Electrochromism applications, electrochromic materials

A large number of materials show electrochromic properties however, only a few of them are interesting for practical applications since for this purpose, additional requirements must be fulfilled (1) as discussed above, for most applications the materials need to have a colorless (bleached) state (2) the electrochromic transformations should occur without parasitic reactions, such as gas evolution or material degradation (3) to be considered practical, the electrochromic materials should survive at least 10 coloring-bleaching cycles (4) the rate of electrochromic transformation needs to be sufficiently fast (1 s for most applications) and (5) due to problems with leaking and diffusion in iiquids, it is preferabie to have insoiubie soiid efec-trochromic materiais. 

It is important, however, to realize that whilst many types of chemical species exhibit electro-chromism, only those with favorable electrochromic performance parameters1 are potentially useful in commercial applications. Thus, most applications require electrochromic materials with a high contrast ratio, coloration efficiency, cycle life, and write-erase efficiency.1 Some performance parameters are application dependent displays need low response times, whereas smart windows can tolerate response times of up to several minutes. 

Electrochromic materials, which change color when subjected to an electric field, are widely explored for application as smart windows that control the amount of light reflected or transmitted. Color production is linked to defect formation in an otherwise colorless matrix. 

Stabilization of the redox cycle is relatively important in construction of potentially useful electrochromic materials, because the molecules needed for application require high redox-stability. Recently, S. Hiinig et al. proposed the concept of violene-cyanine hybrid to produce stabilized organic electrochromic materials (3). The hybrid is constructed by a violene-type redox system containing delocalized closed-shell polymethine dyes as end groups. The hybrid is expected to exhibit the color of a cyanine dye, by an overall two-electron... 

Electrochromic materials are electroactive compounds whose visible spectra depend on the oxidation state. Possible applications are smart windows, displays, mirrors, and so on. Among the most important performance aspects in electrochromic materials, the reversibility and lifetime of the material to repeated cycles, the time of response (usually in order of seconds), the colors of the oxidized/reduced forms and the change in absorbance upon redox switching (contrast) are of interest. 

As shown, optical electron-transfer transitions in polynuclear complexes are certainly not a new phenomenon and have been studied very broadly and thoroughly [119-122]. However, most of the examples are strictly solution based and the attachment to an electrode has not been addressed. As a result, then-applicability to devices has not been investigated. Overall, it is quite obvious that the tremendous potential of the field for the discovery of electrochromic materials has hardly been touched. 

Metal oxides which undergo proton insertion reactions find extensive application in batteries and are currently being investigated as potential electrochromic materials. The properties of battery oxides, e.g. manganese dioxide [107-110] and nickel [111-114] have been extensively reviewed in the literature and will therefore not be discussed here. Rather, the properties of electrochemically grown, electrochromic oxide films will be described since this is a relatively new and interesting field. 

The film as deposited is amorphous and the better crystallisation was observed from the change in the predominant plane of crystallisation (222) and (400) plane after heat treatment. The sharp peaks of (211), (222), (400), (411), (431), (440), (611), (622) and (800) peaks of In Oj emerged from the amorphous XRD at 300 °C annealing in air for 1 hour. The film that is improved in Rs after low annealing T less than 250 °C is still amorphous. The film annealed at 220 °C suits for the application of smart windows of electrochromic material or other ftinctional multilayer films. 

These novel properties are the basis for a number of application including polymer light emitting diodes (LEDs), polymer light-emitting electrochemical cells (LECs), conducting polymers as electrochromic materials, polymer photodetectors and polymer photovoltaic cells. These application areas are discussed in detail in Section VII. 

Electrochromic materials are of interest for displays, smart windows, sunroofs, etc., and are characterized by the reversible change in their color upon application of light or electrical inputs. The electrochromic effect is generally associated with the ingress/issue of electrons and metal cations. 

We devote a significant part of this chapter to the method of chemical vapour deposition, the development of which has been closely tied to the need to deposit thin films of a range of metals and inorganic materials for use in semiconducting devices, ceramic coatings and electrochromic materials. Table 27.3 lists some applications of selected thin film materials. Part of the challenge of the successful production of thin films is to find suitable molecular precursors, and there is much research interest in this area. 

Electrochromic applications of the PAn and substituted PAn films [307a-h], composite films of PAn or its derivatives with other electrochromic materials [307i-m], silanized PAn films [306o] and photo-induced electrochromic reactions on semiconductor particles [307p]. Again, most other polymers can be used for this purpose as long as their oxidized and reduced states are stable and have different colors from that of neutral polymers. 

The electrochromic materials first drew interest for the realization of large area display panels, able to be read at any view angle (which is not the case for liquid crystals). However, the problems related to multiplexing in this kind of device remain in a large part uiuesolved and the two real main applications are in architecture and the automotive industry. 

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