Surface and interface analysis of thin film solar cell components
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. 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.
Alkali treatment of CIGSe
The focus of the current research is understanding the impact of alkali metals and their fluorides on the performance of CIGSe devices. While numerous publications have treated the so-called post deposition treatment of CIGSe using KF or RbF, there is no agreement on the exact mechanism that leads to an improvement of the resulting device. To simplify this problem, we use the direct deposition of either alkali metals or their fluorides on CIGS in a UHV environment and a subsequent surface analysis with XPS and UPS without breaking the vacuum, either in the integrated CISSY system or at a BESSY II end station. 
1. Ümsür, B.; Calvet, W.; Steigert, A.; Lauermann, I.; Gorgoi, M.; Prietzel, K.; Greiner, D.; Kaufmann, C.A.; Unold, T.; Lux-Steiner, M.Ch.: Investigation of the potassium fluoride post deposition treatment on the CIGSe/CdS interface using hard X-ray photoemission spectroscopy - a comparative study , Physical Chemistry Chemical Physics 18 (2016), p. 14129-14138, doi:10.1039/c6cp00260a.
Interdiffusion at the buffer/CIGSe interface
It is well known that cations diffuse into the CIGSe absorber material during or after formation of the hetero-contact between buffer materials like CdS or Zn(S,O) and CIGSe. We examine the diffusion kinetics, their dependence on the stoichiometry of the absorber and specific surface pretreatments. In order to obtain depth-resolved information, we use high energy photoelectron spectroscopy (HAXPES) at the HIKE beamline at BESSY II. 
 Ümsür, B.; Calvet, W.; Höpfner, B.; Steigert, A.; Lauermann, I.; Gorgoi, M.; Prietzel, K.; Navirian, H.; Kaufmann, C.; Unold, T.; Lux-Steiner, M.: Investigation of Cu-poor and Cu-rich Cu(In,Ga)Se2/CdS interfaces using hard X-ray photoelectron spectroscopy Thin Solid Films 582 (2015), p. 366-370 doi:10.1016/j.tsf.2014.08.049.
Secondary phase formation at the back contact
In collaboration with industry partners we examine the formation of unwanted secondary phases at the back contact between Mo and CIGSe. In order to do so we use a variety of imaging and locally resolved spectroscopies.