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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 (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:SnO 2 (FTO), while preserving the optoelectronic properties of the FTO substrate and preventing the formation of a resistive SnS x interfacial layer. We demonstrate that such attributes can be achieved via closespace 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 H 2 SO 4 on the 2.05 eV CuInGaS 2 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.

A new material CuBiW2O8 is reported here which is suitable for photocatalysts for solar-to-hydrogen generation by splitting water through photoelectrochemical approach. By density functional theory total energy calculations along with extensive mineral database search of relevant oxides, the crystal structures of CuBiW2O8 has been determined, which agrees well with the experimental result. We have analyzed the thermodynamical stability of this material. Its stability was found to be comparable to other well-known oxides, such as CuWO4. The band structure calculation reveals that it has a suitable band gap. In addition to this, density of states and optical absorption calculations show favorable features of a photocatalyst.

In the present communication, we report our efforts to integrate chalcogenide-based photoelectrochemical (PEC) materials into a standalone device capable of water-splitting using sunlight as the only source of energy. More specifically, the PEC performances of copper gallium diselenide are presented. First, a brief introduction to the material microstructural characteristics is presented. Then, the PEC properties are discussed, including incident-photonto-current efficiency (>60% in the visible), Faradaic efficiency (uncatalyzed, 86%) and durability (400 hours). Finally, we report the solar-to-hydrogen benchmark efficiency (3.7%) of a device made of a CuGaSe2 photocathode and a-Si solar cells measured in a 2-electrode configuration using a RuO2 counter electrode.