In this study, we have shown the power conversion efficiency of flexible organic thin film solar cells utilizing conjugated polymer/fullerene bulk hetero junction device structures. We used poly[2-methoxy-5-(3,7-dimethyloctyloxy-1,4-phenylenevinylene)] (MDMO-PPV) as an electron donor, [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as an electron acceptor, and PEDOT:PSS used as a Hole Injection Layer (HIL). These materials were coated by spin coating method on the flexible substrates. And a small amount of LiF was deposited between the organic layer and the Al contact layer. The properties of fabricated flexible organic solar cell (OSC) devices have been analyzed as a function of post-annealing temperatures from 25 degrees C (RT) to 150 degrees C. The electrical characteristics of the fabricated devices were investigated by means of J-V, fill factor (FF) and power conversion efficiency (PCE). We observed the highest PCE of 0.75% after annealed at 110 degrees C, which is 1.9 times better than the device without post-annealing (0.398%) and the lowest PCE of 0.027% after annealed at 150 degrees C. (C) 2010 Elsevier B.V. All rights reserved.