Admittance spectroscopy of rectifying junctions is known to be a powerful technique for the characterization of bulk or interface defect properties in crystalline semiconductor based devices. In the case of non-crystalline semiconductors, e.g. polycrystalline or amorphous ones, specific models were developed to take into account the disordered structure which is typically the bulk of these materials. Different models were proposed in the literature, depending on the investigated materials. In this work, these models are reviewed and their basic assumptions are discussed and compared. This discussion will more particularly focus on the models, developed for the characterization of two important candidates on thin film solar cells, namely hydrogenated amorphous silicon and copper indium diselenide based materials. The benefits of frequency and temperature dependent admittance spectroscopy are illustrated with results, concerning the evaluation of some technological processes relevant to these materials for photovoltaic applications.