TWI912055B - Five-particle electrophoretic medium with improved black optical state - Google Patents

Abstract

A color electrophoretic display with improved black optical state includes an electrophoretic medium having five types of charged electrophoretic pigment particles in a non-polar fluid: a first particle having a first optical property and first charge polarity; a second particle having a second optical property and second opposite charge polarity with a first charge magnitude; a third particle having a third optical property and second charge polarity with a second charge magnitude smaller than the first charge; a fourth particle having a fourth optical property and second charge polarity with a third charge magnitude smaller than the second charge; and a fifth particle having a fifth optical property and second charge polarity with a fourth charge magnitude greater than the first charge. The first and fifth particles are white and black, respectively, and the second, third, and fourth particles are each a different one of cyan, magenta, and yellow.

Description

Five-particle electrophoretic medium with improved black optical state

Related Applications This application is a partial continuation of U.S. Patent Application No. 18/140,161, filed April 27, 2023, which is a continuation of U.S. Patent Application No. 17/474,582, filed September 14, 2021, which 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. This application also claims priority to U.S. Provisional Patent Application No. 63/613,889, filed December 22, 2023. Each of the foregoing applications is incorporated herein by reference in its entirety. All patents and announcements disclosed herein are incorporated herein by reference in their entirety.

Electrophoretic displays (EPDs) alter colors by changing the position of charged, colored particles relative to a light-transmitting viewing surface. These EPDs are typically called "electronic paper" or "ePaper" because the resulting display offers high contrast and can be read in sunlight, much like ink on paper. EPDs are widely used in e-readers such as Amazon Kindle® because they provide a book-like reading experience, consume little power, and allow users to carry hundreds of books in a lightweight handheld device.

For many years, electrophoretic displays have only included two types of charged color particles: black and white (it is certain that "color" as used herein includes both black and white). White particles are typically light-scattering and contain, for example, titanium dioxide, while black particles are absorbent in the visible spectrum and may contain carbon black or absorbing metal oxides, such as copper chromite. In its simplest sense, a monochrome electrophoretic display requires only a transparent electrode on the viewing surface, a back electrode, and an electrophoretic medium containing white and black particles with opposite charges. When a voltage of one polarity is applied, white particles move to the viewing surface, and when a voltage of the opposite polarity is applied, black particles move to the viewing surface. If the back electrode includes controllable areas (pixels)—either segmented electrodes or an active matrix of pixel electrodes controlled by transistors—then a pattern can be electronically displayed on the viewing surface. For example, this pattern could be the text of a book.

Recently, electrophoretic displays have been commercially available with a variety of color options, including three-color displays (black, white, and red; black and white, and yellow) and four-color displays (black, white, red, and yellow). Similar to the operating principle of a black-and-white electrophoretic display, an electrophoretic display with three or four reflective particles operates much like a simple black-and-white display, as the desired color particles are driven onto the viewing surface. The driving scheme is far more complex than that of just black and white, but ultimately the optical function of these particles is the same.

Advanced Color Electronic Paper (ACeP™) also includes four types of particles, but the cyan, yellow, and magenta particles are subtractive rather than reflective, allowing thousands of colors to be produced per pixel. The coloring process is functionally equivalent to printing methods long used in offset and inkjet printers. Specific colors are produced by using the correct proportions of cyan, yellow, and magenta against a bright white paper background. In the ACeP example, the relative positions of the cyan, yellow, magenta, and white particles relative to the viewing surface determine the color of each pixel. While this type of electrophoretic display allows for thousands of colors per pixel, the key is careful control of the position of each pigment (50 to 500 nanometers in size) within a working space approximately 10 to 20 micrometers thick. Obviously, changes in the position of the particles will cause a given pixel to display incorrect colors. Therefore, this system requires precise voltage control. Further details of this system can be found 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.

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

The terms "bistable" and "bistable" are used here in their conventional sense within this art field to refer to a display containing display elements having first and second display states that differ in at least one optical characteristic, such that after any given element has been driven to its first or second display state by an addressing pulse of finite duration, that 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 illustrates particle-based electrophoretic displays capable of displaying grayscale that are stable not only at their extreme black and white states but also at intermediate grayscale states, as are some other types of electro-optic displays. This type of display is correctly termed a multistable display rather than a bistable display, although for convenience, the term "bistable" may be used here to encompass both bistable and multistable displays.

When used to refer to driving an electrophoretic display, the term "pulse" here refers to the integral of the applied voltage with respect to time during the period when the display is being driven.

Particles that absorb, scatter, or reflect broadband or selected wavelengths of light are referred to herein as colored or pigment particles. Various materials other than pigments that absorb or reflect light (strictly speaking, this term refers to insoluble colored materials), such as dyes or photonic crystals, can also be used in the electrophoretic medium and displays of this invention.

Particle-based electrophoretic displays have been a subject of in-depth research and development for many years. In these displays, multiple charged particles (sometimes called pigment particles) move through a fluid under the influence of an electric field. Compared to liquid crystal displays, electrophoretic displays offer advantages such as better brightness and contrast, wider viewing angles, state bistable operation, and lower power consumption. However, long-term image quality issues have hindered their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in a shorter lifespan for these displays.

As mentioned above, the electrophoresis medium requires the presence of a fluid. In most existing electrophoresis media, this fluid is a liquid, but gaseous fluids can be used to produce the electrophoresis medium; for example, see Kitamura, T. et al., “Electroporation of Toner for Electronic Paper Displays,” Japanese IDW, 2001, Paper HCS1-1, and Yamaguchi, Y. et al., “Toner Display Using Tribocharged Insulating Particles,” Japanese IDW, 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 for such settling, such as in a marker where the medium is placed in a vertical plane, such gas-based electrophoresis media appear to be susceptible to the same types of problems as liquid-based electrophoresis media. In fact, the problem of particle sedimentation in gas-based electrophoresis media appears to be more serious than that in liquid-based electrophoresis media, because the lower viscosity of gaseous suspensions allows for faster sedimentation of electrophoretic particles compared to liquid-based electrophoresis media.

The patents and applications transferred to MIT and E Ink Corporation, or in their name, describe various techniques used in encapsulating electrophoretic media and other electro-optic media. These encapsulating media comprise a plurality of small capsules, each capsule containing an internal phase in a fluid medium containing electrophoretically moving particles, and a capsule wall surrounding this internal phase. Typically, the capsules themselves are held in a polymer binder to form a coherent layer positioned between two electrodes. The technologies described in these patents and applications include: (a) electrophoretic particles, fluids, and fluid additives; see, for example, U.S. Patents 7,002,728 and 7,679,814; (b) capsules, adhesives, and encapsulation processes; see, for example, U.S. Patents 6,922,276 and 7,411,719; (c) micro-unit structures, wall materials, and methods of forming micro-units; see, for example, U.S. Patents 7,072,095 and 9,279,906; (d) methods of filling and sealing micro-units; see, for example, U.S. Patents 7,144,942 and 7,715,088; (e) Thin films and subassemblies containing electro-optic materials; see, for example, U.S. Patents 6,982,178 and 7,839,564; (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; (g) Color forming and color adjustment; see, for example, U.S. Patents 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; 7,07 5,502; 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; 7,952,790; 7,956,841; 7,982,94 1; 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; 8,466,852; 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；8,917,4 39; 8,964,282; 9,013,783; 9,116,412; 9,146,439; 9,164,207; 9,170,467; 9,170,468; 9,182,646; 9,195,111; 9,199,441; 9,268,191; 9,285,649; 9,293,511; 9 ,341,916; 9,360,733; 9,361,836; 9,383,623; and 9,423,666; and U.S. Patent Application Publication Nos. 2008/0043318; 2008/0048970; 2009/0225398; 2010/0156780; 2011/0043543; 2012/03 26957; 2013/0242378; 2013/0278995; 2014/0055840; 2014/0078576; 2014/0340430; 2014/0340736; 2014/0362213; 2015/0103394; 2015/0118390; 2015/012434 5; 2015/0198858; 2015/0234250; 2015/0268531; 2015/0301246; 2016/0011484; 2016/0026062; 2016/0048054; 2016/0116816; 2016/0116818; and 2016/0140909; (h) A method for driving a display; for example, see U.S. Patent Nos. 5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970; 6,900,851; 6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,061,166; 7,061,662; 7,116,466; 7,119,772; 7,177,066; 7,193,625; 7,202,847; 7,242,5 14; 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；9,22 4,342; 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 Application Publication Nos. 2003/0102858; 2004/0246562; 2005/0253777; 2007/0091418; 2007/0103427; 2007/0176912; 2008/0024429; 2008/0024 482; 2008/0136774; 2008/0291129; 2008/0303780; 2009/0174651; 2009/0195568; 2009/0322721; 2010/0194733; 2010/0194789; 2010/0220121; 2010/0265561; 2010/0283804; 2011/0063314; 2011/0175875; 2011/0193840; 2011/0193841; 2011/0199671; 2011/02 21740; 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/ Patents and applications numbered 0340734; 2015/0070744; 2015/0097877; 2015/0109283; 2015/0213749; 2015/0213765; 2015/0221257; 2015/0262255; 2015/0262551; 2016/0071465; 2016/0078820; 2016/0093253; 2016/0140910; and 2016/0180777 (these patents and applications may be referred to below as MEDEOD (Method for Driving an Electro-Optical Display) applications); (i) Applications to displays; see, for example, U.S. Patents 7,312,784 and 8,009,348; and (j) Non-electrophoretic displays, as described in U.S. Patent 6,241,921; and U.S. Patent Application Publication No. 2015/0277160; and U.S. Patent Application Publication Nos. 2015/0005720 and 2016/0012710.

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

A related type of electrophoretic display is the so-called microcell electrophoresis display. In a microcell electrophoresis display, charged particles and fluids are not encapsulated in 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.

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

Encapsulated electrophoretic displays typically do not suffer from the aggregation and sedimentation failure modes of conventional electrophoresis apparatus 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 intended to include all forms of printing and coating, including, but not limited to: pre-measured coating, such as patch-type die coating, slotted or extruded coating, sliding or stepped coating, curtain coating; roller coating, such as blade roller coating, forward and reverse roller 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.

As mentioned above, most simple existing electrophoretic media essentially display only two colors. These media either use a single type of electrophoretic particle with the first color in a colored 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 separated from the viewing surface), or use first and second types 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, these two colors are black and white. For full-color display, an array of color filters can be deposited above the viewing surface of a monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color mixing to create color stimuli. Available display areas are shared among three or four primary colors, such as red/green/blue (RGB) or red/green/blue/white (RGBW), and the filters can be arranged in one-dimensional (stripes) or two-dimensional (2x2) repeating patterns. Other choices of three or more 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 intended viewing distance, they visually blend together to form a single pixel with a uniform color stimulus ("color mixing"). The inherent drawback of area sharing is that the colorant is always present, and the color can only be adjusted by switching the corresponding pixel of the underlying monochrome display to white or black (turning the corresponding three primary colors on or off). For example, in an ideal RGBW display, red, green, blue, and white each occupy one-quarter of the display area (one of the four sub-pixels), making the white sub-pixel as bright as the white of the underlying monochrome display, and the brightness of each colored sub-pixel does not exceed one-third of the brightness of the white of the monochrome display. The overall brightness of white displayed by the display cannot exceed half the brightness of the white sub-pixel (the white area of the display is generated by displaying one of the four sub-pixels, and each colored sub-pixel is equivalent to one-third of the white sub-pixel in its color form, so the combined contribution of the three colored sub-pixels does not exceed that of one white sub-pixel). By switching color pixels to black areas for sharing, the brightness and saturation of colors can be reduced. Area sharing is particularly problematic when mixing yellow, as it is lighter than any other color of equal brightness, and saturated yellow is almost as bright as white. Switching blue pixels (a quarter of the display area) to black makes yellow too dark.

U.S. Patents 8,576,476 and 8,797,634 describe a multicolor electrophoretic display having a single back plane comprising independently addressable pixel electrodes and a common, transparent front electrode. Multiple electrophoretic layers are disposed between the back plane and the front electrode. The displays described in these applications are capable of displaying any of the primary colors (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel location. However, using multiple electrophoretic layers between a single set of addressable electrodes has some disadvantages. The electric field experienced by particles in a particular layer is lower than in the case of a single electrophoretic layer addressed with the same voltage. Furthermore, optical losses in the electrophoretic layer closest to the viewing surface (e.g., caused by light scattering or unwanted absorption) can affect the appearance of the image formed in the underlying electrophoretic layer.

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 9,697,778 describes a display in which a stained 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 high voltage. A full-color display is possible when the white particles and the stained solvent are combined with two additional particles of opposite charge to the white particles. However, the color state of patent '778 is unacceptable for applications such as word readers. In particular, some of the stained fluid will always separate the white scattering particles from the viewing surface, resulting in a tinted white state in the display.

U.S. Patent No. 9,921,451 describes a second type of electrophoretic medium capable of displaying any color at any pixel location. In Patent No. 451, the electrophoretic medium comprises four types of particles: white, cyan, magenta, and yellow, two of which are positively charged and two are negatively charged. However, the display of Patent No. 451 also suffers from a color problem involving mixing with the white state. Because one type of particle has the same charge as the white particles, when a desired color state is desired, a certain number of particles with the same charge will move toward the viewing surface along with the white particles. Although this unwanted color problem can be overcome with complex waveforms, such waveforms greatly increase the display's refresh rate and, in some cases, cause unacceptable "flickering" between images.

According to one aspect of the present invention, a color electrophoretic display providing an improved black optical state includes a light-transmitting electrode layer on a viewing surface, a back electrode layer, and an electrophoretic medium disposed therebetween. The electrophoretic medium comprises a nonpolar fluid and a multi-color particle system, the multi-color particle system comprising five types of charged electrophoretic color particles dispersed in the nonpolar fluid. The five types of charged electrophoretic pigment particles include: Type I particles, possessing a first optical property and a first charge polarity; Type II particles, possessing a second optical property and a second charge polarity with a first charge amount opposite to the first charge polarity; Type III particles, possessing a third optical property and a second charge polarity with a second charge amount less than the first charge amount; Type IV particles, possessing a fourth optical property and a second charge polarity with a third charge amount less than the second charge amount; and Type V particles, possessing a fifth optical property and a second charge polarity with a fourth charge amount greater than the first charge amount. Type I particles are white, Type V particles are black, and Types II, III, and IV particles are each a different shade of cyan, magenta, and yellow.

In one or more embodiments, the second, third, and fourth types of particles are cyan, magenta, and yellow, respectively.

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

As is well known in the field of electrophoresis, charge (i.e., charge density per unit area of the pigment particle surface) is approximately proportional to the zeta potential (potential in the shear plane used for electrophoresis), thus giving a constant Debye length and dielectric constant. The values of charge (i.e., charge density) provided below are zeta potentials measured in hydrocarbon solvents in the presence of surfactants (charge control agents).

In one or more embodiments, the first charge is about 80 mV to 100 mV.

In one or more embodiments, the second charge is approximately 40 mV to 50 mV.

In one or more embodiments, the third charge is about -5 mV to about 5 mV.

In one or more embodiments, the fourth charge is approximately 100 mV.

In one or more embodiments, the first type of particle has a charge of about -55 mV to about -70 mV.

In one or more embodiments, the back electrode layer includes an array of thin-film transistors coupled to pixel electrodes. Each thin-film transistor comprises a layer of metal oxide semiconductor.

In one or more embodiments, the metal oxide semiconductor is indium gallium zinc oxide (IGZO).

In one or more embodiments, the thin-film transistor comprising the metal oxide semiconductor layer is capable of switching control voltages greater than 25V and less than -25V, while the phototransmitting electrode layer is maintained at a constant voltage for changing the optical state of the pixels of the electrophoretic display.

In one or more embodiments, the black particles include a polymer shell grafted onto the particle surface.

In one or more embodiments, the third type of particle is magenta and includes a polymer shell coated on the particle by dispersion polymerization.

The various embodiments disclosed herein relate to a five-particle electrophoretic medium in a display device that can provide an improved, more saturated black optical state on the viewing surface of the device.

U.S. Patent No. 11,686,989 discloses a four-particle electrophoretic medium comprising a first particle of a first polarity and three other particles of opposite polarity but different charges, the entire contents of which are incorporated herein by reference. Typically, this system includes a negatively charged white particle and positively charged yellow, magenta, and cyan particles, which are subtractive primary colors. Furthermore, some particles can be designed such that their electrophoretic mobility is nonlinear with respect to the strength of the applied electric field. Therefore, when a high electric field of the correct polarity (e.g., 20 V or higher) is applied, one or more particles will experience a decrease in their electrophoretic mobility. This four-particle system is schematically shown in Figure 1, and it can provide white, yellow, red, magenta, blue, cyan, green, and black in each pixel.

As shown in Figure 1, each of the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) corresponds to a different arrangement of four types of particles in the electrophoretic medium, so that the viewer only sees the colored particles on the viewing side of the white particles (i.e., particles that can only scatter light). To achieve a wide range of colors, additional voltage levels must be used for more precise control of the particles. In this arrangement, the first (typically negative) particle is a reflective particle (typically white), while the other three particles with opposite charges (typically positive) include three types of particles that do not actually scatter light ("SNLS"). Using SNLS particles allows for color mixing and provides more color effects than can be achieved with the same number of scattering particles. These thresholds must be sufficiently separated to avoid crosstalk, and this separation necessitates the use of high addressing voltages for some colors. The revealed four-particle electrophoretic medium can also be updated more quickly, requiring "less flickering" transitions and producing a more pleasing color spectrum (and therefore more commercially valuable). Furthermore, the revealed scheme provides rapid updates between black and white pixels (e.g., less than 500 ms, less than 300 ms, less than 200 ms, less than 100 ms), thereby enabling fast page turning of black text on a white background.

The viewing surface of the display in Figure 1 is at the top (as indicated by the eye symbol in the figure), meaning the user views the display from this direction, and light is incident from this direction. As already noted, only one of the four types of particles used in the electrophoretic medium actually scatters light, and in Figure 1, this particle is assumed to be white pigment. This white particle that scatters light forms a white reflector, through which any particle above the white particles is viewed (as illustrated in Figure 1). Light entering the viewing surface of the display passes through these particles, is reflected by the white particles, passes back through these particles, and appears on the display. Therefore, the particles above the white particles can absorb various colors, and the colors seen by the user are the combination of colors originating from the particles above the white particles. Any particles positioned below the white particles (behind the user's viewpoint) are obscured by the white particles and do not affect the displayed colors. Since the second, third, and fourth particles do not actually scatter light, their order or arrangement relative to each other is not important, but for the reasons already stated, their order or arrangement relative to the white (scattering light) particles is crucial.

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

It is possible to present a subtractive primary color system using particles that scatter light, such that the display would contain two types of light-scattering particles: white particles and colored particles. However, in this case, the position of the scattering colored particles relative to the other colored particles covering the white particles is important. For example, when presenting black (when all three types of colored particles are above the white particles), the scattering colored particles cannot be above the non-scattering colored particles (otherwise they would be partially or completely hidden behind the scattering particles, and the color presented would be the color of the scattering colored particles, not black).

Figure 1 illustrates an idealized scenario where color is uncontaminated (i.e., the white particles scattering light completely obscure any particles behind them). In reality, the obscuring effect of the white particles may not be perfect, resulting in some light absorption by particles that would ideally be completely obscured. This contamination typically reduces both the brightness and chromaticity of the resulting color. In the electrophoretic medium of this invention, this color contamination should be minimized to the extent that the resulting color conforms to industry standards for color reproduction. One particularly favored standard is SNAP (the standard used in newspaper advertising), which specifies the 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, as shown in Figure 1).

Figures 2A-2E show schematic cross-sectional views of a display layer with a four-particle type. The display layer includes a first (viewing) surface 13 on the viewing side and a second surface 14 on the opposite side of the first surface 13. An electrophoretic medium is disposed between the two surfaces. Each space between the two vertical dashed lines marks a pixel. Within each pixel, the electrophoretic medium can be addressed, and the viewing surface 13 of each pixel can achieve the color state shown in Figure 1 without the need for additional layers or a filter array.

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

The newly developed active matrix backplane incorporates thin-film transistors made of metal oxide materials such as tungsten oxide, tin oxide, indium oxide, and zinc oxide, or more complex metal oxides such as indium gallium zirconium oxide. In these applications, using these metal oxide materials to form a channel formation region for each transistor allows for faster switching at higher voltages. These metal oxide transistors also allow for less leakage in the "off" state of the thin-film transistor (TFT) than that achievable with, for example, amorphous silicon TFTs. In a typical scanned TFT backplane containing n lines, the transistors will be in the "off" state for approximately (n-1)/n of the time required to refresh each line of the display. Any charge leakage from the storage capacitors associated with each pixel will degrade the electro-optical performance of the display. TFTs typically include a gate electrode, a gate insulating film (typically _SiO2 ), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film on the gate insulating film, at least partially overlapping the gate electrode, source electrode, and drain electrode. These backplanes can be obtained from manufacturers such as Sharp/Foxconn, LG, and BOE. These backplanes are capable of providing a drive voltage of ±30V (or higher). In some embodiments, an intermediate voltage driver may be included, such that the resulting drive waveform may be five-bit, seven-bit, nine-bit, or more.

A preferred metal oxide material for these 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, it is possible to provide voltages greater than 30V via a suitable display driver. Furthermore, source drivers capable of supplying at least five-bit, and preferably seven-bit, driving paradigms are provided for four-particle electrophoretic display systems. In one embodiment, there will be two positive voltages, two negative voltages, and zero volts. In another embodiment, there will be three positive voltages, three negative voltages, and zero volts. In one embodiment, there will be four positive voltages, four negative voltages, and zero volts. These levels can be selected in the range of approximately -27V to +27V, without being subject to the limitations imposed by the top-plane switch as described above.

Figures 2A-2E illustrate an electrophoretic medium comprising four types of electrophoretic particles in a nonpolar fluid 17. The first type of particle (W-*; open circle) is negatively charged and can be surface-treated such that the electrophoretic mobility of the first type of particle depends on the strength of the driving electric field (discussed in more detail below). In these cases, the electrophoretic mobility of the particle actually decreases under the influence of a stronger electric field, which is somewhat counterintuitive. The second type of particle (M++*; dark circles) is positively charged and can be surface-treated (or intentionally left untreated) so that its electrophoretic mobility 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 reverses, the unpacking rate of the second type of particle aggregate is slower than that of the third and fourth type of particle aggregates. The third type of particle (Y+; checkerboard circles) is positively charged but has a smaller charge value than the second type of particle. Furthermore, the third type of particle can be surface-treated, but not in a way that causes its electrophoretic mobility to depend on the strength of the driving electric field. That is, the third type of particle can have a surface treatment, but this surface treatment does not cause the aforementioned decrease in electrophoretic mobility with increasing electric field. The fourth type of particle (C++; gray circle) has the highest positive charge and its surface treatment is the same as that of the third type of particle. As indicated in Figure 2A, the particles are nominally white, magenta, yellow, and cyan to produce the colors shown in Figure 1. These color combinations are not limited to one reflective particle and three absorptive particles. For example, the system may include one black absorptive particle and three reflective particles of red, yellow, and blue with appropriately matched reflective spectra to produce a process white state when all three reflective particles are mixed and can be viewed on the surface.

In one example, the first type of particle (negative) is white and scattering. The second type of particle (positive, medium charge value) is magenta and absorptive. The third type of particle (positive, low charge value) is yellow and absorptive. The fourth type of particle (positive, high charge value) is cyan and absorptive. Table 1 below shows the diffuse reflectance of exemplary yellow, magenta, cyan, and white particles useful in electrophoretic media, and the ratio of their absorptance to scattering coefficients according to Kubelka-Munk analysis of these materials, such as those dispersed in a polyisobutylene matrix. Table 1. Preferred diffuse reflectance of yellow, magenta, cyan, and white particles. Diffuse reflectance of a 1μm layer on 0% black Absorption/scattering ratio color Volume fraction 450 nm 550 nm 650 nm K/S 450nm K/S 550nm K/S 650nm Yellow (Y1) 0.097 4.5% 0.9% 0.5% 9.67 0.38 0.63 Yellow (Y1) 0.147 4.4% 0.9% 0.4% 9.84 0.25 0.02 Magenta(M1) 0.115 2.8% 3.8% 0.7% 10.01 10.85 1.27 Magenta(M1) 0.158 3.2% 4.1% 1.0% 10.00 10.75 1.64 Magenta(M1) 0.190 3.4% 4.1% 1.3% 10.09 10.80 1.03 Cyan (C1) 0.112 1.3% 3.7% 4.3% 7.27 11.17 10.22 Cyan (C1) 0.157 1.5% 3.8% 4.3% 7.41 11.30 10.37 Cyan (C1) 0.202 1.7% 3.9% 4.3% 7.21 11.56 10.47 White (W1) 0.147 8.1% 6.2% 4.8% 0.0015 0.0020 0.0026 White (W1) 0.279 24.9% 20.6% 17.0% 0.0003 0.0003 0.0004 White (W1) 0.339 26.3% 21.7% 18.1% 0.0001 0.0002 0.0002

The electrophoretic medium can be any of the forms described above. Therefore, the electrophoretic medium can be unencapsulated, encapsulated in a dispersed capsule surrounded by a capsule wall, encapsulated in sealed microcells, or in the form of a polymer dispersion. The pigment is described in detail elsewhere, as in U.S. Patents 9,697,778 and 9,921,451. In short, the white particle W1 is a silanol-functionalized light-scattering pigment (titanium dioxide), as described in U.S. Patent 7,002,728, to which a polymeric material containing lauryl methacrylate (LMA) monomers has been attached. White particles W2 are polymer-coated titanium dioxide substantially produced as described in Example 1 of U.S. Patent No. 5,852,196, having a polymer coating comprising lauryl methacrylate and 2,2,2-trifluoroethyl methacrylate in a ratio of approximately 99:1. Yellow particles Y1 are C.I. Pigment Yellow 180, usable without coating, and dispersed by grinding in the presence of Solsperse 19000, as substantially as described in U.S. Patent No. 9,697,778. Yellow particles Y2 are C.I. Pigment Yellow 155, usable without coating, and dispersed by grinding in the presence of Solsperse 19000, as substantially as described in U.S. Patent No. 9,697,778. Yellow particles Y3 are C.I. Pigment Yellow 139, which can be used without coating and is dispersed by grinding in the presence of Solsperse 19000, as generally described in U.S. Patent No. 9,697,778. Yellow particles Y4 are C.I. Pigment Yellow 139, as described in Example 4 of U.S. Patent No. 9,921,451, coated by dispersion polymerization, incorporating trifluoroethyl methacrylate, methyl methacrylate, and a monomer containing dimethylsiloxane. Magenta particles M1 are positively charged magenta materials (dimethylquinacridone, C.I. Pigment Red 122) coated with vinylbenzyl chloride and LMA, as described in Example 5 of U.S. Patent Nos. 9,697,778 and 9,921,451.

Magenta particles M2 are a C.I. Pigment Red 122, which is coated by dispersion polymerization, methyl methacrylate, and a monomer containing dimethylsiloxane, as described in Example 6 of U.S. Patent No. 9,921,451. Cyan particles C1 are a copper phthalocyanine material (C.I. Pigment Blue 15:3), which is coated by dispersion polymerization, as described in Example 7 of U.S. Patent No. 9,921,451, incorporating methyl methacrylate and a monomer containing dimethylsiloxane. In some embodiments, it has been found that the color gamut can be improved by using Inkjet Yellow 4GC (Clariant) as the core yellow pigment and incorporating a methyl methacrylate surface polymer. The zeta potential of this yellow pigment can be adjusted by adding 2,2,2-trifluoroethyl methacrylate (TFEM) monomers and monomethacrylate-terminated poly(dimethylsiloxane). The black particles can be formed from CI pigment black 26 or 28 or similar substances (e.g., manganese ferrite black spinel or copper chromite black spinel) or carbon black.

The charge polarity and charge amount of any of the aforementioned particles can be designed using various surface treatment methods. Furthermore, surface treatment can improve the compatibility of the core particles with monomers in the reaction medium, or chemically bond them to monomers, thereby forming the shell of composite color particles. For example, surface treatment can be performed using organosilanes having functional groups such as acrylate, vinyl, _-NH₂ , -NCO-, -OH, etc. These functional groups can undergo chemical reactions with monomers. Color core particles can also be surface-treated using inorganic materials such as silica, alumina, zinc oxide, or combinations thereof. Sodium silicate or tetraethoxysilane can be used as common precursors for silica coatings. In the case of inorganic treatments, the coating structure can be porous to reduce density. To increase the negative charge on the particles, various fluorinated acrylates or fluorinated methacrylates (especially 2,2,2-trifluoroethyl methacrylate, hereinafter referred to as "TFEM") can be incorporated into the polymer shell, or for example, during the production of composite organic pigment particles. Other fluorinated monomers (i.e., 2,2,3,4,4,4-hexafluorobutyl acrylate and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecylfluorooctyl acrylate) can also be used to adjust the zeta potential of the particles.

Electrophoretic medium additives and surface treatments for promoting different electrophoretic migration rates are discussed in detail in U.S. Patent No. 9,697,778, the entire patent of which is incorporated herein by reference. In this electrophoretic medium, one way to control the interactions between various types of particles is by controlling the type, number, and thickness of the polymer coating on the particles. For example, to control particle properties so 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 of a third species, type II particles can be treated with polymer surfaces, while type III and IV particles either are not treated with polymer surfaces or are treated with polymer surfaces, resulting in a lower mass coverage per unit area on their particle surfaces than type II particles. More generally, the Hamaker constant (a measure of the strength of the van der Waals interaction between two particles, whose potential is proportional to the Hamaker constant and inversely proportional to the sixth power of the distance between the two particles) and/or the interparticle spacing need to be adjusted by carefully selecting the polymer coatings on the particles of the three species.

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 distance between the pathways of oppositely charged particles is maximized by a three-dimensional barrier (typically a polymer grafted or adsorbed onto the surface of one or two particles). The polymer shell can be a covalently bonded polymer produced by a grafting process or by chemisorption as well known in the art, or it can be physically adsorbed onto the particle surface. For example, the polymer can be a block copolymer comprising insoluble and soluble segments. Alternatively, the polymer shell can be dynamic, wherein it is a loose network of free polymer derived from an electrophoretic medium, complexed with pigment particles in the presence of an electric field and a sufficient amount and type of charge control agent (CCA, discussed below). Therefore, depending on the strength and polarity of the electric field, particles can have more associated polymers, causing the particles to interact differently with the container (e.g., microcapsules or microcells) and other particles. The extent of the polymer shell can be conveniently assessed by thermogravimetric analysis (TGA), 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 fraction of polymer particles can be measured and converted into a volume fraction using the known density of the core pigment and the polymer attached to them. Conditions under which the polymer coating is lost but the core pigment is retained can be found (these conditions depend on the precise core pigment particles used). Various polymer combinations can be made to work as described below with respect to Figures 2A-2E. For example, the particles (typically the first and/or second type of particles) may have a covalently attached polymer shell that interacts strongly with the container (e.g., a microcell or microcapsule). Meanwhile, other particles of the same charge lack a polymer coating or are complexed with free polymer in solution, such that those particles have virtually no interaction with the container. In other embodiments, one type of particle (typically the first and/or second type of particles) will have no surface coating, making it easier for the particle to form a charged bilayer and undergo reduced electrophoretic migration in the presence of a strong field.

The fluid 17, containing four types of particles, is transparent and colorless. This fluid contains charged electrophoretic particles that move through the fluid under the influence of an electric field. A preferred suspension has a low dielectric constant (approximately 2), high volume resistivity (approximately ^10¹⁵ Ohm·cm), low viscosity (less than 5 mPas), low toxicity and environmental impact, low water solubility (less than 10 parts per million (ppm) if conventional aqueous encapsulation methods are used; however, it should be noted that this requirement may be relaxed for unencapsulated or certain micro-unit displays), high boiling point (greater than approximately 90°C), and low refractive index (less than 1.5). This last requirement stems from the use of high-refractive-index scattering (typically white) pigments, whose scattering efficiency depends on the mismatch in refractive indices between the particles and the fluid.

Organic solvents such as saturated linear or branched hydrocarbons, silicone oils, halogenated organic solvents, and low molecular weight halogenated polymers are some useful fluids. These fluids may contain a single component or a mixture of more than one component to allow for the tuning of their chemical and physical properties. Reactants or solvents used in microencapsulation processes (if used), such as oil-soluble monomers, may also be contained in the fluid.

For high particle mobility, the fluid preferably has low viscosity and a dielectric constant in the range of about 2 to about 30, more preferably about 2 to about 15. Suitable dielectric fluids include hydrocarbons such as Isopar®, DECALIN, 5-ethylene-2-norbornene, fatty oils, paraffin oils, and silicone liquids; aromatic hydrocarbons such as toluene, xylene, phenoxyethane, dodecylbenzene or alkylnaphthalene; halogenated solvents such as perfluoronaphthene, perfluorotoluene, perfluoroxylene, dichlorotrifluorotoluene, 3,4,5-trichlorobenzotrifluoro, chloropentafluorobenzene, dichlorononane or pentachlorobenzene; perfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M (St. Paul, Minnesota); low molecular weight halogenated polymers of poly(perfluoropropylene oxide) from TCI America (Portland, Oregon); and hydrocarbons such as those from River, New Jersey. Edge's halogenated hydrocarbon products include poly(trifluorochloroethylene) halogenated hydrocarbon oils, such as Galden's perfluoropolyalkyl ethers from Ausimont, Krytox oils and greases from DuPont Delaware (K-Fluid series), and polydimethylsiloxane silicone oil (DC-200) from Dow Corning.

Electrophoretic media typically include one or more charge control agents (CCAs) and may also include charge conductors. CCAs and charge conductors typically comprise low-molecular-weight surfactants, polymers, or mixtures of one or more components, and function to stabilize or otherwise modify the sign and/or magnitude of the charge on the electrophoretic particles. CCAs are typically molecules containing ionic or other polar groups, hereinafter referred to as head groups. At least one positively or negatively ionic head group is preferably attached to a nonpolar chain (typically a hydrocarbon chain), hereinafter referred to as a tail group. It is believed that CCAs form reverse micelles in the inner phase, and that these are a small group of charged reverse micelles, resulting in conductivity in polar and nonpolar fluids typically used as electrophoretic fluids.

The addition of CCAs provides the means to generate reverse micelles, comprising a highly polar core surrounded by nonpolar tail groups of CCA molecules, the size of which can vary from 1 nanometer to tens of nanometers (and can have spherical, cylindrical, or other geometric shapes). In the electrophoretic medium, it can typically be divided into three phases: solid particles with a surface, a highly polar phase distributed in the form of extremely small droplets (reverse micelles), and a continuous phase containing the fluid. When an electric field is applied, both charged particles and charged reverse micelles can move through the fluid, and thus there are two parallel electrical conduction paths through the fluid (which typically has almost negligible conductivity).

It is believed that the polar core of CCA influences the charge on the surface through adsorption. In electrophoretic displays, this adsorption can occur on the surface of electrophoretic particles or the inner walls of microcapsules (or another solid phase, such as the walls of microcells), forming a structure similar to reverse micelles, referred to below as hemimicelles. When one ion of an ion pair is more firmly attached to the surface than the other (e.g., by covalent bonding), ion exchange between the hemimicelle and the unbound reverse micelle can lead to charge separation, in which the more firmly bound ion remains associated with the particle, and the unbound ion becomes incorporated into the core of the free reverse micelle.

Ionic materials forming CCA head groups can also induce ion pair formation on particle (or other) surfaces. Therefore, CCAs can perform two basic functions: generating charges on the surface and separating charges from the surface. Charge generation can originate from acid-base reactions between acid-base reactions and particle surfaces, either present in the CCA molecule or otherwise incorporated into the reverse micelle core or fluid. Therefore, useful CCA materials are those capable of participating in this reaction, or any other charging reaction known in the art.

Non-limiting classes of charge control agents useful in the media of this invention include organic sulfates or sulfonates, metal soaps, block or comb copolymers, organoamides, 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 alkaline barium dinonylnaphthalenesulfonate, neutral or alkaline calcium dinonylnaphthalenesulfonate, sodium dodecylbenzenesulfonate, and ammonium dodecyl sulfate. Useful metal soaps include, but are not limited to, alkaline or neutral barium petroleum sulfonates, calcium petroleum sulfonates, cobalt, calcium, copper, manganese, magnesium, nickel, zinc, aluminum, and iron salts of carboxylic acids such as cycloalkanoic acid, 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 ethyl 2-(N,N-dimethylamino)methacrylate quaternized with methyl p-toluenesulfonate and (B) AB diblock copolymers of poly(2-ethylhexyl methacrylate), and comb graft copolymers having an oil-soluble tail of poly(12-hydroxystearic acid) and a molecular weight of about 1800, side-attached to an oil-soluble anchoring group of poly(methyl methacrylate-methacrylic acid). Useful organoamines/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 Wycliffe Road, Ohio: Solsperse is a registered trademark), and N-vinylpyrrolidone polymers. Useful organozonants include, but are not limited to, lecithin. Useful organophosphates and phosphonates include, but are not limited to, sodium salts of phosphorylated monoglycerides and diglycerides having saturated and unsaturated acid substituents. Useful tail groups for CCAs include olefinic polymers with molecular weights in the range of 200-10,000, such as poly(isobutylene). Head groups can be sulfonic, phosphoric, or carboxylic, or amide groups, or alternatively, amine groups such as primary, secondary, tertiary, or quaternary ammonium groups. One class of CCAs useful in the disclosed four-particle electrophoresis media is disclosed in U.S. Patent Publication No. 2017/0097556, the entire contents of which are incorporated herein by reference. These CCAs typically include a quaternary ammonium head group and an unsaturated polymer tail, i.e., including 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 CCA is 12,000 g/mol or larger, for example, between 14,000 g/mol and 22,000 g/mol.

As described in more detail below, the charge adjuvants used in the present invention's medium can deflect the charge on the surface of electrophoretic particles. Such charge adjuvants can be Brønsted acids, Lewis acids, or alkalis. Exemplary charge adjuvants are disclosed in U.S. Patents 9,765,015, 10,233,339, and 10,782,586, all of which are incorporated herein by reference in their entirety. Exemplary adjuvants 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, anthranilol, 5-amino-1-pentanol, and tetra(2-hydroxyethyl)ethylenediamine. In some embodiments, the electrocharged adjuvant is present in the electrophoretic display medium at a concentration of about 1 to about 500 milligrams per gram of particle mass (“mg/g”), and more preferably at a concentration of about 50 to about 200 mg/g.

Particle dispersion stabilizers may be added to prevent particle flocculation or adhesion to capsules or other walls or surfaces. For typical high resistivity liquids used as fluids in electrophoretic displays, non-aqueous surfactants may be used. These surfactants include, but are not limited to, ethylene glycol ethers, alkynyl glycols, alkanolamides, sorbitol derivatives, alkylamines, quaternary ammonium compounds, imidazolines, dialkyl oxides, and sulfosuccinates.

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 exceeding about 20,000 in the fluid, which is substantially non-absorbent 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 fixed charges 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 species. Outside this layer is a diffusion layer containing charged (opposite) micelles containing CCA molecules in the fluid. In traditional direct current electrophoresis, the applied electric field exerts a force on the stationary surface charge and an opposite force on the movable countercharge, causing slippage within the diffusion layer and particle motion relative to the fluid. The potential in the slip plane is known as the zeta potential.

Therefore, some particle types within the electrophoretic medium exhibit different electrophoretic mobility depending on the electric field strength traversing the medium. For example, when a first (low intensity, i.e., approximately ±10V or lower) electric field is applied to the electrophoretic medium, particles of type I move in one direction relative to the electric field. However, when a second (high intensity, i.e., approximately ±20V or higher) electric field with the same polarity as the first field is applied, particles of type I begin to move in the opposite direction relative to the electric field. The theoretical basis for this behavior is that it originates from conduction within a highly nonpolar fluid mediated by charged reverse micelles or oppositely charged electrophoretic particles. Thus, any electrochemically generated protons (or other ions) can be transported through the nonpolar fluid within the micelle core or adsorbed onto the electrophoretic particles. For example, as illustrated in Figure 5B of U.S. Patent No. 9,697,778, a positively charged reverse micelle can approach a negatively charged electrophoretic particle traveling in the opposite direction, wherein the reverse micelle is incorporated into an electric bilayer surrounding the negatively charged particle. (The electric bilayer comprises a charge diffusion layer with enhanced anti-ion concentration and a semi-micelle surface adsorption coating on the particle; in the latter case, the reverse micelle charge becomes associated with the particle within the slip cladding, which, as described above, defines the zeta potential of the particle.) Through this mechanism, a positively charged electrochemical current flows through the electrophoretic fluid, and the negatively charged particle becomes biased toward more positive charges. Therefore, for example, the electrophoretic mobility of the first type of negatively charged particle 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.

Furthermore, as described in U.S. Patent No. 9,697,778, positively charged particles can be prepared, which exhibit different electrophoretic migration rates depending on the applied electric field. In some embodiments, a secondary (or common-stage) CCA can be added to the electrophoretic medium to adjust the zeta potential of various particles. Careful selection of the common-stage CCA allows for variations in the zeta potential of one type of particle while keeping the zeta potentials of other particles essentially constant, allowing for close control of both the electrophoretic velocity of various particles and the interactions between particles during switching.

In some embodiments, part of the 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 the addition of a quaternary ammonium charge control agent during polymer grafting results in a certain amount of CCA misalignment with the particles. (This can be confirmed by removing the particles from the electrophoretic fluid and subsequently peeling the surface species from the THF-containing pigment to remove all adsorbed species. When the THF extract is evaluated by 1H NMR, it is clear that a large amount of CCA is adsorbed to the pigment particles or misaligned with the surface polymer.) Experiments show that, in the presence of a strong electric field, a high CCA loading in the surface polymer of the particles favors the formation of a charged bilayer around the particles. For example, in the presence of a high positive electric field, magenta particles with a charge control agent (CCA) of greater than 200 mg per gram of finished magenta particles exhibit excellent retention performance (e.g., as described in Figure 2C and 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 electrophoresis medium have high CCA loadings, it is important that particles with consistent electrophoretic migration rates do not have excessively high CCA loadings, for example, less than 50 mg of charge control agent (CCA) per gram of finished particles, or for example, less than 10 mg of charge control agent (CCA) per gram of finished particles.

In other embodiments, the electrophoretic medium comprising four types of particles in Solsperse 17000 contained in Isopar E benefits from the addition of, for example, a small amount of acidic material, such as an aluminum salt like di-tert-butyl salicylic acid (Bontron E-88, available from Orient Corporation, Kenilworth, New Jersey). The addition of the acidic material shifts the zeta potential of many particles (though not all) to a corrected 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 a Lewis acidic material like an aluminum salt will depend on the details of the particle surface chemistry.

Table 2 shows exemplary relative zeta potentials for three types of colored particles and a single white particle. Table 2. Relative zeta potentials of colored particles in the presence of white particles. White zeta potential (mV) -30 0 10 20 Cyan zwitterion potential (mV) 80 110 80 70 60 Magenta zeta potential (mV) 40 70 40 30 20 + Yellow zeta potential (mV) 20 50 20 10 0 - Yellow zeta potential (mV) -20 10 -20 -30 -40

Negatively charged (white) particles have a zeta potential of -30 mV, and the other three types of particles are all positively charged relative to white particles. Therefore, a display containing positively charged 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, bringing the white particles closest to the viewer and preventing the viewer from perceiving the other three types of particles. In contrast, when white particles have a zeta potential of 0 V, negatively charged yellow particles are the most negatively charged of all particles, and therefore a display containing this particle will switch between yellow and blue states. This will also occur if the white particles are positively charged. However, positively charged yellow particles will carry more positive charge than white particles unless their zeta potential exceeds +20 mV.

The electrophoretic medium of this invention behaves in accordance with the migration rate of white particles (represented as zeta potential in Table 2), depending on the applied electric field. Therefore, in the example illustrated in Table 2, when addressed with a low voltage, a white particle may behave as if its zeta potential is -30 mV, but when addressed with a higher voltage, it may behave as if its zeta potential is more positively charged, possibly even as high as +20 mV (matching the zeta potential of yellow particles). Therefore, 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.

Figures 2B-2E illustrate the motion of various particles in the presence of high electric fields (e.g., ±H, ±20V, ±25V) and low electric fields (e.g., ±L, ±5V, ±10V). For illustrative purposes, each box defined by the dashed lines represents a pixel defined by the top transparent electrode 21 and the bottom electrode 22, which can be a pixel electrode of an active matrix, but it can also be a transparent electrode or a segmented electrode, etc. As shown in Figures 2B-2E, the electrophoretic medium can be driven from the first state condition to four different optical states, in which all positively charged particles are present on the viewing surface (nominally black). This results in a white optical state (Fig. 2B), a magenta optical state (Fig. 2C), a yellow optical state (Fig. 2D), and a red optical state (Fig. 2E). It should be obvious that the other four optical states in Fig. 1 can be achieved by reversing the initial state and the order of the driving electric field, as illustrated in Fig. 5.

When addressing with low voltage, as shown in Figure 2B, in the case of applying a negative voltage to the backplane, the behavior of the particles depends on their relative zeta potential, having a relative velocity 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 position of the particles because their movement is already restricted by the walls of the enclosure. The second (negative) pulse swaps the positions of the colored and white particles, thus switching the display between black and white states, although the transient color reflects the relative migration rate of the colored particles. Reversing the starting position and polarity of the pulse allows a transition from white to black. Therefore, compared to other black and white formulations that achieve black and white by processing black or white with most colors, it provides a black and white update that requires lower voltage (and consumes less power).

In Figure 2C, the first (positive) pulse has a high positive voltage, sufficient to reduce the migration rate of the magenta particles (i.e., the particles with the middle migration rate among the three positively charged colored particles). Due to the reduced migration rate, the magenta particles are essentially frozen in place, and the 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, with the negatively charged white particles positioned 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), this pulse sequence will produce green (i.e., a mixture of yellow and cyan particles).

In Figure 2D, the first pulse is a low voltage that does not significantly reduce the migration rate of either the magenta or white particles. However, the second pulse is a high negative voltage that reduces the migration rate of the white particles. This allows for more efficient competition among the three positively charged particles, ensuring that the slowest type of particle (yellow in this example) remains visible in front of the white particles, while the movement of the white particles is diminished by 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 pulse are reversed (equivalent to viewing the display from the side opposite to the viewing surface, i.e., through electrode 22), this pulse sequence will produce blue (i.e., a mixture of magenta and cyan particles).

Finally, Figure 2E shows that when both pulses are at high voltage, the migration rate of magenta particles is weakened by the first high positive pulse, and the competition between cyan and yellow is enhanced by the weakening of the white migration rate caused by the second high negative pulse. 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), this pulse sequence will produce cyan.

To achieve a high-resolution display, individual pixels of the display should be addressable and unaffected by interference from adjacent pixels. One way to achieve this is to provide an array of nonlinear elements, such as transistors or diodes, such that at least one nonlinear element is associated with each pixel to create an "active matrix" display. The address of a pixel, or pixel electrode, 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 transistor's drain, and this arrangement will be assumed in the following description, although it is essentially arbitrary and the pixel electrode can be connected to the transistor's source. As is customary, in high-resolution arrays, pixels are arranged in a two-dimensional array of rows and columns, such that any particular pixel is uniquely defined by the intersection of a particular row and a particular column. The sources of all transistors in each column are connected to a single column electrode, and the gates of all transistors in each row are connected to a single row electrode; furthermore, the allocation of sources to rows and gates to columns is conventional, but essentially arbitrary, and can be reversed if desired. The row electrodes are connected to the row driver, which essentially ensures that only one row is selected at any given time. This means a selection voltage is applied to the selected row electrode to ensure that all transistors in the selected row are conductive, while a non-selection voltage is applied to all other rows to ensure that all transistors in these non-selected rows are non-conductive. The column electrodes are connected to the column driver, which places a selected voltage on each column electrode to drive the pixels in the selected row to their desired optical state. (This voltage is relative to a common front electrode, which is conventionally located on the side opposite the electro-optical dielectric and nonlinear array and extends across the entire display.) After a pre-selected interval known as the "line addressing time", the selected row is deselected, the next row is selected, and the voltage on the column driver is changed, causing the next line of the display to be written. This process is repeated so that the entire display is written line by line.

By convention, each pixel electrode is associated with a capacitor electrode, such that the pixel electrode and the capacitor electrode form a capacitor; for example, see International Patent Application WO 01/07961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) may be used to form the transistor, and the "selection" and "non-selection" voltages applied to the gate electrode may be positive and negative, respectively.

Figure 3 of the accompanying figures illustrates an exemplary equivalent circuit for a single pixel of an electrophoretic display. As illustrated, the circuit includes a capacitor 10 formed between the pixel electrode and the capacitor electrode. The electrophoretic dielectric 20 is shown as a capacitor and resistor connected in parallel. In some cases, a direct or indirect coupling capacitor 30 (commonly referred to as a "parasitic capacitor") between the gate electrode of the transistor associated with the pixel and the pixel electrode can introduce unwanted noise into the display. Typically, the parasitic capacitor 30 is much smaller than the parasitic capacitance of the storage capacitor 10, and when a pixel row of the display is selected or deselected, the parasitic capacitor 30 can cause a small negative bias voltage to the pixel electrode, also known as a "reverse voltage" typically less than 2 volts. In some embodiments, to compensate for unwanted "backlash voltage", a common potential V _com can be supplied to the top planar electrode and capacitor electrode associated with each pixel, such that when V _com is set to a value equal to the backlash voltage (V _KB ), each voltage supplied to the display can be biased by the same amount and will not experience net DC imbalance.

However, problems can arise when V _{_com} is set to an uncompensated backpressure voltage. This can occur when it is desired to apply a higher voltage to the display than that obtained solely from the backplane. As is well known in the art, for example, if the backplane offers a nominal choice of +V, 0, or -V, the maximum voltage applied to the display can be doubled, for example, when V _{_com} is supplied with -V. In this case, the maximum voltage experienced is +2V (i.e., the voltage relative to the top plane on the backplane), while the minimum voltage is zero. If a negative voltage is required, the V _{_com} potential must rise to at least zero. Therefore, the waveform of a display using top-plane switching to address positive and negative voltages must have a specific frame assigned to each of the more than one V _com voltage settings.

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 voltages, three negative voltages, and zero voltage. However, in some cases, the maximum voltage used in these waveforms is higher than that that can be handled by amorphous silicon thin-film transistors. In such cases, a suitable high voltage can be obtained by using top-plane switching. When V _{_com} is intentionally set to V _{_KB} (as described above), a separate power supply can be used. However, when using top-plane switching, using as many separate power supplies as V _{_com} is both expensive and inconvenient. Furthermore, the top plane switching is well known to increase impact, thereby reducing the stability of color states.

The display device can be manufactured using the electrophoretic fluid of this invention in several ways known in the art. The electrophoretic fluid can be encapsulated in microcapsules or incorporated into microcell structures, which are 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 supporting conductive material. This assembly can be laminated to a backplate supporting pixel electrodes using a conductive adhesive layer. Alternatively, the electrophoretic fluid can be directly dispensed onto a thin, open-cell mesh already arranged on a backplate including an active matrix of pixel electrodes. The filled mesh can then be top-sealed with an integrated protective sheet/transparent electrode.

Figure 4 shows a schematic, cross-sectional view (not to scale) of a display structure 200 including an electrophoretic medium. In the display 200, the electrophoretic fluid is illustrated as confined to microcells, although an equivalent structure incorporating microcapsules may also be used. Pixel electrodes 204, which may be glass or plastic, support a substrate 202. These pixel electrodes are individually addressed segments or associated with thin-film transistors in an active matrix arrangement. (The combination of substrate 202 and electrodes 204 is conventionally referred to as the backplane of the display.) Layer 206 is an optional dielectric layer applied to the backplane according to the invention. (The 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 plane of the display includes a transparent substrate 222 supporting a transparent, conductive coating 220. An optional dielectric layer 218 covers the electrode layer 220. Layer (or layers) 216 is a polymer layer that may include a primer layer for adhering microcells to the transparent electrode layer 220 and some residual polymer containing the bottom of the microcells. The walls of the microcells 212 are used to hold electrophoretic fluid 214. The microcells are sealed with layer 210, and the entire front plane structure is adhered to a backplate using a conductive adhesive layer 208. The fabrication process used to form micro-units is described in the prior art, for example, in U.S. Patent No. 6,930,818. In some cases, the depth of the micro-unit is less than 20 μm, for example less than 15 μm, for example less than 12 μm, for example approximately 10 μm, for example approximately 8 μm.

Due to the wider availability of manufacturing facilities 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 the supply allows gate voltages that are higher than approximately +/-15V. Nevertheless, as described below, ACeP performance is improved when high positive and negative voltage values exceed +/-15V. Therefore, as previously disclosed, improved performance is achieved by additionally changing the bias voltage of the top transparent electrodes relative to the bias voltage on the backplane pixel electrodes, also known as top-plane switching. Therefore, if a voltage of +30V (relative to the backplane) is required, the top surface can be switched to -15V, while the appropriate backplane pixels are switched to +15V. For example, U.S. Patent No. 9,921,451 describes in more detail a method for driving four particle electrophoresis systems by switching the top plane.

These waveforms need to drive each pixel of the display at five different addressing voltages, labeled +V _high , +V _low , 0, -V _low , and -V _high , and described as 30V, 15V, 0, -15V, and -30V. In practice, it is preferable to use a larger number of addressing voltages. If only three voltages are available (i.e., +V _high , 0, and -V _high ), it is possible to achieve the same result as addressing at a lower voltage (e.g., V _high / n, where n is a positive integer greater than 1) by addressing with pulses of voltage V _high but with a duty cycle of 1/n.

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

Although Figure 5 shows the simplest dipole required to form color, it will be understood that actual waveforms can use multiple repetitions of these patterns, or other non-periodic patterns, and use more than five voltage levels.

Of course, achieving the desired color using the driving pulses of Figure 5 depends on the particles starting the process from a known state, and this color is unlikely to be the last color displayed on the pixel. Therefore, a series of reset pulses precede the driving pulses, increasing 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 address) and any pauses (i.e., the zero-voltage cycles between them) can be selected so that the entire waveform (i.e., the voltage integral with respect to time) is DC balanced (i.e., the voltage integral with time is essentially zero). DC balance can be achieved by adjusting the lengths of the pulses and pauses in the reset phase so that the magnitude of the net pulse supplied in the reset phase is equal and the sign is opposite to the sign of the net pulse supplied in the address phase, during which the display is switched to a specific desired color. However, as shown in Figures 2B-2E, the initial state used for the eight primary colors is black or white, which can be achieved using continuous low-voltage drive pulses. The ease of achieving this initial state further shortens the update time between states, which is more satisfactory for the user and also reduces the amount of power consumed (thus extending battery life).

Furthermore, the aforementioned discussion of waveforms, and specifically the discussion of DC balance, neglects the issue of reverse voltage. In reality, as previously stated, each backplane voltage is biased by the voltage supplied by the power supply by a number equal to the reverse voltage _VKB . Therefore, if the power supply provides three voltages: +V, 0, and -V, the backplane will actually receive V+ _VKB , _VKB , and -V+ _VKB (note that in the case of amorphous silicon TFTs, _VKB is typically negative). However, the same power supply will supply +V, 0, and -V to the front electrode without any reverse voltage bias. Therefore, for example, when -V is supplied to the front electrode, the display will experience a maximum voltage of 2V+ _VKB and a minimum voltage of _VKB . Instead of using a separate power supply to power the front _electrode (which can be both expensive and inconvenient), the waveform can be divided into segments, where the front electrode is powered by positive voltage, negative voltage, and _VKB . [Provides an improved black optical state for five-particle electrophoresis media]

According to the present invention, a five-particle electrophoretic medium can provide a plurality of colored optical states on each pixel, including a modified black optical state. The five-particle electrophoretic medium is similar to the four-particle electrophoretic medium disclosed above, but further incorporates black particles.

The five-particle electrophoretic medium includes a first white particle with a first polarity and four other particles with opposite polarities and different charges (including a black particle). The medium preferably includes a negatively charged white particle and positively charged yellow, magenta, cyan, and black particles, which contain subtractive primary colors. Furthermore, as mentioned above, some particles can be designed, as discussed earlier, such that their electrophoretic mobility is nonlinear with respect to the applied electric field strength. Therefore, with a high electric field of the correct polarity (e.g., 20V or higher), one or more particles can experience a decrease in electrophoretic mobility. This five-particle system is schematically shown in Figure 6, and it can provide white, yellow, red, magenta, blue, cyan, green, and black for each pixel.

As shown in Figure 6, each of the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) corresponds to a different arrangement of the five particles, so that the viewer only sees the colored particles located on the viewing side of the white particles (which are particles that only scatter light). These colors can be achieved using the same or substantially the same waveforms described above relative to Figure 5. It should be noted that, compared to other subtractive particles, because the relative number of black particles is smaller, the color state of magenta does not become black, but has a reduced L* value, and due to the specific absorption spectrum of black particles, there may be some shift in a* and b*. In some embodiments, the reduced L* value can be offset by increasing the intensity of the LEDs fed into the front light panel between the viewer and the display.

Figures 7A-7E show cross-sectional schematics of a display layer with five particle types similar to those in Figures 2A-2E, illustrating a four-particle system. The display layer includes a first (viewing) surface 13 on the viewing side and a second surface 14 on the opposite side of the first surface 13. A five-particle electrophoretic medium is disposed between the two surfaces. Each space between the two vertical dashed lines marks a pixel. Within each pixel, the electrophoretic medium can be addressed, and the viewing surface 13 of each pixel can achieve the color state shown in Figure 6 without the need for additional layers or a filter array.

Depending on the specific implementation, the white, yellow, cyan, and magenta particles in the five-particle system are the same as or similar to the corresponding particles in the four-particle medium described above regarding charge polarity, zeta potential, surface treatment, and behavior under different electric fields.

For example, in a five-particle electrophoretic medium, the white particles (W-*) are negatively charged and can be surface-treated so that their electrophoretic mobility depends on the strength of the driving electric field. In such cases, the electrophoretic mobility of the white particles actually decreases in the presence of a stronger electric field, which is somewhat counterintuitive.

Magenta particles (M++*) are positively charged and can be surface-treated (or intentionally left untreated), so that the electrophoretic migration rate of magenta particles depends on the strength of the driving electric field. Alternatively, when the direction of the electric field is reversed, after the magenta particle aggregate has been driven to the side of the cavity containing the particles, its unpacking rate is slower than that of yellow and cyan particles.

Yellow particles (Y+) are positive but have a smaller charge than magenta particles. Furthermore, yellow particles can be surface-treated, but in a way that does not cause their electrophoretic mobility to depend on the strength of the driving electric field. In other words, yellow particles can have a surface treatment, but this treatment will not lead to a decrease in electrophoretic mobility when the aforementioned electric field increases.

Cyan particles (C+++) have a higher positive charge than magenta particles and have the same type of surface treatment as yellow particles.

Black particles (K++++) have the highest positive charge. Black particles have a relatively low concentration in the electrophoretic medium. For example, by weight, black particles may comprise 0.5-2% of the entire internal phase (including solvent, dispersed polymer, CCA, and other pigment particles). The black to magenta particle ratio (black:magenta) is typically between 1:10 and 1:5, for example, 1:7.

Although the particles disclosed herein are nominally white, magenta, yellow, black, and cyan in order to produce the colors shown in Figure 6, it should be understood that the invention is not limited to this specific set of colors, nor to a single reflective particle and four absorptive particles.

The electrophoretic medium for a five-particle system can be any of the above-mentioned forms. Therefore, the electrophoretic medium can be unencapsulated, encapsulated in a dispersed capsule surrounded by capsule walls, encapsulated in a sealed microcell, or in the form of a polymer dispersion medium.

Table 3 below shows exemplary zeta potentials of particles used in five-particle electrophoresis media in one or more embodiments. Table 3. Zeta Potentials of Particles in Five-Particle Media color Zeta potential White (W) Approximately -55mV to -70mV Cyan (C) Approximately +80 to +100mV Magenta (M) Approximately +40 to +50mV Yellow (Y) Approximately -5mV to +5mV Black (K) About +100mV

Starting from the first state, where all positive particles are present on the viewing surface (nominally black), the electrophoretic medium can be driven to several different optical states, including the four optical states shown in Figures 7B-7E: white optical state (Figure 7B), magenta optical state (Figure 7C), yellow optical state (Figure 7D), and red optical state (Figure 7E). The other four optical states in Figure 6 can be achieved by reversing the initial state and the order of the driving electric fields, as illustrated in the shorthand in Figure 5.

When addressed with low voltage, as shown in Figure 7B, particles behave according to their relative zeta potentials, their relative velocities illustrated by arrows, in the case of a negative voltage applied to the backplane. Therefore, in this example, black particles move faster than cyan particles, cyan particles move faster than magenta particles, and magenta particles move faster than yellow particles. The first (positive) pulse does not change the particle positions because their movement is already restricted by the walls of the enclosure. The second (negative) pulse alternates the positions of colored and white particles, thus switching the display between black and white states, although the instantaneous color reflects the relative migration rate of the colored particles. Reversing the starting position and polarity of these pulses allows for a transition from white to black. Therefore, compared to other black-and-white formulations achieved by processing black or white with a majority of colors, this embodiment provides a black-and-white update that requires lower voltage (and consumes less power).

In Figure 7C, the first (positive) pulse is a high positive voltage, sufficient to reduce the migration rate of magenta and black particles. Due to the reduced migration rate, the magenta and black particles are essentially frozen in place, and the subsequent low-voltage pulse in the opposite direction causes cyan, white, and yellow particles to move more than the magenta and black particles, thus producing magenta on the viewing surface, with the negative white particles positioned behind the magenta and black particles. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through electrode 22), this pulse sequence will produce green (i.e., a mixture of yellow and cyan particles).

In Figure 7D, the first pulse is a low voltage, which does not significantly reduce the migration rates of the magenta, black, and white particles. However, the second pulse is a high negative voltage, which reduces the migration rates of the black and magenta particles. This allows for more efficient racing among the four positive particles, keeping the slowest type of particle (yellow in this example) visible in front of the white particles, while the movement of the white particles is weakened by 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 these pulses are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., via electrode 22), this pulse sequence will produce blue (i.e., a mixture of magenta and cyan particles).

Finally, Figure 7E illustrates that when both pulses are at high voltage, the first high positive pulse reduces the migration rate of magenta and black particles, while the second high negative pulse reduces the migration rate of white particles, thus enhancing the race between cyan and yellow. This produces red. Importantly, if the starting position and polarity of these pulses are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., via electrode 22), this pulse sequence will produce cyan.

Therefore, as summarized in Figure 6, when the display is in red, magenta, yellow, and black optical states, the black particles are visible on the viewing surface of the display (i.e., in front of the white particles).

In a black optical state, the presence of black particles on the viewing surface provides an improved, more saturated black optical state. It can also shift the chromaticity of black so that the viewer perceives a "purer" black. This is especially important when reading white text on a black background using "Dark Mode".

In red, magenta, and yellow optical states, the presence of black particles reduces the brightness of the displayed color. However, since the concentration of black particles in the electrophoretic medium is relatively low compared to other colored particles, this effect is limited. Furthermore, the reduction in the brightness of the displayed color can be mitigated by changing the illumination intensity or spectrum of the lamp used to illuminate the viewing surface in the display device.

Therefore, having described several aspects and embodiments of the technology of this application, it should be understood that various changes, modifications, and improvements will be readily apparent to those skilled in the art. These changes, modifications, and improvements are intended to be within the spirit and scope of the technology described in this application. For example, those skilled in the art will readily conceive of various other means and/or structures for implementing the functions described herein and/or obtaining the results and/or one or more advantages described herein, and each of these changes and/or modifications is considered to be within the scope of the embodiments described herein. Those skilled in the art will recognize, or determine using no more than conventional experimental methods, many equivalent solutions to the embodiments described herein. Therefore, it should be understood that the foregoing embodiments are presented by way of example only, and that embodiments of the invention can 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 is included within the scope of this disclosure if such features, systems, articles, materials, kits, and/or methods do not conflict with each other.

10: Capacitor 11: Common Electrode 12: Electrode Layer 13: First Surface 14: Second Surface 15: Pixel Electrode 17: Non-polar Fluid 20: Electrophoretic Medium 21: Top Transparent Electrode 22: Bottom Electrode 30: Coupling Capacitor 200: Display 202: Substrate 204: Electrode 206: Layer 208: Adhesive Layer 210: Layer 212: Microunit 214: Electrophoretic Fluid 216: Layer 218: Dielectric Layer 220: Electrode Layer 222: Substrate

Additional details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the following description. Other features, appearances, and advantages of the subject matter will become apparent from the description and drawings contained herein. It should be emphasized that the drawings are schematic and not drawn to scale. In particular, for ease of explanation, the thicknesses of the layers in the drawings do not correspond to their actual thicknesses. The thicknesses of the layers relative to their lateral dimensions are also not drawn to scale. Generally, for ease of explanation, components with similar structures are labeled with the same reference numerals throughout the drawings. However, the specific properties and functions of the components in different embodiments may not be entirely identical. Furthermore, the drawings are only intended to illustrate the subject matter. The accompanying figures do not illustrate the various embodiments described and do not limit the scope of this disclosure or the claims.

Figure 1 is a schematic cross-sectional view showing the positions of various colored particles in a four-particle electrophoresis medium when displaying black, white, subtractive color mixing and additive color mixing.

Figure 2A is a general illustration of an electrophoretic display with four types of particles in a nonpolar fluid, wherein a full range of colors can be provided at each pixel electrode.

Figure 2B illustrates the transition of a four-particle display between a first optical state and a second optical state. In the first optical state, all particles of the first charge polarity are on the viewing surface, and in the second optical state, the particles on the viewing surface have a second (opposite) polarity.

Figure 2C illustrates the transition between the first and third optical states of a four-particle display. In the first optical state, all particles of the first charge polarity are on the viewing surface, and in the third optical state, particles with the second (opposite) polarity are positioned behind the middle charged particles of the first polarity, which are located on the viewing surface.

Figure 2D illustrates the transition between the first and fourth optical states of a four-particle display. In the first optical state, all particles of the first charge polarity are on the viewing surface, and in the fourth optical state, particles with the second (opposite) polarity are positioned behind the low-charged particles of the first polarity, which are located on the viewing surface.

Figure 2E illustrates the transition between the first and fifth optical states of a four-particle display. In the first optical state, all particles of the first charge polarity are on the viewing surface, while in the fifth optical state, particles with the second (opposite) polarity are positioned behind a combination of low-charged and intermediate-charged particles of the first polarity, which are located on the viewing surface.

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

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

Figure 5 illustrates an exemplary push-pull drive scheme for addressing an electrophoretic medium comprising three subchromatic particles and a scattering (white) particle.

Figure 6 is a schematic cross-sectional view showing the positions of various colored particles in a five-particle electrophoresis medium according to one or more embodiments, when displaying black, white, subtractive color mixing and additive color mixing.

Figure 7A is a general illustration of an electrophoretic display having five types of particles in a nonpolar fluid, wherein a full range of colors can be provided at each pixel electrode. It will 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, one type of positively charged particle is black, and one type of positively charged particle is cyan. However, the invention is not limited to this exemplary color set.

Figure 7B illustrates the transition of a five-particle display between a first optical state and a second optical state. In the first optical state, all particles of the first charge polarity are on the viewing surface, and in the second optical state, the particles on the viewing surface have a second (opposite) polarity.

Figure 7C illustrates the transition between the first and third optical states of a five-particle display. In the first optical state, all particles of the first charge polarity are on the viewing surface, and in the third optical state, particles with the second (opposite) polarity are positioned behind the middle charged particles of the first polarity, which are located on the viewing surface.

Figure 7D illustrates the transition between the first and fourth optical states of a five-particle display. In the first optical state, all particles of the first charge polarity are on the viewing surface, while in the fourth optical state, particles with the second (opposite) polarity are positioned behind the low-charged particles of the first polarity, which are located on the viewing surface.

Figure 7E illustrates the transition between the first and fifth optical states of a five-particle display. In the first optical state, all particles of the first charge polarity are on the viewing surface, while in the fifth optical state, particles with the second (opposite) polarity are positioned behind a combination of low-charged and intermediate-charged particles of the first polarity, which are located on the viewing surface.

without.

Claims (14)

A color electrophoretic display includes: a light-transmitting electrode layer located on a viewing surface; a back electrode layer; and an electrophoretic medium disposed between the light-transmitting electrode layer and the back electrode layer, the electrophoretic medium comprising: a non-polar fluid; and a multi-color particle system comprising five types of charged electrophoretic color particles dispersed in the non-polar fluid, the five types of charged electrophoretic color particles comprising: a first type of particle having a first optical property and a first charge polarity; a second type of particle having a second optical property and a second charge polarity having a first charge amount opposite to the first charge polarity; and a third type of particle having a third optical property and a second charge polarity having a second charge amount less than the first charge amount. A fourth type of particle has a fourth optical property and a second charge polarity with a third charge amount less than the second charge amount; and a fifth type of particle has a fifth optical property and a second charge polarity with a fourth charge amount greater than the first charge amount; wherein the first type of particle is white, the fifth type of particle is black, and the second, third, and fourth types of particles are each one of cyan, magenta, and yellow. For example, in the color electrophoresis display of claim 1, the second, third, and fourth types of particles are cyan, magenta, and yellow, respectively. The color electrophoretic display of any of claims 1 to 2, wherein the first charge polarity is negative and the second charge polarity is positive. For example, in the color electrophoretic display of claim 1, the first charge is about 80 mV to about 100 mV. For example, in the color electrophoretic display of claim 1, the second charge is about 40 mV to about 50 mV. For example, in the color electrophoretic display of claim 1, the third charge is approximately -5 mV to approximately 5 mV. For example, in the color electrophoresis display of claim 1, the fourth charge is approximately 100 mV. As in the color electrophoretic display of claim 1, wherein the first type of particle has a charge of about -55 mV to about -70 mV. As in the color electrophoretic display of claim 1, the back electrode layer includes a thin-film transistor array coupled to pixel electrodes, each thin-film transistor comprising a layer of metal oxide semiconductor. For example, in the color electrophoretic display of claim 9, the metal oxide semiconductor is indium gallium zinc oxide (IGZO). For example, in the color electrophoretic display of claim 9, the thin-film transistor of the metal oxide semiconductor layer is capable of switching control voltages greater than 25V and less than -25V, while the light-transmitting electrode layer is maintained at a constant voltage for changing the optical state of the pixels of the electrophoretic display. As in claim 1, a color electrophoretic display wherein the fifth type of particle includes a polymer shell grafted onto the surface of the particles. The color electrophoretic display of claim 1, wherein the third type of particles is magenta and includes a polymer shell coated on the particles by dispersion polymerization. An electrophoretic medium for a color electrophoretic display, the electrophoretic medium comprising: a nonpolar fluid; and a multi-color particle system comprising five types of charged electrophoretic color particles dispersed in the nonpolar fluid, the five types of charged electrophoretic color particles comprising: a first type of particle having a first optical property and a first charge polarity; a second type of particle having a second optical property and a second charge polarity having a first charge amount opposite to the first charge polarity; a third type of particle having a third optical property and a second charge polarity having a second charge amount less than the first charge amount; and a fourth type of particle having a fourth optical property and a second charge polarity having a third charge amount less than the second charge amount; and The fifth type of particle has a fifth optical property and a second charge polarity with a fourth charge greater than the first charge; wherein the first type of particle is white, the fifth type of particle is black, and the second, third, and fourth type of particles are each one of cyan, magenta, and yellow.