Optical properties of thin films and electrochromic devices

Abstract

Electrochromic devices (ECDs) are electrochemical cells composed of thin films and electrolyte that have a property to change the color of transmitted light upon applied potential and, consequently, altering their transmittance and reflectance. This property is useful not only for the display industry but also for windows in buildings because it can promote savings of energy spent on air conditioning and lighting. ECDs are composed of two electrodes (ITO coated glasses), an electrochromic layer (WO3, Nb2O5, Prussian blue thin film etc.), a counter electrode (CeO2-TiO2 thin film), and an electrolyte that can be liquid, solid, or gel. As electrolyte the most interesting is gel because of its semi-solid state, volume and shape flexibility, good ionic conductivity, and in most cases simplicity in preparation. Gels can be obtained from synthetic and natural macromolecules such as polysaccharides, proteins, and DNA. These macromolecules can be used in their natural form or can be subjected to chemical or physical modifications such as crosslinking or plasticization processes. Different thin films and ECDs, beside the electrochemical characterization, were tested for their optical properties. It was found that the transmittance of thin films depends strongly on their composition. That of ECDs depends of both thin films and the electrolyte. For example, a 40.2% change between the colored and discolored states was observed for the ECD with gelatin-LiClO4 and 35.2% for the ECD with agar-Eu(CF3SO3) electrolyte. The results show that the thin films and electrolytes are equally important parts of ECDs. Their composition and properties influence the device optical performance. Therefore they need to be more deeply investigated.