Electrochromism Electroluminescence

Indium-tin oxide (ITO, or tin-doped indium oxide) is a solid miture of indium(III) oxide (Iu203) and tin(IV) oxide (Sn02). It is transparent and colorless in thin layers. In bulk form, it is yellowish to grey. It is a transparent conducting material that is usually used in thin coating form. ITO is commonly used in applications such as touch panels, electrochromic, electroluminescent and LCD displays, plasma displays, field emission displays, heat reflective coatings, energy efficient windows, gas sensors and photovoltaics. 

Reprinted from F. Carpi, D. De Rossi, Colors from electroactive polymers electrochromic, electroluminescent and laser devices based on organic materials. Opt. Laser Technol. 38, 292 (2006), Copyright 2006, with permission from Elsevier. 

The apphcation of a high electric field across a thin conjugated polymer film has shown the materials to be electroluminescent (216—218). Until recentiy the development of electroluminescent displays has been confined to the use of inorganic semiconductors and a limited number of small molecule dyes as the emitter materials. Expansion to the broad array of conjugated polymers available gives advantages in control of emission frequency (color) and facihty in device fabrication as a result of the ease of processibiUty of soluble polymers (see Chromogenic materials,electrochromic). 

As a result, an insolnble transparent blue polymer film forms on the electrode. Electrochemical oxidation of the film in acetonitrile initiates a rapid color change from bine to pale gray, while redaction to the first or second cathodic waves causes the film to become pale green or orange, respectively. These electrochromic effects are stable and reversible when air and water are excluded, even after 30,000 rednction cycles. The material has potential uses in electrochromic or electroluminescent devices. 

The examples of polyacetylenes whose main chain is directly bonded to heteroaromatic rings (e.g., silole, carbazole, imidazole, tetrathiafulvalene, ferrocene) are increasing in number. Such polymers are usually obtained by one of catalysts (W, Mo, and Rh). The formed polymers are expected to display interesting (opto)electronic properties such as electrochromism, cyclic voltammetry, electroluminescence, and so on. 

High-resolution circuitry and active devices employing Langmuir-Blodgett film techniques or polymer-based transistors are being considered for the sophisticated electronics required in future vehicles. Temperature or energy balance in the vehicle could be controlled through conductive polymers or semiconductor deposits on electrochromic windows. Electroluminescent liquid crystals and fluorescent and electrochromic materials used for visual displays show promise for future development. 

Lincot, Froment, and Cachet review the chemical and mechanistic aspects of chemical bath deposition of chalcogenide compounds with special emphasis on structural properties associated with epitaxial growth. Present applications of chemically deposited films are reviewed and several characteristic advantages are identified that may be exploited in the future for applications such as small band gap semiconductors, large area electrochromic devices, electroluminescence, quantum sized films, and films with spatially modulated composition and structure. 

Conducting polymers such as polyacetylene, polypyrrole, polyaniline, polythiophene, etc. have been actively studied for use in various fields due to their interesting properties batteries,46 electrochromic displays,47 materials for supercapacitors,48 corrosion protection,49 protecting layers for static electricity,50 materials for organic electroluminescence displays,51 sensing materials,52 etc. Polypyrrole is reported to be extremely rigid, with a semi-crystalline structure. 

Gas discharge displays, electrochromic and electroluminescent devices [161] are used for similar purposes as liquid crystals, but in place of the liquid crystal, a plasma (gas discharge like in a Ne tube) or solid state materials are used. The optical and electrical requirements are very similar. With gas discharge display electrodes, a high temperature stability is also required (450° C cycling). 

While less intensively studied than other conducting polymers covered in this chapter, polycarbazoles have found the most use as electroluminescent polymers. In the area of electrochromics, carbazoles have most commonly been utilized as a unit in electrochromic hybrid heterocycle systems, which will be covered later in this chapter. Most commonly, substitution occurs at the M-position. While this can offer increased solubility in a variety of solvents, there is not much variation in the colors achieved. 

---