High Performance Polymer Thin Film Transistors Array Printed on a Flexible Polycarbonate Substrate
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High Performance Polymer Thin Film Transistors Array Printed on a Flexible Polycarbonate Substrate Sung Kyu Park, Jeong In Han, Dae Gyu Moon, Won Keun Kim, and Yong Hoon Kim Information Display Research Center, Korea Electronics Technology Institute, Pyungtaek, Kyunggi, Korea. ABSTRACT High performance poly (3-hexylthiophene) (P3HT) thin film transistors (TFTs) array was fabricated on a polycarbonate substrate by micro-contact printing method. A thin polyimide layer (40 nm) was applied before silicon oxide deposition to improve the electrical properties of the TFT device. Also, the effects of O2 plasma treatment on the field effect mobility and output current behaviors of the devices were investigated. By plasma treatment, the surface roughness of gate dielectric was improved which accounts for the increased field effect mobility and the hole Schottky barrier height in electrode/semiconductor interface was lowered resulting in large drain current in the device. Based on the experiments, we fabricated P3HT TFTs array with 0.025 cm2/V·s in saturation field effect mobility and on/off current ratio of 103 ∼ 104 on a polycarbonate substrate. INTRODUCTION The performance of organic thin film transistors (OTFTs) based on conjugated polymers has continuously increased in the last decade [1-2]. Conjugated polymers are usually soluble in most organic solvents, such as C6H5Cl, CHCl3 and THF. Thus solution-based processes, like ink-jet printing [3], spin coating, dip coating and stamp printing are easily adaptable. However, ink-jet printing lacks in high-resolution printing and usually requires wells to produce any patterns on the device. Spin coating and dip coating are also possible but photolithographic patterning would require new photo-sensible polymer materials to be developed. However, using pre-patterned silicone elastomer stamps, high-resolution (sub-micron) polymer layers can be printed without any lithographic process, which is suitable for low-cost display devices. In this research, we have fabricated OTFT devices with P3HT as active layer on flexible polycarbonate substrates by using micro-contact printing method. Also, the effects of O2 plasma treatment on the electrical properties of OTFT devices were investigated. EXPERIMENTAL DETAILS Polycarbonate (PC) films with thickness of 100 µm and 200 µm were used as the substrates. Prior to device fabrication, the substrates were pre-annealed at 120 °C for several hours for the reduction of shrinkage during the process. Aluminum (Al) gate electrode was then deposited on the substrate at room temperature. As gate dielectric, polyimide-SiO2 dual layer was applied. The surface of bare PC substrate is quite rough (3 ~ 4 nm) thus to improve the surface roughness of the gate dielectric, thin layer of polyimide (40 nm) was spin coated before SiO2 deposition. SiO2 deposition was carried out by e-beam evaporation, which is favored for hydrogen free process and low ion damage at relatively low temperature (120 °C) [4]. Detailed experiments on the gate dielectric layer were
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