Deaggregation driving sequence for four-particle electrophoretic display

Abstract

The present invention provides an improved drive method for four-particle electrophoretic displays which improves the performance of such displays when the displays are deployed in low temperature environments and when the displays are required to be updated when positioned vertically (i.e., the drive electric field is substantially perpendicular to the direction of the Earth's gravity). Methods are provided for displaying each of the colors at each pixel as desired, with minimal interference (contamination) from the other particles.

Description

[Technical field]

[0001] (Reference to Related Application) This application claims priority to U.S. Provisional Patent Application No. 63 / 181,514, filed April 29, 2021. The entire contents of all patents and publications mentioned below are incorporated herein by reference in their entirety.

[0002] FIELD OF THEINVENTION The present invention is directed to an improved driving method for a color electrophoretic display device in which each pixel is capable of displaying at least four high quality color states. [Background technology]

[0003] Electrophoretic displays (electronic paper, e-paper, etc.), such as those commercially available from E Ink Holdings (Hsinchu, Taiwan), have the advantage that they consume very little power, are lightweight, durable, and environmentally friendly. The technology is being incorporated into electronic book readers (e.g., electronic books, e-books) and other display environments (e.g., phones, tablets, electronic shelf tags, hospital signs, road signs, public transport timetables). The combination of low power consumption and readability in sunlight has enabled rapid growth in so-called "non-plug-and-play" operation, where electronic signage systems are simply attached to a surface, interfaced with existing communications networks, and provide periodic updates of information or images. Because the displays are powered using batteries or solar collectors, there is no need for wiring or even the need to have a plug dangling from the display.

[0004] A variety of color options for electrophoretic displays have recently become available, ranging from advanced color filter arrays to complex subtractive pigment sets to high fidelity color options that rely on multiple sets of reflective color particles. The modern systems have been widely accepted for commercial signage such as grocery stores, clothing stores, and electronics retailers. In particular, three-color electrophoretic displays of the type described in U.S. Patent Application No. 2020 / 0379312 have been rapidly adopted for outdoor and indoor signage, and for room temperature and refrigerated food segments. U.S. Patent Application No. 2020 / 0379312 is incorporated herein by reference in its entirety.

[0005] While the three-particle electrophoretic displays of U.S. Patent Application No. 2020 / 0379312 and U.S. Patent Nos. 8,717,664, 10,162,242, and 10,339,876 have been deployed in millions of individual displays worldwide, there is a strong demand to add a fourth particle with a fourth color, such as those described in U.S. Patent Nos. 9,285,649, 9,513,527, and 9,812,073. Such four-color displays are not currently commercially available. While it is hoped that such four-particle electrophoretic displays can be "dropped" into the same retail environment, initial testing suggests that four-particle electrophoretic systems of the above type have unique quirks that differ from three-particle systems, not only depending on the temperature of operation, but also depending on the display orientation, i.e., horizontal (charged pigments are driven up and down along the Earth's gravitational field) versus vertical (charged pigments are driven back and forth across the Earth's gravitational field). One surprising effect observed is that when such four-particle electrophoretic displays are used in cold environments (e.g., refrigerated or frozen food sections), the particles aggregate in an unexpected way, resulting in black pixels with intermittent contamination by other colors (e.g., white, yellow, and red). Interestingly, this phenomenon cannot be fully reproduced when the display is driven horizontally at low temperatures. Clearly, there is a need for improved driving sequences to de-aggregate the pigments prior to addressing the need to achieve desired color performance and to meet customer demand for pure and vibrant colors in electronic digital signage. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] US Patent Application Publication No. 2020 / 0379312 [Patent Document 2] U.S. Pat. No. 8,717,664 Summary of the Invention [Means for solving the problem]

[0007] The driving method disclosed herein overcomes the drawbacks described above to accommodate four-particle electrophoretic displays at lower temperatures in a typical environment, i.e., where the display panel is vertically oriented. In a first aspect, there is provided a method of driving a display layer, the display layer being disposed between a viewing surface including a light-transmissive electrode and a second surface on an opposite side of the display layer from the viewing surface, the second surface including the driving electrode, the display layer including 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 optical properties respectively different from one another, the first and third types of particles having charges of a first polarity and the second and fourth types of particles having charges of a second polarity opposite to the first polarity, the first type of particles and the third type of particles not having the same charge magnitude and the second type of particles and the fourth type of particles not having the same charge magnitude, the method comprising, in order, the steps of: (i) applying a first electric field having a large magnitude and a first or second polarity to drive a first or second type of particle towards the viewing surface, thereby causing the display layer to display a first or second optical property at the viewing surface; (ii) applying a second electric field having a large magnitude and a negative polarity; (iii) applying a shaking pulse comprising at least four periods of a large-scale electric field at a first polarity and at least four periods of a large-scale electric field at a second polarity; (iv) applying a second electric field, large scale and having the same polarity as in step (i), again driving the first or second type of particles towards the viewing surface, thereby causing the display layer to again display the first or second optical property at the viewing surface; (v) applying a third electric field, on a smaller scale and having an opposite polarity to step (iv), to drive the fourth or third type of particles towards the viewing surface, thereby causing the display layer to display a fourth or third optical property at the viewing surface; Includes.

[0008] In some embodiments, the first electric field is applied for a longer time than the second electric field, and the third electric field is applied for a longer time than the second electric field. In some embodiments, each of steps (i)-(v) is repeated. In some embodiments, the magnitude of the third electric field is less than 50 percent of the magnitude of the second electric field, and in some embodiments, only the fourth or third optical property is displayed after completion of step (v). In some embodiments, the first electric field is applied for longer than 400 ms. In some embodiments, the second electric field is applied for longer than 100 ms. In some embodiments, the shaking pulse is applied for less than 80 ms. In some embodiments, the shaking pulse is applied for about 40 ms. In some embodiments, a field-free rest period is performed after step (iii), and steps (i)-(iii) are repeated twice before completing steps (iv) and (v). In some embodiments, each electric field is applied in a direction that is substantially perpendicular to the direction of the Earth's gravity.

[0009] In a second aspect, the invention provides a method of driving a display layer, the display layer being disposed between a viewing surface including a light-transmissive electrode and a second surface on an opposite side of the display layer from the viewing surface, the second surface including a driving electrode, the display layer including 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 optical properties respectively different from one another, the first and third types of particles having a charge of a first polarity and the second and fourth types of particles having a charge of a second polarity opposite to the first polarity, the first type of particles and the third type of particles not having the same charge magnitude, and the second and fourth type of particles not having the same charge magnitude, The method comprises the following steps, in order: (i) applying a first electric field having a large magnitude and a first or second polarity to drive a first or second type of particle towards the viewing surface, thereby causing the display layer to display a first or second optical property at the viewing surface; (ii) applying a second electric field having a large magnitude and a negative polarity; (iii) applying a shaking pulse comprising at least four periods of a large-scale electric field at a first polarity and at least four periods of a large-scale electric field at a second polarity; (iv) applying a third electric field having a large scale and opposite polarity to that of step (i) to drive the second or first type of particles towards the viewing surface, thereby causing the display layer to display a second or first optical property at the viewing surface. The present invention provides a method comprising:

[0010] In some embodiments, the first electric field is applied for the same time as the third electric field. In some embodiments, each of steps (i)-(iv) is repeated. In some embodiments, only the second or first optical property is displayed after completion of step (iv). In some embodiments, the first electric field is applied for greater than 400 ms. In some embodiments, the second electric field is applied for greater than 100 ms. In some embodiments, each period of the shaking pulse is applied for less than 80 ms. In some embodiments, each period of the shaking pulse is applied for about 40 ms. In some embodiments, each electric field is applied in a direction that is substantially perpendicular to the direction of the Earth's gravity. The present invention provides, for example, the following items. (Item 1) 1. A method of driving a display layer, the display layer being disposed between a viewing surface including a light-transmissive electrode and a second surface on an opposite side of the display layer from the viewing surface, the second surface including a driving electrode, the display layer including 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 type particles have first, second, third, and fourth optical properties, respectively, different from one another; the first and third type particles have charges of a first polarity, the second and fourth type particles have charges of a second polarity opposite to the first polarity, the first and third type particles do not have the same charge magnitude, and the second and fourth type particles do not have the same charge magnitude; The method comprises: (vi) applying a first electric field having a large magnitude and the first or second polarity to drive the first or second type of particles towards the viewing surface, thereby causing the display layer to display the first or second optical property at the viewing surface; (vii) applying a second electric field having a large magnitude and a negative polarity; (viii) applying a shaking pulse comprising at least four periods of the large-scale electric field at the first polarity and at least four periods of the large-scale electric field at the second polarity; (ix) applying a second electric field at the large scale and having the same polarity as in step (i) to again drive the first or second type of particles towards the viewing surface, thereby again causing the display layer to display the first or second optical property at the viewing surface; (x) applying a third electric field on a smaller scale and having an opposite polarity to step (iv) to drive the fourth or third type of particles towards the viewing surface, thereby causing the display layer to display the fourth or third optical property at the viewing surface. The method includes, in order: (Item 2) 2. The method of claim 1, wherein the first electric field is applied for a longer time than the second electric field and the third electric field is applied for a longer time than the second electric field. (Item 3) 2. The method of claim 1, wherein each of steps (i)-(v) is repeated. (Item 4) 2. The method of claim 1, wherein the magnitude of the third electric field is less than 50 percent of the magnitude of the second electric field. (Item 5) Item 1. The method of item 1, wherein only the fourth or the third optical property is displayed after completion of step (v). (Item 6) 2. The method of claim 1, wherein the first electric field is applied for greater than 400 ms. (Item 7) 2. The method of claim 1, wherein the second electric field is applied for greater than 100 ms. (Item 8) 2. The method of claim 1, wherein each period of the shaking pulse is applied for less than 80 ms. (Item 9) 9. The method of claim 8, wherein each period of the shaking pulse is applied for about 40 ms. (Item 10) 2. The method of claim 1, wherein a field-free rest period is performed after step (iii), and steps (i)-(iii) are repeated twice before completing steps (iv) and (v). (Item 11) 2. The method of claim 1, wherein each electric field is applied in a direction that is substantially perpendicular to the direction of the Earth's gravity. (Item 12) 1. A method of driving a display layer, the display layer being disposed between a viewing surface including a light-transmissive electrode and a second surface on an opposite side of the display layer from the viewing surface, the second surface including a driving electrode, the display layer including 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 type particles have first, second, third, and fourth optical properties that are different from one another, the first and third type particles have charges of a first polarity, the second and fourth type particles have charges of a second polarity opposite to the first polarity, the first and third type particles do not have the same charge magnitude, and the second and fourth type particles do not have the same charge magnitude; The method comprises: (v) applying a first electric field having a large magnitude and the first or second polarity to drive the first or second type of particles towards the viewing surface, thereby causing the display layer to display the first or second optical property at the viewing surface; (vi) applying the second electric field having a large magnitude and a negative polarity; (vii) applying a shaking pulse comprising at least four periods of the large-scale electric field at the first polarity and at least four periods of the large-scale electric field at the second polarity; (viii) applying a third electric field having an opposite polarity to that of step (i) at the large scale to drive the second or first type of particles towards the viewing surface, thereby causing the display layer to display the second or first optical property at the viewing surface. The method includes, in order. (Item 13) Item 13. The method of item 12, wherein the first electric field is applied for the same time as the third electric field. (Item 14) Item 13. The method of item 12, wherein each of steps (i)-(iv) is repeated. (Item 15) Item 13. The method of item 12, wherein only the second or the first optical property is displayed after completion of step (iv). (Item 16) Item 13. The method of item 12, wherein the first electric field is applied for greater than 400 ms. (Item 17) 13. The method of claim 12, wherein the second electric field is applied for greater than 100 ms. (Item 18) Item 13. The method of item 12, wherein each period of the shaking pulse is applied for less than 80 ms. (Item 19) 20. The method of claim 18, wherein each period of the shaking pulse is applied for about 40 ms. (Item 20) 13. The method of claim 12, wherein each electric field is applied in a direction that is substantially perpendicular to the direction of gravity of the Earth. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic cross-section through a display layer that contains four different types of particles and is capable of displaying four different color states.

[0012] [Figure 2-1]2A-2F are schematic cross-sections similar to those of FIG. 1, but illustrating the change in particle position as a result of applying drive sequences of particular charges and polarities. [Figure 2-2] 2A-2F are schematic cross-sections similar to those of FIG. 1, but illustrating the change in particle position as a result of applying drive sequences of particular charges and polarities. [Figure 2-3] 2A-2F are schematic cross-sections similar to those of FIG. 1, but illustrating the change in particle position as a result of applying drive sequences of particular charges and polarities.

[0013] [Diagram 3] 3 shows a generic "oscillating" waveform that may be used in the drive method of the present invention. When used with an active matrix display, the duration of each cycle (+HV to -HV) is at least twice the frame time for that display. However, there are no physical limitations to driving the electrophoretic medium, and the duration of each cycle may be shorter or longer than is typical with active matrix displays.

[0014] [Figure 4] Figure 4A illustrates the horizontal driving of a display of the present invention, and Figure 4B illustrates the vertical driving of a display of the present invention.

[0015] [Figure 5A] FIG. 5A illustrates a driving sequence (waveform) that may be used to cause the display layer shown in FIG. 1 to effect the transition from FIG. 2C to FIG. 2D, thereby displaying the color red at the viewing surface.

[0016] [Figure 5B] FIG. 5B illustrates an improved driving sequence (waveform) of the present invention which provides a transition from FIG. 2C to FIG. 2D, thereby providing better particle separation when displaying the color red at the viewing surface.

[0017] [Figure 6A] FIG. 6A illustrates a driving sequence (waveform) that may be used to cause the display layer shown in FIG. 1 to effect the transition from FIG. 2E to FIG. 2F, thereby displaying the color white at the viewing surface.

[0018] [Figure 6B] FIG. 6B illustrates an improved driving sequence (waveform) of the present invention which provides a transition from FIG. 2E to FIG. 2F, thereby providing better particle separation when displaying the color white at the viewing surface.

[0019] [Figure 7A] FIG. 7A illustrates a driving sequence (waveform) that may be used to cause the display layer shown in FIG. 1 to effect a transition from FIG. 2A to FIG. 2B, thereby displaying the color black at the viewing surface.

[0020] [Figure 7B] FIG. 7B illustrates an improved driving sequence (waveform) of the present invention which provides a transition from FIG. 2A to FIG. 2B, thereby providing better particle separation when displaying the color black on a viewing surface.

[0021] [Figure 8A] FIG. 8A illustrates a driving sequence (waveform) that may be used to cause the display layer shown in FIG. 1 to effect a transition from FIG. 2B to FIG. 2A, thereby displaying the color yellow at the viewing surface.

[0022] [Figure 8B] FIG. 8B illustrates an improved driving sequence (waveform) of the present invention which provides a transition from FIG. 2B to FIG. 2A, thereby providing better particle separation when displaying the color yellow on a viewing surface.

[0023] [Figure 9]9 shows the test protocol with fast driving in horizontal orientation to evaluate the display panel performance, occasional driving in vertical orientation to evaluate the likely commercial use, and final evaluation of specific test points using an electro-optical test bench. For the avoidance of doubt, K=black, W=white, Y=yellow, and R=red. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] As already mentioned, the present invention relates to a driving method for a display layer comprising an electrophoretic medium comprising first, second, third and fourth types of particles, all dispersed in a fluid, all having different optical properties. These optical properties are typically colors perceptible to the human eye, but can be other optical properties (such as light transmittance, reflectance, luminescence or, in the case of displays intended for machine reading, pseudocolor in the sense of changes in reflectance of electromagnetic wavelengths outside the visible range). The present invention broadly encompasses particles of any color, as long as the types of particles are visually distinguishable.

[0025] The four types of particles present in the electrophoretic medium may be considered to comprise two pairs of oppositely charged particles. The first pair (first and second type particles) consists of a first type of positive particle and a first type of negative particle, and similarly, the second pair (third and fourth type particles) consists of a second type of positive particle and a second type of negative particle. Of the two pairs of oppositely charged particles, one pair (first and second particles) carries a stronger charge than the other pair (third and fourth particles). Thus, the four types of particles may also be referred to as high positive particles, high negative particles, low positive particles, and low negative particles.

[0026] The term "charge potential" may be used synonymously with "zeta potential" or electrophoretic mobility in the context of this application. The charge polarity and charge potential level of the particles may be varied (and / or measured in terms of zeta potential) by the methods described in US Patent Application Publication No. 2014 / 0011913. In one embodiment, the zeta potential is determined by a Colloidal Dynamics AcoustoSizer IIM, ESA EN#Attn flow-through cell (K:127) with a CSPU-100 signal processing unit. The 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 (all at the test temperature (25°C)) are entered prior to the test. The pigment sample is dispersed in the solvent (which is usually a hydrocarbon fluid with less than 12 carbon atoms) and diluted to be 5-10% by weight. The samples were prepared using a charge control agent (Solsperse available from Lubrizol Corporation, a subsidiary of Berkshire Hathaway company). TM 17000) 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 a flow-through cell for determination of the zeta potential. Methods and apparatus for the measurement of electrophoretic mobility are well known to those skilled in the art of electrophoretic displays.

[0027] As an example shown in Figure 1, a first black particle (K) and a second yellow particle (Y) are a first pair of oppositely charged particles, in which the black particle is a high positive particle and the yellow particle is a high negative particle. A third red particle (R) and a fourth white particle (W) are a second pair of oppositely charged particles, in which the red particle is a low positive particle and the white particle is a low negative particle.

[0028] In another example, not shown, the black particles can be high positive particles, the yellow particles can be low positive particles, the white particles can be low negative particles, and the red particles can be high negative particles. In another example, not shown, the black particles can be high positive particles, the yellow particles can be low positive particles, the white particles can be high negative particles, and the red particles can be low negative particles. In another example, not shown, the black particles can be high positive particles, the red particles can be low positive particles, the white particles can be high negative particles, and the yellow particles can be high negative particles. Of course, any particular color can be replaced with another color depending on the requirements of the application. For example, if a specific combination of black, white, green, and red particles is desired, the high negative yellow particles shown in FIG. 1 can be replaced with high negative green particles.

[0029] In addition, the color states of the four types of particles can be intentionally mixed. For example, yellow pigments often have a greenish hue in nature, and if a better yellow state is desired, yellow and red particles can be used, in which case both types of particles carry the same charge polarity, with the yellow particles being more highly charged than the red particles. As a result, in the yellow state, a small amount of red particles will be mixed with the greenish yellow particles, making the yellow state have better color purity.

[0030] The particles are preferably opaque in the sense that they should be light reflective, not light transmissive. It is clear to those skilled in color science that if the particles are light transmissive, some of the color states appearing in the following description of specific embodiments of the present invention will be significantly distorted or not obtained. The white particles are, of course, light scattering, not reflective, but care should be taken to ensure that not an excessive amount of light passes through the layer of white particles. For example, in the white state shown in FIG. 2F, discussed below, the layer of white particles allows a significant amount of light to pass through and be reflected from the black and yellow particles behind it, and the brightness of the white state may be significantly reduced.

[0031] In some embodiments, the particles are primary particles without a polymer shell. Alternatively, each particle may comprise an insoluble core with a polymer shell. The core may be either an organic or inorganic pigment, and it may be a single core particle or an aggregate of multiple core particles. The particles may also be hollow particles.

[0032] The white particles may be formed from inorganic pigments such as TiO2, ZrO2, ZnO, Al2O3, Sb2O3, BaSO4, PbSO4, etc. The black particles may be formed from Cl pigment black 26 or 28, etc. (e.g., manganese ferrite black spinel or copper chromite black spinel), or carbon black. The other colored particles (which are non-white and non-black) may be red, green, blue, magenta, cyan, yellow, or any other desired color, and may be formed from, for example, 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 Handbooks "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 L 3630, Cinquasia Red L 4100 HD, and Irgazin Red L 3660 HD, Sun Chemical phthalocyanine blue, phthalocyanine green, diarylide yellow, or diarylide AAOT yellow. The colored particles can also be inorganic pigments such as red, green, blue, and yellow. Examples can include, but are not limited to, CI pigment blue 28, CI pigment green 50, and CI pigment yellow 227.

[0033] The fluid in which the four types of particles are dispersed can be 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 isoparaffin, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oil, 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, and 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), poly(chlorotrifluoroethylene) from Halocarbon Product Corp. (Riverside, MO), and the like. Examples of suitable silicone oils include halocarbon oils manufactured by Edge, NJ; perfluoropolyalkyl ethers such as Galden manufactured by Ausimont or Krytox Oils; Greases K-Fluid Series manufactured by DuPont (Delaware); and polydimethylsiloxane-based silicone oil (DC-200) manufactured by Dow-corning.

[0034] The proportions of different types of particles in the fluid may vary: for example, one type of particle may occupy 0.1%-10%, preferably 0.5%-5%, by volume of the electrophoretic fluid, another type of particle may occupy 1%-50%, preferably 5%-20%, by volume of the fluid, and each of the remaining types of particle may occupy 2%-20%, preferably 4%-10%, by volume of the fluid.

[0035] Various types of particles may have different particle sizes. For example, smaller particles may have sizes ranging from about 50 nm to about 800 nm. Larger particles may have sizes that are about 2 to about 50 times, more preferably about 2 to about 10 times, the size of the smaller particles.

[0036] An electrophoretic display typically comprises a layer of electrophoretic material and at least two other layers disposed on either side of the electrophoretic material, one of these two layers being 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 at right angles to the row electrodes, the pixels being 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 and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electrophoretic display, intended for use with a display and a separate stylus, print head, or similar movable electrode, only one of the layers adjacent to the electrophoretic layer comprises an electrode; the layer opposite the electrophoretic layer is typically a protective layer intended to prevent the movable electrode from damaging the electrophoretic layer.

[0037] Numerous patents and applications assigned to or in the name of Massachusetts Institute of Technology (MIT), E Ink Corporation, E Ink (California), LLC, E Ink Holdings, Prime View International, and affiliates, describe various technologies used in encapsulated electrophoretic media, microcell electrophoretic media, and other electro-optic media. Encapsulated electrophoretic media comprises a multiplicity of small capsules, each of which itself consists of an internal phase containing electrophoretically mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held in a polymeric binder to form a coherent layer positioned between two electrodes. In microcell electrophoretic displays, the charged particles and fluid are not encapsulated in microcapsules, but instead are held in a plurality of cavities formed in a carrier medium, typically a polymeric film. Technologies described in these patents and applications include the following: (a) electrophoretic particles, fluids, and fluid additives (see, e.g., U.S. Pat. Nos. 7,002,728 and 7,679,814); (b) capsules, binders, and encapsulation processes (see, e.g., U.S. Pat. Nos. 6,922,276 and 7,411,719); (c) microcell structures, wall materials, and methods of forming the microcells (see, e.g., U.S. Pat. Nos. 7,072,095 and 9,279,906); (d) methods for filling and sealing the microcells (see, e.g., U.S. Pat. Nos. 7,144,942 and 7,715,088); (e) films and subassemblies containing electro-optical materials (see, e.g., U.S. Pat. Nos. 6,982,178 and 7,839,564); (f) Backplanes, adhesive layers and other auxiliary layers, and methods used in displays (see, e.g., U.S. Pat. Nos. 7,116,318 and 7,535,624) (g) color formation and color control (see, e.g., U.S. Pat. Nos. 7,075,502 and 7,839,564); (h) methods for driving displays (see, e.g., U.S. Pat. Nos. 7,012,600 and 7,453,445); (i) display applications (see, e.g., U.S. Pat. Nos. 7,312,784 and 8,009,348); (j) non-electrophoretic displays, as described in U.S. Pat. No. 6,241,921 and U.S. Patent Application Publication No. 2015 / 0277160, and non-display applications of encapsulation and microcell technology (see, e.g., U.S. Patent Application Publication Nos. 2015 / 0005720 and 2016 / 0012710);

[0038] Many of the aforementioned patents and applications recognize that the walls surrounding the separate microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thus producing so-called "polymer-dispersed electrophoretic displays," in which the electrophoretic medium comprises a plurality of separate droplets of electrophoretic fluid and a continuous phase of polymeric material, and that the separate droplets of electrophoretic fluid in such polymer-dispersed electrophoretic displays may be considered capsules or microcapsules even though a separate capsule membrane is not associated with each individual droplet (see, for example, aforementioned 2002 / 0131147). Thus, for purposes of this application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

[0039] A related type of electrophoretic display is the so-called "microcell electrophoretic display," in which the charged particles and suspending fluid are not encapsulated in microcapsules, but instead are held within a plurality of cavities formed in a carrier medium, typically a polymer film (see, e.g., International Application Publication No. WO 02 / 01281 and U.S. Pat. No. 6,788,449).

[0040] Preferred embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

[0041] Figure 1 is a schematic cross-section through a display layer that can be driven by the method of the present invention. The display layer has two major surfaces, a first viewing surface 13 (upper surface as illustrated in Figure 1) where the user views the display, and a second surface 14 on the opposite side of the display layer from the first surface 13. The display layer comprises a fluid and an electrophoretic medium comprising first black particles (K) with a high positive charge, second yellow particles (Y) with a high negative charge, third red particles (R) with a low positive charge, and fourth white particles (W) with a low negative charge. The display layer comprises electrodes as known in the art to apply an electric field across the display layer, i.e., it comprises two electrode layers, the first of which is a light-transmitting or transparent common electrode layer 11 that extends across the entire viewing surface 13 of the display layer. This electrode layer 11 can be formed from indium tin oxide (ITO) or a similar light-transmitting conductor. The other electrode layer 12 is a layer of separate pixel electrodes 12a on the second surface 14, which electrodes 12a define the individual pixels of the display, which are indicated by dotted vertical lines in FIG. 1. Alternatively, the other electrode layer 12 may be a solid electrode, for example a metal foil or graphite plane, or a conductive polymer. Alternatively, the electrode layer 12 may also be a light-transmitting or transparent electrode layer, similar to the transparent common electrode layer 11. An electric field is generated for a pixel by a potential difference between a voltage applied to the common electrode and a voltage applied to the corresponding pixel electrode. The pixel electrodes 12a may form part of an active matrix drive system, for example with a thin film transistor (TFT) backplane, but other types of electrode addresses may also be used, provided that the electrodes provide the required electric field across the display layer.

[0042] The pixel electrodes may be described in U.S. Patent No. 7,046,228. The pixel electrodes 12a may form part of an active matrix thin film transistor (TFT) backplane, although other types of electrode address may be used, provided the electrodes provide the required electric field across the display layer.

[0043] In one embodiment, the charge carried by the "low charge" particles may be less than about 50%, preferably about 5% to about 30%, of the charge carried by the "high charge" particles. In another embodiment, the "low charge" particles may be less than about 75%, or about 15% to about 55% of the charge carried by the "high charge" particles. In further embodiments, the comparison of charge levels as shown applies to two types of particles with the same charge polarity. The charge on the "high positive" particles and the charge on the "high negative" particles may be the same or different. Similarly, the amplitude of the "low positive" particles and the amplitude of the "low negative" particles may be the same or different. In any particular electrophoretic fluid, the two pairs of high-low charge particles may have different levels of charge difference. For example, in one pair, the low positive charged particles may have a charge intensity that is 30% of the charge intensity of the high positive charged particles, and in another pair, the low negative charged particles may have a charge intensity that is 50% of the charge intensity of the high negative charged particles.

[0044] Figures 2A-2F illustrate the four color states, and the transitions between them, that may be displayed at the viewing surface of each pixel of the display layer shown in Figure 1. As mentioned above, high positive particles are black (K), high negative particles are yellow (Y), low positive particles are red (R), and low negative particles are white (W).

[0045] In Figures 2A and 2B, a high negative drive voltage (V H2When a negative voltage (hereinafter referred to as "positive voltage", e.g. -15V, e.g. -30V) is applied to pixel electrode 22a (hereinafter it will be assumed that common electrode 21 is maintained at 0V, and thus in this case the common electrode is strongly positive with respect to the pixel electrode) for a sufficiently long period of time, an electric field is generated causing the highly negative yellow particles to be driven adjacent to the common electrode 21 and the highly positive black particles to be driven adjacent to the pixel electrode 22a, resulting in the state of FIG. 2A.

[0046] The low positive red R and low negative white W particles, because they carry a weaker charge, move slower than the more highly charged black and yellow particles, and as a result, they remain in the center of the pixel, and the white particles are above the red particles, both of which are masked by the yellow particles and therefore not visible at the viewing surface. Thus, the yellow color is displayed at the viewing surface.

[0047] Conversely, a high positive drive voltage (V H1 When a negative voltage (e.g., +15V, e.g., +30V) is applied to pixel electrode 22a (such that common electrode 21 is strongly negative with respect to the pixel electrode) for a long enough period of time, an electric field is generated that causes the highly positive black particles to be driven adjacent to common electrode 21 and the highly negative yellow particles to be driven adjacent to pixel electrode 22a. The resulting state in Figure 2B is the opposite of Figure 2A, and the color black is displayed at the viewing surface.

[0048] 2C and 2D illustrate how low positive (red) particles are displayed at the viewing surface of the display layer shown in FIG. 1. The process begins with the (yellow) condition shown in FIG. 2A and repeated as FIG. 2C. A low positive voltage (V L1, e.g., +3V, e.g., +5V, e.g., +10V) is applied to pixel electrode 22a for a long enough period of time (i.e., common electrode 21 is made slightly negative with respect to the pixel electrode), causing the highly negative yellow particles to move towards pixel electrode 22a while the highly positive black particles to move towards common electrode 21. However, as shown in FIG. 2D, when the yellow and black particles meet halfway between the pixel and common electrodes, they remain in the intermediate position because the electric field generated by the low driving voltage is not strong enough to counteract the attractive forces between them. As shown, the yellow and black particles remain halfway between the pixel and common electrodes in a mixed state.

[0049] The term "attractive force" as used herein encompasses electrostatic interactions, which are linearly dependent on the particle charge potential, and which can be further strengthened by other forces such as van der Waals forces, hydrophobic interactions, etc.

[0050] Obviously, attractive forces also exist between the low positive red particles and the high negative yellow particles, and between the low negative white particles and the high positive black particles. However, these attractive forces are not as strong as those between the black particles and the yellow particles, so the weak attractive forces for the red and white particles can be countered by the electric field generated by the low driving voltage, so that the low charged particles and the highly charged particles of the opposite polarity can be separated. The electric field generated by the low driving voltage is also sufficient to separate the low negative white particles and the low positive red particles, so that the red particles move adjacent to the common electrode 21 and the white particles move adjacent to the pixel electrode 22a. As a result, as shown in FIG. 2D, the pixel displays red, while the white particles are closest to the pixel electrode.

[0051] 2E and 2F illustrate how low negative (white) particles are displayed at the viewing surface of the display layer shown in FIG. 1. The process begins from the (black) state of FIG. 2B and is repeated as FIG. 2E. A low negative voltage (V L2, e.g., −3V, e.g., −5V, e.g., −10V) is applied to the pixel electrode for a long enough period (i.e., the common electrode is made slightly positive with respect to the pixel electrode), causing the highly positive black particles to move towards the pixel electrode 22a while the highly negative yellow particles to move towards the common electrode 21. However, when the yellow and black particles meet halfway between the pixel and common electrodes, as shown in FIG. 2F, they remain in the intermediate position because the electric field generated by the low driving voltage is not strong enough to counteract the attractive forces between them. Thus, as discussed above with reference to FIG. 2D, the yellow and black particles remain halfway between the pixel and common electrodes in a mixed state.

[0052] As discussed above with reference to Figures 2C and 2D, attractive forces also exist between the low positive red particles and the high negative yellow particles, and between the low negative white particles and the high positive black particles. However, these attractive forces are not as strong as those between the black and yellow particles, and therefore the weak attractive forces for the red and white particles can be countered by the electric field generated by the low driving voltage, thereby separating the low charged particles from the highly charged particles of the opposite polarity. The electric field generated by the low driving voltage is sufficient to separate the low negative white particles from the low positive red particles, thereby moving the white particles adjacent to the common electrode 21 and the red particles adjacent to the pixel electrode 22a. As a result, as shown in Figure 2F, the pixel displays white, while the red particles are closest to the pixel electrode.

[0053] In the display layer shown in Figures 1 and 2A-2F, the black particles (K) carry a high positive charge, the yellow particles (Y) carry a high negative charge, the red particles (R) carry a low positive charge, and the white particles (W) carry a low negative charge, however, in principle the highly positive, or highly negative, or low positive, or low negative particles can be of any color. All of these variations are intended to be within the scope of the present application.

[0054] It should also be noted that the low potential difference applied to reach the color state of Figures 2D and 2F can be about 5% to about 50% of the high potential difference required to drive the pixel from a highly positive color state to a highly negative color state, or vice versa (i.e., as shown in Figures 2A and 2B).

[0055] 1 and 2A-2F show the display layer as being unencapsulated, but the electrophoretic fluid may be filled into the display cells, which may be cup-like microcells as described in U.S. Patent No. 6,930,818. The display cells may also be other types of microcontainers, such as microcapsules, microchannels, or equivalents, regardless of their shape or size. All of these are within the scope of this application.

[0056] It will be readily apparent to one skilled in imaging science that if "pure" fully saturated colors are to be obtained in the various color states illustrated in Figures 2A-2F, all non-black and non-white particles used in the electrophoretic medium should be light reflective, not light transmissive (white particles are essentially light scattering, while black particles are essentially light absorbing). For example, in the red state of Figure 2D, if the red particles were substantially light transmissive, a significant percentage of the light entering the electrophoretic layer through the viewing surface would pass through the red particles, and a percentage of this transmitted light would be reflected back from the yellow particles "behind" the red particles (i.e., below, as illustrated in Figure 2D). The overall effect would be a severe "contamination" of the desired red color with a yellow tint, a highly undesirable result.

[0057] To ensure both color brightness and color purity, an oscillating waveform may be applied prior to driving the display layer from one color state to another. FIG. 3 is a voltage versus time graph of such an oscillating waveform. The oscillating waveform may consist of a pair of opposite driving pulses repeated over many cycles. When used with an active matrix display, each positive or negative pulse is at least the frame width of an update. For example, each pulse width may be about 16 milliseconds when the display is updated at 60 Hz. However, in practice, the frame time is typically slightly longer due to various charge and decay times for the capacitive elements of the backplane. For example, as shown in FIG. 3, the oscillating waveform may consist of a +15V pulse for 20 milliseconds and a -15V pulse for 20 milliseconds, and this pair of pulses may be repeated 50 times. The total duration of such an oscillating waveform would be 2,000 milliseconds. For ease of illustration, FIG. 3 illustrates only seven pairs of pulses.

[0058] The pulse width does not have to be limited to a frame time, and each pulse may include multiple frames, e.g., a pulse width of 40 milliseconds, e.g., a pulse width of 60 milliseconds, e.g., a pulse width of 80 milliseconds, e.g., a pulse width of 100 milliseconds. In some embodiments, the pulse width of each element of the vibration pulse may be 80 milliseconds or less, e.g., 60 milliseconds or less, e.g., 40 milliseconds or less, e.g., 20 milliseconds or less. In practice, there may be at least four repetitions (i.e., four pairs of positive and negative pulses), e.g., at least six repetitions, e.g., at least eight repetitions, e.g., at least ten repetitions, e.g., at least twelve repetitions, e.g., at least fifteen repetitions. Similarly, all subsequent figures showing vibration waveforms simplify the vibration waveform in the same manner. The vibration waveform may be applied regardless of the optical state prior to the application of the driving voltage. After the vibration waveform is applied, the optical state (either at the viewing surface or, if visible, at the second surface) will not be a pure color, but a mixture of colors of various types of pigment particles. In some cases, multiple shaking pulses will be delivered with pauses of 0 V between the shaking pulses to allow the electrophoretic medium to equilibrate and / or for charge built up on the electrodes to dissipate.

[0059] Each of the drive pulses in the oscillating waveform is applied for a time not exceeding 50% (or not exceeding 30%, 10%, or 5%) of the drive time required to drive from a high positive particle color state to a high negative particle color state, or vice versa. For example, if it takes 300 milliseconds to drive a display device from the color state of FIG. 2B to the high positive particle, i.e., color state of FIG. 2A, or vice versa, the oscillating waveform may consist of positive and negative pulses, each applied for a time not exceeding 150 milliseconds. In practice, it is preferred that the pulses be as short as possible.

[0060] For this purpose, a high drive voltage (V H1 or V H2 ) is defined as the drive voltage that is sufficient to drive a pixel from a high positive color state to a high negative color state and vice versa (see Figures 2A and 2B).L1 or V L2 ) is defined as the drive voltage that may be sufficient to drive a pixel from a high charge carrier color state to a low charge carrier color state (see Figures 2D and 2F). In general, V L The scale of (e.g., V L1 or V L2 ) is V H The amplitude of (e.g., V H1 or V H2 ) is less than 50%, preferably less than 40%.

[0061] As mentioned in the background art, the orientation of the electrophoretic medium with respect to gravity affects the purity of the resulting color state, especially when the display is operated at lower temperatures, e.g., below 5°C, e.g., below 0°C, e.g., below -5°C, e.g., below -10°C, e.g., below -15°C. As shown in Fig. 4A, horizontal driving is when the electric field gradient provided by the electrodes (11 and 12a) is along the direction of gravity (G). In contrast, vertical driving is when the electric field gradient provided by the electrodes (11 and 12a) is transverse to the direction of gravity (G).

[0062] Experimental measurements of the black state using the CIELAB color space (e.g., L*, a*, b*), for example, as illustrated in Figures 2A and 2B, show that driven black pixels have consistently higher L* for black pixels driven in a vertical orientation compared to the same display driven in a horizontal orientation (lower L* is better for the black state, i.e., less reflectivity). In addition, using an eye loupe or similar magnification, a viewer may see extra specks of white, yellow, and red pigments that contaminate the black state. Using a predefined test pattern, the L* value for black is typically about 3L* higher for a vertically driven 4-particle panel driven at 0°C compared to a horizontally driven 4-particle panel driven at 0°C. Although this is not noticeable, it is observed that all color states have increased contamination when driven in a vertical orientation, especially at low temperatures. The cause of this color contamination is not completely understood, however, it may result from density differential separation of various components in the electrophoretic medium, including pigments, charge control agents, and other additives.

[0063] FIG. 5A illustrates a standard waveform that may be used to effect the yellow to red (high negative to low positive) transition of FIGS. 2C and 2D. In the waveform of FIG. 5A, a high negative drive voltage (V H2 , e.g., −15V) is applied for a period of t1, driving the pixel towards a yellow state (see FIG. 2C). This initial application of a high negative drive voltage is known as a balance phase, which is included to ensure that the entire waveform of FIG. 5A is DC balanced (the term “DC balanced” is used herein to mean that the integral of the drive voltage applied to the pixel over the time taken for the entire waveform is substantially zero). The balance pulse of t1 may last for 500 milliseconds or more, e.g., more than 1 second. An oscillating waveform (aka a mixed waveform) is then applied, driving the pixel towards a yellow state (V H2) continues for a period of t2, which places the pixel in the yellow state shown in FIG. 2C. The width of period t2 is typically less than t1, e.g., half as long, e.g., about 200 ms, or about 250 ms, or about 500 ms. In some embodiments of FIG. 5A, each pulse of the shaking pulse may be about 80 ms wide, however, longer or shorter pulse widths are acceptable. From this yellow state, the pixel is turned on by application of a low positive drive voltage (V L1 , e.g., +3V), resulting in a yellow-to-red transition (shown in FIG. 2C to FIG. 2D). H2 is applied, it is sufficient to drive the pixel to the yellow state, and the period t3 is L1 is applied is sufficient to drive the pixel from a yellow state to a red state. Period t3 is typically longer than t2, e.g., about 300 ms, e.g., about 400 ms, e.g., about 600 ms. It should be understood that the waveform of FIG. 5A is a "base" waveform for preparation of red at the viewing surface. Portions of the waveform may be repeated, e.g., balance and shake pulses may be repeated before the first drive pulse is applied. In some embodiments, there may be a 0V pause between repeated portions of the waveform (i.e., balance, shake, pause, balance, shake). In addition, a decontamination pulse may be added to the waveform, as described in U.S. Pat. No. 10,586,499 (incorporated by reference in its entirety).

[0064] However, as discussed above, the waveform of FIG. 5A does not provide sufficient initial separation of the aggregated pigments to achieve pure optical states, especially when driven at low temperatures (e.g., 0° C.) and in a vertical orientation. That is, after driving with the waveform of FIG. 5A, black, yellow, and white pigment contamination can be seen in the red pixels. Surprisingly, it has been found that this contamination can be countered by adding a simple high negative deaggregation pulse at time t1′, as shown in FIG. 5B. Although this extra high negative time appears as an extension of the equilibration pulse t1, the deaggregation pulse t1′ has been found to be effective in preparing all color states for the described four-particle electrophoretic display system, for example of the type described above with respect to FIGS. 1 and 2A-2F. The period t1′ is typically between 100 ms and 700 ms, for example, about 400 ms, or about 500 ms, or 400-500 ms.

[0065] Although we do not wish to be bound by the following proposed mechanism, we speculate that the positively charged black and red particles develop aggregates after sustained actuation (especially at lower temperatures). Particle agglomeration may be promoted by charge control agents in the electrophoretic medium, however, the effect does not appear to be sensitive to the particular type of charge control agent. When a negative deagglomeration pulse, i.e., t1' in FIG. 5B, is added, the red and black particles are driven closer to the actuation electrode (22a), which results in a higher dispersive force (i.e., a sharper "kick") for the positive particle aggregates when the vibration pulse begins. Hence, the positive particles are better separated and respond better to the subsequent actuation (i.e., address) pulse. When a deagglomeration pulse is added, there is less color mixing and the resulting colors are more consistent when evaluated using electro-optical metrology (see example).

[0066] In a similar manner, Figures 6A and 6B illustrate waveforms that may be used to effect the black to white (high positive to low negative) transition of Figures 2E to 2F. The waveform of Figure 6A is a standard waveform, while the waveform of Figure 6B has been modified to include a de-agglomeration pulse t4' to reduce contamination in the resulting white state. The waveform of Figure 6A, which is essentially an inverted version of the waveform of Figure 5A, uses a high positive drive voltage (V H1 , e.g., +15V) is applied as a balancing pulse for a period of t4. An oscillatory waveform is then applied, and a high positive drive voltage (V H1 ) continues for a period of t5, thus ensuring that the pixel is in the black state shown in FIG. 2E. From this black state, the pixel is driven with a low negative drive voltage (V L2 , e.g., −3V), resulting in the black-to-white transition shown in FIG. 2E through FIG. 2F. Period t5 is H1 is applied, it is sufficient to drive the pixel to the black state, and the period t6 is V L2 is applied is sufficient to drive the pixel from the black state to the white state. The de-agglomeration pulse t4' shown in Figure 6B improves the purity of the final white state, especially when the display is driven in a vertical orientation at low temperatures. The period t4' is typically between 100 ms and 700 ms, for example about 400 ms, or about 500 ms, or between 400 ms and 500 ms.

[0067] FIG. 7A illustrates a standard waveform that may be used to effect the yellow to black (high negative to high positive) transition of FIGS. 2A to 2B. A balance pulse of width t7 and having a high negative voltage is delivered prior to the oscillating waveform. The balance pulse achieves DC balance for the entire waveform, and the oscillating pulse is included to ensure color brightness and purity. Following the balance and oscillating pulse, a high positive driving voltage (V H1 , +15V, +30V) is applied for period t8, after the oscillating waveform, driving the pixel towards the black state.

[0068] As detailed above and explained in the examples below, the waveform of Fig. 7A does not achieve the desired black purity when the display is in a vertical orientation, especially for low temperature operation. Therefore, in a manner similar to the waveforms of Figs. 5B and 6B, the addition of a high negative pulse for an intermediate time t7' has been found to achieve particle deagglomeration and result in improved black state electro-optical performance. As in Figs. 5B and 6B, the period t7' is typically between 100 ms and 700 ms, for example about 400 ms, or about 500 ms, or between 400 ms and 500 ms.

[0069] FIG. 8A illustrates a standard waveform that may be used to effect the black to yellow (high positive to high negative) transition of FIGS. 2B to 2A. A balance pulse of width t9 and having a high negative voltage is delivered prior to the oscillating waveform. The balance pulse achieves DC balance for the entire waveform, and the oscillating pulse is included to ensure color brightness and purity. Following the balance and oscillating pulse, a high negative drive voltage (V H1 , +15V, +30V) is applied for period t10, after the oscillating waveform, driving the pixel towards the yellow state.

[0070] As detailed above, the waveform of Fig. 8A does not achieve the desired yellow purity when the display is in the vertical orientation, especially for low temperature operation. Therefore, it has been found that the addition of a high negative pulse for an intermediate time t9' in a manner similar to the waveform of Fig. 7B achieves particle de-agglomeration, resulting in improved black state electro-optical performance. As in Fig. 7B, the period t9' is typically between 100 ms and 700 ms, for example about 400 ms, or about 500 ms, or between 400 ms and 500 ms.

[0071] The waveforms described so far are intended to display one of the four optical states shown in Figures 2A-2F, essentially the color of one of the four types of particles present in the display layer. It will be seen from the above that while the embodiments of the invention described above allow the display of any one of the four colors at each pixel, they do not provide an easy way to reproducibly control the gray level of each color or its saturation. Thus, if it is desired to use the invention to provide a grayscale color image, it will be necessary to dither (area-modulate) the pixels of the display to provide the required grayscale. For example, a non-saturated red (pink) color can be displayed by setting alternating pixels of the display to red and white. Area modulation effectively trades an increased number of gray levels (because each pixel is effectively used as a subpixel of a larger pixel capable of gray level display) for a reduction in display resolution, the loss in resolution being limited by increasing the number of reproducible color states (primaries) that can be displayed at each pixel. In the method of the present invention, it has been found that the number of primary colors available from each pixel can be increased by driving each pixel to a color represented by a mixture of low positive (red) and high negative (yellow) particles (orange in the embodiment shown in the drawings), and / or a color represented by a mixture of low negative (white) and high positive (black) particles (gray).

[0072] It has been found that reproducible mixed colors can only be obtained by first driving the display to the color of the low charged particles required in the mixed color, and then applying a high drive voltage of a polarity that causes the appropriate high charged particles to mix with the low charged particles to form the desired mixed color. More specifically, to provide a reproducible orange color, it is required to start from a red state. To transition from this red state 2 to the orange state (i.e., mixed red and yellow), a high negative drive voltage (V H2A high driving voltage (e.g., -15V) is applied to the pixel electrode (22a) for a short period of time (i.e., the common electrode is made strongly positive with respect to the pixel electrode). The high driving voltage is sufficient to cancel the interaction between the black and yellow particles previously aggregated halfway between the pixel electrode and the front electrode, so that the negatively charged yellow particles start to move rapidly towards the front electrode (21), while the positively charged black particles start to move towards the pixel electrode (22a). At the same time, the positively charged red particles start to move away from the front electrode (21), toward the pixel electrode (22a), while the negatively charged white particles start to move away from the pixel electrode (22a), toward the front electrode (21). However, since the electrophoretic mobilities of the low-charged red and white particles are smaller than those of the high-charged black and yellow particles, the red and white particles move slower than the black and yellow particles. The length of the drive pulse is adjusted so that a mixture of red and yellow particles is present adjacent the front electrode (21), so that orange is seen at the viewing surface. A mixture of black and white particles is present adjacent the pixel electrode (22a), so that when this surface is viewed, grey will be seen through the second surface of the display.

[0073] (Example) A four-particle electrophoretic medium, containing black, white, yellow, and red particles of the type described above with reference to FIG. 1, was prepared, filled into an array of transparent microcells, and sealed with an acrylate seal layer. The array of microcells was laminated to a front transparent electrode (PET-ITO) and subsequently bonded to a thin film transistor (TFT) backplane. The resulting display was placed on an optical bench using a temperature-controlled chuck that allows for horizontal and vertical positioning of the test display. As shown in FIG. 9, the panel was first driven in a horizontal orientation through various patterns with little or no dwell time between successive patterns. The horizontal test patterns were recorded with video to ensure reliable switching between states and to check for "dead pixels" or other defects that may result from improper filling or sealing. To ensure that the panel is functioning properly, after driving in the horizontal patterns, the panel is reoriented to a vertical position and fired up with multiple updates with long dwell times between updates. This position and test sequence is intended to mimic real-world conditions where panels are typically mounted in a vertical position and updated only occasionally. In this test, the dwell time was 30 minutes, but it could be 60 minutes or longer. The total time of evaluation in the vertical orientation was 3 days. After 3 days of vertical operation, the display is evaluated for electro-optical performance using a spectrophotometric detector that measures L* and b* values ​​at multiple measurement points on the display, as shown in the right-most schematic diagram of FIG. 9.

[0074] As shown in Table 1 below, test runs were performed at 0° C. using waveforms of the type shown in FIGS. 5A, 6A, 7A, and 8A, and the black measurement points have a large amount of variance after extended vertical drive at low temperatures. When viewed through an eye loupe or similar magnification, it is apparent that this variability is primarily due to the black state being inadequately contaminated (tinted) with white, yellow, and red pigments. However, when the panel is driven with waveforms of the type shown in FIGS. 5B, 6B, 7B, and 8B, the resulting L* values ​​of the black measurement points are lower and the final L* values ​​have less variance (from high to low). In addition, the b* values ​​are closer to zero with much less variance. This data suggests that the waveforms of FIGS. 5B, 6B, 7B, and 8B are superior to the waveforms of FIGS. 5A, 6A, 7A, and 8A for driving a four-particle electrophoretic display in a vertical orientation at low temperatures. [Table 1]

[0075] Although the present invention has been described with reference to specific embodiments thereof, it should be understood by those skilled in the art that various modifications may be made and equivalents substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step, or steps to the objective and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.

Claims

1. 1. A method of driving a display layer, the display layer being disposed between a viewing surface including a light-transmissive electrode and a second surface on an opposite side of the display layer from the viewing surface, the second surface including a driving electrode, the display layer including 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 have first, second, third, and fourth optical properties that are different from one another, respectively; the first and third types of particles have charges of a first polarity, the second and fourth types of particles have charges of a second polarity opposite to the first polarity, the first and third types of particles do not have the same charge magnitude, and the second and fourth types of particles do not have the same charge magnitude; The method comprises: (i) applying a first electric field having a first electric field strength magnitude and positive or negative polarity between the light-transmissive electrode and the drive electrode to drive the first or second type of particles towards the viewing surface, thereby causing the display layer to display the first or second optical property at the viewing surface; (ii) applying a second electric field having the first electric field magnitude and the negative polarity between the light transmissive electrode and the drive electrode; (iii) applying a vibration pulse between the light transmissive electrode and the drive electrode, the vibration pulse including at least four periods of the first electric field strength magnitude at the negative polarity and at least four periods of the first electric field strength magnitude at the positive polarity; (iv) applying a third electric field between the light-transmissive electrode and the drive electrode, the third electric field having the magnitude of the first electric field strength and the positive or negative polarity, again driving the first or second type of particles towards the viewing surface, thereby again causing the display layer to display the first or second optical property at the viewing surface; (v) applying a fourth electric field having a magnitude of the second electric field strength and a polarity opposite to that of step (iv) and driving the fourth or third type of particles towards the viewing surface, thereby causing the display layer to display the fourth or third optical property at the viewing surface. a field-free rest period is performed after step (iii), and steps (i)-(iii) are repeated twice before completing steps (iv) and (v); The method of claim 1, wherein the magnitude of the first electric field strength is greater than the magnitude of the second electric field strength.

2. The method of claim 1 , wherein the first electric field is applied for a longer time than the second electric field and the third electric field is applied for a longer time than the second electric field.

3. The method of claim 1 , wherein each of steps (i) through (v) is repeated.

4. The method of claim 1 , wherein a magnitude of the second electric field strength of the fourth electric field is less than 50 percent of a magnitude of the first electric field strength of the third electric field.

5. The method of claim 1 , wherein only the fourth or the third optical property is displayed after completion of step (v).

6. The method of claim 1 , wherein the first electric field is applied for greater than 400 ms.

7. The method of claim 1 , wherein the second electric field is applied for greater than 100 ms.

8. The method of claim 1 , wherein each period of the shaking pulses is applied for less than 80 ms.

9. The method of claim 8 , wherein each period of the shaking pulse is applied for 40 ms.

10. The method of claim 1 , wherein each electric field is applied in a direction that is substantially perpendicular to the direction of the Earth's gravity.

11. 1. A method of driving a display layer, the display layer being disposed between a viewing surface including a light-transmissive electrode and a second surface on an opposite side of the display layer from the viewing surface, the second surface including a driving electrode, the display layer including 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 have first, second, third, and fourth optical properties that are different from one another, respectively, the first and third types of particles have charges of a first polarity, the second and fourth types of particles have charges of a second polarity opposite to the first polarity, the first and third types of particles do not have the same charge magnitude, and the second and fourth types of particles do not have the same charge magnitude; The method comprises: (i) applying a first electric field having a first electric field strength magnitude and positive or negative polarity between the light-transmissive electrode and the drive electrode to drive the first or second type of particles towards the viewing surface, thereby causing the display layer to display the first or second optical property at the viewing surface; (ii) applying a second electric field having the first electric field magnitude and the negative polarity between the light transmissive electrode and the drive electrode; (iii) applying a vibration pulse between the light transmissive electrode and the drive electrode, the vibration pulse including at least four periods of the first electric field strength magnitude at the positive polarity and at least four periods of the first electric field strength magnitude at the negative polarity; (iv) applying a third electric field having a magnitude of the first electric field strength and a polarity opposite to that of step (i) to drive the second or first type of particles towards the viewing surface, thereby causing the display layer to display the second or first optical property at the viewing surface. in order, wherein a field-free rest period is performed after step (iii), and steps (i)-(iii) are repeated twice before completing step (iv).

12. The method of claim 11 , wherein the first electric field is applied for the same time as the third electric field.

13. The method of claim 11 , wherein each of steps (i) to (iv) is repeated.

14. The method of claim 11 , wherein only the second or the first optical property is displayed after completion of step (iv).

15. The method of claim 11 , wherein the first electric field is applied for greater than 400 ms.

16. The method of claim 11 , wherein the second electric field is applied for greater than 100 ms.

17. The method of claim 11 , wherein each period of the shaking pulses is applied for less than 80 ms.

18. 20. The method of claim 17, wherein each period of the shaking pulse is applied for 40 ms.

19. The method of claim 11 , wherein each electric field is applied in a direction that is substantially perpendicular to the direction of the Earth's gravity.

Images