Electrochromism of Nanocrystal-in-Glass Tungsten Oxide Thin Films under Various Conduction Cations

The nanocrystal-in-glass (nanocrystals embedded amorphous matrix) tungsten oxide (WO₃) thin films with a nanoporous characteristic were prepared via an electron beam evaporation technique. The e-beam evaporated WO₃ thin films present a fast colored/bleached time of 16/11, 16/14, and 12/12 s, a large optical modulation of 92, 91, and 87% at 633 nm, and a high coloration efficiency of 61.78, 62.04, and 67.59 cm² C^-1 in Li⁺, Na⁺, and Al³⁺ electrolytes, respectively. On one hand, the improved electrochromic performance is mainly attributed to the short diffusion distance and buffering effect in the host matrix, which facilitates the ion insertion/extraction and alleviates the structural collapse of the framework. On the other, owing to the strong electrostatic interactions between the trivalent cations and the host, the WO₃ thin films in Al³⁺ possess a shallow diffusion depth and long cycle life. The individual contribution from the capacitance- or diffusion-controlled process is comprehensively demonstrated. Pseudocapacitive behavior in the nanocrystal-in-glass WO₃ thin films is in favor of fast kinetics response and sound cycling stability. Our work offers an in-depth insight of the electrochromic mechanism for nanocrystal-in-glass WO₃ thin films in various electrolytes and sheds light on the fundamental principle in the electrochromic devices.