In compound thin film solar cells like those based on chalcopyrites, interfaces between the different layers that constitute the device (Fig.1) are crucial for the solar cell performance. Interfaces between p-type CIGSe-absorber and n-type CdS-buffer layer lead to the formation of the charge-separating built-in voltage and their quality is directly linked to the efficiency of the device. Furthermore, interfaces exist between buffer and window layer and at the back contact between the molybdenum and the absorber. The complexity increases with tandem solar cells which have even more interfaces. These too can have an impact on the quality of the entire device. Understanding these interfaces in terms of electronic and chemical properties is therefore necessary to further improve existing and to develop new device concepts. In order to examine surfaces of the individual layers, surface sensitive x-ray and ultraviolet photo electron spectroscopy (XPS, UPS) are utilized at PVcomB, either using an in-house laboratory-based system "CISSY" or dedicated end-stations at the BESSY II storage ring.
