Ternary chalcogenide-based photoelectrochemical cells III. n-CuIn₅S₈/aqueous polysulfide

The photoelectrochemistry of the spinel phase CuIn₅S₈ in polysulfide electrolyte was studied. A sequence of annealing in S and in vacuum was found to give optimal material for this purpose. The performance of such material could be improved by acid etching and photoelectrochemical etching; both of these treatments deplete the top layer of Cu. Notwithstanding obvious kinetic limitations in polysulfide, the title system shows a negative temperature dependence. The output stability which is inferior to that of n-CuInS₂ systems in polysulfide, is limited by increasing dark currents. These may be due to the formation of CuS on the surface. While, for all examples studied, the indirect bandgap around 1.3 eV was observed, the direct gap around 1.5 eV was seen clearly only for photoelectrochemically etched electrodes. It is suggested that the poor photovoltaic performance and mediocre output stability behaviour of CuIn₅S₈ photoanodes is related to the inability of the material to tolerate significant Cu depletion and to the absence of a homogeneous indium oxide surface layer. This latter finding may be connected with the fact that most of the indium is octahedrally, rather than tetrahedrally coordinated, in contrast to what is the case for the chalcopyrite type disulfide.