The active interface of Cu(In,Ga)Se, solar cells is the key to further improvements of their performance. The formation of this interface is not yet well understood. It depends on the initial state of the CIGS layer, then on the evolution of the interfacial chemistry during the US deposition. We present a contribution to its understanding using XPS studies at different steps of the interface formation. Attention has been brought to the surface preparation and to the buried interface CIGS/CdS. Buried interfaces were studied after gradual sputtering. Well-resolved spectra have been obtained. We aimed at a clarification of the role of the various segregated/intermixed phases at the interface. To achieve this, admittance spectroscopy and Kelvin probe measurements have been performed on the same devices to correlate the chemical composition to the electrical responses associated with the buried interface.