High-performance heptacyclic ladder-type heteroarene-based electron acceptors enabled by bulky neighboring side-chains and end-group fluorination

A banana-shape heptacyclic heteroarene core without sp 3 -hybridized bridging atoms is designed and synthesized for constructing novel nonfullerene acceptors. Through introducing bulky neighboring side-chains and fluorinated terminals, molecular aggregation and crystallinity of the nonfullerene acceptors can be effectively modulated, thus leading to improved charge transport and device performance. • Non-fullerene acceptors with sp 3 -hybridized-carbon-free skeletons are developed. • Bulky neighboring side-chains leads to acceptors with suitable aggregation. • End-group fluorination leads to a 6-fold increase in the efficiency of acceptor. • An “A-to-D” type J -aggregation is observed for the acceptor. Although ladder-type heteroarene building blocks without sp 3 -hybridized bridging atoms have been widely used for the synthesis of polymer donors, they have rarely been applied for constructing non-fullerene acceptors (NFAs) mainly due to the lack of strategies to effectively modulate self-aggregation and crystallinity of the resulting acceptors. Herein, a novel sp 3 -hybridized-carbon-free ladder-type skeleton, benzo[2,1-b:3,4-b']bis(4H-dithieno[3,2-b:2′,3′-d]pyrrole) featuring with two sets of neighboring side-chains, is designed and successfully synthesized as a donor core for developing NFAs ( cis -MH and cis -MF). The single-crystal result shows that the cis -MF forms slip-stacked J -type aggregates and packs in an anti-parallel “face-to-face” manner due to the bulkiness effect of neighboring side-chains. This molecular stacking mode effectively prevents the formation of oversized aggregates in the corresponding active layer as evidenced by atomic force microscopy analyses. Introduction of the fluorinated terminals broadens the absorption range and increases the crystallinity of the acceptor, thus greatly promoting its carrier transport. As a result, the cis -MF-based device exhibits a 6-fold increase in power conversion efficiency compared to the cis -MH-based device. Overall, this work provides a new molecular design strategy as well as a novel sp 3 -hybridized-carbon-free ladder-type building block for developing efficient NFAs that can be used to further advance the PCEs of polymer solar cells.