Effects of Ti-doping amount and annealing temperature on electrochromic performance of sol-gel derived WO3.

Fine control of structural and morphological features in electrochromic materials is of paramount importance for realizing practical electrochromic devices (ECDs), which can dynamically adjust indoor light and temperature of buildings. To this end, herein we investigate impacts of two variants such as Ti-doping amount and the annealing temperature on physical and chemical properties of sol-gel derived electrochromic WO3 films. We use a wide range of titanium coupling agents (TCAs) as Ti-dopants ranging from 0 wt% to 20 wt% and vary the annealing temperature between 200 °C and 400 °C with 50 °C interval. Both variants greatly influence the physical properties of the resulting WO3 films, resulting in different crystallinities and morphologies. Through complementary analytical techniques, we find that the WO3 film featuring an amorphous phase with nano-porous morphology enhances the electrochemical and electrochromic performances. The specific TCA used in this study helps stabilize the amorphous WO3 structure and generate the nano-pores during the following thermal treatment via its thermal decomposition. As a result, the WO3 film having an optimal 8 wt% TCA annealed at 300 °C shows a high optical density of 73.78% in visible light (400-780 nm), rapid switching speed (t c = 5.12 s and t b = 4.74 s), and high coloration efficiency of 52.58 cm2 C-1 along with a superior cyclic stability. Thus, understanding a structure-property relationship is of paramount importance in engineering the advanced electrochromic WO3 for use in practical ECDs and other optoelectronic applications.