Wide-Bandgap Cu(In,Ga)S2 Photocathodes Integrated on Transparent Conductive F:SnO2 Substrates for Chalcopyrite-Based Water Splitting Tandem Devices

Whole-chalcopyrite-based tandem devices for photoelectrochemical (PEC) water splitting have emerged as a promising route for obtaining similar to 20% solar-to-hydrogen efficiencies. Here we pursue this approach by demonstrating integration of the top cell wide-bandgap (E-G) 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)S-2 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 E-G Cu(In,Ga)Se-2 precursors, coevaporated on FTO at low temperature. Depending on Cu(In,Ga)Se-2 precursors' Ga and In content, the resulting Cu(In,Ga)S-2 solar absorbers have E-G 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 E-G and band edge positions that impact PEC performance. Photoelectrochemical measurements performed under AM1.5(G) illumination in 0.5 M H2SO4 on the 2.05 eV CuInGaS2 photocathode revealed a saturation photocurrent density (J(SAT)) of -5.25 mA/cm(2), a value corresponding to 38% of the absorber's optical limit. We further concluded that such low J(SAT) originates from subpar optical absorption of Cu(In,Ga)S-2 absorbers. Future improvements of the CSS process are expected to improve material quality toward our end goal of achieving whole-chalcopyrite tandem PEC devices.