Colored electrophoretic media with four pigment particle systems addressable by waveforms with four voltage levels

Abstract

A colored electrophoretic display includes a light transmissive electrode at a viewing surface, a back electrode, and an electrophoretic medium therebetween. The electrophoretic medium includes a non-polar fluid and a multi-pigment particle system having four types of charged electrophoretic pigment particles in the fluid. The particles include first, second, third, and fourth types of particles having different optical characteristics. The second, third, and fourth types of particles have a charge polarity opposite to that of the first type of particles. The multi-pigment particle system is directly addressable by a push-pull waveform applied to the back electrode, the push-pull waveform having a voltage level selected from a set of exactly four different voltage levels to exhibit any of eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) at each pixel, and meanwhile, the voltage on the light-transmitting electrode is kept constant. The voltage levels include a first positive voltage, a lower second positive voltage, a near zero voltage, and a negative voltage.

Description

Cross-references to related applications

[0001] This application claims priority to U.S. Provisional Patent Application No. 6,347,1148, filed June 5, 2023, entitled “COLOR ELECTROPHORETIC MEDIUM HAVING FOUR PIGMENT PARTICLE SYSTEM ADDRESSABLE BY WAVEFORMS HAVING FOURVOLTAGE LEVELS,” the entire contents of which are incorporated herein by reference. Background Technology

[0002] Electrophoretic displays (EPDs) change color by adjusting the position of charged colored particles relative to a light-transmitting viewing surface. These electrophoretic displays are often called "electronic paper" or "ePaper" because the resulting displays have high contrast and are readable in sunlight, much like ink on paper. Electrophoretic displays are widely used in e-readers such as the Amazon Kindle® because they provide a book-like reading experience, consume little 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 colored particles: black and white. (Of course, "colored" as used herein includes both black and white.) White particles are often light-scattering and include, for example, titanium dioxide; while black particles are absorbing in the visible spectrum and may include carbon black or absorbing metal oxides (such as copper chromite). In its simplest form, a monochrome EPD only requires a transparent electrode at the viewing surface, a back electrode, and an electrophoretic medium containing white and black particles with opposite charges. When a voltage of one polarity is applied, the white particles move to the viewing surface; 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)—segmented electrodes or an active matrix of pixel electrodes controlled by transistors—patterns can be electronically rendered at the viewing surface. For example, the pattern could be the text of a book.

[0004] Recently, various color options have become commercially available for EPDs, including tri-color displays (black, white, red; black, white, yellow), four-color displays (black, white, red, yellow), and color filter displays that rely on the aforementioned black / white particles. EPDs with three or four reflective particles operate similarly to traditional monochrome displays because the desired colored particles are driven onto the viewing surface. The driving scheme is far more complex than with only black and white, but ultimately, the optical function of the particles is the same: to reflect incident light back to the observer in the correct color.

[0005] Advanced Color Electronic Paper (ACeP™) also contains four types of particles, but the cyan, yellow, and magenta particles are subtractive rather than reflective, thus allowing thousands of colors to be produced at each pixel. This color process is functionally equivalent to printing methods long used in offset and inkjet printers. A given color is produced by using the correct proportions of cyan, yellow, and magenta on a bright white paper background. In the ACeP instance, the relative positions of the cyan, yellow, magenta, and white particles relative to the viewing surface determine the color at each pixel. While this type of EPD allows thousands of colors to be displayed at each pixel, the key lies in carefully controlling the position of each pigment (50 to 500 nanometers in size) within a working space approximately 10 to 20 micrometers thick. Obviously, variations in particle position will result in an incorrect color being displayed at a given pixel. Therefore, such systems require precise voltage control. Further details of this system are available in the following U.S. patents, all of which are incorporated herein by reference in their entirety: U.S. Patent Nos. 9,361,836, 9,921,451, 10,276,109, 10,353,266, 10,467,984, and 10,593,272.

[0006] The term grayscale state is used in this document in its conventional sense within the imaging field, referring to a state intermediate between two extreme optical states of a pixel, and does not necessarily imply a black-to-white transition between these two extreme states. For example, several Eink patents and published applications mentioned below describe EPDs with extreme states of white and dark blue, so the intermediate grayscale state is actually light blue. In fact, as already mentioned, changes in optical state may not be color changes at all. The terms black and white can be used below to refer to two extreme optical states of a display and should be understood to generally include extreme optical states that are not strictly black and white, such as the white and dark blue states mentioned above.

[0007] The terms bistable and bistable are used herein in their conventional sense to refer to a display comprising display elements having a first display state and a second display state that differ in at least one optical characteristic, such that after any given element is driven to present its first or second display state by an addressing pulse of finite duration, the state persists for at least several times (e.g., at least four times) the shortest duration of the addressing pulse required to change the state of the display element after the addressing pulse terminates. As shown in U.S. Patent No. 7,170,670, some particle-based EPDs capable of supporting grayscale are stable not only in their extreme black and white states but also in their intermediate grayscale states, as are some other types of electro-optical displays. Such displays are more appropriately referred to as multistable than bistable, but for convenience, the term bistable may be used herein to encompass both bistable and multistable displays.

[0008] The term impulse, when used in relation to driving an EPD, is used herein to refer to the integral of the voltage applied during driving the display with respect to time.

[0009] Particles that absorb, scatter, or reflect light over a wide band or at a selected wavelength are referred to herein as colored particles or pigment particles. Various materials that absorb or reflect light, other than pigments (the term strictly refers to insoluble colored materials), such as dyes or photonic crystals, can also be used in the electrophoretic media and displays of this invention.

[0010] Particle-based EPDs have been a subject of in-depth research and development for many years. In these displays, multiple charged particles (sometimes called pigment particles) move through a fluid under the influence of an electric field. Compared to liquid crystal displays (LCDs), EPDs offer advantages such as good brightness and contrast, wide viewing angles, state bistability, and low power consumption. However, long-term image quality issues have hindered their widespread adoption. For example, the particles constituting the EPD are prone to settling, leading to a shorter lifespan for these displays.

[0011] As mentioned 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 made 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. When the medium is used in a direction that allows such sedimentation (e.g., in a sign, where the medium is in a vertical plane), such gas-based electrophoretic media, like liquid-based electrophoretic media, exhibit the same type of problem due to particle sedimentation. In fact, particle sedimentation presents a more serious problem in gas-based electrophoresis media compared to liquid-based electrophoresis media because the lower viscosity of gaseous suspensions compared to liquids causes electrophoretic particles to settle more quickly.

[0012] Numerous patents and applications, assigned or registered in the name of MIT and Einkel, describe various techniques for encapsulating electrophoretic and other electro-optic media. Such encapsulation media comprise numerous small capsules, each capsule itself including an inner phase and a capsule wall surrounding the inner phase, wherein the inner phase contains electrophoretically mobile particles in a fluid medium. Typically, the capsules themselves are contained in a polymer binder to form a coherent layer located between two electrodes. The techniques described in these patents and applications include:

[0013] (a) Electrophoretic particles, fluids, and fluid additives; see, for example, U.S. Patent Nos. 7,002,728 and 7,679,814;

[0014] (b) Encapsulation, adhesives, and encapsulation processes; see, for example, U.S. Patent Nos. 6,922,276 and 7,411,719;

[0015] (c) Microunit structures, wall materials, and methods of forming microunits; see, for example, U.S. Patent Nos. 7,072,095 and 9,279,906;

[0016] (d) Methods for filling and sealing microcells; see, for example, U.S. Patent Nos. 7,144,942 and 7,715,088;

[0017] (e) Thin films and subassemblies containing electro-optic materials; see, for example, U.S. Patent Nos. 6,982,178 and 7,839,564;

[0018] (f) Backplates, adhesive layers and other auxiliary layers used in displays, and methods thereof; see, for example, U.S. Patent Nos. 7,116,318 and 7,535,624.

[0019] (g) Color formation and color adjustment; see, for example, U.S. Patents 6017584, 6545797, 6664944, 6788452, 6864875, 6914714, 6972893, 7038656, 7038670, 7046228, 7052571, 7075502***, 7167155, 7385751, 7492505, 7667684, 7684108, 7791789, 7800813, 7821702, 7839564***, 7910175, 7952790, 7956841, 7982941, 8040594, 8 054526, 8098418, 8159636, 8213076, 8363299, 8422116, 8441714, 8441716, 8466852, 8503063, 8576470, 8576475, 8593721, 8605354, 8649084, 8670174, 8704756, 8717664, 8786935, 8797634, 8810899, 8830559, 8873129, 8902153, 8902491, 8917439, 8964282, 9013783, 9116412, 914 6439, 9164207, 9170467, 9170468, 9182646, 9195111, 9199441, 9268191, 9285649, 9293511, 9341916, 9360733, 9361836, 9383623 and 9423666, and U.S. Patent Application Publications 2008 / 0043318, 2008 / 0048970, 2009 / 0225398, 2010 / 0156780, 2011 / 0043543, 2012 / 0326957, 2013 / 0242378, 2013 / 0278995, 2 Numbers 014 / 0055840, 2014 / 0078576, 2014 / 0340430, 2014 / 0340736, 2014 / 0362213, 2015 / 0103394, 2015 / 0118390, 2015 / 0124345, 2015 / 0198858, 2015 / 0234250, 2015 / 0268531, 2015 / 0301246, 2016 / 0011484, 2016 / 0026062, 2016 / 0048054, 2016 / 0116816, 2016 / 0116818 and 2016 / 0140909.

[0020] (h) A method for driving a display; see, for example, U.S. Patent Nos. 5,930,026, 6,445,489, 6,504,524, 6,512,354, 6,531,997, 6,753,999, 682,5970, 6,900,851, 6,995,550, 7012,600, 7023,420, 7034,783, 7061,166, 7061,662, 711,6466, 711,9772, 717,7066, 7193,625, 720,2847, 724,2514, 7259,744, 7304,787, 7 312794, 7327511, 7408699, 7453445, 7492339, 7528822, 7545358, 7583251, 7602374, 7612760, 7679599, 7679813, 7683606, 7688297, 7729039, 7733311, 7733335, 7787169, 7859742, 7952557, 7956841, 7982479, 7999787, 8077141, 812550 1. 8139050, 8174490, 8243013, 8274472, 8289250, 8300006, 8305341, 8314784, 8373649, 8384658, 8456414, 8462102, 8514168, 8537105, 8558783, 8558785, 8558786, 8558855, 8576164, 8576259, 8593396, 8605032, 8643595, 8665206, 868 Numbers 1191, 8730153, 8810525, 8928562, 8928641, 8976444, 9013394, 9019197, 9019198, 9019318, 9082352, 9171508, 9218773, 9224338, 9224342, 9224344, 9230492, 9251736, 9262973, 9269311, 9299294, 9373289, 9390066, 9390661, and 9412314And U.S. Patent Application Publications Nos. 2003 / 0102858, 2004 / 0246562, 2005 / 0253777, 2007 / 0091418, 2007 / 0103427, 2007 / 0176912, 2008 / 0024429, 2008 / 0024482, 2008 / 0136774, 2008 / 0291129, 2008 / 0303780, 2009 / 0174651, 2009 / 0195568, and 2009 / 032272 1. 2010 / 0194733, 2010 / 0194789, 2010 / 0220121, 2010 / 0265561, 2010 / 0283804, 2011 / 0063314, 2011 / 0175875, 2011 / 0193840, 2011 / 0193841, 2011 / 0199671, 2011 / 0221740, 2012 / 0001957, 2012 / 0098740, 2013 / 0063333, 2013 / 019 4250, 2013 / 0249782, 2013 / 0321278, 2014 / 0009817, 2014 / 0085355, 2014 / 0204012, 2014 / 0218277, 2014 / 0240210, 2014 / 0240373, 2014 / 0253425, 2014 / 0292830, 2014 / 0293398, 2014 / 0333685, 2014 / 0340734, 2015 / 0070744, 2015 Patents and applications numbered / 0097877, 2015 / 0109283, 2015 / 0213749, 2015 / 0213765, 2015 / 0221257, 2015 / 0262255, 2015 / 0262551, 2016 / 0071465, 2016 / 0078820, 2016 / 0093253, 2016 / 0140910 and 2016 / 0180777 (these patents and applications may be referred to below as MEDEOD (method for driving an electro-optical display) applications);

[0021] (i) Applications of displays; see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348; and

[0022] (j) Non-electrophoretic displays, as described in U.S. Patent No. 6,241,921, U.S. Patent Application Publication No. 2015 / 0277160, and U.S. Patent Application Publications Nos. 2015 / 0005720 and 2016 / 0012710.

[0023] Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thereby producing a so-called polymer dispersion electrophoretic display (EPD), wherein the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymeric material, and the discrete droplets of electrophoretic fluid within such a polymer dispersion electrophoretic display can be considered as capsules or microcapsules, even without a discrete capsule membrane associated with each individual droplet; see, for example, U.S. Patent No. 6,866,760. Therefore, for the purposes of this application, such polymer dispersion electrophoretic media are considered a subclass of encapsulated electrophoretic media.

[0024] One related type of EPD is the so-called micro-unit EPD. In micro-unit EPDs, charged particles and fluids are not encapsulated in microcapsules, but are retained in multiple cavities formed within a carrier medium (typically a polymer film). See, for example, U.S. Patents 6,672,921 and 6,788,449.

[0025] Although electrophoretic media are often opaque (e.g., in many electrophoretic media, particles substantially block visible light from passing through the display) and operate in reflective mode, many EPDs can be configured to operate in a so-called shutter mode, in which one display state is substantially opaque and another display state is transparent. 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. Dielectrophoretic displays are similar to EPDs but rely on variations in electric field strength and can operate in a similar mode. See U.S. Patent 4,418,346. Other types of electro-optic displays may also be able to operate in shutter mode. Electro-optic media operating in shutter mode can be used in multilayer structures of full-color displays; in such structures, at least one layer adjacent to the display's viewing surface operates in shutter mode to expose or conceal a second layer farther from the viewing surface.

[0026] Encapsulated EPDs are generally not plagued by the aggregation and sedimentation failure modes of conventional electrophoretic equipment and offer further advantages, such as the ability to print or coat displays on a wide range of flexible and rigid substrates. (The word "print" is intended to encompass all forms of printing and coating, including but not limited to: pre-metering coating, such as patch die coating, slot or extrusion coating, slide or stack coating, curtain coating; roll coating, such as blade roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; screen printing; electrostatic printing; thermal printing; inkjet printing; electrophoretic deposition (see U.S. Patent No. 7,339,715); and other similar techniques.) Therefore, the final display can be flexible. Furthermore, because the display medium can be printed (using a variety of methods), the manufacture of the display itself is not expensive.

[0027] As described above, the simplest prior art electrophoretic media essentially displays only two colors. Such electrophoretic media use a single type of electrophoretic particle with a first color, located in a colored fluid with a different second color (in this case, the first color is displayed when the particle is near the viewing surface of the display, and the second color is displayed when the particle is far from the viewing surface); or they use first and second types of electrophoretic particles with different first and second colors, located in a colorless fluid (in this case, the first color is displayed when the first type of particle is near the viewing surface of the display, and the second color is displayed when the second type of particle is near the viewing surface). Typically, these two colors are black and white. If a full-color display is desired, an array of color filters can be deposited on the viewing surface of a monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color mixing to generate color stimuli. Available display areas are 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 (stripes) or two-dimensional (2 x 2) repeating pattern. Other choices of primary colors, or choices of more than three primary colors, are also known in the art. The selected three (for RGB) or four (for RGBW) subpixels are small enough that at the intended viewing distance, they visually blend together to form a single pixel with uniform color stimulation (“color mixing”). An inherent drawback of area sharing is that colorant is always present, and colors can only be modulated by switching the corresponding pixel of the underlying monochrome display to white or black (turning the corresponding primary color on or off). For example, in an ideal RGBW display, each of the red, green, blue, and white primary colors occupies one-quarter of the display area (one of the four subpixels), while the brightness of the white subpixel is the same as the white of the underlying monochrome display, and the brightness of each colored subpixel does not exceed one-third of the brightness of the white of the monochrome display. The overall brightness of the white color displayed by the display cannot exceed half the brightness of the white subpixel (the white area of ​​the display is created by displaying one white subpixel out of every four subpixels, and each colored subpixel, when displayed in its colored form, is equivalent to one-third of the white subpixel, so the combined contribution of three colored subpixels does not exceed that of one white subpixel). Switching colored pixels to black reduces the brightness and saturation of colors through area sharing. Area sharing 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 the blue pixels (a quarter of the display area) to black makes the yellow too dark.

[0028] U.S. Patents 8,576,476 and 8,797,634 describe a multicolor EPD with a single backplate containing individually addressable pixel electrodes and a shared, transparent front electrode. Multiple electrophoretic layers are disposed between the backplate and the front electrode. The displays described in these patents are capable of displaying any primary color (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel location. However, there are drawbacks to using multiple electrophoretic layers located between a single set of addressable electrodes. The electric field experienced by particles in a particular layer is lower than that of a single electrophoretic layer addressed with the same voltage. Furthermore, optical losses in the electrophoretic layer closest to the viewing surface (e.g., caused by light scattering or unwanted absorption) can affect the appearance of the image formed in the underlying electrophoretic layer.

[0029] Two other types of EPD systems offer a single electrophoretic medium capable of displaying any color at any pixel location. Specifically, U.S. Patent No. 9,697,778 describes a display combining a dyeing solvent with white (light-scattering) particles, where the particles move in a first direction when addressed with a low applied voltage and in the opposite direction when addressed with a higher voltage. Full-color display can be achieved when the white particles and the dyeing solvent, along with two additional particles of opposite charge to the white particles, are combined. However, the color state of the '778 patent is not acceptable for applications such as text readers. Specifically, there is always some dyeing fluid separating the white scattering particles from the viewing surface, which causes a color shift in the display's white state.

[0030] U.S. Patent No. 9,921,451 describes a second electrophoretic medium capable of displaying any color at any pixel location. In the '451 patent, the electrophoretic medium contains four particles: white, cyan, magenta, and yellow, with two particles carrying a positive charge and two carrying a negative charge. However, the display of the '451 patent also suffers from color mixing with the white state. Because one of the particles has the same charge as the white particle, a certain number of particles with the same charge will move toward the viewing surface along with the white particle when a white state is desired. While this unwanted color shift can be overcome by driving complex waveforms in the display, such waveforms significantly increase the display's update time and, in some instances, cause unacceptable "flickering" between images. Summary of the Invention

[0031] This document discloses an improved full-color EPD that can be addressed by drive waveforms with four voltage levels. The EPD comprises an electrophoretic medium with four types of charged electrophoretic pigment particles, including three essentially non-light-scattering colored pigments, preferably corresponding to subtractive primary colors (cyan, magenta, and yellow), and a fourth pigment, light-scattering white. These three colored pigments have one charge polarity, while the white pigment has the opposite charge polarity. Preferably, the white pigment is negatively charged, while the three colored pigments are positively charged. The display is addressed by waveforms with exactly four different voltage levels to render any primary color (white, black, cyan, magenta, yellow, red, green, and blue) at any pixel location. Preferably, the voltage levels include high and low positive voltages, a single negative voltage, and near-zero voltage. Because this EPD uses exactly four voltage levels and can be addressed by two bits of data per pixel (instead of three or more bits used in prior art devices), device costs can be reduced while providing excellent color quality without significantly sacrificing color gamut.

[0032] In a first aspect, the present invention provides a colored electrophoretic display, comprising a light-transmitting electrode, a back electrode, and an electrophoretic medium disposed between the light-transmitting electrode and the back electrode at an observation surface. The electrophoretic medium comprises a nonpolar fluid and a multi-pigment particle system containing 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 characteristic and a first charge polarity; a second type of particle having a second optical characteristic and a second charge polarity opposite to the first charge polarity; a third type of particle having a third optical characteristic and the second charge polarity; and a fourth type of particle having a fourth optical characteristic and the second charge polarity. The first, second, third, and fourth optical characteristics are different from each other. The multi-pigment particle system is directly addressable by a push-pull waveform applied to the back electrode, the push-pull waveform having a pulse voltage level selected from a set of exactly four different voltage levels to present any one of eight primary colors—red, green, blue, cyan, magenta, yellow, black, and white—at each pixel of the electrophoretic medium, while maintaining a constant voltage on the light-transmitting electrode. The voltage levels include a first positive voltage, a lower second positive voltage, a near-zero voltage, and a negative voltage.

[0033] In a second aspect, the present invention provides a method comprising providing a colored electrophoretic display, including a transparent electrode, a back electrode, and an electrophoretic medium disposed between the transparent electrode and the back electrode at an observation surface. The electrophoretic medium comprises a nonpolar fluid and a multi-pigment particle system containing 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 the second charge polarity; and a fourth type of particle having a fourth optical property and the second charge polarity. The first, second, third, and fourth type optical properties are different from each other. The method also includes directly addressing the multi-pigment particle system by applying a push-pull waveform to the back electrode, the push-pull waveform having a voltage level selected from exactly four different voltage levels to render any one of eight primary colors—red, green, blue, cyan, magenta, yellow, black, and white—at each pixel of the electrophoretic medium, while maintaining a constant voltage on the light-transmitting electrode. The four different voltage levels include a first positive voltage, a lower second positive voltage, a near-zero voltage, and a negative voltage.

[0034] 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.

[0035] In one or more embodiments, the first positive voltage is 24V, the second positive voltage is 10V, and the negative voltage is -24V.

[0036] In one or more embodiments, the push-pull waveform includes an aperiodic voltage sequence.

[0037] In one or more embodiments, the first type of particle is a light-scattering particle, and the second, third, and fourth type of particles are light-absorbing particles.

[0038] In one or more embodiments, the first type of particle is white, and the second, third, and fourth type of particles are selected from cyan, magenta, and yellow.

[0039] In one or more embodiments, the first type of particle is white, the second type of particle is cyan, the third type of particle is magenta, and the fourth type of particle is yellow.

[0040] In one or more embodiments, the first charge is negative and the second charge is positive.

[0041] In one or more embodiments, the multi-pigment particle system has exactly four types of charged electrophoretic particles.

[0042] In one or more embodiments, the nonpolar fluid includes a charge-controlling additive.

[0043] In one or more embodiments, the electrophoretic medium is encapsulated in a capsule or contained in a sealed microcell.

[0044] In one or more embodiments, the electrophoretic display is configured to be incorporated into a book reader, portable computer, tablet computer, monitor, telephone, smart card, signage, watch, jewelry, shelf label, vehicle panel, or flash drive.

[0045] In one or more embodiments, the back electrode includes segmented electrodes or a backplate containing a pixel electrode array.

[0046] In one or more embodiments, the backplane includes an array of thin-film transistors coupled to the pixel electrode.

[0047] In one or more embodiments, the backplane includes an array of thin-film transistors coupled to the pixel electrode.

[0048] In one or more embodiments, a trained computer model is used to determine the push-pull waveform.

[0049] In one or more embodiments, a system is disclosed including a colored electrophoretic display, wherein the back electrode includes a backplate comprising an array of pixel electrodes. The system also includes a source driver; at least one power supply; and a controller for transmitting two bits per pixel data to the source driver to control the source driver to apply a selection voltage from the at least one power supply to a selected pixel electrode. Attached Figure Description

[0050] Figure 1 It is a schematic cross-sectional view showing the position of various colored particles in the electrophoretic medium when displaying black, white, the three subtractive primary colors, and the three additive primary colors.

[0051] Figure 2A This is a simplified illustration of an exemplary EPD, which has four types of particles (white, yellow, magenta, and cyan) in a nonpolar fluid, enabling it to display a full range of colors at each pixel electrode.

[0052] Figure 2B The transition between a first optical state and a second optical state is shown, wherein in the first optical state all particles with a first charge polarity are located on the observation surface, while in the second optical state particles with a second (opposite) charge polarity are located on the observation surface.

[0053] Figure 2CThe transition between a first optical state and a third optical state is shown, wherein in the first optical state all particles with a first charge polarity are located on the observation surface, while in the third optical state particles with a second (opposite) charge polarity are located after the moderately charged particles with a first charge polarity on the observation surface.

[0054] Figure 2D The transition between a first optical state and a fourth optical state is shown, wherein in the first optical state all particles with a first charge polarity are located on the observation surface, while in the fourth optical state particles with a second (opposite) charge polarity are located after low-charged particles with a first charge polarity on the observation surface.

[0055] Figure 2E The transition between a first optical state and a fifth optical state is shown, wherein in the first optical state all particles with a first charge polarity are located on the observation surface, while in the fifth optical state particles with a second (opposite) charge polarity follow a combination of low-charged particles and medium-charged particles with a first charge polarity located on the observation surface.

[0056] Figure 3 An exemplary equivalent circuit for a single pixel of the EPD is shown.

[0057] Figure 4 This is a simplified diagram showing the layers of an exemplary EPD.

[0058] Figure 5 An exemplary push-pull drive waveform with five voltage levels is shown for addressing a four-particle electrophoretic medium with white, yellow, magenta, and cyan particles.

[0059] Figure 6A and 6B An exemplary push-pull drive waveform with four voltage levels for addressing a four-particle colored electrophoretic medium is shown according to one or more embodiments.

[0060] Figure 7 This is a schematic diagram of an exemplary drive system for controlling a four-particle colored electrophoretic medium according to one or more embodiments.

[0061] Figure 8 It is a graph comparing the color gamut volume produced by waveforms with different numbers of driving voltage levels.

[0062] Figure 9 The color gamut achievable using waveforms with different numbers of drive voltage levels is shown. Detailed Implementation

[0063] Various embodiments of the invention disclosed herein relate to a full-color EPD addressable via drive waveforms having four voltage levels. The EPD comprises an electrophoretic medium having four types of charged electrophoretic pigment particles, including three substantially non-light-scattering colored pigments, preferably corresponding to subtractive primary colors (cyan, magenta, and yellow), and a fourth pigment, light-scattering white. These three colored pigments have one charge polarity, while the white pigment has the opposite charge polarity. Preferably, the white pigment is negatively charged, while the three colored pigments are positively charged. The display is addressed via waveforms having exactly four different voltage levels to display any primary color (white, black, cyan, magenta, yellow, red, green, and blue) at any pixel location. The voltage levels are specified by two bits of data and preferably include high and low positive voltages, a single negative voltage, and a near-zero voltage. As used herein, "near-zero voltage" refers to -0.5V to 0.5V. Because this EPD uses exactly four voltage levels and can be addressed with 2 bits of data per pixel instead of 3 or more bits used in existing devices, it can reduce device costs while providing excellent color quality without significantly sacrificing color gamut, as described below.

[0064] As background, U.S. Patent Application Publication No. 20220082896 (the entire contents of which are incorporated herein by reference) discloses a four-particle electrophoretic medium comprising a first particle having a first polarity and three other particles having opposite polarities and different charges. Typically, such a system includes negatively charged white particles and positively charged yellow, magenta, and cyan particles having subtractive primary colors. Furthermore, some particles can be designed such that their electrophoretic mobility is non-linearly related to the applied electric field strength. Therefore, when a high-voltage electric field of the correct polarity (e.g., 20 V or higher) is applied, one or more particles will experience a decrease in electrophoretic mobility. Such a four-particle system, as... Figure 1 As shown, it can provide white, yellow, red, magenta, blue, cyan, green, and black at each pixel.

[0065] like Figure 1As shown, each of the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) corresponds to a different arrangement of four particles, so that the observer can only see the colored particles on the observation side of the white particle (i.e., the only particle that scatters light). To achieve a wide range of colors, additional voltage levels are used to control the particles more finely. In the scheme described, the first particle (usually negatively charged) is reflective (usually white), while the other three particles with opposite charges (usually positively charged) contain three types of substantially non-light-scattering (SNLS) particles. Using SNLS particles allows for color mixing and provides more color effects than can be achieved using the same number of scattering particles. These thresholds must be sufficiently separated to avoid crosstalk, and this separation requires high addressing voltages for some colors. The four-particle electrophoretic medium can also be updated more quickly, requiring “less flickering” transitions and producing a more pleasing color spectrum for the observer (and therefore, greater commercial value). Furthermore, the disclosed solution provides fast updates between black and white pixels (e.g., less than 500 milliseconds, e.g., less than 300 milliseconds, e.g., less than 200 milliseconds, e.g., less than 100 milliseconds), thereby enabling fast page turning of black text on a white background.

[0066] exist Figure 1 In this scenario, we assume 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 incident from this direction. As mentioned earlier, only one of the four particles used in the electrophoretic medium significantly scatters light, and... Figure 1 In this context, these particles are assumed to be white pigment. These light-scattering white particles form white reflectors, and any particles located above the white particles (such as...) Figure 1 All of these particles (as shown) will be visible. Light entering the display's viewing surface passes through these particles, is reflected by the white particles, passes through these particles again, and exits the display. Therefore, particles above the white particles can absorb various colors, and the color presented to the user is the result of the combination of particles above the white particles. Any particles located below the white particles (behind them from the user's viewing point) are blocked by the white particles and do not affect the displayed color. Because the second, third, and fourth particles essentially do not scatter light, their order or arrangement relative to each other is unimportant, but for the reasons already explained, their order or arrangement relative to the white (light-scattering) particles is crucial.

[0067] More specifically, when cyan, magenta, and yellow particles are located below white particles ( Figure 1In case [A]), there are no particles above the white particles, and the pixel displays only white. When a single particle is above a white particle, the color of that single particle is displayed, respectively. Figure 1 The colors displayed are yellow, magenta, and cyan under conditions [B], [D], and [F]. When two particles are positioned above a white particle, the displayed color is a combination of the colors of those two particles; in... Figure 1 In case [C], magenta and yellow particles appear red; in case [E], cyan and magenta particles appear blue; and in case [G], yellow and cyan particles appear green. Finally, when all three colored particles are above the white particle ( Figure 1 In the case of [H], all incident light is absorbed by these three subtractive primary color particles, and the pixel displays black.

[0068] It is possible for a subtractive primary color to be rendered by particles that scatter light, so the display would contain two types of light-scattering particles, one for white and one for color. However, in this case, the position of the light-scattering color particles relative to other color particles covering the white particles becomes important. For example, when rendering black (when all three color particles are above the white particles), the scattering color particles cannot be above the non-scattering color particles (otherwise they would be partially or completely hidden behind the scattering particles, and the color rendered would be the color of the scattering color particles, not black).

[0069] Figure 1 This illustrates the ideal situation where color is uncontaminated (i.e., light-scattering white particles completely cover any particles located behind them). In reality, the coverage of white particles may be imperfect, so even particles that are ideally completely covered may absorb a small amount of light. This type of contamination typically reduces the brightness and chromaticity of the resulting color. Such color contamination should be minimized to ensure that the resulting color conforms to industry standards for color reproduction. A particularly favored standard is SNAP (Newsprint Advertising Specification), which specifies the L*, a*, and b* values ​​for each of the eight primary colors mentioned above. (The term "primary color" will be used below to refer to...) Figure 1 The eight colors shown are: black, white, three subtractive primary colors, and three additive primary colors.

[0070] Figures 2A-2E(Also disclosed in U.S. Patent Application Publication No. 20220082896) Schematic cross-sectional representations of four particle types are shown. The display layer employing an improved electrophoretic medium includes a first (observation) surface 13 on the observation side and a second surface 14 located on the opposite side of the first surface 13. The electrophoretic medium is located between these two surfaces. Each space between two vertical dashed lines represents a pixel. Within each pixel, the electrophoretic medium can be addressed, and the observation surface 13 for each pixel can be implemented without additional layers and without a color filter array. Figure 1 The color status shown.

[0071] As with the standard EPD, the first surface 13 includes a common electrode 11, which is transparent and may be, for example, composed of a PET plate with indium tin oxide (ITO) disposed thereon. An electrode layer 12 is present on the second surface 14, which includes a plurality of pixel electrodes 15. Such pixel electrodes are described in U.S. Patent No. 7,046,228, the entire contents of which are incorporated herein by reference. It should be noted that while the use of a thin-film transistor (TFT) backplane to drive an active matrix for the pixel electrode layer is mentioned, other types of electrode addressing may be used, provided the electrodes can achieve the desired function. For example, the top and bottom electrodes may be continuous or segmented. Furthermore, pixel electrode backplanes different from those described in the '228 patent are also applicable and may include active matrix backplanes capable of providing higher drive voltages than those typically found using amorphous silicon TFT backplanes.

[0072] The newly developed active matrix backplanes incorporate thin-film transistors (TFTs) with 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, each transistor forms a channel formation region using such metal oxide materials, enabling faster switching at higher voltages. The leakage current allowed by such metal oxide transistors in the "off" state of a thin-film transistor (TFT) is less than that achievable with, for example, amorphous silicon TFTs. In a typical scanning TFT backplane containing n lines, the transistors will be in the "off" state for approximately (n-1) / n of the time required to refresh each line of the display. Any charge leakage from the storage capacitors associated with each pixel will cause a degradation in the electro-optical performance of the display. TFTs typically consist of a gate electrode, a gate insulating film (typically SiO2), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film that overlaps at least partially with the gate, source, and drain electrodes above the gate insulating film. Such backplanes are available from manufacturers such as Sharp / Foxconn, LG, and BOE. Such backplanes can provide drive voltages of ±30V (or higher). Intermediate voltage drivers may be included so that the resulting drive waveform can include five, seven, nine, or more stages.

[0073] A preferred metal oxide material for such applications is indium gallium zinc oxide (IGZO). The electron mobility of IGZO-TFTs is 20 to 50 times that of amorphous silicon. By using IGZO TFTs in the active matrix backplane, voltages higher than 30V can be provided via a suitable display driver. This allows for at least five, and possibly seven, drive voltage levels to be provided using source drivers (switched in the EPD to determine which voltage to apply to each column electrode in a given selected row of the display). In one example, there are two positive voltages, two negative voltages, and zero volt. In another example, there are three positive voltages, three negative voltages, and zero volt. In yet another example, there are four positive voltages, four negative voltages, and zero volt. These levels can be selected in the range of approximately -27V to +27V, without the limitations imposed by the top-panel switching as described above.

[0074] like Figures 2A-2E The electrophoretic medium shown contains four types of electrophoretic particles in a nonpolar fluid 17. The first particle (W-*; hollow circle) is negatively charged and can be surface-treated such that its electrophoretic mobility depends on the strength of the driving electric field (discussed in more detail below). In this case, the electrophoretic mobility of the particle actually decreases when a stronger electric field is present, which is somewhat counterintuitive. The second particle (M++*; dark circle) is positively charged and can also be surface-treated (or intentionally left untreated), also such that its electrophoretic mobility depends on the strength of the driving electric field; or, when the electric field direction is reversed, the second particle's aggregation unpacking rate is slower than that of the third and fourth particles after being driven to one side of the cavity containing the particles; or, this particle forms a Coulomb aggregate with the first particle (W- in this example), which can be separated by applying a high electric field but not by applying a low electric field. The third particle (Y+; circle with a checkered pattern) is positively charged, but has a smaller charge than the second particle. Furthermore, the third particle can undergo surface treatment, but the treatment method should not cause its electrophoretic mobility to depend on the strength of the driving electric field. In other words, the third particle can be surface-treated, but such surface treatment will not cause the aforementioned decrease in electrophoretic mobility with increasing electric field. The fourth particle (C++; the gray circle) has the highest amount of positive charge, and its surface treatment type is the same as the third particle. Figure 2A As shown, these particles are described in terms of color as white, magenta, yellow, and cyan, thus producing... Figure 1The system is not limited to this specific set of colors, nor is it limited to one reflective particle and three absorbing particles. For example, the system could contain one black absorbing particle and three reflective particles (red, yellow, and blue) with appropriately matched reflectance spectra, producing a process white state when all three reflective particles are mixed and visible on the surface.

[0075] The first particle (negatively charged) is white and has scattering properties. The second particle (positively charged, medium charge) is magenta and has absorptive properties. The third particle (positively charged, low charge) is yellow and has absorptive properties. The fourth particle (positively charged, high charge) is cyan and has absorptive properties. Table 1 below shows the diffuse reflectance of exemplary yellow, magenta, cyan, and white particles available in the electrophoretic medium of the present invention, and the ratio of absorption coefficient to scattering coefficient obtained from Kubelka-Munk analysis of these materials dispersed in a polyisobutylene matrix.

[0076] Table 1. Diffuse reflectance of preferred yellow, magenta, cyan, and white particles.

[0077]

[0078] The electrophoretic medium can take any of the above forms. Therefore, the electrophoretic medium can be unencapsulated, encapsulated in individual capsules surrounded by capsule walls, encapsulated in sealed microunits, or exist in the form of a polymer dispersion medium. Pigments are described in more detail in other literature, such as U.S. Patents 9,697,778 and 9,921,451. In short, white particles W1 are silanol-functionalized light-scattering pigments (titanium dioxide) bonded to a polymeric material containing lauryl methacrylate (LMA) monomers, as described in U.S. Patent 7,002,728. White particles W2 are polymer-coated titanium dioxide, prepared in a manner substantially the same as that described in Example 1 of U.S. Patent 5,852,196, wherein the polymer coating comprises lauryl methacrylate and 2,2,2-trifluoroethyl methacrylate in a ratio of approximately 99:1. Yellow particles Y1 are CI Pigment Yellow 180, unencapsulated, and dispersed by milling in the presence of Solsperse 19000, as described in U.S. Patent 9,697,778. Yellow particle Y2 is CI Pigment Yellow 155, uncoated, and dispersed by milling in the presence of Solsperse 19000, as described in U.S. Patent No. 9,697,778. Yellow particle Y3 is CI Pigment Yellow 139, uncoated, and dispersed by milling in the presence of Solsperse 19000, as described in U.S. Patent No. 9,697,778. Yellow particle Y4 is CI Pigment Yellow 139, coated by dispersion polymerization, incorporating trifluoroethyl methacrylate, methyl methacrylate, and a monomer containing dimethylsiloxane, as described in Example 4 of U.S. Patent No. 9,921,451. Magenta particle M1 is a positively charged magenta material (dimethylquinacridone, CI Pigment Red 122), coated with vinylbenzyl chloride and LMA, as described in Example 5 of U.S. Patent Nos. 9,697,778 and 9,921,451.

[0079] Magenta particles M2 are CI Pigment Red 122, which are coated with methyl methacrylate and a dimethylsiloxane-containing monomer via dispersion polymerization, as described in Example 6 of U.S. Patent No. 9921451. Cyan particles C1 are copper phthalocyanine material (CI Pigment Blue 15:3), which are coated with methyl methacrylate and a dimethylsiloxane-containing monomer via dispersion polymerization, as described in Example 7 of U.S. Patent No. 9921451. In some embodiments, it has been found that using inkjet yellow 4GC (Clariant) as the core yellow pigment improves the color gamut while incorporating a methyl methacrylate surface polymer. The zeta potential of this yellow pigment can be adjusted by adding 2,2,2-trifluoroethyl methacrylate (TFEM) monomer and monomethacrylate-terminated poly(dimethylsiloxane).

[0080] U.S. Patent No. 9,697,778 discusses in detail the additives and surface treatments used to promote different electrophoretic mobilities in electrophoretic media, and the proposed mechanisms for the interaction between the surface treatment and the surrounding charge control agent and / or free polymer, the entire contents of which are incorporated herein by reference. In such electrophoretic media, one method of controlling the interactions between various types of particles is to control the type, quantity, and thickness of the polymer coating on the particles. For example, to control particle properties such that the particle-particle interaction between a second type of particle and third and fourth type particles is less than, for example, the interaction between a third type of particle and a fourth type particle, the second type of particles are polymer-surface-treated, while the third and fourth type particles are not polymer-surface-treated, or are polymer-surface-treated but with a lower mass coverage per unit area of ​​particle surface area than the second type of particles. More generally, the Hamaker constant (a measure of the strength of the van der Waals interaction between two particles, which is proportional to the potential energy and inversely proportional to the sixth power of the distance between the two particles) and / or the spacing between particles need to be adjusted by the appropriate selection of polymer coatings on these three types of particles.

[0081] As described in U.S. Patent No. 9,921,451, different types of polymers may contain different types of polymer surface treatments. For example, Coulomb interactions may be weakened when particles with opposite charges maximize their closest contact distance through steric hindrance (typically by grafting or adsorbing polymers onto the surface of one or both particles). The polymer shell can be a covalently bonded polymer obtained by grafting processes or chemisorption well known in the art, or it can be physically adsorbed onto the particle surface. For example, the polymer shell can be a block copolymer containing insoluble and soluble segments. Alternatively, the polymer shell can be dynamic, i.e., a loose network of free polymers from an electrophoretic medium complexed with pigment particles in the presence of an electric field and a sufficient number and variety of charge control agents (CCAs—discussed below). Thus, depending on the strength and polarity of the electric field, particles may have more associated polymers, which can lead to different interactions between particles and containers (e.g., microcapsules or microunits) and other particles. The extent of the polymer shell can be conveniently assessed by thermogravimetric analysis (TGA), a technique that involves raising the temperature of a particle-dried sample and measuring the change in mass loss due to pyrolysis with temperature. Using TGA, the proportion of polymer in the particle mass can be measured, and then converted into a volume fraction by the known density of the core pigment and the polymer to which the pigment is attached. Conditions under which the polymer coating is lost but the core pigment is retained can be identified (these conditions depend on the exact core pigment particles used). Figures 2A-2EThe following can be achieved using various polymer combinations. For example, in some embodiments, the particles (typically the first and / or second particles) may have a covalently bonded polymer shell that strongly interacts with the container (e.g., microunits or microcapsules). Meanwhile, other particles with the same charge have no polymer coating or form complexes with free polymers in solution, thus these particles have minimal interaction with the container. In other embodiments, the particles (typically the first and / or second particles) will have no surface coating, thus such particles are more likely to form a charge bilayer and experience reduced electrophoretic mobility in the presence of a strong electric field.

[0082] The fluid 17 dispersing these four types of particles is clear and colorless. This fluid contains charged electrophoretic particles that move through the fluid under the influence of an electric field. The preferred suspension fluid has a low dielectric constant (approximately 2) and a high volume resistivity (approximately 10⁻⁶). 15 The requirements include: low ohm-cm, low viscosity (less than 5 mPas), low toxicity and environmental impact, low water solubility (less than 10 parts per million (ppm) if conventional aqueous encapsulation methods are used; however, this requirement may be relaxed for unencapsulated or certain micro-cell displays), high boiling point (greater than approximately 90°C), and low refractive index (less than 1.5). The last requirement stems from the use of high-refractive-index scattering pigments (typically white), whose scattering efficiency depends on the refractive index mismatch between the particles and the fluid.

[0083] Saturated linear or branched hydrocarbons, silicone oils, halogenated organic solvents, and organic solvents containing low molecular weight halogenated polymers are some useful fluids. Fluids can contain a single component or a mixture of more than one component to modulate their chemical and physical properties. Reactants or solvents used in microencapsulation processes (if used), such as oil-soluble monomers, can also be included in the fluid.

[0084] The fluid preferably has low viscosity and a dielectric constant in the range of about 2 to about 30, and preferably in the range of about 2 to about 15 in order to achieve high particle mobility. Examples of suitable dielectric fluids include hydrocarbons (such as Isopar®, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils, and silicone fluids), aromatic hydrocarbons (such as toluene, xylene, phenyl dimethyl ethane, dodecylbenzene, or alkylnaphthalene), halogenated solvents (such as perfluoronaphthene, perfluorotoluene, perfluoroxylene, dichlorotrifluorotoluene, 3,4,5-trichlorotrifluorotoluene, chloropentafluorobenzene, dichlorononane, or pentachlorobenzene), and perfluorinated solvents (such as FC-43, FC-70, or FC-5060 from 3M (St. Paul, Minnesota), low molecular weight halogenated polymers (such as poly(perfluoropropylene oxide) from TCI America (Portland, Oregon), and poly(chlorotrifluoroethylene) (such as from Halocarbon Product Corp. (River, New Jersey)). Halocarbon oil from Edge), perfluoropolyethers such as Galden from Ausimont, or Krytox oil and Greases K-Fluid series from DuPont, Delaware, and polydimethylsiloxane silicone oil (DC-200) from Dow-corning.

[0085] Electrophoretic media typically also contain one or more charge control agents (CCAs) and may also contain charge directing agents. CCAs and charge directing agents typically comprise low-molecular-weight surfactants, polymerizing agents, or mixtures of one or more components used 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 head groups). At least one positive or negative ionic head group is preferably bonded to a nonpolar chain (typically a hydrocarbon chain) (hereinafter referred to as a tail group). It is believed that CCAs form reverse micelles in the inner phase, and that it is precisely the small amount of charged reverse micelles that results in conductivity in the strongly nonpolar fluids commonly used as electrophoretic fluids.

[0086] The addition of CCA enables the formation of reverse micelles containing highly polar cores, the size of which can vary from 1 nanometer to tens of nanometers (and can be spherical, cylindrical, or other geometries), surrounded by nonpolar tail groups of the CCA molecule. In the electrophoretic medium, three phases are typically distinguished: solid particles with surfaces, a highly polar phase distributed as extremely small droplets (reverse micelles), and a continuous phase containing the fluid. When an electric field is applied, both charged particles and charged reverse micelles can move through the fluid, thus creating two parallel paths for electrical conduction across the fluid (which itself typically has very low conductivity).

[0087] The polar cores of CCA are thought to influence the charge on the surface through adsorption. In EPD, this adsorption can occur on the surface of electrophoretic particles or on the inner wall of microcapsules (or other solid phases, such as microunit walls), forming structures similar to reverse micelles, referred to below as hemimicelles. When one ion in an ion pair is more strongly bonded to the surface than the other (e.g., through covalent bonding), ion exchange between hemimicelles and unbound reverse micelles can lead to charge separation, where the more strongly bound ion remains bound to the particle, while the less strongly bound ion is incorporated into the core of the free reverse micelle.

[0088] The ionic materials constituting the head groups of CCA can also induce ion pair formation at the particle (or other) surface. Therefore, CCA can perform two fundamental functions: generating charge at the surface and separating charge from the surface. Charge generation may occur due to acid-base reactions or ion exchange reactions between certain portions of the CCA molecule present in the antimicelle core or fluid and the particle surface. Therefore, useful CCA materials are those capable of participating in such reactions or any other charged reactions known in the art.

[0089] Non-limiting types of charge control agents useful in electrophoretic media include organic sulfates or sulfonates, metal soaps, block or comb copolymers, organic amides, organic zwitterions, and organophosphates and phosphonates. Useful organic sulfates and sulfonates include, but are not limited to, sodium bis(2-ethylhexyl)sulfosuccinate, calcium dodecylbenzenesulfonate, calcium petroleum sulfonate, neutral or basic barium dinonylnaphthalenesulfonate, neutral or basic calcium dinonylnaphthalenesulfonate, sodium dodecylbenzenesulfonate, and ammonium dodecyl sulfate. Useful metal soaps include, but are not limited to, basic or neutral barium petroleum sulfonate, calcium petroleum sulfonate, and cobalt, calcium, copper, manganese, magnesium, nickel, zinc, aluminum, and iron salts of carboxylic acids (such as naphthenic acids, 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 (poly(12-hydroxystearic acid)) and a molecular weight of approximately 1800, attached to oil-soluble anchoring groups composed of poly(methyl methacrylate-methacrylic acid). Useful organic amides / amines include, but are not limited to, polyisobutylene succinimide, such as OLOA 371 or 1200 (available from Chevron Oronite Company LLC (Houston, Texas), or SOLSPERSE 17000 or 19000 (available from Lubrizol (Wickliffe, Ohio: Solsperse is a registered trademark), and N-vinylpyrrolidone polymers. Useful organic zwitterions include, but are not limited to, lecithin. Useful organophosphates and phosphonates include, but are not limited to, sodium salts of phosphorylated monoglycerides and diglycerides with saturated and unsaturated acid substituents. Useful tail groups for CCAs include olefin polymers, such as polyisobutylene with a molecular weight in the range of 200 to 10,000. The head group can be a sulfonic acid, phosphoric acid, or carboxylic acid or amide, or alternatively, an amino group, such as a primary ammonium, secondary ammonium, tertiary ammonium, or quaternary ammonium group. U.S. Patent Publication No. 2017 / 0097556 discloses a class of CCAs for the disclosed four-particle electrophoresis media, the entire contents of which are incorporated herein by reference. Such CCAs typically contain a quaternary ammonium head group and an unsaturated polymer tail, i.e., containing at least one C / C 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 the CCA is 12,000 g / mol or higher, for example, between 14,000 g / mol and 22,000 g / mol.

[0090] The charge aids used in the medium can deflect the charge on the surface of electrophoretic particles, as detailed below. Such charge aids can be Bronsted acids or bases, or Lewis acids or bases. Exemplary charge aids are disclosed in U.S. Patent Nos. 9,765,015, 1,023,339, and 10,782,586, the entire contents of which are incorporated herein by reference. Exemplary aids may include polyhydroxy compounds containing at least two hydroxyl groups, including, but not limited to, ethylene glycol, 2,4,7,9-tetramethyldecyn-4,7-diol, polypropylene glycol, pentaethylene glycol, tripropylene glycol, triethylene glycol, glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), glycerol monohydroxystearate, and ethylene glycol monohydroxystearate. Examples of amino alcohol compounds containing at least one alcohol group and one amino 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 tetra(2-hydroxyethyl)ethylenediamine. In some embodiments, the charge aid is present in the EPD medium at a concentration between about 1 and about 500 milligrams (“mg / g”) per gram of particle mass, more preferably between about 50 and about 200 mg / g.

[0091] Particle dispersion stabilizers can be added to prevent particle flocculation or adhesion to the capsule or other walls or surfaces. For the typical high resistivity liquid in the EPD, non-aqueous surfactants can be used. These surfactants include, but are not limited to, ethylene glycol ethers, alkynyl glycols, alkanolamides, sorbitol derivatives, alkylamines, quaternary ammoniums, imidazolines, dialkyl oxides, and sulfosuccinates.

[0092] As described in U.S. Patent No. 7,170,670, the bistable nature of the electrophoretic medium can be improved by adding a polymer with a number-average molecular weight exceeding about 20,000 (such polymers are essentially non-adsorbent to electrophoretic particles) to the fluid; polyisobutylene is a preferred polymer for this purpose. Furthermore, as described in U.S. Patent No. 6,693,620, particles with a fixed charge on their surface form an electric bilayer with opposite charges in the surrounding fluid. The ionic head groups of CCA can ion-pair with charged groups on the surface of the electrophoretic particles, thereby forming a fixed or partially fixed layer of charged material. Beyond this layer is a diffusion layer comprising charged (anti)micelles containing CCA molecules in the fluid. In conventional DC electrophoresis, the applied electric field exerts a force on the fixed surface charge and an opposite force on the moving anti-charge, causing slippage within the diffusion layer, and the particles move relative to the fluid. The potential at the slip surface is called the zeta potential.

[0093] Therefore, some particle types in an electrophoretic medium exhibit different electrophoretic mobilities depending on the strength of the electric field across the medium. For example, when a first (low intensity, i.e., about ±10 V or lower) electric field is applied to the electrophoretic medium, particles of type I begin to move relative to the field in one direction; however, when a second (high intensity, i.e., about ±20 V or higher) electric field is applied, and the polarity of this field is the same as the first field, particles of type I begin to move relative to the field in the opposite direction. Theoretically, this behavior is caused by conduction within a highly nonpolar fluid mediated by charged reverse micelles or electrophoretic particles with opposite charges. Therefore, any electrochemically generated protons (or other ions) can potentially be transported through the nonpolar fluid within the micelle core or adsorbed onto electrophoretic particles. For example, as in U.S. Patent No. 9,697,778... Figure 5 As shown in B, positively charged reverse micelles can approach negatively charged electrophoretic particles traveling in the opposite direction, where the reverse micelles are incorporated into an electric bilayer around the negatively charged particles. (The electric bilayer comprises both a charge-diffused layer with an enhanced concentration of counterions and a semi-micelle surface adsorption coating on the particles; in the latter case, the reverse micelle charge will bind to the particles within a slip envelope defined above the particle zeta potential.) Through this mechanism, a positively charged electrochemical current flows through the electrophoretic solution, while negatively charged particles may be biased towards the more positively charged charge. Thus, the electrophoretic mobility of, for example, a type I negatively charged particle is a function of the magnitude of the electrochemical current and the residence time of the positive charge near the particle surface, the latter being a function of the electric field strength.

[0094] Furthermore, as described in U.S. Patent No. 9,697,778, positively charged particles can be prepared, which also exhibit different electrophoretic mobilities depending on the applied electric field. In some embodiments, a secondary (or co-)CCA can be added to the electrophoretic medium to modulate the zeta potential of various particles. Careful selection of the co-CCA can alter the zeta potential of one particle while keeping the zeta potentials of other particles substantially constant, thereby enabling precise control over both the electrophoretic velocities of various particles and the interactions between particles during switching.

[0095] In some embodiments, a portion of the charge control agent used in the final scheme is added during the synthesis of electrophoretic particles to control the desired zeta potential and influence the reduction in electrophoretic mobility caused by a strong electric field. For example, it has been observed that the addition of a quaternary ammonium charge control agent during polymer grafting results in a certain amount of CCA complexing with the particles. (This can be confirmed by removing the particles from the electrophoretic fluid and subsequently stripping the surface material of the pigment with THF to remove all adsorbed material. When the THF extract is evaluated using 1H NMR, it is clear that a large amount of CCA has been adsorbed onto the pigment particles or complexed with the surface polymer.) Experiments show that a high CCA content in the surface polymer of the particles in the presence of a strong electric field is beneficial for the formation of a charge bilayer around the particles. For example, magenta particles containing more than 200 mg of charge control agent (CCA) per gram of finished magenta particles exhibit excellent retention performance in the presence of a high positive electric field. (See, for example...) Figure 2C (As described above.) In some embodiments, the CCA comprises a quaternary ammonium head group and a fatty acid tail. In some embodiments, the fatty acid tail is unsaturated. When some particles in the electrophoretic medium contain high CCA content, it is important that particles with consistent electrophoretic mobility do not have significant CCA content, for example, less than 50 mg of charge control agent (CCA) per gram of finished particles, or for example, less than 10 mg of charge control agent (CCA) per gram of finished particles.

[0096] Alternatively, in the presence of Solsperse 17000 in Isopar E, an electrophoretic medium containing all four particle types can benefit from the addition of small amounts of acidic substances (such as aluminum di-tert-butylsalicylate, Bontron E-88, available from Orient Corporation (Kenilworth, NJ)). The addition of acidic substances shifts the zeta potential of many particles (though not all) towards more positively charged values. In one case, approximately 1% acidic substance and 99% Solsperse 17000 (based on the total weight of both substances) shifted the zeta potential of the third type of particle (Y+) from -5 mV to approximately +20 mV. Whether the zeta potential of a particular particle will be altered by Lewis acidic substances like aluminum salts will depend on the details of the particle surface chemistry.

[0097] Table 2 shows exemplary relative zeta potentials for three types of colored particles and a single white particle in the preferred embodiment.

[0098] Table 2. Relative zeta potentials of colored particles in the presence of relative zeta potentials of white particles.

[0099]

[0100] Negatively charged (white) particles have a zeta potential of -30 mV, while the other three types of particles are positively charged relative to white particles. Therefore, a display containing positively charged cyan, magenta, and yellow particles can switch between a black state (all colored particles are in front of the white particles relative to the viewing surface) and a white state (white particles are closest to the observer and block the observer's perception of the other three particles). In contrast, when white particles have a zeta potential of 0 V, negatively charged yellow particles are the most negatively charged of all particles, so a display containing these particles will switch between a yellow and blue state. This also occurs if the white particles are positively charged. However, positively charged yellow particles will be more positively charged than white particles unless the zeta potential of the yellow particles exceeds +20 mV.

[0101] The behavior of the electrophoretic medium indicates that the mobility of white particles (represented by zeta potential in Table 2) is related to the applied electric field. Therefore, in the examples shown in Table 2, when using low-voltage addressing, the white particles might appear to have a zeta potential of -30 mV, but when using higher-voltage addressing, they might appear to have a more positive zeta potential, possibly even as high as +20 mV (consistent with the zeta potential of yellow particles). Consequently, when using low-voltage addressing, the display switches between black and white states, but when using higher-voltage addressing, the display switches between blue and yellow states.

[0102] Particles with opposite charges may also form Coulomb aggregates. The particle mobility within the aggregates may differ from the mobility measured for the aggregate components. Therefore, for example, aggregates may form between negatively charged white particles and any pigment with an opposite charge. In some embodiments, it may be preferable that the electric field required to separate Coulomb aggregates is higher for some pigment combinations than for others. In this example, the electric field required to separate aggregates formed between magenta and white pigments may be greater than the electric field required to separate aggregates formed between cyan and white or yellow and white particles.

[0103] The charge of various pigment particles can be affected by the presence of other charged pigments in their environment. Therefore, the charge (or zeta potential) of a pigment in its final form is not necessarily the same as the charge measured on the pigment itself when dispersed in a solvent in the presence of CCA.

[0104] Figure 2B-2EThe diagram illustrates the motion of various particles under one possible assumption in the presence of high electric fields (e.g., "±H", e.g., ±20V, e.g., ±25V) and low electric fields (e.g., "±L", e.g., ±5V, e.g., ±10V). For illustrative purposes, each box surrounded by dashed lines represents a pixel surrounded by a top transparent electrode 21 and a bottom electrode 22. The bottom electrode can be a pixel electrode of an active matrix; however, it can also be a transparent electrode or a segmented electrode, etc. Starting from the first state, all positively charged particles appear at the observation surface (nominally black), and the electrophoretic medium can be driven to four different optical states, such as... Figure 2B-2E As shown. In a preferred embodiment, this produces a white optical state ( Figure 2B Magenta optical state ( Figure 2C ), yellow optical state ( Figure 2D ) and red optical state ( Figure 2E Obviously, Figure 1 The other four optical states can be achieved by reversing the order of the initial state and the driving electric field, such as... Figure 5 As shown in the simplified version.

[0105] like Figure 2B As shown, when using low-voltage addressing, the behavior of the particles depends on their relative zeta potential; the arrows indicate the relative velocities when a negative voltage is applied to the backplane. Therefore, in this example, cyan particles move faster than magenta particles, and magenta particles move faster than yellow particles. The first (positive) pulse does not change the particle positions because their movement is already restricted by the housing walls. The second (negative) pulse swaps the positions of the colored and white particles, thus switching the display between black and white states, but the transient color reflects the relative mobility of the colored particles. The starting position and polarity of the inverted pulse allow for the transition from white to black. Therefore, compared to other black and white schemes achieved via process black or process white, the black-to-white update provided in this embodiment requires lower voltage (and consumes less power).

[0106] exist Figure 2C In the sequence, the first (positive) pulse has a high positive voltage, sufficient to reduce the mobility of the magenta particles (i.e., the particles with moderate mobility among the three positively charged colored particles). Due to the reduced mobility, the magenta particles remain essentially stationary, while the subsequent reverse low-voltage pulse causes the cyan, white, and yellow particles to move more than the magenta particles, thus producing a magenta color at the observation surface, while the negatively charged white particles are behind the magenta particles. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to observing the display from the opposite side of the observation surface, i.e., through electrode 22), this pulse sequence will produce a green color (i.e., a mixture of yellow and cyan particles).

[0107] exist Figure 2D In this sequence, the first pulse is a low voltage that does not significantly reduce the mobility of either the magenta or white particles. However, the second pulse is a high negative voltage that reduces the mobility of the white particles. This allows for more efficient competition among the three types of positive particles, ensuring that the slowest type of particle (yellow in this example) remains visible in front of the white particles, whose migration is diminished by the earlier negative pulse. Notably, the yellow particles are unable to reach the top surface of the cavity containing the particles. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to observing the display from the opposite side of the observation surface, i.e., through electrode 22), this pulse sequence will produce a blue color (i.e., a mixture of magenta and cyan particles).

[0108] Figure 2E As shown, when both pulses are high voltage, the first high positive pulse reduces the mobility of magenta particles, while the second high negative pulse causes a decrease in white mobility, thus enhancing the competition between cyan and yellow. This produces the red color. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to observing the display from the opposite side of the observation surface, i.e., through electrode 22), this pulse sequence will produce the cyan color.

[0109] To achieve a high-resolution display, each pixel of the display should be addressable without interference from adjacent pixels. One way to achieve this is to provide an array of nonlinear elements, such as transistors or diodes, with each pixel associated with at least one nonlinear element, thus forming an "active matrix" display. The addressing electrode, or pixel electrode, for addressing a pixel is connected to an appropriate voltage source via 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 arrangement will be assumed in the following description, although it is inherently arbitrary, and the pixel electrode may be connected to the source of the transistor. Traditionally, 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 a specified row and a specified 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 source-to-row assignment and the gate-to-column assignment are conventional but inherently arbitrary and can be reversed if necessary. Row electrodes are connected to row drivers, which essentially ensures that only one row is selected at any given time. That is, a selection voltage is applied to the selected row electrode to ensure that all transistors in the selected row are turned on, while a non-selection voltage is applied to all other rows to ensure that all transistors in those non-selected rows remain off. Column electrodes are connected to column drivers, which apply selection voltages to individual column electrodes to drive the pixels in the selected row to their desired optical state. (These voltages are relative to a common front electrode, which is typically provided on the side of the electro-optical medium opposite the nonlinear array and extends across the entire display.) After a preselection interval called the “line addressing time,” the selected row is deselected, the next row is selected, and the voltage on the column driver is changed to write the next line of the display. This process is repeated to write the entire display line by line.

[0110] Traditionally, each pixel electrode is associated with a capacitor electrode, such that the pixel electrode and the capacitor electrode form a capacitor; see, for example, International Patent Application WO2001007961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) can be used to construct the transistor, and the “select” voltage and “non-select” voltage applied to the gate electrode can be positive and negative, respectively.

[0111] In the attached diagram Figure 3An exemplary equivalent circuit for a single pixel of an EPD is shown. As shown, the circuit includes a capacitor 10 formed between a pixel electrode and a capacitor electrode. The electrophoretic dielectric 20 is represented as a capacitor and a resistor connected in parallel. In some instances, a direct or indirect coupling capacitance 30 (often referred to as a “parasitic capacitance”) between the gate electrode of the pixel-associated transistor and the pixel electrode can generate unwanted noise for the display. Typically, the parasitic capacitance 30 is much smaller than the capacitance of the storage capacitor 10, and when a pixel row of the display is selected or deselected, the parasitic capacitance 30 can generate a small negative offset voltage, also known as a “recoil voltage,” on the pixel electrode, which is typically less than 2 volts. In some embodiments, to compensate for the unwanted “recoil voltage,” a common potential V can be provided to the top plate electrode and the capacitor electrode associated with each pixel. com , so that when V com Set to be related to the recoil voltage (V) KB When the values ​​are equal, each voltage supplied to the display can be canceled out by the same amount, and no net DC imbalance will be experienced.

[0112] However, when V com Problems may arise when the voltage is set to not compensate for recoil voltage. This can happen when the voltage expected to be applied to the display is higher than what the backplane voltage alone can provide. This is well known in the art, for example, if the backplane is powered by a nominal +V, 0, or -V option (e.g., when V...). com When powered by -V voltage, the maximum voltage applied to the display can be doubled. In this case, the maximum voltage experienced is +2V (i.e., the voltage at the back panel relative to the top panel), while the minimum voltage is zero. If a negative voltage is required, then V... com The potential must rise to at least zero. Therefore, the waveform used to address the display using positive and negative voltages via top-panel switching must therefore have more than one V. com A specific frame is allocated based on voltage settings.

[0113] A waveform set for driving a colored EPD with 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 are applied to the pixel electrodes: three positive voltages, three negative voltages, and zero voltage. However, in some cases, the maximum voltage used in these waveforms is higher than the voltages that amorphous silicon thin-film transistors can handle. In such instances, a suitable high voltage can be obtained by using a top-plate switch. When (as described above) V com Intentionally set to V KB At times, a separate power supply can be used. However, when using the top panel switch, a power supply compatible with V is required. comSetting up a single power supply with the same number of components is costly and inconvenient. Furthermore, it is known that top-plate switching increases backlash, thereby reducing the stability of color states.

[0114] Display devices can be constructed using electrophoretic fluids through several methods known in the prior art. The electrophoretic fluid can be encapsulated in microcapsules or incorporated into microcell structures, then sealed with a polymer layer. The microcapsules or microcell layers can be coated or imprinted onto a plastic substrate or film with a transparent conductive material coating. The assembly can be laminated to a backplane containing pixel electrodes using a conductive adhesive. Alternatively, the electrophoretic fluid can be directly dispersed onto a thin, open-cell mesh already arranged on a backplane containing an active matrix of pixel electrodes. The filled mesh can then be top-sealed with an integrated protective plate / transparent electrode.

[0115] Figure 4 A schematic cross-sectional view (not drawn to scale) of a display structure 200 containing an electrophoretic medium is shown. In the display 200, the electrophoretic fluid is described as being confined within microcells, but other structures, such as microcapsules, may also be used. A substrate 202, which may be glass or plastic, carries pixel electrodes 204, which may be individual addressable segments or associated with thin-film transistors in an active matrix arrangement. (The combination of substrate 202 and electrodes 204 is generally referred to as the backplane of the display.) Layer 206 is an optional dielectric layer applied to the backplane according to the invention. (A method for depositing a suitable dielectric layer is described in U.S. Patent Publication No. 2020 / 0348576, which is incorporated herein by reference.) The front plane of the display includes a transparent substrate 222 with a transparent conductive coating 220. Overlying the electrode layer 220 is an optional dielectric layer 218. Layer (or multiple layers) 216 is a polymer layer that may include an undercoat layer for adhering the microcells to the transparent electrode layer 220 and some residual polymer forming the bottom of the microcells. The walls of microcell 212 are used to contain electrophoretic fluid 214. The microcell is sealed using layer 210, while the entire front planar structure is adhered to the backplate using a conductive adhesive layer 208. The process for forming the microcell is described in the prior art, for example in U.S. Patent No. 6,930,818. In some instances, the depth of the microcell is less than 20 μm, for example less than 15 μm, for example less than 12 μm, for example approximately 10 μm, for example approximately 8 μm.

[0116] Due to the widespread availability of manufacturing facilities and the cost of various starting materials, most commercial EPDs use amorphous silicon based on thin-film transistors (TFTs) in the construction of the active matrix backplane (202 / 024). Unfortunately, amorphous silicon TFTs become unstable when a gate voltage is provided that would allow voltage switching above approximately + / - 15 V. Nevertheless, as described below, the performance of ACeP is improved when the magnitudes of high positive and high negative voltages exceed + / - 15 V. Therefore, as previously disclosed, the performance improvement is achieved by additionally changing the bias of the top transparent electrode relative to the bias on the backplane pixel electrode (also known as top-plate switching). Thus, if a voltage of +30 V (relative to the backplane) is required, the top plate can be switched to -15 V while the corresponding backplane pixel is switched to +15 V. A method for driving a four-particle electrophoresis system using top-plate switching is described in more detail, for example, in U.S. Patent No. 9,921,451.

[0117] These waveforms require that each pixel of the display can be driven at five different addressing voltages, denoted as +V. high +V low , 0, -V low and -V high The diagram shows 30 V, 15 V, 0, -15 V, and -30 V. In practice, it may be preferable to use a larger number of addressing voltages. If only three voltages are available (i.e., +V...) high 0 and -V high Then, by using a voltage V with a duty cycle of 1 / n... high Pulse addressing enables communication with lower voltages (e.g., V). high / n, where n is a positive integer greater than 1, has the same effect as addressing.

[0118] Figure 5 The diagram shows a typical waveform (simplified form) used to drive the aforementioned four-particle colored electrophoretic display system. Such waveforms have a "push-pull" structure: that is, they consist of dipoles containing two pulses of opposite polarity. The amplitude and length of these pulses determine the resulting color. The waveform has five such voltage levels. Figure 5 The display shows high and low positive voltages, high and low negative voltages, and zero volt. Generally, "low" (L) refers to a range of approximately 5–15 V, while "high" (H) refers to a range of approximately 15–30 V. Generally, the higher the amplitude of the "high" voltage, the better the color gamut the display achieves. An additional "medium" (M) level can also be used, typically around 15 V; however, the value of M depends to some extent on the composition of the particles and the environment of the electrophoretic medium.

[0119] although Figure 5The simplest dipoles required to form color are shown, but it is understood that actual waveforms can use these patterns repeatedly, or use other non-periodic patterns.

[0120] Of course, using Figure 5 The desired color achieved by the drive pulse depends on the particles starting the process from a known state, which is unlikely to be the last color displayed on the pixel. Therefore, a series of reset pulses precede the drive pulse, increasing the time required for the pixel to update from the first color to the second. The reset pulses are described in more detail in U.S. Patent No. 10,593,272, which is incorporated herein by reference. The lengths of these pulses (refresh and address) and any pauses (i.e., the periods of zero voltage between them) are selectable 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 essentially zero). DC balance can be achieved by adjusting the lengths of the pulses and pauses during the reset phase such that the net impulse provided during the reset phase is equal in magnitude and opposite in sign to the net impulse provided during the addressing phase, in which the display switches to a specific desired color. However, as... Figure 2B-2E As shown, the initial state of the eight primary colors is either black or white, which can be achieved using a continuous low-voltage drive pulse. The simplicity of achieving this initial state further reduces the time spent updating between states, which is more satisfactory for the user and also reduces power consumption (thus extending battery life).

[0121] Furthermore, the preceding waveform discussion, especially the DC balance discussion, neglected the issue of recoil voltage. In reality, as mentioned earlier, each backplane voltage will deviate from the voltage supplied by the power supply, and this deviation is equal to the recoil voltage V. KB Therefore, if the power supply used provides three voltages: +V, 0, and –V, the backplane will actually receive voltage V+V. KB V KB and –V+V KB (Note that, for amorphous silicon TFTs, V) KB (Typically negative). However, the same power supply will provide +V, 0, and –V to the front electrode without any recoil voltage offset. Therefore, for example, when the current electrode is powered by –V, the display will experience a maximum voltage of 2V+V. KB and minimum voltage V KB No separate power supply is required to provide V to the front electrode. KB (This could be both expensive and inconvenient), but instead, the waveform can be divided into individual parts to provide positive voltage, negative voltage, and V to the front electrode. KB .

[0122] Four pigment EPDs addressable by four voltage levels

[0123] According to one or more embodiments of the present invention, a full-color EPD that can be addressed by waveforms having exactly four voltage levels is disclosed. The EPD comprises an electrophoretic medium having four types of charged electrophoretic pigment particles, including three substantially non-light-scattering colored pigments, preferably corresponding to subtractive primary colors (cyan, magenta, and yellow), and a fourth pigment that scatters light, white. These three colored pigments have one charge polarity, while the white pigment has the opposite charge polarity. Preferably, the white pigment is negatively charged, while the three colored pigments are positively charged. The display, addressed by waveforms having exactly four different voltage levels, can display any primary color (white, black, cyan, magenta, yellow, red, green, and blue) at any pixel location without using top-plate switching (i.e., simultaneously maintaining a constant voltage on the light-transmitting electrodes of the EPD). The voltage levels are specified by two bits of data per pixel and preferably include high and low positive voltages, a single negative voltage, and a near-zero voltage.

[0124] Because this EPD uses exactly four voltage levels and can be addressed by two bits per pixel instead of the three or more bits used in existing colored EPDs, and does not use top-panel switching, it can reduce device costs while providing excellent performance and color quality without significantly sacrificing color gamut, as described below.

[0125] As described above, U.S. Patent Nos. 9,697,778 and 10,678,111 disclose two different types of EPDs capable of displaying any color at any pixel location. The first patent discloses a display in which a white (light-scattering) pigment moves in a first direction when addressed using an applied low voltage, and in the opposite direction when addressed using a high voltage. The second patent discloses a full-color EPD comprising four pigments: white, cyan, magenta, and yellow. U.S. Patent No. 10,276,109 and other patents disclose methods for addressing such displays using waveforms having at least five different voltage levels.

[0126] Existing monochrome EPDs typically use only three different voltage levels: a positive voltage, a negative voltage, and zero voltage. The source driver of a monochrome EPD receives two bits of data per pixel from the display controller to specify which power supply is connected to which column.

[0127] When five voltage levels are required (the minimum number for addressing existing full-color multi-pixel displays), the module architecture must be modified in one of two conventional ways: (a) maintain a 2-bit per pixel architecture but perform top-panel switching to change the potential difference between the back-panel electrode and the top-panel (transparent) common electrode for each pixel in the display; or (b) provide a source driver capable of selecting between at least five different power supplies. In the latter case, two bits of data result in insufficient information, and the controller must provide at least three bits of data per pixel to the source driver.

[0128] Both methods have significant drawbacks. Because adjusting the top plate of the display affects each pixel, the flexibility of possible waveform structures available for addressing the display is limited. For example, it is impossible to simultaneously apply the maximum positive and maximum negative voltages across the electrophoretic fluid in two separate pixels. Therefore, the waveform becomes longer, typically about twice as long. Requiring the controller to provide at least three bits of data per pixel requires faster data transfer rates (or longer transfer times), which increases controller cost or reduces the frame time that can be supported. Furthermore, many controllers in the current EPD ecosystem cannot provide three bits of data per pixel. Therefore, for many applications, providing at least five different voltage levels can result in unacceptably high module costs or excessively long waveforms.

[0129] The desired approach is to address colored EPDs using two bits of data per pixel, eliminating the need for top-panel switching. Two bits of data per pixel can be used to specify four voltage levels, not just three. Dynamically reallocating power per frame does not offer an equally flexible alternative, as this reallocation would be applied to every pixel when updating a specific frame, leading to the same trade-offs as top-panel switching.

[0130] In EPDs containing negatively charged white pigments and positively charged cyan, magenta, and yellow pigments, the complementary colors yellow / blue and magenta / green are best represented by waveforms consisting of dipoles (high-voltage and low-voltage pulses of opposite polarity). In existing devices, waveforms represent these colors using a series of series dipoles in near-DC equilibrium. Therefore, if a dipole comprises a first pulse with voltage V1 and duration t1, and a second pulse with voltage –V2 and duration t2, then DC equilibrium is achieved if V1t1 = V2t2.

[0131] When V1 is positive and |V1|>|V2|, the display oscillates 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 oscillates 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 voltage levels, as well as the zero-volt option, will be needed to render all colors at each pixel.

[0132] However, it has been found that displays can be addressed and all colors rendered at each pixel location using more complex, aperiodic voltage sequences with exactly four voltage levels (preferably high and low positive voltages, a single negative voltage, and near-zero voltage). (The term "aperiodic" refers to voltage sequences that do not have a precisely repeating structure.) Tuning such waveforms is far more complex and difficult than the easily parameterizable dipole structures in the prior art; so much so that tuning directly using an actual display panel would take a very long time. However, it has been found that tuning such complex waveforms can be made much easier using a two-stage computer modeling process. In the first stage, a set of arbitrary voltage lists is provided to the display, and the optical response is recorded at the end of each frame. Based on this training set, a model is built using machine learning methods to accurately predict the effects of applying voltage lists not present in the original training set (e.g., with an accuracy of approximately 1–2 dE*). In the second stage, the model is used to tune the ideal waveform, which allows for testing a much larger number of waveforms than the physical display module can handle in a given time.

[0133] Figure 6A and 6B Exemplary waveforms for driving a four-particle colored EPD with four voltage levels are shown according to one or more embodiments. The figure illustrates an exemplary pulse sequence for forming eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) at each pixel. The amplitude and length of these pulses determine the resulting color. In one or more embodiments (e.g., as shown in the figure), Figure 6A As shown), these four voltage levels include high and low positive voltages, near-zero volts, and high negative voltages (denoted as +V). high +V low 0 and –V high In one or more alternative embodiments (e.g., as...), Figure 6B As shown), these four voltage levels include high and low negative voltages, near-zero volts, and high positive voltage (denoted as -V). high -V low 0 and +V high Typically, "low" voltages range from approximately 5 to 15V, while "high" voltages range from approximately 15 to 30V. Generally, the higher the amplitude 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.

[0134] Figure 7This is a schematic diagram of an exemplary driving system 300 for controlling a pixel electrode array 302 of a four-particle colored electrophoretic medium according to one or more embodiments. The driving 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 multiple data lines and multiple 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 a gate line and a data line.

[0135] The controller shown includes a microcontroller 304 containing control logic and switching logic. It receives input data from input data line 322. The microcontroller has an output port for each data line of the matrix to provide a data signal. Data signal line 306 connects each output port to the data line of the matrix. The microcontroller 304 also has an output port for each gate line of the matrix to provide a gate line select signal. Gate signal line 308 connects each output port to the gate line of the matrix. Data line driver 310 and gate line driver 312 are arranged on each data line and gate signal line, respectively. Only the signal lines for those data lines and gate lines shown in the figure are shown. The gate line driver can be integrated into a single integrated circuit. Similarly, the data line driver can be integrated into a single integrated circuit (as a source driver). This integrated circuit can include a complete driver assembly as well as the microcontroller.

[0136] The data line driver provides a signal level corresponding to the voltage of the selected drive pixel. The gate line driver provides a signal for selecting the gate line of the electrode to be driven. Figure 7 A voltage sequence for one of the data line drivers 210 is shown. When there is a sufficiently large positive voltage on the gate line, the impedance between the data line and the pixel is low, so the voltage on the data line is transferred to the pixel. When there is a negative voltage on the TFT gate, the TFT exhibits high impedance, and the voltage is stored in the pixel capacitor unaffected by the voltage on the data line. The figure shows four columns labeled n to n+3 and five rows labeled n to n+4.

[0137] like Figure 7 As shown, line-by-line addressing is used, where one gate line n is high and all other gate lines are low. Signals on all data lines are then transmitted to all pixels in row n. At the end of the row scan, the signal on gate line n goes low, and the next gate line n+1 goes high, so data for the next row is transmitted to the TFT pixels in row n+1. All gate lines continue to be scanned sequentially, thus driving the entire matrix. Almost all active-matrix LCDs (such as mobile phone screens, laptop screens, and LCD TVs) use the same method, and the same method is also used in active-matrix EPDs because TFT control maintains the voltage across the liquid crystal layer.

[0138] Example

[0139] The advantages of addressing the full-color EPD using waveforms with four voltage levels have been experimentally demonstrated.

[0140] The experiment included the above types (and) Figures 2A-2E The electrophoresis was performed on an EPD module containing white, cyan, magenta, and yellow particles, respectively, represented by W-, C+++, M++, and Y+. The electrophoretic fluid was contained in a microcup with a thickness of 8 micrometers.

[0141] Experiments were conducted using waveforms with different numbers of voltage levels from Table 3.

[0142] Table 3. Waveform Voltage Levels

[0143]

[0144] The waveform was adjusted using the computer modeling method described above.

[0145] The EPD module was addressed using each different waveform, and the results were recorded and shown in Tables 4 to 8.

[0146] Table 4. Level 3 Waveform Results

[0147]

[0148] Table 5. Level 4 waveform results

[0149]

[0150] Table 6. Waveform Results of Level 4*

[0151]

[0152] Table 7. Level 5 Waveform Results

[0153]

[0154] Table 8. Waveform Results of Level 7

[0155]

[0156] The results show that the "4-level" driving scheme (Table 5), with two negative voltage levels, one positive voltage level, and one near-zero voltage level, offers only a slight improvement in gamut volume compared to the "3-level" driving scheme (Table 4). The "4-level*" driving scheme (Table 6), with two positive voltage levels, one negative voltage level, and one near-zero voltage level, approaches the quality achievable with the "5-level" driving scheme (Table 7). There is only a negligible difference between the "5-level" and "7-level" driving schemes (Table 8).

[0157] Table 9 shows detailed information on the colors obtained using various driving voltage schemes.

[0158] Table 9. Overall Results

[0159]

[0160] When using a 4-stage* driving scheme with two positive voltages, one negative voltage, and one near-zero voltage, magenta and blue exhibit a decrease in quality compared to the 5-stage driving scheme, as expected. However, this quality reduction is tolerable because the magenta in the 5-stage driving scheme is particularly unbalanced compared to other colors, especially yellow and green. Therefore, sacrificing some magenta quality in exchange for improved yellow and green quality is acceptable. Figure 9 This trade-off is highlighted, and the figure shows the color gamut available for various drive voltage schemes.

[0161] Therefore, excellent color quality is provided by a four-particle pigment EPD with waveform addressing of exactly four voltage levels, while reducing the cost of the display by using 2 bits of data per pixel instead of 3 bits.

[0162] Therefore, having described various aspects and embodiments of the technology described herein, it should be understood that various alternatives, modifications, and improvements will readily occur to those skilled in the art. Such alternatives, modifications, and improvements are intended to fall within the spirit and scope of the technology described herein. For example, various other means and / or structures will readily occur to those skilled in the art to perform the functions described herein and / or obtain the results and / or one or more advantages described herein, and each such change and / or modification is considered to fall within the scope of the embodiments described herein. Those skilled in the art will recognize, or can determine, many equivalents of the specific embodiments described herein through routine experimentation alone. Therefore, it should be understood that the foregoing embodiments are presented by way of example only, and that inventive embodiments may be practiced in ways other than those specifically described within the scope of the appended claims and their equivalents. Furthermore, any combination of two or more features, systems, articles, materials, kits, and / or methods described herein (if such features, systems, articles, materials, kits, and / or methods do not contradict each other) is included within the scope of this disclosure.

Claims

1. A colored electrophoresis display, comprising: The light-transmitting electrode is observed on the surface. Back electrode; as well as The electrophoretic medium disposed between the transparent electrode and the back electrode includes: Nonpolar fluids; and A multi-pigment particle system comprising four types of charged electrophoretic pigment particles dispersed in the nonpolar fluid, the four types of charged electrophoretic pigment particles including: The first type of particle possessing first optical properties and first electric 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 possessing the third optical property and the second charge polarity; and A fourth type of particle possessing the fourth optical property and the second charge polarity; The first optical property, the second optical property, the third optical property, and the fourth optical property are different from each other; The multi-pigment particle system is directly addressable by a push-pull waveform applied to the back electrode, the push-pull waveform having a pulse voltage level selected from exactly four different voltage levels to present any one of the eight primary colors—red, green, blue, cyan, magenta, yellow, black, and white—at each pixel of the electrophoretic medium, while maintaining a constant voltage on the light-transmitting electrode, 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.

2. The colored electrophoretic display according to claim 1, 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.

3. The colored 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 colored electrophoretic display according to claim 1, wherein the push-pull waveform comprises an aperiodic voltage sequence.

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

9. The colored electrophoretic display according to claim 1, wherein the multi-pigment particle system has exactly four types of charged electrophoretic particles.

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

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

12. The colored electrophoretic display of claim 1, wherein the electrophoretic display is configured to be incorporated into a book reader, portable computer, tablet computer, monitor, telephone, smart card, signage, watch, jewelry, shelf label, vehicle panel, or flash drive.

13. The colored electrophoretic display according to claim 1, wherein the back electrode comprises segmented electrodes or an active matrix backplate comprising a pixel electrode array.

14. The colored electrophoretic display according to claim 13, wherein, The active matrix backplane includes a thin-film transistor array coupled to the pixel electrodes.

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

16. A system comprising: A colored electrophoretic display according to any one of claims 1-15, wherein the back electrode comprises an active matrix backplate including a pixel electrode array; Source driver; At least one power source; as well as A controller is configured to transmit two bits of data per pixel to the source driver to control the source driver to apply a selection voltage from the at least one power source to the selected pixel electrode.

17. A method comprising: A colored electrophoretic display is provided, comprising a transparent electrode, a back electrode, and an electrophoretic medium disposed between the transparent electrode and the back electrode at an observation surface. The electrophoretic medium comprises a nonpolar fluid and a multi-pigment 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 the second charge polarity; and a fourth type of particle having a fourth optical property and the second charge polarity; wherein the first optical property, the second optical property, the third optical property, and the fourth optical property are different from each other. The multi-pigment particle system is directly addressed by a push-pull waveform applied to the back electrode, the push-pull waveform having a voltage level selected from exactly four different voltage levels to present any one of the eight primary colors—red, green, blue, cyan, magenta, yellow, black, and white—at each pixel of the electrophoretic medium, while maintaining a constant voltage on the light-transmitting electrode. 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.

18. The method according to any one of the preceding 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 of claim 17, wherein the first positive voltage is 24V, the second positive voltage is 10V, and the negative voltage is -24V.

20. The method of claim 17, wherein the push-pull waveform comprises an aperiodic voltage sequence.

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

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

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

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

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

26. The method of claim 17, wherein the nonpolar fluid comprises a charge control additive.

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

28. The method of claim 17, wherein the electrophoretic display is configured to be incorporated into a book reader, portable computer, tablet computer, monitor, telephone, smart card, signage, watch, jewelry, shelf label, vehicle panel, or flash drive.

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

30. The method of claim 29, wherein the active matrix backplane comprises a thin-film transistor array coupled to the pixel electrode.

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

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