Solvent-polarity-induced active layer morphology control in crystalline diketopyrrolopyrrole-based low band gap polymer photovoltaics

Solvent effects on the morphology of diketopyrrolopyrrole (DPP)-based low band gap polymer (PDPPBT):phenyl-C71-butyric acid methyl ester (PC ₇₁BM) 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. Mixed solvent systems of various polarities significantly change the behavior of phase separation process between the diketopyrrolopyrrole (DPP)-based low band gap polymer (PDPPBT) and phenyl-C71-butyric acid methyl ester (PC₇₁BM). Unfavorable interactions between PC₇₁BM and non-polar secondary solvents cause significant reduction in power conversion efficiencies, while reduced interfibrillar distances in blend films cast from polar and low vapor pressure secondary solvents give substantial enhancement in device performance.