Electrochromic material

Electrochromic materials are cheap and simple to produce and they can provide reasonable contrast for a variety of colors [13], The increasing popularity of electrochromic materials is due to their rapid response times, long lifetimes, high optical contrasts and their ability to be modified in order to create multicolored electrochromics [7], 

The alteration of the optical properties for an electrochromic material involves the insertion or extraction of charge. These polymers can be classified into three types, depending on their specific optical states (1) absorption/transmission-type materials made of metal oxides, viologens or polymers such as PEDOT, including at least one colored and one bleached state for smart windows, (2) display-type materials made of polythiophenes without a transmissive state and (3) materials composed of blends, laminates and copolymers including more than two colored states [7], 

The importance of an electrochromic material can be measured by the intensity of the color change between its colored and bleached states. Usually, the intensity of the color change is shown in terms of the thickness-dependent properties such as the change in transmittance (AT), the contrast ratio (CR) or the change in absorbance (AA). Hence the values reported for the same electrochromic material can vary from device to device, depending on the preparation conditions and device design. Using PEDOT, Lim et al. proposed a very convenient and systematic method to predict the thickness for maximum contrast (Lm) of an EC film that is independent of the device characteristics. The ATmax of PEDOT was estimated by Lim et al. to be 48.2 % at ca. 3 cycles [14]. 

Coloration efficiency (electrochromic efficiency). This is the amount of electronic charge (2d) which is necessary to produce an optical density change (AOD). The coloration efficiency (/ ) of an electrochromic material can be defined at the absorbing wavelength and is given by 

Sufficiently high coloration efficiencies of electrochromic materials can be used to develop very rapid, efficient displays and light modulators. Eor example, the unusually high coloration efficiency of poly(3,4-ethylenedioxythiophene-didodecyloxybenzene) (PEB) could be effected in the presence of tungsten trioxide (WO3) as a counter electrode. This PEB-WO3 device was reported to have a maximum coloration efficiency of 1250 cm C at 552 nm [11]. The coloration properties and efficiencies for the PEB cell can be compared with those of other thiophene-based systems, such as PEDOT, as shown in Table 20.1 [11]. 

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Miscellaneous. Iridium dioxide, like RUO2, is useful as an electrode material for dimensionally stable anodes (DSA) (189). SoHd-state pH sensors employing Ir02 electrode material are considered promising for measuring pH of geochemical fluids in nuclear waste repository sites (190). Thin films (qv) ofIr02 ate stable electrochromic materials (191). 

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

Polymers, large molecules made up of smaller molecules in a repeating pattern, are used for many electrochromic materials. Conjugating polymers, which have alternating single and double bonds, are particularly suitable. Figure B shows the electrochemical oxidation of the conjugated polymer, polythiophene. Oxidation (in which electrons are removed) produces a semiconductive polymer. The neutral (unoxidized) polythiophene is red in color, whereas the semiconductive polythiophene (oxidized) is blue. In their neutral... 

Figure 33.1a illustrates the idea of the smart window. In this device a layer of electrochromic material and a layer of a transparent ion-conducting electrolyte are sandwiched between two optically transparent electrodes (OTEs). Indium-doped tin oxide on glass is used most commonly as the OTE. This material has very low... 

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. 

Presented beiow are several electrochromic materials that showed significant promise for practical devices. 

Metal Oxides Tungsten trioxide, undoubtedly the most widely studied electrochromic material, is used in several types of commercial electrochromic devices. 

If an anodically colored electrochromic material (e.g., Ir02) is used as one electrode in the device in Eig. 33.1fi and a cathodically colored (e.g., WO3) is used as the other electrode, a much larger change in transmission per charge supplied can be seen compared to the case when only one electrode is electrochromic. Also, the use of an intercalation material as the counter electrode may be advantageous for the device shown in Eig. 33.1a, as it can minimize undesired reactions on the counter electrode. 

Other Inorganic Compounds Prussian Blue represents another type of inorganic mixed valence electrochromic materials. This material is known in three states ... 

Metal Complexes as Dyes for Optical Data Storage and Electrochromic Materials... 

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 (either as an electroactive surface film or an electroactive solute) are generally first studied at a single working electrode, under potentiostatic or galvanostatic control, using three-electrode circuitry.1 Traditional electrochemical techniques such as cyclic voltametry (CV), coulometry, and chronoamperometry, all partnered by in situ spectroscopic measurements... 

Since PB and WO32 are respectively anodically and cathodically coloring electrochromic materials, they can be used together in a single device121-125 so that their electrochromic reactions are complementary (Equation (15)) ... 

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