Size Effect of Steric Hindrance Groups on Non‐Fused Ring Electron Acceptors

ABSTRACT Three new non‐fused ring electron acceptors (NFREAs), denoted as 4T‐EH, 4T‐PH, and 4T‐2PH, are designed with 2‐ethylhexyl, 4‐hexylbenzyl, and bis(4‐hexylphenyl)methyl substituents as side chains, respectively. From 4T‐EH to 4T‐PH, and 4T‐2PH, the planarity of the molecular backbone is progressively enhanced, leading to a corresponding evolution in aggregation states. 4T‐2PH with a maximally bulky bis(4‐hexylphenyl)methyl group possesses a desired J ‐aggregation and ordered molecular stacking, which suppress face‐to‐face parallel stacking of the central cores. Consequently, moderate crystallinity and face‐on molecular orientation are formed in 4T‐2PH film. The rational crystallization characteristics of 4T‐2PH afforded better morphologies, enhanced face‐on orientation and shortest π ‐ π stacking distance in the bulk‐heterojunction (BHJ) blends, which further impacts the exciton and charge dynamic behaviors. As a result, the corresponding organic photovoltaics based on D18:4T‐2PH achieved an impressive power conversion efficiency (PCE) of 13.70 %. Our findings demonstrate that the steric hindrance strategy can assist NFREAs to achieve a planar molecular backbone, and optimize the molecular arrangements, ultimately gain high photovoltaic performance.