Surface Off-Stoichiometry of CuInS $_{2}$ Thin-Film Solar Cell Absorbers

X-ray photoelectron and x-ray-excited Auger electron spectroscopy was used to investigate the chemical surface structure of CuInS 2 thin-film solar cell absorbers. We find that the [In]/[Cu] surface composition can vary between 1.6 (± 0.4) and 3.7 (± 0.7), depending on relatively minor variations in the absorber formation process and/or whether additional wet-chemical treatments are performed. These variations are primarily due to differences in the Cu surface concentration. The corresponding change of the modified In Auger parameter is interpreted as being indicative for a change of the chemical environment of In as a function of Cu off-stoichiometry.