Transition-driven mode for impulse equilibrium when switching between global color mode and direct update mode for electrophoretic displays.

Abstract

A method for driving a multi-pixel electrophoretic display designed to enable at least three colors, e.g., eight colors, at each pixel. The method uses a first drive scheme capable of producing transitions between all of the color states that can be displayed at each pixel, and a second drive scheme containing only transitions that end in white or black, which is very useful for drawing black lines on a white page, reading black text on a white page, or reading white text on a black page. Two intermediate transition modes are added to control the amount of impulse potential stored at each pixel during switching between drive modes.

Description

[Background technology]

[0001] (Related applications) This application claims priority to U.S. Provisional Application No. 63 / 401,110, filed on 25 August 2022. All patents and publications disclosed herein are incorporated together by reference.

[0002] (background) Electrophoretic displays (EPDs) change color by modifying the position of charged colored particles on a light-transmitting viewing surface. Such electrophoretic displays are typically referred to as "electronic paper" or "e-paper" because the resulting display has high contrast and is sunlight-readable, much like ink on paper. Electrophoretic displays have enjoyed widespread adoption in e-readers such as Amazon Kindle® because they offer a book-like reading experience, use less power, and allow users to carry a library of hundreds of books in a lightweight, handheld device.

[0003] For many years, electrophoretic displays have contained only two types of charged color particles, namely black and white (to be clear, “color” as used herein includes black and white). White particles are often light-scattering and consist of, for example, titanium dioxide, while black particles are absorptive across the visible spectrum and may consist of carbon black or absorptive metal oxides such as copper chromite. In its simplest sense, a monochrome electrophoretic display requires only an electrophoretic medium containing a light-transmitting electrode on the viewing surface, a back electrode, and oppositely charged white and black particles. When a voltage of one polarity is provided, the white particles move to the viewing surface, and when a voltage of the opposite polarity is provided, the black particles move to the viewing surface. If the back electrode contains controllable regions (pixels), i.e., an active matrix of segmented electrodes or pixel electrodes controlled by transistors, a pattern can be fabricated to appear electronically on the viewing surface. This pattern could be, for example, the text of a book.

[0004] More recently, a variety of color options, including three-color displays (black, white, red, and black, white, yellow) and four-color displays (black, white, red, yellow), have become commercially available for electrophoretic displays. Similar to the operation of a monochrome electrophoretic display, an electrophoretic display with three or four reflective particles operates similarly to a simple monochrome display, as the desired color particles are driven toward the viewing surface. While this driving scheme is far more complex than that of a monochrome-only system, the optical properties of the particles are ultimately identical.

[0005] Advanced Color Electronic Paper (ACeP®) also contains four particles, but the cyan, yellow, and magenta particles are subtractive rather than reflective, thereby enabling the production of thousands of colors in each pixel. This color process is functionally equivalent to the printing method that has long been used in offset printing and inkjet printers. A given color is produced by using the correct ratio of cyan, yellow, and magenta on a bright white paper background. In the case of ACeP, the relative positions of the cyan, yellow, magenta, and white particles with respect to the viewing surface will determine the color in each pixel. While this type of electrophoretic display enables thousands of colors in each pixel, it is crucial to carefully control the position of each pigment (50-500 nanometers in size) within a working space of approximately 10-20 microns in thickness. Obviously, variations in particle position will result in the display of the wrong color in a given pixel. Therefore, precise voltage control is required for such a system. Further details of this system are available in the following U.S. Patents, namely U.S. Patents 9,361,836, 9,921,451, 10,276,109, 10,353,266, 10,467,984, and 10,593,272, all of which are incorporated as a whole by reference.

[0006] The colors generated using such color electrophoretic displays may be subject to various "error accumulation" phenomena. These errors may be due to minute fluctuations in the driving voltage during a series of transitions, residual voltages accumulated on the driving pixels, or temperature fluctuations in the electrophoretic medium. As a result, the desired color state may deviate from the displayed color state due to the voltage accumulated on the pixels and / or the disorder of the internal phase medium adjacent to those pixels. For example, a pixel that returns to a black state after 100 consecutive color transitions in one area of ​​the display may have an L* of 6 (where L* has the following standard CIE definition): L*=116(R / R0) 1 / 3 -16

[0007] (In the formula, R is the reflectance, and R0 is the standard reflectance value.) Meanwhile, another adjacent pixel that did not change during those 100 consecutive transitions started as black and remained so, after the same 100 transitions, has an L* of 4. A deviation of 2L* is noticeable to the average observer and detracts from the overall experience of the display.

[0008] This accumulation of error phenomena applies to color states as well as black and white states. Again, due to the interaction of electrophoretic particles, slight changes in local voltage or the electrophoretic medium environment can result in different colors being displayed on the screen. Furthermore, with respect to certain color states, particularly yellow and green, the human eye is more sensitive to changes in saturation, and even slight fluctuations in color state can cause physical discomfort. For example, skin tones with a greenish tint can be very disorienting to the viewer. Therefore, general grayscale / color image flow requires very precise control of the applied impulses to produce good results.

[0009] To further complicate matters, in some situations it may be desirable for a single display to utilize multiple driving schemes. For example, a display capable of producing many colors at each pixel may typically operate in “Globally Complete” (“GC mode”), where each color pixel has the ability to transition from the first color to the second color while each image is updated. Naturally, such updates can be time-consuming (e.g., one second or more), especially when DC equilibrium and residual voltage management are required to achieve the highest quality color, as described, for example, in U.S. Patent No. 10,657,869. However, in other cases such as drawing with a stylus or turning pages of text, very rapid updates are desired, and users will readily sacrifice color fidelity in exchange for a faster update experience. Such faster update schemes are typically known as “Direct Update” (“DU mode”) and typically involve simply driving an electrophoretic medium through the black and white range. See, for example, U.S. Patent No. 9,672,766. For higher-performance products such as color electronic readers / tablets, multiple types of modes may exist depending on the displayed content. Additional modes, such as video (known as "A2 mode"), may also be included, and the display controller may be programmed to automatically switch between modes in response to the displayed content or user actions, such as touching with a stylus.

[0010] As discussed in U.S. Patent No. 11,686,989, in multi-particle systems used to process black or white, driving between white and black states can require very different impulse potentials (voltages accumulated over time). For example, in ACeP®, which includes one negative white particle and three differently charged positive particles, both of which produce a black state, a much larger impulse potential may be required to achieve a better black state than a good white state. That is, to produce white, the negative white particle simply needs to be placed between the viewer and the colored particles, whereas to achieve a good black state, all the positively charged particles must be driven to the viewing surface and mixed, and all the white particles must be driven behind the positive colored particles. In a typical "GC" mode, the white and black states are achieved using DC equilibrium such that the round trip for both the black and white states does not result in any impulse potentials related to those color states. However, this is done at the expense of longer waveforms, typically around 1 second, e.g., 500ms to 3 seconds, e.g., 700ms to 1 second. For DU mode, preferably, the switching time between the black and white states should be much shorter, e.g., less than 500ms, e.g., less than 300ms, e.g., around 250ms. However, due to the difference in impulse potential between the black and white states, the impulse potential must be carefully managed to prevent charge accumulation on the pixels, which would lead to a poor color state later on. This condition can be encountered in an electronic reader where the reader views 20 pages of text and then a full-color image.

[0011] The term “gray state” is used herein in its conventional sense in imaging technology and refers to an intermediate state between the two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, some of E INK’s patents and published applications, referenced below, describe electrophoretic displays in which the extreme states are white and dark blue, so that the intermediate gray state is actually light blue. In fact, as already described, a change in optical state may not be a change in color at all. The term “black and white” may be used herein hereafter to refer to the two extreme optical states of a display and should generally be understood to include extreme optical states that are not strictly black and white, such as the aforementioned white and dark blue color states.

[0012] The terms “bistable” and “bistable” are used herein to refer to a display having a display element having a first and second display state having at least one different optical property, wherein after any given element is driven with a finite-duration addressing pulse to exhibit either the first or second display state, the state will persist for at least several times, e.g., at least four times, the minimum duration of the addressing pulse required to change the state of the display element after the addressing pulse has terminated. U.S. Patent No. 7,170,670 shows that several particle-based electrophoretic displays with grayscale capabilities are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true for several other types of electro-optic displays. These types of displays are more appropriately called “multistable” rather than “bistable,” but for convenience, the term “bistable” may be used herein to encompass both bistable and multistable displays.

[0013] The term "impulse," when used, refers to the driving of an electrophoretic display and the integral of the applied voltage over time during the cycle in which the display is driven.

[0014] Particles that absorb, scatter, or reflect light in a broadband or selected wavelength are referred to herein as colored or pigmented particles. Various materials other than pigments (in the strict sense of the term meaning insoluble coloring materials) that absorb or reflect light, such as dyes or photonic crystals, may also be used in the electrophoretic media and displays of the present invention.

[0015] Particle-based electrophoretic displays have been the subject of vigorous research and development for many years. In such displays, multiple charged particles (sometimes also called pigment particles) move through a fluid under the influence of an electric field. Compared to liquid crystal displays, electrophoretic displays can have attributes such as good brightness and contrast, wide viewing angles, state bistability, and low power consumption. Nevertheless, problems associated with the long-term image quality of these displays have hindered their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in a poor usable lifespan for these displays.

[0016] As noted above, an electrophoretic medium requires the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, although electrophoretic media can also be produced using a gaseous fluid. See, for example, Kitamura, T., et al., Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1 and Yamaguchi, Y., et al., Toner display using insulative particles charged triboelectrically, IDW Japan, 2001, Paper AMD4-4. Also see U.S. Pat. Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media are thought to be susceptible to the same type of problems due to particle sedimentation as liquid-based electrophoretic media when, for example, the media are used in an orientation where the media are disposed in a vertical plane and the media permit such sedimentation. In fact, particle sedimentation is thought to be a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media due to the lower viscosity of the gaseous suspension fluid compared to the viscosity of the liquid that permits faster sedimentation of the electrophoretic particles.

[0017] A number of patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various techniques used within encapsulated electrophoretic media and other electro-optic media. Such encapsulated media comprise a number of small capsules, each of which itself comprises an internal phase containing particles movable by electrophoresis within a fluid medium and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held within a polymeric binder to form a coherent layer positioned between two electrodes. The techniques described in these patents and applications include the following. (a) Electrophoretic particles, fluid, and fluid additives (see, e.g., U.S. Patent Nos. 7,002,728 and 7,679,814) (b) Capsules, binders, and encapsulation processes (see, e.g., U.S. Patent Nos. 6,922,276 and 7,411,719) (c) Microcell structures, wall materials, and methods of forming microcells (see, e.g., U.S. Patent Nos. 7,072,095 and 9,279,906) (d) Methods for filling and sealing microcells (see, e.g., U.S. Patent Nos. 7,144,942 and 7,715,088) (e) Films and subassemblies containing electro-optical materials (see, e.g., U.S. Patent Nos. 6,982,178 and 7,839,564) (f) Backplanes, adhesive layers, other auxiliary layers, and methods used in displays (see, e.g., U.S. Patent Nos. 7,116,318 and 7,535,624) (g) Color formation and color adjustment (e.g., U.S. Patent Nos. 6,017,584, 6,545,797, 6,664,944, 6,788,452, 6,864,875, 6,914,714, 6,972,893, 7,038,656, 7,038,670, 7,046,228, 7,052,571, 7,075,502***, 7,167,155, 7,385,751, 7,492,505, 7,667,68 No. 4, No. 7,684,108, No. 7,791,789, No. 7,800,813, No. 7,821,702, No. 7,839,564***, No. 7,910,175, No. 7,952,790, No. 7,956,841, No. 7,982, No. 941, No. 8,040,594, No. 8,054,526, No. 8,098,418, No. 8,159,636, No. 8,213,076, No. 8,363,299, No. 8,422,116, No. 8,441,714, No. 8,441,7 No. 16, No. 8,466,852, No. 8,503,063, No. 8,576,470, No. 8,576,475, No. 8,593,721, No. 8,605,354, No. 8,649,084, No. 8,670,174, No. 8,704,7 No. 56, No. 8,717,664, No. 8,786,935, No. 8,797,634, No. 8,810,899, No. 8,830,559, No. 8,873,129, No. 8,902,153, No. 8,902,491, No. 8,917,43 Nos. 9, 8,964,282, 9,013,783, 9,116,412, 9,146,439, 9,164,207, 9,170,467, 9,170,468, 9,182,646, 9,195,111, 9,199,441, 9,268,191, 9,285,649, 9,293,511, 9,341,916, 9,360,733, 9,361,836, 9,383,623, and 9,423,U.S. Patent Application Publication No. 666, and U.S. Patent Application Publication Nos. 2008 / 0043318, 2008 / 0048970, 2009 / 0225398, 2010 / 0156780, 2011 / 0043543, 2012 / 0326957, 2013 / 0242378, 2013 / 0278995, 2014 / 0055840, 2014 / 0078576, 2014 / 0340430, 2014 / 0340736, and 2014 / 0362 See issues 213, 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). (h) Methods for driving a display (e.g., U.S. Patent Nos. 5,930,026, 6,445,489, 6,504,524, 6,512,354, 6,531,997, 6,753,999, 6,825,970, 6,900,851, 6,995,550, 7,012,600, 7,023,420, 7,034,783, 7,061,166, 7,061,662, 7,116,466, 7,119,772, 7,177,066, 7,193,625, 7,2 No. 02,847, No. 7,242,514, No. 7,259,744, No. 7,304,787, No. 7,312,794, No. 7,327 ,511, No. 7,408,699, No. 7,453,445, No. 7,492,339, No. 7,528,822, No. 7,545,3 No. 58, No. 7,583,251, No. 7,602,374, No. 7,612,760, No. 7,679,599, No. 7,679,813 No. 7,683,606, No. 7,688,297, No. 7,729,039, No. 7,733,311, No. 7,733,335, No. No. 7,787,169, No. 7,859,742, No. 7,952,557, No. 7,956,841, No. 7,982,479, No. 7, No. 999,787, No. 8,077,141, No. 8,125,501, No. 8,139,050, No. 8,174,490, No. 8,24 No. 3,013, No. 8,274,472, No. 8,289,250, No. 8,300,006, No. 8,305,341, No. 8,314, No. 784, No. 8,373,649, No. 8,384,658, No. 8,456,414, No. 8,462,102, No. 8,514,168 No. 8,537,105, No. 8,558,783, No. 8,558,785, No. 8,558,786, No. 8,558,855, No. 8,576,164, No. 8,576,259, No. 8,593,396, No. 8,605,032, No. 8,643,595, No. 8 ,665,206, No.8,681,191, No.8,730,153, No.8,810,525, No.8,928,562, No.8,9 No. 28,641, No. 8,976,444, No. 9,013,394, No. 9,019,197, No. 9,019,198, No. 9,019,Nos. 318, 9,082,352, 9,171,508, 9,218,773, 9,224,338, 9,224,342, 9,224,344, 9,230,492, 9,251,736, 9,262,973, 9,269,311, 9,299,294, 9,373,289, 9,390,066, 9,390,661, and 9,412,Patent No. 314, and U.S. Patent Application Publications No. 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 / 0322721 , No. 2010 / 0194733, No. 2010 / 0194789, No. 2010 / 0220121, No. 2010 / 0265561, No. 2010 / 0283804, No. 2011 / 0063314, No. 2011 / 0175875, No. 2011 / No. 0193840, No. 2011 / 0193841, No. 2011 / 0199671, No. 2011 / 0221740, No. 2012 / 0001957, No. 2012 / 0098740, No. 2013 / 0063333, No. 2013 / 0194250 , No. 2013 / 0249782, No. 2013 / 0321278, No. 2014 / 0009817, No. 2014 / 0085355, No. 2014 / 0204012, No. 2014 / 0218277, No. 2014 / 0240210, No. 2014 / No. 0240373, No. 2014 / 0253425, No. 2014 / 0292830, No. 2014 / 0293398, No. 2014 / 0333685, No. 2014 / 0340734, No. 2015 / 0070744, No. 2015 / 0097877 (See patents No. 2015 / 0109283, No. 2015 / 0213749, No. 2015 / 0213765, No. 2015 / 0221257, No. 2015 / 0262255, No. 2015 / 0262551, No. 2016 / 0071465, No. 2016 / 0078820, No. 2016 / 0093253, No. 2016 / 0140910, and No. 2016 / 0180777) (These patents and applications may hereafter be referred to as MEDEOD (Method for Driving Electro-Optical Displays) applications), (i) Application of the display (see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348) (j) Non-electrophoretic displays as described in U.S. Patent No. 6,241,921, and U.S. Patent Application Publication No. 2015 / 0277160, and U.S. Patent Application Publication Nos. 2015 / 0005720 and 2016 / 0012710.

[0018] Many of the aforementioned patents and applications recognize that the wall surrounding discrete microcapsules in an encapsulated electrophoretic medium may be replaced by a continuous phase, thus producing a so-called "polymer-dispersed electrophoretic display," in which the electrophoretic medium consists of a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymer material, and that discrete droplets of electrophoretic fluid in such a polymer-dispersed electrophoretic display may be considered capsules or microcapsules even if no discrete capsule membrane is associated with each individual droplet. See, for example, U.S. Patent No. 6,866,760. Therefore, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a variant of encapsulated electrophoretic media.

[0019] A related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, charged particles and fluids are not encapsulated within microcapsules, but instead are retained within a carrier medium, typically a polymer film, containing multiple cavities. See, for example, U.S. Patents 6,672,921 and 6,788,449.

[0020] Electrophoretic media are often impermeable (for example, in many electrophoretic media, the particles substantially block the transmission of visible light through the display) and can operate in reflective mode. However, many electrophoretic displays can be manufactured to operate in a so-called "shielding mode," where one display state is substantially impermeable and the other is light-transmitting. See, for example, U.S. Patents 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. Dielectric displays, similar to electrophoretic displays but relying on variations in electric field intensity, can operate in a similar mode. See, for example, U.S. Patent 4,418,346. Other types of electro-optical displays may also be capable of operating in shielding mode. Electro-optical media operating in shielding mode can be used in multilayer structures for full-color displays. In such a structure, at least one layer adjacent to the viewing surface of the display operates in shielding mode, exposing or concealing a second layer located further away from the viewing surface.

[0021] Encapsulated electrophoretic displays typically do not suffer from the clustering and sedimentation failure modes of conventional electrophoretic devices and offer further advantages such as the ability to print or coat displays on a wide variety of flexible and rigid substrates. (The use of the term “printing” is intended to include, but is not limited to, all forms of printing and coating, including pre-metering coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coatings such as knife over-roll coating, forward and reverse roll coating; gravure coating; immersion coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silkscreen printing processes; electrostatic printing processes; thermal printing processes; inkjet printing processes; electrophoretic deposition (see U.S. Patent No. 7,339,715); and other similar techniques.) Therefore, the resulting displays can be flexible. Furthermore, since the display medium can be printed (using various methods), the displays themselves can be manufactured inexpensively.

[0022] As shown above, the simplest prior art electrophoretic media essentially display only two colors. Such electrophoretic media use either a single type of electrophoretic particle having a first color in a colored fluid having a second distinct color (in which case the first color is displayed when the particle is adjacent to the visible surface of the display, and the second color is displayed when the particle is separated from the visible surface), or first and second types of electrophoretic particles having different first and second colors in an uncolored fluid (in which case the first color is displayed when the first type of particle is adjacent to the visible surface of the display, and the second color is displayed when the second type of particle is adjacent to the visible surface). Typically, the two colors are black and white. If a full-color display is desired, a color filter array may be deposited over the visible surface of a monochrome (black and white) display. A display with a color filter array creates a color stimulus by relying on area sharing and color mixing. The available display area is shared among three or four primary colors, such as red / green / blue (RGB) or red / green / blue / white (RGBW), and the filters can be arranged in a one-dimensional (striped) or two-dimensional (2x2) repeating pattern. Other alternative primary colors or more than three primary colors are also known in the art. Three (for RGB displays) or four (for RGBW displays) subpixels are selected to be small enough that, at the intended viewing distance, they visually blend together into a single pixel with a uniform color stimulus ("color blending"). An inherent disadvantage of area sharing is that the colorants are always present, and the color can only be modulated by switching the corresponding pixels of the underlying monochrome display to white or black (switching the corresponding primary colors on or off). For example, in an ideal RGBW display, the primary colors red, green, blue, and white each occupy one-quarter of the display area (one of the four subpixels), the white subpixel is as bright as the white of the underlying monochrome display, but each of the colored subpixels is no brighter than one-third of the white of the monochrome display.Overall, the brightness of white displayed by the display cannot exceed half the brightness of a single white subpixel (the white area of ​​the display is produced by displaying one of each of the four white subpixels plus each colored subpixel in its coloring form, which is equivalent to one-third of the white subpixel; therefore, the three combined colored subpixels do not contribute more than one white subpixel). The brightness and saturation of colors are reduced by area sharing with color pixels that are switched to black. Area sharing is particularly problematic when mixing yellow, as it is brighter than any other color of equal brightness, and saturated yellow is almost as bright as white. Switching from blue pixels (one-quarter of the display area) to black significantly darkens yellow.

[0023] U.S. Patents 8,576,476 and 8,797,634 describe a multicolor electrophoretic display having a single backplane, comprising independently addressable pixel electrodes and a common light-transmitting front electrode. Multiple electrophoretic layers are arranged between the backplane and the front electrode. The displays described in these applications are capable of rendering any of the primary colors (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel location. However, there are disadvantages to using multiple electrophoretic layers located between a single set of addressable electrodes. The electric field exerted by particles in a particular layer is lower than that would be the case for a single electrophoretic layer addressed using the same voltage. In addition, optical losses in the electrophoretic layer closest to the viewing surface (e.g., caused by light scattering or undesirable absorption) can affect the appearance of the image formed in the underlying electrophoretic layers.

[0024] A second form of electrophoretic medium capable of rendering any color at any pixel location is described in U.S. Patent No. 9,921,451. In Patent No. 451, the electrophoretic medium comprises four particles, namely white, cyan, magenta, and yellow, of which two are positively charged and two are negatively charged. However, the display of Patent No. 451 also suffers from color mixing with the white state. Because one of the particles has the same charge as the white particle, when the white state is desired, a certain number of identically charged particles move toward the viewing surface along with the white. It is conceivable that this unwanted color mixing could be overcome using complex waveforms, but such waveforms would significantly increase the display refresh time and, in some cases, result in unacceptable "flashes" between images.

[0025] As discussed above with respect to U.S. Patent No. 11,686,989, one solution is to use an electrophoretic medium comprising four particles, namely white, cyan, magenta, and yellow, where three of the particles are positively charged and the negatively charged particle is white. Charging all non-white particles opposite to the white particles helps reduce color contamination in the white state, but this combination results in an unbalanced waveform when transitioning from the color state to the white or black state. In particular, the black state typically requires a sustained, highly positive drive to ensure that all positively charged particles are moved to the visible surface. [Prior art documents] [Patent Documents]

[0026] [Patent Document 1] U.S. Patent No. 9,361,836 [Patent Document 2] U.S. Patent No. 9,921,451 [Non-patent literature]

[0027] [Non-Patent Document 1] Kitamura, T., et al., Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1 [Non-Patent Document 2] Yamaguchi, Y., et al., Toner display using insulative particles charged triboelectrically, IDW Japan, 2001, Paper AMD4-4 [Overview of the project] [Means for solving the problem]

[0028] (summary) A first aspect of the present invention is a method for driving an electrophoretic display having a plurality of pixels, each pixel capable of displaying at least three optical states, including white, black, and a color that is neither white nor black. The method includes driving an electrophoretic display using a first drive mode that enables transitions between all optical states, driving an electrophoretic display using a second drive mode that includes only transitions between a black optical state and a white optical state, wherein in the second drive mode, the impulse potential overlaid by a pixel transitioning from a white state to a black state is equal to and inversely equal to the impulse potential overlaid by a pixel transitioning from a black state to a white state, and vice versa, driving an electrophoretic display using a first transition mode that enables transitions from a color state of the first drive mode to a white or black state of the second drive mode, wherein the first transition mode compensates for any excess impulse potential that would be delivered to the pixels in the second drive mode, and driving an electrophoretic display using a second transition mode that enables transitions from a white or black state of the second drive mode to a color state of the first drive mode, wherein the second transition mode compensates for any excess impulse potential delivered to the pixels in the second drive mode. In one embodiment, in the first drive mode, the impulse potential covered by a pixel transitioning from a white state to a black state is not equal to, and vice versa, the impulse potential covered by a pixel transitioning from a black state to a white state. In one embodiment, the first and second transition modes do not have identical impulse potential compensation between the color state of the first drive mode and the white state of the second drive mode, and between the color state of the first drive mode and the black state of the second drive mode. In one embodiment, the first and second transition modes do not have identical waveforms between the color state of the first drive mode and the white state of the second drive mode, and between the color state of the first drive mode and the black state of the second drive mode. In one embodiment, in the second drive state, the waveform causing the transition from a white state to a black state includes at least five frames of maximum positive voltage.In one embodiment, the waveform that causes a transition from a black state to a white state in the second driving state includes at least five frames of maximum negative voltage. In one embodiment, the first driving mode is DC balanced. In one embodiment, the first and second transition modes are not DC balanced. In one embodiment, each pixel is capable of displaying at least eight optical states, and the first transition mode allows a transition from each of six non-black and non-white optical states to the white or black state of the second driving mode. In one embodiment, the eight optical states are black, white, red, magenta, yellow, green, cyan, and blue.

[0029] On another note, the display controller is configured to perform one of the methods described above.

[0030] In another aspect, the electrophoretic display is configured to implement one of the methods described above. In one embodiment, the electrophoretic display includes an electrophoretic medium comprising at least three types of particles having different electrophoretic mobilities. In one embodiment, at least two of the three types of particles have the same electrical charge but have different charge magnitudes. In one embodiment, one of the particle types is negatively charged and white in color. In one embodiment, the display includes three types of positively charged particles, each type of positively charged particle being partially absorbent and having a different color from the other types of positively charged particles. In one embodiment, the electrophoretic medium is confined within a plurality of capsules or a plurality of microcells. The present invention provides, for example, the following: (Item 1) A method for driving an electrophoretic display having multiple pixels, wherein each pixel is capable of displaying at least three optical states, including white, black, and a color that is neither white nor black, and the method is The electrophoretic display is driven using a first drive mode that enables transitions between all of the aforementioned optical states, Driving the electrophoretic display using a second driving mode that includes only transitions between a black optical state and a white optical state, wherein in the second driving mode, the impulse potential covered by a pixel transitioning from the white state to the black state is equal to and inversely equal to the impulse potential covered by a pixel transitioning from the black state to the white state. Driving the electrophoretic display using a first transition mode that enables a transition from the color state of the first drive mode to the white state or the black state of the second drive mode, wherein the first transition mode compensates for excess impulse potentials that would be delivered to the pixels in the second drive mode. The electrophoretic display is driven using a second transition mode that enables a transition from the white state or the black state of the second drive mode to the color state of the first drive mode, wherein the second transition mode compensates for excess impulse potentials delivered to the pixels in the second drive mode. Methods that include... (Item 2) The method according to item 1, wherein in the first driving mode, the impulse potential covered by the pixels transitioning from the white state to the black state is not equal to, and vice versa, the impulse potential covered by the pixels transitioning from the black state to the white state. (Item 3) The method according to item 1, wherein the first transition mode and the second transition mode do not have the same impulse potential compensation between the color state of the first drive mode and the white state of the second drive mode, and between the color state of the first drive mode and the black state of the second drive mode. (Item 4) The method according to item 1, wherein the first transition mode and the second transition mode do not have the same waveform between the color state of the first drive mode and the white state of the second drive mode, and between the color state of the first drive mode and the black state of the second drive mode. (Item 5) The method according to item 1, wherein in the second driving state, the waveform causing the transition from the white state to the black state includes at least five frames of maximum positive voltage. (Item 6) The method according to item 1, wherein in the second driving state, the waveform causing the transition from the black state to the white state includes at least five frames of the maximum negative voltage. (Item 7) The first drive mode is DC balanced, as described in item 1. (Item 8) The first and second transition modes are not DC balanced, as described in item 1. (Item 9) The method according to item 1, wherein each pixel is capable of displaying at least eight optical states, and the first transition mode enables a transition from each of the six non-black and non-white optical states to the white state or the black state of the second drive mode. (Item 10) The method according to item 9, wherein the eight optical states are black, white, red, magenta, yellow, green, cyan, and blue. (Item 11) A display controller configured to perform the actions described in item 1. (Item 12) An electrophoretic display configured to implement the method described in item 1. (Item 13) The electrophoretic display according to item 12, comprising an electrophoretic medium comprising at least three types of particles having different electrophoretic mobilities. (Item 14) The display according to item 13, wherein at least two of the three types of particles have the same electrical charge but different magnitudes of charge. (Item 15) One of the particle types is negatively charged and white in color, as described in item 13 for the display. (Item 16) The display described in item 15 further comprises three types of positively charged particles, each type of positively charged particle being partially absorbent and having a different color from the other types of positively charged particles. (Item 17) The electrophoretic medium is enclosed in a plurality of capsules or a plurality of microcells, as described in item 12. [Brief explanation of the drawing]

[0031] [Figure 1] Figure 1 is a schematic cross-sectional view showing the positions of various colored particles in the electrophoretic medium of the present invention when displaying black, white, subtractive primary colors, and additive primary colors.

[0032] [Figure 2A]Figure 2A is a general illustration of an electrophoretic display having four types of particles in a nonpolar fluid, with the full range of colors available at each pixel electrode. In some embodiments, one type of negatively charged particle is white, one type of positively charged particle is yellow, one type of positively charged particle is magenta, and one type of positively charged particle is cyan; however, it should be understood that the present invention is not limited to the exemplary color set or combination of charge polarity and size.

[0033] [Figure 2B] Figure 2B illustrates the transition between a first optical state, in which all particles of the first charge polarity are present on the visible surface, and a second optical state, in which particles with the second (opposite) polarity are present on the visible surface.

[0034] [Figure 2C] Figure 2C illustrates the transition between a first optical state, in which all particles of the first charge polarity are present on the visible surface, and a third optical state, in which particles with the second (opposite) polarity are present behind the intermediate charged particles of the first polarity located on the visible surface.

[0035] [Figure 2D] Figure 2D illustrates the transition between a first optical state, in which all particles of the first charge polarity are present on the visible surface, and a fourth optical state, in which particles with the second (opposite) polarity are present behind the low-charge particles of the first polarity located on the visible surface.

[0036] [Figure 2E] Figure 2E illustrates the transition between a first optical state, in which all particles of the first charge polarity are present on the visible surface, and a fifth optical state, in which particles with the second (opposite) polarity are present behind a combination of low-charged and medium-charged particles of the first polarity located on the visible surface.

[0037] [Figure 3]Figure 3 illustrates an exemplary equivalent circuit for a single pixel of an electrophoretic display.

[0038] [Figure 4] Figure 4 shows the layers of an exemplary electrophoretic color display.

[0039] [Figure 5] Figure 5 shows an exemplary push-pull drive scheme for addressing an electrophoretic medium containing three subtractive and scattering (white) particles. Such an addressing pulse is typically accompanied by a DC equilibrium clearing pulse, allowing each color state to be switched to all other color states.

[0040] [Figure 6] Figure 6 illustrates a drive mode workflow that can be performed by a display controller using the method of the present invention. In particular, the display drive mode changes based on the need for color content of the image to be displayed and / or the need for fast page turning and / or stylus refresh. In some embodiments, the controller switches between modes automatically. In other embodiments, the mode switch requires user input.

[0041] [Figure 7] Figure 7 is a generalized schematic diagram of the present invention, showing DUin and DUout transition modes that compensate for impulse potentials when moving between the global perfect ("GC") mode and the DU mode and vice versa.

[0042] [Figure 8] Figure 8 illustrates waveforms that may be used to provide fast transitions between the white and black optical states in direct update ("DU") mode. Other waveforms may also be used, provided that they result in offset impulse potentials when progressing between the black and white states.

[0043] [Figure 9A] Figure 9A shows a series of waveforms as voltage frames for several transitions in DUin mode (the mode of the present invention), which enables impulse potential-compensated transitions between GC mode and DU mode. Of note, "duK" and "duW" correspond to the black and white optical states in DU mode, respectively. K, W, GC2, and GC3 correspond to black, white, and generalized second and third colors in GC mode, respectively. Typically, in GC mode, K is the first color and W is the last color.

[0044] [Figure 9B] Figure 9B shows the actual visual transitions associated with each frame in DUin mode, that is, corresponding to the waveform shown in Figure 9A.

[0045] [Figure 10A] Figure 10A shows a series of waveforms as voltage frames for several transitions in DUout mode, which enables impulse potential compensation transitions between DU mode and GC mode.

[0046] [Figure 10B] Figure 10B shows the actual visual transitions associated with each frame in DUout mode, i.e., corresponding to the waveform shown in Figure 10A.

[0047] [Figure 11] Figure 11 shows an illustrative example of how the impulse potential is compensated when moving between GC mode and DU mode. It is important to note that the impulse compensation from GC to DU (i.e., DUIn) is different from the impulse compensation from DU to GC (i.e., DUout). [Modes for carrying out the invention]

[0048] (Detailed explanation) The present invention provides a method for driving a multi-pixel electrophoretic display, designed to enable at least three colors, e.g., eight colors, at each pixel. The method uses a first driving scheme capable of bringing about transitions between all colors that can be displayed at each pixel, and a second driving scheme that includes only transitions ending in white or black, which is highly useful for drawing black lines on a white page, reading black text on a white page, or reading white text on a black page. The second driving scheme is intended to enable a fast response of the display to user input, e.g., a user "writing" on the display with a stylus, incorporating a touchscreen or electromagnetic resonance (EMR) or other form of stylus or touch interaction. The present invention further provides a transition driving scheme for switching between the first and second driving schemes.

[0049] The present invention comprises an improved four-particle electrophoretic medium, comprising a first particle of a first polarity and three other particles of the opposite polarity, but with different charge magnitudes. Typically, such a system includes a negatively charged white particle and positively charged particles of subtractive primary colors: yellow, magenta, and cyan. In addition, some particles can be engineered so that their electrophoretic mobility is nonlinear with respect to the intensity of the applied electric field. Thus, one or more particles will suffer a decrease in electrophoretic mobility upon application of a high electric field (e.g., 20V or greater) of the correct polarity. Such a four-particle system is schematically shown in Figure 1, which can provide white, yellow, red, magenta, blue, cyan, green, and black in every pixel.

[0050] As shown in Figure 1, the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) each correspond to different arrangements of four particles, so that the viewer sees only the colored particles (i.e., only the light-scattering particles) on the viewing side of the white particles. To achieve a wide range of colors, additional voltage levels must be used for more subtle control of the particles. In the described formulations, the first (typically negative) particles are reflective (typically white), while the other three particles, i.e., the oppositely charged (typically positive) particles, include three substantially non-light-scattering ("SNLS") particles. The use of SNLS particles allows for color mixing and provides more color results than can be achieved with the same number of scattering particles. These thresholds must be sufficiently separated to avoid crosstalk, and this separation necessitates the use of high addressing voltages for some colors. The disclosed four-particle electrophoretic medium can refresh faster, require "less flashing" transitions, and produce a more viewer-satisfying (and therefore more commercially beneficial) color spectrum. In addition, the disclosed formulation provides faster refresh between black and white pixels (e.g., less than 500 ms, e.g. less than 300 ms, e.g. less than 200 ms, e.g. less than 100 ms), thereby enabling faster page turning for black on white text.

[0051] In Figure 1, it is assumed that the viewing surface of the display is at the top (as shown in the figure), i.e., the user views the display from this direction, and light is incident from this direction. As already stated, in a preferred embodiment, only one of the four particles used in the electrophoretic medium of the present invention substantially scatters light, and in Figure 1, it is assumed that this particle is a white pigment. This light-scattering white particle forms a white reflector, and any particle above the white particle is visible to it (as shown in Figure 1). Light that passes through these particles and enters the viewing surface of the display is reflected by the white particle, passes back through these particles, and emerges from the display. Thus, particles above the white particle may absorb various colors, and the color that appears to the user is the result of the combination of particles above the white particle. Any particle positioned below the white particle (behind the user's viewpoint) is masked by the white particle and does not affect the displayed color. Since the second, third, and fourth particles are substantially non-light-scattering, their order or arrangement relative to each other is not important; however, for the reasons already stated, their order or arrangement relative to the white (light-scattering) particles is important.

[0052] More specifically, when cyan, magenta, and yellow particles are located below the white particle (situation [A] in Figure 1), no particles are located above the white particle, and the pixel simply displays white. When a single particle is located above the white particle, the color of that single particle is displayed as yellow, magenta, and cyan in situations [B], [D], and [F] in Figure 1, respectively. When two particles are located above the white particle, the displayed color is a combination of those of the two particles. That is, in situation [C] in Figure 1, the magenta and yellow particles display red; in situation [E], the cyan and magenta particles display blue; and in situation [G], the yellow and cyan particles display green. Finally, when all three colored particles are located above the white particle (situation [H] in Figure 1), all incident light is absorbed by the subtractive primary colored particles, and the pixel displays black.

[0053] One possibility is that a single subtractive primary color can be rendered by light-scattering particles, thereby giving the display two types of light-scattering particles, one of which is white and the other is colored. However, in this case, the position of the light-scattering colored particles relative to the other colored particles that cover the white particles will be important. For example, when rendering black (when all three colored particles are present covering the white particles), the scattering colored particles cannot be present covering the non-scattering colored particles (otherwise they would be partially or completely hidden behind the scattering particles, and the rendered color would be that of the scattering colored particles, not black).

[0054] Figure 1 illustrates the ideal situation where the color is not contaminated (i.e., the light-scattering white particles completely mask any particles present behind them). In practice, the masking by white particles may be imperfect, thereby allowing for slight absorption of light by particles that would ideally be completely masked. Such contamination typically reduces both the brightness and saturation of the rendered color. In the electrophoretic media of the present invention, such color contamination should be minimized to the extent that the resulting color is comparable to industrial standards for color rendering. A particularly preferred standard is snap (a standard for newspaper advertising production), which specifies L*, A*, and b* values ​​for each of the eight primary colors referred to above (hereafter, "primary colors" will be used to refer to the eight colors, namely black, white, the subtractive primary colors, and the additive primary colors, as shown in Figure 1).

[0055] Figures 2A-2E show schematic cross-sectional representations of four particle types used in the present invention. Utilizing an improved electrophoretic medium, the display layer includes a first (visible) surface 13 on the viewing side and a second surface 14 opposite the first surface 13. The electrophoretic medium is positioned between the two surfaces. Each space between the two vertical dotted lines points to a pixel. Within each pixel, the electrophoretic medium can be addressed, and the visible surface 13 of each pixel can achieve the color state shown in Figure 1 without requiring an additional layer or involving a color filter array.

[0056] As a standard for electrophoretic displays, the first surface (13) is light-transmitting and includes a common electrode (11) constructed from, for example, a sheet of PET with indium tin oxide (ITO) placed thereon. On the second surface (14) there is an electrode layer (12) containing a plurality of pixel electrodes (15). Such pixel electrodes are described in U.S. Patent No. 7,046,228, the contents of which are incorporated herein by reference as a whole. With respect to the pixel electrode layer, an active matrix driven together with a thin-film transistor (TFT) backplane is described, but it should be noted that the scope of the present invention also includes addressing other types of electrodes, as long as the electrodes perform the desired function. For example, the top and bottom electrodes may be continuous. In addition, a pixel electrode backplane different from that described in Patent No. 228 is also preferred and may include an active matrix backplane that can provide a higher driving voltage than that typically found with amorphous silicon thin-film transistor backplanes.

[0057] Newly developed active matrix backplanes may include thin-film transistors incorporating metal oxide materials such as tungsten oxide, tin oxide, indium oxide, zinc oxide, or more complex metal oxides such as indium gallium zirconium oxide. In these applications, channel-forming regions are formed for each transistor using such metal oxide materials, enabling faster switching of higher voltages. Such metal oxide transistors also allow for less leakage in the "off" state of the thin-film transistors (TFTs), which can be achieved, for example, by amorphous silicon TFTs. In a typical scanning TFT backplane with n rows, the transistors will be in the "off" state for approximately a ratio of (n-1) / n of the time required to refresh all rows of the display. Any leakage of charge from the storage capacitor associated with each pixel will result in a degradation of the electro-optical performance of the display. A TFT typically includes a gate electrode, a gate insulating film (typically SiO2), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film covering the gate insulating film, which polymerizes at least partially with the gate electrode, source electrode, and drain electrode. Such backplanes are available from manufacturers such as Sharp / Foxconn, LG, and BOE. Such backplanes can provide drive voltages of ±30V (or higher). In some embodiments, an intermediate voltage driver is included, thereby the resulting drive waveform may include five levels, seven levels, nine levels, or more.

[0058] One preferred metal oxide material for such applications is indium gallium zinc oxygen (IGZO). IGZO-TFTs have an electron mobility 20 to 50 times greater than amorphous silicon. By using IGZO TFTs in an active matrix backplane, it is conceivable that voltages greater than 30V can be provided via a suitable display driver. Furthermore, a source driver capable of supplying at least five, preferably seven, levels provides different driving paradigms for a four-particle electrophoretic display system. In one embodiment, there may be two positive voltages, two negative voltages, and zero volts. In another embodiment, there may be three positive voltages, three negative voltages, and zero volts. In one embodiment, there may be four positive voltages, four negative voltages, and zero volts. These levels can be selected within a range of approximately -27V to +27V, without the limitations imposed by the switching of the top plane as described above.

[0059] The electrophoretic medium of the present invention contains four types of electrophoretic particles in a nonpolar fluid 17, as shown in Figures 2A-2E. The first particles (W-*; open circle) are negatively charged and can be surface-treated such that the electrophoretic mobility of the first particles depends on the strength of the driving electric field (discussed in more detail below). In such cases, the electrophoretic mobility of the particles actually decreases in the presence of a stronger electric field, which is somewhat counterintuitive. The second particles (M++*; dark circle) are positively charged and can also be surface-treated (or not deliberately treated) such that the electrophoretic mobility of the second particles depends on the strength of the driving electric field, or, depending on the reversal of the direction of the electric field, the rate of expansion of the aggregate of the second particles after being driven to one side of the cavity containing the particles is slower than the rate of expansion of the aggregate of the third and fourth particles. The third particle (Y+; checkered circle) is positive but has a smaller charge magnitude than the second particle. In addition, the third particle can be surface-treated, but not in a way that makes its electrophoretic mobility dependent on the strength of the driving electric field. That is, the third particle has a surface treatment, however such a surface treatment does not result in the aforementioned decrease in electrophoretic mobility with increasing electric field. The fourth particle (C+++; gray circle) has the largest positive charge magnitude and the same type of surface treatment as the third particle. As shown in Figure 2A, the particles are nominally white, magenta, yellow, and cyan in order to produce the colors shown in Figure 1. However, the present invention is not limited to this specific set of colors, nor is it limited to one reflective particle and three absorptive particles. For example, the system may include one black absorbing particle and three red, yellow, and blue reflective particles, each with a reflectance spectrum suitably matched to produce a process white state, when all three reflective particles are mixed and visible on the surface.

[0060] In a preferred embodiment, the first particle (negative) is white and scattering. The second particle (positive, moderate charge) is magenta and absorptive. The third particle (positive, low charge) is yellow and absorptive. The fourth particle (positive, high charge) is cyan and absorptive. Table 1 below shows the ratio of absorption and scattering coefficients, as determined by Kuberkamunk analysis, of exemplary yellow, magenta, cyan, and white particles useful in the electrophoretic medium of the present invention, along with their diffuse reflectance. [Table 1]

[0061] The electrophoretic medium of the present invention may be any of the forms discussed above. Therefore, the electrophoretic medium may be in the form of an unencapsulated medium, encapsulated in separate capsules surrounded by a capsule wall, encapsulated in a sealed microcell, or a polymer dispersion medium. The pigments are described in detail elsewhere, e.g., U.S. Patent Nos. 9,697,778 and 9,921,451. Briefly, the white particles W1 are silanol-functionalized light-scattering pigments (titanium dioxide) to which a polymer material containing lauryl methacrylate (LMA) monomer is attached, as described in U.S. Patent No. 7,002,728. The white particles W2 are polymer-coated titania, substantially produced as described in Example 1 of U.S. Patent No. 5,852,196, with a polymer coating containing lauryl methacrylate and 2,2,2-trifluoroethyl methacrylate in a ratio of about 99:1. Yellow particle Y1 is CI pigment Yellow 180, used without coating as generally described in U.S. Patent No. 9,697,778, and dispersed by abrasion in the presence of Solsperse 19000. Yellow particle Y2 is CI pigment Yellow 155, used without coating as generally described in U.S. Patent No. 9,697,778, and dispersed by abrasion in the presence of Solsperse 19000. Yellow particle Y3 is CI pigment Yellow 139, used without coating as generally described in U.S. Patent No. 9,697,778, and dispersed by abrasion in the presence of Solsperse 19000. Yellow particle Y4 is CI pigment Yellow 139, which is coated by dispersion polymerization and incorporates trifluoroethyl methacrylate, methyl methacrylate, and dimethylsiloxane-containing monomers as described in Example 4 of Patent No. 9,921,451. The magenta particles M1 are a positively charged magenta material (dimethylquinacridone, CI pigment red 122) and are coated using vinyl benzyl chloride and LMA, as described in Example 5 of U.S. Patent Nos. 9,697,778 and 9,921,451.

[0062] Magenta particles M2 are CI pigment red 122, which is coated by dispersion polymerization and incorporates methyl methacrylate and dimethylsiloxane-containing monomers as described in Example 6 of U.S. Patent No. 9,921,451. Cyan particles C1 are copper phthalocyanine material (CI pigment blue 15:3), which is coated by dispersion polymerization and incorporates methyl methacrylate and dimethylsiloxane-containing monomers as described in Example 7 of U.S. Patent No. 9,921,451. In some embodiments, the color gamut has been found to be improved by using inkjet yellow 4GC (Clariant) as the core yellow pigment, with the incorporation of a methyl methacrylate surface polymer. The zeta potential of this yellow pigment can be adjusted with the addition of 2,2,2-trifluoroethyl methacrylate (TFEM) monomer and monomethacrylate-terminated poly(dimethylsiloxane).

[0063] Additives and surface treatments to electrophoretic media for enhancing differential electrophoretic mobility, as well as proposed mechanisms for the interaction between the surface treatment and surrounding charge control agents and / or free polymers, are discussed in detail in U.S. Patent No. 9,697,778, which, by reference, are incorporated as a whole. In such electrophoretic media, one method of controlling the interaction between different types of particles is by controlling the type, amount, and thickness of the polymer coating on the particles. For example, to control particle properties such that particle-particle interactions are less between a second type of particle and third and fourth type particles than, for example, between a third type of particle and a fourth type particle, the second type of particle may carry a polymer surface treatment, while the third and fourth type particles may not carry a polymer surface treatment, or may carry a polymer surface treatment with a lower mass coverage per unit area of ​​particle surface than the second type of particle. More generally, the Hamarka constant (a measure of the strength of the van der Waals interaction between two particles, where the pair potential is proportional to the Hamarka constant and inversely proportional to the sixth power of the distance between the two particles) and / or the spacing between particles must be adjusted by a wise selection of polymer coatings on the third type of particles.

[0064] As discussed in U.S. Patent No. 9,921,451, different types of polymers may involve different types of polymer surface treatments. For example, Coulomb force interactions may be weakened when the closest approach distance of oppositely charged particles is maximized by a steric barrier (typically a polymer grafted or adsorbed onto the surface of one or both particles). The polymer shell may be a covalent polymer, produced by grafting or chemisorption, as is well known in the art, or it may be physically adsorbed onto the particle surface. For example, the polymer may be a block copolymer having insoluble and soluble compartments. Alternatively, the polymer shell may be dynamic in that it is a loose network of free polymers from the electrophoretic medium, compounded with pigment particles in the presence of an electric field and a sufficient amount and type of charge control agent (CCA - discussed later). Thus, depending on the strength and polarity of the electric field, the particles may have more associated polymers, which may cause the particles to interact differently with the container (e.g., microcapsules or microcells) and other particles. [The range of the polymer shell is conveniently assessed by thermogravimetric analysis (TGA), a technique in which the temperature of a dry sample of particles is increased and the mass loss due to thermal decomposition is measured as a function of temperature. Using TGA, the ratio of particle mass to polymer can be measured, which can be converted to a volume fraction using the known densities of the core pigment and the polymer attached to them.] Conditions can be found in which the polymer coating is lost but the core pigment remains (these conditions depend on the precise core pigment particles used). Various polymer combinations can be fabricated to act as described below with respect to Figures 2A-2E. For example, in some embodiments, particles (typically first and / or second particles) may have a covalently attached polymer shell that interacts significantly with the container (e.g., a microcell or microcapsule). At the same time, other particles of the same charge may not have a polymer coating, or may compound with a free polymer in solution such that those particles have less interaction with the container.In other embodiments, the particles (typically the first and / or second particles) do not have a surface coating, thereby making it easier for the particles to form a charge bilayer and undergo a reduction in electrophoretic mobility in the presence of a strong field.

[0065] Fluid 17, in which four types of particles are dispersed, is transparent and colorless. This fluid contains charged electrophoretic particles, which move through the fluid under the influence of an electric field. Preferred suspension fluids have a low dielectric constant (about 2) and a high volume resistivity (about 10). 15 It has a viscosity of ohms / cm, low viscosity (less than 5 mPas), low toxicity and environmental impact, low water solubility (less than 10 ppm (parts per million) if conventional encapsulation methods are used, however, it should be noted that this requirement may not be relaxed for unencapsulated or secure microcell displays), a high boiling point (higher than approximately 90°C), and a low refractive index (less than 1.5). The last requirement arises from the use of high refractive index scattering (typically white) pigments, the scattering efficiency of which depends on the refractive index mismatch between the particles and the fluid.

[0066] Organic solvents such as saturated linear or branched hydrocarbons, silicone oils, halogenated organic solvents, and low molecular weight halogen-containing polymers are some useful fluids. The fluid may consist of a single component or a mixture of one or more components to adjust its chemical and physical properties. Reactants or solvents for microencapsulation processes, such as oil-soluble monomers, may be included in the fluid (if used).

[0067] The fluid preferably has low viscosity and a dielectric constant in the range of about 2 to about 30, preferably about 2 to about 15, for high particle mobility. Examples of suitable dielectric fluids include hydrocarbons such as Isopar®, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils, silicone solutions, aromatic hydrocarbons such as toluene, xylene, phenylxylethane, dodecylbenzene or alkylnaphthalene, halogenating solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluorobenzene, dichlorononane or pentachlorobenzene, perfluorating solvents such as FC-43, FC-70, or FC-5060 from 3M Company, St. Paul MN, low molecular weight halogen-containing polymers such as poly(perfluoropropylene oxide) from TCI America, Portland, Oregon, and Halocarbon from Halocarbon Product Corp., River Edge, NJ It contains poly(chlorotrifluoroethylene) such as Oils, and perfluoropolyalkyl ethers such as Galden from Ausimont, or polydimethylsiloxane-based silicone oils from Dupont, Delaware (Krytox Oils and Greases K-Fluid Series), and Dow-corning (DC-200).

[0068] Electrophoretic media typically contain one or more charge control agents (CCAs) and may also contain charge directors. CCAs and charge directors typically comprise low molecular weight surfactants, polymers, or mixtures of one or more components, and play a role in stabilizing or otherwise modifying the sign and / or magnitude of the charge on the electrophoretic particles. CCAs are typically molecules containing an ionic group or other polar group, hereafter referred to as the head group. At least one of the positive or negative ionic head groups is preferably attached to a nonpolar chain (typically a hydrocarbon chain), hereafter referred to as the tail group. CCAs are thought to form inverse micelles within the inner phase, which are small collections of charged inverse micelles that lead to conductivity in hypernonpolar fluids, typically used as electrophoretic fluids.

[0069] The addition of CCA provides the production of inverse micelles containing a highly polar core, which can vary in size from 1 nm to tens of nanometers (and may have spherical, cylindrical, or other geometric shapes), surrounded by the nonpolar tail groups of the CCA molecule. In electrophoretic media, three phases can typically be distinguished: a solid phase with a surface, a highly polar phase (inverse micelles) distributed in the form of extremely small droplets, and a continuous phase containing fluid. Both charged particles and charged inverse micelles can move through the fluid in response to the application of an electric field, and therefore there are two parallel paths for electrical conduction through the fluid (which itself typically has a conductivity close to zero).

[0070] The polar core of CCA is thought to influence the charge on the surface through adsorption onto the surface. In electrophoretic displays, such absorption exists on the surface of electrophoretic particles or on the inner wall of microcapsules (or other solid phases such as the walls of microcells) to form structures similar to inverse micelles, and these structures will hereafter be referred to as hemimicelles. When one ion of an ion pair is attached more firmly to the surface than the other (e.g., by covalent bonding), ion exchange between hemimicelles and unbonded inverse micelles can lead to charge separation, in which the more firmly bonded ion remains associated with the particle, while the less firmly bonded ion is incorporated into the core of the free inverse micelle.

[0071] The ionic material forming the head group of the CCA may also potentially induce ion pair formation on the particle (or other) surface. Therefore, the CCA can perform two basic functions: charge generation on the surface and charge separation from the surface. Charge generation can result from acid-base or ion-exchange reactions between certain parts present in the CCA molecule, or otherwise incorporated into an inverse micelle core or fluid, and the particle surface. Thus, a useful CCA material is one capable of participating in such reactions or any other charge reactions known in the art.

[0072] A non-limiting classification of charge control agents useful in the medium of the present invention includes organic sulfuric acids or sulfonates, metal soaps, block or comb copolymers, organic amides, organic zwitterions, organic phosphoric acids, and phosphonates. Useful organic sulfuric acids and sulfonates include, but are not limited to, sodium bis(2-ethylhexyl) sulfosuccinate, calcium dodecylbenzenesulfonate, calcium petroleum sulfonate, neutral or basic barium dinonylnaphthalene sulfonate, neutral or basic calcium dinonylnaphthalene sulfonate, sodium dodecylbenzenesulfonate, and ammonium lauryl sulfate. Useful metal soaps include, but are not limited to, basic or neutral barium petrolates, calcium petrolates, cobalt, calcium, copper, manganese, magnesium, nickel, zinc, aluminum, and iron salts of carboxylic acids such as naphthenic acid, octanoic acid, oleic acid, palmitic acid, stearic acid, myristic acid, and equivalents. 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 a molecular weight of about 1,800, with an oil-soluble tail of poly(12-hydroxystearic acid) pendanted on an oil-soluble anchor group of poly(methyl methacrylate-methacrylic acid). Useful organic amides / amines include, but are not limited to, polyisobutylene succinimides such as OLOA371 or 1200 (available from Chevron Oronite Company LLC, Houston, Tex.), or SOLSPERSE 17000 or 19000 (available from Lubrizol, Wickliffe, OH; Solsperse is a registered trademark), and N-vinylpyrrolidone polymers. Useful organic zwitterions include, but are not limited to, lecithin. Useful organic phosphoric acids and phosphonates include, but are not limited to, sodium salts of phosphorylated mono and di-glycerides with saturated and unsaturated acid substituents.Useful tail groups for CCAs include olefin polymers such as poly(isobutylene) with molecular weights in the range of 200 to 10,000. The head group may be a sulfonic acid, phosphoric acid, or carboxylic acid, or an amide, or alternatively, an amino group such as a primary, secondary, tertiary, or quaternary ammonium group. One classification of CCAs useful in the disclosed four-particle electrophoresis medium is disclosed in U.S. Patent Publication 2017 / 0097556, which is incorporated herein by reference as a whole. Such a CCA typically comprises a quaternary amine head group and an unsaturated polymer tail, i.e., at least one CC 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 greater, for example, 14,000 g / mol to 22,000 g / mol.

[0073] The charge enhancers used in the medium of the present invention can bias the charge on the electrophoretic particle surface, as will be described in more detail below. Such charge enhancers may be Brønsted or Lewis acids, or bases. Exemplary charge enhancers are disclosed in U.S. Patents 9,765,015, 10,233,339, and 10,782,586, all of which are incorporated as a whole by reference. Exemplary charge enhancers may include, but are not limited to, polyhydroxy compounds containing at least two hydroxyl groups, including ethylene glycol, 2,4,7,9-tetramethyldecine-4,7-diol, poly(propylene glycol), pentaethylene glycol, tripropylene glycol, triethylene glycol, glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), propyleneglycerol monohydroxystearate, and ethylene glycol monohydroxystearate. Examples of amino alcohol compounds containing at least one alcohol function and one amine function within the same molecule include, but are not limited to, triisopropanolamine, triethanolamine, ethanolamine, 3-amino-1-propanol, o-aminophenol, 5-amino-1-pentanol, and tetrakis(2-hydroxyethyl)ethylenediamine. In some embodiments, the charge enhancer is present in the electrophoretic display medium in an amount of about 1 to about 500 milligrams ("mg / g") per gram of particle mass, more preferably about 50 to about 200 mg / g.

[0074] Particle dispersion stabilizers may be added to prevent particle aggregation or adhesion to capsules or other walls or surfaces. For typical high-resistance liquids used as fluids in electrophoretic displays, non-aqueous surfactants may be used. These include, but are not limited to, glycol ethers, acetylene glycols, alkanolamides, sorbitol derivatives, alkylamines, quaternary amines, imidazolines, dialkyl oxides, and sulfosuccinates.

[0075] As described in U.S. Patent No. 7,170,670, the bistability of an electrophoretic medium can be improved by including a polymer in the fluid having an average molecular weight value exceeding approximately 20,000, wherein the polymer is essentially non-absorbent on the electrophoretic particles, and poly(isobutylene) is a preferred polymer for this purpose. Also, as described in U.S. Patent No. 6,693,620, for example, particles with immobilized charges on their surface form an electrical bilayer of opposite charges in the surrounding fluid. The ionic head groups of CCA may be ionic pairs with charged groups on the surface of the electrophoretic particles, which can form an immobilized or partially immobilized layer of charged species. Outside this layer, there is a diffusion layer comprising charged (reverse) micelles containing CCA molecules in the fluid. In conventional DC electrophoresis, the applied electric field exerts a force on fixed surface charges and opposing forces on movable charge pairs, causing sliding within the diffusion layer and resulting in particle movement relative to the fluid. The potential in the sliding plane is known as the zeta potential.

[0076] As a result, some particle types within the electrophoretic medium have different electrophoretic mobilities across the medium, depending on the strength of the electric field. For example, when a first electric field (low strength, i.e., about ±10V or less) is applied to the electrophoretic medium, the first type of particle moves in one direction relative to the electric field; however, when a second electric field (high strength, i.e., about ±20V or more) with the same polarity as the first electric field is applied, the first type of particle begins to move in the opposite direction relative to the electric field. This behavior is theorized to result from conduction in a highly nonpolar fluid, mediated by charged inverse micelles or charged electrophoretic particles. Therefore, any electrochemically generated protons (or other ions) are probably transported through the nonpolar fluid within the micelle core or adsorbed onto the electrophoretic particles. For example, as illustrated in Figure 5B of U.S. Patent No. 9,697,778, a positively charged inverse micelle may approach a negatively electrophoretic particle moving in the opposite direction, and the inverse micelle may be incorporated into an electrical bilayer around the negatively charged particle (the electrical bilayer includes both a charge diffusion layer with an enhanced counterion concentration and a hemimicelle surface adsorption coating on the particle, in which case the inverse micelle charge would be associated with the particle in a sliding envelope that defines the particle's zeta potential, as mentioned above). Through this mechanism, the electrochemical current of the positively charged ion may flow through the electrophoretic fluid, and the negatively charged particle may be biased toward a more positive charge. As a result, for example, the electrophoretic mobility of a first negative type of particle is a function of the magnitude of the electrochemical current and the residence time of the positive charge near the particle surface, which is a function of the electric field strength.

[0077] Furthermore, as described in U.S. Patent No. 9,697,778, positively charged particles can also be prepared to exhibit different electrophoretic mobilities depending on the applied electric field. In some embodiments, secondary (or co)CCA can be added to the electrophoretic medium to adjust the zeta potentials of various particles. Co-CCA selection allows for modification of the zeta potential of one particle while keeping the zeta potentials of other particles essentially unchanged, enabling precise control of both the electrophoretic velocities of various particles during switching and the interactions between particles.

[0078] In some embodiments, a portion of the charge control agents intended for the final formulation are added during the synthesis of electrophoretic particles to engineer the desired zeta potential and influence the reduction of electrophoretic mobility due to a strong electric field. For example, it has been observed that adding a quaternary amine charge control agent during polymer grafting results in a certain amount of CCA being compounded into the particles (this can be confirmed by removing the particles from the electrophoretic fluid to remove all adsorbed species, followed by fragmentation of surface species from the pigment with THF. When the THF extraction is evaluated with 1H NMR, it is clear that a good amount of CCA was adsorbed onto the pigment particles or compounded with the surface polymer). Experiments suggest that high CCA, filling the space between the surface polymers of the particles, promotes the formation of a charge bilayer around the particles in the presence of a strong electric field. For example, magenta particles having more than 200 mg of charge control agent (CCA) per gram of finished magenta particles exhibit excellent retention properties in the presence of a high positive electric field (see, e.g., Figure 2C and the above description). In some embodiments, the CCA comprises a quaternary amine head group and a fatty acid tail. In some embodiments, the fatty acid tail is unsaturated. When some of the particles in the electrophoretic medium contain high CCA packing, it is important that particles for which consistent electrophoretic mobility is desired do not have a large amount of CCA packing, 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.

[0079] In other embodiments, an electrophoretic medium containing four types of particles benefits from the addition of a small amount of acidic entity, such as aluminum salt of di-t-butylsalicylic acid (available from Bontron E-88, Orient Corporation, Kenilworth, NJ), in the presence of Solsperse 17000 of Isopar E. The addition of the acidic material shifts the zeta potential of many, but not all, particles to a more positive value. In one embodiment, about 1% of the acidic material and 99% of Solsperse 17000 (based on the total weight of the two materials) shifts the zeta potential of a third type of particle (Y+) from -5mV to about +20mV. Whether the zeta potential of a particular particle is altered by a Lewis acidic material such as an aluminum salt will depend on the details of the particle's surface chemistry.

[0080] Table 2 shows exemplary relative zeta potentials for three types of colored particles and a single white particle in a preferred embodiment. [Table 2]

[0081] In one embodiment, a negative (white) particle has a zeta potential of -30mV, while the remaining three particles are all positive relative to the white particle. Therefore, a display with positively charged cyan, magenta, and yellow particles can switch between a black state (where all colored particles are in front of the white particle relative to the viewing surface) and a white state, with the white particle being closest to the viewer and blocking the viewer from perceiving the remaining three particles. In contrast, when the white particle has a zeta potential of 0V, the negatively charged yellow particle is the most negative of all particles, and therefore, a display with this particle will switch between a yellow and a blue color state. This would also occur if the white particle were positively charged. However, a positively charged yellow particle will be more positive than the white particle unless its zeta potential exceeds +20mV.

[0082] The behavior of the electrophoretic medium of the present invention is consistent with the mobility of white particles (shown as zeta potential in Table 2), which depends on the applied electric field. Therefore, in the examples illustrated in Table 2, when addressed using a low voltage, the white particles may behave as if their zeta potential were -30mV, but when addressed using a higher voltage, they may behave as if their zeta potential were more positive, perhaps as high as +20mV (matching the zeta potential of yellow particles). Therefore, when addressed using a low voltage, the display will switch between a black state and a white state, but when addressed using a higher voltage, it will switch between a blue color state and a yellow color state.

[0083] The motion of various particles in the presence of high (e.g., "±H", e.g., ±20V, e.g., ±25V) and low (e.g., "±L", e.g., ±5V, e.g., ±10V) electric fields is shown in Figures 2B-2E. For illustrative purposes, each box, bounded by a dashed line, represents a pixel, coupled by an upper light-transmitting electrode 21 and a bottom electrode 22, which may be an active matrix pixel electrode, however, this may also be a light-transmitting electrode or a segmented electrode, etc. Starting from a first state in which all positive particles are present on the visible surface (nominally black), the electrophoretic medium can be driven to four different optical states, as shown in Figures 2B-2E. In a preferred embodiment, this results in a white optical state (Figure 2B), a magenta optical state (Figure 2C), a yellow optical state (Figure 2D), and a red optical state (Figure 2E). It is clear that the remaining four optical states in Figure 1 can be achieved by reversing the order of the initial states and the driving electric field, as shown in the abbreviated representation in Figure 5.

[0084] As in Figure 2B, when addressed using a low voltage, the particles behave according to their relative zeta potential, with relative velocity illustrated by arrows, for the case where a negative voltage is applied to the backplane. Thus, in this embodiment, cyan particles move faster than yellow particles and magenta particles. The first (positive) pulse does not change the position of the particles, as their movement is already restricted by the enclosure walls. The second (negative) pulse swaps the positions of the colored and white particles, and the display switches between a black state and a white state, with the transition color reflecting the relative mobility of the colored particles. Reversing the starting position and pulse polarity allows for a transition from white to black. Thus, this embodiment provides a black-white refresh that requires lower voltage (and consumes less power) compared to other black and white formulations achieved with multiple colors, either via process black or process white.

[0085] In Figure 2C, the first (positive) pulse is a high positive voltage sufficient to reduce the mobility of the magenta particles (i.e., the particles with intermediate mobility among the three positively charged colored particles). Due to the reduced mobility, the magenta particles remain essentially frozen in place, and subsequent reverse pulses of low voltage move the cyan, white, and yellow particles more than the magenta particles, thereby producing magenta on the viewing surface with negatively charged white particles behind the magenta particles. Importantly, if the starting position and pulse polarity are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through electrode 22), this pulse sequence will produce green (i.e., a mixture of yellow and cyan particles).

[0086] In Figure 2D, the first pulse is a low voltage that does not significantly reduce the mobility of 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 effective competition among the three positive particles, thereby allowing the slowest type of particle (yellow in this embodiment) to remain visible in front of the white particles whose movement has been reduced using the preceding negative pulse. In particular, the yellow particles do not reach the upper surface of the cavity containing the particles. Importantly, if the starting position and pulse polarity are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through electrode 22), this pulse sequence will produce blue (i.e., a mixture of magenta and cyan particles).

[0087] Finally, Figure 2E shows that when both pulses are high voltage, the mobility of magenta particles will be reduced by the first high positive pulse, and the competition between cyan and yellow will be amplified by the reduction in the mobility of white particles, caused by the second high negative pulse. This produces red. Importantly, if the starting position and pulse polarity are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through electrode 22), this pulse sequence will produce cyan.

[0088] To obtain a high-resolution display, each pixel of the display must be addressable without interference from neighboring pixels. One way to achieve this objective is to provide an array of nonlinear elements, such as transistors or diodes, with at least one nonlinear element associated with each pixel, in order to produce an "active matrix" display. The addressing or pixel electrode that addresses a single pixel is connected to a suitable voltage source through the associated nonlinear element. Typically, when the nonlinear element is a transistor, the pixel electrode is connected to the drain of the transistor, and although this arrangement will be assumed in the following description, it is essentially arbitrary, and the pixel electrode may also be connected to the source of the transistor. Conventionally, in high-resolution arrays, pixels are arranged in a two-dimensional array of rows and columns such that any particular pixel is uniquely defined by the intersection of one defined row and one defined column. The sources of all transistors in each column are connected to a single column electrode, while the gates of all transistors in each row are connected to a single row electrode. Again, the assignment of sources to rows and gates to columns is conventional but essentially arbitrary and can be reversed as desired. Row electrodes are connected to row drivers, which essentially ensure that at any given moment, only one row is selected, i.e., a selection voltage is applied to the selected row electrodes to ensure that all transistors in the selected row are conductive, while a deselection voltage is applied to all other rows to ensure that all transistors in those unselected rows remain nonconductive. Column electrodes are connected to column drivers, which apply selected voltages to various column electrodes to drive the pixels in the selected rows to their desired optical state (the aforementioned voltages are conventionally provided on the opposite side of the nonlinear array of the electro-optic medium and are relative to a common front electrode that extends across the entire display). After a pre-selected interval known as the "line address time," the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed so that the next line of the display is written.This process is repeated until the entire display is written in a line-by-line format.

[0089] Conventionally, each pixel electrode has a capacitor electrode associated with it such that the pixel electrode and the capacitor electrode form a capacitor. See, for example, International Patent Application WO 01 / 07961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) may be used to form a transistor, and the “selective” and “deselective” voltages applied to the gate electrode can be positive and negative, respectively.

[0090] Figure 3 of the accompanying drawings depicts an exemplary equivalent circuit of a single pixel in an electrophoretic display. As shown, the circuit includes a capacitor 10 formed between the pixel electrode and the capacitor electrode. The electrophoretic medium 20 is represented as a capacitor and resistor in parallel. In some cases, direct or indirect coupling capacitance 30 (commonly referred to as “parasitic capacitance”) between the gate electrode of a transistor associated with the pixel and the pixel electrode can introduce undesirable noise into the display. Typically, the parasitic capacitance 30 is much smaller than that of the storage capacitor 10, and when a row of pixels in the display is selected or deselected, the parasitic capacitance 30 can introduce a small negative offset voltage to the pixel electrode, also known as “kickback voltage,” which is typically less than 2 volts. In some embodiments, to compensate for the undesirable “kickback voltage,” V com The kickback voltage (V KB When set to a value equal to ), all voltages supplied to the display are offset by the same amount, so that net DC unbalance cannot be introduced, a common potential V com However, this may be supplied to the top plane electrode and capacitor electrode associated with each pixel.

[0091] However, V comProblems can occur when it is set to a voltage that is not compensated for the kickback voltage. This can occur when it is desired to apply a voltage to the display that is higher than what is available from the backplane alone. For example, when the maximum voltage applied to the display is, while the backplane is supplied with options such as nominal +V, 0, or -V, V com is supplied with -V, it is well known in the art that it may be doubled. In this case, the maximum voltage encountered is +2V (i.e., in the backplane with respect to the top plane), while the minimum voltage is zero. If a negative voltage is required, the V com potential must be raised to at least zero. The waveforms used to address the display with positive and negative voltages using top plane switching thus have to have a specific frame allocated to each of the V com voltage settings.

[0092] A set of waveforms for driving a color electrophoretic display having four particles is described in U.S. Patent No. 9,921,451, which is incorporated herein by reference. In U.S. Patent No. 9,921,451, seven different voltages are applied to the pixel electrodes, i.e., three positive, three negative, and zero. However, in some embodiments, the maximum voltage used in these waveforms is higher than what can be handled by amorphous silicon thin film transistors. In such cases, a suitable high voltage can be obtained by using top plane switching. (As described above) V com is, V KB When set programmatically to, a separate power supply may be used. However, when top plane switching is used, using the same number of separate power supplies as the V com settings is costly and inconvenient. Further, top plane switching is known to increase the kickback, thereby degrading the stability of the color states.

[0093] Display devices may be constructed using the electrophoretic fluid of the present invention in several ways known in the prior art. The electrophoretic fluid may be encapsulated in microcapsules or incorporated into a microcell structure and then sealed with a polymer layer. The microcapsules or microcell layers may be coated or embossed onto a plastic substrate or film supporting a transparent coating of conductive material. The assembly may be laminated onto a backplane supporting pixel electrodes using a conductive adhesive. Alternatively, the electrophoretic fluid may be dispensed directly onto a thin, continuous cell grid arranged on the backplane, containing the active matrix of the pixel electrodes. The filled grid may then be top-sealed together with an integrated protective sheet / light-transmitting electrode.

[0094] Figure 4 shows a schematic cross-sectional view (not to scale) of a display structure 200 suitable for use in conjunction with the present invention. In the display 200, the electrophoretic fluid is shown to be confined in a microcell, but an equivalent structure incorporating microcapsules may also be used. A substrate 202, which may be glass or plastic, supports pixel electrodes 204, which are either individually addressable compartments or associated with thin-film transistors in an active matrix array (the combination of the substrate 202 and electrodes 204 is conventionally referred to as the backplane of the display). Layer 206 is an optional dielectric layer according to the present invention, applied to the backplane (a method for depositing a suitable dielectric layer is described in U.S. Patent Application No. 16 / 862,750, incorporated by reference). The frontplane of the display comprises a transparent substrate 222 supporting a transparent conductive coating 220. The covering electrode layer 220 is an optional dielectric layer 218. The layer (or multiple layers) 216 is a polymer layer which may include a primer layer for bonding the microcell to the transparent electrode layer 220, and some residual polymer constitutes the bottom of the microcell. The walls of the microcell 212 are used to contain the electrophoretic fluid 214. The microcell is sealed together with the layer 210, and the entire front plane structure is bonded to the back plane using a conductive adhesive layer 208. The process for forming the microcell is described in the prior art, for example, in U.S. Patent No. 6,930,818. In some cases, the microcell is less than 20 μm deep, e.g., less than 15 μm deep, e.g., less than 12 μm deep, e.g., about 10 μm deep, e.g., about 8 μm deep.

[0095] Most commercial electrophoretic displays use amorphous silicon-based thin-film transistors (TFTs) in the construction of active matrix backplanes (202 / 204) due to the wider availability of processing equipment and the cost of various starting materials. Unfortunately, amorphous silicon thin-film transistors become unstable when supplied with gate voltages that would allow switching of voltages higher than approximately + / -15V. Nevertheless, as described below, the performance of ACeP is improved when the magnitude of high positive and negative voltages is allowed to exceed + / -15V. Therefore, improved performance is achieved by additionally changing the bias of the upper light-transmitting electrode relative to the bias on the backplane pixel electrode, which is also known as top-plane switching, as described in the prior disclosure. Thus, when a voltage of +30V (relative to the backplane) is required, the top plane can be switched to -15V, while the appropriate backplane pixels are switched to +15V. A method for driving a four-particle electrophoresis system using top-plane switching is described in detail, for example, in U.S. Patent No. 9,921,451.

[0096] These waveforms indicate that each pixel on the display is +V high , +V low , 0, -V low , and -V high It is required that it be able to be driven at five different addressable voltages, specified as 30V, 15V, 0, -15V, and -30V. In practice, it may be preferable to use more addressable voltages. Three voltages (i.e., +V high , 0, and -V high If only ) is available, then voltage V high It involves a pulse, but with a duty cycle of 1 / n, and by addressing, a lower voltage (e.g., V) high It may be possible to achieve the same result as addressing in / n (where n is a positive integer > 1).

[0097] Figure 5 shows a typical waveform (in a simplified form) used to drive the four-particle color electrophoresis display system described above. Such waveforms have a “push-pull” structure, i.e., they consist of dipoles with two pulses of opposite polarity. The magnitude and length of these pulses determine the color obtained. At a minimum, there should be five such voltage levels. Figure 5 shows high and low positive and negative voltages, as well as zero volts. Typically, “low” (L) refers to a range of about 5–15V, while “high” (H) refers to a range of about 15–30V. In general, the higher the magnitude of the “high” voltage, the better the color gamut achieved by the display. In some embodiments, an additional “medium” (M) level is used, which is typically about 15V, however, the value for M will depend somewhat on the particle composition as well as the environment of the electrophoresis medium. In many of the waveforms shown below, +V high / -V high =±24V, +V med / -V med =±17V and +V low / -V low The voltage is ±10V, which is achieved using a power management integrated circuit (PMIC) and a drive backplane incorporating metal oxide transistors such as IGZO, i.e., as discussed above. Suitable controllers are commercially available for integration into the display of the present invention, such as the UltraChip UC8152c or UC8159c, or the Solomon Systech SPD1656.

[0098] Figure 5 shows the simplest dipole required to form a color, but it will be understood that practical waveforms are multiple repetitions of these patterns, or other non-periodic patterns, and can use more than five voltage levels. Typically, such waveforms are shown as a series of impulses corresponding to frames, i.e., the amount of time between new cycles of gate open for the TFTs in an active matrix array. Thus, for example, Figures 9A-10B include frame numbers, and it should be understood that each frame represents approximately 20 ms. However, the size of each frame can vary, for example, due to larger arrays or faster transistors.

[0099] Naturally, achieving the desired color using the drive pulses in Figure 5 is particle-dependent, starting the process from a known state, and is unlikely to be the last color displayed on the pixel. Therefore, a series of reset pulses precede the drive pulses, increasing the amount of time required to update the pixel from the first color to the second color. The reset pulses are incorporated by reference to U.S. Patent No. 10,593,272. The length of these pulses (refresh and address) and any rest (i.e., the period of zero voltage between them) may be selected such that the entire waveform (the integral of voltage over time across the entire waveform) is DC balanced (i.e., the integral of voltage over time is substantially zero). DC balance can be achieved by adjusting the pulse and rest lengths in the reset phase such that the net impulse supplied in the reset phase is equal in magnitude and opposite in sign to the net impulse supplied in the address phase, during which the display switches to a particular desired color. However, as shown in Figure 2B-2E, the starting state for the eight primary colors is either black or white, which can be achieved using sustained low-voltage drive pulses. The simplicity of achieving this starting state further reduces the update time between states, which is more satisfying for the user and also reduces power consumption (and thus increases battery life).

[0100] Furthermore, the aforementioned discussion of waveforms, specifically the discussion of DC equilibrium, ignores the problem of kickback voltage. In practice, as described above, all backplane voltages are affected by the kickback voltage V KB It is offset by an amount equal to that from the voltage supplied by the power source. Therefore, if the power source used provides three voltages, +V, 0, and -V, the backplane is actually offset by the voltage V+V KB , V KB , and -V+V KB It will receive (V KB Note that in the case of amorphous silicon TFTs, this is usually a negative number. However, the same power source will supply +V, 0, and -V to the front electrode without any kickback voltage offset. Therefore, for example, when the front electrode is supplied with -V, the display will have a maximum voltage of 2V+V KB and minimum voltage V KB This would incur costs and inconvenience, and may involve using a separate power source. KB Instead of supplying voltage to the front electrode, the waveform is such that the front electrode receives positive voltage, negative voltage, and V KB It may be supplied, and may be divided into sections.

[0101] An exemplary controller workflow is shown in Figure 6. The workflow above represents a conventional workflow for a two-particle system as described in U.S. Patent No. 9,672,766, where the black / white and white / black transitions in GC mode are nearly symmetrical. Therefore, it is very easy to switch between GC mode and DU mode as needed, depending on the use case (e.g., menu, scrolling, stylus writing, etc.). There is virtually no accumulation of impulse potential, and therefore, an intermediate transition mode is not necessary in a two-particle system.

[0102] However, in multi-particle systems such as ACeP(registered trademark), the electrophoretic medium suffers from a large difference in blooming-based afterimages when symmetric white / black and black / white transitions are used in GC mode. Therefore, the GC waveform is asymmetric when conventional transition rules are used, resulting in impulse potential accumulation when the device is switched to DU (Direct Update) mode. However, using the present invention, i.e., when switching between GC mode and DU mode for a multi-particle system, as illustrated in the workflow below Figure 6, the transition mode (DU_IN;DU_OUT) is used to compensate for the unbalanced impulse potential when moving between black and white states in GC mode. In one embodiment, the DU (Direct Update) mode sets the black and white states to very high impulse potentials (approximately 400V* frames), thereby achieving good black states and balanced impulse potentials when the display operates in DU mode. Essentially, the present invention maintains a consistent black state impulse potential between DU mode and GC mode, while driving the white state in DU mode using the same and opposite impulse potential as the black state, allowing the white state to remain optically matched, but the impulse potential is not matched when transitioning between GC mode and DU mode.

[0103] A generalized visualization of the DUin and DUout modes is shown in Figure 7. In Figure 7, each set of GC states is shown to transition to the DU mode via a set of DUin waveforms that compensate for the impulse difference. Furthermore, when leaving the DU mode, the display transitions in reverse to the GC mode via the DUout waveform, i.e., in the DUout mode. The schematic in Figure 7 is general-purpose and equally suitable for GC modes including 8, 16, 32, 64, or more color states. In most cases, the DU mode will include only the white and black states, however, alternative DU modes can also be achieved, for example, using green and white as DU states, which is feasible with the correct selection of colored particles.

[0104] Figure 8 shows a suitable exemplary waveform for the DU mode in a 4-particle ACEP® system, including negative white particles and three positive particles (charged differently) of cyan, yellow, and magenta. Although the waveform is not symmetrical (or opposite), the accumulated impulse (voltage* frame) is approximately 400V* frame for both the white-to-black transition (upper right waveform) and the black-to-white transition (lower left waveform). Therefore, as the pixel transitions between white and black in DU mode, the accumulated impulse potential will be negligible. In some embodiments, duK and duW are separate controller states, i.e., transitions that A) maintain round-trip balance and B) match optical performance with the corresponding GC optical state. Furthermore, due to high-voltage driving for most of the DU waveform, it is possible to achieve black-to-white or white-to-black pixel updates within approximately 250ms. While still slightly slower than the best monochrome (only) electrophoretic displays, this refresh rate is sufficient for page turning and stylus input.

[0105] As discussed above, the need for transition modes (DU_IN;DU_OUT) exists to allow the display to move between GC mode and DU mode without excessive impulse potentials that can manifest as afterimages and shorten the lifespan of backplane electronics. The structure imposed in these transitions is such that, when using DUin, the transition to duK is direct, i.e., without a flash effect and a single pulse, while an optical state match between duK and K is enforced. In contrast, the conventional DU mode leaves K->K as an empty transition. In the DUin scheme, duK->duK is empty, but K->duK is filled and involves a direct transition, as shown in Figure 9A. Figure 9A shows a set of potential transition waveforms for DU_IN, with voltages on the y-axis and frame numbers on the x-axis. It should be noted that many of the transitions are actually empty, i.e., no pulse is delivered. Generally, DUin, intended to be applied when transitioning between text mode states (K, GT2, GT3, W) and DU states (duK, duW), applies a net impulse equal to the duK impulse potential to the transition in order to become duK. This is because, due to the associated clearing pulse, the impulse potential of K in GC mode is zero. The impulse potential of the W state in GC is also zero. All transitions between white states are selected to maintain a net impulse potential of 0. Finally, the transition from gray to DU black and white retains the original text mode net impulse potential. To get a sense of the actual optical transitions being overlaid, the waveforms in Figure 9A were invoked on a simulator of a 4-particle ACEP® system including negative white particles and three positive particles (charged differently) of cyan, yellow, and magenta. The resulting transitions are shown in Figure 9B. For empty transitions, there is no color change. However, for some transitions, such as GC2 to duK and GC2 to duW, the display will "flash" bright colors (e.g., red, blue) for several frames. However, since the DUin transition is about 365ms, the color transition is not very noticeable.In the same scheme, Figures 10A and 10B represent various waveform embodiments that can be used to transition from DU to GC, i.e., in DU_OUT mode. Again, some of the transitions, such as from duK to GC2, will transition through the light color, but the transitions are not noticeable because they are fast.

[0106] The round-trip impulse equilibrium between GC mode and DU mode is preserved by adjusting the DUout transition to have a specific net impulse potential. In particular, the transition between the DU state (duK, duW) and the GC mode state (K, GC2, GC3, W) has a net impulse potential equal to a negative duK IP, as shown in Figure 10A. The transition from duW to W / K has a net impulse of 0. In general, all transitions from the DU state to the GC state using DUout will appear as if they are switching to white before transitioning to the text mode state, as can be seen in Figure 10B. A complete transition from DU to GC using DU_OUT is typically about 576 ms.

[0107] An example of impulse potential calculation is shown in Figure 11. In Figure 11, four display pixels are represented as squares in the upper box, and the matrix for impulse potential equilibrium is shown in the lower box (i.e., the lower box is not a pixel). The starting and ending states are GC mode, with two black pixels at the top and two white pixels at the bottom. As shown in the leftmost square, the four pixels end the last GC update with an impulse potential of zero. In DU mode, represented by the two lower matrices in the center, an impulse of +1 arises from the transition from duW to duK, while an impulse of -1 arises from the transition from duK to duW. It should be understood that +1 and -1 are arbitrary units. Using the waveform in Figure 8, +1 would correspond to approximately 400V* frames. The important point is that in DU mode, the transitions are of the same impulse potential but opposite signs.

[0108] However, to adapt to DU mode calculations, the transition from the black state in GC mode to duK in DU mode, i.e., K, must also be subjected to an impulse potential of +1. This is shown in the lower left matrix. This is not intuitive. In most prior art, the transition from GC to DU between black states does not involve an additional impulse added to the pixel. The transition from W to duK also requires an impulse potential of +1, which is not surprising as the color state of the pixel needs to change. Nevertheless, this amount of impulse would not normally be needed to drive from white to black, but it would be necessary for the increased impulse potential used to switch between states in DU mode. As can be seen from the three boxes in the center of the pixel, as the pixel moves between black and white in DU mode, the impulse potential cycles until it is time to switch back to GC mode. At this point, it is necessary to eliminate any remaining impulse potentials in order to disable afterimages in GC mode. Therefore, the DUout matrix indicates that an impulse potential of -1 is required to move from duK to either black or white.

[0109] From the foregoing, it will be seen that the transition-driven mode method of the present invention can provide an improved update for color electrophoretic displays, thus enabling device designers to create more bidirectional applications and thus increase the usefulness of devices containing such displays. It will be apparent to those skilled in the art that numerous changes and modifications can be made without departing from the scope of the invention in the specific embodiments of the invention described above. Therefore, the entire foregoing description should be interpreted as illustrative rather than restrictive.

Claims

1. A method for driving an electrophoretic display having multiple pixels, wherein each pixel is capable of displaying at least three optical states, the at least three optical states including a white optical state, a black optical state, and an optical state that is neither a white optical state nor a black optical state, and the method is The electrophoretic display is driven in a first drive mode that enables transitions between all of the at least three optical states, The electrophoretic display is driven in a second drive mode that allows only the transition between the black optical state and the white optical state, wherein in the second drive mode, the impulse potential overlaid by a pixel transitioning from the white optical state to the black optical state is equal to and inversely equal to the impulse potential overlaid by a pixel transitioning from the black optical state to the white optical state. The electrophoretic display is driven in a first transition mode that enables a transition from the optical state of the first drive mode to the white optical state or the black optical state of the second drive mode, wherein an impulse potential is delivered to the pixel transitioning to the second drive mode by providing an impulse potential when the pixel remains in the black optical state in both the first drive mode and the second drive mode during the first transition mode, Driving the electrophoretic display in a second transition mode that enables a transition from the white optical state or the black optical state of the second drive mode to the optical state of the first drive mode, wherein an impulse potential equal to and opposite to the impulse potential of the first transition mode is delivered to the pixel transitioning to the first drive mode, by providing an impulse potential during the second transition mode when the pixel remains in the black optical state in both the second drive mode and the first drive mode. Methods that include...

2. The method according to claim 1, wherein in the first driving mode, the impulse potential overlaid by the pixel transitioning from the white optical state to the black optical state is not equal to the impulse potential overlaid by the pixel transitioning from the black optical state to the white optical state, and vice versa.

3. The method according to claim 1, wherein in the second driving mode, the waveform causing the transition from the white optical state to the black optical state includes at least five frames of the maximum positive voltage.

4. The method according to claim 1, wherein in the second driving mode, the waveform causing the transition from the black optical state to the white optical state includes at least five frames of the maximum negative voltage.

5. The method according to claim 1, wherein the first drive mode is DC balanced.

6. The method according to claim 1, wherein each pixel is capable of displaying at least eight optical states, and the first transition mode enables a transition from each of the six non-black and non-white optical states to the white optical state or the black optical state of the second drive mode.

7. The method according to claim 6, wherein the eight optical states are a black optical state, a white optical state, a red optical state, a magenta optical state, a yellow optical state, a green optical state, a cyan optical state, and a blue optical state.

8. A display controller configured to perform the method described in claim 1.

9. An electrophoretic display configured to implement the method described in claim 1.

10. The display according to claim 9, wherein the electrophoretic display includes an electrophoretic medium, and the electrophoretic medium comprises at least three types of particles having a plurality of different electrophoretic mobilities.

11. The display according to claim 10, wherein at least two of the three types of particles have the same electrical charge but have different magnitudes of charge.

12. The display according to claim 10, wherein one of the three types of particles is negatively charged and white in color.

13. The display according to claim 12, further comprising three types of positively charged particles, each of which is partially light-absorbing and has a different color from the other types of positively charged particles.

14. The display according to claim 10, wherein the electrophoretic medium is enclosed in a plurality of capsules or a plurality of microcells.

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