Four-particle electrophoretic medium providing fast, high-contrast optical state switching

Abstract

Improved electrophoretic media comprising first particles of a first polarity and three particles of opposite polarity, wherein the first particles and optionally one of the particles of opposite polarity experience a decrease in electrophoretic mobility with increasing electric field. Such electrophoretic media enable very fast updating of full color displays between white and black pixels.

Description

[0001] Related applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 078,829, filed September 15, 2020, and U.S. Provisional Patent Application No. 63 / 191,075, filed May 20, 2021. All patents and publications disclosed herein are incorporated herein by reference in their entirety.

[0003] background

[0004] Electrophoretic displays (EPDs) change color by altering the position of charged colored particles relative to the light-transmitting viewing surface. Because the resulting display has high contrast and is readable in daylight, much like ink on paper, such electrophoretic displays are often called "electronic paper" or "ePaper." Because electrophoretic displays provide a book-like reading experience, use very little power, and allow users to carry hundreds of books in a lightweight handheld device, they are already being used in e-readers such as Amazon. It has been widely adopted in China.

[0005] For many years, electrophoretic displays have consisted of only two types of charged colored particles: black and white. (Of course, "colored" as used herein includes both black and white). White particles are typically light-scattering and contain, for example, titanium dioxide, while black particles are absorbing across the entire visible spectrum and may contain carbon black or absorbing metal oxides such as copper chromite. In its simplest sense, a black and white electrophoretic display requires only a light-transmitting electrode at the viewing surface, a back electrode, and an electrophoretic medium comprising white and black particles with opposite charges. When a voltage of one polarity is applied, the white particles move to the viewing surface, and when a voltage of the opposite polarity is applied, the black particles move to the viewing surface. If the back electrode comprises controllable regions (pixels)—or segmented electrodes or an active matrix of pixel electrodes controlled by transistors—a pattern can be electronically displayed at the viewing surface. The pattern can be, for example, text in a book.

[0006] Recently, various color options have become commercially available for electrophoretic displays, including tri-color displays (black, white, and magenta; black, white, and yellow) and quad-color displays (black, white, magenta, and yellow). Similar to the operation of black and white electrophoretic displays, since the desired colored particles are driven towards the viewing surface, electrophoretic displays with three or four reflective particles operate similarly to simple black and white displays. The driving scheme is much more complex than that of black and white alone, but ultimately, the optical function of the particles is the same.

[0007] Advanced Color Electronic Paper (ACeP) TMThis also includes four types of particles, but the cyan, yellow, and magenta particles are subtractive rather than reflective, resulting in thousands of colors at each pixel. Color processing is functionally equivalent to printing methods long used in offset and inkjet printers. A given color is produced by using the correct proportions of cyan, yellow, and magenta against a bright white paper background. In the case of ACeP, the relative positions of the cyan, yellow, magenta, and white particles relative to the viewing surface determine the color at each pixel. While this type of electrophoretic display achieves thousands of colors at each pixel, careful control of the position of each pigment (50 to 500 nanometers in size) within a working space approximately 10 to 20 micrometers thick is crucial. Clearly, variations in particle position will result in incorrect color display at a given pixel. Therefore, fine voltage control is required for such a system. Further details of the system are available in the following U.S. patents, all of which are incorporated herein by reference in their entirety: U.S. Patents Nos. 9,361,836, 9,921,451, 10,276,109, 10,353,266, 10,467,984, and 10,593,272.

[0008] The term "gray state" is used herein in its conventional sense within the field of imaging, referring to the state intermediate between two extreme optical states of a pixel, and does not necessarily imply a black-to-white transition between these two extreme states. For example, several E Ink patents and published applications mentioned below describe electrophoretic displays where the extreme states are white and dark blue, such that the intermediate gray state would actually be a light blue. In fact, as already mentioned, a change in optical state may not be a color change at all. The terms "black" and "white" can be used below to refer to two extreme optical states of a display and should be understood to generally include extreme optical states that are not strictly black and white, such as the aforementioned white and dark blue states.

[0009] The terms bistable and bistable are used herein in their conventional sense in the art to refer to a display comprising display elements having first and second display states that are different in at least one optical property, such that after any given element has been driven to present its first or second display state by an addressing pulse of finite duration, the state will persist for at least several times, for example, 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 some particle-based electrophoretic displays capable of displaying grayscale are stable not only in their extreme black and white states but also in their intermediate gray states, and this is also true for some other types of electro-optical displays. This type of display is properly referred to as multistable rather than bistable; however, for convenience, the term bistable may be used herein to encompass both bistable and multistable displays.

[0010] The term pulse, when used to refer to driving an electrophoretic display, is used herein to refer to the integral of the voltage applied during the driving cycle of the display with respect to time.

[0011] Particles that absorb, scatter, or reflect light in a broadband or at a selected wavelength are referred to herein as coloring particles or pigment particles. Various materials other than pigments (in the strict sense of the term, referring to 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.

[0012] Particle-based electrophoretic displays have been a subject of intensive research and development for many years. In such displays, various charged particles (sometimes called pigment particles) move through a fluid under the influence of an electric field. Compared to liquid crystal displays, electrophoretic displays can offer advantages such as good brightness and contrast, wide viewing angles, state bistableness, and low power consumption. However, long-term image quality issues have hindered their widespread use. For example, the particles constituting an electrophoretic display are prone to settling, leading to a short lifespan for these displays.

[0013] As mentioned above, the electrophoretic medium requires the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be prepared using a gaseous fluid; see, for example, Kitamura, T. et al., Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y. et al., Toner display using insulative particles charged triboelectrically, IDW Japan, 2001, Paper AMD4-4). See also U.S. Patents 7,321,459 and 7,236,291. When the medium is used in a direction that allows particle settling as in liquid-based electrophoretic media, such as in a sign where the medium is set in a vertical plane, such gas-based electrophoretic media are prone to the same type of problems due to such settling. In fact, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media because the lower viscosity of the gaseous suspension compared to the liquid suspension causes the electrophoretic particles to settle more quickly.

[0014] Numerous patents and applications assigned to or attributed to MIT and E Ink describe various techniques used in encapsulated electrophoretic media and other electro-optic media. Such encapsulated media comprise a plurality of small capsules, each capsule containing an inner phase in a fluid medium containing electrophoretically moving particles and a capsule wall surrounding the inner phase. Typically, the capsules themselves are held within a polymer binder to form an adhesive layer located between two electrodes. The techniques described in these patents and applications include:

[0015] (a) Electrophoretic particles, fluids, and fluid additives; see, for example, U.S. Patents 7,002,728 and 7,679,814;

[0016] (b) Encapsulation, adhesives, and encapsulation methods; see, for example, U.S. Patents 6,922,276 and 7,411,719;

[0017] (c) Microunit structures, wall materials, and methods of forming microunits; see, for example, U.S. Patents 7,072,095 and 9,279,906;

[0018] (d) A method for filling and sealing microcells; see, for example, U.S. Patents 7,144,942 and 7,715,088;

[0019] (e) Films and subassemblies containing electro-optic materials; see, for example, U.S. Patents 6,982,178 and 7,839,564;

[0020] (f) Backplanes, adhesive layers, and other auxiliary layers and methods used in displays; see, for example, U.S. Patents 7,116,318 and 7,535,624;

[0021] (g) Color formation and color adjustment; see, for example, 6,017,584; 6,545,797; 6,664,944; 6,788,452; 6,864,875; 6,914,714; 6,972,893; 7,038,656; 7,038,670; 7,046,228; 7,052,571; 7,075,502***; 7,167,155; 7,385,751; 7,492,505; 7,667,684; 7,684,108; 7,791,789; 7,800,813; 7,821,702; 7,839,564***; 7,910,175; 7,95 2,790; 7,956,841; 7,982,941; 8,040,594; 8,054,526; 8,098,418; 8,159,636; 8,213,076; 8,363,299; 8,422,116; 8,441,714; 8,441,716; 8,466,85 2; 8,503,063; 8,576,470; 8,576,475; 8,593,721; 8,605,354; 8,649,084; 8,670,174; 8,704,756; 8,717,664; 8,786,935; 8,797,634; 8,810,899; 8, 830,559;8,873,129;8,902,153;8,902,491;8,917,439;8,964,282;9,013,783;9,116,412;9,146,439;9,164,207;9,170,467;9,170,468;9,182, U.S. Patents Nos. 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 9,423,666; and U.S. Patent No. 2008 / 0043318. ; 2008 / 0048970; 2009 / 0225398; 2010 / 0156780; 2011 / 0043543; 2012 / 0326957; 2013 / 0242378; 2013 / 0278995; 2014 / 0055840; 2014 / 0078576; 2014 / 0 340430; 2014 / 0340736; 2014 / 0362213; 2015 / 0103394; 2015 / 0118390; 2015 / 0124345; 2015 / 0198858; 2015 / 0234250; 2015 / 0268531; 2015 / 0301246;U.S. Patent Application Publications Nos. 2016 / 0011484; 2016 / 0026062; 2016 / 0048054; 2016 / 0116816; 2016 / 0116818; and 2016 / 0140909;

[0022] (h) A method for driving a display; see, for example, sections 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; 7,202,847; 7,242,514; 7,259,744; 7,304,787; 7,312,794; 7 327,511;7,408,699;7,453,445;7,492,339;7,528,822;7,545,358;7,583,251;7,602,374;7,612,760;7,679,599;7,679,813;7,683,606;7,688 297; 7,729,039; 7,733,311; 7,733,335; 7,787,169; 7,859,742; 7,952,557; 7,956,841; 7,982,479; 7,999,787; 8,077,141; 8,125,501; 8,139,050; 8 174,490; 8,243,013; 8,274,472; 8,289,250; 8,300,006; 8,305,341; 8,314,784; 8,373,649; 8,384,658; 8,456,414; 8,462,102; 8,514,168; 8,537 105; 8,558,783; 8,558,785; 8,558,786; 8,558,855; 8,576,164; 8,576,259; 8,593,396; 8,605,032; 8,643,595; 8,665,206; 8,681,191; 8,730,153; 8 810,525;8,928,562;8,928,641;8,976,444;9,013,394;9,019,197;9,019,198;9,019,318;9,082,352;9,171,508;9,218,773;9,224,338;9,224 U.S. Patent Nos. 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 9,412,314; and U.S. Patent 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; 2009 / 0322721; 2010 / 0194733; 2010 / 0 194789; 2010 / 0220121; 2010 / 0265561; 2010 / 0283804; 2011 / 0063314; 2011 / 0175875; 2011 / 0193840; 2011 / 0193841; 2011 / 0199671; 2011 / 0221740; 2012 / 0001957; 2012 / 0098740; 2013 / 0063333; 2013 / 0194250; 2013 / 0249782 ; 2013 / 0321278; 2014 / 0009817; 2014 / 0085355; 2014 / 0204012; 2014 / 0218277; 2014 / 0240210; 2014 / 0240373; 2014 / 0253425; 2014 / 0292830; 2014 / 0293398; 2014 / 0333685; 2014 / 0340734; 2015 / 0070744; 2015 / 0097877; 2015 / U.S. Patent Application Publications Nos. 0109283; 2015 / 0213749; 2015 / 0213765; 2015 / 0221257; 2015 / 0262255; 2015 / 0262551; 2016 / 0071465; 2016 / 0078820; 2016 / 0093253; 2016 / 0140910; and 2016 / 0180777 (these patents and applications may be referred to below as MEDEOD (Method for Driving an Electro-Optical Display) applications);

[0023] (i) Applications of displays; see, for example, U.S. Patents 7,312,784 and 8,009,348; and

[0024] (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 Publications Nos. 2015 / 0005720 and 2016 / 0012710.

[0025] Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thereby producing what are known as polymer-dispersed electrophoretic displays, wherein the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymeric material, and recognize that even if the discrete capsule membrane is not associated with each individual droplet, the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered as capsules or microcapsules; 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 subtype of encapsulated electrophoretic media.

[0026] A related type of electrophoretic display is the so-called microcell electrophoretic display. In a microcell electrophoretic display, charged particles and fluid are not encapsulated within microcapsules, but rather retained within multiple cavities formed within a carrier medium, typically a polymer film. See, for example, U.S. Patents 6,672,921 and 6,788,449.

[0027] Although electrophoretic media are typically opaque (e.g., because in many electrophoretic media, particles essentially block the transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be fabricated to operate in a so-called shutter mode, where one display state is substantially opaque and the other is translucent. See, for example, U.S. Patents 5,872,552; and 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectric electrophoretic displays, similar to electrophoretic displays but dependent on changes in electric field strength, can operate in a similar mode; see U.S. Patent 4,418,346. Other types of electro-optic displays may also be able to operate in shutter mode. Electro-optic media operating in shutter mode can be used in multilayer structures for full-color displays; in such structures, at least one layer adjacent to the viewing surface of the display operates in shutter mode to expose or hide a second layer further away from the viewing surface.

[0028] Encapsulated electrophoretic displays typically do not suffer from the aggregation and sedimentation failure modes of conventional electrophoretic equipment and offer further advantages such as the ability to print or coat displays on a variety of flexible and rigid substrates (the term "printing" is used to include all forms of printing and coating, including but not limited to: volumetric coating, such as patch die coating, slot or extrusion coating, ramp or step coating, curtain coating; roll coating, such as roll-to-roll blade coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; screen printing; electrostatic printing; thermal printing; inkjet printing; electrophoretic deposition (see U.S. Patent No. 7,339,715); and other similar techniques). Therefore, the resulting display can be flexible. Furthermore, because the display medium can be printed (using various methods), the display itself can be manufactured inexpensively.

[0029] As shown above, most simple existing electrophoretic media essentially display only two colors. Such electrophoretic media either use a single type of electrophoretic particles with a first color in a coloring fluid having a second, different color (in this case, the first color is displayed when the particles are near the viewing surface of the display, and the second color is displayed when the particles are spaced apart from the viewing surface), or use a first type and a second type of electrophoretic particles with different first and second colors in a colorless fluid (in this case, the first color is displayed when the first type of particles are near the viewing surface of the display, and the second color is displayed when the second type of particles are near the viewing surface). Typically, the two colors are black and white. If a full-color display is desired, an array of color filters can be deposited on the viewing surface of a monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color mixing to generate color stimuli. Available display areas are shared between three or four primary colors, such as red / green / blue (RGB) or red / green / blue / white (RGBW), and the color filters can be arranged in a one-dimensional (stripes) or two-dimensional (2x2) repeating pattern. Other choices of primary colors, or more than three primary colors, are also known in the art. Three (in the case of RGB displays) or four (in the case of RGBW displays) subpixels are chosen to be small enough that, at the expected viewing distance, they visually blend together to form a single pixel with uniform color stimulation (“color blending”). An inherent drawback of area sharing is that the colorant is always present and can only be modulated by switching the corresponding pixel of the underlying monochrome display to white or black (turning the corresponding primary color on or off). For example, in an ideal RGBW display, each of the red, green, blue, and white primary colors occupies a quarter of the display area (one of the four subpixels), where the white subpixel is as bright as the white of the underlying monochrome display, and each tinted subpixel is no brighter than one-third of the white of the monochrome display. The brightness of white displayed as a whole cannot exceed half the brightness of the white subpixel (the white area of ​​the display is generated by displaying one of the four white subpixels, plus each tinted subpixel of its tinted form is equivalent to one-third of the white subpixel, so the contribution of the combination of three tinted subpixels does not exceed that of a single white subpixel). The brightness and saturation of the color are reduced by sharing the area with the color pixel 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 the blue pixels (a quarter of the display area) to black makes the yellow too dark.

[0030] U.S. Patents 8,576,476 and 8,797,634 describe a multicolor electrophoretic display with a single backplane comprising independently addressable pixel electrodes and a common transparent front electrode. Multiple electrophoretic layers are disposed between the backplane and the front electrode. The displays described in these applications are capable of displaying any primary color (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel location. However, there are drawbacks to using multiple electrophoretic layers located between a single addressable electrode group. The electric field experienced by particles in a particular layer is lower than that would be presented by a single electrophoretic layer addressed with the same voltage. Additionally, optical losses in the electrophoretic layer closest to the viewing surface (e.g., due to light scattering or undesirable absorption) can affect the appearance of the image formed in the underlying electrophoretic layer.

[0031] Two other types of electrophoretic display systems provide a single electrophoretic medium capable of displaying any color at any pixel location. Specifically, U.S. Patent No. 9,697,778 describes a display in which a dyed solvent is combined with white (light-scattering) particles that move in a first direction when addressed with a low applied voltage and in the opposite direction when addressed with a higher voltage. Full-color display is possible when the white particles and the dyed solvent are combined with two other particles carrying the opposite charge to the white particles. However, the color states of the '778 patent are unacceptable for applications such as text readers. In particular, there will always be some dyed fluid that separates the white scattering particles from the viewing surface, resulting in coloration in the white state of the display.

[0032] A second form of electrophoretic medium capable of displaying any color at any pixel location is described in U.S. Patent No. 9,921,451. In the '451 patent, the electrophoretic medium comprises four particles: white, cyan, magenta, and yellow, with two particles carrying a positive charge and two carrying a negative charge. However, the display of the '451 patent also suffers from color mixing with the white state. Because one of the particles has the same charge as the white particles, a certain amount of particles with the same charge move toward the viewing surface along with the white when a white state is desired. While this undesirable color mixing can be overcome with complex waveforms, such waveforms significantly increase the display's refresh time and, in some cases, cause unacceptable "flickering" between images.

[0033] Overview

[0034] This document discloses an improved electrophoretic display system comprising four types of particles. While prior art describes four groups of particles, with two particles for each charge polarity, this invention utilizes a single type of particle with one charge polarity (typically negatively charged) and three types of particles with opposite charge polarities (typically positively charged). This arrangement ensures a fast and clean optical state for the single particle of the first charge polarity (typically white). Furthermore, by selecting particles with opposite charge polarities having suitable complementary colors, high-quality four-color black can be produced, enabling rapid updating of black as white text is turned.

[0035] In a first aspect, the present invention provides an electrophoretic medium comprising a nonpolar fluid and including particles of a first type having a first optical property and a first charge polarity, particles of a second type having a second optical property and a second charge polarity having a first charge amount, the second particles having a polymer surface coating covalently bonded to the particles, particles of a third type having a third optical property and a second charge polarity having a second charge amount less than the first charge amount, and particles of a fourth type having a fourth optical property and a second charge polarity having a third charge amount greater than the first charge amount, wherein both the third type and the fourth type of particles comprise a polymer layer composite with the particles. In some embodiments, the electrophoretic medium is disposed between two electrodes spaced 10-50 micrometers apart, and the first type of particles exhibit a lower electrophoretic mobility when 20V is applied between the two electrodes than when 10V is applied between the electrodes. In some embodiments, the first particles are light-scattering particles, and the second, third, and fourth particles are light-absorbing particles. In some embodiments, the first particles are white, and the second, third, and fourth particles are selected from cyan, magenta, and yellow. In some embodiments, when a solution is prepared by approximately isotropically distributing various particles at 15% volume (particle volume to solution volume) in a nonpolar fluid having a refractive index of less than 1.55, and the solution is set on a black background in a layer approximately 1 micrometer thick, yellow, magenta, and cyan pigments exhibit diffuse reflection at 650, 550, and 450 nanometers, respectively. In some embodiments, the first charge polarity is negative, and the second charge polarity is positive. In some embodiments, an electrophoretic medium is disposed in a color electrophoretic display. The color electrophoretic display includes a light-transmitting electrode at the viewing surface, a backplane including an array of thin-film transistors coupled to pixel electrodes, and the color electrophoretic medium as described above between the light-transmitting electrode and the backplane. Such a display can be incorporated into e-book readers, portable computers, tablet computers, cellular phones, smart cards, signage, watches, shelf labels, or flash drives.

[0036] In a second aspect, the present invention provides an electrophoretic medium comprising a nonpolar fluid and including particles of a first type having a first optical property and a first charge polarity, particles of a second type having a second optical property and a second charge polarity having a first charge amount, the second particles having no polymer surface coating, particles of a third type having a third optical property and a second charge polarity having a second charge amount less than the first charge amount, and particles of a fourth type having a fourth optical property and a second charge polarity having a third charge amount greater than the first charge amount, wherein both the third and fourth type particles comprise a polymer layer composite with the particles. In some embodiments, the electrophoretic medium is disposed between two electrodes spaced 10-50 micrometers apart, and the first type particles exhibit a lower electrophoretic mobility when 20V is applied between the two electrodes than when 10V is applied between the electrodes. In some embodiments, the first particles are light-scattering particles, and the second, third, and fourth particles are light-absorbing particles. In some embodiments, the first particles are white, and the second, third, and fourth particles are selected from cyan, magenta, and yellow. In some embodiments, when a solution is prepared by approximately isotropically distributing various particles at 15% volume (particle volume to solution volume) in a nonpolar fluid having a refractive index of less than 1.55, and the solution is set on a black background in a layer approximately 1 micrometer thick, yellow, magenta, and cyan pigments exhibit diffuse reflection at 650, 550, and 450 nanometers, respectively. In some embodiments, the first charge polarity is negative, and the second charge polarity is positive. In some embodiments, an electrophoretic medium is disposed in a color electrophoretic display. The color electrophoretic display includes a light-transmitting electrode at the viewing surface, a backplane including an array of thin-film transistors coupled to pixel electrodes, and the color electrophoretic medium as described above between the light-transmitting electrode and the backplane. Such a display can be incorporated into e-book readers, portable computers, tablet computers, cellular phones, smart cards, signage, watches, shelf labels, or flash drives.

[0037] Thirdly, the present invention provides an electrophoretic medium comprising a nonpolar fluid and including particles of a first type having a first optical property and a first charge polarity, particles of a second type having a second optical property and a second charge polarity having a first charge amount, wherein for each gram of the second particles in the electrophoretic medium, each second particle has more than 200 milligrams of charge control agent (CCA) adsorbed onto it, particles of a third type having a third optical property and a second charge polarity having a second charge amount less than the first charge amount, and particles of a fourth type having a fourth optical property and a third charge polarity having a second charge amount greater than the first charge amount, wherein for each gram of the third and fourth particles in the electrophoretic medium, each of the third type particles and the fourth type particles has less than 50 milligrams of charge control agent (CCA) adsorbed onto it. In some embodiments, the charge control agent comprises a quaternary ammonium head group and a fatty acid tail. In some embodiments, the electrophoretic medium is disposed between two electrodes spaced 10-50 micrometers apart, and the particles of the first type exhibit a lower electrophoretic mobility when 20V is applied between the two electrodes than when 10V is applied between the electrodes. In some embodiments, the first particle is a light-scattering particle, and the second, third, and fourth particles are light-absorbing particles. In some embodiments, the first particle is white, and the second, third, and fourth particles are selected from cyan, magenta, and yellow. In some embodiments, when a solution is prepared by approximately isotropically distributing various particles at 15% volume (particle volume to solution volume) in a nonpolar fluid having a refractive index of less than 1.55, and the solution is set on a black background in a layer approximately 1 micrometer thick, the yellow, magenta, and cyan pigments exhibit diffuse reflection at 650, 550, and 450 nanometers, respectively. In some embodiments, the first charge polarity is negative, and the second charge polarity is positive. In some embodiments, an electrophoretic medium is disposed in a color electrophoretic display. The color electrophoretic display includes a light-transmitting electrode at the viewing surface, a backplate including an array of thin-film transistors coupled to pixel electrodes, and the color electrophoretic medium as described above between the light-transmitting electrode and the backplate. Such displays can be integrated into e-book readers, laptops, tablets, cellular phones, smart cards, signage, watches, shelf labels, or flash drives.

[0038] Brief description of the attached diagram

[0039] Figure 1 It is a schematic cross-section showing the positions of various colored particles in the electrophoretic medium of the present invention when black, white, three subtractive primary colors and three additive primary colors are displayed.

[0040] Figure 2AThis is a general illustration of an electrophoretic display having four types of particles in a nonpolar fluid, wherein a full range of colors is available at each pixel electrode. It should be understood that 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, the invention is not limited to the example color group.

[0041] Figure 2B This describes the transition between the first optical state of a particle having all the first charge polarities at the viewing surface and the second optical state of a particle having the second (opposite) polarity at the viewing surface.

[0042] Figure 2C This describes the transition between the first optical state of a particle with all first charge polarities at the viewing plane and the third optical state of a particle with a second (opposite) polarity following a moderately charged particle with the first polarity at the viewing plane.

[0043] Figure 2D This describes the transition between the first optical state of a particle having all the first charge polarities at the viewing plane and the fourth optical state of a particle having the second (opposite) polarity after a low-charged particle with the first polarity at the viewing plane.

[0044] Figure 2E This describes the transition between the first optical state of particles having all the first charge polarities at the viewing plane and the fifth optical state of particles having the second (opposite) polarity after a combination of low-charged and medium-charged particles with the first polarity at the viewing plane.

[0045] Figure 3 An exemplary equivalent circuit for a single pixel of an electrophoretic display is shown.

[0046] Figure 4 Showing the layers of an exemplary electrophoretic color display.

[0047] Figure 5 This illustrates an exemplary push-pull drive scheme for addressing an electrophoretic medium comprising three types of subtractive particles and scattering (white) particles.

[0048] Figure 6A The maximum rate of change of optical density in the electrophoretic medium comprising white, yellow, magenta, and cyan particles with zeta potentials of -35 mV, -22 mV, +54 mV, and +70 mV, respectively, is displayed. The measured rate of change for each non-white particle is plotted when a pulse of 500 ms duration is applied with a voltage indicated on the x-axis.

[0049] Figure 6B show Figure 6AThe color of the electrophoretic medium is measured as a function of voltage and time. Electrophoretic media with yellow particles having a zeta potential of -22 mV only produces a clean white state within a narrow voltage and time range.

[0050] Figure 7A The maximum rate of change of optical density in the electrophoretic medium comprising white, yellow, magenta, and cyan particles with zeta potentials of -35 mV, +22 mV, +54 mV, and +70 mV, respectively, is displayed. The measured rate of change for each non-white particle is plotted when a pulse of 500 ms duration is applied with a voltage indicated on the x-axis.

[0051] Figure 7B show Figure 7A The color of the electrophoretic medium as a function of voltage and time is measured. The electrophoretic medium with yellow particles having a zeta potential of +22 mV produces a clean white state over a much wider voltage and time range.

[0052] Figure 8 The colors available in the electrophoretic fluid include white, yellow, magenta, and cyan particles with zeta potentials of -35mV, +22mV, +54mV, and +70mV, respectively.

[0053] Detailed Explanation

[0054] This invention includes an improved four-particle electrophoretic medium comprising a first particle of a first polarity and three other particles of opposite polarity but different charges. Typically, such a system includes negatively charged white particles and positively charged particles of yellow, magenta, and cyan, which are subtractive primary colors. Additionally, some particles can be designed such that their electrophoretic mobility is non-linear with respect to the applied electric field strength. Therefore, with the application of a high electric field of the correct polarity (e.g., 20 V or higher), one or more particles will experience a decrease in electrophoretic mobility. Such a four-particle system... Figure 1 The diagram is shown schematically, and it can provide white, yellow, red, magenta, blue, cyan, green, and black at each pixel.

[0055] like Figure 1As shown, each of the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) corresponds to a different arrangement of four particles, such that the viewer sees only those colored particles on the viewing side of the white particle (i.e., the only particle that scatters light). To obtain a wide range of colors, additional voltage levels must be used for finer control of the particles. In the described formulation, the first (usually negative) particle is reflective (usually white), while the other three particles are particles with opposite charges (usually positive), including three that are essentially non-light-scattering (“SNLS”). The use of SNLS particles enables color mixing and provides more color results than can be achieved with the same number of scattering particles. These boundaries must be sufficiently separated to avoid interference, and this separation requires high addressing voltages for some colors. The disclosed four-particle electrophoretic medium can also be updated more quickly, requires “less flickering” transitions, and produces a more pleasing color spectrum (and is therefore more commercially valuable). In addition, the disclosed formulation provides rapid updates between black and white pixels (e.g., less than 500 milliseconds, e.g., less than 300 milliseconds, e.g., less than 200 milliseconds, e.g., less than 100 milliseconds), thereby enabling fast page turning on black text over white text.

[0056] exist Figure 1 In this context, it is assumed that the viewing surface of the display is at the top (as illustrated), i.e., the user views the display from that direction, and light is incident from that direction. As already noted, in a preferred embodiment, only one of the four particles used in the electrophoretic medium of this invention substantially scatters light, and... Figure 1 The particle is assumed to be white pigment. This light-scattering white particle forms a white reflector, and any particles above the white particle (such as...) can be seen against the white reflector. Figure 1 (As explained). Light entering the viewing surface of the display passes through these particles, is reflected from the white particles, returns through these particles, and exits the display. Therefore, particles above the white particles can absorb various colors, and the color displayed to the user is produced by the combination of particles above the white particles. Any particles positioned below the white particles (behind them from the user's viewing point) are masked by the white particles and do not affect the displayed color. Because the second, third, and fourth particles are essentially 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 crucial.

[0057] More specifically, when cyan, magenta, and yellow particles are located below white particles ( Figure 1In case [A], there are no particles above the white particles, and the pixels simply display white. When a single particle is above a white particle, the color of that single particle is displayed. Figure 1 The colors are yellow, magenta, and cyan in cases [B], [D], and [F], respectively. When two particles are above a white particle, the displayed color is a combination of the colors of those two particles; Figure 1 In case [C], the magenta and yellow particles appear red; in case [E], the cyan and magenta particles appear blue; and in case [G], the yellow and cyan particles appear green. Finally, when all three colored particles are above the white particle ( Figure 1 In the case of [H], all incident light is absorbed by the three subtractive primary color particles, and the pixel displays black.

[0058] One possibility is that a subtractive primary color could be rendered by particles that scatter light, resulting in a display that includes two types of light-scattering particles: one white and the other tinted. However, in this case, the position of the tinted light-scattering particles relative to the other tinted particles covering the white particles will be important. For example, when rendering black (when all three tinted particles are above the white particles), the tinted light-scattering particles cannot be above the non-tinted light-scattering particles (otherwise they would be partially or completely hidden behind the scattering particles, and the color rendered would be the color of the tinted light-scattering particles, not black).

[0059] Figure 1 This represents an idealized scenario where the color is uncontaminated (i.e., light-scattering white particles completely mask any particles located behind them). In reality, masking by white particles may be incomplete, allowing some small absorption of light by particles that would ideally be completely masked. Such contamination typically reduces both the lightness and chromaticity of the resulting color. In the electrophoretic medium of this invention, such color contamination should be minimized to the extent that the resulting color conforms to industry standards used for color representation. A particularly advantageous standard is SNAP (the standard used for newspaper advertising production), which specifies L*, A*, and B* values ​​for each of the eight primary colors mentioned above (hereinafter, "primary color" will be used to refer to the eight colors: black, white, the three subtractive primary colors, and the three additive primary colors, such as...). Figure 1 (As shown).

[0060] Figures 2A-2ESchematic cross-sectional views of the four particle types used in this invention are shown. The display layer utilizing the improved electrophoretic medium includes a first (viewing) surface 13 on the viewing side and a second surface 14 on the opposite side of the first surface 13. The electrophoretic medium is disposed between the two surfaces. Each space between two vertical dashed lines represents a pixel. Within each pixel, the electrophoretic medium can be addressed, and the viewing surface 13 of each pixel can be implemented. Figure 1 The color states shown are achieved without additional layers or color filter arrays.

[0061] As a standard for electrophoretic displays, the first surface 13 includes a light-transmitting common electrode 11, for example, made of a PET sheet on which indium tin oxide (ITO) is disposed. On the second surface (14), there is an electrode layer 12 comprising a plurality of pixel electrodes 15. Such pixel electrodes are described in U.S. Patent No. 7,046,228, the contents of which are incorporated herein by reference in their entirety. It should be noted that while active matrix driving with a thin-film transistor (TFT) backplane is mentioned in relation to the pixel electrode layer, the scope of the invention includes other types of electrode addressing, provided that the electrodes provide the desired functionality. For example, the top and bottom electrodes may be continuous. Additionally, pixel electrode backplanes different from those described in the '228 patent are also suitable and may include active matrix backplanes capable of providing higher drive voltages than those typically found with amorphous silicon TFT backplanes.

[0062] The newly developed active matrix backplane can include thin-film transistors (TFTs) comprising 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, such metal oxide materials are used to form channel formation regions for each transistor, enabling faster switching at higher voltages. Such metal oxide transistors also achieve less leakage in the "off" state of the thin-film transistor (TFT) than that achievable with, for example, amorphous silicon TFTs. In a typical scanning TFT backplane containing n lines, the transistors will be in the "off" state for approximately a fraction (n-1) / n of the time required to refresh each line of the display. Any charge leakage originating from the storage capacitors associated with each pixel will degrade the electro-optical performance of the display. TFTs typically comprise a gate electrode, a gate insulating film (typically SiO2), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film that at least partially overlaps the gate, source, and drain electrodes on the gate insulating film. Such backplanes are available from manufacturers such as Sharp / Foxconn, LG, and BOE. Such a backplane can provide a drive voltage of ±30V (or greater). In some implementations, an intermediate voltage driver is included, such that the resulting drive waveform may include five, seven, nine, or more levels.

[0063] A preferred metal oxide material for such applications is indium gallium zinc oxide (IGZO). IGZO-TFTs have an electron mobility 20-50 times that of amorphous silicon. By using IGZO TFTs in the active matrix backplane, voltages greater than 30V can be provided via suitable display drivers. Furthermore, at least five, and preferably seven, levels of source drivers can be provided to offer different driving paradigms for the four-particle electrophoretic display system. In one embodiment, two positive voltages, two negative voltages, and zero volt will be present. In another embodiment, three positive voltages, three negative voltages, and zero volt will be present. In one embodiment, four positive voltages, four negative voltages, and zero volt will be present. These levels can be selected in the range of approximately -27V to +27V without the limitations imposed by the top-panel switching as described above.

[0064] like Figures 2A-2E As shown, the electrophoretic medium of the present invention includes four types of electrophoretic particles in a nonpolar fluid 17. The first particle (W-*; hollow circle) is negatively charged and may be surface-treated such that the electrophoretic mobility of the first particle depends on the strength of the driving electric field (discussed in more detail below). In this case, the electrophoretic mobility of the particle actually decreases in the presence of a stronger electric field, which is somewhat counterintuitive. The second particle (M++*; black circle) is positively charged and may also be surface-treated (or intentionally left untreated) such that either the electrophoretic mobility of the second particle depends on the strength of the driving electric field, or after being driven to one side of the cavity containing the particle when the electric field direction is reversed, the unpacking rate of a batch of second particles is slower than the unpacking rate of batches of third and fourth particles. The third particle (Y+; square circle) is positive but has a smaller charge than the second particle. Furthermore, the third particle may be surface-treated, but not in a way that makes the electrophoretic mobility of the third particle dependent on the strength of the driving electric field. That is, the third particle can have a surface treatment; however, such a surface treatment does not cause the aforementioned decrease in electrophoretic mobility with increasing electric field. The fourth particle (C++; gray circle) has the largest amount of positive charge and the same type of surface treatment as the third particle. Figure 2A As shown, the particles are labeled with colors such as white, magenta, yellow, and cyan to produce a color like... Figure 1 The color shown. However, the invention is not limited to this particular color group, nor is it limited to one reflective particle and three absorber particles. For example, the system may include one black absorber particle and three reflective particles—red, yellow, and blue—with appropriately matched reflectance spectra to produce a four-color white state when all three reflective particles are mixed and visible on the surface.

[0065] In a preferred embodiment, the first particle (negative) is white and scattering. The second particle (positive, medium charge) is magenta and absorbing. The third particle (positive, low charge) is yellow and absorbing. The fourth particle (positive, high charge) is cyan and absorbing. Table 1 below shows the diffuse reflectance of exemplary yellow, magenta, cyan, and white particles used in the electrophoretic medium of the present invention, and the ratio of their absorption and scattering coefficients according to Kubelka-Munk analysis of these materials dispersed in a poly(isobutylene) matrix.

[0066] Table 1. Diffuse reflectance of preferred yellow, magenta, cyan, and white particles

[0067]

[0068] The electrophoretic medium of the present invention can be any of the forms discussed above. Therefore, the electrophoretic medium can be unencapsulated, encapsulated in discrete capsules surrounded by capsule walls, encapsulated in sealed microunits, or in the form of a polymer dispersion medium. Pigments are described in detail elsewhere, such as in U.S. Patents 9,697,778 and 9,921,451. Briefly, as described in U.S. Patent 7,002,728, the white particle W1 is a silanol-functionalized light-scattering pigment (titanium dioxide) to which a polymer material comprising lauryl methacrylate (LMA) monomer has been attached. The white particle W2 is essentially a polymer-coated titanium dioxide prepared as described in Example 1 of U.S. Patent 5,852,196, having a polymer coating comprising lauryl methacrylate and 2,2,2-trifluoroethyl methacrylate in a ratio of approximately 99:1. As generally described in U.S. Patent No. 9,697,778, yellow particle Y1 is CI Pigment Yellow 180, used without coating, and dispersed by milling in the presence of Solsperse 19000. As generally described in U.S. Patent No. 9,697,778, yellow particle Y2 is CI Pigment Yellow 155, used without coating, and dispersed by milling in the presence of Solsperse 19000. As generally described in U.S. Patent No. 9,697,778, yellow particle Y3 is CI Pigment Yellow 139, used without coating, and dispersed by milling in the presence of Solsperse 19000. As described in Example 4 of U.S. Patent No. 9,921,451, yellow particle Y4 is CI Pigment Yellow 139, coated by dispersion polymerization, comprising trifluoroethyl methacrylate, methyl methacrylate, and a monomer containing dimethylsiloxane. The magenta particles M1 are positively charged magenta materials (dimethylquinacridone, CI Pigment Red 122) coated with vinyl benzyl chloride and LMA as described in Example 5 of U.S. Patent No. 9,697,778 and U.S. Patent No. 9,921,451.

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

[0070] U.S. Patent No. 9,697,778 discusses in detail the electrophoretic medium additives and surface treatments for promoting different electrophoretic mobilities, as well as the mechanism of interaction between the proposed surface treatments and surrounding charge control agents and / or free polymers, which is incorporated herein by reference in its entirety. In such an electrophoretic medium, one way to control the interactions between various 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 the particle-particle interaction between type II particles and type III and IV particles is less than, for example, the particle-particle interaction between type III and type IV particles, type II particles may undergo a polymer surface treatment, while type III and IV particles may not undergo a polymer surface treatment or may undergo a polymer surface treatment with a lower mass coverage per unit area of ​​particle surface than type II particles. More generally, the Hamaker constant (a measure of the strength of the van der Waals interaction between two particles, proportional to the potential and inversely proportional to the sixth power of the distance between the two particles) and / or the interparticle spacing need to be adjusted by judiciously selecting the polymer coatings on the three types of particles.

[0071] As discussed in U.S. Patent No. 9,921,451, different types of polymers can include different types of polymer surface treatments. For example, Coulomb interactions can be weakened when the closest proximity of particles with opposite charges is maximized by a spatial barrier (typically a polymer grafted or adsorbed onto the surface of one or both particles). The polymer shell can be a covalently bonded polymer prepared by grafting methods or chemisorption known in the art, or it can be physically adsorbed onto the particle surface. For example, the polymer can be a block copolymer containing insoluble and soluble segments. Alternatively, the polymer shell can be dynamic, as it is a loose network of free polymer from an electrophoretic medium that, in the presence of an electric field and a sufficient amount and variety of charge control agents (CCA – discussed below), combines with pigment particles. Therefore, depending on the strength and polarity of the electric field, particles can have more polymer bound together, leading to different interactions between the particles and their containers (e.g., microcapsules or microunits) and other particles [the extent of the polymer shell is conveniently evaluated by thermogravimetric analysis (TGA), a technique in which the temperature of a dried sample of particles is raised and the mass loss due to pyrolysis is measured as a function of temperature. Using TGA, the mass proportion of the particles as polymer can be measured, and this can be converted to a volume fraction using the known density of the core pigment and the polymer bonded to it]. Conditions in which the polymer coating is lost but the core pigment is retained can be identified (these conditions depend on the precise core pigment particles used). Various polymer combinations can be made as follows... Figures 2A-2E It works as described. For example, in some embodiments, the particles (typically the first and / or second particles) may have a covalently bonded polymer shell that interacts strongly with the container (e.g., a microunit or microcapsule). Meanwhile, other particles of the same charge have no polymer coating or are complexed with free polymer in solution, so that those particles have little interaction with the container. In other embodiments, the particles (typically the first and / or second particles) will not have a surface coating, making it easier for the particles to form a charge bilayer and experience reduced electrophoretic mobility in the presence of a strong field.

[0072] The fluid 17, in which four types of particles are dispersed, is transparent and colorless. The fluid contains charged electrophoretic particles that move through the fluid under the influence of an electric field. The preferred suspended fluid has a low dielectric constant (approximately 2) and a high volume resistivity (approximately 10⁻⁶). 15The requirements include: low viscosity (less than 5 mPa), low toxicity and environmental impact, low water solubility (less than 10 parts per million (ppm) if conventional aqueous encapsulation methods are used; however, note that this requirement may be relaxed for unencapsulated or certain micro-cell displays), high boiling point (greater than about 90°C), and low refractive index (less than 1.5). The last requirement stems from the use of high-refractive-index scattering (typically white) pigments, whose scattering efficiency depends on the refractive index mismatch between the particles and the fluid.

[0073] Organic solvents such as saturated straight-chain or branched hydrocarbons, silicone oils, halogenated organic solvents, and low molecular weight halogenated polymers are some useful fluids. Fluids can contain a single component or can be blends of more than one component in order to tune their chemical and physical properties. Reactants or solvents used in the microencapsulation process (if used), such as oil-soluble monomers, can also be included in the fluid.

[0074] For high particle mobility, the fluid preferably has low viscosity and a dielectric constant of about 2 to about 30, preferably about 2 to about 15. Examples of suitable dielectric fluids include hydrocarbons such as... Decalene, 5-ethylidene-2-norbornene, fatty oils, paraffin oils, silicone fluids, aromatic hydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzene or alkylnaphthalene, halogenated solvents such as perfluoronaphthalene, perfluorotoluene, perfluoroxylene, dichlorotrifluorotoluene, 3,4,5-trichlorotrifluorotoluene, chloropentafluorobenzene, dichlorononane or pentachlorobenzene, and perfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company, St. Paul, MN, Minnesota, low molecular weight halogenated polymers such as poly(perfluoropropylene oxide) from TCI America, Portland, Oregon, and poly(chlorotrifluoroethylene) such as halogenated hydrocarbon oils from Halocarbon Product Corp., River Edge, New Jersey. Oils), perfluoropolyalkyl ethers such as Krytox Oils and Greases K-Fluid Series from Galden in Ausimont or DuPont in Delaware, and polydimethylsiloxane silicone oil (DC-200) from Dow-Corning.

[0075] Electrophoretic media typically also include one or more charge control agents (CCAs) and may also include charge directors. CCAs and charge directors typically comprise low-molecular-weight surfactants, polymerizers, or blends of one or more components and are used to stabilize or otherwise alter the sign and / or amount of charge on the electrophoretic particles. CCAs are typically molecules comprising ions or other polar groups, hereinafter referred to as head groups. At least one of the positive or negative ion head groups is preferably attached to a nonpolar chain (typically a hydrocarbon chain), hereinafter referred to as a tail group. CCAs are considered to form reverse micelles in the internal phase, and are a small group of charged reverse micelles that produce conductivity in the very nonpolar fluids typically used as electrophoretic fluids.

[0076] The addition of CCA prepares for the preparation of reverse micelles, which comprise a highly polar core of varying sizes (ranging from 1 nanometer to tens of nanometers) surrounded by nonpolar tail groups of CCA molecules (and can have spherical, cylindrical, or other geometries). In electrophoretic media, three phases are typically distinguishable: a solid particle with a surface, a highly polar phase distributed as tiny droplets (reverse micelles), and a continuous phase containing the fluid. Both charged particles and charged reverse micelles can move through the fluid when an electric field is applied, and thus there are two parallel electrical conduction paths for traversing the fluid (which themselves typically have very low conductivity).

[0077] The polar nuclei of CCA are thought to influence the surface charge by adsorbing onto the surface. In electrophoretic displays, such adsorption can occur onto the surface of electrophoretic particles or the inner wall of microcapsules (or other solid phases, such as the walls of microunits), forming structures similar to reverse micelles, hereinafter referred to as hemimicelles. Ion exchange between hemimicelles and unbound reverse micelles can lead to charge separation when one ion of an ion pair is more strongly bound to the surface than the other (e.g., through covalent bonding), where the stronger-bound ion remains bound to the particle and the less strongly bound ion is bound to the nucleus of the free reverse micelle.

[0078] It is also possible that the ionic materials forming the head groups of CCA can induce ion pair formation on the particle (or other) surface. Therefore, CCA can perform two fundamental functions: charge generation at the surface and charge separation from the surface. Charge generation can be caused by acid-base or ion exchange reactions between some portions present in the CCA molecule or otherwise bound to the antimicelle core or fluid and the particle surface. Therefore, useful CCA materials are those capable of participating in such reactions or any other charging reactions known in the art.

[0079] Non-limiting categories of charge control agents useful in the medium of this invention include organic sulfates or sulfonates, metal soaps, block or comb copolymers, organic amides, organic zwitterions, and organophosphates and phosphonates. Useful organic sulfates and sulfonates include, but are not limited to, sodium bis(2-ethylhexyl)sulfosuccinate, calcium dodecylbenzenesulfonate, calcium petroleum sulfonate, neutral or basic barium dinonylnaphthalenesulfonate, neutral or basic calcium dinonylnaphthalenesulfonate, sodium dodecylbenzenesulfonate, and ammonium lauryl sulfate. Useful metal soaps include, but are not limited to, basic or neutral barium petroleum esters, calcium petroleum esters, and cobalt, calcium, copper, manganese, magnesium, nickel, zinc, aluminum, and iron salts of carboxylic acids such as naphthenic acids, octanoic acid, oleic acid, palmitic acid, stearic acid, and myristic acid. Useful block or comb copolymers include, but are not limited to, (A) polymers of 2-(N,N-dimethylamino)ethyl methacrylate quaternized with methyl p-toluenesulfonate and (B) AB diblock copolymers of poly(2-ethylhexyl methacrylate), as well as comb graft copolymers having an oil-soluble tail of poly(12-hydroxystearic acid) dangling over an oil-soluble anchoring group of poly(methyl methacrylate-methacrylic acid) and having a molecular weight of about 1800. Useful organic amides / amines include, but are not limited to, polyisobutylene succinimide, such as OLOA 371 or 1200 (available from Chevron Oronite Company LLC, Houston, Texas) or SOLSPERSE 17000 or 19000 (available from Lubrizol, Wickliffe, OH: Solsperse is a registered trademark), and N-vinylpyrrolidone polymers. Useful organic zwitterions include, but are not limited to, lecithin. Useful organophosphates and phosphonates include, but are not limited to, sodium salts of phosphorylated mono- and di-glycerides having saturated and unsaturated acid substituents. Useful tail groups for CCAs include polymers of olefins, such as poly(isobutylene) with a molecular weight in the range of 200-10000. The head group can be a sulfonic acid, phosphoric acid, or carboxylic acid or amide, or an amino group such as a primary, secondary, tertiary, or quaternary ammonium group. A class of CCAs useful in the disclosed four-particle electrophoresis medium is disclosed in U.S. Patent Publication No. 2017 / 0097556, which is incorporated herein by reference in its entirety. Such CCAs typically comprise a quaternary ammonium head group and an unsaturated polymer tail, i.e., comprising at least one C / C double bond. The polymer tail is typically a fatty acid tail. Various 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.

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

[0081] Particle dispersion stabilizers can be added to prevent particle flocculation or adhesion to the capsule or other walls or surfaces. For liquids with typically high resistivity used as fluids in electrophoretic displays, non-aqueous surfactants can be used. These include, but are not limited to, glycol ethers, alkynyl glycols, alkanolamides, sorbitol derivatives, alkylamines, quaternary ammoniums, imidazolines, dialkyl oxides, and sulfosuccinates.

[0082] As described in U.S. Patent No. 7,170,670, the bistability of the electrophoretic medium can be improved by including a polymer having a number-average molecular weight of more than about 20,000 in the fluid, which is substantially non-adsorbed on the electrophoretic particles; poly(isobutylene) is a preferred polymer for this purpose. Furthermore, as described, for example, in U.S. Patent No. 6,693,620, particles with a fixed charge on their surface establish an electric double layer of opposite charges in the surrounding fluid. The ionic head groups of CCA can pair with charged group ions on the surface of the electrophoretic particles to form a layer of immobilized or partially immobilized charged material. Outside this layer is a diffusion layer containing charged (opposite) micelles containing CCA molecules in the fluid. In conventional DC electrophoresis, the applied electric field exerts a force on the fixed surface charge and an opposite force on the moving opposite charge, causing slippage within the diffusion layer and particle movement relative to the fluid. The potential at the slip plane is called the zeta potential.

[0083] As a result, some particle types within the electrophoretic medium exhibit different electrophoretic mobilities, which depend on the electric field strength across the medium. For example, when a first (low intensity, i.e., approximately ±10 V or lower) electric field is applied to the electrophoretic medium, particles of type I move in one direction relative to the field. However, when a second (high intensity, i.e., approximately ±20 V or higher) electric field with the same polarity as the first field is applied, the particles of type I begin to move in the opposite direction relative to the field. Theoretically, this behavior is caused by conduction within a highly nonpolar fluid mediated by charged reverse micelles or electrophoretic particles with opposite charges. Therefore, any electrochemically generated protons (or other ions) may be transported through the nonpolar fluid in the micelle nucleus or adsorbed onto the electrophoretic particles. For example, as described in U.S. Patent No. 9,697,778. Figure 5 As described in B, positively charged reverse micelles can approach negatively electrophoretic particles traveling in the opposite direction, where the reverse micelles are bound to the electric double layer surrounding the negatively charged particles (the electric double layer comprises a diffused layer with an enhanced counterion concentration and a coating adsorbed on the semi-micelle surface of the particle; in the latter case, the reverse micelle charge will bind to the particle within a sliding envelope, which, as mentioned above, defines the zeta potential of the particle). Through this mechanism, an electrochemical current of positively charged ions flows through the electrophoretic fluid, and negatively charged particles can become biased towards carrying more positive charges. As a result, the electrophoretic mobility, for example, that of particles of the first negatively charged type, is a function of the amount of electrochemical current and the residence time of the positive charge near the particle surface, which is a function of the electric field strength.

[0084] Furthermore, as described in U.S. Patent No. 9,697,778, positively charged particles can be prepared, which also exhibit different electrophoretic mobilities depending on the applied electric field. In some embodiments, a second (or co-)CCA can be added to the electrophoretic medium to adjust the zeta potential of various particles. Careful selection of the co-CCA allows for changing the zeta potential of one particle while keeping the zeta potential of other particles substantially constant, achieving close control over both the electrophoretic velocity of various particles and the interactions between particles during switching.

[0085] In some implementations, a portion of a charge control agent intended for use in the final formulation is added during the synthesis of the electrophoretic particles to design the desired zeta potential and influence the reduction in electrophoretic mobility due to a strong electric field. For example, it has been observed that adding a quaternary ammonium charge control agent during polymer grafting results in a certain amount of CCA complexing onto the particles (this can be confirmed by removing the particles from the electrophoretic fluid and subsequently removing the surface material from the pigment with THF to remove all adsorbed material. When the THF extract is evaluated with 1H NMR, it is clear that a large amount of CCA is adsorbed onto the pigment particles or complexed with the surface polymer). Experiments have shown that high CCA loading in the surface polymer of the particles promotes the formation of a charge bilayer around the particles in the presence of a strong electric field. For example, magenta particles with more than 200 mg of charge control agent (CCA) per gram of finished magenta particles exhibit excellent retention properties in the presence of a high positive electric field (see, for example...). Figure 2C (As described above). In some embodiments, the CCA comprises a quaternary ammonium head group and a fatty acid tail. In some embodiments, the fatty acid tail is unsaturated. When some particles in the electrophoretic medium contain high CCA loadings, it is important that particles with consistent electrophoretic mobility are substantially free of CCA loading, for example, less than 50 mg of charge control agent (CCA) per gram of finished particles, or less than 10 mg of charge control agent (CCA) per gram of finished particles.

[0086] In other embodiments, the electrophoretic medium comprising four types of particles benefits from the addition of a small amount of acidic material, such as, for example, an aluminum salt of di-tert-butylsalicylic acid (Bontron E-88, available from Orient Corporation, Kenilworth, NJ). The addition of the acidic material shifts the zeta potential of many (though not all) particles to a more positive value. In one embodiment, approximately 1% acidic material and 99% Solsperse 17000 (based on the total weight of the two materials) shift the zeta potential of the third type of particle (Y+) from -5 mV to approximately +20 mV. Whether the zeta potential of a particular particle is altered by Lewis acid materials such as aluminum salts will depend on the details of the particle surface chemistry.

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

[0088] Table 2. Relative zeta potentials of colored particles in the presence of relative zeta potentials of white particles.

[0089]

[0090] In one implementation, the negative (white) particles have a zeta potential of -30 mV, and the remaining three types of particles are positive relative to the white particles. Therefore, a display containing positive cyan, magenta, and yellow particles can switch between a black state (where all colored particles are in front of the white particles relative to the viewing surface) and a white state, where the white particles are closest to the viewer and block the viewer's perception of the remaining three types of particles. Conversely, when the white particles have a zeta potential of 0 V, the negatively charged yellow particles are the most negative of all the particles, and therefore a display containing these particles will switch between yellow and blue states. This will also occur if the white particles are positively charged. However, the positively charged yellow particles will carry more positive charge than the white particles unless their zeta potential exceeds +20 mV.

[0091] The behavior of the electrophoretic medium of this invention is consistent with the mobility of white particles (expressed as zeta potential in Table 2) depending on the applied electric field. Therefore, in the examples illustrated in Table 2, when addressed with a low voltage, white particles may behave as if their zeta potential is -30 mV, but when addressed with a higher voltage, they may behave as if their zeta potential is more positive, possibly even as high as +20 mV (matching the zeta potential of yellow particles). Thus, when addressed with a low voltage, the display will switch between black and white states, but when addressed with a higher voltage, the display will switch between blue and yellow states.

[0092] Figure 2B-2E The motion of various particles is shown 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. For illustrative purposes, each box defined by dashed lines represents a pixel defined by a top transparent electrode 21 and a bottom electrode 22, which can be a pixel electrode of an active matrix, however, it can also be a transparent electrode or a segmented electrode, etc. Starting from the first state, where all positive particles are present on the viewing surface (nominal black), the electrophoretic medium can be driven to four different optical states, such as... Figure 2B-2E As shown. In a preferred embodiment, this results in a white optical state ( Figure 2B Magenta optical state ( Figure 2C ), yellow optical state ( Figure 2D ) and red optical state ( Figure 2E Obviously, Figure 1 The remaining four optical states can be achieved by reversing the order of the initial state and the driving electric field, such as... Figure 5 Simplified display.

[0093] When using low-voltage addressing, such as Figure 2BAs shown, when a negative voltage is applied to the backplate, the particles move at relative velocities according to their relative zeta potentials, as indicated by the arrows. Therefore, in this example, cyan particles move faster than magenta particles, and magenta particles move faster than yellow particles. The first (positive) pulse does not change the positions of the particles because they are already confined by the walls of the casing during their motion. The second (negative) pulse swaps the positions of the colored and white particles, and thus the display switches between black and white states, albeit with transient colors reflecting the relative mobility of the colored particles. The inversion of the pulse's start position and polarity achieves the transition from white to black. Therefore, compared to other black and white formulations achieved with multiple colors via process black or process white, this embodiment provides a black-to-white update requiring lower voltage (and consuming less power).

[0094] exist Figure 2C In the process, the first (positive) pulse is a high positive voltage, sufficient to reduce the mobility of the magenta particles (i.e., particles with moderate mobility among the three positively charged coloring particles). Due to the reduced mobility, the magenta particles remain essentially stationary, and subsequent low-voltage pulses in the opposite direction cause the cyan, white, and yellow particles to move more than the magenta particles, thus producing a magenta color on the viewing surface, where the negatively charged white particles are behind the magenta particles. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to viewing the display from the side opposite to the viewing surface, i.e., through electrode 22), the pulse sequence will produce green (i.e., a mixture of yellow and cyan particles).

[0095] exist Figure 2D In this sequence, the first pulse is low voltage, which does not significantly reduce the mobility of either the magenta or white particles. However, the second pulse is high negative voltage, which reduces the mobility of the white particles. This achieves more efficient competition among the three types of positive particles, allowing the slowest type of particle (yellow in this example) to remain visible in front of the white particles, whose movement is diminished with the earlier negative pulse. Notably, the yellow particles do not reach the top surface of the cavity containing the particles. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to viewing the display from the side opposite the viewing surface, i.e., through electrode 22), the pulse sequence will produce blue (i.e., a mixture of magenta and cyan particles).

[0096] at last, Figure 2EThe display shows that when both pulses are high voltage, the first high positive pulse reduces the mobility of magenta particles, and the second high negative pulse, which reduces the mobility of white particles, enhances the competition between cyan and yellow. This produces red. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to viewing the display from the side opposite to the viewing surface, i.e., through electrode 22), the pulse sequence will produce cyan.

[0097] To achieve a high-resolution display, individual pixels must be addressable without interference from neighboring pixels. One way to achieve this is by providing an array of nonlinear elements, such as transistors or diodes, with at least one nonlinear element associated with each pixel to create an "active matrix" display. The addressing or pixel electrode of a pixel is connected to a suitable voltage source via the associated nonlinear element. Typically, when the nonlinear element is a transistor, the pixel electrode is connected to the drain electrode of the transistor, and this arrangement will be presented in the description below, although it is essentially arbitrary and the pixel electrode can be connected to the source electrode of the transistor. Conventionally, in a high-resolution array, pixels are arranged in a two-dimensional array of rows and columns such that any particular pixel is uniquely defined by the intersection of a particular row and a particular column. The source electrodes of all transistors in each column are connected to a single column electrode, and the gate electrodes of all transistors in each row are connected to a single row electrode; furthermore, the assignment of source electrodes to rows and gate electrodes to columns is conventional but essentially arbitrary and can be reversed if desired. Row electrodes are connected to row drivers, which essentially ensure that only one row is selected at any given time. This is achieved by applying a selection voltage to the selected row electrode to ensure all transistors in that selected row are turned on, while a non-selection voltage is applied to all other rows to ensure all transistors in those unselected rows remain off. Column electrodes are connected to column drivers, which apply selected voltages to each column electrode to drive the pixels in the selected row to their desired optical state (the aforementioned voltages are relative to a common front electrode, which is typically located on the side of the electro-optical medium opposite the nonlinear array and extends across the entire display). After a pre-selection interval known as the “line addressing time,” the selected row is deselected, the next row is selected, and the voltage on the column drivers is changed so that the next row of the display is written. This process is repeated so that the entire display is written line by line.

[0098] Conventionally, each pixel electrode has an associated capacitor electrode, such that the pixel electrode and the capacitor electrode form a capacitor; see, for example, International Patent Application WO01 / 07961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) may be used to form the transistor, and the “select” and “non-select” voltages applied to the gate electrode may be positive and negative, respectively.

[0099] In the attached diagram Figure 3 An exemplary equivalent circuit for a single pixel of an electrophoretic display is described. As illustrated, the circuit includes a capacitor 10 formed between a pixel electrode and a capacitor electrode. The electrophoretic dielectric 20 is represented as a capacitor and a resistor connected in parallel. In some cases, the direct or indirect coupling capacitance 30 (commonly referred to as "parasitic capacitance") between the gate electrode of the transistor associated with the pixel and the pixel electrode can introduce undesirable noise into the display. Typically, the parasitic capacitance 30 is much smaller than the capacitance of the storage capacitor 10, and when a pixel row of the display is selected or deselected, the parasitic capacitance 30 can cause a small negative offset voltage, also known as a "recoil voltage," to the pixel electrode, which is typically less than 2 volts. In some embodiments, to compensate for the undesirable "recoil voltage," a common potential V can be... 公共 Provided to the top plate electrode and capacitor electrode associated with each pixel, such that when V 公共 Set to equal to the recoil voltage (V) KB When the value is ), each voltage supplied to the display can be offset by the same amount and does not experience a net DC imbalance.

[0100] However, when V 公共 Problems may arise when the voltage is set to an uncompensated recoil voltage. This can occur when a higher voltage is expected to be applied to the display than is available solely from the backplane. For example, it is known in the art that if a nominal +V, 0, or -V option is supplied to the backplane, while a voltage higher than that available from the backplane is supplied to the V... 公共 If a -V supply is used, the maximum voltage applied to the display can be doubled. In this case, the maximum voltage experienced is +2V (i.e., at the back panel relative to the top panel), while the minimum is zero. If a negative voltage is required, then V... 公共 The potential must rise to at least zero. Therefore, the waveform used for addressing the display with positive and negative voltages via top-panel switching must have a voltage assigned to more than one V. 公共 Each specific frame in the voltage settings.

[0101] U.S. Patent No. 9,921,451 describes a set of waveforms for driving a color electrophoretic display with four particles, which is incorporated herein by reference. In U.S. Patent No. 9,921,451, seven different voltages are applied to the pixel electrodes: three positive, three negative, and zero. However, in some embodiments, the maximum voltage used in these waveforms is higher than the maximum voltage that amorphous silicon thin-film transistors can handle. In such cases, a suitable high voltage can be obtained by using top-plate switching. When (as described above) V 公共 Intentionally set to V KB At times, a separate power supply can be used. However, when using the top panel switch, a power supply compatible with V is used. 公共Setting up as many separate power supplies as possible is expensive and inconvenient. Furthermore, it is known that top-plate switching increases backlash, thereby reducing the stability of color states.

[0102] The electrophoretic fluid of the present invention can be used to construct display devices in several ways known in the prior art. The electrophoretic fluid can be encapsulated in microcapsules or incorporated into a microcell structure subsequently sealed with a polymer layer. The microcapsules or microcell layers can be coated or imprinted onto a plastic substrate or film with a transparent coating of conductive material. The component can be laminated to a backplane with pixel electrodes using a conductive adhesive. Alternatively, the electrophoretic fluid can be directly dispensed onto a thin open cell grid already arranged on a backplane including an active matrix of pixel electrodes. The top of the filled grid can then be sealed with an integrated protective sheet / transparent electrode.

[0103] Figure 4 A schematic cross-sectional view (not to scale) of a display structure 200 suitable for use in this invention is shown. In the display 200, the description of the electrophoretic fluid is limited to microcells, although equivalent structures incorporating microcapsules may also be used. A substrate 202, which may be glass or plastic, carries pixel electrodes 204, which may be individually addressed segments or associated with thin-film transistors in an active matrix arrangement (the combination of substrate 202 and electrodes 204 is generally referred to as the backplane of the display). Layer 206 is an optional dielectric layer applied to the backplane according to the invention (a method for depositing a suitable dielectric layer is described in U.S. Patent Application No. 16 / 862,750, which is incorporated herein by reference). The front panel of the display includes a transparent substrate 222 with a transparent conductive coating 220. Covering electrode layer 220 is an optional dielectric layer 218. Layer (or multiple layers) 216 is a (multilayer) polymer layer that may include a primer layer for adhering microcells to the transparent electrode layer 220 and some residual polymer including the bottom of the microcells. The walls of microcell 212 are used to contain electrophoretic fluid 214. The microcell is sealed with layer 210, and the entire front panel structure is adhered to the back panel using conductive adhesive layer 208. Methods for forming the microcells are described in the prior art, for example, in U.S. Patent No. 6,930,818. In some cases, the depth of the microcell is less than 20 micrometers, for example, less than 15 micrometers, for example, less than 12 micrometers, for example, about 10 micrometers, for example, about 8 micrometers.

[0104] Due to the wider availability of manufacturing equipment and the cost of various starting materials, most commercial electrophoretic displays use amorphous silicon-based thin-film transistors (TFTs) in the construction of the active matrix backplane (202 / 024). Unfortunately, amorphous silicon TFTs become unstable when a gate voltage higher than approximately + / -15V is provided to achieve voltage switching. Nevertheless, as described below, the performance of ACeP is improved when the amount of high positive and high negative voltages allowed exceeds + / -15V. Therefore, as described in previous disclosures, the improved performance is achieved by additionally changing the bias voltage of the top transparent electrode relative to the bias voltage on the backplane pixel electrode, also known as top-plate switching. Thus, if a voltage of +30V (relative to the backplane) is required, the top plate can be switched to -15V while the appropriate backplane pixel is switched to +15V. A method for driving a four-particle electrophoretic system with top-plate switching is described in more detail, for example, in U.S. Patent No. 9,921,451.

[0105] These waveforms require that each pixel of the display can be driven at five different addressing voltages, specified as +V. 高 +V 低 , 0, -V 低 and -V 高 For example, these could be 30V, 15V, 0, -15V, and -30V. In practice, it's preferable to use a larger number of addressing voltages. If only three voltages (i.e., +V) are used... 高 0 and -V 高 If available, then by using voltage V 高 The pulse, but addressing with a 1 / n duty cycle, makes it possible to achieve this at lower voltages (e.g., V). 高 / n, where n is a positive integer > 1) are the same result of addressing.

[0106] Figure 5 The diagram shows the typical waveforms (in simplified form) used to drive the aforementioned four-particle color electrophoretic display system. These waveforms have a "push-pull" structure: they consist of dipoles comprising two pulses of opposite polarities. The amplitude and length of these pulses determine the resulting color. At least five such voltage levels are required. Figure 5 It displays high and low positive and negative voltages, as well as zero volt. Typically, "low" (L) refers to a range of approximately 5-15V, while "high" (H) refers to a range of approximately 15-30V. Generally, the higher the "high" voltage, the better the color gamut achieved by the display. In some implementations, an additional "medium" (M) level is used, typically around 15V; however, the value of M will depend to some extent on the composition of the particles and the environment of the electrophoretic medium.

[0107] although Figure 5The simplest dipoles needed to form colors are shown, but it should be understood that actual waveforms can repeat these patterns multiple times, or be non-periodic and use other patterns with more than five voltage levels.

[0108] Of course, using Figure 5 The desired color achieved by the drive pulses depends on the situation of the particles being processed from a known state, which is unlikely to be the last color displayed on the pixel. Therefore, a series of reset pulses precede the drive pulses, which increases the amount of time required to update the pixel from the first color to the second color. The reset pulses are described in more detail in U.S. Patent No. 10,593,272, which is incorporated herein by reference. The lengths of these pulses (refresh and addressing) and any pause pulses (i.e., the zero-voltage period between them) can be selected such that the entire waveform (i.e., the voltage integral over time) is DC balanced (i.e., the voltage integral over time is substantially zero). DC balance can be achieved by adjusting the pulse lengths and pausing during the reset phase, such that the net pulses provided during the reset phase are equal in magnitude but opposite in sign to the net pulses provided during the addressing phase, during which the display is switched to a specific desired color. However, as Figure 2B-2E As shown, the initial state for the eight primary colors is either black or white, which can be achieved using a continuous low-voltage drive pulse. The simplicity of implementing this initial state further reduces the update time between states, which is more satisfactory for the user and also reduces the amount of power consumed (thus increasing battery life).

[0109] Furthermore, the preceding discussion of waveforms, and especially the discussion of DC balance, neglected the issue of recoil voltage. In fact, as mentioned earlier, the amount by which each backplane voltage deviates from the voltage supplied by the power source is equal to the recoil voltage V. KB Therefore, if the power supply provides three voltages: +V, 0, and -V, the backplane will actually receive voltage V+V. KB V KB and -V+V KB (Note that in the case of amorphous silicon TFTs, V) KB (Typically negative). However, the same power supply will provide +V, 0, and -V to the front electrode without any recoil voltage offset. Therefore, for example, when -V is supplied to the front electrode, the display will experience 2V+V. KB Maximum voltage and V KB The minimum voltage. Use a separate power supply to provide V to the front electrode. KB It might be expensive and inconvenient, but instead, besides recoil, it can divide the waveform into multiple parts, where a positive voltage, a negative voltage, and V are supplied to the forward electrode. KB . Example

[0110] Example 1—Measurement of the rate of change of color density as a function of applied voltage.

[0111] The benefits of this invention have been experimentally proven. In particular, similar to... Figure 4 The structure was described, and two displays were fabricated. One display's microcells were filled with a formulation comprising white, cyan, magenta, and yellow particles with zeta potentials (measured at low applied voltages) of -35 mV, +70 mV, +54 mV, and -22 mV, respectively. The second display had microcells filled with a formulation comprising white, cyan, magenta, and yellow particles with zeta potentials (measured at low applied voltages) of -35 mV, +70 mV, +54 mV, and +24 mV, respectively. Figure 5 The rate of change of density was assessed using a pulse sequence (which is effective for both negative and positive yellow formulations) and an electro-optic measurement stage including a spectrophotometer. See D. Hertel, Optical measurement standards for reflective e-paper to predict colors displayed in ambient illumination environments, Color Research & Application, 43, 6, (907-921), (2018).

[0112] Figure 6A This shows the maximum rate of change in optical density corresponding to each positive particle in the first display when a pulse of 500 milliseconds duration is applied at the voltage indicated on the x-axis. Figure 7A The display shows the maximum rate of change in optical density corresponding to each positive particle in the second display when a pulse of 500 milliseconds duration is applied at the voltage indicated on the x-axis. It can be seen that the maximum rate of change in density for all particles increases with increasing voltage, until it decreases at an applied voltage of approximately + / - 20V. Interestingly, this change in rate of change occurs regardless of whether the yellow particles are positively or negatively charged, and is consistent with the exemplary figures presented in Table 2. While the waveforms successfully achieve approximately the same maximum rate of change as a function of applied voltage for cyan and magenta particles, it is noteworthy that... Figure 6A The curve ratio of yellow particles Figure 7A The surface is much flatter, which means that it is more difficult to "place" negative yellow particles in the proper order among other particles to achieve the desired optical state, compared to positive yellow particles.

[0113] Compared to displays with positive yellow particles, for displays including negative yellow particles, the difference in control between negative and positive yellow manifests as a more yellowish white. Figure 6Band 7B In the diagram, voltage is shown on the x-axis and time on the y-axis, and the obtained colors are plotted. When the yellow particles have a negative charge ( Figure 6A The pattern only switches between black and white when addressed within a narrow voltage range of approximately + / -7 to + / -10 volts (see dashed box). However, when the yellow particles are positively charged ( Figure 7A The display can accordingly display black and white from approximately + / -7V to approximately + / -24V (see dashed box). Therefore, an electrophoretic medium including positively charged yellow particles is excellent for applications requiring periodic updates between black and white pixels, such as e-readers or timetables. That is, a wider window of addressing voltages exists where the display switches between black and white states. Furthermore, as mentioned in Table 2, if the yellow particles are negatively charged above a certain threshold voltage, the display will not simply switch between black and white, but rather between yellow and blue states.

[0114] Example 2—Color gamut of an electrophoretic medium including positive yellow particles.

[0115] Multiple optical states beyond the eight main states of the second display of Example 1 were measured by driving the display with longer and more complex waveforms and measuring the L*A*B* values ​​using a spectrophotometric electro-optic test bench. For example... Figure 8 As shown, the measurement results indicate a color gamut of tens of thousands of colors with an excellent black-to-white contrast ratio.

[0116] Therefore, the present invention provides a full-color electrophoretic medium that provides rapid switching between high-quality white and black states. Thus, several aspects and embodiments of the technology described herein have been described, and it should be understood that various changes, modifications, and improvements will readily occur to those skilled in the art. Such changes, modifications, and improvements are intended to be within the spirit and scope of the technology described herein. For example, those skilled in the art will readily conceive of various other means and / or structures for performing the functions described herein and / or obtaining the results and / or one or more advantages described herein, and each of such changes and / or modifications is considered to be within the scope of the embodiments described herein. Those skilled in the art will recognize or be able to determine many equivalents of the specific embodiments described herein using only conventional experiments. Therefore, it should be understood that the foregoing embodiments are presented by way of example only, and embodiments of the invention may be practiced in ways other than those specifically described within the scope of the appended claims and their equivalents. Furthermore, any combination of two or more features, systems, articles, materials, kits, and / or methods described herein, provided that such features, systems, articles, materials, kits, and / or methods are not contradictory, is included within the scope of this disclosure.

Claims

1. A four-particle electrophoresis medium comprising a nonpolar fluid, and further comprising: The first type of particle possessing the first optical properties and the first electric polarity; A second type of particle having a second optical property and a second charge polarity opposite to the first charge polarity, the second type of particle having a polymer surface coating covalently bonded to the particle; A third type of particle possessing a third optical property and a second charge polarity with a second charge amount less than the first charge amount; as well as A fourth particle possessing a fourth optical property and a second charge polarity with a third charge greater than the first charge. Both the third and fourth types of particles include a polymer layer that is composite with the particles.

2. A four-particle electrophoresis medium comprising a nonpolar fluid and further comprising: The first type of particle possessing the first optical properties and the first electric polarity; A second type of particle having a second optical property and a second charge polarity opposite to the first charge polarity, the second particle having no polymer surface coating; A third type of particle possessing a third optical property and a second charge polarity with a second charge amount less than the first charge amount; as well as A fourth type of particle possessing a fourth optical property and a second charge polarity with a third charge greater than the first charge. Both the third and fourth types of particles include a polymer layer that is composite with the particles.

3. A four-particle electrophoresis medium comprising a nonpolar fluid, and further comprising: The first type of particle possessing the first optical properties and the first electric polarity; The second type of particles have second optical properties and a second charge polarity opposite to the first charge polarity, and for each gram of the second type of particles in the electrophoretic medium, the second type of particles have more than 200 milligrams of charge control agent (CCA) adsorbed onto the second type of particles. A third type of particle possessing a third optical property and a second charge polarity with a second charge amount less than the first charge amount; as well as A fourth type of particle possessing a fourth optical property and a second charge polarity with a third charge greater than the first charge. For each gram of type III and type IV particles in the electrophoresis medium, each type III and type IV particle has less than 50 milligrams of charge control agent (CCA) adsorbed onto the type III and type IV particles, respectively.

4. The four-particle electrophoresis medium of claim 3, wherein the charge control agent comprises a quaternary ammonium head group and a fatty acid tail.

5. The four-particle electrophoretic medium according to any one of claims 1-4, wherein when the electrophoretic medium is disposed between two electrodes spaced 10-50 micrometers apart, the first type of particles have a lower electrophoretic mobility when 20V is applied between the two electrodes than when 10V is applied between the electrodes.

6. The four-particle electrophoretic medium according to any one of claims 1-4, wherein the first type of particles are light-scattering particles, and the second, third and fourth types of particles are light-absorbing particles.

7. The four-particle electrophoretic medium according to any one of claims 1-4, wherein the first type of particles are white, and the second, third and fourth types of particles are selected from cyan, magenta and yellow.

8. The four-particle electrophoretic medium of claim 7, wherein when the various particles are approximately isotropically distributed in a nonpolar fluid having a refractive index of less than 1.55 at a volume percentage (particle volume to solution volume) to prepare the solution, and the solution is set on a black background in a layer of about 1 micrometer thickness, the yellow, magenta and cyan pigments exhibit diffuse reflection at 650, 550 and 450 nanometers, respectively.

9. The four-particle electrophoretic medium according to any one of claims 1-4, wherein the first charge polarity is negative and the second charge polarity is positive.

10. A color electrophoresis display, comprising: Transparent electrodes at the viewing surface; Including a backplane of a thin-film transistor array coupled to pixel electrodes; and The four-particle electrophoretic medium according to any one of claims 1-9 is disposed between the light-transmitting electrode and the backplate.

11. An e-book reader, portable computer, tablet computer, cellular phone, smart card, signage, watch, shelf label, or flash drive comprising the color electrophoretic display of claim 10.

12. A four-particle electrophoretic medium for a color electrophoretic display, comprising: non-polar fluid; and A four-particle system comprising four types of electrophoretic particles dispersed in the nonpolar fluid, the four types of particles including: The first type of particle possessing the first optical properties and the first electric polarity; A second type of particle having a second optical property and a second charge polarity having a first charge amount, the second charge polarity being opposite to the first charge polarity; A third type of particle possessing a third optical property and a second charge polarity of a second charge amount less than the first charge amount; and A fourth type of particle possessing a fourth optical property and a second charge polarity with a third charge greater than the first charge; Among them, the first type of particles and only one of the second, third and fourth types of particles are configured to experience a decrease in electrophoretic mobility as the electric field increases, and The first type of particles is white, while the second, third, and fourth types of particles are selected from cyan, magenta, and yellow.

13. The electrophoretic medium of claim 12, wherein the second type of particles are magenta, the third type of particles are yellow, and the fourth type of particles are cyan.

14. The electrophoretic medium of claim 12, wherein the first charge is negative and the second charge is positive.

15. The electrophoretic medium of claim 12, wherein the four-particle system comprises exactly four types of electrophoretic particles.

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