Oxidation State and Properties of Electrochromic Tungsten Oxide Films
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OXIDATION STATE AND PROPERTIES OF ELECTROCHROMIC TUNGSTEN OXIDE FILMS J. D. KLEIN AND A. YEN EIC Laboratories, Norwood, MA 02062 ABSTRACT Tungsten oxide films were deposited by reactively sputtering a metallic W target in OAr atmospheres. Plasma emission spectra were observed as a function of the oxygen content of the chamber inlet gas using an optical multichannel analyser. The gradual loss of W lines as the 02 content was increased to 10% was accompanied by a drastic reduction in deposition rate. Cyclic voltammetry of the films deposited on ITO-coated substrates reveals that films produced under high oxygen conditions have low charge capacities and retain substantial Li upon initial cycling. The photopic coloration efficiency increased with increasing oxygen in the inlet gas. The electrochemical and electrochromic data exhibit breaks in behavior at the 10% 02 break point defined by optical spectroscopy. INTRODUCTION There is ongoing interest in the development of electrochromic films for energy conservation and optical display applications [1,2]. Most electrochromic window designs can be described as thin film batteries in which one or more of the electrodes exhibits a visible state of charge. The motion of intercalation species between electrodes governs the transmittance of the sandwich structure. For example, coloration of a tungsten oxide layer proceeds according to the cathodic reaction (1) (transparent) WO 3 +xLid+xe- cathodic > LWO blue) Reversing the potential to favor the anodic reaction bleaches the film to restore the high transmittance state. In a WO 3-Ir0 2 complementary structure the transfer of a small cation from the 1102 film to the W0 3 film is accompanied by the simultaneous darkening of both layers. Reversing the reaction switches both layers to their transparent states. In practice small cations such as HI [3,4] and Li+ [5] can be intercalated in glassy [3,4] or crystalline [5] layers. It is known that the electrochromic properties of the films can be sensitive to the oxidizing characteristics of the deposition process used to generate them [6,7]. Properties thought to be sensitive to the oxidation state of the film include charge capacity, reversibility of intercalation, and photopic coloration efficiency. The research described in this paper seeks to determine the process sensitivity and explore optical methods for obtaining precise control. Our previous spectroscopic monitoring studies [8,9] have indicated the impact of process conditions on the cation stoichiometry of superconducting oxides. Tungsten oxide provides an excellent opportunity for examining the role of an oxidizing plasma in determining oxide film properties. EXPERIMENTAL All deposition runs were performed in a Microscience IBEX-2000 deposition chamber evacuated by a diffusion pump fitted with a liquid nitrogen trap and backed by a mechanical pump. An Ion Tech, Ltd. B315 planar magnetron sputter source was used in the sputter-up configuration with either OjAr or 02 atmospheres. The 5.1 cm diameter metallic tungsten sputter ta
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