We investigated the degradation of the poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C-61-butyric acid methyl ester (PCBM) organic solar cells (OSCs) induced by several agents (ambient air and humid air in dark, and photo-oxidation in ambient air). Most of previous studies have focused on such a high degree of the organic film degradation which was far behind the OSC functionality. This study correlates degradation of OSC electrical parameters in time with defect states measured with charge deep-level transient spectroscopy on the same samples. The basic electrical parameters of the OSC, and the generation rate and the dissociation probability of exciton were determined from dark and light current-voltage measurements. Degradation agents were applied to the complete samples or directly to the active P3HT: PCBM layer before back Ca/Ag contact evaporation with the aim to discriminate contributions of particular solar cell regions to the degradation. The results show that a decrease of the OSC power conversion efficiency is mainly caused by the photocurrent loss in all kinds of degradation treatment. We observed the formation of oxygen-related defect states with the energy around 0.1 eV above highest occupied molecular orbital of P3HT, which is in accordance with other published data. The increase of the defect states with the degradation time correlates with the photocurrent loss and enhancing exciton recombination. The OSC performance is substantially influenced by the degradation of the back contact and its interface with the active layer. This effect is even more serious than the P3HT:PCBM layer exposure to the ambient or humid air in dark. The fastest degradation was observed in the case of the active layer photo-oxidation. (C) 2016 Published by Elsevier B.V.