The influence of film deposition conditions during vacuum co-evaporation, including the flux ratio and the partial pressure of hydrogen, on the structural and photoelectrochemical properties of copper gallium selenide (CGSe) films was examined. The flux ratio between Se and the other metals strongly affected both surface morphology and crystallite size. After ZnS modification, CGSe films grown at flux ratios (J Se /J Cu+Ga ) of 3 to 5 had the highest energy conversion efficiency under AM 1.5G irradiation. In addition, introducing hydrogen during the growth of CGSe films induced the selective growth of the (112) plane and an increase in the grain size. ZnS-modified photocathodes using CGSe with an increased grain size due to hydrogen introduction had high solar energy conversion efficiencies because of their improved structural properties.

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Polycrystalline thin films of CuGaSe2-related Cu-deficient materials were prepared by vacuum co-evaporation. The composition was adjusted in order to prepare copper gallium selenide, abbreviated as CGSe, with an optimal band gap and valence band maximum position for photoelectrochemical water splitting. The effect of the Ga/Cu ratio on the photoelectrochemical properties of CGSe was also studied. With increasing Ga/Cu ratio, the band gap of CGSe became larger, and the valence band maximum position became deeper against the vacuum level. However, an analysis of the photocurrent and onset potential indicated that the Ga/Cu ratio should be less than 3.5 for optimal performance. A Pt-deposited CGSe electrode with a Ga/Cu ratio of 3 showed an onset potential of about 1.1 V vs RHE and an energy conversion efficiency of 0.35% under AM 1.5G light illumination in a 0.1 M Na2SO4 solution with pH 9.5.