CIS solar cells show the best efficiencies of all thin film solar cell technologies. So far all the laboratory cells and commercial modules are based on Cu-poor CIS. However, all basic electrical and optical investigations show, that CIS grown under Cu-rich conditions has better semiconductor properties in terms of transport and recombination. On the other hand these solar cells suffer from interface recombination. In a previous project we have successfully demonstrated that the interface recombination is reduced by controlling the doping level, which is in turn controlled by the Se pressure during preparation – and can be completely avoided by a surface treatment that turns the surface of the “Cu-rich” bulk Cu-poor. Meanwhile it has been shown in the literature that a potassium based postdeposition treatment of Cu-poor CIS turns the surface very Cu-poor. In this new project we will investigate the effects of this K-postdeposition treatment on Cu-rich CIS absorbers and solar cells. The results will on the one hand improve the basic knowledge of the material in general and will help improve efficiencies also of Cu-poor solar cells. Since our main approach will be to study Cu-rich CIS, which is rarely studied, this project will illuminate the effects of the K-treatment from a different angle than most other investigations and will therefore allow unique insights. Based on our previous experience with Cu-rich solar cells we expect to obtain a better controllable process for the surface treatment and finally Cu-rich solar cells which are better than our Cu-poor solar cells.