Impact of RbF post deposition treatment on CdS/CIGSe and Zn(O,S)/CIGSe interfaces – A comparative HAXPES study

In conventional Cu(In,Ga)Se2 (CIGSe) solar cells a chemical bath deposited CdS thin film is used as a buffer layer. However, it is desired to replace CdS due to the toxicity of cadmium and the rather narrow bandgap energy of CdS. Zn(O,S) is considered to be one of the most attractive candidates as an alternative, non-toxic buffer layer with a larger bandgap. This paper aims to compare the properties of the CdS/CIGSe and the Zn(O,S)/CIGSe interfaces depending on the absorber composition and the application of an RbF post deposition treatment (PDT). Synchrotron-based hard X-ray photoelectron spectroscopy revealed a strong correlation of Cd diffusion and concentration of V_Cu in CIGSe before the PDT. Additionally, the RbF-PDT enhanced the Cd diffusion into the CIGSe. On the other hand, it was found that Zn atoms are not as easily incorporated into the CIGSe as Cd atoms. As a result, we consider the formation of donor-like Zn_Cu+ defects at the interface to be less likely than the formation of Cd_Cu+ defects. Moreover, we observed that the Zn(O,S)/CIGSe interface is less sensitive to changes of the CIGSe composition and to the RbF-PDT compared to the CdS/CIGSe interface.