Chemical insights into the Cd2+/NH3 treatment—An approach to explain the formation of Cd-compounds on Cu(In,Ga)(S,Se)2 absorbers

When replacing the generally used CdS-buffer in chalcopyrite-based solar cells by an ILGAR-ZnO layer, the absorber has to be pre-treated in a Cd2+/NH3-containing solution in order to yield high efficiency devices. The purpose of this article is to explain the observed formation of Cd-compounds on top of the Cd2+/NH3-treated Cu(In,Ga)(S,Se)2 absorber surface. Within this framework, results from X-ray-based electron spectroscopies of the modified absorber surfaces and an X-ray diffraction and UV–Vis investigation of the film formed at the surface of the treatment solution are discussed.