Effect of 1,8-diiodooctane on the performance of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) solar cells with glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/P3HT: PCBM/Ca/Al structure was studied. The morphology and thickness of the active layer were investigated using atomic force microscopy (AFM). The UV-visible spectroscopy and X-ray diffraction (XRD) analysis were used to study the absorption behavior (of the solutions and coated layers) and crystallinity of the active layer, respectively. The results show that the existence of 1,8-diiodooctane reduced the open circuit voltage from 0.81 to 0.52 V and increased the short circuit current by about three folds; the fill factor (FF) and power conversion efficiency were increased from 36.0 to 54.1% and 0.47% to 1.54%, respectively. These changes can be attributed to the enhanced crystallinity of P3HT or the doping effect of 1,8-diiodooctane on P3HT chains. UV-visible analysis demonstrated that the addition of 1,8-diiodooctane to the solution did not change the absorption onset, whereas in the coated layers, the maximum absorption peak shifted to higher wavelengths. The XRD analyses demonstrated the enhancement of crystallinity of P3HT upon the introduction of 1,8-diiodooctane.