In this research, organic photovoltaic devices based on bulk-heterojunction (BHJ) from Poly(3-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl C-71 butyric acid methyl ester (PC71BM) were successfully fabricated to determine the optimization of solvent additive of 1,8-diiodooctane (DIO). The BHJ devices with DIO show improved short circuit current (J(SC)) correlated to the improved device performance, since the DIO in P3HT: PC71BM BHJ affects the morphology in terms of the surface roughness and crystal size. Furthermore, the DIO contributes to the enhanced charge carrier transport of the BHJ device, because the decreased fluorescence and increased hole mobility as found through Raman spectra analysis and SCLC measurement, respectively. The device with optimized concentration (1 vol %) of DIO accomplishes 5% improved photovoltaic performance due to the favorable structural property and enhanced carrier mobility. (C) 2016 Elsevier B.V. All rights reserved.