Excellent electrochromic properties of tungsten oxide films with a mesoporous structure

Sol–gel derived tungsten oxide (WO₃) films have been deposited by dip coating route using a small amount of polyethylene glycol (PEG) introduced into peroxotungstic acid precursor sol as the precursor solution, followed by thermal treatment in air. The influence of PEG additive and annealing temperature on the structural and electrochromic properties have been characterized. The PEG plays an important role in hindering the formation of large crystals and inter-connected polycrystalline structure, and also plays the role of a structure-directing agent, providing an amorphous WO₃ film when annealed at 300 °C. Compared with neat WO₃ film, the PEG-modified WO₃ film exhibits much higher optical contrast and higher coloration efficiency. More importantly, WO₃/PEG film shows excellent cycling stability because its mesoporous structure acts as a buffer layer to accommodate the volume change brought by ion insertion/extraction, and therefore the film can expand and shrink much more easily without causing degradation of WO₃ in long-term electrochemical cycling.