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

Abstract 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 2 ). Our findings underscore the potential of isomerism in 3D dimer acceptors to enhance the performance of eco‐friendly OSCs.