Investigation on the coloring and bleaching processes of WO3-x photochromic thin films

WO3-x thin films and WO3-x / Ta2O5 composite films exhibit a huge photochromic effect with 60% of near-infrared transmission modulation, in comparison to literature reviews. The colouring efficiency of such film results from both the stoichiometry of the particles (oxygen/tungsten deficient ratio) and the quality of the films (films are thin, homogeneous, constituted of nanoparticles about 5 nm diameter and so with low scattering). Additionally, the high colouring efficiency obtained with a low fluence UV lamp, is followed by a nearly complete bleaching phenomenon after few hours in dark. Beyond the attractiveness of our films toward multiple applications, and especially as films able to regulate the solar flow through smart windows, the optical properties and the colouring and bleaching kinetics were deeply investigated. Whatever the WO3-x thin films or the WO3-x / Ta2O5 composite films, colouring under irradiation as well as bleaching under dark, are phenomena which both proceed with a complex kinetic combining ultra-fast process (from charge transfers, with characteristic time about the minute) and slower process (linked to ionic diffusion, with characteristic time about the hour or even longer).