[en] Studying the pseudo-binary phase diagrams of Cu₂S-In₂S₃ and CuS-InS in the ternary system Cu-In-S showed that the CuInS₂ compound semiconductor exhibits a melting point at T=1093(±3) C and a homogeneity region at room temperature ranging from 2 mol% in In₂S₃ to 1.5 mol% in the CuS direction. This material, which crystallizes in the chalcopyrite-type structure, occurs in nature as the mineral roquesite. Investigating the influence of inert gas pressure on the crystallization of a stoichiometric melt, a disproportionation of the crystallized material into a Cu₂S-rich bulk material of composition Cu_1.06In_0.98S₂ epitaxially intergrown with ∝1.5μm thick lamellae of the thiospinel phase CuIn₅S₈ was observed. Structure and composition of the bulk material were characterized by applying scanning Auger microscopy (SAM) and scanning electron microscopy (SEM) as well as differential thermal analysis (DTA) and X-ray diffractometry (XRD). In photoluminescence measurements (PL), Cu-rich bulk material is distinguished by sharp emission lines representing excitons close to the band to band transition at 1.55 eV and a broadband emission in the range from 1.3 eV to 1.45 eV. Exciton peaks of highest intensity were found at a Cu:In ratio =1.0524. Investigating the spectral response of thin film chalcopyrite solar cells, a significant change of the optical gap from 1.45 eV for a ''Cu-rich'' to 1.37 eV for an ''In-rich'' absorber layer was observed. The results show that the homogeneity region of the material can be varied in the percentage range. A shift of the optical band gap can be influenced by stoichiometry (at% sulphur: at% metal) and the crystallization conditions of the adamantine-type structure. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)