The knowledge of band energy diagrams of solar cells is essential for a fundamental understanding of their function. We have used photoelectron spectroscopy (PES) as a powerful tool for a systematic study of the formation of interfaces of CdTe solar cells in which the different layers CdS/SnO2. CdTe/CdS and Te/CdTe are deposited step by step by thermal evaporation in model experiments. The results of these studies show that in contrast to other investigations the energy converting heterojunction is not a simple n-CdS/p-CdTe contact. Although depth profiling reveals a homogeneously intrinsic CdTe bulk layer, contact formation and CdCl2-activation are assumed to form an n-i-p CdTe absorber. Such non-ideal conditions may strongly affect optimization processes of conversion efficiencies. The main limitations are evidently due to back-contact formation. Our results do not confirm that the electrochemically formed Te layer produces a good ohmic contact between the CdTe layer and the metallic back contact. From model experiments, we assume the formation of a tunneling contact, instead.