Improving Cu₂ZnSnS₄-Based Photocathodes for Solar Water Splitting via SnO₂ Overlayers

Cu2ZnSnS4 has the appropriate optical properties required for photocathodes in solar water splitting systems; however, its performance is limited by poor carrier separation efficiency and stability. Herein, we use SnO2 to modify the CdS/Cu2ZnSnS4 photocathode by a straightforward spin-coating process, significantly improving the photoelectrode efficiency and stability. The optimal Pt/SnO2/CdS/Cu2ZnSnS4 photocathode maintained a stable −22.01 mA/cm2 photocurrent density for more than 30 h with a high applied bias photon current efficiency (ABPE) conversion efficiency (4.86%). Furthermore, a tandem cell (Pt/SnO2/CdS/Cu2ZnSnS4∥BiVO4) was constructed by combining this photocathode with a BiVO4 photoanode, which exhibited a high unbiased STH of 2.61% with no decrease over 10 h. Detailed investigations indicate a dual role of the SnO2: (1) as an electron acceptor to facilitate photoelectron transfer to Pt and (2) as a protective layer that blocks the electrolyte, thus avoiding the erosion of the inner layers. This approach offers a promising low-cost strategy for the design of advanced photoelectrodes with high activity and long durability.