Electrophoretic display device

Abstract

To provide a suitable electrophoretic display device.SOLUTION: The electrophoretic medium comprises a fluid and first, second, third and fourth types of particles (W, Y, R, B) having four different colors. The first and third particles have charges of one polarity, the second and fourth particles have charges of the opposite polarity, the first particles have a greater zeta potential or electrophoretic mobility than the third particles, and the second particles have a greater zeta potential or electrophoretic mobility than the fourth particles. One of the particles is white (W), one of the non-white particles (B) partially transmits light, and the remaining two of the non-white particles (R, Y) reflect light. Particles having a third light reflective property (G) may be added to create a five particle medium.SELECTED DRAWING: Figure 3A

Description

[Background technology]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS)

[0002] This application is related to U.S. Patent Nos. 9,170,468, 9,361,836, 9,513,527, 9,541,814, 9,640,119, 9,812,073, 9,922,603, 10,147,366, 10,234,742, 10,431,168, 10,509,293, 10,586,499, and 10,782,586.

[0003] The entire contents of the aforementioned provisional applications and patents, as well as all other US patents and published co-pending applications mentioned below, are incorporated herein by reference.

[0004] BACKGROUND OF THE INVENTION

[0005] The aforementioned patents and published applications describe electrophoretic media, methods for driving such media, and electrophoretic display devices incorporating such media. The electrophoretic medium comprises a fluid and first, second, third, and fourth types of particles dispersed in the fluid; such media may hereinafter be referred to as "four-particle electrophoretic media." In some cases, the medium also comprises a fifth type of particles. The four or five types of particles have optical properties (typically color) that differ from one another. The first type of particles carry a high positive charge, and the second type of particles carry a high negative charge. The third type of particles carry a low positive charge, and the fourth type of particles carry a low negative charge (charge strength measured in terms of zeta potential). The fifth type of particles carry an intermediate charge of either polarity. In electrophoretic display devices, the electrophoretic medium is disposed between a front electrode and a rear electrode, and the display is typically viewed from the front electrode (viewing) side. In a typical multi-pixel display, the front electrode is continuous and extends across multiple pixels, and typically the entire display, while a separate rear electrode is provided for each pixel, allowing the displayed color to be controlled on a pixel-by-pixel basis.

[0006] The optical properties of the first and second types of particles can (in principle) be displayed at the view side by applying a high electric field of appropriate polarity across the electrophoretic medium for a period sufficient to allow the first or second type of particles to be positioned adjacent to the front electrode. To display the optical properties of the third type of particles, the second type of particles are first driven to the view surface by applying a high electric field of appropriate polarity, and then a low electric field of opposite polarity is applied to position the third type of particles adjacent to the view surface while the first, second, and fourth types of particles are moved away from this surface. (Note that the second part of this sequence involves a change from the optical properties of high negative particles (second type particles) to the optical properties of low positive particles (third type particles).) Similarly, to display the optical properties of a fourth type of particle, the first type of particle is first driven to the viewing surface by applying a high electric field of the appropriate polarity, then a low electric field of the opposite polarity is applied to place the fourth type of particle adjacent to the viewing surface while the first, second, and third type particles are moved away from the viewing surface (note again that the second part of this sequence involves a change from the optical properties of highly positive particles (first type of particle) to the optical properties of low negative particles (fourth type of particle)). To display the color of medium-charged particles, low-charged particles of opposite polarity to the medium-charged particles are first driven to the viewing surface, then a medium electric field of a polarity is applied that drives the medium-charged particles toward the viewing surface (again, the final step involves a change from the optical properties of low-charged particles to the optical properties of medium-charged particles of the opposite polarity).In practice, to achieve optimal separation of various particle types, waveforms (sequences of drive pulses) may be significantly more complex than the simple outline above suggests, and may include any one or more of the following: (a) repetition of one or two basic drive pulses already described; (b) periods of zero voltage between drive pulses; (c) the use of shake pulses (rapidly alternating positive and negative pulses) intended to uniformly mix the various particle types; and (d) direct current (DC) balance pulses intended to bring the overall impulse of the waveform to zero or near zero (it is known that repeated application of DC-unbalanced waveforms to electrophoretic displays can ultimately cause damage to the display, which can reduce the quality of the displayed image and ultimately completely destroy the display). For all of the foregoing waveform features, see, for example, the aforementioned U.S. Patent No. 9,640,119.

[0007] Although not explicitly stated in most cases, four- and five-particle electrophoretic media use light-scattering ("reflective") particles rather than light-transmitting particles, so that the color (or other optical property) seen on the view side is determined only by the color of the particles immediately adjacent to the front electrode, regardless of the relative positions of the other particles. Thus, although such electrophoretic media display only four or five independent optical states, they may also display a "particle mixture" state (typically grayish) in which various types of particles are randomly mixed, and other mixed states in which two types of particles are placed adjacent on the view side; for example, orange can be produced by mixing red and yellow particles adjacent to the view surface.

[0008] This limitation of four-particle electrophoretic media to four independent optical states is a serious practical disadvantage, since in many applications, such as electronic signage such as electronic shelf labels, it is desirable to be able to display black, white, and three primary colors, e.g., red, green, and blue, or blue, red, and yellow. While good black and white states are important for text, the three primary colors allow for full-color display through dithering. Until now, four-particle electrophoretic media have typically either had good black and white with two "highlight" colors (usually red and yellow), or had white and three primary colors, relying on a mixture of the three primary colors to produce a (often poor) "process" black.

[0009] It is known to overcome the aforementioned disadvantages of four-particle electrophoretic media by incorporating a fifth (and optionally a sixth) type of particle into the electrophoretic medium; see, for example, U.S. Patent Nos. 9,541,814 and 9,922,603. However, increasing the number of particle types in the electrophoretic medium makes it more difficult to select appropriate particles due to the increased need for strict control of the charge on the various particles, the increased potential for interactions between the various particles (which can result in increased color contamination), and longer waveforms. The five- and six-particle electrophoretic media described in U.S. Patent Nos. 9,541,814 and 9,922,603 ​​require at least one three-step waveform, whereby to display the color of medium-charged particles of one polarity, first display the color of highly charged particles of one polarity, then display the color of low-charged particles of the opposite polarity, and finally display the color of medium-charged particles of one polarity.

[0010] The inventors have now discovered that by selecting one of the types of partially optically transparent particles, a four-particle electrophoretic medium can be made to display five distinct optical states. A fifth particle may be added to enable the display of six optical states, e.g., black, white, red, green, blue, and yellow. (In practice, such a sixth optical state is typically desirable because a black / white / red / green / blue system does not provide a sufficiently saturated yellow, while a black / white / blue / red / yellow system does not provide a sufficiently saturated green.) [Prior art documents] [Patent documents]

[0011] [Patent Document 1] U.S. Patent No. 9,640,119 [Patent Document 2] U.S. Patent No. 9,541,814 [Patent Document 3] U.S. Patent No. 9,922,603 Summary of the Invention [Means for solving the problem]

[0012] (Summary of the Invention)

[0013] Thus, the present invention provides an electrophoretic medium comprising a fluid and first, second, third, and fourth types of particles dispersed in the fluid, the first, second, third, and fourth types of particles having first, second, third, and fourth colors, respectively, different from one another; the first and third types of particles having a charge of one polarity and the second and fourth types of particles having a charge of the opposite polarity; the first type of particles having a higher zeta potential or electrophoretic mobility than the third type of particles; the second type of particles having a higher zeta potential or electrophoretic mobility than the fourth type of particles; one of the types of particles being white; one of the types of non-white particles being partially light-transmitting; and the remaining two types of non-white particles being light-reflective. The electrophoretic medium may further comprise a fifth type of charged particles having a fifth color different from all of the first, second, third, and fourth colors. In some embodiments, the fifth type of particles carries a charge of the same polarity as the light-reflective, partially light-transmitting type of particles, and in some embodiments, the fifth type of particles has a greater zeta potential or electrophoretic mobility than the other two types of particles carrying a charge of the same polarity.

[0014] In some embodiments of the present invention, the white-type particles are third or fourth-type particles, i.e., one of the low-charge type particles. Also, if the white particles are one of the low-charge type particles, the partially light-transmitting type particles can be highly charged (or, if five charged particles are present, medium-charge) type particles of the opposite polarity to the white particles. In this case, it is advantageous for the light-reflective type particles (or one of them in a five-particle system) carrying the same charge as the partially light-transmitting type particles to have optical properties such that the mixture of the two types of particles absorbs substantially all visible radiation, i.e., provides process black.

[0015] In the electrophoretic medium of the present invention, the fourth particle may be white, the second particle may be yellow in color, and the first and third particles may be red and blue (in either order), although it may be advantageous for the first particle to be red and partially light-transmitting for reasons discussed below with reference to Figures 6A-6F. The fifth particle, if present, may be, for example, green, cyan, or magenta, although green is generally preferred for the reasons discussed above.

[0016] The four-particle electrophoretic medium of the present invention can display at least five colors (not considering a completely mixed state in which all four particle types are randomly mixed). The colors of the white particles and the two light-reflective types of particles can be displayed simply by adjacent each type of particle to the viewing surface. The color of the partially light-transmissive type of particles can be displayed by adjacent a mixture of this type of particle and white particles to the viewing surface such that light entering through the viewing surface is scattered by the white particles and passes through the partially light-transmissive particles, eventually re-emerging from the viewing surface along with the color of the light-transmissive type of particles (see the discussion below with reference to Figure 3A for practical details of this color formation). The fifth color displayed is process black, displayed by adjacent optically transparent particles to the viewing surface with reflective particles of the same polarity charge directly behind them (i.e., directly on the opposite side of the optically transparent particles from the viewing surface) so that light entering through the viewing surface passes through the optically transparent type particles and is then essentially totally absorbed by the reflective type particles directly behind the optically transparent particles. Clearly, for this process black to be satisfactory, the combined absorption by the two types of particles needs to extend across the entire visible spectrum, since the two types of particles are preferably red and blue (in either order), and it is relatively easy to arrange red and blue particles so that they together absorb all visible light. An example of the absorption spectrum over which red and blue pigments are capable of producing a good process black is given below. Some four-particle electrophoretic media of the present invention can usefully display a sixth color by adjacent the viewing surface a mixture of two non-white light-reflective types of particles; when these two types of particles are yellow and blue, this produces a green color. The five particle medium of the present invention can display six colors, the additional color being the color of the third non-white reflective particle.

[0017] As previously indicated, in the electrophoretic media of the present invention, one type of particle is white, another type is partially light-transmitting, while the remaining two or three types of particles are light-reflective (i.e., light-scattering). In practice, of course, there is no such thing as a completely light-scattering particle or a completely non-light-scattering, light-transmitting particle, and the minimum light scattering degree of a light-scattering particle and the maximum acceptable light scattering degree of a light-transmitting particle may vary somewhat depending on factors such as the exact pigment used, its refractive index and size, its color, the thickness of the particle layer (which itself depends on the thickness of the electrophoretic medium layer and the loading of each type of particle in the medium), and the user's ability or tolerance to tolerate some deviation from the ideal desired color. The scattering and absorption properties of a pigment may be assessed by measuring the diffuse reflectance of a sample of the pigment dispersed in an appropriate matrix or liquid against a white and dark background. Results from such measurements can be interpreted according to several models well known in the art, such as the one-dimensional Kubelka-Munk process.

[0018] The light transmittance of a pigment is most conveniently measured by the contrast ratio, which (for the purposes of this application) is defined as the ratio of the luminous reflectance of a sample against a background of black material of a specified reflectance (Rb) to the reflectance of the same sample against a background of white material of a specified reflectance (Rw). CR=Rb / Rw The contrast ratio (CR) is an indicator of opacity and, of course, varies with the thickness of the pigment layer present in the electrophoretic medium as well as the type of pigment used. Generally, full opacity is obtained at a CR of 0.98. The hiding power of a paint is understood to be its ability to eliminate the contrast between a black and a white substrate to the extent that the reflectance obtained over the black substrate is 98% of that obtained over the white substrate. The light-transmitting pigment layer used in this electrophoretic medium should have a contrast ratio of about 0.5 or less, preferably 0.3 or less. The blue pigment used in the experiments described below has a contrast ratio of about 0.2. The reflective pigment should have a contrast ratio of about 0.6 or more, preferably about 0.7 or more.

[0019] The electrophoretic medium of the present invention may be encapsulated or non-encapsulated. If encapsulated, the electrophoretic medium may be contained within a plurality of microcells, as described in U.S. Pat. No. 6,930,818, the contents of which are incorporated herein by reference in their entirety. Regardless of their shape or size, the display cells may also be other types of microcontainers, such as microcapsules, microchannels, or equivalents. Alternatively, the electrophoretic medium may be encapsulated within a capsule or in the form of a so-called polymer-dispersed electrophoretic medium, comprising a plurality of individual droplets of electrophoretic fluid and a continuous phase of polymer material. Individual droplets of electrophoretic fluid in such a polymer-dispersed electrophoretic display may be considered capsules or microcapsules, even if no individual capsule membrane is associated with each individual droplet. See, e.g., U.S. Pat. No. 6,866,760.

[0020] The invention extends to an electrophoretic display device comprising a layer of an electrophoretic medium of the invention, with front and rear electrodes disposed on opposite sides of the layer of electrophoretic medium, and with voltage control means arranged to control the potential of the front and rear electrodes. The invention also extends to a front plane laminate, inverted front plane laminate, or double release film comprising a layer of an electrophoretic medium of the invention. See below for definitions of front plane laminate, inverted front plane laminate, and double release film.

[0021] The present invention also extends to methods of driving the electrophoretic display device of the present invention. A first driving method comprises: (i) applying a first drive voltage having a polarity to drive partially optically transmissive particles and optically reflective particles carrying a charge of the same polarity as the partially optically transmissive particles toward a viewing surface for a first period of time; (ii) after step (i), not applying the drive voltage for a second period of time that is longer than the first period of time; (iii) repeating steps (i) and (ii), thereby causing the color (preferably black) of a mixture of the two types of particles to appear on the viewing surface; Includes.

[0022] The second driving method is (i) applying a second driving voltage having a polarity to drive the partially optically transmissive particles toward the viewing surface for a third period of time; (ii) applying a third drive voltage having a polarity opposite to and a magnitude less than the second drive voltage for a fourth time period longer than the third time period; (iii) repeating steps (i) and (ii); and (iv) after repeating steps (i) and (ii), applying a third drive voltage for a fifth period of time; and (v) not applying a driving voltage for a sixth period of time; (vi) repeating steps (iv) and (iv); (vii) after repeating steps (iv) and (v), applying a second drive voltage for a seventh period of time; (viii) applying a fourth drive voltage having the same polarity as the third drive voltage but a magnitude less than the third drive voltage for an eighth time period longer than the seventh time period; (ix) repeating steps (vii) and (viii), except that the repetition of step (viii) terminates without a subsequent repetition of step (vii), thereby displaying the color of the light-transmitting type particles at the viewing surface. Includes.

[0023] In this second driving method, a period of zero voltage may be inserted between each step (i) and the subsequent step (ii) and / or between each step (vii) and the subsequent step (viii).

[0024] The third driving method of the present invention is (i) applying a fifth driving voltage having a polarity to drive two types of particles of one polarity toward the viewing surface for a ninth period of time; (ii) applying a sixth drive voltage having a polarity opposite to and a magnitude less than the fifth drive voltage for a tenth period longer than the ninth period; (iii) repeating steps (i) and (ii); and (iv) after repeating steps (i) and (ii), applying a sixth drive voltage having the same polarity as the fourth drive voltage, wherein the magnitude and / or duration of application of the sixth drive voltage is not sufficient to drive the display to display the color of the second type of particles at the viewing surface, thereby displaying the color of a mixture of the two non-white light-reflecting particles at the viewing surface; Includes.

[0025] In this third driving method, a period of zero voltage may be inserted between each step (i) and the subsequent step (ii).

[0026] Each of the drive methods of the present invention may be followed by one or more periods of a shake waveform and / or one or more periods of a DC balance waveform (i.e., periods during which a non-zero voltage is applied to the display so as to reduce or eliminate the overall impulse of the total waveform applied).

[0027] In any of the driving methods of the present invention, when the sequence of driving pulses is repeated, the repetition may be at least four times, and preferably at least eight times. The present specification also provides, for example, the following items: (Item 1) 1. An electrophoretic medium comprising a fluid and first, second, third, and fourth types of particles dispersed in the fluid, the first, second, third, and fourth types of particles having first, second, third, and fourth colors, respectively, different from one another; the first and third types of particles having an electric charge of one polarity and the second and fourth types of particles having an electric charge of the opposite polarity; the first type of particles having a larger zeta potential or electrophoretic mobility than the third type of particles; the second type of particles having a larger zeta potential or electrophoretic mobility than the fourth type of particles; one of the types of particles being white; one of the types of non-white particles being partially light-transmitting; and the remaining two types of non-white particles being light-reflective. (Item 2) Item 2. An electrophoretic medium according to item 1, wherein the white-type particles are the third or fourth type particles. (Item 3) 3. An electrophoretic medium according to item 2, wherein the partially light-transmitting type of particles is a highly charged type of particles of opposite polarity to the white particles. (Item 4) 4. An electrophoretic medium according to item 3, wherein the light-reflective type particles, which carry the same charge as the partially light-transmitting type particles, have optical properties such that a mixture of the two types of particles absorbs substantially all visible radiation. (Item 5) Item 10. The electrophoretic medium of item 1, wherein the first type of particles are optically transparent, the second type of particles are yellow, the fourth type of particles are white, and the first and third types of particles are red and blue (in any order). (Item 6) Item 10. The electrophoretic medium of item 1, wherein the light-transmitting pigment layer has a contrast ratio of about 0.5 or less. (Item 7) Item 10. An electrophoretic medium according to item 1, wherein the light-transmitting pigment layer has a contrast ratio of about 0.3 or less. (Item 8) Item 10. The electrophoretic medium of item 1, further comprising a fifth type of charged particles having a fifth color different from all of the first, second, third, and fourth colors. (Item 9) 9. An electrophoretic medium according to item 8, wherein the fifth type of particles is light reflective and carries a charge having the same polarity as the partially light transmissive type of particles. (Item 10) 10. The electrophoretic medium according to item 9, wherein the fifth type of particles has a higher zeta potential or electrophoretic mobility than the other two types of particles carrying a charge of the same polarity. (Item 11) 10. An electrophoretic display device comprising a layer of electrophoretic medium according to item 1, wherein front and rear electrodes are arranged on opposite sides of the layer of electrophoretic medium, and voltage control means are arranged to control the potential of said front and rear electrodes. (Item 12) 2. A front plane laminate, an inverted front plane laminate, or a double release film comprising a layer of the electrophoretic medium of item 1. (Item 13) Item 12. A method for driving an electrophoretic display device according to item 11, said method comprising: (i) applying a first drive voltage having a polarity to drive partially optically transmissive particles and optically reflective particles carrying a charge of the same polarity as the partially optically transmissive particles toward a viewing surface for a first period of time; (ii) after step (i), not applying the driving voltage for a second period longer than the first period; (iii) repeating steps (i) and (ii), thereby causing a mixture of the colors of the two types of particles to be displayed on the viewing surface; and A method comprising: (Item 14) Item 12. A method for driving the electrophoretic display device according to item 11, comprising: (i) applying a second driving voltage having a polarity to drive the partially optically transmissive particles toward the viewing surface for a third period of time; (ii) applying a third drive voltage having a polarity opposite to and a magnitude less than the second drive voltage for a fourth time period longer than the third time period; (iii) repeating steps (i) and (ii); and (iv) applying the third drive voltage for a fifth period after said repeating of steps (i) and (ii); and (v) not applying a driving voltage for a sixth period of time; (vi) repeating steps (iv) and (iv); (vii) applying the second drive voltage for a seventh period after said repeating of steps (iv) and (v); and (viii) applying a fourth drive voltage having the same polarity as the third drive voltage but a magnitude less than the third drive voltage for an eighth period longer than the seventh period; repeating steps (vii) and (viii), but terminating the repetition of step (viii) without following iteration of step (vii), thereby displaying the color of the light-transmitting type particles at the viewing surface; and A method comprising: (Item 15) 15. The method according to item 14, wherein a period of zero voltage is inserted between each step (i) and the subsequent step (ii) and / or between each step (vii) and the subsequent step (viii). (Item 16) Item 12. A method for driving the electrophoretic display device according to item 11, comprising: (i) applying a fifth driving voltage having a polarity to drive two types of particles of one polarity toward the viewing surface for a ninth period of time; (ii) applying a sixth drive voltage having a polarity opposite to and a magnitude less than the fifth drive voltage for a tenth period longer than the ninth period; (iii) repeating steps (i) and (ii); and (iv) after said repeating of steps (i) and (ii), applying a sixth drive voltage having the same polarity as said fourth drive voltage, wherein the magnitude and / or duration of application of said sixth drive voltage is insufficient to drive said display to display the color of said second type of particles at said viewing surface, thereby displaying the color of a mixture of two non-white light-reflective particles at said viewing surface; A method comprising: (Item 17) 17. The method according to item 16, wherein a period of zero voltage is inserted between each step (i) and the subsequent step (ii). [Brief explanation of the drawings]

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] [Figure 1]Figure 1 of the accompanying drawings is a schematic cross section through a four particle display device of the present invention.

[0030] [Figure 2] FIG. 2 shows the absorption spectrum of preferred pigment particles for use in the display device of FIG.

[0031] [Figure 3A] 3A-3F are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 may undergo. [Figure 3B] 3A-3F are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 may undergo. [Figure 3C] 3A-3F are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 may undergo. [Figure 3D] 3A-3F are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 may undergo. [Figure 3E] 3A-3F are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 may undergo. [Figure 3F] 3A-3F are schematic cross-sections similar to that of FIG. 1, but illustrating various optical transitions that the display device of FIG. 1 may undergo.

[0032] [Figure 4] FIG. 4 illustrates DC balance and shake waveforms that can be incorporated into the driving method of the present invention.

[0033] [Figure 5A] 5A-5F show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5B] 5A-5F show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5C] 5A-5F show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5D] 5A-5F show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5E] 5A-5F show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively. [Figure 5F] 5A-5F show waveforms that may be used to implement the transitions shown in FIGS. 3A-3F, respectively.

[0034] [Figure 6A] 6A-6F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions within a display device using red partially light-transmitting particles. [Figure 6B] 6A-6F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions within a display device using red partially light-transmitting particles. [Figure 6C] 6A-6F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions within a display device using red partially light-transmitting particles. [Figure 6D] 6A-6F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions within a display device using red partially light-transmitting particles. [Figure 6E] 6A-6F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions within a display device using red partially light-transmitting particles. [Figure 6F] 6A-6F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions within a display device using red partially light-transmitting particles.

[0035] [Figure 7A]7A-7F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions that a five-particle display device of the present invention can undergo. [Figure 7B] 7A-7F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions that a five-particle display device of the present invention can undergo. [Figure 7C] 7A-7F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions that a five-particle display device of the present invention can undergo. [Figure 7D] 7A-7F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions that a five-particle display device of the present invention can undergo. [Figure 7E] 7A-7F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions that a five-particle display device of the present invention can undergo. [Figure 7F] 7A-7F are schematic cross-sections similar to those of FIGS. 3A-3F, but illustrating various optical transitions that a five-particle display device of the present invention can undergo.

[0036] [Figure 8A] 8A and 8B show waveforms that may be used to implement the transitions shown in FIGS. 7E and 7F, respectively. [Figure 8B] 8A and 8B show waveforms that may be used to implement the transitions shown in FIGS. 7E and 7F, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0037] (Detailed explanation)

[0038] As noted above, in one aspect, the present invention provides a four-particle electrophoretic medium that can be fabricated to display five distinct optical states. The electrophoretic medium comprises a fluid and first, second, third, and fourth types of particles dispersed in the fluid, with all four types of particles having different colors. The first and third types of particles carry a charge of one polarity, while the second and fourth types of particles carry a charge of the opposite polarity. The first type of particles has a higher zeta potential or electrophoretic mobility than the third type of particles, and the second type of particles has a higher zeta potential or electrophoretic mobility than the fourth type of particles. (Thus, in two pairs of oppositely charged particles, one pair carries a stronger charge than the other pair. Thus, the four types of particles may also be referred to as highly positive particles, highly negative particles, low positive particles, and low negative particles.) One type of particle is white. One of the non-white particle types is partially light-transmitting, while the other two types of non-white particles are light-reflective.

[0039] As an example shown in Figure 1, blue particles (B) and yellow particles (Y) are a first pair of oppositely charged particles, in which the blue particles are highly positive and the yellow particles are highly negative. Red particles (R) and white particles (W) are a second pair of oppositely charged particles, in which the red particles are low positive and the white particles are low negative. Of course, it should be understood that the aforementioned charges can be reversed in polarity and the display can continue to function in the same manner, except that the polarity of the drive waveforms described below needs to be reversed.

[0040] The white particles may be formed from inorganic pigments such as TiO2, ZrO2, ZnO, Al2O3, Sb2O3, BaSO4, PbSO4, or the like.

[0041] The non-white and non-black particles are independent of color, such as red, green, blue, magenta, cyan, or yellow. Pigments for the color particles may include, but are not limited to, CI pigments PR254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY83, PY138, PY150, PY155, or PY20. These are commonly used organic pigments described in the Color Index Handbook "New Pigment Application Technology" (CMC Publishing Co., Ltd., 1986) and "Printing Ink Technology" (CMC Publishing Co., Ltd., 1984). Specific examples include Clariant Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm Green GNX, BASF Irgazine red L3630, Cinquasia Red L 4100 HD, and Irgazin Red L 3660 HD, Sun Chemical Phthalocyanine Blue, Phthalocyanine Green, Diarylide Yellow, or Diarylide AAOT Yellow. A preferred partially light-transmitting blue pigment for use in the display of Figure 1 is Kremer 45030, "Ultramarine Blue, greenish extra," a sodium aluminum sulfosilicate pigment, CI Pigment Blue 29:77007, available from Kremer Pigmente GmbH & Co. KG, Hauptstr. 41-47, DE-88317 (Aichstetten, Germany). This light-transmitting blue pigment may be usefully used in combination with the aforementioned Hostaperm Red D3G 70 pigment.

[0042] As illustrated in Figure 2, this blue pigment has a peak transmittance at about 450 nm and substantial transmittance across the visible range of 400 to about 530 nm. On the other hand, the Hostaperm Red D3G 70 pigment is essentially non-reflective below about 555 nm. Thus, when the two pigments are arranged as shown in Figure 3A, with the light-transmitting blue pigment adjacent to the viewing surface and the reflective red pigment directly opposite the blue pigment from the viewing surface, all visible radiation that enters through the viewing surface and passes through the blue pigment is absorbed by the red pigment, and the viewing surface appears black.

[0043] The non-white particles may be inorganic pigments such as red, green, blue, and yellow. Examples may include, but are not limited to, CI Pigment Blue 28, CI Pigment Green 50, and CI Pigment Yellow 227.

[0044] In addition to color, the four types of particles may have other different optical properties such as optical transmittance, reflectance, luminescence, or, in the case of displays intended for machine reading, pseudocolor in the sense of changes in reflectance at electromagnetic wavelengths outside the visible range.

[0045] A display layer utilizing the display fluid of the present invention has two surfaces, a first surface (13) on the view side and a second surface (14) opposite the first surface (13), as shown in Figure 1. The display fluid is sandwiched between the two surfaces. On the first surface (13) side is a common electrode (11), which is a transparent electrode layer (e.g., ITO), extending across the entire top of the display layer. On the second surface (14) side is an electrode layer (12) comprising a plurality of pixel electrodes (12a). However, the present invention is not limited to any particular electrode configuration.

[0046] Pixel electrodes are described in U.S. Patent No. 7,046,228, the contents of which are incorporated herein by reference in their entirety. It should be noted that although active matrix drive using a thin film transistor (TFT) backplane is described with respect to a layer of pixel electrodes, the scope of the present invention encompasses other types of electrode addressing, so long as the electrodes perform the desired function.

[0047] Each space between two vertical dotted lines in Figure 1 represents a pixel. As shown, each pixel has a corresponding pixel electrode. An electric field is generated about the pixel due to the potential difference between the voltage applied to the common electrode and the voltage applied to the corresponding pixel electrode. (Note that in the various waveforms illustrated in the accompanying figures, the plotted potential difference is that applied to pixel electrode 12a, and the common electrode is assumed to be held at ground voltage, as is typically the case. The color displayed by a pixel depends on the particles adjacent to common electrode 11, so when a positive potential difference is shown in the figures, the common electrode is negative with respect to the pixel electrode, and positively charged particles are attracted to the common electrode.)

[0048] The solvent in which the four types of particles are dispersed is clear and colorless, and preferably has a low viscosity for high particle mobility and a dielectric constant in the range of about 2 to about 30, preferably about 2 to about 15. Examples of suitable dielectric solvents are hydrocarbons such as Isopar®, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils, silicone fluids, aromatic hydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzene, or alkylnaphthalenes, halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluorobenzene, dichlorononane, or pentachlorobenzene, as well as perfluorinated solvents such as FC-43, FC-70, or FC-5060 from 3M Company (St. Paul, MN), low molecular weight halogen containing polymers such as poly(perfluoropropylene oxide) from TCI America (Portland, Oregon), and Halocarbon from Halocarbon Product Corp. (River Edge, NJ). Poly(chlorotrifluoroethylene) Oils such as Galden from Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont (Delaware), perfluoropolyalkyl ethers such as polydimethylsiloxane-based silicone oils from Dow-corning (DC-200).

[0049] In one embodiment, the charge carried by a "low charge" particle may be less than about 50%, preferably about 5% to about 30%, of the charge carried by a "high charge" particle. In another embodiment, a "low charge" particle may be less than about 75%, or about 15% to about 55%, of the charge carried by a "high charge" particle. In further embodiments, the comparison of charge levels as shown applies to two types of particles having the same charge polarity.

[0050] Charge strength may be measured in terms of zeta potential. In one embodiment, zeta potential is determined using a Colloidal Dynamics AcoustoSizer IIM with a CSPU-100 signal processing unit and an ESA EN# Attn flow-through cell (K:127). Instrument constants, such as the density of the solvent used in the sample, the dielectric constant of the solvent, the speed of sound in the solvent, and the viscosity of the solvent, are all entered prior to testing at the test temperature (25°C). The pigment sample is dispersed in a solvent (typically a hydrocarbon fluid with fewer than 12 carbon atoms) and diluted to 5-10% by weight. The sample also contains a charge control agent (Solsperse 19K, available from Lubrizol Corporation, a subsidiary of Berkshire Hathaway; "Solsperse" is a registered trademark) with a 1:10 weight ratio of charge control agent to particles. The mass of the diluted sample is determined, and the sample is then loaded into the flow-through cell for zeta potential determination.

[0051] The amplitudes of "highly positive" and "highly negative" particles can be the same or different. Similarly, the amplitudes of "lowly positive" and "lowly negative" particles can be the same or different. However, the zeta potential of "highly positive" or positive particles with greater charge strength or greater charge magnitude is greater than the zeta potential of "lowly positive" or positive particles with less charge strength or smaller charge magnitude, and the same logic follows for high-negative and low-negative particles. In the same medium under the same field, more highly charged particles have greater electrophoretic mobility, i.e., they traverse the same distance in less time than less charged particles.

[0052] It should also be noted that in the same fluid, two pairs of high-low charged particles may have different levels of charge difference: for example, in one pair, the low positively charged particle may have a charge strength that is 30% of the charge strength of the high positively charged particle, and in another pair, the low negatively charged particle may have a charge strength that is 50% of the charge strength of the high negatively charged particle.

[0053] The following examples illustrate display devices utilizing such display fluids.

[0054] (Example) This example is demonstrated in Figures 3A-3F. The highly positive light-transmitting particles are blue (B), the highly negative particles are yellow (Y), the low positive particles are red (R), and the low negative particles are white (W). The transition shown in Figure 3A starts from a fully mixed state, which is created by applying a shake pulse, indicated by "(M)," as described below. When alternating pulses of high positive potential difference (e.g., +15V) and no potential difference (0V) are applied to the pixel electrode 22a for a sufficiently long period, the blue (B) and red (R) particles are driven toward the common electrode (21) or view side, and the yellow and white particles are driven toward the pixel electrode 22a side. The red (R) and white (W) particles carry a weaker charge and therefore move slower than the highly charged blue and yellow particles. As a result, the blue particles are positioned directly adjacent to the common electrode, with the red particles directly below the blue particles (as shown in Figure 3A). For reasons already discussed above, this causes the pixel to appear black, which is indicated by "(K)" in Figure 3A, and the white and yellow particles are obscured by the reflective red particles and do not affect the displayed color.

[0055] Similarly, the transition shown in FIG. 3B starts from a fully mixed state (M), which is produced by applying a shake pulse, as described below. When alternating pulses of a high negative potential difference (e.g., −15 V) and no voltage (0 V) are applied to the pixel electrode 22a for a sufficiently long period, the blue (B) and red (R) particles are driven toward the pixel electrode 22a, and the yellow and white particles are driven toward the common electrode. The red (R) and white (W) particles move slower than the highly charged blue and yellow particles because they carry a weaker charge. As a result, the reflective yellow particles are placed directly adjacent to the common electrode, thus causing the pixel to appear yellow, which is indicated by “(Y)” in FIG. 3B. The white, red, and blue particles are all obscured by the reflective yellow particles and do not affect the displayed color. In principle, yellow could be produced by alternating pulses of -15V and 0V, but in practice more complex waveforms are preferred, as explained below with reference to Figure 5B.

[0056] The transition shown in Figure 3C starts from a fully mixed state (M). When alternating pulses of a high negative potential difference (e.g., -15 V) and a low positive potential difference (e.g., +8 V) are applied to pixel electrode 22a for a sufficiently long period, with the low positive pulses much longer than the high negative pulses, red (R) particles are driven toward the common electrode 21, and white (W) particles are driven toward pixel electrode 22a. The effect of the oscillating electric field is to cause the highly charged blue and yellow particles to repeatedly pass each other in the center of the electrophoretic layer thickness, while the strong electric attraction between the highly charged positive and negative particles tends to greatly slow their movement and keep them in the center of the electrophoretic layer thickness. However, the electric field generated by the low positive pulse is sufficient to separate the low-charged white and red particles, thereby allowing the low-positive red particles (R) to migrate to the edge of the common electrode 21 and the low-negative white particles to migrate toward pixel electrode 22a. As a result, the reflective red particles are placed directly adjacent to the common electrode, thus causing the pixel to appear red, indicated by "(R)" in Figure 3C, while the white, yellow, and blue particles are all obscured by the reflective red particles and do not affect the displayed color. Importantly, the present system allows for the separation of weaker charged particles from more strongly charged particles of the opposite polarity.

[0057] The transition shown in Figure 3D starts from a fully mixed state (M). Alternating pulses of a high positive potential difference (e.g., +15 V) and a low negative potential difference (e.g., -8 V) are applied to pixel electrode 22a for a long enough period, with the low negative pulses being much longer than the high positive pulses, to drive the red (R) particles toward pixel electrode 22a and the white particles (W) toward the common electrode 21. As with the transition shown in Figure 3C, the effect of the oscillating electric field is to cause the highly charged blue and yellow particles to remain together in the center of the electrophoretic layer thickness. However, the electric field generated by the low negative pulse is sufficient to separate the less charged white and red particles, thereby allowing the less positive red particles (R) to move to the edge of pixel electrode 22a and the less negative white particles to move toward the common electrode 21. As a result, the white particles are placed directly adjacent to the common electrode, thus causing the pixel to appear white, which is indicated by "(W)" in Figure 3D; the red, yellow, and blue particles are all obscured by the white particles and do not affect the displayed color. In principle, white can be produced by alternating pulses of +15V and -8V, but in practice, more complex waveforms such as those described below with reference to Figure 5D are preferred.

[0058] The transition shown in Figure 3E begins from the white state (W) shown in Figure 3D. With the device in this state, a positive potential difference pulse is applied, the overall impulse of which is insufficient to drive the device to the black state (K) shown in Figure 3A. The positive pulse causes the highly charged blue particles to migrate toward the common electrode 21 and the white particles to migrate toward the pixel electrode 22a. However, because the highly charged blue particles migrate more quickly than the less charged white particles, a mixture of the blue and white particles is visible through the viewing surface, causing the pixel to appear blue.

[0059] At first glance, Figure 3E appears to show a significant reduction in blue saturation at the viewing surface due to reflection from the white pigment located directly adjacent to the front electrode. However, it should be understood that Figure 3E (and also Figures 3A-3D, 3F, 6A-6F, and 7A-7F) are all highly schematic. In practice, pigment particles are not spherical (e.g., crystalline pigments used preferentially fracture along certain crystal planes; e.g., rutile titania, commonly used as a white pigment in electrophoretic media, is tetragonal and tends to form regular square prisms), particles vary significantly in size, the "reflection" from the white particles is essentially Lambertian light scattering rather than specular reflection, and several more layers of particles than those illustrated in Figure 3E are present (the exact number of layers will, of course, depend on the particle loading in the electrophoretic medium, the thickness of this medium, and the size of the individual particles, but in practice, at least 5-10 layers are typically present). The overall effect of all the aforementioned factors is that only a very small percentage of visible light entering the electrophoretic medium through the viewing surface is reflected directly back through the viewing surface by the white particles, and in practice, a fully saturated blue color can be achieved.

[0060] Also, while Figure 3A shows blue and red particles in completely separate layers, while Figure 3E shows a complete mixture of blue and white particles, it should be understood that these represent two extremes, and that in practice there may be a continuum between completely separate layers and complete mixture. The present invention is not limited to any theoretical interpretation of the exact location of the particles and their degree of mixture with other particles, provided that the desired color is obtained.

[0061] Finally, the transition shown in Figure 3F begins from the red state (R) shown in Figure 3C. With the device in this state, a negative potential difference pulse is applied, the overall impulse being insufficient to drive the device into the yellow state (Y) shown in Figure 3B. The negative pulse causes the highly negative yellow particles to migrate toward the common electrode (21), while the low positive red particles migrate much more slowly toward the pixel electrode (22a). The result is that a mixture of red and yellow particles is visible through the common electrode 21, and the pixel appears orange.

[0062] To ensure both color brightness and color purity, a DC balance and / or shake waveform may be used prior to any of the transitions discussed above. The shake waveform consists of a pair of opposing drive pulses repeated over many cycles. For example, the shake waveform may consist of a +15V pulse for 20 milliseconds and a -15V pulse for 20 milliseconds, with such a pair of pulses repeated 50 times. The total time of such a shake waveform is 2,000 milliseconds. In practice, there may be at least 10 repetitions of the shake pulse (i.e., 10 pairs of positive and negative pulses). The shake waveform may be applied before the drive voltage is applied, regardless of the optical state (black, white, red, or yellow). After the shake waveform is applied, the optical state is neither pure white, pure black, pure yellow, nor pure red. Instead, the color state results from a mixture of four types of pigment particles.

[0063] Each of the drive pulses in the shake waveform is applied for no more than 50% (or no more than 30%, 10%, or 5%) of the drive time required for a complete transition from one highly charged particle color to another highly charged particle color (in this example, from blue to yellow, or vice versa). For example, if it takes 300 milliseconds to drive a display device from a fully black state to a fully yellow state (or vice versa), the shake waveform may consist of positive and negative pulses each applied for 150 milliseconds or less. In practice, shorter pulses are preferred. The shake waveform as described may be used in the drive method of the present invention. In all of the figures throughout this application, the shake waveform is shortened (i.e., the number of pulses is less than the actual number).

[0064] A DC-balanced waveform is designed to reduce the overall impulse (i.e., the integral of voltage over time) of the overall waveform to a small value, zero if possible. For example, as discussed in U.S. Patent Nos. 6,531,997 and 6,504,524, problems can be encountered in that the useful life of a display can be reduced if the method used to drive the display does not result in a zero or near-zero net time-averaged applied electric field across the electro-optic medium. Waveforms that result in a zero net time-averaged applied electric field across the electro-optic medium are conveniently referred to as "direct current balanced" or "DC balanced" waveforms.

[0065] Figure 4 illustrates a combined DC balance / shake waveform comprising a DC balance section 42 followed by a shake section 44. While Figure 4 illustrates the DC balance section 42 as having a high positive potential difference, it should be understood that the DC balance section may have a high or low positive or negative potential difference, or zero potential difference, depending on the remaining impulses of the applied waveform.

[0066] Additionally, while FIG. 4 illustrates a single DC balance segment followed by a single shake segment, the combined DC balance / shake waveform may include multiple DC balance segments and multiple shake segments alternating with each other, and may begin and end with either a DC balance segment or a shake segment. The use of multiple DC balance segments may be advantageous in that it may be possible to achieve a closer approach to a zero overall waveform impulse than with a single DC balance segment by (for example) setting one or more DC balance segments to a high voltage and one or more to zero. Multiple DC balance segments may vary from each other in both duration and applied potential difference. Similarly, multiple shake segments may differ from each other in duration, magnitude of potential difference, and frequency.

[0067] 5A illustrates the waveforms used to effect the transition of FIG. 3A to produce the black optical state. This waveform is an example of a first driving method of the present invention. After a DC balance section and shake section S of duration t1 at a high negative voltage VH2 to achieve mixed state M (the durations of both t1 and shake section S are greatly reduced in FIG. 5A ; multiple DC balance and shake sections could, of course, be used), (i) a period of zero voltage of duration t2, (ii) a period of high positive driving voltage VH1 of duration t3, (iii) a period of zero voltage of duration t4 significantly greater than t3, and (iv) several repetitions (typically 4-8 repetitions) of (ii) and (iii) are applied to the pixel electrode.

[0068] FIG. 5B illustrates a waveform used to effect the transition of FIG. 3B to produce a yellow optical state (i.e., the color of the second particle). As already indicated, in principle, yellow can be produced by applying alternating pulses of a high negative potential difference (e.g., −15 V) and no voltage (0 V) to pixel electrode 22a for a sufficiently long period of time. However, to ensure a pure yellow color, a much more complex waveform, as shown in FIG. 5B, is preferred. After a DC balance section and a shake section S of duration t1, which are essentially the same as those already described with reference to FIG. 5A, the waveform of FIG. 5B comprises a period of zero voltage of duration t5, followed by (i) a short period of time t6 of a high negative potential difference VH2, (ii) a period of zero voltage of duration t7, and (iii) a period of time t8 of a low positive potential difference VL1 for a period t8 longer than t6. Typically, the magnitude of VL1 is about half that of VH2, t7 is comparable in length to t6, and t8 is about 10 times larger than t6. For example, t6 and t7 may each be 50 milliseconds, while t8 may be 500 milliseconds. Steps (i), (ii), and (iii) are then repeated several times, as indicated by "[xm]" in FIG. 5B. Typically, these steps may be repeated four to six times. Following these repetitions, (iv) a high negative potential difference VH2 is applied for a period t9 longer than t6, and then (v) a low positive potential difference VL3 lower than VL1 (typically about one-third of VH1) is applied for a period t10 shorter than t8. Steps (iv) and (v) are then repeated, as indicated by "[xn]" in FIG. 5B. Typically, these steps may be repeated two to three times. The final portion of the waveform in FIG. 5B comprises application of a high negative potential difference VH2 for a period t11 longer than t9, a period of zero voltage for duration t12, and a second application of a high negative potential difference VH2 for period t11. As will be readily apparent, the number of VH2 applications and duration t11 in this portion of the waveform can be adjusted experimentally.

[0069] FIG. 5C illustrates a waveform used to effect the transition of FIG. 3C to produce the red optical state (i.e., the color of the second particle). The waveform shown in FIG. 5C resembles the initial portion of the waveform shown in FIG. 5B. After a DC balance section of duration t1 and a shake section S, which are essentially the same as those already described with reference to FIG. 5A, the waveform of FIG. 5C comprises a period of zero voltage of duration t13 followed by (i) a short period of duration t14 of a high negative voltage difference VH2, (ii) a period of zero voltage of duration t15, and a period of low positive voltage difference VL1 for a duration t16 longer than t14. Typically, the magnitude of VL1 is about half that of VH2, t15 is comparable in length to t14, and t16 is about 10 times longer than t14. For example, t6 and t7 may each be 50 milliseconds, while t8 may be 500 milliseconds. Steps (i), (ii), and (iii) are then repeated several times, as shown in FIG. 5C; typically, these steps may be repeated 6-10 times. The waveform is terminated by a transition from the last application of VL1 to 0 V to ensure a good red color. As will be readily apparent, the number of applications of VH2 and VL1 in this waveform, and the durations t14 and t16, can be adjusted experimentally.

[0070] FIG. 5D illustrates the waveform used to effect the transition of FIG. 3D to produce the white optical state (i.e., the color of the fourth particle). Not surprisingly, the initial portion of the waveform shown in FIG. 5D resembles the “red” waveform shown in FIG. 5C (albeit with a change in polarity), and after a DC balance section of duration t1′ (the DC balance section is high positive in this example) and shake section S, which are essentially the same as those already described with reference to FIG. 5A, the waveform of FIG. 5D comprises (i) a short period of time t17 of a high positive potential difference VH1 (note that in this example, there is no period of zero voltage between the application of shake section S and the high drive potential difference), (ii) a period of zero voltage of duration t18, and a period of a low negative potential difference VL2 for a period t19 longer than t17. Typically, the magnitude of VL2 is about half that of VH1, t18 is comparable in length to t17, and t19 is about 10 times larger than t17. For example, t6 and t7 may each be 50 milliseconds, while t8 may be 500 milliseconds. Steps (i), (ii), and (iii) may then be repeated several times, as shown in FIG. 5D ; typically, these steps may be repeated 6 to 10 times. However, to ensure a pure white color, it has been found advantageous to repeat steps (i), (ii), and (iii) with a period of zero potential difference of duration t20, followed by (v) application of a low negative potential difference VL2 for period t21, and repetition of steps (iv) and (v). Typically, steps (iv) and (v) are repeated 6 to 10 times, with t20 being equal to t18 and t21 being shorter than t19. As will be readily apparent, the number of VH1 and VL2 applications and the durations t17, t18, t19, t20, and t21 in this waveform can be adjusted experimentally.

[0071] FIG. 5E illustrates a waveform used to effect the transition of FIG. 3E to produce a blue optical state (i.e., the color of the first particles), which is an example of a second driving method of the present invention. Not surprisingly, the initial portion of the waveform shown in FIG. 5E is the same as the “white” waveform shown in FIG. 5D. However, after the repetition of steps (iv) and (v) discussed in the previous section, the waveform of FIG. 5E continues with (vi) application of a high positive potential difference VH1 for a period t22 shorter than t17, (vii) application of a zero potential difference for a period t23 shorter than t18, (viii) application of a low negative potential difference VL4 having a magnitude smaller than VL2 for a period t24 shorter than t19 or t21, and a repetition of steps (vi)-(viii), but without a repetition of step (vi) being followed by a repetition of step (viii), i.e., terminating with a final positive driving pulse, as described above with reference to FIG. 3E. Typically, the magnitude of VL4 is about 75 percent of VL2, and steps (vi)-(viii) may be repeated 10-20 times. As will be readily apparent, the number of VH1 and VL4 applications and durations t22, t23, and t24 in this waveform can be adjusted experimentally.

[0072] Finally, Figure 5F shows the waveform used to effect the transition shown in Figure 3F to produce the orange optical state, which is an example of a third driving method of the present invention. The waveform shown in Figure 5F is the same as the "red" waveform shown in Figure 5C, except for the addition of a final short low negative potential difference pulse SP, the impulse of which is insufficient to drive the electrophoretic medium from the red optical state (R) to the yellow optical state (Y) (see Figure 3B). The magnitude and duration of the pulse SP can be widely varied, and the optical combination of magnitude and duration may be determined experimentally.

[0073] A four-particle electrophoretic medium as shown in Figure 1 was formulated using the aforementioned Kremer 45030 as a partially light-transmitting blue pigment, rutile titania white pigment, light-reflecting 1254 DPP Red 254 (available from DCL Corporation), and Novoperm Yellow HR 70-EDS (available from Clariant Corporation, Holden MA) in Isopar E with the addition of a charge control agent pigment. Additionally, the following five colors were produced using a non-optimized waveform: [Table 1]

[0074] The electrophoretic medium shown in Figures 1, 3A-3F, and 5A-5F comprises a first type of partially transparent, highly positive blue particles, a second type of reflective, highly negative yellow particles, a third type of reflective, low positive red particles, and a fourth type of low negative white particles. However, other colors can be produced by using red partially transparent, highly positive particles and blue reflective, low positive particles. Specifically, this combination allows the electrophoretic medium to display black, white, red, blue, and green, as shown in Figures 6A-6F below. Thus, the system is capable of displaying black, white, and the three regular additive colors, and additional colors may be generated by area modulation (dithering).

[0075] Figure 6A shows a transition similar to that shown in Figure 3A, starting from a fully mixed state (M). In the final state (K), the red light-transmitting particles are placed directly adjacent to the common electrode 21 with the blue light-reflective particles directly below the red light-transmitting particles (as shown in Figure 6A), such that a black optical state is still generated despite the reversal of the positions of the red and blue particles compared to Figure 3A.

[0076] Figure 6B shows a transition similar to that shown in Figure 3B. A yellow state is again produced, as the reversal of the positions of the red and blue particles in Figure 6B compared to Figure 3B has no effect on the optical state because the yellow particles adjacent to the common electrode obscure the white, red, and blue particles.

[0077] Figure 6C shows a transition similar to that shown in Figure 3C. Like Figure 3C, the transition in Figure 6C places low positive particles adjacent to the common electrode, and because these low positive particles are now reflective blue particles, a blue optical state is produced rather than the red optical state of Figure 3C.

[0078] Figure 6D shows a transition similar to that shown in Figure 3D. A white state is again produced, as the reversal of the positions of the red and blue particles in Figure 6D compared to Figure 3D has no effect on the optical state because the white particles adjacent to the common electrode obscure the yellow, red, and blue particles.

[0079] Figure 6E shows a transition similar to that shown in Figure 3E. Like Figure 3E, the transition in Figure 6E places a mixture of highly positive and white particles adjacent to a common electrode, and because these highly positive particles are now light-transmitting red particles, a red optical state (R) is produced rather than the blue optical state of Figure 3E.

[0080] Finally, Figure 6F shows a transition similar to that shown in Figure 3F. The initial state is one in which low-positive particles are placed adjacent to the common electrode, which in the medium in Figure 6F is a blue optical state (B). Therefore, the final state has a mixture of low-positive blue particles and high-negative yellow particles adjacent to the common electrode, and therefore a green optical state (G) is displayed.

[0081] Each of the waveforms described above with reference to Figures 5A-5F for producing the transitions of Figures 3A-3F, respectively, can also be used to produce the corresponding transitions of Figures 6A-6F, respectively.

[0082] Figures 7A-7F are schematic cross-sections similar to those of Figures 3A-3F, but illustrating various optical transitions that a five-particle display device of the present invention can undergo. The electrophoretic medium shown in Figures 7A-7F is generally similar to that shown in Figures 3A-3F, and thus comprises highly negatively charged yellow particles (Y), low-charged negative white particles (W), and low-charged positive red particles (R), as well as light-transmitting positively charged blue particles (B) having a higher zeta potential than the red particles. However, the electrophoretic medium shown in Figures 7A-7F also comprises highly charged positive green particles (G) having a higher zeta potential than the blue particles (B), so that the blue particles are merely moderately positive particles. The yellow, white, and green particles are all light-reflective.

[0083] The transition shown in FIG. 7A starts from a fully mixed state, which is created by applying a shake pulse, indicated by "(M)," as described below. This transition is similar to that shown in FIG. 3A, and involves applying alternating pulses of high positive potential difference (e.g., +15 V) and no potential difference (0 V) to the pixel electrode 22a for a long enough period of time, thereby driving the green (G), blue (B), and red (R) particles toward the common electrode (21) or view side, and the yellow (Y) and white (W) particles toward the pixel electrode 22a side. Because the green particles are more highly charged than the blue and red particles, they are placed directly adjacent to the common electrode (21) with the blue and red particles directly below them (as shown in FIG. 7A), thus causing the pixel to appear green; this is indicated by "(G)" in FIG. 7A; the blue, red, white, and yellow particles are all obscured by the reflective red particles and do not affect the displayed color.

[0084] The transition shown in Figure 7B is an exact analog of that shown in Figure 3B, in that when alternating pulses of a high negative potential difference (e.g., -15V) and no voltage (0V) are applied to the pixel electrode 22a of a pixel in the fully mixed state (M), the yellow and white particles are driven toward the common electrode, and the red, blue, and green particles are driven toward the pixel electrode. Because the yellow particles are more highly charged than the white particles, the reflective yellow particles are placed directly adjacent to the common electrode, thus causing the pixel to appear yellow, which is indicated by "(Y)" in Figure 7B; the white, red, blue, and green particles are all obscured by the reflective yellow particles and do not affect the displayed color.

[0085] The transition shown in Figure 7C starts from the yellow state (Y) shown in Figure 7B. When alternating pulses of a high negative potential difference (e.g., -15 V) and a low positive potential difference (e.g., +8 V) are applied to pixel electrode 22a for a sufficiently long period, with the low positive pulses much longer than the high negative pulses, red (R) particles are driven toward common electrode 21 and white (W) particles are driven toward pixel electrode 22a. The effect of the oscillating electric field is to cause the highly charged green and yellow particles and the moderately charged blue particles to repeatedly pass each other in the center of the thickness of the electrophoretic layer, and the strong electric attraction between these charged positive and negative particles tends to greatly slow down the movement of these particles and keep them in the center of the thickness of the electrophoretic layer. However, the electric field generated by the low positive pulse is sufficient to separate the low charged white and red particles, thereby allowing the low positive red particles (R) to move to the edge of the common electrode 21 and the low negative white particles to move to the pixel electrode 22a. As a result, the reflective red particles are placed directly adjacent to the common electrode, thus causing the pixel to appear red, which is indicated by "(R)" in Figure 7C; the white, yellow, blue, and green particles are all obscured by the reflective red particles and do not affect the displayed color.

[0086] The transition shown in Figure 7D starts from the green state (G) shown in Figure 7A. When alternating pulses of a high positive potential difference (e.g., +15 V) and a low negative potential difference (e.g., -8 V) are applied to the pixel electrode 22a for a sufficiently long period, with the low negative pulses being much longer than the high positive pulses, the red (R) particles are driven toward the pixel electrode 22a, and the white particles (W) are driven toward the common electrode 21. As with the transition shown in Figure 7C, the effect of the oscillating electric field is to cause the highly charged green and yellow particles and the moderately charged blue particles to remain together in the center of the thickness of the electrophoretic layer. However, the electric field generated by the low negative pulse is sufficient to separate the low-charged white and red particles, thereby allowing the low-positive red particles (R) to move to the edge of the pixel electrode 22a and the low-negative white particles to move toward the common electrode 21. As a result, the white particles are placed directly adjacent to the common electrode, thus causing the pixel to appear white, which is indicated by "(W)" in Figure 7D; the red, yellow, blue, and green particles are all obscured by the white particles and do not affect the displayed color.

[0087] The transition shown in Figure 7E begins with the white state (W) shown in Figure 7D. Alternating pulses of a high positive potential difference (e.g., +15 V) and a moderate negative potential difference (e.g., -10 V) are applied to pixel electrode 22a for a long enough period, with the negative pulses much longer than the positive pulses, causing the blue particles to "escape" from the green / yellow / blue particle aggregates and gradually migrate toward common electrode 21. The white particles are similarly migrated in the opposite direction, causing a mixture of blue and white particles to form adjacent to the common electrode, resulting in a blue color, denoted "(B)" in Figure 7E, visible through the viewing surface. The red particles migrate away from the pixel electrode. While Figure 7E illustrates red particles intermixed with yellow and green particles, whether such a mixture forms or whether the red particles remain in a separate layer adjacent to the pixel electrode is irrelevant to the displayed color, because the blue and white particles adjacent to the common electrode obscure the yellow, green, and red particles. The considerations made above with respect to FIG. 3E regarding blue saturation and the question of whether the blue and white particles are mixed or in separate layers also apply to the blue state illustrated in FIG. 7E.

[0088] Finally, the transition shown in Figure 7F begins from the green state (G) shown in Figure 7A. Application of an appropriate waveform (discussed below) causes the positively charged green, blue, and red particles to move away from the common electrode 21. However, because the green particles are more strongly charged than the blue and red particles, they move more quickly than the blue and red particles, thus leaving the blue and red particles adjacent to the common electrode, either in separate layers as illustrated in Figure 7F or as a mixed layer. In either case, a black state, designated "(K)" in Figure 7F, is produced, for reasons already discussed with reference to Figure 3A. The exact location of the green, yellow, and white particles does not affect the apparent color because the red and blue particles obscure them, but because both the white and yellow particles move away from the pixel electrode 22a with the yellow particles moving more quickly than the white, the most likely situation is that illustrated in FIG. 7F, with the green and yellow particles concentrated in the center of the electrophoretic layer and the white particles adjacent to the pixel electrode.

[0089] The transitions shown in Figures 7A and 7B can be produced using the waveforms shown in Figures 5A and 5B, respectively (it should be understood that some optimization of these waveforms may be required depending on the exact electrophoretic mobilities of the particles used, the charge thereon, and similar factors). Similarly, the overall transition from the mixed state (M) to the yellow state (Y) shown in Figure 7B, and from there to the red state (R) shown in Figure 7C, can be produced using the waveform shown in Figure 5C. Also, the overall transition from the mixed state (M) to the green state (G) shown in Figure 7A, and from there to the white state (W) shown in Figure 7D, can be produced using the waveform shown in Figure 5D.

[0090] The white / blue transition shown in Figure 7E may be produced using the waveform shown in Figure 8A. This waveform comprises a short, high, positive pulse of voltage VH1 of duration t25 followed by a period of zero voltage of duration t26 and a period of medium negative voltage VL1 of duration t27, where t27 is significantly greater than t25. This sequence is then repeated several times; although five repetitions are illustrated in Figure 8A, the number of repetitions can vary widely. It should be understood that to produce the blue state (B) starting from the mixed state (M), the waveform shown in Figure 8A should be applied after the waveform shown in Figure 5D.

[0091] The green / black transition shown in FIG. 7F can often be produced by a single short high-voltage negative pulse (or a short series of high-voltage negative pulses separated by periods of zero voltage), which is sufficient to move the green particles under the blue and red particles, thereby leaving the mixture of red and blue particles adjacent to the common electrode and producing the desired black color. Thus, using one or more high-voltage negative pulses following the waveform of FIG. 3A, black can be produced from the mixed state (M) shown in FIG. 7A. However, in some cases, depending on the precise electrophoretic mobilities of the green, blue, and red particles, the use of such one or more high-voltage negative pulses may tend to place the blue particles, on average, further away from the common electrode than the red particles, resulting in a color ranging from dark red to reddish. If this problem occurs, the green / black transition of FIG. 7F may also be produced using the waveform of FIG. 8B.

[0092] The waveform of FIG. 8B begins with a short, high-negative pulse of duration t28, followed by a period of zero voltage of duration t29, where t29 is greater than t28. This sequence of high-negative pulses and zero voltage is repeated, followed by a third high-negative pulse of duration t28; of course, a greater or lesser number of pulses may be used if desired. By comparing FIG. 8B with FIG. 5A, it can be seen that the effect of the three negative pulses partially reverses the drive of the green particles to the common electrode and the drive of the yellow particles to the pixel electrode, thereby placing both the green and yellow particles near the center of the electrophoretic layer, where they may aggregate as previously described. The second portion of the waveform shown in FIG. 8B comprises three medium-voltage positive pulses of duration t30, each separated by a period of zero voltage of duration t31, where t30 is greater than t28. The applied durations t28 and t30 and driving voltages are selected so that the impulses applied by the positive pulses are substantially equal in magnitude to those applied by the negative pulses, but opposite in polarity. While the positive pulses do not substantially change the positions of the yellow and green particles because the moderate voltages employed are insufficient to disrupt the green / yellow aggregates, these positive pulses substantially restore the red, blue, and white particles to the positions they occupied prior to application of the waveform of Figure 8B, thus producing the final particle positions shown at (K) in Figure 7F.

[0093] From the foregoing, it can be seen that the present invention can provide a four-particle electrophoretic medium capable of generating at least five, and in some cases six, useful colors using only four different types of particles. A five-particle electrophoretic medium of the present invention can generate at least six useful colors. It should be noted that in practice, the green color produced by (for example) the five-particle system shown in FIG. 7A is the natural color of the pigment particles, while the green color produced by the four-particle system shown in FIG. 6F can vary somewhat depending on the exact location of the blue and yellow particles, with the former being more consistent than the latter, which may be difficult to precisely maintain over the life of the display.

[0094] Electrophoretic media and devices of the present invention may utilize any of the particles, fluids, encapsulation materials, and electrophoretic device designs described in the prior art, for example, as described below. (a) Electrophoretic particles, fluids, and fluid additives (see U.S. Patent Nos. 7,002,728 and 7,679,814). (b) Capsules, binders, and encapsulation processes (see U.S. Patent Nos. 6,922,276 and 7,411,719). (c) Microcell structures, wall materials, and methods of forming the microcells (see U.S. Patent Nos. 7,072,095 and 9,279,906). (d) Methods of filling and sealing microcells (see, for example, U.S. Patent Nos. 7,144,942 and 7,715,088). (e) Films and subassemblies containing electro-optical materials (e.g., U.S. Patent Nos. 6,825,829, 6,982,178, 7,112,114, 7,158,282, 7,236,292, 7,443,571, 7,513,813, 7,561,324, 7, No. 636,191, No. 7,649,666, No. 7,728,811, No. 7,729,039, No. 7,791,782, No. 7,826,129 No. 7,839,564, No. 7,843,621, No. 7,843,624, No. 8,034,209, No. 8,068,272, No. 8,07 No. 7,381, No. 8,177,942, No. 8,390,301, No. 8,482,835, No. 8,786,929, No. 8,830,553, No. 8,854,721, No. 9,075,280, No. 9,238,340, No. 9,470,950, No. 9,554,495, No. 9,563, (See U.S. Patent Application Publication Nos. 2007 / 0237962, 2009 / 0168067, and 2011 / 0164301, as well as U.S. Patent Application Publication Nos. 2009 / 0237962, 2009 / 0168067, and 2011 / 0164301). (f) Backplanes, adhesive layers, and other auxiliary layers and methods used in displays (see, for example, US Pat. Nos. 7,116,318 and 7,535,624). (g) color formation and color control (e.g., U.S. Patent Nos. 6,017,584, 6,545,797, 6,664,944, 6,788,452, 6,864,875, 6,914,714, 6,972,893, 7,038,656, 7,038,670, 7,046,228, 7,052,571, 7,075,502, 7,167,155, 7,385,751, 7,492,505, 7,667,684, 7,684,108, 7,791,789, 7,800,813, No. 7,821,702, No. 7,839,564, No. 7,910,175, No. 7,952,790, No. 7,956,841, No. 7 ,982,941, No.8,040,594, No.8,054,526, No.8,098,418, No.8,159,636, No.8,21 No. 3,076, No. 8,363,299, No. 8,422,116, No. 8,441,714, No. 8,441,716, No. 8,466, No. 852, No. 8,503,063, No. 8,576,470, No. 8,576,475, No. 8,593,721, No. 8,605,354 No. 8,649,084, No. 8,670,174, No. 8,704,756, No. 8,717,664, No. 8,786,935, No. No. 8,797,634, No. 8,810,899, No. 8,830,559, No. 8,873,129, No. 8,902,153, No. 8, No. 902,491, No. 8,917,439, No. 8,964,282, No. 9,013,783, No. 9,116,412, No. 9,14 No. 6,439, No. 9,164,207, No. 9,170,467, No. 9,170,468, No. 9,182,646, No. 9,195,1 No. 11, No. 9,199,441, No. 9,268,191, No. 9,285,649, No. 9,293,511, No. 9,341,916 No. 9,360,733, No. 9,361,836, No. 9,383,623, No. 9,423,666, No. 9,436,056, No. No. 9,459,510, No. 9,513,527, No. 9,541,814, No. 9,552,780, No. 9,640,119, No. 9,6 No. 46,547, No. 9,671,668, No. 9,697,778, No. 9,726,959, No. 9,740,076, No. 9,759,No. 981, No. 9,761,181, No. 9,778,538, No. 9,779,670, No. 9,779,671, No. 9,812,073, No. 9,829,764 , No. 9,921,451, No. 9,922,603, No. 9,989,829, No. 10,032,419, No. 10,036,929, No. 10,036,931, No. No. 10,332,435, No. 10,339,876, No. 10,353,266, No. 10,366,647, No. 10,372,010, No. 10,380,931 , No. 10,380,955, No. 10,431,168, No. 10,444,592, No. 10,467,984, No. 10,475,399, No. 10,509,293 Nos. 10,514,583, and U.S. Patent Application Publication Nos. 2008 / 0043318, 2008 / 0048970, 2009 / 0225398, 2010 / 0156780, 2011 / 0043543, 2012 / 0326957, 2013 / 0242378, 2013 / 0278995, 2 (See Nos. 014 / 0055840, 2014 / 0078576, 2015 / 0103394, 2015 / 0118390, 2015 / 0124345, 2015 / 0268531, 2015 / 0301246, 2016 / 0026062, 2016 / 0048054, and 2016 / 0116818). (h) Methods for driving displays (see, for example, U.S. Patent Nos. 7,012,600 and 7,453,445). (i) Display applications (see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348).

[0095] Electrophoretic displays typically comprise a layer of electrophoretic material and at least two other layers disposed on opposite sides of the electrophoretic material, one of which is an electrode layer. In most such displays, both layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongated row electrodes and the other into elongated column electrodes extending perpendicular to the row electrodes, with pixels defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode, while the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display.

[0096] The manufacture of a three-layer electrophoretic display typically involves at least one lamination operation. For example, some of the aforementioned patents and applications describe a process for manufacturing an encapsulated electrophoretic display, in which an encapsulated electrophoretic medium comprising capsules in a binder is coated onto a flexible substrate comprising an indium tin oxide (ITO) or similar conductive coating on a plastic film (which serves as one electrode in the final display). The capsule / binder coating is then dried to form a coherent layer of electrophoretic medium firmly adhered to the substrate. Separately, a backplane is prepared containing an array of pixel electrodes and an appropriate arrangement of conductors for connecting the pixel electrodes to drive circuitry. To form the final display, the substrate with the capsule / binder layer thereon is laminated to the backplane using a lamination adhesive. In one preferred form of such a process, the backplane itself is flexible and is prepared by printing pixel electrodes and conductors on a plastic film or other flexible substrate. The obvious lamination technique for mass production of displays by this process is roll lamination using a lamination adhesive.

[0097] As discussed in the aforementioned U.S. Patent No. 6,982,178 (see paragraph 3, line 63 to paragraph 5, line 46), many of the components used in electrophoretic displays and the methods used to manufacture such displays are derived from the technology used in liquid crystal displays (LCDs). For example, electrophoretic displays utilize an active matrix backplane comprising an array of transistors or diodes and a corresponding array of pixel electrodes on a transparent substrate, and a "continuous" front electrode (in the sense of an electrode that extends across multiple pixels and typically the entire display), and these components can be essentially identical to those of LCDs. However, the methods used to assemble LCDs cannot be used with encapsulated electrophoretic displays. LCDs are typically assembled by forming the backplane and front electrode on separate glass substrates, then adhesively bonding these components together while leaving a small opening between them, placing the resulting assembly under vacuum, and immersing the assembly in a bath of liquid crystals, which flow through the opening between the backplane and front electrode. Finally, with the liquid crystal in place, the opening is sealed to provide the final display.

[0098] This LCD assembly process cannot be easily transferred to encapsulated electrophoretic displays. Because the electrophoretic material is typically solid (i.e., has a solid outer surface), it must be present between the backplane and the front electrode before these two components are bonded to one another. Furthermore, in contrast to liquid crystalline materials, which are simply placed between the front electrode and the backplane without being attached to either, the solid electro-optic medium typically needs to be bonded to both. In most cases, the solid electro-optic medium is formed on the front electrode, which is generally easier than forming the medium on a backplane containing circuitry. Therefore, the front electrode / electro-optic medium combination is then typically laminated to the backplane by coating the entire surface of the electro-optic medium with an adhesive and laminating under heat, pressure, and possibly vacuum. Thus, most prior art methods for the final lamination of solid-state electrophoretic displays are essentially batch methods in which the electro-optic medium, lamination adhesive, and backplane are (typically) directly combined prior to final assembly; it would be desirable to provide a method better suited to mass production.

[0099] The aforementioned U.S. Patent No. 6,982,178 describes a method for assembling solid electro-optic displays (including encapsulated electrophoretic displays) that are well adapted for mass production. Essentially, this patent describes a so-called "front plane laminate" ("FPL") that comprises, in order, a light-transmitting conductive layer, a layer of solid electro-optic medium in electrical contact with the conductive layer, an adhesive layer, and a release sheet. Typically, the light-transmitting conductive layer is carried on a light-transmitting substrate that is preferably flexible in the sense that the substrate can be manually wrapped around a 10-inch (254 mm) diameter drum (for example) without permanent deformation. The term "light-transmitting" is used in this patent and herein to mean that the layer so designated transmits sufficient light to allow an observer looking through the layer to observe a change in the display state of the electro-optic medium, typically viewed through the conductive layer and adjacent substrate (if present); if the electro-optic medium exhibits a change in reflectance at non-visible wavelengths, the term "light-transmitting" should, of course, be interpreted to refer to transmission of the relevant non-visible wavelengths. The substrate is typically a polymer film, usually having a thickness within the range of about 1 to about 25 mils (25 to 634 μm), preferably about 2 to about 10 mils (51 to 254 μm). The conductive layer may conveniently be a thin metal or metal oxide layer, e.g., aluminum or ITO, or a conductive polymer. Aluminum or ITO coated poly(ethylene terephthalate) (PET) films are commercially available, for example as "aluminized Mylar" ("Mylar" is a registered trademark) from EI du Pont de Nemours & Company (Wilmington, Del.), and such commercially available materials can be used with good results in front plane laminates.

[0100] Assembly of an electro-optic display using such a front plane laminate may be effected by removing the release sheet from the front plane laminate and contacting the backplane and adhesive layer under conditions effective to adhere the adhesive layer to the backplane, thereby affixing the adhesive layer, layer of electro-optic medium, and conductive layer to the backplane. This process is well adapted for mass production, typically using roll / roll coating techniques, because the front plane laminate can be mass-produced and then cut into pieces of any size required for use with a particular backplane.

[0101] U.S. Patent No. 7,561,324 describes a so-called "dual release sheet," which is essentially a simplified version of the front plane laminate of the aforementioned U.S. Patent No. 6,982,178. One form of the double release sheet comprises a layer of solid electrophoretic medium sandwiched between two adhesive layers, one or both of which are covered by a release sheet. Another form of the double release sheet comprises a layer of solid electrophoretic medium sandwiched between two release sheets. Both forms of the double release film are generally intended for use in processes similar to the process for assembling electro-optic displays from the front plane laminate already described, but involving two separate laminations; typically, in a first lamination, the double release sheet is laminated to a front electrode to form a front subassembly, and then, in a second lamination, the front subassembly is laminated to a backplane to form the final display, although the order of these two laminations can be reversed if desired.

[0102] U.S. Patent No. 7,839,564 describes a so-called "inverted front plane laminate," a variation of the front plane laminate described in the aforementioned U.S. Patent No. 6,982,178. This inverted front plane laminate comprises, in order, at least one of a light-transmitting protective layer and a light-transmitting conductive layer, an adhesive layer, a layer of solid electrophoretic medium, and a release sheet. This inverted front plane laminate is used to form electro-optic displays having a layer of laminating adhesive between the electrophoretic layer and the front electrode or front substrate, with a second, typically thin, layer of adhesive sometimes being present between the electrophoretic layer and the backplane. Such electro-optic displays can combine good resolution with good low-temperature performance.

Claims

**Claim 1**: A method of driving at least a portion of an electrophoretic display device to a process black color state, wherein the electrophoretic display device comprises: (i) a layer of an electrophoretic medium having a first viewing side and an opposite second side, the electrophoretic medium comprising a non-polar fluid and first, second, third, and fourth types of charged electrophoretic pigment particles dispersed in the non-polar fluid, the first, second, third, and fourth types of particles having different colors from each other, the first and third types of particles having a charge of one polarity, the second and fourth types of particles having a charge of the opposite polarity, the first type of particles having a greater zeta potential or electrophoretic mobility than the third type of particles, the second type of particles having a greater zeta potential or electrophoretic mobility than the fourth type of particles, the first type of particles being partially light-transmissive, the third type of particles being light-reflective, the first and third types of particles being different colors of red and blue respectively, a portion of the incident light being reflected by the first type of particles that are partially light-transmissive, and another portion of the incident light passing through the first type of particles that are partially light-transmissive; a layer of electrophoretic medium; (ii) a front electrode layer superimposed on the first viewing side of the electrophoretic medium; (iii) a rear electrode layer superimposed on the opposite second side of the electrophoretic medium; and (iv) a control unit coupled to the front and rear electrode layers for changing the optical state of the electrophoretic display device by applying a selected voltage pulse sequence between the front and rear electrode layers, the method comprising: **(a)** promoting mixing of the first, second, third, and fourth types of particles dispersed in the non-polar fluid by applying a shake voltage pulse sequence between the front and rear electrode layers to form a mixed optical state; (b) After step (a), by applying a driving voltage pulse sequence between the front and rear electrode layers, driving the first type of particles to form a layer adjacent to the first view side of the electrophoretic medium, driving the third type of particles to form a layer adjacent to the layer of the first type of particles on the side opposite to the layer of the first type of particles on the first view side of the electrophoretic medium, and driving the second and fourth types of particles between the third type of particles and the opposite second side of the electrophoretic medium, so that visible light incident on the first view side of the electrophoretic medium is substantially absorbed by the first and third types of particles to produce a process black color on the first view side. A method comprising. **Claim 2**: The method according to claim 1, wherein one of the second and fourth types of particles is white and the other of the second and fourth types of particles is yellow. **Claim 3**: The first type of particles that are partially light-transmissive have a contrast ratio of 0.5 or less, and the contrast ratio is defined as the ratio of the apparent reflectance Rb of the same sample with a black material having a defined reflectance as the background to the apparent reflectance Rw of the sample with a white material having a defined reflectance as the background, and the sample comprises the first type of particles that are partially light-transmissive. The method according to claim 1. **Claim 4**: The first type of particles that are partially light-transmissive have a contrast ratio of 0.3 or less, and the contrast ratio is defined as the ratio of the apparent reflectance Rb of the same sample with a black material having a defined reflectance as the background to the apparent reflectance Rw of the sample with a white material having a defined reflectance as the background, and the sample comprises the first type of particles that are partially light-transmissive. The method according to claim 1. **Claim 5**: The method according to claim 1, wherein the first, second, third, and fourth types of particles are red, yellow, blue, and white, respectively. **Claim 6**: The method according to claim 1, wherein the first, second, third, and fourth types of particles are blue, yellow, red, and white, respectively. **Claim 7**: The method according to claim 1, wherein one polarity is positive and the opposite polarity is negative.

8. The method according to claim 1, wherein the shake voltage pulse sequence comprises pairs of alternating drive pulses of opposite polarity.

9. The method according to claim 8, wherein each pair of drive pulses of opposite polarity comprises a +15V pulse and a -15V pulse.

10. The method according to claim 1, wherein the drive voltage pulse sequence comprises a plurality of voltage pulses of one polarity separated by 0V pauses.

11. The method according to claim 10, wherein the plurality of voltage pulses comprises +15V pulses.

12. The method according to claim 1, further comprising reducing or eliminating the overall impulse of the drive voltage pulse sequence by applying a DC balance waveform between the front and rear electrode layers.

13. An electrophoretic display device, the electrophoretic display device comprising a layer of electrophoretic medium having a first viewing side and an opposite second side, the electrophoretic medium comprising a non-polar fluid and first, second, third, and fourth types of charged electrophoretic pigment particles dispersed in the non-polar fluid, the first, second, third, and fourth types of particles having different colors from each other, the first and third types of particles having a charge of one polarity, the second and fourth types of particles having a charge of the opposite polarity, the first type of particles having a greater zeta potential or electrophoretic mobility than the third type of particles, the second type of particles having a greater zeta potential or electrophoretic mobility than the fourth type of particles, the first type of particles being partially light-transmissive, the third type of particles being light-reflective, a portion of the incident light being reflected by the first type of particles that are partially light-transmissive, another portion of the incident light passing through the first type of particles that are partially light-transmissive, the first and third types of particles each being a different color of red and blue, respectively, a layer of electrophoretic medium; a front electrode layer superimposed on the first viewing side of the electrophoretic medium; a rear electrode layer superimposed on the opposite second side of the electrophoretic medium; a control unit coupled to the front and rear electrode layers configured to change the optical state of the electrophoretic display device by applying a selected voltage pulse sequence between the front and rear electrode layers comprising (a) a shake voltage pulse sequence for promoting mixing of the first, second, third, and fourth types of particles dispersed in the nonpolar fluid to form a mixed optical state; (b) a drive voltage pulse sequence applied after the shake voltage pulse sequence for driving the first type of particles to form a layer adjacent to the first view side of the electrophoretic medium, driving the third type of particles to form a layer adjacent to the layer of the first type of particles on the opposite side of the layer of the first type of particles from the first view side of the electrophoretic medium, and driving the second and fourth types of particles between the third type of particles and the opposite second side of the electrophoretic medium so that visible light incident on the first view side of the electrophoretic medium is substantially absorbed by the first and third types of particles to produce a process black color on the first view side An electrophoretic display device comprising.

14. The electrophoretic display device according to claim 13, wherein one of the second and fourth types of particles is white and the other of the second and fourth types of particles is yellow.

15. The first type of particles that are partially light transmissive have a contrast ratio of 0.5 or less, and the contrast ratio is defined as the ratio of the visual reflectance Rb of a black material having a defined reflectance to the visual reflectance Rw of the same sample with a white material having a defined reflectance as the background, and the sample comprises the first type of particles that are partially light transmissive. The electrophoretic display device according to claim 13.

16. The electrophoretic display device according to claim 13, wherein the first, second, third, and fourth types of particles are each red, yellow, blue, and white, or each blue, yellow, red, and white.

17. The electrophoretic display device according to claim 13, wherein the shake voltage pulse sequence comprises repeating pairs of opposite drive pulses, and each pair of opposite drive pulses comprises a +15V pulse and a -15V pulse.

18. The electrophoretic display device according to claim 13, wherein the drive voltage pulse sequence includes a plurality of voltage pulses having one of the polarities separated by a 0V pause.

19. The electrophoretic display device according to claim 18, wherein the plurality of voltage pulses include +15V pulses.

20. The electrophoretic display device according to claim 13, wherein the control unit is further configured to reduce or eliminate the overall impulse of the drive voltage pulse sequence by applying a DC balance waveform between the front and rear electrode layers.

Images