JP2007264285A - Electrophoretic display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prescribe the correlation of the numbers of two kinds of floating particles for display differing in particle size, display response speeds, etc., of an electrophoretic display device having the respective floating particles dispersed in a migration medium. <P>SOLUTION: The migration medium is held between opposed substrates having electrodes formed on internal surfaces, and first floating particles having a mean particle radius r1 μm and second floating particles having a mean particle radius r2 μm are dispersed in the migration medium; and r2≤0.15×r1 and t2≤t1 are satisfied, where t1 is the display response speed of the first floating particles and t2 is the display response speed of the second floating particles. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electrophoretic display device that performs display by dispersing charged display particles on an electrophoretic medium such as a solution and moving the particles in an electric field direction.

  Conventionally, an electrophoretic display device 100 shown in FIG. 5 is known. Two transparent substrates 101 and 102 on which transparent electrodes 104 and 105 are formed are arranged to face each other through a seal 109, and an electrophoresis medium 106 that is an insulator is placed in a gap formed by the two transparent electrodes 104 and 105. Encapsulate. In the electrophoresis medium 106, charged display particles 107 are dispersed. Further, the migration medium 106 is colored in a color different from that of the display particles 107 with a dye or a pigment. In a state where no voltage is applied between the transparent electrode 104 and the transparent electrode 105, the display particles 107 are dispersed in the migration medium 106.

  When the switch 108 is turned on and an electric field is applied, the display particles 107 dispersed in the electrophoresis medium 106 move to the transparent electrode 104. When viewed from above, the color of the display particles 107 is displayed. Then, by reversing the polarity of the voltage applied to the transparent electrodes 104 and 105, the display particles 107 move away from the transparent electrode 104 and move toward the transparent electrode 105, so that the display particles 107 are shielded by the migration medium 106 and migrated. The color of the medium 106 is displayed.

  The electrophoretic display device can maintain the memory characteristics by adjusting the specific gravity of the display particles 107 and the specific gravity of the electrophoretic medium 106. Further, if the migration medium 106 is black and the display particles 107 are white, high contrast can be expected. In addition, by forming the transparent electrodes 104 and 105 into a plurality of orthogonal stripe electrodes and having a matrix configuration, characters and patterns can be displayed. Since there is no need to use a polarizing plate as in a liquid crystal display device, a reflective display device with good visibility and less eye strain as printed on paper can be obtained.

In order to improve the memory performance, it is necessary to reduce the specific gravity difference between the electrophoresis medium 106 and the display particles 107. However, if the diameter of the display particles is reduced, the light transmittance of the display particles increases, and high contrast cannot be obtained. Further, when the display particle diameter is increased in order to increase the contrast, the gap between the display particles is increased, the color of the display particles 107 is shielded by the colored migration medium 106, and the display contrast is also deteriorated. Therefore, an electrophoretic display device in which display particles 107 having a small particle diameter and display particles 107 having a large particle diameter are mixed has been proposed (see Patent Document 1). When an electric field is applied to the electrophoretic medium 106 and display particles are attached to one electrode, the display particles 107 having a small particle diameter fill the gaps between the display particles 107 having a large particle diameter, thereby improving the display density. That's it.
JP 2002-287179 A

  However, even if two kinds of display particles are mixed, if a small particle moves to a substrate on the display surface side that displays an image before a large particle when a voltage is applied to the electrode, the small particle is displayed. There is a problem that the display density cannot be improved because the substrate is sandwiched between the substrate on the surface side and large particles.

In order to solve the above-described problems, the present invention has the following configuration.
(1) In one aspect of the present invention, electrophoresis is performed by sandwiching a migration medium between opposing substrates having electrodes formed on the inner surface, and having migration particles dispersed in the migration medium and moving in the direction of an electric field. In the display device, the migrating particles include first migrating particles having an average particle radius of r1 μm and second migrating particles having an average particle radius of r2 μm, and the display response speed of the first migrating particles. Is t1, and the display response speed of the second migrating particles is t2, r2 ≦ 0.15 × r1, t2 ≦ t1
Thus, an electrophoretic display device satisfying the above relationship is obtained.
(2) In the electrophoretic display device of the above (1), the r2 is an electrophoretic display device satisfying a relationship of 0.025 ≦ r2.
(3) In the electrophoretic display device of the above (1) or (2), the electrophoretic display device satisfying the relationship of 0.5 × t1 ≦ t2 between t1 and t2.
(4) In the electrophoretic display device according to any one of (1) to (3), the second electrophoretic particles dispersed in the electrophoretic medium are average particles per square μm in the display region of the substrate. The electrophoretic display device satisfying the relationship 1 / (√3 (r1) ² ) ≦ n, where n is the number.
(5) In the electrophoretic display device according to any one of (1) to (4), the second electrophoretic particles dispersed in the electrophoretic medium are average particles per square μm in the display region of the substrate. The electrophoretic display device satisfying the relationship of n ≦ 0.071 / (r2) ³ , where n is the number.
(6) The electrophoretic display device according to any one of (1) to (5), wherein the r1 is 0.2 to 10.
(7) In the electrophoretic display device according to any one of the above (1) to (6), the electrophoretic display device satisfying a relationship of r2 ≦ 0.09 × r1 between r1 and r2.
(8) In the electrophoretic display device according to any one of (1) to (7), the electrophoretic display device satisfying a relationship of r2 ≦ 0.03 × r1 between r1 and r2.

  According to the invention of (1) above, the first migrating particles are imaged by defining the relative relationship between the first migrating particles which are display particles having a large particle size and the second migrating particles having a small particle size. Then, the second migrating particles move so as to fill the space between the first migrating particles. For this reason, since the second migrating particles are not sandwiched between the substrate and the first migrating particles, there is an advantage that the display density is improved.

  According to the invention of (2) above, there are advantages that the display density can be prevented from being lowered and the production of the particles is easy.

  According to the invention of (3) above, when the polarity of the voltage applied to the electrode is reversed, the second migrating particles hinder the movement of the first migrating particles when the first migrating particles are separated from the electrode surface. Has the advantage of not.

  According to the invention of (4), when the first migrating particles are packed most closely, the number of the second migrating particles can be inserted into the gap between the first migrating particles, thereby preventing a decrease in display density. Has the advantage of being able to.

  According to the invention of (5) above, there is an advantage that the movement of the first electrophoretic particles is not hindered due to the excessive number of the second electrophoretic particles.

  According to the invention of (6) above, it is possible to prevent an increase in driving voltage due to an increase in the particle diameter of the first migrating particles that are display particles, and a decrease in display density due to a decrease in the particle diameter. Can be prevented.

  According to the invention of (7) above, there is an advantage that the second migrating particles are easily inserted into the particle gaps of the first migrating particles, and the display density can be further increased.

  The above invention (8) has the advantage that the second migrating particles can be more easily inserted into the particle gaps of the first migrating particles, and the display density can be further increased.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a schematic longitudinal sectional view of the electrophoretic display device 1. In FIG. 1A, an upper electrode 4 and a lower electrode 5 are formed on the inner surfaces of an upper substrate 2 and a lower substrate 3, respectively. The upper substrate 2 is a transparent substrate, and the upper electrode 4 is a transparent electrode. The upper electrode 4 and the lower electrode 5 are both divided into a large number of stripe-shaped electrodes and are orthogonal to each other to form a matrix, and each intersection of the matrix functions as one pixel. The two substrates are bonded to each other by a seal portion 6 so as to form a predetermined gap. Between the two substrates, the electrophoretic medium 9, the first electrophoretic particles 7 having a large particle diameter, and the second electrophoretic particles 8 having a small particle diameter are dispersed in the electrophoretic medium 9. An image signal is given between the upper electrode 4 and the lower electrode 5 from a drive circuit (not shown). Specifically, one of the upper electrode 4 and the lower electrode 5 is a scanning electrode and the other is a signal electrode, a scanning signal is given to the scanning electrode, and an image signal is synchronized with the scanning signal. The display data is written in all the pixels by sequentially selecting the scanning electrodes.

  Here, the upper substrate 2 and the lower substrate 3 use glass or a transparent plastic material. The seal part 6 uses an epoxy resin. The upper electrode 4 is formed by depositing ITO (indium, tin oxide) or ZnO (zinc oxide) by sputtering or vapor deposition, and patterning by lithography. The lower electrode 5 may be formed as a transparent electrode in the same manner as the upper electrode 4 or may be an opaque conductor film formed by patterning a metal thin film. In order to maintain the distance between the upper substrate 2 and the lower substrate 3, a gap material made of a divinylbenzene polymer is introduced into the gap, but is omitted in FIG. The electrophoresis medium 9 uses an insulating liquid such as alcohols, hydrocarbons, and silicone oil. The first migrating particles 7 and the second migrating particles 8 are used by mixing titanium oxide or carbon black as a coloring pigment with a resin material. Alternatively, inorganic oxides such as zinc oxide, zirconium oxide, and aluminum oxide can be used as the color pigment. In addition, other organic compounds can be used. Examples of the resin material for particles include polyvinyl alcohol resin, polystyrene resin, polyacrylic resin, and fluororesin. Other resins can also be used.

  When white titanium oxide is used as a pigment to be mixed with the first migrating particles 7 and the second migrating particles 8, the migrating medium 9 is colored black with a pigment or dye. FIG. 1A shows a state in which no voltage is applied between the upper electrode 4 and the lower electrode 5. Both the first migrating particles 7 and the second migrating particles 8 are dispersed, and when the electrophoretic display device 1 is viewed from above, the first migrating particles 7 and the second migrating particles 8 are shielded by the black migrating medium 9 and migrated. The black color of the medium 9 is visible. FIG. 1B is a schematic cross-sectional view showing a state when a voltage equal to or higher than a threshold is applied between the upper electrode 4 and the lower electrodes 5b, 5d, and 5f. The first migrating particles 7 and the second migrating particles 8 between the lower electrodes 5 b, 5 d and 5 f and the upper electrode 4 migrate to the upper electrode 4 and adhere to the upper electrode 4. As a result, only the portions of the lower electrodes 5b, 5d and 5f appear to be white with the first migrating particles 7 and the second migrating particles 8 attached thereto. The other parts are shielded by the migration medium 9 and appear black. This makes it possible to display a document or an image.

  Next, the reason for mixing the first migrating particles 7 having a large particle diameter and the second migrating particles 8 having a small particle diameter will be described. When the particle diameter of the first migrating particles 7 is reduced, the light transmittance of the particles is increased. For this reason, when it is applied to the electrophoretic display device 1, the display density is lowered. Therefore, the particle size cannot be made too small. Further, when the particle diameter is increased, even when the first migrating particles 7 and the adjacent first migrating particles 7 are closely packed, the volume of the gap between the particles becomes larger. As a result, the first migrating particles 7 is shielded by the migration medium 9, and the color of the first migration particles 7 is hidden. As a result, the display density is lowered and the display has a low contrast.

  FIG. 2 (a) is a side view schematically showing a state in which the first migrating particles having a large particle diameter are arranged closest to the transparent upper electrode 4. FIG. When the first migrating particles 7 are viewed from above the upper electrode 4, the color of the first migrating particles 7 can be seen because there is almost no thickness of the migrating medium 9 in the region 10. However, in the region 11, the color of the first migrating particles 7 is shielded by the presence of the migrating medium 9. Therefore, if the small second migrating particles 8 can be arranged in the region 11, shielding by the migrating medium 9 can be prevented.

  FIG. 2B is a plan view schematically showing the arrangement of the maximum diameter that can be taken by the second migrating particles 8 when the first migrating particles 7 are packed most closely. The average particle radius of the first migrating particles 7 is r1 μm, and the average particle radius of the second migrating particles 8 is r2 μm. In the case where the first migrating particles 7 are packed most closely, three first migrating particles 7 indicated by the broken line 12 and four firsts of the first migrating particles 7 arranged on the triangle of the broken line 12 are used. This is a case surrounded by the migrating particles 7. From the geometric calculation, the average particle radius r2 μm of the second migrating particles 8 is expressed by the equation (e1).

r2 = (2√3 / 3-1) × r1 = 0.15 × r1 (e1)
If the radius of the second migrating particles becomes larger than this, the first migrating particles 7 cannot be packed most closely. As a result, a gap is generated between the first migrating particles 7. This gap reduces the display density. Therefore, the condition (e1) is the maximum average particle radius of the second migrating particles 8. This requirement is expressed by equation (e2).

r2 ≦ 0.15 × r1 (e2)
Further, when the particle diameter of the second migrating particles 8 is reduced, there are processing limitations. According to experiments, the lower limit was approximately 0.025 μm. Therefore, the average particle radius r2 μm of the second migrating particles 8 is expressed by the equation (e2).

r2 ≧ 0.025 (e3)
As already described, when the particle diameter of the first migrating particles 7 is reduced, light is easily transmitted and the color of the migrating medium 9 is easily reflected. As a result, the display density cannot be increased. According to experiments, the minimum average particle radius of the first migrating particles 7 was approximately 0.2 μm. Therefore, the lower limit of the average particle radius r1 μm of the first migrating particles is set to 0.2 μm. Further, if the particle diameter of the first migrating particles 7 as display particles is to be increased, the electrode interval must be increased, and the drive voltage increases. According to the experiment, when the average particle radius r1 μm of the display particles is larger than 10 μm, the response speed is significantly reduced. Therefore, it was appropriate to set the first migrating particles 7 for display to a range represented by the formula (e4).

0.2 ≦ r1 ≦ 10 (e4)
Although the average particle radius of the second migrating particles 8 is defined by the formula (e2), the average particle radius r2 μm that is easily filled by the gap between the first migrating particles 7 is preferably 0.09 × r1 μm or less. More preferably, it is 0.03 × r1 μm or less.

  FIG. 3 is an explanatory diagram for explaining the display response speed of the migrating particles. FIG. 3A shows that the display response speed t1 of the first migrating particles 7 is higher than the display response speed t2 of the second migrating particles 8. 3B schematically shows a case where the display response speed t2 of the second migrating particles 8 is larger than the display response speed t1 of the first migrating particles 7. Even if the particle diameter between the first migrating particle 7 and the second migrating particle 8 is appropriate, if the display response speed of both the migrating particles when the voltage is applied is not appropriate, two types of particles are selected. The mixed effect cannot be demonstrated. In FIG. 3, when a driving voltage is applied between the upper electrode 4 and the lower electrode 5, the display response speed of the first migrating particles 7 is t1, and the display response speed of the second migrating particles 8 is t2. . The display response speed is the measurement of the reflected light intensity on the display surface of the particle and the reflected light intensity before applying the voltage and the reflected light intensity at which the reflected light on the display surface of the particle is saturated after the voltage is applied. The difference is defined as 100%, and the time from when the voltage is applied until the difference in reflected light intensity on the display surface of the particles reaches 90%.

  When the display response speed t2 of the second migrating particles 8 is higher than the display response speed of the first migrating particles 7, the second migrating particles 8 reach the upper electrode 4 faster as shown in FIG. The first migrating particles 7 arrive at the first. The second migrating particles 8 have a smaller particle diameter and lower display density than the first migrating particles 7. As a result, a display with low display density and low contrast is obtained. In addition, as the display response speed of the first migrating particles 7 decreases, voltage application must be maintained until the first migrating particles 7 reach the vicinity of the upper electrode 4. Then, a seizure phenomenon in which the second migrating particles 8 stick to the surface of the upper electrode 4 occurs. When the burn-in phenomenon occurs, there is a problem that even if the display screen is switched, the previously displayed display screen remains as an afterimage. Therefore, the display response speed t2 of the second migrating particles 8 may be set so as not to exceed the display response speed of the first migrating particles 7. When the display response speed t2 of the second migrating particles 8 and the display response speed t1 of the first migrating particles 7 are equal, the first migrating particles 7 and the second migrating particles 8 reach the upper electrode 4 with almost equal probability. can do. As a result, the probability that the second migrating particle 8 enters the gap between the first migrating particles 7 shown in the region 11 of FIG. 2 can be increased. This condition is expressed by equation (e5).

t2 ≦ t1 (e5)
Note that the display response speed of the first migrating particles 7 and the second migrating particles 8 is controlled by controlling the type of charging material and the amount of charging material contained in each particle, thereby changing the charge amount of each particle. It can be realized by having it. For example, the value of the equation (e5) can be satisfied by setting the charge amount of the first migrating particles 7 with a zeta potentiometer to 100 mV, and the charge amount of the second migrating particles 8 with the zeta potentiometer to 70 mV. realizable.

  Next, as the display response speed t <b> 2 of the second migrating particles 8 becomes slower, the first migrating particles 7 reach before the second migrating particles 8 reach the surface of the upper electrode 4. As a result, as shown in FIG. 3A, the migration medium 9 is filled in the gaps between the first migrating particles, and the display density of the first migrating particles 7 is lowered. Furthermore, if the voltage is continuously applied to the electrodes until the second migrating particles 8 reach the vicinity of the upper electrode 4, a burn-in phenomenon occurs as described above. Therefore, the display response speed t2 of the second migrating particles 8 should not be less than ½ of the display response speed t1 of the first migrating particles 7. This is represented by formula (e6).

0.5 × t1 ≦ t2 (e6)
FIG. 4 is an explanatory diagram for explaining a method for obtaining the optimum average particle number n of the second migrating particles 8. FIG. 4A shows a state in which the first migrating particles 7 are closely packed on the surface of the transparent upper electrode 4 and one second migrating particle 8 is in the gap with the upper electrode 4. FIG. 4B shows that the first migrating particles 7 are closely packed on the surface of the upper electrode 4, and the second migrating particles 8 having a small average particle radius r 2 μm are packed in the gap with the upper electrode 4. Indicates the state that has been performed. Note that the average particle number n described below is the average number of particles per unit area, and the average particle radii r1 and r2 have dimensions of μm, so the unit area is 1 square micrometer (μm) ² .

  When the number of the second migrating particles 8 is smaller than that in the state of FIG. 4A, the gap between the first migrating particles 7 may not be filled with the second migrating particles 8. In that case, the gap between the upper electrode 4 and the first migrating particles 7 is filled and shielded by the migrating medium 9. As a result, the display density decreases. Therefore, it is preferable that the lower limit of the number of the second migrating particles 8 is a case where the gap between the first migrating particles 7 and the upper electrode 4 is filled with one second migrating particle 8. Further, as shown in FIG. 4 (b), any number that allows the second migrating particles 8 to fill the gap between the first migrating particles 7 and the upper electrode 4 is acceptable. The particles 8 are deposited below the first migrating particles 7 to hinder the movement of the first migrating particles 7, or the close-packed arrangement of the first migrating particles 7 becomes difficult. Therefore, as shown in FIG. 4B, the number of second migrating particles 8 is preferably set to an upper limit that can fill the gap between the upper electrode 4 and the first migrating particles 7 arranged in the closest packing. .

Next, the preferable lower limit number of the second migrating particles 8 will be described. FIG. 2B is a top view showing a state in which the first migrating particles 7 are arranged closest to the upper electrode 4, and the area S of the rhombic region 14 indicated by a broken line 13 connecting the centers of the four first migrating particles 7. Is 2√3 (r1) ² (μm) ² . The preferred lower limit of the second migrating particles 8 is the number that can be put in the gap between the first migrating particles 7 that are closely packed. In the three-dimensional space, two second electrophoretic particles 8 are assigned to each first electrophoretic particle 7. Therefore, the number of the second migrating particles 8 per unit area is 2 / (2√3 (r1) ² ). This is expressed by the formula (e7) as a preferable average number of particles per unit area.

n ≧ 2 / (2√3 (r1) ² ) (e7)
Next, the preferable upper limit number of the second migrating particles 8 will be described. FIG. 4C is a schematic perspective view in which the four first migrating particles 7 constituting the rhombic region 14 shown in FIG. 2B are extracted. A broken line 15 shown in FIG. 4C represents a solid having the rhombus region 14 as a base and a height r1 μm. The filling rate Q that the first migrating particles 7 occupy in this three-dimensional volume can be expressed by the equation (e8).

Q = {(1/2) (4/3) π (r1) ³ } / (2√3 (r1) ³ ) (e8)
The filling rate is about 60%, and the void ratio occupied by this solid is (1-Q), so it is about 40%. Further, the closest packing ratio in the three-dimensional space of the sphere is about 74%. Therefore, the average number n of particles per unit area in which the second migrating particles 8 having an average particle radius of r2 μm enter the voids of the first migrating particles 7 in FIG. 4C can be expressed by the equation (e9).

n = (0.4 × 0.74) / ((4/3) π (r2) ³ ) (e9)
Therefore, a preferable upper limit of the second migrating particles 8 can be represented by the formula (e10).

n ≦ 0.071 / (r2) ³ ... (e10)
The average number n of the second migrating particles 8 in the formulas (e7) and (e10) is specifically the unit area on the display surface, 1 square μm, and the gap between the upper electrode 4 and the lower electrode 5. Represents the average number of the second migrating particles 8 included in the three-dimensional object created by the above.

1 is a schematic cross-sectional view illustrating an electrophoretic display device according to an embodiment of the present invention. It is explanatory drawing showing the state in which the 1st electrophoretic particle which concerns on embodiment of this invention was arranged closely. It is explanatory drawing for demonstrating the electrophoretic particle which concerns on embodiment of this invention. It is explanatory drawing for demonstrating the electrophoretic particle which concerns on embodiment of this invention. It is typical sectional drawing showing a conventionally well-known electrophoretic element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrophoretic display device 2 Upper board | substrate 3 Lower board | substrate 4 Upper electrode 5 Lower electrode 6 Seal part 7 1st phoretic particle 8 2nd phoretic particle 9 Electrophoresis medium

Claims (8)

An electrophoretic display device having electrophoretic particles sandwiched between opposing substrates having electrodes formed on the inner surface, dispersed in the electrophoretic medium and moving in the direction of an electric field,
The electrophoretic particles include first electrophoretic particles having an average particle radius of r1 μm and second electrophoretic particles having an average particle radius of r2 μm,
The display response speed of the first migrating particles is t1, and the display response speed of the second migrating particles is t2.
r2 ≦ 0.15 × r1
t2 ≦ t1
An electrophoretic display device satisfying the relationship: The r2 is
0.025 ≦ r2
The electrophoretic display device according to claim 1, wherein the relationship is satisfied. The t1 and t2 are
0.5 × t1 ≦ t2
The electrophoretic display device according to claim 1, wherein the relationship is satisfied. The second migrating particles dispersed in the migrating medium have an average number of particles per square μm in the display area of the substrate as n,
1 / (√3 (r1) ² ) ≦ n
The electrophoretic display device according to claim 1, wherein the relationship is satisfied. The second migrating particles dispersed in the migrating medium have an average number of particles per square μm in the display area of the substrate as n,
n ≦ 0.071 / (r2) ³
The electrophoretic display device according to claim 1, wherein the relationship is satisfied.   The electrophoretic display device according to claim 1, wherein the r1 is 0.2 to 10. The r1 and r2 are
r2 ≦ 0.09 × r1
The electrophoretic display device according to claim 1, wherein the relationship is satisfied. The r1 and r2 are
r2 ≦ 0.03 × r1
The electrophoretic display device according to claim 1, wherein the relationship is satisfied.

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