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Abstract: Whole-chalcopyrite-based tandem devices for photoelectrochemical (PEC) water splitting have emerged as a promising route for obtaining ∼20% solar-to-hydrogen efficiencies. Here we pursue this approach by demonstrating integration of the top cell wide-bandgap (EG) chalcopyrite onto a transparent conductor, which is a critical step in the realization of tandem devices. We report specifically on our efforts to synthesize photoactive Cu(In,Ga)S2 thin films on transparent conductive F:SnO2 (FTO), while preserving the optoelectronic properties of the FTO substrate and preventing the formation of a resistive SnSx interfacial layer. We demonstrate that such attributes can be achieved via close-space sulfurization (CSS) of lower EG Cu(In,Ga)Se2 precursors, coevaporated on FTO at low temperature. Depending on Cu(In,Ga)Se2 precursors’ Ga and In content, the resulting Cu(In,Ga)S2 solar absorbers have EG energies spanning from 2.05 to 2.45 eV. The CSS process, which includes a low-temperature annealing in sulfur vapor followed by a high-temperature crystallization under inert atmosphere, allowed for up to 95% Se substitution with S in the chalcopyrite lattice, tuning both EG and band edge positions that impact PEC performance. Photoelectrochemical measurements performed under AM1.5G illumination in 0.5 M H2SO4 on the 2.05 eV CuInGaS2 photocathode revealed a saturation photocurrent density (JSAT) of −5.25 mA/cm2, a value corresponding to 38% of the absorber’s optical limit. We further concluded that such low JSAT originates from subpar optical absorption of Cu(In,Ga)S2 absorbers. Future improvements of the CSS process are expected to improve material quality toward our end goal of achieving whole-chalcopyrite tandem PEC devices.