This study was undertaken to investigate the effect of ultraviolet (UV) irradiation on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl (P3HT:PCBM) solar cells before the thermal annealing of its active layer. Aggregation size of the active layer was observed to increase after UV irradiation, according to the transmission electron microscopy images. Characterization of X-ray diffraction and atomic force microscopy demonstrated that UV irradiation can improve the crystallinity of P3HT phase and optimize the roughness of the active layer. Simulation results indicated that UV irradiation of appropriate time can improve physics parameters of the devices. As a result, the maximum power conversion efficiency of the devices was increased from 3.5% to 4.5%. (C) 2015 Published by Elsevier B.V.