Solution-deposited PEDOT for transparent conductive applications
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Introduction The first thoroughly investigated, intrinsically conductive polymer (ICP), polyacetylene, was discovered in 1977.1,2 Due to the availability of free charge carriers in ICPs, they can conduct current in contrast to traditional polymers such as polyethylene or polypropylene.3,4 While polypropylene is an insulator and can be used as a cable coating to confine current to a metallic wire, ICPs have metallic properties and can be used to conduct current themselves. Furthermore, ICPs have the advantage of easy processing, such as printing and drying at low temperatures. However, the conductivity of ICPs is much lower than that of metals. Nonetheless, due to their specific properties as polymers, they have found a wide range of industrial applications within the last 30 years. The specific properties of ICPs arise since these materials are conjugated polymers with alternating single and double bonds. Figure 1 shows some important ICPs in their neutral form. Free charge carriers in ICPs can be introduced by oxidation or reduction. As a result, the electrical behavior of these materials changes from a typical semiconductor to a conductor. For example, when polyacetylene is oxidized by AsF5, the conductivity increases from 10–5 S/cm to 200 S/cm.5 This process is often referred to as doping, and the resulting polymer is called a doped polymer. The conductivity is due to the mobility of free charge carriers (i.e., electrons and holes). Polyacetylene has not penetrated into the marketplace, since the material is unstable under ambient conditions. However,
other ICPs are very stable. For example, polypyrrole is used in polyelectrolyte capacitors, and polyaniline is used for antistatic coatings on plastic films. Poly(3,4-ethylenedioxythiophene) (PEDOT) is a highly stable ICP, and PEDOT films are exceptionally transparent and conductive. It was first discovered in 1988,6 and its first application was its use as an antistatic agent in photographic films. Since then, it has penetrated into other markets, including solid electrolyte capacitors, printed wiring boards, packaging films, and as hole transport layers in organic light-emitting diodes (OLEDs) and organic photovoltaics (OPV).7–9 However, in this article, the focus is on the use of PEDOT as a transparent electrode. Therefore, the properties of the PEDOT dispersion, the physics of PEDOT films, and applications of PEDOT as a transparent electrode (e.g., in electroluminescent lamps, touch screens, or OLEDs) will be described. The availability of PEDOT in a solution processable form is a great advantage, and the way in which such a solution can be obtained is described in the following section.
Solution-processable PEDOT When PEDOT was first discovered, it was obtained by the oxidation of ethylenedioxythiophene by iron salts.6 The resulting PEDOT forms a dark blue precipitate, which cannot be re-dissolved, re-dispersed, or used in any other way for transparent conductive coatings. However, if the reaction mixture is deposited onto a substrate and the polymerization takes place o
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