Structural ordering in F8T2 polyfluorene thin film transistors

Abstract

We have used thermal treatment and rubbed polyimide alignment layers to produce large domains of poly(9,9-dioctylfluorene-co-bithiophene) alternating copolymer (F8T2). 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 showed that thermal annealing at temperatures ranging from 150 to 350°C modifies the polymer structure from an amorphous to ordered phase as observed by X-ray diffraction. Polarized light optical microscopy showed 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 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.