Photoemission study of evaporated CuInS₂ thin films. II. 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.