In thin film polycrystalline Cu(In,Ga)Se2/CdS solar cells, the intrinsic and interface defect properties are of primary importance for their performance. In this paper, we have performed Capacitance Spectroscopy, XPS and Kelvin probe measurements in order to gain a better understanding of the heterointerface properties at different stages of its formation. All investigations were carried out on Cu(In,Ga)Se2 co-evaporated samples elaborated in pilot-line conditions onto which CdS layers with different thickness were deposited by CBD. From the capacitance measurements versus frequency and temperature, the gap states density evolution was derived using the Walter et al. method [1]. A shallow interface defect was identified with a strong evolution of its density of states with the CdS layer thickness. XPS profiles of the heterojunction were obtained by sputter-etch so as to study the composition at the buried interface. Kelvin probe measurements under dark and illumination showed that the interface is not built until ZnO is deposited and that significant charge is trapped in CdS under illumination. The results obtained by these different characterization techniques at different stages of the heterojunction formation were correlated to the evolution of the solar cells electrical performances.