A short overview on thin film solar cells is given, and the complexity of their electronic structure is illustrated. Several physical mechanisms that give rise to a decay of the capacitance from a low-frequency value C-LF to a high-frequency value C-HF are discussed. A key of interpreting features in measured admittance spectroscopy (AS) spectra is a careful analysis Of C-LF and C-HF and the temperature dependence (activation energy) of the transition frequency between them. As a case study, AS measurements of thin film CdTe/CdS cells are analyzed in dependence of the activation treatment applied to the CdTe layer, and the structure of the CdTe contact. Also the relation with Deep Level Transient Spectroscopy (DLTS) measurements is studied. The measurements explain that CdTe layers treated in air are more robust to variations of the CdTe contact properties, than those treated in vacuum. (c) 2005 Elsevier B.V. All rights reserved.