Two asymmetric small molecule acceptors with aromatic and non-aromatic ring side chains were developed for organic solar cells

Abstract

Asymmetric side-chain engineering has been demonstrated to be an effective approach for modifying the solubility and morphology of non-fullerene small molecule acceptors (NF-SMAs). However, prior research has mostly concentrated on optimizing the length and branching locations of alkyl chains. Phenyl and cyclohexyl groups suspended on the side chains have been shown to be effective for tuning the crystallinity and self-assembly of symmetric organic molecules for efficient organic solar cells. To investigate the impact of the aromatic and non-aromatic ring side chains on the performance of asymmetric NF-SMAs, two asymmetric acceptors, Y-Ph4F and Y–CH4F, are designed with phenyl and cyclohexyl as two distinct side chains, respectively, in this work. Due to their common backbone, the two NF-SMAs exhibit similar absorption spectra and energy levels as determined by UV–vis absorption, cyclic voltammetry, and density functional theory calculations. Differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) demonstrate that Y–CH4F with 4-cyclohexylbutyl side chains has stronger molecular packing and crystallinity in the solid state compared to Y-Ph4F with 4-phenylbutyl side chains. By pairing these NFAs with a well-known polymer donor PM6, both PM6:Y-Ph4F and PM6:Y-Ph4F-based organic solar cells (OSCs) achieve power conversion efficiencies (PCEs) of more than 15%. When fabricated with a 1 cm² active area, the Y-Ph4F and Y–CH4F-based devices can also obtain exceptional PCEs of 15.05% and 14.10%, respectively. These findings suggest that the aromatic and non-aromatic ring side chains can effectively tune the molecular packing and crystallinity for efficient OSCs.