Charge Carrier Separation Enhancement Mechanism in Eco‐Friendly CZTSSe/(Zn, Sn)O Thin‐Film Photocathodes for Highly Efficient Solar‐To‐Hydrogen Evolution

Abstract Kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe)‐based photocathodes present promising solutions for solar hydrogen evolution, owing to their non‐toxic, cost‐effective nature and exceptional photoelectrochemical (PEC) properties. Traditionally, the development of CZTSSe‐based photocathodes for PEC water splitting have utilized CdS as the electron transport layer (ETL) owing to its favorable band alignment with the CZTSSe light‐absorbing thin film. However, its environmental concerns pose a significant challenge. Therefore, it is crucial to identifying an eco‐friendly ETL that ensures effective band alignment with kesterite materials. In this study, the Zn/Sn ratio and the thickness of the (Zn,Sn)O buffer layer is optimized to fabricate a Cd‐free CZTSSe/(Zn,Sn)O/TiO₂/Pt photocathode. This design not only promotes environmental safety but also establishes optimal spike‐like band alignments with the CZTSSe thin film. The optimized photocathode demonstrates excellent charge carrier separation and transfer, resulting in a photocurrent density of 29.80 mA cm − 2 at 0 V RHE and a half‐cell solar‐to‐hydrogen (HC‐STH) conversion efficiency of 4.0% in a 0.5 M H₂SO₄ electrolyte. As research continues to optimize the alternative materials, Cd‐free CZTSSe/(Zn, Sn)O‐based photocathodes along with their eco‐friendly nature hold great promise for achieving competitive efficiencies in sustainable solar‐to‐hydrogen energy applications.