Novel Method of Pentacene Alignment Using Photoaligned Polyimide and Its Application in Thin-Film Transistors

Pentacene films with submicron thickness, deposited by molecular beam deposition, were fabricated and characterized using X-ray diffraction, atomic force microscopy, and Raman spectroscopy. Organic thin-film transistors (OTFTs) were fabricated on a p-type silicon substrate, using an active layer of highly ordered small pentacene molecules grown at room temperature. Pentacene film was aligned using the photo aligned polyimide method, which is usually employed to align liquid crystals. Improved field-effect mobility, in the range 0.4-0.75 cm^2/V.s, was obtained in pentacene-based OTFTs with photoaligned polyimide layers in which the pentacene molecules were aligned parallel to the direction of the transport of the carriers. The mobility was improved when the current flowed parallel or perpendicular to the orientation of the pentacene films with anisotropic ratios of 2.7-8.3. The thin-film structures of pentacene on the photoaligned polyimide surface depend on the dose of UV light that irradiates on the polyimide surface. X-ray analyses reveal that the structure of the highly ordered pentacene films includes only a single "thin-film phase", generated by optimizing the energy of irradiation of the photoaligned polyimide. However, the poorly aligned films include two phases: a "single-crystal phase" and a "thin-film phase".