This thesis addresses the question whether the nanometer-scaled morphology of the bulk heterojunction (BHJ) in polymer-fullerene solar cells is stable and, if not, what the ramifications are on the device performance and stability. Using a novel measurement concept, in which grazing incidence small angle X-ray scattering and current-voltage tracking are combined to probe simultaneously and in-situ a running solar cell, changes in the BHJ are evidenced for the first time. These changes are found as a consequence of solar cell operation within the first hours of operation. Different material systems processed with and without additive are investigated and different models are proposed that directly link the morphological changes observed with the deterioration of the photovoltaic performance. Further investigations are performed on how different additives affect the morphology of polymer-fullerene blend thin films. This thesis reveals fundamental information for the proper design of stable organic photovoltaics.
