Morphology Control Of The Electrochromic Effect In Tungsten Oxide Thin Films
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MORPHOLOGY CONTROL OF THE ELECTROCHROMIC EFFECT IN TUNGSTEN OXIDE THIN FILMS H. S. WITHAM, P. CHINDAUDOM, R. MESSIER*, AND K. VEDAM** Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802. *Also in Department of Engineering Science and Mechanics **Also in Department of Physics ABSTRACT Electrochromic tungsten oxide thin films have been prepared by reactive dc-magnetron sputtering under different deposition conditions. Through the use of scanning electron microscopy, spectroscopic ellipsometry, and electrochromic coloration experiments, the effect of preparation conditions on film morphology and electrochromic properties has been studied. The results of this study are consistent with a previous report which found that tungsten oxide thin films are dominated by a hierarchy of dense columnar regions and less dense void regions. This morphology not only dominates the structure of tungsten oxide thin films but also strongly controls the electrochromic properties. From spectroscopic ellipsometry data and Bruggeman effective medium approximation models presented here, it seems reasonable that a cermet model of absorption can accurately describe the tungsten oxide preparationelectrochromic property relationship. INTRODUCTION Electrochromism is a phenomenon in which the optical absorption or transmission properties of a material are changed by a dual injection or ejection of ions and electrons under small voltages (1-2 volts dc). Such materials are of technological interest for displays [1-4], rearview mirrors [5], and for use in variable transmittance electrochromic (EQ windows which enhance energy efficiency in cars (6] and buildings [7-9]. Electrochromism occurs in many organic materials and in transition metal oxides based on W, V, Ni, Mo, Ti, Ir, etc. Of this group, tungsten oxide (WO3) is probably the most prominent and widely studied. It is the primary material used as the active layer in electrochromic windows. Stoichiometric WO3 is optically transparent in the visible region. When an electric field is applied to WO. in contact with an alkali metal ionic conductor, the WOQ undergoes an optical absorption shift and turns blue. The WO 3can be switched back to the optically transparent state by reversing the electric field. It is generally accepted that the optical absorption change is due to the injection of electrons and alkali metal ions (H÷. Li*, Na÷, and K÷) into the WO,3 forming a tungsten bronze [I], as shown: xM++xe'+WO e4 MWO Transparent " Blue "
(1)
where M+ are small alkali ions and 0._5 x 5 0.5 and 0 • y!•0.03. Although it has been shown that the morphology of WO3 thin films is dominated by dense highly columnar regions and an anisotropic void network surrounding the columns [10], there have been essentially no other reports on the microstructure of WO 3 as it relates to the EC properties with few exceptions [ 11,12]. This is somewhat surprising since in a number of other areas of thin film science (e.g. magneto-optic materials [13] and a-Si:H solar cells [14]), thin
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