Flexible polymer solar cells with an ITO/PEDOT/P3HT:PCBM/Al structure were fabricated using regioregular poly(3-hexylthiophene) (P3TH) polymer:(6,6)-phenyl C61-butyric acid methyl ester (PCBM) fullerene polymer as the photovoltaic (PV) bulk hetero-junction layer. The P3HT and PCBM used as the electron donor and electron acceptor materials were spin cast on indium tin oxide (ITO) coated polyethylene naphthalate (PEN) substrates. The optimum mixing concentration ratio of the P3HT:PCBM PV layer was found to be 4:4 wt.%, at which the maximum short circuit current density (J(SC)), open circuit voltage (V(OC)), fill factor (FF) and power conversion efficiency (PCE) values were about 3.8 mA/cm(2), 427 mV, 36.6% and 0.66%, respectively. To investigate the effects of the post annealing treatment, the as-prepared flexible polymer solar cells were post annealed at temperatures ranging from 150 degrees C to 180 degrees C for 5 min. The J(SC) and V(OC) values increased with increasing post annealing temperature from 150 degrees C to 170 degrees C, which may be due to the improvement of the light absorption coefficient of P3HT and improved ohmic contact between the PV layer and Al electrode film. The maximum J(SC), V(OC), FF and PCE values of the flexible polymer solar cell, which was post annealed at 170 degrees C for 5 min, were found to be about 4.3 mA/cm(2), 616 mV, 32.6% and 0.86%, respectively. (C) 2010 Elsevier B.V. All rights reserved.