Optical properties of amorphous tungsten oxide films: Effect of stoichiometry

The optical properties of sputter deposited amorphous tungsten oxide films have been measured in-situ during slow electrochemical cycling in a lithium containing electrolyte. Amorphous films exhibit coloration under Li insertion and bleaching under Li extraction. Substoichiometric films show almost reversible optical changes already in the first electrochemical cycle and are completely reversible thereafter. Tungsten oxide films sputtered in a large excess of oxygen were found to be slightly overstoichiometric, as determined by Rutherford Backscattering Spectrometry. They exhibit irreversible charge transfer and coloration in the first cycle. Thereafter they cannot be completely bleached and exhibit transmittance contrast between coloured and partially bleached states. The irreversible colouration of the stoichiometric films is associated with a feature at 2.6 to 2.9 eV vs. Li in electrochemical measurements. Possible chemical reactions giving rise to this behaviour are discussed.