Spectroscopic investigation of the deeply buried Cu(In,Ga)(S,Se)2/Mo interface in thin-film solar cells

The Cu(In,Ga)(S,Se)(2)Mo interface in thin-film solar cells has been investigated by surface-sensitive photoelectron spectroscopy, bulk-sensitive x-ray emission spectroscopy, and atomic force microscopy. It is possible to access this deeply buried interface by using a suitable lift-off technique, which allows us to investigate the back side of the absorber layer as well as the front side of the Mo back contact. We find a layer of Mo(S,Se)(2) on the surface of the Mo back contact and a copper-poor stoichiometry at the back side of the Cu(In,Ga)(S,Se)(2) absorber. Furthermore, we observe that the Na content at the Cu(In,Ga)(S,Se)(2)Mo interface as well as at the inner grain boundaries in the back contact region is significantly lower than at the absorber front surface.