PHOTOEMISSION-STUDY OF EVAPORATED CUINS(2) THIN-FILMS .2. ELECTRONIC SURFACE-STRUCTURE

The electronic surface structure of evaporated CuInS2 films with different stoichiometries was investigated by photoelectron spectroscopy [x-ray and ultraviolet (XPS and UPS, respectively)], The Fermi level position is observed to vary from E(F)-E(V)=1.3 eV for the In-rich to E(F)-E(V)=0.0 eV for the Cu-rich films. Valence band spectra of In-rich films can be interpreted according to theoretical band structure calculations in the literature. The electronic surface structure of Cu-rich films is determined by a CuS phase segregation which is displayed by an additional valence band emission structure. This phase is responsible for the semimetallic character of the Cu-rich films. A comparison of the energy level positions of Cu-rich and In-rich films is given according to a simple model. The removal of CuS by a KCN treatment is indicated by the complete recovery of the CuInS2 valence band structure. As a consequence highly efficient solar-to-energy conversion can be obtained from CuInS2 films of a wide as-deposited Cu-rich composition range.