Dirk Hauschild1 2 Mary Blankenship3 Victor van Maris1 2 Thomas Dalibor4 Patrick Eraerds4 Thomas Niesen4 Jörg Palm4 Monika Blum3 5 Wanli Yang5 Clemens Heske1 2 3 Lothar Weinhardt1 2 3

1, Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, , Germany
2, Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Karlsruhe, , Germany
3, Department of Chemistry and Biochemistry, University of Nevada, Las Vegas (UNLV), Las Vegas, Nevada, United States
4, AVANCIS GmbH, Munich, , Germany
5, Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, Berkeley, California, United States

The efficiency of Cu(In,Ga)(S,Se)2-based (CIGSSe) thin-film solar cells has significantly increased over the last few years, both on the lab- as well as the module-scale. In order to achieve even higher efficiencies, larger open circuit voltages are desired, which can, in principle, be realized by increasing the absorber band gap, e.g., by increasing the Ga/(Ga+In) (GGI) and/or S/(S+Se) (SSSe) ratio of the absorber. Traditionally, the AVANCIS GmbH absorber exhibits a high SSSe and no Ga at the surface. Recently, the GGI at the absorber surface has also been increased. Consequently, it becomes an area of interest whether and how the band alignment of the buffer/absorber interface is modified by this approach. Hence, we have conducted a detailed study of the chemical and electronic structure of this interface.
In this contribution, we present results for the heterojunction between a sputtered Zn(O,S) buffer and the CIGSSe absorber using a unique combination of spectroscopies, namely x-ray and UV photoelectron spectroscopy (XPS and UPS), x-ray emission spectroscopy (XES), and inverse photoemission spectroscopy (IPES). We find a significant increase of the absorber band gap at the surface for increasing GGI and SSSe ratios. Furthermore, our results paint a detailed picture of the electronic structure at the Zn(O,S)/CIGSSe interface, which will be discussed in view of the observed efficiency improvements and previously published results obtained for similar heterojunctions with different preparation processes and parameters.