Building-up an interrelationship between isomeric benzyl inner side chains within nonfullerene acceptors and isomeric xylene solvents for non-chlorinated solvent-processed organic solar cells

Abstract

The side-chain engineering of A-DA'D-A-type nonfullerene acceptors (NFAs), well known as the Y6 series, is an efficient approach for retaining the physical properties of the Y6 series but allowing the manipulation of the solubility, crystallization, intermolecular packing, and orientation. This can permit further fine-tuning of their structures for high power conversion efficiencies (PCEs) of organic solar cells (OSCs). Inspired by recent results regarding the critical roles of "inner side-chain modulation " in the aforementioned features, this study designed and synthesized three isomeric BzY-series NFAs (o-BzY, m-BzY, and p-BzY) using o-, m-, and p-hexylbenzyl side chains on the pyrrole motif of the Y6 core framework. This design concept implants a benzene ring at the branching position of the inner side chain. The interrelationship of the isomeric inner side chains within the BzY-series NFAs and isomeric xylene solvents (o-, m-, and p-xylenes) was examined by UV-vis spectroscopy, 1D/2D-nuclear magnetic resonance spectroscopy, and morphological characterization. In-depth studies of 3 (BzY-series NFAs) x 3 (solvents) pair systems of non-chlorinated solvent-processed OSCs found that the best PCEs were achieved from the additive and thermal annealing-free OSCs fabricated with the same structural isomeric pairs of BzY-series NFAs and solvents (i.e., o-BzY with o-xylene, m-BzY with m-xylene, and p-BzY with p-xylene cases). These findings show that the high structural compatibility between the side chains and processing solvents has great potential for improving the OSC performance.