Depth-resolved band gap in Cu(In,Ga)(S,Se)2 thin films

The surface composition of Cu(In,Ga)(S,Se)(2) (CIGSSe) thin films intrinsically deviates from the corresponding bulk composition, which also modifies the electronic structure and thus the optical properties. We used a combination of photon and electron spectroscopies with different information depths to gain depth-resolved information on the band gap energy (E-g) in CIG(S)Se thin films. We find an increasing E-g with decreasing information depth, indicating the formation of a surface region with significantly higher E-g. This E-g-widened surface region extends further into the bulk of the sulfur-free CIGSe thin film compared to the CIGSSe thin film.