Tungsten trioxide films during cation insertion : in situ optical characterization

At the moment WO3 is the most suitable material as cathodic film for electrochromic devices, mainly because of its good mechanical and physical properties, and efficiency in coloring and bleaching upon cation intercalation. We studied the effect of the electrochemical insertion of small cations (H+, Li+ or Na+) in WO3 thin films by means of in situ transmittance and reflectance measurements. As result we observed for Na+ and Li+ a behavior depending on the concentration of the cation in the film: under a given limit (around 14 mC cm-2 for a 100 nm thick film) the optical changes were only related to the number of electrons entering the film from the external circuit for electrical charge neutralization. Beyond this limit a phase transition was observed with the formation of the so called tungsten bronzes and the electrochemical process was no longer reversible. With regard to the intercalation of H+ , beyond the above mentioned limit the saturation of the optical density was observed, but no bronze formation was detected even for charge insertion amounts as large as 50 mC cm-2. In order to prevent irrevesible changes during the insertion of Li+ and Na+ the current density must not exceed a maximum value that also depends on the diffusion of cations within the film. For the WO3 samples studied during this work we found the diffusion coefficients for Li+ and Na+ to be related to the cation concetration and varying in the range 10(-11)-10(-9) cm2 s-1.