Dipole Moment Modulation of Terminal Groups Enables Asymmetric Acceptors Featuring Medium Bandgap for Efficient and Stable Ternary Organic Solar Cells.

Abstract

This study puts forth a novelterminal groupdesign todevelopmedium-bandgap Y-series acceptors beyond conventional side-chain engineering.We focused onthe strategical integration of an electron-donating methoxy group and an electron-withdrawing halogen atomatbenzene-fusedterminal groups. This combination preciselymodulatedthedipole moment and electron density of terminal groups, effectively attenuating intramolecular charge transfereffect, and widening the bandgap ofacceptors. The incorporation of theseterminalgroups yielded two asymmetric acceptors, named BTP-2FClO and BTP-2FBrO, both of which exhibited open-circuit voltage (VOC) as high as 0.96 V in binary devices, representing thehighestVOCs among the asymmetric Y-seriessmall molecule acceptors. More importantly, both BTP-2FClO and BTP-2FBrO exhibitmodestaggregation behaviorsandmolecular crystallinity,making themsuitable as a third component to mitigate excess aggregation of the PM6: BTP-eC9 blend and optimize thedevices'morphology.As a result,the optimized BTP-2FClO-based ternary organic solar cells (OSCs) achieved a remarkable power conversionefficiency (PCE)of 19.34%, positioning it among the highest-performing OSCs.Ourstudyhighlights themolecular designimportance on manipulating dipole moments and electron densityindevelopingmedium-bandgap acceptors,andoffersa highly efficient third component forhigh-performance ternary OSCs.