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.

Conjugated donor−acceptor polymers are a large category of cutting edge semiconductor materials with obvious advantages such as easy processing, light weight, and mechanical flexibility. By rational design of electronwithdrawing and electron-donating moieties, the intermolecular interactions between conjugated polymers can be precisely tuned, realizing the efficient control of polymer aggregation in solution and subsequent polymer packing orientation in solid states. In this Review article, the recent advances on the effect of molecular design on the polymer packing orientation are summarized. The behavior and possible mechanism of molecular orientation are first introduced. Then, the key structural factor, side chains, is systematically discussed with the main focuses on their shape (linear or branched), length, density, position, and symmetry, all of which critically affect the polymer packing. Furthermore, the appropriate substitution of conjugated segments in polymer backbone allows the modulation of the π−π interaction and conjugation length, contributing to the transition between edge-on and face-on arrangements in solid states. Such transition can also be achieved by other influencing factors such as molecular weight, additive, post-treatment, and solvent. In addition, the impact of polymer packing on charge transport in field-effect transistors is reviewed, providing further understanding of structure−property relations for high-performance organic electronic devices.

Main observation and conclusion A few monolayers of organic semiconductors adjacent to the dielectric layer are of vital importance in organic field‐effect transistors due to their dominant role in charge transport. In this report, the 2‐nm‐thick polymer monolayers based on poly(3‐hexylthiophene) with different molecular weights (Mn) were fabricated using dip‐coating technique. During the monolayer (solid state) formation from the solution, a disorder‐to‐order transition of polymer conformation is observed through UV‐vis absorption measurement. Meanwhile, high Mn polymer monolayer generates higher crystalline fibrillar microstructure than the low Mn one due to the stronger π–π intermolecular packing between polymers. More importantly, the solution aging procedure is utilized to further improve the morphology of polymer monolayers. It is obvious that after aging for 6 d, both fiber dimension and density as well as conjugation length are significantly increased under the same processing conditions in comparison to the fresh solution, and consequently the field‐effect mobilities are remarkably enhanced by 2—4 times. Note that the maximum mobility of 0.027 cm2·V–1·s–1 is among the highest reported values for poly(3‐hexylthiophene) monolayer transistors. These results demonstrate a simple but powerful strategy for boosting the device performance of polymer monolayer transistors.