Isomerism Effect of 3D Dimeric Acceptors for Non-Halogenated Solvent-Processed Organic Solar Cells with 20 % Efficiency.

Organic photovoltaic materials that can be processed via non-halogenated solvents are crucial for the large-area manufacturing of organic solar cells (OSCs). However, the limited available of electron acceptors with adequate solubility and favorable molecular packing presents a challenge in achieving efficient non-halogenated solvent-processed OSCs. Herein, inspired by the three-dimensional dimeric acceptor CH8-4, we employed a molecular isomerization strategy to synthesize its isomers, CH8-4A and CH8-4B, by tuning the position of fluorine (F) atom in the central unit. The differing intramolecular fluorine-sulfur non-covalent interactions among these isomers led to differences in molecular pre-aggregation abilities (CH8-4B<CH8-4<CH8-4A) in o-xylene (o-XY) solution, which significantly influence the film-forming process and the resultant morphological characteristics. Among these, the blend film of CH8-4, characterized by moderate molecular pre-aggregation, achieved optimal bi-continuous donor/acceptor phase separation. Consequently, the o-xylene processed PM6 : CH8-4 device achieved a power conversion efficiency (PCE) of 18.1 %, outperforming that of two other devices. By incorporating L8-BO-D as a guest acceptor, we attained an impressive PCE of 20.0 % for the CH8-4-based ternary device, alongside a high PCE nearing 16 % for the mini-module (13.5 cm). Our findings underscore the potential of isomerism in 3D dimer acceptors to enhance the performance of eco-friendly OSCs.