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.

A Transparent, High-Performance, and Stable Sb2 S3 Photoanode Enabled by Heterojunction Engineering with Conjugated Polycarbazole Frameworks for Unbiased Photoelectrochemical Overall Water Splitting Devices.

Developing low-cost, high-performance, and durable photoanodes is essential in solar-driven photoelectrochemical (PEC) energy conversion. Sb2 S3 is a low-bandgap (≈1.7 eV) n-type semiconductor with a maximum theoretical solar conversion efficiency of ≈28% for PEC water splitting. However, bulk Sb2 S3 exhibits opaque characteristics and suffers from severe photocorrosion, and thus the use of Sb2 S3 as a photoanode material remains underexploited. This study describes the design and fabrication of a transparent Sb2 S3 -based photoanode by conformably depositing a thin layer of conjugated polycarbazole frameworks (CPF-TCzB) onto the Sb2 S3 film. This structural design creates a type-II heterojunction between the CPF-TCzB and the Sb2 S3 with a suitable band-edge energy offset, thereby, greatly enhancing the charge separation efficiency. The CPF-TCzB/Sb2 S3 hybrid photoanode exhibits a remarkable photocurrent density of 10.1 mA cm-2 at 1.23 V vs reversible hydrogen electrode. Moreover, the thin CPF-TCzB overlayer effectively inhibits photocorrosion of the Sb2 S3 and enables long-term operation for at least 100 h with ≈10% loss in photocurrent density. Furthermore, a standalone unbiased PEC tandem device comprising a CPF-TCzB/Sb2 S3 photoanode and a back-illuminated Si photocathode can achieve a record solar-to-hydrogen conversion efficiency of 5.21%, representing the most efficient PEC water splitting device of its kind.

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.

Revealing composition and structure dependent deep-level defect in antimony trisulfide photovoltaics.

Antimony trisulfide (Sb2S3) is a kind of emerging light-harvesting material with excellent stability and abundant elemental storage. Due to the quasi-one-dimensional symmetry, theoretical investigations have pointed out that there exist complicated defect properties. However, there is no experimental verification on the defect property. Here, we conduct optical deep-level transient spectroscopy to investigate defect properties in Sb2S3 and show that there are maximum three kinds of deep-level defects observed, depending on the composition of Sb2S3. We also find that the Sb-interstitial (Sbi) defect does not show critical influence on the carrier lifetime, indicating the high tolerance of the one-dimensional crystal structure where the space of (Sb4S6)n ribbons is able to accommodate impurities to certain extent. This study provides basic understanding on the defect properties of quasi-one-dimensional materials and a guidance for the efficiency improvement of Sb2S3 solar cells.

In Situ Surface Fluorination of TiO2 Nanocrystals Reinforces Interface Binding of Perovskite Layer for Highly Efficient Solar Cells with Dramatically Enhanced Ultraviolet-Light Stability.

Low-temperature solution-processed TiO2 nanocrystals (LT-TiO2) have been extensively applied as electron transport layer (ETL) of perovskite solar cells (PSCs). However, the low electron mobility, high density of electronic trap states, and considerable photocatalytic activity of TiO2 result in undesirable charge recombination at the ETL/perovskite interface and notorious instability of PSCs under ultraviolet (UV) light. Herein, LT-TiO2 nanocrystals are in situ fluorinated via a simple nonhydrolytic method, affording formation of Ti─F bonds, and consequently increase electron mobility, decrease density of electronic trap states, and inhibit photocatalytic activity. Upon applying fluorinated TiO2 nanocrystals (F-TiO2) as ETL, regular-structure planar heterojunction PSC (PHJ-PSC) achieves a champion power conversion efficiency (PCE) of 22.68%, which is among the highest PCEs for PHJ-PSCs based on LT-TiO2 ETLs. Flexible PHJ-PSC devices based on F-TiO2 ETL exhibit the best PCE of 18.26%, which is the highest value for TiO2-based flexible devices. The bonded F atoms on the surface of TiO2 promote the formation of Pb─F bonds and hydrogen bonds between F- and FA/MA organic cations, reinforcing interface binding of perovskite layer with TiO2 ETL. This contributes to effective passivation of the surface trap states of perovskite film, resulting in enhancements of device efficiency and stability especially under UV light.

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.

Aqueous solution processed MoS3 as an eco-friendly hole-transport layer for all-inorganic Sb2Se3 solar cells.

Here we report a solution processed environmentally friendly MoS3 hole-transport material for Sb2Se3 solar cells, where MoS3 exhibits a matched energy level relative to Sb2Se3. In the synthesis, H2S produced by the thermal decomposition of (NH4)2MoS4 is found to efficiently eliminate the antimony oxide impurity formed on the Sb2Se3 surface. Finally, the all-inorganic Sb2Se3 solar cell delivers an efficiency of 6.86% with excellent stability.

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.