Ultrathin ZnS Electron Transport Layer Enables Efficient p-Si Photocathode for Photoelectrochemical Hydrogen Production

Silicon (Si) is considered one of the most promising semiconductor materials for photocathodes in photoelectrochemical (PEC) hydrogen production, owing to its narrow bandgap, excellent optoelectronic properties, and earth abundance. To achieve efficient charge separation and transport during PEC operation, selecting an appropriate electron transport layer (ETL) on p-Si is essential for constructing a buried junction. However, the development of p-Si heterojunction photocathodes with high photovoltage and efficient interfacial carrier transfer is often hindered by unfavorable band alignment between the absorber and ETL. In this study, an ultrathin ZnS layer was employed as the ETL for p-Si due to its low electron affinity, which enables a high theoretical photovoltage for the heterojunction photocathode. Furthermore, a sulfidation process was applied to passivate the ZnS layer. After sulfidation at 400 °C, the p-Si/ZnS/Pt photocathode exhibited a positive onset potential of 0.5 VRHE and an ABPE of 2.3% at 0.2 VRHE. EDS and XPS analyses confirmed the removal of oxidized species and the formation of sulfur vacancies in suitable amounts, which led to optimized carrier concentration and barrier height, thereby enhancing charge transfer kinetics and suppressing carrier recombination.