JPS6159489B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to an image display device that utilizes color development and fading phenomena based on electrochemical redox reactions. 2. Description of the Related Art In recent years, attention has been paid to devices that display images by externally supplying electricity to a substance that develops and fades in color through an electrochemical redox reaction. That is, this display device basically consists of a display electrode, a hidden counter electrode, and a color-developing medium in contact with these electrodes. And the display electrode (-)
When the polarity is turned on and energized, "writing" is performed, and when the polarity is reversed, "erasing" is performed. Such a device is called an electrochromy device, and unlike conventional display devices (such as fluorescent display tubes, glass discharge tubes, plasma display devices, or LEDs), it is a non-emissive (passive) display, so it cannot be used in bright places. It has the advantage that it looks reasonably good, and your eyes won't get tired even if you look at it for a long time. Also, unlike the same non-emissive type liquid crystal display device, it has advantages such as no viewing field dependence and display with high image contrast. Furthermore, since the image once displayed by such an element remains even when the driving power is turned off (memory function), the driving power can be reduced. However, many problems remain in such devices regarding their repeated durability when put into practical use. That is, if the cycle of "writing → memory → erasing" is repeated, phenomena such as poor coloring, unerasable areas, and changes in color tone occur, and the display effect is significantly impaired. These phenomena are intricately related and cause device deterioration, but in any case, during the display cycle, electron transfer and accompanying chemical reactions occur at the contact interface between the electrode and the coloring/decoloring medium. This has to do with the fact that it is not reversible. For example, when a displayed image is memorized, colored species remain deposited on the display electrode. In an electrochromy element, when a reduction reaction occurs on the display electrode, an oxidation reaction occurs on the counter electrode, and when an oxidation reaction occurs on the display electrode, a reduction reaction occurs on the counter electrode. Ideally, this should be done alternately and evenly on the display and counter electrodes, but as mentioned above, if colored species remain on the display electrodes for a long time, this balance will be disrupted.
It is thought that the remaining color may remain, resulting in poor color development or contamination of the electrodes. Further, the electrode surface is generally likely to be partially contaminated due to the presence of impurities in the coloring/decoloring medium. Once the electrode surface becomes contaminated, coloring does not occur uniformly and "visibility" deteriorates. In addition, excessive transient current flows when voltage is applied, or once the electrode surface becomes contaminated, the potential distribution on the electrode surface becomes partially high, etc.
Water decomposition occurs, producing H ₂ or O ₂ bubbles. Furthermore, when KBr is used as the electrolyte and a viologen bromide color-developing substance, an anodic reaction of Br ^- ions occurs, forming a dark yellow colored precipitate on the electrode surface, which significantly impairs the display effect. Therefore, many proposals have been made for the purpose of improving the various drawbacks mentioned above and improving the repeated durability of the device. For example, a method is known in which a reference electrode is used to detect the potential difference between the display and reference electrodes, and a feedback circuit is provided to avoid excessive voltage application, thereby suppressing side reactions. Additionally, as an improvement to the above method, it is known that numerous small holes with a diameter of approximately 10 μm are made on the electrode surface, and these holes function as a reference electrode to prevent side reactions in the same manner as in the above example. ing. Furthermore, in order to prevent excessive transient current from flowing when voltage is applied and causing side reactions, it is possible to devise a driving waveform and gradually apply voltage, or to take advantage of the fact that the colored species has a back electromotive force. There is also known a method (Japanese Patent Application Laid-Open No. 144662/1983) in which the driving potential is lowered during decoloring than when developing color. However, at present, a material that is practical and has sufficient durability has not been obtained. For example, a method of detecting the potential difference between the display electrode and the reference electrode using a reference electrode does not work effectively during erasing, and results in residual erasure. That is, in order to keep the potential between the display electrode and the reference electrode constant, a feedback current is passed between the display electrode and the counter electrode, so the potential between the display electrode and the counter electrode becomes high, and the electricity of the water or auxiliary electrolyte increases. The decomposition voltage is exceeded and OH ^- ions increase. It is thought that this slows down the erasing speed and causes some residue. Furthermore, in these methods, when seven segment electrodes are used, a feedback circuit is required for each of the seven segment electrodes, resulting in a complicated configuration. Furthermore, the method of making numerous holes in the electrode is extremely impractical because it requires special processing of the electrode. The present invention aims to remedy all the above-mentioned drawbacks. That is, an object of the present invention is to provide an image display device that can be driven at a low potential. Another object of the present invention is to provide an image display device that does not leave any residual image. Still another object of the present invention is to provide an image display device that does not require a complicated drive circuit or the like. Yet another object of the present invention is to provide an image display device with a simple electrode configuration. Another object of the present invention is to provide an image display device that does not cause electrode deterioration. The present invention, which achieves the above object, is an image display device in which a coloring/decolorizing medium that causes a visible change through an electrochemical redox reaction and a current-carrying electrode in contact with the medium are housed in a cell. The image display device is characterized in that the electrodes include a display electrode, a coloring counter electrode used when coloring the display electrode, and a color erasing counter electrode used when color erasing the display electrode. The structure, operation, etc. of the present invention will be explained in detail below. A color developing and erasing substance used in the image display device of the present invention,
Regarding the electrolyte or inert solvent, all those commonly used in electromechanical devices may be used. The electrochemical coloring/decolorizing substance used in the present invention is not particularly limited, and can include a wide range of redox-reactive inorganic and organic substances. I will explain this using an example. Examples of redox-reactive organic substances include pyridinium compounds having a quaternary ammonium salt structure, such as 1,1'-dimethyl-4,4'-bipyridinium.
Dibromide 1,1'-diethyl-4,4'-bipyridinium
Dibromide 1,1'-diheptyl-4,4'-bipyridinium dibromide 1,1'-dibenzyl-4,4'-bipyridinium dibromide N,N'-di(P-cyanophenyl)-4,4' −
Bipyridinium dibrolide 2,2'-(diethyl)bipyridinium dichloride N,N'-diethyl-2,7-diazapyrenium dichloride N-benzyl-4-cyano-pyridinium dibromide Also, as a redox indicator, Safranin T New TRALURED Indigo monosulfonic acid Diphenyl amine Diphenyl amine-P-sulfonic acid P-nitro diphenyl amine Diphenyl amine-2,3'-dicarboxylic acid Diphenyl amine-2・2'-dicarboxylic acid, etc. can be mentioned. The solvent used in the present invention is generally water, but other solvents include methyl alcohol, ethyl alcohol, ethylene glycol, propylene carbonate, ethylene glycol monomethyl ether acetate, tetrahydrofuran, dioxane,
Examples include non-aqueous solvents such as dimethylformamide and dimethyl sulfoxide, and mixed systems of water and these non-aqueous solvents. As an electrolyte, lithium chloride, potassium chloride,
Sodium chloride, potassium bromide, potassium sulfate,
Potassium acetate, potassium phosphate, ammonium sulfate, ferrous sulfate, ammonium phosphate, lithium perchlorate, zinc ammonium sulfate, magnesium ammonium sulfate, ferrous ammonium sulfate, and the like can be preferably used. In addition, in the color developing and decoloring device of the present invention, when storing the color developing and decoloring medium in which these components are dissolved in a suitable housing, the dissolved oxygen concentration in the medium may be adjusted to 5 ppm or less. desirable. By reducing dissolved oxygen in the medium, side reactions in the medium can be suppressed. Next, the electrode structure, etc. of the present invention will be explained in comparison with a conventional image display device. First, the basic configuration of the conventional device is as shown in FIG. 1, in which a display electrode 1 and a counter electrode 2 are arranged in a cell 3 made of glass or the like, and both electrodes 1 and 2 are connected to a power source 4. Inside the cell 3, a coloring/erasing medium 5 made of a coloring/erasing substance, an electrolyte, an inert solvent, etc. is enclosed. The materials for the electrodes 1 and 2 include chemically stable metals such as Pt, Pd, and Au, or SnO ₂ , In ₂ O ₃ ,
Metal compounds such as TiO ₂ are used. On the other hand, the basic configuration of the image display device of the present invention is as shown in FIG. 2a. That is, the display electrode 1, the cell 3, the power source 4, and the coloring/decoloring medium 5 are the same as those of the conventional device, but when the display electrode 1 is colored, the counter electrode 2 (hereinafter referred to as coloring When decoloring the counter electrode (referred to as the counter electrode) and the display electrode,
It has a counter electrode 2' (hereinafter referred to as a decoloring counter electrode) which is turned into a (-) pole and is energized. Furthermore, 2
It has a switch 4' for controlling the polarity of the two opposing electrodes 2 and 2'. In the state shown in Figure 2a,
Since the display electrode 1 is energized with the (-) pole and the coloring counter electrode 2 is the (+) pole, color development occurs on the display electrodes. If the switch 4' is switched as shown by the broken line, the display electrode 1 becomes the (+) pole and the counter electrode 2' for erasing becomes the (-) pole, and the colored species on the display electrode disappears. Color development is occurring on the counter electrode 2'. Although the manual changeover switch 4' has been described here for the sake of explanation, when putting the display device into practical use, an appropriate control device or drive circuit that automatically switches the polarity of the electrodes will be connected to the power supplies 4 and 3. inserted between the two electrode leads. In the present invention, by dividing the counter electrode into a coloring electrode and a coloring electrode, the durability of the electrochromic element can be improved.
That is, in general, in electrome elements, when the display electrode or the counter electrode is set to the (-) pole, a reduction reaction occurs on the electrode surface, and when the display electrode or the counter electrode is set to the (+) pole, an oxidation reaction occurs. However, when a single counter electrode is provided to the display electrode as in the past, even if the counter electrode is made of noble metal, both oxidation and reduction reactions occur on this single counter electrode, resulting in electrode deterioration. was likely to occur. In contrast, in the present invention, the above-mentioned drawbacks can be overcome by dividing the counter electrode into one for color development and one for reduction. Furthermore, in the present invention, materials that are resistant to oxidation and reduction reactions can be selected for the coloring and decolorizing counter electrodes, respectively. For example, oxide electrodes such as transparent electrodes SnO ₂ , In ₂ O ₃ or TiO ₂ are more resistant to oxidation reactions than reduction reactions, and metal electrodes are more resistant to reduction reactions than to oxidation reactions. They tend to be highly resistant. Therefore, if a material such as SnO ₂ or In ₂ O ₃ is used for the coloring counter electrode, and a metal electrode is used for the decoloring counter electrode, the repeated durability can be improved. Another advantage of the present invention is that during the color development/discoloration cycle, it is easy to obtain a state in which no coloring species is deposited on either one of the opposing electrodes. Alternatively, in the present invention, even if the same material is used for the coloring and decoloring counter electrodes, an oxide electrode such as SnO ₂ or In ₂ O ₃ is used, and the resistance value is higher than that of the coloring counter electrode. By increasing the resistance value of the color counter electrode, it is possible to obtain the same effect as a method in which a special circuit is used to drive at a lower potential during decoloring than during coloring. The arrangement, shape, etc. of the electrodes are appropriately selected depending on the specifications of the device. For example, the display electrode 1, the counter electrode 2 for color development, or the counter electrode 2' for decolorization are shown in FIG. 2b,
Two opposing substrates 6 and 7 as shown in c or d
It may be placed on the same substrate 6, or it may be placed on the periphery of two substrates or on the spacer 8 as shown in FIG. 2e. In these figures, the substrate 6 is an opaque or transparent substrate, 7 is a transparent substrate, 8 is a spacer, 9 is a blinding plate, and 10 is an ion-permeable screen. Further, the shape of the electrode may be a segment electrode or a pattern electrode. Next, a method of driving such a device will be explained.
First, "writing" is performed by applying an external voltage so that the display electrode is negative and the coloring opposite electrode is positive. Then, the memory effect is exhibited with the external voltage turned off and the circuit open. "Erasing" is achieved by applying a voltage with the polarity reversed from "writing", but in this case a counter electrode for erasing is used. Figure 3 shows a typical drive pulse application method,
10 represents a write step, 11 a memory step, and 12 an erase step. The driving method described above is the simplest two-pole constant voltage method, but in the present invention, a two-pole constant current method and a three-pole constant potential driving method may also be used. Note that the three-electrode constant potential method uses a reference electrode in addition to the display electrode 1 and the counter electrodes 2 and 2' to detect changes in the potential of the display electrodes and automatically adjust the applied voltage. This is a drive method that keeps the potential constant. Although this method requires a complicated circuit when displaying segments, it may be used in the present invention except for this point. Note that specific effects of the present invention will be explained in more detail in Examples. Example 1 Cells were created using the following method, and the durability of each cell was examined for repeated use. A rod-shaped electrode was placed in the glass cell as shown in FIG. 2b. A tin oxide transparent electrode with a resistance value of 15 Ω/cm ² was used as a counter electrode for coloring and decolorizing the display electrode, and the electrode size was each a rectangle of 2 mm x 3 mm. Further, the distance between both electrodes was 2 mm. The solution composition of this cell was as follows. Electrochemical coloring/decolorizing substance: Heptyl viologen bromide (0.1M/) Electrolyte: Potassium bromide (0.3M/) Solvent: Water A DC power source was connected between the working electrode and counter electrode of the various cells obtained above. -2V (2sec) → 0V
It was driven by applying a potential cycle of (0.5 sec) → +2V (2 sec), and the durability was checked by repeating coloring → memory → decoloring. The device of this example has approximately 2×10 ⁶
It had durability over multiple cycles. The repeated durability referred to here is an evaluation based on the number of times the process of coloring → memory → decoloring is repeated using the above-mentioned driving method until there is a fading mark or uneven coloring on the electrode. . Examples 2 to 5 Using the same method as in Example 1, the durability was repeatedly checked by changing the material and resistance value of the dedicated counter electrode. The results are shown in Table 1 together with the results of Example 1 and the following comparative examples. Comparative Example 1 A rod-shaped display electrode and a counter electrode as shown in Fig. 1 are arranged facing each other as shown in Fig. 2b (however, the electrode material is
(SnO ₂ ), and the durability was repeatedly checked in the same manner as in Example 1 except for that. It showed durability after being repeated about 5×10 ⁵ times. Comparative Example 2 Durability was repeatedly checked in exactly the same manner as Comparative Example 1 except that an Au vapor-deposited electrode was used as the counter electrode. The durability was shown to be repeated approximately 5 to 8×10 ⁵ times.

[Table] As can be seen from Table 1 above, according to the present invention, the repeated durability is greatly improved in all the examples. Good results were also obtained in terms of "visibility" or response characteristics.

[Brief explanation of the drawing]

Figure 1 is a basic configuration diagram of an electrochemical color developing and decoloring device.
FIG. 2a is a schematic diagram illustrating the basic configuration of the device of the present invention, FIGS. 2b, c, d, and e are sectional views showing examples of electrode arrangement, and FIG. 3 is a waveform diagram of a drive signal. 1 is a display electrode, 2 is a counter electrode for color development, 2' is a counter electrode for decolorization, 3 is a cell, 4 is a power supply, 4' is a changeover switch, 5 is a color development/decolorization medium, 9 is a blinding plate, and 10 is a Ion-permeable screen, 11 is a writing step, 12 is a memory step, 13
is the erase step.

Claims (1)

[Claims] 1. In an image display device in which a coloring/decolorizing medium that causes a visible change through an electrochemical redox reaction and a current-carrying electrode in contact with the medium are housed in a cell, the electrode causes the display electrode to develop color. A counter electrode for coloring made of an oxide of SnO ₂ or In ₂ O ₃ which is sometimes used, and a counter electrode for decoloring made of the same material as the coloring electrode used to decolor the display electrode. An image display device comprising an electrode, and the resistance value of the decoloring counter electrode is larger than that of the coloring counter electrode.