Tailoring grain boundary defects via Cd doping in CZTSSe for efficient solar-driven hydrogen evolution under neutral conditions

Cu₂SnZn(S, Se)₄ (CZTSSe) has emerged as a sustainable, earth-abundant alternative to photoelectrochemical (PEC) water splitting devices as well as conventional copper indium gallium selenide (CIGS) photovoltaics for solar fuel production. However, limitations in the performance of CZTSSe persist due to intrinsic material challenges involving secondary phase segregation, grain boundary defects, and associated carrier recombination losses. This work demonstrates the fabrication of Pt/TiO₂/CdS/CZTSSe-Cd/Mo photocathode with strategic Cd doping via the chemical bath deposition (CBD). With combined sputter and CBD processes, the obtained high-quality CZTSSe-Cd films effectively suppress Zn defect clusters and alleviate Zn-related antisite defects at grain boundaries. As a result, the photovoltaic device comprised of CZTSSe-Cd achieved a maximum power conversion efficiency of 9.26 %. Moreover, as a photocathode for solar-driven hydrogen evolution, the CZTSSe-Cd delivered a record photocurrent density of 19.05 mA/cm² (at 0 V_RHE) in a neutral electrolyte (pH 7), representing a 41.4 % enhancement over pristine CZTSSe (∼13.47 mA/cm²). The Cd-induced defect passivation reduces the non-radiative recombination and modifies the band alignment enhancing charge extraction efficiency, further contributing to the overall boost in device efficiency. The results demonstrate that cation doping is a critical pathway for unlocking the full potential of kesterite absorbers in practical solar fuel applications.