Reduction of Electric Current Loss by Aggregation-Induced Molecular Alignment of a Non-Fullerene Acceptor in Organic Photovoltaics.

Y6 is a recently developed non-fullerene electron acceptor (NFA) with dithienothiophen[3.2-]-pyrrolobenzothiadiazole as the central unit and improves the performance of organic photovoltaics (OPVs) in combination with many electron-donor polymers. Although Y6 has desirable electronic properties for OPVs, the origin of its superiority as an acceptor is unclear. This study empirically investigates why Y6 is an excellent acceptor by comparing Y6 with F8IC, an analogue with a similar electronic structure, in bulk heterojunction (BHJ) OPVs with various electron-acceptor concentrations. The difference in the performance between Y6 and F8IC appears only at high concentrations, suggesting that it originates from the aggregation structures of the NFAs in the BHJs. Electric current loss analysis reveals that the exciton loss and non-geminate recombination are suppressed more strongly in Y6 OPVs than in F8IC OPVs. Variable angle spectroscopic ellipsometry shows that Y6 molecules align in the in-plane direction at high concentrations, contributing to the high charge generation rate via efficient exciton collection.