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.