Delocalized Electronic States Induced by Phosphorus Doping Suppress Charge Recombination in Cu₂ZnSnS₄ Photocathodes

Kesterite Cu₂ZnSnS₄ (CZTS)-a non-toxic and low-cost material for photoelectrochemical water splitting toward hydrogen production- has long been challenged by severe charge recombination and poor carrier transport due to its strong electron localization. Herein, we introduce an anion doping strategy through phosphorus (P) incorporation into CZTS crystals to induce delocalization of electronic states. P-doping significantly enhances the electron transport kinetics of CZTS-P by effectively reconstructing its electronic structure, which benefits from the transition of the conduction band from localized to continuous states, accompanied by a narrowed bandgap and a downshifted Fermi level. Furthermore, P-doping effectively suppresses the localized trapping of carriers by intrinsic defects within the material, thereby enhancing the interband transitions. When coupled with a CdS layer, the CZTS-P-based photocathode forms a stronger built-in electric field and more efficient conductive pathways compared to pristine CZTS, thereby significantly enhancing bulk carrier separation and transport efficiency. Consequently, the optimized Pt/SnO₂/TiO₂/CdS/CZTS-P photocathode exhibited a stable photocurrent of -29.34 mA/cm² at 0 V vs the reversible hydrogen electrode (RHE) for over 100 h, with a high applied bias photon-to-current efficiency (ABPE) of 6.16%. This work provides an effective approach to overcoming electron localization and provides valuable insights for the design of high-performance photoelectrodes.