Role of the Side Chain in the Phase Segregation of Polymer:Fullerene Bulk Heterojunction Composites

The most effi cient class of polymer materials (as donors) follows a push-pull molecular design that utilizes internal charge transfer (ICT) from electron-rich units to electron-defi cient moieties within the fundamental repeating unit of π -conjugated polymers ( π -CPs). [ 4–6 ] However, when push-pull π -CP donors form composites with fullerene acceptors to create BHJ active layers, undesirable phase segregation between the two components is observed on the scale of a few hundred nanometers. Because large-scale phase segregation increases the probability of exciton (electron-hole pair) recombination in the BHJ active layer due to the relatively short diffusion length of the exciton in the polymers (approximately 20 nm), [ 7 ] the PCE is drastically reduced in BHJ solar cells with large-scale phase segregation. [ 8,9 ]