Benzimidazole-Fused A-D-A'-D-A Type Non-Fullerene Acceptors for Efficient Organic Solar Cells.

The molecular engineering of the A-D-A'-D-A type non-fullerene acceptors (NFAs) at the core has generated a diverse of high-performance acceptor materials. Recently, transformation of the benzothiadiazole (BT) core to benzimidazole (Im), which is a widely used unit in constructing functional framework, cluster or carbene based materials, has been explored with a series of Im-based NFAs, which displayed attractive optoelectronic properties. However, the more general synthetic route remains necessary for further exploration of this kind of NFAs. Herein, we applied the typical approach for Im structures by using o-benzoquinone and aldehyde in the presence of ammonium acetate yielding the target NFAs effectively, and three new Im-based acceptors were obtained by adjusting the hanging groups with benzene, 4-fluorobenzene and 3.5-difluorobenzene. These NFAs displayed tunable molecular dipole moments, UV-vis absorption, and frontier energy levels. When used as the guest acceptor in the PM6:BTP-eC9 based organic solar cells, the fluorinated NFAs can effectively improve the device efficiency to 19.48%, which is significantly higher than 18.42% of the binary control and also among top efficiencies for cells using Im-based NFAs. This work provides an effective protocol for Im-based NFAs with tunable molecular scaffolds and properties, which can be of great potential for optoelectronics.