Effect of Na from soda-lime glass substrate and as post-deposition on Cu(In,Ga)Se2 absorbers: A photoelectron spectroscopy study in ultra-high vacuum

[Display omitted] •Auger parameter analysis utilized to detect Na speciation at Cu(In,Ga)Se2 surfaces.•Na from post deposition treatment formed (NaxCu1-x)(InyOz) complex compound.•No interaction between Na from soda-lime glass and Cu(In,Ga)Se2 elements was found.•Na from post deposition treatment might not always be beneficial for solar cells.•Na from soda-lime glass resulted in type-inversion at the absorber/buffer interface. Sodium is known to play a crucial role in thin-film solar cells based on Cu(In,Ga)Se2 (CIGSe) absorbers, however, the chemical state of Na in CIGSe is still under discussion. In this work, an Auger parameter analysis has been carried out to identify the differences in the chemical states of Na in CIGSe, originating from two different sources: the soda-lime glass substrate (Na-SLG) and from a post-deposition treatment (Na-PDT). In Na-PDT CIGSe, a Na state with extra positive charges, an InyOz phase, a reduced [Cu]/[In] or [Cu]/[Se] ratio at the surface and most favorable formation of NaCu defects indicates the possible formation of a (NaxCu1-x)(InyOz) complex compound. In Na-SLG CIGSe, only Na states possibly located at the grain boundaries at the absorber surface could be identified. Both CIGSe with sputtered Zn(O,S) buffer have been studied to gain an insight on the influence of the absorber surface states on the energy band line-up at the respective absorber/buffer interfaces.