Selective halogenation of central and end-units of nonfullerene acceptors enables enhanced molecular packing and photovoltaic performance

Halogenation of nonfullerene acceptors (NFAs) is a general and effective strategy to improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Although end-group halogenation has achieved great success, central-unit halogenation has not been systematically studied due to the lack of substitution points in traditional Y-series acceptors. Herein, based on recently developed quinoxaline (Qx)-series acceptors, a series of NFAs, Qx-o-4F, Qx-m-4F, Qx-p-4F, and Qx-p-4Cl, were developed by changing the substitution positions of fluorine atoms on the central unit and the type of terminal halogen atoms. These isomeric fluorinated central units and different end groups result in altered local dipole moments, thus affecting molecular stacking modes and photoelectronic properties of NFAs. Qx-p-4Cl with the para-fluorinated central unit and chlorinated end groups exhibits red-shifted absorption, decreased energy loss, ordered molecular packing, and a favorable blend morphology, which are conducive to charge generation and transport. As a result, OSCs based on PM6:Qx-p-4Cl exhibit a high PCE of 18.06%, which could be further improved to 18.78% by interface optimization. This work underlines the importance of selective halogenation of central units and end groups in manipulating molecular packing and boosting the photovoltaic performance of OSCs.