A series of poly(3-hexylthiophene) (P3HT)/(6,6)-phenyl C-60 butyric acid methyl ester (PCBM) bulk hetero-junction polymer solar cells were fabricated with different iodine (I-2) doping concentrations. The short circuit current density (J(sc)) was increased to 8.7 mA/cm(2) from 4 mA/cm(2), meanwhile the open circuit voltage (V-oc) was decreased to 0.52 V from 0.63 V when the iodine doping concentration is 5%. The optimized power conversion efficiency of polymer solar cells (PSCs) with iodine doping is about 1.51%, which should be attributed to the better charge carrier transport and collection, and the more photon harvesting due to the red shift of absorption peaks and the widened absorption range to the longer wavelength. The morphology and phase separation of polymer thin films were measured by atomic force microscopy (AFM). The phase separation of P3HT and PCBM has been distinctly increased, which is beneficial to the exciton dissociation. The photocurrent density of PSCs with iodine doping was increased compared with the PSCs without iodine doping under the same effective voltage. (C) 2011 Elsevier Ltd. All rights reserved.