Photovoltaic Performances of Fused Ring Acceptors with Isomerized Ladder-Type Dipyran Cores

We precisely design and synthesize two A-π-D-π-A type dipyran-cored nonfullerene acceptors (NFAs) -OT and -OT with oxygen atoms at the outer and inner positions, respectively. 3-Hexyloxythiophene is used as the π-spacer to expand the effective conjugation length of the acceptors. These two NFAs possess similar optical band gaps and energy levels. However, the position of the oxygen atom at the dipyran core can markedly influence the molecular packing and aggregation behavior of the resulted acceptors. -OT with a strong intermolecular affinity tends to form larger aggregates blending with PBDB-T, leading to a lower photovoltaic performance; -OT presents good miscibility with PBDB-T and the blend films preferentially adopt a face-on orientation. -OT-based devices display high and balanced hole and electron mobilities, leading to an optimal power conversion efficiency of 11.38%, which is much higher than those of -OT-based ones (7.60%). Moreover, -OT-based devices also exhibit a lower nonradiative recombination voltage loss of 0.268 eV. Our work demonstrates that the π-spacer and chemical structure of the core unit can greatly influence the molecular packing and the morphology of blend films, which are critical to the photovoltaic performance of devices.