Phase separation in donor/acceptor bulk heterojunction (BHJ) significantly affects the exciton dissociation and charge collection in organic solar cells (OSCs). In this work, the dynamics of phase separation in 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 (PC70BM) based BHJ is analyzed. Solvent additive diiodooctane selectively dissolves PC70BM aggregation to form homogeneous distribution in PTB7 matrix. Quantitative analyses show that the size of PC70BM domains decreases from 67 nm to 38 nm with the addition of solvent additive. The vertical phase separation in PTB7:PC70BM BHJ was studied by the X-ray photoelectron spectroscopy measurement and its effect on the performance of the regular and inverted OSCs was examined. Compare to the vertical phase separation occurred in the poly(3-hexylthiophene):PCBM system, the homogeneous vertical phase separation observed in the PTB7:PC70BM BHJ favors an enhanced exciton dissociation process for attaining high performing OSCs. (C) 2015 Elsevier B.V. All rights reserved.