Rubidium Fluoride Post-Deposition Treatment: Impact on the Chemical Structure of the Cu(In,Ga)Se 2 Surface and CdS/Cu(In,Ga)Se 2 Interface in Thin-Film Solar Cells

We present a detailed characterization of the chemical structure of the Cu(In,Ga)Se thin-film surface and the CdS/Cu(In,Ga)Se interface, both with and without a RbF post-deposition treatment (RbF-PDT). For this purpose, x-ray photoelectron and Auger electron spectroscopy, as well as synchrotron-based soft x-ray emission spectroscopy have been employed. Although some similarities with the reported impacts of light-element alkali PDT (i.e., NaF- and KF-PDT) are found, we observe some distinct differences, which might be the reason for the further improved conversion efficiency with heavy-element alkali PDT. In particular, we find that the RbF-PDT reduces, but not fully removes, the copper content at the absorber surface, and does not induce a significant change in the Ga/(Ga + In) ratio. Additionally, we observe an increased amount of indium and gallium oxides at the surface of the treated absorber. These oxides are partly (in the case of indium) and completely (in the case of gallium) removed from the CdS/Cu(In,Ga)Se interface by the chemical bath deposition of the CdS buffer.