Computation-Guided Placement of Nonfullerene Acceptor Core Halogenation for High-Performance Organic Solar Cells

The strategic molecular design of nonfullerene acceptors (NFAs) plays a crucial role in enhancing the efficiency of organic solar cells (OSCs). Here, working from first-principles theoretical computation, we report a new series of quinoxaline-based NFAs (<b>Qx-PhHal</b>, where Hal = F, Cl, or Br) with varying halogen substitution on the central acceptor core of the molecules to investigate their impact on OSC performance. Notably, OSCs incorporating the brominated NFA demonstrate a significantly higher power conversion efficiency (PCE = 17.58%) than those with fluorinated or chlorinated NFAs (∼14%). Theoretical and experimental analyses reveal that bromination enhances electrostatic interactions, donor-acceptor miscibility, crystallinity, and fibrillar film morphology versus the other halogenated NFAs, thereby enhancing exciton dissociation efficiency, more balanced hole/electron mobility, and reduced exciton recombination rates in the corresponding OSCs. Additionally, ternary solar cells incorporating the brominated NFA as the third component achieve a very high PCE of 20.14%. These findings provide valuable insights into the molecular design of future high-performance NFAs for OSC applications.