EFFET DE LA VARIATION DU DEBIT D’OXYGENE SUR LES PROPRIETES MULTIPHYSIQUES ET LES PERFORMANCES ELECTROCHROMES DE FILMS MINCES DE WOX ELABORES PAR PULVERISATION CATHODIQUE MAGNETRON : APPLICATION AUX « VITRES INTELLIGENTES »

Résumé

Tungsten oxide (WOx) thin films are receiving more attention due to their novel multiphysical properties. These films are able to transit reversibly between transparent and colored optical states when subjected to a potential difference and ions are inserted into them, qualified as electrochromic. The objectives of this work were, on the one hand, to verify and complete current knowledge of the optical and electrical (multiphysical) properties of WOx thin films. Secondly, to correlate these multiphysical properties with the electrochromic properties (ability to be transparent or colored) as a function of oxygen concentration, rate of introduction and frequency of introduction (continuous or by periodic pulsation). A whole range of WOx films with varying oxygen composition were synthesized by magnetron sputtering under reactive environment. Oxygen was introduced either continuously (CP: Conventional process) or by periodic pulsing (RGPP: Reactive Gas Pulsing Process) with exponential signals. Films with a lower oxygen content are opaque or reflective, behave like electrical conductors and have a higher deposition rate (14 nm min-1 < Vd < 48 nm min-1). In contrast, sufficiently oxygen-rich films are transparent (T ≈ 80%), electrically insulating and are obtained at low deposition rates (4 nm mn-1 < Vd < 6 nm mn-1). Electrical measurements indicate a reduction in interface defects for transparent WOx films close to WO3 stoichiometry. Furthermore, an increase in these defects correlated with an increase in positive ion density and sub-stoichiometry, which is observed for sub-stoichiometric and transparent films, favors an improvement in coloring efficiency. The study of electrochromic performance was therefore carried out on transparent films with chemical compositions ranging from WO2.70 to WO3. The results reveal that proper adjustment of defects, ion density and oxygen stoichiometry improve the coloring efficiency of the films, which can reach ~51 cm2/C at 632 nm. This color efficiency value was obtained on a sample prepared using the RGPP process.

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