Manipulating Heavy Halogenated Asymmetric Terminals Affords Regio-Regular Hetero-Fluorinated/Brominated Dual Asymmetric Acceptor With a Binary Photovoltaic Efficiency of 20.3%

Asymmetric terminal engineering of small molecular acceptors (SMAs) is one promising approach to efficient organic solar cells (OSCs), while synthesizing regio-regular dual asymmetric terminal-based SMAs for boosting binary OSCs remains a critical challenge, and heavier halogenated effects are still underexplored. Herein, three global asymmetric SMAs (AY2F-ClF, AY2F-BrF, and AY2F-IF) and symmetric SY-2(FBr), featuring one or two regio-regular, locally asymmetric hetero-dihalogenated terminals with successively heavier halogens, were successfully synthesized by applying the dual asymmetric terminal strategy. Asymmetric AY2F-BrF presents a compact 3D molecular packing with the strongest and most multidimensional electronic coupling, which promotes exciton delocalization, and enhanced crystallinity and electron mobility in pure film. Remarkably, D18:AY2F-BrF OSCs deliver a champion binary PCE of 20.26% with energy loss (E_loss) of 0.512 eV, surpassing the PCEs of AY2F-ClF (19.74%), AY2F-IF (19.15%), and SY-2(FBr) (19.40%)-based OSCs, setting a new record for asymmetric terminal SMAs-based binary BHJ-OSCs, which is attributed to the optimized phase separation morphology and enhanced and compact crystallinity and faster charge transfer and transport. Our innovative work demonstrates that integrating global asymmetric terminal molecular engineering with a regioisomer-free hetero-fluorinated/brominated terminal strategy provides an ingenious dual asymmetric terminal strategy for achieving champion PCE and suppressed E_loss of binary OSCs incorporating asymmetric terminal-based SMAs.