Modular Design of Polymer Donors Regulates Solution Aggregation and Stretchability of Organic Solar Cells

The use of nonhalogenated solvents in organic solar cell (OSC) manufacturing is crucial for environmental sustainability but remains hindered by difficulties in achieving optimal thin-film morphologies. In particular, controlling solution-state aggregation to form well-defined fibrillar networks that enable high device performance is a central challenge. Here, we present a modular molecular design strategy based on the PTzBI polymer platform, enabling simultaneous tuning of energy levels, crystalline packing, and chain flexibility to address this limitation. The resulting polymer, PTzBI-dF-Si, integrates a fluorinated backbone and siloxane-terminated side chains, showing balanced solubility and controlled solution-state aggregation in o-xylene. These network-like solution aggregates translate into enhanced molecular crystallinity and a well-organized fibrillar morphology in the solid film. The corresponding PTzBI-dF-Si:L8-BO blends achieve a power conversion efficiency (PCE) of 19.9% in rigid OSCs. Moreover, PTzBI-dF-Si exhibits ductile deformation with a fracture strain of ∼20%, leading to intrinsically stretchable OSCs with a PCE over 16%, retaining >80% of the initial PCE under 40% strain. These results highlight the promise of rational, modular polymer design in advancing nonhalogenated-solvent processed, high-efficiency, and mechanically robust OSCs for scalable and wearable electronic applications.