| Summary: | 博士 === 國立成功大學 === 微電子工程研究所碩博士班 === 101 === In this study, research on application of modification layer in both conjugated polymer and small molecule in organic thin film transistors was presented. For the polymer-based OTFTs, a depletion-mode polymer transistor with the composite channel structure consisting of polymer electrolyte poly(ethyleneimine) (PEI) modification layer and active material polyaniline-camphor sulfonic acid (PANI-CSA) was proposed. From the experimental results observed, the PANI-CSA device without the electrolyte PEI exhibits a resistor behavior. With the insertion of the PEI electrolyte layer between poly-4-vinylphenol (PVP) polymer dielectric and PANI-CSA, the device behaves as a transistor working in depletion mode. This is due to the NH3+ cations of PEI stuffed into PANI-CSA to compensate the SO3- anions of PANI-CSA, thereby resulting in dedoping process and current modulation by applying gate voltage. However, PANI-CSA OTFTs only exhibit p-type transistor behavior. To further simplify the design of organic complementary metal oxide semiconductor (CMOS) integrated circuit, the ambipolar transistor through the ion-assisted electrochemical doping mechanism and poly(vinyl alcohol) (PVA) surface modification layer was proposed. With PVA modification layer, the OTFT with polyaniline-poly (styrene sulfonic acid) (PANI-PSS) as active layer exhibit the obvious ambipolar transistor characteristics. The hole and electron mobility of PANI-PSS OTFTs is 3.36 and 1.19 cm2/Vs respectively. Although the polymer-based OTFTs have been demonstrated, the electrical characteristic is not always stable during the device operation. Sometimes, the devices show the undesired output characteristic or even breakdown directly. After tracing the data obtained from the instrument, we find the unstable characteristics or breakdown phenomenon result from the high leakage current due to the unreliable insulator property. From the scanning electron microscope (SEM) results, some cracks and valleys existed on the surface of PVP dielectric. To solve this problem, dual-layer non-cross-linked PVP (NCPVP)/cross-linked PVP (CPVP) dielectrics structure was applied to small molecule pentacene-based OTFTs. The NCPVP/CPCP specific structure could not only strengthen the polymer insulator property but also modify the surface energy to facilitate the deposition of pentacene film. The experimental results suggest that the leakage current of the device with dual-layer NCPVP/CPVP dielectrics can be improved at least one order magnitude smaller than that with a single-layer CPVP, which may be ascribed to the reduction of cracks and valleys generated during the thermal treatment process. Besides, the lower surface energy on the surface of dual-layer polymer dielectrics results in better pentacene film morphology and larger crystalline size, contributing to better output characteristics and more stable properties of OTFTs. Another two effective ways to improve device performance are also presented. One is to enhance the injection efficiency from source/drain to active layer; another is to improve active film quality. In the former, ITO/C60 source/drain contact applied in transparent organic thin-film transistors could resulting in the increased mobility by more than four times as compare to the device with ITO electrode only. The experimental results suggest the introduction of C60 layer could modify the work function of ITO, contributing to the barrier lowering of the S/D contacts. Even after the introduction of the 3.5nm-thick C60 layer, the average transmittance of the device in the visible region could remain at 62.98%. In the latter, sandwich structure pentacene/C60/pentacene was applied in the active channel layer. The introduction of ultra-thin C60 modification layer could smooth the surface on the lower pentacene film, facilitating the deposition of pentacene molecules in a smaller tilt angle (the angle between pentacene molecule and normal vector of the substrate). This will delay phase transformation in pentacene film and result in the higher order active film structure, contributing to the stronger intermolecular coupling and, therefore, to improved device performance by more than six times.
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