HK40067704B - Methods for driving electro-optic displays - Google Patents

Description

Method for driving electro-optic displays

Citation of relevant applications

This application relates to and claims priority to U.S. Provisional Application 62/936,914, filed November 18, 2019.

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

Technical Field

This invention relates to reflective electro-optic displays and materials for such displays. More specifically, this invention relates to displays with reduced residual voltage and driving methods for reducing residual voltage in electro-optic displays.

Background Technology

Electro-optic displays driven by DC-imbalanced waveforms may generate residual voltages, which can be determined by measuring the open-circuit electrochemical potential of the display pixels. Residual voltages have been found to be more prevalent, both causally and consequentially, in electrophoretic displays and other impulse-driven electro-optic displays. DC imbalances have also been found to contribute to long-term degradation in some electrophoretic displays.

The term "residual voltage" is sometimes used conveniently to refer to the overall phenomenon. However, the switching behavior of impulse-driven electro-optic displays is based on the application of a voltage impulse (the voltage integral over time) across the electro-optic medium. The residual voltage may peak immediately after the application of the driving pulse and then decay substantially exponentially thereafter. The persistence of the residual voltage over a long period exerts a "residual impulse" on the electro-optic medium, and strictly speaking, it is this residual impulse, rather than the residual voltage, that is likely the cause of the effects on the optical state of electro-optic displays that are generally considered to be caused by the residual voltage.

Theoretically, the effect of residual voltage should directly correspond to residual impulse. However, in practice, impulse switching models lose accuracy at low voltages. Some electro-optic media have a threshold such that a residual voltage of approximately 1V may not cause a significant change in the optical state of the medium after the drive pulse ends. However, for other electro-optic media, including the preferred electrophoretic medium used in the experiments described herein, a residual voltage of approximately 0.5V may cause a significant change in the optical state. Therefore, two equivalent residual impulses may differ in practical results, and increasing the threshold of the electro-optic medium may help reduce the effect of residual voltage. ENKOOL has produced electrophoretic media with a “small threshold” sufficient to prevent residual voltage experienced in certain situations from immediately altering the displayed image after the drive pulse ends. If the threshold is insufficient or the residual voltage is too high, the display may exhibit recoil/self-erasure or self-improvement phenomena. The term “optical recoil” is used herein to describe changes in the optical state of a pixel that occur at least in part in response to the discharge of residual pixel voltage.

Even when residual voltages are below a small threshold, they can still significantly impact image switching if they persist into the next image update. For example, suppose a +/-15V drive voltage is applied to move electrophoretic particles during an image update on an electrophoretic display. If a +1V residual voltage persists from the previous update, the drive voltage will effectively shift from +15V/-15V to +16V/-14V. As a result, depending on whether the pixel has a positive or negative residual voltage, it will be biased towards a darker or whiter state. Furthermore, this effect varies over time due to the decay rate of the residual voltage. Electro-optical materials in a pixel switching to white immediately after a previous image update using a 15V, 300ms drive pulse might actually experience a waveform closer to 16V, 300ms, while materials in a pixel switching to white one minute later using the exact same drive pulse (15V, 300ms) might actually experience a waveform closer to 15.2V, 300ms. Therefore, the pixel may exhibit noticeably different shades of white.

If a previous image has already created residual voltage fields (e.g., dark lines on a white background) on multiple pixels, the residual voltages can also be arranged in a similar pattern on the display. In fact, the most noticeable effect of residual voltage on display performance may be ghosting. This issue complements the previously mentioned problem that DC imbalance (e.g., 16V/14V instead of 15V/15V) can be a cause of the slow degradation of the electro-optical dielectric lifetime.

If the residual voltage decays slowly and is nearly constant, its effect on the offset waveform will not vary with different image updates and may actually produce less ghosting than a rapidly decaying residual voltage. Therefore, updating one pixel after 10 minutes and another after 11 minutes will result in far less ghosting than updating one pixel immediately and another after 1 minute. Conversely, a residual voltage that decays so rapidly to near zero before the next update may actually not cause detectable ghosting.

Residual voltage has multiple potential sources. It is believed (although some embodiments are not limited to this view) that a significant cause of residual voltage is ionic polarization within the materials forming the various layers of the display.

In summary, residual voltage, as a phenomenon, can manifest as image ghosting or visual artifacts in various ways, and its severity varies with the time elapsed between image updates. Residual voltage can also cause DC imbalance and shorten the ultimate lifespan of a display. Therefore, the effects of residual voltage can be detrimental to the quality of electrophoresis or other electro-optical devices, and it is desirable to minimize the residual voltage itself and the sensitivity of the device's optical state to its effects.

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

Summary of the Invention

The present 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 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 applying a third voltage to the transistor for a third duration to drain residual voltage from the display pixel.

Attached Figure Description

Figure 1 is a circuit diagram showing an electrophoretic display according to the subject matter disclosed herein;

Figure 2 shows a circuit model of the electro-optic imaging layer according to the subject matter disclosed herein;

Figure 3 illustrates an exemplary driving method based on the subject matter disclosed herein;

Figure 4 illustrates another driving approach based on the subject matter disclosed herein;

Figure 5 illustrates yet another driving approach based on the subject matter disclosed herein;

Figure 6 illustrates an additional driving approach based on the subject matter disclosed herein;

Figure 7 illustrates alternative driving methods based on the subject matter disclosed herein; and

Figure 8 illustrates another driving approach based on the subject matter disclosed herein.

Detailed Implementation

The term "electro-optic," used here in the context of materials or displays, refers to a material having first and second display states, where at least one optical property differs, and the material is changed from its first display state to its second display state by applying an electric field. While the optical property is typically color perceptible to the human eye, it can be another optical property, such as light transmission, reflection, emission, or, in the case of machine-reading displays, a pseudocolor in the sense of a change in reflectivity at electromagnetic wavelengths outside the visible light range.

The term "gray state" is used here in its conventional meaning in the imaging field, referring to a state between two extreme optical states of a pixel, but not necessarily a black-and-white transition between these two extreme states. For example, several IENK patents and publications mentioned below describe electrophoretic displays where the extreme states are white and dark blue, making the intermediate "gray state" actually a light blue. In fact, as already mentioned, a change in optical state may not be a color change at all. The terms "black" and "white" may be used below to refer to the two extreme optical states of a display, and should be understood to generally include extreme optical states that are not strictly black and white, such as the white and dark blue states mentioned above. The term "monochrome" may be used below to refer to a driving scheme that drives pixels only to their two extreme optical states without an intermediate gray state.

Much of the following discussion focuses on methods for driving one or more pixels of an electro-optic display via a transition from an initial grayscale to a final grayscale (which may be different from or the same as the initial grayscale). The term "waveform" will be used to describe a curve of the entire voltage over time used to achieve the transition from a particular initial grayscale to a particular final grayscale. Typically, such a waveform will include multiple waveform elements; where these elements are substantially rectangular (i.e., where a given element includes the application of a constant voltage over a time period); the elements may be referred to as "pulses" or "drive pulses." The term "drive scheme" refers to a set of waveforms sufficient to achieve all possible transitions between grayscales of a particular display. A display may utilize more than one drive scheme; for example, as taught in the aforementioned U.S. Patent No. 7,012,600, 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 can provide multiple different drive schemes for use at different temperatures, etc. A set of drive schemes used in this way may be referred to as a "set of related drive schemes." As described in several of the aforementioned MEDEOD applications, more than one driving scheme can be used simultaneously in different areas of the same display, and a group of driving schemes used in this manner can be referred to as a "group of simultaneous driving schemes".

In the sense that a material has a solid outer surface, some electro-optic materials are solid, although the material may and often does indeed have internal spaces filled with liquid or gas. For convenience, such displays using solid electro-optic materials may be referred to as "solid-state electro-optic displays" below. Therefore, the term "solid-state electro-optic display" includes rotating dual-color component displays, encapsulated electrophoretic displays, microcell electrophoretic displays, and encapsulated liquid crystal displays.

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

Several types of electro-optic displays are known. One type of electro-optic display is the rotating bicolor component type, as described, for example, in 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 often referred to as a "rotating bicolor sphere" display, the term "rotating bicolor component" is preferred as it is more accurate because in some of the patents mentioned above, the rotating component is not spherical). This display uses a number of small bodies (typically spherical or cylindrical) and internal dipoles, said bodies comprising two or more parts with different optical properties. These bodies are suspended within liquid-filled bubble chambers within a matrix, the bubble chambers being filled with liquid to allow the bodies to rotate freely. The appearance of a display is altered by applying an electric field to the display, thereby rotating the subject to various positions and changing which part of the subject is seen through the viewing surface. This type of electro-optic medium is typically bistable.

Electro-optic displays, a type of display that has been the subject of intensive research and development for many years, are particle-based electrophoretic displays, in which multiple charged particles move through a fluid under the influence of an electric field. Compared to liquid crystal displays (LCDs), electrophoretic displays can offer advantages such as good brightness and contrast, wide viewing angles, state bistability, and low power consumption. However, long-term image quality issues have hindered their widespread use. For example, the particles constituting an electrophoretic display are prone to settling, resulting in a short lifespan for these displays.

As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be generated using a gaseous fluid; see, for example, 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 impulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Patent Nos. 7,321,459 and 7,236,291. When such gas-based electrophoretic media are used in a direction that allows particle sedimentation, such as in a sign where the medium is arranged in a vertical plane, they are susceptible to the same type of problems as liquid-based electrophoretic media due to the same particle sedimentation. In fact, particle sedimentation is more severe in gas-based electrophoretic media than in liquid-based electrophoretic media because the lower viscosity of gaseous suspensions compared to liquids causes electrophoretic particles to settle more quickly.

Numerous patents and applications transferred to or in the name of MIT and Einkel describe various techniques for encapsulating electrophoretic and other electro-optic media. These encapsulated media comprise a plurality of small capsules, each capsule comprising an inner phase and a capsule wall surrounding the inner phase, wherein the inner phase contains electrophoretically mobile particles in a fluid medium. Typically, the capsules themselves are held in a polymer binder to form a coherent layer located between two electrodes. Techniques described in these patents and applications include:

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

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

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

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

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

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

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

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

(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 technologies other than displays; see, for example, U.S. Patent Application Publications Nos. 2015/0005720 and 2016/0012710; and

Methods for driving a display; see, for example, U.S. Patent Nos. 5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970; 6,900,851; 6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,061,166; 7,061,662; 7,116,466; 7,119,772; 7,177,066; 7,193,625; 7,202,847; 7,242,5 14; 7,259,744; 7,304,787; 7,312,794; 7,327,511; 7,408,699; 7,453,445; 7,492,339; 7,528,822; 7,545,358; 7,583,251; 7,602,374; 7,612,760; 7,679,599; 7,679,813; 7,683,606; 7,688,297; 7,729,039; 7,733,311; 7,733,335; 7,787,169; 7,859,742; 7,952, 557；7,956,841；7,982,479；7,999,787；8,077,141；8,125,501；8,139,050；8,174,490；8,243,013；8,274,472；8,289,250；8,300,006；8,305,341；8,314,784；8,373,649；8,384,658；8,456,414；8,462,102；8,537,105；8,558,783；8,558,785；8,558,786；8,558 855；8,576,164；8,576,259；8,593,396；8,605,032；8,643,595；8,665,206；8,681,191；8,730,153；8,810,525；8,928,562；8,928,641；8,976,444；9,013,394；9,019,197；9,019,198；9,019,318；9,082,352；9,171,508；9,218,773；9,224,338；9,224,342；9,224 ,344; 9,230,492; 9,251,736; 9,262,973; 9,269,311; 9,299,294; 9,373,289; 9,390,066; 9,390,661; and 9,412,314; and U.S. Patent Application Publication Nos. 2003/0102858; 2004/0246562; 2005/0253777; 2007/0070032; 2007/0076289; 2007/0091418; 2007/0103427; 2007/0176912; 2007/0 296452; 2008/0024429; 2008/0024482; 2008/0136774; 2008/0169821; 2008/0218471; 2008/0291129; 2008/0303780; 2009/0174651; 2009/0195568; 2009/0322721; 2010/0194733; 2010/0194789; 2010/0220121; 2010/0265561; 2010/0283804; 2011/0063314; 2011 /0175875; 2011/0193840; 2011/0193841; 2011/0199671; 2011/0221740; 2012/0001957; 2012/0098740; 2013/0063333; 2013/0194250; 2013/0249782; 2013/0321278; 2014/0009817; 2014/0085355; 2014/0204012; 2014/0218277; 2014/0240210; 2014/0240373; 20 14/0253425; 2014/0292830; 2014/0293398; 2014/0333685; 2014/0340734; 2015/0070744; 2015/0097877; 2015/0109283; 2015/0213749; 2015/0213765; 2015/0221257; 2015/0262255; 2016/0071465; 2016/0078820; 2016/0093253; 2016/0140910; 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 so-called polymer-dispersed electrophoretic displays, wherein the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymeric material, and the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered as capsules or microcapsules, even without a discrete capsule film associated with each individual droplet; see, for example, the aforementioned 2002/0131147. Therefore, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a subclass of encapsulated electrophoretic media.

One related type of electrophoretic display is the so-called "micro-unit electrophoretic display." In a micro-unit electrophoretic display, charged particles and suspended fluid are not encapsulated within microcapsules, but rather held within multiple cavities formed within a carrier medium (e.g., a polymer film). See, for example, International Application Publication No. WO 02/01281 and U.S. Application No. 2002/0075556, both assigned to Sipix Imaging, Inc.

Many of the aforementioned Einkel and MIT patents and applications also consider microcell electrophoretic displays and polymer-dispersed electrophoretic displays. The term "encapsulated electrophoretic display" can refer to all such display types, and can also be collectively referred to as "microcavity electrophoretic display" to encompass the morphology of the entire wall.

Another type of electro-optic display is the electrowetting display developed by Philips, described in Hayes, R.A. et al., “Video-Speed Electronic Paper Based on Electrowetting,” Nature, 425, 383-385 (2003). It is shown in co-pending application sequence No. 10/711,802, filed October 6, 2004, that this electrowetting display can be fabricated to be bistable.

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

While electrophoretic media can be opaque (because, for example, in many electrophoretic media, particles essentially block visible light from passing through the display) and operate in reflective mode, some electrophoretic displays can be made to operate in a so-called "shutter mode," in which one display state is substantially opaque and the other is transmissive. See, for example, U.S. Patent Nos. 6,130,774 and 6,172,798 and U.S. Patent Nos. 5,872,552, 6,144,361, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays, similar to electrophoretic displays but dependent on changes in electric field strength, can operate in a similar mode; see U.S. Patent No. 4,418,346. Other types of electro-optic displays are also capable of operating in shutter mode.

High-resolution displays can include addressable, individual pixels that are independent of interference from adjacent pixels. One way to obtain such pixels is to provide an array of nonlinear elements (such as transistors or diodes), with at least one nonlinear element associated with each pixel to produce an "active matrix" display. The addressing or pixel electrode used to address a pixel is connected to an appropriate voltage source via the associated nonlinear element. When the nonlinear element is a transistor, the pixel electrode can be connected to the drain of the transistor, and this arrangement will be used in the description below, although it is essentially arbitrary and the pixel electrode can be connected to the source of the transistor. In a high-resolution array, pixels can 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 can be connected to a single column electrode, while the gates of all transistors in each row can be connected to a single row electrode; furthermore, the source-to-row and gate-to-column arrangements can be reversed as needed.

The display can be written line by line. Row electrodes are connected to a row driver that applies a voltage to the selected row electrode, for example, to ensure that all transistors in the selected row are turned on, while applying a voltage to all other rows, for example, to ensure that all transistors in the unselected rows remain off. Column electrodes are connected to a column driver that applies a voltage to each column electrode, the voltage being selected to drive the pixels in the selected row to their desired optical state. (The aforementioned voltage is relative to a common front electrode, which may be positioned on the opposite side of the electro-optic medium from the nonlinear array and extend across the entire display. As is known in the art, voltage is relative and is a measurement of the charge difference between two points. One voltage value is relative to another voltage value. For example, zero voltage (“0V”) means there is no voltage difference relative to another voltage.) After a pre-selection interval known as the “row address time,” the selected row is deselected, another row is selected, and the voltage on the column driver is changed so that the next row of the display is written.

However, in use, certain waveforms may produce residual voltages on the pixels of the electro-optic display, and as is evident from the discussion above, these residual voltages produce several unwanted optical effects, which are generally undesirable.

As described herein, an “offset” in an optical state associated with an addressing pulse refers to a situation where a particular addressing pulse is first applied to an electro-optic display resulting in a first optical state (e.g., a first grayscale), and the same addressing pulse is subsequently applied to the electro-optic display resulting in a second optical state (e.g., a second grayscale). 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 an offset in the optical state.

The "drift" of the optical state of a display over time refers to the change in the optical state of an electro-optic display when the display is stationary (e.g., during a period when no addressing pulse is applied to the display). Since the optical state of a pixel may depend on the pixel's residual voltage, and the residual voltage of a pixel may decay over time, residual voltage can cause the optical state to drift.

As mentioned above, "ghosting" refers to the situation where traces of a previous image remain visible after an electro-optic display has been rewritten. Residual voltage can cause "edge ghosting," a type of ghosting where the outline (edge) of a portion of the previous image remains visible.

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

Figure 1 illustrates 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, the imaging film 110 may be bistable. In some embodiments, the 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 rear 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 rear electrode 104.

Pixel 100 may be one of a plurality of pixels. These 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," wherein each pixel is associated with at least one nonlinear circuit element 120. The nonlinear circuit element 120 may be coupled between a backplane electrode 104 and an address electrode 108. In some embodiments, the nonlinear element 120 may include diodes and/or transistors, including but not limited to metal-oxide-semiconductor field-effect transistors (MOSFETs). 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 address electrode 108, and the gate 106 of the MOSFET may be coupled to a driver and configured to control the 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. However, those skilled in the art will recognize that in some embodiments, the source and drain of the MOSFET may be interchanged.)

In some embodiments of the active matrix, the addressing 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 that can select one or more rows of pixels by applying a voltage sufficient to activate the nonlinear elements 120 of all pixels 100 in the selected row. The column electrodes may be connected to a column driver that can apply a voltage suitable for driving the pixel to a desired optical state on the transistor gates 106 of the selected (activated) pixel. The voltage applied to the addressing electrodes 108 may be relative to the voltage applied to the front panel electrodes 102 of the pixel (e.g., approximately zero volts). In some embodiments, the front panel 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 can be written row by row. For example, a row driver can select a row of pixels, and a column driver can apply a voltage to the pixel corresponding to the desired optical state of that row of pixels. After a pre-selection interval known as "row address time," the selected row can be deselected, another row can be selected, and the voltage on the column driver can be changed so that another row of the display is written.

Figure 2 illustrates a circuit model of an electro-optic imaging layer 110 according to the subject matter presented herein, disposed between a front electrode 102 and a rear electrode 104. Resistors 202 and capacitors 204 may represent the resistance and capacitance of the electro-optic imaging layer 110, the front electrode 102, and the rear electrode 104 (including any adhesive layers). Resistors 212 and capacitors 214 may represent the resistance and capacitance of the laminated adhesive layer. Capacitor 216 may represent the capacitance that may be formed between the front electrode 102 and the rear electrode 104, for example, at the interfacial contact region between layers, such as the interface between the imaging layer and the laminated adhesive layer and/or the interface between the laminated adhesive layer and the backplane electrode. The voltage Vi on the imaging film 110 of a pixel may include the residual voltage of the pixel.

The discharge of residual voltage in a pixel can 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 in Figures 3 and 4-8 below.

Figure 3 illustrates an exemplary driving method 300 according to the subject matter disclosed herein. Typically, post-drive discharge of 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 sufficiently increases the transconductance of the pixel transistor, thereby allowing 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 sufficiently large and applied to the gate of the transistor 120 associated with the display pixel such that the transistor conducts current and drives the display pixel) for selecting rows 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 by reference, the value of the discharge voltage may be selected to be small in amplitude but large enough to induce sufficient pixel transistor conductance to allow residual voltage to drain from the display pixel. The discharge voltage may be constant or may vary over time. 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 over a specified post-drive discharge time. Specifically, the gate voltage can be set to the desired discharge voltage over two or more time periods during the post-drive time span, and at different voltages for the remainder of the post-drive discharge time. In practice, in some embodiments, multiple alternating voltages can be used instead of a single different voltage. However, it should be understood that these alternating voltages may be expected to be less induced in the pixel thin-film transistors compared to when the discharge voltage is applied. In use, this means that the different voltage or alternating voltage values are somewhere in the range between the discharge voltage and the gate cutoff voltage used during a typical display scan, including the gate cutoff voltage. While a convenient alternating voltage could be zero volts, in which case zero volts is the same voltage held by the source line during this discharge time period, it may be advantageous to have the alternating voltages have opposite signs or polarities to the discharge voltage. The advantage here is that voltages with opposite signs can at least partially offset the stress caused by the voltage applied to the transistor by the drive voltage.

The subject matter disclosed herein introduces several advantages, one of which is the reduction of TFT transconductance stress during residual voltage discharge when a discharge voltage is applied to the TFT gate. TFT transconductance stress accumulates over time and leads to display performance degradation. The driving method described herein can reduce the combined time of the discharge voltage applied to the TFT, and to some extent, preserves the effectiveness of post-drive discharge better than alternatives, such as reducing discharge voltage stress simply by reducing the time of post-drive discharge.

Furthermore, by dividing the post-discharge process into multiple segments with different voltage values, in some cases, one of these segments may have a voltage level carrying an amplitude opposite to that of the discharge segment (e.g., a negative voltage compared to the positive voltage during the TFT discharge segment). In this configuration, at least part of the accumulated transconductance stress can be backed up or reduced, thereby improving the reliability and performance of the TFT.

As shown in Figure 3, one embodiment of a driving method for discharging residual charge to reduce residual voltage may include three driving sections or time intervals 302, 304, and 306. In time interval 302, a discharge voltage V _{_PDD} 308 may be applied to the pixel transistor to create a conduction path for discharging residual charge. In some embodiments, the value of the discharge voltage V _{_PDD} 308 may be small in amplitude but large enough to induce sufficient pixel transistor conductance to allow residual voltage to drain from the pixel. During time interval 302, when the discharge voltage V _{_PDD} 308 is applied, the pixel voltage V _{_pixel} may be zero during this time interval, and the residual charge dissipates from the pixel through the _discharge current J. Subsequently, during a dwell time period 304, the discharge voltage V _{_PDD} may be set to equal the nominal gate cutoff voltage 310, which makes the pixel voltage V _{_pixel} a zero current value, and at this time, the pixel current J _discharges to zero and no residual charge dissipates. After this dwell time period 304, the pixel voltage V _{_PDD} 308 may be turned on again to the nominal discharge voltage 312 in another discharge time period 306. During this second discharge period, additional residual charge can be dissipated.

In some other embodiments, instead of changing the pixel voltage _VPDD to the nominal gate cutoff voltage as described above, the pixel voltage _VPDD can be set to zero volts, and the discharge cycle can oscillate between the nominal discharge voltage and the zero-volt level, as shown in Figure 4. It should be understood that the segment durations and dwell times of the discharge cycle can vary depending on the application. For example, as shown in Figure 5, discharge cycle 404 can be preset to have a 40% duty cycle (i.e., the complete duty cycle can be the sum of cycles 402 and 404).

In some other embodiments, the nominal gate cutoff voltage may have a longer duration than the discharge voltage V _PDD . For example, as shown in FIG6, the nominal gate cutoff voltage 604 may have a 60% duty cycle, while the discharge voltage V _PDD 602 may have a 40% duty cycle.

In yet another embodiment, the driving scheme may include discharge voltage V _PDD and nominal gate cutoff voltage of different durations. This means that within a driving sequence, the discharge voltage V _PDD period and/or the gate cutoff voltage period can differ in duration to suit a specific display application. For example, as shown in FIG7, the duration of discharge voltage period 702 can be longer than the duration of discharge voltage period 706. Similarly, the duration of the gate cutoff voltage period can also differ. For example, as shown in FIG8, not only do the discharge voltage V _PDD periods have different durations (e.g., period 802 has a longer duration than period 806, and period 806 itself has a longer duration than period 808), the gate cutoff voltage periods can also have different durations (e.g., period 810 has a longer duration than period 804). Furthermore, the duration variations within the aforementioned periods can be inherently irregular.

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. Therefore, the foregoing description as a whole should be interpreted as exemplary rather than restrictive.

Claims (19)

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: A first voltage is applied to the transistor associated with the display pixel for a first duration to drain residual voltage from the display pixel; A second voltage is applied to the transistor during a second duration to stop residual voltage discharge from the display pixel; and A third voltage is applied to the transistor during a third duration to remove residual voltage from the display pixel; The length of the second duration is configured to reduce stress on the transistor. 2. The method according to claim 1, wherein the first voltage is a gate turn-on voltage. 3. The method according to claim 2, wherein the third voltage is a gate turn-on voltage. 4. The method according to claim 1, wherein the second voltage is zero volts. 5. The method of claim 1, wherein the length of the first duration is the same as the second duration. 6. The method of claim 1, wherein the length of the first duration is the same as the third duration. 7. The method of claim 1, wherein the length of the second duration is the same as the third duration. 8. The method of claim 1, wherein the length of the first duration is different from the second duration. 9. The method of claim 1, wherein the length of the first duration is different from the third duration. 10. The method of claim 1, wherein the second voltage has a voltage polarity opposite to that of the first voltage. 11. The method of claim 1, wherein the second voltage has a voltage polarity opposite to that of the third voltage. 12. The method of claim 1, wherein the second voltage is a nominal gate cutoff voltage. 13. The method of claim 1, further comprising applying a fourth voltage to the transistor for a fourth duration to stop residual voltage discharge from the display pixel. 14. The method of claim 13, wherein the length of the fourth duration is configured to reduce stress in the transistor. 15. The method of claim 13, further comprising applying a fifth voltage to the transistor for a fifth duration to drain residual voltage from the display pixel. 16. The method of claim 15, wherein the fourth duration has a different length than the fifth duration. 17. The method of claim 15, wherein the length of the fourth duration is the same as that of the fifth duration. 18. The method of claim 15, wherein the fourth duration has a length different from the second duration. 19. The method of claim 15, wherein the length of the fourth duration is the same as the second duration.