An Asymmetry Strategy to Reduce Excessive Aggregation of Brominated Non-Fullerene Acceptors for Enhanced Efficiency of Organic Solar Cells

Halogenation of non-fullerene electron acceptors (NFAs) is an effective approach to modulate molecular interaction, packing as well as optoelectronic properties to tune photovoltaic performance. Whilst fluorination and chlorination are widely explored, bromination of NFAs has received less attention. In this work, we synthesized brominated and symmetric ITCC-Br, and observed redshifted optical spectrum, downshifted energy levels and reduced bandgap. However, the strong intermolecular interaction leads to excessive aggregation of ITCC-Br in its photovoltaic blends with PBDB-T-2F or PBDB-T-2Cl, leading to inferior device performance. This excessive molecular aggregation can be suppressed by introducing asymmetry to the NFA, with the newly synthesized asymmetric IDTT-Br-2F showing further redshifted absorption, downshifted energy levels and reduced bandgap. The PBDB-T-2F:IDTT-Br-2F OSC obtains a PCE of 12.1% with an open-circuit voltage (V-OC) of 0.94 V, a shortcircuit current (J(SC)) of 18.39 mA cm(-2) and a fill factor (FF) of 68.70%, much higher than the 10.7% PCE of PBDB-T-2F:ITCC-Br OSC. Similar improvements can be observed in PBDB-T-2Cl:NFA OSCs. Our results demonstrate that asymmetry tailoring is an effective approach to tune the aggregation behavior of halogenated NFAs toward higher performance.