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Abstract

Abstract

Development of Solution-Processable Gate Insulators for Organic Thin Film Transistor Using Polymer Composites

Submitted for the Ph. D. degree - Doctor of Philosophy Lee, Eunkyung Department of Chemistry Hanyang University

Organic thin film transistors (OTFTs) represent a promising alternative to silicon-based materials, specifically adapted to flexible, foldable and large-area electronic applications. Within the fame of low-cost electronics, OTFT would be an important component of the future generation of organic light emitting diode (OLED), liquid crystal displays (LCD), electronic papers, and radio-frequency identification tags (RFID) produced with the solution process. The organic gate insulating layer at the bottom (bottom gate structure) or the top (top gate structure) of the organic semiconductor layer is a critical part controlling the overall performance of the OTFT device. While being compatible with the solution process, the organic gate insulator should keep a high breakdown strength, low leakage current and high capacitance for large drain current when operating at low biases with enough thickness. In this thesis, I report on three different new approaches for solution-processable organic gate insulators with a high dielectric strength. In chapter 2, poly(N-(4-hydroxyphenyl)maleimide-co-4-vinylphenol) (PHPMIVP) and its derivatives were developed for polymer gate dielectrics exhibiting the high chemical resistance to the various organic solvents and the hysteresis free operations in OTFT. PHPMIVP were modified with photo-reactive side-groups including cinnamoyl (PHPMIVP-C), methacroyl (PHPMIVP-M) or 4-(6-(7-coumarinyloxyl)hexyloxy)benzoyl (PHPMIVP-CHB). Especially, PHPMIVP-CHB exhibited the high thermal stability and the very strong chemical resistance to the various organic solvents, which are beneficial for forming dielectric layers and semiconducting layers by sequential spin-casting processes without deterioration of device performance. Neither breakdown-voltage shift nor change in the leakage current density curve was observed. The field-effect transistors fabricated by sequential spin-casting of PHPMIVP-CHB insulating layers and PQTBTz-C12 semiconducting layers showed a charge mobility of 0.029 cm2/Vs and an on/off ratio of 106. In chapter 3, polysiloxane/nanosilica composite (PSR-NSC) was developed as a hybrid organic-inorganic gate insulator material for high dielectric strength gate insulator layer in OTFT, by reinforcing a polysiloxane matrix with nanosilica. The nanosilica is added to increase the density the film by filling the interstitial voids of the polysiloxane networks, and which leads to enhance the dielectric strength, and the thermal resistance without altering the flexibility of the layer. The PSR-NSC film showed a low leakage current and no observable breakdown voltage up to 4.3 MV/cm. Thin film transistor devices fabricated using PSR-NSC as a gate insulator and dibenzothiopheno[6,5-b:6’,5’-f]thieno[3,2-b]thiophene (DTBTT) as an active semiconductor, showed a charge mobility of 2.5 cm2/Vs and an on/off ratio of 106. In chapter 4, it was demonstrated that the dielectric constant of gate insulators can be modulated by improved by making hybrid composites with high-k nanoparticles. The crucial factor for achieving the high performance of the gate insulator was the homogenous distribution of nanoparticles in the polymer resin. We showed that the direct surface modification with reactive methacrylate monomers is effective for homogeneous dispersion of nanoparticles in polymer resin. Hybrid composites based on PSR-Al2O3, PSR-ZrO2 and PSR-TiO2 were demonstrated for gate insulator layers with high dielectric constant (k > 5). The gate insulator layers were transparent without haze and showed the high breakdown voltage and low leakage current. Thin film transistor device based on dibenzothiopheno[6,5-b:6',5'-f]thieno[3,2-b]thiophene (DTBTT) and hybrid gate insulator showed a mobility of 2.3 cm2/Vs and an on/off ratio of 107. The three organic gate insulators described in this thesis showed a good processability for solution coating, a good film formability, a high resistance and especially a high dielectric strength. Among them, coumarin polymer (PHPMIVP-CHB) shows a high stability even in ambient atmosphere, but the layer cannot be treated with a self-assembled monolayer (SAM) such as ODTS (octadecyltrichlorosilane) to increase the mobility of the device. In contrast, hybrid gate insulators based on polysiloxane and nanoparticles (PSR-NSC, PSR-SNPs) showed the high compatibility with SAM surface treatment for a further increase of the device performance. In chapter 4, the dielectric constant of polysiloxane nanocomposite gate insulator layers in OTFT can be improved by inserting high-k nanoparticles in a polysiloxane resin. A crucial point for high performance is the homogenous distribution of nanoparticles in the resin matrix, which was achieved here through their surface treatment with reactive methacrylate groups. By controlling the contents of the high-k nanoparticles in the resin, PSR-Al2O3, PSR-ZrO2 and PSR-TiO2 gate insulator layers with a dielectric constant over 5 was achieved, which represent a new promising threshold considering the low temperature process. And the gate insulator layers are furthermore transparent without haze and high breakdown voltage and low leakage current. Thin film transistor devices were then fabricated by depositing in vacuum a dibenzothiopheno[6,5-b:6',5'-f]thieno[3,2-b]thiophene (DTBTT) for the semiconductor layer. The OTFT device shows a mobility of 2.3 cm2/Vs and an on/off ratio of 107. The three organic gate insulators developed in this thesis have a good processability for solution coating, a good film formability, a high resistance and especially a high dielectric strength. Among them, coumarin polymer (PHPMIVP-CHB) shows a high stability even in ambient atmosphere, but the layer cannot be treated with a self-assembled monolayer (SAM) such as ODTS (octadecyltrichlorosilane) to increase the mobility of the device. In contrast, modified polysiloxanes with nanoparticle (PSR-NSC, PSR-SNPs), while having a lower stability, has the advantage of being compatible with SAM surface treatment for a further increase of the device performance.