Band-bending engineering in Cu₂ZnSnS₄ photocathodes using a composite (Zn,Ti)O electron transport layer for solar...

The development of efficient and stable photocathodes from earth-abundant materials is a critical challenge for photoelectrochemical (PEC) hydrogen production. While Cu₂ZnSnS₄ (CZTS) is a promising absorber due to its ideal bandgap and non-toxic constituents, its performance is plagued by severe interfacial recombination. This work addresses this limitation through the precise engineering of a zinc-titanium oxide (Zn,Ti)O electron transport layer (ETL) in Mo/CZTS/CdS/(Zn,Ti)O/Pt photocathodes. We systematically investigate the influence of atomic-layer-deposited (Zn,Ti)O on the heterojunction's microstructure, band alignment, and charge transfer kinetics. A suite of characterization studies reveals that a suitable ETL thickness forms a dense, conformal coating and establishes an optimal spike-like conduction band offset at the CdS/(Zn,Ti)O interface, which minimizes bulk and interfacial charge-transfer resistance. Consequently, the optimized photocathode achieves a remarkable photocurrent density of 29.2 mA cm^-2 at 0 V_RHE and a high half-cell solar-to-hydrogen efficiency (HC-STH) of 7.02%. This study demonstrates that fine control of composite ETLs is a powerful strategy to unlock the potential of CZTS and related earth-abundant absorbers for scalable solar fuel production.