Essential role of oxygen vacancy in electrochromic performance and stability for WO3-y films induced by atmosphere annealing

Tungsten oxide was extensively studied in electrochromism (EC) due to its extraordinary electrochromic performance. It is widely accepted that the cyclic stability is correlated with crystallinity of the films, while the EC performance are determined by structure features and oxygen vacancy level. However, the relationship between oxygen vacancy and stability is far from clear. In this paper we report on the impact of oxygen vacancy on optical properties, EC performance and cyclic stability of the WO3-y films. Firstly, the as-deposited a-WO3-y films were fabricated by reactive direct current magnetron sputtering at room temperature and then annealed for 2 h at 350 °C in Ar and O2 respectively. The X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy was used to verify that the Ar and O2 annealed films exist different oxygen vacancy concentration with similar crystallinity level due to the thermal treatment. The cyclic voltammetry (CV) and in-situ optical variation measurement results reveal the benefits of EC properties due to the disordered structures and high oxygen vacancy concentration for WO3-y films. The long-term CV cycles experiment proves that O2 annealed films shows the higher stability with the lower oxygen vacancy concentration compared with Ar annealed films which indicates that the oxygen vacancy may go against the cyclic stability of WO3-y films.