Structure dependent electrochromic behavior of WO3 thin films under dry lithiation

electrochromic (EC) property, reversible coloration of certain materials under double injection of ions and electrons, of transition metal oxides (TMO) such as tungsten trioxide is known to depend strongly on the nature and structure of these materials. Possibility exists to tailor the EC behavior of the different TMOs as per the optical modulation needed. In this work the electrochromic performance of three types of tungsten trioxide films deposited under different conditions leading to nanocrystalline, polycrystalline and amorphous films has been studied by day lithiation method. A comparative study of the EC coloration of these three types of films has been carried out, with a special emphasis on the nanocrystalline films. The techniques of spectrophotometry and atomic force microscopy have been employed for this study. Each type of tungsten trioxide (WO3) film exhibits a special nature of coloration indicating the potential for its specific application. The nanocrystalline films seem to exhibit a higher overall coloration efficiency and a selective optical modulation compared to the polycrystalline or amorphous films. These films exhibit a very high degree of transmission in the clear state and a high degree of optical modulation concentrated in the infrared region. Hence, the NC films may be of more interest for smart windows from the point of view of energy efficiency. The amorphous films, and even the polycrystalline films under high degree of lithiation, may be more suited for large area display device application due to their efficient coloration in the visible region of the spectra.