Comparison of Linear- and Star-Shaped Fused-Ring Electron Acceptors

A star-shaped nonfullerene electron acceptor, FBTIC, and a linear-shaped nonfullerene acceptor, FBDIC, with the same electron-deficient end groups and p-hexylphenyl side chains but different fused-ring electron-rich cores (benzotrithiophene and benzodithiophene) are designed, synthesized, and compared. Terminal groups 2FIC in FBTIC form intermolecular pi-pi stacking, leading to the complex 3D conjugated framework, while 2FIC units in FBDIC form double hydrogen bonding. Relative to that of FBDIC, FBTIC shows an 81 nm blue-shifted absorption, a 0.21 eV larger optical bandgap, a 0.13 eV lower highest occupied molecular orbital, a 0.1 eV higher lowest unoccupied molecular orbital, and double electron mobility. Paired with the donor PM6, the optimized PM6/FBDIC-based organic photovoltaic cells show an efficiency of 12.3%, while the cells based on PM6/FBTIC exhibit an efficiency of 10.1% with a fill factor of 75.4%, much better than those of the reported star-shaped fused-ring electron acceptors.