(Zn,Ti)O Electron Transport Layer Enables the Highest Conversion Efficiency in Cd‐Free Sb ₂ Se ₃ Photocathodes for Stable Solar Hydrogen Production

Antimony selenide (Sb₂Se₃) has emerged as a promising photocathode material for photoelectrochemical (PEC)-driven solar hydrogen production due to its low toxicity, cost-effectiveness, and excellent photoelectric properties. Currently, efficient Sb₂Se₃ photocathodes are mostly coupled with CdS electron transport layer (ETL), however, suffering from natural toxicity, parasitic light absorption, and interfacial mismatch and/or instability. In this study, we introduce a Cd-free Sb₂Se₃ photocathode with an atomic layer deposition-processed (Zn,Ti)O ETL, which mitigates the respective limitations of binary oxides (i.e., ZnO and TiO₂). The optimized (Zn,Ti)O ETL enhances electron carrier density, establishes a favorable 'spike-like' band alignment at the Sb₂Se₃/(Zn,Ti)O interface, significantly improves charge separation and transport efficiencies, as well as the stability. Consequently, the champion device achieves an impressive photocurrent density (J_ph) of 31.1 mA/cm², a record half-cell solar-to-hydrogen (HC-STH) efficiency of 5.27% for Cd-free Sb₂Se₃ photocathodes, and the highest unbiased STH efficiency of 2.50% in the Sb₂Se₃-BiVO₄ tandem cell, setting a new benchmark for eco-friendly and high-performance PEC processed green hydrogen production.