Unveiling dynamics evolution mechanism of electrochromic process in WO3-x film with thickness dependence

Abstract

Response time (τ, defined as the time required for 90% change in the full transmittance modulation, including coloring time τc and bleaching time τb) is an important evaluation index for the performance of electrochromic (EC) materials, and it can directly reflect the microscopic rate of EC process. However, the research on the internal relationship between the response time and the process of electrochemical dynamics is rare. Herein, by controlling the thickness of the WO3-x EC film, a complete reversal of the response time (from τc > τb in the thinner film to τb > τc in the thicker film) is observed. Based on the detailed analysis of the characteristics presented in electrochemical impedance spectroscopy (EIS), especially focusing on the changing trend about the arc of different frequency ranges and the slope value of the linear part, it can be found that this reversal is due to the change of the main dynamics process from surface capacitive-controlled to bulk diffusion-controlled with the increase of WO3-x film thickness. Meanwhile, this change with obvious film thickness dependence requires a "trigger", which is reflected by the bleaching threshold potential. Besides, this study reveals the three stages of the EC coloring process from the perspective of dynamics, including the lithium-ions (Li-ions) adsorption charge layer, surface Faraday process, and bulk Faraday process, which also show significant film thickness dependence. The result of this work is beneficial to modulate the balance of response time and to improve the performance of EC films.