Molecular Orientation Control of Liquid Crystal Organic Semiconductor for High-Performance Organic Field-Effect Transistors

The control of molecular orientation and ordering of liquid crystal (LC) organic semiconductor (OSC) for high-performance and thermally stable organic thin-film transistors is investigated. A liquid crystalline molecule, 2-(4-dodecyl thiophenyl)[1]dibenzothiopheno[6,5-b:6′,5′-f]-thieno[3,2-b]thiophene (C12-Th-DBTTT) is synthesized, showing the highly ordered smectic X (SmX) phase, demonstrating molecular reorganization via thermal annealing. The resulting thermally evaporated polycrystalline film and solution-sheared thin film show high charge carrier mobilities of 9.08 and 27.34 cm2 V–1 s–1, respectively. Atomic force microscopy and grazing-incidence X-ray diffraction analyses prove that the random SmA1-like structure (smectic monolayer) is reorganized to the highly ordered SmA2-like structure (smectic bilayer) of C12-Ph-DBTTT at the crystal-SmX transition temperature region. Because of the strong intermolecular interactions between rigid DBTTT cores, the thin film devices of C12-Th-DBTTT show excellent thermal stability up to 300 °C, indicating that LC characterization of conventional OSC materials can obtain high electrical performance as well as superior thermal durability.