AbstractLaboratory‐scale all‐polymer solar cells (all‐PSCs) have exhibited remarkable power conversion efficiencies (PCEs) exceeding 19%. However, the utilization of hazardous solvents and nonvolatile liquid additives poses challenges for eco‐friendly commercialization, resulting in the trade‐off between device efficiency and operation stability. Herein, an innovative approach based on isomerized solid additive engineering is proposed, employing volatile dithienothiophene (DTT) isomers to modulate intermolecular interactions and facilitate molecular stacking within the photoactive layers. Through elucidating the underlying principles of the DTT‐induced polymer assembly on molecular level, a PCE of 18.72% is achieved for devices processed with environmentally benign solvents, ranking it among the highest record values for eco‐friendly all‐PSCs. Significantly, such superiorities of the DTT‐isomerized strategy afford excellent compatibility with large‐area blade‐coating techniques, offering a promising pathway for industrial‐scale manufacturing of all‐PSCs. Moreover, these devices demonstrate enhanced thermal stability with a promising extrapolated T₈₀ lifetime of 14 000 h, further bolstering their potential for sustainable technological advancement.