Modulating Band Alignment and Passivating Defects with In‐Situ Bi‐Substituted Cu ₂ ZnSnS ₄ for Superior Photo‐/Electrochemical Water Splitting

Abstract Tailoring semiconductor properties through cation substitution is a promising strategy to improve the intrinsic properties of a photoabsorber material by improving charge carrier statistics, kinetics, and efficiency of photo‐/electrocatalytic conversions. In the current study, Cu ₂ ZnSnS ₄ (CZTS) thin film photocathodes are investigated by partially replacing Zn with Bi by fabricating Cu ₂ Zn _1−x Bi _x SnS ₄ ( x = 0.02, 0.04, 0.06, and 0.08) (x‐CZBTS; x: 2, 4, 6, and 8) thin films. Incorporating Bi replacing Zn reduce bulk defects, secondary phases, and Cu _Zn antisites and promote ease of crystallization with a decrease in crystallization temperature. Bi‐doping modulates the CZTS film band‐gap with a greater absorption coefficient, red shift, and charge collection. CZBTS coated with CdS/Pt (x‐CZBTS‐cp) films delivered photocurrent density (−4.0, −7.2, −5.2, and −1.3 mA cm ⁻² at 0 V _RHE for x: 2, 4, 6, and 8, respectively). 4‐CZBTS‐cp with the highest J (η _STH = 1.62% at 0.45 V _RHE , 95 µmol cm ⁻² h ⁻¹ H ₂ evolution, F.E. = 90%, and E _onset = 0.71V _RHE ) showed 82% photocurrent density retention after 10h in 1 m kPi buffer (pH 7). DFT and experimental studies underscore improved PEC performance owing to significant Bi contribution in conduction band of CZBTS, lowering bandgap, elongating photogenerated charge carrier lifetime, rapid charge separation/transport, and charge accumulation at the Bi−S site on the surface with optimal Gibbs free energy (ΔG _H* ) for CZBTS on photoexcitation.