Manipulating σ-Hole Interactions in Halogenated Additives for High-Performance Organic Solar Cells with 19.8 % Efficiency.

The incorporation of volatile solid additives has emerged as an effective strategy for enhancing the performance of organic solar cells (OSCs). However, the influence of the electronic structure of these additives on morphological evolution remains insufficiently understood. Herein, 1,4-Dibromobenzene (DBB) and 1,4-Difluoro-2,5-dibromobenzene (DFBB) are introduced as volatile additives into OSCs. Theoretical calculations indicate that DFBB has a higher electrostatic potential extremum and stronger σ-holes interaction compared to DBB, enabling more robust intermolecular interactions with acceptors. The synergistic halogen interactions between DFBB and the active layer matrix balances the differences in crystallinity between the donor and acceptor during the film formation process, promotes the formation of dense molecular packing and ordered orientation, optimizes the vertical composition distribution, and promotes the formation of domain sizes close to the exciton diffusion distance. Consequently, the PM6 : L8-BO-based device treated with DFBB achieves an efficiency of 19.2 % with a fill factor (FF) of 80.8 %, which is superior to the control and DBB. Further validation across various systems, including PM6 : Y6, PM6 : BTP-eC9, and D18 : L8-BO, highlights similar efficiency enhancements, with the D18 : L8-BO system achieving an outstanding PCE of 19.8 %. This work demonstrates that the modulation of σ-hole interactions in volatile additives can effectively optimize multi-scale morphology for high-performance OSCs.