Characterization of ion diffusion and transient electrochromic performance in PECVD grown tungsten oxide thin films

Electrochromic tungsten oxide thin films were synthesized by plasma-enhanced chemical vapor deposition (PECVD). Film density and electrochromic performance were controlled by the degree of ion bombardment. A moderate degree of ion bombardment was optimal, and the refractive index was shown to be a sensitive indicator of electrochromic performance. Chronoamperometry in concert with optical transmission was used to determine diffusion and absorption coefficients using both H+ and Li+ containing electrolytes. The absorption coefficients were similar for both ions, scaling with the degree of intercalation to 50,000 cm−1 in the opaque state. The diffusion coefficients for optimized films were found to be relatively insensitive to the degree of ion intercalation, with values of ∼10−9 and ∼10−10 cm2/s for H+ and Li+, respectively. These values are about an order of magnitude greater than values reported for vacuum-deposited films, which was attributed to low relative density in the PECVD films. The diffusion and absorption coefficients were incorporated into a model that successfully reproduced transient optical performance.