Coloration and depth distribution of cations electrochemically-inserted into electrochromic WO₃ thin films

Li⁺ and H⁺ ions were electrochemically inserted into amorphous WO3 films prepared on an ITO-coated glass substrate by an R.F. magnetron sputtering method under Ar/O2 flow ratios of 4/1 (SP1)and 1/1 (SP2). The cation distribution was estimated indirectly by depth profiles of refractive-index obtained from prism coupler measurements and was evaluated directly by glow discharge spectrometry (GDS). H⁺ ions inserted were segregated only at deeper region around ITO electrode, which was independent to the preparing condition. In the case of Li⁺ insertion into SP1 film, Li⁺ ions were initially segregated at around ITO electrode, and after further insertions, they were also distributed at around the surface of WO3 film. In SP1 film, Li⁺ ions at around ITO electrode seemed to contribute to coloration. In SP2 film, however, Li⁺ ions subsequently inserted, which were uniformly distributed in the film, were only involved in coloration. The difference in depth distribution and coloration was due to the difference in atomic structure of WO3 films.