HK40067704A - Methods for driving electro-optic displays - Google Patents

Description

Method for driving electro-optic display

Reference to related applications

This application relates to and claims priority from U.S. provisional application 62/936,914 filed on 18/11/2019.

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

Technical Field

The present invention relates to reflective electro-optic displays and materials for such displays. More particularly, the invention relates to displays having reduced residual voltages and driving methods for reducing residual voltages in electro-optic displays.

Background

Electro-optic displays driven by Direct Current (DC) imbalance waveforms may produce a residual voltage that can be determined by measuring the open circuit electrochemical potential of the display pixels. It has been found that residual voltages are a more prevalent phenomenon in electrophoretic displays and other impulse driven electro-optic displays, both for reasons and as a result. It has also been found that dc imbalance may lead to long-term life degradation in some electrophoretic displays.

The term "residual voltage" is also sometimes used as a term to facilitate reference to the overall phenomenon. However, the switching behavior of impulse-driven electro-optic displays is based on the application of a voltage impulse (integral of voltage with respect to time) across the electro-optic medium. The residual voltage may peak immediately after the application of the drive pulse and may decay substantially exponentially thereafter. The continued presence of the residual voltage for a significant period of time imparts a "residual impulse" to the electro-optic medium, and strictly speaking, such a residual impulse, rather than the residual voltage, may be responsible for what is commonly known as the effect on the optical state of an electro-optic display caused by the residual voltage.

Theoretically, the influence of the residual voltage should correspond directly to the residual impulse. However, in practice, the impulse switching model loses accuracy at low voltages. Some electro-optic media have a threshold value such that a residual voltage of about 1V may not cause a significant change in the optical state of the medium after the end of a drive pulse. However, other electro-optic media, including the preferred electrophoretic media used in the experiments described herein, a residual voltage of about 0.5V may result in a significant change in optical state. Thus, the two equivalent residual impulses may differ in practical result, and raising the threshold of the electro-optic medium may help reduce the effect of the residual voltage. The inteck company has produced electrophoretic media with a "small threshold" sufficient to prevent the residual voltage experienced in some cases from changing the displayed image immediately after the end of the drive pulse. If the threshold is insufficient or the residual voltage is too high, the display may exhibit kickback/self-erase or self-improvement phenomena. Wherein the term "optical kickback" is used herein to describe a change in the optical state of a pixel that occurs at least partially in response to a discharge of the pixel's residual voltage.

Even when the residual voltages are below a small threshold, they may have a severe impact on image switching if the residual voltages are still continuously present when the next image update occurs. For example, assume that during an image update of the electrophoretic display a driving voltage of +/-15V is applied to move the electrophoretic particles. If a residual voltage of +1V persists from the previous update, the drive voltage will actually shift from +15V/-15V to + 16V/-14V. As a result, the pixel will be biased towards a dark or white state depending on whether the pixel has a positive or negative residual voltage. Furthermore, this effect may change over time due to the decay rate of the residual voltage. The electro-optic material in a pixel switched to white using a 15V, 300ms drive pulse immediately after a previous image update may actually experience a waveform closer to 16V, 300ms, whereas the material in a pixel switched to white using the exact same drive pulse (15V, 300ms) one minute later may actually experience a waveform closer to 15.2V, 300 ms. Thus, the pixels may show distinctly different shades of white.

The residual voltages can also be arranged in a similar pattern on the display if the previous image has created a residual voltage field (e.g. a dark line on a white background) over a number of pixels. In practice, the most significant effect of residual voltage on display performance may be ghosting. This problem is complementary to the previously mentioned problem, i.e. a dc imbalance (e.g. 16V/14V instead of 15V/15V) may be the cause of slow degradation of the lifetime of the electro-optical medium.

If the residual voltage decays slowly and is almost constant, its effect in the offset waveform does not change from image update to image update and may actually produce less ghosting than a rapidly decaying residual voltage. Thus, the ghosting experienced by updating one pixel after 10 minutes and another pixel after 11 minutes is much less than the ghosting experienced by immediately updating one pixel and another pixel after 1 minute. Conversely, the residual voltage decays so fast that it approaches zero before the next update occurs, may not actually result in a detectable ghost.

The residual voltage has a variety of potential sources. It is believed (although some embodiments are not limited by this perspective) that one important cause of remnant voltage is ionic polarization within the materials forming the various layers of the display.

In summary, the residual voltage as a phenomenon can be presented in various ways as an image ghost or visual artifact, the severity of which varies with the time elapsed between image updates. The residual voltage also creates a dc imbalance and shortens the ultimate lifetime of the display. The effect of the residual voltage may therefore be detrimental to the quality of the electrophoretic or other electro-optical device, and it is desirable to minimise the sensitivity of the residual voltage itself, and the optical state of the device, to the effect of the residual voltage.

Thus, discharging the residual voltage of the electro-optic display can improve the quality of the displayed image even when the residual voltage is already low. The inventors have recognized and appreciated that conventional techniques for discharging the remnant voltage of an electro-optic display may not completely discharge the remnant voltage. That is, conventional techniques for discharging the residual voltage may result in the electro-optic display maintaining at least a low residual voltage. Accordingly, techniques for better discharging the residual voltage from electro-optic displays are needed.

Disclosure of Invention

The invention provides a method for driving an electro-optic display having a plurality of display pixels and each of the plurality of display pixels being associated with a display transistor, the method comprising applying a first voltage to the transistor associated with the display pixel for a first duration to drain a residual voltage from the display pixel, applying a second voltage to the transistor for a second duration to stop the draining of the residual voltage from the display pixel, and applying a third voltage to the transistor for a third duration to drain the residual voltage from the display pixel.

Drawings

Fig. 1 is a circuit diagram representing an electrophoretic display according to the subject matter disclosed herein;

FIG. 2 illustrates a circuit model of an electro-optic imaging layer according to the subject matter disclosed herein;

FIG. 3 illustrates an exemplary driving method according to the subject matter disclosed herein;

FIG. 4 illustrates another driving method according to the subject matter disclosed herein;

FIG. 5 illustrates yet another driving method according to the subject matter disclosed herein;

FIG. 6 illustrates an additional drive method according to the subject matter disclosed herein;

FIG. 7 illustrates an alternative driving method according to the subject matter disclosed herein; and

fig. 8 illustrates another driving method according to the subject matter disclosed herein.

Detailed Description

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 between 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 imperial patents and published applications referred to below 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 dark blue states mentioned above. The term "monochromatic" 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 gray state.

Much of the discussion below focuses on methods for driving one or more pixels of an electro-optic display by transitioning from an initial gray level to a final gray level (which may be different from or the same as the initial gray level). 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); the elements may be referred to as "pulses" or "drive pulses". 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". As described in several of the aforementioned MEDEOD applications, it is also possible to use more than one drive scheme 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".

Some electro-optic materials are solid in the sense that the material has a solid outer surface, although the material may, and often does, have a space filled with a liquid or gas inside. For convenience, such displays using solid electro-optic materials may be referred to hereinafter as "solid electro-optic displays". Thus, the term "solid state electro-optic display" includes rotating bichromal member displays, encapsulated electrophoretic displays, microcell electrophoretic displays, and encapsulated liquid crystal displays.

The terms "bistable" and "bistability" are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical characteristic such that, after any given element is driven to assume either its first or second display state by an addressing pulse of finite duration, after the addressing pulse has terminated, that state will persist for 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. It is shown in U.S. patent No.7,170,670 that some particle-based electrophoretic displays that support gray scale are stable not only in their extreme black and white states, but also in their intermediate gray states, as are 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.

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.

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 than 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. These encapsulated media comprise a plurality of microcapsules, each of which itself comprises 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) microcell structures, wall materials, and methods of forming microcells; see, e.g., U.S. patent nos. 7,072,095 and 9,279,906;

(d) a method for filling and sealing a microcell; see, e.g., U.S. patent nos. 7,144,942 and 7,715,088;

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

(f) 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;

(g) color formation and color adjustment; see, for example, U.S. patent nos. 7,075,502 and 7,839,564.

(h) An application for a display; see, e.g., U.S. patent nos. 7,312,784; 8,009,348, respectively;

(i) non-electrophoretic displays, as described in U.S. patent No.6,241,921 and U.S. patent application publication No. 2015/0277160; and applications of packaging and microcell technology other than displays; see, e.g., U.S. patent application publication nos. 2015/0005720 and 2016/0012710; and

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; 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; 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; 8,139,050, respectively; 8,174,490, respectively; 8,243,013, respectively; 8,274,472; 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; 8,462,102; 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; 8,665,206, respectively; 8,681,191, respectively; 8,730,153; 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; 9,171,508, respectively; 9,218,773, respectively; 9,224,338, respectively; 9,224,342; 9,224,344; 9,230,492, respectively; 9,251,736, respectively; 9,262,973, respectively; 9,269,311, respectively; 9,299,294, respectively; 9,373,289; 9,390,066, respectively; 9,390,661, respectively; and 9,412,314; and U.S. patent application publication No. 2003/0102858; 2004/0246562; 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; 2008/0218471, respectively; 2008/0291129, respectively; 2008/0303780, respectively; 2009/0174651, respectively; 2009/0195568, respectively; 2009/0322721, respectively; 2010/0194733; 2010/0194789; 2010/0220121; 2010/0265561, respectively; 2010/0283804; 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; 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; 2016/0071465, respectively; 2016/0078820, respectively; 2016/0093253, respectively; 2016/0140910, respectively; and 2016/0180777.

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, 2002/0131147, supra. 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 suspending fluid are not encapsulated within microcapsules, but rather are held in a plurality of cavities formed within a carrier medium (e.g., a polymer film). See, for example, international application publication No. wo 02/01281 and published U.S. application No.2002/0075556, both assigned to Sipix Imaging, inc.

Many of the aforementioned yingke and MIT patents and applications also contemplate microcell electrophoretic displays and polymer dispersed electrophoretic displays. The term "encapsulated electrophoretic display" may refer to all such display types, which may also be collectively referred to as "microcavity electrophoretic displays" to generalize the morphology of the entire wall.

Another type of electro-optic display is the electro-wetting display developed by Philips, described in Hayes, R.A. et al, "Video-Speed Electronic Paper Based on electric wetting," Nature,425,383-385 (2003). Which is shown in co-pending application serial No.10/711,802 filed on 6.10.2004, such an electrowetting display can be made bistable.

Other types of electro-optic materials may also be used. Of particular interest, bistable ferroelectric liquid crystal displays (FLCs) are known in the art and exhibit residual voltage behavior.

Although electrophoretic media may be opaque (because, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, some electrophoretic displays may 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. Pat. Nos. 6,130,774 and 6,172,798 and U.S. Pat. Nos. 5,872,552, 6,144,361, 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 a shutter mode.

A high resolution display may include individual pixels that are addressable and not disturbed by adjacent pixels. One way of obtaining such pixels 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. When the non-linear element is a transistor, the pixel electrode may be connected to a drain of the transistor, and this arrangement will be adopted in the following description, although it is arbitrary in nature and the pixel electrode may be connected to a source of the transistor. In a high resolution array, pixels may be arranged in a two-dimensional array of rows and columns such that any particular pixel is uniquely defined by the intersection of a particular row and a particular column. The sources of all transistors in each column may be connected to a single column electrode and the gates of all transistors in each row may be connected to a single row electrode; again, the arrangement of source to row and gate to column may be reversed as desired.

The display can be written in a row-by-row fashion. The row electrodes are connected to a row driver which can apply a voltage to the selected row electrode, for example to ensure that all transistors in the selected row are conductive, while applying a voltage to all other rows, for example 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 various column electrodes which are selected to drive the pixels in the selected row to their desired optical states. (the foregoing voltages are relative to a common front electrode that may be disposed on the opposite side of the electro-optic medium from the non-linear array and extend across the entire display. As is known in the art, voltages are relative and are a measure of the difference in charge between two points.

However, in use, certain waveforms may produce a residual voltage to the pixels of the electro-optic display, and as is apparent from the discussion above, this residual voltage produces several undesirable optical effects, and is generally undesirable.

As described herein, an "offset" in an optical state associated with an addressing pulse refers to a situation in which a particular addressing pulse is first applied to an electro-optic display resulting in a first optical state (e.g., a first gray scale) and the same addressing pulse is subsequently applied to the electro-optic display resulting in a second optical state (e.g., a second gray scale). Since the voltage applied to a pixel of the electro-optic display during the application of the addressing pulse comprises the sum of the residual voltage and the addressing pulse voltage, the residual voltage may cause a shift in the optical state.

"drift" of the optical state of the display over time refers to the situation in which the optical state of an electro-optic display changes when the display is at rest (e.g., during the period of time in which an addressing pulse is not applied to the display). Since the optical state of a pixel may depend on the residual voltage of the pixel, and the residual voltage of the pixel may decay over time, the residual voltage may cause a drift of the optical state.

As mentioned above, "ghosting" refers to the situation where, after rewriting of the electro-optic display, traces of the previous image are still visible. The residual voltage may cause "edge ghosting", a type of ghosting in which the contours (edges) of a portion of the previous image remain visible.

Wherein the term "optical kickback" is used herein to describe a change in the optical state of a pixel that occurs at least partially in response to a discharge of the pixel's residual voltage.

Fig. 1 shows a schematic diagram of a pixel 100 of an electro-optic display according to the subject matter presented herein. Pixel 100 may include an imaging film 110. In some embodiments, imaging film 110 may be bistable. In some embodiments, imaging film 110 may include, but is not limited to, an encapsulated electrophoretic imaging film, which may include, for example, charged pigment particles.

An imaging film 110 may be disposed between the front electrode 102 and the back 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. 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 rear electrode 104 may be formed opposite to 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 include a diode and/or a transistor, including but not limited to a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). The drain (or source) of the MOSFET may be coupled to the backplane electrode 104, the source (or drain) of the MOSFET may be coupled to the addressing electrode 108, and the gate 106 of the MOSFET may be coupled to a driver and configured to control activation and deactivation of the MOSFET. (for simplicity, the terminal of the MOSFET coupled to the backplane electrode 104 will be referred to as the drain of the MOSFET, and the terminal of the MOSFET coupled to the address electrode 108 will be referred to as the source of the MOSFET.

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 gates 106 of all transistors coupled to 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 a voltage across the transistor gates 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 some embodiments, 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 on the imaging film 110 of a pixel may comprise the residual voltage of the pixel.

The discharge of the pixel residual voltage may be initiated and/or controlled by applying any suitable set of signals to the pixel, including but not limited to the set of signals described in more detail below in fig. 3 and 4-8.

Fig. 3 illustrates one exemplary driving method 300 according to the subject matter disclosed herein. In general, the post-drive discharge of the residual voltage may involve applying a discharge voltage (e.g., a voltage applied to the gate 106 of the transistor 120 associated with each display pixel) that substantially increases the pixel transistor transconductance, thereby allowing the residual voltage to drain from the display pixel. In some embodiments, the discharge voltage value may be selected to be the same as the gate-on voltage (i.e., a voltage that is large enough and applied to the gate of the transistor 120 associated with the display pixel so that the transistor conducts current and drives the display pixel) for selecting a row of display pixels during active matrix scanning. Alternatively, as described in U.S. patent application No.15/266,554, the entire contents of which are incorporated herein, the value of the discharge voltage may be selected to be of a small magnitude but of a large enough amplitude to induce sufficient pixel transistor conductance to allow residual voltage to drain from the display pixel. The discharge voltage may be constant or may be time-varying. For example, the discharge voltage may be designed to decay approximately exponentially during the post-drive discharge phase. In some other embodiments, the discharge voltage may be applied intermittently for a specified post-drive discharge time. Specifically, the gate voltage may be set to a desired discharge voltage for two or more periods during the post-drive time range, and at different voltages for the remainder of the post-drive discharge time. Indeed, in some embodiments, instead of a single distinct voltage, there may be multiple alternating voltages. It will be appreciated, however, that these alternative voltages may be expected not to be as sensitive to the pixel thin film transistors as when the discharge voltage is applied. In use, this means that the different or alternating voltage values are somewhere in the range between the discharge voltage and the gate-off voltage employed during a typical display scan, including the gate-off voltage. While a convenient alternating voltage may be zero volts, in which case zero volts is the same as the voltage held by the source line during this discharge period, it may be advantageous to have the alternating voltage and the discharge voltage have opposite signs or polarities. An advantage here is that voltages of opposite sign may at least partially counteract the stress caused by the voltage applied to the transistor by the drive voltage.

The subject matter disclosed herein introduces several advantages, one being a reduction in the transconductance stress of the TFT when a discharge voltage is applied to the TFT gate during residual voltage discharge. TFT transconductance stress can accumulate over time and cause display performance degradation. The driving method described herein can reduce the integration time of the discharge voltage applied to the TFT, in a way that preserves the efficacy of the post-drive discharge better than alternatives, such as reducing the discharge voltage stress by only reducing the time of the post-drive discharge.

Further, by dividing the post-drive discharge into multiple portions having different voltage values, in some cases, one of the portions may have a voltage level that carries an amplitude that is opposite to the amplitude of the discharge portion (e.g., a negative voltage, as compared to a positive voltage during the TFT discharge portion). In this configuration, at least a portion of the accumulated transconductance stress may be rolled back or reduced, thereby improving the reliability and performance of the TFT.

As shown in fig. 3, one embodiment of a driving method for discharging residual charge to reduce residual voltage may include three driving portions or time intervals 302, 304, and 306. In time interval 302, discharge voltage V_PDD308 may be applied to the pixel transistor to create a conduction path for discharging residual charge. In some embodiments, the discharge voltage V_PDDThe value of 308 may be a small magnitude but large enough in amplitude to induce sufficient pixel transistor conductance to allow residual electricityThe pressure is drained from the pixel. In this time interval 302, when the discharge voltage V is applied_PDDAt 308, the pixel voltage V_PixelDuring this time interval 302 may be zero and the residual charge passes through the current J_{Discharge of electricity}Dissipates from the pixel. Subsequently, during the dwell period 304, the discharge voltage V may be set_PDDSet equal to the nominal gate-off voltage 310, which results in the pixel voltage V_PixelIs zero current value, and at this time, the pixel current J_{Discharge of electricity}Becomes zero and no residual charge dissipates. After the dwell period 304, the pixel voltage V_PDD308 may be turned on again to the nominal discharge voltage 312 in another discharge period 306. During this second discharge period, additional residual charge may be dissipated.

In some other embodiments, the pixel voltage V is not applied as described above_PDDBecomes the nominal gate-off voltage and the pixel voltage V can be set_PDDSet to zero volts and the discharge period may oscillate between the nominal discharge voltage and the zero volt level as shown in fig. 4. It should be understood that the segment duration and dwell period of the discharge cycle may vary depending on the application. For example, as shown in fig. 5, discharge period 404 may be preset to have a duty cycle of 40% (i.e., the full duty cycle may be the sum of periods 402 and 404).

In some other embodiments, the nominal gate-off voltage may have a specific discharge voltage V_PDDLonger duration. For example, as shown in FIG. 6, nominal gate-off voltage 604 may have a duty cycle of 60% while discharge voltage V is_PDD602 has a duty cycle of 40%.

In yet another embodiment, the drive scheme may include discharge voltages V of different durations_PDDAnd a nominal gate-off voltage. This means that, in the drive sequence, the discharge voltage V_PDDThe period and/or the gate-off voltage period may be varied in duration to suit a particular display application. For example, as shown in FIG. 7, the duration of discharge voltage period 702 may be longer than the duration of discharge voltage period 706. In addition, the duration of the gate-off voltage period may also be the sameDifferent. For example, as shown in FIG. 8, not only the discharge voltage V_PDDThe periods have different durations (e.g., period 802 is longer in duration than period 806, and period 806 itself is longer in duration than period 808), and the gate-off voltage period may also have different durations (e.g., period 810 is longer in duration than period 804). And the variation in duration of the above cycles may be irregular in nature.

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. The foregoing description, therefore, is to be considered in all respects as illustrative and not restrictive.

Claims (20)

1. A method for driving an electro-optic display, the display having a plurality of display pixels and each of the plurality of display pixels being associated with a display transistor, the method comprising:

applying a first voltage to a transistor associated with a display pixel for a first duration to drain a residual voltage from the display pixel;

applying a second voltage to the transistor for a second duration to stop draining residual voltage from the display pixel; and

a third voltage is applied to the transistor for a third duration to drain a residual voltage from the display pixel.

2. The method of claim 1, wherein the first voltage is a gate-on voltage.

3. The method of claim 2, wherein the third voltage is a gate-on voltage.

4. The method of claim 1, wherein the second voltage is zero volts.

5. The method of claim 1, wherein the first duration is the same length as the second duration.

6. The method of claim 1, wherein the length of the second duration is configured to reduce stress on the transistor.

7. The method of claim 1, wherein the first duration is the same length as the third duration.

8. The method of claim 1, wherein the second duration is the same length as the third duration.

9. The method of claim 1, wherein the first duration is different in length than the second duration.

10. The method of claim 1, wherein the first duration is different in length than the third duration.

11. The method of claim 1, wherein the second voltage has a voltage polarity opposite the first voltage.

12. The method of claim 1, wherein the second voltage has an opposite voltage polarity to the third voltage.

13. The method of claim 1, wherein the second voltage is a nominal gate-off voltage.

14. The method of claim 1, further comprising applying a fourth voltage to the transistor for a fourth duration to stop draining residual voltage from the display pixel.

15. The method of claim 14, wherein a length of the fourth duration is configured to reduce stress in the transistor.

16. The method of claim 14, further comprising applying a fifth voltage to the transistor for a fifth duration to drain a residual voltage from the display pixel.

17. The method of claim 16, wherein the fourth time duration has a different length than the fifth time duration.

18. The method of claim 16, wherein the fourth duration is the same length as the fifth duration.

19. The method of claim 16, wherein the fourth time duration has a different length than the second time duration.

20. The method of claim 16, wherein the fourth duration is the same length as the second duration.