Noncovalent molecular interactions, charge transport and photovoltaic performance of asymmetric M-series acceptors with dichlorinated end groups

Abstract

Utilization of chlorinated end groups is an efficient strategy for developing small molecule nonfullerene acceptors (NFAs) with narrow bandgaps. Previous studies on nonfullerene acceptors with chlorinated end groups mainly focus on the modulation of either heterohalogenated or monochlorinated end groups, and the structure–property relationships in a series of isomeric NFAs are poorly understood. In this work, three A–D–A (acceptor–donor–acceptor)-type isomeric NFAs (MQ7-i, MQ7-o, and MQ7-m) are designed and synthesized by using an asymmetric ladder-type heteroheptacene core flanked by two dichlorinated end groups with various chlorine atom positions. Based on grazing incidence wide-angle X-ray scattering (GIWAXS) results and crystallographic analysis, the relationships between molecular structures, π–π stacking motifs, and charge transport behaviors of the three isomeric acceptors are systematically revealed. Both the twisted conformations and enlarged π–π stacking distances of MQ7-o and MQ7-m undermine their efficient charge transport, leading to the low power conversion efficiencies (PCEs) for MQ7-o- and MQ7-m-based polymer solar cells. In contrast, MQ7-i shows a significantly higher electron mobility and therefore an enhanced PCE of 16.23%. The improved performance of MQ7-i can be attributed to the more efficient charge transport channels in its 3D stacking network, which comprises the more planar molecular structure with the more favorable π–π interaction and the largest charge transfer integrals (18.5 and 24.8 meV).