Stability of Organic Thin Film Transistors

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Stability of Organic Thin Film Transistors Jeong In Han, Yong Hoon Kim, Sung Kyu Park, Dae Gyu Moon Won Keun Kim Information Display Research Center, Korea Electronics Technology Institute, Pyungtaek, Kyunggi, Korea.

ABSTRACT The stability of organic thin film transistors (OTFTs) has become one of the most vital issues in this area of research. In this report, we investigated the stability of rubber stamp printed OTFTs. The electrical properties such as saturation field effect mobility, threshold voltage and on/off current ratio change significantly in ambient air condition. In order to analyze the degradation of the device, transistors were measured in vacuum, dry N2 and air environment as a function of time. In vacuum and dry N2 atmosphere, saturation field effect mobility and threshold voltage variations are relatively small compared to those measured in ambient condition. To realize an air stable device, we applied a passivation layer which protects the device from oxygen or water molecules which is believed to be the source of the degradation. With the passivation layer, the threshold voltage shift was reduced suggesting that a proper passivation layer is a prerequisite in organic-based electronics.

INTRODUCTION Electronic devices based on organic materials are now widely investigated as an alternative to conventional inorganic devices. Especially in flexible display applications, organic materials are preferred because these organic materials show excellent mechanical stability, whereas cracks and failures are easily occurred using inorganic materials. However most of the functional organic materials are very sensitive to air, leading to significant degradation or failure of the device. Thus in order to achieve an air-stable and reliable device, the degradation process by the air must be clarified beforehand. Several physical mechanisms have been proposed for the degradation recently, such as trapping of charges at semiconductor-insulator interface [1] or at the grain boundary [2] and chemical degradation of the organic material. In this report, we investigated the stability of OTFT in various conditions using P3HT as the active layer. The device was relatively stable when measured in vacuum condition, however, as the air was introduced the mobility decreased by an order of 102. Also, the threshold voltage was also found to be sensitive to the time of exposure to air.

EXPERIMENTAL DETAILS The gate electrode, Al or Ni, was deposited by rf-magnetron sputter on glass and polycarbonate (PC) film substrates at room temperature. The thickness of the gate electrode was 100 nm. The gate insulator, silicon dioxide (SiO2) with thickness of 250 nm was deposited by ebeam evaporator at 110°C. The source and drain electrodes which consist of Cr (20 nm) and Au (70 nm), were deposited sequentially by e-beam evaporator and thermal evaporator, respectively.

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The channel length (L) and width (W) were 25 and 500 µm, respectively. Figure 1 shows a schematic image of a bottom-contact type OTFT device and structu

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