Principles of Electrochromism as Related to Display Applications

Abstract

In electrochromic materials an electric current induces a persistent and reversible change in optical properties. The elementary process of coloration consists either in a valency change of one of the constituent ions or in the formation of a color center associated with a lattice defect. Both can be regarded as electrochemical processes and Smakula’s equation directly predicts the electrical charge required to achieve a given contrast. For charge neutrality reasons two species of charge carriers (electrons and ions) are needed to induce coloration. In order to make bulk electrochromism feasible, the material has to be both electronically and ionically conducting. The underlying general physical principles provide clear guidelines in materials research and cell design and will be discussed with emphasis on WO₃ and solid state technology.