1. Field of the Invention
The present invention relates to an electrochromic display device used for reflective displays, electronic paper and the like.
2. Description of the Related Art
Recently, much research effort has been made to electronic paper as an electronic medium that is expected to displace paper. In contrast to conventional displays including CRTs and liquid crystal displays (LCDs), electronic paper requires the following characteristics: being a reflective display device; high white reflectivity and contrast ratio; high definition display; memory effect; low-voltage drive capability; slimness; lightness; and inexpensiveness, for example. In particular, electronic paper is required to have as excellent white reflectivity and contrast ratio as paper with respect to its display characteristics, and the development of a display device offering all of these characteristics is far from easy. Although electronic paper technologies, such as reflective liquid crystal devices, electrophoretic display devices and toner electrophoretic displays, have heretofore been proposed, they are poor in white reflectivity.
A phenomenon in which electrochromic materials applied with a voltage show a reversible color change during the electrochemical redox reaction is called electrochromism. Electrochromic (hereinafter sometimes abbreviated as “EC”) display devices, which utilize color change in EC compounds causing such a phenomenon, have emerged as a candidate for electronic paper because they serve as a reflective display device, have high white reflectivity as well as a memory effect, and can be driven at low voltage. For example, Japanese Patent Application Laid-Open (JP-A) No. 2000-506629, Japanese Patent Application Laid-Open (JP-A) No. 2001-510590, Japanese Patent Application Laid-Open (JP-A) No. 2003-511837, Japanese Patent Application Laid-Open (JP-A) No. 2002-328401, and Japanese Patent Application Laid-Open (JP-A) No. 2004-537743 disclose an EC device in which organic EC compounds are attached to the surfaces of particles made of, for example, titanium oxide.
Such EC devices are known for their ability of efficiently changing colors by utilizing a surface-area effect of a metal oxide, as well as for their excellent durability. The EC device is a current-driven device and therefore requires high electrical power consumption, thereby posing problems relating to an improvement in the efficiency of color production.