This contribution discusses the importance of the back contact of Cu (In, Ga) Se2 thin film solar cells with respect to the device performance and to the device stability. Usually Mo is applied as a back contact material whereas the direct contact to Cu (In, Ga) Se2 is formed by a MoSex interfacial layer. Ideally, an ohmic contact is required, however, a moderate barrier at the back contact only affects the device performance at low temperatures. Nevertheless, the development of devices with a diffusion length in the order of the film thickness and solar cells with a thin absorber layer can enhance the impact of the back contact on the device performance. We shall show and discuss that such a Schottky barrier at the back contact can be interpreted in terms of a phototransistor model. At low temperatures such a barrier can limit the achievable open circuit voltage (Voc). It will be demonstrated that the barrier height at the back contact can be extracted from temperature dependent Voc measurements. Furthermore, it will be concluded that accelerated ageing can lead to an increase of this back barrier. The proposed phototransistor model will be supported by simulations and temperature dependent device measurements.