Advanced Strategy for High-Performance A-D-A'-D-A Type Non-Fused Ring Electron Acceptors with Nitrogen Heterocyclic Cores.

Abstract

The development of nonfused ring electron acceptors (NFREAs) has garnered significant attention due to their simplified molecular design and cost-effectiveness. Recent advancements have pushed the power conversion efficiency (PCE) of NFREAs beyond 19%. Despite these advantages, most NFREAs adopt A-D-A structures, where the electron-donating core is typically a benzene ring substituted with fluorine or alkoxy groups. This design restricts the tunability of energy levels, and the selection of substituents for benzene rings as central units is relatively constrained, which hampers further optimization of material properties. In this work, we designed three A-D-A'-D-A structured fully NFREAs featuring distinct nitrogen heterocyclic cores: linear-shaped TT, star-shaped TYT, and quad-rotor-shaped TTVP. The nitrogen-containing aromatic units, typically strong electron-withdrawing groups, enable precise tuning of energy levels. Moreover, these electron-withdrawing cores enhance molecular rigidity, facilitating efficient π-π stacking and improving electron mobility. Although these NFREAs share identical π-bridges and terminal groups, their unique nitrogen heterocyclic cores exert divergent effects on photovoltaic performance. Theoretical calculations reveal that TT and TTVP exhibit higher electron affinity, greater absorption intensity, lower exciton binding energy, and higher electron mobility compared to the high-performance reference NFREA, TBT-26. Notably, TTVP, featuring an electron-withdrawing core and four terminal groups, exhibits exceptional electronic properties. It achieves the highest electron affinity, the narrowest bandgap of 1.76 eV, and a predicted electron mobility of 4.43 × 10^-4 cm² V^-1 s^-1, surpassing TBT-26. These findings underscore the potential of nitrogen heterocyclic cores in diversifying NFREA design and advancing the development of next-generation high-performance NFREAs.