Isomerized Dithienopyrazine‐Based Solid Additive Enables Organic Solar Cells With 20.5% Efficiency

ABSTRACT Solid additives have emerged as a widely adopted strategy in organic solar cells (OSCs) due to their efficacy in modulating the film morphology and regulating the aggregation behavior of the photoactive layer, which is crucial for achieving high power conversion efficiencies (PCEs). However, the chemical structure of these solid additives fundamentally dictates their performance. Herein, we designed two isomeric solid additives, syn ‐dithieno[2,3‐ b :3',2'‐ e ]pyrazine ( syn ‐DTPy) and anti ‐dithieno[2,3‐ b :2',3' ‐e ]pyrazine ( anti ‐DTPy), to enhance OSCs’ performance. When incorporated into the D18:L8‐BO system, anti ‐DTPy establishes directional intermolecular interactions that optimize phase separation and charge transport pathways. This approach enabled OSCs to achieve a remarkable PCE of 20.5%, with a high fill factor (FF) of 81.9%, ranking among the highest reported values. This study reveals a distinct isomer‐dependent conformational effect for morphology control, providing profound insights into structure‐property relationships and guiding the design of advanced solid additives.