| Summary: | 碩士 === 國立成功大學 === 光電科學與工程研究所 === 96 === In this study, the structural and electronic transport properties of polycrystalline pentacene thin-films grown on various gate dielectric surfaces, including inorganic and polymeric insulators, were investigated using organic thin films transistors (OTFTS) with top contact configuration. The study was divided into two parts; in the first part we studied the structural properties of a series of pentacene films grown on inorganic and polymeric insulator surfaces. The polymeric insulator was used as a modification layer upon silicon dioxide (SiO2), including polymethylmethacrylate (PMMA), poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVNP), polyfluorene (PF), poly(vinylidene fluoride) (PVDF), polyimide (PI), polyethyleneimine (PEI), and polystyrene (PS). For the second part of this study we investigated the influence of these polymeric modification layers on the electronic characteristics of pentacene-based OTFT devices.
Part I: The influence of surface properties of various gate dielectric surfaces on the surface morphology, crystal structure, surface free energy, molecular vibrational characteristics and microstructure, as well as π-π* electronic transitions of a series of polycrystalline pentacene films have been investigated using atomic force microscopy (AFM), X-ray diffraction (XRD), contact angle meter, Raman spectroscopy, and absorption spectroscopy. We found that the interfacial adhesive force between the pentacene layer and the dielectric surface are independent of the total surface energy of polymeric surfaces, but dependent on the dispersion force components of polymeric surface free energy. When pentacene was grown on different polymeric surfaces, the pentacene films exhibited variety grainy texture morphology. Among the, these morphologies, pentacene film on PVNP has the largest grain size. We therefore suggest that the polymeric surface properties have a great influence on the first stage of pentacene growth. XRD results indicated the pentacene films on PI, PMMA, and PEI have better crystal quality than that on other polymeric surfaces. Raman analysis results indicated that the pentacene films on PI, PMMA, PVNP, and PEI have large intermolecular coupling energy (w1) and better film homogeneity. Furthermore, we observed a positive relationship between the w1 and the crystal quality along the vertical direction of substrate of these pentacene films on various surfaces. Analysis of the absorption spectrum indicated that these pentacene films have a similar value as the π-π* gap. However, pentacene film on PMMA has the largest Davydov splitting, thus stronger intermolecular interactions. When compared to the pentacene films on other dielectric surfaces, we suggest that the film on PMMA could have superior carrier transport properties due to better crystal quality, larger intermolecular interactions, and homogeneous microstructure.
Part 2: The pentacene films with thicknesses of 600 A were grown on polymeric buffer layers, which deposited upon the SiO2 dielectrics with different surface properties, and the corresponding pentacene-based OTFTs were studied. When the polymeric insulators modified the rough SiO2 surface, the corresponding pentacene-based OTFTs showed better field-effect mobilities (μ) as compared to that on a smooth SiO2 surface. We could not observe clear relationships between the �� and the crystal quality, grain size, w1, and interfacial adhesive force of pentacene on the polymeric surfaces. Moreover, we found that the insulation ability of these polymeric insulators play an important role in the electronic transport properties of pentacene-based OTFTs. To make the OTFTs with a polymeric modification layer upon gate dielectrics, the polymeric properties should be considered on a case by case basis for different surface characteristics of gate dielectrics. For example, the PS suit modifies smooth SiO2 surfaces and the PVNP modifies rough SiO2 surfaces, thus improved charge transport efficiency of pentacene-based OTFTs were obtained. Based on the concepts, we have successfully fabricated pentacene-based OTFTs with a high mobility up to 1-2 cm2/Vs.
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