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.