Some Perspectives On Electrochromic Device Research

The electrochromic phenomenon consists of changing the optical absorption properties of certain materials by an externally applied electric field or current. In the unactivated state, these materials do not absorb in the visible range of the spectrum and hence they are colorless. However, on applying a moderate DC electric field, the material develops an absorption band in the visible region which results in a color change. The coloration remains for various lengths of time even after the external field is removed. The system returns to its original state when the polarity of the external field is reversed. Consequently this basic phenomenon can he utilized for a great variety of technologically important applications such as light modulation, display devices, optical information recording, electrophotography, and the like. The term "electrochromism" can he applied to a broad range of phenomena in which any kind of color change is brought about by an electric field or a current in a solid or semi-solid system. A great deal of interest in this type of phenomenon arose primarily as a result of an observation that large densities of color-centers can be generated in a controlled and reversible manner in thin films of certain transition metal oxides, such as W03, Mo03, etc., which are ordinarily large band gap insulators. In this paper we will present some historical perspectives on this effect and assess, to a limited extent, the current status of this technology.