Copolymerized Plasticizer Enables Halogen‐Free Processing Toward 20.78% and 17.83% Efficient Organic Solar Cells and Modules

Nonhalogenated solvents are promising for scalable fabrication of organic solar cells (OSCs). However, the intrinsically low solubility of high‐molecular‐weight donor polymers in nonhalogenated solvents, particularly those with tightly packed, highly crystalline backbones such as the pinnacle of donor materials (D18), disrupts multiscale morphology evolution and severely limits device performance. Here, without compromising the polymer's molecular weight or its intrinsic propensity for ordered packing, we establish a copolymerized “plasticizer” strategy that enhances the solvation interaction between a nonhalogenated solvent (toluene) and an appended plasticizing comonomer, thereby improving the processability. By integrating a plasticizing unit (bis(2‐(2‐methoxyethoxy)ethyl) thieno[3,2‐ b ]thiophene‐3,6‐dicarboxylate) into the D18 backbone, the resulting D18‐O exhibits markedly enhanced solubility through dipolar interactions with toluene. These interactions suppress excessive pre‐aggregation, slow down crystallization kinetics of the donor, and ultimately guide the formation of well‐defined nanocrystals and a hierarchy of optimized morphologies during nonhalogenated processing. This approach enables both OSCs and modules processed from toluene to achieve record power conversion efficiencies of 20.78% (certified 20.40%) and 17.83% (certified 17.29%), respectively, along with exceptional stability.