We report that n-type single crystals of n-CuInSe2 in aqueous polysulfide solution exhibit high quantum efficiencies of 0.8–0.9 in the wavelength region between 600–1150 nm when used as photoanodes in photoelectrochemical cells. Photocurrents of 28 mA/cm2 were measured on a ’’winter’’ day in Rehovot (71 mW/cm2) which corresponds to 40 mA/cm2 under AM1 (Air Mass 1) conditions (100-mW/cm2 sunlight). Stability tests of n-CuInSe2 photoanodes (20 000-C/cm2 photocharge passage) at short circuit current densities of 40 mA/cm2 have shown no deterioration whatsoever of the measured photocurrents.

The photoelectrochemistry of n-CuInS2 and CuInSe2 in polysulfide electrolyte is studied with particular emphasis on the pretreatments of the electrodes and on their output stability. The use of Cd doping, (photoelectro)chemical etching, and mild air oxidation all were found to improve electrode performance. The effect of air oxidation was reproducible only for the diselenide, where it improved the fill factor and, because of a negative shift of the flatband potential, the open-circuit voltage. Optimized cells showed, at elevated temperatures, conversion efficiencies around 5 and 7.5% for the sulfide and selenide, respectively. The positive temperature dependence of the photo-I-V characteristics at both low and high illumination intensities, the existence of optimal polysulfide solution compositions, the linear dependence of the photocurrent on the light intensity, and the effects of temperature, solution composition, and initial current density on the photocurrent decrease during the first minute of operation of the cells, are ascribed to limitations of the charge-transfer process across the solid/liquid interface. Thermally activated rates of ad-and desorption of sulfur containing solution species on the semiconductor surface and/or the presence of adsorption-induced electronic states in the bandgap are postulated as causes for this behavior. Notwithstanding these limitations the cells are resistant to photocorrosion, after the initial decrease is arrested, in contrast to what is known for similar Cd-chalcogenide-based systems. We suggest that this stability, which persists under load and at high light intensities, is due to the strength and character of the bonds in CuIn-dichalcogenides, or to the presence of a top layer of indium oxide in which recombination will take place, or to both.

No abstract