KINETICS STUDY AND MODELING OF THE ELECTROCHROMIC PHENOMENON IN AMORPHOUS TUNGSTEN TRIOXIDE THIN-FILMS IN ACID AND LITHIUM ELECTROLYTES

The coupling of electrochemical and optical measurements allows us to point out that the kinetics of coloration of amorphous tungsten trioxi de thin films (a-WO3) is slower than the kinetics of the electrochemic al reaction. However it is shown that the electrochemical reaction is controlled by a resistance R which depends on the conductivities of th e electrolyte, the MxWO3 bronze, and the transparent electronic conduc ting sublayer. The molar absorption coefficients (at lambda=633 nm) of both LixWO3 and HxWO3 are determined. From the experimental results, a model is proposed to describe the mechanism of the electrochromic pr ocess in a-WO3. According to this model, the kinetics of coloration is found to be controlled by the diffusion into the oxide of an intermed iate species (M+, e-). The oxidation of this species by a W6+ ion lead s to the formation of an absorbing center and to the coloration of the oxide. The diffusion coefficients of (Li+, e-) and (H+, e-) as well a s the rate constants k of the corresponding redox reactions are determ ined.