Temperature dependent optical absorption in electrochromic tungsten and molybdenum oxides

The optical absorption in thin films of electrochromic oxides of tungsten and molybdenum has been measured over the solar spectral range at temperatures from about 100 K to 365 K. The films were deposited on glass at room temperature by RF plasma enhanced chemical vapor deposition and then converted to the lithium-ion injection compounds LixWO3 and Lix MoO3 by reaction with n-butyl lithium in a water-free and oxygen-free environment to duplicate their colored electrochromic state. The optical absorption in the LixWO3 films consists of a broad peak centered near 1.7 eV and a temperature dependent Urbach (exponential) band edge absorption near 2.77 eV. The absorption in the LixMoO3 films consists of a broad band centered near 1.3 eV, a temperature dependent Urbach band edge near 3 eV plus a very broad tailing absorption connecting these two features. The optical absorption in these materials is interpreted in terms of a small polaron absorption process and temperature dependent structural disorder