Gasochromic switching of tungsten oxide films: a correlation between film properties and coloration kinetics

Optically switchable coatings based on tungsten oxide offer a wide variety of promising applications, such as the dynamic variation of transmission in smart windows. Our research focuses on the gasochromic switching of amorphous tungsten oxide (WO,) films with a palladium catalyst cap-layer, where coloration takes place in a gas atmosphere with diluted hydrogen and bleaching can be achieved by an oxygen containing gas. The samples are produced by DC-magnetron-sputtering and show a laterally inhomogeneous switching behaviour where the coloration speed varies over more than one order of magnitude. To understand this phenomenon, the films were analyzed by optical spectroscopy with spatial resolution and subsequent simulation of their dielectric function. The switching kinetics of the films could be correlated to the index of refraction n: regions with high n did not switch, whereas regions with lower n showed fast switching behaviour. Since n is linked to the film density via the Lorenz-Lorentz equation, this implies that films with a low-density switch much faster than high-density films. Finally, a model to explain the observed lateral inhomogeneous deposition and coloration velocity is presented. (C) 2002 Elsevier Science B.V. All rights reserved.