Abstract Temperature is one of the most specific external parameters that can accelerate the degradation rate in polymer:fullerene solar cells. To detect modifications of the active layer materials, electric noise spectroscopy is a sensitive experimental technique. A detailed characterization of the dc electric transport and voltage–noise properties shows how thermal ageing is detrimental for the investigated bulk heterojunction photovoltaic system. In particular, an increase of the energy barrier height at the interface between the metal contact and the blend, and a simultaneous decrease of the charge carrier zero-field mobility, evaluated through the analysis of the flicker noise component, are observed as a consequence of a thermal treatment. These effects can be related to morphological changes of the solar cell active layer and interface, and are revealed by monitoring the noise level.