Patent Publication Number: US-2005141074-A1

Title: Electrochromic display device

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to electrochromic display devices, and particularly to an electrochromic display device having an electrochromic layer that is a porous nanostructured film.  
      2. Description of the Prior Art  
      Electrochromic materials change color upon application of a voltage. Electrochromic devices are commonly used in windows, automobile rear view mirrors, and flat panel displays.  
      The change in color of an electrochromic material is usually due to an oxidation/reduction process within the electrochromic material. Most electrochromic devices are responsive in the visible light region. Electrochromic materials active in the visible spectral region include metal oxides such as tungsten trioxide (WO 3 ), molybdenum trioxide (MoO 3 ), and nickel oxides. Metal oxides typically range in color from highly colored, such as dark blue, to transparent.  
      U.S. Pat. No. 4,304,465 issued on Dec. 8, 1981 discloses an electrochromic display device. Referring to  FIG. 2 , this represents a schematic cross-section of the electrochromic display device. A panel  10  consists of two parallel sheets  12  and  14 , which are made of glass or another suitable material. The sheets  12  and  14  are sealed together at peripheries thereof by a sealing glass  16  or another suitable bonding material, thereby hermetically enclosing a space  17  containing electrolyte.  
      A thin polymer film  18  is deposited on display electrodes  19  by conventional methods, such as the method described in IBM Technical Disclosure Bulletin Vol. 22, No. 4, page 1639, September 1979, which is incorporated herein by reference. The display electrodes  19  are on an inner surface of the sheet  12 . Counter electrodes  11  are on an inner surface of the sheet  14 . The electrolyte in the space  17  is a conventional electrolyte. Electrical connection between the display electrodes  19  and the counter electrode  11  is effected by means of a conventional edge connection arrangement.  
      The polymer film  18  needs to be sufficiently insoluble in the electrolyte so that the polymer film  18  remains on the display electrodes  19 . Typically, the polymer film  18  is of high molecular weight and insoluble in conventional solvents. If the polymer film  18  is formed by a deposition method, it has good electrical contact with the display electrodes  19 . A typical composition of a polymer film formed by the deposition method is described in detail in J.C.S., Chem. Comm., page 635, 1979, which is incorporated herein by reference. Such compositions contain approximately 70% polymer and 30% anion salt. The anion salt may, for example, be BF4.  
      The thickness of the polymer film  18  is between 0.01 μm and 5 μm, and preferably between 0.05 μm and 1 μm. Typically, there is a tradeoff between color contrast and switching speed. The thinner the polymer film  18 , the faster the switching speed. The thicker the polymer film  18 , the higher the color contrast.  
      In the writing step, the polymer film  18  is oxidized to a colored, nontransparent form. In the erasing step, the polymer film  18  is reduced to a neutral transparent form. The writing and erasing steps are effected by varying or switching the potential of the display electrodes  19 .  
      One drawback of the electrochromic display device is that it uses liquid electrolyte. This requires very painstaking packaging when the electrochromic display device is manufactured.  
      U.S. Pat. No. 5,995,273 issued on Nov. 30, 1999 discloses another kind of electrochromic display device. Referring to  FIG. 3 , the electrochromic display device  20  includes a first substrate  21 , a second substrate  22 , a frame  23 , a conductive connector  24 , a counter electrode  25 , an electrolyte layer  26 , and an electrochromic layer  27 . The electrolyte layer  26  is liquid or solid. If the electrolyte layer  26  is solid, this overcomes the above-described problem of U.S. Pat. No. 4,304,465.  
      However, because of the characteristics of the material of the electrochromic layer  27 , relatively little electrochromic material adheres to the electrochromic layer  27 . As a result, upon application of a voltage, the electrochromism of the electrochromic display device  20  is diminished. In addition, the electrochromic display device  20  operates according to only the single electrochromic layer  27 . This means that the optical density differences are difficult to adjust, and the color changes are less pronounced. That is, the electrochromism is diminished.  
     SUMMARY OF THE INVENTION  
      It is therefore an objective of the present invention to provide an electrochromic display device having improved electrochromism characteristics.  
      In order to achieve the above objective, an electrochromic display device in accordance with the present invention generally includes a first sheet and a second sheet. A bottom electrode, an ion storage layer, an electrolyte layer, an electrochromic layer, and a top electrode are interposed between the first sheet and the second sheet in that order from bottom to top. An external voltage source connects the bottom electrode and the top electrode. The electrochromic layer is a porous nanostructured film, and adheres to the top electrode.  
      Because the electrochromic layer of the electrochromic display device is a porous nanostructured film, it has a very large surface area. Therefore much more electrochromic material adheres to the electrochromic layer, and the electrochromism characteristics of the electrochromic display device are greatly improved.  
      Other objects, advantages, and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic, side cross-sectional view of an electrochromic display device of the present invention;  
       FIG. 2  is a schematic, side cross-sectional view of a conventional electrochromic display device; and  
       FIG. 3  is a schematic, side cross-sectional view of another conventional electrochromic display device.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to  FIG. 1 , an electrochromic display device  30  and an assembly thereof in accordance with a preferred embodiment of the present invention includes a top first sheet  31  and a bottom second sheet  32 . The first and second sheets  31 ,  32  are made of glass or another suitable material. A bottom electrode  33 , an ion storage layer  34 , an electrolyte layer  35 , an electrochromic layer  36 , and a top electrode  37  are interposed between the first sheet  31  and the second sheet  32  in that order from bottom to top. The electrochromic display device  30  is driven by an external voltage source (not shown) that connects the bottom electrode  33  and the top electrode  37 .  
      At least one of the sheets  31 ,  32  is a transparent glass sheet for displaying images. The bottom electrode  33  and the top electrode  37  are both transparent indium tin oxide (ITO) films, and respectively act as a counter electrode and a display electrode of the electrochromic display device  30 . The ITO films are formed respectively on the first sheet  31  and the second sheet  32  by sputtering a target containing approximately 20% indium trioxide (In 2 O 3 ) and 80% tin oxide (SnO).  
      The ion storage layer  34  is made of niobium pentoxide (Nb 2 O 5 ), and is formed on the bottom electrode  33  by electron beam evaporation or ion beam deposition. Niobium pentoxide is an electrochromic material. Thus, the ion storage layer  34  is regarded as a secondary electrochromic layer so that it can adjust optical density differences of the electrochromic display device  30  and provide various color changes.  
      The electrolyte layer  35  is made of tantalic oxide (Ta 2 O 5 ) doped with hydrogen ions. The electrolyte layer  35  is formed on the electrochromic layer  36  by electron beam evaporation or ion beam deposition, and provides stimulant hydrogen ions for the electrochromic layer  36 .  
      The electrochromic layer  36  is a monolayer or multilayer porous nanostructured film, and adheres to the top electrode  37 . The electrochromic layer  36  is made of nickel oxide (NiO). Because the electrochromic layer  36  is a nanostructured film, it has a surface area approximately 1000 times larger than that of a conventional electrochromic layer. Therefore much more electrochromic material adheres to the electrochromic layer  36 , so that the electrochromism characteristics are greatly improved.  
      The electrochromic display device  30  changes color upon application of a voltage. It has been found that a light transmittance of the electrochromic display device  30  in a dark environment is kept within the range from 70˜90%, and a light transmittance of the electrochromic display device  30  in a bright environment is kept below 40%. That is, the electrochromic display device  30  achieves excellent electrochromism.  
      In summary, because the nanostructured electrochromic layer  36  has a very large surface area, more electrochromic material adheres to the electrochromic layer  36 . This improves the electrochromism characteristics of the electrochromic display device  30 . In addition, the ion storage layer  34  is made of an electrochromic material; namely, niobium pentoxide. Thus the ion storage layer  34  is regarded as a secondary electrochromic layer of the electrochromic display device  30 . That is, the ion storage layer  34  improves the electrochromism characteristics of the electrochromic display device  30 .  
      The present invention may have other alternative embodiments as follows. The electrolyte layer  35  can be made of a polymer electrolyte, a polyelectrolyte, or an inorganic electrolyte. The polymer electrolyte is preferably a polymer doped with lithium hypochlorite (LiClO) or phosphoric acid (H 3 PO 4 ). The polyelectrolyte is a polymer that can provide ions. The inorganic electrolyte is generally a transition metal oxide or an alkali salt; for example, lithium tantalite (LiTaO 3 ). The electrochromic layer  36  can be made of titanium oxide (TiO 2 ).  
      It is to be understood that even though numerous characteristics and advantages of the present invention have been set out in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.