Correlation between lithium storage and diffusion properties and electrochromic characteristics of WO3 thin films

Abstract

As essential electrochromic (EC) materials are related to energy savings in fenestration technology, tungsten oxide (WO3) films have been intensively studied recently. In order to achieve better understanding of the mechanism of EC properties, and thus facilitate optimization of device performance, clarification of the correlation between cation storage and transfer properties and the coloration performance is needed. In this study, transparent polycrystalline and amorphous WO3 thin films were deposited on SnO2:F-coated glass substrates by the pulsed laser deposition technique. Investigation into optical transmittance in a wavelength range of 400–800 nm measured at a current density of 130 μA · cm−2 with the applied potential ranging from 3.2 to 2.2 V indicates that polycrystalline films have a larger optical modulation of ∼ 30% at 600 nm and a larger coloration switch time of 95 s in the whole wavelength range compared with amorphous films (∼ 24% and 50 s). Meanwhile, under the same conditions, polycrystalline films show a larger lithium storage capacity corresponding to a Li/W ratio of 0.5, a smaller lithium diffusion coefficient (2×10−12 cm2·s−1 for Li/W=0.24) compared with the amorphous ones, which have a Li/W ratio of 0.29 and a coefficient of ∼2.5×10−11 cm2·s−1 as Li/W=0.24. These results demonstrate that the large optical modulation relates to the large lithium storage capacity, and the fast coloration transition is associated with fast lithium diffusion.