Electrochromic devices types

Construction of a dual-polymer electrochromic device (type iii memory device) that has two face-to-face polymer layers in each cell producing 400 (20 x 20) different combinations of absorptions was shown by Sonmez and Sonmez [84], In this study, several examples of this 3x3 pixel dual polymeric electrochromic device composed of red poly(3-alkylthiophene), green poly(2,3-di(thien-3-yl)-5, 7-di(thien-2-yl)thieno[3,4-fc]pyrazine] and blue PEDOT polymers, which switch at different wavelengths, were presented. These distinct absorption states have led to speculation that electrochromic polymers could be used for memory storage devices and computing functionalities. 

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

Stability tests performed in sandwich-type cells containing the dyes adsorbed on Sn02/Sb electrodes demonstrated a high stability, with optical density changes lower than 2% after cycling the electrochromic device 20000 times between -0.5 and +0.5 V. 

Anofher application of CPs with equal significance fo fhe milifary is in the area of electrochromic devices, which may be of two types ... 

S.I. Cho, R. Xiao, and S.B. Lee, Electrochemical synthesis of poly(3,4-ethylenedioxythiophene) nanotubes towards fast window-type electrochromic devices. Nanotechnology 18, 405705 (2007). 

Similarly, introduction of carbazole in 174 also leads to a polymer presenting distinct redox processes and hence optical states [297,298]. Unlike electrochromic materials based on pure PEDOT, these compounds are colored in the oxidized state and colorless in the neutral one. The exploitation of the complementary properties of these two types of polymers has led to electrochromic devices exhibiting a large variety of colors [122],... 

Many typical electrochromic devices are composed of seven layers, as shown in Figure 20.4 [37]. One alternative to this classical model is a simplified five-layer electrochromic device, where transparent conducting layers were eliminated by Mecerreyes et al. (Figure 20.5) [37]. This type of configuration was successfully tested by using a solution of poly(ethylene oxide-lithium triflate) (PEO -I- CEsSOsLi) in tetrahydrofuran (THF) as a polymer electrolyte, while PEDOT was used as an electroactive polymer by Carpi and De Rossi [29]. 

Poly (thiophene)s are of particular interest as electfochromic materials owing to their chemical stability, ease of synthesis and processability. For the most part, current research has been focused on composites, blends and copolymer formations of several conjugated polyheterocyclics, polythiophene and its derivatives, especially PEIX)T. In one example, poly(3,4-ethylenedioxythiophene) (PEDOT)/poly(2-acrylamido-2-methyl-l-propanesulfonate) (PAMPS) composite films were prepared by Sonmez et al. for alternative electrochromic applications [50]. Thin composite films comprised of PEDOT/PAMPS were reported to switch rapidly between oxidized and neufial states, in less than 0.4 s, with an initial optical contrast of 76% at A.max. 615 nm. Nanostructured blends of electrochromic polymers such as polypyrrole and poly(3,4-ethylenedioxythiophene) were developed via self-assembly by Inganas etal. for application as an electrochromic window [26]. Uniir etal. developed a graft-type electrochromic copolymer of polythiophene and polytetrahydrofuran for use in elecfiochromic devices [51]. Two EDOT-based copolymers, poly[(3,4-ethylenedioxythiophene)-aZ/-(2,5-dioctyloxyphenylene)] and poly[(3,4-ethylenedioxythiophene)-aft-(9,9 -dioctylfluorene)] were developed by Aubert et al. as other candidates for electrochromic device development [52],... 

Dual-type polymer electrochromic devices based on copolymers of 2-benzyl-5,12-dihydro-27f-pyrrolo [3, 4 2,3] [1, 4]dioxocino[6,7-6]quinoxaline (DPOQ) and 5,12-dihydrothieno[3, 4 2,3] [1, 4]dithiocino [6,7- >]quinoxaline (DTTQ) with bithiophene were developed. P (DPOQ-co-BT) and P(DTTQ-co-BT) were used as the anodically coloring and PEDOT as the cathodically coloring electrochromic materials [81]. Each device performed with a favorable switching time, optical contrast, open-circuit memory and stability. 

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],... 

Impedance Spectroscopy. Impedance spectroscopy has been carried out on devices with WO3 as the cathodic electrochromic layer, counter electrodes of iridium oxide, polyaniline or Prussian blue, and polymers as electrolytes (Katsube et al [1986], Friestad et al [1997]). The equivalent circuit for a whole device becomes very complicated. In the works quoted above simplified, Randles-type circuits were used for the two electrochromic layers, while the ion conductor was modeled by a pure resistance, or neglected. Extraction of device parameters from the data fitting was reported. However, it is clear that in many cases it will be difficult to distinguish the contributions from the different layers in a device, in particular if the migration impedances, ion diffusion impedances, etc. are of the same order of magnitude. When it comes to characterizing electrochromic devices, impedance spectroscopy is a very time-consuming process, since a spectrum down to low frequencies should be taken at a number of equilibrium potentials. Thus we believe that transient current measurements in many cases offer a faster alternative that sometimes allows a simple determination of diffusion coefficients. 

Electrochromic devices can be categorized according to several principles. Following earlier work of ours, separate discussions are given for devices based on different types of electrolytes liquid, solid inorganic, and sohd organic (polymeric). Almost all of the devices incorporate an electrochromic W oxide film. 

FIGURE 16.13. Reflectance as a function of time for an electrochromic device of the type shown in the inset. The bleached state reflectance was set to 100%. (From Granqvist, C., Handbook of Inorganic Electrochromic Materials, Elsevier Science, 1995. With permission.)... 

Transmitting electrochromic devices can incorporate a thin film that serves as an ion storage. Obviously, this film has to be transparent at least when the W oxide film is in its bleached state, so the carbon-containing layers used successfully in the earlier discussed display-type devices now are useless. A simple approach is to use two W oxide films with different crystallinities. These films color differently under ion intercalation, and when charge is shuttled from a heavily disordered film into a crystalhne film the overall coloration is lowered in the luminous wavelength range (but increased in the infrared), and when charge is moved back into the disordered film the optical effect is reversed. Devices of this type have been discussed by Matsuhiro and Masuda" and Rauh et al. °... 

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