Conjugated polymers electrochromic applications

The application of a high electric field across a thin conjugated polymer film has shown the materials to be electroluminescent (216—218). Until recendy 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 facility in device fabrication as a result of the ease of processibility of soluble polymers (see CHROMOGENIC MATERIALS, electrochromic). 

Conjugated polymers, discovered in the late 1970s, have attracted a variety of attentions because of their unique properties, such as electrical conductivity and color versatility. The conjugated polymers with different colors can be used as ideal electrochromic materials, which have potential applications in sensors, mirrors, displayers, and textiles. Most of their reversible electrochromic behaviors are caused by the electro-induced oxidation-reduction, that is, the reversible change of a chemical species between two redox states under a certain voltage (Niklasson and Granqvist, 2007 Beaujuge et al., 2010). 

PEDOT, one of the most popular jr-conjugated polymers, has been intensively studied for developing nanoscale materials as well as for application to various nanodevices such as biosensors and electrochromic devices, and for drug delivery [82-84]. However, studies on PEDOT nanomaterials and bulk films have mainly focused on their electrical and structural properties and on the various applications of the conducting form of the material (i.e., doped PEDOT systems). The light-emitting characteristics of doped and de-doped PEDOT nanomaterials were first reported by Park et al. in 2008 [43]. 

Among the conjugated polymers, polypyrrole (PPy) is the most representative one for its easy polymerization and wide application in gas sensors, electrochromic devices and batteries. Polypyrrole can be produced in the form of powders, coatings, or films. It is intrinsically conductive, stable and can be quite easily produced also continuously. The preparation of polypyrrole by oxidation of pyrrole dates back to 1888 and by electrochemical polymerization to 1957. However, this organic p>-system attracted general interest and was foimd to be electrically conductive in 1963. Polypyrrole has a high mechanical and chemical stability and can be produced continuously as flexible film (thickness 80 mm trade name Lutamer, BASF) by electrochemical techniques. Conductive polypyrrole films are obtained directly by anodic polymerization of pyrrole in aqueous or organic electrolytes. 

While an invaluable tool in producing conjugated polymers on conducting substrates, electropolymerization has limitations that include a lack of primary structure verification and characterization along with the inability to synthesize large quantities of processable polymer. To overcome the insolubility of PEDOT, a water-soluble polyelectrolyte, poly(styrenesulfonate) (PSS) was incorporated as the counterion in the doped PEDOT to yield the commercially available PEDOT/PSS (Baytron P) (39), which forms a dispersion in aqueous solutions [140]. While this polymer finds most of its application as a conductor for antistatic films, solid state capacitors, and organic electronic devices, its electrochromism is distinct and should not be ignored. 

K-Conjugated polymers and oligomers are organic materials with many interesting and useful properties [1, 2], Examples of this class of materials include polyacetylene, polythiophene, polypyrrole, poly(phenylenevinylene) and their derivatives. Electronic conductivity, luminescence and nonlinear optical behavior are all observed in these materials and these properties have been exploited in applications such as electroluminescent devices (polymer light-emitting devices or PLEDs), electrostatic coatings, electrochromic windows, chemical sensors and memory devices [3-9]. 

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