-treatment to modify the CdS ETL for improving the Sb2(S,Se)3 solar cell efficiency. We found that KCl plays dual roles that reduce roughness and enhance conductivity of the CdS films, thus acquiring a maximum efficiency of 9.98%, which is 9.2% higher than the control device. This study provides a new method for the surface engineering of CdS layer to improve the morphological and electrical properties, which is significant for improving the performance of CdS-based thin-film solar cells.

Thermally Driven Point Defect Transformation in Antimony Selenosulfide Photovoltaic Materials.

Thermal treatment of inorganic thin films is a general and necessary step to facilitate crystallization and, in particular, to regulate the formation of point defects. Understanding the dependence of the defect formation mechanism on the annealing process is a critical challenge in terms of designing material synthesis approaches for obtaining desired optoelectronic properties. Herein, a mechanistic understanding of the evolution of defects in emerging Sb2 (S,Se)3 solar cell films is presented. A top-efficiency Sb2 (S,Se)3 solar-cell film is adopted in this study to consolidate this investigation. This study reveals that, under hydrothermal conditions, the as-deposited Sb2 (S,Se)3 film generates defects with a high formation energy, demonstrating kinetically favorable defect formation characteristics. Annealing at elevated temperatures leads to a two-step defect transformation process: 1) formation of sulfur and selenium vacancy defects, followed by 2) migration of antimony ions to fill the vacancy defects. This process finally results in the generation of cation-anion antisite defects, which exhibit low formation energy, suggesting a thermodynamically favorable defect formation feature. This study establishes a new strategy for the fundamental investigation of the evolution of deep-level defects in metal chalcogenide films and provides guidance for designing material synthesis strategies in terms of defect control.

Efficient In Situ Sulfuration Process in Hydrothermally Deposited Sb2S3 Absorber Layers.

Sulfuration plays a decisive role in enhancing crystal growth and passivate defects in the fabrication of high-efficiency metal-sulfide solar cells. However, the traditional sulfuration process always suffers from high-price professional equipment, tedious processes, low activity of S, or high toxicity of H2S. Here, we develop a desired in situ sulfuration by introducing tartaric acid additive into the hydrothermal deposition process of Sb2S3. Tartaric acid, sodium thiosulfate, and potassium antimony tartaric can form Sb2Sx-contained (x > 3) as-prepared films. Encouragingly, the annealing becomes an inspiring in situ sulfuration process, which can obtain a more compact absorber layer. In addition, the crystallinity and defect property of the Sb2S3 film are also improved significantly. Finally, we achieve a high-performance Sb2S3 solar cell with a power conversion efficiency of 6.31%, which shows an encouraging enhancement of ∼15% compared with the traditional hydrothermal process. This study provides an innovative way to prepare high-efficiency Sb2S3 solar cells and provides a desirable guide to realize the in situ sulfuration process.

Post-Treatment of TiO2 Film Enables High-Quality Sb2Se3 Film Deposition for Solar Cell Applications.

The TiO2 thin film is considered as a promising wide band gap electron-transporting material. However, due to the strong Ti-O bond, it displays an inert surface characteristic causing difficulty in the adsorption and deposition of metal chalcogenide films such as Sb2Se3. In this study, a simple CdCl2 post-treatment is conducted to functionalize the TiO2 thin film, enabling the induction of nucleation sites and growth of high-quality Sb2Se3. The interfacial treatment optimizes the conduction band offset of TiO2/Sb2Se3 and leads to an essentially improved TiO2/Sb2Se3 heterojunction. With this convenient interface functionalization, the power conversion efficiency of the Sb2Se3 solar cell is remarkably improved from 2.02 to 6.06%. This study opens up a new avenue for the application of TiO2 as a wide band gap electron-transporting material in antimony chalcogenide solar cells.

Dopant-free hole-transporting materials for stable Sb2(S,Se)3 solar cells.

Herein, we demonstrate that a thiophene-modified quinoxaline core small molecule can be applied in Sb2(S,Se)3 solar cells. We reveal that the interaction between thiophene and Sb2(S,Se)3 through the Sb-S bond essentially improves the interfacial hole-extraction ability. This study provides a cost-effective dopant-free hole-transporting material for inorganic thin film solar cell applications with excellent stability.

Distinctive Deep-Level Defects in Non-Stoichiometric Sb2 Se3 Photovoltaic Materials.

Characterizing defect levels and identifying the compositional elements in semiconducting materials are important research subject for understanding the mechanism of photogenerated carrier recombination and reducing energy loss during solar energy conversion. Here it shows that deep-level defect in antimony triselenide (Sb2 Se3 ) is sensitively dependent on the stoichiometry. For the first time it experimentally observes the formation of amphoteric SbSe defect in Sb-rich Sb2 Se3 . This amphoteric defect possesses equivalent capability of trapping electron and hole, which plays critical role in charge recombination and device performance. In comparative investigation, it also uncovers the reason why Se-rich Sb2 Se3 is able to deliver high device performance from the defect formation perspective. This study demonstrates the crucial defect types in Sb2 Se3 and provides a guidance toward the fabrication of efficient Sb2 Se3 photovoltaic device and relevant optoelectronic devices.

Direct Hydrothermal Deposition of Antimony Triselenide Films for Efficient Planar Heterojunction Solar Cells.

Antimony selenide (Sb2Se3) has attracted increasing attention in photovoltaic applications due to its unique quasi-one-dimensional crystal structure, suitable optical band gap with a high extinction coefficient, and excellent stability. As a promising light-harvesting material, the available synthetic methods for the fabrication of a high-quality film have been quite limited and seriously impeded both the fundamental study and the efficiency improvement. Here, we developed a facile and low-cost hydrothermal method for in situ deposition of Sb2Se3 films for solar cell applications. In this process, we apply KSbC4H4O7 and Na2SeSO3 as the antimony and selenium sources, respectively, in which thiourea (TU) serves as an additive to suppress the formation of Sb2O3 impurities. As a result, improved phase purity and enhanced crystallinity of the Sb2Se3 film are thus obtained, along with decreased trap states. Finally, the planar heterojunction Sb2Se3 solar cell delivered a power conversion efficiency of 7.9%, which is thus far the highest reported efficiency among solution-processed Sb2Se3 solar cells. This simple procedure and efficiency achievement demonstrate the great potential of the hydrothermal deposition process for the fabrication of high-efficiency Sb2Se3 solar cells.

Sequential Coevaporation and Deposition of Antimony Selenosulfide Thin Film for Efficient Solar Cells.

Antimony selenosulfide (Sb2 (S,Se)3 ) is an emerging low-cost, nontoxic solar material with suitable bandgap and high absorption coefficient. Developing effective methods for fabricating high-quality films would benefit the device efficiency improvement and deepen the fundamental understanding on the optoelectronic properties. Herein, equipment is developed that allows online introduction of precursor vapor during the reaction process, enabling sequential coevaporation of Sb2 Se3 and S powders for the deposition of Sb2 (S,Se)3 thin films. With this unique ability, it is revealed that the deposition sequence manipulates both the interfacial properties and optoelectronic properties of the absorber film. A power conversion efficiency of 8.0% is achieved, which is the largest value in vapor-deposition-derived Sb2 (S,Se)3 solar cells. The research demonstrates that multi-source sequential coevaporation is an efficient technique to fabricate high-efficiency Sb2 (S,Se)3 solar cells.

Probing the trap states in N-i-P Sb2(S,Se)3 solar cells by deep-level transient spectroscopy.

In this study, we provide fundamental understanding on defect properties of the Sb2(S,Se)3 absorber film and the impact on transmission of photo-excited carriers in N-i-P architecture solar cells by both deep level transient spectroscopy (DLTS) and optical deep level transient spectroscopy (ODLTS) characterizations. Through conductance-voltage and temperature-dependent current-voltage characterization under a dark condition, we find that the Sb2(S,Se)3 solar cell demonstrates good rectification and high temperature tolerance. The DLTS results indicates that there are two types of deep level hole traps H1 and H2 with active energy of 0.52 eV and 0.76 eV in the Sb2(S,Se)3 film, and this defect property is further verified by ODLTS. The two traps hinder the transmission of minority carrier (hole) and pinning the Fermi level, which plays a negative role in the improvement of open-circuit voltage for Sb2(S,Se)3 solar cells. This research suggests a critical direction toward the efficiency improvement of Sb2(S,Se)3 solar cells.

In-Situ Surface Reconstruction of InN Nanosheets for Efficient CO2 Electroreduction into Formate.

Probing and understanding the intrinsic active sites of electrocatalysts is crucial to unravel the underlying mechanism of CO2 electroreduction and provide a prospective for the rational design of high-performance electrocatalysts. However, their structure-activity relationships are not straightforward because electrocatalysts might reconstruct under realistic working conditions. Herein, we employ in-situ measurements to unveil the intrinsic origin of the InN nanosheets which served as an efficient electrocatalyst for CO2 reduction with a high faradaic efficiency of 95% for carbonaceous product. During the CO2 electroreduction, InN nanosheets reconstructed to form the In-rich surface. Density functional theory calculations revealed that the reconstruction of InN led to the redistribution of surface charge that significantly promoted the adsorption of HCOO* intermediates and thus benefited the formation of formate toward CO2 electroreduction. This work establishes a fundamental understanding on the mechanism associated with self-reconstruction of heterogeneous catalysts toward CO2 electroreduction.

Interfacial Engineering by Indium-Doped CdS for High Efficiency Solution Processed Sb2(S1- xSe x)3 Solar Cells.

Sb2(S1- xSe x)3 alloy material is a kind of encouraging material for realistically apposite solar cell because it benefits from high absorption coefficient, suitable bandgap, superior stability, and plentiful elemental storage. Interfacial engineering is vital for effective charge carrier transport in solar cells, which could upgrade the photoelectric conversion efficiency (PCE). Herein, as an interlayer, indium-doped CdS thin film fabricated by chemical bath deposition is found to remarkably enhance the photovoltaic performance of Sb2(S1- xSe x)3 solar cells. Mechanistic investigations show that the interlayer can both optically and electrically optimize the device quality. With that a PCE of 6.63% is obtained, which is the highest efficiency among the planar heterojunction solar cells and slightly higher than the reported record efficiency of mesoscopic Sb2(S1- xSe x)3-sensitized solar cells. This research provides an efficient interfacial engineering for high performance Sb2(S1- xSe x)3 solar cells.

Composition engineering of Sb2S3 film enabling high performance solar cells.

Sb2S3 is a kind of stable light absorption materials with suitable band gap, promising for practical applications. Here we demonstrate that the engineering on the composition ratio enables significant improvement in the device performance. We found that the co-evaporation of sulfur or antimony with Sb2S3 is able to generate sulfur- or antimony-rich Sb2S3. This composition does not generate essential influence on the crystal structure, optical band and film formability, while the carrier concentration and transport dynamics are considerably changed. The device investigations show that sulfur-rich Sb2S3 film is favorable for efficient energy conversion, while antimony-rich Sb2S3 leads to greatly decreased device performance. With optimizations on the sulfur-rich Sb2S3 films, the final power conversion efficiency reaches 5.8%, which is the highest efficiency in thermal evaporation derived Sb2S3 solar cells.

n-Type Doping of Sb2S3 Light-Harvesting Films Enabling High-Efficiency Planar Heterojunction Solar Cells.

Sb2S3 is a kind of new light-absorbing material possessing high stability in ambient environment, high absorption coefficient in the visible range, and abundant elemental storage. To improve the power conversion efficiency of Sb2S3-based solar cells, here we control the defect in Sb2S3 absorber films. It is found that the increase of sulfur vacancy is able to upgrade photovoltaic properties. With the increase in sulfur vacancy, the carrier concentrations are increased. This n-type doping gives rise to an upshift of the Fermi level of Sb2S3 so that the charge transport from Sb2S3 to the electron selection material becomes dynamically favorable. The introduction of ZnCl2 in film fabrication is also found to regulate the film growth for enhanced crystallinity. Finally, the photovoltaic parameters, short-circuit current density, open-circuit voltage, and the fill factor of the device based on the Sb2S3 film are all considerably enhanced, boosting the final power conversion efficiency from 5.15 to 6.35%. This efficiency is the highest value in planar heterojunction Sb2S3 solar cells and among the top values in all kinds of Sb2S3 solar cells. This research provides a fundamental understanding regarding the properties of Sb2S3 and a convenient approach for enhancing the performance of Sb2S3 solar cells.

V2O5 as Hole Transporting Material for Efficient All Inorganic Sb2S3 Solar Cells.

This research demonstrates that V2O5 is able to serve as hole transporting material to substitute organic transporting materials for Sb2S3 solar cells, offering all inorganic solar cells. The V2O5 thin film is prepared by thermal decomposition of spin-coated vanadium(V) triisopropoxide oxide solution. Mechanistic investigation shows that heat treatment of V2O5 layer has crucial influence on the power conversion efficiency of device. Low temperature annealing is unable to remove the organic molecules that increases the charge transfer resistance, while high temperature treatment leads to the increase of work function of V2O5 that blocks hole transporting from Sb2S3 to V2O5. Electrochemical and compositional characterizations show that the interfacial contact of V2O5/Sb2S3 can be essentially improved with appropriate annealing. The optimized power conversion efficiency of device based on Sb2S3/V2O5 heterojunction reaches 4.8%, which is the highest power conversion efficiency in full inorganic Sb2S3-based solar cells with planar heterojunction solar cells. Furthermore, the employment of V2O5 as hole transporting material leads to significant improvement in moisture stability compared with the device based organic hole transporting material. Our research provides a material choice for the development of full inorganic solar cells based on Sb2S3, Sb2(S,Se)3, and Sb2Se3.

Aqueous-Solution-Based Approach Towards Carbon-Free Sb2 S3 Films for High Efficiency Solar Cells.

Sb2 S3 is a new kind of photovoltaic material that is promising for practical application in solar cells owing to its suitable bandgap, earth-abundant elements, and excellent stability. Here, we report on an aqueous-solution-based approach for the synthesis of Sb2 S3 films from easily accessible Sb2 O3 as antimony source. In this reaction, 3-mercaptopropionic acid was applied as both solvent and sulfur precursor, aqueous ammonia was employed as a solvent. After simple annealing at a temperature as low as 270 °C, the spin-coated precursor solution can generate compact, flat, uniform, and well-crystallized Sb2 S3 film. Mechanistic study showed that the formation of Sb-complex with ammonium carboxylates leads to the successful dissolution of Sb2 O3 powder. A suitable annealing process was able to generate carbon-free Sb2 S3 films. Planar heterojunction solar cell based on the as-prepared Sb2 S3 film delivered a power conversion efficiency of 5.57 %, which is the highest efficiency of solution-processed planar heterojunction Sb2 S3 solar cells and a high value in all kinds of Sb2 S3 solar cells. This research provides a convenient approach for the fabrication of device-quality Sb2 S3 films, and highlights solution processing of carbon-free metal chalcogenide thin films as a suitable process for application in optoelectronic devices.

A fast chemical approach towards Sb2S3 film with a large grain size for high-performance planar heterojunction solar cells.

A facile chemical method is developed for the fabrication of Sb2S3 film with a lateral grain size as large as ∼12 µm. A solar cell based on this Sb2S3 film achieves a power conversion efficiency of 4.3%, which is the highest value in solution processed planar heterojunction solar cells based on Sb2S3 film.

Kesterite Cu2ZnSnS4 as a Low-Cost Inorganic Hole-Transporting Material for High-Efficiency Perovskite Solar Cells.

Kesterite-structured quaternary semiconductor Cu2ZnSnS4 (CZTS) has been commonly used as light absorber in thin film solar cells on the basis of its optimal bandgap of 1.5 eV, high absorption coefficient, and earth-abundant elemental constituents. Herein we applied CZTS nanoparticles as a novel inorganic hole transporting material (HTM) for organo-lead halide perovskite solar cells (PSCs) for the first time, achieving a power conversion efficiency (PCE) of 12.75%, which is the highest PCE for PSCs with Cu-based inorganic HTMs reported up to now, and quite comparable to that obtained for PSCs based on commonly used organic HTM such as 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD). The size of CZTS nanoparticles and its incorporation condition as HTM were optimized, and the effects of CZTS HTM on the optical absorption, crystallinity, morphology of the perovskite film and the interface between the perovskite layer and the Au electrode were investigated and compared with the case of spiro-MeOTAD HTM, revealing the role of CZTS in efficient hole transporting from the perovskite layer to the top Au electrode as confirmed by the prohibited charge recombination at the perovskite/Au electrode interface. On the basis of the effectiveness of CZTS as a low-cost HTM competitive to spiro-MeOTAD in PSCs, we demonstrate the new role of CZTS in photovoltaics as a hole conductor beyond the traditional light absorber.