Electro-optic displays and methods for discharging remnant voltage using backlight

Abstract

A method and apparatus for reducing residual voltage in an electro-optic display. The display includes a transparent front electrode; an electro-optic material layer; a pixel electrode; a thin-film transistor positioned adjacent to the electro-optic material; a light source positioned adjacent to the thin-film transistors; and a display driver coupled to the light source and a gate line and a source line of each thin-film transistor. Each thin-film transistor includes a photosensitive semiconductor region. The electro-optic material layer and the light source are disposed on opposite sides of the thin-film transistors. The display driver is configured to apply substantially the same voltage to the front electrode and the source line of the thin-film transistors and to initiate a drive signal to the light source to emit light of sufficient intensity to activate the thin-film transistors, thereby establishing a conduction path for discharging residual voltage.

Description

Technical Field

[0001] [Reference Materials for Comparison of Related Applications] All contents of any patent, published application or other published work mentioned herein are incorporated herein by reference.

[0002] This invention relates to reflective electro-optic displays and materials used in such displays. More specifically, this invention relates to displays with reduced residual voltage and methods for reducing residual voltage in electro-optic displays. Prior Technology

[0003] Electro-optic displays driven by DC unbalanced waveforms may generate residual voltage, which can be determined by measuring the open-circuit electrochemical potential of the display pixels. In some cases, the residual voltage may drift in the measured optical state of the display when it receives the same update command after a period of time. It has been found that, causally, residual voltage is a more prevalent phenomenon in electrophoretic and other pulse-driven electro-optic displays. It has also been found that DC imbalance may lead to a reduced long-term lifetime of electrophoretic displays, for example, due to electrochemical reactions at the interface between the pixel electrodes and the electro-optic medium or adhesive layer.

[0004] The term "residual voltage" is sometimes used as a convenient term to refer to the overall phenomenon. However, the switching behavior of pulse-driven electro-optic displays is based on the application of voltage pulses (the integral of voltage with respect to time) onto the electro-optic medium. After the application of the driving pulse, the residual voltage may immediately reach its peak, and then decay substantially exponentially. The persistent presence of residual voltage over a considerable period of time applies a "residual pulse" to the electro-optic medium. Strictly speaking, this residual pulse, rather than the residual voltage itself, is likely the cause of the optical state of the electro-optic display, and it is usually considered to be caused by the residual voltage.

[0005] Theoretically, the effect of residual voltage should directly correspond to the residual pulse. However, in practice, pulse switching patterns lose accuracy at low voltages. Some electro-optic media have a critical value such that a residual voltage of approximately 1V after a drive pulse may not cause a significant change in the optical state of the medium. However, for other electro-optic media (including the preferred electrophoretic media used in the experiments described herein), a residual voltage of approximately 0.5V may cause a significant change in the optical state. Therefore, the actual results of two equivalent residual pulses may differ, and this may help to increase the critical value of the electro-optic media to reduce the effect of residual voltage. E Ink has produced electrophoretic media with a “small critical value” sufficient to prevent residual voltage experienced in certain situations from immediately altering the displayed image after a drive pulse. If the critical value is insufficient or if the residual voltage is too high, the display may exhibit kickback / self-erase or self-improvement phenomena. Here, 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 the discharge of the pixel’s residual voltage.

[0006] Even when residual voltages are below a small critical value, they can still significantly impact image switching if they persist during the next image update. For example, suppose a + / -15V drive voltage is applied during an image update of an electrophoretic display to move electrophoretic particles. If a +1V residual voltage persists from the previous update, the drive voltage will effectively transition from +15V / -15V to +16V / -14V. As a result, depending on whether the pixel has a positive or negative residual voltage, the pixel will be biased towards a darker or whiter state. Furthermore, this effect varies over time due to the decay rate of residual voltage. Electro-optical material in a pixel switched to white immediately after the last image update using a 15V, 300ms drive pulse might actually experience a waveform close to 16V for 300ms, while material in a pixel switched to white one minute later using the exact same drive pulse (15V, 300ms) might actually experience a waveform close to 15.2V for 300ms. Therefore, the pixel might display a noticeably different white hue.

[0007] If a residual voltage field is generated on multiple pixels by a previous image (e.g., a dark line on a white background), the residual voltage may also be configured on the display in a similar pattern. Furthermore, in practice, the most noticeable effect of residual voltage on display performance may be ghosting. This is in addition to the previously mentioned problem (i.e., DC imbalance (e.g., 16V / 14V instead of 15V / 15V) can be the cause of a slow decline in the lifetime of the electro-optical medium).

[0008] Of course, rapid residual voltage discharge can lead to unexpected behavior. For example, if the residual voltage decays slowly and almost constantly, its effect on waveform shift will not differ with image updates, and the display may actually experience less ghosting (also known as image persistence or staining) than with rapidly decaying residual voltage. Therefore, the ghosting experienced when updating one pixel after 10 minutes and another after 11 minutes is far less than the ghosting experienced when updating one pixel immediately and another after 1 minute. Conversely, if the residual voltage decays so quickly that it approaches zero before the next update occurs, it may not actually cause detectable ghosting.

[0009] There are several potential sources of residual voltage. One major cause of residual voltage is believed (although some embodiments are not limited to this view) to be ionic polarization within the layers of materials that make up the display.

[0010] In summary, residual voltage can manifest as image ghosting or visual artifacts in various ways, and its severity varies with the elapsed time 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 impair the quality of electrophoresis or other electro-optical devices, and it is desirable to minimize the sensitivity of residual voltage itself and the optical state of the device to its influence. Thus, even when the residual voltage is already very low, discharging the residual voltage in an electro-optical display can improve the quality of the displayed image.

[0011] Traditional techniques for reducing residual voltage involve setting the front and rear electrodes of a pixel to approximately the same voltage and simultaneously activating the pixel's transistors for a specific period and / or until the amount of residual voltage remaining in the pixel is less than a critical value. However, the select line control circuitry of a standard active matrix driver chip typically does not include control signals to drive all gate lines to the level required to bring all pixel transistors to the aforementioned on state. Therefore, conventional techniques must utilize specially designed select line driver chips with input control lines that allow external signals to impose a condition where all select line outputs receive the voltage supplied to the select driver, thus turning on all pixel transistors. In practice, this dedicated voltage control and related electronics increase design overhead and significantly increase the cost of enabling the discharge mechanism. Therefore, there is a need for easier-to-implement and more cost-effective techniques for discharging residual voltage in electro-optical displays. Summary of the Invention

[0012] The subject matter presented herein provides a display with reduced residual voltage and a method for reducing residual voltage in an electro-optical display.

[0013] In one embodiment, the present invention features an electro-optic display comprising a transparent front electrode; an electro-optic material layer; a pixel electrode array; and a thin-film transistor array positioned adjacent to a surface of the electro-optic material layer. Each thin-film transistor includes a photosensitive semiconductor region, and each pixel electrode is coupled to a single thin-film transistor. The electro-optic display also includes at least one light source positioned adjacent to the thin-film transistor array. The electro-optic material layer and the at least one light source are disposed on opposite sides of the thin-film transistor array. The electro-optic display also includes a display driver coupled to a gate line and a source line of the at least one light source and each thin-film transistor. Only one thin-film transistor is operatively connected between the display driver and each pixel electrode. The display driver is configured to apply substantially the same voltage to the transparent front electrode and the source line of each thin-film transistor. The display driver is also configured to initiate a drive signal to the at least one light source to emit light of sufficient intensity to activate the thin-film transistor array, thereby establishing a conduction path for discharging residual voltage.

[0014] In another embodiment, the invention is characterized by a method for reducing residual voltage in an electro-optic display. The method includes: providing an electro-optic display comprising a transparent front electrode; an electro-optic material layer; a pixel electrode array; and a thin-film transistor array positioned adjacent to a surface of the electro-optic material layer. Each thin-film transistor includes a photosensitive semiconductor region, and each pixel electrode is coupled to a single thin-film transistor in the thin-film transistor array. The electro-optic display also includes at least one light source positioned adjacent to the thin-film transistor array; and a display driver coupled to a gate line and a source line of each thin-film transistor. The electro-optic material layer and the at least one light source are disposed on opposite sides of the thin-film transistor array, and only one thin-film transistor is operatively connected between the display driver and each pixel electrode. The method also includes applying substantially the same voltage from the display driver to the transparent front electrode and the source line of each thin-film transistor. The method also includes initiating a drive signal from the display driver to the at least one light source to emit light of sufficient intensity to activate the thin-film transistor array, thereby establishing a conduction path for discharging residual voltage.

[0015] The present invention may include one or more of the following features. In some embodiments, the electro-optic material layer includes an electrophoretic dielectric. In some embodiments, the electrophoretic dielectric includes charged particles that move in the presence of an electric field. In some embodiments, the thin-film transistor array includes n-type thin-film transistors, and the display driver applies a ground potential to the front-transmitting electrode and the source line of each thin-film transistor. In some embodiments, the thin-film transistor array includes p-type thin-film transistors, and the display driver applies a positive voltage to the front-transmitting electrode and the source line of each thin-film transistor.

[0016] In some embodiments, the display driver applies a voltage between 5V and 15V to the front light-transmitting electrode and the source line of each thin-film transistor.

[0017] In some embodiments, the thin-film transistor array is formed on a light-transmitting substrate. In some embodiments, the at least one light source is positioned to emit light onto the thin-film transistor array. In some embodiments, the at least one light source performs optical communication with the photosensitive semiconductor region of each thin-film transistor. In some embodiments, the at least one light source is positioned to emit light onto the gate line of each thin-film transistor. In some embodiments, the at least one light source emits light onto an unshielded portion of each thin-film transistor. In some embodiments, the at least one light source emits light onto a portion of each thin-film transistor that is not shielded by light-shielding material.

[0018] In some embodiments, the electro-optic display includes a light guide positioned between the at least one light source and the thin-film transistor array. In some embodiments, the electro-optic display includes an optical diffuser positioned between the at least one light source and the thin-film transistor array. In some embodiments, the electro-optic display includes one or more of a light guide plate, a diffuser sheet, a prism film, and a polarizing plate positioned between the at least one light source and the thin-film transistor array.

[0019] In some embodiments, the drive signal is one of a DC voltage, an AC voltage, and a pulse modulation signal.

[0020] In some embodiments, the at least one light source is configured to emit light with an intensity of at least 900 lux in response to the drive signal. In some embodiments, the at least one light source is configured to emit light with an intensity of at least 1300 lux in response to the drive signal. In some embodiments, the at least one light source is configured to emit light with an intensity of at least 1700 lux in response to the drive signal.

[0021] In some embodiments, the at least one light source includes a plurality of light-emitting diodes positioned along an outer edge of the electro-optical display device. In some embodiments, the at least one light source includes an array of light-emitting diodes positioned in conjunction with the thin-film transistor array. Simple Explanation of the Diagram

[0022] Figure 1 is a schematic diagram of an electrophoresis display; Figure 2 shows the circuit model of the electro-optic imaging layer; Figure 3A shows a cross-sectional view illustrating an exemplary embodiment of an electrophoretic display with a side-lit backlight for an object presented herein; Figure 3B is a cross-sectional view illustrating an exemplary embodiment of an electrophoretic display with an array-type backlight based on the subject matter presented herein; Figure 4A is a schematic diagram of a driving embodiment according to an exemplary embodiment of the present invention, wherein thin-film transistors (TFTs) are exposed to light between display updates; Figure 4B is a schematic diagram of a driving embodiment according to the present invention, wherein after the display is updated, the TFTs are successively exposed to light and enter a floating state; Figure 4C is a schematic diagram of a driving embodiment according to the present invention, wherein after a display update, the TFTs are brought into a floating state and then exposed to light; and Figure 5 is a graph illustrating the residual voltage discharge performance of various discharge methods. Implementation

[0023] The term "electro-optic," applied to materials or displays, is used here in its conventional sense in imaging technology to refer to a material having first and second display states that differ in at least one optical property, wherein the material is changed from its first display state to its second display state by applying an electric field to the material. While the optical property is typically color perceptible to the human eye, it can be other optical properties, such as light transmission, reflectivity, and luminance, or, in the case of a display intended for machine reading, pseudo-color in the sense of a change in reflectivity at electromagnetic wavelengths outside the visible light range.

[0024] The term "gray state" is used here in its conventional sense in imaging technology to refer to the state between the two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several E Ink patents and published applications mentioned below describe electrophoretic displays, where the extreme states are white and dark blue, such that the intermediate "gray state" is actually light blue. More precisely, as mentioned earlier, the change in optical state may not be a color change at all. The terms "black" and "white" can 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 black and white at all, such as the aforementioned white and dark blue states. The term "monochrome" is used here to refer to a driving scheme that drives pixels only to their two extreme optical states that do not have an intermediate gray state.

[0025] The electro-optical display described herein includes one or more pixels whose optical state can be driven by 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 refer to the entire voltage-to-time curve used to achieve the transition from a particular initial grayscale to a particular final grayscale. Typically, such a waveform will include a plurality of waveform elements; these elements are substantially rectangular (i.e., a given element includes the application of a fixed voltage for a period of time); these 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. In some embodiments, a waveform or a drive waveform may include a plurality of drive pulses configured to drive a display pixel to a desired optical state. Between the plurality of drive pulses, the display pixel may be kept in a floating state. In some embodiments, when the display is in a floating state, the transistor of the display pixel (e.g., see element 120 below Figure 1) may be in a non-conductive state; for example, the gate voltage of the pixel's transistor may be low.

[0026] In practice, a display can use more than one driving scheme; for example, U.S. Patent No. 7,012,600 teaches that a driving scheme may need to be modified based on parameters such as the display's temperature or the duration of its operation during use, thus the display can provide multiple different driving schemes for different temperatures, etc. A set of driving schemes used in this way can be called a "set of related driving schemes". As described in the aforementioned MEDEOD applications, more than one driving scheme can also be used simultaneously in different areas of the same display, and a set of driving schemes used in this way can be called a "set of synchronized driving schemes".

[0027] Some electro-optic materials are solid in the sense that they have a solid outer surface, but they can and often have internal liquid or gas-filled spaces. For convenience, such displays using solid electro-optic materials will be referred to as "solid-state electro-optic displays" in the following text. 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.

[0028] The terms "bistable" and "bistability" are used herein in their conventional sense to refer to a display comprising display elements having at least one different optical characteristic in first and second display states, such that after any given element has been driven by an addressing pulse of finite duration, it presents its first or second display state; after the addressing pulse is terminated, that state persists at least several times (e.g., at least four times); and a minimum duration of the termination pulse is required to change the state of the display element. U.S. Patent No. 7,170,670 shows that some particle-based electrophoretic displays with grayscale capability 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 called "multistable" rather than bistable, but for convenience, the term "bistable" may be used herein to encompass both bistable and multistable displays.

[0029] Several types of electro-optic displays are known. One type of electro-optic display is, for example, the rotating bichromal member type described in U.S. Patents 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 called a "rotating bichromatic ball" display, the term "rotating bichromal member" is more accurate and preferred because, in some of the aforementioned patents, the rotating member is not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) having two or more regions with different optical properties and an internal dipole. These objects are suspended within liquid-filled vacuoles (which are filled with liquid) within a matrix, allowing them to rotate freely. The appearance of the display can be altered by applying an electric field, thus rotating the objects to different positions and changing which areas of the objects are visible through a viewing surface. This type of electro-optic medium is typically bistable.

[0030] Particle-based electrophoretic displays have been a subject of intense research for years, in which multiple charged particles move through a liquid under the influence of an electric field. Compared to liquid crystal displays (LCDs), electrophoretic displays offer advantages such as good brightness and contrast, wide viewing angles, dual stability, and low power consumption. However, long-term image quality issues have hindered their widespread adoption. For example, the particles constituting an electrophoretic display tend to settle, leading to a shorter lifespan for these displays.

[0031] As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid system is liquid, but gaseous fluids can be used to manufacture electrophoretic media; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Patents 7,321,459 and 7,236,291. When the medium is used in an orientation that allows such sedimentation (e.g., in the performance of media arranged in a vertical plane), such gas-based electrophoretic media appear to be susceptible to the same type of problems as liquid-based electrophoretic media due to particle sedimentation. In fact, the problem of particle sedimentation appears to be more severe in gas-based electrophoresis media than in liquid-based electrophoresis media, because the lower viscosity of gaseous suspensions allows for faster sedimentation of electrophoretic particles.

[0032] Numerous patents and applications, transferred to or under the name of the Massachusetts Institute of Technology (MIT) and E Ink Corporation, describe various techniques used in encapsulating electrophoretic and other electro-optic media. Such encapsulation media comprise a plurality of microcapsules, each microcapsule comprising an internal phase of electrophoretically moving particles contained in a fluid medium and a capsule wall surrounding the internal phase. Typically, these capsules are themselves contained within a polymeric binder to form a coherent layer between two electrodes. The techniques described in these patents and applications include:

[0033] (a) Electrophoretic particles, fluids, and fluid additives; see, for example, U.S. Patent Nos. 7,002,728 and 7,679,814;

[0034] (b) Capsules, adhesives, and encapsulation processes; see, for example, U.S. Patent Nos. 6,922,276 and 7,411,719;

[0035] (c) Microunit structures, wall materials, and methods of forming microunits; see, for example, U.S. Patent Nos. 7,072,095 and 9,279,906;

[0036] (d) Methods for filling and sealing microcells; see, for example, U.S. Patent Nos. 7,144,942 and 7,715,088;

[0037] (e) Thin films and subassemblies containing electro-optic materials; see, for example, U.S. Patent Nos. 6,982,178 and 7,839,564;

[0038] (f) Backplanes, adhesive layers, and other auxiliary layers used in displays, and methods thereof; see, for example, U.S. Patent Nos. 7,116,318 and 7,535,624;

[0039] (g) Color formation and color adjustment; see, for example, U.S. Patent Nos. 7,075,502 and 7,839,564;

[0040] (h) Applications to displays; see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348;

[0041] (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

[0042] 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; 7,242,514; 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;7,68 8,297;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;8,5 58,783;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;8,810,52 5; 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 No. 2003 / 0102858;2004 / 0246562; 2005 / 0253777; 2007 / 0070032; 2007 / 0076289; 2007 / 0091418; 2007 / 0103427; 2007 / 0176912; 2007 / 0296452; 2008 / 0024429; 2008 / 0024482; 2008 / 0136774; 2008 / 0169821; 2008 / 0218471; 2008 / 0291129; 2008 / 03 03780;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;2 012 / 0001957;2012 / 0098740;2013 / 0063333;2013 / 0194250;2013 / 0249782;2013 / 0321278;2014 / 0009817;2014 / 0085355;2014 / 0204012;2014 / 0218277;2014 / 0240210;2014 / 0240373;2014 / 0253425;2014 / 0292830;2014 / 029 3398;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.

[0043] Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thereby producing a so-called polymer dispersion electrophoretic display, wherein the electrophoretic medium comprises electrophoretic fluids of a plurality of discrete droplets and a continuous phase of polymeric material, and even without discrete capsule membranes associated with each individual droplet, the electrophoretic fluids of the discrete droplets within such a polymer dispersion electrophoretic display can be considered as capsules or microcapsules; see, for example, U.S. Publication No. 2002 / 0131147. Thus, for the purposes of this application, such polymer dispersion electrophoretic media are considered a subtype of encapsulated electrophoretic media.

[0044] One related type of electrophoretic display is the so-called "microcell electrophoretic display." In a microcell electrophoretic display, instead of encapsulating charged particles and suspensions in microcapsules, they are held in a plurality of cavities formed within a carrier medium (e.g., a polymeric membrane). See, for example, International Application Publication No. WO 02 / 01281 and U.S. Application Publication No. 2002 / 0075556, both patents assigned to Sipix Imaging Inc.

[0045] Many of the aforementioned E Ink and MIT patents and applications also consider microcell electrophoretic displays and polymer dispersion 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.

[0046] Another type of electro-optic display is the electro-wetting display developed by Philips, described in Hayes, RA, et al., “Video-Speed ​​Electronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003). U.S. Patent No. 7,420,549 shows that such an electro-wetting display can be fabricated to be bistable.

[0047] Other types of electro-optic materials can also be used. Of particular interest are bistable ferroelectric liquid crystal displays (FLCs), which are known in this technology and have exhibited residual voltage behavior.

[0048] Although the electrophoretic medium may be opaque (because, for example, in many electrophoretic media, particles substantially block the transmission of visible light through the display) and operate in reflective mode, some electrophoretic displays can operate in a so-called "shutter mode," in which one display state is substantially opaque and another display state is transparent. See, for example, U.S. Patents 6,130,774 and 6,172,798 and U.S. Patents 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 4,418,346. Other types of electro-optic displays can also operate in grating mode.

[0049] High-resolution displays may include individual pixels that can be addressed without interference from adjacent pixels. One method of obtaining such pixels provides an array of nonlinear circuit elements (e.g., transistors or diodes), with at least one nonlinear circuit element associated with each pixel to produce an "active matrix" display. An address or pixel electrode is used to address a pixel, the address or pixel electrode being connected to a suitable voltage source via the associated nonlinear circuit element. When the nonlinear circuit element is a transistor, the pixel electrode may be connected to the drain of the transistor, and this configuration will be assumed in the following description, but it is essentially arbitrary, and the pixel electrode may be connected to the source of the transistor. In a high-resolution array, pixels are configured in a two-dimensional array of columns and rows, such that any particular pixel is uniquely defined by the intersection of a particular column and a particular row. The sources of all transistors in each row are connected to a single row electrode, and the gates of all transistors in each column are connected to a single column electrode; furthermore, the source-to-column and gate-to-row assignments may be reversed if desired.

[0050] The display can be written column by column. The column electrodes are connected to a column driver that applies voltage to a selected column electrode to ensure all transistors in the selected column are conducting, while simultaneously applying voltage to all other columns to ensure all transistors in the unselected columns remain de-conducting. The row electrodes are connected to row drivers that apply voltages selected to drive pixels in a selected column to their desired optical state to different row electrodes. (The aforementioned voltages are relative to a common front electrode, which is located on the opposite side of the electro-optic medium away from the nonlinear array and extends across the entire display. As known in this art, voltages are relative and are a measure 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 preselection interval called "line address time," a selected column is deselected, the next column is selected, and the voltage on the row drivers is changed to write the next line of the display.

[0051] However, during use, certain waveforms may generate residual voltages on the pixels of the electro-optic display. As can be clearly seen from the discussion above, these residual voltages produce several unwanted optical effects and are generally undesirable.

[0052] As illustrated here, "transition" in the optical state associated with an addressing pulse refers to a situation where applying a specific addressing pulse to an electro-optic display for the first time results in a first optical state (e.g., a first grayscale), and subsequently applying the same addressing pulse to the electro-optic display results in a second optical state (e.g., a second grayscale). Residual voltage may cause the transition of optical states because the voltage applied to a pixel of the electro-optic display during the application of an addressing pulse includes the sum of the residual voltage and the voltage of the addressing pulse.

[0053] The "drift" of the optical state of a display over time refers to a situation where the optical state of an electro-optic display changes when the display is at rest (e.g., during a period of time when no addressing pulse is applied to the display). Residual voltage can cause the drift of the optical state because 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.

[0054] As mentioned above, "ghosting" refers to a situation where traces of the previous image are still visible after the electro-optical display has been rewritten.

[0055] "Edge ghosting" is another type of ghosting where the outline (edge) of a portion of the previous image remains visible. This type of artifact is caused by inter-pixel effects (so-called "image blooming"). For example, in monochrome and color systems, the electric field generated by the pixel electrodes often affects a wider electro-optical medium area than the pixel electrodes themselves, such that the optical state of one pixel actually diffuses to parts of adjacent pixels. Furthermore, in some cases, driving adjacent pixels can cause the final optical state of the region between pixels to differ from the optical state achieved by any one of the adjacent pixels themselves. The final optical state of the region between adjacent pixels is caused by the electric field experienced by the region between pixels, which is the average of the electric fields applied to adjacent pixels. It has been found that using a DC unbalanced waveform to drive an electro-optical display can reduce edge ghosting. However, as mentioned above, a DC unbalanced waveform may produce residual voltage.

[0056] In this document, the term "optical back-kick" is used to describe the change in the optical state of a pixel that occurs at least in part due to the discharge of the pixel's residual voltage.

[0057] Figures 1 and 2 respectively provide a high-order schematic diagram of a pixel of an electro-optic display and a corresponding equivalent circuit model of the components and interface of an electro-optic imaging layer that may have residual voltage.

[0058] Figure 1 shows a simplified 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 contain, for example, charged pigment particles.

[0059] 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 surface of the display. In some embodiments, the front electrode 102 may be transparent and 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, parasitic capacitance (not shown) may be formed between the front electrode 102 and the rear electrode 104.

[0060] Pixel 100 can be one of a plurality of pixels. The plurality of pixels can be configured in a two-dimensional array of columns and rows to form a matrix such that any particular pixel is uniquely defined and / or driven by the intersection of a specified column and a specified row. In some embodiments, the pixel matrix can be an "active matrix" in which each pixel is associated with at least one nonlinear circuit element 120. The nonlinear circuit element 120 can be coupled between the back electrode 104 and the address electrode 108. In some embodiments, the nonlinear circuit element 120 can include diodes and / or transistors, including but not limited to MOSFETs. The drain (or source) of the MOSFET can be coupled to the back electrode 104, the source (or drain) of the MOSFET can be coupled to the address electrode 108, and the gate of the MOSFET can be coupled to a driver electrode 106 configured to control the start and stop of the MOSFET. (For simplicity, the terminal of the MOSFET coupled to the rear 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 can be interchanged.)

[0061] In some embodiments of the active matrix, the addressing electrodes 108 of all pixels in each row can be connected to the same row electrode, and the driver electrodes 106 of all pixels in each column can be connected to the same column electrode. The column electrodes can be connected to a column driver that can select one or more columns of pixels by applying a voltage sufficient to activate the nonlinear circuitry element 120 of all pixels 100 in the selected column to the selected column electrode. The row electrodes can be connected to a row driver that can apply a voltage suitable for driving the selected (activated) pixel to a desired optical state on the addressing electrode 108. The voltage applied to the addressing electrode 108 can be relative to the voltage applied to the front electrode 102 of the pixel (e.g., approximately zero volts). In some embodiments, the front electrodes 102 of all pixels in the active matrix can be coupled to a common electrode.

[0062] In some embodiments, the pixels 100 of the active matrix can be written column by column. For example, the column driver can select a column of pixels, and the row drivers can apply a voltage corresponding to the desired optical state of the pixels in that column to those pixels. After a preselection interval called "line addressing time", the selected column can be deselected, another column can be selected, and the voltage on the row drivers can be changed to write another line of the display.

[0063] Figure 2 shows a circuit model of an electro-optic imaging layer configured between the front electrode 102 and the rear 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, the front electrode 102, and the rear electrode 104 (including any adhesive layers). Resistor 212 and capacitor 214 may represent the resistance and capacitance of a 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 interlayer interface contact region, such as between the imaging layer and the laminated adhesive layer and / or between the laminated adhesive layer and the backplane electrode. The voltage Vi across the imaging film 110 of the pixel may include the residual voltage of the pixel.

[0064] Using the electro-optical display described below with reference to Figures 3A and 3B, the discharge of residual voltage of a pixel can be initiated and / or controlled by applying any suitable set of signals (including, but not limited to, the set of signals described in more detail in Figures 4A-4C below) to the pixel.

[0065] Figures 3A and 3B are cross-sectional views illustrating exemplary embodiments of the electro-optic display 300 of the subject matter presented herein. Both Figures 3A and 3B show the electro-optic display 300 including a transparent front electrode 302, an electrophoretic medium 310, and a plurality of rear electrodes 304. Figure 3A also includes a backlight 370 arranged in a side-lit configuration, while Figure 3B includes a backlight 370 arranged in an array configuration.

[0066] The transparent front electrode 302 represents the viewing side of the electro-optic display 300 and therefore includes a light-transmitting transparent conductor (e.g., indium tin oxide (ITO)), which in some cases may be deposited on a transparent substrate (e.g., polyethylene terephthalate (PET)). In some embodiments, the transparent front electrode 302 includes a transparent conductor deposited on a glass, plastic, or polyester substrate. The electro-optic display 300 may also include a protective layer (not shown), which may simply protect the transparent front electrode 302 from damage or may surround the entire electro-optic display 300 to prevent liquids such as water from entering.

[0067] Each back electrode 304 is associated with at least one nonlinear circuit element 320. For example, the pixels of the active matrix are typically configured in a two-dimensional array of columns and rows, such that any particular pixel is uniquely defined by the intersection of a designated column and a designated row. The source lines of all nonlinear circuit elements 320 (e.g., 108 in FIG. 1) are connected to a single row (scan or address) line, while the gate lines of all nonlinear circuit elements 320 (e.g., 106 in FIG. 1) are connected to a single column (gate) line. In such a configuration, each drain line of each nonlinear circuit element 320 is connected to a single back electrode 304. (The source-to-column and gate-to-row assignments are conventional but inherently arbitrary and can therefore be reversed if desired. Furthermore, in some embodiments, the connections of the drain and source lines can be reversed.)

[0068] In some embodiments, the nonlinear circuit element 320 and the back electrode 304 can be formed or disposed on the substrate 350 using conventional microfabrication techniques (e.g., lithography, metal deposition, ion implantation, etc.). The substrate 350 can be a rigid or flexible structure made of various plastics, polymer films, metal foils, and glass. In some embodiments, the substrate 350 is a plastic film or other flexible material, and the nonlinear circuit element 320 and the back electrode 304 are printed onto the substrate 350.

[0069] Although each nonlinear circuit element 320 is depicted in Figures 3A and 3B as being placed side-by-side with its corresponding rear electrode 304, other configurations of the backplate 360 ​​are also within the scope of this disclosure. As an example, the backplate 360 ​​may be formed such that each nonlinear circuit element 320 is located below its corresponding rear electrode 304, with an insulating material placed between them.

[0070] The electro-optical display 300 may also include circuitry (not shown) for controlling the timing and voltage levels of waveforms that drive each display pixel to a desired optical state to display an image. This control circuitry may be included in a display controller or processor, in addition to the "selector or gate driver" and "source driver" integrated circuits (ICs) or chips. These devices may be separate chips mounted on the display module, or they may be integrated into a single chip. In some embodiments, the gate line and source line control circuitry is integrated into a single display driver chip, which is coupled to a separate display controller.

[0071] The control circuit described above is electrically coupled to the gate lines and source lines of each non-linear circuit element 320, and changes the voltages applied to these lines to place each non-linear circuit element 320 in an on state (e.g., for an n-type non-linear circuit element 320, VGS > VGS(th); for a p-type non-linear circuit element 320, VGS < VGS(th)), or to place each non-linear circuit element 320 in an off state (e.g., for an n-type non-linear circuit element 320, VGS < VGS(th); for a p-type non-linear circuit element 320, VGS > VGS(th)).

[0072] According to the subject matter presented herein, each non-linear circuit element 320 can be a thin film transistor formed using a photosensitive semiconductor, which can turn on the non-linear circuit element 320 when exposed to a sufficient amount of light. For example, the non-linear circuit element 320 can be an n-type or p-type transistor whose gate can respond simultaneously to an electric field and incident light to which it is exposed, such that a sufficient change in either can control the current flowing through the transistor. In some embodiments, the non-linear circuit element 320 is a phototransistor or a photoFET. In some embodiments, the non-linear circuit element 320 is fabricated such that the incident light used to change the conductivity of the non-linear circuit element 320 is absorbed by one or more of the gate, the semiconductor coupled to the gate, and the junction between the gate and the drain (or source, depending on whether an n-type or p-type transistor is used).

[0073] To prevent ambient light entering the electro-optical display 300 from changing the state of the non-linear circuit element 320 at an unwanted time (e.g., when the display controller actively drives the non-linear circuit element 320 to update the optical state of the display), the components of the non-linear circuit element 320 are typically covered or shielded with an opaque light-blocking material. According to an embodiment of the present invention, the non-linear circuit element 320 can include an unshielded portion 325 that allows light to enter each non-linear circuit element 320 only from the side facing the backlight 370. Additionally, the size and position of the unshielded portion 325 can be designed to allow light to reach only certain photosensitive regions of the non-linear circuit element 320 (e.g., the gate, the semiconductor coupled to the gate, and the junction between the gate and the drain or source, depending on whether an n-type or p-type transistor is used). Further, the backlight 370 can include an opaque or reflective portion to prevent the photosensitive regions of the non-linear circuit element 320 from being exposed to light that is not emitted by the light source 380 (e.g., light emitted by a front light, ambient light, etc.), which could interfere with the operation of the electro-optical display 300.

[0074] Electrophoretic medium 310 contains at least one electrophoretic particle; however, a second, third, fourth, or more electrophoretic particles are also possible. Electrophoretic medium 310 typically includes a solvent (e.g., isoparaffin) and may also include a dispersible polymer and a charge control agent to promote state stability (e.g., bistable), that is, the ability to maintain an electro-optical state without inputting any additional energy.

[0075] Although microcapsules are shown in Figures 3A and 3B, as described above, the electrophoretic medium 310 can be segmented by microcapsules or microcell walls. The electrophoretic medium layer 310 can be coated or imprinted onto a plastic substrate or film with a transparent conductive material coating (e.g., a transparent front electrode 302). This assembly can be laminated to a structure consisting of a substrate 350, nonlinear circuit elements 320, and a rear electrode 304 (collectively referred to as the backplate 360) using a conductive adhesive. Alternatively, the electrophoretic medium layer 310 can be directly dispensed onto an open-cell grid already disposed on the backplate 360. The filled grid can then be top-sealed using an integrated protective sheet / transparent electrode (e.g., the transparent front electrode 302).

[0076] Although not shown, the electro-optic display 300 may also include one or more adhesive layers (e.g., between the transparent front electrode 302 and the electrophoretic medium 310, between the electrophoretic medium 310 and the backplate 360, and between the backplate 360 ​​and the backlight 370) and / or sealing layers as needed. In some embodiments, the adhesive layer may contain a primer component to enhance adhesion to the transparent front electrode 302, or a separate primer layer may be used (not shown in Figures 3A-3B). The structure and components of the electrophoretic display, pigments, adhesives, electrode materials, etc., have been described in numerous patents and patent applications published by E Ink Corporation (e.g., U.S. Patents 6,922,276, 7,002,728, 7,072,095, 7,116,318, 7,715,088, and 7,839,564, the entire contents of which are incorporated herein by reference).

[0077] The backlight 370 includes a light source 380 and an optical modulator 385. The light source 380 is typically one or more light-emitting diodes (LEDs) configured in a side-lit (Fig. 3A) or array (Fig. 3B) configuration. However, those skilled in the art will understand that the light source 380 may include other types of light sources (e.g., electroluminescent panels (ELP), lasers, hot or cold cathode fluorescent lamps, etc.).

[0078] The electro-optical display 300 may also include circuitry (not shown) for controlling the timing and amplitude of the voltage applied to each light source 380 to turn the light source on and off and adjust the intensity of the emitted light. In some embodiments, this control circuitry is included within a display controller or gate and source drivers. In some embodiments, this control circuitry is included within a separate IC coupled to the display controller and / or gate and source drivers. For example, general purpose input / output (GPIO) controlled by the display controller can be used to enable and disable the light source control chip. In some embodiments, the light source control chip is controlled by the display controller via a low-pin-count peripheral interface (e.g., internal integrated circuit (I2C), serial peripheral interface (SPI), controller area network (CAN) bus, etc.). In some embodiments, the timing and amplitude of the voltage applied to the light source 380 can be individually controlled for each device.

[0079] As described above, the backlight 370 includes components for providing light emitted by the light source 380 in a uniform manner. In some embodiments, the light intensity emitted by the backlight 370 is low enough not to distract or imperceptibly attract the attention of the user of the electro-optical display 300. Operating the backlight 370 to emit low-intensity light also reduces power consumption during operation. In some embodiments, the backlight 370 emits light with an illuminance or intensity of approximately 900, 1300, or 1700 lux. In some embodiments, the backlight 370 emits light with an intensity in the range of 200 to 1000 lux.

[0080] Figure 3A shows a backlight 370 with a side-lit configuration, meaning that the light source 380 is configured as a line or row of light sources along the outer edge (e.g., top edge, bottom edge, left edge, right edge) of the electro-optical display 300. Figure 3B shows a backlight 370 with an array configuration, meaning that the light source 380 comprises a plurality of light sources uniformly arranged behind the backlight 360. In some embodiments, the light source 380 is a single light-emitting component, and the elements of the optical modulator 385 uniformly guide and diffuse the light 390 emitted from the light source 380 behind the backlight 360.

[0081] The optical modulator 385 includes one or more components for manipulating and guiding light 390 emitted from the light source 380. For example, the optical modulator 385 may include one or more of the following: a light guide plate, a diffuser sheet, a prism film, and a polarizing plate.

[0082] As shown in Figure 3A, light source 380 emits light 390 at a generally oblique angle to the surface of backplate 360 ​​adjacent to backlight source 370. Therefore, optical modulator 385 may include a light guide to change the direction of the light 390 entering the light guide, so that when it exits the light guide, the light leaves the light guide substantially perpendicular to the surface of backplate 360 ​​adjacent to backlight source 370. Figure 3A shows the light 390 being guided to each nonlinear circuit element 320 of backplate 360. However, in some embodiments, the light guide uniformly guides the light 390 to all or substantially all of the surface of backplate 360 ​​adjacent to backlight source 370. The light guide may be made of an optically transparent material (e.g., polymers such as polycarbonate, polymethyl methacrylate (PMMA), and other acrylic polymers, or materials with similar properties). In addition to the light guide plate, the embodiment of the optical modulator 385 shown in FIG3A may also include one or more of the following: a diffuser sheet, a prism film, and a polarizing plate for further modulating the light 390, as described in more detail below with reference to FIG3B.

[0083] As shown in Figure 3B, each light source 380 emits light 390 substantially perpendicular to the surface of the backplate 360 ​​adjacent to the backlight 370. In some embodiments, the light 390 initially emitted by each light source 380 is sufficient to turn on the nonlinear circuit element 320, as described in more detail below, without the need for additional elements in the optical modulator 385 to adjust the light. However, depending on considerations such as the location and number of light sources 380 present in the backlight 370, the desired uniformity of light, and the overall size of the area illuminated by the emitted light, other elements may be present in the optical modulator 385.

[0084] In some embodiments, the optical modulator 385 includes a diffuser sheet that functions to extend the light at an angle 390 and distribute it uniformly over a wider area of ​​the surface of the backplate 360, as shown by the distribution light 391 in FIG3B. The diffuser sheet may be made of PMMA, other acrylic polymers, polymers such as polycarbonate, polystyrene-based materials, PET, or materials with similar properties.

[0085] In some embodiments, the optical modulator 385 also includes a prism or film located between the diffuser sheet and the backplate 360. The prism film may have ridges on its surface adjacent to the backplate 360 ​​for focusing and redirecting off-axis distributed light 391 and emitting light 392 at an optimal angle (e.g., substantially perpendicular to the surface of the backplate 360 ​​adjacent to the backlight 370) (FIG. 3B) to activate the nonlinear circuit element 320, as described in more detail below. The prism film may be made of PMMA, other acrylic polymers, polymers such as polycarbonate, polystyrene-based materials, PET, or materials with similar properties. In some embodiments, the prism film is formed from acrylic resin on a PET substrate and has a microprism ridge structure with a height of about 10 to 100 μm. In some embodiments, the optical modulator 385 includes one or more polarizing plates for further filtering light entering the backplate 360 ​​from the backlight 370.

[0086] Figure 4A is a schematic diagram 400a of an exemplary embodiment of the present invention, in which TFTs are exposed to light between display updates. As shown, after one or more drive waveforms or signals (i.e., one or more positive and / or negative voltage pulses) are applied to the display pixels to update their optical state (represented as "update time" in Figure 4A), one or more drive signals are transmitted to the backlight 370 to enable the light source 380 to begin emitting light at a predetermined intensity (represented as "exposure time" in Figure 4A). As described above, the drive signals and the associated circuitry for controlling the operation of the light source 380 can be implemented in various ways, for example, by a display controller controlling GPIO or low pin-count peripheral interfaces, which can operate the light source control chip. Those skilled in the art will understand that these exemplary control schemes are not limiting, and other control schemes are also within the scope of this disclosure.

[0087] Immediately before or simultaneously with activating the light source 380, approximately the same voltage is applied to the source lines of the transparent front electrode 302 and the nonlinear circuit element 320. For example, the voltage applied to the transparent front electrode 302 and the source lines of the nonlinear circuit element 320 (commonly referred to as "VCOM") can be approximately 0V or ground. Furthermore, at this time, the gate line of each nonlinear circuit element 320 is set to a voltage suitable for stopping the nonlinear circuit element 320 (e.g., below the threshold voltage VGS(th) of the pixel transistor), thereby effectively interrupting the conduction path or channel between the source and drain lines. In some embodiments, the gate line of each nonlinear circuit element 320 is set to approximately 0V or ground.

[0088] As described above, each nonlinear circuit element 320 may include an unshielded portion 325, the size and location of which are designed to guide light emitted from the light source 380 to the photosensitive semiconductor region of the nonlinear circuit element 320 (e.g., the gate, the semiconductor coupled to the gate, and the gate-source or source junction, depending on whether an n-type or p-type transistor is used). Therefore, although the gate line is set to a voltage suitable for stopping the nonlinear circuit element 320, exposing the photosensitive semiconductor region to light emitted from the light source 380 can activate the nonlinear circuit element 320, thereby effectively establishing a conduction path for discharging residual voltage from the electro-optic display 300.

[0089] As shown in Figure 4A, the "exposure time" of residual voltage discharge can be maintained until the optical state of the electro-optic display 300 is updated at a subsequent update time. Therefore, any post-update residual voltage present on the pixels of the electro-optic display 300 can be eliminated or significantly reduced to a level that will not impair the quality or lifespan of the display.

[0090] Therefore, the electrophoretic display and the corresponding method for reducing residual voltage presented in this paper have advantages over traditional displays and driving methods.

[0091] For example, conventional displays and techniques for reducing residual voltage must utilize specially designed select line driver chips that drive all select line outputs to a voltage level that turns on all pixel transistors, while simultaneously setting the front and rear electrodes of the pixels to approximately the same voltage. For instance, a display with n-type transistors for the nonlinear circuit element 320 must use a select driver with an "Xon" control line input that simultaneously channels a "gate-high" voltage to all select lines. These dedicated driver chips increase design overhead and unit cost to achieve conventional discharge methods. In contrast, as described above, the circuitry controlling the backlight 370 in the electro-optic display 300 can use only GPIO or simple interface connections already widely used on standard display controllers.

[0092] Furthermore, it is known that applying a voltage to the gate of the nonlinear circuit element 320 for an extended period (e.g., by conventional techniques for reducing residual voltage) results in positive bias stress, which causes significant changes in the transfer function characteristics of the nonlinear circuit element 320. These changes represent a shift in the gate critical voltage, a decrease in the subcritical swing, and / or a deterioration in the overall transistor performance. Because the invention and technology described herein do not rely on applying a positive voltage to the gate of the nonlinear circuit element 320, the occurrence of positive bias stress can be eliminated or significantly reduced, thereby improving the accuracy and lifespan of the electro-optical display 300.

[0093] Figure 4B is a schematic diagram 400b of another exemplary embodiment of the present invention, wherein after a display update, nonlinear circuit elements 320 are successively exposed to light and enter a floating state. As shown in schematic diagram 400b, after the initial "exposure time," the pixels of the electro-optic display 300 may be in an electrically floating state (represented by a "floating time"). Any suitable technique can be used to put the pixels in an electrically floating state, including but not limited to setting the gate line voltage of each nonlinear circuit element 320 to a value suitable for interrupting the conduction path (e.g., below the critical voltage value of the pixel transistor) and putting the transparent front electrode 302 in a high-impedance state. Furthermore, during the "floating time," the light source 380 may be turned off.

[0094] As shown in Figure 4B, the "float time" period can be followed by an "exposure time" period and then another "float time" period. The electro-optical display 300 can alternate between the "exposure time" period and the "float time" period any number of times before the next update of the display's optical state (e.g., "update time") occurs. Because the light source 380 is off, the nonlinear circuit element 320 is not actively driven during the "float time" period, so this embodiment can effectively save system power during periods of long time intervals between display updates or during periods when the display is in standby or rest mode (represented by "rest time").

[0095] Figure 4C is a schematic diagram 400c of an exemplary embodiment of the present invention, wherein after a display update, the nonlinear circuit element 320 is brought into a floating state and then exposed to light. In fact, when the nonlinear circuit element 320 is in the floating state, the leakage current is so low that it can be ignored, as if there were an open circuit connection between the nonlinear circuit element 320 and any conduction path. Therefore, especially for the nonlinear circuit element 320, whose semiconductor region is highly sensitive to light, entering the "floating time" period before the "exposure time" period effectively mitigates the short-term backflush after the "update time" period.

[0096] Figure 5 is a graph 500 illustrating the residual voltage discharge performance of various discharge methods. Graph 500 contains three curves showing the relationship between residual voltage (mV) and time (seconds). Curve 501 shows the residual voltage discharge situation without the use of discharge technology. As shown, the residual voltage decays to below 100mV within several hundred seconds.

[0097] Curve 503 illustrates the case of discharging residual voltage using the aforementioned conventional discharge technique, where all gate lines are driven to a voltage that turns on all pixel transistors. Finally, curve 502 illustrates the case of discharging residual voltage using the inventive technique described herein, where the nonlinear circuit element 320 is turned on by exposing its photosensitive semiconductor region to light, thereby reducing the residual voltage. In this case, the emitted light has an intensity of 250 to 750 lux.

[0098] As shown in graph 500, the conventional technique initially reduced the residual voltage at a faster rate than the inventive technique described herein, but over time, the two techniques reduced the residual voltage by roughly the same amount. If needed, the discharge rate of the inventive technique can be increased by increasing the intensity of the light emitted by the backlight 370, while still retaining its advantages over conventional displays and the discharge technique described herein.

[0099] It will be apparent to those skilled in the art that many changes and modifications can be made to the specific embodiments of the present invention described above without departing from the scope of the invention. Therefore, the entire description above should be interpreted in an illustrative rather than restrictive manner.

[0100] This disclosure provides the following specifications and embodiments:

[0101] Clause 1: An electro-optic display comprising: a light-transmitting front electrode; an electro-optic material layer; a pixel electrode array; a thin-film transistor array positioned adjacent to a surface of the electro-optic material layer, each thin-film transistor including a photosensitive semiconductor region, wherein each pixel electrode is coupled to a single thin-film transistor; at least one light source positioned adjacent to the thin-film transistor array, wherein the electro-optic material layer and the at least one light source are disposed on opposite sides of the thin-film transistor array; and a display driver coupled to a gate line and a source line of each thin-film transistor, wherein only one thin-film transistor is operatively connected between the display driver and each pixel electrode, the display driver being configured to: apply substantially the same voltage to the light-transmitting front electrode and the source line of each thin-film transistor; and initiate a drive signal to the at least one light source to emit light of sufficient intensity to activate the thin-film transistor array to establish a conduction path for discharging residual voltage.

[0102] Clause 2: An electro-optical display as described in Clause 1, wherein the thin-film transistor array comprises (i) n-type thin-film transistors and the display driver applies a ground potential to the front-transmitting electrode and the source line of each thin-film transistor, or (ii) p-type thin-film transistors and the display driver applies a positive voltage to the front-transmitting electrode and the source line of each thin-film transistor.

[0103] Clause 3: An electro-optical display as described in Clause 1 or 2, wherein the at least one light source communicates optically with the photosensitive semiconductor region of each thin-film transistor.

[0104] Clause 4: An electro-optical display as described in Clause 3, wherein the at least one light source is positioned to emit light onto the gate line of each thin-film transistor.

[0105] Clause 5: An electro-optical display as described in Clause 4, wherein the at least one light source emits light onto an unshielded portion of each thin-film transistor.

[0106] Clause 6: An electro-optical display as described in any of Clauses 1 to 5 further includes a light guide positioned between the at least one light source and the thin-film transistor array.

[0107] Clause 7: An electro-optical display of any of Clauses 1 to 5 further includes an optical diffuser positioned between the at least one light source and the thin-film transistor array.

[0108] Clause 8: An electro-optic display as described in any of Clauses 1 to 4 further includes one or more of a light guide plate, a diffuser sheet, a prism film, and a polarizing plate, positioned between the at least one light source and the thin-film transistor array.

[0109] Clause 9: An electro-optical display as described in any of Clauses 1 to 8, wherein the driving signal is one of a DC voltage, an AC voltage, and a pulse modulation signal.

[0110] Clause 10: An electro-optical display as described in any of Clauses 1 to 9, wherein the at least one light source is configured to emit light with an intensity of at least 900 lux in response to the drive signal.

[0111] Clause 11: An electro-optical display as described in any of Clauses 1 to 9, wherein the at least one light source is configured to emit light with an intensity of at least 1300 lux in response to the drive signal.

[0112] Clause 12: An electro-optical display as described in any of Clauses 1 to 9, wherein the at least one light source is configured to emit light with an intensity of at least 1700 lux in response to the drive signal.

[0113] Clause 13: An electro-optical display as described in any of Clauses 1 to 12, wherein the at least one light source comprises a plurality of light-emitting diodes positioned along an outer edge of the electro-optical display device.

[0114] Clause 14: An electro-optical display as described in any of Clauses 1 to 12, wherein the at least one light source comprises an array of light-emitting diodes positioned in accordance with the thin-film transistor array.

[0115] Clause 15: A method for reducing residual voltage in an electro-optic display, the method comprising: providing an electro-optic display including a light-transmitting front electrode; an electro-optic material layer; a pixel electrode array; a thin-film transistor array positioned adjacent to a surface of the electro-optic material layer, each thin-film transistor including a photosensitive semiconductor region, wherein each pixel electrode is coupled to a single thin-film transistor in the thin-film transistor array; at least one light source positioned adjacent to the thin-film transistor array, wherein the electro-optic material layer and the at least one light source are disposed on opposite sides of the thin-film transistor array; and a display driver coupled to a gate line and a source line of each thin-film transistor, wherein only one thin-film transistor is operatively connected between the display driver and each pixel electrode; applying substantially the same voltage from the display driver to the light-transmitting front electrode and the source line of each thin-film transistor; and initiating a drive signal from the display driver to the at least one light source to emit light of sufficient intensity to activate the thin-film transistor array to establish a conduction path for discharging residual voltage.

[0116] Clause 16: The method of Clause 15, wherein the thin-film transistor array comprises (i) an n-type thin-film transistor and the display driver applies a ground potential to the front-transmitting electrode and the source line of each thin-film transistor, or (ii) a p-type thin-film transistor and the display driver applies a positive voltage to the front-transmitting electrode and the source line of each thin-film transistor.

[0117] Clause 17: The method of Clause 15 or 16, wherein the at least one light source communicates optically with the photosensitive semiconductor region of each thin-film transistor.

[0118] Clause 18: The method of Clause 17, wherein the at least one light source is positioned to emit light onto the gate line of each thin-film transistor.

[0119] Clause 19: The method of Clause 18, wherein the at least one light source emits light onto an unshielded portion of each thin-film transistor.

[0120] Clause 20: The method of Clause 18, wherein the electro-optical display further includes a light guide positioned between the at least one light source and the thin-film transistor array.

[0121] Clause 21: The method of any one of Clauses 15 to 19, wherein the electro-optic display further includes an optical diffuser positioned between the at least one light source and the thin-film transistor array.

[0122] Clause 22: The method of any one of Clauses 15 to 19, wherein the electro-optic display further includes one or more of a light guide plate, a diffuser sheet, a prism film and a polarizing plate, positioned between the at least one light source and the thin-film transistor array.

[0123] Clause 23: The method of any one of Clauses 15 to 22, wherein the drive signal is one of a DC voltage, an AC voltage and a pulse modulation signal.

[0124] Clause 24: The method of any one of Clauses 15 to 23, wherein initiation further comprises the emission of light having an intensity of at least 900 lux by the at least one light source in response to the drive signal.

[0125] Clause 25: The method of any one of Clauses 15 to 23, wherein initiation further comprises the emission of light having an intensity of at least 1300 lux by the at least one light source in response to the drive signal.

[0126] Clause 26: The method of any one of Clauses 15 to 23, wherein initiation further comprises the emission of light having an intensity of at least 1700 lux by the at least one light source in response to the drive signal.

[0127] Clause 27: The method of any one of Clauses 15 to 20 or 22 to 27, wherein the at least one light source comprises a plurality of light-emitting diodes positioned along an outer edge of the electro-optical display device.

[0128] Clause 28: The method of any one of Clauses 15 to 19 or 20 to 27, wherein the at least one light source comprises an array of light-emitting diodes positioned in accordance with the thin-film transistor array.

[0129] Clause 29: A method for reducing residual voltage in an electro-optic display, the method comprising: providing an electro-optic display including a light-transmitting front electrode; an electro-optic material layer; a pixel electrode array; a thin-film transistor array positioned adjacent to a surface of the electro-optic material layer, each thin-film transistor including a photosensitive semiconductor region, wherein each pixel electrode is coupled to a single thin-film transistor in the thin-film transistor array; at least one light source positioned adjacent to the thin-film transistor array, wherein the electro-optic material layer and the at least one light source are disposed on opposite sides of the thin-film transistor array. The display driver is coupled to the at least one light source and a gate line and a source line of each thin-film transistor, wherein only one thin-film transistor is operatively connected between the display driver and each pixel electrode; the display driver applies substantially the same voltage to the front light-transmitting electrode and the source line of each thin-film transistor; and the photosensitive semiconductor region of each thin-film transistor is exposed to light emitted from the at least one light source, wherein the light has sufficient intensity to activate the thin-film transistor array, thereby establishing a conduction path for discharging residual voltage.

[0130] Clause 30: The method of Clause 29, wherein exposing the photosensitive semiconductor region of each thin-film transistor to light emitted from the at least one light source includes using a light guide to direct the light emitted from the at least one light source to the photosensitive semiconductor region of each thin-film transistor.

[0131] Clause 31: The method of Clause 29, wherein exposing the photosensitive semiconductor region of each thin-film transistor to light emitted from the at least one light source comprises using an optical diffuser to uniformly distribute the light emitted from the at least one light source across the entire thin-film transistor array.

[0132] Clause 32: The method of Clause 31 further includes using a prism film to focus and redirect light emitted from the diffuser sheet at an angle substantially perpendicular to the photosensitive semiconductor region of each thin-film transistor.

[0133] Clause 33: The method of any one of Clauses 29 to 32, wherein the display driver applies substantially the same voltage to the front light-transmitting electrode and the source line of each thin-film transistor, further comprising setting the gate line of each thin-film transistor to a voltage suitable for stopping each thin-film transistor.

[0134] Clause 34: The method of any one of Clauses 29 to 33, wherein exposing the photosensitive semiconductor region of each thin-film transistor to light emitted from the at least one light source comprises initiating a drive signal by the display driver to cause the at least one light source to emit light.

[0135] Clause 35: The method of Clause 34 further includes placing each thin-film transistor in a floating state after exposing the photosensitive semiconductor region of each thin-film transistor to light emitted from the at least one light source.

[0136] Clause 36: The method of Clause 35, wherein placing each thin-film transistor in a floating state includes stopping the drive signal by the display driver.

[0137] Clause 37: The method of any one of Clauses 29 to 36, wherein the thin-film transistor array comprises (i) an n-type thin-film transistor and the display driver applies a ground potential to the front-transmitting electrode and the source line of each thin-film transistor, or (ii) a p-type thin-film transistor and the display driver applies a positive voltage to the front-transmitting electrode and the source line of each thin-film transistor.

[0138] Clause 38: The method of any one of Clauses 29 to 37, wherein the at least one light source communicates optically with the photosensitive semiconductor region of each thin-film transistor.

[0139] Clause 39: The method of any one of Clauses 29 to 38, wherein the at least one light source emits light onto an unshielded portion of each thin-film transistor.

[0140] Clause 40: The method of Clause 30, wherein the light guide is positioned between the at least one light source and the thin-film transistor array.

[0141] Clause 41: The method of Clause 31, wherein the optical diffuser is positioned between the at least one light source and the thin-film transistor array.

[0142] Clause 42: The method of any one of Clauses 29 or 33 to 39, wherein the electro-optic display further includes one or more of a light guide plate, a diffuser sheet, a prism film and a polarizing plate, positioned between the at least one light source and the thin-film transistor array.

[0143] Clause 43: The method of any one of Clauses 34 to 42, wherein the drive signal is one of a DC voltage, an AC voltage and a pulse modulation signal.

[0144] Clause 44: The method of any one of Clauses 34 to 43, wherein the at least one light source emits light with an intensity of at least 900 lux in response to the drive signal.

[0145] Clause 45: The method of any one of Clauses 34 to 43, wherein the at least one light source emits light with an intensity of at least 1300 lux in response to the drive signal.

[0146] Clause 46: The method of any one of Clauses 34 to 43, wherein the at least one light source emits light with an intensity of at least 1700 lux in response to the drive signal.

[0147] Clause 47: The method of any one of Clauses 29, 31 to 40 or 43 to 46, wherein the at least one light source comprises a plurality of light-emitting diodes positioned along an outer edge of the electro-optical display device.

[0148] Clause 48: The method of any one of Clauses 29, 30, 33 to 40 or 43 to 46, wherein the at least one light source comprises an array of light-emitting diodes positioned in accordance with the thin-film transistor array.

[0149] 100 pixels 102: Front electrode 104: Rear Electrode 106: Driver Electrode 108: Addressing Electrode 110: Imaging membrane 120: Nonlinear circuit elements 202: Resistor 204: Capacitor 212: Resistor 214: Capacitor 216: Capacitor 300: Electro-optical display 302: Transparent front electrode 304: Rear Electrode 310: Electrophoretic medium 320: Nonlinear circuit elements 325: Uncovered portion 350:Substrate 360: Backplate 370: Backlight 380: Light source 385: Optical Adjuster 390: Light 391: Distributed Light 392: Light 400a: Drive diagram 400b: Drive diagram 400c: Drive diagram 500: Curve Graph 501: Curve 502: Curve 503: Curve Vi: Voltage

Claims

1. An electro-optical display, comprising: One transparent front electrode; One electro-optic material layer; One-pixel electrode array; A thin-film transistor array positioned adjacent to a surface of the electro-optic material layer, each thin-film transistor including a photosensitive semiconductor region, wherein each pixel electrode is coupled to a single thin-film transistor; at least one light source positioned adjacent to the thin-film transistor array, wherein the electro-optic material layer and the at least one light source are disposed on opposite sides of the thin-film transistor array; and a display driver coupled to a gate line and a source line of each thin-film transistor, wherein only one thin-film transistor is operatively connected between the display driver and each pixel electrode, the display driver being configured to: apply substantially the same voltage to the front-transmitting electrode and the source line of each thin-film transistor; and initiate a drive signal to the at least one light source to emit light of sufficient intensity to activate the thin-film transistor array to establish a conduction path for discharging residual voltage.

2. The electro-optical display of claim 1, wherein the thin-film transistor array comprises (i) n-type thin-film transistors and the display driver applies a ground potential to the front-transmitting electrode and the source line of each thin-film transistor, or (ii) p-type thin-film transistors and the display driver applies a positive voltage to the front-transmitting electrode and the source line of each thin-film transistor.

3. The electro-optical display of claim 1, wherein the at least one light source communicates optically with the photosensitive semiconductor region of each thin-film transistor.

4. The electro-optical display of claim 3, wherein the at least one light source is positioned to emit light onto the gate line of each thin-film transistor.

5. The electro-optical display of claim 4, wherein the at least one light source emits light onto an unshielded portion of each thin-film transistor.

6. The electro-optical display of claim 1 further includes a light guide positioned between the at least one light source and the thin-film transistor array.

7. The electro-optical display of claim 1 further includes an optical diffuser positioned between the at least one light source and the thin-film transistor array.

8. The electro-optic display of claim 1 further includes one or more of a light guide plate, a diffuser sheet, a prism film and a polarizing plate, positioned between the at least one light source and the thin-film transistor array.

9. The electro-optical display as claimed in claim 1, wherein the drive signal is one of a DC voltage, an AC voltage, and a pulse-wave modulation signal.

10. The electro-optical display of claim 1, wherein the at least one light source is configured to emit light with an intensity of at least 900 lux in response to the drive signal.

11. The electro-optical display of claim 1, wherein the at least one light source is configured to emit light with an intensity of at least 1300 lux in response to the drive signal.

12. The electro-optical display of claim 1, wherein the at least one light source is configured to emit light with an intensity of at least 1700 lux in response to the drive signal.

13. The electro-optical display of claim 1, wherein the at least one light source comprises a plurality of light-emitting diodes positioned along an outer edge of the electro-optical display device.

14. The electro-optical display of claim 1, wherein the at least one light source comprises an array of light-emitting diodes positioned in conjunction with the thin-film transistor array.

15. A method for reducing residual voltage in an electro-optic display, the method comprising: An electro-optic display is provided, which includes a light-transmitting front electrode; One electro-optic material layer; One-pixel electrode array; A thin-film transistor array positioned adjacent to a surface of an electro-optic material layer, each thin-film transistor including a photosensitive semiconductor region, wherein each pixel electrode is coupled to a single thin-film transistor in the thin-film transistor array; at least one light source positioned adjacent to the thin-film transistor array, wherein the electro-optic material layer and the at least one light source are disposed on opposite sides of the thin-film transistor array; and a display driver coupled to the at least one light source and a gate line and a source line of each thin-film transistor, wherein only one thin-film transistor is operatively connected between the display driver and each pixel electrode; the display driver applies substantially the same voltage to the front light-transmitting electrode and the source line of each thin-film transistor; and the display driver initiates a drive signal to the at least one light source to emit light of sufficient intensity to activate the thin-film transistor array, thereby establishing a conduction path for discharging residual voltage.

16. The method of claim 15, wherein the thin-film transistor array comprises (i) an n-type thin-film transistor and the display driver applies a ground potential to the front-transmitting electrode and the source line of each thin-film transistor, or (ii) a p-type thin-film transistor and the display driver applies a positive voltage to the front-transmitting electrode and the source line of each thin-film transistor.

17. The method of claim 15, wherein the at least one light source performs optical communication with the photosensitive semiconductor region of each thin-film transistor.

18. The method of claim 17, wherein the at least one light source is positioned to emit light onto the gate line of each thin-film transistor.

19. The method of claim 18, wherein the at least one light source emits light onto an unshielded portion of each thin-film transistor.

20. The method of claim 18, wherein the electro-optic display further includes a light guide positioned between the at least one light source and the thin-film transistor array.

21. The method of claim 15, wherein the electro-optic display further includes an optical diffuser positioned between the at least one light source and the thin-film transistor array.

22. The method of claim 15, wherein the electro-optic display further includes one or more of a light guide plate, a diffuser sheet, a prism film and a polarizing plate, positioned between the at least one light source and the thin-film transistor array.

23. The method of claim 15, wherein the drive signal is one of a DC voltage, an AC voltage, and a pulse modulation signal.

24. The method of claim 15, wherein initiation further comprises the at least one light source emitting light having an intensity of at least 900 lux in response to the drive signal.

25. The method of claim 15, wherein initiation further comprises the at least one light source emitting light having an intensity of at least 1300 lux in response to the drive signal.

26. The method of claim 15, wherein initiation further comprises the at least one light source emitting light having an intensity of at least 1700 lux in response to the drive signal.

27. The method of claim 15, wherein the at least one light source comprises a plurality of light-emitting diodes positioned along an outer edge of the electro-optical display device.

28. The method of claim 15, wherein the at least one light source comprises an array of light-emitting diodes positioned in accordance with the thin-film transistor array.

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