Studies on the correlation between electrochromic colouration and the relative density of tungsten trioxide (WO<inf>3-x</inf>) thin films prepared by electron beam evaporation

The colouration efficiency in the electrochromic devices depends upon the amount of intercalating charge in the tungsten oxide (WO3-x) thin films. One of the physical properties of the film that significantly influence the intercalating charge is the film density. In this paper, the dependence of intercalated charge in WO3-x thin films on the film density is reported. The amorphous tungsten oxide thin films have been prepared by electron beam evaporation at three substrate temperatures: 300, 470 and 570 K. With increasing substrate temperature, the refractive indices of the films increase. The relative film density calculated from the refractive index using the Lorentz-Lorentz relationship increases with the substrate temperature. With increasing density of the WO3 thin films, the diffusion coefficients of protons and the amount of intercalated charge decrease. The intercalated charge in the coloured electrochromic thin films brings about a change in the surface work function of the thin film; the difference in the surface work function (between the coloured and the bleached states) is measured by the Kelvin probe. In the present investigation, an intercalating charge of 59, 48 and 39 mC cm-2 brings about a change of 0.24, 0.16 and 0.13 eV in the surface work function of the WO3 thin films. © 2009 IOP Publishing Ltd.