Electrochromism of W-oxide-based thin films : Recent advances

We summarize some recent work related to electrochromic tungsten-oxide-based thin films. The electronic structure of crystalline cubic (perovskite) WO3 and HWO3 was calculated from first principles. It was found, that hydroxide formation was energetically favoured. Experimental studies were made on heavily disordered films prepared by reactive magnetron sputtering in Ar + O2 with and without CF4 addition and substrate bias. Structural studies indicated that the electron bombardment associated with a positive substrate bias led to grain growth and partial crystallization while maintaining a high density of WO double bonds presumably on grain boundaries. Tandem films, with a thin protective layer of electron bombarded oxide covering a thicker oxyfluoride layer, were able to combine rapid dynamics of the electrochromism with good electrochemical durability. Oblique angle sputtering in Ar + O2 led to films whose microstructure comprised inclined columns. Pronounced angular selective transmittance was found to coexist with electrochromism.