Energy loss near edge structure of sulfur in non-stoichiometric CuInS(2)

Non-stoichiometric CuInS(2) is a novel material for thin film solar cells. Electron energy loss spectrometry analysis shows that there are considerable lateral inhomogeneities on a scale of less than 100nm. It is as yet unknown whether the Cu or In surplus comes in nanoprecipitates irregularly distributed in the otherwise stoichiometric matrix, or forms supercells where particular atoms are replaced by their complements. This question bears important consequences for the understanding of the local electronic structure and the mechanism of undesired recombination processes in the cell. Since the energy-loss near edge structure of ionisation edges (ELNES) mirrors the local density of unoccupied states it should be different in the two cases. We compare the sulfur L(2,3) ELNES of a Cu-rich and of an In-rich specimen with simulations of supercells based on the TELNES/WIEN97 code. Faint differences in the edge shape can be made visible using difference spectra. They agree reasonably well with simulations. The results show that the surplus species exists as a separate non-stoichiometric phase and not merely as a precipitate of elemental Cu or In.