Study of WO3 films with textured surfaces for improved electrochromic performance

We present a study of electrochromic (EC) devices based on WO3 thin films with textured surfaces. Surface patterns were generated by electron gun deposition of WO3 stripes (173-mum wide and 150-nm high) on a previously grown 'flat' WO, film, 150-nm thick. The films were characterized by SEM, XPS and FTIR. They are amorphous and near-sloichiometric and have a packing density of 0.8. The textured films were used for the fabrication of EC devices of the configuration: K-glass/WO3/1 M LiClO4 in PC/K-glass. These devices were characterized by optical and electrochemical techniques such as transmission spectroscopy, cyclic voltammetry and GITT. They exhibit considerable reduction of transmittance in the visible with the application of a potential of 4 V for 30 s, stability and reversibility of their electrochromic properties with an open-circuit memory of more than a week. Compared to equivalent EC devices with 'flat' WO3 films, the textured ones exhibit nearly double diffusion coefficient for Li+ and up to 30% more current during cyclic voltammetry tests. Hence, they present a significant improvement over 'flat' films, due to better access of Li ions into the WO3 matrix. The textured WO3 films are suitable for EC applications as they exhibit sharp contrast between regions with different shades of blue in the colored state. Other. more complicated texture patterns are possible. (C) 2001 Elsevier Science B.V. All rights reserved.