Circular Flow Alignment of Polymeric Semiconductor Thin Films on Air-Liquid Interfaces

Orientational control of polymeric semiconductors (PSs) is a fundamental technology for understanding and improving the carrier transport properties. Although PS thin films have been fabricated through facile solution processes, complex convection flows during solvent evaporation often limit the scalability and reproducibility of orientational control. To address these problems, we developed a circular flow alignment method for PS thin films. PS solutions were dropped on glycerol flowing in a circular motion inside a container to obtain thin films at the air-liquid interface. The resulting thin films showed alignment of the main chains along the flow direction, which suppressed the effects of convection flows during solvent evaporation. Anisotropic characteristics were observed in the thin-film structure as evaluated by X-ray diffraction measurements, optical absorption, and carrier transport properties, which serve as supporting evidence for the uniaxial alignment of the PS main chains. In field-effect transistors, a mobility of 0.13 cm²  V^-1  s^-1 was observed under atmospheric conditions, which was four times higher than that of spin-coated thin films. Considering that macroscopic liquid flows are easily controlled, the proposed flow alignment method may serve as a scalable and facile method for fabricating highly aligned PS thin films for various applications.