We fabricated bulk heterojunction organic solar cells based on 1,2-dichlorobenzene solutions of poly[[4,8-bis[(2-ethylhexyl) oxy]benzo[1,2-b:4,5-b'] dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno [3,4-b]-thiophenediyl]] (PTB7): [6,6]-phenyl-C71-butyric-acid-methyl-ester (PTB7:PC₇₁BM) with additional dimethyl sulfoxide (DMSO) and surfactants diiodooctane (DIO). The optimal weight ratios of DMSO and DIO relative to the total weight of PTB7:PC₇₁BM were 13% and 3%, respectively, and the resulting solar cells exhibited an open circuit voltage of 0.72 V, short circuit current density of 15.63 mA/cm², fill factor of 0.49, and power conversion efficiency of 5.47%. The surfaces of the active layers deposited with added DMSO and DIO were smoother than those of the layers without the added surfactants and consisted of smaller nanograins, which may have resulted in the improved solar cell performance.