Synthesis, Characterization And Application Of Polymer Materials For The Gate Insulator Of Organic Thin-film Transistors

Organic thin film transistors (OTFTs) are transistors device using organic semiconductor materials as active layers and have attracted a great deal of attention because of their flexibility, light weight, low cost, and easy processability. Compared with common inorganic transistors, because of the relatively low mobility of the organic semiconducors, organic thin film transistors can not rival the performance of TFTs based on single-crystalline inorganic senmicondutors, such as Si and Ge, which have charge carrier mobilities about three orders of magnitude higher. So OTFTs are not suitable for use in applications requiring high switching speeds. Because they can be fabricated simply and low-costly at room temperature, which can be competitive for novel TFT applications to meet the requirements of large area, flexibility, low-temperature processing and especially low-cost, so OTFTs are of interest for the fabrication of large area displays and low-end electronic devices, such as electronic identification tags and smart cards, as well as large-area sensing devices. Since the first reported OTFTs in 1986, there have been many on-going efforts to research the OTFTs. The mobility of OTFTs with vacuum-deposited pentacene films at room temperature reached as high as 2.7 cm2/Vs, and the on/off current ratio was larger than 106, which was close to that of hydrogenated amorphous silicon thin-film transistor. However, relative to the impressive advances that have been made in OTFTs, little work has been reported on gate dielectrics, which are extremely vital for the commercialization of the high-performance OTFTs. Ta2O5, ZrO2, TiO2, and ferroelectric metal oxide and so on have been employed as dielectric layers. These materials with high-k would afford comparable or greater capacitance values, and therefore comparable or greater surface charge densities at the TFT semiconductor-dielectric interface at greater insulator thicknesses with lower leakage currents. But these dielectrics are typically deposited in a vacuum and they are brittle such that they are not suitable for flexible device applications. Polymeric materials as gate insulators have been of great attention and considered as one of the strongest candidates due to such advantages: First, they can be fabricated simply and low-costly using solution-based process, such as spin coating, solution casting, dip coating, etc. at room temperature, and exhibiting good characteristics. Second, they can generally produce smooth surfaces and have good interfacial compatibility with organic semiconductors . Third, they have different chemical structures and their characters can be tuned by the design of the monomer precursors or polymerization reaction conditions. Recent process in materials, fabrication processes, device designs, applications related to organic thin film transistors have been reviewed in chapter 1. And the theories of the organic thin film transistors such as carriers transport mechanics have been introduced.In this dissertation, we can define four general approaches to achieving novel and high performance materials for the gate insulator of organic thin film transistors, such as photosensitive poly(MMA-CO-EMC), photosensitive polyurethane, photosensitive hybrid material and crosslinkable polyimide.In chapter 2, we have synthesized a photosensitive Poly(methyl methacrylate-co- ethylene methylacrylate cinnamoylate). The structures of the resulting copolymers were characterized using FT-IR, 1H NMR, gel permeation chromatography(GPC), and differential scanning calorimeter(DSC). And they have high photosensitivity, good solubility and film-forming properties and in addition, after crosslinking they show excellent resistance towards solvents and good compatibility with organic semiconductor and substrates. The photoinitiation of the copolymer films were recorded by UV spectra. The surface morphology of the films before and after UV irradiation was investigated using AFM. Results indicated that the spin-coated films had smooth surfaces with the root-mean-square (RMS) surface roughness was 0.23 nm, 0.41 nm, respectively. Finally, we have investigated the characteristics of vanadyl-phthalocyanine (VOPc) OTFTs with the photosensitive copolymer as gate insulator and found that carrier mobility was 0.25 cm2/Vs, on/off ratio was 104.In chapter 3, A novel photosensitive, solution-processable and low-temperature processable polyurethane as the gate insulator of organic thin film transistors through the one-step condensation polymerization of the monomers Bis(2-hydroxyethyl)terephthalate, 4, 4'-Methylenebis(cyclohexyl isocyanate) and 2, 2-Bis(hydroxymethyl)butyl cinnamoylate was designed and synthesized. The resulting polyurethane displays excellent thermal stability and good adhesion on substrate after crosslinking. The photosensitive polyurethane with the molecular weights (Mn:10100 g mol-1) and polydispersities (1.1) was useful for the fabrication of organic thin film transistors because of its good solubility, low-temperature in common organic solvent and photopatternability at room temperature. The polyurethane was characterized by FT-IR and 1H NMR. The polyurethane film had good electrical characteristics, and the gate insulator leakage of the film was less than 1×10-10 A cm-2. In addition, we have successfully fabricated VOPc OTFTs with the polyurethane as gate insulator, and found the OTFTs exhibiting good performance with mobility of 0.13 cm2/Vs, and on/off ratio of 104.In chapter 4, In order to further improve the polyurethane performance, TiO2 (ZrO2) was covalently incorporated into the thiethoxysilane-capped photosensitive polyurethane through an in situ sol-gel method. The thiethoxysilane-capped photosensitive polyurethane was synthesized through the two-step condensation polymerization of the monomers Bis(2-hydroxyethyl)terephthalate, 4, 4'-Methylenebis(cyclohexyl isocyanate), 2, 2-Bis(hydroxymethyl)butyl cinnamoylate, Abstract 2-ethyl-2-(hydroxymethyl)propane-,3-diol, and 3-isocyanatopropyl thiethoxysilane. The polyurethane was characterized by FTIR, 1HNMR, gel permeation chromatography(GPC). The TiO2 or ZrO2 content in the hybrid materials was adjusted from 30% to 70% by the feed ratio of precursor Ti(OBu)4 or Zr(OBu)4 to polyurethane. Both FT-IR, DSC and TGA analyses indicated that there was chemical bonding between the inorganic domain and the polyurethane. The surface morphology of the hybrid films were investigated using AFM and the root-mean-square (RMS) surface roughness was 0.35 nm, 0.48 nm, 0.62 nm , respectively. The dielectric constant and capacitance of the hybrid materials was increased with the increasing of TiO2 or ZrO2 content in the hybrid materials. Finally, we have investigated the characteristics of VOPc OTFTs with the hybrid materials as gate insulator and found that carrier mobility was 0.08 cm2/Vs, on/off ratio was 103.In chapter 5, solution-processable photosensitive and the surface-modified polyimide was synthesized based on 2, 4-amino phenoxyethyl methacrylate, 1-biphenyl-4-ylmethyl dodecanoate-12-(2', 4'-amino phenoxy) ethyl dodecanoate and 4, 4'-(hexafluoroisopropylidene) diphthalic anhydride. The polyimide had good solubility in most of the organic solvents such as dichloromethane, chloroform, tetrahydrofuran,dioxane and N,N-dimethylformamid. The structures of the polyimide was characterized using FT-IR, 1H NMR, GPC and TGA. The film which was made by spinning coated had good UV light lithograph sensitivity. In addition, we have successfully fabricated VOPc OTFTs with inorganic/organic double gate insulators, in which the inorganic gate insulator was SiO2, the organic gate insular was crosslinked polyimide. And we found the OTFTs exhibiting very good performance with mobility of 1.2 cm2/Vs, on/off ratio was 106.