Aggregation of Small Molecule and Polymer Acceptors with 2D-Fused Backbones in Organic Solar Cells

Substitution of two-dimensional (2D) extended conjugation is considered to be an effective strategy to modify the optoelectronic properties of molecules, but the effects of these fused 2D-expansion are not clearly understood. Here, phenanthrene and acenaphthene were applied in A-DAD-A systems to study the impacts of different fused 2D-expansions. It was found that the aggregation behaviors in small molecules are quite different from those of polymer acceptors. The reasons can be explained by theoretical calculations with various configurations of monomers and dimers. This phenomenon leads to obvious diversities in morphologies in blend films, resulting in discrepancies with the performance of mobilities, exciton splitting, and bimolecular recombination in devices with the final organic solar cells (OSCs). The phenanthrene- and acenaphthene-based OSC devices, PTIC-HD-4Cl and PATIC-HD-Th, with appropriate aggregation states achieve superior power conversion efficiencies (PCEs) of 13.71% and 12.47%, while inferior PCEs of 7.29% and 6.57% are observed in OSC devices based on the ATIC-HD-4Cl and PPTIC-HD-Th with excessive aggregations. Our studies demonstrate that the packing arrangements of small molecules are quite different from that of polymer acceptors, and the fused 2D-expansion was found to be an effective strategy with which to adjust the aggregation behaviors for the design of high-crystallinity materials.