Electrochromic devices transmittance

An electrochromic device embodies a number of superimposed layers on a transparent substrate or between two transparent substrates, and optical transmittance is altered when an electrical potential is applied so that charge is shuttled between layers serving in the same way as anodes and cathodes in an electrical battery. One specific design with a five-layer construction shown in Figure 30 uses cathodically coloring WO3 and anodically coloring nickel oxide joined by an ion-conducting electrolytic laminate. A potential of a few volts, preferably supplied by solar cells, is applied between... 

Compared with PEO electrolytes, PDVF, and PMMA electrolytes exhibited higher ionic conductivities. In particular, PMMA has attracted increasing attentions due to its low cost, high solvent retention ability, high transparency, and processibility. The first allpolymer electrochromic device was obtained based on a gel electrolyte and PEDOT-PSS [poly(styrene sulfonate)] electrochromic material (Argun et al., 2003). The fabricated device exhibited a maximum transmittance change of 51% at 540 nm. In addition, this device was fairly stable and only 5% contrast loss was observed after 32,000 cycles. 

The interest in electrochromic devices lies in the fact that they have a number of specific advantages, such as high optical contrast with continuous variation of the transmittance and no dependence on viewing angle, optical memory, UV stability, and wide operation temperature ranges. These favourable characteristics may ultimately overcome the well known deficiencies of liquid crystal displays and thus place electrochromic devices in a prominent position for the production of large visual angle panels. 

Pol mier devices such as sensors, organic field effect transmitters, printed circuit boards, electrochromic devices, nonvolatile memory devices, or photovoltaic devices can be fabricated (22). 

FIGURE 16.15. Spectral transmittance at three different states of coloration for an electrochromic device of the type shown in the inset. (From Granqvist, C., Handbook of Inorganic Electrochromic Materials, Elsevier Science, 1995. With permission.)... 

Inorganic ion conductors have been stndied in several different prototype electrochromic devices. Reflectance changes have been reported in devices with bulk-type ion conductors, and transmittance as well as reflectance changes have been reported in devices with thin-film ion conductors. Most of the work on bulk-type ion conductors have dealt with proton conductors, initially with (W03)i2 29H20 (phosphotungstic acid) and Zr0(H2P04)2-7H20 (zirconium phosphate). Durability problems were noted with these materials. 

Optical transmittance as a function of wavelength for the electrochromic device in bleached and colored state using (a) d-U(900)5LiTFSI and (b) d-U(600)25LiBF4 di-ureasils. Adapted from Rodrigues et al. ... 

Figure 2.21 Electrochromic switching behavior of glass/FTO/WO /ion-conductive-polymer-membrane/Niji jO/PANI/FTO/glass device (a) cychc voltammograms after the 50th and 300th cycles between -0.8 and +1.7 V at scan rate 20 m V/s (b) chronocoulometry curves and optical transmittance changes at X = 480 nm measured after 201 switches between -1.7 and +1.0 V and (c) the in situ optical spectra for bleached and colored states measured after 201 switches between -1.7 and +1.0 V and corresponding coloration efficiency. Reprinted with permission from [233]. Copyright (2012) Elsevier.

Polyaniline films have not only been shown to exhibit electrochromism in the visible region, but also in the microwave and far-IR regions of the electromagnetic spectrum. A polyaniline film doped with camphorsulfonic add and incorporated into a sohd state microwave shutter demonstrated that the transmittance and reflectance of X-band microwave energy could be modulated [6]. At a wavelength of 10 GHz, the shutter could be switched between 4.8% transmission when the polymer is oxidized and 42% transmission when the polymer is neutral. When utilized in a reflective device configuration in combination with poly(diphenylamine), polyamline yields a high reflective modulation in the far-IR [119,120]. This device shows a reflectance contrast of 53% at 10.5 p,m, 28% at 16.5 p,m, and 46% at 620 nm. 

The device by Corradini et al uses an ITO counter-electrode at which occurs, without significant colour change, an electrochemical process that is presumably due to lithium ion insertion and involves up to ca. 7.5mCcm [29]. Figure 7.7 shows the transmittance in the visible and IR regions of the electrochromic-electrode in the undoped and doped states as well as of the ITO counter-electrode before and after lithium insertion. 

Reynolds et al. [279] reported electrochromic behavior of self-doped propanesulfonated poly(3,4-propylenedioxypyrrole). The polymer has not only shown interesting electrochromic properties in the visible, but, upon doping, also exhibits a very strong absorption in the near infrared with changes in transmittance up to 97 %, extending the use of the polymer as the active layer in a visible/near infrared switchable device. Viinikanoja etal. [280] reported the electrochromism and pH-induced halochromism of self-doped poly(3-(3 -thienyloxy)propanesulfonate) multilayers. 

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]. 

Another example of a galvanic cell is an electrochromic (EC) device, which is a multilayer construction where one of the layers shows electrochromic properties (Baucke, 1991 Greenberg, 1994 Granqvist, 1999). Certain compounds, especially oxides of polyvalent metals, exhibit coloration that depends on the oxidation state oftheir cations. This property leads to electrochromism, which is a reversible and visible change in transmittance. The oxidation-reduction reactions are electrochemically induced using low voltages, on the order 1 Vdc. 

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