Modulating Surface Potential and Electron/Hole Overlap of Singlet Excited State in Asymmetry End-Capped Dimeric Acceptors for Efficient and Stretchable Organic Solar Cells.

Back-to-back dimeric acceptors have attracted widespread attention for organic solar cells (OSCs) due to their exceptional stability and unique three-dimensional (3D) charge transport channels. However, these dimers suffer from inferior intermolecular interactions and molecular packing, limiting the development of OSCs. Here, we first employed an asymmetry end-group strategy to develop a novel asymmetry back-to-back dimer DQx-FCl. Breaking structural symmetry in DQx-FCl alters the electrostatic surface potential to strengthen intermolecular π-π interactions. Meanwhile, it also reduces the overlap of electron and hole in the singlet excited state to promote charge separation. Thus, the asymmetric DQx-FCl-based binary device achieved a superior power conversion efficiency (PCE) of 19.11% along with improved stability, relative to its symmetric DQx-F. More notably, DQx-FCl-based ternary device achieves a record PCE of 20.27% among reported back-to-back dimer-based OSCs. Furthermore, the reinforced intermolecular interactions also enhance the mechanical robustness of OSCs. Flexible devices based on the PM6:L8-BO:DQx-FCl attain a PCE of 17.27% with a crack-onset strain of 14.6%, while the intrinsically stretchable OSC retains 80% of its initial efficiency under a tensile strain exceeding 23%. This study demonstrates the great potential of asymmetric back-to-back dimeric acceptors for improving efficiency, stability, and mechanical flexibility toward high-performance OSCs.