Low-Voltage Organic Thin Film Transistors on Flexible Plastic Substrates with Anodized Ta 2 O 5 Gate Insulators
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Low-Voltage Organic Thin Film Transistors on Flexible Plastic Substrates with Anodized Ta2O5 Gate Insulators Youji Inoue, Yoshihide Fujisaki, Yoshiki Iino, Hiroshi Kikuchi, Shizuo Tokito and Fumio Sato Science and Technical Research Laboratories of NHK, 1-10-11 Kinuta, Setagaya-ku, Tokyo, 157-8510, Japan ABSTRACT Organic thin film transistors (OTFTs) that operate at low voltage were fabricated using a tantalum oxide (Ta2O5) gate insulator on plastic substrates. The Ta2O5 insulator was prepared by anodizing a Ta gate electrode. The high dielectric constant of the Ta2O5 enabled the OTFTs to operate at a lower voltage than those of previous devices. The OTFTs exhibited a high field-effect mobility of 0.35 - 0.51 cm2/Vs, and a current on/off ratio of 105 -106 at gate voltages of less than 5 V. The threshold voltage of -1.1 V and the subthreshold slope of 0.2 V/decade were the best among those reported to date. We also demonstrated operation of a phosphorescent organic light-emitting device (OLED) with the OTFT operating at a low voltage. INTRODUCTION Organic electronic devices have been attracting a lot of attention because of their potential use in flexible displays; organic materials are compatible with polymeric substrates that can be fabricated with a low-temperature process. Combination of organic light emitting device (OLED) and organic thin-film transistor (OTFT) will lead to economical, lightweight, and mechanically flexible displays. Recent progress in OLED technology has been impressive [1,2], and passive matrix flexible displays based on small molecule and polymer OLED have been demonstrated [3,4]. However, the active-matrix, needed for a high-resolution, long-lifetime and high-efficiency display is only in the development stage [5,6], as high-performance OTFTs have yet to be fabricated on plastic substrates. Driving an OLED pixel requires a high mobility, a low threshold voltage and good stability. The performance of OTFTs has been improved through various efforts, such as the development of new organic semiconductors and the optimization of the deposition conditions for controlling the ordering of the molecules [7-9]. Pentacene OTFTs have achieved a mobility larger than 1 cm2/Vs and a current on/off ratio of 108 [10], values which are comparable to those of amorphous silicon (α-Si) TFTs. However, the relatively higher operating voltage (more than 30 V) for those OTFTs to achieve a high enough mobility was too high for practical display panels. To achieve a high mobility at a low operating voltage, gate insulators with high dielectric constants have been employed [11,12]. Barium zirconate titanate was deposited by radio-frequency (RF) sputtering at room temperature, and high mobility was obtained at operating voltages as low as 4 V. Although RF sputtering is suitable for preparing metal oxide films, it requires high process temperatures in order to form a high-quality insulator. Anodization of metal is a much more suitable method to fabricate dielectric thin films with high dielectric constants, high
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