Drive sequence for removing previous state information from a color electrophoretic display

Abstract

Methods for efficiently clearing previous state information when driving a multi-particle color electrophoretic medium, e.g., where at least two particles are colored and at least one particle is scattering. Typically, such a system includes white particles as well as cyan, yellow, and magenta subtractive primary particles. The clearing pulse can include two different portions of alternating impulses and the overall waveform can be direct current balanced.

Description

[0001] Related Applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 108,713, filed November 2, 2020. The contents of all patents and publications disclosed herein are incorporated by reference in their entirety. TECHNICAL FIELD

[0003] The present invention relates to methods for driving electro-optic displays, particularly but not exclusively electrophoretic displays capable of presenting more than two colors using a single layer of electrophoretic material comprising a plurality of colored particles, for example white, cyan, yellow and magenta absorbing particles, in which two of the particles are positively charged, two of the particles are negatively charged, and one positively charged particle and one negatively charged particle have thick polymer shells. BACKGROUND

[0004] Other color and charge combinations are also possible, and can be more suitable for different applications. For example, three-color displays (black, white, red; black, white, yellow), four-color displays (black, white, red, yellow) and five-color displays (black, white, red, yellow, blue) can be implemented. In some embodiments, all of these particles (except black) are reflective / scattering, resulting in reliable and saturated colors, which are well suited for advertising. In other embodiments, only one of the particles is reflective (e.g., white), while the remaining particles are absorbing, resulting in a larger color gamut. In some applications, the number of positive particle sets is not balanced with the number of negative particle sets. For example, a four-particle display can include one negative white particle set and three positive particle sets, which can be, for example, cyan, magenta and yellow.

[0005] Operation of electrophoretic displays with three or four reflective pigments is similar to that of black-and-white electrophoretic displays, in that the desired color particles are driven to the viewing surface. The driving scheme is far more complex than with just black and white, but ultimately the optical function of the particles is the same.

[0006] Advanced color electronic paper ACeP is a special type of color electrophoretic display sold by E Ink Corporation that includes four types of particles, where the cyan, yellow, and magenta particles are subtractive rather than reflective, allowing thousands of colors to be produced at each pixel. (An ACeP-type display refers to a four-particle system where one particle is scattering and the other particles are subtractive.) The color process is functionally equivalent to the printing method that has been used for decades in offset printing and inkjet printers. A given color is produced by using the correct proportions of cyan, yellow, and magenta on a bright white paper background. In the case of ACeP, the relative positions of the cyan, yellow, magenta, and white particles relative to the viewing surface will determine the color at each pixel. While this type of electrophoretic display allows thousands of colors at each pixel, it is critical to carefully control the position of each (50-500 nanometer size) pigment within the working space of about 10-20 microns in thickness. Obviously, a change in pigment position will result in an incorrect color being displayed at a given pixel. Therefore, precise voltage control is required for this system. More details of this system can be found in the following U.S. patents, all of which are incorporated by reference in their entirety: U.S. Patent Nos. 9,361,836, 9,921,451, 10,276,109, 10,353,266, 10,467,984, and 10,593,272.

[0007] U.S. Patent No. 10,593,272 describes specific waveforms for erasing the previous state in an ACeP-type electrophoretic display, particularly an ACeP-type electrophoretic display that uses top plate switching. The drive diagram from U.S. Patent No. 10,593,272 is reproduced as FIG. 1. In FIG. 1, a general waveform for addressing an ACeP-type color electrophoretic display is shown, where the abscissa represents time (arbitrary units) and the ordinate represents the voltage difference between the pixel electrode and the common front electrode. The amplitudes of the three positive voltages used in the drive scheme shown in FIG. 1 can be between about +3 V and +30 V, and the amplitudes of the three negative voltages can be between about -3 V and -30 V.

[0008] The prior art teaches two different phases in which pulses of +V max and -V max are provided for erasing the previous image presented on the display (i.e., to “reset” the display). The lengths of these pulses (t1 and t3) and the lengths of the rests (i.e., the periods of zero voltage between them (t2 and t4)) can be chosen such that the entire waveform (i.e., the integral of the voltage over the entire waveform as shown in FIG. 1 with respect to the entire time) is direct current balanced (i.e., the integral of the voltage over time is essentially zero). Notably, the amplitudes of +V max and -V max are matched, allowing for easier direct current balancing.

[0009] The prior art reset pulse of the type shown in FIG. 1 uses a phase comprising a high voltage dipole to set the initial conditions of the waveform and to balance the entire transition to zero DC. This zero DC balancing phase precedes the pulse sequence used to form the desired final color. The zero DC balancing portion of the waveform consists of at least two pulses: one positive pulse and one negative pulse, and the net impulse of the zero DC balancing portion of the waveform is equal and opposite to the net impulse of the color forming pulse that follows it.

[0010] Over time, it became apparent that waveforms of the type shown in FIG. 1 had certain deficiencies. In particular, waveforms similar to FIG. 1 had a high level of “ghosting” (i.e., the influence of the previous state on the final optical state). Thus, many transitions from a first optical state to a second optical state required an additional pre-clear, such as by inserting a transition to a white state between two color image transitions. The additional “trip” through the white state resulted in very long total transition times, up to 20 seconds. In addition, in experiments using a single push-pull sequence to produce colors (i.e., using a waveform that lacked the reset / zero DC balancing phase of conventional waveforms), it was found that only certain direct transitions from one color to another were possible. In particular, it was not possible to directly transition from any color with a yellow component (i.e., yellow, green, red, and black) to any color without a yellow component (i.e., blue, cyan, and white) except for magenta.

[0011] One proposed mechanism for ghosting and colorless transitions was the unwanted polarization of one or more of the pigments during the previous update. In particular, as described in U.S. Patent No. 9,697,778, incorporated by reference, it was shown that negatively charged polymer-functionalized titania particles could be polarized to the extent that their electrophoretic mobility actually reversed. This behavior can be due to the accumulation of charge control agents on the surface of the white pigment as well as other non-linear behavior, including the temporary formation of local counter-charge spheres around the titania particles. Thus, in theory, the addition of a transition to the white state during all waveform updates normalizes the electrophoretic mobility of the particles and helps the electrophoretic material achieve a neutral state from which all subsequent colors are available. Other sets of particles are also polarized after being driven to the previous color state, and driving the electrophoretic medium to the white state also counteracts these unwanted polarizations.

[0012] A subject of intense research and development for many years has been particle-based electrophoretic displays. In such displays, a plurality of electrically charged particles (sometimes referred to as pigment particles) move through a fluid under the influence of an electric field. In comparison to liquid crystal displays, electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption. However, problems with long-term image quality of these displays have hindered their widespread use. For example, the particles that make up electrophoretic displays are prone to settling, resulting in inadequate service life of these displays.

[0013] As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can work with gaseous fluids; see for example Kitamura, T. et al., "Electronic paper based on electrophoretic light modulation", Nature, Vol. 415, 2002, p. 541; Kitamura, T. et al., "Electron Paper Driven by Using Tribal Chips with Electrophoretic Property and Plastic Electrode", SID 02 Digest, 2002, pp 1320 - 1323; Yamaguchi, Y. et al., "Toner display using insulative particles charged triboelectrically", IDW Japan, 2001, Paper AMD4-4; and Yamaguchi, Y. et al., "Electronic paper

[0014] A number of patents and applications assigned to, or in the name of, the Massachusetts Institute of Technology (MIT) and E Ink Corporation, and references therein, describe various technologies for encapsulated electrophoretic and other electroluminescent media. These encapsulated media comprise a plurality of small capsules, each of which includes an inner phase and a capsule wall surrounding the inner phase, wherein the inner phase comprises electrically charged particles suspended in a fluid medium. 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:

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

[0016] (b) Encapsulants, adhesives and packaging processes; see for example U.S. Patents Nos. 6,922,276 and 7,411,719;

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

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

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

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

[0021] (g) Color formation and color adjustment; see for example U.S. Patent Nos. 6,017,584; 6,545,797; 6,664,944; 6,788,452; 6,864,875; 6,914,714; 6,972,893; 7,038,656; 7,038,670; 7,046,228; 7,052,571; 7,075,502; 7,167,155; 7,385,751; 7,492,505; 7,667,684; 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 U.S. Patent Application Publication No.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; 2015 / 0234250; 2015 / 0268531; 2015 / 0301246; 2016 / 0011484; 2016 / 0026062; 2016 / 0048054; 2016 / 0116816; 2016 / 0116818; and 2016 / 0140909.

[0022] (h) methods for driving a display; see for example U.S. Patents 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; 7,242,514; 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 Publication Nos.2003 / 0102858; 2004 / 0246562; 2005 / 0253777; 2007 / 0091418; 2007 / 0103427; 2007 / 0176912; 2008 / 0024429; 2008 / 0024482; 2008 / 0136774; 2008 / 0291129; 2008 / 0303780; 2009 / 0174651; 2009 / 0195568; 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; 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 can be referred to herein as MEDEOD (Method for Driving Electro-Optic Displays) applications).

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

[0024] (j) Non-electrophoretic displays, as described in U.S. Pat. No. 6,241,921 and U.S. Pat. App. Pub. Nos. 2015 / 0277160, and 2015 / 0005720 and 2016 / 0012710.

[0025] Many of the foregoing patents and applications recognize that the walls surrounding discrete microcapsules in a encapsulated electrophoretic medium can be replaced by a continuous phase, thereby creating a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymer material, and the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered capsules or microcapsules, even though no discrete capsule membranes are associated with each separate droplet; see for example U.S. Patent No. 6,866,760. Accordingly, for the purposes of the present application, such a polymer-dispersed electrophoretic medium is considered to be a sub-class 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 are held in suspension within a carrier medium (typically a liquid) in a plurality of microcells formed within a thin film. See for example U.S. Patents Nos. 6,672,921 and 6,788,449, both to the assignee of the present application.

[0027] Although electrophoretic media are often opaque (as, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in so-called shutter mode, in which one display state is substantially opaque and the other is light-transmissive. See for example U.S. Patents 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, similar to electrophoretic displays but relying on changes in the electrical field strength rather than particle charge, can operate in similar modes; see U.S. Patent No. 4,418,346. Other types of electro-optic displays are also capable of operating in shutter mode. An electro-optic medium operating in shutter mode can be used in a multilayer structure for a full-color display; in such a structure, at least one layer adjacent the viewing surface of the display operates in shutter mode to expose or hide a second layer farther from the viewing surface.

[0028] Encapsulated electrophoretic displays are generally free from the aggregation and settling failure modes of conventional electrophoretic devices and provide additional benefits such as the ability to print or coat displays on a variety of flexible and rigid substrates. (The use of the word "print" is intended to encompass all forms of printing and coating, including but not limited to: pre-metered coating such as stamp coating, slit or extrusion coating, slide or curtain coating, curtain coating; roll coating such as roll blade coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brushing; air knife coating; screen printing processes; electrostatic printing processes; thermal printing processes; inkjet printing processes; electrophoretic deposition (see U.S. Patent No. 7,339,715); and other like techniques.) Thus, the resulting displays can be flexible. In addition, because the display media can be printed (using a variety of methods), the displays themselves can be manufactured inexpensively.

[0029] As noted above, the simplest prior art electrophoretic medium displays essentially only two colors. Such an electrophoretic medium uses either 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 particles are near the viewing surface of the display, and the second color is displayed when the particles are distanced from the viewing surface), or first and second types of electrophoretic particles having first and second, different colors in a colorless fluid (in which 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. If a full color display is desired, a color filter array can be deposited on the viewing surface of the monochrome (black and white) display. A display with a color filter array relies on area sharing and color mixing to create color stimuli. The available display area is shared between three or four primary colors such as red / green / blue (RGB) or red / green / blue / white (RGBW), the filters can be arranged in a one-dimensional (stripe) or two-dimensional (2x2) repeating pattern. Other choices of primary colors or more than three primary colors are also known in the art. The three (in the case of an RGB display) or four (in the case of an RGBW display) sub-pixels are chosen to be small enough that at the intended viewing distance they visually mix together into a single pixel with a uniform color stimulus ("color mixing"). An inherent disadvantage of area sharing is that the colorant is always present and can only modulate the color by switching the corresponding pixel of the underlying monochrome display to white or black (turning on or off the corresponding primary color). For example, in an ideal RGBW display, the red, green, blue and white primary colors each occupy one quarter of the display area (one quarter of the sub-pixels), the white sub-pixels are as bright as the underlying monochrome display white, and each color sub-pixel is no brighter than one third of a monochrome display white. The brightness of white displayed by the display as a whole cannot exceed half the brightness of the white sub-pixels (the white area of the display is created by displaying one white sub-pixel out of every four white sub-pixels, plus the color form of each color sub-pixel is equivalent to one third of a white sub-pixel, so the total contribution of three color sub-pixels does not exceed one white sub-pixel). The brightness and saturation of the colors are reduced by the area sharing with color pixels switched to black. Area sharing is especially problematic when mixing yellow, since it is brighter than any other color of the same brightness, and a saturated yellow is almost as bright as white. Switching a blue pixel (one quarter of the display area) to black makes the yellow too dark.

[0030] Multi-layer stacked electrophoretic displays are known in the art; see for example J. Heikenfeld, P. Drzaic, J-S Yeo and T. Koch, Journal of the SID, 19(2), 201 1, pages 129-156. In such displays, ambient light passes through an image of each of the three subtractive primary colors, much like traditional color printing. U.S. Patent No. 6,727,873 describes a stacked electrophoretic display in which three layers of switchable cells are placed on a reflective background. Similar displays are known in which the colored particles move laterally (see International Application No. WO 2008 / 065605), or are sequestered into microcells using a combination of vertical and lateral motion. In both cases, each layer is provided with electrodes for concentrating or dispersing the colored particles on a pixel-by-pixel basis, so that each of the three layers requires a thin film transistor (TFT) layer (two of the three layers of TFTs must be substantially transparent) and a light-transmissive counter electrode. This complex electrode arrangement is expensive to manufacture and it is difficult in the prior art to provide a sufficiently transparent pixel electrode plane, especially when the white state of the display must be viewed through several layers of electrodes. Multi-layer displays also suffer from parallax issues when the thickness of the display stack approaches or exceeds the pixel size.

[0031] U.S. Application Publication Nos. 2012 / 0008188 and 2012 / 0134009 describe a multi-color electrophoretic display having a single backplane that includes independently addressable pixel electrodes and a common light-transmissive front electrode. A plurality of electrophoretic layers are disposed between the backplane and the front electrode. The displays described in these applications are capable of rendering any of the primary colors (red, green, blue, cyan, magenta, yellow, white and black) at any pixel location. However, there are disadvantages to using multiple electrophoretic layers between a single set of addressable electrodes. The electric field experienced by the particles in a particular layer is lower than would be the case for a single electrophoretic layer addressed with the same voltage. In addition, optical losses (e.g., caused by light scattering or unwanted absorption) in the electrophoretic layer closest to the viewing surface can affect the appearance of the image formed in the underlying electrophoretic layers.

[0032] Attempts have been made to use a single electrophoretic layer to provide a full color electrophoretic display. For example, U.S. Patent Application Publication No. 2013 / 0208338 describes a color display including an electrophoretic fluid including one or two types of pigment particles dispersed in a transparent and colorless or colored solvent, the electrophoretic fluid disposed between a common electrode and a plurality of pixel or driving electrodes. The driving electrodes are arranged to expose a background layer. U.S. Patent Application Publication No. 2014 / 0177031 describes a method for driving a display cell filled with an electrophoretic fluid including two types of charged particles with opposite charge polarity and having two contrasting colors. The two types of pigment particles are dispersed in a colored solvent or in a solvent having uncharged or lightly charged colored particles dispersed therein. The method includes driving the display cell to display the color of the solvent or the color of the uncharged or lightly charged colored particles by applying a driving voltage that is about 1% to about 20% of a full driving voltage. U.S. Patent Application Publications Nos. 2014 / 0092465 and 2014 / 0092466 describe an electrophoretic fluid and a method for driving an electrophoretic display. The fluid includes first, second, and third types of pigment particles, all of which are dispersed in a solvent or solvent mixture. The first and second types of pigment particles have opposite charge polarity, and the third type of pigment particles has a charge level that is less than about 50% of the charge level of the first or second type of pigment particles. 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 of the term as used hereinafter.

[0033] U.S. Patent Application Publication No. 2007 / 0031031 describes an image processing apparatus for processing image data for displaying an image on a display medium, in which each pixel is capable of displaying white, black, and one other color. U.S. Patent Application Publication Nos. 2008 / 0151355, 2010 / 0188732, and 2011 / 0279885 describe a color display in which mobile particles move through a porous structure. U.S. Patent Application Publication Nos. 2008 / 0303779 and 2010 / 0020384 describe a display medium including first, second, and third particles of different colors. The first and second particles can form aggregates, and the smaller third particles can move through the voids left between the aggregated first and second particles. U.S. Patent Application Publication No. 2011 / 0134506 describes a display apparatus including: an electrophoretic display element including a plurality of types of particles of different optical properties and different migration speeds and / or different threshold values for an electric field for movement, which are encapsulated between a pair of substrates, a semi-transparent display-side electrode, a first back electrode, and a second back electrode, at least one of the substrates being semi-transparent, and each of the respective plurality of types of particles carrying an electric charge of the same polarity, the semi-transparent display-side electrode being provided on a substrate side on which the semi-transparent substrate is disposed, the first back electrode being provided on a side of the other substrate facing the display-side electrode, and the second back electrode being provided on a side of the other substrate facing the display-side electrode; and a voltage control portion that controls voltages applied to the display-side electrode, the first back electrode, and the second back electrode, so that a particle type having the fastest migration speed among the plurality of types of particles or a particle type having the lowest threshold value among the plurality of types of particles is moved to the first back electrode or the second back electrode in turn through each of the different types of particles, and then the particles moved to the first back electrode are moved to the display-side electrode. U.S. Patent Application Publication Nos. 2011 / 0175939, 2011 / 0298835, 2012 / 0327504, and 2012 / 0139966 describe a color display relying on multiple particle aggregation and threshold voltage. U.S. Patent Application Publication No. 2013 / 0222884 describes an electrophoretic particle including a colored particle containing a polymer having a charged group and a colorant, and a branched siloxane-based polymer attached to the colored particle and containing a reactive monomer and at least one monomer selected from a specific group of monomers as a copolymer component. U.S. Patent Application Publication No. 2013 / 0222885 describes a dispersion liquid for an electrophoretic display including: a dispersion medium; a group of colored electrophoretic particles dispersed in the dispersion medium and migrating in an electric field; a group of non-electrophoretic particles that do not migrate and have a color different from the group of electrophoretic particles; and a compound having a neutral polar group and a hydrophobic group, which is included in the dispersion medium at a ratio of about 0.01 to about 1 mass percent based on the entire dispersion liquid.U.S. Patent Application Publication No. 2013 / 0222886 describes a dispersion for a display comprising self-floating particles, the self-floating particles comprising: a plurality of core particles including a colorant and a hydrophilic resin; and a shell covering a surface of each of the core particles and including a hydrophobic resin having a difference in solubility parameter of 7.95 (J / cm. 3 1 / 2 or more. U.S. Patent Application Publication Nos. 2013 / 0222887 and 2013 / 0222888 describe electrophoretic particles having a specific chemical composition. Finally, U.S. Patent Application Publication No. 2014 / 0104675 describes a particle dispersion including first colored particles and second colored particles that move in response to an electric field, and a dispersion medium, the second colored particles having a larger diameter than the first colored particles and the same charging characteristics as the first colored particles, and wherein a ratio (Cs / Cl) of a charge amount Cs of the first colored particles per unit area of the display to a charge amount Cl of the second colored particles is less than or equal to 5. Some of the above-described displays do provide full color, but at the cost of requiring a lengthy and cumbersome addressing method.

[0034] U.S. Patent Application Publication Nos. 2012 / 0314273 and 2014 / 0002889 describe an electrophoretic device including a plurality of first and second electrophoretic particles included in an insulating liquid, the first particles and the second particles having different charging characteristics different from each other; the device further including a porous layer included in the insulating liquid and formed of a fibrous structure. These patent applications are not full color displays in the sense of the term used below.

[0035] See also U.S. Patent Application Publication No. 2011 / 0134506 and the above-mentioned Application Serial No. 14 / 277,107; the latter describes a full color display using three different types of particles in a colored fluid, but the presence of the colored fluid limits the quality of the white state that can be achieved by the display.

[0036] ​In order to obtain a high-resolution display, the individual pixels of the display must be addressable without interference from adjacent pixels. One way of achieving this is to provide an array of non-linear elements, such as transistors or diodes, at least one non-linear element being associated with each pixel to create an "active matrix" display. The addressing or pixel electrode of an addressed pixel is connected to the appropriate voltage source through the associated non-linear element. Typically, when the non-linear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this arrangement will be assumed in the following description, although it is essentially arbitrary and the pixel electrode could be connected to the source of the transistor. Typically, in a high-resolution array, the pixels are arranged in a two-dimensional array of rows and columns, such that any particular pixel is uniquely defined by the intersection of a specified row and a specified column. The sources of all the transistors in each column are connected to a single column electrode, while the gates of all the transistors in each row are connected to a single row electrode; again, 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 row drivers, which essentially ensure that only one row is selected at any given time, i.e. a selection voltage is applied to the selected row electrode to ensure that all the transistors in the selected row are turned on, while a non-selection voltage is applied to all other rows to ensure that all the transistors in these unselected rows remain turned off. The column electrodes are connected to column drivers, which apply voltages across the various column electrodes, the voltages being chosen to drive the pixels in the selected row to their desired optical state. (The voltages referred to above are relative to a common front electrode, which is typically provided on the side of the electro-optic medium opposite the non-linear array and extends across the entire display.) After a preselected interval, referred to as the "line addressing time", the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed in order to write the next line of the display. This process is repeated in order to write the entire display in a row-by-row fashion.

[0037] Conventionally, each pixel electrode has a capacitor electrode associated with it, such that the pixel electrode and the capacitor electrode form a capacitor; see, for example, International Patent Application WO 01 / 07961. In some embodiments, an N-type semiconductor (e.g. amorphous silicon) can be used to form the transistors, and the "selection" and "non-selection" voltages applied to the gate electrode can be positive and negative, respectively. SUMMARY

[0038] The reset of the display ideally erases any memory of the previous state, including residual voltages and pigment configurations specific to the previous display color, and allows the electrophoretic display to reliably achieve the same color state using the same driving scheme. In general, the reset pulse of the present invention comprises a high (amplitude) positive voltage followed by a lower (amplitude) negative voltage, and the (time) length of the positive voltage pulse must be shorter than the (time) length of the negative voltage pulse. The clearing waveform can be direct current balanced or direct current unbalanced. The reset pulse can comprise two or more sets of different high (amplitude) positive voltages and two or more sets of different lower (amplitude) negative voltages. For example, the first part of the reset pulse can comprise a push-pull sequence to achieve a universal color initial state, such as magenta, to achieve the best possible clearing. The second part comprises a neutral state similar to white, i.e. a state from which all subsequent colors can be produced by simplified push-pull color transitions.

[0039] In one aspect, the present invention comprises a method of driving an electrophoretic display comprising an electrophoretic medium having four different types of electrophoretic particles, each type of electrophoretic particle having a different color and a different combination of charge polarity and charge magnitude. The method comprises providing a clearing pulse comprising a first set of pulses having an amplitude Vi and a length ti, alternating with a pulse having an amplitude V2 and a length t2, and a second set of pulses having an amplitude V3 and a length t3, alternating with a pulse having an amplitude V4 and a length t4, wherein Vi is a positive voltage and V2 is a negative voltage having an amplitude lower than Vi, wherein the ratio V 1· t1 / V 2· t2 is greater than the ratio V 3· t3 / V 4· t4; and providing a push-pull color addressing pulse having a push pulse having an amplitude V5 and a length ts, and a pull pulse having an amplitude V6 and a length t6, wherein V5 and V6 have opposite polarity.

[0040] In some embodiments, the electrophoretic display includes two types of positively charged electrophoretic particles having different amounts of charge, and two types of negatively charged electrophoretic particles having different amounts of charge. In some embodiments, the two types of positively charged particles are cyan and magenta in color, and the two types of negatively charged particles are white and yellow in color. In some embodiments, the method further includes providing at least three impulses of amplitude Vi and length ti, at least three impulses of amplitude V2 and length t2, at least three impulses of amplitude V3 and length t3, and at least three impulses of amplitude V4 and length t4. In some embodiments, at least two impulses of amplitude Vi and length ti are interspersed with impulses of amplitude V2 and length t2, and at least two impulses of amplitude V3 and length t3 are interspersed with impulses of amplitude V4 and length t4. In some embodiments, the method further includes providing a DC balance pulse, the DC balance pulse including a first DC balance impulse of amplitude V7 and length t7 and a second DC balance impulse of amplitude V8 and length t8, where V7 and V8 have opposite polarity, where the sum of the voltage-time areas of all positive voltage pulses equals the sum of the voltage-time areas of all negative voltage pulses. Such features can be represented as

[0041]

[0042] where n = 1-8, and V n and t n are as defined above. In some embodiments, the DC balance pulse precedes the clearing pulse and the push-pull color addressing pulse. In some embodiments, the DC balance pulse is between the clearing pulse and the push-pull color addressing pulse.

[0043] In some embodiments, the electrophoretic display includes a first light-transmissive electrode layer, a second electrode layer including a plurality of pixel electrodes, and an electrophoretic layer including an electrophoretic medium disposed between the first light-transmissive electrode layer and the second electrode layer. In some embodiments, the electrophoretic layer includes a plurality of microcells containing the electrophoretic medium. In some embodiments, the electrophoretic layer includes a plurality of microcapsules containing the electrophoretic medium. BRIEF DESCRIPTION OF DRAWINGS

[0044] FIG. 1 shows a prior art clearing waveform for an ACeP type electrophoretic display. FIG. 1 is equivalent to FIG. 2 of U.S. Patent No. 10,593,572. Figure 6 .

[0045] Figure 2 is a cross-sectional schematic showing the positions of various colored particles in the electrophoretic medium of the present invention when displaying black, white, three subtractive primary colors, and three additive primary colors.

[0046] Figure 3A Four types of different pigment particles used in a multi-particle electrophoretic medium are shown in schematic form.

[0047] Figure 3B Four types of different pigment particles used in a multi-particle electrophoretic medium are shown in schematic form.

[0048] Figure 3C Four types of different pigment particles used in a multi-particle electrophoretic medium are shown in schematic form.

[0049] Figure 4 An exemplary equivalent circuit for a single pixel of an electrophoretic display is shown.

[0050] Figure 5 Layers of an exemplary electrophoretic color display are shown.

[0051] Figure 6 A simple push-pull waveform that can be used to implement a set of primary colors in an optimized system comprising one reflective (white) particle and three subtractive (cyan, yellow, magenta) particles, where two particles are negatively charged but have different sizes, and two particles are positively charged but have different sizes, is shown.

[0052] Figure 7A is a generalized clearing pulse, with alternating pulses of Vi, ti and V2, t2. Vi has a larger amplitude than V2, while ti is smaller than t2.

[0053] Figure 7B is a generalized clearing pulse, comprising a first portion with alternating pulses of Vi, ti and V2, t2, and a second portion with alternating pulses of V3, t3 and V4, t4.

[0054] Figure 8 is a specific example of a clearing pulse, comprising a first portion to drive an ACeP-type medium toward a magenta state, and a second portion to drive the ACeP-type medium toward a neutral white state.

[0055] Figure 9 An exemplary clearing pulse of the present invention coupled to a push-pull waveform adapted to achieve a desired color in an ACeP-type electrophoretic medium is shown when incorporated in a display having a metal oxide backplane and using a seven-level driver. DETAILED DESCRIPTION

[0056] This invention details a method for efficiently removing previous state information when driving a multi-particle color electrophoretic medium, for example, wherein at least two types of particles are colored and subtractive, and at least one particle is scattering. Typically, such a system includes white particles and subtractive primary color particles of cyan, yellow, and magenta. Such a system in… Figure 2 The image is schematically shown, and it can provide white, yellow, red, magenta, blue, cyan, green, and black at each pixel.

[0057] In the ACeP example, each of the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) corresponds to a different arrangement of the four pigments, such that the observer can only see those colored pigments located on the observation side of the white pigment (i.e., the only pigment scattering light). It has been found that classifying the four pigments into appropriate configurations to produce the waveforms of these colors requires at least five voltage levels (high positive, low positive, zero, low negative, high negative). See also Figure 2 To achieve a wider color gamut, additional voltage levels must be used for better control of the pigment, such as seven voltage levels, or even nine voltage levels. However, as discussed in the background section, it has been found that not all second color states are derived from all first color states, resulting in more complex (and longer) drive waveforms. The invention described herein overcomes this difficulty by providing a push-pull clear waveform with mismatched voltages. This type of clear waveform allows for rapid transitions between most color states without noticeable flickering.

[0058] The term "color" as used herein includes both black and white. White particles are typically light-scattering. The term "gray state" is used herein in its conventional meaning in the imaging field, referring to a state between the two extreme optical states of a pixel, but not necessarily a black-and-white transition between these two extremes. For example, several patents and publications of IENK, mentioned below, describe such electrophoretic displays where the extreme states are white and dark blue, making the intermediate gray state actually a pale blue. In fact, as already 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 white and dark blue states mentioned above.

[0059] The terms "bistable" and "bistability" as used herein refer to displays comprising display elements having first and second display states and which do not require a continuous supply of power to maintain the display in one of its two states once the element has been supplied with an appropriate voltage pulse to change its state. In the bistable display of an electrophoretic medium, which is the most representative example of the class of optical-electrical-optical displays, these elements are generally in the form of suspended particles dispersed in a suitable fluid, and exhibit either a light-transmissive state like that shown in Fig. 1A or a light-absorbing state, as shown in Fig. 1B. The display elements are in their stable states and remain there until such time as they are addressed by an electric field varing with time to change their state. In this particular example, the particles change states as a result of the forces exerted on the particles by the time-varying field applied across the display elements. Since the particles are suspended in a fluid, they do not change state unless and until they are addressed by a suitable electric field. Furthermore, the particles remain in their new state even after the electric field has been removed.

[0060] The term "impulse" as used herein in reference to driving an electrophoretic display refers to the integral with respect to time of the voltage applied during the period of time in which the display is driven.

[0061] Particles that absorb, scatter, or reflect light of a broad band or a selected wavelength are referred to herein as colored or pigment particles. Various materials other than pigments that absorb or reflect light (in the strict sense of the term, insoluble colored materials) such as dyes or photonic crystals, etc. can also be used in the electrophoretic media and displays of the present application.

[0062] The three particles providing the three subtractive primary colors can be substantially non-light scattering ("SNLS") for an ACeP system, for example. The use of SNLS particles allows mixing of colors and provides more color results than can be achieved using the same number of scattering particles. These thresholds must be sufficiently separated with respect to the voltage drive levels to avoid cross-talk between particles, and this separation requires the use of high addressing voltages for some colors. In addition, addressing the colored particles with the highest threshold also moves all the other colored particles, and these other particles must then be switched to their desired positions at lower voltages. This stepwise color addressing scheme results in undesirable color flicker and longer transition times.

[0063] The figures Figure 2 are cross-sectional schematic views showing the positions of various particles in an ACeP-type electrophoretic medium when displaying black, white, three subtractive primary colors, and three additive primary colors. In Figure 2In this embodiment, it is assumed that the viewing surface of the display is at the top (as shown in the figure), i.e., the user views the display from this direction, and light is incident from this direction. As already noted, in the preferred embodiment, only one of the four particles used in the electrophoretic medium of the present invention substantially scatters light, and... Figure 2 In this context, the particle is assumed to be a white pigment. This light-scattering white particle forms a white reflector, through which any particle above the white particle (such as...) can reflect light. Figure 2 All of these particles (as shown) can be observed. Light entering the display's observation surface passes through these particles, is reflected by the white particles, passes through these particles again, returns, and exits from the display. Therefore, the particles above the white particles can absorb various colors, and the colors presented to the user are produced by the combination of particles above the white particles. Any particles located below the white particles (behind them from the user's perspective) are masked by the white particles and do not affect the displayed colors. Because the second, third, and fourth particles are essentially non-light-scattering, their order or arrangement relative to each other is not important; however, for the reasons already stated, their order or arrangement relative to the white (light-scattering) particles is crucial.

[0064] More specifically, when cyan, magenta, and yellow particles are located below white particles ( Figure 2 In case [A], there are no particles above the white particle and the pixel displays only white. When a single particle is above a white particle, the color of that single particle is... Figure 2 In cases [B], [D], and [F], the colors are yellow, magenta, and cyan, respectively. When two particles are positioned above a white particle, the displayed color is a combination of the colors of those two particles; Figure 2 In case [C], magenta and yellow particles appear red; in case [E], cyan and magenta particles appear blue; and in case [G], yellow and cyan particles appear green. Finally, when all three colored particles are above the white particle ( Figure 2 In the case of [H], all incident light is absorbed by the three subtractive primary color particles and the pixel displays black.

[0065] A subtractive primary color can be represented by a single type of light-scattering particle, so the display would include two types of light-scattering particles, one white and one colored. However, in this case, the position of the light-scattering colored particles relative to other colored particles covering the white particles will be important. For example, when displaying black (when all three 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 displayed would be the color of the scattering colored particles, not black).

[0066] If more than one type of colored particle scatters light, it is not easy to produce black.

[0067] Figure 2 An idealized scenario is illustrated where the color is uncontaminated (i.e., light-scattering white particles completely mask any particles located behind them). In reality, the masking of white particles may not be perfect, so particles that would ideally be completely masked may still have some light absorption. This contamination typically reduces the brightness and chromaticity of the resulting color. In the electrophoretic medium of this invention, this color contamination should be minimized to a degree that the resulting color is commensurate with industry standards for color reproduction. A particularly popular standard is SNAP (Newspaper Advertising Production Standard), which assigns L*, a*, and b* values ​​to each of the eight primary colors mentioned above. (Hereinafter, "primary color" will be used to refer to...) Figure 3A The eight colors shown are black, white, the three subtractive primary colors, and the three additive primary colors.

[0068] Figure 3A and 3B Cross-sectional schematic diagrams of four pigment types (1-4; 5-8) used in ACeP-type electrophoretic displays are shown. Figure 3A In this design, the polymer shell adsorbed onto the core pigment is represented by a dark shading, while the core pigment itself appears unshaded. Various forms can be used for the core pigment: spherical, needle-like, or other anisoaxial shapes; aggregates of smaller particles (i.e., “grape clusters”); composite particles comprising small pigment particles or dyes dispersed in a binder; and so on, as is known in the art. The polymer shell can be a covalently bonded polymer prepared by grafting processes or chemisorption known in the art, or it can be physically adsorbed onto the particle surface. For example, the polymer can be a block copolymer comprising both insoluble and soluble segments.

[0069] exist Figure 3B In the embodiments, the first and second particle types preferably have a more robust polymer shell than the third and fourth particle types. The light-scattering white particles belong to either the first or second type (negatively or positively charged). In the following discussion, it is assumed that the white particles are negatively charged (i.e., belonging to type 1), but those skilled in the art will understand that the general principles described will apply to a group of particles in which the white particles are positively charged.

[0070] In addition, such as Figure 3B As shown, compared to the third and fourth particle types, the first and second particle types do not require different polymer shells. Figure 3AAs shown, the sufficient differential charge on the four types of particles will allow electrophoretic control of the particles and create the desired color at the viewing surface. For example, particle 5 can have a greater magnitude of negative charge than particle 7, while particle 6 has a greater magnitude of positive charge than particle 8. Other combinations of polymer functionality and charge (or particle size) can be used; however, it must be the case that all four types of particles can be separated from each other in the presence of a suitable electric field, such as a low voltage electric field that can be generated with commercial digital electronics.

[0071] In Figure 3A the system, the electric field required to separate the aggregates formed between the first and second types of particles is less than the electric field required to separate the aggregates formed between the first and fourth particles or the second and third particles (and of course less than the electric field required to separate the third and fourth particles).

[0072] In Figure 3A , the core pigments comprising the particles are shown as having approximately the same size, and the electrokinetic potential of each particle (although not shown) is assumed to be approximately the same. What is different is the thickness of the polymer shell surrounding each core pigment. As Figure 3A shown, the polymer shell of the type 1 and 2 particles is thicker than the polymer shell of the type 3 and 4 particles.

[0073] In the present invention, not all colored pigments must behave as described above with reference to Figure 3C and 3B . As Figure 3A shown, the third particle can have a robust polymer shell, and can have a wide range of charges, including a weak positive charge. In this case, the surface chemistry of the third particle must be different from the surface chemistry of the first particle. For example, the first particle can have a covalently attached silane shell, which is grafted with a polymer that can consist of acrylic or styrene monomers that are preferably hydrophobic. The third particle can include a polymer shell that is not covalently attached but is deposited on the surface of the core particle by dispersion polymerization. In this case, the present invention is not limited to the mechanisms described above with reference to Figure 4 and 3B .

[0074] In order to obtain a high-resolution display, the individual pixels of the display must be addressable without interference from adjacent pixels. One way of achieving this is to provide an array of non-linear elements, such as transistors or diodes, at least one non-linear element being associated with each pixel to create an "active matrix" display. The addressing or pixel electrode of an addressed pixel is connected to the appropriate voltage source through the associated non-linear element. Typically, when the non-linear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this arrangement will be assumed in the following description, although it is essentially arbitrary and the pixel electrode could be connected to the source of the transistor. Typically, in a high-resolution array, the pixels are arranged in a two-dimensional array of rows and columns, such that any particular pixel is uniquely defined by the intersection of a specified row and a specified column. The sources of all the transistors in each column are connected to a single column electrode, while the gates of all the transistors in each row are connected to a single row electrode; again, 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 row drivers, which essentially ensure that only one row is selected at any given time, i.e. a selection voltage is applied to the selected row electrode to ensure that all the transistors in the selected row are turned on, while a non-selection voltage is applied to all other rows to ensure that all the transistors in these unselected rows remain turned off. The column electrodes are connected to column drivers, which apply voltages across the various column electrodes, the voltages being chosen to drive the pixels in the selected row to their desired optical state. (The voltages referred to above are relative to a common front electrode, which is typically provided on the side of the electro-optic medium opposite the non-linear array and extends across the entire display.) After a pre-selection interval, referred to as the "line addressing time", the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed in order to write the next line of the display. This process is repeated in order to write the entire display in a row-by-row fashion. The time between addressing in the display is referred to as a "frame". Thus, a display updated at 60 Hz has a frame of 16 milliseconds.

[0075] Conventionally, each pixel electrode has a capacitor electrode associated therewith, such that the pixel electrode and the capacitor electrode form a capacitor; see, for example, International Patent Application WO 01 / 07961. In some embodiments, an N-type semiconductor (e.g. amorphous silicon) can be used to form the transistors, and the "selection" and "non-selection" voltages applied to the gate electrode can be positive and negative, respectively.

[0076] BRIEF DESCRIPTION OF DRAWINGS Figure 5An exemplary equivalent circuit for a single pixel of an electrophoretic display is depicted. As shown, the circuit includes a capacitor 10 formed between a pixel electrode and a capacitor electrode. The electrophoretic dielectric 20 is represented as a capacitor and a resistor connected in parallel. In some instances, the direct or indirect coupling capacitance 30 between the gate electrode of the transistor associated with the pixel and the pixel electrode (often referred to as "parasitic capacitance") can introduce unwanted noise into the display. Typically, the parasitic capacitance 30 is much smaller than the capacitance of the storage capacitor 10, and when a pixel row of the display is selected or deselected, the parasitic capacitance 30 can cause a small negative offset voltage, also referred to as "recoil voltage," to the pixel electrode, typically less than 2 volts. In some embodiments, to compensate for the unwanted "recoil voltage," a common potential V can be provided to the top plate electrode and the capacitor electrode associated with each pixel. com , so that when V com Set to equal to the recoil voltage (V) KB When the value is ), each voltage supplied to the display can be offset by the same amount, and no net DC imbalance is experienced.

[0077] U.S. Patent No. 9,921,451 describes a set of waveforms for driving a color electrophoretic display with four types of 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 embodiments, the maximum voltage used in these waveforms is higher than the voltage that amorphous silicon thin-film transistors can handle. In these instances, a suitable high voltage can be obtained by using top-plate switching. However, when using top-plate switching, using voltages related to V... com Setting up the same number of independent power supplies is both expensive and inconvenient. Furthermore, it is known that top-panel switching increases backlash, thereby reducing the stability of color states.

[0078] Methods for manufacturing ACeP-type electrophoretic displays have been discussed in the prior art. The electrophoretic fluid can be encapsulated in microcapsules or incorporated into a microcell structure subsequently sealed with a polymer layer. The microcapsules or microcell layers can be coated or imprinted onto a plastic substrate or film with a transparent conductive material coating. The assembly can be laminated to a backplane containing pixel electrodes using a conductive adhesive. Alternatively, the electrophoretic fluid can be directly dispensed onto a thin, open-cell mesh already arranged on a backplane including an active matrix of pixel electrodes. The filled mesh can then be top-sealed with an integrated protective sheet / transparent electrode.

[0079] Figure 6A cross-sectional schematic (not to scale) of a display structure 200 of an ACeP type electrophoretic display is shown. In the display 200, the electrophoretic fluid is shown as confined in microcups, although equivalent structures containing microcapsules can also be used. A substrate 202, which can be glass or plastic, carries pixel electrodes 204, which are either individually addressable segments or associated with thin film transistors in an active matrix arrangement. (The combination of the substrate 202 and the electrodes 204 is often referred to as the backplane of the display.) A layer 206 is an optional dielectric layer applied to the backplane in accordance with the present invention. (Methods for depositing suitable dielectric layers are described in U.S. Patent Application No. 16 / 862,750, incorporated by reference.) The frontplane of the display includes a transparent substrate 222 with a transparent conductive coating 220. The overlying electrode layer 220 is an optional dielectric layer 218. One layer (or multiple layers) 216 is a polymer layer, which can include a base layer for adhering the microcups to the transparent electrode layer 220 and some residual polymer constituting the bottom of the microcups. The walls of the microcups 212 serve to contain the electrophoretic fluid 214. The microcups are sealed with a layer 210, and the entire frontplane structure is adhered to the backplane using a conductive adhesive layer 208. Processes for forming the microcups are described in the prior art, for example in U.S. Patent No. 6,930,818. In some examples, the depth of the microcups is less than 20 pm, for example, the depth is less than 15 pm, for example, the depth is less than 12 pm, for example, the depth is about 10 pm, for example, the depth is about 8 pm.

[0080] Due to the widespread 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 thin film transistors become unstable when providing gate voltages that allow for voltage switching above about + / - 15 V. Nonetheless, as described below, the performance of ACeP is improved when the magnitude of the allowed high positive and negative voltages is more than + / - 15 V. Thus, as described in previous disclosures, improved performance is achieved by additionally changing the bias on the top light-transmissive electrode relative to the bias on the backplane pixel electrode, which is also referred to as top plate switching. Thus, if a voltage of +30 V (relative to the backplane) is needed, the top plate can be switched to -15 V, while the appropriate backplane pixel is switched to +15 V. Methods for driving four-particle electrophoretic systems with top plate switching are described in more detail, for example, in U.S. Patent No. 9,921,451.

[0081] The top plate switching method has several drawbacks. First, when (as is usually the case) the top plate is not pixelated but a single electrode that extends over the entire surface of the display, its potential affects every pixel in the display. If it is set to match one of the voltages of maximum amplitude available from the back plate (e.g. the maximum positive voltage), there will be no net voltage on the ink when that voltage is in effect on the back plate. When any other available voltage is provided to the back plate, a voltage of negative polarity will always be provided to any pixel in the display. Thus, if a waveform requires a positive voltage, it cannot be provided to any pixel until the top plate voltage is changed. A typical waveform for a multi-color display of the third embodiment uses multiple pulses of positive and negative polarity, and the lengths of these pulses differ from those used to produce waveforms of different colors. Furthermore, the phases of waveforms of different colors can differ: in other words, for some colors, a positive pulse can precede a negative pulse, while for other colors, a negative pulse can precede a positive pulse. To accommodate this, a "rest" (i.e. pause) must be inserted in the waveform. In practice, this results in waveforms that are much longer (up to twice as long) than they need to be.

[0082] Second, in top plate switching, there is a limit to the voltage levels that can be selected. If the voltages applied to the top plate are denoted V t+ and V t- respectively, and the voltages applied to the back plate are denoted V b+ and V b- respectively, in order to achieve a zero volt condition in the electrophoretic fluid, it must be that |V t+ | = |V b+ | and |V t- | = |V b- |. However, the amplitudes of the positive and negative voltages need not be the same.

[0083] In the previous embodiments of advanced color electronic paper The waveforms (voltage versus time curves) applied to the pixel electrodes of the back plate of the displays of the present invention are described and plotted, assuming that the front electrode is grounded (i.e. at zero potential). The electric field experienced by the electrophoretic medium is of course determined by the potential difference between the back plate and the front electrode and the distance between them. The display is usually viewed through its front electrode, and it is the particles adjacent to the front electrode that control the color displayed by the pixel, and it is sometimes easier to understand the optical transformations involved if the potential of the front electrode relative to the back plate is taken into account; this can simply be done by reversing the waveforms discussed below.

[0084] Figure 6A typical waveform (simplified form) for driving the four-particle color electrophoretic display system described above is shown. This waveform has a simple "push-pull" structure: that is, it consists of a dipole comprising two pulses of opposite polarity. The amplitude and length of these pulses determine the color obtained. There should be at least five such voltage levels. Figure 6 High and low positive and negative voltages are shown, as well as zero volts. In general, "low" (L) refers to a range of about 5-15 V, while "high" (H) refers to a range of about 15-30 V. In general, the higher the amplitude of the "high" voltage, the better the color gamut achieved by the display. The "medium" (M) level is typically around 15 V; however, the value of M will depend somewhat on the composition of the particles and the environment of the electrophoretic medium. If only three voltages are available (i.e., +V high , 0 and -V high ), the same results as addressing at a lower voltage (e.g., V high / n, where n is a positive integer greater than 1) can be achieved by addressing with pulses of voltage V high but with a duty cycle of 1 / n. The electrophoretic particle set that can be used with Figure 6 push-pull waveforms can include negatively charged white particles, negatively charged yellow particles, positively charged magenta particles, and positively charged cyan particles.

[0085] In an alternative embodiment, a seven-level driver can be used to directly address each pixel without the need for top plate switching. However, it is difficult to implement a seven-level driver with sufficient voltage amplitude using a standard amorphous silicon backplane. It has been found that using control transistors made from less common materials with higher electron mobility allows the transistors to switch larger control voltages, e.g., + / - 30 V, as needed to implement a seven-level drive. Newly developed active matrix backplanes can include thin film transistors incorporating metal oxide materials such as tungsten oxide, tin oxide, indium oxide, and zinc oxide. In these applications, the channel formation region is formed for each transistor using such a metal oxide material, allowing faster switching of higher voltages, e.g., in the range of about -27 V to +27 V. Such transistors typically include a gate electrode, a gate insulating film (typically Si02), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film over the gate insulating film that at least partially overlaps the gate, source, and drain electrodes. Such backplanes are available from manufacturers such as Sharp / Foxconn, LG, and BOE. One preferred metal oxide material for such applications is indium gallium zinc oxide (IGZO). IGZO-TFTs have an electron mobility that is 20-50 times that of amorphous silicon. By using IGZO TFTs in an active matrix backplane, voltages greater than 30 V can be provided by a suitable display driver.

[0086] In this document, the term "frame" refers to a single update of all rows in a display. It will be clear to those skilled in the art that in a display of the present invention driven using a thin film transistor (TFT) array, Figure 6 The available time increments on the abscissa will typically be quantized by the frame rate of the display. Also, it will be clear that the display is addressed by changing the potential of the pixel electrode relative to the front electrode, and this can be achieved by changing the potential of either the pixel electrode or the front electrode or both. In the prior art, there is typically a matrix of pixel electrodes on a backplane, while the front electrode is common to all pixels. Thus, when the potential of the front electrode is changed, it affects the addressing of all pixels. The basic structure of the waveforms described above with reference to Figure 7A is the same whether or not a varying voltage is applied to the front electrode.

[0087] The reset (i.e. clearing pulse) of the display ideally erases any memory of the previous state, including residual voltages and pigment configurations specific to the previous display color, and allows the multi-particle electrophoretic display to reliably achieve the same color state using the same driving scheme. Figure 7B A generalized clearing pulse is shown in FIG. 1. In general, the reset pulse of the present invention comprises a high positive voltage (V H ), followed by a lower negative voltage (V L ), and the length of the positive voltage pulse (t) must be shorter than the length of the negative voltage pulse (t'). Each period of voltage as a function of time can be referred to as a "kick," and each kick i can be described as having a voltage V i and a time t i The waveforms can be direct current (DC) balanced or DC unbalanced. The waveforms can be made DC balanced by adding additional kicks, which would make the sum of the voltage-time areas of all positive voltage pulses equal to the sum of the voltage-time areas of all negative voltage pulses. Mathematically, DC balance can be expressed as

[0088]

[0089] where n = 1-8, and V n and t n are as defined above. Furthermore, the amplitude and duration of each kick in the push-pull sequence can vary, and optional "rests" (i.e. periods of no applied voltage) can be inserted between kicks. In some embodiments.

[0090] In other instances, the clearing waveform comprises two parts, each designed specifically for a particular purpose. For example, as Figure 8The first part of the previous state clearing phase implements a push-pull sequence to achieve a universal color initial state, e.g., magenta, to achieve the best possible clearing. The second part includes a neutral state, e.g., a state from which all subsequent colors can be produced by a simplified push-pull color transition. In some embodiments, the first part includes pulses of amplitude Vi and length ti alternating with pulses of amplitude V2 and length t2, where Vi is a positive voltage and V2 is a negative voltage of lower magnitude than Vi. In such embodiments, the second part includes pulses of amplitude V3 and length t3 alternating with pulses of amplitude V4 and length t4. The ratio V 1· t1 / V 2· t2 is greater than the ratio V 3· t3 / V 4· t4.

[0091] An exemplary waveform can drive the electrophoretic medium from an unknown state to a magenta optical state, and then to a neutral white state, e.g., as Figure 8 shown. As noted above, in Figure 6 the case of a sequence that presents a magenta state, the impulse difference, i.e., |Vi·ti| - |V2·t2|, is greater than in the case of a sequence that presents a white state.

[0092] The previous state clearing pulses of the present invention can be combined with all color waveforms, e.g., as Figure 9 shown. For example, in some waveforms described in the above-mentioned U.S. Patent No. 9,921,451, seven different voltages can be applied to the pixel electrode: three positive voltages, three negative voltages, and zero voltage. Figure 9 One such waveform is schematically depicted, for clearing and then displaying a desired color at a pixel. As Figure 9 shown, the waveform for each color has the same basic form: (A) a preliminary series of frames to “reset” the display to a state from which any color can be reproducibly obtained, and during which a DC imbalance is provided that is equal and opposite to the DC imbalance of the rest of the waveform, and (B) a series of frames specific to the color to be presented. If a DC balance pulse is included in the waveform, it can occur before the clearing pulse, or after the clearing pulse but before the push-pull color addressing pulses. It should be appreciated that Figure 7B is a generalization of a clearing pulse combined with color addressing pulses. Figure 9 The two-part clearing pulse of the type shown in Figure 9 may be used as a substitute for the one-part clearing pulse shown in ​ Thus, in

[0093] For comparison, see Figure 1 [prior art]. In phase A (a reset phase), it can be seen that this phase is divided into two parts of equal duration (as indicated by the dashed lines). When using top plate switching, the top plate will be held at one potential in the first of these parts, and at the opposite polarity potential in the second part. In the particular case of Figure 1, in the first such part, the top plate will be held at V p H, while the back plate will be held at V n H, to achieve a potential drop over the electrophoretic fluid of V n H-V p H (where it is agreed to use the back plate potential with reference to the top plate potential). In the second part, the top plate will be held at V n H, while the back plate is held at V p H. As shown, in the second part, the electrophoretic fluid will be subjected to a potential of V p H-V n H, which is the highest potential available. However, for the reproduction of certain colors, exposure to such high voltages can result in an initial pigment arrangement from which it is difficult to achieve the ideal final configuration. For example, as described in the prior art, in order to render a cyan color, it is necessary to combine magenta pigments (which have the same polarity of charge as cyan pigments) with yellow pigments into an aggregate. Such an aggregate will be split by the high potential applied, so the magenta color will not be controlled and will contaminate the cyan color.

[0094] The foregoing discussion of waveforms, and in particular the discussion of DC balance, ignores the problem of kickback voltage. In fact, as noted previously, each back plate voltage is offset from the voltage supplied by the power supply by an amount equal to the kickback voltage V KB . Thus, if the power supply used supplies three voltages +V, 0, and -V, the back plate will in fact receive voltages V+V KB , V KB , and -V+V KB (please note that V KB is typically negative in the case of amorphous silicon TFTs). However, the same power supply would supply +V, 0, and -V to the front electrode without any kickback voltage offset. Thus, for example, when the front electrode is supplied with -V, the display will experience a maximum voltage of 2V+V KB and a minimum voltage of V KB . Instead of using a separate power supply to supply V KB to the front electrode (which can be expensive and inconvenient), it is possible to divide the waveforms into parts in which the front electrode is supplied with a positive voltage, a negative voltage, and V KB .

[0095] Accordingly, the present application provides erase waveforms for multi-particle electrophoretic displays. Having thus described several aspects of the technology of the present application, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described in the present application. For example, it will be readily apparent to one of ordinary skill in the art that the various features, aspects, and / or functions described herein can be rearranged and / or combined into a single procedure, without departing from the scope of the present application. It will also be apparent that the various features, aspects, and / or functions described herein can be implemented on a variety of software- and / or hardware-only systems. Additionally, it should be noted that any combination of two or more features, systems, articles, materials, kits, and / or methods described herein, where such features, systems, articles, materials, kits, and / or methods are not mutually inconsistent, is within the scope of the present disclosure.

Claims

1. A method for driving an electrophoretic display, the electrophoretic display comprising a metal oxide backplane using a seven-stage actuator providing three positive voltages, three negative voltages, and zero voltage, the electrophoretic display comprising an electrophoretic medium having four different types of electrophoretic particles, each type of electrophoretic particle having a different color and a different combination of charge polarity and charge quantity, the method comprising: A clearing pulse is provided, the clearing pulse comprising a first set of impulses with amplitude V1 and length t1 alternating with impulses of amplitude V2 and length t2, and a second set of impulses with amplitude V3 and length t3 alternating with impulses of amplitude V4 and length t4, wherein V1 is a positive voltage and V2 is a negative voltage with an amplitude lower than V1, wherein V3 is a positive voltage and V4 is a negative voltage with an amplitude lower than V3, wherein the ratio (V 1· t1) / (V 2· t2) is greater than the ratio (V) 3· t3) / (V 4· t4); and A push-pull color-addressing pulse is provided, which has a push impulse of amplitude V5 and length t5 and a pull impulse of amplitude V6 and length t6, wherein V5 and V6 have opposite polarities.

2. The method according to claim 1, wherein, The electrophoretic display includes two types of positively charged electrophoretic particles with different charge amounts, and two types of negatively charged electrophoretic particles with different charge amounts.

3. The method according to claim 2, wherein, The two types of positively charged particles are cyan and magenta in color, and the two types of negatively charged particles are white and yellow in color.

4. The method of claim 1, further comprising providing at least three impulses with amplitude V1 and length t1, at least three impulses with amplitude V2 and length t2, at least three impulses with amplitude V3 and length t3, and at least three impulses with length V4 and length t4.

5. The method according to claim 4, wherein, An impulse of amplitude V2 and length t2 is interspersed among at least two impulses of amplitude V1 and length t1, and an impulse of amplitude V4 and length t4 is interspersed among at least two impulses of amplitude V3 and length t3.

6. The method according to claim 4, further comprising providing a DC balancing pulse, the DC balancing pulse comprising a first DC balancing impulse with an amplitude of V7 and a length of t7 and a second DC balancing impulse with an amplitude of V8 and a length of t8, wherein, V7 and V8 have opposite polarities, and the sum of the voltage-time areas of all positive voltage pulses is equal to the sum of the voltage-time areas of all negative voltage pulses.

7. The method of claim 6, wherein n = 1-8, and V n and t n As defined above.

8. The method according to claim 6, wherein, The DC balancing pulse precedes the clear pulse and the push-pull color addressing pulse.

9. The method according to claim 6, wherein, The DC balancing pulse is between the clear pulse and the push-pull color addressing pulse.

10. The method according to claim 1, wherein, The electrophoretic display includes a first light-transmitting electrode layer, a second electrode layer including a plurality of pixel electrodes, and an electrophoretic layer, wherein the electrophoretic layer includes an electrophoretic medium disposed between the first light-transmitting electrode layer and the second electrode layer.

11. The method according to claim 10, wherein, The electrophoretic layer includes a plurality of micro-units containing the electrophoretic medium.

12. The method according to claim 10, wherein, The electrophoretic layer includes a plurality of microcapsules containing the electrophoretic medium.

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