JP2005257888A - Method of driving electrophoretic display element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce power consumption and to obtain stable high contrast. <P>SOLUTION: In the method of driving the electrophoretic display element equipped with first and second substrates 1, 2 placed opposite to each other, and an electrophoretic layer 4 having migrating particles 4a sealed in between the substrates and having a specified charge and an insulating fluid to disperse the migrating particles therein, and when obtaining a colored state of the electrophoretic layer, being a state in which the color of the migrating particles is visually recognized, an AC voltage is applied to the electrophoretic layer, and at least one out of an amplitude of the AC voltage and an offset voltage is varied during the period of the colored state. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for driving an electrophoretic display element.

  Expectations for reflective display devices are increasing from the standpoint of reducing power consumption and reducing the burden on the eyes. Until now, an electrophoretic display element has been known as one of reflective display elements. This electrophoretic display element comprises an electrophoretic layer composed of charged electrophoretic particles and an insulating liquid and a pair of electrodes facing each other with the electrophoretic layer interposed therebetween, and an electric field is applied to the electrophoretic layer through the electrode. By applying the electrophoretic particles, the electrophoretic particles are moved onto an electrode having a polarity opposite to that of the charge, and display is performed. The contrast color of the electrophoretic particles is borne by the above-described insulating liquid in which the dye is dissolved. More specifically, when the electrophoretic particles adhere to the surface of the first electrode close to the observer, the color of the electrophoretic particles is observed, while the surface of the second electrode is far from the observer. In the case of adhering to the surface, the color of the electrophoretic particles is concealed by the insulating liquid and the contrasting color of the insulating liquid is observed. Although the electrophoretic display element has the advantages of a wide viewing angle, high contrast, and low power consumption, the dye dissolved in the insulating liquid is adsorbed to the electrophoretic particles, and the electrode surface on which the electrophoretic particles are adsorbed Due to the penetration of the insulating liquid between the electrophoretic particles, there is a big problem that it is difficult to achieve both high reflectivity, that is, brightness and high contrast.

  As one of the attempts to solve this problem, the transparent state is used in an area where the observer can visually recognize the electrophoretic particles by concealing the electrophoretic particles with a shielding layer without dissolving the pigment in the insulating liquid. It is known to obtain a colored state by adsorbing electrophoretic particles on a certain electrode (see, for example, Patent Document 1). However, there are cases where the electrophoretic particles are not uniformly adsorbed on the electrode in a colored state, and in such a case, there is a problem in that the transmittance at the time of coloring increases, resulting in a decrease in contrast.

As one of attempts to solve this problem, although it is limited to a transmissive electrophoretic display element, it is known to obtain a colored state of the display element by applying an alternating voltage (for example, Patent Documents). 2). However, when the AC voltage is continuously applied, there arises a problem that the power consumption increases, which is not preferable as a reflective display element aiming at low power consumption. In addition, Patent Document 2 also mentions that the power is turned off after applying an AC voltage, or that the DC voltage is applied to adhere to the electrode. It becomes difficult to maintain a uniform dispersion state stably. In addition, when a DC voltage is applied to adhere to the electrode, the adhesion to the electrode may be insufficient depending on the setting of the voltage magnitude, and eventually it will be uneven. It turned out to be dispersed. That is, the problem of obtaining a stable high contrast still remains. Furthermore, when an AC voltage is applied to obtain a colored state, both the transition from the transparent state to the colored state and the transition from the colored state to the transparent state have a slower response speed than when applying the DC voltage of the conventional method. There was a problem of becoming. Furthermore, Patent Document 2 does not mention matrix driving of a display element composed of a plurality of pixels.
JP 9-2111499 A Japanese Patent Laid-Open No. 3-91722

  As described above, it has been difficult to realize an electrophoretic display element having a stable high contrast and low power consumption.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for driving an electrophoretic display element that has low power consumption and can obtain a stable high contrast.

  According to a first aspect of the present invention, there are provided first and second substrates arranged opposite to each other, electrophoretic particles encapsulated between the substrates and having a predetermined charge, and an insulating fluid that disperses the electrophoretic particles. An electrophoretic layer comprising: an electrophoretic layer comprising: an electrophoretic layer comprising: an electrophoretic layer, wherein the electrophoretic layer has a colored state in which a color of the electrophoretic particle is visually recognized. An AC voltage is applied, and at least one of the amplitude of the AC voltage and the offset voltage is changed during the coloring state.

  Further, the second aspect of the present invention provides the first and second substrates disposed opposite to each other, the migrating particles encapsulated between these substrates and having a predetermined charge, and the insulation for dispersing these migrating particles. An electrophoretic layer having an electrophoretic fluid, wherein the electrophoretic layer has a colored state in which the color of the electrophoretic particles is visually recognized. A DC voltage is first applied to the layer, and then an AC voltage is applied. The application time of the DC voltage is longer than a half cycle of the AC voltage.

  According to a third aspect of the present invention, there are provided first and second substrates disposed opposite to each other, electrophoretic particles encapsulated between these substrates and having a predetermined charge, and insulation for dispersing the electrophoretic particles. An electrophoretic layer having an electrophoretic fluid, wherein the electrophoretic layer has a colored state in which the color of the electrophoretic particles is visually recognized. An AC voltage is first applied to the layer, and then a DC voltage larger than the amplitude of the AC voltage is applied.

  According to a fourth aspect of the present invention, there are provided first and second substrates arranged opposite to each other, a matrix formed between the first and second substrates, each of which is a pixel electrode. An electrophoretic display device comprising: a pixel including an electrophoretic layer having electrophoretic particles having a predetermined charge enclosed between the first and second substrates; and an insulating fluid that disperses the electrophoretic particles. A driving method of applying an AC voltage or a DC voltage to the electrophoretic layer of each pixel, and applying time of high voltage and low voltage of the AC voltage or applying time of the DC voltage is set to m, n. The natural number is m / n times the frame period.

  According to the present invention, low power consumption can be realized and stable high contrast can be obtained.

  A method for driving an electrophoretic display device according to an embodiment of the present invention will be described.

  In the present embodiment, the “colored state” of the electrophoretic layer refers to a state in which an observer visually recognizes the color carried by the electrophoretic particles. A chromatic color is generally used for the electrophoretic particles, but when white particles are used, the colored state is white. On the other hand, what the observer sees in the transmission state of the electrophoretic layer is reflected light from a substance located behind the electrophoretic layer as viewed from the observer or transmitted light from the rear. For example, when a layer responsible for the contrast color is provided behind the display element as viewed from the observer, the observer visually recognizes the contrast color when the electrophoretic layer is in a transmissive state. Further, the layer responsible for the contrasting color only needs to be provided at the rear of the electrophoretic layer, and can be incorporated in the display element. For example, it is conceivable that the substrate itself that exists far from the observer becomes a layer responsible for the contrasting color, or a layer that exhibits the contrasting color is formed on the electrode surface on the substrate. Thus, the driving method according to the present embodiment can be applied to both transmissive and reflective electrophoretic display elements.

  In the present embodiment, the AC voltage amplitude means 1/2 of the fluctuating voltage. The AC voltage can be defined by a voltage amplitude and an offset voltage which is a DC component. The offset voltage is a voltage of a DC component included in AC.

  The driving method of the electrophoretic display device according to the present embodiment will be described with reference to FIGS. FIGS. 1 to 3 are waveform diagrams showing driving waveforms of the electrophoretic display element used in the driving method according to the present embodiment. FIGS. 4 to 6 show one example of the electrophoretic display element used in the driving method according to the present embodiment. It is sectional drawing which shows the state of one pixel. In this pixel, two substrates 1 and 2 arranged opposite to each other, a spacer 3 that keeps the substrates 1 and 2 at a predetermined distance, and charged electrophoretic particles 4 a are enclosed between the substrates 1 and 2. The electrophoretic layer 4 and the electrodes 6 and 7 are provided. The electrode 6 is provided between the substrate 1 and the spacer 3, and the electrode 7 is provided on the substrate 2. The electrophoretic particles 4a are colored and have a negative charge. The electrophoretic layer 4 includes electrophoretic particles 4a and an insulating fluid 4b. The driving waveforms shown in FIGS. 1 to 3 show the potential change of the electrode 7 when the electrode 6 is taken as a reference. FIG. 1 shows a driving waveform in which a first AC voltage is applied, and then a second AC voltage having a smaller amplitude than the first AC voltage is applied. FIG. 2 shows a driving waveform in which a DC voltage is first applied and then an AC voltage having an amplitude smaller than that of the DC voltage is applied. FIG. 3 shows a drive waveform in which an AC voltage is applied and then a DC voltage having a larger amplitude than the AC voltage is applied.

  FIG. 6 shows a transmission state of the electrophoretic layer 4, and the electrophoretic particles 4 a are attached to the electrode 6. When driven with the drive waveform shown in FIG. 1 in this state, the electrophoretic particles 4a go back and forth between the electrodes 6 and 7 by an alternating current having a large voltage amplitude, and the electrophoretic layer 4 as shown in FIG. A uniform coloring state can be obtained. Once a uniform dispersion state is obtained, the uniform dispersion state shown in FIG. 4 can be stably maintained even with an alternating current having a small voltage amplitude in order to suppress power consumption. That is, high contrast and low power consumption can be realized.

  Next, when the driving state shown in FIG. 2 is driven with respect to the transmission state shown in FIG. 6, the electrophoretic particles 4 a start moving from the electrode 6 toward the electrode 7 at a high speed by the initial DC voltage. Compared with the case where an alternating voltage is applied from the beginning, the speed is larger because the Coulomb force that returns the electrophoretic particles 4a to the electrode 6 does not work. If the DC voltage is continuously applied as it is, the electrophoretic particles 4a are not uniformly attached to the electrode 7 or are not uniformly dispersed, and a good colored state cannot be obtained. Therefore, a uniform dispersion state as shown in FIG. 4 can be obtained by switching the drive from direct current to alternating current. That is, high contrast and high-speed response can be realized. Further, as described above, the power consumption can be reduced by reducing the AC voltage amplitude after uniform dispersion.

  Next, when the transmission state shown in FIG. 6 is driven with the drive waveform shown in FIG. 3, the electrophoretic particles 4a move back and forth between the electrode 6 and the electrode 7 by the initial AC voltage, as shown in FIG. A uniform coloring state can be obtained. Subsequently, when a direct current having a voltage higher than the alternating voltage amplitude is applied, a colored state in which the electrophoretic particles 4a are uniformly attached to the electrode 7 can be stably realized as shown in FIG. That is, high contrast can be realized with low power consumption. In addition, when shifting to the colored state as described above, if a DC voltage is applied for a time longer than the half cycle of the alternating current and less than the response time of the electrophoretic particles 4a, a uniform colored state can be realized and the response can be achieved. Both speeding up is possible.

  An example of the configuration of the electrophoretic display element to which the driving method according to the present embodiment is applied is shown in FIG. The display element includes a pixel group 11, a signal line driving circuit 12, a scanning line driving circuit 13, and a driving signal generator 14 arranged in a matrix. Each pixel constituting the pixel group 11 has an electrophoretic layer 4 sealed between substrates.

  A specific example of the case where each pixel constituting the pixel group 11 includes a thin film transistor is shown in FIG. Each pixel includes a signal line 21, a scanning line 22, a pixel electrode 23, a thin film transistor 24, an auxiliary capacitance electrode 26, and an auxiliary capacitance line 27. The auxiliary capacitance electrode 26 is formed on the glass substrate on the back side of the paper surface from the pixel electrode 23. The size of the auxiliary capacitor is desirably a size that can suppress the decrease in the potential of the pixel electrode 23 during the holding period as small as possible. When used as a transmissive electrophoretic display element, it is preferable to use a transparent electrode material for the auxiliary capacitance electrode 26 in order to ensure a higher aperture ratio.

  9 and 10 show cross sections when one pixel is cut along the cutting line 60 shown in FIG. FIG. 9 is a diagram illustrating an example in which the area of the common electrode 25 is smaller than that of the pixel electrode 23. FIG. 10 is a diagram illustrating an example in which the area of the common electrode 25 is larger than that of the pixel electrode 23 and the pixel electrode 23 in FIG. 8 is long in the scanning line direction. Each of the display elements shown in FIGS. 9 and 10 is a case of a reflection type electrophoretic display element, and a contrasting color layer 41 that bears the contrasting color of the electrophoretic particles 4 a is formed on the pixel electrode 23. The common electrode 25 applies the same potential to all the pixels. An auxiliary capacitance electrode 26 is formed on the substrate 2, an insulating film 40 is formed so as to cover the auxiliary capacitance electrode 26, and a pixel electrode 23 is formed on the insulating film 40. The observer views the display element from the substrate 1 side on which the common electrode 25 is formed. The auxiliary capacitance electrode 26 is set to the same potential as the common electrode 25.

  Next, see FIGS. 13 to 18 for examples of drive waveforms applied to the electrophoretic display element which is composed of the pixel group 11 arranged in a matrix form shown in FIG. 7 and whose cross section is shown in FIG. Will be described in more detail.

  It is assumed that the electrophoretic particles 4a are negatively charged and colored black. The contrast color layer 41 is assumed to be white as a contrast color. The observer observes from the substrate 1 side. Vcom is a potential of the common electrode 25, Vsig is a potential of the signal line 21, Vg is a potential of the scanning line 22, Vsig-c is a center potential of the signal line 21, and Vcom-c is a common potential when changing the potential of the common electrode 25. This is the center potential of the electrode 25. The auxiliary capacitance line 27 is given the same potential as the common electrode. The pixel electrode 23 is supplied with the potential Vsig of the signal line 21 through the thin film transistor 24.

  13, 15, and 17, Vcom is set lower than Vsig-c. However, this is a case where only the feedthrough voltage of the thin film transistor 24 is considered, and this is the case when an alternating current having an offset voltage is given as Vsig. The Vcom is set according to the electrophoretic layer, the distance between the electrodes, and the like. The potential Vg of the scanning line 22 has a longer period than other electrode potentials, and it is preferable to negatively charge the electrophoretic particles 4a in order to eliminate the influence as much as possible and obtain good display characteristics. .

  FIG. 14 is a waveform diagram showing an example of drive waveforms when writing is performed in which the full screen is displayed in white and FIG. 13 is written in which the full screen is displayed in black. In the white display, as shown in FIG. 14, the thin film transistor 24 is turned on when Vg becomes a high voltage in each frame, and a potential of Vsig lower than Vcom is applied to the pixel electrode 23, and thereafter, the thin film transistor 24 In the holding period, that is, in the holding period, the potential of the pixel electrode 23 is repeatedly decreased by a time constant determined from the resistance value and capacitance of the electrophoretic layer 4 and the auxiliary capacitance of the auxiliary capacitance line 27, thereby repeating the potential of the pixel electrode 23. Gradually approaches the potential Vsig of the signal line 21. At this time, the electrophoretic particles 4 a adhere to the common electrode 25, and the observer observes the white color of the contrast color layer 41. In black display, as shown in FIG. 13, the potential Vsig is applied to the pixel electrode 23 when the thin film transistor 24 is turned on while changing the potential Vsig of the signal line 21 with respect to Vcom in each frame. Write. The change in potential of the pixel electrode 23 during the holding period is as described above. At this time, the electrophoretic particles 4a are uniformly dispersed in the electrophoretic layer 4, and the observer observes the black color of the electrophoretic particles 4a.

  FIG. 16 is a waveform diagram showing another example of a drive waveform in the case where writing is performed so that the entire screen is displayed in white and the entire screen is displayed in black. By changing the potential Vcom of the common electrode 25 in synchronization with the potential Vsig of the signal line 21, low voltage driving of the electrophoretic element 4a having a relatively high driving voltage can be realized. However, as can be seen from FIG. 16, the electrophoretic particles 4a are not easily held by the common electrode 25 in a frame in which the potential difference (Vcom−Vsig) between the common electrode 25 and the signal line 21 is small during white display. It is more preferable that the electrophoretic particles 4 a have a memory property such that the electrophoretic particles 4 a are held by the common electrode 25.

  FIG. 18 is a waveform diagram showing another example of a drive waveform when writing is performed in which the entire screen is displayed in white and FIG. 17 is written in which the entire screen is displayed in black. Unlike the case shown in FIG. 13, black display is performed by changing the potential Vsig of the signal line 21 every two frames, that is, twice the frame period. As described above, the potential of the pixel electrode 23 gradually decreases with a time constant determined from the resistance value and capacitance of the electrophoretic layer 4 and the auxiliary capacitance of the auxiliary capacitance line 27. In the case of an electrophoretic display element, the gradual decrease amount is larger than that of a liquid crystal display element, and the auxiliary capacity is increased by increasing the area of the auxiliary capacity electrode 26 or increasing the dielectric constant of the insulating layer 40. In many cases, it is difficult to suppress the gradual decrease. Therefore, when m and n are natural numbers, the potential Vsig of the signal line 21 is not changed every frame period, but is changed every m / n period, so that the potential of the pixel electrode 23 becomes the potential Vsig of the signal line 21. It is preferable to drive under conditions that are sufficiently asymptotic.

  Note that when attention is paid to pixels in a colored state, the polarity of the common electrode 25 with respect to the potential Vcom is not limited to frame inversion that is inverted in a frame, but is a row inversion or signal line that is inverted for each pixel line parallel to the scanning line 22. 21 may be driven by column inversion which inverts every pixel line in the direction parallel to 21. When the potential Vcom of the common electrode 25 is changed, it can be used in combination with frame inversion or row inversion. From the viewpoint of reducing flicker and crosstalk, an appropriate inversion method is selected.

  FIG. 10 is a cross-sectional view showing an example in which the area of the pixel electrode 23 is smaller than that of the common electrode 25. In this case, the electrophoretic particles 4 a are collected on the pixel electrode 23 to realize the transmission state of the electrophoretic layer 4. Therefore, a DC potential that makes the pixel electrode 23 positive with respect to the common electrode 25 may be applied to the signal line. The variation in the potential of the pixel electrode 23 during the holding period and the fact that the electrophoretic particles 4a are desirably negatively charged are the same as described with reference to FIG.

  The electrophoretic particles 4a used in the driving method of the present embodiment are stably dispersed in a fluid, have a single polarity, and have a small particle size distribution, so that the lifetime of the display element, contrast, resolution, and the like are considered. Desirable from. From the viewpoint of response speed, the particle diameter is preferably 0.1 μm to 5 μm. Examples of the material for the electrophoretic particles 4a include inorganic pigments such as titanium oxide, zinc oxide, zirconium oxide, iron oxide, aluminum oxide, cadmium selenide, carbon black, barium sulfate, lead chromate, zinc sulfide, cadmium sulfide, or the like. Organic pigments such as phthalocyanine blue, phthalocyanine green, Hansa yellow, watching red, and diary ride yellow can be used.

  In the present embodiment, the fluid 4b in which the electrophoretic particles 4a are dispersed has a low solubility in the electrophoretic particles 4a, can stably disperse the electrophoretic particles 4a, does not contain ions, and does not generate ions when a voltage is applied. An insulating material is desirable. Furthermore, in order to prevent the electrophoretic particles 4a from floating and sinking, it is preferable that the specific gravity is substantially equal to that of the electrophoretic particles 4a and the viscosity is low in terms of mobility of the electrophoretic particles 4a when a voltage is applied. Examples of the insulating liquid that can be used for a relatively large number of electrophoretic particles 4a include hexane, decane, hexadecane, kerosene, toluene, xylene, olive oil, tricresyl phosphate, isopropanol, trichlorotrifluoroethane, and dibromo. Examples thereof include tetrafluoroethane and tetrachloroethylene. Note that a mixed fluid can also be used when specific gravity matching with the electrophoretic particles 4a is performed in order to prevent the electrophoretic particles 4a from rising and falling. Moreover, gas can also be used as a fluid. Examples of the gas include air, nitrogen, and argon.

  In the present embodiment, the mixing weight ratio of the electrophoretic particles 4a in the electrophoretic layer 4 is not particularly limited as long as the electrophoretic properties of the electrophoretic particles 4a are not hindered and the reflection control of the dispersion layer can be sufficiently performed. For example, 1 to 20% by weight is preferable.

  In this embodiment, in order to increase the charge of the electrophoretic particles 4a or to have the same polarity, an additive such as a resin or a surfactant is added to the above-described fluid as necessary, or the electrophoretic particles 4a can be pretreated with additives.

  In the present embodiment, the thickness of the electrophoretic layer 4 is larger than the diameter of the electrophoretic particles 4a and is not particularly limited as long as the movement of the electrophoretic particles 4a is not hindered. It is desirable to be as thin as possible. From such a viewpoint, the preferred thickness of the electrophoretic layer 4 is 5 μm to 200 μm.

  In the present embodiment, the amplitude of the AC voltage and the magnitude of the DC voltage may be selected appropriately from -50V to + 50V depending on the gap between the substrates and the distance between the electrodes. An alternating current with a large voltage amplitude only needs to have a voltage amplitude sufficient to uniformly disperse the electrophoretic particles 4a, and an alternating current with a small voltage amplitude only needs to have a voltage amplitude that can maintain the dispersion state. It is appropriately selected depending on the gap between the substrates and the distance between the electrodes. The application time of alternating current with a large voltage amplitude is sufficient for the electrophoretic particles 4a to be uniformly dispersed, and is 0.1 to 5 seconds depending on the gap between the substrates and the distance between the electrodes. Is appropriately selected.

  In the present embodiment, when the AC voltage amplitude is periodically changed, the AC voltage amplitude may be changed to an AC voltage having a large voltage amplitude before it becomes difficult to maintain a dispersed state. The period to be changed is appropriately selected between 5 seconds and several minutes depending on the gap between the substrates, the distance between the electrodes, and the like.

  In the present embodiment, the drive waveform shown in FIG. 1 changes the amplitude of the AC voltage, but the AC offset voltage may be changed. In this case, the amount of change and the period may be set according to the gap between the substrates and the distance between the electrodes so that the dispersed state of the electrophoretic particles 4a is maintained. Further, at least one of the amplitude of the AC voltage and the offset voltage may be periodically changed.

  In this embodiment, the AC waveform is basically a rectangular wave, but a sine wave or a triangular wave may be used in some cases.

  In the present embodiment, when the drive waveform shown in FIG. 2 is used, the magnitude of the direct-current voltage that is first applied when obtaining the colored state of the electrophoretic layer 4a may be such that a sufficient response speed can be obtained. The application time is set longer than a half cycle of the subsequent alternating current. The upper limit of the application time is the response time of the electrophoretic particles 4a. Note that a DC voltage may be applied again after the AC voltage having the drive waveform shown in FIG. 2 is applied, or a DC voltage and an AC voltage may be applied periodically.

  In this embodiment, as shown in FIG. 3, when an AC voltage is first applied and then a DC voltage larger than the amplitude of the AC voltage is applied, the voltage is sufficient for the electrophoretic particles 4a to stably adhere to the electrodes. What is necessary is just to select a suitable value between -50V- + 50V according to the gap between board | substrates or the distance between electrodes.

  In the present embodiment, when alternating current and direct current are changed periodically, it is sufficient to use alternating current drive before it becomes difficult to maintain a dispersed state, and if the period is determined according to the gap between the substrates and the distance between the electrodes. Good.

  In the present embodiment, it is possible to use an AC voltage in which the application time of the high voltage of the AC voltage is different from the application time of the low voltage, that is, the duty ratio is set to a value other than 1: 1. In this case, the dispersion state of the electrophoretic particles may be set so as to be most stably maintained, and the response speed between the electrodes of the electrophoretic particles may be equal between the colored state → the transmission state and the transmission state → the colored state. Depending on the gap between the substrates, the distance between the electrodes, etc., and also according to the offset voltage, it is appropriately selected between 1: 9 and 9: 1. FIG. 11 shows an example of an AC waveform whose duty ratio is not 1: 1.

  In this embodiment, the AC offset voltage may be set so that the dispersed state of the electrophoretic particles can be maintained most stably, and the response speed between the electrodes of the electrophoretic particles is colored state → transmission state and transmission state → coloration. It is set between −50 V and +50 V in accordance with the gap between the substrates, the distance between the electrodes, and the like, and also considering the above-described duty ratio so as to be equal in the state. FIG. 12 shows an example of an AC waveform having an offset voltage, that is, the offset voltage is not 0V.

  In the present embodiment, the electrophoretic display element composed of a plurality of pixels in a matrix form may be driven by simple matrix driving, or may be operated by active matrix driving by installing a thin film transistor or a thin film diode in each pixel. The frame period is a period for rewriting the screen and may be set in a range where flicker is not visually recognized. The reciprocal of the frame period is the frame frequency, and the frame frequency is set to 0.01 Hz to 120 Hz, for example. When m and n are natural numbers, the potential of the pixel electrode is set when the application time of the high voltage and the low voltage of the AC voltage and the application time of the DC voltage in each pixel are m / n times the frame period. M and n are set so that the potential is at least 50% or more, preferably 80% or more.

  In the present embodiment, when the potential of the common electrode is changed by m / n times the frame period, it is possible to drive even in the case of an electrophoretic display element that requires a high driving voltage. The natural numbers m and n are set in consideration of the application time of the high voltage and the low voltage of the AC voltage and the application time of the DC voltage in each pixel described above. When the potential of the common electrode is changed at m / n times the frame period, the potential change amount is preferably set to an AC potential change amount for obtaining a colored state, that is, not more than twice the voltage amplitude. When attention is paid to a pixel that obtains a transmissive state, an AC voltage having an offset voltage is applied to the electrophoretic layer of the pixel. Therefore, in order to realize a more stable transmission state, it is desirable that the electrophoretic display element has a memory property.

  Examples of the present invention will be described below.

(First embodiment)
First, in an electrophoretic display element in which one pixel has a structure similar to the structure shown in FIG. 4, selection, formation, and setting of each member were performed as follows. As the substrates 1 and 2, transparent glass plates having a thickness of 0.7 mm were used. After the sputter film formation, the electrode 6 made of ITO having a cross-shaped shape was formed on the substrate 1 according to a standard lithography method. The height of the electrode 6 was about 0.1 μm and the width was 10 μm. On the other hand, an electrode 7 made of ITO was formed on the substrate 2 in the same manner. The height of the electrode 7 was about 0.1 μm and was a quadrangle with a side of 120 μm. For example, a cross-shaped spacer 3 made of a negative resist was formed on the substrate 2 on which the electrode 7 was formed. The height of the spacer 3 was set to 30 μm.

  The electrophoretic layer 4 was prepared as follows. First, black resin toner (particle size 1 μm) is used as the electrophoretic particles 4a, isopropanol is used as the fluid 4b, and both are mixed so that the mixing weight ratio of the electrophoretic particles 4a is 10%. A small amount of surfactant was added for improvement, and the electrophoretic layer 4 was prepared. In this case, the surface of the electrophoretic particle 4a is negatively charged.

  Finally, the electrophoretic layer 4 was sealed, and the spacer 3 and the electrode 6 were bonded together.

  For the electrophoretic display device thus obtained, first, a DC voltage of 10 V is applied between the electrode 6 and the electrode 7 so that the electrode 6 has a polarity opposite to that of the electrophoretic particles 4a. The transmission state shown in Fig. 6 was obtained. Subsequently, the coloring state was realized by the driving waveform shown in FIG. That is, a rectangular wave having a voltage of 10 V and a frequency of 30 Hz was applied between the electrode 6 and the electrode 7 for 3 seconds to obtain the colored state shown in FIG. Subsequently, the colored state shown in FIG. 4 was maintained by applying a rectangular wave with a voltage of 0.5 V and a frequency of 30 Hz.

  The display element was evaluated by measuring the transmittance and the current consumption in the transmissive state and the colored state. In the driving method of this example, the transmittance in a colored state was 30%, the current consumption was 10 V on average, and the average was 5 μA, and in the case of 0.5 V, the average was 0.2 μA. The transmittance in the transmissive state was 70%.

  In the following, the response time from the transparent state to the colored state is defined as the time τ when the rate of change in transmittance is from 0% to 90%. The response time τ in this example was 1.5 seconds.

(Comparative Example 1)
As a comparative example, direct current driving was performed at a voltage of 10 V on the electrophoretic display element described in the first example. In the case of this comparative example, the transmittance in the transmissive state was 70%, and the transmittance in the colored state was 40%.

  Thereby, in the case of a present Example, it has confirmed that a high contrast was realizable compared with a comparative example.

(Comparative Example 2)
Further, as compared with the case of Comparative Example 2 in which AC driving is always performed at 10 V as in the driving method described in Patent Document 2 (Japanese Patent Laid-Open No. 3-91722), the driving method of this embodiment is apparent as described above. In addition, low power consumption was achieved. In addition, Patent Document 2 (Japanese Patent Laid-Open No. 3-91722) also describes that the power is turned off after AC driving. However, in this case, the distribution of the electrophoretic particles 4a becomes gradually non-uniform and high contrast is maintained in this embodiment. It was inferior to. Patent Document 2 also describes that a DC voltage is applied to attach electrophoretic particles to the electrode to maintain a colored state. However, depending on the setting of the DC voltage, the maintenance is insufficient, and a high contrast is also required. It was inferior to the present Example in the point of long-time maintenance. The response time τ of this comparative example 2 was 1.5 seconds.

(Second embodiment)
Next, the driving method of the second embodiment will be described.

  First, an electrophoretic display element is obtained in the same manner as described in the first embodiment. For the electrophoretic display device thus obtained, first, a DC voltage of 10 V is applied between the electrode 6 and the electrode 7 so that the electrode 6 has a polarity opposite to that of the electrophoretic particles 4a. The transparent state shown in 6 was obtained. Subsequently, the coloring state was realized by the driving waveform shown in FIG. That is, a DC voltage of 10 V is applied between the electrode 6 and the electrode 7 so that the electrode 6 has a polarity opposite to that of the electrophoretic particle 4 a, and the electrophoretic particle 4 a is emitted from the electrode 6 toward the electrode 7. The DC voltage time was 1 second. Subsequently, a colored state shown in FIG. 4 was obtained by applying a rectangular wave having a voltage of 10 V and a frequency of 30 Hz between the electrode 6 and the electrode 7.

  Evaluation of the display element was performed in the same manner as in the first embodiment. In the case of this example, similarly to the first example, the transmittance in the colored state was 30% compared to 40% in the case of Comparative Example 1 of the first example, and as a result, a high contrast could be realized. The response time of this example was τ = 0.7 seconds.

  On the other hand, in the case of the driving method described in Patent Document 2 (Japanese Patent Laid-Open No. 3-91722), the transmittance during coloring is 30% as in the present embodiment, but the response time τ is 1.5 seconds. Slower than the present embodiment. That is, in the case of the present Example, it has confirmed that a high contrast and a high-speed response were realizable.

(Third embodiment)
First, an electrophoretic display element was obtained in the same manner as described in the first example. For the electrophoretic display device thus obtained, first, a DC voltage of 10 V is applied between the electrode 6 and the electrode 7 so that the electrode 6 has a polarity opposite to that of the electrophoretic particles 4a. The transparent state shown in 6 was obtained. Subsequently, the coloring state was realized by the driving waveform shown in FIG. That is, a rectangular wave having a voltage of 10 V and a frequency of 30 Hz was applied between the electrode 6 and the electrode 7 for 3 seconds to obtain the colored state shown in FIG. Subsequently, the coloring state shown in FIG. 5 was maintained by applying a DC voltage of 40V.

  The evaluation of the display element was performed in the same manner as in the first example. The transmittance in the colored state in the driving method of the present embodiment is 31%, which is suppressed to a lower transmittance than the 40% of the driving method of the comparative example 1 that performs DC driving at a voltage of 10 V described in the first embodiment. I was able to. Moreover, it has confirmed that a coloring state could be maintained stably.

  That is, in the case of the present Example, it has confirmed that stable high contrast was realizable. The response time τ in this example was 1.5 seconds.

(Fourth embodiment)
Each pixel has a thin film transistor (TFT), a TFT array electrode substrate 2 (0.7 mm thick) and a cross-shaped electrode substrate 1 (with a thickness of 1024 pixels and a vertical size of 768 pixels) arranged in a matrix. An electrophoretic display element in which the cross section of one pixel has the structure shown in FIG. The cross-shaped common electrode 25 is a chromium / chromium oxide electrode having a width of 15 μm, and the height of the cross-beam spacer 3 is 20 μm and the width is 8 μm. As the contrasting color layer 41, titanium dioxide was used. The size of one pixel was a quadrangle with a side of 127 μm. The electrophoretic layer 4 was prepared as follows. First, a black resin toner (particle size: 1 μm) is used as the electrophoretic particles 4a, and isopropanol is used as the fluid 4b. A small amount of surfactant was added for improvement, and the electrophoretic layer 4 was prepared. In this case, the surface of the electrophoretic particle 4a is negatively charged.

  When the reflectance was measured by performing white display on the entire surface with the driving waveform shown in FIG. 18, the reflectance was 75%. When the reflectance was measured by performing black display on the entire surface with the driving waveform shown in FIG. %Met. The low potential value of the potential Vg of the scanning line 22 is −9 V, the high potential value is +20 V, the low potential value of the potential Vsig of the signal line 21 is +1 V, the high potential value is +10 V, and the potential Vcom of the common electrode 25 is +4. The frame period was 5V and the frame period was 16.7 ms (frame frequency 60 Hz).

  By the driving method of this embodiment, stable high contrast and low power consumption can be realized.

The wave form diagram which shows an example of the drive waveform of the drive method by one Embodiment of this invention. The wave form diagram which shows an example of the drive waveform of the drive method by one Embodiment of this invention. The wave form diagram which shows an example of the drive waveform of the drive method by one Embodiment of this invention. Sectional drawing which shows the coloring state of the electrophoretic layer of 1 pixel of an electrophoretic display element used for the drive method by one Embodiment of this invention. Sectional drawing which shows the coloring state of the electrophoretic layer of 1 pixel of an electrophoretic display element used for the drive method by one Embodiment of this invention. FIG. 4 is a cross-sectional view showing a transmission state of an electrophoretic layer of one pixel of an electrophoretic display element used in a driving method according to an embodiment of the present invention. The block diagram which shows the structure of the electrophoretic display element which has the pixel arrange | positioned at the matrix form used for the drive method by one Embodiment of this invention. The figure which shows the structure of the pixel which has a thin-film transistor of the display element used for the drive method by one Embodiment of this invention. FIG. 3 is a cross-sectional view illustrating a configuration of an electrophoretic display element used in a driving method according to an embodiment of the present invention. FIG. 3 is a cross-sectional view illustrating a configuration of an electrophoretic display element used in a driving method according to an embodiment of the present invention. The wave form diagram which shows an example of the alternating current waveform whose duty ratio from which application time differs by high voltage and low voltage is not 1: 1. The wave form diagram which shows an example of the alternating current waveform which has an offset voltage. The wave form diagram which shows an example of the matrix drive waveform used for the drive method by one Embodiment of this invention. The wave form diagram which shows an example of the matrix drive waveform used for the drive method by one Embodiment of this invention. The wave form diagram which shows an example of the matrix drive waveform used for the drive method by one Embodiment of this invention. The wave form diagram which shows an example of the matrix drive waveform used for the drive method by one Embodiment of this invention. The wave form diagram which shows an example of the matrix drive waveform used for the drive method by one Embodiment of this invention. The wave form diagram which shows an example of the matrix drive waveform used for the drive method by one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Substrate 3 Spacer 4 Electrophoretic layer 4a Electrophoretic particle 4b Insulating fluid 6 Electrode 7 Electrode 11 Pixel group 12 Signal line drive circuit 13 Scan line drive circuit 14 Drive signal generation circuit 21 Signal line 22 Scan line 23 Pixel electrode 24 Thin film transistor 25 Common electrode 26 Auxiliary capacitance electrode 27 Auxiliary capacitance line 40 Insulating film 41 Contrasting color layer 60 Cutting line

Claims (12)

An electrophoretic layer having first and second substrates disposed opposite to each other, electrophoretic particles encapsulated between the substrates and having a predetermined charge, and an insulating fluid that disperses the electrophoretic particles. A method for driving an electrophoretic display element comprising:
When obtaining the colored state of the electrophoretic layer in which the color of the electrophoretic particles is visually recognized, an alternating voltage is applied to the electrophoretic layer, and at least one of the amplitude of the alternating voltage and the offset voltage is the colored state. A method for driving an electrophoretic display element, wherein the electrophoretic display element is changed during the period. An electrophoretic layer having first and second substrates disposed opposite to each other, electrophoretic particles encapsulated between the substrates and having a predetermined charge, and an insulating fluid that disperses the electrophoretic particles. A method for driving an electrophoretic display element comprising:
When obtaining a colored state of the electrophoretic layer in which the color of the electrophoretic particles is visually recognized, a DC voltage is first applied to the electrophoretic layer, then an AC voltage is applied, and the DC voltage is applied. A method for driving an electrophoretic display element, characterized in that the time is longer than a half cycle of the AC voltage.   3. The electrophoretic display element according to claim 1, wherein the alternating voltage is applied by applying a second alternating voltage having an amplitude smaller than that of the first alternating voltage after the first alternating voltage is applied. Driving method.   3. The method for driving an electrophoretic display element according to claim 1, wherein the application of the AC voltage periodically changes at least one of an amplitude and an offset voltage.   3. The method for driving an electrophoretic display element according to claim 2, wherein the DC voltage is applied again after the AC voltage is applied.   6. The method for driving an electrophoretic display element according to claim 5, wherein the magnitude of the DC voltage applied after application of the AC voltage is larger than the amplitude of the AC voltage.   3. The method for driving an electrophoretic display element according to claim 2, wherein a DC voltage and an AC voltage are periodically applied after the application of the AC voltage. An electrophoretic layer having first and second substrates disposed opposite to each other, electrophoretic particles encapsulated between the substrates and having a predetermined charge, and an insulating fluid that disperses the electrophoretic particles. A method for driving an electrophoretic display element comprising:
When obtaining a colored state of the electrophoretic layer in which the color of the electrophoretic particles is visually recognized, an AC voltage is first applied to the electrophoretic layer, and then a DC voltage greater than the amplitude of the AC voltage is applied. A method for driving an electrophoretic display element.   9. The method of driving an electrophoretic display element according to claim 1, wherein the AC voltage has an offset voltage, or a high voltage application time and a low voltage application time are different. The first and second substrates arranged opposite to each other and the first and second substrates are arranged in a matrix, and each is enclosed between the pixel electrode and the first and second substrates. An electrophoretic display element including a pixel including an electrophoretic layer having electrophoretic particles having a predetermined charge and an insulating fluid that disperses the electrophoretic particles,
When an AC voltage or a DC voltage is applied to the electrophoretic layer of each pixel, and the application time of the high voltage and the low voltage of the AC voltage, or the application time of the DC voltage is m and n are natural numbers, A method of driving an electrophoretic display element, wherein the driving ratio is m / n times.   A thin film transistor is provided in each pixel, the pixel electrode is provided on the first substrate, and a common electrode for applying a potential common to all pixels is provided on the second substrate. Item 11. A driving method of an electrophoretic display element according to Item 10.   12. The method of driving an electrophoretic display element according to claim 11, wherein when applying a voltage to the electrophoretic layer, the potential of the common electrode is changed at m / n times the frame period.

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