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 a result we observed for Li+ and Na+ a behavior depending on the cation concentration: under a given limit (around 14 mC/cm2 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 the electric charge neutralization. Beside this limit a phase transition was observed with the formation of the so called tungsten bronze and the electrochemical process was no longer reversible. With regard to the intercalation of H+ beside the above mentioned limit the saturation of the optical density was observed, but no bronze formation was detected even for charge insertion as large as 50 mC/cm2. As a consequence, during the insertion of Li+ and Na+ the current density must not exceed a maximum value that also depends on the diffusion of cations inside the film. For the WO3 samples studied during this work we found the diffusion coefficient for Li+ and Na+ to be related to the cation concentration and varying in the range of 10-9 divided by 10-11 cm2/s.