Reminiscences on the discovery of electrochromic phenomena in transition metal oxides

It has long been known that transition metal oxides of the types WO₃, MoO₃, TiO₂, V₂O₅, etc., show considerable variation in stoichiometry, and that very often the non-stoichiometric compounds are characterized by strong coloration. This observation led to the fabrication of highly disordered thin films of some of these oxides—ones that showed strong coloration upon optical excitation. Back in the 1960s, there was considerable interest in optically bistable phenomena for high-density information recording, and these oxide films were investigated for that purpose. However, the difficulty encountered in the bleaching process led to the proposition that the optically induced color centers could be bleached by an electric field. Subsequent experiments clearly demonstrated that it was not only possible to form color centers by optical excitation and to bleach them by an electric field, but that it was possible to color and bleach by an electric field alone by simply reversing the polarity. This constituted the first successful demonstration of eletrochromic phenomena in thin films of transition metal oxides. This was quickly followed by the development of various innovative device structures for practical application. In this paper, an attempt will be made to reminisce the discovery of the electrochromic phenomena in this type of material, ultimately leading to the current state of this technology.