Efficiency of Li doping on electrochromic WO 3 thin films

Abstract

Electrochromic materials show a variation in their optical properties associated with reduction/oxidation reactions and the injection and extraction of positive ions in their crystalline lattice to keep overall charge neutrality. In this work, lithium was inserted into WO 3 thin films in the gas phase during the deposition process. This lithium in the solid WO 3 oxidises, Li→Li ++e −, and the afforded electron plays the role of the reducing agent for the W atoms. We observed a clear relation between the amount of lithium inserted and the variation of the optical properties; the highest variation was obtained for lithium-to-tungsten atomic ratios near 0.4. In order to calculate how many of these ions can be extracted from the layer, and thus how much is ‘active’ to change the optical properties of the layer, cyclic voltammetry and chronoamperometry measurements were performed. From the study of the amount of lithium inserted and that extracted electrically, the efficiency of the doping process was established. A series of WO 3 samples with lithium-to-tungsten atomic ratios ranging from 0.3 to 0.5 were deposited onto ITO (indium tin oxide) coated glass. The optical and electrical properties were recorded simultaneously during the lithium extraction. The results showed a slight reduction of the doping efficiency as the ratio was increased. We attempted to find a correlation between the doping ratio and the optical properties of the WO 3 layer. These results help us to understand the role of lithium in coloration, and to solve some of the problems associated with doping, such as a suitable level, and diffusion of lithium ions in the bulk of the WO 3 layer and to the surface.