Isomeric asymmetric polymerized small molecule acceptors enable halogen-free organic solar cells with a power conversion efficiency of 19.6%

The development of polymerized small-molecule acceptors (PSMAs) offers a promising route toward high-performance and stable organic solar cells (OSCs). Herein, we designed two isomeric PSMAs, PAS1 and PAS2, based on an asymmetric A1-DAD-A2 Y-shaped molecular backbone, incorporating indandione (A1) and 3-dicyanomethylene-1-indanone (A2) as distinct terminal groups. Structural isomerism has been shown to significantly influence the physical, chemical, and optoelectronic properties of the two PSMAs. Moreover, isomerism markedly influences their dielectric constants, with the PAS2-based active layer exhibiting a higher value (3.10 vs. 2.59). This enhancement reduces the charge-transfer state binding energy by approximately 16%, thereby promoting more efficient exciton dissociation. When blended with the donor JD40-BDD20 and processed using the non-halogenated solvent o-xylene, the PAS2-based film forms a more ordered nanoscale morphology, which promotes efficient charge transport and suppresses recombination losses. As a result, the PAS2-based device achieves a power conversion efficiency (PCE) of 15.5%, with an open circuit voltage (VOC) of 0.95 V, short circuit current (JSC) of 21.9 mA cm‒2, and fill factor (FF) of 74.1%, outperforming the PAS1-based device, which exhibits a PCE of 13.5%. The asymmetric structure endows a blue-shifted absorption and an elevated lowest unoccupied molecular orbital (LUMO) energy level, making it suitable as a secondary acceptor for Y-type narrow-bandgap acceptors, and as a third component in the D18:L8-BO system. Notably, the ternary devices processed with o-xylene can be significantly improved and achieve a power conversion efficiency of 19.6%. These results underscore the potential of asymmetric isomeric PSMAs as versatile and efficient acceptors for realizing high-performance, halogen-free organic solar cells.