P. V. Ashrit, Zakaria Kabore, Bassel A. Samad, Université de Moncton, Moncton, New Brunswick Canada
Tungsten trioxide (WO₃) thin films are very well known for their efficient electrochromic properties under the double insertion and extraction of various suitable ions and electrons. Although both the amorphous and polycrystalline WO3 thin films exhibit an efficient coloration in the visible and near-infrared spectral regions from an initial transparent state under the double insertion, the underlying phenomenon leading to this coloration is quite different. In the amorphous films it is the absorption modulation while it is reflectance modulation in the polycrystalline state. In the former case, the coloration takes place due to the intervalence transfer of electrons from one site to the other in the mixed valence WO₃ films through the absorption of light (polaron absorption). However, the coloration in the polycrystalline films is due to the reflectance arising from the free electrons injected into the host material (plasma frequency edge displacement). However, in most commonly used deposition conditions in which a polycrystalline film is expected to be formed, it can be surmised that the WO₃ film is composed of polycrystallites held in an amorphous host material. This leads to an electrochromic coloration arising from a combination of absorption and reflectance modulation. In the present work, we have undertaken a systematic study of the electrochromic evolution of WO₃ films deposited under different conditions and subjected to dry lithiation using lithium niobate (LiNbO₃) powder under controlled heat treatment. The structural, electrical and optical properties of the as-deposited and colored films have been measured using an Atomic Force Microscope (AFM), four-point probe and a UV-Vis-NIR double beam spectrophotometer. Hall effect measurements have been carried out to calculate the free electron density (ne) inserted into the WO₃ film and correlated with the effective lithium film thickness measured by quartz crystal method. Preliminary results of this work are presented here.