HK40045242A - Electro-optic displays and driving methods - Google Patents

Description

Electro-optic display and driving method

Reference to related applications

This application relates to and claims priority from U.S. provisional application 62/773,609 filed on 30/11/2018.

The entire disclosure of the aforementioned application is incorporated herein by reference.

Technical Field

The subject matter presented herein relates to driving methods for electro-optic display devices.

Background

In order to realize color display, color filters are often used. The most common approach is to add color filters on top of the black/white sub-pixels of the pixelated display to display red, green and blue. When red is desired, the green and blue subpixels change to a black state so that the only color displayed is red. When blue is desired, the green and red sub-pixels change to a black state so that the only color displayed is blue. When green is desired, the red and blue subpixels change to a black state so that the only color displayed is green. When a black state is desired, all three subpixels become in the black state. When a white state is desired, the three sub-pixels become red, green and blue, respectively, and as a result, the viewer sees a white state.

The biggest disadvantage of this technique is that the white state is rather dark, since the reflectivity of each sub-pixel is about one third of the desired white state (1/3). To compensate for this, a fourth sub-pixel capable of displaying only black and white states may be added, such that the white level is doubled at the expense of the red, green or blue level (where each sub-pixel now occupies only a quarter of the pixel area). Brighter colors can be achieved by increasing the light from the white pixels, but this is done at the expense of color gamut, resulting in very bright and unsaturated colors. Similar results can be achieved by reducing the color saturation of the three sub-pixels. Even with these methods, the white level is typically much less than half of the white level of a black and white display, making it an unacceptable choice for display devices (e.g., e-readers or displays that require well readable black and white brightness and contrast).

Disclosure of Invention

The present invention provides a driving method for driving a pixel of an electrophoretic display comprising a first surface on a viewing side, a second surface on a non-viewing side and an electrophoretic fluid arranged between a first light transmissive electrode and a second electrode, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles and a fourth type of particles, all of which are dispersed in a solvent, wherein the four types of pigment particles have different optical properties, the first type of particles and the third type of particles being positively charged, wherein the first type of particles have a larger positive charge than the third particles, and the second type of particles and the fourth type of particles being negatively charged, wherein the second type of particles have a larger negative charge than the fourth particles, the method comprising the steps of: (i) applying a first drive voltage to the pixels of the electrophoretic display at a first amplitude for a first period of time to drive the pixels to a color state of the fourth type of particles at the viewing side; and (ii) applying a second drive voltage to the pixels of the electrophoretic display for a second period of time, the second drive voltage being opposite to the first drive voltage and the second amplitude being smaller than the first amplitude, to drive the second type of particles towards the non-viewing side.

Drawings

Various aspects and embodiments of the present application will be described with reference to the following drawings. It should be understood that the drawings are not necessarily drawn to scale. Items appearing in multiple figures are identified by the same reference numeral in all of the appearing figures.

FIG. 1 shows a schematic representation of an electro-optic display according to the subject matter presented herein;

FIG. 2 shows an equivalent circuit representing the electro-optic display shown in FIG. 1;

FIG. 3 illustrates a cross-sectional view of an electro-optic display according to the subject matter presented herein;

fig. 4a and 4b illustrate a display from yellow to red according to the subject matter presented herein;

FIG. 5 illustrates one embodiment of driving an electro-optic display according to the subject matter presented herein;

FIG. 6 illustrates a red to yellow electro-optic display according to the subject matter presented herein;

FIG. 7 illustrates yet another embodiment of a driving method for driving an electro-optic display according to the subject matter presented herein;

FIG. 8 illustrates another embodiment of a driving method for driving an electro-optic display according to the subject matter presented herein;

FIG. 9 illustrates yet another embodiment of a driving method for driving an electro-optic display according to the subject matter presented herein;

FIG. 10 shows experimental results of a Qsun test according to the subject matter presented herein; and

fig. 11 shows experimental results of RA testing according to the subject matter presented herein.

Detailed Description

The present invention relates to a method of driving an electro-optic display, in particular a bi-stable electro-optic display, in a dark mode, and to an apparatus for use in such a method. More particularly, the invention relates to a driving method that may allow for reduced "ghosting" and edge artifacts and reduced flicker in such displays when displaying white text on a black background. The invention is particularly, but not exclusively, intended for use with a particle-based electrophoretic display in which one or more types of charged particles are present in a fluid and move through the fluid under the influence of an electric field to change the appearance of the display.

As applied to materials or displays, the term "electro-optic" is used herein in its conventional sense in the imaging arts to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first display state to its second display state by application of an electric field to the material. Although the optical property is typically a color perceptible to the human eye, it may be another optical property, such as light transmission, reflection, luminescence, or, in the case of a display for machine reading, a false color in the sense of a change in reflectivity of electromagnetic wavelengths outside the visible range.

The term "gray state" is used herein in its conventional sense in the imaging art to refer to a state intermediate two extreme optical states of a pixel, but does not necessarily imply a black-and-white transition between the two extreme states. For example, several of the patents and published applications by the incorporated of lngk referred to above describe electrophoretic displays in which the extreme states are white and dark blue, so that the intermediate "gray state" is effectively pale blue. In fact, as already mentioned, the change in optical state may not be a color change at all. The terms "black" and "white" may be used hereinafter to refer to the two extreme optical states of the display and should be understood to generally include extreme optical states that are not strictly black and white, such as the white and deep blue states mentioned above. The term "monochrome" may be used hereinafter to denote a driving scheme in which a pixel is driven only to its two extreme optical states, without an intermediate grey state.

Much of the discussion below will focus on methods of driving one or more pixels of an electro-optic display by transitioning from an initial gray level (or "gray tone") to a final gray level (which may or may not differ from the initial gray level). The terms "gray state," "gray level," and "gray tone" are used interchangeably herein and include extreme optical states as well as intermediate gray states. The number of possible gray levels in current systems is typically 2-16 due to limitations such as the discreteness of the drive pulses applied by the frame rate of the display driver and temperature sensitivity. For example, in a black and white display having 16 gray levels, normally, gray level 1 is black and gray level 16 is white; however, the designation of black and white gray levels may be reversed. Here, gray tone 1 will be used to designate black. As the gray tone progresses toward gray tone 16 (i.e., white), gray tone 2 will be a lighter black.

The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property such that, after any given element is driven to assume its first or second display state using an addressing pulse of finite duration, that state will last at least several times (e.g. at least 4 times) the minimum duration of the addressing pulse required to change the state of that display element after the addressing pulse has terminated. It is shown in U.S. patent No.7,170,670 that some particle-based electrophoretic displays that support gray scale can be stabilized not only in their extreme black and white states, but also in their intermediate gray states, as well as some other types of electro-optic displays. This type of display is properly referred to as "multi-stable" rather than bi-stable, but for convenience the term "bi-stable" may be used herein to cover both bi-stable and multi-stable displays.

The term "impulse" is used herein in its conventional sense, i.e., the integral of a voltage with respect to time. However, some bistable electro-optic media act as charge converters, and for such media an alternative definition of impulse, i.e. the integral of the current with respect to time (which is equal to the total charge applied) may be used. Depending on whether the medium is used as a voltage-time impulse converter or as a charge impulse converter, a suitable impulse definition should be used.

The term "waveform" will be used to denote the entire voltage versus time curve used to effect a transition from one particular initial gray level to a particular final gray level. Typically, such a waveform will include a plurality of waveform elements; wherein the elements are substantially rectangular (i.e., wherein a given element comprises applying a constant voltage over a period of time); this element may be referred to as a "pulse" or "drive pulse". The term "drive scheme" denotes a set of waveforms sufficient to achieve all possible transitions between gray levels for a particular display. The display may utilize more than one drive scheme; for example, the aforementioned U.S. patent No.7,012,600 teaches that the drive scheme may need to be modified according to parameters such as the temperature of the display or the time it has been operating during its lifetime, and thus the display may be provided with a plurality of different drive schemes for use at different temperatures or the like. A set of drive schemes used in this manner may be referred to as a "set of correlated drive schemes". More than one drive scheme may also be used simultaneously in different regions of the same display, and a set of drive schemes used in this manner may be referred to as a "set of simultaneous drive schemes", as described in several of the aforementioned MEDEOD applications.

Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type, as described in, for example, U.S. patent nos. 5,808,783, 5,777,782, 5,760,761, 6,054,071, 6,055,091, 6,097,531, 6,128,124, 6,137,467, and 6,147,791 (although this type of display is commonly referred to as a "rotating bichromal ball" display, the term "rotating bichromal member" is preferably more accurate because in some of the patents mentioned above, the rotating member is not spherical). Such displays use a number of small bodies (usually spherical or cylindrical) comprising two or more parts with different optical properties and an internal dipole. These bodies are suspended in liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed by: an electric field is applied to the display, thereby rotating the body to various positions and changing which part of the body is seen through the viewing surface. This type of electro-optic medium is generally bistable.

Another type of electro-optic display uses an electrochromic medium, for example in the form of a nano-electrochromic (nanochromic) film comprising electrodes formed at least in part of a semiconducting metal oxide and a plurality of dye molecules capable of reverse color change attached to the electrodes; see, e.g., O' Regan, b. et al, Nature 1991,353,737; and Wood, d., Information Display,18(3),24 (3 months 2002). See also Bach, u. et al, adv.mater, 2002,14(11), 845. Nano-electrochromic films of this type are described, for example, in U.S. patent nos. 6,301,038; 6,870,657, respectively; and 6,950,220. This type of media is also generally bistable.

Another type of electro-optic display is the electro-wetting display developed by Philips, which is described in Hayes, R.A. et al, "Video-Speed Electronic Paper Based on electric wetting", Nature,425,383-385 (2003). Such electrowetting displays can be made bistable as shown in us patent No.7,420,549.

One type of electro-optic display that has been the subject of intensive research and development for many years is a particle-based electrophoretic display in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays may have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption compared to liquid crystal displays. However, problems with the long-term image quality of these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in insufficient lifetime of these displays.

As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, the fluid is a liquid, but the electrophoretic medium can be produced using a gaseous fluid; see, e.g., Kitamura, T. et al, "Electronic Toner movement for Electronic Paper-like display", IDW Japan,2001, Paper HCS 1-1, and Yamaguchi, Y. et al, "Toner display using insulating substrates charged triangular display", IDW Japan,2001, Paper AMD 4-4). 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 the particles to settle, such as in signs where the media are arranged in a vertical plane, such gas-based electrophoretic media are susceptible to the same type of problems due to the same settling of particles as liquid-based electrophoretic media. In fact, the problem of particle settling in gas-based electrophoretic media is more severe than in liquid-based electrophoretic media, because the lower viscosity of gaseous suspending fluids allows faster settling of the electrophoretic particles compared to liquids.

A number of patents and applications assigned to or in the name of the Massachusetts Institute of Technology (MIT) and yingke corporation describe various techniques for encapsulating electrophoretic and other electro-optic media. Such encapsulated media comprise a plurality of microcapsules, each microcapsule itself comprising an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsule itself is held in a polymeric binder to form a coherent layer between the two electrodes. The techniques described in these patents and applications include:

(a) electrophoretic particles, fluids, and fluid additives; see, e.g., U.S. Pat. Nos. 7,002,728 and 7,679,814;

(b) capsule, adhesive and packaging process; see, e.g., U.S. patent nos. 6,922,276 and 7,411,719;

(c) films and sub-assemblies comprising electro-optic material; see, e.g., U.S. Pat. Nos. 6,982,178 and 7,839,564;

(d) backsheets, adhesive layers, and other auxiliary layers and methods for use in displays; see, e.g., U.S. patent nos. 7,116,318 and 7,535,624;

(e) color formation and color adjustment; see, e.g., U.S. patent No.7,075,502 and U.S. patent application publication No. 2007/0109219;

(f) a method for driving a display; see, e.g., U.S. Pat. Nos. 5,930,026; 6,445,489, respectively; 6,504,524; 6,512,354, respectively; 6,531,997, respectively; 6,753,999, respectively; 6,825,970, respectively; 6,900,851, respectively; 6,995,550, respectively; 7,012,600; 7,023,420, respectively; 7,034,783, respectively; 7,061,166, respectively; 7,061,662, respectively; 7,116,466, respectively; 7,119,772; 7,177,066, respectively; 7,193,625, respectively; 7,202,847, respectively; 7,242,514, respectively; 7,259,744; 7,304,787, respectively; 7,312,794, respectively; 7,327,511, respectively; 7,408,699, respectively; 7,453,445, respectively; 7,492,339, respectively; 7,528,822, respectively; 7,545,358, respectively; 7,583,251, respectively; 7,602,374, respectively; 7,612,760, respectively; 7,679,599, respectively; 7,679,813, respectively; 7,683,606, respectively; 7,688,297, respectively; 7,729,039, respectively; 7,733,311, respectively; 7,733,335, respectively; 7,787,169, respectively; 7,859,742, respectively; 7,952,557, respectively; 7,956,841, respectively; 7,982,479, respectively; 7,999,787, respectively; 8,077,141, respectively; 8,125,501, respectively; 8,139,050, respectively; 8,174,490, respectively; 8,243,013, respectively; 8,274,472, respectively; 8,289,250, respectively; 8,300,006, respectively; 8,305,341, respectively; 8,314,784, respectively; 8,373,649, respectively; 8,384,658, respectively; 8,456,414, respectively; 8,462,102, respectively; 8,537,105, respectively; 8,558,783, respectively; 8,558,785, respectively; 8,558,786, respectively; 8,558,855, respectively; 8,576,164, respectively; 8,576,259, respectively; 8,593,396, respectively; 8,605,032, respectively; 8,643,595, respectively; 8,665,206, respectively; 8,681,191, respectively; 8,730,153, respectively; 8,810,525, respectively; 8,928,562, respectively; 8,928,641, respectively; 8,976,444, respectively; 9,013,394, respectively; 9,019,197, respectively; 9,019,198, respectively; 9,019,318, respectively; 9,082,352, respectively; 9,171,508, respectively; 9,218,773, respectively; 9,224,338, respectively; 9,224,342, respectively; 9,224,344, respectively; 9,230,492, respectively; 9,251,736, respectively; 9,262,973, respectively; 9,269,311, respectively; 9,299,294, respectively; 9,373,289, respectively; 9,390,066, respectively; 9,390,661, respectively; and 9,412,314; and U.S. patent application publication No. 2003/0102858; 2004/0246562, respectively; 2005/0253777, respectively; 2007/0070032, respectively; 2007/0076289, respectively; 2007/0091418, respectively; 2007/0103427, respectively; 2007/0176912, respectively; 2007/0296452, respectively; 2008/0024429, respectively; 2008/0024482, respectively; 2008/0136774, respectively; 2008/0169821, respectively; 2008/0218471, respectively; 2008/0291129, respectively; 2008/0303780, respectively; 2009/0174651, respectively; 2009/0195568, respectively; 2009/0322721, respectively; 2010/0194733, respectively; 2010/0194789, respectively; 2010/0220121, respectively; 2010/0265561, respectively; 2010/0283804, respectively; 2011/0063314, respectively; 2011/0175875, respectively; 2011/0193840, respectively; 2011/0193841, respectively; 2011/0199671, respectively; 2011/0221740, respectively; 2012/0001957, respectively; 2012/0098740, respectively; 2013/0063333, respectively; 2013/0194250, respectively; 2013/0249782, respectively; 2013/0321278, respectively; 2014/0009817, respectively; 2014/0085355, respectively; 2014/0204012, respectively; 2014/0218277, respectively; 2014/0240210, respectively; 2014/0240373, respectively; 2014/0253425, respectively; 2014/0292830, respectively; 2014/0293398, respectively; 2014/0333685, respectively; 2014/0340734, respectively; 2015/0070744, respectively; 2015/0097877, respectively; 2015/0109283, respectively; 2015/0213749, respectively; 2015/0213765, respectively; 2015/0221257, respectively; 2015/0262255, respectively; 2016/0071465, respectively; 2016/0078820, respectively; 2016/0093253, respectively; 2016/0140910, respectively; and 2016/0180777;

(g) an application for a display; see, e.g., U.S. patent nos. 7,312,784; and U.S. patent application publication No. 2006/0279527; and

(h) non-electrophoretic displays, such as those described in U.S. patent nos. 6,241,921; 6,950,220, respectively; and 7,420,549 and U.S. patent application publication No. 2009/0046082.

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-dispersed 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 polymer-dispersed electrophoretic displays can be considered capsules or microcapsules, even if no discrete capsule film is associated with each individual droplet; see, for example, the aforementioned U.S. patent No.6,866,760. Accordingly, for the purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media.

A related type of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and the fluid are not encapsulated within microcapsules, but are held in a plurality of cavities formed within a carrier medium, typically a polymer film. See, for example, U.S. patent nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, inc.

Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, the particles substantially block the transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "shutter mode" in which one display state is substantially opaque and one display state is light-transmissive. 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. A dielectrophoretic display similar to an electrophoretic display but relying on a change in electric field strength may operate in a similar mode; see U.S. patent No.4,418,346. Other types of electro-optic displays can also operate in the shutter mode. Electro-optic media operating in shutter mode may be used in the multilayer structure of full color displays; in this configuration, at least one layer adjacent to the viewing surface of the display operates in a shutter mode to expose or hide a second layer further from the viewing surface.

Encapsulated electrophoretic displays are generally not plagued by the aggregation and settling failure modes of conventional electrophoretic devices and provide further benefits such as the ability to print or coat the display on a variety of flexible and rigid substrates. (the use of the word "printing" is intended to include all forms of printing and coating including, but not limited to, pre-metered coating such as slot or extrusion coating, slide or stack coating, curtain coating, roll coating such as knife coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, electrophoretic deposition (see U.S. patent No.7,339,715), and other similar techniques.) thus, the resulting display may be flexible. In addition, because the display medium can be printed (using a variety of methods), the display itself can be inexpensively manufactured.

Other types of electro-optic media may also be used in the displays of the present invention.

The bistable or multistable behaviour of particle-based electrophoretic displays and other electro-optic displays exhibiting similar behaviour (such displays may be referred to hereinafter for convenience as "impulse-driven displays") are in sharp contrast to conventional liquid crystal ("LC") displays. Twisted nematic liquid crystals are not bistable or multistable but act as voltage converters, and therefore the application of a given electric field to a pixel of such a display produces a particular grey scale at that pixel, irrespective of the grey scale previously present at the pixel. Furthermore, LC displays are driven in only one direction (from non-transmissive or "dark" to transmissive or "bright"), with the reverse transition from the lighter state to the darker state being achieved by reducing or eliminating the electric field. Finally, the gray levels of the pixels of an LC display are insensitive to the polarity of the electric field, only to the amplitude of the electric field, and in fact commercial LC displays usually reverse the polarity of the drive field at frequent intervals for technical reasons. In contrast, a bistable electro-optic display functions approximately as an impulse transducer, so that the final state of a pixel depends not only on the applied electric field and the time at which it is applied, but also on the state of the pixel before the electric field is applied.

Whether or not the electro-optic medium used is bistable, in order to achieve a high resolution display, individual pixels of the display must be addressable without interference from adjacent pixels. One way to achieve this is to provide an array of non-linear elements (e.g. transistors or diodes) with at least one non-linear element associated with each pixel to produce an "active matrix" display. The addressing or pixel electrode used to address a pixel is connected to a suitable voltage source via an associated non-linear element. In general, when the non-linear element is a transistor, a pixel electrode is connected to a drain of the transistor, and this arrangement will be assumed in the following description, although it is arbitrary in nature and the pixel electrode may be connected to a source of the transistor. Conventionally, in a high resolution array, pixels may be arranged in a two-dimensional array having rows and columns such that any particular pixel is uniquely defined by the intersection of one particular row and one 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; likewise, the source to row and gate to column assignments are conventional but arbitrary in nature and can be reversed if desired. The row electrodes are connected to a row driver which essentially ensures that only one row is selected at any given time, i.e. a voltage is applied to the selected row electrode, e.g. to ensure that all transistors in the selected row are conductive, while a voltage is applied to all other rows, e.g. to ensure that all transistors in these non-selected rows remain non-conductive. The column electrodes are connected to a column driver which applies voltages to the different column electrodes which are selected to drive the pixels in the selected row to their desired optical states. (the aforementioned voltages are relative to a common front electrode, which is conventionally disposed on the opposite side of the non-linear array of electro-optic medium and extends across the display.) after a preselected interval called the "row address time", the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed so that the next row of the display is written. This process is repeated to write the entire display in a row-by-row fashion.

It should be understood that even though the various embodiments presented below use electrophoretic materials with microcells to illustrate the working principle according to the subject matter presented herein, the same principle can easily be employed for electrophoretic materials with microencapsulated particles (e.g. pigment particles). Electrophoretic materials having microcells are used herein for illustration and not for limitation.

FIG. 1 shows a schematic model of a display pixel 100 of an electro-optic display according to the subject matter presented herein. The pixel 100 may include an imaging film 110. In some embodiments, the imaging film 110 may be a layer of electrophoretic material and is bistable in nature. The electrophoretic material may include a plurality of charged colored pigment particles (e.g., black, white, yellow, or red) disposed in a fluid and capable of moving through the fluid under the influence of an electric field. In some embodiments, imaging film 110 may be an electrophoretic film having microcells with charged pigment particles. In some other embodiments, imaging film 110 may include, but is not limited to, an encapsulated electrophoretic imaging film, which may include, for example, charged pigment particles. It should be understood that the driving methods set forth below can be readily applied to any type of electrophoretic material (e.g., encapsulated or microcell-containing films).

In some embodiments, the imaging film 110 may be disposed between the front electrode 102 and the back or pixel electrode 104. The front electrode 102 may be formed between the imaging film and the front of the display. In some embodiments, the front electrode 102 may be transparent and light transmissive. In some embodiments, the front electrode 102 may be formed of any suitable transparent material, including but not limited to Indium Tin Oxide (ITO). The back electrode 104 may be formed on a side of the imaging film 110 opposite the front electrode 102. In some embodiments, a parasitic capacitance (not shown) may be formed between the front electrode 102 and the back electrode 104.

The pixel 100 may be one of a plurality of pixels. The plurality of pixels may be arranged in a two-dimensional array of rows and columns to form a matrix such that any particular pixel is uniquely defined by the intersection of a particular row and a particular column. In some embodiments, the matrix of pixels may be an "active matrix" in which each pixel is associated with at least one non-linear circuit element 120. A non-linear circuit element 120 may be coupled between the backplane electrode 104 and the address electrode 108. In some embodiments, the non-linear element 120 may be a diode and/or a transistor, including but not limited to a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) or a Thin Film Transistor (TFT). The drain (or source) of the MOSFET or TFT may be coupled to the backplane or pixel electrode 104, the source (or drain) of the MOSFET or TFT may be coupled to the addressing electrode 108, and the gate of the MOSFET or TFT may be coupled to a driver electrode 106, the driver electrode 106 being configured to control activation and deactivation of the MOSFET or TFT. (for simplicity, the terminal of the MOSFET or TFT coupled to the backplane electrode 104 will be referred to as the drain of the MOSFET or TFT, and the terminal of the MOSFET or TFT coupled to the address electrode 108 will be referred to as the source of the MOSFET or TFT

In some embodiments of the active matrix, the address electrodes 108 of all pixels in each column may be connected to the same column electrode, and the driver electrodes 106 of all pixels in each row may be connected to the same row electrode. The row electrodes may be connected to a row driver which may select one or more rows of pixels by applying a voltage to the selected row electrodes, the voltage being sufficient to activate the non-linear elements 120 of all pixels 100 in the selected row. The column electrodes may be connected to a column driver which may apply voltages on the address electrodes 106 of selected (activated) pixels suitable for driving the pixels to a desired optical state. The voltage applied to the address electrode 108 can be relative to the voltage applied to the front plate electrode 102 of the pixel (e.g., a voltage of about zero volts). In some embodiments, the front plate electrodes 102 of all pixels in the active matrix may be coupled to a common electrode.

In use, the pixels 100 of the active matrix may be written in a row-by-row manner. For example, a row driver may select a row of pixels, and a column driver may apply voltages to the pixels corresponding to the desired optical states of the row of pixels. After a pre-selection interval called "row address time", the selected row may be deselected, another row may be selected, and the voltage on the column driver may be changed to cause another row of the display to be written.

Fig. 2 illustrates a circuit model of an electro-optic imaging layer 110, the electro-optic imaging layer 100 disposed between a front electrode 102 and a back electrode 104, according to the subject matter presented herein. Resistor 202 and capacitor 204 may represent the resistance and capacitance of the electro-optic imaging layer 110, front electrode 102, and back electrode 104 (including any adhesive layers). Resistor 212 and capacitor 214 may represent the resistance and capacitance of the laminating adhesive layer. The capacitor 216 may represent a capacitance that may be formed between the front electrode 102 and the back electrode 104, for example, an interfacial contact area between layers, such as an interface between an imaging layer and a lamination adhesive layer and/or an interface between a lamination adhesive layer and a backplane electrode. The voltage Vi of the imaging film 110 of the pixel may include a residual voltage of the pixel.

Fig. 3 illustrates a cross-sectional view of an exemplary imaging film 300 (e.g., an electrophoretic film) similar to imaging layer 110 shown in fig. 1. As shown in fig. 3, the black particles (K) and the yellow particles (Y) are a first pair of oppositely charged particles, and in this pair, the black particles are highly positive particles and the yellow particles are highly negative particles. The red particles (R) and the white particles (W) are a second pair of oppositely charged particles, and in this pair the red particles are low positive particles and the white particles are low negative particles.

In another example not shown, the black particles may be highly positive particles; the yellow particles may be low positive particles; the white particles may be low negative particles and the red particles may be high negative particles.

In addition, the color states of the four types of particles may be intentionally mixed. For example, since yellow pigments generally have a green hue in nature, and if a better yellow state is desired, yellow particles and red particles can be used, where both types of particles carry the same charge polarity, and the yellow particles are more charged than the red particles. As a result, in the yellow state, a small amount of red particles are mixed with the yellow particles in the green color, thereby providing a better color purity in the yellow state.

It should be understood that the scope of the present invention broadly encompasses particles of any color, so long as the four types of particles have visually distinguishable colors.

For white particles, they may be made of, for example, TiO₂、ZrO₂、ZnO、Al₂O₃、Sb₂O₃、BaSO₄、PbSO₄And the like.

For black particles, they may be formed of Cl pigment black 26 or 28 or the like (e.g., iron manganese black or copper chromium black) or carbon black.

The non-white and non-black particles are independent of colors such as red, green, blue, magenta, cyan or yellow. Pigments for colored particles may include, but are not limited to, CI pigments PR254, PR122, PR149, PG36, PG58, PG7, PB28, PB 15: 3. PY83, PY138, PY150, PY155, or PY 20. These are the commonly used organic pigments described in the Pigment index handbook "New Pigment Application Technology" (CMC Publishing Co. Ltd.1986) and "Printing Ink Technology" (CMC Publishing Co. Ltd. 1984). Specific examples include Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV fast Red D3G, Hostaperm Red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm Green GNX, of Clariant; irgazine Red L3630, Cinqasia Red L4100 HD and Irgazin Red L3660 HD from BASF; phthalocyanine blue, phthalocyanine green, aniline yellow or benzidine yellow of Sun Chemical (Sun Chemical).

The colored particles may also be inorganic pigments, such as red, green, blue and yellow. Examples may include, but are not limited to, CI pigment blue 28, CI pigment green 50, and CI pigment yellow 227.

In addition to color, the four types of particles may have other different optical properties, such as light transmission, reflection, luminescence, or, in the case of displays for machine reading, pseudo-color in the sense of variations in reflectivity of electromagnetic wavelengths outside the visible range.

The display layer utilizing the display fluid of the present invention has two surfaces, a first surface (313) on the viewing side and a second surface (314) on the opposite side of the first surface (313). The fluid is shown sandwiched between two surfaces. On the side of the first surface (313) there is a common electrode (311), which is a transparent electrode layer (e.g. ITO), extending over the entire top of the display layer. On the side of the second surface (314), there is an electrode layer (312) comprising a plurality of pixel electrodes (312 a). It should be noted that the display layers shown in fig. 3 and discussed herein may be capsule-based or cup-based electrophoretic materials, and the working principles set forth herein may be applied to any of these materials.

Pixel electrodes are described in U.S. patent No.7,046,228, the entire contents of which are incorporated herein by reference. It should be noted that although the use of Thin Film Transistor (TFT) backplanes to drive the active matrix is mentioned for the pixel electrode layer, the scope of the invention includes other types of electrode addressing, as long as the electrodes provide the desired function.

Each interval between two vertical dotted lines in fig. 1 represents one pixel. As shown, each pixel has a corresponding pixel electrode. An electric field is generated for a pixel by a potential difference between a voltage applied to the common electrode and a voltage applied to the corresponding pixel electrode.

The solvent in which the four types of particles are dispersed is transparent and colorless. To achieve high particle mobility, the solvent preferably has a low viscosity and a dielectric constant in the range of about 2 to about 30, preferably in the range of about 2 to about 15. Examples of suitable dielectric solvents includeDECALIN (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils, hydrocarbons of silicon fluids; aromatic hydrocarbons such as toluene, xylene, diarylethane, dodecylbenzene, or alkylnaphthalene; halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorotrifluorotoluene, 3,4, 5-trichlorotrifluorotoluene, chloropentafluorobenzene, dichlorononane or pentachlorobenzene; and perfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M company of St.Paul MN; low molecular weight halogen-containing polymers such as poly (perfluoropropylene oxide) from TCI America of Portland, Oregon; poly (chlorotrifluoroethylene), e.g. from the LivereQi region of New JerseyHalocarbon Oils of Halocarbon Product corp. of (River Edge, NJ); perfluoropolyalkyl ethers, such as Galden from Monte corporation (Ausimont) or the Krytox Oils and Greases K-Fluid series from DuPont, Delaware, Dow-corning, polydimethylsiloxane-based silicone Oils (DC-200).

In one embodiment, the "low charge" particles may carry about 50% less charge than the "high charge" particles, preferably about 5% to about 30%. In another embodiment, the "low charge" particles may carry about 75% less charge than the "high charge" particles, or about 15% to about 55%. In a further embodiment, the indicated comparison of charge levels is applicable to two types of particles having the same charge polarity.

The charge intensity can be measured in terms of zeta potential (zeta potential). In one embodiment, the electromotive potential is determined by a colloid Dynamics Acoustosizer IIM, ESA EN # Attn flow-through electrolytic cell (K:127) with CSPU-100 signal processing unit. Instrument constants at the test temperature (25 ℃) such as the density of the solvent used in the sample, the dielectric constant of the solvent, the speed of sound in the solvent, the viscosity of the solvent are input before the test. The pigment sample is dispersed in a solvent (which is typically a hydrocarbon fluid having less than 12 carbon atoms) and diluted to 5-10% by weight. The sample also contained a charge control agent (Solsperse)Available from the Lubrizol Corporation of Berkshire Hathaway; "Solsperse" is a registered trademark) having 1: 10 by weight. The mass of the diluted sample is determined and the sample is then loaded into a flow-through electrolytic cell to determine the zeta potential.

The magnitudes of the "high positive" particles and the "high negative" particles may be the same or different. Likewise, the magnitudes of the "low positive" particles and the "low negative" particles may be the same or different.

It should also be noted that two pairs of high-low charge particles may have different levels of charge difference in the same fluid. For example, in one pair, the charge intensity of the low positively charged particles may be 30% of the charge intensity of the high positively charged particles, while in the other pair, the charge intensity of the low negatively charged particles may be 50% of the charge intensity of the high negatively charged particles.

Fig. 4a and 4b show examples of display devices using such display fluids. As shown in fig. 4a and 4b, the high positive particles are black (K); high negative particles are yellow (Y); low positive particles are red (R); and the low negative particles are white (W).

In operation, as shown in fig. 4a, when a high negative voltage potential difference (e.g., -15V) is applied to the pixel for a sufficiently long period of time, an electric field is generated such that the yellow particles (Y) are pushed to the common electrode (421) side and the black particles (K) are pulled to the pixel electrode (422a) side. The red (R) and white (W) particles move more slowly than the more highly charged black and yellow particles because they carry a weaker charge, and therefore they remain in the middle of the pixel, with the white particles above the red ones. In this case, yellow color was seen at the observation side.

Further, as shown in fig. 4b, when a relatively low positive voltage potential difference (e.g., +6V or +3V) is applied to the pixel of fig. 4a for a sufficiently long period of time (i.e., driven from the yellow state), an electric field is generated such that the yellow particles (Y) move toward the pixel electrode (422a) and the black particles (K) move toward the common electrode (421). However, when they meet in the middle of the pixel, they slow down significantly and remain there because the electric field generated by the low drive voltage is not strong enough to overcome the strong attraction between them. On the other hand, the electric field generated by the low driving voltage is sufficient to separate the less strongly charged white and red particles, so that the low positive red particles (R) move all the way to the common electrode (421) side (i.e., the viewing side) and the low negative white particles (W) move to the pixel electrode (422a) side. As a result, red color was seen. It should also be noted that in this figure there is also an attractive force between the less strongly charged particles (e.g., R) and the more strongly charged particles of opposite polarity (e.g., Y). However, these attractive forces are not as strong as those between the two types of strongly charged particles (K and Y), and therefore can be overcome by the electric field generated by the low drive voltage. In other words, weakly charged particles and more strongly charged particles of opposite polarity can be separated.

An exemplary waveform for completing this yellow to red transition is shown in fig. 5.

Referring now to fig. 5, in an initial step, a high negative drive voltage (V) is applied_H2E.g., -15V) to push the yellow particles toward the viewing side, and then a positive driving voltage (+ V') is applied for a time t8, which pulls down the yellow particles and pushes the red particles toward the viewing side.

The amplitude of + V' is lower than V_H(e.g. V)_H1Or V_H2) The amplitude of (d). In one embodiment, the amplitude of + V' is less than V_H(e.g., V)_H1Or V_H2) 50% of the amplitude of (c).

In one embodiment, t8 is greater than t 7. In one embodiment, t7 may be in the range of 20-400 milliseconds, and t8 may be ≧ 200 milliseconds.

The waveform of FIG. 5 may repeat for at least 2 cycles (N ≧ 2), preferably at least 4 cycles, and more preferably at least 8 cycles. After each drive cycle, the red color becomes more intense.

Similarly, as shown in FIG. 6, the display may be driven from the red state to the yellow state. In practice, to drive a display pixel to the yellow state, a short negative fifteen volt pulse may be applied after the red waveform, as shown in FIG. 7. The yellow particles are preferably particles that carry a high negative charge, and are strongly negative, and will be pushed to the viewing side by a negative 15 volt pulse (e.g., t1 shown in fig. 7).

However, in practice, the yellow state described above may be the most heat sensitive and the RA decay is greatest. In some cases, the yellow state may have a lower b, while L is very good. This means that there may not be enough yellow particles on the viewing side. The yellow particles are mixed with or even behind the white particles.

Wherein b and L are the international commission on illumination or CIE color coordinates, where L denotes the luminance and b is the yellow/blue coordinates.

In some embodiments, a waveform may be used to ameliorate this problem. As shown in fig. 8, to separate the white particles from the yellow particles, a weak positive voltage V2802 may be applied after the-15V pulse (i.e., t 1804). Wherein the white particles have a weak negative charge and can be pulled further to the bottom by a weak positive voltage. Subsequently, another-15V pulse is applied to pull the yellow particles further up towards the viewing side, and then another weak positive voltage V2 is applied. Furthermore, by adding additional periods, more white particles can be pulled to the bottom, while strongly negatively charged yellow particles can stay on the viewing side. The exact number of drive cycles and pulse widths can be optimized based on the physical properties of the display medium.

In one embodiment, a driving method according to the subject matter presented herein may be summarized as a driving method for driving a pixel of an electrophoretic display comprising a first surface on a viewing side, a second surface on a non-viewing side and an electrophoretic fluid arranged between a first light transmissive electrode and a second electrode, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles and a fourth type of particles, all of which are dispersed in a solvent, wherein

a. The four types of pigment particles have different optical properties;

b. the first type of particles and the third type of particles are positively charged, wherein the first type of particles has a greater positive charge than the third particles; and

c. the second type of particles and the fourth type of particles are negatively charged, wherein the second type of particles has a larger negative charge than the fourth particles;

the method comprises the following steps:

(i) applying a first drive voltage to the pixels of the electrophoretic display at a first amplitude for a first period of time to drive the pixels to a color state of the fourth type of particles at the viewing side; and

(ii) a second drive voltage is applied to the pixels of the electrophoretic display for a second period of time, the second drive voltage being opposite to the first drive voltage and the second amplitude being smaller than the first amplitude, to drive the second type of particles towards the non-viewing side.

In another embodiment, a driving method according to the subject matter presented herein may be summarized as a driving method for driving a pixel of an electrophoretic display comprising a first surface on a viewing side, a second surface on a non-viewing side and an electrophoretic fluid arranged between a first light transmissive electrode and a second electrode, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles and a fourth type of particles, all of which are dispersed in a solvent, wherein

a. The four types of pigment particles have different optical properties;

b. the first type of particles and the third type of particles are positively charged, wherein the first type of particles has a greater positive charge than the third particles; and

c. the second type of particles and the fourth type of particles are negatively charged, wherein the second type of particles has a larger negative charge than the fourth particles;

the method comprises the following steps:

(i) applying a first drive voltage to the pixels of the electrophoretic display at a first amplitude for a first period of time to drive the pixels to a color state of the third type of particles at the viewing side;

(ii) applying a second drive voltage to the pixels of the electrophoretic display at a second amplitude for a second period of time to drive the pixels to the color state of the fourth type of particles at the viewing side; and

(iii) applying a third drive voltage to the pixels of the electrophoretic display for a third period of time, the third drive voltage being opposite to the second drive voltage and the third amplitude being smaller than the second amplitude, to drive the second type of particles towards the non-viewing side.

In some embodiments, the voltage v2 may be different from the PP voltage VR used to drive the pixel to the red state. In some other embodiments, as shown in fig. 9, the red waveform period 902 and the latency between the red waveform and the t1 waveform may be further optimized to reduce the overall waveform length. For example, in some embodiments, latency may be greatly reduced. In another embodiment, the latency may be removed entirely.

TABLE 1 EO before and after reliability test

Shown in table 1 above are experimental data showing the yellow state optical performance before and after reliability testing using the old and new waveforms. It can be seen that the yellow CIE index b is improved by using the new yellow waveform proposed herein.

Fig. 10 and 11 show data for 30 samples with different particle dispersion formulations subjected to Qsun (fig. 10) and 70C RA (fig. 11) tests. Both tests showed that b was improved using the new waveform.

It will be apparent to those skilled in the art that many changes and modifications can be made to the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the entire foregoing description is to be construed in an illustrative and not a restrictive sense.

Claims (12)

1. A driving method for driving a pixel of an electrophoretic display comprising a first surface on a viewing side, a second surface on a non-viewing side and an electrophoretic fluid arranged between a first light-transmissive electrode and a second electrode, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles and a fourth type of particles, all of which are dispersed in a solvent, wherein

a. The four types of pigment particles have different optical properties;

b. the first and third types of particles are positively charged, wherein the first type of particles has a greater positive charge than the third particles; and

c. the second and fourth types of particles are negatively charged, wherein the second type of particles has a larger negative charge than the fourth particles;

the method comprises the following steps:

(i) applying a first drive voltage to the pixel of the electrophoretic display at a first amplitude for a first period of time to drive the pixel to a color state of the fourth type of particles at the viewing side; and

(ii) applying a second drive voltage to the pixels of the electrophoretic display for a second period of time, the second drive voltage being opposite to the first drive voltage and a second amplitude being smaller than the first amplitude, to drive the second type of particles towards the non-viewing side.

2. The driving method according to claim 1, wherein the second period in step (ii) is longer than the first period in step (i).

3. The driving method according to claim 1, further comprising repeating steps (i) and (ii).

4. The driving method according to claim 3, wherein the steps (i) and (ii) are repeated at least 3 times.

5. The driving method according to claim 1, further comprising not applying a voltage to the pixel for a period of time before step (i).

6. The driving method according to claim 1, wherein the amplitude of the second driving voltage is less than 50% of the amplitude of the first driving voltage.

7. A driving method for driving a pixel of an electrophoretic display comprising a first surface on a viewing side, a second surface on a non-viewing side and an electrophoretic fluid arranged between first, light-transmissive, electrodes and second electrodes, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles and a fourth type of particles, all of which are dispersed in a solvent, wherein

a. The four types of pigment particles have different optical properties;

b. the first and third types of particles are positively charged, wherein the first type of particles has a greater positive charge than the third particles; and

c. the second and fourth types of particles are negatively charged, wherein the second type of particles has a larger negative charge than the fourth particles;

the method comprises the following steps:

(i) applying a first drive voltage to the pixel of the electrophoretic display at a first amplitude for a first period of time to drive the pixel to a color state of the third type of particles at the viewing side;

(ii) applying a second drive voltage to the pixel of the electrophoretic display at a second amplitude for a second period of time to drive the pixel to a color state of the fourth type of particles at the viewing side; and

(iii) applying a third drive voltage to the pixels of the electrophoretic display for a third period of time, the third drive voltage being opposite to the second drive voltage and a third amplitude being smaller than the second amplitude to drive the second type of particles towards the non-viewing side.

8. The driving method according to claim 7, further comprising not applying the driving voltage to the display pixel for a fourth period of time after the step (i).

9. The driving method according to claim 7, wherein the third period in step (iii) is longer than the second period in step (ii).

10. The driving method according to claim 7, wherein the step (i) is repeated at least 5 times.

11. The driving method according to claim 3, wherein the steps (ii) and (iii) are repeated at least 3 times.

12. The driving method according to claim 7, wherein the amplitude of the third driving voltage is less than 50% of the amplitude of the second driving voltage.