Influence of coloring voltage and thickness on electrochromical properties of e-beam evaporated WO3 thin films

In this investigation, the effect of coloring voltage and thickness on optical and also electrochromical properties of WO3 thin films has been studied. The WO3 thin films were grown on glass and indium tin oxide coated conducting glass substrates by e-beam evaporation at different thicknesses of 200, 400, and 700 nm. Optical properties of the deposited samples were characterized in the ultraviolet-visible range (300 - 1100 nm). The optical bandgap energy of the WO3 was obtained in a range of 3.3 - 3.5 eV showing its increase by decreasing the film thickness. The refractive index of the WO3 films was measured around 2 in the visible range. Surface chemical states of the films were studied by X-ray photoelectron spectroscopy, which showed the stoichiometry of our deposited tungsten oxide thin films is acceptable. Atomic force microscopy was used for studying surface morphology of the deposited films. The electrochromic properties of the WO3 films were characterized using a lithium-based electrolyte. It was shown that there is an optimum coloring voltage for each film thickness, which maximizes the change in optical density during electrochromic process. The coloration efficiency of the samples at the optimum voltage was linearly improved by increasing the film thickness at a constant wavelength (500 nm). (c) 2005 The Electrochemical Society.