Electrochromic Devices Visible-Region

Figure 19.9 illustrates the principle of surface modification of oxide nanoparticles with dyes. Dyes that absorb in the visible and/or IR region become colorless (or change to another color) upon undergoing reduction (or oxidation) under the influence of an electrochemical bias. The feasibility of this concept has already been demonstrated in our laboratory by binding a thiazine dye to nanostructured Sn02 film [156]. Reverse electrochromism obtained using such dye systems provides an alternate approach to conventional oxide-based electrochromic devices. 

Other CPs are produced in large quantities inhouse by many companies for marketing as products for specific applications. Milliken Research Corporation, for example, supplies PPy-coated textiles for microwave absorption, ESD, and other uses. Ashwin-Ushas Corporation markets several different electrochromic devices for the visible, IR, and microwave regions. Sigma-Aldrich, Inc. offers a wide range of CPs from the main polymer classes and a complete complement of monomer precursors. 

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 absorptive/transmissive-type ECD operates with a reversible switching of the electrochromic materials between a colored state and a bleached state. Both working electrode and counter electrode are transparent so that light can pass through the device [4,5,15,250]. For flexible devices, ITO, SWNT, or PEDOT/PSS deposited onto a plastic such as poly(ethylene terepthalate) (PET) have been used [258,259]. When deposited in the doped form and dried, PEDOT/PSS films, to a thickness of 300 nm, are relatively transmissive in the visible region ( 75% T), have a relatively low resistivity (500 fi/D), and adhere to the plastic substrate in most common electrolyte solutions. The polymer films were demonstrated to be useable over the operating range of the device with no loss in conductivity or transmissivity. 

Small bandgap polymers have il ax of longer than 550 nm in the neutral state and are transparent in the visible region upon doping. A transparent conductive film can be cast from solutions of these polymers. The smallest bandgap is below 0.5 eV. These polymers are not only potentially useful electrochromic devices and switching windows but will also play an important role in the development of science of synthetic metals. 

Therefore rational design and synthesis of the monomer structure and the electropolymerisation experimental conditions play an important role in tailoring the properties of the conjugated polymers for application as electrochromic and fluorescent materials. Several general principles should be kept in mind, including band gap and absorption in the visible region behaviour, HOMO and LUMO energy levels and presence of side chains to enhance the solubility and processability when desirable. These factors are dependent on each other and must be comprehensively considered in pursuit of ideal polymers for application in optical devices. 

From an analysis of contributions in each spectral region, identify properties within a CP system most conducive to a) a high-contrast, large-area Visible-region electrochromic window b) a space satellite thermal coating c) a communication device in the far-IR d) a camoufiage panel in the Visible-NIR, far-IR and microwave regions. 

Because of the demonstrated success achieved with electrochromism in CPs, there is an ongoing search for a CP-based device capable of modulating the entire spectrum from the visible through the IR to the micro-wave region. 

The electrochromic properties of PANi from the visible to the microwave region can be improved by copolymerization of aniline with other aromatic amines such as diphenyl amine and A,A -diphenyl-benzidine. Devices with solid electrolytes, rapid switching, high contrast, and acceptable cycling stability have been discovered and successfully exploited by Chandrasekhar. 

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. 

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