Structural Modification of Electrochromic Thin Films towards Optimized Mixed Ionic and Electronic Conduction

Abstract

Smart windows based on electrochromic materials offer adjustable shading which can reduce the energy consumption required to cool or heat a building and increase the indoor occupant comfort. Commercial smart windows currently available on the market, however, may require up to several minutes to switch from one optical state to another. Therefore, special attention must be given to the enhancement of the switching kinetics of the electrochromic materials for the optimal use in smart windows. The switching kinetics are strongly dependent on the transfer of electrons and the diffusion of charge-balancing ions in the electrochromic material and, hence, on the presence of suitable pathways for the transport of electrons and ions in the electrochromic thin films. One part of this work focused on tungsten oxide as a well-known electrochromic material. Thin films of tungsten oxide were prepared by spin-coating with optional addition of different polymers as structure-directing agents to allow for fine-tuning of the porosity and the internal structure of the films. Tungsten oxide thin films prepared with optimum compositions of the polymers provided accessible porous networks which enabled facilitated diffusion of the intercalating ions through the film and, thus, improvement in the rate and modulation of the switching between two transmittance states of the films. Another part of this work dealt with new substituted phthalocyanines as an alternative group of electrochromic materials. Thin films composed of different fluorinated phthalocyanines were fabricated via physical vapor deposition. The intermolecular coupling of the molecules in the solid state is influenced by the degree of fluorination. The aimed moderate degree of intermolecular coupling that allowed a well-balanced, equally fast transport of electrons and ions could be obtained for films consisting of either one type of specifically substituted phthalocyanine molecules or an appropriate mixture of two well-established phthalocyanine molecules. The rate of the electron and/or ion transport could be enhanced by versatile approaches that allowed proper modifications of the film properties of thin films of tungsten oxide as well as of phthalocyanines leading to fast and stable electrochromic switching.