Solvent effects on the morphology of diketopyrrolopyrrole (DPP)-based low band gap polymer (PDPPBT):phenyl-C71-butyric acid methyl ester (PC71BM) blends are studied systematically using a mixture of a non-aromatic polar primary solvent with high boiling point (b.p.) secondary solvents of increasing polarities. An unfavorable solvent-PC71BM interaction, due to a polarity mismatch, leads to significantly different morphology, also affecting the growth process of polymer crystallites. Non-aromatic polar solvent produces large PC71BM aggregates that increase in size with the addition of non-polar secondary solvents. The size scales of the aggregates decrease markedly when polar solvents are instead used as the secondary solvents. This processing method fundamentally changes the behavior of phase separation, creating a percolated fibrillar type network structure. Moreover, polar secondary solvents with lower vapor pressures reduce the interfibrillar distances that enhance the device performance even more. Power conversion efficiencies (PCE) of 0.03% to 5% are obtained, depending on the solvent system used.