Doping electrochromic devices

Further research on the substitution of the thiophene 3-position with phenyl groups containing electron-withdrawing or electron-donating groups (such as methyl, methoxy, fluoro, chloro, bromo, trifluoromethyl, sulfoxy) in the para position have lead to polymers with unique features [57]. The electron-withdraw-ing groups allow the formation of a radical anion and thus stabilize the n-doped state. As a result, such conducting polymers can be reversibly oxidized and reduced and electrochromic devices can be built with identical anode and cathode materials [58]. 

Indium-tin oxide (ITO) is indium oxide doped with tin oxide. Thin films of ITO have commercially valuable properties it is transparent, electrically conducting and reflects IR radiation. Applications of ITO are varied. It is used as a coating material for flat-panel computer displays, for coating architectural glass panels, and in electrochromic devices. Coating motor vehicle and aircraft windscreens and motor vehicle rear windows allows them to be electrically heated for de-icing... 

Critical properties of TCO coatings are electrical resistance and transparency [3], but for solar cell applications very often texture and large haze factors, i.e., ratio of diffuse to total transmission, have similar importance. Large haze factors have been shown to influence positively the efficiency of silicon solar cells, because the reflection at the TCO-silicon interface is reduced and the scattering increases the pathway of light inside the active material. The preparation and characteristics of several TCO materials have been reviewed by Chopra et al. [92] and Dawar and Joshi [93]. The optical and electrical properties of ITO and aluminum doped zinc oxide have been studied in detail by Granqvist and coworkers [94, 95], but these films were prepared by sputtering and not by CVD. Very recently they also published an overview of transparent conductive electrodes for electrochromic devices [7]. 

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]. 

Solid-state systems of particular interest in this book are conductive polymers with the ability to occlude dopants entering the bulk of the polymer sample, thus conferring on it its special electrical properties. Some of these properties are a consequence of the mobility of the dopant ions in the host polymer material, and these properties are responsible for such technological applications as battery electrodes,ion gates, and electrochromic devices, which depend on a field-induced oxidation of the polymer specified by its doping level. Various diffraction methods and tunnelling electron microscopy reveal that these... 

Differendy linked carbazole derivatives possessing selenophene and pyrene (the latter not covered in this review) moieties were electropoly-merized (14EA430). The electropolymerized polymers displayed unusual properties upon electrochemical doping suggesting their potential use as electroactive layers in electrochromic devices. A typical synthesis of 2,7-diselenophenylcarbazole is shown below. 

Optical spectra (UV/ViyNIR) of electrochemically polymerized PEDT in both its reduced and fully oxidized state have been published as early as 1994 [10,103], demonstrating the nearly colorless, sky-blue conductive doped state. In addition, optical spectra of the PEDT PSS complex have been measured in the same maimer, exhibiting a value of about 600 run for the reduced form [26]. The exact chemical nature of the deep blue, reduced state of PEDT and PEDT PSS used in electrochromic devices has long been under discussion. As previously discussed in this review, fiilly undoped, reduced PEDT is not blue, but is reddish purple, and it cannot be readily obtained by electrochemical means [27,28]. Some recent... 

Self-doped polyanilines are advantageous due to properties such as solubility, pH independence, redox activity and conductivity. These properties make them more promising in various applications such as energy conversion devices, sensors, electrochromic devices, etc. (see Chapter 1, section 1.6). Several studies have focused on the preparation of self-doped polyaniline nanostructures (i.e., nanoparticles, nanofibers, nanofilms, nanocomposites, etc.) and their applications. Buttry and Tor-resi et al. [51, 244, 245] prepared the nanocomposites from self-doped polyaniline, poly(N-propane sulfonic acid, aniline) and V2O5 for Li secondary battery cathodes. The self-doped polyaniline was used instead of conventional polyaniline to minimize the anion participation in the charge-discharge process and maximize the transport number of Li". In lithium batteries, it is desirable that only lithium cations intercalate into the cathode, because this leads to the use of small amounts of electrolyte... 

C. H. Yang, Y. K. Chih, W. C. Wu, C. H. Chen, Molecular assembly engineering of self-doped polyaniline film for application in electrochromic devices, Electrochemical and Solid State Letters 2006, 9, C5. 

In this section, the constmction of electrochromic devices whose principle electrochromic materials are polythiophenes is discussed. 3-Substituted polythiophenes are the most popular electrochromic materials for such devices. A solid-state electrochromic device was assembled by using poly[3-[12-(p-methoxyphenoxy)dodecyl]thiophene] and poly(3,4-ethylenedioxythiophene) doped with poly(styrene-sulfonate), which showed a color variation from red in the reduced state to blue in the oxidized state during ca. 500 charge-discharge cycles [80]. 

Dual-type absorptive/transmissive polymer electrochromic devices based on poly[thiophen-3-ylacetic acid 4-pyrrol-1-ylphenyl ester (TAPE)-co-A -methylpyrrole (NMPy)] and PEDOT have been assembled, which exhibit good optical memory, stability with moderate switching times and light yellow and green colors upon doping and dedoping, respectively [82],... 

Ethylenedioxythiophene (EDOT) (34) is now commercially available. Hey-wang and Jonas [234-236] first polymerized EDOT and found that the relevant polymer could be useful for antistatic coatings. Heinze and co-workers [237] subsequently studied the spectroscopic and electrochemical properties of poly(EDOT) the optical band gap of the polymer is about 16 eV (760-780 nm). Inganas and colleagues [238] showed the usefulness of poly(EDOT) as a potential material for electrochromic devices because of its ability to cycle between an opaque blue-black in the reduced (undoped) state and a transparent sky blue in the oxidized (doped) state. Recently, poly(EDOT) has been used for the construction of rapidly switching solid-state electrochromic devices based on complementary conducting polymer films [239]. 

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