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

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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.

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