Molecular Chemistry-Controlled Hybrid Ink-Derived Efficient Cu₂ZnSnS₄ Photocathodes for Photoelectrochemical Water Splitting

To realize economically competitive hydrogen production through photoelectrochemical (PEC) water splitting, it is essential to develop an efficient photoelectrode consisting of earth-abundant constituents in conjunction with low-cost solution processing. Cu₂ZnSnS₄ (CZTS) has received significant attention as a promising photocathode owing to its abundance and good absorption properties. However, the efficiency of the solution-processed CZTS photocathode is not yet comparable to its counterparts. Here, a hybrid ink, obtained by careful control of precursor mixing order, was used to produce a highly efficient CZTS photocathode. The molecular chemistry-controlled hybrid ink formulation, particularly the roles of thiourea-Sn²⁺ complexation, was elucidated by liquid Raman spectroscopy. The hybrid ink-derived CZTS thin films modified with conformal coating of an n-type TiO₂/CdS double layer and a Pt electrocatalyst achieved an exceptionally high photocurrent of 13 mA cm^-2 at -0.2 V versus a reversible hydrogen electrode under 1 sun illumination. The modified photocathodes showed relatively stable H₂ production with faradaic efficiency close to unity.