Oxygen heterocycle-fused indacenodithiophenebithiophene enables an efficient non-fullerene molecular acceptor

To improve the power conversion efficiency (PCE) of solar cells based on non-fullerene acceptors (NFAs), designing low bandgap NFAs of enhanced light-harvesting capability is desirable. Herein, a new set of NFAs based on an oxygen heterocycle-fused indacenodithiophenebithiophene core, were designed and synthesized, all of which achieved low bandgaps around 1.3 eV. An optimal power conversion efficiency of similar to 8.4% was realized by systematic sidechain engineering. Detailed comparison based on the designed NFAs clarifies that the optimal molecule, IDTODT-3, contributes to a moderate phase separation with balanced charge transport and reduced recombination in the blend, and thus performs best. This work verifies the effectiveness of oxygen heterocycle-fusion on realizing LBG, which provides guidelines for designing efficient NFAs via molecule-engineering.