1. Field of the Invention
The present invention relates to an electrochromic element used in various kinds of display units and dimmers.
2. Description of the Prior Art
Electrochromic elements known in the art include electrode pairs having at least one electrode made of a transparent electrode film, a color forming film made of tungsten oxide laminated on each electrode, and an electrolyte. When voltage is applied to the electrodes, the color forming film is chemically reduced and becomes blue. When reverse current is applied, the blue film is oxidized and becomes transparent.
With this type of electrochromic element, the use of tungsten oxide for the color forming film traditionally has been problematic, because even when voltage is not applied to the electrodes, the film becomes colored when it is irradiated with ultraviolet rays. As indicated by the following formulas: ##STR1## when a tungsten oxide film is irradiated with ultraviolet radiation, electrons (e.sup.-) are excited and positive holes (h.sup.+) are formed. The excited electrons (e.sup.-) combine with lithium ions Li.sup.+ and with hydrogen ions H.sup.+ to four tungsten bronze (LixWO.sub.3). The formation of this material is believed to cause coloring problems.
For the reasons described above, when ultraviolet rays contact a transparent film of tungsten oxide, the film becomes colored. Furthermore, charging balances of the coloring and de-coloring reactions are affected, thereby causing deterioration of the electrochromic element.