A color electrophoretic medium having a four-pigment particle system addressable by a waveform with four voltage levels.

Abstract

A color electrophoretic display includes a light-transmitting electrode on the viewing surface, a back electrode, and an electrophoretic medium placed between them. The electrophoretic medium includes a nonpolar fluid and a multipigment particle system having four types of charged electrophoretic pigment particles in the fluid. The particles include a first type of particle, a second type of particle, a third type of particle, and a fourth type of particle, each having different optical properties. The second type of particle, the third type of particle, and the fourth type of particle have a charge polarity opposite to that of the first type of particle.

Description

[Background technology]

[0001] (Related applications) This application claims priority to U.S. Provisional Patent Application No. 63,471,148, filed on 5 June 2023, entitled "COLOR ELECTROPHORETIC MEDIUM HAVING FOUR PIGMENT PARTICLE SYSTEM ADDRESSABLE BY WAVEFORMS HAVING FOUR VOLTAGE LEVELS," which is incorporated herein by reference in its entirety.

[0002] Electrophoretic displays (EPDs) change color by altering the position of charged colored particles on a light-transmitting viewing surface. Because the resulting display has high contrast, similar to ink on paper, and is readable in sunlight, these electrophoretic displays are commonly called "electronic paper" or "ePaper." Electrophoretic displays are widely used in e-readers such as the Amazon Kindle® because they offer a book-like reading experience, consume less power, and allow users to carry libraries of hundreds of books in a lightweight handheld device.

[0003] For many years, EPDs have contained only two types of charged color particles: black and white. (As used herein, “color” includes both black and white.) White particles are often light-scattering and may include, for example, titanium dioxide, while black particles are absorbent across the entire visible spectrum and may include carbon black or absorbent metal oxides such as copper chromite. In its simplest sense, black and white EPDs require only a light-transmitting electrode on the viewing surface, a back electrode, and an electrophoretic medium containing positively and negatively charged white and black particles. When a voltage of one polarity is applied, the white particles move to the viewing surface, and when a voltage of the opposite polarity is applied, the black particles move to the viewing surface. If the back electrode contains controllable regions (pixels) (either segmented electrodes or an active matrix of transistor-controlled pixel electrodes), a pattern can be electronically displayed on the viewing surface. The pattern could be, for example, text from a book.

[0004] Recently, a variety of color options for EPDs have become commercially available, including tricolor displays (black, white, red; black, white, yellow), quadcolor displays (black, white, red, yellow), and color filter displays that rely on the aforementioned black / white particles. EPDs using three or four reflective particles operate similarly to conventional monochrome displays, as they drive the desired color particles onto the viewing surface. While the driving mechanism is far more complex than simple monochrome, the optical function of the particles is ultimately the same, reflecting incident light to the viewer in the correct color.

[0005] Advanced Color Electronic Paper (ACeP) TMAlthough it also contains four particles, the cyan, yellow, and magenta particles are subtractive rather than reflective, making it possible to generate thousands of colors in each pixel. The color process is functionally equivalent to the printing methods that have been used for many years in offset printing and inkjet printers. A specific color is produced by using the appropriate ratio of cyan, yellow, and magenta on a bright white paper background. In the ACeP example, the relative positions of the cyan, yellow, magenta, and white particles to the viewing surface determine the color of each pixel. In this type of EPD, thousands of colors are envisioned in each pixel, but it is crucial to carefully control the position of each pigment (50-500 nanometers in size) within a working space of approximately 10-20 micrometers in thickness. Naturally, variations in particle position will cause the wrong color to be displayed in a particular pixel. Therefore, precise voltage control is required in such systems. Further details of this system are described in the following U.S. patents, all incorporated together by reference: U.S. Patents No. 9,361,836, No. 9,921,451, No. 10,276,109, No. 10,353,266, No. 10,467,984, and No. 10,593,272.

[0006] The term "gray state" is used herein in its conventional sense in imaging techniques to refer to an intermediate state between two extreme optical states of a pixel, and does not necessarily mean a black-and-white transition between these two extreme optical states. For example, some of the E Ink patents and published applications cited below describe EPDs where the extreme states are white and deep blue, so the intermediate gray state would actually be light blue. In fact, as already mentioned, a change in optical state may not be a change in color at all. The terms black and white may be used below to refer to two extreme optical states of a display, and should be understood to usually include extreme optical states that are not strictly black and white (e.g., the white and deep blue states mentioned above).

[0007] The terms bistable and bistable are used herein in their conventional sense in imaging skills to refer to a display having a display element having a first and second display state of at least one distinct optical property, wherein after a given element is driven to take either the first or second display state using addressing pulses at finite intervals, the state persists for at least several times, e.g., at least four times, the minimum interval of addressing pulses required to change the state of the display element after the addressing pulses have finished. U.S. Patent No. 7,170,670 shows that some grayscale-capable particle-based EPDs are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true for some other types of electro-optical displays. While this type of display device is more accurately called polystable rather than bistable, for convenience, the term bistable may be used herein to encompass both bistable and polystable displays.

[0008] When the term "impulse" refers to the driving of an EPD, it is used herein to refer to the integral of the applied voltage with respect to time during the period in which the display is driven.

[0009] Particles that absorb, scatter, or reflect light over a broadband or selected wavelength are referred to herein as colored particles or pigment particles. In the electrophoretic media and displays of the present invention, a variety of materials other than pigments (strictly speaking, meaning insoluble coloring materials), such as dyes and photonic crystals, may also be used to absorb or reflect light.

[0010] Particle-based EPDs have been the subject of intense research and development for many years. In such displays, multiple charged particles (sometimes referred to as pigment particles) move through a fluid under the influence of an electric field. Compared to liquid crystal displays, EPDs can offer advantages such as better brightness and contrast, wide viewing angles, state bistability, and low power consumption. Nevertheless, problems with the long-term image quality of these displays have hindered their widespread use. For example, the particles that make up EPDs tend to settle, resulting in insufficient lifespan for these displays.

[0011] As described above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can also be produced using a gaseous fluid. See, for example, Kitamura, T., et al., Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., Toner display using insulative particles charged triboelectrically, IDW Japan, 2001, Paper AMD4-4. See also U.S. Patents 7,321,459 and 7,236,291. Such gas-based electrophoretic media are considered susceptible to the same types of particle sedimentation problems as liquid-based electrophoretic media when the medium is used in an orientation that allows such sedimentation, for example, when the medium is positioned in a label within a vertical plane. In fact, particle sedimentation is considered a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media, due to the lower viscosity of gaseous suspension fluids compared to the viscosity of liquids, which allows for faster sedimentation of electrophoretic particles.

[0012] A number of patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies used within encapsulated electrophoretic media and other electro-optic media. Such encapsulated media comprise a number of small capsules, each of which itself comprises an internal phase containing particles movable by electrophoresis within a fluid medium and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in these patents and applications include the following. (a) Electrophoretic particles, fluids, and fluid additives, see, for example, U.S. Pat. Nos. 7,002,728 and 7,679,814. (b) Capsules, binders, and encapsulation processes, see, for example, U.S. Pat. Nos. 6,922,276 and 7,411,719. (c) Microcell structures, wall materials, and methods of forming microcells, see, for example, U.S. Pat. Nos. 7,072,095 and 9,279,906. (d) Methods for filling and sealing microcells, see, for example, U.S. Pat. Nos. 7,144,942 and 7,715,088. (e) Films and subassemblies containing electro-optic materials, see, for example, U.S. Pat. Nos. 6,982,178 and 7,839,564. (f) Backplanes, adhesive layers and other auxiliary layers, and methods used within displays, see, for example, U.S. Pat. Nos. 7,116,318 and 7,535,624. (g) Color formation and color adjustment (e.g., U.S. Pat.

Chem.

Chem.

[0013] Many of the aforementioned patents and applications recognize that the wall surrounding discrete microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thus producing a so-called polymer-dispersed EPD, in which the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymer material, and that discrete droplets of electrophoretic fluid in such a polymer-dispersed EPD may be considered capsules or microcapsules even if no discrete capsule membrane is associated with each individual droplet. See, for example, U.S. Patent No. 6,866,760. Therefore, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a variant of encapsulated electrophoretic media.

[0014] A related type of EPD is the so-called microcell EPD. In microcell EPDs, charged particles and suspension fluids are not encapsulated within microcapsules, but instead are retained within multiple cavities formed within a carrier medium, usually a polymer film. See, for example, U.S. Patent Nos. 6,672,921 and 6,788,449.

[0015] Electrophoretic media are often impermeable (for example, in many electrophoretic media, the particles substantially block the transmission of visible light through the display) and operate in reflective mode, but many EPDs can be made to operate in a so-called shutter mode, where one display state is substantially impermeable and the other is light-transmitting. See, for example, U.S. Patents 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. Dielectric displays, similar to EPDs but relying on variations in electric field strength, can operate in a similar mode. See, for example, U.S. Patent 4,418,346. Other types of electro-optical displays may also be able to operate in shielding mode. An electro-optical medium operating in shutter mode can be used in a multilayer structure for a full-color display, in which at least one layer adjacent to the viewing surface of the display operates in shutter mode, exposing or concealing a second layer further away from the viewing surface.

[0016] Encapsulated EPDs typically offer further advantages, such as the ability to print or coat displays on a wide variety of flexible and rigid substrates, without being plagued by the clustering and sedimentation failure modes of conventional electrophoretic devices. (The use of the term "printing" is intended to include all forms of printing and coating, including but not limited to: pre-measured coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coatings such as knife-over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silkscreen printing process; electrostatic printing process; thermal printing process; inkjet printing process; electrophoretic deposition (see U.S. Patent No. 7,339,715); and other similar techniques.) Thus, the resulting displays can be flexible. Furthermore, since the display medium can be printed (using various methods), the displays themselves can be manufactured inexpensively.

[0017] As described above, the simplest prior art electrophoretic media essentially display only two colors. Such electrophoretic media use either a single type of electrophoretic particle having a first color in a colored fluid having a second distinct color (in which case the first color is displayed when the particle is adjacent to the viewing surface of the display, and the second color is displayed when the particle is separated from the viewing surface), or first and second types of electrophoretic particles having different first and second colors in an uncolored fluid (in which case the first color is displayed when the first type of particle is adjacent to the viewing surface of the display, and the second color is displayed when the second type of particle is adjacent to the viewing surface). Typically, the two colors are black and white. If a full-color display is desired, a color filter array may be deposited on top of the screen of a monochrome (black and white) display. Displays with a color filter array rely on area sharing and color mixing to create color stimuli. The available display area is shared among three or four primary colors, such as red / green / blue (RGB) or red / green / blue / white (RGBW), and the filters can be arranged in a one-dimensional (striped) or two-dimensional (2x2) repeating pattern. Other primary colors or selections of three or more primary colors are also known in the art. The three (for RGB displays) or four (for RGBW displays) subpixels are selected to be small enough that, at the intended viewing distance, they visually blend together into a single pixel with a uniform color stimulus ("color blending"). An inherent drawback of area sharing is that a colorant is always present, and color can only be modulated by switching the corresponding pixels of the underlying monochrome display to white or black (switching the corresponding primary color on or off). For example, in an ideal RGBW display, the red, green, blue, and white primary colors each occupy one-quarter of the display area (one of the four subpixels), the white subpixel is as bright as the white of the underlying monochromatic display, and each of the colored subpixels is no brighter than one-third of the white of the monochromatic display.The brightness of white displayed by the display cannot, in whole, exceed half the brightness of the white subpixels (the white area of ​​the display is generated by displaying one of each of four white subpixels, and in addition, each colored subpixel in its colored form is equivalent to one-third of a white subpixel; therefore, three combined colored subpixels contribute to one or fewer white subpixels). The brightness and saturation of a color are reduced by sharing space with color pixels switched to black. This sharing space is particularly problematic when mixing yellow, because yellow is brighter than any other color of equal brightness, and saturated yellow is almost as bright as white. Switching blue pixels (one-quarter of the display area) to black makes yellow too dark.

[0018] U.S. Patents 8,576,476 and 8,797,634 describe a multicolor electrophoretic display having a single backplane with independently addressable pixel electrodes and a common light-transmitting front electrode. Multiple electrophoretic layers are positioned between the backplane and the front electrode. The displays described in these patents are capable of rendering any primary color (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel position. However, the use of multiple electrophoretic layers positioned between a set of address electrodes has drawbacks. The electric field experienced by particles in a particular layer is lower than that of a single electrophoretic layer addressed at the same voltage. In addition, optical losses in the electrophoretic layer closest to the viewing surface (e.g., caused by light scattering or undesirable absorption) can affect the appearance of the image formed in the underlying electrophoretic layers.

[0019] The other two types of EPD systems provide a single electrophoretic medium capable of rendering any color at any pixel position. Specifically, U.S. Patent No. 9,697,778 describes a display combining a stained solvent with white (light-scattering) particles, which move in one direction when driven by a low applied voltage and in the opposite direction when driven by a higher voltage. By combining the white particles and the staining solvent with two additional particles that have the opposite charge to the white particles, a full-color display can be rendered. However, the color state described in Patent No. 778 is not suitable for applications such as text readers. In particular, there is always some portion of the staining fluid that separates the white-scattering particles from the viewing surface, which leads to a tint in the white state of the display.

[0020] A second embodiment of an electrophoretic medium capable of rendering any color at any pixel position is described in U.S. Patent No. 9,921,451. In Patent No. 451, the electrophoretic medium comprises four particles, namely white, cyan, magenta, and yellow, two of which are positively charged and two are negatively charged. However, the display of Patent No. 451 also suffers from color mixing with the white state. Since one of the particles has the same charge as the white particles, when a white state is desired, a certain amount of the same-charged particles move toward the viewing surface along with the white. It is possible to overcome this undesirable coloring using complex waveforms to drive the display, but such waveforms significantly increase the display refresh time and, in some cases, result in unacceptable "flickering" between images. [Prior art documents] [Patent Documents]

[0021] [Patent Document 1] U.S. Patent No. 9,361,836 [Non-patent literature]

[0022] [Non-Patent Document 1] Kitamura,T.,et al.,Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1 [Overview of the project] [Means for solving the problem]

[0023] This specification discloses an improved full-color EPD addressable by a drive waveform having four voltage levels. The EPD includes an electrophoretic medium having four types of charged electrophoretic pigment particles, comprising three colored, substantially non-light-scattering pigments and a fourth light-scattering white pigment, preferably corresponding to subtractive color primary colors (cyan, magenta, and yellow). The three colored pigments have one charge polarity, while the white pigment has the opposite charge polarity. Preferably, the white pigment is negatively charged and the three colored pigments are positively charged. The display renders any primary color (white, black, cyan, magenta, yellow, red, green, and blue) at any pixel position by being addressable by a waveform having just four different voltage levels. The voltage levels preferably include a high positive voltage, a low positive voltage, a single negative voltage, and a near-zero voltage. Because EPDs utilize just four voltage levels and can be addressed with 2 bits of data per pixel (instead of 3 bits or more used in conventional devices), they can reduce device costs while providing superior performance and color quality without significantly sacrificing color gamut.

[0024] In a first embodiment, the present invention provides a color electrophoretic display comprising a light-transmitting electrode on a viewing surface, a back electrode, and an electrophoretic medium disposed between the light-transmitting electrode and the back electrode. The electrophoretic medium comprises a nonpolar fluid and a polypigment particle system comprising four types of charged electrophoretic pigment particles dispersed in the nonpolar fluid. The four types of charged electrophoretic pigment particles include a first type of particle having a first optical property and a first charge polarity, a second type of particle having a second optical property and a second charge polarity opposite to the first charge polarity, a third type of particle having a third optical property and a second charge polarity, and a fourth type of particle having a fourth optical property and a second charge polarity. The first, second, third, and fourth optical properties are distinct from each other. The multi-pigment particle system is directly addressable by a push-pull waveform applied to the back electrode, which has pulse voltage levels selected from a set of just four different voltage levels to render one of eight primary colors—red, green, blue, cyan, magenta, yellow, black, and white—at each pixel of the electrophoretic medium, while keeping the voltage of the light-transmitting electrode constant. The voltage levels include a first positive voltage, a lower second positive voltage, a near-zero voltage, and a negative voltage.

[0025] In a second embodiment, the present invention provides a method for providing a color electrophoretic display, the color electrophoretic display comprising a light-transmitting electrode on a viewing surface, a back electrode, and an electrophoretic medium disposed between the light-transmitting electrode and the back electrode. The electrophoretic medium comprises a nonpolar fluid and a polypigment particle system comprising four types of charged electrophoretic pigment particles dispersed in the nonpolar fluid. The four types of charged electrophoretic pigment particles include a first type of particle having a first optical property and a first charge polarity, a second type of particle having a second optical property and a second charge polarity, wherein the second charge polarity is opposite to the first charge polarity, a third type of particle having a third optical property and a second charge polarity, and a fourth type of particle having a fourth optical property and a second charge polarity. The first, second, third, and fourth optical properties are distinct from each other. The method also involves directly addressing a multiplicative particle system using a push-pull waveform applied to a back electrode, wherein the push-pull waveform has voltage levels selected from a set of exactly four different voltage levels to render one of eight primary colors—red, green, blue, cyan, magenta, yellow, black, and white—at each pixel of the electrophoretic medium, while keeping the voltage of the light-transmitting electrode constant. The four different voltage levels include a first positive voltage, a lower second positive voltage, a near-zero voltage, and a negative voltage.

[0026] In one or more embodiments, the first positive voltage is 15 to 30V, the second positive voltage is 5 to 15V, and the negative voltage is -15 to -30V.

[0027] In one or more embodiments, the first positive voltage is 24V, the second positive voltage is 10V, and the negative voltage is -24V.

[0028] In one or more embodiments, the push-pull waveform includes a non-periodic voltage sequence.

[0029] In one or more embodiments, the first type of particle is a light-scattering particle, and the second, third, and fourth types of particles are light-absorbing particles.

[0030] In one or more embodiments, the first type of particles is white, and the second, third, and fourth types of particles are selected from cyan, magenta, and yellow.

[0031] In one or more embodiments, the first type of particles is white, the second type of particles is cyan, the third type of particles is magenta, and the fourth type of particles is yellow.

[0032] In one or more embodiments, the first charge polarity is negative and the second charge polarity is positive.

[0033] In one or more embodiments, the multiplicative particle system has just four types of charged electrophoretic particles.

[0034] In one or more embodiments, the nonpolar fluid includes a charge control additive.

[0035] In one or more embodiments, the electrophoretic medium is encapsulated within a capsule or contained within a sealed microcell.

[0036] In one or more embodiments, the electrophoretic display is configured for integration into book readers, portable computers, tablet computers, monitors, phones, smart cards, signs, watches, jewelry, shelf labels, vehicle panels, or flash drives.

[0037] In one or more embodiments, the back electrode comprises a backplane including a segmented electrode or an array of pixel electrodes.

[0038] In one or more embodiments, the backplane includes an array of thin-film transistors coupled to pixel electrodes.

[0039] In one or more embodiments, the backplane includes an array of thin-film transistors coupled to pixel electrodes.

[0040] In one or more embodiments, the push-pull waveform is determined using a trained computer model.

[0041] In one or more embodiments, a system is disclosed which comprises a color electrophoretic display having a backplane in which a back electrode includes an array of pixel electrodes. The system further comprises a source driver, at least one power supply, and a controller that transmits 2 bits of data per pixel to the source driver to control the source driver to apply a selected voltage to a selected pixel electrode from at least one power supply. [Brief explanation of the drawing]

[0042] [Figure 1] Figure 1 is a schematic cross-sectional view showing the positions of various colored particles in an electrophoretic medium when displaying the three primary colors of subtractive and additive color mixing: black, white, and white.

[0043] [Figure 2A] Figure 2A is a simplified diagram of an exemplary EPD, which has four types of particles (white, yellow, magenta, and cyan) in a nonpolar fluid capable of rendering the full range of colors at each pixel electrode.

[0044] [Figure 2B] Figure 2B illustrates the transition between a first optical state, in which all particles of the first charge polarity are present on the viewing surface, and a second optical state, in which particles with the second (opposite) polarity are present on the viewing surface.

[0045] [Figure 2C] Figure 2C illustrates the transition between a first optical state, in which all particles of the first charge polarity are present on the viewing surface, and a third optical state, in which particles with the second (opposite) polarity are present behind intermediate charged particles of the first polarity located on the viewing surface.

[0046] [Figure 2D] Figure 2D illustrates the transition between a first optical state, which has all the particles of the first charge polarity on the viewing surface, and a fourth optical state, which has particles with the second (opposite) polarity behind the low-charge particles of the first polarity located on the viewing surface.

[0047] [Figure 2E] Figure 2E illustrates the transition between a first optical state, which has all the particles of the first charge polarity on the viewing surface, and a fifth optical state, which has a combination of low-charged and medium-charged particles of the first polarity located on the viewing surface, with particles of the second (opposite) polarity behind them.

[0048] [Figure 3] Figure 3 illustrates an exemplary equivalent circuit of a single pixel of an EPD.

[0049] [Figure 4] Figure 4 is a simplified diagram showing the layers of an exemplary EPD.

[0050] [Figure 5] Figure 5 shows an exemplary push-pull drive waveform with five voltage levels for addressing a four-particle electrophoresis medium containing white, yellow, magenta, and cyan particles.

[0051] [Figure 6A] Figures 6A and 6B show exemplary push-pull drive waveforms having four voltage levels for addressing a four-particle color electrophoresis medium, according to one or more embodiments. [Figure 6B]Figures 6A and 6B show exemplary push-pull drive waveforms having four voltage levels for addressing a four-particle color electrophoresis medium, according to one or more embodiments.

[0052] [Figure 7] Figure 7 is a schematic diagram of an exemplary drive system for controlling a four-particle color electrophoresis medium according to one or more embodiments.

[0053] [Figure 8] Figure 8 is a graph comparing the color gamut volumes obtained from waveforms with different drive voltage levels.

[0054] [Figure 9] Figure 9 shows the available color gamut using waveforms with different drive voltage levels. [Modes for carrying out the invention]

[0055] Various embodiments of the present invention disclosed herein relate to a full-color EPD addressable by a drive waveform having four voltage levels. The EPD includes an electrophoretic medium having four types of charged electrophoretic pigment particles, comprising three colored, substantially non-light-scattering pigments corresponding to primary colors of subtractive color mixing (cyan, magenta, and yellow), and a fourth light-scattering white pigment. The three colored pigments have one charge polarity, while the white pigment has the opposite charge polarity. Preferably, the white pigment is negatively charged and the three colored pigments are positively charged. The display renders any primary color (white, black, cyan, magenta, yellow, red, green, and blue) at any pixel position by being addressable by a waveform having just four different voltage levels. The voltage levels are specified by 2 bits of data, preferably including a high positive voltage, a low positive voltage, a single negative voltage, and a near-zero voltage. As used herein, “near-zero voltage” means -0.5V to 0.5V. EPDs utilize just four voltage levels and can be addressed with 2 bits of data per pixel, instead of the 3 bits or more used in conventional devices. As a result, they can reduce device costs while providing superior color quality without significantly sacrificing color gamut, as described below.

[0056] As background, U.S. Patent Application Publication No. 20220082896, whose entire content is incorporated herein by reference, discloses a four-particle electrophoretic medium comprising a first particle of a first polarity and three other particles having the opposite polarity and different charge amounts. Typically, such a system includes a negatively charged white particle and positively charged yellow, magenta, and cyan particles having subtractive primary colors. Furthermore, some particles may be manipulated so that their electrophoretic mobility is nonlinear with respect to the intensity of the applied electric field. Thus, one or more particles will suffer a decrease in electrophoretic mobility upon application of a high electric field (e.g., 20V or higher) of the correct polarity. Such a four-particle system is schematically shown in Figure 1 and can provide white, yellow, red, magenta, blue, cyan, green, and black at each pixel.

[0057] As shown in Figure 1, the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) each correspond to different arrangements of four particles, so that the viewer sees only their colored particles on the viewing side of the white particles (i.e., the only particles that scatter light). To achieve a wide range of colors, additional voltage levels are used for finer control of the particles. In the described formulation, the first (typically negative) particle is reflective (typically white), while the other three particles, charged oppositely (typically positive), include three substantially non-light-scattering ("SNLS") particles. The use of SNLS particles allows for color mixing and provides more color results than can be achieved with the same number of scattering particles. These thresholds must be sufficiently separated to avoid crosstalk, and this separation requires the use of higher address voltages for some colors. Four-particle electrophoretic display media can also be updated more quickly, require "less flashing" transitions, and can produce a more pleasing (and therefore more commercially valuable) color spectrum for the viewer. In addition, the disclosed formulation provides fast (e.g., less than 500ms, e.g. less than 300ms, e.g. less than 200ms, e.g. less than 100ms) updates between black and white pixels, thereby enabling fast page turning of black text over white text.

[0058] In Figure 1, the viewing surface of the display is at the top (as shown in the figure), meaning the user views the display from this direction, and light is assumed to be incident from this direction. As already mentioned, only one of the four particles used in the electrophoretic medium substantially scatters light, and in Figure 1, this particle is assumed to be a white pigment. This light-scattering white particle forms a white reflector, and any particles above the white particle are visible relative to this reflector (as shown in Figure 1). Light incident on the viewing surface of the display passes through these particles, is reflected by the white particle, passes through these particles again, and exits the display. Therefore, particles above the white particle may absorb various colors, and the colors that appear to the user result from the combination of particles above the white particle. Any particles positioned below the white particle (behind the user's viewpoint) are masked by the white particle and do not affect the displayed colors. The second, third, and fourth particles are substantially non-light-scattering, so their order or arrangement relative to each other is not important; however, for the reasons already mentioned, their order or arrangement relative to the white (light-scattering) particles is extremely important.

[0059] More specifically, when cyan, magenta, and yellow particles are below the white particle (situation [A] in Figure 1), there are no particles above the white particle, and the pixel simply displays white. When a single particle is above the white particle, the color of that single particle is displayed as yellow, magenta, and cyan in situations [B], [D], and [F] in Figure 1, respectively. When two particles are positioned above the white particle, the displayed color is a combination of the colors of these two particles; in Figure 1, in situation [C], the magenta and yellow particles display red; in situation [E], the cyan and magenta particles display blue; and in situation [G], the yellow and cyan particles display green. Finally, when all three colored particles are above the white particle (situation [H] in Figure 1), all incident light is absorbed by the three subtractive primary color particles, and the pixel displays black.

[0060] It is possible that a primary color of subtractive color mixing can be rendered by light-scattering particles, and therefore a display could have two types of light-scattering particles, one of which is white and the other is colored. However, in this case, the position of the light-scattering colored particles relative to the other colored particles covering the white particles will be important. For example, when rendering a color as black (when all three colored particles are above the white particles), the scattering colored particles cannot be above the non-scattering colored particles (otherwise they would be partially or completely hidden behind the scattering particles, and the rendered color would be the color of the scattering colored particles, not black).

[0061] Figure 1 shows an ideal situation where color is not contaminated (i.e., white particles that scatter light completely mask the particles behind them). In reality, masking by white particles can be imperfect, as there may be some slight absorption of light by particles that would ideally be completely masked. Such contamination typically reduces both the brightness and saturation of the rendered color. Such color contamination should be minimized to a level where the resulting color meets industry-standard color reproduction. A particularly preferred standard is SNAP (a standard for newspaper advertising production), which specifies L for each of the eight primary colors mentioned above. * a * , and b * Specify the value. (Hereafter, "primary colors" will be used to refer to the eight colors shown in Figure 1: black, white, the three subtractive primary colors, and the three additive primary colors.)

[0062] Figures 2A–2E, also disclosed in U.S. Patent Application Publication No. 20220082896, show schematic cross-sectional views of four particle types. The display layer utilizing the improved electrophoretic medium includes a first (visible) surface 13 on the viewing side and a second surface 14 opposite the first surface 13. The electrophoretic medium is positioned between the two surfaces. Each space between the two vertical dotted lines represents a pixel. Within each pixel, the electrophoretic medium can be addressed, and the visible surface 13 of each pixel can achieve the color state shown in Figure 1 without the need for an additional layer and without a color filter array.

[0063] As is standard with respect to EPDs, the first surface 13 includes a common electrode 11, which is light-transmitting and consists of, for example, a sheet of PET with indium tin oxide (ITO) placed on top thereof. On the second surface 14 is an electrode layer 12 including a plurality of pixel electrodes 15. Such pixel electrodes are described in U.S. Patent No. 7,046,228, the contents of which are incorporated herein by reference in their entirety. While active matrix driving using a thin-film transistor (TFT) backplane is mentioned with respect to the layer of pixel electrodes, it should be noted that other types of pixel electrode addressing may be used as long as the pixel electrodes perform the desired function. For example, the upper and lower electrodes may be continuous or segmented. Furthermore, different pixel electrode backplanes than those described in Patent No. 228 are also suitable and may include an active matrix backplane capable of providing a higher drive voltage than those typically found in amorphous silicon thin-film transistor backplanes.

[0064] Newly developed active matrix backplanes include thin-film transistors incorporating metal oxide materials such as tungsten oxide, tin oxide, indium oxide, zinc oxide, or more complex metal oxides such as indium gallium zirconium oxide. In these applications, a channel-forming region is formed using such metal oxide materials for each transistor, enabling high-speed switching at higher voltages. Such metal oxide transistors also allow for less leakage in the "off" state of the thin-film transistor (TFT) than can be achieved by amorphous silicon TFTs, for example. In a typical scanning TFT backplane with n lines, the transistors are in the "off" state for about (n-1) / n of the time required to refresh each line of the display. Leakage of charge from the storage capacitor associated with each pixel will result in degradation of the electro-optical performance of the display. A TFT typically includes a gate electrode, a gate insulating film (typically SiO2), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film on the gate insulating film that at least partially overlaps the gate electrode, source electrode, and drain electrode. Such backplanes are available from manufacturers such as Sharp / Foxconn, LG, and BOE. Such backplanes are capable of providing drive voltages of ±30V (or higher). Intermediate voltage drivers may be included so that the resulting drive waveform can include five levels, seven levels, nine levels, or more.

[0065] One preferred metal oxide material for such applications is indium gallium zinc oxide (IGZO). IGZO-TFTs have 20–50 times the electron mobility of amorphous silicon. By using IGZO TFTs in the active matrix backplane, it is possible to provide voltages exceeding 30V via a suitable display driver. This allows for the use of a source driver (a switch within the EPD that determines which voltage to apply to each column electrode of a given selected row of the display) that can supply at least five, and possibly seven, drive voltage levels. In one example, there are two positive voltages, two negative voltages, and zero volts. In another example, there are three positive voltages, three negative voltages, and zero volts. In yet another example, there are four positive voltages, four negative voltages, and zero volts. These levels can be selected within a range of approximately -27V to +27V without the limitations imposed by top-plane switching as described above.

[0066] The electrophoretic media shown in Figures 2A-2E contain four types of electrophoretic particles in a nonpolar fluid 17. The first particle (W- * The first particle (M++) can be negatively charged and surface-treated such that the electrophoretic mobility of the first particle depends on the strength of the driving electric field (discussed in more detail below). In such a case, the electrophoretic mobility of the particle actually decreases in the presence of a stronger electric field, which is somewhat counterintuitive. *The particles (black circles) are positively charged and may be surface-treated (or intentionally left untreated) so that the electrophoretic mobility of the second particles depends on the strength of the driving electric field, or so that the unpacking rate of the aggregate of second particles after being driven to one side of the cavity containing the particles is slower than the unpacking rate of the aggregates of third and fourth particles when the direction of the electric field is reversed, or so that the particles form Coulomb aggregates with the first particles (in this case, W-) which are separable by a high applied electric field rather than a low applied electric field. The third particles (Y+; checkered circles) are positive but have a smaller charge magnitude than the second particles. Furthermore, the third particles may be surface-treated, but not in such a way that the electrophoretic mobility of the third particles depends on the strength of the driving electric field. That is, the third particles may have a surface treatment, but such a surface treatment does not result in the aforementioned decrease in electrophoretic mobility due to an increased electric field. The fourth particle (C+++; gray circle) has the largest size, a positive charge, and the same type of surface treatment as the third particle. As shown in Figure 2A, the particles are nominally white, magenta, yellow, and cyan in color, producing the colors shown in Figure 1. However, the system is not limited to this particular set of colors, nor is it limited to one reflective particle and three absorbing particles. For example, the system may include one black absorbing particle and three reflective particles of red, yellow, and blue with suitably matched reflectance spectra, and when all three reflective particles are mixed and visible on the surface, the process may produce a white state.

[0067] The first particle (negative) is white and scattering. The second particle (positive, medium charge) is magenta and absorbent. The third particle (positive, low charge) is yellow and absorbent. The fourth particle (positive, high charge) is cyan and absorbent. Table 1 below shows the diffuse reflectance of representative yellow, magenta, cyan, and white particles useful in the electrophoretic medium of the present invention, along with the ratio of their absorption and scattering coefficients determined by Kuberka-Munk analysis of these substances dispersed in a poly(isobutylene) matrix. [Table 1]

[0068] The electrophoretic medium may be any of the forms described above. Therefore, the electrophoretic medium may be unencapsulated, encapsulated in discrete capsules surrounded by a capsule wall, encapsulated in sealed microcells, or in the form of a polymer dispersion medium. The pigments are described in more detail in other literature, such as U.S. Patent Nos. 9,697,778 and 9,921,451. Briefly, the white particles W1 are silanol-functionalized light-scattering pigments (titanium dioxide) bound to a polymer material containing lauryl methacrylate (LMA) monomer, as described in U.S. Patent No. 7,002,728. The white particles W2 are polymer-coated titania produced substantially as described in Example 1 of U.S. Patent No. 5,852,196, where the polymer coating contains lauryl methacrylate and 2,2,2-trifluoroethyl methacrylate in a ratio of about 99:1. Yellow particle Y1 is CI Pigment Yellow 180, as commonly described in U.S. Patent No. 9,697,778, used without coating and dispersed by friction in the presence of Solsperse 19000. Yellow particle Y2 is CI Pigment Yellow 155, as commonly described in U.S. Patent No. 9,697,778, used without coating and dispersed by friction in the presence of Solsperse 19000. Yellow particle Y3 is CI Pigment Yellow 139, as commonly described in U.S. Patent No. 9,697,778, used without coating and dispersed by friction in the presence of Solsperse 19000. Yellow particle Y4 is CI Pigment Yellow 139, which is coated by dispersion polymerization incorporating trifluoroethyl methacrylate, methyl methacrylate, and dimethylsiloxane-containing monomers, as described in Example 4 of U.S. Patent No. 9,921,451.The magenta particles M1 are positively charged magenta material (dimethylquinacridone, CI Pigment Red 122) coated with vinyl benzyl chloride and LMA, as described in Example 5 of U.S. Patent No. 9,697,778 and U.S. Patent No. 9,921,451.

[0069] Magenta particles M2 are CI Pigment Red 122, which is coated by dispersion polymerization with methyl methacrylate and dimethylsiloxane-containing monomers, as described in Example 6 of U.S. Patent No. 9,921,451. Cyan particles C1 are copper phthalocyanine material (CI Pigment Blue 15:3) coated by dispersion polymerization incorporating methyl methacrylate and dimethylsiloxane-containing monomers, as described in Example 7 of U.S. Patent No. 9,921,451. In some embodiments, the color gamut has been found to be improved by using Ink Jet Yellow 4GC (Clariant) as the core yellow pigment and incorporating a methyl methacrylate surface polymer. The zeta potential of this yellow pigment can be adjusted by the addition of 2,2,2-trifluoroethyl methacrylate (TFEM) monomer and monomethacrylate-terminated poly(dimethylsiloxane).

[0070] Mechanisms proposed for electrophoretic medium additives and surface treatments to facilitate differential electrophoretic mobility, as well as for the interaction between the surface treatment and the surrounding charge control agent and / or free polymer, are discussed in detail in U.S. Patent No. 9,697,778, which is incorporated in whole as a reference. In such electrophoretic media, one way to control the interaction between different types of particles is by controlling the type, amount, and thickness of the polymer coating on the particles. For example, to control particle characteristics such that particle-particle interactions are less between a second type of particle and third and fourth type particles than, for example, between a third type of particle of a third species and a fourth type of particle, the second type of particle may have a polymer surface treatment, while the third and fourth type particles may have no polymer surface treatment or carry a polymer surface treatment having a lower mass coverage per unit area of ​​particle surface than the second type of particle. More generally, the Hamaker constant (a measure of the strength of the van der Waals interaction between two particles, where the pair potential is proportional to the Hamaker constant and inversely proportional to the sixth power of the distance between the two particles) and / or the interparticle spacing must be adjusted by a sensible selection of polymer coatings on the three types of particles.

[0071] As described in U.S. Patent No. 9,921,451, different types of polymers may involve different types of polymer surface treatments. For example, Coulomb interactions can be weakened when the closest approach distance of oppositely charged particles is maximized by a steric barrier (typically a polymer grafted or adsorbed on the surface of one or both particles). The polymer shell may be a covalent polymer fabricated by a grafting process or chemisorption, as is well known in the art, or it may be physically adsorbed on the particle surface. For example, the polymer may be a block copolymer containing insoluble and soluble segments. Alternatively, the polymer shell may be dynamic in that it is a loose network of free polymers from the electrophoretic medium that are complexed with pigment particles in the presence of an electric field and a sufficient amount and type of charge control agent (CCA - discussed below). Thus, depending on the strength and polarity of the electric field, the particles may have more associated polymers, which causes the particles to interact differently with the container (e.g., microcapsules or microcells) and other particles. The degree of polymer shelling is conveniently assessed by thermogravimetric analysis (TGA), a technique that involves increasing the temperature of a dry sample of particles and measuring the mass loss due to thermal decomposition as a function of temperature. Using TGA, the percentage of the particle's mass that is polymer can be measured, which can be converted to a volume fraction using the known densities of the core pigment and the polymer adhering to them. Conditions can be found in which the polymer coating is lost but the core pigment remains (these conditions depend on the exact core pigment particles used). Various polymer combinations can be made functional, as described below with respect to Figures 2A-2E. For example, in some embodiments, particles (typically first and / or second particles) may have a covalently bonded polymer shell that strongly interacts with the container (e.g., a microcell or microcapsule). Other particles of the same charge, on the other hand, do not have a polymer coating or form complexes with free polymer in the solution, and therefore these particles hardly interact with the container.In other embodiments, the particles (typically the first and / or second particles) have no surface coating so that they are more readily able to form a charge bilayer and experience a decrease in electrophoretic mobility in the presence of a strong electric field.

[0072] The fluid 17, in which four types of particles are dispersed, is colorless and transparent. The fluid contains charged electrophoretic particles that move through the fluid under the influence of an electric field. A preferred suspension fluid, when a conventional aqueous encapsulation method is used, has a low dielectric constant (about 2) and a high volume resistivity (about 10). 15 It has a viscosity of less than 5 mPas (Ohm-cm), low toxicity and environmental impact, and low water solubility (less than 10 ppm). However, it should be noted that this requirement may be relaxed for non-encapsulation or some microcell display, high boiling point (above about 90°C), and low refractive index (less than 1.5). The last requirement arises from the use of high refractive index scattering (typically white) pigments, whose scattering efficiency depends on the refractive index mismatch between the particles and the fluid.

[0073] Saturated linear or branched hydrocarbons, silicone oils, halogenated organic solvents, and organic solvents such as low molecular weight halogen-containing polymers are some useful fluids. The fluid may contain a single component or a blend of one or more components to adjust its chemical and physical properties. Reactants or solvents (if used) for microencapsulation processes, such as oil-soluble monomers, may be contained in this fluid.

[0074] The fluid preferably has low viscosity and a dielectric constant in the range of about 2 to about 30, preferably about 2 to about 15, for high particle mobility. Examples of suitable dielectric fluids include hydrocarbons such as Isopar®, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils, and silicon fluids; aromatic hydrocarbons such as toluene, xylene, phenylxylethane, dodecylbenzene, or alkylnaphthalenes; halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluorobenzene, dichlorononane, or pentachlorobenzene; perfluorinated solvents such as FC-43, FC-70, or FC-5060 (manufactured by 3M Corporation, St. Paul MN); low molecular weight halogen-containing polymers such as poly(perfluoropropylene oxide) (manufactured by TCI America, Portland, Oregon); poly(chlorotrifluoroethylene) such as Halocarbon Oil (manufactured by Halocarbon Product Corp., River Edge, NJ); and Galden or DuPont products from Ausimont. Examples include perfluoropolyalkyl ethers such as Krytox Oils and Greases K-Fluid Series from Delaware, and polydimethylsiloxane-based silicone oils from Dow-corning (DC-200).

[0075] Electrophoretic media also typically contain one or more charge control agents (CCAs) and may also contain charge directors. CCAs and charge directors typically contain low molecular weight surfactants, polymers, or blends of one or more components which help to stabilize, or otherwise alter, the sign and / or magnitude of the charge on the electrophoretic particles. CCAs are typically molecules containing ionic or other polar groups (hereinafter referred to as heads). At least one of the positive or negative ionic heads is preferably bonded to a nonpolar chain (typically a hydrocarbon chain) referred to below as a tail. CCAs form inverse micelles in the inner phase, and it is thought that small collections of charged inverse micelles are responsible for the conductivity in the highly nonpolar fluids typically used as electrophoretic fluids.

[0076] The addition of CCA provides the generation of inverse micelles containing a highly polar core, which can vary in size from 1 nm to tens of nanometers (and may have spherical, cylindrical, or other shapes), surrounded by the nonpolar tails of the CCA molecules. In electrophoretic media, three phases can typically be distinguished: a solid particle with a surface, a highly polar phase distributed in the form of extremely small droplets (inverse micelles), and a continuous phase containing fluid. Both charged particles and charged inverse micelles can move through the fluid when an electric field is applied, and therefore there are two parallel paths for electrical conduction through the fluid (typically having negligibly small electrical conductivity themselves).

[0077] The polar core of CCA is thought to influence the surface charge through adsorption onto the surface. In EPD, such adsorption may occur on the surface of the electrophoretic particle or on the inner wall of the microcapsule (or other solid phase such as the wall of the microcell), forming a structure similar to an inverse micelle, which will be referred to as hemimicelle hereafter. If one ion of an ion pair is more strongly attached to the surface than the other (e.g., by covalent bonding), ion exchange between the hemimicelle and the unbonded inverse micelle may result in charge separation, where the more strongly bonded ion remains associated with the particle, while the less strongly bonded ion is incorporated into the core of the free inverse micelle.

[0078] The ionic material forming the head of the CCA may also induce ion pair formation on the particle (or other) surface. Therefore, CCAs can perform two fundamental functions: charge generation at the surface and charge separation from the surface. Charge generation can arise from acid-base or ion exchange reactions between a portion present in the CCA molecule, or otherwise incorporated into the inverse micelle core or fluid, and the particle surface. Thus, a useful CCA material is one that can participate in such reactions, or any other charging reactions known in the art.

[0079] A non-limiting class of charge control agents useful in electrophoretic media includes organic sulfates or sulfonates, metal soaps, block copolymers or comb copolymers, organic amides, organic zwitterions, and organic phosphates and phosphonates. Useful organic sulfates and sulfonates include, but are not limited to, sodium bis(2-ethylhexyl) sulfosuccinate, calcium dodecylbenzenesulfonate, calcium petrolium sulfonate, neutral or basic barium dinonylnaphthalene sulfonate, neutral or basic calcium dinonylnaphthalene sulfonate, sodium dodecylbenzenesulfonate, and ammonium lauryl sulfate. Useful metal soaps include, but are not limited to, basic or neutral barium petronates, calcium petronates, cobalt, calcium, copper, manganese, magnesium, nickel, zinc, aluminum, and iron salts of carboxylic acids such as naphthenic acid, octanoic acid, oleic acid, palmitic acid, stearic acid, and myristic acid. Useful block or comb copolymers include, but are not limited to, AB diblock copolymers of (A) a polymer of 2-(N,N-dimethylamino)ethyl methacrylate quaternized with methyl p-toluenesulfonate and (B) poly(2-ethylhexyl methacrylate), and comb graft copolymers having an oil-soluble tail of poly(12-hydroxystearic acid), a molecular weight of about 1800, and suspended on an oil-soluble anchor group of poly(methyl methacrylate-methacrylic acid). Useful organic amides / amines include, but are not limited to, polyisobutylene succinimide, e.g., OLOA 371 or 1200 (available from Chevron Oronite Company LLC, Houston, TX), or SOLSPERSE 17000 or 19000 (available from Lubrizol, Wickliffe, OH; Solsperse is a registered trademark), and N-vinylpyrrolidone polymers. Lecithin is one example of a useful organic zwitterion, but it is not limited to this.Useful organic phosphates and phosphonates include, but are not limited to, sodium salts of phosphorylated mono- and di-glycerides having saturated and unsaturated acid substituents. Useful tails for CCAs include olefin polymers such as poly(isobutylene) with molecular weights ranging from 200 to 10,000. The head may be a sulfonic acid, phosphoric acid, or carboxylic acid or amide, or an amino group such as a primary, secondary, tertiary, or quaternary ammonium group. One class of CCAs useful in the disclosed four-particle electrophoretic media is disclosed in U.S. Patent Publication 2017 / 0097556 (incorporated herein in whole by reference). Such a CCA typically comprises a quaternary amine head and an unsaturated polymer tail, i.e., at least one CCA double bond. The polymer tail is typically a fatty acid tail. A variety of CCA molecular weights can be used. In some embodiments, the molecular weight of CCA is 12,000 grams / mol or more, for example, between 14,000 grams / mol and 22,000 grams / mol.

[0080] Charge enhancers used in the medium can bias the charge on the electrophoretic particle surface, as described in more detail below. Such charge enhancers may be Brønsted acids or bases or Lewis acids or bases. Exemplary charge enhancers are disclosed in U.S. Patents 9,765,015, 10,233,339, and 10,782,586, all of which are incorporated in their entirety by reference. Exemplary enhancers include, but are not limited to, polyhydroxy compounds containing at least two hydroxyl groups, such as ethylene glycol, 2,4,7,9-tetramethyldecine-4,7-diol, poly(propylene glycol), pentaethylene glycol, tripropylene glycol, triethylene glycol, glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), propyleneglycerol monohydroxystearate, and ethylene glycol monohydroxystearate. Examples of amino alcohol compounds containing at least one alcohol functional group and one amine functional group in the same molecule include, but are not limited to, triisopropanolamine, triethanolamine, ethanolamine, 3-amino-1-propanol, o-aminophenol, 5-amino-1-pentanol, and tetrakis(2-hydroxyethyl)ethylenediamine. In some embodiments, the charge booster is present in the EPD medium in an amount of about 1 to about 500 milligrams ("mg / g"), more preferably about 50 to about 200 mg / g per gram of particle mass.

[0081] Particle dispersion stabilizers may be added to prevent particle aggregation or adhesion to capsules or other walls or surfaces. For typical high-resistivity liquids used as fluids in EPDs, non-aqueous surfactants may be used. These include, but are not limited to, glycol ethers, acetylene glycols, alkanolamides, sorbitol derivatives, alkylamines, quaternary amines, imidazolines, dialkyl oxides, and sulfosuccinates.

[0082] As described in U.S. Patent No. 7,170,670, the bistability of an electrophoretic medium can be improved by including a polymer in the fluid having a number-average molecular weight greater than approximately 20,000, which is essentially non-absorbent to the electrophoretic particles, and poly(isobutylene) is a preferred polymer for this purpose. Also, as described in U.S. Patent No. 6,693,620, for example, particles with fixed charges on their surface form an electrical bilayer of opposite charges in the surrounding fluid. The ionic heads of CCA can form ion pairs with charged groups on the surface of the electrophoretic particles, forming a layer of immobilized or partially immobilized charged species. Outside this layer is a diffusion layer containing charged (reverse) micelles containing CCA molecules in the fluid. In conventional DC electrophoresis, a sliding surface occurs within the diffusion layer, and the applied electric field exerts a force on the fixed surface charges and an opposite force on the mobile pair charges, causing the particles to move relative to the fluid. The potential at the sliding surface is known as the zeta potential.

[0083] As a result, some particle types within the electrophoretic medium have different electrophoretic mobilities depending on the strength of the electric field across the medium. For example, when a first electric field (low intensity, i.e., approximately ±10V or less) is applied to the electrophoretic medium, particles of the first type move in one direction relative to the electric field. However, when a second electric field (high intensity, i.e., approximately ±20V or more) with the same polarity as the first electric field is applied, particles of the first type begin to move in the opposite direction relative to the electric field. This behavior is theorized to arise from conduction in a highly nonpolar fluid mediated by charged inverse micelles or inversely charged electrophoretic particles. Thus, any electrochemically generated protons (or other ions) are likely transported through the nonpolar fluid within the micelle core or adsorbed onto the electrophoretic particles. For example, as illustrated in Figure 5B of U.S. Patent No. 9,697,778, a positively charged inverse micelle can approach a negatively electrophoretic particle moving in the opposite direction, where the inverse micelle is incorporated into an electric bilayer around the negatively charged particle. (The electric bilayer includes both a charge diffusion layer with an enhanced counterion concentration and a semi-micelle surface adsorption coating on the particle. In the latter case, the inverse micelle charge will associate with the particle in a slip envelope that defines the particle's zeta potential, as described above.) Through this mechanism, an electrochemical current of positively charged ions flows through the electrophoretic fluid, and negatively charged particles can be biased toward more positive charges. As a result, for example, the electrophoretic mobility of a first negative type of particle is a function of the magnitude of the electrochemical current and the residence time of the positive charge near the particle surface, which is a function of the electric field strength.

[0084] Furthermore, as described in U.S. Patent No. 9,697,778, positively charged particles exhibiting different electrophoretic mobilities depending on the applied electric field can be prepared. In some embodiments, secondary (or co)CCA can be added to the electrophoretic medium to adjust the zeta potentials of various particles. Careful selection of co-CCA may allow modification of the zeta potential of one particle without essentially altering the zeta potentials of other particles, enabling precise control of both the electrophoretic velocities and interparticle interactions of various particles during switching.

[0085] In some embodiments, a portion of the charge control agents intended for the final formulation are added during the synthesis of electrophoretic particles to manipulate the desired zeta potential and influence the reduction of electrophoretic mobility under a strong electric field. For example, it has been observed that when quaternary amine charge control agents are added during polymer grafting, some amount of CCA is complexed onto the particles. (This can be confirmed by removing the particles from the electrophoretic fluid and then stripping the surface species from the pigment using THF to remove all adsorbed species. Evaluation of the THF extract by 1H NMR reveals that a considerable amount of CCA was adsorbed onto the pigment particles or formed complexes with the surface polymers.) Experiments suggest that a high CCA loading between the surface polymers of the particles promotes the formation of a charge bilayer around the particles in the presence of a strong electric field. For example, magenta particles with more than 200 mg of charge control agent (CCA) per gram of finished magenta particles exhibit excellent retention properties in the presence of a high positive electric field. (See, for example, Figure 2C and the description above.) In some embodiments, the CCA includes a quaternary amine head and a fatty acid tail. In some embodiments, the fatty acid tail is unsaturated. When some of the particles in the electrophoretic medium contain high CCA loadings, it is important that particles for which consistent electrophoretic mobility is desired do not have a substantial CCA loading, e.g., less than 50 mg of charge control agent (CCA) per gram of finished particle, e.g., less than 10 mg of charge control agent (CCA) per gram of finished particle.

[0086] Alternatively, an electrophoretic medium containing four types of particles in the presence of Solsperse 17000 in Isopar E can be enhanced by adding a small amount of an acidic substance, such as aluminum salt of di-t-butylsalicylic acid (Bontron E-88, available from Orient Corporation in Kenilworth, New Jersey). The addition of an acidic substance shifts the zeta potential of many (but not all) particles to a more positive value. In some cases, about 1% of the acidic substance and 99% of Solsperse 17000 (based on the total weight of the two materials) shifts the zeta potential of a third type of particle (Y+) from -5mV to about +20mV. Whether the zeta potential of a particular particle is altered by a Lewis acidic substance such as aluminum salt depends on the details of the particle's surface chemistry.

[0087] Table 2 shows exemplary relative zeta potentials of three types of colored and single-white particles in a preferred embodiment. [Table 2]

[0088] The negative (white) particle has a zeta potential of -30mV, while the remaining three particles are all positive relative to the white particle. Therefore, a display with positively charged cyan, magenta, and yellow particles can switch between a black state (all colored particles are in front of the white particle relative to the viewing surface) and a white state, with the white particle being closest to the viewer and preventing the viewer from perceiving the other three particles. In contrast, when the white particle has a zeta potential of 0V, the negatively charged yellow particle is the most negative of all particles, and therefore a display with this particle will switch between a yellow state and a blue state. This would also occur if the white particle were positively charged. However, the positively charged yellow particle would be more positive than the white particle unless its zeta potential exceeds +20mV.

[0089] The behavior of the electrophoretic medium is consistent with the mobility of the white particles (shown as zeta potential in Table 2), which depends on the applied electric field. Therefore, in the embodiments illustrated in Table 2, when addressed at a low voltage, the white particles may behave as if their zeta potential is -30mV, but when addressed at a higher voltage, they may behave as if their zeta potential is more positive, possibly even as high as +20mV (matching the zeta potential of the yellow particles). Consequently, when addressed at a low voltage, the display will switch between a black state and a white state, but when addressed at a higher voltage, it will switch between a blue state and a yellow state.

[0090] Particles with opposite charges can also form Coulomb aggregates. In aggregate form, the mobility of the particles may differ from that which can be measured for the components of the aggregate. Therefore, for example, aggregates may form between negatively charged white particles and either of the oppositely charged pigments. In certain embodiments, the electric field required to separate the Coulomb aggregates may preferably be higher for some pigment pairs than for others. In this example, the electric field required to separate the aggregates formed between magenta and white pigments is likely to be greater than the electric field required to separate the aggregates formed between cyan and white or yellow and white particles.

[0091] The charge of various pigment particles can be influenced by the presence of other charged pigments in their environment. Therefore, it is not necessarily true that the charge (or zeta potential) of pigments in the final formulation is the same as that measured for the pigment itself dispersed in the solvent in the presence of CCA.

[0092] The motion of various particles, according to one possible hypothesis, in the presence of high (e.g., "±H", e.g., ±20V, e.g., ±25V) and low (e.g., "±L", e.g., ±5V, e.g., ±10V) electric fields, is shown in Figures 2B-2E. For illustrative purposes, each box enclosed by a dashed line represents a pixel enclosed by an upper light-transmitting electrode 21 and a lower electrode 22, which may be an active matrix pixel electrode, but may also be a light-transmitting electrode or a segmented electrode, etc. Starting from a first state (nominally black) where all positive particles are present on the viewing surface, the electrophoretic medium can be driven to four different optical states, as shown in Figures 2B-2E. In a preferred embodiment, this results in a white optical state (Figure 2B), a magenta optical state (Figure 2C), a yellow optical state (Figure 2D), and a red optical state (Figure 2E). It is clear that the remaining four optical states in Figure 1 can be achieved by reversing the order of the initial state and the driving electric field, as briefly shown in Figure 5.

[0093] As shown in Figure 2B, when addressed at a low voltage, the particles behave according to their relative zeta potential, with the relative velocity when a negative voltage is applied to the backplane indicated by the arrows. Thus, in this embodiment, cyan particles move faster than magenta particles, and magenta particles move faster than yellow particles. The first (positive) pulse does not change the position of the particles because their movement is already restricted by the enclosure walls. The second (negative) pulse swaps the positions of the colored and white particles, and the display switches between a black state and a white state, with transient colors reflecting the relative mobility of the colored particles. Reversing the starting position and polarity of the pulses allows for a transition from white to black. Thus, this embodiment provides a black-and-white update that requires lower voltages (and consumes less power) compared to other black-and-white formulations achieved with multiple colors via either process black or process white.

[0094] In Figure 2C, the first (positive) pulse is a sufficiently high positive voltage to reduce the mobility of the magenta particles (i.e., particles with intermediate mobility among the three positively charged colored particles). Due to the reduced mobility, the magenta particles remain essentially frozen in place, and subsequent pulses in the opposite direction with a lower voltage move more cyan, white, and yellow particles than magenta particles, thereby producing magenta on the viewing surface with negative white particles behind the magenta particles. Importantly, if the starting position and pulse polarity are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through electrode 22), this pulse sequence will produce green (i.e., a mixture of yellow and cyan particles).

[0095] In Figure 2D, the first pulse is a low voltage that does not significantly reduce the mobility of magenta or white particles. However, the second pulse is a high negative voltage that reduces the mobility of white particles. This allows for more effective racing among the three positive particles, and thus the slowest type of particle (yellow in this embodiment) remains visible in front of the white particles, its movement reduced by the earlier negative pulse. In particular, the yellow particles do not reach the top surface of the cavity containing the particles. Importantly, if the starting position and pulse polarity are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through electrode 22), this pulse sequence will produce blue (i.e., a mixture of magenta and cyan particles).

[0096] Figure 2E shows that when both pulses are high voltage, the magenta particle mobility will be reduced by the first high positive pulse, and the race between cyan and yellow will be improved by the reduction in white mobility caused by the second high negative pulse, which produces red. Importantly, if the starting position and pulse polarity are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through electrode 22), this pulse sequence produces cyan.

[0097] To obtain a high-resolution display, each pixel of the display should be addressable without interference from adjacent pixels. One way to achieve this objective is to provide an array of nonlinear elements, such as transistors or diodes, where at least one nonlinear element is associated with each pixel, producing an "active matrix" display. The addressing or pixel electrode that addresses a single pixel is connected to a suitable voltage source through the associated nonlinear element. Typically, when the nonlinear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this array, though assumed in the following description, is essentially arbitrary, and the pixel electrode may also be connected to the source of the transistor. Conventionally, in high-resolution arrays, pixels are arranged in a two-dimensional array of rows and columns such that any particular pixel is uniquely defined by the intersection of one designated row and one designated column. The sources of all transistors in each column are connected to a single column electrode, while the gates of all transistors in each row are connected to a single row electrode. Again, the assignment of sources to rows and gates to columns is conventional but essentially arbitrary and can be reversed as needed. The row electrodes are connected to a row driver, which essentially ensures that at any given moment, only one row is selected, i.e., a selection voltage is applied to the selected row electrodes such that all transistors in the selected row are conductive, while a deselection voltage is applied to all other rows such that all transistors in those unselected rows remain non-conductive. The column electrodes are connected to a column driver, which applies a selected voltage to various column electrodes to drive the pixels in the selected row to their desired optical state. (The aforementioned voltages are for a common front electrode, conventionally supplied from a nonlinear array to the opposite side of the electro-optical medium and extending throughout the display.) After a pre-selected interval known as the "line address time," the selected row is deselected, the next row is selected, and the voltage on the column driver is changed so that the next line of the display is written.This process is repeated such that the entire display is written row by row.

[0098] Conventionally, each pixel electrode has an associated capacitor electrode such that the pixel electrode and the capacitor electrode form a capacitor. See, for example, International Patent Application No. WO2001007961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) may be used to form the transistor, and the “select” and “non-select” voltages applied to the gate electrode can be positive and negative, respectively.

[0099] FIG. 3 of the accompanying drawings depicts an exemplary equivalent circuit of a single pixel of an EPD. As shown, the circuit includes a capacitor 10 formed between the pixel electrode and the capacitor electrode. The electrophoretic medium 20 is represented as a parallel capacitor and resistor. In some cases, the direct or indirect coupling capacitance 30 (commonly referred to as “parasitic capacitance”) between the gate electrode of the transistor associated with the pixel and the pixel electrode can generate undesirable noise in the display. Typically, the parasitic capacitance 30 is much smaller than that of the storage capacitor 10, and when the pixel row of the display is selected or deselected, the parasitic capacitance 30 can typically result in a small negative offset voltage to the pixel electrode, also known as the “kickback voltage,” which is less than 2 volts. In some embodiments, to compensate for the undesirable “kickback voltage,” a common potential V com may be supplied to the top electrode and the capacitor electrode associated with each pixel, and thus, when V com is set to a value equal to the kickback voltage (V KB ), all voltages supplied to the display are offset by the same amount, and the net DC imbalance may not be affected.

[0100] However, V comProblems can arise when the voltage is set to a level that is not compensated for kickback voltage. This can occur when it is desired to apply a higher voltage to the display than is available from the backplane alone. For example, the maximum voltage applied to the display is, for example, V com It is well known in the art that if -V is supplied while the backplane is supplied with nominal options of +V, 0, or -V, it can be doubled. In this case, the maximum voltage experienced is +2V (i.e., at the backplane relative to the top surface), while the minimum voltage is zero. If a negative voltage is required, V com The potential must be raised to at least zero. Therefore, the waveforms used to address the display with positive and negative voltages using top-plane switching must be one more V com Each voltage setting must have a specific frame assigned to it.

[0101] A set of waveforms for driving a color EPD having four particles is described in U.S. Patent No. 9,921,451, which is incorporated herein by reference. In U.S. Patent No. 9,921,451, seven different voltages, namely three positive voltages, three negative voltages, and a zero voltage, are applied to the pixel electrodes. However, in some cases, the maximum voltage used with these waveforms is higher than the voltage that amorphous silicon thin-film transistors can handle. In such cases, a suitable high voltage can be obtained by using top-plane switching. (As described above) V com But intentionally V KB If set to this, a separate power supply may be used. However, when top-plane switching is used, V com Using the same number of individual power supplies as the number of settings is costly and inconvenient. Furthermore, top-plane switching is known to increase kickback, thereby degrading the stability of color states.

[0102] Display devices can be constructed using electrophoretic fluids in several ways known in the prior art. The electrophoretic fluid may be encapsulated in microcapsules or incorporated into microcell structures, which are then sealed with a polymer layer. The microcapsules or microcell layers may be coated or embossed onto a plastic substrate or film having a transparent coating of conductive material. This assembly may be laminated onto a backplane having pixel electrodes using a conductive adhesive. Alternatively, the electrophoretic fluid may be dispensed directly onto a thin open-cell grid arranged on a backplane containing the active matrix of pixel electrodes. The filled grid can then be sealed on top with an integrated protective sheet / light-transmitting electrode.

[0103] Figure 4 shows a schematic cross-sectional view (not to exact scale) of a display structure 200 including an electrophoretic medium. In the display 200, the electrophoretic fluid is shown as being confined in a microcell, but other structures, e.g., microcapsules, may also be used. A substrate 202, which may be glass or plastic, supports pixel electrodes 204 that are individually addressed segments or associated with thin-film transistors in an active matrix array. (The combination of the substrate 202 and electrodes 204 is conventionally called the backplane of the display.) Layer 206 is an optional dielectric layer according to the present invention applied to the backplane. (A method for depositing a suitable dielectric layer is described in U.S. Patent Application Publication No. 2020 / 0348576, incorporated by reference.) The front of the display includes a transparent substrate 222 supporting a transparent conductive coating 220. Above the electrode layer 220 is an optional dielectric layer 218. Layer (or more layers) 216 is a polymer layer which may include a primer layer for bonding the microcell to the transparent electrode layer 220 and some residual polymer including the bottom of the microcell. The walls of the microcell 212 are used to contain the electrophoretic fluid 214. The microcell is sealed with layer 210 and the entire front plane structure is bonded to the back plane using a conductive adhesive layer 208. The process for forming the microcell is described in the prior art, e.g., U.S. Patent No. 6,930,818. In some examples, the microcell is less than 20 μm deep, e.g., less than 15 μm deep, e.g., less than 12 μm deep, e.g., about 10 μm deep, e.g., about 8 μm deep.

[0104] Most commercially available EPDs use amorphous silicon-based thin-film transistors (TFTs) in the construction of the active matrix backplane (202 / 024) due to the wider availability of manufacturing equipment and the cost of various starting materials. Unfortunately, amorphous silicon thin-film transistors become unstable when supplied with gate voltages that would allow switching of voltages higher than approximately + / -15V. Nevertheless, as described below, the performance of ACePs is improved when the magnitude of high positive and negative voltages exceeding + / -15V is tolerated. Thus, as described in previous disclosures, the improved performance is achieved by further varying the bias of the upper light-transmitting electrode relative to the bias on the backplane pixel electrode, which is also known as top-plane switching. Thus, if a voltage of +30V (relative to the backplane) is required, the top plane can be switched to -15V while the appropriate backplane pixels are switched to +15V. A method for driving a four-particle electrophoresis system using top-plane switching is described in detail, for example, U.S. Patent No. 9,921,451.

[0105] These waveforms illustrate each pixel of the display as 30V, 15V, 0, -15V, and -30V, +V high , +V low , 0, -V low , and -V high It is required that it can be driven by five different addressing voltages, as specified. In practice, it may be preferable to use more addressing voltages. Three voltages (i.e., +V high , 0, and -V high If only ) is available, then voltage V high However, by addressing using pulses with a duty cycle of 1 / n, it is possible to use lower voltages (e.g., V high It may be possible to achieve the same result as addressing in / n (where n is a positive integer > 1).

[0106] Figure 5 shows a typical waveform (in a simplified form) used to drive the aforementioned four-particle color electrophoresis display system. Such waveforms have a “push-pull” structure, meaning they consist of a dipole with two pulses of opposite polarity. The magnitude and length of these pulses determine the resulting color. The waveform has five such voltage levels. Figure 5 shows positive and negative voltages for high and low voltages, as well as zero volts. Typically, “low” (L) refers to a range of approximately 5–15V, and “high” (H) refers to a range of approximately 15–30V. Generally, the greater the magnitude of the “high” voltage, the better the color gamut achieved by the display. An additional “medium” (M) level, typically about 15V, may also be used, although the value of M will depend to some extent on the particle composition and the environment of the electrophoresis medium.

[0107] Figure 5 shows the simplest dipole required to form a color, but it should be understood that actual waveforms may use multiple repetitions of these patterns or other non-periodic patterns.

[0108] Naturally, achieving the desired color using the drive pulses in Figure 5 is conditional on starting the process from a known state where the particle is unlikely to be the last color displayed on the pixel. Therefore, a series of reset pulses precede the drive pulses, which increase the amount of time required to update the pixel from the first color to the second color. The reset pulses are described in detail in U.S. Patent No. 10,593,272 and are taken by reference. The length of these pulses (refresh and address) and any pauses (i.e., periods of zero voltage between them) may be chosen so that the entire waveform (i.e., the integral of voltage over time over the entire waveform) is DC balanced (i.e., the integral of voltage over time is substantially zero). DC balance can be achieved by adjusting the length of the pulses and pauses in the reset phase so that the net impulse supplied in the reset phase is equal in magnitude and opposite in sign to the net impulse supplied in the address phase, when the display is switched to a particular desired color. However, as shown in Figures 2B-2E, the starting state for the eight primary colors is either black or white, which can be achieved using a sustained low-voltage drive pulse. The ease of achieving this starting state further reduces the update time between states, which is more comfortable for the user and also reduces the amount of power consumed (and thus increases battery life).

[0109] Furthermore, the aforementioned discussion of waveforms, specifically the discussion of DC equilibrium, ignores the kickback voltage problem. In reality, as mentioned above, all backplane voltages are affected by the kickback voltage V KB It is offset by an amount equal to the voltage supplied by the power supply. Therefore, if the power supply used provides three voltages, +V, 0, and -V, the backplane is actually offset by the voltage V+V KB , V KB , and -V+V KB (V KBNote that in the case of amorphous silicon TFTs, this is usually a negative number. However, the same power supply will supply +V, 0, and -V to the front electrode without any kickback voltage offset. Therefore, for example, if the front electrode is supplied with -V, the display will have a maximum voltage of 2V+V KB and minimum V KB It will incur costs and inconvenience. KB Instead of using a separate power supply to provide voltage to the front electrode, the waveform is such that the front electrode receives positive voltage, negative voltage, and V KB It may be divided into supplied portions.

[0110] (Four-pigment EPD addressed by four voltage levels) According to one or more embodiments of the present invention, a full-color EPD addressable by a waveform having exactly four voltage levels is disclosed. The EPD includes an electrophoretic medium having four types of charged electrophoretic pigment particles, comprising three colored, substantially non-light-scattering pigments corresponding to primary colors of subtractive color mixing (cyan, magenta, and yellow), and a fourth light-scattering white pigment. The three colored pigments have one charge polarity, while the white pigment has the opposite charge polarity. The white pigment is preferably negatively charged, and the three colored pigments are positively charged. The display renders any primary color (white, black, cyan, magenta, yellow, red, green, and blue) at any pixel position without utilizing top-plane switching (i.e., while keeping the voltage of the light-transmitting electrodes of the EPD constant) by being addressable by a waveform having exactly four different voltage levels. The voltage levels are specified by 2 bits of data per pixel, and preferably include a high positive voltage, a low positive voltage, a single negative voltage, and a near-zero voltage.

[0111] Because EPDs utilize just four voltage levels and can be addressed with 2 bits of data per pixel, instead of the 3 or more bits used in conventional color EPDs that do not utilize top-plane switching, they can reduce device costs while providing superior performance and color quality without significantly sacrificing color gamut, as described below.

[0112] As described above, U.S. Patents 9,697,778 and 10,678,111 disclose two different types of EPDs capable of rendering any color at any pixel position. The first of these patents discloses a display in which a white (light-scattering) pigment moves in a first direction when addressed with a low applied voltage and in the opposite direction when addressed with a higher voltage. The second patent discloses a full-color EPD containing four pigments: white, cyan, magenta, and yellow. U.S. Patents 10,276,109 and others disclose a method for addressing such a display using a waveform having at least five different voltage levels.

[0113] Conventional monochrome EPDs typically use only three different voltage levels: one positive voltage level, one negative voltage level, and a zero voltage level. The source driver of a monochrome EPD receives 2 bits of data per pixel from the display controller, specifying which power supply to connect to which column.

[0114] When five voltage levels (the minimum number used to address conventional full-color multiply pigment displays) are required, the module architecture must be modified in one of two common ways: (a) maintaining a 2-bit per pixel architecture but implementing top-plane switching to change the potential difference between the backplane electrodes and the top (transparent) common electrode for all pixels in the display, or (b) providing a source driver that can select at least five different power supplies. In the latter case, 2 bits is insufficient information, and at least 3 bits of data per pixel must be supplied to the source driver by the controller.

[0115] Both of these approaches have significant drawbacks. Adjusting the top surface of the display affects all pixels, thus limiting the flexibility of possible waveform structures that could be used to address the display. For example, it is impossible to simultaneously assert the maximum positive and negative voltages across the electrophoretic solutions of two separate pixels. As a result, the waveform is typically about twice as long. Requiring the controller to supply at least 3 bits of data per pixel necessitates faster data transfer rates (or longer transfer times), which increases the cost of the controller or reduces the frame time that can be supported. In addition, many controllers in the current EPD ecosystem cannot supply 3 bits of data per pixel. Therefore, providing at least five different voltage levels can lead to unacceptably high module costs or excessively long waveforms for many applications.

[0116] It is preferable to address color EPDs using 2 bits of data per pixel without using top-plane switching. Using 2 bits of data per pixel allows for the specification of four voltage levels, not just three. Dynamically reallocating power per frame does not offer an equally flexible alternative, as the reallocation applies to all pixels when updating a particular frame, leading to the same kind of compromise as top-plane switching.

[0117] In an EPD having negatively charged white pigment and positively charged cyan, magenta, and yellow pigments, the complementary colors yellow / blue and magenta / green are best represented by a waveform with dipoles (high and low voltage pulses of opposite polarity). In conventional devices, the waveform used a nearly DC-balanced coupled dipole sequence to render these colors. Thus, if the dipole has a first pulse of voltage V1 over time t1 and a second pulse of voltage -V2 over time t2, DC equilibrium is achieved when V1t1 = V2t2.

[0118] If V1 is positive and |V1|>|V2|, the display fluctuates between blue (at the end of a positive pulse) and magenta (at the end of a negative pulse). Conversely, if the polarity of each pulse is reversed, the display fluctuates between yellow (at the end of a negative pulse) and green (at the end of a positive pulse). Therefore, it is expected that all four of these voltage levels, plus the zero-volt option, may be required to render all colors at each pixel.

[0119] However, it has been discovered that by using a more complex aperiodic voltage sequence having exactly four voltage levels (preferably a high positive voltage, a low positive voltage, a single negative voltage, and a near-zero voltage), it is possible to address the display and create all colors at any pixel position. (The term "aperiodic" refers to a voltage sequence that does not have a precisely repeating structure.) Tuning such a waveform is more complex and difficult than the easily parameterizable dipole structures of prior art, and therefore, performing tuning directly on an actual display panel can require an impractically long time. However, it has been found that such complex waveforms can be tuned more easily using a two-stage computer modeling process. In the first stage, an arbitrary set of voltage lists is fed to the display, and the optical response is recorded at the end of each frame. From this training set, machine learning techniques are used to accurately (e.g., about 1-2 dB) determine the effect of applying a voltage list that was not present in the original training set. * A model is constructed that predicts (with accuracy). In the second stage, the model is used to achieve tuning of the ideal waveform, which makes it possible to test far more waveforms in a given time than is possible with a physical display module.

[0120] Figures 6A and 6B show exemplary waveforms having four voltage levels for driving a four-particle color EPD according to one or more embodiments. These figures show exemplary pulse sequences used to form eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) in each pixel. The magnitude and length of these pulses determine the resulting color. In one or more embodiments (for example, as depicted in Figure 6A), the four voltage levels are a high positive voltage, a low positive voltage, near zero volts, and a high negative voltage (+V). high , +V low , 0, and -V highThis includes (as specified). In one or more alternative embodiments (for example, as depicted in Figure 6B), the four voltage levels are high negative voltage and low negative voltage, near zero volts, and high positive voltage (-V). high , -V low , 0, and +V high This includes (as specified). Typically, the "low" voltage ranges from approximately 5 to 15V, and the "high" voltage ranges from approximately 15 to 30V. Generally, the larger the magnitude of the "high" voltage, the better the color gamut achieved by the display. In one particular embodiment, the four voltage levels are 24V, 10V, 0, and -24V. In another particular embodiment, the four voltage levels are -24V, -10V, 0, and +24V.

[0121] Figure 7 is a schematic diagram of an exemplary drive system 300 for controlling a pixel electrode array 302 of a four-particle color electrophoresis medium, according to one or more embodiments. The drive system 300 may be in the form of an integrated circuit. The elements of the array 302 are arranged in the form of a matrix having a plurality of data lines and a plurality of gate lines. Each element of the matrix includes a TFT for controlling the electrode potential of the corresponding electrode, and each TFT is connected to one of the gate lines and one of the data lines.

[0122] The illustrated controller comprises a microcontroller 304 containing control logic and switching logic. It receives input data from input data line 322. The microcontroller has outputs for each data line of the matrix and provides data signals. Data signal line 306 connects each output to the data line of the matrix. The microcontroller 304 also has outputs for each gate line of the matrix and provides gate line selection signals. Gate signal line 308 connects each output to the gate line of the matrix. Data line drivers 310 and gate line drivers 312 are located for each data signal line and each gate signal line, respectively. The figure shows only the signal lines for the data lines and gate lines shown in the figure. The gate line drivers may be integrated into a single integrated circuit. Similarly, the data line drivers may be integrated into a single integrated circuit (as source drivers). The integrated circuit may include a complete driver assembly together with the microcontroller.

[0123] The data line driver supplies a signal level corresponding to the voltage selected to drive the pixel. The gate line driver provides a signal to select the gate line on which the electrodes are actuated. A sequence of one voltage from the data line driver 210 is shown in Figure 7. When a sufficiently large positive voltage is present on the gate line, a low impedance exists between the data line and the pixel, so the voltage on the data line is transferred to the pixel. When there is a negative voltage on the TFT gate, the TFT has high impedance, and the voltage is stored in the pixel capacitor and is not affected by the data line voltage. The figure shows four columns labeled n through n+3 and five rows labeled n through n+4.

[0124] As illustrated in Figure 7, simultaneous line addressing is used, where one gate line n is high and all others are low. Then, the signals on all data lines are transferred to all pixels in row n. At the end of line time, the signal on gate line n goes low, the next gate line n+1 goes high, and as a result, the data for the next line is transferred to the TFT pixel in row n+1. This is repeated until all gate lines are scanned sequentially and the entire matrix is ​​driven. This is the same method used in almost all active-matrix LCDs such as mobile phone screens, laptop screens, and LCD-TVs, as well as in active-matrix EPDs, where the TFT controls the voltage maintained across the liquid crystal layer.

[0125] (Examples) The advantages of the aforementioned full-color EPD, which is addressed by a waveform with four voltage levels, have been experimentally confirmed.

[0126] The experiment was performed on an EPD module equipped with an electrophoretic medium containing white particles, cyan particles, magenta particles, and yellow particles of the types described above in relation to Figures 2A to 2E, namely, white particles, cyan particles, magenta particles, and yellow particles, respectively, represented by W-, C+++, M++, and Y+. The electrophoretic fluid was contained in a microcup with a thickness of 8 microns.

[0127] The experiment was conducted using waveforms with various voltage levels listed in Table 3. [Table 3-1] [Table 3-2]

[0128] The waveforms were adjusted using the computer modeling method described above.

[0129] The EPD module is addressed by each of the different waveforms, and the results are recorded and shown in Tables 4-8. [Table 4] [Table 5] [Table 6-1] [Table 6-2] [Table 7] [Table 8]

[0130] The results show that the "4-level" driving scheme (Table 5), with two negative voltage levels, one positive voltage level, and a near-zero voltage level, provides only a slight improvement in color gamut volume compared to the "3-level" driving scheme (Table 4), while the "4-level" driving scheme (Table 5), with two positive voltage levels, one negative voltage level, and a near-zero voltage level, provides only a slight improvement in color gamut volume. * This means that the "5-level" drive scheme (Table 6) approaches the quality achievable using the "7-level" drive scheme (Table 7). There is only a very small difference between the "5-level" drive scheme and the "7-level" drive scheme (Table 8).

[0131] Table 9 shows the details of the colors obtained with various drive voltage schemes. [Table 9]

[0132] 4-level with two positive voltages, one negative voltage, and a near-zero voltage. *When the drive scheme is used, magenta and blue are reduced in quality compared to the 5-level drive scheme, as expected. However, this quality degradation is acceptable because magenta in the 5-level drive scheme is particularly unbalanced with other colors, especially yellow and green. Thus, some sacrifice in magenta quality is acceptable in exchange for improved yellow and green quality. This trade-off is highlighted in Figure 9, which shows the available color gamut for various drive voltage schemes.

[0133] Therefore, a four-particle pigment EPD, addressed by a waveform with just four voltage levels, provides superior color quality while reducing the cost of the display by using 2 bits instead of 3 bits per pixel data.

[0134] While several aspects and embodiments of the art of this application have been described herein, it should be understood that various changes, modifications, and improvements will be readily conceivable to those skilled in the art. Such modifications, modifications, and improvements are intended to be within the spirit and scope of the art described herein. For example, those skilled in the art will readily conceive of various other means and / or structures for performing the functions described herein and / or obtaining the results and / or one or more advantages, and each of such variations and / or modifications will be considered within the scope of the embodiments described herein. Those skilled in the art will be able to recognize or confirm many equivalents to the particular embodiments described herein by means of routine experimentation alone. Therefore, it should be understood that the embodiments described herein are presented only as examples, and within the scope of the appended claims and their equivalents, embodiments of the present invention may be carried out in ways different from those specifically described. In addition, any combination of two or more features, systems, articles, materials, kits, and / or methods described herein is included within the scope of this disclosure, provided that such features, systems, articles, materials, kits, and / or methods are not contradictory to each other.

Claims

1. A color electrophoresis display, Light-transmitting electrodes on the viewing surface, Back electrode and An electrophoretic medium disposed between the light-transmitting electrode and the back electrode, wherein the electrophoretic medium is Nonpolar fluids and A multi-pigment particle system comprising four types of charged electrophoretic pigment particles dispersed in the nonpolar fluid, wherein the four types of charged electrophoretic pigment particles are A first type of particle having a first optical property and a first charge polarity, A second type of particle having a second optical property and a second charge polarity, wherein the second charge polarity is opposite to the first charge polarity, A third type of particle having a third optical property and the second charge polarity, A fourth type of particle having a fourth optical property and the second charge polarity Includes, The first optical properties, the second optical properties, the third optical properties, and the fourth optical properties are different from each other, and are a multi-pigment particle system. Electrophoretic media including Equipped with, A multi-pigment particle system is directly addressable using a push-pull waveform applied to the back electrode, the push-pull waveform having pulse voltage levels selected from a set of exactly four different voltage levels to render one of eight primary colors—red, green, blue, cyan, magenta, yellow, black, and white—at each pixel of the electrophoretic medium while keeping the voltage on the light-transmitting electrode constant, the voltage levels including a first positive voltage, a second positive voltage lower than the first positive voltage, a near-zero voltage, and a negative voltage, in a color electrophoretic display.

2. The color electrophoretic display according to claim 1, wherein the first positive voltage is 15 to 30 V, the second positive voltage is 5 to 15 V, and the negative voltage is -15 to -30 V.

3. The color electrophoretic display according to claim 1, wherein the first positive voltage is 24V, the second positive voltage is 10V, and the negative voltage is -24V.

4. The color electrophoretic display according to claim 1, wherein the push-pull waveform includes a non-periodic voltage sequence.

5. The color electrophoretic display according to claim 1, wherein the first type of particle is a light-scattering particle, and the second type of particle, the third type of particle, and the fourth type of particle are light-absorbing particles.

6. The color electrophoretic display according to claim 1, wherein the first type of particles is white, and the second type of particles, the third type of particles, and the fourth type of particles are selected from cyan, magenta, and yellow.

7. The color electrophoretic display according to claim 1, wherein the first type of particles is white, the second type of particles is cyan, the third type of particles is magenta, and the fourth type of particles is yellow.

8. The color electrophoretic display according to claim 1, wherein the first charge polarity is negative and the second charge polarity is positive.

9. The color electrophoretic display according to claim 1, wherein the multiplicative particle system has exactly four types of charged electrophoretic particles.

10. The color electrophoretic display according to claim 1, wherein the nonpolar fluid includes a charge control additive.

11. The color electrophoretic display according to claim 1, wherein the electrophoretic medium is encapsulated within a capsule or contained within a sealed microcell.

12. The color electrophoretic display according to claim 1, wherein the electrophoretic display is configured for integration into a book reader, portable computer, tablet computer, monitor, telephone, smart card, sign, wristwatch, jewelry, shelf label, vehicle panel, or flash drive.

13. The color electrophoretic display according to claim 1, wherein the back electrode comprises an active matrix backplane including a segmented electrode or an array of pixel electrodes.

14. The color electrophoretic display according to claim 13, wherein the active matrix backplane includes an array of thin-film transistors coupled to the pixel electrodes.

15. The color electrophoretic display according to claim 1, wherein the push-pull waveform is determined using a trained computer model.

16. A system, wherein the system is A color electrophoresis display according to any one of claims 1 to 15, The aforementioned back electrode comprises a color electrophoretic display having an active matrix backplane including an array of pixel electrodes, Source driver and, At least one power supply, A controller for transmitting 2 bits of data per pixel to the source driver in order to control the source driver to apply a selected voltage to a selected pixel electrode from at least one of the power supplies, A system that includes these features.

17. A method, wherein the said method is The present invention provides a color electrophoretic display comprising a light-transmitting electrode on a viewing surface, a back electrode, and an electrophoretic medium disposed between the light-transmitting electrode and the back electrode, wherein the electrophoretic medium comprises a nonpolar fluid and a multipigment particle system comprising four types of charged electrophoretic pigment particles dispersed in the nonpolar fluid, wherein the four types of charged electrophoretic pigment particles comprise a first type of particle having a first optical property and a first charge polarity, a second type of particle having a second optical property and a second charge polarity, wherein the second charge polarity is opposite to that of the first charge polarity, a third type of particle having a third optical property and a second charge polarity, and a fourth type of particle having a fourth optical property and a second charge polarity, wherein the first, second, third, and fourth optical properties are different from each other. The method involves directly addressing the multiplicative particle system using a push-pull waveform applied to the back electrode, wherein the push-pull waveform has voltage levels selected from a set of exactly four different voltage levels to render one of eight primary colors—red, green, blue, cyan, magenta, yellow, black, and white—at each pixel of the electrophoretic medium, while keeping the voltage on the light-transmitting electrode constant, and the four different voltage levels include a first positive voltage, a second positive voltage lower than the first positive voltage, a near-zero voltage, and a negative voltage. Methods that include...

18. The method according to any one of the claims, wherein the first positive voltage is 15 to 30V, the second positive voltage is 5 to 15V, and the negative voltage is -15 to -30V.

19. The method according to claim 17, wherein the first positive voltage is 24V, the second positive voltage is 10V, and the negative voltage is -24V.

20. The method according to claim 17, wherein the push-pull waveform includes a non-periodic voltage sequence.

21. The method according to claim 17, wherein the first type of particle is a light-scattering particle, and the second type of particle, the third type of particle, and the fourth type of particle are light-absorbing particles.

22. The method according to claim 17, wherein the first type of particles is white, and the second type of particles, the third type of particles, and the fourth type of particles are selected from cyan, magenta, and yellow.

23. The method according to claim 17, wherein the first type of particles is white, the second type of particles is cyan, the third type of particles is magenta, and the fourth type of particles is yellow.

24. The method according to claim 17, wherein the first charge polarity is negative and the second charge polarity is positive.

25. The method according to claim 17, wherein the multiply pigment particle system has exactly four types of charged electrophoretic particles.

26. The method according to claim 17, wherein the nonpolar fluid includes a charge control additive.

27. The method according to claim 17, wherein the electrophoretic medium is encapsulated in a capsule or contained in a sealed microcell.

28. The method according to claim 17, wherein the electrophoretic display is configured for integration into a book reader, portable computer, tablet computer, monitor, telephone, smart card, sign, wristwatch, jewelry, shelf label, vehicle panel, or flash drive.

29. The method according to claim 17, wherein the back electrode comprises an active matrix backplane including a segmented electrode or an array of pixel electrodes.

30. The method according to claim 29, wherein the active matrix backplane includes an array of thin-film transistors coupled to the pixel electrodes.

31. The method according to claim 17, further comprising determining the push-pull waveform using a trained computer model.

Images