Azaphenanthrene-based polycyclic acceptors regulated by N/halogen engineering achieving over 20% efficiency in binary organic solar cells

Abstract

The molecular core-expansion strategy has proved effective in reducing energy loss (E_loss) and improving acceptor packing behavior, thereby achieving a better balance of open-circuit voltage (V_oc), short-circuit current density (J_sc) and fill factor (FF). However, most reported core-expansion designs still focus on quinoxaline-fused frameworks, whereas investigations into larger multi-ring systems (n ⩾ 5) remain scarce. The limited progress stems mainly from the fact that extensive ring fusion often disrupts molecular packing, leading to poor device performance. To address this challenge, we employed a heteroatom-guided design strategy by constructing a series of azaphenanthrene-fused acceptors and systematically incorporating halogen atoms (F, Cl, and Br) into the central core. As the halogen radius increases, both the crystallinity and packing order of the acceptors are progressively enhanced. Ultimately, the Br-substituted azaphenanthrene-fused acceptor CHNBr delivers an impressive fill factor of 78.84%, enabling the binary organic solar cells to surpass the 20% power conversion efficiency (PCE) among multi-ring-expanded acceptor systems.