There has been great interest in recent years in using solution processible conjugated organic polymers as the active layers in semiconducting devices including field-effect transistors (FETs), light emitting diodes (LEDs), and photovoltaics (PVs). The structure of the polymer film is very important to device performance at the interfaces where charge separation and collection occur, as well as in the bulk of the film, where it has been shown that alignment of the polymer backbone can increase the pi-orbital overlap, thus enhancing charge carrier mobility. In this work, we report on the temperature-dependent alignment of liquid crystalline fluorene-thiophene copolymer thin film surfaces using the near-edge X-ray fine absorption structure (NEXAFS) technique. Partial electron yield spectra were recorded over a range of temperatures to observe directly the bond orientation in various polymer phases. In addition, samples were annealed under varying processing conditions and spectra were taken at room temperature on these heat-treated samples. The NEXAFS data shows: a) in thin polyfluorene films, the polymer backbone lies flat in the plane of the substrate, b) that along the main chain axis, biphenyl and thiophene rings have a preference to lie flat in the plane of the surface, c) the orientation of the polymer backbone can be controlled using a rubbed polyimide alignment layer as a template for liquid crystal orientation, and d) under proper annealing conditions, there is strong temperature-dependent alignment of the copolymer main-chain axis to the rubbing direction with dichroic ratio (R) reaching R=0.7.
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.