HK40102717A - Coordinated top electrode - drive electrode voltages for switching optical state of electrophoretic displays using positive and negative voltages of different magnitudes - Google Patents

Description

A coordinated top electrode – drive electrode voltage – uses different positive and negative voltages to switch the optical state of the electrophoresis display.

Related applications

This application claims priority to U.S. Provisional Patent Application No. 63/320524, filed March 16, 2022. It also claims priority to U.S. Patent Application No. 17/474375, filed September 14, 2021. The entire contents of the patents and publications disclosed herein are incorporated herein by reference.

Background Technology

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

For many years, electrophoretic displays have contained only two types of charged colored (black and white) particles. (Of course, "colored" as used here includes both black and white.) White particles are typically light-scattering and include, for example, titanium dioxide, while black particles are absorbing across the entire visible spectrum and can include carbon black or absorbing metal oxides (e.g., copper chromite). In its simplest sense, a monochrome electrophoretic display requires only a transparent electrode at the viewing surface, a back electrode, and an electrophoretic medium containing white and black particles with opposite charges. When a voltage of one polarity is applied, the white particles move to the viewing surface, and when a voltage of the opposite polarity is applied, the black particles move to the viewing surface. If the back electrode includes controllable regions (pixels) (an active matrix or segmented electrode of pixel electrodes controlled by transistors), a pattern can appear electronically on the viewing surface. This pattern could, for example, be the text of a book.

Recently, several color options have become commercially viable for electrophoretic displays, including tri-color displays (black, white, and red; black, white, and yellow) and quad-color displays (black, white, red, and yellow). Similar to the operation of a black-and-white electrophoretic display, the operation of an electrophoretic display with three or four reflective pigments is analogous to a simple black-and-white display, as the desired colored particles are driven to the viewing surface. The driving scheme is far more complex than that of just black and white, but the final optical function of the particles remains the same.

Advanced Color Electronic Paper (ACeP ^™ ) also contains four types of particles, but the cyan, yellow, and magenta particles are subtractive rather than reflective, thus allowing for thousands of colors to be produced at each pixel. This color processing is functionally comparable to printing methods long used in offset and inkjet printing. A given color is produced by using the appropriate proportions of cyan, yellow, and magenta on a bright white paper background. In ACeP, for example, the relative positions of the cyan, yellow, magenta, and white particles relative to the viewing surface determine the color of each pixel. While this type of electrophoretic display allows for thousands of colors to be displayed at each pixel, careful control of the position of each pigment (50 to 500 nanometers in size) within a working space approximately 10 to 20 micrometers thick is crucial. Clearly, variations in pigment position will result in incorrect color display at a given pixel. Therefore, such a system requires precise voltage control. Further details of this system can be found in the following U.S. patents, all of which are incorporated herein by reference in their entirety: U.S. Patent Nos. 9,361,836, 9,921,451, 10,276,109, 10,353,266, 10,467,984, and 10,593,272.

This invention relates to color electrophoretic displays, and particularly, but not limited to, electrophoretic displays capable of displaying more than two colors using a single-layer electrophoretic material comprising multiple colored particles (e.g., white, cyan, yellow, and magenta particles). In some instances, two particles are positively charged and two particles are negatively charged. In some instances, three particles are positively charged and one particle is negatively charged. In some instances, one positively charged particle has a thick polymer shell, and one negatively charged particle has a thick polymer shell.

The term "gray state" is used in this document in its conventional sense within the imaging field, referring to the state between two extreme optical states of a pixel, but not necessarily implying a black-to-white transition between these two extreme states. For example, several patents and publications of IENK, discussed below, describe such electrophoretic displays where the extreme states are white and dark blue, making the intermediate gray state actually a pale blue. In fact, as already mentioned, a change in optical state may not be a color change at all. The terms "black" and "white" may be used below to refer to the two extreme optical states of a display and should be understood to generally include extreme optical states that are not strictly black and white, such as the aforementioned white and dark blue states.

The terms "bistable" and "bistable" are used herein in their conventional sense to refer to a display comprising display elements having first and second display states, the first and second display states having at least one different optical characteristic, and such that after any given element is driven to present its first or second display state using an addressing pulse of finite duration, the state will persist for at least several times (e.g., at least four times) the shortest duration of the addressing pulse required to change the state of the display element. As shown in U.S. Patent No. 7,170,670, some particle-based electrophoretic displays capable of achieving grayscale are stable not only in their extreme black and white states but also in intermediate gray states, and so are some other types of electro-optical displays. This type of display is aptly referred to as multistable rather than bistable, but for convenience, the term "bistable" is used herein to encompass both bistable and multistable displays.

The term "impulse" is used in this document to refer to the integral of the voltage applied during the driving cycle of an electrophoretic display.

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

Particle-based electrophoretic displays have been a subject of intensive research and development for several years. In such displays, multiple charged particles (sometimes called pigment particles) move through a fluid under the influence of an electric field. Compared to liquid crystal displays (LCDs), electrophoretic displays offer superior brightness and contrast, wide viewing angles, bistable operation, and low power consumption. However, issues concerning the long-term image quality of these displays have hindered their widespread adoption. For example, the particles constituting an electrophoretic display tend to settle, leading to a shorter 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., Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1 and Yamaguchi, Y., et al., Toner display using impulative particles charged triboelectrically, IDW Japan, 2001, Paper AMD4-4. Also see U.S. Patents 7,321,459 and 7,236,291. When such a gas-based electrophoretic medium is used in orientations where particle settling is permitted (e.g., in a signboard where the medium is arranged in a vertical plane), this medium appears to be susceptible to the same type of problems caused by particle settling as those in 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 the lower viscosity of gas suspensions allows for faster sedimentation of electrophoretic particles compared to liquid suspensions.

Numerous patents and applications attributed to or under 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 comprising an inner phase and a capsule wall surrounding the inner phase, the inner phase containing particles capable of electrophoretic movement in a fluid medium. Typically, the capsules themselves are held in a polymer binder to form a coherent layer located between two electrodes. Such techniques described in these patents and patent 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) A method 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) Backplates, adhesive layers, and other auxiliary layers and methods used in displays; 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. 6,017,584, 6,545,797, 6,664,944, 6,788,452, 6,864,875, 6,914,714, 6,972,893, 7038,656, 7038,670, 7046,228, 7052,571, 7075,502, 7167,155, 7385,751, 7492,505, 766,7684, 7684,108, 7791,789, 7800,813, 7821,702, 7839,564, 7910,175, 7952,790, 7956,841, 7982,941, 8040,594, 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, 9146 439, 9164207, 9170467, 9170468, 9182646, 9195111, 9199441, 9268191, 9285649, 9293511, 9341916, 9360733, 9361836, 9383623, and 9423666; and U.S. Patent Application Publications 2008/0043318, 2008/0048970, 2009/0225398, 2010/0156780, 2011/0043543, 2012/0326957, 2013/0242378, 2013/0278995, 2 Numbers 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. Patents 5,930,026, 6,445,489, 6,504,524, 6,512,354, 6,531,997, 6,753,999, 682,5970, 6,900,851, 6,995,550, 7012,600, 7023,420, 7034,783, 7061,166, 7061,662, 711,6466, 711,9772, 717,7066, 7193,625, 720,2847, 724,2514, 7259,744, 7304,787, 73 12794, 7327511, 7408699, 7453445, 7492339, 7528822, 7545358, 7583251, 7602374, 7612760, 7679599, 7679813, 7683606, 7688297, 7729039, 7733311, 7733335, 7787169, 7859742, 7952557, 7956841, 7982479, 7999787, 8077141, 8125501, 81 39050, 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 Patent applications numbered 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 Publications. Numbers: 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, 201 0/0194789, 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, Patents and applications numbered 2015/0213765, 2015/0221257, 2015/0262255, 2015/0262551, 2016/0071465, 2016/0078820, 2016/0093253, 2016/0140910 and 2016/0180777 (these patents and applications may be referred to below as MEDEOD (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, as described in U.S. Patent No. 6,241,921, U.S. Patent Application Publication No. 2015/0277160, and U.S. Patent Application Publications 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 droplets of electrophoretic fluid and a continuous phase of polymeric material, and the discrete droplets of electrophoretic fluid within such a polymer dispersion electrophoretic display can be considered as capsules or microcapsules, even without a discrete capsule film 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, instead of encapsulating charged particles and fluids in microcapsules, they are held within multiple cavities formed within a carrier medium (typically a polymer film). See, for example, U.S. Patents 6,672,921 and 6,788,449.

Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, particles essentially block visible light from passing through the display) and operate in reflective mode, many electrophoretic displays can be fabricated to operate in a so-called "shutter mode," in which one display state is substantially opaque and another display state is transparent. See, for example, U.S. Patents 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971 and 6,184,856. Dielectrophoretic displays (which are similar to electrophoretic displays but depend on changes in electric field strength) can operate in a similar mode; see U.S. Patent 4,418,346. Other types of electro-optic displays are also capable of operating in shutter mode. Electro-optic media operating in shutter mode can be used in multilayer structures of 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 are generally free from the clustering and sedimentation failure modes of conventional electrophoretic equipment and offer more beneficial effects, such as the ability to print or coat displays onto 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-volume coating (e.g., patch die coating, slot or extrusion coating, slide or stack coating, curtain coating); roll coating (e.g., roller-lined blade coating and 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, because the display medium can be printed (using various methods), the display itself can be manufactured inexpensively.

As mentioned above, most simple existing electrophoretic media essentially display only two colors. Such electrophoretic media use a single type of electrophoretic particles of 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, a color filter array can be deposited 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 among three or four primary colors, such as red/green/blue (RGB) or red/green/blue/white (RGBW), and the color filters can be arranged in a one-dimensional (stripes) or two-dimensional (2x2) repeating pattern. The choice of other primary colors or more than three primary colors is also known in the art. Three (in the case of RGB displays) or four (in the case of RGBW displays) subpixels are chosen to be small enough that at the intended viewing distance, these subpixels visually blend together to form a single pixel with a uniform color stimulus (“color mixing”). An inherent drawback of area sharing is that the colorant is always present, and the color can only be modulated by switching the corresponding pixel of the underlying monochrome display to white or black (turning the corresponding primary color on or off). For example, in an ideal RGBW display, each of the red, green, blue, and white primary colors occupies a quarter of the display area (one of the four subpixels), the white subpixel is as bright as the white of the underlying monochrome display, and each colored subpixel is no brighter than one-third the white of the monochrome display. The overall white brightness displayed on a monitor cannot exceed half the brightness of a white subpixel (the white area of a monitor is produced by displaying one white subpixel out of every four subpixels plus each colored subpixel whose color form is equivalent to one-third of the white subpixel, so the combined contribution of three colored subpixels does not exceed that of one white subpixel). Color brightness and saturation are reduced by sharing areas with color pixels switched to black. Area sharing is particularly problematic when mixing yellow, because yellow is brighter than any other color of the same brightness, and saturated yellow is almost as bright as white. Switching a blue pixel (one-quarter of the display area) to black makes yellow too dark.

A commonly used system for quantifying the color characteristics (including both luminance and hue) of a display is the CIELAB system. Under the CIE standard illuminant D65 (e.g., a color temperature of 6500K), it specifies the color coordinate values (i.e., L*, a*, b*) corresponding to the colors displayed by a typical color reflective display device. L* represents luminance from black to white on a scale of 0 to 100, while a* and b* represent chromaticity without specific numerical limitations. Negative a* corresponds to green, positive a* to red, negative b* to blue, and positive b* to yellow. L* can be converted to reflectance using the following formula: L* = 116(R/ _R0 ) ^{^(1/3)} - 16, where R is reflectance and _R0 is the standard reflectance value.

U.S. Patents 8,576,476 and 8,797,634 describe a multicolor electrophoretic display having a single backplate containing independently addressable pixel electrodes and a common transparent front electrode. The common transparent front electrode is also referred to as the top electrode. Multiple electrophoretic layers are disposed between the backplate and the front electrode. The displays described in these applications can display any primary color (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel location. However, there are drawbacks to using multiple electrophoretic layers located between a single set of addressable electrodes. 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.

Attempts have been made to provide full-color electrophoretic displays using a single-layer electrophoretic layer. For example, U.S. Patent No. 8,917,439 describes a color display comprising an electrophoretic fluid containing one or two types of pigment particles dispersed in a transparent, colorless or colored solvent, the electrophoretic fluid being disposed between a common electrode and a plurality of pixels or driving electrodes. The driving electrodes are configured to expose a background layer. U.S. Patent No. 9,116,412 describes a method for driving a display unit filled with an electrophoretic fluid containing two types of charged particles with opposite charge polarities and two contrasting colors. The two types of pigment particles are dispersed in a colored solvent or in a solvent having uncharged or slightly charged colored particles. The method includes driving the display unit by applying a driving voltage of about 1 to about 20% of the full driving voltage to display the color of the solvent or the color of the uncharged or slightly charged colored particles. U.S. Patents Nos. 8,717,664 and 8,964,282 describe an electrophoretic fluid and a method for driving an electrophoretic display. The fluid comprises pigment particles of types one, two, and three, all of which are dispersed in a solvent or solvent mixture. The first and second types of pigment particles carry opposite charge polarities, and the third type of pigment particles has a charge level approximately 50% lower than that of the first or second type pigment particles. These three types of pigment particles have different critical voltage levels or different mobility levels, or both. None of these patent applications discloses a full-color display, as used hereinafter in the sense of the term, capable of displaying at least eight independent colors (white, red, green, blue, cyan, yellow, magenta, and black).

Summary of the Invention

This document discloses improved methods for driving full-color electrophoretic displays and full-color electrophoretic displays using these driving methods. In one aspect, the invention relates to a color electrophoretic display comprising: a light-transmitting electrode at a viewing surface; a backplate including an array of thin-film transistors coupled to pixel electrodes, wherein each thin-film transistor includes a layer of metal-oxide-semiconductor; and a color electrophoretic medium disposed between the light-transmitting electrode and the backplate. The color electrophoretic medium comprises (a) a fluid; (b) a plurality of first particles and a plurality of second particles dispersed in the fluid, the first and second particles carrying opposite charges, the first particles being light-scattering particles, and the second particles having a subtractive primary color; and (c) a plurality of third particles and a plurality of fourth particles dispersed in the fluid, the third and fourth particles carrying opposite charges, and both the third and fourth particles having a subtractive primary color different from each other and different from the second particles.

In some embodiments, the first electric field required to separate aggregates formed by third and fourth types of particles is greater than the second electric field required to separate aggregates formed by the other two types of particles. In some embodiments, at least two of the second, third, and fourth types of particles are non-light-scattering. In some embodiments, the first particle is white, and the second, third, and fourth particles are non-light-scattering. In some embodiments, the first and third particles are negatively charged, and the second and fourth particles are positively charged. In some embodiments, the colors of the first, second, third, and fourth particles are white, cyan, yellow, and magenta, respectively, with white and yellow particles being negatively charged and magenta and cyan particles being positively charged. In some embodiments, when the pigments are approximately isotropically distributed at 15% volume in a 1 μm thick layer comprising the pigment and a liquid with a refractive index less than 1.55, the yellow, magenta, and cyan pigments exhibit diffuse reflectances of less than 2.5% measured against a black background at 650, 550, and 450 nm, respectively. In some embodiments, the fluid is a non-polar fluid with a dielectric constant less than about 5. In some embodiments, the fluid has been dissolved or dispersed in a polymer having a number-average molecular weight exceeding about 20,000 and being inherently non-absorbent of particles. In some embodiments, the metal oxide semiconductor is indium gallium zinc oxide (IGZO). The invention described above can be incorporated into e-book readers, portable computers, tablet computers, mobile phones, smart cards, signs, watches, shelf labels, or flash drives.

In another aspect, a color electrophoretic display includes: a controller; a light-transmitting electrode at a viewing surface; and a backplane including an array of thin-film transistors coupled to pixel electrodes, each thin-film transistor including a layer of metal-oxide-semiconductor. A color electrophoretic medium is disposed between the light-transmitting electrode and the backplane, the color electrophoretic medium including (a) a fluid; (b) a plurality of first particles and a plurality of second particles dispersed in the fluid, the first and second particles carrying opposite charges, the first particles being light-scattering particles, and the second particles having a subtractive primary color; and (c) a plurality of third particles and a plurality of fourth particles dispersed in the fluid, the third and fourth particles carrying opposite charges, each of the third and fourth particles having a subtractive primary color different from each other and different from the second particles. The controller is configured to provide a plurality of driving voltages to the pixel electrodes such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed at each pixel electrode while maintaining the light-transmitting electrode at a constant voltage. In some embodiments, the controller is configured to provide voltages greater than 25 volts and less than -25 volts to the pixel electrodes. In some embodiments, the controller is configured to additionally provide a voltage between 25V and 0V and a voltage between -25V and 0V. In some embodiments, the metal oxide semiconductor is indium gallium zinc oxide (IGZO).

On the other hand, a color electrophoretic display includes: a controller; a light-transmitting electrode at a viewing surface; a back electrode; and a color electrophoretic medium disposed between the light-transmitting electrode and the back electrode. The color electrophoretic medium includes (a) a fluid; (b) a plurality of first particles and a plurality of second particles dispersed in the fluid, the first and second particles carrying opposite charges, the first particles being light-scattering particles, and the second particles having a subtractive primary color; and (c) a plurality of third particles and a plurality of fourth particles dispersed in the fluid, the third and fourth particles carrying opposite charges, and both the third and fourth particles having a subtractive primary color different from each other and different from the second particles. The controller is configured to provide a first high voltage and a first low voltage to the light-transmitting electrode, and a second high voltage, zero voltage, and a second low voltage to the back electrode, such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed at the viewing surface, wherein at least one of the first high voltage, the first low voltage, the second high voltage, and the second low voltage has a different magnitude. In some embodiments, the magnitude of the first high voltage and the second high voltage are the same. In some embodiments, the magnitude of the first low voltage and the magnitude of the second low voltage are the same, and the magnitude of the first high voltage and the magnitude of the first low voltage are different.

On the other hand, a color electrophoretic display includes: a controller; a light-transmitting electrode at a viewing surface; a back electrode; and a color electrophoretic medium disposed between the light-transmitting electrode and the back electrode. The color electrophoretic medium includes (a) a fluid; (b) a plurality of first particles and a plurality of second particles dispersed in the fluid, the first and second particles carrying opposite charges, the first particles being light-scattering particles, and the second particles having a subtractive primary color; and (c) a plurality of third particles and a plurality of fourth particles dispersed in the fluid, the third and fourth particles carrying opposite charges, and both the third and fourth particles having a subtractive primary color different from each other and different from the second particles. The controller is configured to display colors such as white, yellow, red, magenta, blue, cyan, green, and black at the viewing surface by providing one of a plurality of time-dependent driving voltages to the back electrode, while simultaneously providing the light-transmitting electrode with one of the following driving voltages: 1) a high voltage at a first time, a low voltage at a second time, and a high voltage at a third time, or 2) a low voltage at a first time, a high voltage at a second time, and a low voltage at a third time.

On the other hand, a system for driving an electrophoretic medium includes an electrophoretic display and a power supply capable of providing positive and negative voltages of different magnitudes; and a controller coupled to a top electrode driver, a first driving electrode driver, and a second driving electrode driver. The electrophoretic medium includes: a light-transmitting top electrode at a viewing surface; a first driving electrode; a second driving electrode; and an electrophoretic medium disposed between the top electrode and the first and second driving electrodes. The controller is configured to: A) in a first frame, provide a positive voltage to the top electrode, a negative voltage to the first driving electrode, and a positive voltage to the second driving electrode; B) in a second frame, provide a negative voltage to the top electrode, a negative voltage to the first driving electrode, and a negative voltage to the second driving electrode; C) in a third frame, provide a ground voltage to the top electrode, a ground voltage to the first driving electrode, and a positive voltage to the second driving electrode; and D) in a fourth frame, provide a positive voltage to the top electrode, a positive voltage to the first driving electrode, and a positive voltage to the second driving electrode. In one embodiment, the controller is further configured to: E) provide a negative voltage to the top electrode, a ground voltage to the first driving electrode, and a negative voltage to the second driving electrode in a fifth frame; and F) provide a ground voltage to the top electrode, a ground voltage to the first driving electrode, and a ground voltage to the second driving electrode in a sixth frame. In one embodiment, the electrophoretic medium is encapsulated in a plurality of microcapsules, and the microcapsules are dispersed in a polymer adhesive between the top electrode and the first and second driving electrodes. In one embodiment, the electrophoretic medium is encapsulated in a micro-unit array with openings, wherein the openings are sealed with a polymer adhesive, and the micro-unit array is disposed between the top electrode and the first and second driving electrodes. In one embodiment, the electrophoretic medium comprises a nonpolar fluid and four groups of particles with different optical properties. In one embodiment, the first and second groups of particles have opposite charges, the third and fourth groups of particles have opposite charges, the first group of particles are light-scattering particles, and the second, third, and fourth groups of particles are all subtractive primary colors different from each other. In one embodiment, the controller is configured to provide a combination of positive voltage, negative voltage, and ground voltage to the top electrode and the first drive electrode, such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. In one embodiment, the first and second groups of particles carry charges of opposite polarities, the third and fourth groups of particles carry the same charge as the second group of particles, the first group of particles are light-scattering particles, and the second, third, and fourth groups of particles are all subtractive primary colors distinct from each other. In another embodiment, the controller is configured to provide a combination of positive voltage, negative voltage, and ground voltage to the top electrode and the first drive electrode, such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. In one embodiment, the positive voltage is +15V and the negative voltage is -9V. In yet another embodiment, the positive voltage is +9V and the negative voltage is -15V.

On the other hand, a system for driving an electrophoretic medium includes an electrophoretic display and a power supply capable of providing positive and negative voltages of different magnitudes; and a controller coupled to a top electrode driver, a first driving electrode driver, and a second driving electrode driver. The electrophoretic medium includes: a light-transmitting top electrode at a viewing surface; a first driving electrode; a second driving electrode; and an electrophoretic medium disposed between the top electrode and the first and second driving electrodes. The controller is configured to: A) provide a positive voltage to the top electrode, a negative voltage to the first driving electrode, and a positive voltage to the second driving electrode in a first frame; B) provide a negative voltage to the top electrode, a negative voltage to the first driving electrode, and a negative voltage to the second driving electrode in a second frame; C) provide a ground voltage to the top electrode, a ground voltage to the first driving electrode, and a ground voltage to the second driving electrode in a third frame; and D) provide a positive voltage to the top electrode, a positive voltage to the first driving electrode, and a positive voltage to the second driving electrode in a fourth frame. In one embodiment, the controller is further configured to: E) provide a negative voltage to the top electrode, a ground voltage to the first driving electrode, and a negative voltage to the second driving electrode in a fifth frame; and F) provide a ground voltage to the top electrode, a ground voltage to the first driving electrode, and a ground voltage to the second driving electrode in a sixth frame. In one embodiment, the electrophoretic medium is encapsulated in a plurality of microcapsules, and the microcapsules are dispersed in a polymer adhesive between the top electrode and the first and second driving electrodes. In one embodiment, the electrophoretic medium is encapsulated in a micro-unit array with openings, wherein the openings are sealed with a polymer adhesive, and the micro-unit array is disposed between the top electrode and the first and second driving electrodes. In one embodiment, the electrophoretic medium comprises a nonpolar fluid and four groups of particles with different optical properties. In one embodiment, the first and second groups of particles have opposite charges, the third and fourth groups of particles have opposite charges, the first group of particles are light-scattering particles, and the second, third, and fourth groups of particles are all subtractive primary colors different from each other. In one embodiment, the controller is configured to provide a combination of positive voltage, negative voltage, and ground voltage to the top electrode and the first drive electrode, such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. In one embodiment, the first and second groups of particles carry charges of opposite polarities, the third and fourth groups of particles carry the same charge as the second group of particles, the first group of particles are light-scattering particles, and the second, third, and fourth groups of particles are all subtractive primary colors distinct from each other. In another embodiment, the controller is configured to provide a combination of positive voltage, negative voltage, and ground voltage to the top electrode and the first drive electrode, such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. In one embodiment, the positive voltage is +15V and the negative voltage is -9V. In yet another embodiment, the positive voltage is +9V and the negative voltage is -15V.

Attached Figure Description

Figure 1 is a schematic cross-sectional view of one embodiment of an encapsulated electrophoretic display suitable for use with the method of the present invention.

Figure 2 is a schematic cross-sectional view of one embodiment of an encapsulated electrophoretic display suitable for use with the method of the present invention.

Figure 3 shows an exemplary equivalent circuit for a single pixel of an electrophoretic display, where the voltage on the single pixel is controlled by a transistor. The circuit in Figure 3 is commonly used in active matrix backplanes.

Figure 4 illustrates how a positive voltage source and a negative voltage source are applied to the top electrode and two separate drive electrodes to achieve the desired drive voltage on the two separate drive electrodes.

Figure 5 is a schematic cross-sectional view showing the positions of various colored particles in a colored electrophoretic medium when displaying black, white, the three subtractive primary colors, and the three additive primary colors.

Figure 6 illustrates an exemplary push-pull drive scheme for addressing an electrophoretic medium that includes tri-color subtractive particles and scattering (white) particles.

Figure 7 depicts a simplified top-plane driven waveform for generating eight colors in an electrophoretic medium that includes subtractive particles and scattering (white) particles.

Figure 8 illustrates an exemplary driving mode that achieves a green optical state at the viewing surface above the first driving electrode and a yellow optical state at the viewing surface above the second driving electrode using only two voltage sources.

Figure 9A shows the variation of the L*a*b* values of the eight color indices when the same four-particle electrophoretic medium is driven by seven independent driving voltages or by two voltage sources and using a coordinated top electrode voltage cycle.

Figure 9B shows the state of simulated color in the curve graph of Figure 9A.

Detailed Implementation

A system for simplified driving of electrophoretic media using positive and negative voltage sources, wherein the voltage sources have different magnitudes, and a controller cycles the top electrode between two voltage sources and a ground voltage while coordinating the driving of at least two driving electrodes opposite the top electrode. The resulting system achieves approximately the same color state compared to providing six independent drive levels and a ground voltage to each driving electrode. Therefore, the system simplifies the required electronics with only a marginal loss in color gamut. This system is particularly useful for addressing electrophoretic media containing four different sets of particles, for example, where three sets of particles are colored and subtractive, and one set of particles is light-scattering.

This invention proposes an improved method for driving electro-optic media devices using a so-called top-plane switching method, wherein the voltage on the top (common) electrode varies during device update processes. In some embodiments, the invention is used with an electrophoretic medium comprising four particles, wherein two particles are colored and subtractive, and at least one particle is scattering. Typically, such a system comprises white particles and subtractive primary color particles of cyan, yellow, and magenta. In some embodiments, two particles are positively charged and two particles are negatively charged. In some embodiments, three particles are positively charged and one particle is negatively charged. In some embodiments, one particle is positively charged and three particles are negatively charged. Such a system is schematically illustrated in FIG5 and can provide white, yellow, red, magenta, blue, cyan, green, and black at each pixel.

Display devices can be constructed using the electrophoretic fluid of the present invention through several methods known in the prior art. The electrophoretic fluid can be encapsulated in microcapsules or contained within microcell structures, subsequently sealed with a polymer layer. The microcapsules or microcell layers can be coated or imprinted onto a plastic substrate or film with a transparent coating of 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 disposed 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.

Regarding Figures 1 and 2, an electrophoretic display (101, 102) typically includes a top transparent electrode 110, an electrophoretic medium 120, and bottom driving electrodes 130/135, which are typically pixel electrodes of an active pixel matrix controlled by thin-film transistors (TFTs). Alternatively, the bottom driving electrodes 130/135 may be directly wired to a controller or some other switch that supplies voltage to the bottom driving electrodes 130/135 to change the optical state of the electrophoretic medium 120, i.e., segmented electrodes. Importantly, the junctions between the driving electrodes 130/135 do not need to correspond to the intersections of microcapsules or the walls of microcells 127. Because the electrophoretic medium 120 is thin enough and the capsules or microcells are wide enough, when the display is viewed from a viewing surface, the pattern of the driving electrodes (square, circular, hexagonal, wavy, text, or other) will be displayed, rather than the pattern of the container. Electrophoretic medium 120 contains at least one electrophoretic particle 121; however, a second electrophoretic particle 122, a third electrophoretic particle 123, a fourth electrophoretic particle 124, or more particles are feasible. [It should be noted that the third group of electrophoretic particles 123 and the fourth electrophoretic particle 124 may be included in the microcapsule 126 of FIG. 1, but have been omitted for clarity.] The electrophoretic medium 120 typically includes a solvent, such as isoalkanes, and may also include a dispersing polymer and a charge control agent to promote state stability, such as bistableness, i.e., the ability to maintain an electro-optical state without inputting any additional energy.

The electrophoretic medium 120 is typically separated by the walls of microcapsules 126 or microunits 127. The entire display stack is typically disposed on a substrate 150, which may be a rigid or flexible substrate. The displays (101, 102) also typically include a protective layer 160, which may simply protect the top electrode 110 from damage, or may surround the entire display (101, 102) to prevent water ingress, etc. The electrophoretic displays (101, 102) may also include one or more adhesive layers 140, 170 and/or sealing layers 180 as needed. In some embodiments, the adhesive layer may include a primer component to improve adhesion to the electrode layer 110, or a separate primer layer (not shown in Figure 1 or 2) may also be used. The structure of the electrophoretic display and its components, pigments, binders, electrode materials, etc., can be found in numerous patents and patent applications published by Einkel (e.g., U.S. Patents 6,922,276, 7,002,728, 7,072,095, 7,116,318, 7,715,088, and 7,839,564), the entire contents of which are incorporated herein by reference.

Each pixel electrode or drive electrode of a thin-film transistor (TFT) backplane typically has only one transistor. 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 WO 01/07961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) can be used to form the transistor, and the “select” and “non-select” voltages applied to the gate electrode can be positive and negative, respectively.

As shown in Figure 3, each transistor (TFT) is connected to a gate line, a data line, and a pixel electrode (push electrode). When the positive voltage on the TFT gate is large enough (or negative voltage, depending on the type of transistor), there is a small impedance between the scan line and the pixel electrode coupled to the TFT drain (i.e., Vg "ON" or "OPEN"), and the voltage on the scan line is thus transferred to the pixel electrode. However, when there is a negative voltage on the TFT gate, there is a high impedance, and the voltage is stored in the pixel storage capacitor and is unaffected by the voltage on the scan line when other pixels are addressed (i.e., Vg "OFF" or "CLOSED"). Therefore, ideally, the TFT is used as a digital switch. In practice, the TFT still has some resistance when it is in the "ON" setting, so the pixel needs some time to charge. In addition, when the TFT is in the "OFF" setting, voltage may leak from V_{_S} to V_{_pix} , causing crosstalk. Increasing the capacitance of the storage capacitor _CS can reduce crosstalk, but at the cost of making the pixel more difficult to charge and increasing the charging time. As shown in Figure 3, a separate voltage ( _VTOP ) is provided to the top electrode, thereby establishing an electric field between the top electrode and the pixel electrode ( _VFPL ). Ultimately, the value of _VFPL determines the optical state of the associated electro-optic medium. When the first side of the storage capacitor is coupled to the pixel electrode, the second side of the storage capacitor is coupled to a separate line ( _Vcomal ), enabling the removal of charge from the pixel electrode. See, for example, U.S. Patent No. 7,176,880, the entire contents of which are incorporated herein by reference. [In some embodiments, an N-type semiconductor (e.g., amorphous silicon) may be used to form the transistor, and the “select” and “non-select” voltages applied to the gate electrode may be positive and negative, respectively.] In some embodiments, _VComal may be grounded; however, there are many different designs for draining charge from the charging capacitor, such as those described in, for example, U.S. Patent No. 10,037,735, the entire contents of which are incorporated herein by reference.

The problem with traditional amorphous silicon TFTs is that their operating voltage is limited to approximately ±15V, causing transistors to begin leaking and eventually fail. While an operating voltage range of ±15V is suitable for many two-particle electrophoresis systems, it is found that a wider voltage range makes it easier to separate particles with different zeta potentials, resulting in faster updates and more reproducible colors in advanced electrophoretic displays. The solution to increasing the pixel electrode voltage range is to utilize top-plane switching, where the voltage on the top (common) electrode varies over time.

The principle of top-plane switching is illustrated in Figure 4. An exemplary electrophoretic display 401 includes an electrophoretic medium 420 disposed between a top electrode 410 and a (bottom) drive electrode 430. The electrophoretic medium 420 in Figure 4 is shown to have four different types of electrophoretic particles; however, the electrophoretic medium 420 may have fewer or more different types of particles than shown. In the simplified embodiment of Figure 4, both the top electrode 410 and the drive electrode 430 are powered by two different power supplies 440 and 460, which may be from the same power supply (not shown). Additionally, a ground voltage 470 is available. Typically, one power supply is positive relative to ground, and the other is negative relative to ground. A controller 470 controls which power supply (or ground voltage) is connected to which electrode within a given unit of time (one frame). This controller may be, for example, a commercial electrophoretic display controller manufactured by UltraChip, or a research controller (HULK controller, ARC30 ^™ controller) provided by Einkel, or a virtual controller used, for example, to control the output of a voltage plate.

As shown in the equation below the electrophoretic display 401 in Figure 4, each combination of voltages supplied to the top electrode 410 and the drive electrode 430 produces a voltage difference ΔV = V(drive electrode) - V(top electrode) on the electrophoretic medium 420. From this equation (described below), it can be seen that a wider dynamic range of voltage can be achieved on the electrophoretic medium 420 by modifying the voltage on the top electrode. Furthermore, intermediate differential voltage values on the electrophoretic medium can be obtained when the sizes of 440 and 460 are different. As shown in Figure 4, seven different voltages can be supplied to the electrophoretic medium 420 by carefully coordinating when the top electrode 410 and the drive electrode 430 are connected to which power source.

Although Figure 4 shows only a single driving electrode 430, it is understood that this principle can be extended to systems with many driving pixels, such as those provided by an active matrix backplane. However, as the number of pixels increases, coordinating the necessary top electrode voltages to achieve the desired voltage difference at a particular pixel becomes very complex very quickly. In practice, this involves using independent voltage controllers for the top plane and pixel electrodes via top plane switching of the active matrix backplane, and requires continuous multi-frame top electrode voltage cycling while switching individual pixel electrodes to produce the desired waveform. Further details of this method can be found in U.S. Patent No. 10,593,272, the entire contents of which are incorporated herein by reference.

In the example, each of the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) corresponds to a different arrangement of the four pigments, such that the viewer only sees the colored pigments located on the viewing side of the white pigment (i.e., the pigment that only scatters light). More specifically, when cyan, magenta, and yellow particles are below white particles (case [A] in Figure 5), there are no particles above the white particles, and the pixel displays only white. When there is only one type of particle above the white particles, the color of that particle is displayed, which is yellow, magenta, and cyan in cases [B], [D], and [F] of Figure 5, respectively. When two particles are above white particles, the displayed color is a combination of the two particles; in case [C] of Figure 5, 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 the white particles (case [H] in Figure 5), all incident light is absorbed by the three subtractive primary color particles, and the pixel displays black.

A subtractive primary color might be represented by particles that scatter light, resulting in a display containing two types of light-scattering particles: white and colored. However, in this case, the position of the scattering colored particles relative to other colored particles covering the white particles becomes important. 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, the non-scattering colored particles would be partially or completely hidden behind the scattering particles, and the displayed color would be the color of the scattering colored particles, not black). If more than one type of colored particle scatters light, displaying black becomes more complex.

It has been found that classifying the four pigments into appropriate configurations to produce these colors is best achieved using waveforms with at least seven voltage levels (high positive, medium positive, low positive, zero, low negative, medium negative, and high negative). Figure 6 shows a typical waveform (in a simplified manner) used to drive the aforementioned 4-particle color electrophoresis display system. These waveforms have a "push-pull" structure, i.e., they consist of dipoles comprising pulses of two opposite polarities. The magnitude and length of these pulses determine the color achieved. Generally, the higher the "high" voltage, the better the color gamut achieved by the display. This "high" voltage is typically between 20V and 30V, more typically around 25V, for example, 24V. The "medium" (M) level is typically between 10V and 20V, more typically around 15V, for example, 15V or 12V. The "low" (L) voltage is typically between 3V and 10V, more typically around 7V, for example, 9V or 5V. Of course, the values of H, M, and L depend to some extent on the composition of the particles and the environment of the electrophoresis medium. In some applications, H, M, and L can be set by the cost of the components used to generate and control these voltage levels.

As shown in Figure 6, if the top electrode is kept at a constant voltage (i.e., non-top-plane switching), even the system's "simple" waveform requires the driving electronics to supply seven different voltages (+H, +M, +L, 0, -L, -M, -H) to the data lines during selected pixel updates of the display. While multi-stage source drivers capable of transmitting seven different voltages can be provided, most commercially available source drivers for electrophoretic displays only allow three different voltages to be transmitted during a single frame (typically positive, zero, and negative).

Of course, achieving the desired color using the drive pulses of Figure 6 depends on the particles starting the process from a known state, which is unlikely to be the last color displayed on the pixel. Therefore, a series of reset pulses takes precedence over the drive pulses, increasing the amount of time required to update the pixel from the first color to the second color. A more detailed description of this reset pulse can be found in U.S. Patent No. 10,593,272, which is incorporated herein by reference. The lengths of these pulses (refresh and address) and any rest periods (i.e., the zero-voltage periods between them) can be selected such that the entire waveform (i.e., the integral of voltage over time over the entire waveform) is DC balanced (i.e., the integral of voltage over time is approximately zero). DC balance can be achieved by adjusting the pulse lengths and the rest periods in the reset phase such 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 particular desired color.

Furthermore, the preceding discussion of waveforms, particularly the discussion of DC balance, neglected the issue of recoil voltage. In fact, as mentioned earlier, the offset of each backplane voltage from the voltage supplied by the power supply is equal to the recoil voltage _VKB . Therefore, if the power supply used provides these three voltages: +V, 0, and -V, the voltages actually received by the backplane are V+ _VKB , _VKB , and -V+ _VKB (note that in the case of amorphous silicon TFTs, _VKB is typically negative). However, the same power supply provides +V, 0, and -V to the front electrode without any recoil voltage offset. Therefore, for example, when -V is supplied to the front electrode, the display experiences a maximum voltage of 2V+ _VKB and a minimum voltage of _VKB . The waveform can be divided into portions supplying positive, negative, and _VKB to the front electrode, rather than using a separate power supply to supply _VKB to the front electrode (which could be both expensive and inconvenient). This is in addition to the recoil voltage.

High-voltage addressing is achieved using a metal oxide backplane.

While modifying the rail voltage offers some flexibility in achieving electro-optical properties different from those of a four-particle electrophoresis system, it still introduces several limitations in top-plane switching. For example, it is generally preferred that, in order to make the display of the present invention white, the lower negative voltage V _{_M-} is less than half of the maximum negative voltage V_{_H-}. However, as shown in the equations above, top-plane switching requires that the lower positive voltage is always at least half of the maximum positive voltage, and typically higher than half.

An alternative solution to the complexity of top-plane switching can be provided by fabricating control transistors using uncommon materials with higher electron mobility, allowing transistors to directly switch larger control voltages, such as +/-30V. Newly developed active matrix backplanes include thin-film transistors comprising metal oxide materials (e.g., tungsten oxide, tin oxide, indium oxide, and zinc oxide). In these applications, this metal oxide material is used to form channel formation regions for each transistor, allowing for faster switching of higher voltages. Such transistors typically include a gate electrode, a gate insulating film (typically _SiO₂ ), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film above the gate insulating film, at least partially overlapping the gate, source, and drain electrodes. Such backplanes are available from manufacturers such as Sharp/Foxconn, LG, and BOE.

A preferred metal oxide material for such applications is indium gallium zinc oxide (IGZO). The electron mobility of IGZO-TFTs is 20 to 50 times that of amorphous silicon. By using IGZO TFTs in the active matrix backplane, voltages greater than 30V can be provided via suitable display drivers. Furthermore, at least five (preferably seven) source drivers can be provided to offer different driving paradigms for four-particle electrophoretic display systems. In one embodiment, two positive voltages, two negative voltages, and 0 volts are provided. In another embodiment, three positive voltages, three negative voltages, and 0 volts are provided. In one embodiment, four positive voltages, four negative voltages, and 0 volts are provided. These levels can be selected in the range of approximately -27V to +27V without the limitations imposed by the top-plane switching as described above.

Using an advanced backplane, such as a metal oxide backplane, each pixel can be addressed directly with a suitable push-pull waveform, as shown in Figure 6. This significantly reduces the time required to update each pixel, converting a six-second update to less than one second in some cases. However, in some situations, a reset pulse may be needed to establish the starting point for addressing; the reset can be completed faster at higher voltages. Furthermore, in four-color electrophoretic displays with a reduced color set, a specific waveform only slightly longer than the push-pull waveform shown in Figure 6 can be used to drive directly from the first color to the second color.

Simplified top plane switching

To reduce update time and flicker, the complexity of the front-plane switching can be reduced in exchange for a smaller number of available colors. Furthermore, because particles have a finite velocity in the electrophoretic medium, the amount of time spent applying dipoles also affects the size of the color gamut.

Figure 7 illustrates a solution using a simplified top-plane switching pulse sequence (top left), with a simplified backplane pulse sequence (bottom left) matched to a single top-plane sequence, thus providing at least different colors. The top plane switches between two voltages (one positive and one negative), while the backplane can take three different voltages: positive, negative, and zero. (In Figure 7, the voltage levels are relative (i.e., 1, 0, and -1), but in many examples, they are actually 15V, 0, and -15V, as they are typically used with commercially available backplanes that include amorphous silicon thin-film transistors.) Note that the eight color sequences in Figure 6 (right side of Figure 7) can be achieved by subtracting the top-plane pulse sequence from the backplane pulse sequence (left side of Figure 7). It can be understood that for the pulse sequences in Figures 6 and 7, the electrophoretic fluid includes negatively charged white pigment, positively charged magenta and cyan pigments, and yellow pigment that can be positively charged, negatively charged, or substantially neutral. Other color/charge combinations are possible, and the waveforms can be adjusted accordingly.

As described above, at least five different voltages are required in the waveform of Figure 7. In an active matrix drive environment, this can be achieved in any of the following ways: (a) providing five different voltage options to the column when a specific row is selected at a specific time; or (b) providing fewer (e.g., three) different voltage options to the column when a specific row is selected at a first time, and providing a different set of voltages when the same row is selected at a second time; or (c) providing the same three voltage options to the column simultaneously at the first and second times, but changing the potential of the front electrode between the first and second times. Option (c) is particularly useful when at least one voltage needs to be provided that is higher than the voltage that the backplane electronics can support.

Because switching using the top plane makes it impossible to simultaneously determine high positive and high negative potentials, the +/- dipoles of the top plane must be offset relative to the -/+ dipoles of the backplane. In the waveform shown in Figure 7, only one dipole is used for each transition. This provides the fewest possible "flash" waveforms, as each dipole would produce two visible optical changes to the display. If five different voltage levels can be supplied to the backplane electrodes when selecting each row, and the backplane electronics can support the highest voltage required, then offsetting the dipoles as shown in Figure 7 is unnecessary.

Using cyclic top plane voltage drive

For the driving sequence in Figure 7, the voltages applied to the top plane are denoted as V _{_t+} and V _{_t-} , and the voltages applied to the backplane are denoted as V _{_b+} and V _{_b-, respectively, and |V t+}|＝|V_t-|＝|V _{_b+}|＝|V _{_b-} |＝V. Therefore, when the maximum supply voltage is +/-15 volts (typically as with commercially available backplanes), the voltage across the electrophoretic medium becomes 30V, 28V, 0V, -28V, and -30V.

However, the magnitudes of the maximum voltages (i.e., the "rails") of the top and back electrodes do not need to be the same. For example, the rail voltage offset can be calculated from a nominal maximum voltage value V. The offset of each rail can be expressed as w, x, y, and z, while assuming that the zero-voltage rail remains zero and is not applied to the top plane.

therefore:

V _t+ ＝V+w

V _t- ＝-V+x

V _t0 = 0

Vb ₊ = V+y

V _b- ＝-V+z

V _b0 = 0

Referring to the backplate voltage, when the top plane is set to Vt ₊ , three different negative voltages of high, medium and low can be applied to the electrophoretic medium, which are represented as _VH- , _VM- and _VL- respectively (i.e., _Vb - _Vt , where _Vb can take any of the three values shown above).

These voltages are:

V _H- ＝-2V+zw

V _M -＝-Vw

V _L- ＝yw

When the voltage at the top plane is set to _Vt- , the available voltage is:

V _H+ ＝2V+yx

V _M+ = Vx

V _L+ ＝zx

When the voltage at the top plane is set to 0, the available voltage is:

V _H0 = V+y

V _M0 = 0

V _L0 ＝-V+z

As can be seen from the above, when w = y and x = z, the zero-voltage condition can be maintained regardless of whether the top plane is set to Vt ₊ , _Vt- , or 0. In reality, to obtain optimal color, the waveform requires significantly greater complexity and length. Therefore, the complexity required for the top plane switching mode is thus greater than that shown in Figure 7. However, difficulties arise in applications where simultaneous and staggered updates are needed in different areas of the display, with start times less than the length of a single waveform. Because the top plane potential is established across the entire display, a new update may not be able to be initiated in one area of the display until the update previously initiated in another location has finished.

The problem of coordinating multiple simultaneous updates requiring top plane switching can be solved by cyclically stretching the waveform with the top plane voltage, as shown in Figure 8. ( _VTE = top electrode voltage, _VDE1 = first drive electrode voltage, _VDE2 = second drive electrode voltage, _ΔVDE1 = voltage difference on the electrophoretic medium between the first drive electrode and the top electrode, _ΔVDE2 = voltage difference on the electrophoretic medium between the second drive electrode and the top electrode.) The green and yellow waveforms previously established for a seven-level backplane capable of providing +/-24V, +/-15V, +/-9V, or 0V at any pixel location in any frame have been modified for cyclic top plane driving. The controller provides consecutive frames of +15V, -9V, and 0V to the top electrode (i.e., V = 15V, w = y = 0V, and x = z = 6V in the above equations), as shown in Figure 9. By stretching the waveform, coordinating the voltages provided to the first and second drive electrodes with the top voltage cyclically, simultaneous color updates can be generated at two different drive electrodes using top switching.

When the top electrode is +15V, the available voltage differences across the electrophoretic medium are -24V, -15V, and -0V. When the top electrode is -9V, the available voltage differences across the electrophoretic medium are 24V, 9V, and 0V. In the third frame, when the top electrode is grounded (0V), the available voltage differences across the electrophoretic medium are 15V, 0V, and -9V. [Conventionally, this voltage difference is ΔV = V(drive electrode) - V(top electrode).] Therefore, seven voltage levels can be provided: +/-24V, +/-15V, +/-9V, and 0V. It should be noted that when a particular drive electrode needs to "wait" for the next top electrode frame, that drive electrode is set to the same voltage as the top electrode, such that for that frame, the voltage difference across the electrophoretic medium is zero. Clearly, this makes the waveform longer, and each "simple" waveform now requires three times the update time of the original multi-stage waveform.

A model of a four-particle electrophoresis system was used, with cyclic driving of the top electrode at +15V, -9V, and 0V. This was tested against the same system with seven individual drive levels and a static top electrode. The results are shown in Tables 1 and 2 below, and are represented in the plot of Figure 9A and the analog color table of Figure 9B.

Table 1. L*a*b* values calculated for the modeled ACeP system using a dedicated seven-level driver.

Table 2. L*a*b* values calculated for the modeled ACeP system using top electrode cycling and +15V and -9V power supplies.

Comparing Tables 1 and 2, it appears that there is a slight loss in top electrode cycling, aside from the longer update time. In fact, the color gamut (color space) of the top electrode cycling method is actually slightly larger. The difference between the two methods can be further visualized by considering Figures 9A and 9B. In Figure 9A, solid circles represent L*a*b* measurements for a seven-level driver, while hollow circles represent L*a*b* measurements for a cyclic top electrode driver. As can be seen from Figures 9A and 9B, the resulting primary color states are very similar. (Compare the positions of the hollow and solid circles.) The greatest variation can be seen in the green primary color (the left center portion of Figure 9A), where the green primary color shifts significantly towards yellow. The difference in the color state of the green primary color is also evident in Figure 9B.

Therefore, the present invention provides a full-color electrophoretic display capable of directly addressing the electrophoretic medium, with and without top-plane switching, and the waveform of such an electrophoretic display. Thus, several aspects and embodiments of the technology described herein have been described, and it should be understood that various changes, modifications, and improvements will readily occur to those skilled in the art. These changes, modifications, and improvements are intended to fall within the spirit and scope of the technology described herein. For example, various other means and/or structures for performing functions and/or obtaining results and/or one or more advantages described herein will readily occur to those skilled in the art, and each of these changes and/or modifications is considered to be within the scope of the embodiments described herein. Those skilled in the art will recognize or be able to determine many equivalents of the specific embodiments described herein using only conventional experimentation. Therefore, it should be understood that the foregoing embodiments are given by way of example only, and that inventive embodiments may be practiced in ways other than those specifically described within the scope of the appended claims and their equivalents. Furthermore, any combination of two or more features, systems, articles, materials, kits, and/or methods described herein is included within the scope of this disclosure, provided that these features, systems, articles, materials, kits, and/or methods are not inconsistent with each other.

Claims (20)

1. A system for driving an electrophoretic medium, comprising: Electrophoresis display, comprising: The top electrode that transmits light at the viewing surface. First driving electrode, Second driving electrode, An electrophoretic medium disposed between the top electrode and the first and second driving electrodes; A power source capable of providing positive and negative voltages, wherein the positive and negative voltages are of different magnitudes; and A controller, coupled to a top electrode driver, a first drive electrode driver, and a second drive electrode driver, is configured to: In the first frame, the positive voltage is provided to the top electrode, the negative voltage is provided to the first driving electrode, and the positive voltage is provided to the second driving electrode. In the second frame, the negative voltage is provided to the top electrode, the negative voltage is provided to the first driving electrode, and the negative voltage is provided to the second driving electrode. In the third frame, a ground voltage is provided to the top electrode, a ground voltage is provided to the first driving electrode, and the positive voltage is provided to the second driving electrode. In the fourth frame, the positive voltage is provided to the top electrode, the positive voltage is provided to the first driving electrode, and the positive voltage is provided to the second driving electrode. 2. The system of claim 1, wherein the controller is further configured to: In the fifth frame, the negative voltage is provided to the top electrode, a ground voltage is provided to the first driving electrode, and the negative voltage is provided to the second driving electrode. In the sixth frame, a ground voltage is provided to the top electrode, a ground voltage is provided to the first drive electrode, and a ground voltage is provided to the second drive electrode. 3. The system of claim 1, wherein the electrophoretic medium is encapsulated in a plurality of microcapsules, and the microcapsules are dispersed in a polymer adhesive between the top electrode and the first and second driving electrodes. 4. The system of claim 1, wherein the electrophoretic medium is encapsulated in a micro-unit array having an opening, wherein the opening is sealed with a polymer adhesive, and the micro-unit array is disposed between the top electrode and the first and second drive electrodes. 5. The system according to claim 1, wherein the electrophoretic medium comprises a nonpolar fluid and four groups of particles with different optical properties. 6. The system according to claim 5, wherein the first and second groups of particles have opposite charges, the third and fourth groups of particles have opposite charges, the first group of particles are light scattering particles, and the second, third and fourth groups of particles are all subtractive primary colors that are different from each other. 7. The system of claim 6, wherein the controller is configured to provide a combination of the positive voltage, the negative voltage, and the ground voltage to the top electrode and the first drive electrode such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. 8. The system according to claim 5, wherein the first and second groups of particles have opposite charges, the third and fourth groups of particles have the same charge as the second group of particles, the first group of particles are light scattering particles, and the second, third and fourth groups of particles are all subtractive primary colors different from each other. 9. The system of claim 8, wherein the controller is configured to provide a combination of the positive voltage, the negative voltage, and the ground voltage to the top electrode and the first drive electrode such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. 10. The system of claim 1, wherein the positive voltage is +15V and the negative voltage is -9V. 11. The system according to claim 1, wherein the positive voltage is +9V and the negative voltage is -15V. 12. A system for driving an electrophoretic medium, comprising: Electrophoresis display, comprising: The top electrode that transmits light at the viewing surface. First driving electrode, Second driving electrode, An electrophoretic medium disposed between the top electrode and the first and second driving electrodes; A power source capable of providing positive and negative voltages, wherein the positive and negative voltages are of different magnitudes; and A controller, coupled to a top electrode driver, a first drive electrode driver, and a second drive electrode driver, is configured to: In the first frame, the positive voltage is provided to the top electrode, the negative voltage is provided to the first driving electrode, and the positive voltage is provided to the second driving electrode. In the second frame, the negative voltage is provided to the top electrode, the negative voltage is provided to the first driving electrode, and the negative voltage is provided to the second driving electrode. In the third frame, a ground voltage is provided to the top electrode, a ground voltage is provided to the first driving electrode, and a ground voltage is provided to the second driving electrode. In the fourth frame, the positive voltage is provided to the top electrode, the positive voltage is provided to the first driving electrode, and the positive voltage is provided to the second driving electrode. 13. The system of claim 12, wherein the controller is further configured to: In the fifth frame, the negative voltage is provided to the top electrode, a ground voltage is provided to the first driving electrode, and the negative voltage is provided to the second driving electrode. In the sixth frame, a ground voltage is provided to the top electrode, a ground voltage is provided to the first drive electrode, and a ground voltage is provided to the second drive electrode. 14. The system of claim 12, wherein the electrophoretic medium is encapsulated in a plurality of microcapsules, and the microcapsules are dispersed in a polymer adhesive between the top electrode and the first and second drive electrodes. 15. The system of claim 12, wherein the electrophoretic medium is encapsulated in a microcell array having an opening, wherein the opening is sealed with a polymer adhesive, and the microcell array is disposed between the top electrode and the first and second drive electrodes. 16. The system of claim 12, wherein the electrophoretic medium comprises a nonpolar fluid and four groups of particles with different optical properties. 17. The system of claim 16, wherein the first and second groups of particles carry charges of opposite polarities, the third and fourth groups of particles carry charges of opposite polarities, the first group of particles are light scattering particles, and the second, third and fourth groups of particles are all subtractive primary colors that are different from each other. 18. The system of claim 17, wherein the controller is configured to provide a combination of the positive voltage, the negative voltage, and the ground voltage to the top electrode and the first drive electrode such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface. 19. The system of claim 16, wherein the first and second groups of particles carry charges of opposite polarities, the third and fourth groups of particles carry the same charge as the second group of particles, the first group of particles are light scattering particles, and the second, third and fourth groups of particles are all subtractive primary colors distinct from each other. 20. The system of claim 19, wherein the controller is configured to provide a combination of the positive voltage, the negative voltage, and the ground voltage to the top electrode and the first drive electrode such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed on the viewing surface.