Understanding the Structure–Performance Relationship for Organic Thin-Film Transistors Using Liquid-Crystalline Organic Semiconductors

Abstract Liquid-crystalline organic semiconductors are rapidly gaining attention as next-generation materials for organic field-effect transistors (OFETs) due to their high charge mobility and thermal durability. However, thermal treatment at temperatures exceeding the transition point of the liquid-crystalline phase degrades device performance, and the relationship between structure and performance remains unclear. In the present study, we investigate 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene, which exhibits outstanding charge-carrier transport properties among liquid-crystalline organic semiconductors. The aggregation structure in thin films is analyzed by combining X-ray diffraction, infrared spectroscopy, and atomic force microscopy, depending on the heating conditions. Meanwhile, the OFET mobility is measured in comparison with this structure. A comparative analysis identifies crystallographic and conformational disorders responsible for the degradation in OFET performance. This is due to the fact that, even after cooling to room temperature, flipped molecules resulting from the liquid-crystalline phase transition partially remain in the polycrystalline film. In other words, the OFET mobility of liquid-crystalline organic semiconductors is governed by the film crystallinity, for which the conformational ordering of the alkyl side chains is a prerequisite but not a sufficient condition. The results demonstrate a clear structure–performance relationship in liquid-crystalline organic semiconductors, thus facilitating the fabrication of high-mobility OFET devices.