Enhanced contrast ratio of an electrochromic window (ECW) based on a V2O5-TiO2 composite thin film as a counter electrod
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Enhanced contrast ratio of an electrochromic window (ECW) based on a V2O5-TiO2 composite thin film as a counter electrode *Sooyeun Kim1, Chunye Xu2, and Minoru Taya1,2 1 Materials Science & Engineering, University of Washington, Seattle, WA, 98115 2 Mechanical Engineering, University of Washington, Seattle, WA, 98115 ABSTRACT An Electrochromic window (ECW) is a strong candidate for an optical shutter. An ECW is composed of the conductive polymer as a working electrode and V2O5 as a counter electrode. The ECW technology switches between colored and bleached stages. In this paper the modification of a V2O5 thin film as a counter electrode is studied to increase contrast ratio of ECWs by adding TiO2 into V2O5. The dependence of the intercalation properties and transmissivity of V2O5-TiO2 composite films on various chemical compositions, a preparation and characterization of the films for use as a transparent ion storage layer in organic ECWs are focused on this paper. A V2O5-TiO2 composite film was synthesized with sol-gel electrophoresis deposition method. Indium Tin Oxide (ITO) coated glass was used as an electrically conductive and transparent substrate. A V2O5-TiO2 composite film with V/Ti ratio of 70/30 in the reduced state showed 6 to13% improvement of transmissivity between 380 nm and 800 nm wavelengths as compared to a V2O5 thin film in the same state. Li+ intercalation rate was also observed to be higher. The composite film exhibited twice of capacitance than that of the V2O5 film. An ECW with the composite film exhibited 6% higher contrast ratio than that with the V2O5 film. The contrast ratio of the ECW, measured as ∆%T, was equal to 56%, and ranged from 2% to 58% between the colored and bleached state measured at 580 nm wavelength. The color change response speed was also faster, i.e., only two seconds between fully colored and bleached stages. Key words: electrochromic window (ECW), V2O5-TiO2 composite film, contrast ratio, transmissivity, Li+ intercalation rate
INTRODUCTION An ECW technology has been developed in our laboratory with aim of applying it to architectural, vehicular and aircraft windows, skylights, sunroofs, eyeglasses and numerous types of displays.1 An electrochromic window (ECW), referred to as ‘an optical window’ is one of switching devices based on electrochromic (EC) conducting polymers. Exhibition of high contrast ratio, Tmax-Tmin, between its colored (Tmin) and bleached state (Tmax) is a key performance as a switching device.2 An ECW is composed of three primary components: (1) an electrochromic layer (as a working electrode, WE) that changes color or transparency when subjected to charged electrode, (2) an ion storage layer (as a counter electrode, CE) to balance charges, (3) an ion conducting layer (electrolyte) to deliver charges. We have been developed an ECW as an effective optical shutter with high contrast ratio. Vanadium oxide has been used as an ion storage material for a counter electrode. Vanadium oxide is a good ion storage material. However, A V2O5 thin fil
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