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

Sodium is known to play a crucial role in thin-film solar cells based on Cu(In,Ga)Se-2 (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 Na-Cu 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.