JPS5945995B2 - Display device driving method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an electrochromic device that reversibly changes light absorption characteristics depending on an electric current applied between two support plates, at least one of which is transparent, in contact with at least two electrodes. The present invention relates to a method of driving a display device using a substance.

First, a display device (hereinafter abbreviated as ECD) using an electrochromic substance will be outlined.

It is known that there are roughly two types of ECD. (
For example, L. A. Goodman, ``Passive Li.
quidDisplays'', RCAReport6
13258 reference J- uses an inorganic solid membrane, and its typical structure is as shown in FIG. In FIG. 1, 1 is a layer of carbon powder added with a binder (trade name: Aquadac), 2 is a stainless steel plate, and these two form a counter electrode. 3 is a spacer, 4 is a transparent electrode, 5 is a glass substrate, 6
7 is an inorganic solid film exhibiting an electrochromic phenomenon, and 7 is an electrolyte.

The most commonly used inorganic material 6 is tungsten oxide (WO3), and its film thickness is approximately 1 μm.

The electrolytic solution 7 is a mixture of sulfuric acid, alcohol such as glycerin, and fine white powder such as titanium oxide. The alcohol is to dilute the acid and the powder is to provide a white background for coloring phenomena. The thickness of the electrolyte layer is usually 17lgt.
That's about it. A material suitable for functioning as a display device is selected for the counter electrode. Amorphous tungsten oxide is colored blue when the transparent electrode is brought to a negative potential with respect to the counter electrode.

The applied voltage at that time is about several volts. If the polarity of the applied voltage is reversed, the tungsten oxide film returns to its original colorless and transparent state (hereinafter referred to as decolorization). This coloration depends on the injection of electrons and protons into the tungsten oxide film.

Further, decolorization occurs because electrons and protons return to their original state. If a decolorizing voltage is not applied, the colored state will persist for several days even after the coloring voltage is removed (it has a memory effect). Another type of ECD is to reduce a colorless liquid through an electrochemical reaction to form a colored, insoluble film on the cathode. If there is no oxygen, this colored film will not be decolored unless an electric current is applied. However, if oxygen remains, the color will gradually discolor (hereinafter referred to as fading). If the voltage polarity is reversed, the colored film will dissolve and the color will disappear at the same time.

This type of ECD material includes potassium bromide as the supporting electrolyte and heptyl bromide as the substance that produces the colored film.
There is an aqueous solution using viologen bromide. The operating voltage is about 1 volt. The basic cell structure of this type is shown in FIG. In the figure, 8 is a glass substrate, 9 is a counter electrode, 10 is a display electrode, 11 is a viologen mixture, 12
is a spacer and 13 is a sealing material. The thickness of the liquid 11 is usually 1WI. That's about it. ECD using viologen
It can be used as a transmissive type by using transparent electrodes for both electrodes, or as a reflective type by mixing a reflective pigment in the liquid. What has been described above is the simple operating principle of ECD.

Such an ECD has the following characteristics. (1) Wide viewing angle. (2) Several colors can be selected.

(3) Energy consumption is estimated to be several times per cycle of coloring.
It is several tens of mJ/CTl and increases in proportion to the number of cycles.

(4) Even after the coloring voltage is removed, if it is kept electrically open, it has a memory effect in that the colored state persists for several hours to several days.

Of course, in this memory state, there is no need to apply external power. Now, EC with the above principles and characteristics
For example, D is the seven display elements (segments) shown in Figure 3.
FIG. 4 is a simple explanatory diagram of one driving circuit when used as a numeric display device consisting of the following. For simplicity, there are three segments, Sl, S2, and S3. B is power supply, SW
Ol, SWO2 are interlocking voltage polarity switching switches, SW
1, SW2, and SW3 are segment switches. First, let's talk about when to color.

First, move both SWOL and SWO2 switches downward, and turn on only the segment switch for the segment to be colored. Current then flows from the counter electrode 9 through the electrolyte and out of the segment. Thus, the segments through which current flowed become colored, and the segments through which no current flowed remained in their previous state. When the segment is colored to a sufficient density, if at least one of the switches SWOl and SWO2 is set to the neutral position, no current flows and the colored segment is placed in a memory state. Also switch SWO
Even if the segment switch is turned off while both L and SWO2 are pushed down, the segment can be placed in the memory state. In this case, if you turn off the segment switches at different times instead of turning them off at the same time, the segments that were on for a long time will be colored darker, and the segments that were on for a short time will be colored lighter and placed in a memory state. It turns out. That is, by controlling the time during which the segment switch is on, the concentration in the loading and unloading state can be controlled. Next, the case of bleaching will be described. First, both the switches SWOl and SWO2 are turned upward to reverse the coloring and battery connections, and only the segment switch for the segment to be bleached is turned on. Then, a current flows through the segment to be bleached in the opposite direction to that used during coloring, and bleaching is performed. During decolorization, the degree of decolorization can be controlled by controlling the length of time that the segment switch is on. It goes without saying that the switch depicted in FIG. 4 can be easily implemented with an electronic switch such as a transistor analog switch. The present invention briefly relates to an improvement of the above-mentioned driving method, and is characterized in that the rear end of the driving voltage waveform during coloring is made higher. Hereinafter, the merits of the case will be explained using the case of WO8 as an example. First, we will discuss ECD, with particular emphasis on the reaction at the counter electrode. The reactions directly involved in display have already been briefly mentioned, but the current that flows during these reactions must also flow to the counter electrode. In other words, there must be some kind of charge exchange between the opposing electrodes as well. The most commonly used material for the counter electrode is carbon, as shown in FIG. However, the fundamental drawback of this material is that, due to the inertness of the carbon, the electrolyte participates in the reaction at the counter electrode. Since this reaction is generally irreversible,
This raises questions about the reliability of ECD. For example, in the case of water, which is often used in addition to alcohol, it is thought that oxygen is generated at the counter electrode during coloring, and hydrogen is generated during decolorization. Furthermore, even when the irreversible reaction does not impair the reliability of the ECD, a large voltage must be applied to cause the exchange of charges in order to flow sufficient current for coloring and decoloring. This leads to an undesirable increase in driving power when used as an ECD. Therefore, the following two methods can be considered to solve this problem with the counter electrode. One is to add a substance that reacts reversibly at the counter electrode to the electrolyte, and the other is to add a substance that reacts reversibly at the counter electrode. The method is to use a reactive electrode itself.

The present invention is applied to the latter case, using a tungsten oxide film as the reactive electrode. The basic structure in this case is shown in FIG. 14 is a transparent electrode, 15 is a tungsten oxide film, both of which form a counter electrode, and the transparent electrode 4,
It has the same structure as the display electrode consisting of two tungsten oxide films 6.

From this, the following can be said. In other words, considering the direction of the current, when the display electrode is colored, the counter electrode is bleached, and conversely, when the display electrode is bleached, the counter electrode is colored. . One theory of coloring: Color reactions are reversible reactions, so all reactions that occur in a system consisting of a display electrode and a counter electrode are reversible reactions, and this is expected to improve the lifespan of ECDs, which has been confirmed by our experiments. There is. As mentioned above, this proposal relates to the drive waveform of an ECD that also uses a tungsten oxide film for the counter electrode.

Generally speaking, ECD drive methods include two-pole drive and three-pole drive.
There are two methods of polar drive, but three-pole drive avoids side reactions and has better response characteristics. It is considered desirable for the following reasons. However, in the case of three-pole drive, since a third electrode called a reference electrode is required in addition to the two electrodes facing each other in the display, it is thought that the three-pole drive is inferior to the two-pole drive in the following respects. First, the complexity of the ECD manufacturing process increases manufacturing costs, and second, there are difficulties associated with the need for linear amplifiers, which are more difficult to manufacture than digital circuits. This means that the cost will be high due to the difficulty. Therefore, the present invention is characterized in that the trailing end of the drive waveform is made high in order to apply the waveform obtained by three-pole drive to the ECD in a pseudo manner using two poles.

FIG. 6 shows a simple circuit diagram of three-pole drive, and FIG. 7 shows applied voltage waveforms when an ECD using tungsten oxide as the counter electrode is driven with three poles. In FIG. 6, 9 is a counter electrode, S is a display electrode, and 11 is a reference electrode. To enter the memory state, just open the switch SW. In FIG. 7, VS is a set voltage, Vr is a reference electrode potential, and Vc is a counter electrode potential. i is the current flowing through the ECD. Also, when the voltage is positive, it means coloring, and when it is negative, it means bleaching. As can be seen from Figures 5 and 6, the purpose of three-pole one drive is to keep the reference electrode potential always at the optimal value for the desired reaction.
This is to adjust the counter electrode voltage Vc using lp. In other words, if the reference electrode voltage r is too low, the desired reaction may not occur, and even if it occurs, the response will be slow.
Moreover, if it is too high, other side reactions may occur, causing a problem in the life of the ECD. When a rectangular set voltage Vs shown in FIG. 7 is applied to the input terminal Vs in FIG. 6, the reference electrode potential Vr takes approximately the same value as the set voltage Vs because the resistance in the opposing electrode is small.

In this example, the difference between the two potentials Vs (5vr) is about 0.1 mV, and the amplifier Amp with a gain of 10,000 times is used, so the output of the amplifier Amp, that is, the counter electrode potential Vc, is as shown in the figure. The voltage is about 1 V.The tungsten oxide film exhibits a low resistance value when colored, and almost becomes an electrical insulator when bleached.In other words, when coloring an ECD, the tungsten oxide film in the display electrode becomes low in resistance, whereas , the tungsten oxide of the counter electrode becomes high in resistance. Therefore, if coloring progresses now on the display electrode S, the other counter electrode 9
In this case, decolorization progresses and resistance increases. As a result, the difference between the set electrode potential Vs and the reference electrode potential Vr increases at the rear end of writing, and the counter electrode potential Vc exhibits a waveform in which the rear end becomes higher as shown in the figure. As a result, although the value of current 1 decreases at the rear end of writing, it does not become negligible and a certain amount of current continues to flow. Since the three-pole drive works to keep the reference electrode potential constant by passing current through the display electrode, which has become low in resistance, the counter electrode voltage increases so that current passes through the counter electrode, which has high resistance. In the case of decolorization, the opposite electrode is colored, so no peak appears. <Configuration of the Invention> Based on the above findings, the present invention aims to obtain the same effects as described above with a simple configuration.

In the above configuration, a three-pole structure using a reference electrode is used in order to obtain a certain write current value even at the rear end of writing. However, from the practical point of view of industrially manufacturing EC cells, a two-pole structure without a reference electrode is practically preferable to a three-pole structure, which has a complex cell structure and takes time and effort to create. The present invention has been made with the aim of providing a driving method that allows a certain amount of current to continue flowing even at the rear end of writing in a two-pole structure by providing an appropriate external circuit, as in the case of the three-pole structure described above. . An embodiment of the present invention is shown in FIGS. 8 and 9.

In the circuit of FIG. 8, transistors Tr, Tr2, T
The switch circuit consisting of r3 and Tr4 is opened and closed appropriately to create a write pulse with a raised trailing end. When the potential of the terminal C changes from a low potential to a high potential, the switch Tr4 becomes conductive, and the voltage divided by R1 and R2 is applied to the base of the transistor Trl.
1 is the conductivity corresponding to this. At this time, terminal A becomes low potential and switch Tr2
is closed, the terminal B becomes a low potential, the switch Tr3 is opened, and the counter electrode 9 becomes a write potential w. After that, at the rear end of the write pulse, the terminal C becomes low potential and the switch Tr4 is opened, so that the base of the transistor Trl becomes +
Vcc potential, the conductivity of this transistor Trl increases, and the high portion VW.Vcc at the rear end of the write pulse. high is formed. Then switch Tr2. Tr3 is inverted,
An erase pulse Ve is generated. At this time, if the switch SW is closed, the segment S is written or erased accordingly, but if it is opened, the previous state is simply maintained. In FIG. 9, positive voltage means coloring and negative voltage means decoloring. In this way, avoid applying a large voltage at the rising edge of the positive voltage. This is because, as can be inferred from the current waveform in FIG. 7, this is to prevent the instantaneous current from becoming excessive and to prevent side reactions from occurring. However, at the time when the counter electrode becomes considerably decolored and becomes highly resistive, making it difficult to flow current, the applied voltage is increased and V
The trailing edge of the pulse became high as shown in waveform c. During decolorization, the applied voltage is kept constant without changing. This negative voltage is the final voltage when decolorizing the waveform c in FIG. 7. It is true that a negative peak can be seen in the initial stage of decolorization in the waveform c in FIG. 7, but at this time, as can be seen from the current waveform, a large current is flowing. Therefore, by creating the waveform shown in Figure 9, there is a possibility that the negative current value will be smaller and the bleaching speed will be slower, but bleaching is faster than coloring, and a slight delay in response is a practical problem. It is not. On the contrary, a reduction in the peak current value reduces the burden on the power supply circuit, which is considered to be preferable. As stated above, depending on the merits of the case, the EC
By increasing the rear end of the voltage during D coloring in a stepwise manner, EC using tungsten oxide for the counter electrode is also possible.
D can be driven with two poles, and at this time, even without a reference electrode, the same effect as when a reference electrode is provided can be achieved.

Moreover, the present invention can be realized with a simple circuit. It goes without saying that the present invention is applicable to all ECDs in which a tungsten oxide film is used as the counter electrode at least in order to make the counter electrode a reactive electrode.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing the basic construction principle of a solid-state ECD. FIG. 2 is a sectional view showing the basic construction principle of a liquid ECD. FIG. 3 is a segment layout diagram of the Japanese character type numeric display pattern. FIG. 4 is a basic two-pole one drive circuit diagram of the ECD. FIG. 5 is a sectional view showing the basic configuration when a tungsten dioxide film is also used for the counter electrode. FIG. 6 is a simple circuit diagram of three-pole drive, and FIG. 7 shows voltage and current waveforms at various parts in three-pole drive. FIG. 8 is a circuit diagram of one embodiment of the present invention, and FIG. 9 is a signal and voltage waveform diagram of each part in FIG. 8. 1...Aqua Datsuku (product name).

Claims (1)

[Claims] 1. Applying a positive voltage with respect to the display electrode to the counter electrode of an electrochromic display device consisting of a counter electrode and a display electrode comprising a transition metal oxide having an electrochromic phenomenon, and an electrolyte filled between them. In the method of writing and driving the display electrodes, a voltage having a waveform in which the rear end of the positive voltage is raised is applied to the counter electrode by opening and closing a plurality of switch means at appropriate timings. A method for driving a featured electrochromic display device.