A variety of measurement techniques including photothermal deflection spectroscopy (PDS), auger electron spectroscopy (AES), (sub-bandgap) external quantum efficiency (EQE), and impedance spectroscopy are applied to poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10, 30-benzothiadiazole (PCDTBT)/[6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) films and devices to probe the stability under thermal annealing. Upon annealing, solar cell performance is drastically decreased for temperatures higher than 140 C. Detailed investigation indicate changes in polymer:fullerene interactions resulting in the formation of a polymer wetting layer upon annealing at temperatures higher than 140 C. Upon device completion this wetting layer is located close to the metal electrode and therefore leads to an increase in recombination and a decrease in charge carrier extraction, providing an explanation for the reduced fill factor (FF) and power conversion efficiency (PCE). Because long-term solar cell stability may be influenced by morphological reorganization, the effect of thermal annealing on bulk heterojunction solar cells composed of poly[N-900-hepta-decanyl-2,7-carbazole- alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole (PCDTBT):[6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) is investigated. Using photothermal deflection spectroscopy (PDS), Auger electron spectroscopy, and impedance spectroscopy, it is demonstrated that annealing at high temperatures leads to formation of a polymer wetting layer close to the metal electrode.