Tetrathiophene-based fully non-fused ring electron acceptors via asymmetric side chain engineering

Side-chain engineering has been considered as one of the most promising strategies to optimize non-fullerene acceptors. In this work, we use side-chain engineering to synthesize three fully non-fused electron acceptors (FNEAs) i.e. two symmetric acceptors (4T-BE and 4T-TO) and one asymmetric acceptor (4T-BOE) by different side chain combination onto the tetrathiophene unit, which could effectively tune the molecular conformations, electronic properties, charge carrier transport, film morphology, and photovoltaic properties. From 4T-BE to 4T-BOE and 4T-TO, the molecules present more red-shifted absorption, smaller optical bandgaps, initially rising and then declining LUMO energy levels and stronger intermolecular stacking. When blended with polymer donor PBDB-T, asymmetric 4T-BOE with alkoxy and ester side chains demonstrate a champion PCE of 9.57% with a short-circuit current density (Jsc) of 16.28 mA/cm2, an open circuit voltage (Voc) of 0.87 V, and a fill factor (FF) of 67.50%, which was higher than that of the devices based on PBDB-T:4T-BE and PBDB-T:4T-TO. These results demonstrate that asymmetric side-chain engineering is an especially crucial and effective approach for the design of highly efficient FNEAs.