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.