High-Performance Ladder-Type Heteroheptacene-Based Nonfullerene Acceptors Enabled by Asymmetric Cores with Enhanced Noncovalent Intramolecular Interactions.

Noncovalent intramolecular interactions play an important role in the photovoltaic performance of nonfullerene acceptors. Here, three nonfullerene acceptors (MQ3, MQ5 and MQ6) are designed and synthesized by using asymmetric and symmetric ladder-type heteroheptacene cores which have one and two selenophene heterocycles, respectively. Although MQ3 and MQ5 are constructed with the same number of selenophene heterocycles, the heteroheptacene core of MQ5 is end-capped with selenophene rings while that of MQ3 is flanked with thiophene rings. With the enhanced noncovalent interaction of O···Se in comparison with that of O···S, MQ5 shows a bathochromically shifted absorption band and greatly improved carrier transport thereby leading to a higher power conversion efficiency (PCE) of 15.64% in comparison with MQ3 which shows a PCE of 13.51%. Additionally, based on the asymmetric heteroheptacene core, MQ6 shows a further improved carrier transport induced by the reduced π-π stacking distance which could be related with the increased dipole moment in comparison with the nonfullerene acceptors based on symmetric cores. Therefore, MQ6 exhibits an outstanding PCE of 16.39% with a V OC of 0.88 V, a FF of 75.66% and a J SC of 24.62 mA cm -2 . These results suggest an important strategy to improve the performance of nonfullerene acceptors by using asymmetric cores with enhanced noncovalent intramolecular interactions.