Tantalum oxide (Ta2O5) thin films prepared by reactive magnetron sputtering are investigated for their potential use as the ionic conducting layer in all solid state monolithic electrochromic devices. The paper focuses on the influence of Ta2O5 coatings on the electrochromic response of amorphous tungsten oxide (a-WO3), the methods employed for charging during monolithic device fabrication and associated charge losses caused by subsequent deposition of the component layers of monolithic (all solid state) electrochromic devices. The dependence of the electrochromic response and related charge loss processes have been studied for Ta2O5 films of different thicknesses prepared on a-WO3 electrochromic films.
The dependence of the infrared spectral optical constants, n(lambda ) and k(lambda ), of electrochromic tungsten oxide thin films, prepared by reactive magnetron sputtering at different substrate temperatures, on the quantities of charge, q, inserted has been determined from spectrophotometric measurements of transmittance and reflectance. For WO3 films deposited at temperatures greater than 200 degree(s)C significant infrared absorption is observed to rise with increasing levels of charge insertion. For the most crystalline films prepared at temperatures greater than 350 degree(s)C the rates of change dn(lambda )/dq and dk(lambda )/dq are greatest. The results provide essential design data for variable reflectance/emittance devices. Rear surface devices, which mask the reflectance of a highly reflective rear surface mirror through a charge dependent increase in k(lambda ), have been successfully demonstrated using either liquid or solid electrolytes. Monolithic devices have been prepared using a hybrid combination of sputtered and electron-beam evaporated coatings.
While designing a thermal control device based upon crystalline tungsten oxide it is vital to have knowledge of both n, the real part of the refractive index and k, the complex part. With this knowledge it is possible to predict the performance of an electrochromic device at different wavelengths of radiation. We performed a series of experiments on crystalline samples of tungsten oxide where both n and k were carefully determined from spectral reflectance and transmittance measurements after set amounts of charge were inserted electrochemically, thus reducing the tungsten oxide. The trends for n and k are different, which has implementations for device performance and design. We also investigated the effect of deposition temperature on the optical properties of crystalline tungsten oxied. This has implications for both the substrate that may be u sed in a device and the construction of the device. The final data we shall present are some electromechanical studies of insertion into crystalline tungsten oxide flake samples and powder samples. The flakes and powder are deposited in a similar way to a graphite paste electrodes used in Li ion battery studies. This is a possible route to a flexible eleectrochromic device based upon crystalline tungsten oxide.
A novel approach to tailor the electrochromic properties of polythiophenes utilizing a silyl bridge to form discrete electrochromes is described. The monomer bis[2-(5,2'- dithienyl)]dimethyl silane was chemically synthesized and electrochemically polymerized. Electrochromic properties of devices incorporating this polymer as active material were studied and analyzed following CIE L*a*b* formalism. This polymer exhibits a yellow to green electrochromism from neutral to oxidized state.
Electrochromic materials are of great interest, owing to their potential application in large area displays, active camouflage and energy saving smart windows. The effectiveness of devices fabricated for most applications depends in part on the ability to tailor the observed color in a predictable manner. Several color-tailoring strategies such as polymer blends, copolymers, and layered composites have been investigated recently. Another technique utilizing patterns of electrochromes is currently under development in our labs and affords a false way to modify a device's observed color. The pattern is composed of materials that have different observed colors; at least one of which is an electrochrome. When the pattern is viewed at a distance, the observer perceives a different color than those of the materials comprising the pattern. This `confusion' is due to diffraction. By knowing the color of the patterning materials the observed color can be predicted in a straight forward manner by color subtraction theory. The patterns are produced by screen-printing the electrochrome and other materials onto a Mylar/ITO substrate that is then used as the working electrode in a device. Following this strategy, geometrical patterns composed of thick stripes, fine stripes, small dots, and checkerboards were studied using different materials as the foreground and background colors. We will report on the fabrication of these patterned devices and their characterization by spectrocolorimetry.
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