“In situ” optical and electrochemical characterization of electrochromic phenomena into tungsten trioxide thin films

Abstract

The results of “in situ” measurements of the electrochemical and optical properties of tungsten trioxide (WO3) thin films obtained by vacuum evaporation, anodic oxidation of W and chemical vapor deposition, are presented. Optical measurements were performed at λ = 633 nm with a HeNe laser. Cyclic voltammetry and chronoamperometry have been used as electrochemical techniques. The determination of the variation of the chemical potential of the inserted species (ions and electrons) during insertion into the oxide has been carried out using an electrochemical method. This variation is of great importance in electrochromic kinetics. Based on the assumption of a double injection of electrons and ions into the oxide and on experimental observations, we propose a model to describe the electrochromic phenomena and the different reactions leading to the coloration of the oxide. From this model and from the experimental curves, J = f(t) and OD = f(t), the ionic diffusion coefficient into the oxide is obtained. The electrode-electrolyte interface was also investigated in this study. We used the technique of impedance spectroscopy to analyse this interface. Both charge transfer and diffusion coefficient have been determined from the Nyquist diagrams. The values obtained were in good agreement with those obtained from the above proposed model.