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

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 correlation between the 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 wavelength range of 400–800 nm measured at the current density of 130 μA/cm² with the applied potential ranging from 3.2 to 2.2 V indicates that the 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 the amorphous films (～24% and 50 s). Meanwhile, under the same conditions, the 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·^–1 for Li/W=0.24) compared with the amorphous ones with the Li/W ratio of 0.29 and the 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 associates with the fast lithium diffusion.