Enhanced push-pull (EPP) waveforms for achieving primary color sets in multi-color electrophoretic displays

Abstract

To provide favorable enhanced push-pull (EPP) waveforms for achieving primary color sets in multi-color electrophoretic displays.SOLUTION: The invention relates to enhanced push pull driving waveforms for driving a four-particle electrophoretic medium, the four-particle electrophoretic medium includes four different types of particles, e.g., a set of scattering particles and three sets of subtractive particles. The invention also relates to methods for identifying a preferred waveform for a target color state when using a voltage driver having at least five different voltage levels.SELECTED DRAWING: Figure 8

Description

Background Art

[0001] (Related Application) This application claims the benefit of priority of U.S. Provisional Patent Application No. 63 / 108,521, filed Nov. 2, 2020. All patents and publications disclosed herein are hereby incorporated by reference in their entirety.

[0002] An electrophoretic display (EPD) changes color by modifying the position of charged color particles relative to a light-transmissive viewing surface. Such electrophoretic displays are typically referred to as "electronic paper" or "e-paper" because the resulting display has high contrast and is sunlight-readable, much like ink on paper. Electrophoretic displays have been widely adopted in e-book readers such as AMAZON KINDLE (registered trademark) because they provide a book-like reading experience, use low power, and allow users to carry a library of hundreds of books in a lightweight, portable device.

[0003] For many years, electrophoretic displays have included only two types of charged color particles, namely, black and white only (to be sure, "color" includes black and white as used herein). White particles are often light-scattering and, for example, comprise titanium dioxide, while black particles are absorptive across the visible spectrum and may comprise carbon black or an absorptive metal oxide such as copper chromite. In the simplest sense, black and white electrophoretic displays require only a light-transmissive electrode on the viewing surface, a back electrode, and an electrophoretic medium containing oppositely charged white and black particles. When a voltage of one polarity is provided, the white particles move to the viewing surface, and when a voltage of the opposite polarity is provided, the black particles move to the viewing surface. If the back electrode includes either a controllable area (pixel), i.e., a segmented electrode, or an active matrix of pixel electrodes controlled by transistors, a pattern can be electronically presented on the viewing surface. This pattern can be, for example, the text for a book.

[0004] More recently, various color options, including three-color displays (black, white, red, and black, white, yellow) and four-color displays (black, white, red, yellow), have become commercially available for electrophoretic displays. Similar to the operation of black and white electrophoretic displays, electrophoretic displays with three or four reflective pigments operate in the same manner as simple black and white displays, as the desired color particles are driven towards the viewing surface. This drive scheme is much more complex than black and white only, but ultimately, the optical function of the particles is the same.

[0005] Advanced Color ePaper (ACeP TM) also includes four particles, but the cyan, yellow, and magenta particles are subtractive rather than reflective, thereby enabling thousands of colors to be generated at each pixel. This color process is functionally equivalent to the printing method that has long been used in offset printing and inkjet printers. A given color is generated by using the correct ratios of cyan, yellow, and magenta on a bright white paper background. In the case of ACeP, the relative positions of the cyan, yellow, magenta, and white particles with respect to the viewing surface will determine the color at each pixel. This type of electrophoretic display enables thousands of colors at each pixel, but it is extremely important to carefully control the position of each of the pigments (sized 50 - 500 nanometers) within a working space that is approximately 10 - 20 microns thick. Clearly assuming white, variations in the position of the pigments will result in incorrect colors being displayed at a given pixel. Therefore, precise voltage control is required for such a system. Further details of the system are available in the following U.S. patents, all of which are incorporated by reference in their entirety: U.S. Patent Nos. 9,361,836 (Patent Document 1), 9,921,451, 10,276,109, 10,353,266, 10,467,984, and 10,593,272.

[0006] The present invention relates to waveforms for driving a color electrophoretic display, and more particularly, but not exclusively, to an electrophoretic display capable of rendering two or more colors using a single layer of an electrophoretic material comprising a plurality of colored particles, such as 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 has a thick polymer shell and one negatively charged particle has a thick polymer shell.

[0007] The term "gray state" is used herein in its conventional meaning in imaging technology and refers to a state intermediate between the optical states of two extreme pixels and does not necessarily imply a black-white transition between these two extreme states. For example, some of the E Ink patents and published applications referred to below describe electrophoretic displays where the extreme states are white and dark blue, such that the intermediate gray state would actually be light blue. In fact, as already described, a change in optical state may not be a color change at all. The terms "black and white" may hereinafter be used herein to refer to the two extreme optical states of a display and are generally to be understood to include extreme optical states that are not strictly black and white, such as the white and dark blue states described above.

[0008] The terms "bistable" and "bistability" are used herein in their conventional meaning in the art to refer to a display having a display element with first and second display states with at least one different optical property, such that after any given element is driven using an address pulse of finite duration to indicate either its first or second display state, the state of the display element will persist for at least several times, e.g., at least four times, the minimum duration of the address pulse required to change its state after the address pulse has ended. Some particle-based electrophoretic displays with grayscale capability are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is shown in U.S. Patent No. 7,170,670 (Patent Document 2) to be true for several other types of electro-optical displays. This type of display is appropriately referred to as "multi-stable" rather than "bistable", but for convenience, the term "bistable" may be used herein to encompass both bistable and multi-stable displays.

[0009] As used herein, the term "impulse" refers to the integral of the applied voltage with respect to time during the period when the electrophoretic display is driven. The term "waveform" is used herein to describe a series of voltages, or the pattern thereof, provided to the electrophoretic medium over a given period (seconds, frames, etc.) to produce a desired optical effect in the electrophoretic medium when used to refer to the driving of an electrophoretic display.

[0010] Particles that absorb, scatter, or reflect light, either in a broad band or at a selected wavelength, are referred to herein as colored or pigment particles. Various materials other than pigments such as dyes or photonic crystals (in the strict sense of the term as meaning insoluble coloring materials) that absorb or reflect light can also be used in the electrophoretic media and displays of the present invention. and in the displays.

[0011] Particle-based electrophoretic displays have been the subject of intensive research and development over the years. In such displays, a plurality of charged particles (sometimes also referred to as pigment particles) move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, bistable states, and low power consumption when compared to liquid crystal displays. Nevertheless, problems associated with the long-term image quality of these displays have hindered their widespread use. For example, the particles that make up an electrophoretic display tend to sediment, resulting in an inadequate lifespan for these displays.

[0012] As described above, an electrophoretic medium requires the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, although electrophoretic media can also be produced using a gaseous fluid. See, for example, Kitamura, T., et al., Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1 and Yamaguchi, Y., et al., Toner display using insulative particles charged triboelectrically, IDW Japan, 2001, Paper AMD4-4. Also see U.S. Patent Nos. 7,321,459 (Patent Document 3) and 7,236,291. Such gas-based electrophoretic media are likely to be affected by the same type of problems due to particle sedimentation as liquid-based electrophoretic media when, for example, the medium is used in an orientation that allows such sedimentation in a sign where the medium is disposed in a vertical plane. In fact, particle sedimentation is considered to be a more serious problem in gas-based electrophoretic media than in liquid-based ones because the lower viscosity of the gaseous suspension fluid allows for faster sedimentation of the electrophoretic particles compared to that of a liquid.

[0013] A number of 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 electrophoretic media and other electro-optic media. Such encapsulated media comprise a number of small capsules, each of which itself comprises an internal phase containing particles movable by electrophoresis in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder, which forms a coherent layer positioned between two electrodes. The techniques described in these patents and applications include the following. (a) Electrophoretic particles, fluids, and fluid additives (see, e.g., U.S. Pat. Nos. 7,002,728 and 7,679,814) (b) Capsules, binders, and encapsulation processes (see, e.g., U.S. Pat. Nos. 6,922,276 and 7,411,719) (c) Microcell structures, wall materials, and methods of forming microcells (see, e.g., U.S. Pat. Nos. 7,072,095 and 9,279,906) (d) Methods of filling and sealing microcells (see, e.g., U.S. Pat. Nos. 7,144,942 and 7,715,088) (e) Films and subassemblies containing electro-optical materials (see, e.g., U.S. Pat. Nos. 6,982,178 and 7,839,564) (f) Backplanes, adhesive layers, and other auxiliary layers, and methods of use in displays (see, e.g., U.S. Pat. Nos. 7,116,318 and 7,535,624) (g) Color formation and color adjustment (see, e.g., U.S. Pat. No. 6,017,584, 6, Nos. 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,No. 666, and U.S. Patent Application Publication Nos. 2008 / 0043318, 2008 / 0048970, 2009 / 0225398, 2010 / 0156780, 2011 / 0043543, 2012 / 0326957, 2013 / 0242378, 2013 / 0278995, 2014 / 0055840, 2014 / 0078576, 2014 / 0340430, 2014 / 0340736, 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 (see), (h) Method of driving a display (e.g., U.S. Patent Nos. 5,930,026, 6,445,489, 6,504,524, 6,512,354, 6,531,997, 6,753,999, 6,825,970, 6,900,851, 6,995,550, 7,012,600, 7,023,420, 7,034,783, 7,061,166, 7,061,662, 7,116,466, 7,119,772, 7,177,066, 7,193,625, 7,202,847, 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, No. 558,783, No. 8,558,785, No. 8,558,786, No. 8,558,855, No. 8,576,164, No. 8,576,259, No. 8,593,396, No. 8,605,032, No. 8,643,595, No. 8,665,206, No. 8,681,191, No. 8,730,153, No. 8,810,525, No. 8,928,562, No. 8,928,641, No. 8,976,444, No. 9,013,394, No. 9,019,197, No. 9,019,198, No. 9,019,318, No. 9,082,352, No. 9,171,508, No. 9,218,773, No. 9,224,338, No. 9,224,342, No. 9,224,344, No. 9,230,492, No. 9,251,736, No. 9,262,973, No. 9,269,311, No. 9,299,294, No. 9,373,289, No. 9,390,066, No. 9,390,661, and No. 9,412,No. 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 may hereinafter be referred to as MEDEOD (Methods for Driving Electro-Optical Displays) applications), (i) Use of the display (see, for example, U.S. Pat. Nos. 7,312,784 and 8,009,348) (j) For example, a non-electrophoretic display as described in U.S. Patent No. 6,241,921, and U.S. Patent Application Publication Nos. 2015 / 0277160, 2015 / 0005720, and 2016 / 0012710

[0014] Many of the aforementioned patents and applications recognize that the walls surrounding separate microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, and thus, an electrophoretic medium can produce a so-called "polymer-dispersed electrophoretic display" comprising a plurality of separate droplets of an electrophoretic fluid and a continuous phase of a polymeric material. The separate droplets of the electrophoretic fluid within such a polymer-dispersed electrophoretic display can be regarded as capsules or microcapsules even though no separate capsule membranes are associated with each individual droplet. See, for example, U.S. Patent No. 6,866,760. Thus, for the purposes of the present application, such a polymer-dispersed electrophoretic medium is regarded as a sub-species of an encapsulated electrophoretic medium.

[0015] A related type of electrophoretic display is the so-called "microcell electrophoretic display" ". In a microcell electrophoretic display, charged particles and a fluid are not encapsulated within microcapsules, but instead are held within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, U.S. Patents Nos. 6,672,921 and 6,788,449.

[0016] Electrophoretic media are often opaque (e.g., in many electrophoretic media, the particles substantially block the transmission of visible light through the display), and operate in a reflective mode. However, many electrophoretic displays can be fabricated to operate in a so-called "shutter mode" where one display state is substantially opaque and one is light transmissive. See, for example, U.S. Pat. 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 are similar to electrophoretic displays but rely on variations in electric field strength and can operate in similar modes. See U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in a shutter mode. Electro-optic media operating in a shutter mode can be used in a multilayer structure for a full-color display. In such a structure, at least one layer adjacent to the viewing surface of the display operates in a shutter mode to expose or conceal a second layer that is more remote from the viewing surface.

[0017] Encapsulated electrophoretic displays typically suffer no clustering and settling failure modes of conventional electrophoretic devices and offer additional advantages such as the ability to print or coat displays on a wide variety of flexible and rigid substrates (the use of the term "print" is intended to include, but is not limited to, pre-metered coating, such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating, etc., roll coating, such as knife over roll coating, forward and reverse roll coating, etc., gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, silk screen printing process, electrostatic printing process, thermal printing process, inkjet printing process, electrophoretic deposition (see U.S. Patent No. 7,339,715), and other similar techniques, including any form of printing and coating). Thus, the resulting display can be flexible. Further, since the display medium can be printed (using various methods), the display itself can be manufactured inexpensively.

[0018] As described above, the simplest prior art electrophoretic media essentially display only two colors. Such electrophoretic media use either a single type of electrophoretic particle having a first color in a colored fluid having a second different color (in which case the first color is displayed when the particles are adjacent to the viewing surface of the display and the second color is displayed when the particles are spaced from the viewing surface), or first and second types of electrophoretic particles having different first and second colors in an uncolored fluid (in which case the first color is displayed when the first type of particles are adjacent to the viewing surface of the display and the second color is displayed when the second type of particles are adjacent to the viewing surface). Typically, the two colors are black and white. If a full-color display is desired, a color filter array can be deposited over the viewing surface of a monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color mixing to create color stimuli. 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 (2×2) repeating pattern. Other alternative primary colors or more than four primary colors are also applicable It is known in the art. Three (in the case of an RGB display) or four (in the case of an RGBW display) subpixels are selected to be small enough so that, at the intended viewing distance, they visually blend together into a single pixel with a uniform color stimulus (“color mixing”). A particular disadvantage of area sharing is that the colorants are always present and the color can only be modulated by switching the corresponding pixel of the underlying monochrome display to white or black (switching the corresponding primary color on or off). For example, in an ideal RGBW display, each of the red, green, blue, and white primary colors occupies one quarter (one of four subpixels) of the display area, and the white subpixel is as bright as the white of the underlying monochrome display, but each of the colored subpixels is not brighter than one third of the white of the monochrome display. The brightness of the white shown by the display as a whole cannot exceed half of the brightness of the white subpixel (the white area of the display is generated by displaying each colored subpixel in its colored form equivalent to one third of the white subpixel in addition to one white subpixel out of each four, so the three colored subpixels combined do not contribute more than one white subpixel). The brightness and saturation of the color are reduced by area sharing with the color pixel switched to black. Area sharing is particularly problematic when mixing yellow, as it is brighter than any other color of equal brightness and a saturated yellow is almost as bright as white. Switching the blue pixel (one quarter of the display area) to black makes the yellow too dark.

[0019] U.S. Patents Nos. 8,576,476 and 8,797,634 describe a multicolor electrophoretic display having a single backplane with independently addressable pixel electrodes and a common optically 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 associated with the use of a plurality of electrophoretic layers positioned between a single set of address 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 using the same voltage. In addition, optical losses (e.g., due to 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.

[0020] Attempts have been made to provide a full-color electrophoretic display using a single electrophoretic layer. For example, U.S. Patent No. 8,917,439 describes a color display comprising an electrophoretic fluid, the electrophoretic fluid comprising one or two types of pigment particles dispersed in a clear and colorless or colored solvent, the electrophoretic fluid being disposed between a common electrode and a plurality of pixels or drive electrodes. The drive electrodes are arranged to expose a background layer. U.S. Patent No. 9,116,412 describes a method of driving a display cell filled with an electrophoretic fluid, the electrophoretic fluid having two types of charged particles with opposite charge polarities and two contrast colors. The two types of pigment particles are dispersed in a colored solvent or in a solvent with dispersed uncharged or weakly charged colored particles. The method includes driving the display cell by applying a driving voltage that is about 1% to about 20% of the total driving voltage to display the color of the solvent or the color of the uncharged or weakly charged colored particles. U.S. Patent Nos. 8,717,664 and 8,964,282 describe an electrophoretic fluid and a method of driving an electrophoretic display. The fluid comprises 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 polarities, and the third type of pigment particle has a charge level that is less than about 50% of the charge level of the first or second type. 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 hereinafter.

Prior Art Documents

Patent Documents

[0021]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Means for Solving the Problems

[0022] Disclosed herein are improved driving methods for full-color electrophoretic displays and methods for identifying waveforms for full-color electrophoretic displays using these methods. In one aspect, a method of driving an electrophoretic display is disclosed. The driving method includes providing an electrophoretic medium comprising four sets of particles, each set of particles having different optical properties and different charge properties; disposing the electrophoretic medium between a first light-transmissive electrode and a second electrode; providing a voltage driver configured to provide at least five voltage levels, namely a high negative voltage, a medium negative voltage, a zero voltage, a medium positive voltage, and a high positive voltage; and driving the electrophoretic medium to a desired optical state by providing a push-pull waveform. Such a push-pull waveform includes a first positive portion consisting of a first pulse and a second pulse, the first pulse having a first positive magnitude and a first time width, and the second pulse having a second positive magnitude and a second time width. The push-pull waveform further includes a second negative portion consisting of a third pulse and a fourth pulse, the third pulse having a first negative magnitude and a third time width, and the fourth pulse having a second negative magnitude and a fourth time width. All of the first positive magnitude, the second positive magnitude, the first negative magnitude, and the second negative magnitude are non-zero, and at least three of the first, second, third, and fourth time widths are non-zero. In certain embodiments, the first set of particles is reflective and the second, third, and fourth sets of particles are subtractive. In certain embodiments, two of the sets of particles are positively charged and two of the sets of particles are negatively charged. In certain embodiments, one of the sets of particles is positively charged and three of the sets of particles are negatively charged. In certain embodiments, three of the sets of particles are positively charged and one of the sets of particles is negatively charged. In certain embodiments, the second electrode comprises a plurality of pixel electrodes disposed in an array. In certain embodiments, the second electrode is light-transmissive.In one embodiment, the high negative voltage is from -30V to -20V, the medium negative voltage is from -20V to -2V, the medium positive voltage is from 2V to 20V, and the high positive voltage is from 20V to 30V.

[0023] In another aspect, a method for identifying an enhanced push-pull waveform is disclosed. The method for identifying an enhanced push-pull waveform includes selecting a finite set of voltages for driving an electrophoretic display, the set including at least five different voltage levels, selecting a finite time width for a candidate waveform, and calculating all waveforms having a first positive portion consisting of a first pulse and a second pulse, the first pulse having a first positive magnitude and a first time width, the second pulse having a second positive magnitude and a second time width, a second negative portion consisting of a third pulse and a fourth pulse, the third pulse having a first negative magnitude and a third time width, the fourth pulse having a second negative magnitude and a fourth time width. Each of the first positive magnitude, the second positive magnitude, the first negative magnitude, and the second negative magnitude has a value from the finite set of voltages, and the sum of the first pulse width, the second pulse width, the third pulse width, and the fourth pulse width is equal to the finite time width. The final step comprises four sets of particles Using a model of an electrophoretic display having an electrophoretic medium, calculating an optical state generated by each of a set of candidate waveforms, wherein each set of particles has different optical properties and different charge properties, and the electrophoretic medium is disposed between a first optically transparent electrode and a second electrode, and selecting a waveform for generating a targeted optical state. In certain embodiments, selecting includes comparing a target color to a predicted output color. In certain embodiments, the selected waveform is input to a physical electrophoretic display including an electrophoretic medium having four sets of particles, wherein each set of particles has different optical properties and different charge properties, and the electrophoretic medium is disposed between a first optically transparent electrode and a second electrode. In certain embodiments, the color output of the physical electrophoretic display is evaluated and compared to the target color. In certain embodiments, a finite set of voltages includes a high negative voltage from -30V to -20V, a medium negative voltage from -20V to -2V, a medium positive voltage from 2V to 20V, and a high positive voltage from 20V to 30V. In certain embodiments, a finite set of voltages includes -27V, 0V, and +27V. In certain embodiments, a first set of particles is reflective and second, third, and fourth sets of particles are subtractive. In certain embodiments, two of the sets of particles are positively charged and two of the sets of particles are negatively charged. In certain embodiments, one of the sets of particles is positively charged and three of the sets of particles are negatively charged. In certain embodiments, three of the sets of particles are positively charged and one of the sets of particles is negatively charged. This specification also provides, for example, the following items. (Item 1) A method of driving an electrophoretic display, the method comprising: providing an electrophoretic medium having four sets of particles, wherein each set of particles has different optical properties and different charge properties, and disposing the electrophoretic medium between a first optically transparent electrode and a second electrode, and To provide a voltage driver configured to provide at least five voltage levels, wherein the at least five voltage levels are a high negative voltage, a medium negative voltage, a zero voltage, a medium positive voltage, and a high positive voltage, To drive the electrophoretic medium to a desired optical state by providing a push-pull waveform comprising wherein the push-pull waveform has a first positive portion consisting of a first pulse and a second pulse, wherein the first pulse has a first positive magnitude and a first time width, and the second pulse has a second positive magnitude and a second time width, a second negative portion consisting of a third pulse and a fourth pulse, wherein the third pulse has a first negative magnitude and a third time width, and the fourth pulse has a second negative magnitude and a fourth time width, and has wherein all of the first positive magnitude, the second positive magnitude, the first negative magnitude, and the second negative magnitude are non-zero, and at least three of the first, second, third, and fourth time widths are non-zero, a method. (Item 2) The method according to item 1, wherein the first set of the particles is reflective, and the second, third, and fourth sets of the particles are subtractive. (Item 3) The method according to item 2, wherein two of the sets of the particles are positively charged, and two of the sets of the particles are negatively charged. (Item 4) The method according to item 2, wherein one of the sets of the particles is positively charged, and three of the sets of the particles are negatively charged. (Item 5) The method according to item 2, wherein three of the sets of the particles are positively charged, and one of the sets of the particles is negatively charged. (Item 6) The method according to item 1, wherein the second electrode comprises a plurality of pixel electrodes arranged in an array. (Item 7) The method according to item 1, wherein the second electrode is light-transmissive. (Item 8) The method according to item 1, wherein the high negative voltage is -30V to -20V, the medium negative voltage is -20V to -2V, the medium positive voltage is 2V to 20V, and the high positive voltage is 20V to 30V. (Item 9) The method according to item 1, wherein the voltage driver is configured to provide seven voltage levels: a high negative voltage, a medium negative voltage, a low negative voltage, a zero voltage, a low positive voltage, a medium positive voltage, and a high positive voltage. (Item 10) A method for identifying an enhanced push-pull waveform, the method comprising: selecting a finite set of voltages for driving an electrophoretic display, the set including at least five different voltage levels; selecting a finite time width for a candidate waveform; calculating all waveforms, all of the waveforms comprising: a first positive portion consisting of a first pulse and a second pulse, the first pulse having a first positive magnitude and a first time width, and the second pulse having a second positive magnitude and a second time width; a second negative portion consisting of a third pulse and a fourth pulse, the third pulse having a first negative magnitude and a third time width, and the fourth pulse having a second negative magnitude and a fourth time width; each of the first positive magnitude, the second positive magnitude, the first negative magnitude, and the second negative magnitude having a value from the finite set of voltages; the sum of the first pulse width, the second pulse width, the third pulse width, and the fourth pulse width being equal to the finite time width. Using a model of an electrophoretic display having an electrophoretic medium comprising four sets of particles, calculating an optical state generated by each of the candidate waveforms, wherein each set of particles has different optical properties and different charge properties, and the electrophoretic medium is disposed between a first light-transmissive electrode and a second electrode; selecting a waveform for generating a targeted optical state; A method comprising. (Item 11) The method according to item 10, wherein selecting comprises comparing a target color with a predicted output color. (Item 12) The method according to item 11, further comprising inputting the selected waveform into a physical electrophoretic display comprising an electrophoretic medium comprising four sets of particles, each set of particles having different optical properties and different charge properties, and the electrophoretic medium is disposed between a first light-transmissive electrode and a second electrode. (Item 13) The method according to item 12, further comprising evaluating the color output of the physical electrophoretic display and comparing the color output with the target color. (Item 14) The method according to item 11, wherein the finite set of voltages includes a high negative voltage of -30V to -20V, a medium negative voltage of -20V to -2V, a medium positive voltage of 2V to 20V, and a high positive voltage of 20V to 30V. (Item 15) The method according to item 10, wherein the finite set of voltages includes -27V, 0V, and +27V. (Item 16) The method according to item 10, wherein the set includes seven voltage levels: high negative voltage, medium negative voltage, low negative voltage, zero voltage, low positive voltage, medium positive voltage, and high positive voltage. (Item 17) The method according to item 10, wherein the first set of particles is reflective and the second, third, and fourth sets of particles are subtractive. (Item 18) The method according to item 17, wherein two of the sets of particles are positively charged and two of the sets of particles are negatively charged. (Item 19) The method according to item 17, wherein one of the sets of particles is positively charged and three of the sets of particles are negatively charged. (Item 20) The method according to item 17, wherein three of the sets of particles are positively charged and one of the sets of particles is negatively charged.

Brief Description of the Drawings

[0024]

Figure 1

[0025]

Figure 2A

[0026]

Figure 2B

[0027]

Figure 2C

[0028]

Figure 3

[0029]

Figure 4

[0030]

Figure 5

[0031]

Figure 6

[0032]

Figure 7

[0033]

Figure 8

[0034]

Figure 9

[0035]

Figure 10

[0036]

Figure 11

[0037]

Figure 12

[0038]

Figure 13

DETAILED DESCRIPTION OF THE INVENTION

[0039] The present invention details a method for identifying enhanced push-pull waveforms for driving a multi-color electrophoretic medium, where at least two of the particles are colored and subtractive, and at least one of the particles is scattering. Typically, such a system includes white particles and cyan, yellow, and magenta subtractive primary color particles. Such a system is schematically shown in FIG. 1, which can provide white, yellow, red, magenta, blue, cyan, green, and black in all pixels.

[0040] In the case of ACeP, each of the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) corresponds to a different arrangement of four pigments, such that the viewer sees only those colored pigments that are on the viewing side of the white pigment (i.e., the only pigment that scatters light). To create these colors, waveforms for arranging the four pigments in the appropriate configuration have been found to require at least five voltage levels (high positive, low positive, zero, low negative, high negative). See FIG. 1. To achieve a wide range of colors, additional voltage levels must be used for finer control of the pigments, e.g., seven voltage levels, e.g., nine voltage levels. The present invention provides a method for identifying enhanced push-pull waveforms to drive such an electrophoretic medium such that pixel color refresh is faster, there is less flashing, and a more satisfactory color spectrum is provided for the viewer.

[0041] For example, the three particles providing the subtractive primary colors for an ACeP system can be substantially non-light scattering (“SNLS”). The use of SNLS particles enables color mixing and provides more color results than can be achieved using the same number of scattering particles. These thresholds must be relatively well separated with respect to the voltage drive levels in order to avoid crosstalk between the particles, and this separation necessitates the use of high address voltages for some colors. Additionally, addressing the coloring particles with the highest threshold also moves all of the other coloring particles, and these other particles must subsequently be switched to their desired positions at a lower voltage. Such a stepwise color addressing scheme produces unwanted color flashing and long transition times.

[0042] As already described, FIG. 1 of the accompanying drawings is a schematic cross section showing the positions of the various particles within an ACeP type electrophoretic medium when displaying black, white, the subtractive primary colors, and the additive primary colors. In FIG. 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 incident from this direction. As already described, in a preferred embodiment, only one of the four particles used in the electrophoretic medium of the present invention substantially scatters light. In FIG. 1, this particle is assumed to be a white pigment. This light-scattering white particle forms a white reflector, and any particles above the white particle (as shown in FIG. 1) are visible. The light entering the viewing surface of the display passing through these particles is reflected from the white particles, passes back through these particles, and exits the display. Therefore, the particles above the white particle can absorb various colors, and the color appearing to the user is generated from the combination of the particles above the white particle. Any particles disposed below the white particle (behind the user's viewpoint) are masked by the white particle and do not affect the displayed color. Since the second, third, and fourth particles are substantially non-light-scattering, their relative order or arrangement with respect to each other is not important, but for the reasons already described, their order or arrangement with respect to the white (light-scattering) particle is important.

[0043] More specifically, when cyan, magenta, and yellow particles are present below the white particle (situation [A] in FIG. 1), there are no particles above the white particle, and the pixel simply displays white. When a single particle is above the white particle, the color of that single particle is displayed as yellow, magenta, and cyan in situations [B], [D], and [F] in FIG. 1, respectively. When two particles are present above the white particle, the displayed color is the combination of these two particles. That is, in FIG. 1, in situation [C], the magenta and yellow particles display red, in situation [E], the cyan and magenta particles display blue, and in situation [G], the yellow and cyan particles display green. Finally, when all three colored particles are present above the white particle (situation [H] in FIG. 1), all incident light is absorbed by the subtractive primary color colored particles, and the pixel displays black.

[0044] One subtractive primary color can be rendered by particles that scatter light, such that the display comprises two types of light-scattering particles, one of which is white and the other will be colored. However, in this case, the position of the light-scattering colored particles relative to the other colored particles covering the white particles will be important. For example, in rendering black (when all three colored particles are present covering the white particles), the scattered colored particles cannot be present covering the non-scattered colored particles (otherwise they would be partially or fully hidden behind the scattering particles and the color rendered would be that of the scattered colored particles and not black).

[0045] Rendering black will not be easy when two or more types of colored particles scatter light.

[0046] Figure 1 shows an ideal situation where the color is not contaminated (i.e., the light-scattering white particles completely mask any particles present behind the white particles). In reality, the masking by the white particles can be incomplete, whereby there can be a slight absorption of light by particles that would ideally be completely masked. Such contamination typically reduces both the lightness and chroma of the rendered color. In the electrophoretic media of the present invention, such color contamination should be minimized to the point where the colors formed are the same as the industrial standards for color rendering. A particularly preferred standard is SNAP (the standard for newspaper advertisement generation), which defines the L * , a * , and b * values (hereinafter, "primary colors" will be used to refer to the eight colors as shown in Figure 1, namely, black, white, the subtractive primary colors, and the additive primary colors).

[0047] Figures 2A and 2B show four pigments used in an ACeP type electrophoretic display Schematic cross-sectional representations of types (1-4, 5-8) are shown. In FIG. 2A, the polymer shell adsorbed on the core pigment is shown by the dark shading, while the core pigment itself is shown as unshaded. As is well known in the art, various forms can be used for the core pigment, such as aggregates of smaller spherical, needle-shaped, or otherwise anisotropic particles (i.e., "bunches of grapes"), composite particles comprising small pigment particles or dyes dispersed in a binder, etc. The polymer shell can be a covalently bonded polymer made by a grafting process or chemisorption, as is well known in the art, or can be physically adsorbed on the particle surface. For example, the polymer can be a block copolymer having insoluble and soluble compartments.

[0048] In the embodiment of FIG. 2A, the first and second particle types preferably have a more sufficient polymer shell 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 the following discussion, the white particles are assumed to be negatively charged (i.e., type 1), but it will be apparent to those skilled in the art that the general principles explained will also apply to a set of particles where the white particles are positively charged.

[0049] In addition, as depicted in FIG. 2B, the first and second particle types are not required to have a distinguishable polymer shell compared to the third and fourth particle types. As shown in FIG. 2B, sufficient distinguishable charges on the four particles will enable electrophoretic control of the particles and creation of the desired color on the visible surface. For example, particle 5 can have a larger scale of negative charge than particle 7, while particle 6 can have a larger scale of positive charge compared to particle 8. Other combinations of polymer functionality and charge (or particle size) can also be used, however, all four particles must be separable from each other in the presence of a suitable electric field, e.g., a lower voltage electric field that can be generated using commercial digital electronic equipment.

[0050] In the system of FIG. 2A of the present invention, the electric field required to separate aggregates formed from a mixture of Type 3 and 4 particles in a suspension solvent containing a charge control agent is greater than that required to separate aggregates formed from any other combination of the two types of particles. On the other hand, the electric field required to separate aggregates formed between the first and second types of particles is smaller than that required to separate aggregates formed between the first and fourth particles or between the second and third particles (and of course smaller than that required to separate the third and fourth particles).

[0051] In FIG. 2A, the core pigments constituting the particles are shown to have approximately the same size, and the zeta potential of each particle is also assumed to be approximately the same, although not shown. What varies is the thickness of the polymer shell surrounding each core pigment. As shown in FIG. 2A, this polymer shell is thicker for Type 1 and 2 particles than for Type 3 and 4 particles. In the present invention, it is not necessary for all colored pigments to behave as described above with reference to FIGS. 2A and 2B. As shown in FIG. 2C, the third particle may have a sufficient polymer shell and may have a wide range of charges including a weak positive. In this case, the chemical nature of the surface of the third particle must be different from that of the first particle. For example, the first particle may be grafted with a polymer that may consist of acrylic or styrene monomers that are preferably hydrophobic, with a covalently attached silane shell. The third particle may have a polymer shell that is deposited on the surface of the core particle by dispersion polymerization, although not covalently attached. In such a case, the present invention is not limited to the mechanism described above with reference to FIGS. 2A and 2B.

[0052] To obtain a high-resolution display, the individual pixels of the display must be addressable without interference from adjacent pixels. To achieve this purpose One way is to provide an array of non-linear elements such as transistors or diodes, with at least one non-linear element associated with each pixel in order to generate an "active matrix" display. An address or pixel electrode that addresses one pixel is connected through the associated non-linear element to an appropriate voltage source. 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, but is essentially arbitrary, and the pixel electrode could also be connected to the source of the transistor. Conventionally, in a high-resolution array, the pixels are arranged in a two-dimensional array of rows and columns such that any given pixel is uniquely defined by the intersection of one defined row and one defined column. The sources of all the transistors within each column are connected to a single column electrode, while the gates of all the transistors within each row are connected to a single row electrode. Again, the assignment of source to row and gate to column is conventional but is essentially arbitrary and could be reversed if desired. The row electrodes are connected to a row driver, which applies a selection voltage to the selected row electrode such that only one row is selected at any given instant (i.e., all the transistors within the selected row are conductive), while applying a non-selection voltage to all the other rows to ensure that all the transistors within these non-selected rows remain non-conductive. The column electrodes are connected to a column driver, which applies a voltage selected to drive the pixels within the selected row into their desired optical state on the various column electrodes (the aforementioned voltage is conventionally relative to a common front electrode provided on the opposite side of the non-linear array of the electro-optic medium and extending over the entire display). After a pre-selected interval known as the "line address time", the selected row is deselected and the next row is selected, and the voltage on the column driver is changed so that the next line of the display is written. This process is repeated so that the entire display is written in a row-by-row fashion. The time between addresses within the display is known as a "frame".Therefore, a display updated at 60 Hz has frames that are 16 milliseconds.

[0053] 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 No. WO01 / 07961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) can be used to form a transistor, and the “select” and “non-select” voltages applied to the gate electrode can be positive and negative, respectively.

[0054] FIG. 3 of the accompanying drawings depicts an exemplary equivalent circuit of a single pixel of an electrophoretic display. As shown, the circuit includes a capacitor 10 formed between the pixel electrode and the capacitor electrode. The electrophoretic medium 20 is represented as a parallel capacitor and resistor. In some cases, the direct or indirect coupling capacitance 30 (commonly referred to as “parasitic capacitance”) between the gate electrode of the transistor associated with the pixel and the pixel electrode can introduce unwanted noise into the display. Typically, the parasitic capacitance 30 is much smaller than that of the storage capacitor 10, and when a pixel row of the display is selected or deselected, the parasitic capacitance 30 can introduce a small negative offset voltage, also known as the “kickback voltage,” to the pixel electrode, which is typically less than 2 volts. In some embodiments, to compensate for the unwanted “kickback voltage,” a common potential V com can be supplied to the top plane electrode and the capacitor electrode associated with each pixel, whereby V com is set to a value equal to the kickback voltage (V KB ), all voltages supplied to the display are offset by the same amount, and no net DC imbalance is experienced.

[0055] A set of waveforms for driving a color electrophoretic display having four particles is described in U.S. Patent No. 9,921,451, which is incorporated herein by reference. This is the case. In U.S. Patent No. 9,921,451, seven different voltages are applied to the pixel electrodes: three positive, three negative, and zero. However, in some embodiments, the maximum voltage used in these waveforms is higher than what can be handled by amorphous silicon thin-film transistors. In such cases, a suitable high voltage can be obtained by using top-plane switching. However, when top-plane switching is used, V com Using the same number of separate power supplies as the settings is costly and inconvenient. Furthermore, top-plane switching is known to increase kickback, thereby degrading the stability of the color state.

[0056] Methods of fabricating ACeP-type electrophoretic displays have been discussed in the prior art. The electrophoretic fluid can be encapsulated within microcapsules or incorporated into a microcell structure and then sealed with a polymer layer. The microcapsule or microcell layer can be coated or embossed onto a plastic substrate or film having a transparent coating of a conductive material. This assembly can be laminated to a backplane having pixel electrodes using a conductive adhesive. Alternatively, the electrophoretic fluid can be directly dispensed onto a thin continuous cell grid disposed on a backplane including an active matrix of pixel electrodes. The filled grid can then be top-sealed using an integrated protective sheet / light-transmissive electrode.

[0057] FIG. 4 shows a schematic cross-sectional view (not to scale) of a display structure 200 of an ACeP type electrophoresis display. In display 200, the electrophoretic fluid is shown as being confined within microcups, although an equivalent structure incorporating microcapsules could also be used. A substrate 202, which can be glass or plastic, supports pixel electrodes 204 that are either individually addressed compartments or are associated with thin film transistors in an active matrix arrangement (the combination of substrate 202 and electrode 204 is, in the conventional method, referred to as the backplane of the display). Layer 206 is an optional dielectric layer according to the present invention applied to the backplane (a method for depositing a suitable dielectric layer is described in U.S. Patent Application No. 16 / 862,750, which is incorporated by reference). The front plane of the display comprises a transparent substrate 222 that supports a transparent conductive coating 220. The overlying electrode layer 220 is an optional dielectric layer 218. Layer (or layers) 216 is a polymer layer that can comprise a primer layer for the adhesion of the microcups to the transparent electrode layer 220, and some residual polymer forms the bottom of the microcups. The walls of the microcups 212 are used to contain the electrophoretic fluid 214. The microcups are sealed together with layer 210, and the entire front plane structure is adhered to the backplane using a conductive adhesive layer 208. A process for forming the microcups is described in the prior art, for example, in U.S. Patent No. 6,930,818. In some cases, the microcups have a depth of less than 20 μm, for example, less than 15 μm, for example, 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 structure of the active matrix backplane (202 / 024) due to the wider availability of fabrication facilities and the cost of various starting materials. Unfortunately, amorphous-silicon thin-film transistors become unstable when supplied with a gate voltage that would enable switching of voltages higher than about + / - 15V. Nevertheless, as described below, the performance of the ACeP is improved when the magnitudes of the high positive and negative voltages are allowed to exceed + / - 15V. Thus, as described in the previous disclosure, improved performance is achieved by additionally varying the bias of the top light-transmissive electrode relative to the bias on the backplane pixel electrode, also known as top-plane switching. Therefore, when a voltage of +30V (relative to the backplane) is required, the top plane can be switched to -15V while the appropriate backplane pixel is switched to +15V. A method of driving a four-particle electrophoretic system using top-plane switching is described in more detail, for example, in U.S. Patent No. 9,921,451. When such is the case, the top plane can be switched to -15V while the appropriate backplane pixel is switched to +15V. A method of driving a four-particle electrophoretic system using top-plane switching is described in more detail, for example, in U.S. Patent No. 9,921,451.

[0059] There are several disadvantages associated with the top plane switching approach. First, when the top plane is (as is typical) a single electrode that extends across the entire surface of the display without being pixelated, its potential affects all the pixels within the display. If it is set to match one of the maximum available voltages from the backplane (e.g., the maximum positive voltage), when this voltage is asserted on the backplane, there will be no net voltage across the ink. 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. Thus, 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 the multicolor display of the third embodiment uses multiple pulses of both positive and negative polarities, and the lengths of these pulses are not the same in the waveforms used to create different colors. In addition, the phase of the waveform may also be different for different colors. In other words, the positive pulse may precede the negative pulse for some colors, while for other colors, the negative pulse may precede the positive pulse. To cope with such cases, "rests" (i.e., pauses) have to be built into the waveform. In fact, it results in a waveform that is much longer (about twice as long) than ideally required.

[0060] Second, in top plane switching, there are limitations on the voltage levels that can be selected. If the voltages applied to the top plane are represented as V t+ and V t- respectively, and the voltages applied to the backplane are represented as V b+ and V b- respectively, then |V t+ | = |V b+ | and |V t- | = |V b- | must hold in order to achieve zero-volt conditions across the electrophoretic fluid. However, the magnitudes of the positive and negative voltages do not have to be the same.

[0061] In a prior embodiment of the high-color electronic paper (ACeP TM ), the waveform (voltage vs. time curve) applied to the pixel electrodes of the backplane of the display of the present invention is 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 backplane and the front electrode and the distance separating them. The display is typically viewed through its front electrode, whereby it is the particles adjacent to the front electrode that control the color displayed by the pixel, and sometimes it is easier to understand the associated optical transitions when the potential of the front electrode relative to the backplane is considered. This can be done simply by inverting the waveforms discussed below.

[0062] FIG. 5 shows a typical waveform (in simplified form) used to drive the four-particle color electrophoretic display system described above. Such waveforms have a "push-pull" structure, i.e., they consist of dipoles with two pulses of opposite polarity. The magnitude and length of these pulses determine the color obtained. At a minimum, five such voltage levels should exist. FIG. 5 shows high and low positive and negative voltages, and zero 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" voltage, the better the color gamut achieved by the display. The "medium" (M) level is typically about 15 V, however, for M The value will somewhat depend on the particle composition and the environment of the electrophoretic medium. In some embodiments, a high negative voltage is from -30V to -20V, a medium negative voltage is from -20V to -2V, a medium positive voltage is from 2V to 20V, and a high positive voltage is from 20V to 30V. For example, a high negative voltage is -27V, a medium negative voltage is -15V, a medium positive voltage is 15V, and a high positive voltage is 27V. If only three voltages (i.e., +V high , 0, and -V high ) are available, it may be possible to achieve the same result as an address at a lower voltage (e.g., V high / n, where n is a positive integer > 1) by using an address with a pulse of voltage V high but with a duty cycle of 1 / n.

[0063] An enhanced push-pull (EPP) waveform can be achieved using more drive levels. For example, a 7-level driver can provide seven different voltages (e.g., V H , V H ’, V H ’’, 0, V L ’’, V L ’, VL, e.g., +V H , +V M , +V L , 0, -V L , -V M , -V H ) to the data line during the update of a selected pixel of the display. The spacing between drive levels may be the same or different depending on the formulation of the electrophoretic medium. For example, +V H = 27V, +V M = 15V, +V L = 5V, 0, -V L = -5V, -V M = -15V, -V H = -27V. For example, +VH = 30V, +V M = 20V, +V L = 10V, 0, -V L = -10V, -V M = -20V, -V H=-30V. In any case, when driving an active matrix backplane with a 7-level driver and having a single controller, the controller can only update one frame of a given pixel at a time. Therefore, any enhanced push-pull waveform consists of a combination of pulses, each of which persists for a certain frame period, i.e., persists as shown in FIG. 6. The resulting waveform is used to achieve the desired optical state in the medium, and such a waveform is assumed to either have each of the pulses of FIG. 6 or have a certain number n and is composed of a combination of the pulses of FIG. 6.

[0064] Implementing a 7-level driver with sufficient voltage amplitude is difficult when using a standard amorphous silicon backplane. Using control transistors from less common materials with higher electron mobility has been found to enable the transistors to switch larger control voltages, such as + / - 30V, which are required to implement 7-level driving. The newly developed active matrix backplane may 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 metal oxide materials, enabling faster switching within a higher voltage range, for example, in the range of approximately -27V to +27V. Such 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, and the metal oxide semiconductor film covers the gate insulating film and at least partially overlaps the gate electrode, source electrode, and drain electrode. 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 20 to 50 times that of amorphous silicon. By using IGZO TFTs within an active matrix backplane, it is possible to provide a voltage greater than 30V via a suitable display driver.

[0065] For example, when using a 7-level driver, an enhanced push-pull (EPP) waveform may use significantly more space and duration of the waveform shape to achieve the desired optical performance. The EPP waveform is limited to consist of a finite number of pulses, either positive or negative, where N P is a manageable number, N is the number of possible voltage levels, and P is the number of pulses. Refer to Figure 6. For example, when N = 7, P < 5. For a set of voltage level options, fixed waveform length, and number of pulses, all possible waveforms can be enumerated. For each pulse, we have each of the N voltage levels, which can lead to N P unique voltage permutations (with substitutions), and P is the number of pulses. Regarding the pulse length, these can be selected subject to the constraint that the total length M of the waveform is fixed. Considering the scenario of using P pulses, assuming that adjacent pulses cannot be of the same length (which would be P - 1 pulses), there are N * (N - 1) P options for selecting unique voltage levels for the P pulses. Then, the number of pulse lengths can be

Number

Number

Number

Number

Number

[0066] Naturally, identifying the "optimal" waveform is not a trivial task. Assuming N = 7, P = 3, and M = 42, the total number of unique waveforms is 206,640. Each of these 206,640 waveforms needs to be tested against a given set of environmental conditions (e.g., light source and temperature) and enhanced using a prefix waveform to provide appropriate erasure (e.g., vibration pulses) such that the initial state of the medium matches the expected starting state for the waveform.

[0067] A more efficient way to identify the preferred EPP waveform is to virtually execute each proposed EPP waveform in a surrogate model representing the final display structure. A particular electrophoretic display structure can be represented by a transfer function. Its simplest form is as follows:

Number

Number

[0068] [Mathematics] Once established, each enhanced push-pull (EPP) waveform can be evaluated on a surrogate model with respect to computable quantities such as the color values of the final optical state achieved, and intermediate states (optical trace information), and afterimage performance, voltage sensitivity, appearance of transitions (e.g., "flashiness"), and temperature sensitivity, etc. Any or all of these metrics can be combined into a total cost function that identifies the preferred EPP waveform, which is subsequently verified on an actual electrophoretic display being tested. These subsequent measurements on the actual electrophoretic display are fed back into the deep learning model, [Mathematics] can provide further refinement. This complete process is described in block form in FIG. 7. It should be recognized that the method described in FIG. 7 is exhaustive within its parameterization, i.e., all possible permutations are searched. Thus, the method necessarily overcomes the general problem of parameterization, i.e., the method by which an optimization algorithm ensures sufficient sampling of the parameter space. The combination of an active matrix driven with a set clock cycle and a driver with a finite voltage level greatly reduces the parameter space, and the output waveform is still meaningful and immediately applicable in a physical display. Thus, the method of adjusting the EPP is mathematically exhaustive and may not require additional optimization when adjusting the final waveform for the generated display.

[0069] As shown in FIG. 7, this process begins with a step (710) of selecting a waveform length. As discussed above, limitations such as frame width, customer application, and power consumption can constrain this calculation. Nevertheless, the method can be used for various waveform lengths from tens of milliseconds to several seconds. In steps (720) and (730), the number of pulses and the number of total voltage and voltage levels are selected respectively, which can again be limited by commercial generation limitations such as the cost and availability of the storage medium for the waveform, and the surplus cost of the variable power supply source relative to the cost of multiple power supply sources. When all these factors are accumulated, in step (740), a unique base set of waveforms is generated, and accordingly, each of the waveforms is evaluated against a color target in step (750). The color target can be, for example, an RGB color code or a hexadecimal code for a digital image. Alternatively, the color target can be a Pantone color or a CMYK printing standard. The waveform that achieves the closest result to the color target is output as a candidate waveform in step (760). This waveform is actually fed into a real 4-particle electrophoresis display corresponding to the modeled display, whereby the results are measured using a calibrated optical bench and compared to the target. In some embodiments, these measurements are fed back into the model via step (770). Further details regarding a suitable calibrated optical bench for evaluating the output of a 4-particle electrophoresis display are incorporated by reference in its entirety in "Optical measurement standards for reflective e-paper to predict colors displayed in ambient illumination environments", Color Research and Application , which can be found in vol. 43, issue 6, pages 907 - 921 (2018).

[0070] Using the method described above, a portion of the color waveform for a faster and less flushing ACeP type system is quickly isolated for further testing. Such push-pull waveforms can include dipoles that are actually bifurcated (or trifurcated) into certain combinations of pulse height and relative polarity width. For example, as shown in FIGS. 8 and 9, the enhanced push-pull waveform has a negative dipole's first part with magnitude V L and a first width t1, and a second part of the negative dipole with magnitude V L ' and a second width t2. 。 The positive part of the dipole can be, for example, a single pulse with magnitude VH and a third width t3, or the positive part of the dipole can be bifurcated or trifurcated as determined by the model

Number

[0071] Of course, achieving the desired color using push-pull drive pulses depends on the particles starting the process from a known state, which is unlikely to be the last color displayed on the pixel. Thus, a series of reset pulses precede the drive pulses, which increases the amount of time required to update the pixel from the first color to the second color. The reset pulses are described in more detail by U.S. Patent No. 10,593,272, which is incorporated by reference. The lengths of these pulses (refresh and address) and any rest periods between them (i.e., the period of zero voltage) are such that the overall waveform (the integral of voltage over time across the entire waveform) is DC balanced (i.e., the integral of voltage over time The score can be selected such that it is substantially zero. The DC balance can be achieved by adjusting the length of the pulses and the rests during the reset phase, such that the net impulse supplied during the reset phase is equal in magnitude and opposite in sign to the net impulse supplied during the address phase, and during that phase, the display is switched to a particular desired color.

[0072] The use of an EPP waveform is superior to an unconstrained waveform in that the appearance of the transition is limited to a set of maxima of P abrupt color changes. An unconstrained waveform can be designed to reduce the number of color changes or to have a satisfactory appearance of the transition, but this is a technically difficult problem that requires better analysis of the training data and more computational power. This is much easier when using a selected EPP waveform, as described herein. Further, this EPP adjustment method has historically provided a good trade-off between a simple waveform structure with a controlled appearance of the transition and the complexity of the optimization, allowing for a comprehensive enumeration of square pulse-based waveforms. Preventing single-frame drive and a large number of transient currents is also likely to make the resulting EPP waveform more robust in other ways (robustness over temperature sensitivity, voltage sensitivity, manufacturing variability).

[0073] (Example) The method described above was used to construct a model function that describes a metal oxide AM-TFT backplane and a four-particle electrophoretic medium that includes one reflective (white) particle and three subtractive particles (cyan, magenta, and yellow). For a 42-frame waveform at 85 Hz (0.5 seconds), each three-pulse EPP waveform (a total of 206,640 unique waveforms) was tested. Eight color targets corresponding to black, white, magenta, blue, cyan, green, yellow, and red were selected. 10,000 waveforms with the final color state closest to each of these eight targets were selected for further evaluation. These 10,000 final color state points are plotted on the a*-b* plot in FIG. 10.

[0074] Interestingly, the method of this specification provides better insights when searching for other prominent features such as afterimages or DC balance. As shown in FIG. 11, it is possible to achieve many of the same color states using a DC balanced (triangle) or DC unbalanced (circular) waveform. Note the overlap between the DC unbalanced EPP waveform (circular) and the DC balanced EPP waveform (triangle) in the representative color states within FIG. 11. However, looking at the actual waveforms, it is notable that in some cases, the DC balanced waveform and the DC unbalanced waveform are very similar in shape. For example, compare FIGS. 12A and 12B corresponding to the square in FIG. 11 with FIGS. 13A and 13B corresponding to the star in FIG. 11. In the case of FIGS. 12A and 12B, there is a very slight difference between the DC balanced waveform and the DC unbalanced waveform, while in FIGS. 13A and 13B, the difference between the DC balanced waveform and the DC unbalanced waveform is quite significant.

[0075] It is noted that in FIGS. 10 and 11, a preferred target color (the "X" in FIG. 11) may not be achievable in a given ACeP type electrophoresis display construction using an EPP waveform. This phenomenon is reproduced in a physical display.

[0076] So far, some aspects and embodiments of the technology of this application have been described, but it should be understood that various modifications, corrections, and improvements will readily occur to those skilled in the art. Such modifications, corrections, and improvements are intended to be within the spirit and scope of the technology described in this application. For example, those skilled in the art can easily envision various other means and / or structures for implementing the functions described in this specification and / or for obtaining one or more of the results and / or advantages, and each of such variations and / or corrections is considered to be within the scope of the embodiments described in this specification. One of ordinary skill in the art would be able to recognize, or ascertain, many equivalents to the specific embodiments described herein using only routine experimentation. Accordingly, the foregoing embodiments are presented by way of example only, and it is to be understood that the embodiments of the present invention may be practiced otherwise than as specifically described within the scope of the appended claims and their equivalents. In addition, any combination of two or more of the features, systems, articles, materials, kits, and / or methods described herein is also within the scope of the present disclosure if such features, systems, articles, materials, kits, and / or methods do not mutually conflict.

Claims

A method for determining a push-pull waveform for driving an electrophoretic display, the method comprising: Determining a set of candidate waveforms for driving the electrophoretic display; Estimating an optical state generated by each of the candidate waveforms using a model representing the electrophoretic display, the model comprising: O(t) = f(V(t), x(0)) where t is time, O(t) is the optical state of the electrophoretic display as a function of t, V(t) is the voltage applied to the electrophoretic display as a function of t, x(0) is the initial optical state of the electrophoretic display at t = 0, and f is a function of V(t) and x(0); Identifying a push-pull waveform for generating a normalized optical state based on the estimated optical states generated by the candidate waveforms; A method comprising: The method according to claim 1, further comprising driving the electrophoretic display to generate a color output using the push-pull waveform identified for generating the normalized optical state. The method according to claim 2, further comprising evaluating the color output of the electrophoretic display and comparing the color output with a target color. The method according to claim 2, further comprising using the color output and associated waveforms as training data for the model. The determining of the set of candidate waveforms comprises: Selecting a finite set of at least five different voltage levels for a waveform for driving the electrophoretic display; Selecting a finite time width for the waveform; Identifying a set of waveforms each having a positive part and a negative part; Including Each of the positive and negative parts comprises at least one pulse, at least one of the positive and negative parts comprises two pulses having different voltage magnitudes each corresponding to one of the at least five different voltage levels, and the sum of the pulse widths of the positive and negative parts is equal to the finite time width. The method according to claim 1. **Claim 6**: The method according to claim 5, wherein selecting the finite time width includes comparing the target color with a predicted output color. **Claim 7**: The method according to claim 5, wherein the finite set of at least five different voltage levels includes a high negative voltage of -30V to -20V, a medium negative voltage of -20V to -2V, a medium positive voltage of 2V to 20V, and a high positive voltage of 20V to 30V. **Claim 8**: The method according to claim 5, wherein the finite set of at least five different voltage levels includes -27V, 0V, and +27V. **Claim 9**: The method according to claim 5, wherein the finite set of at least five different voltage levels includes seven voltage levels: a high negative voltage, a medium negative voltage, a low negative voltage, a zero voltage, a low positive voltage, a medium positive voltage, and a high positive voltage. **Claim 10**: Determining the set of candidate waveforms comprises selecting a finite set of voltages for driving the electrophoretic display, the set of voltages including at least five different voltage levels, selecting a finite time width of time for the candidate waveforms, calculating all the waveforms, and all the waveforms have a first positive portion consisting of a first pulse and a second pulse, the first pulse having a first positive magnitude and a first time width, the second pulse having a second positive magnitude and a second time width, a second negative portion consisting of a third pulse and a fourth pulse, the third pulse having a first negative magnitude and a third time width, the fourth pulse having a second negative magnitude and a fourth time width, the first positive magnitude, the second positive magnitude, the first negative magnitude, and the second negative magnitude each have a value from the finite set of voltages, the sum of the first pulse width, the second pulse width, the third pulse width, and the fourth pulse width is equal to the finite time width, the method according to claim 1. **Claim 11**: The electrophoretic display comprises an electrophoretic medium disposed between a first light-transmissive electrode and a second electrode, the electrophoretic medium including four sets of particles, each set of particles having different optical properties and different charge properties from other sets of particles in the electrophoretic medium, the method according to claim 1. **Claim 12**: The method according to claim 11, wherein the four sets of particles comprise a first, a second, a third, and a fourth set of particles, the first set of particles being reflective and the second, third, and fourth sets of particles being subtractive. **Claim 13**: The method according to claim 12, wherein two of the four sets of particles are positively charged and two of the four sets of particles are negatively charged. **Claim 14**: The method according to claim 12, wherein one of the four sets of particles is positively charged and three of the four sets of particles are negatively charged. **Claim 15**: The method according to claim 12, wherein three of the four sets of particles are positively charged and one of the four sets of particles is negatively charged. **Claim 16**: The method according to claim 1, wherein the model is a differentiable deep learning model based on a recurrent neural network architecture.

Images