HK40117400A - Transitional driving modes for impulse balancing when switching between global color mode and direct update mode for electrophoretic displays - Google Patents

Description

A transition-driven mode used for impulse balancing when switching between the global color mode and direct update mode of an electrophoretic display.

Related applications

This application claims priority to U.S. Provisional Application No. 63/401110, filed August 25, 2022. All patents and publications disclosed herein are incorporated herein by reference in their entirety.

Background Technology

Electrophoretic displays (EPDs) change color by modifying the position of charged colored particles relative to a light-transmitting viewing surface. These electrophoretic displays are often called "electronic paper" or "ePaper" because the resulting displays have high contrast and are readable in sunlight, much like ink on paper. Electrophoretic displays are widely used in e-readers, such as Amazon's Kindle®, because they provide a book-like reading experience, consume little power, and allow users to carry a library of hundreds of books in a lightweight handheld device.

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

Recently, multiple color options have been commercially used in electrophoretic displays, including tri-color displays (black, white, red; black, white, yellow) and quad-color displays (black, white, red, yellow). Similar to the operation of a black-and-white electrophoretic display, electrophoretic displays with three or four reflective particles operate much like a simple black-and-white display because the desired colored particles are driven onto the viewing surface. The driving scheme is far more complex than that with only black and white, but ultimately, the light function of the particles is the same.

Advanced Color Electronic Paper (ACEP®) also includes four types of particles, but the cyan, yellow, and magenta particles are subtractive rather than reflective, allowing for the production of thousands of colors per pixel. Color processing is functionally equivalent to the printing methods long used in offset and inkjet printers. A given color is produced by using the correct ratio 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 determine the color at each pixel. While this type of electrophoretic display allows for the display of thousands of colors per pixel, careful control of the position of each pigment within the pigment (50 to 500 nanometers in size) within a working space approximately 10 to 20 micrometers thick is crucial. Clearly, variations in particle position will result in incorrect color display at a given pixel. Therefore, this system requires precise voltage control. Further details of the system are available in the following U.S. patents, all of which are incorporated herein by reference in their entirety: U.S. Patent Nos. 9,361,836, 9,921,451, 10,276,109, 10,353,266, 10,353,266, 10,467,984, and 10,593,272.

Colors produced using this type of color electrophoretic display may encounter various "error accumulation" phenomena. These errors may be due to small perturbations in the driving voltage, residual voltage accumulated on the driving pixels, or temperature variations in the electrophoretic medium during a series of transitions. As a result, the desired color state may vary due to accumulated voltage on the pixel and/or disturbances in the inner phase medium of adjacent pixels. For example, after 100 consecutive color transitions in a region of the display, the L* of a pixel returning to a black state may be 6 (where L* has the usual CIE definition:

L* = 116 (R/R ₀ ) ^1/3 - 16,

Where R is reflectance, and R _₀ is the standard reflectance value), while another adjacent pixel remains unchanged during these 100 consecutive transitions, starting and remaining black, and after the same 100 transitions, its L* is 4. For the average viewer, an L* deviation of 2 is noticeable and detrimental to the overall viewing experience.

This error accumulation phenomenon applies to both color and black and white states. Similarly, due to the interaction of electrophoretic particles, small changes in local voltage or the electrophoretic medium environment can cause different colors to be displayed on the monitor. Furthermore, for specific color states, especially yellow and green, the human eye is more sensitive to changes in chromaticity, and subtle changes in color state can be uncomfortable. For example, skin tones appearing greenish can be very uncomfortable for the viewer. Therefore, general grayscale/color image streaming requires very precise control of the applied impulse to produce good results.

Complicating matters further, in some cases, it may be desirable for a single display to use multiple driving schemes. For example, a display capable of producing multiple colors per pixel can typically operate in “Global Complete” (GC mode), where each colored pixel has the ability to transition from one color to another during each image update. Of course, as described in U.S. Patent No. 10,657,869, such updates can be time-consuming (e.g., one second or more), especially when DC balancing and residual voltage management are required to achieve the highest quality colors. However, in other instances, such as when drawing with a stylus or turning pages, very fast updates are needed, and users are willing to sacrifice color fidelity for a faster update experience. This faster update scheme is often referred to as “Direct Update” (DU mode) and typically only requires driving the electrophoretic medium to the black and white extremes. See, for example, U.S. Patent No. 9,672,766. For more advanced end products, such as color e-readers/tablets, there may be multiple modes for each, depending on the content being displayed. In addition, other modes may be included, such as animation (also known as “A2 mode”), and the display controller may be programmed to automatically switch between modes based on the content displayed or the user’s actions (e.g., touching with a stylus).

As discussed in U.S. Patent 11,686,989, in multi-particle systems using process black or process white, the driving between white and black states can require entirely different impulse potentials (voltages that accumulate over time). For example, in ACeP®, which includes one negative white particle and three positive particles with different charges that together produce a black state, achieving a good black state may require a much larger impulse potential than a good white state. That is, to produce white, one only needs to place a negative white particle between the viewer and the colored particles; however, to achieve a good black state, all positively charged particles must be driven to the viewing surface and mixed, and all white particles must be driven behind the positively charged colored particles. In a typical “GC” mode, white and black states are achieved through DC balancing so that the round trip between black and white states does not generate impulse potentials for these color states. However, this comes at the cost of a longer waveform, typically around 1 second, for example, between 500 ms and 3 seconds, or for example, between 700 ms and 1 second. For DU mode, the desired switching time between black and white states needs to be much shorter, for example, less than 500 milliseconds, less than 300 milliseconds, or approximately 250 milliseconds. However, due to the difference in impulse potential between the black and white states, the impulse potential must be carefully managed to prevent charge buildup on the pixels, which could lead to poor color performance later in the time frame. This situation may occur in e-readers where a reader reads 20 pages of text and then sees a full-color image.

The term "grayscale state" is used herein in its conventional meaning within the field of imaging, referring to a state between two extreme optical states of a pixel, and does not necessarily imply a black-to-white transition between these two extreme states. For example, several IENK patents and published applications mentioned below describe electrophoretic displays where the extreme states are white and dark blue, making the intermediate "grayscale state" actually a pale blue. In fact, as previously stated, a change in optical state may not be a change in color at all. The terms "black" and "white" can be used below to refer to two extreme optical states of a display and should be understood to generally include extreme optical states that are not strictly black and white, such as the white and dark blue states mentioned above.

The terms “bistable” and “bistable” are used herein in their conventional sense to refer to a display comprising display elements having a first display state and a second display state that are different in at least one optical characteristic, such that after any given element is driven to present its first or second display state by means of an addressing pulse of finite duration, the state will persist for at least several times (e.g., at least four times) the shortest addressing pulse duration required to change the state of the display element after the addressing pulse terminates. As shown in U.S. Patent No. 7,717,0670, some particle-based electrophoretic displays capable of supporting grayscale are stable not only in their extreme black and white states but also in their intermediate grayscale states, as are some other types of electro-optical displays. This type of display is properly referred to as “multistable” rather than bistable, although for convenience, the term “bistable” may be used herein to encompass both bistable and multistable displays.

The term "impulse" is used to refer to driving an electrophoretic display, and refers to the integral of the applied voltage with respect to time during the period when the display is driven.

Particles that absorb, scatter, or reflect light across a wide band or a selected wavelength are referred to herein as colored or pigment particles. Various light-absorbing or light-reflecting materials other than pigments (strictly speaking, the term refers to insoluble colored materials), such as dyes or photonic crystals, may also be used in the electrophoretic media and displays of this invention.

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

As mentioned above, the electrophoretic medium requires the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but a gaseous fluid can be used to generate the electrophoretic medium; see, for example, Kitamura, T. et al., “Electricaltoner movement for electronic paper-like display,” IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y. et al., “Toner display using insulative particles charged triboelectrically,” IDW Japan, 2001, Paper AMD4-4. See also U.S. Patent Nos. 7,321,459 and 7,236,291. When such gas-based electrophoretic media are used in a direction that allows particle settling, such as in the case of a medium located in a vertical plane, such media appear to be susceptible to the same type of particle settling problem as liquid-based electrophoretic media. In fact, particle sedimentation appears to be a more serious problem in gas-based electrophoresis media than in liquid-based electrophoresis media because gaseous suspensions have lower viscosity than liquid suspensions, allowing electrophoretic particles to settle more quickly.

Numerous patents and applications assigned to or in the name of the Massachusetts Institute of Technology (MIT) and Einkel Corporation describe various techniques used in encapsulated electrophoresis and other electro-optic media. Such encapsulation media comprise a plurality of small capsules, each capsule containing an inner phase and a capsule wall surrounding that inner phase, which contains electrophoretically moving particles in a fluid medium. Typically, the capsules themselves are held in a polymer binder to form an adhesive layer located between two electrodes. The techniques described in these patents and applications include:

(a) Electrophoretic particles, fluids, and fluid additives; see, for example, U.S. Patent Nos. 7,002,728 and 7,679,814;

(b) Encapsulation, adhesives, and encapsulation processes; see, for example, U.S. Patent Nos. 6,922,276 and 7,411,719;

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

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

(e) Thin films and subassemblies containing electro-optic materials; see, for example, U.S. Patent Nos. 6,982,178 and 7,839,564;

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

(g) Color formation and color adjustment; see, for example, U.S. Patent Nos. 6017584; 6545797; 6664944; 6788452; 6864875; 6914714; 6972893; 7038656; 7038670; 7046228; 7052571; 7075502; 7167155; 7385751; 7492505; 7667684; 7684108; 7791789; 7800813; 7821702; 7839564; 7910175; 7952790; 7956841; 7982941; 8040594; 805 4526; 8098418; 8159636; 8213076; 8363299; 8422116; 8441714; 8441716; 8466852; 8503063; 8576470; 8576475; 8593721; 8605354; 8649084; 8670174; 8704756; 8717664; 8786935; 8797634; 8810899; 8830559; 8873129; 8902153; 8902491; 8917439; 8964282; 9013783; 9116412; 914643 9; 9164207; 9170467; 9170468; 9182646; 9195111; 9199441; 9268191; 9285649; 9293511; 9341916; 9360733; 9361836; 9383623; and 9423666; and U.S. Patent Application Publication Nos. 2008/0043318; 2008/0048970; 2009/0225398; 2010/0156780; 2011/0043543; 2012/0326957; 2013/0242378; 2013/0278995; 2 014/0055840; 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;

(h) A method for driving a display; see, for example, U.S. Patent Nos. 5930026; 6445489; 6504524; 6512354; 6531997; 6753999; 6825970; 6900851; 6995550; 7012600; 7023420; 7034783; 7061166; 7061662; 7116466; 7119772; 7177066; 7193625; 7202847; 7242514; 7259744; 7304787; 7312794; 7327511; 7408699; 7453445; 7492339; 7528822; 7545358; 7583251; 7602374; 7612760; 7679599; 7679813; 7683606; 7688297; 7729039; 7733311; 7733335; 7787169; 7859742; 7952557; 7956841; 7982479; 7999787; 8077141; 8125501; 8 139050; 8174490; 8243013; 8274472; 8289250; 8300006; 8305341; 8314784; 8373649; 8384658; 8456414; 8462102; 8514168; 8537105; 8558783; 8558785; 8558786; 8558855; 8576164; 8576259; 8593396; 8605032; 8643595; 8665206; 8681191; 87 30153; 8810525; 8928562; 8928641; 8976444; 9013394; 9019197; 9019198; 9019318; 9082352; 9171508; 9218773; 9224338; 9224342; 9224344; 9230492; 9251736; 9262973; 9269311; 9299294; 9373289; 9390066; 9390661; and 9412314; and U.S. Patent Application Publication. No. 2003/0102858; 2004/0246562; 2005/0253777; 2007/0091418; 2007/0103427; 2007/0176912; 2008/0024429; 2008/ 0024482; 2008/0136774; 2008/0291129; 2008/0303780; 2009/0174651; 2009/0195568; 2009/0322721; 2010/0194733; 20 10/0194789; 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; 2013/032 1278; 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; 2 015/0213765; 2015/0221257; 2015/0262255; 2015/0262551; 2016/0071465; 2016/0078820; 2016/0093253; 2016/0140910; and 2016/0180777 (these patents and patent applications may be referred to below as MEDEOD (Methods for Driving Electro-optic Displays) applications).

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

(j) Non-electrophoretic displays, such as those described in U.S. Patent No. 6,241,921; and U.S. Patent Application Publication No. 2015/0277160; and U.S. Patent Application Publication Nos. 2015/0005720 and 2016/0012710.

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

One related type of electrophoretic display is the so-called microcell electrophoretic display. In a microcell electrophoretic display, charged particles and fluid are not encapsulated in microcapsules, but rather retained within multiple cavities formed within a carrier medium (typically a polymer membrane). See, for example, U.S. Patent Nos. 6,672,921 and 6,788,449.

Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, particles essentially block visible light transmission through the display) and operate in a reflective mode, many electrophoretic displays can operate in a so-called shutter mode, where one display state is essentially opaque and the other is translucent. See, for example, U.S. Patent Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, similar to electrophoretic displays but dependent on changes in electric field strength, can operate in a similar mode; see U.S. Patent No. 4,418,346. Other types of electro-optic displays can also operate in shutter mode. Electro-optic media operating in shutter mode can be used in multilayer structures for full-color displays; in such structures, at least one layer adjacent to the viewing surface of the display operates in shutter mode to expose or conceal a second layer farther from the viewing surface.

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

As mentioned above, most simple existing electrophoretic media essentially display only two colors. Such electrophoretic media either use a single type of electrophoretic particles with a first color in a colored fluid with a second, different color (in this case, the first color is displayed when the particles are near the viewing surface of the display, and the second color is displayed when the particles are spaced apart from the viewing surface), or use first and second types of electrophoretic particles with different first and second colors in a colorless fluid (in this case, the first color is displayed when the first type of particles are near the viewing surface of the display, and the second color is displayed when the second type of particles are near the viewing surface). Typically, these two colors are black and white. If a full-color display is required, an array of color filters can be placed on the viewing surface of a monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color mixing to generate color stimuli. The available display area is shared by 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 (stripes) or two-dimensional (2x2) repeating pattern. Other options for primary colors or more than three primary colors are also known in the art. Three (in the case of RGB displays) or four (in the case of RGBW displays) subpixels are chosen to be small enough that, at the expected viewing distance, they visually blend together to form a single pixel with uniform color stimulation (“color mixing”). An inherent drawback of area sharing is that colorant is always present, and colors can only be modulated by switching the corresponding pixels of the underlying monochrome display to white or black (turning the corresponding primary color on or off). For example, in an ideal RGBW display, red, green, blue, and white primary colors each occupy a quarter of the display area. One of the four subpixels, the white subpixel, is as bright as the monochrome white displayed below, and the brightness of each colored subpixel does not exceed one-third of the monochrome white. The total white brightness of the entire display cannot exceed half the brightness of the white subpixel (the white area of the display is created by displaying one white subpixel out of every four subpixels, plus the fact that the colored form of each colored subpixel is equivalent to one-third of the white subpixel, so the combined three colored subpixels contribute no more than one white subpixel). By switching colored pixels to black, area sharing reduces the brightness and saturation of the color. Area sharing is particularly problematic when mixing yellow, because yellow is lighter than any other color of equal brightness, and saturated yellow is almost as bright as white. Switching the blue pixel (one-quarter of the display area) to black makes yellow too dark.

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

U.S. Patent No. 9,921,451 describes a second form of electrophoretic medium capable of displaying any color at any pixel location. In Patent No. 9,921,451, the electrophoretic medium comprises four particles: white, cyan, magenta, and yellow, two of which are positively charged and two are negatively charged. However, the display of Patent No. 9,921,451 also suffers from color mixing problems in the white state. Because one of the particles carries the same charge as the white particles, when a white state is needed, a certain amount of the same-charged particles will move towards the viewing surface along with the white particles. While this unwanted hue can be overcome with complex waveforms, such waveforms significantly increase the display's refresh rate and, in some instances, cause unacceptable "flickering" between images.

As discussed above, regarding US Patent 11,686,989, one solution uses an electrophoretic medium comprising four particles: white, cyan, magenta, and yellow, three of which are positively charged and the white particle is negatively charged. While all non-white particles carry the opposite charge to the white particles, helping to reduce color contamination in the white state, this combination results in waveform imbalance when transitioning from a colored state to a white or black state. In particular, the black state typically requires a continuously high positive charge drive to ensure that all positively charged particles are moved to the viewing surface.

Summary of the Invention

In a first aspect of the invention, a method for driving an electrophoretic display having a plurality of pixels, each pixel capable of displaying at least three optical states, including white, black, and a color that is neither white nor black. The method includes driving the electrophoretic display with a first driving mode that allows transitions between all optical states; driving the electrophoretic display with a second driving mode that includes transitions only between black and white optical states, wherein in the second driving mode, the impulse potential experienced by a pixel transitioning from a white state to a black state is equal in magnitude and opposite in sign to the impulse potential experienced by a pixel transitioning from a black state to a white state; driving the electrophoretic display with a first transition mode that allows transitions from a color state of the first driving mode to a white state or a black state of the second driving mode, wherein the first transition mode compensates for excess impulse potential to be transferred to the pixel in the second driving mode; and driving the electrophoretic display with a second transition mode that allows transitions from a white state or a black state of the second driving mode to a color state of the first driving mode, wherein the second transition mode compensates for excess impulse potential already transferred to the pixel in the second driving mode. In one embodiment, in the first driving mode, the impulse potential experienced by a pixel transitioning from a white state to a black state is unequal in magnitude and opposite in sign to the impulse potential experienced by a pixel transitioning from a black state to a white state. In one embodiment, the first transition mode and the second transition mode do not have the same impulse potential compensation between the color state of the first driving mode and the white state of the second driving mode, and between the color state of the first driving mode and the black state of the second driving mode. In one embodiment, the first transition mode and the second transition mode do not have the same waveform between the color state of the first driving mode and the white state of the second driving mode, and between the color state of the first driving mode and the black state of the second driving mode. In one embodiment, in the second driving state, the waveform causing the transition from a white state to a black state includes at least five frames of maximum positive voltage. In one embodiment, in the second driving state, the waveform causing the transition from a black state to a white state includes at least five frames of maximum negative voltage. In one embodiment, the first driving mode is DC balanced. In one embodiment, the first transition mode and the second transition mode are not DC balanced. In one embodiment, each pixel is capable of displaying at least eight optical states, and the first transition mode allows transitions from each of six non-black and non-white color optical states to either a white state or a black state in the second driving mode. In one embodiment, the eight optical states are black, white, red, magenta, yellow, green, cyan, and blue.

On the other hand, a display controller is configured to perform any of the above methods.

In another aspect, an electrophoretic display is configured to implement any of the methods described above. In one embodiment, the electrophoretic display includes an electrophoretic medium comprising at least three types of particles with different electrophoretic mobilities. In one embodiment, at least two of the three particles have the same charge, but different charge amounts. In one embodiment, one of the particles is negatively charged and white. In one embodiment, the display comprises three types of positively charged particles, each of which is partially light-absorbing and has a different color from the other types of positively charged particles. In one embodiment, the electrophoretic medium is confined within a plurality of capsules or a plurality of microunits.

Attached Figure Description

Figure 1 is a schematic cross-sectional view showing the positions of various colored particles in the electrophoretic medium of the present invention when displaying black, white, three subtractive primary colors and three additive primary colors.

Figure 2A is a schematic diagram of an electrophoretic display with four types of particles in a nonpolar fluid, where a full range of colors can be obtained on each pixel electrode. It will be understood that in some embodiments, one negatively charged particle is white, one positively charged particle is yellow, one positively charged particle is magenta, and one positively charged particle is cyan; however, the invention is not limited to the exemplary set of colors or combinations of charge polarity and magnitude.

Figure 2B illustrates the transition between a first optical state and a second optical state, where the first optical state places all first-charge polarity particles at the viewing surface, and the second optical state places second (opposite) polarity particles at the viewing surface.

Figure 2C illustrates the transition between a first optical state and a third optical state, where the first optical state places all first-charge polar particles at the viewing surface, and the third optical state places second (opposite) polar particles behind the first polar moderately charged particles located at the viewing surface.

Figure 2D illustrates the transition between a first optical state and a fourth optical state, where the first optical state places all first-charge polar particles at the viewing surface, and the fourth optical state places second (opposite) polar particles behind the first low-charge polar particles located at the viewing surface.

Figure 2E illustrates the transition between a first optical state and a fifth optical state, where the first optical state places all first-charge polarity particles at the viewing surface, and the fifth optical state places second (opposite) polarity particles behind a combination of low-charged and medium-charged particles of the first polarity located at the viewing surface.

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

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

Figure 5 illustrates an exemplary push-pull drive scheme for addressing an electrophoretic medium comprising three subtractive particles and one scattering (white) particle. This addressing pulse is typically accompanied by a DC balance clearing pulse to allow each color state to switch to various other color states.

Figure 6 illustrates the workflow of the driving modes that can be executed by a display controller using the method of the present invention. Specifically, the driving mode of the display changes according to the color content of the image to be displayed and/or the need for rapid page turning and/or the need for stylus updates. In some embodiments, the controller automatically switches between modes. In other embodiments, switching modes requires user input.

Figure 7 is a schematic diagram of the present invention, showing the DUin and DUout switching modes for compensating the impulse potential when moving back and forth between the "Global Completion" ("GC") mode and the DU mode.

Figure 8 illustrates a waveform that can be used to provide a rapid transition between a white optical state and a black optical state in Direct Update (“DU”) mode. Other waveforms may also be used, as long as they generate counteracting impulse potentials when transitioning between the black and white states.

Figure 9A shows a series of waveform voltage frames for multiple transitions in the DUin mode (one mode of the present invention), which achieves impulse potential compensation transitions between the GC mode and the DU mode. It is noteworthy that "duK" and "duW" correspond to the black and white optical states in the DU mode, respectively. K, W, GC2, and GC3 correspond to black, white, the generalized second color, and the third color in the GC mode, respectively. Typically, in the GC mode, K is the first color, and W is the last color.

Figure 9B shows the actual visual transitions for each frame in DUin mode, corresponding to the waveform shown in Figure 9A.

Figure 10A shows a series of waveforms as voltage frames for multiple transitions in DUout mode, which realizes the impulse potential compensation transition between DU mode and GC mode.

Figure 10B shows the actual visual transition for each frame in DUout mode, corresponding to the waveform shown in Figure 10A.

Figure 11 is a graphical example of how the impulse potential is compensated when switching between GC mode and DU mode. It is worth noting that the impulse compensation from GC to DU (i.e., DUin) is different from the impulse compensation from DU to GC (i.e., DUout).

Detailed Implementation

This invention provides a method for driving a multi-pixel electrophoretic display designed to display at least three colors, such as eight, on each pixel. The method employs a first driving scheme and a second driving scheme. The first driving scheme enables transitions between all colors that can be displayed on each pixel, while the second driving scheme only includes transitions ending in white or black. This is particularly useful for drawing black lines on a white page, reading black text on a white page, or reading white text on a black page. The second driving scheme is designed to allow the display to respond quickly to user input, such as when a user “writes” on a display incorporating a touchscreen or electromagnetic resonance (EMR) device using a stylus, or other forms of stylus or touch interaction. The invention further provides a transition driving scheme for switching between the first and second driving schemes.

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

As shown in Figure 1, each of the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) corresponds to a different arrangement of four particles, so that the viewer can only see those colored particles located on the viewing side of the white particle (i.e., the only particle that can scatter light). To achieve multiple colors, additional voltage levels must be used to provide finer control over the particles. In the described concept, the first type of particle (typically negatively charged) is reflective (typically white), while the other three particles with opposite charges (typically positively charged) comprise three types of substantially non-light-scattering (SNLS) particles. The use of SNLS particles allows for the mixing of various colors and provides more color results than can be achieved using the same number of scattering particles. These thresholds must be sufficiently separated to avoid crosstalk, and this separation necessitates the use of high addressing voltages for some colors. The disclosed four-particle electrophoretic medium can also be updated more quickly, requiring “less flickering” transitions and producing a color spectrum that is more pleasing to the viewer (and therefore, more commercially viable). Furthermore, the disclosed concept provides rapid updates between black and white pixels (e.g., less than 500 milliseconds, e.g., less than 300 milliseconds, e.g., less than 200 milliseconds, e.g., less than 100 milliseconds), enabling rapid page turning for black text on a white background.

In Figure 1, it is assumed that the viewing surface of the display is located at the top (as shown), i.e., the user views the display from this direction, and light is incident from this direction. As previously mentioned, in the preferred embodiment, only one of the four types of particles used in the electrophoretic medium of the present invention substantially scatters light, and in Figure 1, this particle is assumed to be white pigment. This light-scattering white particle forms a white reflector, through which any particle above the white particle (as shown in Figure 1) can be viewed. Light entering the viewing surface of the display passes through these particles, is reflected from the white particle, passes through these particles again, and exits from the display. Therefore, the particles above the white particles can absorb a variety of colors, and the color presented to the user is the color 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 obscured by the white particles and do not affect the displayed color. Since the second, third, and fourth types of particles substantially do not scatter light, their order or arrangement relative to each other is not important, but for the reasons already explained, their order or arrangement relative to the white (light-scattering) particles is crucial.

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

A subtractive primary color may 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 colored particles scattering the light relative to other colored particles covering the white particles is crucial. For example, when displaying black (when all three types of colored particles are above the white particles), the scattering colored particles cannot be above the non-scattering colored particles (otherwise, these non-scattering particles 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).

Figure 1 illustrates an idealized scenario where the color is uncontaminated (i.e., the light-scattering white particle completely masks any particles behind it). In reality, the masking of white particles may not be perfect, so even in an ideal scenario, completely masked particles 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 ensure that the resulting color conforms to industry standards for color representation. A particularly supported standard is the Standard for Newspaper Advertising Production (SNAP), which specifies the L*, a*, and b* values for each of the eight primary colors mentioned above. (Hereinafter, "primary color" refers to the eight colors shown in Figure 1: black, white, the three subtractive primary colors, and the three additive primary colors).

Figures 2A-2E show cross-sectional schematic diagrams of the four particle types used in this invention. The display layer utilizing the improved electrophoretic medium includes a first (viewing) surface 13 on the viewing side and a second surface 14 on the opposite side of the first surface 13. The electrophoretic medium is located between the two surfaces. Each space between the two dashed vertical lines represents a pixel. Within each pixel, the electrophoretic medium can be addressed, and the viewing surface 13 of each pixel can achieve the color state shown in Figure 1 without the need for additional layers or color filter arrays.

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

The newly developed active matrix backplane may include thin-film transistors (TFTs) comprising metal oxide materials such as tungsten oxide, tin oxide, indium oxide, and zinc oxide, or more complex metal oxides such as indium gallium zirconium oxide. In these applications, each transistor using such a metal oxide material forms a channel formation region, which allows for faster switching at higher voltages. Such metal oxide transistors also allow thin-film transistors (TFTs) to have less leakage in the "off" state compared to, for example, amorphous silicon TFTs. In a typical scanning TFT backplane comprising n rows, the time the transistors are in the "off" state is approximately (n-1)/n of the time required to refresh each row of the display. Any charge leakage from the storage capacitors associated with each pixel will degrade the electro-optical performance of the display. TFTs typically comprise a gate electrode, a gate insulating film (typically _SiO₂ ), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor thin film covering the gate insulating film, at least partially overlapping the gate, source, and drain electrodes. Such backplanes are available from manufacturers such as Sharp/Foxconn, Hyundai, and BOE. This backplane can provide a drive voltage of ±30V (or higher). In some embodiments, an intermediate voltage driver is also included, so the resulting drive waveform may include five, seven, nine or more levels.

A preferred metal oxide material for such applications is indium gallium zinc oxide (IGZO). The electron mobility of IGZO-TFTs is 20-50 times that of amorphous silicon. By using IGZO TFTs in the active matrix backplane, it is possible to provide voltages greater than 30V via suitable display drivers. Furthermore, the ability to supply at least five (preferably seven) levels of source drivers allows for different driving modes in a four-particle electrophoretic display system. In one embodiment, there will be two positive voltages, two negative voltages, and zero voltage. In another embodiment, there will be three positive voltages, three negative voltages, and zero voltage. In one embodiment, there will be four positive voltages, four negative voltages, and zero voltage. These levels can be selected within the range of approximately -27V to +27V, without being limited by the aforementioned top-plane switching.

As shown in Figures 2A-2E, the electrophoretic medium of this invention comprises four types of electrophoretic particles in a nonpolar fluid 17. The first type of particle (W-*; hollow circle) is negatively charged and can be surface-treated such that its electrophoretic mobility depends on the strength of the driving electric field (discussed in more detail below). In this example, the electrophoretic mobility of the particles actually decreases under a stronger electric field, which is somewhat counterintuitive. The second type of particle (M++*; black circle) is positively charged and can also be surface-treated (or intentionally left untreated) such that its electrophoretic mobility depends on the strength of the driving electric field, or that it is driven to one side of the cavity containing the particles when the electric field direction is reversed. The aggregation and separation speed of the second type of particle is slower than that of the third and fourth types. The third type of particle (Y+; square circle) is positively charged, but its charge is less than that of the second type of particle. Furthermore, the third type of particle can be surface-treated, but the treatment method does not cause its electrophoretic mobility to depend on the strength of the driving electric field. In other words, the third type of particle can undergo surface treatment, but this treatment will not lead to a decrease in electrophoretic mobility when the electric field increases. The fourth type of particle (C++; gray circle) carries the highest amount of positive charge, and its surface treatment type is the same as that of the third type of particle. As shown in Figure 2A, the particle colors are nominally white, magenta, yellow, and cyan to produce the color shown in Figure 1. However, the invention is not limited to this specific color combination, nor is it limited to one reflective particle and three absorbing particles. For example, the system may include one black absorbing particle and three reflective particles—red, yellow, and blue—with appropriately matched reflectance spectra, producing a process white state when all three reflective particles are mixed and visible on the surface.

In a preferred embodiment, the first type of particle (negatively charged) is white and scattering. The second type of particle (positively charged, medium charge) is magenta and absorptive. The third type of particle (positively charged, low charge) is yellow and absorptive. The fourth type of particle (positively charged, high charge) is cyan and absorptive. Table 1 below shows the diffuse reflectance of exemplary yellow, magenta, cyan, and white particles that can be used in the electrophoretic medium of the present invention, and the ratio of absorption coefficient to scattering coefficient obtained from Kubelka-Munk analysis of these materials dispersed in a poly(isobutylene) matrix.

Table 1. Diffuse reflection of preferred yellow, magenta, cyan, and white particles.

The electrophoretic medium of the present invention can be any of the above-described forms. Therefore, the electrophoretic medium can be unencapsulated, encapsulated in discrete capsules surrounded by capsule walls, encapsulated in sealed microunits, or in the form of a polymer dispersion medium. Pigments are described in detail elsewhere, such as in U.S. Patent Nos. 9,697,778 and 9,921,451. In short, white particles W1 are a silanol-functionalized light-scattering pigment (titanium dioxide) as described in U.S. Patent No. 7,002,728, which is coated with a polymeric material comprising lauryl methacrylate (LMA) monomers. White particles W2 are polymer-coated titanium dioxide produced substantially according to Example 1 of U.S. Patent No. 5,852,196, the polymer coating comprising lauryl methacrylate and 2,2,2-trifluoroethyl methacrylate in a ratio of approximately 99:1. Yellow particles Y1 are C.I. Pigment Yellow 180, substantially as described in U.S. Patent No. 9,697,778, uncoated, dispersed by milling in the presence of Solsperse 19000. Yellow particles Y2 are C.I. Pigment Yellow 155, generally as described in U.S. Patent No. 9,697,778, uncoated, and dispersed by milling in the presence of Solsperse 19000. Yellow particles Y3 are C.I. Pigment Yellow 139, generally as described in U.S. Patent No. 9,697,778, uncoated, and dispersed by milling in the presence of Solsperse 19000. Yellow particles Y4 are C.I. Pigment Yellow 139, as described in Example 4 of U.S. Patent No. 9,921,451, coated by dispersion polymerization, incorporating trifluoroethyl methacrylate, methyl methacrylate, and a monomer containing dimethylsiloxane. Magenta particles M1 are a positively charged magenta material (dimethylquinacridone, C.I. Pigment Red 122), coated using vinyl benzyl chloride and LMA, as described in U.S. Patent Nos. 9,697,778 and Example 5 of U.S. Patent No. 9,921,451.

Magenta particles M2 are a C.I. Pigment Red 122, as described in Example 6 of U.S. Patent No. 9,921,451, coated via dispersion polymerization, methyl methacrylate, and a monomer containing dimethylsiloxane. Cyan particles C1 are a copper phthalocyanine material (C.I. Pigment Blue 15:3), as described in Example 7 of U.S. Patent No. 9,921,451, coated via dispersion polymerization, combined with a monomer containing methyl methacrylate and dimethylsiloxane. In some embodiments, it has been found that using Ink JetYellow 4GC (Clariant) as the core yellow pigment, combined with a methyl methacrylate surface polymer, can improve the color gamut. The zeta potential of the yellow pigment can be tuned by adding 2,2,2-trifluoroethyl methacrylate (TFEM) monomer and methacrylate terminal polymer (dimethylsiloxane).

U.S. Patent No. 9,697,778 discusses in detail electrophoretic medium additives and surface treatments for promoting different electrophoretic mobilities, and the proposed interaction mechanism between the surface treatment and surrounding charge control agents and/or free polymers, the entire contents of which are incorporated herein by reference. In such electrophoretic media, one method of controlling the interactions between different types of particles is to control the type, quantity, and thickness of the polymer coating on the particles. For example, to control particle properties such that the particle-particle interaction between type II particles and type III and IV particles is less than, for example, the particle-particle interaction between type III and type IV particles, type II particles may be polymer-treated, while type III and IV particles may either not be polymer-treated or have a lower mass coverage per unit area on their particle surface than type II particles. More generally, the Hamaker constant (a measure of the strength of the van der Waals interaction between two particles, the pairing potential being proportional to the Hamaker constant and inversely proportional to the sixth power of the distance between the two particles) and/or the particle spacing need to be adjusted by the judicious selection of the polymer coatings on the three types of particles.

As discussed in U.S. Patent No. 9,921,451, different types of polymers can include different types of polymer surface treatments. For example, Coulomb interactions may be weakened when the nearest approach distance of oppositely charged particles is maximized by spatial barriers (typically polymers grafted or adsorbed onto the surface of one or two particles). The polymer shell can be a covalently bonded polymer produced 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 containing both insoluble and soluble segments. Alternatively, the polymer shell can be dynamic, as it is a loose network of free polymers from an electrophoretic medium that, in the presence of an electric field and a sufficient quantity and type of charge control agent (CCA, discussed below), recombines with the pigment particles. Thus, depending on the strength and polarity of the electric field, the particles can have more associated polymers, resulting in different interactions between the particles and containers (e.g., microcapsules or microunits) and other particles. The extent of the polymer shell can be conveniently assessed using thermogravimetric analysis (TGA), a technique that involves raising the temperature of a dried sample of particles and measuring the pyrolysis mass loss as a function of temperature. Using TGA, the mass percentage of polymer particles can be measured and converted to a volume fraction using the known density of the core pigment and the polymer attached to it. Cases where the polymer coating is lost but the core pigment is retained can be identified (these cases depend on the precise core pigment particles used). Various polymer combinations can be worked as described below with reference to Figures 2A-2E. For example, in some embodiments, the particles (typically the first and/or second type of particles) may have a covalently attached polymer shell that interacts strongly with the container (e.g., microunits or microcapsules). Meanwhile, other particles with the same charge are not polymer-coated or are complexed with free polymer in solution, and therefore those particles have little interaction with the container. In other embodiments, the particles (typically the first and/or second type of particles) will not have a surface coating, making it easier for the particles to form a charged bilayer and experience reduced electrophoretic mobility in the presence of a strong electric field.

The fluid 17, dispersed with four types of particles, is transparent and colorless. The fluid contains charged electrophoretic particles that move within the fluid under the influence of an electric field. Preferred suspension fluids possess low dielectric constant (approximately 2), high volume resistivity (approximately ^10¹⁵ Ohm·cm), low viscosity (less than 5 mPas), low toxicity and environmental impact, low water solubility (less than parts per million, ppm if conventional aqueous encapsulation methods are used; however, note that this requirement may be relaxed for unencapsulated or certain micro-cell displays), high boiling point (greater than approximately 90°C), and low refractive index (less than 1.5). This last requirement is due to the use of a high-refractive-index scattering (typically white) pigment, whose scattering efficiency depends on the refractive index mismatch between the particles and the fluid.

Organic solvents, such as saturated linear or branched hydrocarbons, silicone oils, halogenated organic solvents, and low molecular weight halogenated polymers, are some useful fluids. Fluids can include a single component or a mixture of more than one component to tailor their chemical and physical properties. Fluids may also contain reactants or solvents for microencapsulation processes (if used), such as oil-soluble monomers.

The fluid preferably has low viscosity and a dielectric constant, between about 2 and about 30, preferably between about 2 and about 15, to obtain high particle mobility. Examples of suitable dielectric fluids include hydrocarbons such as Isopar®, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils, silicone oils, aromatic hydrocarbons such as toluene, xylene, phenoxyethane, dodecylbenzene or alkylnaphthalene, halogenated solvents such as perfluoronaphthene, perfluorotoluene, perfluoroxylene, dichlorotrifluorotoluene, 3,4,5-trichlorotrifluorotoluene, chloropentafluorobenzene, dichlorononane or pentachlorobenzene, and perfluorinated solvents such as FC-43, FC-70 or FC-70 manufactured by 3M Corporation (St. Paul, Minnesota). FC-5060 is a low molecular weight halogenated polymer, such as poly(perfluoropropylene oxide) and poly(trifluorochloroethylene) from TCI America in Portland, Oregon; Halocarbon Oils and perfluoropolyalkyl ethers from Halocarbon Product Corp in Riverside, New Jersey; Krytox Oils and Greases K-Fluid series from Galden of Ausimont or DuPont of Delaware; and polydimethylsiloxane silicone oil (DC-200) from Dow Corning.

Electrophoretic media typically also include one or more charge control agents (CCAs), and may also include charge directing agents. CCAs and charge directing agents typically comprise low-molecular-weight surfactants, polymerizing agents, or mixtures of one or more components that function to stabilize or otherwise alter the sign and/or magnitude of the charge on the electrophoretic particles. CCAs are typically molecules, including ionic or other polar groups, hereinafter referred to as head groups. At least one of the positive or negative ionic head groups is preferably attached to a nonpolar chain (typically a hydrocarbon chain), hereinafter referred to as a tail group. It is believed that CCAs form reverse micelles in the inner phase, and it is precisely the small number of charged reverse micelles that contribute to the conductivity of the very nonpolar fluids commonly used as electrophoretic fluids.

The addition of CCA enables the generation of inverted micelles, which consist of highly polar cores ranging in size from 1 nanometer to tens of nanometers (which can be spherical, cylindrical, or other geometries), surrounded by nonpolar tail groups of the CCA molecules. In the electrophoretic medium, there are typically three phases: solid particles with surfaces, a highly polar phase distributed as tiny droplets (inverted micelles), and a continuous phase containing the fluid. When an electric field is applied, both charged particles and charged inverted micelles can move within the fluid, resulting in two parallel electrical conduction paths through the fluid (which typically has very low conductivity).

The polar cores of CCAs are thought to influence the surface charge by adsorbing onto it. In electrophoretic displays, this adsorption can occur on the surface of electrophoretic particles or on the inner walls of microcapsules (or other solid phases, such as micro-unit walls), forming structures similar to reverse micelles, hereinafter referred to as hemimicelles. Ion exchange between hemimicelles and unbound reverse micelles can lead to charge separation when one ion in an ion pair is more firmly attached to the surface than the other (e.g., through covalent bonding). The stronger ion remains associated with the particle, while the weaker ion is incorporated into the core of the free reverse micelle.

It is also possible that the ionic materials forming the CCA head groups may induce ion pair formation on the particle (or other) surface. Therefore, CCA can perform two fundamental functions: generating charge on the surface and separating charge from the surface. Charge generation may result from acid-base reactions or ion exchange reactions between certain portions of the CCA molecule, or those otherwise incorporated into the reverse micelle core or fluid, and the particle surface. Therefore, useful CCA materials are those capable of participating in such reactions or any other charging reactions known in the art.

Non-limiting categories of charge control agents used in the media of this invention include organic sulfates or sulfonates, metal soaps, block or composite copolymers, organic amides, organic zwitterions, and organophosphates and phosphonates. Useful organic sulfates and sulfonates include, but are not limited to, sodium bis(2-ethylhexyl)sulfosuccinate, calcium dodecylbenzenesulfonate, calcium petroleum sulfonate, neutral or basic dinonylnaphthalenesulfonate, neutral or basic dinonylnaphthalenesulfonate, sodium dodecylbenzenesulfonate, and ammonium dodecyl sulfate. Useful metal soaps include, but are not limited to, basic or neutral barium carborate, calcium carborate, carboxylates of cobalt, calcium, copper, manganese, magnesium, nickel, zinc, aluminum, and iron, such as naphthenic acids, octanoic acid, oleic acid, palmitic acid, stearic acid, and myristic acid. Useful block or composite copolymers include, but are not limited to: (A) a polymer of quaternized 2-(N,N-dimethylamino)methacrylate and methyl p-toluenesulfonate and (B) an AB diblock copolymer of poly(2-ethylhexyl methacrylate), and composite graft copolymers having an oil-soluble poly(12-hydroxystearic acid) tail and a molecular weight of approximately 1800, suspended on an oil-soluble anchoring group of poly(methyl methacrylate-methacrylic acid). Useful organic amides/amines include, but are not limited to, polyisobutylene succinimides such as OLOA 371 or 1200 (available from Chevron Oronite LLC, Houston, Texas), or SOLSPERSE 17000 or 19000 (available from Lubrizol, Wycliffe, Ohio: Solsperse is a registered trademark), and N-vinylpyrrolidone polymers. Useful organic zwitterions include, but are not limited to, lecithin. Useful organophosphates and phosphonates include, but are not limited to, sodium salts of phospholipid monoglycerides and diglycerides having saturated and unsaturated acid substituents. Useful tail groups for CCAs include olefin polymers with molecular weights in the range of 200-10,000, such as poly(isobutylene). Head groups can be sulfonic, phosphoric, or carboxylic acids or amides, or amino groups such as primary, secondary, tertiary, or quaternary ammonium groups. One class of CCAs useful in the disclosed four-particle electrophoresis media is disclosed in U.S. Patent Publication No. 2017/0097556, the entire contents of which are incorporated herein by reference. Such CCAs typically comprise a quaternary ammonium head group and an unsaturated polymer tail, i.e., comprising at least one C-C double bond. The polymer tail is typically a fatty acid tail. A variety of CCA molecular weights can be used. In some embodiments, the molecular weight of the CCA is 12,000 g/mol or greater, for example, between 14,000 g/mol and 22,000 g/mol.

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

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

As described in U.S. Patent No. 7,170,670, adding a polymer with a number-average molecular weight exceeding about 20,000 to a fluid can improve the bistable nature of the electrophoretic medium; this polymer essentially does not absorb electrophoretic particles. Poly(isobutylene) is a preferred polymer for this purpose. Furthermore, for example, as described in U.S. Patent No. 6,693,620, particles with a fixed surface charge form an electric double layer with opposite charges in the surrounding fluid. The ionic head groups of CCA can ion-pair with charged groups on the surface of the electrophoretic particles to form a layer of immobilized or partially immobilized charged material. Outside this layer is a diffusion layer comprising charged (reverse) micelles, which include CCA molecules in the fluid. In conventional DC electrophoresis, an applied electric field exerts a force on the fixed surface charge and an opposite force on the moving opposite charge, causing slippage within the diffusion layer, allowing the particles to move relative to the fluid. The potential on the slippage surface is called the zeta potential.

Therefore, certain particle types in an electrophoretic medium exhibit different electrophoretic mobilities depending on the strength of the electric field on the medium. For example, when a first (low intensity, i.e., about ±10 V or lower) electric field is applied to the electrophoretic medium, type I particles move in one direction relative to the field. However, when a second (high intensity, i.e., about ±20 V or higher) electric field is applied, which has the same polarity as the first field, type I particles begin to move in the opposite direction relative to the field. It is presumed that this behavior stems from the fact that conduction in highly nonpolar fluids is mediated by charged reverse micelles or electrophoretic particles with opposite charges. Thus, any electrochemically generated protons (or other ions) are likely to be transported or adsorbed onto electrophoretic particles in the nonpolar fluid of the micelle core. For example, as shown in Figure 5B of U.S. Patent No. 9,697,778, a positively charged reverse micelle may approach a negatively charged electrophoretic particle traveling in the opposite direction, where the reverse micelle is adsorbed into the electric bilayer surrounding the negatively charged particle. (The electric bilayer comprises a charge-diffused layer with an enhanced counterion concentration and a semi-micelle surface adsorption coating on the particle; in the latter case, the countermicelle charge is associated with the particle within a sliding envelope, which, as described above, defines the particle's zeta potential.) Through this mechanism, an electrochemical current carrying positive ions flows through the electrophoretic fluid, and negatively charged particles may be biased towards more positive charges. Thus, the electrophoretic mobility of, for example, a type I negatively charged particle is a function of the magnitude of the electrochemical current and the residence time of positive charges near the particle surface, which in turn is a function of the electric field strength.

Furthermore, as described in U.S. Patent No. 9,697,778, positively charged particles can also be prepared, which will exhibit different electrophoretic mobilities depending on the applied electric field. In some embodiments, a secondary (or common) CCA can be added to the electrophoretic medium to adjust the zeta potential of various particles. Careful selection of the common CCA can change the zeta potential of one particle while keeping the zeta potential of other particles substantially unchanged, allowing close control over the electrophoretic velocities of various particles and the interactions between particles during switching.

In some embodiments, a portion of the charge control agent used in the final formulation is added during the synthesis of electrophoretic particles to design the desired zeta potential and influence the reduction in electrophoretic mobility caused by a strong electric field. For example, it has been observed that the addition of a quaternary ammonium charge control agent during polymer grafting results in a certain amount of CCA complexing with the particles. (This is confirmed by removing the particles from the electrophoretic fluid and then stripping the pigment surface with tetrahydrofuran (THF) to remove all adsorbed material. When the THF extract is evaluated with 1H NMR, a large amount of CCA is clearly seen adsorbed on the pigment particles or complexed with the surface polymer.) Experiments show that a high CCA content in the polymer on the particle surface contributes to the formation of a charge bilayer around the particles under a strong electric field. For example, magenta particles with a charge control agent (CCA) content of more than 200 mg per gram of finished magenta particles exhibit excellent retention properties in a high positive electric field. (See, for example, Figure 2C and the above.) In some embodiments, the CCA comprises a quaternary ammonium head group and a fatty acid tail group. In some embodiments, the fatty acid tail is unsaturated. When some particles in the electrophoretic medium have a high CCA load, it is important that particles with consistent electrophoretic mobility do not have a large CCA load, for example, less than 50 mg of charge control agent (CCA) per gram of finished particles, or less than 10 mg of charge control agent (CCA) per gram of finished particles.

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

Table 2 shows exemplary relative zeta potentials for three types of colored particles and a single white particle in a preferred embodiment.

Table 2. Relative zeta potentials of colored particles for which white particles have relative zeta potentials.

In one embodiment, the negative (white) particle has a zeta potential of -30 mV, and the other three particles are positive relative to the white particle. Therefore, a display including positive cyan, magenta, and yellow particles can switch between a black state (where all colored particles are in front of the white particles relative to the viewing surface) and a white state (where the white particles are closest to the viewer and prevent the viewer from perceiving the other three particles). Conversely, when the white particle has a zeta potential of 0 V, the negatively charged yellow particle is the most negative of all particles, so a display containing this particle will switch between yellow and blue states. This also occurs if the white particle is positively charged. However, a positively charged yellow particle will carry more positive charge than a white particle unless its zeta potential exceeds +20 mV.

The behavior of the electrophoretic medium of this invention is consistent with the mobility of white particles (represented as zeta potential in Table 2) depending on the applied electric field. Therefore, in the examples shown in Table 2, when using low voltage addressing, white particles may behave as if their zeta potential is -30mV, but when using higher voltage addressing, white particles may behave as if their zeta potential is more positive, possibly even as high as +20mV (matching the zeta potential of yellow particles). Thus, when using low voltage addressing, the display switches between black and white states, but when using higher voltage addressing, the display switches between blue and yellow states.

Figures 2B-2E illustrate the motion of various particles under high (e.g., "±H", e.g., ±20V, e.g., ±25V) and low (e.g., "±L", e.g., ±5V, e.g., ±10V) electric fields. For clarity, each dashed box represents a pixel, enclosed by a top transparent electrode 21 and a bottom electrode 22, which may be pixel electrodes of an active matrix, but could also be transparent electrodes or segmented electrodes, etc. As shown in Figures 2B-2E, starting from the first state where all positively charged particles appear at the viewing surface (nominally black), the electrophoretic medium can be driven to four different optical states. In a preferred embodiment, a white optical state (Figure 2B), a magenta optical state (Figure 2C), a yellow optical state (Figure 2D), and a red optical state (Figure 2E) are produced. Clearly, the remaining four optical states in Figure 1 can be achieved by reversing the order of the initial state and the driving electric field, as simplified in Figure 5.

When using low-voltage addressing, as shown in Figure 2B, for the case where a negative voltage is applied to the backplane, the behavior of the particles depends on their relative zeta potential, and their relative velocities are indicated by the arrows. Therefore, in this example, cyan particles move faster than magenta particles, and magenta particles move faster than yellow particles. The first (positive) pulse does not change the position of the particles because their movement is already restricted by the outer casing. The second (negative) pulse swaps the positions of the colored and white particles, thus the display switches between black and white states, although the instantaneous color reflects the relative mobility of the colored particles. The starting position and polarity of the reversing pulse allow for the transition from white to black. Therefore, compared to other black-and-white formulations that obtain multiple colors via process black or process white, this embodiment provides a black-and-white update that requires lower voltage (and consumes less power).

In Figure 2C, the first (positive) pulse has a high positive voltage, sufficient to reduce the mobility of the magenta particles (i.e., the particles with the middle mobility among the three positively charged colored particles). Due to the reduced mobility, the magenta particles are essentially frozen in place, and subsequent low-voltage pulses in the opposite direction cause the cyan, white, and yellow particles to move more than the magenta particles, thus producing a magenta color on the viewing surface, with the negatively charged white particles following the magenta particles. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to viewing the display from the side opposite to the viewing surface, i.e., through electrode 22), this pulse sequence will produce green (i.e., a mixture of yellow and cyan particles).

In Figure 2D, the first pulse is a low voltage, which does not significantly reduce the mobility of either the magenta or white particles. However, the second pulse is a high negative voltage, which reduces the mobility of the white particles. This allows for a more efficient race between the three positively charged particles, keeping the slowest particle (yellow in this example) visible in front of the white particles, whose motion has been attenuated in the previous negative pulse. Notably, the yellow particle does not reach the top surface of the cavity containing the particles. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to viewing the display from the side opposite the viewing surface, i.e., through electrode 22), this pulse sequence will produce a blue (i.e., a mixture of magenta and cyan particles).

Finally, Figure 2E shows that when both pulses are high voltage, the first high positive pulse reduces the mobility of magenta particles, while the second high negative pulse reduces the mobility of white particles, thus enhancing the race between cyan and yellow. This produces red. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to viewing the display from the side opposite to the viewing surface, i.e., through electrode 22), this pulse sequence will produce cyan.

To achieve a high-resolution display, individual pixels must be addressable without interference from neighboring pixels. One way to achieve this is by providing an array of nonlinear elements, such as transistors or diodes, with at least one nonlinear element associated with each pixel, creating an "active matrix" display. Addressing, or pixel electrode, is achieved by connecting to an appropriate voltage source via the associated nonlinear element, thus addressing a pixel. Typically, when the nonlinear element is a transistor, the pixel electrode is connected to the drain of the transistor; this arrangement will be assumed in the following description, although it is inherently arbitrary, and the pixel electrode can be connected to the source of the transistor. Conventionally, in high-resolution arrays, pixels are arranged in a two-dimensional array of rows and columns, such that any particular pixel is uniquely defined by the intersection of a particular row and a particular column. The sources of all transistors in each column are connected to a single column electrode, and the gates of all transistors in each row are connected to a single row electrode; similarly, assigning sources to rows and gates to columns is conventional but essentially arbitrary and can be reversed if desired. Row electrodes are connected to row drivers, which primarily ensure that only one row is selected at any given time. A selection voltage is applied to the selected row electrode to ensure all transistors in that row are conductive, while a non-selection voltage is applied to all other rows to ensure all transistors in those unselected rows are non-conductive. Column electrodes are connected to column drivers, which apply selected voltages to each column electrode to drive the pixels in the selected row to the desired optical state. (These voltages are relative to the common front electrode, which is typically located on the side of the electro-optical medium opposite the nonlinear array and extends throughout the display.) After a preselection interval known as the "row addressing time," the selected row is deselected, the next row is selected, and the voltage on the column driver changes, causing the next row to be written to the display. This process is repeated continuously, writing to the entire display row by row.

Typically, each pixel electrode has an associated capacitor electrode, such that the pixel electrode and the capacitor electrode form a capacitor; see, for example, International Patent Application WO01/07961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) 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.

Figure 3 in the accompanying drawings illustrates an example equivalent circuit for 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 dielectric 20 is represented as a capacitor and 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, parasitic capacitance 30 is much smaller than the parasitic capacitance of the storage capacitor 10, and when a pixel row of the display is selected or deselected, parasitic capacitance 30 can cause a small negative offset voltage, also known as a "bounce voltage," to be generated at the pixel electrode, which is typically less than 2 volts. In some embodiments, to compensate for the unwanted "bounce voltage," a common potential V _{_com} can be provided to the top plane electrode and the capacitor electrode associated with each pixel, such that when V _{_com} is set to a value equal to the bounce voltage (V _{_KB}), each voltage supplied to the display is offset by the same amount and does not experience a net DC imbalance.

However, problems can arise when V _{_com} is set to a voltage without compensation for bounce voltage. This occurs when a higher voltage needs to be applied to the display than is supplied by the backplane alone. It is well known in the art that, for example, if the backplane power supply is selected as nominal +V, 0, or -V, and V_{_com} is supplied as -V, the maximum voltage applied to the display may double. In this case, the maximum voltage is +2V (i.e., the voltage of the backplane relative to the top plane), while the minimum voltage is zero. If a negative voltage is required, the V _{_com} potential must be at least raised to zero. Therefore, the waveform used to address the display with positive and negative voltages using top plane switching must have specific frames assigned to each of more than one V _{_com} voltage setting.

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 voltages that amorphous silicon thin-film transistors can handle. In such instances, a suitable high voltage can be obtained by using top-plane switching. When V _{_com} is intentionally set to V_{_KB} (as described above), a separate power supply can be used. However, using the same number of separate power supplies as V _{_com} is costly and inconvenient when using top-plane switching. Furthermore, top-plane switching is known to increase bounce, thereby reducing the stability of the color state.

Display devices can be constructed using the electrophoretic fluid of this invention in several ways known in the art. The electrophoretic fluid can be encapsulated in microcapsules or incorporated into a microcell structure, which is then sealed with a polymer layer. The microcapsules or microcell layer can be coated or imprinted onto a plastic substrate or film coated with a transparent conductive material. The assembly can be laminated to a backplane with pixel electrodes using a conductive adhesive. Alternatively, the electrophoretic fluid can be directly dispensed onto a thin, open-cell mesh already arranged on a backplane including an active matrix of pixel electrodes. The filled mesh can then be top-sealed using an integrated protective sheet/transparent electrode.

Figure 4 shows a schematic cross-sectional view (not to scale) of a display structure 200 suitable for use with the present invention. In the display 200, electrophoretic fluid is illustrated as being confined within microcells, although an equivalent structure comprising microcapsules may also be used. A substrate 202 may be glass or plastic, on which pixel electrodes 204 are disposed. These electrodes may be individual addressable segments or associated with thin-film transistors in an active matrix arrangement. (The combination of substrate 202 and electrodes 204 is commonly referred to as the backplane of the display). Layer 206 is an optional dielectric layer applied to the backplane according to the present invention. (U.S. Patent Application No. 16/862750 describes a method for depositing a suitable dielectric layer, which is incorporated herein by reference). The front plane of the display includes a transparent substrate 222 on which a transparent conductive coating 220 is disposed. Above the electrode layer 220 is an optional dielectric layer 218. Layer (or multiple layers) 216 is a polymer layer that may include an underlayer for adhering microcells to the transparent electrode layer 220, and some residual polymer including the bottom of the microcells. The walls of microcell 212 are used to contain electrophoretic fluid 214. The microcell is sealed with layer 210, and the entire front planar structure is bonded to the backplate with conductive adhesive layer 208. The process for forming the microcell has been described in the prior art, such as U.S. Patent No. 6,930,818. In some instances, the depth of the microcell is less than 20 micrometers, for example less than 15 micrometers, for example less than 12 micrometers, for example about 10 micrometers, for example about 8 micrometers.

Due to the availability of manufacturing equipment and the cost of various starting materials, most commercial electrophoretic displays use amorphous silicon-based thin-film transistors (TFTs) in constructing the active matrix backplane (202/204). Unfortunately, amorphous silicon TFTs become unstable when the provided gate voltage allows switching voltages higher than approximately +/-15V. Nevertheless, as described below, the performance of ACePs improves when the amplitude of high positive and negative voltages is allowed to exceed +/-15V. Therefore, as previously disclosed, improved performance can be achieved by additionally changing the bias of the top transparent electrode relative to the bias of the backplane pixel electrode (also known as top plane switching). Thus, if a voltage of +30V (relative to the backplane) is required, the top plane can be switched to -15V while the corresponding backplane pixel is switched to +15V. For example, U.S. Patent No. 9,921,451 describes in more detail a method for driving a four-particle electrophoretic system using top plane switching.

These waveforms require that each pixel of the display can be driven at five different addressing voltages: +V _high , +V _low , 0, -V _low , and -V _high , illustrated as 30V, 15V, 0, -15V, and -30V. In practical applications, it may be desirable to use more addressing voltages. If only three voltages are available (i.e., +V _high , 0, and -V _high ), addressing can be achieved using a V _high voltage pulse with a duty cycle of 1/n, thus achieving the same result as addressing with lower voltages (e.g., V _high /n, where n is a positive integer greater than 1).

Figure 5 shows a typical waveform (simplified form) used to drive the four types of particle color electrophoretic display systems described above. This waveform has a "push-pull" structure: that is, the waveform consists of dipoles containing two pulses of opposite polarities. The magnitude and length of these pulses determine the color obtained. At least five such voltage levels are required. Figure 5 shows high positive voltage, low positive voltage, negative voltage, and zero voltage. Typically, "low" (L) refers to a range of approximately 5-15V, while "high" (H) refers to a range of approximately 15-30V. Generally, the higher the magnitude of the "high" voltage, the better the color gamut achieved by the display. In some embodiments, an additional "medium" (M) level is used, typically around 15V; however, the value of M depends to some extent on the composition of the particles and the environment of the electrophoretic medium. In many of the waveforms shown below, +V _high / -V _high = ±24V, +V _med / -V _med = ±17V, +V _low / -V _low = ±10V, which is achieved using a power management integrated circuit (PMIC) and a driver backplane containing metal-oxide transistors (such as IGZO), as described above. Commercially available suitable controllers can be used for the displays of this invention, such as UltraChip UC8152c or UC8159c or Solomon Systech SPD1656.

While Figure 5 shows the simplest dipole required to form color, it is understood that the actual waveforms could be multiple repetitions of these patterns, or other non-periodic patterns using more than five voltage levels. Typically, such waveforms are displayed as a series of impulses corresponding to frames, i.e., the amount of time between new cycles of the gates of the active matrix array TFTs turning on. Thus, Figures 9A-10B, for example, include the number of frames, where each frame can be understood to represent approximately 20 milliseconds. However, the size of each frame can vary, for example, due to a larger array or faster transistor speeds.

Of course, achieving the desired color using the drive pulses in Figure 5 depends on the process by which the particles begin from a known state, which is unlikely to be the last color displayed on the pixel. Therefore, a series of reset pulses precede the drive pulses, increasing the time required for the pixel to update from the first color to the second. U.S. Patent No. 10,593,272 describes the reset pulses in more detail, which is incorporated herein by reference. The lengths of these pulses (refresh and addressing) and any pauses (i.e., the zero-voltage periods between them) can be selected so that the entire waveform (i.e., the integral of voltage with respect to time over the entire waveform) is DC balanced (i.e., the integral of voltage with respect to time is essentially zero). DC balance can be achieved by adjusting the lengths of the pulses and pauses in the reset phase so that the net impulse provided by the reset phase is equal in magnitude and opposite in sign to the net impulse provided by the addressing phase, during which the display is switched to a specific desired color. However, as shown in Figures 2B-2E, the initial state of the eight primary colors is either black or white, which can be achieved using continuous low-voltage drive pulses. The ease of achieving this initial state further reduces the update time between states, which is more pleasant for users and also reduces power consumption (thus extending battery life).

Furthermore, the discussion of waveforms above, particularly regarding DC balance, neglected the issue of bounce voltage. In reality, as mentioned earlier, each backplane voltage is offset from the voltage supplied by the power supply by an amount equal to the bounce voltage _VKB . Therefore, if the power supply provides +V, 0, and -V, the backplane will actually receive three voltages: V+ _VKB , _VKB , and -V+ _VKB (note that for amorphous silicon TFTs, _VKB is typically negative). However, the same power supply will provide +V, 0, and -V to the front electrode without any bounce voltage offset. Thus, for example, when a -V voltage is supplied to the front electrode, the highest voltage of the display is 2V+ _VKB , and the lowest is _VKB . Instead of using a separate power supply to provide _VKB to the front electrode (which is costly and inconvenient), the waveform can be divided into several parts, providing positive, negative, and _VKB voltages to the front electrode separately.

Figure 6 illustrates an exemplary workflow of the controller. The workflow at the top represents a conventional workflow of a two-particle system, as described in U.S. Patent No. 9,672,766, where the black-to-white and white-to-black transitions in GC mode are approximately symmetrical. Therefore, switching between GC and DU modes is very straightforward depending on the usage (e.g., menus, scrolling, stylus writing, etc.), with almost no accumulation of impulse potential, thus eliminating the need for intermediate transition modes in a two-particle system.

However, in some multi-particle systems, such as ACeP®, the electrophoretic medium experiences large differential blur-based ghosting when symmetrical white-to-black and black-to-white transitions are used in GC mode. Therefore, when using conventional transition rules, the GC waveform is asymmetrical when the device switches to DU (Direct Update) mode, leading to impulse potential buildup. However, using the present invention, as illustrated in the workflow at the bottom of Figure 6, transition modes (DU_IN; DU_OUT) are used to compensate for the unbalanced impulse potential when moving between black and white states in GC mode during switching between GC and DU modes in a multi-particle system. In one embodiment, the DU (Direct Update) mode sets the black and white states to very high impulse potentials (approximately 400V*frame), thereby ensuring good black states and balanced impulse potentials when the display operates in DU mode. Essentially, this invention maintains consistency of the impulse potential of the black state between DU and GC modes, while driving the white state in DU mode with an impulse potential of the same magnitude but opposite sign as the black state, and allows the white state to maintain optical matching rather than impulse potential matching when switching between GC and DU modes.

Figure 7 shows a general visualization of DUin and DUout modes. Figure 7 illustrates the transition from each of the GC states to DU mode via a set of DUin waveforms that compensate for the impulse difference. Furthermore, when exiting DU mode, the display transitions back to GC mode, i.e., DUout mode, via DUout waveforms. The schematic diagram in Figure 7 is general and equally applicable to GC modes including 8, 16, 32, 64, or more color states. In most instances, DU modes only include white and black states; however, other DU modes can be implemented by correctly selecting colored particles, such as using green and white as DU states.

Figure 8 shows an example waveform suitable for DU mode in a four-particle ACEP® system, which includes one white negatively charged particle and three positively charged cyan, yellow, and magenta particles (with different charges). Although the waveforms are asymmetrical (or reversed), the cumulative impulse (voltage*frame) for the transition from white to black (top right waveform) and from black to white (bottom left waveform) is approximately 400V*frame. Therefore, in DU mode, the cumulative impulse potential is small when a pixel transitions between white and black. In some embodiments, duK and duW are independent controller states that transition a) maintain round-trip balance and b) match the optical performance of the corresponding GC optical states. Furthermore, since most of the DU waveform is driven by high voltage, pixel updates from black to white or from white to black can be achieved in approximately 250 milliseconds. While this update time is somewhat slow compared to the best (only) monochrome electrophoretic displays, it is sufficient for page turning and stylus input.

As mentioned above, mode transitions (DU_IN; DU_OUT) are required to prevent excessive impulse potentials when the display moves between GC and DU modes, which manifests as ghosting and shortens the lifespan of backplane electronics. The structure applied in these transitions is as follows: when using DUin, the transition to duK is direct, i.e., flicker-free, single-pulse, while enforcing optical state matching between duK and K. In contrast, the conventional DU mode leaves K->K as an empty transition. As shown in Figure 9A, in the DUin scheme, duK->duK is empty, but K->duK is filled and is a direct transition. Figure 9A shows a set of potential transition waveforms for DU_IN, where voltage is on the Y-axis and frame number is on the X-axis. It is noteworthy that many transitions are actually empty, i.e., without pulse output. Generally, DUin is applied during transitions between text mode states (K, GT2, GT3, W) and DU states (duK, duW), applying a transition with a net impulse equal to the duK impulse potential to transition to duK. This is because in GC mode, the impulse potential of K is zero due to the associated purge pulse. The impulse potential of the W state in GC is also zero. Therefore, all transitions between the selected white states maintain a net impulse potential of 0. Finally, the transitions from grayscale to DU black and white maintain the net impulse potential of the original text mode. To understand the actual optical transitions experienced, the waveforms in Figure 9A were run on a simulator of a four-particle ACEP® system, which includes one negatively charged white particle and three positively charged cyan, yellow, and magenta particles (with different charges). The resulting transitions are shown in Figure 9B. For the empty transition, there is no color change. However, for some transitions, such as from GC2 to duK and from GC2 to duW, the display “flickers” for a few frames of bright colors (e.g., red, blue). However, since the DUin transition takes approximately 365 milliseconds, the color transition is not particularly noticeable. Similarly, Figures 10A and 10B represent one embodiment of various waveforms that can be used for the transition from DU to GC (i.e., DU_OUT mode). Similarly, some transitions, such as from duK to GC2, will be achieved through a bright color change, but because the transition is so rapid, they will not be noticeable.

By adjusting the DUout transitions to have a specific net impulse potential, the round-trip impulse balance from GC mode to DU mode and back to GC mode is maintained. Specifically, as shown in Figure 10A, the transitions between DU states (duK, duW) and GC mode states (K, GC2, GC3, W) have a net impulse potential equal to the negative duK IP. The net impulse of the transition from duW to W/K is 0. Generally, all transitions from DU state to GC state using DUout appear as if they switch to white before transitioning to the text mode state, as shown in Figure 10B. The complete transition time from DU to GC using DU_OUT is typically around 576 milliseconds.

Figure 11 shows an example of impulse potential calculation. In Figure 11, the four display pixels are represented as squares in the upper box, and the matrix used for impulse potential balancing is represented in the lower box (i.e., the lower box is not a pixel). The start and final states use GC mode, with two black pixels at the top and two white pixels at the bottom. As shown by the leftmost square, the impulse potential of the four pixels is zero when the last GC update is completed. In DU mode (represented by the two lower matrices in the middle), the transition from duW to duK produces a +1 impulse, while the transition from duK to duW produces a -1 impulse. It can be understood that +1 and -1 are arbitrary units. Using the waveforms in Figure 8, +1 is equivalent to approximately 400V*frame. Importantly, in DU mode, the transitions have the same impulse potential but opposite signs.

However, to accommodate the calculations in DU mode, the transition from K (i.e., the black state in GC mode) to duK in DU mode must also involve an impulse potential of +1, as shown in the matrix at the bottom left. This is not intuitive. In most existing technologies, the transition from GC to DU between black states does not add an extra impulse to the pixel. The transition from W to duK also requires an impulse potential of +1, but this is not surprising, as the color state of the pixel needs to change. Nevertheless, if it weren't for the increased impulse potential used for state switching in DU mode, the transition from white to black would generally not require as much impulse potential. As can be seen from the pixels in the three middle boxes, the impulse potential cycles as the pixel moves from black to white in DU mode until switching back to GC mode. At this point, to eliminate ghosting in GC mode, it is necessary to eliminate any remaining impulse potential. Therefore, the DU output matrix shows that a transition from duK to black or white requires an impulse potential of -1.

As can be seen from the foregoing, the change-drive mode method of the present invention can provide improved updates for color electrophoretic displays, enabling device designers to create more interactive applications and thus improving the usability of devices incorporating such displays. It will be apparent to those skilled in the art that numerous changes and modifications can be made to the specific embodiments of the present invention described above without departing from the scope of the invention. Therefore, all the above descriptions should be interpreted illustratively rather than restrictively.

Claims (17)

1. A method for driving an electrophoretic display having multiple pixels, each pixel being capable of displaying at least three optical states, including white, black, and a color that is neither white nor black, the method comprising: The electrophoretic display is driven by a first driving mode that allows transitions between all optical states; The electrophoretic display is driven by a second driving mode that includes only transitions between black and white optical states, wherein, in the second driving mode, the impulse potential experienced by a pixel when it changes from a white state to a black state is equal in magnitude and opposite in sign to the impulse potential experienced by a pixel when it changes from a black state to a white state. The electrophoretic display is driven by a first transition mode, which allows a transition from a color state in the first driving mode to a white or black state in the second driving mode, wherein the first transition mode compensation will be transferred to the pixel's excess impulse potential in the second driving mode; and The electrophoretic display is driven by a second transition mode, which allows a transition from a white or black state in the second driving mode to a color state in the first driving mode, wherein the second transition mode compensates for excess impulse potential transmitted to the pixel in the second driving mode. 2. The method according to claim 1, wherein, in the first driving mode, the impulse potential experienced by a pixel when it changes from a white state to a black state is not equal in magnitude and has the opposite sign to the impulse potential experienced by a pixel when it changes from a black state to a white state. 3. The method of claim 1, wherein the first transition mode and the second transition mode do not have the same impulse potential compensation between the color state of the first driving mode and the white state of the second driving mode, and between the color state of the first driving mode and the black state of the second driving mode. 4. The method of claim 1, wherein the first transition mode and the second transition mode do not have the same waveform between the color state of the first driving mode and the white state of the second driving mode, and between the color state of the first driving mode and the black state of the second driving mode. 5. The method of claim 1, wherein, in the second driving state, the waveform causing the transition from the white state to the black state comprises at least five frames of maximum positive voltage. 6. The method of claim 1, wherein, in the second driving state, the waveform causing the transition from the black state to the white state comprises at least five frames of maximum negative voltage. 7. The method of claim 1, wherein the first drive mode is DC balanced. 8. The method of claim 1, wherein the first conversion mode and the second conversion mode are not DC balanced. 9. The method of claim 1, wherein each pixel is capable of displaying at least eight optical states, and the first transition mode allows transitions from each of six non-black and non-white color optical states to a white state or a black state of the second driving mode. 10. The method of claim 9, wherein the eight optical states are black, white, red, magenta, yellow, green, cyan, and blue. 11. A display controller configured to perform the method according to claim 1. 12. An electrophoretic display configured to implement the method according to claim 1. 13. The display of claim 12, wherein the electrophoretic display comprises an electrophoretic medium comprising at least three types of particles with different electrophoretic mobilities. 14. The display of claim 13, wherein at least two of the three types of particles have the same charge, but different amounts of charge. 15. The display according to claim 13, wherein one of the particle types is negatively charged and white. 16. The display of claim 15 further comprises three types of positively charged particles, wherein each type of positively charged particle has partial light absorption and its color is different from the other types of positively charged particles. 17. The display of claim 12, wherein the electrophoretic medium is confined within a plurality of capsules or a plurality of microunits.