Non-stoichiometry in thin film dilute tungsten bronzes: MxWO3−y

Thin films of tungsten trioxide have been prepared in various ways which give rise to different optical behaviour. The white light optical absorption versus inserted lithium atom concentration is measured and followed as a function of time. The influence of post-deposition treatment in oxygen at 300°C is studied, and the results of these experiments support the key role of non-stoichiometry. Previous thin film structural studies are re-enforced here by a structural investigation of evaporated films using the atomic force microscope. These optical-insertion data are interpreted by means of a model which proposes that electron trapping occurs at crystalline shear plane boundaries (which account for the non-stoichiometry). The extent and rate of trapping is modelled, and the experimental results are consistent with the trapping rate being proportional to the lithium atom concentration in the films. The importance of minimising non-stoichiometry in electrochromic devices and in specimens of material for use in physical measurements is made clear.