Low temperature deposition of Indium tin oxide (ITO) films on plastic substrates
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Low temperature deposition of Indium tin oxide (ITO) films on plastic substrates Vandana Singh, B. Saswat and Satyendra Kumar Samtel Center for Display Technologies Indian Institute of Technology Kanpur, India Email: [email protected]
ABSTRACT Organic light emitting diodes (OLEDs) require a transparent conducting oxide (TCO) electrode for injection of charge carriers and the emitted light to come out. In order to exploit the full flexibility of organic semiconductor based large area electronic devices, the deposition of TCO on plastic substrates is essential, which prohibits high temperature processing. Therefore, low temperature deposition of Indium tin oxide (ITO) films is very important for flat panel displays and solar cells. Here we have carried out a systematic study of ITO deposition on plastic substrates using RF magnetron sputtering. For the optimization of structural, electrical and optical properties of ITO, various experiments such as X-ray diffractometer, transmission measurements, sheet resistance and AFM were employed. These properties were investigated as a function of substrate temperature, deposition time and RF power. From these experiments, we obtained a reasonably low sheet resistance (~14 Ω /□) and high transmittance (~75%) in the visible region on plastic substrates. We also observed that these films are not much affected by atmosphere and does not degrade with time. These ITO films deposited by RF magnetron sputtering on plastic substrates can be use as anode for flexible organic light emitting displays. Key words: ITO, OLED, Plastic Substrate and Magnetron Sputtering. INTRODUCTION Discovery of electro-luminescence from organic polymer poly (1,4-phenylene vinylene) PPV [1], provided an opportunity to make organic light emitting devices (OLED). These devices are lightweight, flexible and easy to fabricate on large area. Basic structure of an OLED is one or two organic layers (100nm) sandwiched between two electrodes, one of which is transparent. So, any OLED needs a thin film of transparent conducting oxide (TCO) material for injection of charge carriers and extraction of emitted light. This thin film of TCO should have low resistivity, high transmission in the visible range and high work function. ITO, a highly degenerate wide band gap semiconductor and has low resistivity (10-4 Ω cm), high transmission in the visible range (~98%) and high work function (4.6 ev) [2], is most widely used TCO material. However, glass is very brittle, too heavy for large area displays and cannot be deformed. These problems can be overcome by using plastic substrates, which are lightweight, robust and flexible. So plastic substrates have been used in polymeric and molecular OLEDs. For the full flexibility of organic light emitting displays, OLEDs need to be fabricated on plastic (polymer) substrate. For flexible OLED (FOLED), ITO is deposited on plastic substrates, which has relatively high resistivity compared to glass and low glass transition temperature (Tg). Due to temperature constraints, low temp
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