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

Kesterite Cu₂ZnSn(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.