The blend ratio effect on the photovoltaic performance of poly(3-hexyl thiophene) (P3HT):PCBM and poly(3-octylthiophene) (P3OT) solar cells was systematically investigated. In addition to electrical characteristics, the morphology of the blends was examined using Atomic force microscopy (AFM) since the performance of the organic solar cells depends on the morphological organization of donor and acceptor compounds within the bulk heterojunction active layer. The results revealed that decreasing PCBM content, the photovoltaic performances of both organic solar cells improved. The optical microscope images of P3OT:PCBM blend showed that the morphology resembles a needle structure. Both the sizes and the amount of needles decreased with decreasing PCBM loading. An important part of understanding degradation mechanism is proper evaluation of organic solar cell performance as a function of time. In this study, ISOS-D-3 Damp testing was carried for all devices. ISOS-L-1 testing indicated that the efficiencies and short circuit currents of all devices have small variations except for P3OT:PCBM (1:0.6) ratios. (C) 2012 Elsevier B.V. All rights reserved.