The Critical Isomerization Effect of Core Bromination on Nonfullerene Acceptors in Achieving High-Performance Organic Solar Cells with Low Energy Loss

Highly efficient nonfullerene acceptors (NFAs) for organic solar cells (OSCs) with low energy loss (E_loss) and favorable morphology are critical for breaking the efficiency bottleneck and achieving commercial applications of OSCs. In this work, quinoxaline-based NFAs are designed and synthesized using a synergistic isomerization and bromination approach. The π-expanded quinoxaline-fused core exhibits different bromination sites for isomeric NFAs, namely AQx-21 and AQx-22. Theoretical and experimental analyses reveal that the isomerization effect of core bromination significantly influences molecular intrinsic properties, including electrostatic potentials, polarizability, dielectric constant, exciton binding energy, crystallinity, and miscibility with donor materials, thereby improving molecular packing and bulk-heterojunction morphology. Consequently, the AQx-22-based blend exhibits enhanced crystallinity, reduced domain size, and optimized phase distribution, which facilitates charge transport, suppresses charge recombination, and improves charge extraction. The AQx-22-treated OSCs obtain an impressive efficiency of 19.5% with a remarkable open-circuit voltage of 0.970 V and a low E_loss of 0.476 eV. This study provides deep insights into NFA design and elucidates the potential working mechanisms for optimizing morphology and device performance through isomerization engineering of core bromination, highlighting its significance in advancing OSC technology.