In vacuo XPS investigation of Cu(In,Ga)Se2 surface after RbF post-deposition treatment

Latest record efficiencies of Cu(In,Ga)Se2 (CIGSe) solar cells were achieved by means of a rubidium fluoride (RbF) post-deposition treatment (PDT). To understand the effect of the RbF-PDT on the surface chemistry of CIGSe and its interaction with sodium that is generally present in the CIGSe absorber, we performed an X-ray photoelectron spectroscopy (XPS) study on CIGSe thin films as-deposited by a three-stage co-evaporation process and after the consecutive RbF-PDT. The sample transfer from the deposition to the XPS analysis chamber was performed via an ultra-high vacuum transfer system. This allows to minimize air exposure, avoiding oxide formation on the CIGSe surface, especially for alkali-treated absorbers. Beside an expected reduction of Cu- and Ga-content at the surface of RbF-treated CIGSe films, we find that Rb penetrates the CIGSe and, contrary to fluorine, it is not completely removed by subsequent ammonia etching. The remaining Rb contribution at 110.0 eV binding energy, which appears after the RbF-PDT is similar to the one detected on a co-evaporated RbInSe2 reference sample and together with a new Se 3d contribution may hence belong to an Rb-In-Se secondary phase on the CIGSe surface. In addition, Na is driven towards the surface of the CIGSe absorber as a direct result of the RbF-PDT. This proves the ion-exchange mechanism in the absence of moisture and air/oxygen between heavy Rb atoms incorporated via PDT and lighter Na atoms supplied by the glass substrate. A remaining XPS signal of Na 1 s is observed after etching the vacuum transferred RbF-CIGSe sample, indicating that Rb and/or F are not as much a driving force for Na as oxygen usually is.