The effect of precursor aging on the morphology and electrochromic performance of electrodeposited tungsten oxide films

Tungsten oxide films that have larger effective surface area or extensive grain boundaries tend to be more suitable for use in electrochromic devices. We propose in this paper a simple methodology for increasing the roughness and thus the effective surface area of WO3 films. This method is based on the tendency of the peroxytungstate precursor to form large aggregates within its solution with time. To this aim, a systematic study of the precursor aging effect on the resulting WO3 film properties was conducted. It was established that with increasing aging time of the precursor solution, more and larger aggregates are formed, which are then deposited on the film surface. The deposition of the aggregates causes the formation of large cracks on the film surface, thereby increasing its effective area. An optimum of the precursor aging time was found to be around 80 h. Films prepared with such an aged solution were found to have the highest Li+ diffusion coefficient and voltammetrically intercalated charge density per unit film thickness. It was also observed that the coloration efficiency of films prepared using the aforementioned method was higher than that of equivalent electron-gun deposited films throughout the visible spectrum and especially in the near infrared. The enhanced properties of these films indicate their improved electrochromic performance, which is mainly due to their increased surface area. (C) 2011 Elsevier B.V. All rights reserved.