Hierarchical Design Of Electrochromic Glasses
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HIERARCHICAL DESIGN OF ELECTROCHROMIC GLASSES HULYA DEMIRYONT Eyeonics Corporation, 9400 S.W. Tualatin-Sherwood Rd, Tualatin, Oregon 97062
ABSTRACT Layered electrochromic glass was investigated in order to test the feasibility of using hierarchical designs for new electrochromic devices. Matrix-type synthetic electrochromic structures were designed, and two types of matrix devices, polarization and plating, were produced and evaluated. The application and device requirements of electrochromic glasses are discussed.
I. INTRODUCTION Electrochromics (EC) are the most promising chromogenic materials to exhibit controllable color change upon electrochemical oxidation/reduction reactions. The change of color occurs upon oxidation of an anodic EC material and reduction of a cathodic one. Changing the color of an EC material is a very easy laboratory experiment achievable by using a simple electrolytic cell. The difficult task is to produce a solid electrochromic glass capable of switching under harsh environmental conditions without degradation. This paper discusses existing EC device technologies for conventional and hierarchically designed systems.
II. CONVENTIONAL ELECTROCHROMIC GLASS TECHNOLOGY Working principles of a conventional layered EC glass depend on electrochemical oxidation/reduction reactions in the EC and ion storage (IS) layers. An EC element has nonabsorbing and absorbing oxides. The transition from one to the other is provided by receiving (cathodic) or releasing (anodic) ion-electron pairs. Figure 1 shows schematically a conventional fivelayered EC device (ECD).1- 4 The two electrode layers of the ECD provide an electrical field through the color forming system to drive the charges. The color forming system, which is sandwiched between the two electrodes, consists of three layers: (i) an optically active EC layer, (ii) an optically passive or active (complementary color forming element) IS layer, and (iii) an electrolyte layer between the EC and IS layers. The two outer electrode layers, which conduct only electrons, must be transparent for window applications of ECDs. For mirror or display applications, one transparent and one reflecting electrode layer can be used. Within the color forming system, the two layers are separated by an electrolyte film that conducts only ions. The layers on each side of the electrolyte film (EC and IS) transfer ions to each other during the coloring-bleaching process. If the EC and IS layers are both optically
Mat. Res. Soc. Symp. Proc. Vol. 255. @1992 Materials Research Society
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active, they color and bleach simultaneously. This type of IS layer is called complementary film. The EC and IS layers are mixed conductors of both electrons (coming from the electrodes) and ions (from the electrolyte). Figure 2 illustrates the color forming system of a conventional ECD for both optically active (complementary) and passive IS layers. These ECDs have color memory (retain their color) until a color releasing voltage is applied to the device. The color memory of an ECD dep
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