Organic solar cells (OSCs) have been under intensive studies, due to their attractive properties such as large scale, low cost, lightweight. However, their short lifetime, compared with the inorganic counterparts, has placed a severe restriction on the potential applications. To reveal the cause of the short life, here we report the finding of two distinguishable degradation mechanisms, from a typical organic/polymeric solar cell, based on polymer blend of regioregular poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester, fabricated on indium tin oxide-coated glass substrate. The first mode is associated with a localized failed area due to moisture encroachment, where the confronted areas are either completely dead or still 100% functioning. The second mode was found to be related to a mild decrease in power conversion efficiency (PCE) and was induced by a possible interfacial passivation occurred at the organic/cathode interface, due to the presence of residual oxygen, moisture and other impurities. The simulation results agree well with our experimental findings in showing that the degradation due to the localized moisture encroachment may be delayed by encapsulating the OSCs with desiccant. The degradation due to the interfacial passivation could be eliminated by the removal of low work function cathode at the sacrifice of PCE. (C) 2012 Elsevier B.V. All rights reserved.