The optical properties of sputter deposited amorphous tungsten oxide films have been measured in-situ during slow electrochemical cycling in a lithium containing electrolyte. Tungsten oxide is a well-known electrochromic material of interest in the emerging smart window technology. The amorphous films exhibit coloration under Li insertion and bleaching under Li extraction. In this work we compare the optical transmittance of transparent and coloured tungsten oxide films with different stoichiometry. 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 stoichiometric, within experimental uncertainties, as determined by Rutherford Backscattering Spectroscopy. They exhibit irreversible charge transfer and coloration in the first cycle. In the following cycles, they cannot be completely bleached and exhibit only limited transmittance contrast between coloured and partially bleached states.
The electrochemical behaviour of our films was studied by voltammetry and the Galvanostatic Intermittent Titration Technique. The irreversible colouration of the stoichiometric films is associated with a feature at 2.6 eV vs. Li in the electrochemical measurements. Possible chemical reactions giving rise to this behaviour will be discussed. In conclusion, substoichiometric tungsten oxide films are to be preferred for electrochromic applications, because of the large reversible and stable transmittance contrast between the bleached and the coloured states.
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