Surface Compositional Reconstruction of Sb2Se3 Thin Films with Simultaneously Enhanced Charge Transport and Light Trapping Enabling Efficient Photocathodes

Abstract Antimony selenide (Sb2Se3) is a promising photocathode semiconductor for photoelectrochemical (PEC) water splitting thanks to its cost-effectiveness and photoelectric property advantages. However, the performance of Sb2Se3 photocathodes remains limited by a low onset potential (Von), mainly associated with complex deep-level defects that induce photovoltage losses. Herein, we introduce a targeted post-passivation strategy regulated by the diffusion kinetics of a highly reactive gaseous sulfur source. It induces surface compositional reconstruction, mitigating deep-level defects by transforming them into relatively shallow-level defects. Meanwhile, the Sb2Se3(S) layer formed via surface compositional reconstruction enables a favorable spike-like band alignment with the CdS buffer layer and reduces charge transfer resistance. In addition, the resulting Sb2Se3(S) surface exhibits a groove-like light-trapping microstructure, thereby enhancing photon utilization. As a result, the optimized Mo/Sb2Se3(S)/CdS/SnO2/Pt photocathode overcomes the trade-off between photocurrent and photovoltage, achieving the highest reported Von of 0.62 VRHE among Sb2Se3-based photocathodes, together with a high photocurrent density (Jph) of 26.81 mA cm–2, and a half-cell solar to-hydrogen (HC-STH) conversion efficiency of 5.62%. When coupled with a BiVO4 photoanode, it further delivers a record unbiased STH efficiency of 2.72%, setting a benchmark in Sb2Se3-based tandem cells for PEC processed solar hydrogen production.