The solid‐state molecular orientation of conjugated polymers is of vital importance for their charge transport properties, where the edge‐on orientation with π‐stacking direction parallel to the surface is generally preferable to achieving high‐mobility planar field‐effect transistors. However, so far, little is known about the origin of packing‐orientation formation in thin films. Here, it is shown that the solution‐state supramolecular structure of widely studied PffBT4T‐based polymers can be reversibly tuned between 1D worm‐like and 2D lamellar structures for the same polymer/solvent system through solution temperature. Such dimensionality in solution determines the solid‐state packing orientation of the polymer chains, where edge‐on and face‐on textures are generated from solutions with 1D and 2D structures, respectively. More importantly, the transition temperature of solution‐state supramolecular dimensionality is in excellent agreement with that of solid‐state packing orientation. These experimental observations unambiguously demonstrate the predominant roles of solution‐state supramolecular assembly in solid‐state molecular orientation, which is further verified using different molecular weight batches and other two representative polymers. The findings provide new insights into the growth mechanism of polymer semiconductors during transistor fabrication, and open prospective pathways for boosting device performance of solution‐processable plastic electronics.

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Past studies on salt-doped polymer blends indicate that the electrostatic effect is complicated due to the combined effects of translational entropy of dissociated ions, ion solvation, ion–ion correlation, and ion clustering, especially at high salt concentrations. It still remains a challenge to unveil the electrostatic interaction of dissociated ions on phase separation explicitly in experiments. To address this challenge, the key principle of this study is to have the salt at extremely low concentrations, while its effect on phase separation is significant enough to be detected by scattering techniques. This experimental design enables the solvation of dissociated ions as the dominating factor, carefully avoiding complications from other effects. We characterize concentration fluctuations at full length scales by small-angle laser light scattering, X-ray scattering, and neutron scattering. The measured cloud points, spinodal temperatures, correlation lengths, and effective interaction parameters exhibit significant increase with the salt fraction, indicating that the electrostatic interaction promotes phase separation. We further reveal the electrostatic interaction of cations and anions, respectively. The asymmetric deflection of the phase diagram is quantitatively explained by the composition-dependent solvation of dissociated ions. This fundamental research provides experimental support for the progress of recent theoretical work and is helpful to guide the structural control in polymer electrolytes.