This invention relates to an improved display device utilizing electrochromic phenomenon.
Some substances are colored if applied with electric current. This coloring is called "electrochromic phenomenon", and substances undergoing this phenomenon are called "electrochromic material". The color of an electrochromic material is bleached by flow of current in the reverse direction. The electrochromic phenomenon is generally thought to accompany formation of color centers or oxidation-reduction reaction.
Organic and inorganic electrochromic materials are known. U.S. Pat. No. 3,806,229 names a viologen salt as a typical organic electrochromic material. U.S. Pat. No. 3,712,710 discloses oxides of transition metals such as tungsten oxide and molybdenum oxide as typical inorganic electrochromic material. Generally, these metal oxides are used together with electrolytes such as sulfuric acid, phosphoric acid and lithium perchlorate solution.
Recently, many attempts have been made to apply the electrochromic phenomenon to a display cell. Such a display cell is so constructed as shown in the appended FIG. 1. A coloring region 1 comprising an electrochromic layer and an electrolyte is formed between a pair of electrodes 2 and 3. The electrode 2 is constituted by a transparent conductive film 4 contacting the coloring region 1 and a transparent substrate 5 on which the film 4 is formed. The film 4 is cut to have a pattern denoting numerals, letters, symbols, etc, and the substrate 5 is a glass pate or the like. Likewise, the electrode 3 is constituted by a substrate 6 and a conductive film 7 formed on the substrate 6 and contacting the coloring region 1. A spacer 8 of an insulating material is provided between the electrodes 2 and 3 and contacting the periphery of the coloring region 1. Between the electrodes 2 and 3, voltage is applied reversibly by actuating a switch 9 connected to a power source 10, thereby to color and bleach the coloring region 1 alternatively. The display cell becomes a transmission type if the substrate 6 and film 7 of the electrode 3 are formed of transparent material. It becomes a reflection type if the electrode 3 is formed of opaque material or if a colored background sheet is stretched within the electrolyte. If the conductive film 7 is made transparent, indium oxide, tin oxide etc. are suitable materials. If the film 7 is made opaque, stainless steel, tantalum, platinum etc. may be used.
In general, an electrochromic material absorbs ambient light to perform a so-called "passive display" which depends on the color density. Owing to the passive display, eyes do not get tired, and the letters etc. displayed can be seen clearly regardless of visual angle. In addition, the color displayed does not disappear even if the display cell has been turned off because the display cell works as a memory device. Because of these advantages, the electrochromic display device is expected to be employed in various fields.
The conventional electrochromic display device is, however, not satisfactorily durable. This is because that part of the electrochromic material (e.g. WO.sub.3) in which electric field is likely to concentrate will be dissolved after the switch has been actuated about 10.sup.6 times to color and bleach the electrochromic material. As a result, the electrodes are exposed to the electrolyte and eventually reduced and dissolved by the electrolyte, whereby the display cell ceases functioning.