The formation of the electrically active interface of chalcopyrite based solar cells in the course of the buffer layer and TCO deposition was studied by photoluminescence, X-ray photoelectron spectroscopy and elastic recoil detection analysis. Changes in absorber properties, namely defect chemistry and composition, were observed after each buffer and TCO deposition step. After the completed cell processing a strong intermixing between absorber, buffer and TCO material in the interface region was found.