Rationally regulating the pi-bridge of small molecule acceptors for efficient organic solar cells

Nonfullerene acceptors have boosted the power conversion efficiencies (PCE) of organic solar cells (OSCs). Rigid fused-ring molecular skeletons enable decent molecular planarity and stacking orientations, but suffer from tedious synthesis procedures for those with large pi-backbones. Promisingly, the pi-bridge strategy provides an alternative to readily manipulate the light utilization, but sometimes increases the conformation complexity. To rationally regulate the performance of pi-bridged acceptors, herein, we report two new acceptors WA1 and WA2, functionalized with single or dual pi-bridges flanking the phenylalkyl modified indacenodithiophene (IDT) backbone. The unilateral pi-bridge endows WA1 with an asymmetric electron cloud distribution, affording a greater dipole moment and well-organized molecular orientation. In contrast, the bilateral pi-bridges endow WA2 with two disparate molecular dipoles and disordered stacking orientations, generating inferior PCEs below 10%. Notably, the PM6:WA1 device shows a remarkable PCE of up to 15.45% with an impressive fill factor (FF) of up to 79.31%, both of which rank among the highest values of OSCs with pi-bridged acceptors. More importantly, the PM6:Y6:WA1 ternary organic solar cell (TOSC) affords a high PCE of up to 18.15%, which is the best efficiency among Y6-based bulk-heterojunction TOSCs and the first example of an efficient ternary cell featuring a pi-bridged guest acceptor.