Improved driving voltages for advanced color electrophoretic displays and displays with improved driving voltages

Abstract

An improved method for driving a four-particle electrophoretic medium comprising scattering particles and at least two dechromatic particles is provided. These methods enable displays such as a color electrophoretic display comprising a backplane having an array of thin-film transistors, wherein each thin-film transistor comprises a layer of metal oxide semiconductor. The metal oxide transistors allow for faster and higher voltage switching, and thus allow for direct color switching of the four-particle electrophoretic medium without the need for top-plane switching. As a result, the color electrophoretic display can be updated at a higher speed, and colors are reproduced more reliably.

Description

Technology Field

[0001] Related applications

[0002] This application claims priority to U.S. Provisional Application No. 63 / 078,829 filed September 15, 2020. All patents and disclosures disclosed herein are incorporated by reference in their entirety. Background Technology

[0003] Electrophoretic displays (EPDs) change color by altering the position of charged colored particles on a light-transmitting viewing surface. These electrophoretic displays are commonly referred to as "electronic paper" or "ePaper" because the resulting display has high contrast and is light-readable, very similar to ink on paper. Electrophoretic displays are widely adopted in eReaders such as the Amazon Kindle® because they provide a book-like reading experience, consume very little power, and allow users to carry hundreds of books in a lightweight handheld device.

[0004] For many years, electrophoretic displays have contained only two types of charged colored particles: black and white. (Certainly, "color" as used herein includes black and white.) White particles are often of the light-scattering type and include, for example, titanium dioxide, while black particles are absorbent across the visible spectrum and may include carbon black, or absorbent metal oxides, such as copper chromite. In the simplest sense, a black and white electrophoretic display requires only an electrophoretic medium containing a light-transmitting electrode on the viewing surface, a back electrode, and oppositely charged white and black particles. When a voltage of one polarity is applied, white particles move toward the viewing surface, and when a voltage of the opposite polarity is applied, black particles move toward the viewing surface. If the back electrode contains controllable regions (pixels)—an active matrix of pixel electrodes controlled by transistors or divided electrodes—a pattern can be made to appear electronically on the viewing surface. The pattern could be, for example, text on a book.

[0005] More recently, various color options, including three-color displays (black, white, red; black, white, yellow) and four-color displays (black, white, red, yellow), have become commercially available for electrophoretic displays. Similar to the operation of black and white electrophoretic displays, electrophoretic displays with three or four reflective pigments operate similarly to simple black and white displays because the desired colored particles are driven to the viewing surface. Although the driving mechanism is much more complex than that of black and white, the optical function of the particles is ultimately the same.

[0006] ACeP™ (Advanced Color Electronic Paper) also contains four particles, but the cyan, yellow, and magenta particles are subtractive rather than reflective, thereby enabling the generation of thousands of colors in each pixel. The color process is functionally equivalent to the printing methods long used in offset printing and inkjet printers. A given color is produced by using precise ratios of cyan, yellow, and magenta against a bright white paper background. In the case of ACeP, the relative positions of the cyan, yellow, magenta, and white particles with respect to the viewing surface determine the color in each pixel. While this type of electrophoretic display allows for thousands of colors in each pixel, it is crucial to carefully control the position of each pigment (50 to 500 nanometers in size) within a working space approximately 10 to 20 micrometers thick. Obviously, variations in the position of the pigments will result in the display of inaccurate colors in a given pixel. Therefore, sophisticated voltage control is required for this system. More detailed information regarding this system is available in the following U.S. Patents No. 9,361,836, 9,921,451, 10,276,109, 10,353,266, 10,467,984, and 10,593,272, all of which are incorporated herein by reference in their entirety.

[0007] The present invention relates to color electrophoretic displays, and in particular to electrophoretic displays capable of rendering more than two colors using a single layer of an electrophoretic material comprising, non-exclusively, a plurality of colored particles, e.g., white, cyan, yellow, and magenta particles. In some cases, two of the particles will be positively charged and two of the particles will be negatively charged. In some cases, one positively charged particle will have a thick polymer shell and one negatively charged particle will have a thick polymer shell.

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

[0009] The terms bistable and bistable are used in their conventional sense in the art to refer to a display comprising display elements having first and second display states having at least one optical characteristic that is different, and thus, any given element is driven by an addressing pulse of a finite duration to take either of its first or second display state, and after the addressing pulse is terminated, the state will continue for at least several times, e.g., at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is indicated in U.S. Patent No. 7,170,670 that some grayscale-capable particle-based electrophoretic displays are stable in their extreme black and white states as well as in their intermediate gray states, and that the same applies to other types of electro-optical displays. Although it is more appropriate to call this type of display polystable rather than bistable, for convenience, the term bistable may be used in this specification to cover both bistable and polystable displays.

[0010] In this document, the term impulse is used to refer to the integral of the applied voltage over time during the period in which the display is driven, when used to refer to driving an electrophoretic display.

[0011] Particles that absorb, scatter, or reflect light in either a broadband or selected wavelength as described herein are referred to as coloring or pigment particles. Various materials other than pigments that absorb or reflect light (in the strict sense of the term meaning insoluble coloring materials), such as dyes or photonic crystals, may also be used in the electrophoretic medium and display of the present invention.

[0012] Particle-based electrophoretic displays have been the subject of intensive research and development for many years. In these displays, multiple charged particles (sometimes referred to as pigment particles) move through a fluid under the influence of an electric field. Compared to liquid crystal displays, electrophoretic displays can possess properties such as good brightness and contrast, wide viewing angles, state bistable, and low power consumption. Nevertheless, issues regarding the long-term image quality of these displays have hindered their widespread use. For example, the particles constituting electrophoretic displays tend to settle, resulting in an insufficient service life for these displays.

[0013] As noted above, electrophoretic media require the presence of a fluid. In most conventional electrophoretic media, this fluid is a liquid, but electrophoretic media can be prepared using a gaseous fluid; see, for example, Kitamura, T. et al., Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y. et al., Toner display using insulative particles charged triboelectrically, IDW Japan, 2001, Paper AMD4-4. See also U.S. Patent Nos. 7,321,459 and 7,236,291. These gaseous electrophoretic media appear to be prone to the same types of problems caused by particle sedimentation as liquid electrophoretic media when the medium is used as a sign placed in a vertical plane, for example, in an orientation where the medium allows such sedimentation. In fact, particle sedimentation appears to be a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media, because the viscosity of gas suspension fluids is lower compared to liquid electrophoretic media, allowing electrophoretic particles to settle more rapidly.

[0014] Numerous patents and applications assigned to or in the name of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various techniques used in encapsulated electrophoresis and other electro-optical media. These encapsulated media comprise a number of small capsules, each comprising an inner phase containing particles that are electrophoretically movable in a fluid medium and a capsule wall surrounding the inner phase. Typically, the capsules are retained within a polymer binder to form a coherent layer placed between two electrodes. The techniques described in these patents and applications include the following:

[0015] (a) Electrophoretic particles, fluids and fluid additives; see, exe., U.S. Patents No. 7,002,728 and 7,679,814;

[0016] (b) capsules, binders, and encapsulation processes; see, exe., U.S. Patents No. 6,922,276 and 7,411,719;

[0017] (c) Microcell structures, wall materials and methods for forming microcells; see, for example, U.S. Patents No. 7,072,095 and 9,279,906;

[0018] (d) Methods for microcell filling and sealing; see, for example, U.S. Patent Nos. 7,144,942 and 7,715,088;

[0019] (e) Films and subassemblies containing electro-optical materials; see, for example, U.S. Patents No. 6,982,178 and 7,839,564;

[0020] (f) a method used for backplanes, adhesive layers and other auxiliary layers and displays; see, for example, U.S. Patents No. 7,116,318 and 7,535,624;

[0021] (g) Color forming color control; e.g., U.S. Patent No. 6,017,584; 6,545,797; 6,664,944; 6,788,452; 6,864,875; 6,914,714; 6,972,893; 7,038,656; 7,038,670; 7,046,228; 7,052,571; 7,075,502***; 7,167,155; 7,385,751; 7,492,505; 7,667,684; 7,684,108; 7,791,789; 7,800,813; 7,821,702; 7,839,564***; 7,910,175; 7,952,790; 7,956,841; 7,982,941; 8,040,594; 8,054,526; 8,098,418; 8,159,636; 8,213,076; 8,363,299; 8,422,116; 8,441,714; 8,441,716; 8,466,852; 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; 8,917,439; 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 USPatent Application Publication Nos. 2008 / 0043318; 2008 / 0048970; 2009 / 0225398; 2010 / 0156780; 2011 / 0043543; 2012 / 0326957; 2013 / 0242378; 2013 / 0278995; 2014 / 0055840; 2014 / 0078576; 2014 / 0340430; 2014 / 0340736; 2014 / 0362213; 2015 / 0103394; 2015 / 0118390; 2015 / 0124345; 2015 / 0198858; See 2015 / 0234250; 2015 / 0268531; 2015 / 0301246; 2016 / 0011484; 2016 / 0026062; 2016 / 0048054; 2016 / 0116816; 2016 / 0116818; and 2016 / 0140909.

[0022] (h) a method for driving a display; exemplified by U.S. Patent No. 5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970; 6,900,851; 6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,061,166; 7,061,662; 7,116,466; 7,119,772; 7,177,066; 7,193,625; 7,202,847; 7,242,514; 7,259,744; 7,304,787; 7,312,794; 7,327,511; 7,408,699; 7,453,445; 7,492,339; 7,528,822; 7,545,358; 7,583,251; 7,602,374; 7,612,760; 7,679,599; 7,679,813; 7,683,606; 7,688,297; 7,729,039; 7,733,311; 7,733,335; 7,787,169; 7,859,742; 7,952,557; 7,956,841; 7,982,479; 7,999,787; 8,077,141; 8,125,501; 8,139,050; 8,174,490; 8,243,013; 8,274,472; 8,289,250; 8,300,006; 8,305,341; 8,314,784; 8,373,649; 8,384,658; 8,456,414; 8,462,102; 8,514,168; 8,537,105; 8,558,783; 8,558,785; 8,558,786; 8,558,855; 8,576,164; 8,576,259; 8,593,396; 8,605,032; 8,643,595; 8,665,206; 8,681,191; 8,730,153; 8,810,525; 8,928,562; 8,928,641; 8,976,444; 9,013,394; 9,019,197; 9,019,198; 9,019,318; 9,082,352; 9,171,508; 9,218,773; 9,224,338; 9,224,342; 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 Publications No. 2003 / 0102858; 2004 / 0246562; 2005 / 0253777; 2007 / 0091418; 2007 / 0103427; 2007 / 0176912; 2008 / 0024429; 2008 / 0024482; 2008 / 0136774; 2008 / 0291129; 2008 / 0303780; 2009 / 0174651; 2009 / 0195568; 2009 / 0322721; 2010 / 0194733; 2010 / 0194789; 2010 / 0220121; 2010 / 0265561; 2010 / 0283804; 2011 / 0063314; 2011 / 0175875; 2011 / 0193840; 2011 / 0193841; 2011 / 0199671; 2011 / 0221740; 2012 / 0001957; 2012 / 0098740; 2013 / 0063333; 2013 / 0194250; 2013 / 0249782; 2013 / 0321278; 2014 / 0009817; 2014 / 0085355; 2014 / 0204012; 2014 / 0218277; 2014 / 0240210; 2014 / 0240373; 2014 / 0253425; 2014 / 0292830; 2014 / 0293398; 2014 / 0333685; 2014 / 0340734; 2015 / 0070744; 2015 / 0097877; See 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 hereinafter be referred to as MEDEOD (MEthods for Driving Electro-optic Displays) applications);

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

[0024] (j) A non-electrophoretic display as described in U.S. Patent No. 6,241,921; and U.S. Patent Application Publication No. 2015 / 0277160; and U.S. Patent Application Publications No. 2015 / 0005720 and 2016 / 0012710.

[0025] Many of the aforementioned patents and applications recognize that a wall surrounding a microcapsule separated in an encapsulated electrophoretic medium may be replaced by a continuous phase, thereby producing a so-called polymer-dispersed electrophoretic display comprising a plurality of separated droplets of electrophoretic fluid and a continuous phase of a polymer material, and that the droplets of separated electrophoretic fluid within such a polymer-dispersed electrophoretic display may be considered as capsules or microcapsules even if the separated capsule membrane is not associated with each individual droplet; see, for example, U.S. Patent No. 6,866,760. Accordingly, for the purposes of this application, such a polymer-dispersed electrophoretic medium is considered a subspecies of an encapsulated electrophoretic medium.

[0026] A related type of electrophoretic display is a so-called microcell electrophoretic display. In a microcell electrophoretic display, charged particles and fluids are not encapsulated within microcapsules, but instead are held within a plurality of cavities formed within a carrier medium, typically a polymer film. See, for example, U.S. Patent Nos. 6,672,921 and 6,788,449.

[0027] Electrophoretic media are often opaque (for example, in many electrophoretic media, particles substantially block the transmission of visible light through the display) and operate in a reflective mode, but many electrophoretic displays can be made to operate in a so-called shutter mode where one display state is substantially opaque and another is light-transmitting. See, for example, U.S. Patent Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but depend on variations in electric field strength, may operate in a similar mode; see, for example, U.S. Patent No. 4,418,346. Other types of electro-optical displays may also be capable of operating in a shutter mode. Electro-optical media operating in a shutter mode may be used in multilayer structures for full-color displays; In these structures, at least one layer adjacent to the visible surface of the display operates in shutter mode to expose or hide a second layer further away from the visible surface.

[0028] Encapsulated electrophoretic displays typically do not suffer from the clustering and settling failure modes of conventional electrophoretic devices and provide additional advantages such as the ability to print or coat displays on a wide variety of flexible and rigid substrates. (The use of the word "printing" is intended to include all forms of printing and coating, and, without limitation, pre-measurement coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coatings such as knife-over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; inkjet printing processes; electrophoretic deposition methods (see U.S. Patent No. 7,339,715) and other similar techniques.) Accordingly, the resulting display may be flexible. In addition, since the display medium can be printed (using various methods), the display itself can be manufactured at a low cost.

[0029] As described above, the simplest conventional electrophoretic media essentially display only two colors. Such electrophoretic media may use a single type of electrophoretic particle having a first color in a colored fluid having a second, different color (in which case the first color is displayed when the particle is placed adjacent to the viewing surface of the display, and the second color is displayed when the particle is moved away from the viewing surface), or use first and second types of electrophoretic particles having a first or second color in an uncolored fluid (in which case the first color is displayed when the first type of particle is placed adjacent to the viewing surface of the display, and the second color is displayed when the second type of particle is placed adjacent to the viewing surface). Generally, the two colors are black and white. If a full-color display is desired, a color filter array may be deposited on the viewing surface of a monochromatic (black and white) display. A display with a color filter array creates color stimuli based on area sharing and color blending. The available display area is shared among three or four primary colors, such as Red / Green / Blue (RGB) or Red / Green / Blue / White (RGBW), and the filters can be arranged in a one-dimensional (stripe) or two-dimensional (2x2) repeating pattern. Other selections of primary colors or more than three primary colors are also known in the art. Three (in the case of an RGB display) or four (in the case of an RGBBW display) subpixels are selected ('color blending') that are small enough to be visually blended together into a single pixel having uniform color stimuli at an intended viewing distance.The inherent disadvantage of area sharing is that colorants are always present, and colors can only be adjusted by switching the corresponding pixels of the underlying monochrome display to white or black (switching the corresponding primary color on or off). For example, in an ideal RGBW display, the primary colors red, green, blue, and white each occupy one-quarter of the display area (one of four subpixels), the white subpixel has the same luminance as the white of the underlying monochrome display, and each of the colorant subpixels is no brighter than one-third of the white of the monochrome display. Overall, the luminance of the white color displayed by the display cannot be greater than half the luminance of the white subpixel (the white areas of the display are created by displaying one of four white subpixels and each colorant subpixel having a coloration equivalent to one-third of the white subpixel, and thus the three combined colorant subpixels contribute no more than a single white subpixel). The luminance and saturation of colors are reduced by area sharing, which involves color pixels switched to black. Area sharing is particularly problematic when mixing yellow because it is brighter than any other color of the same luminance, and saturated yellow has nearly the same luminance as white. Switching blue pixels (one-quarter of the display area) to black makes the yellow too dark.

[0030] U.S. Patent Nos. 8,576,476 and 8,797,634 describe a multicolor electrophoretic display having a single backplane comprising independently addressable pixel electrodes and a common, light-transmitting front electrode. Multiple electrophoretic layers are disposed between the backplane and the front electrode. The displays described in these applications can render any of the primary colors (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel location. However, there are disadvantages to the use of multiple electrophoretic layers positioned between a single set of addressing electrodes. The electric field experienced by a particle in a particular layer is lower than in the case of a single electrophoretic layer addressed with the same voltage. Additionally, optical loss in the electrophoretic layer closest to the viewing surface (caused, for example, by light scattering or unwanted absorption) may affect the appearance of the image formed on the underlying electrophoretic layers.

[0031] Attempts have been made to provide full-color electrophoretic displays using a single electrophoretic layer. For example, U.S. Patent No. 8,917,439 describes a color display comprising one or two types of pigment particles dispersed in a clear and colorless or colored solvent, wherein the electrophoretic fluid is positioned between a common electrode and a plurality of pixels or driving electrodes. The driving electrodes are arranged to expose a background layer. U.S. Patent No. 9,116,412 describes a method for driving a display cell charged with an electrophoretic fluid comprising two types of charged particles having two contrasting colors and opposite charge polarities. The two types of pigment particles are dispersed in the colored solvent or in a solvent in which uncharged or slightly charged colored particles are dispersed therein. The method comprises the step of driving the display cell to display the color of the solvent or the color of the uncharged or slightly charged colored particles by applying a driving voltage that is about 1 to 20% of the total driving voltage. U.S. Patents No. 8,717,664 and 8,964,282 describe an electrophoretic fluid and a method for driving an electrophoretic display. The fluid comprises pigment particles of types 1, 2, and 3, all of which are dispersed in a solvent or a solvent mixture. The pigment particles of types 1 and 2 have opposite charge polarities, and the pigment particles of type 3 have a charge level less than about 50% of the charge level of type 1 or 2. The three types of pigment particles have different levels of threshold voltage, or different levels of mobility, or both. None of these patent applications disclose a full-color display in the sense in which the term is used below. means of solving the problem

[0032] In this specification, an improved driving method for a full-color electrophoretic display and full-color electrophoretic displays using the driving method are disclosed. In one embodiment, the present invention comprises a color electrophoretic display, which comprises a light-transmitting electrode on a viewing surface, a backplane comprising an array of thin-film transistors coupled to pixel electrodes—each thin-film transistor comprising a layer of metal oxide semiconductor—and a color electrophoretic medium disposed between the light-transmitting electrode and the backplane. A color electrophoretic medium comprises (a) a fluid, (b) a plurality of first particles and a plurality of second particles dispersed in the fluid - the first particles and the second particles have opposite polarity charges, the first particles are light-scattering particles, and the second particles have one of subtractive primary colors -, (b) a plurality of third particles and a plurality of fourth particles dispersed in the fluid - the third particles and the fourth particles have opposite polarity charges, and each of the third particles and the fourth particles has a subtractive primary color that is different from each other and different from the second particles -.

[0033] In some embodiments, the first electric field required to separate aggregates formed by the third and fourth types of particles is greater than the second electric field required to separate aggregates formed from any two other types of particles. In some embodiments, at least two of the second, third, and fourth particles are non-light-scattering. In some embodiments, the first particles are white, and the second, third, and fourth particles are non-light-scattering. In some embodiments, the first and third particles are negatively charged, and the second and fourth particles are positively charged. In some embodiments, the first, second, third, and fourth particles are each white, cyan, yellow, and magenta, respectively, and the white and yellow particles are negatively charged, and the magenta and cyan particles are positively charged. In some embodiments, yellow, magenta, and cyan pigments exhibit diffuse reflectances of less than 2.5% at 650, 550, and 450 nm, respectively, measured on a black background when the pigments are distributed approximately isotropically at 15 volume% in a 1 µm thick layer containing a liquid and pigment having a refractive index of less than 1.55. In some embodiments, the liquid is a nonpolar liquid having a dielectric constant of about 5. In some embodiments, the fluid dissolves or disperses a polymer that is essentially non-absorbent on particles having a number average molecular weight exceeding about 20,000. In some embodiments, the metal oxide semiconductor is indium gallium zinc oxide (IGZO). The above inventions may be incorporated into electronic book readers, portable computers, tablet computers, cellular phones, smart cards, signs, watches, shelf labels, or flash drives.

[0034] In another embodiment, the color electrophoretic display comprises a controller, a light-transmitting electrode on a viewing surface, and a backplane comprising an array of thin-film transistors coupled to pixel electrodes, each thin-film transistor comprising a layer of metal oxide semiconductor. A color electrophoretic medium is disposed between the light-transmitting electrode and the backplane, and the color electrophoretic medium comprises (a) a fluid, (b) a plurality of first particles and a plurality of second particles dispersed in the fluid—the first particles and the second particles have charges of opposite polarity, the first particles are light-scattering particles, and the second particles have one of subtractive primary colors—and (c) a plurality of third particles and a plurality of fourth particles dispersed in the fluid—the third particles and the fourth particles have charges of opposite polarity, and the third particles and the fourth particles each have subtractive primary colors that are different from each other and different from the second particles. The controller is configured to provide multiple driving voltages to the pixel electrodes, so that white, yellow, red, magenta, blue, cyan, green, and black may be displayed at each pixel electrode while maintaining the light-transmitting electrode at a constant voltage. In some embodiments, the controller is configured to provide the pixel electrodes a voltage greater than 25 volts and less than -25 volts. In some embodiments, the controller is configured to additionally provide a voltage between 25 V and 0 V and a voltage between -25 V and 0 V. In some embodiments, the metal oxide semiconductor is indium gallium zinc oxide (IGZO).

[0035] In another embodiment, the color electrophoretic display comprises a controller, a light-transmitting electrode on a viewing surface, a backplane electrode, and a color electrophoretic medium disposed between the light-transmitting electrode and the backplane electrode. The color electrophoretic medium comprises (a) a fluid, (b) a plurality of first particles and a plurality of second particles dispersed in the fluid—the first particles and the second particles have charges of opposite polarity, the first particles are light-scattering particles, and the second particles have one of subtractive primary colors—and (c) a plurality of third particles and a plurality of fourth particles dispersed in the fluid—the third particles and the fourth particles have charges of opposite polarity, and the third particles and the fourth particles each have subtractive primary colors that are different from each other and different from the second particles. The controller is configured to apply a first high voltage and a first low voltage to a light-transmitting electrode and a second high voltage, a zero voltage, and a second low voltage to a backplane electrode so that colors, white, yellow, red, magenta, blue, cyan, green, and black, can be displayed on a visible surface, wherein at least one of the first high voltage, the first low voltage, the second high voltage, and the second low voltage is not the same. In some embodiments, the magnitude of the first high voltage is the same as the magnitude of the second high voltage. In some embodiments, the magnitude of the first low voltage and the magnitude of the second low voltage are the same, and the magnitude of the first high voltage and the magnitude of the first low voltage are not the same.

[0036] In another embodiment, the color electrophoretic display comprises a controller; a light-transmitting electrode on a viewing surface, a backplane electrode, and a color electrophoretic medium disposed between the light-transmitting electrode and the backplane electrode. The color electrophoretic medium comprises (a) a fluid, (b) a plurality of first particles and a plurality of second particles dispersed in the fluid, wherein the first particles and the second particles have opposite polarity charges, the first particles are light-scattering particles, and the second particles have one of subtractive primary colors; and (c) a plurality of third particles and a plurality of fourth particles dispersed in the fluid—the third particles and the fourth particles have opposite polarity charges, and the third particles and the fourth particles each have a subtractive primary color that is different from each other and different from the second particles. The controller is configured to provide one of a plurality of time-dependent driving voltages to the backplane electrode, while providing the following driving voltages to the light-transmitting electrode, 1) high voltage during the first time, low voltage during the second time, and high voltage during the third time, or 2) low voltage during the first time, high voltage during the second time, and high voltage during the third time, so as to cause colors, white, yellow, red, magenta, blue, cyan, green, and black, to be displayed on the visible surface. Brief explanation of the drawing

[0037] FIG. 1 is a schematic cross-sectional view showing the positions of various colored particles in the electrophoretic medium of the present invention when displaying black, white, three subtractive primary, and three additive primary colors. Figure 2a illustrates four different types of pigment particles used in a multi-particle electrophoresis medium in a schematic form. Figure 2b illustrates, in schematic form, four different types of pigment particles used in a multi-particle electrophoresis medium. Figure 3 shows an exemplary equivalent circuit of a single pixel of an electrophoretic display. Figure 4 illustrates the layers of an exemplary electrophoretic color display. FIG. 5 illustrates an exemplary push-pull driving scheme for addressing an electrophoretic medium containing three photosensitive particles and scattering (white) particles. FIG. 6 is an exemplary waveform from the prior art including a two-part reset phase (A) and a color transition phase (B), which is achieved using top-plane switching. Figure 7 illustrates the correlation between the available color gamut and the number of dipoles ("flash") in an update of a 4-particle full-color electrophoretic display. Figure 8 illustrates simplified top-plane driving waveforms for generating eight colors in an electrophoretic medium containing three photosensitive particles and scattering (white) particles. Figure 9 illustrates the calculated effect of modifying the top-plane voltage rails by +2V in an electrophoretic medium containing three subtractive particles and scattering (white) particles. Figure 10 illustrates the calculated effect of modifying backplane voltage rails by +2V in an electrophoretic medium containing three subtractive particles and scattering (white) particles. Figure 11 illustrates the calculated effect of modifying both the top-plane voltage rails and the back-plane voltage rails by +2V and -2V in an electrophoretic medium containing three subtractive particles and scattering (white) particles. Specific details for implementing the invention

[0038] The present invention comprises improved methods for driving a 4-particle electrophoretic medium, wherein at least two of the particles are colored and decolored and at least one of the particles is scattered. Typically, such a system comprises white particles and particles colored with cyan, yellow, and magenta decolored primary colors. Such a system is schematically illustrated in FIG. 1, which can provide white, yellow, red, magenta, blue, cyan, green, and black at all pixels.

[0039] In the case of ACeP, each of the eight major colors (red, green, blue, cyan magenta, yellow, black, and white) corresponds to a different arrangement of four pigments, and thus the viewer sees only these colored pigments located on the viewing side of the white pigment (i.e., the only pigment that scatters light). It has been found that the waveform for classifying the four pigments into the appropriate configuration to create these colors requires at least five voltage levels (high positive, low positive, zero, low negative, and high negative levels). Refer to Fig. 1. To realize a wider range of colors, additional voltage levels must be used for finer control of the pigments. The present invention provides several improved methods for driving such electrophoretic media so that they refresh pixel colors at a faster speed and with less flash, resulting in a more satisfactory color spectrum for the viewer.

[0040] The three particles providing the three subtractive primary colors may be substantially non-light-scattering ("SNLS"). The use of SNLS particles enables the mixing of colors and provides more color results than can be obtained 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. Furthermore, addressing the colored particle with the highest threshold also shifts all other colored particles, and these other particles must subsequently be switched to desired positions at lower voltages. This stepwise color addressing method results in the flashing of unwanted colors and long transition times. The present invention does not require the use of such stepwise waveforms, and addressing for all colors can be achieved with only two positive and two negative voltages as described below (i.e., only five different voltages, two positive voltages, two negative voltages, and zero voltage are required for the display, but as described below in a certain embodiment, it may be preferable to use more different voltages to address the display).

[0041] As previously mentioned, Figure 1 of the attached drawings is a schematic cross-sectional view showing the positions of various particles in the electrophoretic medium of the present invention when displaying black, white, three subtractive primary colors, and three additive primary colors. In Figure 1, the viewing surface of the display is assumed to be at the top (as shown), that is, the user views the display from this direction, and light is assumed to be incident from this direction. As previously mentioned, in a preferred embodiment, only one of the four particles used in the electrophoretic medium of the present invention substantially scatters light, and in Figure 1, this particle is assumed to be a white pigment. This light-scattering white particle forms a white reflector and, in contrast to it, any particles on top of the white particle are visible (as shown in Figure 1). Light entering the viewing surface of the display passes through these particles, is reflected from the white particles, passes through these particles again, and exits the display. Thus, the particles on top of the white particles may absorb various colors, and the colors appearing to the user are derived from the combination of particles on top of the white particles. Any particle placed below the white particles (behind from the user's viewpoint) is obscured by the white particles and does not affect the displayed color. Since the second, third, and fourth particles are substantially non-light-scattering, their order or arrangement relative to each other is not important, but for the reasons already mentioned, their order or arrangement relative to the white (light-scattering) particles is important.

[0042] More specifically, when cyan, magenta, and yellow particles are placed below the white particles (Situation [A] in FIG. 1), there are no particles above the white particles, and the pixel simply displays a white color. When a single particle is placed above the white particles, the color of that single particle is displayed as yellow, crimson, and cyan in Situations [B], [D], and [F] in FIG. 1, respectively. When two particles are placed above the white particles, the displayed color is a combination of the colors of these two particles; in FIG. 1, in Situation [C], the crimson and yellow particles display a red color, in Situation [E], the cyan and crimson particles display a blue color, and in Situation [G], the yellow and cyan particles display a green color. Finally, when all three colored particles are placed above the white particles (Situation [H] in FIG. 1), all incident light is absorbed by the three subtractive primary colored particles, and the pixel displays a black color.

[0043] Since a single subtractive primary color can be rendered by light-scattering particles, it is possible for the display to contain two types of light-scattering particles, one white and the other colored. However, in this case, the position of the light-scattering colored particles relative to the other colored particles placed above the white particles is important. For example, when rendering a color as black (when all three colored particles are placed above the white particles), the scattering colored particles cannot be placed above the non-scattering colored particles (otherwise, they would be partially or completely hidden behind the scattering particles, and the rendered color would be the color of the scattering colored particles, not black).

[0044] It is not easy to render a color black when more than one type of colored particle scatters light.

[0045] FIG. 1 illustrates an ideal situation where colors are uncontaminated (i.e., light-scattering white particles completely obscure any particles behind them). In reality, obscuration by white particles is imperfect, so there may be some light absorption by particles that are ideally completely obscured. Such contamination typically reduces both the lightness and chroma of the rendered colors. In the electrophoretic medium of the present invention, such color contamination must be minimized to the extent that the colors formed correspond to industry standards for color rendition. A particularly preferred standard is SNAP (the standard for newspaper advertising production), which specifies L*, a*, and b* values ​​for each of the eight primary colors mentioned above. (Hereafter, "primary colors" are used to refer to the eight colors, black, white, three subtractive primary colors, and three additive primary colors as illustrated in FIG. 1.)

[0046] FIGS. 2a and 2b illustrate schematic cross-sectional views of four pigment types (1-4; 5-8) used in preferred embodiments of the present invention. In FIG. 2a, the polymer shell adsorbed onto the core pigment is shown in dark shading, while the core pigment itself is shown unshaded. A wide range of forms may be used for the core pigment: spherical, acicular, or otherwise non-isoaxial, aggregates of smaller particles (i.e., "grape clusters"), and composite particles containing small pigment particles or dyes dispersed in a binder are widely known in the art. The polymer shell may be a covalently bonded polymer prepared by grafting processes or chemical adsorption as widely known in the art, or it may be physically adsorbed onto the surface of the particles. For example, the polymer may be a block copolymer containing insoluble and soluble segments. Several methods of attaching the polymer shell to the core pigments are described in the examples below.

[0047] In the embodiment of FIG. 2a, the first and second particle types preferably have more substantial polymer shells than the third and fourth particle types. The light-scattering white particles are of the first or second type (charged either negatively or positively). In subsequent discussion, the white particles are assumed to have a positive charge (i.e., to be of type 1), but it will be obvious to those skilled in the art that the described general principles apply to the set of particles in which the white particles are positively charged.

[0048] Additionally, as shown in FIG. 2b, the first and second particle types do not need to have differential polymer shells compared to the third and fourth particle types. As shown in FIG. 2b, sufficient differential charge on the four particles will allow for electrophoretic control of the particles and the generation of the desired color on the visible surface. For example, particle 5 may have a larger negative charge than particle 7, while particle 6 may have a larger positive charge compared to particle 8. It is also possible to use other combinations of polymer functional groups and charges (or particle sizes); however, this must be the case where all four particles can be separated from each other in the presence of appropriate electric fields, for example, a lower voltage low field that can be manufactured with commercial digital electronic devices.

[0049] In the system of FIG. 2a, the electric field required to separate aggregates formed from a mixture of particles of types 3 and 4 in a suspension solvent containing a charge control agent is greater than the electric field required to separate aggregates formed from any other combination of particles of the two types. On the other hand, the electric field required to separate aggregates formed between particles of types 1 and 2 is smaller than the electric field required to separate aggregates formed between particles of types 1 and 4 or between particles of types 2 and 3 (and, of course, smaller than the electric field required to separate particles of types 3 and 4).

[0050] In FIG. 2a, the core pigments containing the particles are shown to have approximately the same size, and the zeta potential of each particle is assumed to be approximately the same, although not shown. The thickness of the polymer shell surrounding each core pigment is different. As shown in FIG. 2a, this polymer shell is thicker for Type 1 and 2 particles than for Type 3 and 4 particles.

[0051] To obtain a high-resolution display, individual pixels of the display must be addressable without interference from adjacent pixels. One method to achieve this objective is to provide an array of nonlinear elements, such as transistors or diodes, having at least one nonlinear element associated with each pixel, in order to manufacture an "active matrix" display. An addressing or pixel electrode addressing a single pixel is connected to a suitable voltage source through the associated nonlinear element. Typically, when the nonlinear element is a transistor, the pixel electrode is connected to the drain of the transistor, and while this arrangement will be assumed in the following description, it is essentially arbitrary, and the pixel electrode may be connected to the source of the transistor. Typically, in a high-resolution array, pixels are arranged in a two-dimensional array of rows and columns, so that any particular pixel is uniquely defined by the intersection of one designated row and one designated column. The sources of all transistors in each column are connected to a single column electrode, while the gates of all transistors in each row are connected to a single row electrode; Again, the assignment of sources to rows and gates to columns is conventional but essentially arbitrary and can be reversed if desired. The row electrode is connected to a row driver, which essentially ensures that only one row is selected at any given moment, that is, that a selection voltage is applied to the selected row electrode to ensure that all transistors in the selected row are conducting, while a non-selection voltage is applied to all other rows to ensure that all transistors in these non-selected rows remain in a non-conducting state.Column electrodes are connected to column drivers, which apply selected voltages to various column electrodes to drive pixels in a selected row to a desired optical state. (The aforementioned voltages are provided on the opposite side of the electro-optical medium from a conventional non-linear array and are for a common front electrode extending across the entire display.) After a pre-selected interval known as the "line address time," the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed to write the next line of the display. This process is repeated so that the entire display is written row by row.

[0052] Conventionally, each pixel electrode has an associated capacitor electrode such that the pixel electrode and the capacitor electrode form a capacitor; see, for example, International Patent Application WO 01 / 07961. In some embodiments, a transistor may be formed using an N-type semiconductor (e.g., amorphous silicon), and the “select” and “non-select” voltages applied to the gate electrode may be positive and negative, respectively.

[0053] FIG. 3 of the attached drawings illustrates an exemplary equivalent circuit of a single pixel of an electrophoretic display. As indicated, the circuit includes a capacitor (10) formed between the pixel electrode and the capacitor electrode. The electrophoretic medium (20) is represented as a capacitor and a resistor in parallel. In some examples, the direct or indirect coupling capacitance (30) (commonly referred to as "parasitic capacitance") between the gate electrode of the transistor associated with the pixel and the pixel electrode may generate unwanted noise in the display. Typically, the parasitic capacitance (30) is much smaller than that of the storage capacitor (10), and when pixel rows of the display are selected or deselected, the parasitic capacitance (30) may result in a small negative offset voltage on the pixel electrode, also known as a "kickback voltage," which is usually less than 2 volts. In some embodiments, to compensate for the unwanted "kickback voltage," a common potential (V com ) may also be supplied to the capacitor electrode and top-plane electrode associated with each pixel, V com This kickback voltage (V KB When set to the same value, all voltages supplied to the display may be offset by the same amount, and a net DC-imbalance is not experienced.

[0054] However, V com Problems may occur when this kickback voltage is set to an uncompensated voltage. This may happen when attempting to apply a voltage to the display higher than is available only at the backplane. If the backplane is supplied with a choice of nominal +V, 0, or -V, for example, V comIt is well known in the art that while -V is supplied, the maximum voltage applied to the display may double. In this case, the maximum voltage experienced (i.e., from the backplane relative to the top plane) is +2V, while the minimum voltage is 0. If a negative voltage is required, V com The potential must rise to at least zero. Therefore, the waveforms used to address the display with positive and negative voltages using top-plane switching are more than one V com Each voltage setting must have specific frames assigned to it.

[0055] A set of waveforms for driving a color electrophoretic display having four particles is described in U.S. Patent No. 9,921,451, incorporated herein by reference. In U.S. Patent No. 9,921,451, seven different voltages: three positive voltages, three negative voltages, and a voltage of 0 are applied to the pixel electrodes. However, in some embodiments, the maximum voltage used for these waveforms is higher than that which can be handled by an amorphous silicon thin-film transistor. In these cases, a suitable high voltage can be obtained by using top-plane switching. (As described above) V com This is intentionally V KB When set to , a separate power supply may be used. However, when top-plane switching is used, V com Using a large number of separate power supplies is costly and inconvenient given the number of settings. Additionally, top-plane switching is known to increase kickback and thereby degrade the stability of color states.

[0056] A display device may be constructed using the electrophoretic fluid of the present invention by various methods known in the prior art. The electrophoretic fluid may be encapsulated in microcapsules or contained within microcell structures, which may then be sealed with a polymer layer. Microcapsule or microcell layers may be coated or embossed on a plastic substrate or film having a transparent coating of an electrically conductive material. This assembly may be laminated to a backplane having pixel electrodes using an electrically conductive adhesive. Alternatively, the electrophoretic fluid may be directly dispersed onto a thin open-cell grid arranged on a backplane containing an active matrix of pixel electrodes. The charged grid may then be top-sealed with an integrated protective sheet / light-transmitting electrode.

[0057] FIG. 4 illustrates a schematic cross-sectional view (not shown in scale) of a display structure (200) suitable for use with the present invention. In the display (200), the electrophoretic fluid is exemplified as being limited to microcups, but equivalent structures including microcapsules may also be used. A substrate (202), which may be glass or plastic, supports pixel electrodes (204) associated with thin-film transistors in an individually addressed segment or active matrix array. (The combination of the substrate (202) and the electrodes (204) is typically referred to as the back-plane of the display.) A layer (206) is an optional dielectric layer according to the present invention applied to the backplane. (A method for forming a suitable dielectric layer is described by reference in U.S. Patent Application No. 16 / 862,750.) The front-plane of the display comprises a transparent substrate (222) supporting a transparent electrically conductive coating (220). The electrode layer (220) placed on top is an optional dielectric layer (218). The layer (or layers) (216) is a polymer layer(s) that may include a primer layer for attaching microcups to the transparent electrode layer (220) and some residual polymer comprising the bottom of the microcups. The walls of the microcups (212) are used to contain an electrophoretic fluid (214). The microcups are sealed with the layer (210), and the entire front-plane structure is attached to the backplane using an electrically conductive adhesive layer (208). A method for forming the microcups is described in the prior art, for example, U.S. Patent No. 6,930,818. In some cases, the microcups have a depth of less than 20 µm, for example, a depth of less than 15 µm, for example, a depth of less than 12 µm, for example, a depth of about 10 µm, for example, a depth of about 8 µm.

[0058] Most commercial electrophoretic displays use amorphous silicon-based thin-film transistors (TFTs) in the configuration of active matrix backplanes (202 / 024) due to the cost of various starting materials and the wide availability of manufacturing facilities. Unfortunately, amorphous silicon thin-film transistors become unstable when supplied with gate voltages that allow switching of voltages higher than approximately + / - 15 V. Nevertheless, as described below, the performance of ACeP is improved when the magnitude of high positive and negative voltages is allowed to exceed + / - 15 V. Thus, improved performance is achieved by additionally changing the bias of the top light-transmitting electrode with respect to the bias on the backplane pixel electrodes, also known as top-plane switching, as described in previous disclosures. Thus, if a voltage of +30 V (relative to the backplane) is required, the top-plane can be switched to -15 V, while the appropriate backplane pixel is switched to +15 V. Methods for driving a 4-particle electrophoresis system having top-plane switching are described in more detail, for example, in U.S. Patent No. 9,921,451.

[0059] There are several disadvantages to the top-plane switching approach. First, when the top-plane is not pixelated (as is typical) and is a single electrode extending across the entire surface of the display, its electrical potential affects all pixels of the display. If this voltage is set to match one of the maximum voltages available from the backplane (e.g., the maximum positive voltage), there will be no net voltage across the link ends when this electrode is asserted on the backplane. When any other available voltage is supplied to the backplane, there will always be a negative polarity voltage supplied to any pixel within the display. Therefore, if a waveform requires a positive voltage, it cannot be supplied to any pixel until the top-plane voltage is changed. A typical waveform for use in a multicolor display of the third embodiment uses multiple pulses of both positive and negative polarity, and the lengths of these pulses are not the same as the lengths in the waveforms used to create different colors. Furthermore, the phase of the waveform can differ for different colors: that is, a positive pulse may lead a negative pulse for some colors, while a negative pulse may lead a positive pulse for others. To accommodate these cases, "stops" (i.e., pauses) must be constructed into these waveforms. In practice, this results in waveforms that are much longer (by a factor of 2) than are ideally required.

[0060] Secondly, in top-plane switching, there are limitations on the voltage levels that can be selected. The voltages applied to the top plane are V t+ and V t- It is individually denoted as, and the voltage applied to the backplane is V b+ and V b- If denoted as respectively, |V to realize zero-volt conditions across both ends of the electrophoretic fluid t+ | = |V b+ | and |Vt- | = |V b- This must be true. However, the magnitudes of the positive and negative voltages do not necessarily have to be the same.

[0061] In the prior embodiments of ACeP (Advanced Color electronic Paper), the waveform (voltage versus time curve) applied to the pixel electrode of the backplane of the display of the present invention is described and plotted, while the front electrode is assumed to be grounded (i.e., at zero potential). The electric field experienced by the electrophoretic medium is determined by the potential difference between the backplane and the front electrode and the distance separating them. The display is typically viewed through its front electrode, where particles adjacent to the front electrode control the color displayed by the pixel, and where it is easier to understand the optical transitions that accompany the potential of the front electrode relative to the backplane; this can be done simply by inverting the waveforms discussed below.

[0062] These waveforms are +V, where each pixel of the display is exemplified as 30V, 15V, 0, -15V, and -30V. high , +V low , 0, -V low and -V high It requires that it be able to be driven at five different addressing voltages specified. In practice, using a larger number of addressing voltages may be desirable. If only three voltages are available (i.e., +V high , 0, and -V high ), lower voltage (i.e., V high The same result as addressing at / n, where n is a positive integer > 1, voltage V high However, it may be possible to achieve this by addressing with pulses, but with a duty cycle of 1 / n.

[0063] Figure 5 illustrates the typical waveforms (in a simplified form) used to drive the 4-particle color electrophoretic display system described above. These waveforms have a "push-pull" structure: that is, they consist of a dipole containing two pulses of opposite polarity. The magnitude and length of these pulses determine the acquired color. At least five such voltage levels must be present. Figure 5 illustrates high and low positive and negative voltages as well as 0 volts. Typically, "Low" (L) refers to a range of about 5–15 V, while "High" (H) refers to a range of about 15–30 V. Generally, the higher the magnitude of the "High" voltages, the better the color gamut achieved by the display. The addition "medium" (M) level is typically about 15 V; however, the value of M will depend somewhat on the composition of the particles as well as the environment of the electrophoretic medium.

[0064] Although Fig. 5 illustrates the simplest dipoles required to form colors, it is understood that actual waveforms may be multiple repetitions of these patterns or other patterns that are non-periodic and use more than five voltage levels.

[0065] Therefore, typical driving voltages require driving electronics to provide as many as seven different voltages (+H, +M, +L, 0, -L, -M, -H) to the data lines during the update of a selected pixel of the display. Although multi-level source drivers capable of delivering seven different voltages are available, the most commercially available source drivers for electrophoretic displays allow only three different voltages (typically positive voltage, zero, and negative voltage) to be delivered during a single frame. Thus, as previously discussed, if the three voltages (typically +V, 0, and -V) supplied to the backplane can be changed from one frame to the next, it is necessary to modify the typical waveforms of Fig. 5 to accommodate a three-level source driver architecture. The remaining voltage levels can be realized by using a "top-plane switching" driving method, where the light-transmitting (top-plane) common electrode can be switched between -V, 0, and +V, while the voltage applied to the pixel electrode can also vary from -V, 0 to +V, where pixel transitions in one direction are handled when the common electrode is at 0, and transitions in the other direction are handled when the common electrode is at +V.

[0066] Of course, achieving the desired color with the driving pulses of FIG. 5 depends on the particles starting the process from a known state, which is unlikely to be the last color displayed on the pixel. Therefore, a series of reset pulses precedes the driving pulses, which increases the amount of time required to update the pixel from the first color to the second color. The reset pulses are described in more detail in U.S. Patent No. 10,593,272 incorporated by reference. The lengths of these pulses (refresh and address) and the lengths of any remaining ones (i.e., periods of zero voltage between them) may be selected so that the entire waveform (i.e., the integral of voltage over time over the entire waveform) becomes DC balanced (i.e., the integral of voltage over time becomes zero). DC balance can be achieved by adjusting the pulse lengths and the remainder in the reset phase, so that the net impulse supplied in the reset phase has the same magnitude and opposite sign to the net impulse supplied in the address phase, and during that page, the display switches to a specific desired color.

[0067] Additionally, the preceding discussion of waveforms, and in particular the discussion of DC balance, ignores the kickback voltage issue. In fact, as previously explained, all backplane voltages are equal to the kickback voltage V KB It is offset from the voltage supplied by the power supply by the same amount. Therefore, if the power supply used provides three voltages +V, 0, and -V, the backplane is actually V+V KB , V KB , and -V+V KB Receives voltage (V KB Note that is usually negative for amorphous silicon TFTs). However, the same power supply provides +V, 0, and -V to the front electrode without kickback voltage offset. Therefore, for example, when -V is supplied to the front electrode, the display is 2V + V KBThe maximum voltage and V KB You will experience the minimum value of. Using a separate power supply, which can be costly and inconvenient, V on the front electrode. KB Instead of supplying, positive voltage, negative voltage, and V to the front electrode KB The waveform can also be divided into supplied sections. In addition to kickback,

[0068] In commercial embodiments using amorphous silicon transistor control, seven different voltages: three positive, three negative, and zero voltages may be applied to the pixel electrodes. Once the complexity of top-plane switching, reset pulses, and DC balancing is taken into account, the resulting waveforms are somewhat complex. For example, FIG. 6, taken from U.S. Patent No. 10,593,272, schematically illustrates one such waveform used to display a single color. As illustrated in FIG. 6, for each color, the waveforms have the same basic forms: that is, the waveform is uniquely DC-balanced and may include the following two sections or phases; (1) a preliminary series of frames provided with a DC impedance equal to and opposite to the DC impedance of the remainder of the waveform, used to provide a "reset" of the display to a state where any color can be reproducibly obtained, and (2) a series of frames specific to the color to be rendered; For comparison, refer to sections A and B of the waveform shown in Fig. 6.

[0069] During the first "Reset" phase, the display reset ideally erases any memory of the previous state, including residual voltage and pigment configurations characteristic of previously displayed colors. This erasure is most effective when the display is addressed at the maximum possible voltage during the "Reset / DC Balancing" phase. Additionally, sufficient frames may be allocated during this phase to enable balancing of the most unbalanced color transitions. Because some colors require positive DC balance in the second section of the waveform and others require negative balance in about half of the frames of the "Reset / DC Balancing" phase, the front electrode voltage V com is V pH It is set to (allowing the maximum possible negative voltage between the backplane and the front electrode), and in the remainder, V com This V nH It is set to (allowing the maximum possible amount of voltage between the backplane and the front electrode). Empirically, V com = V nH Frames V com = V pH It was found that it is desirable to lead by frames.

[0070] The "desired" waveform (i.e., the curve of the actual voltage over the time desirable to apply across the electrophoretic medium) is exemplified in the lower part of FIG. 6, and its implementation by top-plane switching is illustrated above, where the potentials (V) applied to the front electrode and the backplane (BP) are com ) is exemplified. The following voltages: V pH , V nH (Generally the highest positive and negative voltage in the range of ± 10-15V), V pL , V nLIt is assumed that a column driver is used, connected to a power supply capable of supplying (typically lower positive and negative voltages in the range of ± 1 - 10V), and zero. In addition to these voltages, the kickback voltage (V KB ) (a small value specific to the particular backplane used, measured, for example, as described in U.S. Patent No. 7,034,783) can be supplied to the front electrode by an additional power supply. As shown in FIG. 6, all backplane voltages are V from the voltage supplied by the power supply. KB Although offset by (shown as a negative number), the front electrode voltage is, as previously mentioned, explicitly V of the front electrode KB Except when set to , it is not offset in that way.

[0071] Based on feedback from potential users, it was determined that the driving pulses (waveforms) as illustrated in FIG. 6 are too long for A) and too flashy for B). ("Flash" refers to an excessive number of end-state pigment problems known as "dipoles" during an update. As the number of dipoles per update increases, the viewer is likely to perceive the display as "flashing" even though no light is emitted from the display.)

[0072] Simplified top-plane switching

[0073] To reduce the update time length and the number of flashes, the complexity of front-plane switching can be reduced by exchanging for a smaller number of available colors. (The calculation of the available color gamut as a function of the number of dipoles ("flashes") is presented in FIG. 7. Additionally, since particles have a finite velocity within the electrophoretic medium, the amount of time the dipole is applied also affects the size of the color gamut.

[0074] Fig. 8 illustrates this solution (top left panel) using a simplified top-plane switching pulse sequence, and the simplified backplane pulse sequence (left; bottom) is matched to a single top-plane sequence to provide at least discrete colors. The top plane switches between two voltages, one positive and one negative, while the bottom plane can take three different voltages: positive voltage, negative voltage, and zero voltage. (In Fig. 8, the voltage levels are relative, i.e., 1, 0, -1, but in many cases, they would actually be 15V, 0, and -15V, as is typical with commercial backplanes containing amorphous silicon thin-film transistors.) The eight color sequences of Fig. 5 are achieved by subtracting the top-plane pulse sequence from the backplane pulse sequence (left side of Fig. 8) (right side of Fig. 8). For the pulse sequences of FIGS. 5 and FIGS. 8, it is understood that the electrophoretic fluid comprises a negatively charged white pigment, a positively charged magenta pigment, and a cyan pigment, and that the yellow pigment may be positively or negatively charged or be essentially neutral. Other color / charge combinations are possible, and the waveforms can be adjusted accordingly.

[0075] As previously discussed, at least five different voltages are required in the waveforms of FIG. 8. In an active matrix driving environment, this can be realized by (a) supplying a selection of five different voltages to the columns when a specific row is selected at a specific time, or by (b) providing a selection of several (i.e., three) different voltages to the columns when a specific row is selected at the first time, and a selection of a different set of voltages to the columns when the same row is selected at the second time, or by (c) providing the same selection of three voltages to the columns at both the first and second times, but changing the potential of the front electrode between the first and second times. Option (c) is particularly useful when at least one of the voltages required to be supplied is higher than what the backplane electronics can support.

[0076] Since high positive and high negative potentials cannot be asserted simultaneously with top-plane switching, it is necessary to offset the + / - dipoles of the top-plane relative to the - / + dipoles of the backplane. In the waveform shown in FIG. 8, only one dipole exists per transition. This provides the smallest possible "flash" waveform, as each dipole results in two visible optical changes to the display. In cases where five different voltage levels can be supplied to the backplane electrodes when each row is selected, and the backplane electronics can support the required maximum voltages, it is not necessary to offset the dipoles in the manner shown in FIG. 8.

[0077] Drive using modified rail voltage

[0078] For the driving sequence of Fig. 8, the voltages applied to the top plane are V, respectively. t+ and V t- It is denoted as , and the voltage applied to the backplane is V b+ and V b- If, |V t+ | = |Vt- | = |V b+ | = |V b- | = V. Accordingly, as is typical in commercial backplanes, the maximum supply voltage is + / - 15 volts, and the voltages across the electrophoretic medium are 30 V, 28 V, 0 V, -28 V, and -30 V.

[0079] However, the maximum voltage magnitudes (i.e., "rails") of the top-plane electrode and the back-plane electrode do not need to be identical. For example, rail voltage offsets can be calculated from some nominal maximum voltage magnitude value V. The offset for each rail may be denoted as w, x, y, and z, while the zero voltage rail is considered to remain at 0 and not applied to the top plane. (That is, as shown in FIG. 8, the backplane is high, low, and zero, whereas the top plane is only high and low.)

[0080] Therefore, it is as follows:

[0081]

[0082]

[0083] Referring to the backplane voltage, the three different negative voltages of high, medium, and low magnitudes are that the top plane is V H- , V M- , and V L- V marked as t+ It may also be applied to the electrophoretic ink when set to (i.e., V b - V t , here, V b It can take any of the three values ​​shown above).

[0084] These voltages are as follows.

[0085]

[0086] The top plane is V t- When set to , the available voltages are as follows:

[0087]

[0088] Intermediate voltage V when w = x = y = z M+ and V M- It is evident that only [the voltage] is affected by the offset. Therefore, while the intermediate voltages are each reduced by the amount of the offset (assumed to be positive), the high voltage magnitudes can be maintained at 2V and the zero voltages at 0. The difference between the two intermediate voltages will always be 2V.

[0089] For example, in a 5-level drive system, if the top-plane rail(s) increase (or decrease) by some amount δ, i.e., w = δ or x = δ, while the backplane rails remain the same, the overall magnitude of some drive voltage will increase (or decrease), and the offset is V L+ and V L- As the state differs, a new drive level will be generated. The effect of this change can be calculated when w = δ or x = δ and δ = +2V, and is graphed in Fig. 9. Fig. 9 illustrates the change of eight primary colors according to the change in drive voltage. In Fig. 9, the open square represents the "base," uniformly distributed drive levels; the filled triangle represents an additional +2V for the highest drive level, while all other levels remain the same; and the filled circle represents an additional +2V for the mid-upper drive voltage, while all other states remain as in the base drive case. As can be seen from Fig. 9, modifying only the individual top-plane rails by +2V produces only minor effects. Compare the position of the open circle with that of the closed triangle and the closed circle. The largest change is observed in the white state (center of Fig. 9) when a positive offset is applied to the positive rail, in which case b* is (undesirably) raised.

[0090] In a similar manner, the backplane rail(s) can be increased (or decreased) by some positive δ, i.e., y = δ or z = δ, while the topplane rail remains the same. The effect of a similar backplane rail adjustment of y = z = δ = +2V is illustrated in Fig. 10. Fig. 10 illustrates the change of eight primary colors according to the change in driving voltage. In Fig. 10, the open square represents the "base," uniformly distributed driving levels; the filled triangle represents an additional +2V for the lowest driving level, while all other levels remain the same; and the filled circle represents an additional +2V for the intermediate lower driving voltage, while all other states remain as in the base driving case. Although the change in performance is not pronounced, the yellow color gradually turns green when a positive or negative rail is modified by +2V. This is typically undesirable because it causes, for example, the flash tones to appear green. However, in some digital signage applications, it may be desirable to trade yellow tones for stronger green tones.

[0091] Surprisingly, when the same offset δ = w = x = y = z is applied to all four rail voltages, a more distinct shift occurs in the electro-optical performance, providing an opportunity to adjust the color performance of the electrophoretic medium as required by specific applications. Figure 11 illustrates the change in the eight primary colors according to the change in driving voltage. In Figure 11, the open square represents the "base" and indicates uniformly distributed driving levels, the filled triangle represents an additional +2V for all driving levels, and the filled circle represents an additional -2V for all driving levels. In Figure 11, electro-optical performance was calculated for cases where δ is -2V, 0, and +2V, and the shift in the electro-optical response is indicated by arrows. It can be seen that some states (yellow, red, cyan) are affected only very slightly by changes in the offset, while the white and magenta states shift very substantially toward a more negative b* as the offset becomes more negative. As can be seen from the equations above, what makes the offset more negative is V M- to less negative and V M+ It makes the amount more. The most important of these effects is the reduction of b* in the white state.

[0092] Therefore, when considered together, these results suggest that the most useful adjustment is to apply the same offset to all four voltage rails. This allows for the intermediate voltage (V) without affecting the high voltage magnitude or zero. M ) results in the ability to change levels. This allows the white state to be adjusted to be more neutral, especially in the b* dimension.

[0093] Higher voltage addressing using metal oxide backplanes

[0094] Modifying the rail voltage provides some flexibility in achieving electro-optical performance different from that of a 4-particle electrophoretic system, but there are many limitations introduced by top-plane switching. For example, to create a white state with the display of the present invention, a lower negative voltage (V M- ) is the maximum negative voltage (V H- It is generally desirable that it be smaller than half of ). However, as shown in the equations above, top-plane switching requires that the lower positive voltage is always at least half, typically greater than half, of the maximum positive voltage.

[0095] An alternative solution to the complexities of top-plane switching can be provided by fabricating control transistors from less common materials with higher electron mobility, thereby enabling the transistors to directly switch larger control voltages, e.g., + / - 30 V. Newly developed active matrix backplanes may include thin-film transistors incorporating metal oxide materials, such as tungsten oxide, tin oxide, indium oxide, and zinc oxide. In these applications, a channel-forming region is formed for each transistor using these metal oxide materials to allow for higher voltage and higher-speed switching. These transistors 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, with the gate electrode, source electrode, and drain electrode at least partially overlapping. Such backplanes are available from manufacturers such as Sharp / Foxconn, LG, and BOE.

[0096] One preferred metal oxide material for these applications is indium gallium zinc oxide (IGZO). IGZO-TFTs have an electron mobility 20 to 50 times higher than that of amorphous silicon. By using IGZO TFTs in the active matrix backplane, it is possible to provide voltages greater than 30 V through a suitable display driver. Additionally, a source driver capable of supplying at least five, preferably seven, levels provides different driving paradigms for a four-particle electrophoretic display system. In one embodiment, there will be two positive voltages, two negative voltages, and zero volts. In another embodiment, there are three positive voltages, three negative voltages, and zero volts. In one embodiment, there are four positive voltages, four negative voltages, and zero volts. These levels can be selected within a range of about -27 V to +27 V without limitations imposed by top-plane switching as described above.

[0097] By using an improved backplane, such as metal oxide backplanes, that is, as described in FIG. 5, it is possible to directly address each pixel with a suitable push-pull waveform. This significantly reduces the time required to update each pixel, in some cases where a 6-second update is converted to less than 1 second. In some cases, it may be necessary to use reset pulses to set the starting point for addressing, but the reset can be performed at higher voltages at a faster speed. Additionally, in four-color electrophoretic displays with reduced color sets, it is possible to drive directly from the first color to the second color with a specific waveform that is only slightly longer than the push-pull waveforms shown in FIG. 5.

[0098] Accordingly, the present invention provides full-color electrophoretic displays capable of directly addressing an electrophoretic medium without top-plane switching, as well as waveforms for such electrophoretic displays. Accordingly, while various aspects and embodiments of the technology of this application have been described, it should be understood by those skilled in the art that various changes, modifications, and improvements may readily occur. Such changes, modifications, and improvements are intended to be within the spirit and scope of the technology described in the application. For example, those skilled in the art will readily conceive various other means and / or structures to perform a function and / or obtain the results and / or one or more advantages described herein, and each of such variations and / or modifications is considered to be within the scope of the embodiments described herein. Those skilled in the art will be able to recognize and identify many equivalents to the specific embodiments described herein using only routine experiment. Accordingly, it should be understood that the foregoing embodiments are presented merely as examples, and within the scope of the appended claims and their equivalents, embodiments of the invention may be practiced differently from those specifically described. Additionally, any combination of two or more features, systems, articles, materials, kits and / or methods described herein is included within the scope of the present disclosure, provided that such features, systems, articles, materials, kits and / or methods do not correspond to one another.

Claims

Claim 1 A color electrophoretic display comprising: a controller; a light-transmitting electrode on a viewing surface; a backplane comprising an array of thin-film transistors coupled to pixel electrodes, wherein each thin-film transistor comprises a layer of a metal oxide semiconductor; and a four-particle color electrophoretic medium disposed between the light-transmitting electrode and the backplane, wherein the four-particle color electrophoretic medium comprises: (a) a fluid; (b) a plurality of first and a plurality of second particles dispersed in the fluid, wherein the first and second particles have charges of opposite polarity, the first particles are light-scattering particles, and the second particles have one of subtractive primary colors; and (c) a plurality of third and fourth particles dispersed in the fluid, wherein the third and fourth particles have charges of opposite polarity, and the third and fourth particles each have a subtractive primary color that is different from each other and different from the second particles, and the controller is configured to provide driving voltages to the pixel electrodes at five or more different voltage levels including voltages greater than 25 V and less than -25 V while maintaining the light-transmitting electrode at a constant voltage that enables switching of the color optical state of the pixels of the 4-particle color electrophoretic medium among the plurality of primary colors, a color electrophoretic display. Claim 2 A color electrophoretic display according to claim 1, wherein the controller is configured to provide at least one voltage between -25 V and 0 V or at least one voltage between 25 V and 0 V. Claim 3 A color electrophoretic display according to claim 1, wherein the metal oxide semiconductor is indium gallium zinc oxide (IGZO). Claim 4 A color electrophoretic display according to claim 1, wherein the plurality of primary colors include red, green, blue, cyan, magenta, yellow, black, and white. Claim 5 A color electrophoretic display according to claim 1, wherein the first electric field required to separate aggregates formed by particles of the third and fourth types is greater than the second electric field required to separate aggregates formed from particles of any two other types. Claim 6 A color electrophoretic display according to claim 1, wherein at least two of the second, third, and fourth particles are non-photo-scattering. Claim 7 A color electrophoretic display according to claim 6, wherein the first particles are white and the second, third, and fourth particles are non-photo-scattering. Claim 8 A color electrophoretic display according to claim 1, wherein the first and third particles are negatively charged and the second and fourth particles are positively charged. Claim 9 In claim 8, the color electrophoretic display wherein the first, second, third, and fourth particles each have colors of white, cyan, yellow, and magenta, and the white and yellow particles are negatively charged and the magenta and cyan particles are positively charged. Claim 10 A color electrophoretic display according to claim 9, wherein the yellow, magenta, and cyan particles exhibit a diffuse reflectance of less than 2.5% at 650, 550, and 450 nm, respectively, measured on a black background when the particles are isotropically distributed at 15 volume% in a 1 μm thick layer containing a liquid and particles having a refractive index of less than 1.

55. Claim 11 A color electrophoretic display according to claim 1, wherein the fluid is a nonpolar liquid having a dielectric constant of less than 5. Claim 12 In claim 11, the fluid dissolves or disperses a polymer that is essentially non-absorbent on particles having a number average molecular weight exceeding 20,000, in the color electrophoretic display. Claim 13 An electronic book reader including a color electrophoretic display according to claim 1. Claim 14 A color electrophoretic display according to claim 1, wherein the thin-film transistors each comprise a gate electrode, a gate insulating film, a metal source electrode, and a metal drain electrode, and a metal oxide semiconductor layer is disposed on the gate insulating film and at least partially disposed on the gate electrode, the metal source electrode, and the metal drain electrode. Claim 15 A color electrophoretic display according to claim 1, wherein the five or more different voltage levels include 30 V, 15 V, 0 V, -15 V, and -30 V. Claim 16 A color electrophoretic display according to claim 1, wherein the thin-film transistors comprising the layer of the metal oxide semiconductor enable direct switching of the color optical state of the color electrophoretic medium among the plurality of primary colors without a reset phase. Claim 17 A method comprising the step of providing a color electrophoretic display, wherein the color electrophoretic display comprises: a controller; a light-transmitting electrode on a viewing surface; a backplane comprising an array of thin-film transistors coupled to pixel electrodes, wherein each thin-film transistor comprises a layer of a metal oxide semiconductor; and a four-particle color electrophoretic medium disposed between the light-transmitting electrode and the backplane, wherein the four-particle color electrophoretic medium comprises: (a) a fluid; (b) a plurality of first and a plurality of second particles dispersed in the fluid, wherein the first and second particles have charges of opposite polarity, wherein the first particles are light-scattering particles, and the second particles have one of subtractive primary colors; and (c) a plurality of third and fourth particles dispersed in the fluid, wherein the third and fourth particles have charges of opposite polarity, and the third and fourth particles each have a subtractive primary color that is different from each other and different from the second particles, the method comprising the plurality of third and fourth particles; and the step of applying driving voltages to the pixel electrodes at five or more different voltage levels including voltages greater than 25 V and less than -25 V while maintaining the light-transmitting electrode at a constant voltage that switches the optical states of the pixels of the 4-particle color electrophoretic medium among the plurality of primary colors. Claim 18 In claim 17, the driving voltages applied to the pixel electrodes include voltages between 25 V and 0 V and voltages between -25 V and 0 V. Claim 19 In claim 17, the method wherein at least two of the second, third, and fourth particles are non-photo-scattering. Claim 20 In claim 19, the first particles are white, and the second, third, and fourth particles are non-light-scattering, method. Claim 21 A method according to claim 17, wherein the first and third particles are negatively charged and the second and fourth particles are positively charged. Claim 22 A method according to claim 21, wherein the first, second, third, and fourth particles each have colors of white, cyan, yellow, and magenta, and the white and yellow particles are negatively charged and the magenta and cyan particles are positively charged. Claim 23 A method according to claim 22, wherein the yellow, magenta, and cyan particles exhibit a diffuse reflectance of less than 2.5% at 650, 550, and 450 nm, respectively, measured on a black background when the particles are isotropically distributed at 15 volume% in a 1 μm thick layer containing a liquid and particles having a refractive index of less than 1.

55. Claim 24 In claim 17, the method wherein the fluid is a nonpolar liquid having a dielectric constant of less than 5. Claim 25 In claim 24, the fluid has a number average molecular weight exceeding 20,000 and has a polymer that is essentially non-absorbent dissolved or dispersed within the particles. Claim 26 In claim 17, the metal oxide semiconductor is indium gallium zinc oxide (IGZO), method. Claim 27 In claim 17, the method comprises at least five different voltages including 30 V, 15 V, 0 V, -15 V, and -30 V. Claim 28 In claim 17, the color electrophoretic display is integrated into an electronic book reader, portable computer, tablet computer, cellular phone, smart card, sign, watch, shelf label, or flash drive. Claim 29 A method according to claim 17, wherein the optical states of the 4-particle color electrophoretic medium are directly switched among the plurality of primary colors without a reset phase. Claim 30 In claim 17, the plurality of primary colors includes red, green, blue, cyan, magenta, yellow, black, and white. Claim 31 A sign including a color electrophoretic display according to claim 1. Claim 32 A shelf label comprising a color electrophoretic display according to claim 1.

Images