Structural ordering and enhanced carrier mobility in organic polymer thin film transistors

We have used thermal treatment and rubbed polyimide alignment layers to produce large domains of poly(9,9-dioctylfluorene-co-bithiophene) alternating copolymer (FM). The direction of rubbing on the polyimide surface determines the orientation of these domains, allowing us to create thin-film transistors with channel lengths parallel and perpendicular to the liquid-crystal polymer director. We show that thermal annealing at 280degreesC brings F8T2 into a mesophase as observed by X-ray diffraction and differential scanning calorimetry. Polarized optical microscopy and polarized UV-vis absorption spectroscopy show that this ordered phase is associated with very large ordered domains and corresponds to a thermotropic, nematic liquid-crystal phase. We investigated thermal annealing effects on both F8T2 structural ordering and the associated anisotropic electrical properties of the thin-film transistors (TFTs). Enhanced mobility of holes is observed with ordering. Field-effect mobility parallel to the polymer backbone is as much as 6.5 times greater than the perpendicular configuration. (C) 2004 Elsevier B.V. All rights reserved.