Electrochromic properties of polycrystalline thin films of tungsten trioxide prepared by chemical vapour deposition

The electrochromic properties of polycrystalline thin films of tungsten trioxide prepared by chemical vapour deposition were studied using cyclic voltametry and chronoamperometry measurements. Two kinds of films were investigated depending on the conditions of preparation. Although the composition of the layers obtained after pyrolysis of W(CO)₆ is influenced by the presence of oxygen flow, the final annealing of these products leads to the same polycrystalline structure. Electrochromic properties have been investigated in acid and hydro-organic electrolytes. Cyclic voltametry shows that both colouration and bleaching of the films are associated with electrochemical reactions. The optical efficiencies and the response times were studied in both media and compared with amorphous WO₃ thin films prepared by vacuum evaporation. Best results have been obtained in acid electrolyte for films prepared by pyrolysis of W(CO)₆ in the presence of oxygen flow. We also observed that cycling greatly enhanced the response time. Current injection during colouration was found to depend strongly on time and to be mainly controlled by the resistance of the electrolyte at short times (f<200 ms).