We investigate the degradation of bulk heterojunction solar cells based on the polymer poly((4,8-bis (octyloxy) benzo (1,2-b:4,5-b') dithiophene-2,6-diyl) (2-((dodecyloxy) carbonyl) thieno(3,4-b) thiophenediyl)) (PTB1) and the fullerene 16,6]-phenyl C-61-butyric acid methyl ester (PCBM) under different environments: dry nitrogen atmosphere (H2O < 0.1 ppm, O-2 < 0.1 ppm), air (60 +/- 5% relative humidity) and under encapsulation in accordance with established ISOS-D-1 protocols. The evolution of the electrical measurements under dark and illumination conditions is used to analyze the degradation process during 5300 h and its relationship with the physical mechanisms. The degradation in the efficiency of the solar cells is mainly due to a reduction of short circuit current density (J(SC)) and fill factor (FF) while open circuit voltage (V-OC) is the most stable parameter in all studied conditions. The T-S80 lifetimes for power conversion efficiency (PCE) of samples exposed to nitrogen and air environments and under encapsulation were 990 h, 4 h and 48 h, respectively. The analysis of the PCE decay permits to identify and evaluate the intensity of different degradation mechanisms (intrinsic polymer chemical reactions or by diffused environmental oxygen or water) in the different conditions. (C) 2014 Elsevier B.V. All rights reserved.