Spin-Coated CZTS Film with a Gradient Cu-Deficient Interfacial Layer for Solar Hydrogen Evolution

Kesterite Cu2ZnSnS4-based photocathodes offer promising applications for solar hydrogen evolution due to their nontoxic, cost-effective, and outstanding photoelectrochemical properties. However, the unfavorable band alignment at the CdS/CZTS interface (CBO ≈ 0.5 eV) poses challenges, including high carrier recombination and low minority carrier lifetime, limiting performance enhancement. This study demonstrates that introducing a gradient Cu-deficient interface layer (GCD-layer) significantly reduces the CBO value at the CdS/CZTS interface (to 0.27 eV), improving band alignment and enhancing carrier separation and transport efficiency of the CZTS photocathode. Moreover, the GCD-layer enhances the crystalline quality of the CZTS film and substantially reduces the charge-transfer resistance of the photocathode. Based on these findings, the prepared CZTS-based photocathode achieved an impressive photocurrent density of 24.5 mA·cm–2 (at 0 VRHE), an onset voltage of 0.75 VRHE, and an ABPE of 4.19% in neutral solution (pH 6.5). Furthermore, the CZTS–BiVO4 tandem device presented a record unbiased STH of 3.37%.