ITO nanowires and nanoparticles for transparent films
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Introduction Indium tin oxide (ITO, or tin-doped indium oxide) possesses both a high level of conductivity (≈104 Ω–1cm–1, typical sheet resistance of ≈10 Ω/sq) and transparency (>80% in the visible range) that have made it the most widely used transparent electrode material today, with over 90% of the current market share.1 It is a member of the class of materials known as transparent conducting oxides (TCOs). Although other TCO materials possess properties similar to ITO, industry has chosen ITO as the transparent electrode material of choice due to the combination of conductivity, transparency, and ease of processing offered by ITO as compared to other TCOs. Presently, the largest use of ITO transparent electrodes is in flat panel displays,1 but the material is also used in a variety of other optoelectronic devices, such as inorganic- and organic-based solar cells2 and light-emitting diodes (LEDs).3 ITO itself consists of a solid solution of indium oxide (In2O3) and tin oxide (SnO2), typically 5% to 10% SnO2 by weight, in which tin acts as an impurity dopant. The relatively large amount of tin content in ITO causes a degenerate doping condition (doping densities of ≈1021 cm–3 are typical), which in turn gives rise to large conductivity and metallic behavior.4 Moreover, since both SnO2 and In2O3 are large bandgap (> 3 eV) materials, ITO itself possesses a large bandgap (3.3 eV to 4.3 eV, depending on the preparation method used), which results in substantial transparency to visible light.5 In addition,
ITO possesses several advantages beneficial to manufacturing needs: it has good chemical durability (including resistance to moisture and stability in air) and is readily patterned by wet chemical etching.6 ITO has been extensively studied and utilized for industrial purposes for over 50 years.4 Currently, it is most often used in the form of polycrystalline thin films that are deposited by physical vapor deposition (PVD) methods, in particular, sputter deposition. Such sputter-deposited thin films, although widely used in industry, possess two key disadvantages: (1) physical incompatibility with flexible substrates and (2) high cost. Thus far, deficiencies have restricted the use of ITO to rigid substrates and have prevented its usage on less expensive, flexible substrates (e.g., plastics or paper). This incompatibility is a major problem; electronic devices such as smart phones and tablet computers fabricated on low-cost, flexible substrates are expected to eventually displace the more expensive, rigid silicon and glass substrates used in conventional electronics.7 Two reasons account for the physical incompatibility of sputter-deposited ITO with most flexible substrates: (1) the high deposition temperatures (>300°C) that are required by sputtering to achieve adequate film quality,8 and (2) the relatively brittle nature of the films, which when deposited on flexible substrates such as plastic sheets are prone to cracking under repeated or extreme bending.9 Besides these physical incompatibilities, the sputter deposition of ITO
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