JP2007512579A - Low residual voltage electro-optic display - Google Patents

Abstract

The present invention provides materials and methods (including driving methods) that reduce the effects of residual voltage in electro-optic displays. For example, in a method of driving a bistable electro-optic display having a plurality of pixels capable of displaying at least two gray levels, each pixel of the display has a waveform determined by the initial gray level and the final gray level of the pixel Applying at least one change from a particular initial gray level to a particular final gray level if the pixel remains at that initial gray level for less than a predetermined interval. The method is characterized in that if one waveform is used and the pixel remains at its initial gray level for longer than the predetermined interval, a second waveform different from the first waveform is used.

Description

  The present invention relates to a low residual voltage electro-optic display and a method for reducing the residual voltage of an electro-optic display. As used herein, the term “residual voltage” refers to the persistent electric field or the residual electric field that remains in some electro-optic displays after the addressing pulse (the voltage pulse used to change the optical state of the electro-optic medium) has ended. Refers to the decaying electric field. It has been found that such residual voltage pulses can have undesired effects on electro-optic displays, in particular, residual voltage can lead to a so-called “ghost” phenomenon. Therefore, the traces of the previous image are still visible after rewriting to the display. The present invention is specifically intended for use in electro-optic displays, but is not intended to be limited thereto.

  The electro-optic display comprises a layer of electro-optic material. The term electro-optic material is used herein in the conventional sense used in imaging technology. A material having first and second display states, having at least one different optical property in both states, and changing from a first display state to a second display state when an electric field is applied to the material. . The optical property is typically color recognition by the human eye, but in addition to optical properties such as light transmission, reflection, luminescence, or in displays intended for machine reading, In the meaning of the wavelength reflectance change of the electromagnetic wave beyond the visual range, it may be a pseudo-color.

  In the display of the present invention, the electro-optic medium is typically solid (such a display may hereinafter be referred to as a “solid electro-optic display” for convenience). This is typically a solid in the sense that an electro-optic medium can have and often has a space filled with a liquid or gas inside, but has a solid outer surface. Thus, the term “solid-state electro-optic display” includes encapsulated electrophoretic displays, encapsulated liquid crystal displays, and other types of displays discussed below.

  The term “gray state” is used herein in the conventional sense used in video technology. That is, it indicates an intermediate state between the extreme optical states of the pixel, but does not necessarily mean a black-white change between the extreme states of black and white. For example, in the patents and published applications referenced below, an electrophoretic display is described in which the extreme states are white and dark blue and the intermediate “gray state” is substantially light blue. There are also some things. In fact, as already mentioned, the change in the extremes may not be a color change at all.

  The terms “bistable” and “bistable” are used herein to refer to a display that has first and second display states and also includes display components that have at least one different optical characteristic in both states. Used in the conventional sense in the field used. When the addressing pulse ends after any given component has been driven to assume either its first or second display state using a limited duration addressing pulse means The state continues at least several times (eg, at least four times), ie, the minimum duration of the addressing pulse required to change the state of the display component. As shown in US 2002/0180687, particle-based electrophoretic displays that can be gray scaled are not only black and white at their extremes, but also intermediate between them. It is stable even in gray. The same is true for some other types of electro-optic displays. Although this type of display is more precisely “multi-stable” than bistable, for simplicity, the term “bistable” is used herein to refer to bistable, multistable. It may be used covering both.

  The term “impulse” is used herein to mean the integration of voltage with respect to time and in the sense of conventional video technology. However, some bistable electro-optic media function as charge transducers, in which an alternative definition of impulse, ie the integration of current over time (which is applied Equal to total charge) can also be used. An appropriate definition of impulse should be used depending on whether the medium functions as a voltage-time impulse transducer or a charge impulse transducer.

  Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type. For example, U.S. Pat. Nos. 5,808,783, 5,777,782, 5,760,761, 6,054,071, and 6,055,091. 6,097,531, 6,128,124, 6,137,467, and 6,147,791 (this type of display is described as “Rotating Often referred to as a “rotating bichromal ball” display, in some of the above-mentioned patents, the rotating member is not spherical, so it is more accurate to call it a “rotating bichromal member”). Such displays use a number of small bodies (typically spherical or cylindrical). This body has two or more parts with different optical properties and an internal dipole. These bodies are suspended in liquid-filled vacuoles in the matrix, and the vacuoles are filled with liquid so that the bodies can rotate freely. When an electric field is applied to the display, the body rotates to various positions and the portion of the body visible through the screen changes, thus changing the appearance of the display. This type of electro-optic medium is typically bistable.

  Another type of electro-optic display uses electrochromic media. For example, an electrochromic medium in the form of a nanochromic film comprising at least a portion of an electrode formed of a semiconductor metal oxide and a plurality of reversibly color-changing dye molecules attached to the electrode. For example, O'Regan, B.I. Nature 1991, 353, 737, and Wood, D. et al. Information Display, 18 (3), page 24 (March 2002). Also, Bach, U.S. Adv. Mater. See also 2002, 14 (11), page 845. This type of nanochromic membrane is disclosed, for example, in US Pat. No. 6,301,038, International Application Publication No. WO01 / 27690, and US Patent Application 2003/0214695. This type of medium is typically bistable.

  Another type of electro-optic display that has been the subject of active research and development for many years is the particle-based electrophoretic display. This is because a plurality of charged particles move through a suspended fluid under the influence of an electric field. An electrophoretic display may have characteristics of good brightness and contrast, wide viewing angle, clear bistability, and low power consumption compared to a liquid crystal display. Nevertheless, these displays have long-term image quality problems that have hindered their widespread use. For example, the particles that make up electrophoretic displays tend to settle, and as a result, the lifetime of such displays is insufficient.

  As mentioned above, the electrophoretic medium requires the presence of a suspending fluid. In most prior art electrophoretic media, this suspending fluid is a liquid, but electrophoretic media can also be produced using a gaseous suspending fluid. For example, Kitamura, T .; "Electrical toner movement for electronic paper-like display", IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y. et al. See “Toner display using insulating particles charged trielectrically”, IDW Japan, 2001, Paper AMD4-4. In addition, European Patent Application Nos. 1,429,178, 1,462,847, 1,482,354, and 1,484,625, and International Application Publication No. WO2004 / 090626, WO 2004/077942, WO 2004/077140, WO 2004/059379, WO 2004/055586, WO 2004/008239, WO 2004/006006, WO 2004/001498 No., WO 03/091799, and WO 03/088895. Such gas-based electrophoretic media are subject to the same problems as liquid-based electrophoretic media due to particle fixation. That is when the media is used in one direction that allows such fixing, for example when used in signs where the media is placed in a vertical plane. In fact, particle fixing becomes a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media. This is because the gas suspension fluid has a lower viscosity than the liquid suspension fluid, so that the electrophoretic particles can be fixed faster.

  A number of recently published patents and patent applications assigned to or under the name of Massachusetts Institute of Technology (MIT) and E Ink Corporation describe encapsulated electrophoretic media. Such encapsulating media include a number of small capsules, each of which includes an internal phase containing electrophoretically movable particles suspended in a liquid suspending medium and a capsule wall surrounding the internal phase. Typically, the capsule is encapsulated in a polymer binder that forms a coherent layer placed between the two electrodes. Examples of this type of encapsulating medium include, for example, U.S. Pat. Nos. 5,930,026, 5,961,804, 6,017,584, 6,067,185, and 6, 118,426, 6,120,588, 6,120,839, 6,124,851, 6,130,773, 6,130,774 No. 6,172,798, No. 6,177,921, No. 6,232,950, No. 6,249,721, No. 6,252,564, No. 6, 262,706, 6,262,833, 6,300,932, 6,312,304, 6,312,971, 6,323,989, No. 6,327,072, No. 6,376,828, No. 6,377,387, 6,392,785, 6,392,786, 6,413,790, 6,422,687, 6,445,374, 6,445, No. 489, No. 6,459,418, No. 6,473,072, No. 6,480,182, No. 6,498,114, No. 6,504,524, No. 6,506,438, 6,512,354, 6,515,649, 6,518,949, 6,521,489, 6,531,997 No. 6,535,197, No. 6,538,801, No. 6,545,291, No. 6,580,545, No. 6,639,578, No. 6 652,075, 6,657,772, 6,664,944, 6,680, No. 25, No. 6,683,333, No. 6,704,133, No. 6,710,540, No. 6,721,083, No. 6,727,881, 6,738,050, 6,750,473, 6,753,999, 6,816,147, 6,819,471 and 6,822,782 U.S. Patent Application Publication Nos. 2002/0019081, 2002/0060321, 2002/0060321, 2002/0063661, 2002/0090980, 2002/0113770, 2002/0130832, 2002/0131147, 2002/0171910, 2002/0180687, 2002/01806 No. 88, No. 2003/0011560, No. 2003/0020844, No. 2003/0025855, No. 2003/0053189, No. 2003/0102858, No. 2003/0132908, No. 2003/0137521 2003/0137717, 2003/0151702, 2003/0214695, 2003/0214697, 2003/0222315, 2004/0008398, 2004/0012839 No. 2004/0014265, No. 2004/0027327, No. 2004/0075634, No. 2004/0094422, No. 2004/0105036, No. 2004/0112750 and No. 2004 /. No. 119681, and International Application Publication Nos. WO99 / 67678, WO00 / 05704, WO00 / 38000, WO00 / 38001, WO00 / 36560, WO00 / 67110, WO00 / 67327, WO01 / 07961, WO01 / 08241, WO03 / 107,315, WO2004 / 023195, WO2004 / 049045, WO2004 / 05378, These are described in WO 2004/088002, WO 2004/088395, and WO 2004/090857.

  Many of the aforementioned patents and patent applications state that the wall surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, and the electrophoretic medium is an electrophoretic fluid The ability to produce so-called polymer-dispersed electrophoretic displays with multiple discrete droplets and a continuous phase of polymer material, even if the discrete capsule membranes are not associated with individual droplets. Discrete droplets of a dispersed electrophoretic fluid can be considered as capsules or microcapsules. For example, see the aforementioned 2002/0131147. Thus, for the purposes of this application, such polymer dispersed electrophoretic media are considered as sub-species of encapsulated electrophoretic media.

  A related type of electrophoretic display is the so-called “microcell electrophoretic display”. In microcell electrophoretic displays, charged particles and suspending fluid are not encapsulated in microcapsules but are held in multiple cavities formed in a carrier medium (typically a polymer film). ing. See, for example, International Application Publication No. WO 02/01281, and US Patent Application Publication No. 2002/0075556 (both by Sipix Imaging, Inc.).

  Many of the aforementioned E Ink and MIT patents and patent applications discuss microcell electrophoretic displays and polymer dispersed electrophoretic displays. The term “encapsulated electrophoretic display” is sometimes referred to for all such display types. Moreover, in order to generalize about the form of a wall, it can also be described generically as a “microcavity electrophoretic display”.

  Another type of electro-optic display is the electrowetting display by Philips, which is described in the article “Nature” published on September 25, 2003, under the title “Performing Pixels: Moving Images on Electronic Paper”. ing. Also, copending application Ser. No. 10 / 711,802 (filed Oct. 6, 2004) shows that such an electrowetting display can be bistable.

  Other types of electro-optic materials can also be used in the present invention. Of particular interest is the bistable ferroelectric liquid crystal display (FLC) known in the industry, which exhibits residual voltage behavior.

  Electrophoretic media are often opaque (because, for example, in many electrophoretic media, particles substantially block the transmission of visible light through the screen) and operate in reflective mode, but often The electrophoretic display can be manufactured to operate in a so-called “shutter mode” where one display state is substantially opaque and one display state is light transmissive. For example, the aforementioned US Pat. Nos. 6,130,774 and 6,172,798, and US Pat. Nos. 5,872,552, 6,144,361, 6,271, See 823, 6,225,971 and 6,184,856. A dielectrophoretic display is similar to an electrophoretic display, but responds to changes in field strength and can operate in a similar mode. See U.S. Pat. No. 4,418,346. Other types of electro-optic displays can also be operable in shutter mode.

  Encapsulated electrophoretic displays or microcell electrophoretic displays typically do not suffer from the failure modes of conventional electrophoretic devices such as clustering and fusing, and as an added benefit on a wide variety of flexible and rigid substrates The ability to print or coat on the display. (The term “printing” is intended to include, but is not limited to, any form of printing or coating: patch die coating, slot or extrusion coating, slide or cascade. Pre-measurement coating such as coating, curtain coating; roll application such as roll knife coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; rotation Painting; Brush painting; Air knife coating; Silk screen printing process; Electrostatic printing process; Thermal printing process Ink jet printing processes; electrophoretic deposition;. And other is the same technique) Thus, the display obtained these results may have flexibility. Further, since the display media can be printed (using a variety of methods), the display itself can be manufactured inexpensively.

  Bistable or multi-stable behavior is a particle-based electrophoretic display or other electro-optic display that exhibits similar behavior (such displays may hereinafter be referred to as “impulse driven displays” for convenience) In contrast, the bistable or multistable behavior of conventional liquid crystal (“LC”) displays is in sharp contrast. Twisted nematic liquid crystals are neither bistable nor multistable, but function as voltage transducers. When an arbitrary electric field is applied to a pixel of such a display, a specific gray level is formed in the pixel regardless of the gray level previously present in the pixel. Furthermore, LC displays are only driven in one direction (from non-transparent or “dark” to transparent or “bright”). The reverse change from the bright state to the dark state is achieved by reducing or removing the electric field. In the end, the gray level of the pixels of the LC display does not react to the polarity of the electric field, but only to its magnitude, and in fact, for technical reasons, commercially available LC displays have frequent driving The polarity is reversed. In contrast, a bistable electro-optic display functions as an impulse transducer in the first approximation, and the final state of the pixel depends not only on the applied electric field and its electric field application time, but also before the electric field application. This also depends on the state of the pixel.

  Also, to obtain a high resolution display, individual pixels of the display must be addressable without interference from adjacent pixels. One way to achieve this goal is to provide an array of non-linear elements, such as transistors and diodes, with each pixel such that at least one non-linear element forms an “active matrix” display; It is related. An addressing electrode or pixel electrode designating one pixel is connected to an appropriate power source through an associated non-linear element. Typically, when the nonlinear element is a transistor, the pixel electrode is connected to the drain of the transistor. This is essentially arbitrary and the pixel electrode can be connected to the source of the transistor, but the following description assumes this arrangement. Conventionally, in a high resolution array, the pixels are arranged in a two-dimensional array of rows and columns. This is because only one particular pixel is determined by the intersection of one particular row and one particular column. The sources of all transistors in each column are connected to a single column electrode, and the gates of all transistors in each column are connected to a single row electrode. Also, the arrangement of sources in rows and gates in columns is conventional, but is essentially arbitrary and can be reversed if desired. The row electrode is connected to a row driver, which ensures that only one row is selected at any given moment. That is, a voltage is applied to the selected row electrode to ensure that all transistors in the selected row are conductive, and no voltage is applied to all other row electrodes. Ensure that all transistors in the row are not conductive. The column electrode is connected to a column driver. The column driver assigns various column electrode voltages selected to drive the pixels in the selected row to the desired optical state (the aforementioned voltage is a common front electrode). Conventionally, provided from the non-linear array on the opposite side of the electro-optic medium and spread across the display.) After a preset interval known as “line address time” The selected row is not selected, the next row is selected, and the applied voltage of the column driver is changed to be written to the next line of the display. This process is repeated so that the entire display is written row by row.

  The above-mentioned 2003/0137521 describes how a direct current (DC) unbalanced waveform can generate a residual voltage, and this residual voltage is confirmed by measuring the open circuit electrochemical potential of the display pixel. Is stated.

  For the reasons described in the pending application mentioned above, it is desirable to use a balanced DC drive scheme when driving an electro-optic display. That is, a balanced DC is such that in any sequence of optical states, whenever the final optical state matches the initial optical state, the integral of the applied voltage is zero. . This ensures that the net DC imbalance experienced by the electro-optic layer is limited by known values. For example, a pulse of 15 ms at 300 ms can be used to bring the electro-optic layer from a white state to a black state. After this change, the image layer experiences a DC imbalanced impulse of 4.5V-s. When returning this film to white, if a 300 ms pulse at -15 V is used, the image layer is DC balanced over a continuous transition from white to black and from black to white.

  It has now been found that residual voltage is a more common phenomenon, both in cause and effect, in electrophoretic displays and other impulse driven electro-optic displays. DC imbalance has also been found to cause long-term lifetime degradation of electrophoretic displays.

  Residual voltage has been measured in electrophoretic displays by starting with a sample that has not been switched for an extended period of time (eg, hours or days). A voltmeter is installed across the open circuit of the pixel and the “basic voltage” is measured by reading. An electric field is then applied to the pixel, for example, in a switching waveform. Immediately after the waveform is finished, the voltmeter is used for open circuit potential measurements over a continuous period of time, and the difference between the value read in the measurement and the original base voltage is considered the “residual voltage”.

  The residual voltage decays in a complex manner that can be approximated mathematically as a sum of exponential functions. In a typical experiment, 15V was applied across the electro-optic medium for about 1 second. Immediately after the end of this voltage pulse, a residual voltage between + 3V and -3V is measured, after 1 second a residual voltage between + 1V and -1V is measured, and after 10 minutes (relative to the basic voltage) The voltage was close to zero.

  The term “residual voltage” is sometimes used herein as a convenient term to refer to the overall phenomenon. However, the basis for the switching behavior of impulse driven electro-optic displays is the application of voltage impulses (integration of voltage with respect to time) across the electro-optic medium. Shown in FIG. 1 of the accompanying drawings is a typical graph of time versus residual voltage. Here, immediately after the application of the drive pulse, the residual voltage reaches a peak value indicated by 102 (the time scale of FIG. 1 is basically arbitrary), and then substantially as shown by the curve 104 of FIG. Decay exponentially. The duration of the residual voltage over a considerable time corresponds to the “residual impulse” indicated by the area 106 under the curve 104. Strictly speaking, rather than the residual voltage, this residual impulse affects the optical state of the electro-optic display, which is generally considered to be due to the residual voltage.

  Theoretically, the effect of the residual voltage should directly correspond to the residual impulse. However, in practice, the impulse switching model can be inaccurate at low voltages. Some electro-optic media have small thresholds, including the preferred electrophoretic media used in the experiments described herein. For example, with a residual voltage of about 1 V, there is no noticeable change in the optical state of the medium after the drive pulse ends. Thus, the actual result may be different even with two equivalent residual impulses. Also, increasing the threshold of the electro-optic medium can be useful to reduce the effects of residual voltage. E Ink has adequately produced “small threshold” electrophoretic media to avoid the display changing immediately after the end of the drive pulse due to the residual voltage experienced in typical applications. It is. If the threshold is inappropriate or the residual voltage is too high, the display may show a kickback / self-erasing or self-improvement phenomenon.

  Even if the residual voltage is less than a small threshold, if the residual voltage continues to be maintained until the next image update is performed, image switching is seriously affected. For example, consider a case where a drive voltage of +/− 15 V is applied to move electrophoretic particles when updating an image on an electrophoretic display. If the + 1V residual voltage persists since the last update, the drive voltage is effectively shifted from + 15V / -15V to + 16V / -14V. As a result, the pixel is biased to a dark state or a white state depending on whether the residual voltage of the pixel is positive or negative. Furthermore, this result changes with the elapsed time depending on the decay rate of the residual voltage. Immediately after the previous image update, the electro-optic material of the pixel switched to white using a 15 V, 300 ms drive pulse may actually experience a waveform closer to 16 V, 300 ms. On the other hand, a pixel material that switched to white after 1 minute using exactly the same drive pulse (15V, 300ms) may actually experience a waveform closer to 15.2V, 300ms. As a result, white shading can cause significant differences in both pixels.

  Once the residual voltage field is formed across a number of pixels by a previous image (eg, a dark line on a white background), the residual voltage can be arranged across the display in a similar pattern. Here, in practice, the most prominent influence of the residual voltage on the display performance is ghost. In addition to the aforementioned DC imbalance (for example, 16V / 14V instead of 15V / 15V), this problem can cause the electro-optic medium to gradually deteriorate in life.

Ghosts or similar visual effects can be measured optically with a photometer. On a mobile device display screen, two adjacent pixels with the same target brightness are actual brightness differences of less than 2L ^* (where L ^* is the usual ICE definition,
L ^* = 116 (R / R ₀ ) ^1/3 −16
Where R is the reflectivity and R ₀ is the standard reflectivity value). To avoid user dissatisfaction, it is preferably less than 1 L ^* , ideally less than 0.3 L ^* .

  When the residual voltage decays slowly and is almost constant, the effect of the waveform shift does not change with each image update, and in reality, less ghosting occurs than the rapidly decaying residual voltage. obtain. Thus, a ghost experienced when updating one pixel after 10 minutes and another after 11 minutes is when updating one pixel immediately and another after 1 minute. Very small compared to ghost experience. Conversely, if the residual voltage decays rapidly and approaches zero before the next update, in practice, the sensed ghost may not occur. Thus, for practical purposes, the concern is most aggravated when the residual voltage exceeds about 0.2 V and its duration is between 10 ms and 1 hour, more specifically between 50 ms and 10 minutes.

  As is clear from the above discussion, the effect of residual voltage is reduced by minimizing the residual impulse. As shown in FIG. 1, this can be achieved by reducing the peak residual voltage or increasing the decay rate. Theoretically, if the residual voltage can be measured instantaneously and immediately after the end of the drive pulse, the peak residual voltage is approximately equal in magnitude to the drive pulse voltage, and the sign is reversed. Can be done. In practice, since most of the residual voltage decays very quickly (eg, less than 20 ms), the experimentally measured “peak” residual voltage is much smaller. In this way, the “peak” residual voltage can actually be reduced (1) operating the display at a low voltage, or (2) very much occurring within the first millisecond after image update. By increasing the fast decay, resulting in a very low residual impulse. In essence, besides operating at a low voltage, one main way to reduce the residual impulse is to increase the decay rate.

  There are many potential sources of residual voltage. It is believed that the main cause of the residual voltage is ionic polarization of the various layers of materials that make up the display (but the present invention is by no means bound to this idea).

  Such polarization occurs in various ways. As the first polarization (referred to as “type I” for convenience), an ionic bilayer is formed across or adjacent to the material interface. For example, the anodic potential of indium-tin-oxide (“ITO”) can form a corresponding polarization layer of anions on the adjacent laminated adhesive. The rate of decay of such a polarization layer is related to the recombination of the separated ions in the adhesive stack. The geometry of such a polarization layer is determined by the shape of the interface, but is typically planar in nature.

  As a second type of polarization ("Type II"), nodules, crystals, or other types of material inhomogeneities in a single material result as a region where ions can move slower than the surrounding material. There is something. Due to the different ion transfer rates, the result may be a different degree of charge polarization in the bulk of the medium. Thus, polarization can occur even with a single display component. Such polarization can be substantially localized or distributed throughout the layer.

  As a third type of polarization (“Type III”), polarization can occur at any interface that acts as a barrier to charge transfer for any particular type of ion. An important example of such an interface is the boundary in a microcavity electrophoretic display. It is the boundary between the electrophoretic suspension containing the suspending medium and particles (“inner phase”) and the surrounding medium containing the wall, adhesive and binder (“outer phase”). In many electrophoretic displays, the inner phase is a hydrophobic liquid, while the outer phase is a polymer such as gelatin. Ions present in the inner phase are typically insoluble and non-diffusible in the outer phase, and vice versa. When an electric field perpendicular to such an interface is applied, the positive and negative polarization layers are accumulated on both sides of the interface. When the applied electric field is removed, the resulting non-equilibrium charge distribution can be measured as a residual voltage potential. This potential decays with a relaxation time determined by the ion mobility in the two phases on either side of the interface.

  Polarization typically occurs during drive pulses. Typically, each image update is an event that affects the residual voltage. A positive waveform voltage can form a residual voltage across the electro-optic medium. This residual voltage is either the same polarity or the opposite polarity (or nearly zero) depending on the particular electro-optic display, as described below.

  Polarization occurs at a number of locations within an electrophoretic display or another electro-optic display, primarily at interfaces and material inhomogeneous locations, each of which has its own decay time characteristic spectrum. It becomes clear in the discussion. Here, the arrangement of voltage sources (in other words, polarization charge distribution) with respect to an electroactive component (for example, an electrophoretic suspension), the degree of electrical coupling between each type of charge distribution, and the suspension Depending on the movement of the particles through or other electro-optical activity, the various types of polarization can be more or less detrimental. An electrophoretic display operates by the movement of charged particles and thus essentially causes polarization of the electro-optic layer. In a sense, a preferred electrophoretic display is not a display where the residual voltage present in the display is always zero, but rather a display where the residual voltage does not cause undesirable electro-optical behavior. Ideally, the residual impulse is minimized and the residual voltage is reduced below 1V, preferably below 0.2V, within 1 second, preferably within 50ms. This allows the electrophoretic display to achieve all transitions between optical states without worrying about the effects of residual voltage by introducing a minimum pause during image update. In order to operate an electrophoretic display at video speed or below +/- 15V, these ideal values should be lowered accordingly. Similar considerations apply to other types of electro-optic displays.

  In summary, the phenomenon of residual voltage is a result of ionic polarization occurring at least substantially within the display material component, at its interface or within the material itself. Such polarization is particularly problematic when it persists over a meso time scale of approximately between 50 ms and about 1 hour. The residual voltage can manifest itself as an image ghost or visual effect that occurs in various ways. Further, the degree of seriousness may vary depending on the elapsed time between image updates. Residual voltage can also create a DC imbalance and shorten the final display lifetime. The effect of the residual voltage therefore generally adversely affects the quality of the electrophoretic device or another electro-optic device. Therefore, it is desirable to minimize the residual voltage itself and to minimize the sensitivity that the effect of the residual voltage has on the optical state of the device.

  An approach towards reducing or eliminating ghosts and visual effects resulting from residual voltage is described in the aforementioned E Ink patent application. For example, the aforementioned 2003/0137521 and the co-pending application number PCT / US2004 / 21000 filed on June 29, 2004 (see also the corresponding international patent application PCT / US2004 / 21000) include: A so-called “rail stable” drive scheme is described. In it, the electro-optic medium is driven periodically to one of the “optical rails” (both extreme optical states of the electro-optic medium). Here, the small residual voltage has no clear influence on the optical state. Co-pending application no. 10 / 837,062, filed April 30, 2004 (see also corresponding international patent application PCT / US2004 / 13573), describes the control of capsule height and dye level of electrophoretic media. ing. This is to prevent a noticeable optical change from occurring due to a small residual voltage when switching between black and white.

  Such an approach is effective for monochromatic displays, but does not solve the root cause of residual voltage. Furthermore, although useful for grayscale and color displays, these approaches do not completely solve the problem of addressing the system to gray levels. This is because the gray level of an electrophoretic display typically depends on the mixing ratio of white and black particles without the benefit of a physical wall that corrects for particle velocity differences. Therefore, grayscale addressing is typically more sensitive to small differences between the target waveform and the actual voltage experienced by the electrophoretic medium.

  In the method described in the aforementioned 2003/0137521, the residual voltage is measured, and in order to achieve a state in which the residual voltage is zero, a balanced balancing impulse is applied immediately after each image change or in a period. Is applied to. This is useful for both single color addressing and gray scale addressing. However, it is not always practical to measure the residual voltage using the method described in the aforementioned 2003/0137521.

  The present invention seeks to provide additional addressing methodologies for electro-optic displays. This methodology reduces ghosting caused by residual voltage, but does not require pixel level residual voltage measurement. The present invention also seeks to provide additional addressing methodologies for electro-optic displays. This methodology measures the residual voltage, but improves the measurement method described above and an alternative measurement method for residual voltage. The method of the present invention may be useful in electro-optic displays than in electrophoretic displays. Furthermore, the present invention seeks to provide electro-optic materials, manufacturing methods and designs that minimize residual voltage. Residual voltage reduction may be achieved by reducing peak residual voltage, accelerating the voltage decay rate, or any combination thereof.

  The present invention provides an improved method of addressing for electrophoretic displays and other electro-optic displays where residual voltage is generated. The present invention also provides improved display electronics for electrophoretic displays and other electro-optic displays where residual voltage occurs.

  In one aspect, the present invention provides a method for driving a bistable electro-optic display having a plurality of pixels. Each of these pixels can display at least two gray levels. The method includes applying to each pixel of the display a waveform determined by the pixel's initial gray level and final gray level. In at least one change from a specific initial gray level to a specific final gray level, at least different first and second waveforms are available. According to this aspect of the present invention, the residual voltage of the pixel where the change proceeds is determined before the change, and the first waveform or the second waveform is used for the change depending on the determined residual voltage.

  This aspect of the present invention may hereinafter be referred to as the “waveform selection” method of the present invention for simplicity. As a preferred variant of this waveform selection method (hereinafter referred to as the “dwell time waveform selection” method of the present invention for simplicity), the selection of the first waveform or the second waveform used for the specified change is: , Based on the residence time of the relevant pixels. That is, based on the time that the relevant pixel has been at the initial gray level before the change. If the pixel remains at its initial gray level for less than the predetermined interval, the first waveform is used, and if the pixel remains at its initial gray level for longer than the predetermined interval, the second waveform is used. Is called. The residence time waveform selection method can naturally be used even when more than two waveforms change the same. Thus, in one form of the residence time waveform selection method, related changes use at least the first, second and third waveforms, all of which are different from each other. If the pixel remains at the initial gray level for a shorter period than the first predetermined interval, the first waveform is used and the pixel is initially longer than the first predetermined interval and shorter than the second predetermined interval. If it remains at the gray level, the second waveform is used, and if the pixel remains at its initial gray level for longer than the second predetermined interval, the third waveform is used. The first and second predetermined intervals will naturally vary depending on the particular waveform and electro-optic display used. However, the first predetermined interval can range from about 0.3 seconds to about 3 seconds, and the second predetermined interval can range from about 1.5 seconds to about 15 seconds.

The waveform selection method of the present invention can be executed using a look-up table as described in the aforementioned 2003/0137521. Thus, the waveform selection method is
Storing in each possible change between gray levels of pixels a lookup table containing data representing the one or more waveforms used for the change;
Storing initial state data representing at least the initial state of each pixel;
Storing dwell time data representing the period during which each pixel remains in its initial state;
Receiving an input signal representative of a desired final state of at least one pixel of the display;
Represents the waveform required to convert from the initial state of the one pixel to the desired final state of the one pixel, the lookup of the initial state data, the dwell time data and the output signal depending on the input signal Generating as determined from the table.

  Such a “lookup table waveform selection” method of the present invention may use any of the selective aspects of the lookup table method. In this regard, the aforementioned 2003/0137521 and PCT / US2004 / 21000, as well as 10 / 814,205 filed on March 31, 2004 (corresponding international patent application no. WO 2004/090857). Also, without being bound by the general idea described above, this waveform selection method specifically uses the previous state of the look-up table method described in these copending applications, temperature compensation, and life compensation aspects. obtain. In this manner, the waveform selection method of the lookup table stores data representing at least one previous state of each pixel prior to the initial state of each pixel, and relates to the at least one previous state and the related state. Generating an output signal that depends on both of the initial states of the pixel. Also, the look-up table waveform selection method may include receiving a temperature signal representative of a temperature of at least one pixel of the display and generating an output signal dependent on the temperature signal. Further, the look-up table waveform selection method may include generating a lifetime signal representing the operation time of the related pixel and generating an output signal that depends on the lifetime signal.

The present invention also provides a device controller intended for use in performing the lookup table waveform selection method of the present invention and thus controlling a bistable electro-optic display having a plurality of pixels. To do. Here, each of the pixels can display at least two gray levels. The control device
For each possible change between the gray levels of a pixel, at least one change has at least two different associated waveforms and stores lookup table data representing one or more waveforms used for the change. Storage means arranged as follows:
Storage means further arranged to store initial state data representing at least an initial state of each pixel and dwell time data representing a period during which each pixel remains in the initial state;
Input means for receiving an input signal representative of a desired final state of at least one pixel of the display;
Calculating means for determining from the input signal, the initial state data, the dwell time, and the lookup table a waveform required to change the initial state of the one pixel in the desired final state; ,
Output means for generating an output signal representing the waveform.

  In such a “look-up table waveform selection control device”, the storage means can also be arranged to store data of a previous state representing at least one previous state of each pixel prior to the initial state of each pixel. . Also, the computing means can be arranged to determine a waveform that depends on the input signal, initial state data, dwell time data, previous state data and look-up table. The input means can be arranged to receive a temperature signal representative of the temperature of at least one pixel of the display. The computing means can be arranged to determine a waveform that depends on the input signal, the initial state data, the dwell time data and the temperature signal. The control device further includes a life signal generating means arranged to generate a life signal representing an operation time of the related pixel, and a calculation means for determining a waveform from the input signal, the initial state data, the dwell time data, and the life signal. Can be provided.

  In another aspect, the present invention provides an electro-optic display comprising a layer of electro-optic material and voltage supply means for applying a voltage not greater than a predetermined value in any direction across the layer of electro-optic material. provide. Here, the electro-optic material has a threshold voltage that is greater than zero but less than about one third of the predetermined value.

  This aspect of the invention may hereinafter be referred to as the “low threshold” display of the invention for convenience. Such a low threshold display is intended to reduce the effect of residual voltage on the display. In such a low threshold display, the electro-optic material may have a threshold voltage greater than or equal to about 1/50 and less than about 1/3 of a predetermined value. The electro-optic material of such a low threshold display can be any material of the type previously described. However, low threshold displays are intended to use particle-based materials with suspended fluid and multiple charged particles. The plurality of charged particles are kept in the suspending fluid and are movable through the suspending fluid when a voltage is applied across the layer of electro-optic material. The electrophoretic material can be, for example, an encapsulated electrophoretic material, a polymer-dispersed electrophoretic material, or a microcell electrophoretic material. The suspending fluid can be liquid or gaseous.

  In another aspect, the present invention provides a suspending fluid and a plurality of first types that are retained in the suspending fluid and are movable through the suspending fluid by applying an electric field to the electrophoretic medium. Charged particles and a plurality of second types of charged particles that are kept in the suspension fluid and movable through the suspension fluid by applying an electric field to the electrophoretic medium. The particles provide an electrophoretic medium comprising particles having a charge opposite to that of the first type of particles. Here, the total charge amount of the second type particles is between about one half and about twice the total charge amount of the first type particles.

  This aspect of the present invention may hereinafter be referred to as the “electrophoretic medium with two charge balanced dual particles” display of the present invention for convenience. The electrophoretic medium can be, for example, an encapsulated electrophoretic medium, a polymer-dispersed electrophoretic medium, or a microcell electrophoretic medium. The suspending fluid can be liquid or gaseous. Such a charge balanced two-particle electrophoretic medium has a residual voltage of less than about 1 volt, preferably about 0.2 volt, after 1 second of applying a 300 millisecond square wave addressing pulse at 15 volts. It is desirable to exhibit a residual voltage of less than.

  In another aspect, the present invention provides a suspending fluid and a plurality of first types that are retained in the suspending fluid and are movable through the suspending fluid by applying an electric field to the electrophoretic medium. Charged particles and a plurality of second types of charged particles that are kept in the suspension fluid and movable through the suspension fluid by applying an electric field to the electrophoretic medium. The particles provide an electrophoretic medium comprising particles having a charge opposite to that of the first type of particles. Here, the electrophoretic medium exhibits a residual voltage of less than about 1 volt after 1 second of applying a 300 millisecond square wave addressing pulse at 15 volts.

  This aspect of the present invention may hereinafter be referred to as the “low residual voltage electrophoresis medium” of the present invention for convenience. Such a medium desirably exhibits a residual voltage of less than about 0.2 volts after 1 second of applying a 300 millisecond square wave addressing pulse at 15 volts.

In another aspect, the present invention provides an electrophoretic medium comprising a plurality of discrete droplets of a suspending fluid dispersed in a continuous phase. Here, the droplet further includes a plurality of charged particles that are kept in the suspension fluid and are movable by applying an electric field to the electrophoretic medium. The continuous phase also has a volume resistivity that is less than or equal to about one half of the volume resistivity of the droplets, and both the continuous phase and the droplets have a volume resistivity of less than about 10 ¹¹ Ωcm. This aspect of the invention may hereinafter be referred to as the “volume resistivity balanced electrophoresis medium” of the invention. Also, this aspect can be any type discussed above. Thus, for example, the volume resistivity balanced electrophoretic medium can be an encapsulating medium comprising a capsule surrounding a droplet and a polymer binder surrounding the capsule, a polymer dispersed electrophoretic medium, or a microcell electrophoretic medium. .

  The present invention also provides further improvements and design techniques that reduce the effects of residual voltage in electro-optic media and displays, particularly in electrophoretic displays and media. For example, the present invention provides an electrophoretic display that is selected or designed to allow for low peak residual voltage retention and rapid residual voltage decay (in short, low residual impulse). The present invention also provides an electrophoretic display comprising a material that has been doped, processed, purified, or processed to reduce its ability to carry a residual voltage. Furthermore, the present invention provides an electrophoretic display comprising a binder and a laminated adhesive. The binder and laminated adhesive have similar composition, conductivity, or ion mobility. Furthermore, the present invention introduces an electrophoretic display in which the interface between at least two adjacent components is treated to reduce the residual voltage, or an intervening layer is introduced at least in part to reduce the residual voltage. An electrophoretic display is provided. These aspects of the present invention may hereinafter be collectively referred to as “material selection” inventions.

  Furthermore, the present invention provides an electrophoretic display with a conductive path in the display pixel for the purpose of reducing residual voltage. This aspect of the invention may hereinafter be referred to as a “conducting path” invention.

  Furthermore, the present invention provides an electrophoretic suspension comprising two types of charged particles. The total charge amount for each particle type is selected to reduce the residual voltage. This aspect of the invention may hereinafter be referred to as a “zeta potential” invention.

  Furthermore, the present invention also provides an electrophoretic suspension comprising an additive in the suspending fluid to reduce the residual voltage. This aspect of the invention may hereinafter be referred to as a “suspended fluid additive” invention.

  The present invention also provides an outer phase material for a microcavity electrophoretic display with reduced residual voltage. This aspect of the invention may hereinafter be referred to as a “microcavity outer material” invention.

  Furthermore, the present invention provides an electrophoretic display and other electro-optic display manufacturing method with reduced residual voltage, and a display comprising means for measuring the residual voltage.

  As described above, the present invention provides several different improvements in electro-optic displays and media, as well as several different improvements in the waveforms and controllers that drive such displays. Various aspects of the present invention are described separately (or in related groups) below, but a single display or medium may be used in more than one aspect of the present invention. That should be understood. For example, a single display can also include the volume resistivity balanced electrophoretic medium of the present invention and use the waveform selection method of the present invention to drive the medium.

(Residual voltage determination method, and addressing method and control device for electro-optic display showing residual voltage)
As already mentioned, considering that residual voltage adversely affects the optical performance of electro-optic displays, an addressing method that minimizes the effect of residual voltage when the display is sensitive to such residual voltage Is typically necessary or desirable.

  For a given pixel of an electro-optic display, the residual voltage state is very sensitive to “image history”. That is, the previously applied electric field is thus affected by parameters such as the waveform used, the electric field strength, and the time elapsed between successive image updates.

  One of the addressing methods described in the aforementioned 2003/0137521 and US patent application Ser. No. 10 / 249,973 filed May 23, 2003 (see also International Patent Application No. WO 03/107315) For the most important classifications, the knowledge of the previous image data is used. A lookup table is used. For example, the waveform for a black pixel to be switched to white may vary depending on whether the black pixel was previously white or previously gray. (The change from gray is probably a different waveform that will be formed by different amounts of residual voltage.) In practice, such a “n-state look-up table”. Has been found to tend to reduce ghosts due to residual voltage.

  However, this approach also has some disadvantages. First, while tracking the previous optical state, the algorithm used may not take into account the elapsed time between each image change (optical state change). As a result, the value chosen for the lookup table must be selected taking into account the usage model, for example, an average of one update per second. Second, this method requires additional memory. In order to achieve high accuracy, the size of the lookup table must be increased, and as n increases from 2 to 3, it is required to further increase the amount of memory. As mentioned in the above application, in some cases, a very large look-up table is required, so in some cases it can be difficult to adapt to a mobile device.

  According to the waveform selection method of the present invention, an alternative approach is proposed here. That is, the residual voltage of each pixel is first determined (or estimated using various parameters known to be related to the residual voltage), and then one of the two or more waveforms is determined. Or it is selected based at least in part on the estimated residual voltage. Such a waveform selection method can be utilized as several possible approaches to the estimation or prediction of residual voltage based on known or measured display characteristics. The waveform selection method can also involve direct measurement of residual voltage.

  In a complete manner, the complete update history for each pixel can include both the applied voltage and the elapsed time between image updates. The decay model is used to predict the residual voltage remaining from each previous update. Updates that occur long enough (typically about 10 minutes) before changes are taken into account are ignored and their history can be erased. This is because the contribution to the residual voltage level is essentially reduced to zero. The residual voltage of the pixel can be modeled as a collection of residual voltages from each previous associated update.

  In practice, the preferred approach requiring less memory is to track a single residual voltage value and timestamp for each pixel. Prior to updating each image, the residual voltage of each pixel is decreased by an amount determined by the attenuation function of the display, and the time stamp of that pixel is updated. After each update, the residual voltage value is increased or decreased by an amount based on the waveform actually used, and the time stamp is updated. In this way, the residual voltage is tracked over the whole period, but only two data values have to be stored per pixel.

  The attenuation function and the change function can be calculated in any suitable manner. For example, it can be calculated via analog logic devices, by logic calculations based on formulas and data parameters, or by look-up tables with gradations suitable for display applications. The actual update of the stored residual voltage and time stamp values can occur in any suitable manner, for example, a single step that combines the results of both calculations. If the waveform used to update the image contains continuous pulses over a long period of time (e.g., 300-1000 milliseconds), the residual voltage and / or timestamp value can be set in the interval between image updates itself. It can be advantageous to update.

  The attenuation function in a given display is very sensitive to a number of factors, such as the material of the display, the manufacturing method, and system design features. Therefore, if the electro-optic display is different, it is necessary to change the attenuation function and the function parameter. In practice, electro-optic media and displays are complex, so the display system is experimentally measured for the residual voltage response and decay when applied voltage is applied continuously, and a lookup table is created from the results. It has been found that it is most effective to do or fit a function to the data. It may be beneficial to repeat this measurement step periodically in the manufacturing process, for example when switching to a new material set or changing batches. It may also be beneficial to individually characterize the attenuation function for each display after display assembly and to record the resulting parameters in the display controller.

  The residual voltage decay and response function in a given display can be affected by environmental factors such as temperature and humidity levels. Sensors or user selectable input values can be added to the display (or device that includes the display) to track these factors. Thus, it may be advantageous to use a residual response and decay function or look-up table that allows for these environmental parameters. Regardless of whether the display has been updated, the value of the residual voltage is updated periodically (for example, every 30 to 300 seconds), and the stored value is expected for environmental changes such as temperature and humidity, It can also be advantageous to maintain accuracy.

  If the pixel of the display is small, the total residual voltage of one pixel can be greatly affected by the residual voltage of neighboring pixels. Thus, the function to update the residual voltage can be used to allow for the effects of the external electric field, or pre- or post-processing algorithms can be introduced with such effects in mind. Again, in each pixel, the value of the residual voltage is periodically updated in response to the value of the last pixel's residual voltage, and thus the current value of the residual voltage is appropriately accurate. It may be advantageous to perform the estimation.

  The foregoing discussion has focused on methods for estimating residual voltage based on system input and characteristics. An alternative approach is direct measurement of residual voltage. An electrophoretic display state sensing technique is described in US Pat. No. 6,512,354. Similar techniques can also be used for residual voltage sensing of other electro-optic displays. The aforementioned 2003/0137521 specifically describes the use of a comparator to measure the residual voltage. A direct measurement of the residual voltage can be achieved by updating and modifying the above data values at every image update or periodically.

  The estimation method and the direct measurement method can be used together. For example, an estimation method can be used at each image update, but the value of the residual voltage can be updated periodically based on actual measurements. Because the residual voltage response and decay rate can change with the operating life of the display (which can be a period of several years), the display controller software is able to track and adapt to such changes (eg, current It may be advantageous to use a base algorithm (such as a Bayesian algorithm) that updates the prediction parameters based on the data.

  Similarly, an estimation method is used for each pixel, and the value of the residual voltage can be directly sensed by one or more test pixels. Then, the value of the residual voltage for the remaining pixels is adjusted based at least in part on the difference between the estimated residual voltage and the measured residual voltage for the test pixel. The test pixel can be a pixel that is visible to an observer of the display or a pixel that is not visible.

  At least in some cases, the characteristics of the residual voltage of the display can be very sensitive to the electrical properties of one or more particular layers (eg, the adhesive layer) of the display. Thus, the approach described below for estimating or measuring the residual voltage can be modified. This is to estimate or measure the electrical properties of a particular layer (eg, adhesive layer) that has a significant impact on the residual voltage, and to modify the algorithm appropriately for pixel residual values. . It is possible to use the sensor for probing a particular layer of the display and to probe the material associated with the pixel where the sensor is visible or not. Furthermore, a physical sample of the relevant layer material can be provided outside the display as part of the sensor. Some of the sensors incorporate the material to directly measure the response and change over time.

(Residual voltage to recognize waveform and addressing method)
According to the waveform selection method of the present invention, when the residual voltage (or surrogate variable) is estimated or measured by the above-described method or other appropriate method, at least a part of the estimated or measured current residual voltage or surrogate variable Based on this, an addressing method is selected. The addressing method may be chosen based on the residual voltage of a particular pixel, the residual voltage of that pixel and its surrounding pixels, or the total residual voltage across a display or a larger display than a pixel and its immediate neighboring pixels.

  Various methods can be used to change the standard waveform (ie, the waveform in which the residual voltage is not noticed) to allow for the residual voltage of a particular pixel or group of pixels. For example, the residual voltage can be subtracted from the desired waveform and a reduced voltage applied so that the effective waveform experienced by the pixel is the original desired waveform. Alternatively, magnification or other deformation can be applied to the waveform. Alternatively, the voltage level in the waveform can be maintained unchanged, but its duration can be adjusted. For example, if the standard waveform requires a pulse of 10V and 50ms, but if the pixel has a residual voltage of 2V, the pulse will instead be 40ms at 10V, 50ms at 8V, 44.7ms at 8.94V, or 0V Can result in two pulses of 10 ms at 10 V with a pause in the middle of 10 ms (for simplicity, these examples do not take into account the rate of decay of the residual voltage of 2 V, but the initial value of 2 V Can be adjusted more accurately to meet the expected drop in residual impulses from These waveforms can be adjusted to incorporate the fact that the net impulse is not necessarily ideally perfectly constant. This is because an electro-optic medium can have a small threshold, or its optical response to a pulse voltage or duration can be asymmetric in the waveform.

  This direct calculation of the waveform adjustment in this manner can impose a significant amount of processing on the display controller. To reduce such throughput, the controller may instead choose an addressing waveform, algorithm, formula or look-up table from a series of options, each associated with a range of residual voltage values. In this way, the waveform selection method of the present invention extends to selection from two or more essentially equivalent waveforms (waveforms in which the final optical state of the pixel after completion of the waveform is not substantially different). This is to minimize the change in the residual voltage aggregated within the pixel (that is, to give a very low residual voltage waveform). The optimal waveform can be determined by modeling the decay rate of the electro-optic medium or by direct experimentation and waveform tuning and optimization processes. The waveform selection method of the present invention also selects from essentially equivalent waveforms that produce an equivalent or non-minimum residual voltage, or to select a waveform that brings the net residual voltage of any pixel close to zero. spread. Such a waveform may be referred to as an “offset residual voltage waveform”.

  As described in the above-mentioned WO 2004/090857 and PCT / US2004 / 21000, the waveforms used in the individual changes are DC balanced (as opposed to DC balanced in the overall drive scheme). Certain drive schemes can be used and are often desirable. In other cases, certain portions of the waveform may be DC balanced even though the entire waveform is not DC balanced. For example, shake-up pulses, blanking pulses (see below), and a number of rail-stabilized addressing methods. During such a DC balanced waveform sequence, a pixel that is guided to its extreme optical state (hereinafter referred to as black and white for the sake of simplicity) is included in a part of the sequence. Can select which direction to switch first, white or black. When a switch to one extreme optical state is followed by a switch to the other extreme optical state, the second switch usually has a greater effect on the residual voltage. This is simply because it occurs later in time and the effect of the residual voltage decays with time. In this way, in the sequence of the DC balanced waveform, the selection of whether to switch to black first or to white switches whether the residual voltage for a given pixel has increased slightly or slightly. It can be determined by whether it has decreased. This is another example of the offset residual voltage waveform of the present invention.

  Some drive schemes require blanking pulses that lead the pixels to extreme optical states periodically (typically every 10 minutes or every 10 minutes or every image update). For example, see the aforementioned 2003/0137521. For example, a blanking pulse may switch the display from all white to all black or from all black to all white. In accordance with the waveform selection method of the present invention, these alternatives are made to reduce the residual voltage, thereby reducing the perceived ghost. Alternatively, determining whether the residual voltage of the display pixels as a whole is positive or negative, the appropriate blanking sequence (black / white or white / black) can be used without increasing the image update time. By selection, the total residual voltage of the display can be reduced. As a variation, the determination of which optical rail (extreme optical state) to hit first is not based on the aggregated residual voltage, but on the number of pixels having a high residual voltage in either direction. More generally speaking, which rail should the media be guided first in order to minimize annoying effects such as display anomalies due to residual voltage in the preferences of the user using the intended application? It can be used to determine any suitable algorithm.

  If desired, the algorithm may provide for the introduction of an additional blanking sequence (white-black-white or white-black-white-black) when the residual voltage is extreme in one or both directions. Obviously, the voltage level of the blanking pulse of each pixel can be changed by its duration.

  The waveform selection method according to the present invention further extends to extending the duration of the voltage pulse while the electro-optic medium is already in the extreme optical state (ie, on the optical rail). Therefore, the residual voltage can be increased or decreased without disturbing the optical change. Such a voltage pulse extension opportunity exists every time the pixel is brought into an extreme optical state. The above-described blanking pulse is an example. Therefore, the waveform selection method provides for the blanking pulse duration (or voltage) to change according to the method for each pixel. Based on the calculation of each pixel, the net residual voltage component is applied by extending the pulse in either direction, thus the total residual voltage for that pixel can be reduced or eliminated. Thus, the blanking pulse can be used to reduce the residual voltage across all pixels of the display without any apparent optical effect. In practical terms, the degree to which a pulse is extended can be quantized. That is, the pixels are classified into categories based on the residual voltage range, and the same adjustment can be applied to all pixels within each category.

  The so-called “rail stabilization” drive scheme is known (in the aforementioned 2003/0137521, WO 2004/090857 and PCT / US2004 / 21000), which means that any pixel touches the optical rail. Rather, it allows only a limited number of changes to be received, thus providing for each pixel to be switched to one of its extreme optical states on a frequent basis. For example, because a pixel changes from one gray level to another, the pixel can first be switched to a dark or white state (perhaps for an extended period) and then a pulse can be subsequently applied to achieve the desired Reach the gray level. Such a change is likely to generate a positive or negative residual voltage due to the long period that the pixel is addressed towards extreme optical states. According to the waveform selection method of the present invention, the residual voltage of the pixel can be minimized by changing it using the optical rail. The residual voltage generated by the direction of the switch tends to be opposite in polarity to the residual voltage carried by the pixel just before the change.

  One reason for using a rail-stabilized drive scheme is to mitigate the optical effects of residual voltage. Measurement prediction of the residual voltage level per pixel may reduce the need to use such a rail-stabilized drive scheme, as described above. The hybrid approach uses a rail stabilization method for pixels where the residual voltage is quite high, but switches directly to the desired state if the residual voltage is low and is unlikely to affect the image To do (direct impulse method).

  Another approach to reducing the residual voltage is to identify pixels with extreme residual voltages (i.e., having a magnitude that exceeds some predetermined value), and to such pixels prior to a general image update. Applying an offset voltage to reduce the residual voltage, or preconditioning the residual voltage level across the display. Such preconditioning can shorten the rail stabilization period and achieve a faster recognized image update time. If the offset voltage is small and applied over a sustained period of time, or if the offset voltage tends to help extend the rail stabilization period rather than pulling the particles back from the rail, the residual voltage reduction is noticeably noticeable. Can be achieved without any impact.

  The above discussion has focused on tracking the net residual voltage and selecting an appropriate algorithm that responds to the residual voltage reduction. Another parameter of pixel or display image history is the net DC imbalance. It will be apparent to those skilled in the imaging art that most of the methods described above can be modified to track and correct DC imbalance, either in combination with any residual voltage adjustment, or alone. For example, in the approach described above, DC imbalance can be used when determining which optical rail to select first and which preconditioning is appropriate. For example, even if the residual voltage can be reduced by changing the waveform, if the pixel is already DC imbalanced and the imbalance increases after adjustment of the residual voltage, the drive scheme will omit this correction. obtain. Similarly, any conditioning of the display to achieve a net DC balance or any adjustment of the waveform is allowed when estimating the residual voltage across each pixel.

  Thus, the waveform selection method of the present invention can be generalized as an addressing method for an electro-optic display capable of indicating a residual voltage. Here, the data value corresponding to the residual voltage is determined, and the addressing waveform is selected based at least in part on the value of the residual voltage. In such a method, time and residual voltage values, or data representing each, can typically be tracked. However, it is to be understood that addressing waveforms for electrophoretic displays and other electro-optic displays can implicitly or approximate time and residual voltage values. For example, the aforementioned 2003/0137521, WO 2004/090857 and PCT / US2004 / 21000, and the so-called “state before n” addressing methodology described above may not track time. The optical state history of the previous pixel is tracked. This can be a proxy for time if the designer of the drive scheme has knowledge of typical usage models and normal elapsed time between image updates. Thus, it is now recognized that such methods tend to reduce residual voltage and show improved ghost behavior.

  One practical reason why such a method has been used is that many electro-optic display controls do not have access to clock information that tracks the elapsed time between image updates. is there. This is probably because such elapsed time data is most useful in bistable displays, and bistable displays have never been commercially available. As a preferred form of the waveform selection method of the present invention, the control device includes a clock or an equivalent time mechanism. Alternatively, the controller may be in logical communication with an external information source (eg, a device that uses a display as an output device) that generates an elapsed time value and provides this information to the controller. For example, the device may provide time information with a function call to the display controller or with each new set of image data. Such time information can be quantized (eg, immediately after, 0.5 second, 1 second, 2 seconds, 10 seconds, 30 seconds, 60 seconds, over 60 seconds). As a result, it still provides useful information while reducing data bandwidth. In particular, it provides useful information when the quantized time band is chosen to correspond to a substantially exponential decay of the residual voltage.

  In general, it is most beneficial for the controller to receive elapsed time data for each pixel. This is because some pixels may not change during the update. However, it is still beneficial to receive data corresponding to recent image updates, recent blanking pulses, or elapsed time since a recent update in a set of pixels. Further, the control device may receive data indicating a probable update frequency of the display, for example, a flag indicating that the user is currently entering text. Text can require many updates quickly and continuously over the entire display or in a limited area of the display.

  As another form of rough correction of the residual voltage, the dwell time waveform selection method according to the present invention is used. This method provides a selection from a number of waveforms to perform the image change. Here, the selection from multiple waveforms is based at least in part on the dwell time of the associated pixel in the initial gray state, or some proxy for this dwell time. This time-dependent selection from multiple waveforms implicitly takes into account that the residual voltage decays over time. This is even if the residual voltage is not clearly tracked, inferred or measured.

  For example, the particular dwell time selection method of the present invention may be applied to a display controller with four gray levels using a drive scheme based on a logic change table with a total of 16 entries. Each entry corresponds to a change from one gray level (0, 1, 2, 3) to another gray level (0, 1, 2, 3). The entry selection is based on knowledge of the initial and final gray levels in the desired change. Three waveforms are considered in each entry. The controller selects the first waveform when the image change occurs within 1 second after the previous image is updated, and when the image change occurs between 1 second and 5 seconds when the previous image is updated, The second waveform is selected, and if the image change occurs after more than 5 seconds after the previous image is updated, the third waveform is selected.

  In the dwell time waveform selection method, the waveform is represented by a look-up table (as described above), modified to accommodate changes in environmental conditions (or divided into sub-tables), and display It may be set partially or totally to include specific parameters of individual displays during manufacturing. In short, the waveforms used in this method are the same as the optional components and variations described above in 2003/0137521, WO 2004/090857 and PCT / US2004 / 21000 and above. May be included.

  As described above, in the waveform selection method of the dwell time according to the present invention, even if the residual voltage is not clearly tracked, a specific pixel is determined based on the elapsed time since the display was updated. Even if it may not be based on the elapsed time since the update, the dwell time waveform selection method implicitly approaches both the residual voltage and the pixel update elapsed time, and therefore, compared to prior art drive schemes, It is understood that the ghost behavior is improved.

(Material selection)
As already mentioned, the choice of material used for the electro-optic display can have a significant effect on the residual voltage. In such displays, the residual voltage is present during operation and can thus have a significant impact on the electro-optical performance of such displays.

  Also, as discussed above, certain materials exhibit I-type polarization that contributes to residual voltage when used in electro-optic displays. This polarization is often thought to depend on the mobility and concentration of ions moving through at least one of the constituent materials. (However, the present invention is never bound by this idea).

  The rate of decay of the residual voltage can be measured in any material by providing a test cell that contacts the same interface that the material is in the proposed display. For example, a test cell is prepared that controls the thickness of the laminated adhesive coated on the ITO substrate, and an electric field is applied across the laminated adhesive / ITO interface. The peak value and decay of the residual voltage was then measured by opening the charge circuit and monitoring the voltage across the pixel with a high impedance voltmeter.

It has been found that the higher the ion mobility of the laminated adhesive, the faster the decay rate of the residual voltage. The volume resistivity of the laminated adhesive is preferably less than about 10 ¹¹ Ωcm.

  Previous E Ink patent applications, such as the aforementioned 2003/0011867 and 2003/0025855, and US application 10 / 708,121 (filed February 10, 2004) control resistivity. For example, a Z-axis pressure-sensitive adhesive is described with respect to the laminated pressure-sensitive adhesive, or a heterogeneous or anisotropic and pressure-sensitive laminated pressure-sensitive adhesive. Such an adhesive may provide the additional benefit of reducing residual voltage.

  Laminated adhesives can also exhibit type II polarization. In test cells, it has been found that increasing the thickness of the adhesive increases the residual voltage. Since the polarization at the interface should be independent of film thickness, this result suggests the presence of internal charge polarization sites, a type II polarization effect. Therefore, in the encapsulated electrophoretic display, care must be taken when selecting the thickness of the adhesive and the shape around the capsule. The same test laminate adhesive was heated to drive out suspicious impurities and crystalline regions. Thereafter, a decrease in residual voltage was observed.

  Type I polarization can occur anywhere in the display where there is an interface in the material. Use the same material for the laminate adhesive and binder (ie, the materials used around the capsule to form the capsule in the adhesion layer, these are described in many of the aforementioned E Ink and MIT patents and patent applications. It has been found that the interface is removed and the residual voltage is reduced. Accordingly, the present invention provides an electrophoretic display comprising a microcavity binder and a laminated adhesive. Here, these materials are either the same or similar in composition, or are either electrically conductive or electrically equivalent in ion mobility. Although the materials may differ in their composition, it is desirable to dope materials with low conductivity that can achieve substantially the same ion mobility on both sides of the interface.

  Type I polarization at certain interfaces can be affected by surface roughness. Providing interpenetration of the material on both sides of the interface by planarization or the introduction of some type of interface texture can provide advantages. These techniques can also result in increasing or decreasing polarization at specific interfaces. Either increase or decrease can be a benefit, depending on the particular display you are considering. For example, if the polarization increases in one location and this polarization offsets the polarization elsewhere in the display, it can cause a residual voltage reduction across the electro-optic medium. Typically, if the interface causes a residual voltage strongly associated with the electro-optic medium, it is desirable to reduce the degree of polarization at the interface and reduce its decay rate.

  Type I polarization at the interface can also be affected by surface cleanliness. It is desirable to clean the substrate prior to coating and lamination in order to achieve consistent electrical behavior.

(Conductive path in the electrophoresis layer)
In a microcavity electrophoretic display, cell walls (this term includes the capsule wall of an encapsulated display as used herein) that are electrically parallel to the electrophoretic inner phase (suspended fluid and charged particles). Exists. A current in the form of charged ions can flow through the inner phase or cell wall. The cell wall can be a polymer such as gelatin or any other suitable material. The cell wall is typically further surrounded by a binder as described above. Thus, some of the current may also flow between the electrodes of the display through the binder or cell wall without going through the electrophoresis inner phase. Such currents do not contribute to changes in the electro-optic state of the display or its pixels.

  In the preferred electro-optic displays described in the aforementioned E Ink and MIT patents and patent applications, the conductivity of the cell walls and binder is typically slightly higher than the conductivity of the inner phase. The relaxation of the residual voltage can thus partly occur through the binder and the cell walls.

  During application of an electric field to the electrophoretic medium, the charged particles move toward the two electrodes of the display. If the charged particles near the front electrode (the electrode through which the observer typically sees the display) cluster over a period of time, the corresponding electrons or oppositely charged ions react to the cell wall and / or It can flow through the binder. The charged region thus formed can form a residual voltage that affects subsequent image updates. As a result, the conductivity and ion mobility of the cell walls and binder are important, and their morphology is also important.

The residual voltage for a particular cell / binder configuration can be measured in a manner similar to that described for the laminated adhesive. According to aspects of the volume resistivity balanced electrophoretic medium of the present invention, the binder and the cell wall have a volume resistivity that is at least less than twice the volume resistivity of the electrophoresis internal phase, and both the binder and the cell wall. Preferably have a volume resistivity of less than about 10 ¹¹ Ωcm. More generally, the electrophoretic medium is in the form of a continuous layer (single continuous phase of polymer-dispersed media, a combination of encapsulated electrophoretic media cell walls and binders, or only a cell wall of microcell electrophoretic media. A plurality of discrete droplets of suspending fluid dispersed within, the droplets comprising a plurality of charged particles held in the suspending fluid and applying an electric field to the electrophoretic medium When movable through the suspending fluid, the continuous phase has a volume resistivity not greater than about ½ of the volume resistivity of the droplet, and both the continuous phase and the droplet are less than about 10 ¹¹ Ωcm. It is preferable to have a volume resistivity of In a preferred embodiment, the binder and cell walls occupy between about 5% to about 20% by volume of the electrophoresis medium (the remainder being the electrophoresis inner phase) and the binder is evenly between the capsule walls. Distributed to.

(Consideration of zeta potential and two types of particle electrophoresis media with charge balance)
The preferred type of electrophoretic medium described in many of the aforementioned E Ink and MIT patents and patent applications is the so-called "two particles of opposite polarity" medium. Here, the electrophoretic internal phase includes two different types of particles having opposite polarity charges (see, for example, the discussion of various types of electrophoretic media described in the aforementioned 2002/0171910). The charge amount of each particle is, for example, the above-mentioned 2002/0185378 and the co-pending application No. 10 / 711,829 filed on October 7, 2004 (see also PCT / US2004 / 33188 corresponding thereto) ) Can be controlled by the surface deformation described in (1). The number of particles in each microcavity can be controlled in a predictable manner by selecting the total amount of particles provided to the electrophoretic inner phase prior to encapsulation or implantation in the microcell. By multiplying the average charge amount per particle by the average number of particles per minute cavity, the total charge amount can be estimated for each type of particles in the minute cavity.

  When the total charge amount of multiple types of particles charged to opposite polarities is not nearly balanced, a particularly large polarization is formed in the polarized microcavity, and the corresponding large and slowly decaying polarization continues. It has been found to occur with phase materials. By changing the net total charge amount of multiple types of particles, the system leaves a residual voltage with the same sign (so that subsequent updates in the reverse direction are delayed), that is, a system with little residual voltage. It is possible to change the encapsulated electrophoretic display between a regime in which the electric field leaves a residual voltage of opposite sign (so that subsequent updates in the reverse direction are facilitated).

  According to the aspect of the two types of particles of charge balance according to the present invention, it is preferable that one type of electrophoretic particle does not exceed about twice the total charge amount of other types of particles. Further, according to the aspect of the low residual voltage electrophoretic medium of the present invention, in the electrophoretic display with two kinds of particles charged in opposite polarities, the particle charge amount, particle mass, and particle mobility are set to “low residual voltage”. It is preferably selected to show "behavior". As used herein, “low residual voltage behavior” refers to a measurement of less than about 1V (preferably less than about 0.2V) exactly 1 second after applying a 300 ms square wave DC addressing pulse at 15V. Defined as having a residual voltage.

In order to evaluate the charge balance in the internal phase of electrophoresis due to two kinds of oppositely charged particles, the amount of charge in each particle is analyzed relative to its mass (because the mass can be easily measured during manufacturing). ) Is effective. The ratio of charge amount to mass is the following relationship: q / M is proportional to ζ / d ² (1)
here,
q = particle charge amount,
M = mass,
ζ = zeta potential (mV),
Although it is believed that d = particle diameter can be used to estimate, the present invention is in no way bound to this idea.

  The net total charge of the electrophoresis internal phase should desirably be controlled by carefully and simultaneously optimizing the particle charge, particle mass, particle diameter, and zeta potential.

  In a substantially charge-balanced electrophoretic medium (defined above) that exhibits low residual voltage behavior, such behavior typically ends when any of the following occurs: (A) The average charge amount of any kind of particles varies from about 20% to 100%. (B) The relative mass of one type of particle varies from about 50% to 300%. (C) The average diameter of one type of particle varies from about 30% to 200%. (D) The average mobility of one type of particle changes from about 20% to 100%.

(Additive for suspension fluid)
It is known that adding a surfactant to the suspending fluid of the electrophoretic medium can reduce the residual voltage. For example, when a single-pixel display uses an electrophoretic medium in which exactly the same two kinds of particles are charged with opposite polarities, provided that one of the suspensions is sorbitan trioleate (Span 80). It was shown that the residual voltage decreased when used with (commercially available) added.

  The surfactant is considered to change the balance of the relative charge amount of the two types of electrophoretic particles, but in the present invention, this concept is never limited. Furthermore, it is believed that changing the charge amount relaxation rate of the electrophoresis inner phase can reduce type III polarization and better maintain a balance with the corresponding relaxation rate of the outer phase.

  Thus, the present invention provides an electrophoretic display that exhibits low residual voltage behavior (as defined above). This behavior stops when the concentration of surfactant or charge control agent in the electrophoresis inner phase is changed from about 30% to 200%.

(Material for outer phase of microcavity electrophoretic display)
To obtain the desired residual voltage relaxation rate, it is possible to select materials for the outer phase for the microcavity electrophoretic display, or to mix, dope or condition such materials. As described above, the relaxation rate of the inner phase can be affected by a variety of factors including the choice of electrophoretic particles and the concentration of surfactant or charge control agent. One aspect of the present invention provides a situation where the material of the outer phase and the material of the inner phase are balanced (within factor 2) in the relaxation rate.

  This aspect of the invention provides an electrophoretic display that exhibits low residual voltage behavior (as defined above). This behavior stops when the conductivity of the outer phase is changed from about 30% to 200%.

  In a typical encapsulated electrophoretic display, an important outer phase material is the gelatin capsule wall. Wall conductivity is highly affected by humidity. In a preferred embodiment, the electrophoretic display contains moisture and is resistant to changes in the relative humidity (RH) of the operating environment. In a further preferred embodiment, the final display is electrophoresed (by placing the display in a controlled humidity environment until equilibrium is reached and / or by manufacturing the display in a controlled humidity environment). The display is tuned to achieve a layer between 20% RH and 55% RH, and preferably 35% RH.

  Thus, the present invention provides a method of manufacturing an electrophoretic display that includes RH adjustment of the display material. The electrophoretic display may also comprise a moisture barrier or substrate that is impermeable to water.

(Low threshold electro-optic display)
There are many ways to reduce the threshold in electrophoretic displays or other electro-optic displays. The threshold value can be generated by the attractive force between the particle and the wall or the attractive force between the particles. The attractive force may be an electric one generated from particles charged to the opposite polarity, a physical one such as surface tension, or a magnetic one. The threshold can also arise from the nature of the suspending fluid. That property can be very shear-thinning, or can have a distinct yield stress (eg, for Bingham fluid) or electrorheological properties. Applying an additional electric field to, for example, an in-plane electrode or control grid can be a substitute for the threshold.

For the purpose of this application, the threshold is considered to exist at a certain voltage level. That is the case when a square wave DC pulse lasting 1 second is applied to the display at that voltage level, resulting in an optical change of less than 2L ^* .

  In display technology, it is known that electro-optic media can serve as the basis for a passive addressing scheme. Typically, such a scheme is based on a threshold equal to half of the switching voltage (“V / 2”). In some drive schemes, passive addressing can be achieved with a minimum threshold of one third of the switching voltage (“V / 3”).

  In contrast, as described above, a small threshold of 1V compared to ± 15V switching voltage may be useful in reducing the effect of residual voltage on electro-optic performance. Accordingly, aspects of the low threshold display according to the present invention provide an electro-optic display that operates with a voltage not greater than ± V. Here, the electro-optic material has a threshold voltage greater than zero and less than V / 3.

(Production of electrophoretic display with low residual voltage)
The final aspect of the present invention relates to applying various improvements to electrophoretic display manufacturing to reduce the residual voltage exhibited by displays manufactured to reduce the residual voltage.

  During the manufacture of an encapsulated electrophoretic display, the capsules are typically suspended in a slurry. The slurry includes capsules and polymer binders, and may also include various additives such as water, plasticizers, pH adjusters, biocides, surfactants or charge control agents. For this purpose, such a slurry can be considered to include a “binder” consisting of the non-volatile elements of the slurry, excluding capsules. In some cases, the binder material may be separated during slurry preparation, shipping and storage, and may not necessarily be well mixed prior to coating. As a result, there may be partially non-uniform areas. This can cause type II polarization problems in the final display. In order to reduce such problems, thorough mixing of such binder materials may be appropriate, for example, over a long period of time, such as mixing with a propeller blade or mixing on a roll mill. It is desirable to take appropriate measures.

  The dry binder material should desirably have uniform electrical properties. This is such that after applying a voltage pulse of 15V over 300 ms and a pause of 1 second, the measured residual voltage of the binder material itself is less than about 1V, preferably less than 0.2V.

  As described above, since the space between the capsules occupied by the binder can contribute to the type III polarization, it is desirable to control this amount of space. Electrodeposition can be used to directly control the capsule space. This is described in co-pending application no. 10 / 807,594 filed on March 24, 2004 (see also corresponding international patent application PCT / US2004 / 009421). In microcell electrophoretic displays or photo-patterned electrophoretic media, the microcavity spacing can be directly controlled.

  In a coated encapsulated electrophoretic display, the dry capsule spacing and morphology is the result of a number of controllable factors discussed in several of the aforementioned E Ink and MIT patents and patent applications. In summary, the capsule morphology depends on the thickness and elasticity of the capsule wall, the design of the coating slurry, the height of the coating die away from the substrate, the amount of coating slurry that passes or pumps from substrate to die, the substrate web It can be adjusted by changing the speed and drying conditions (temperature, duration and aeration, etc.) of the wet coated membrane. The following principles are helpful in controlling capsule spacing and morphology.

(A: Influence of characteristics of capsule wall in dry capsule shape)
The properties of the capsule wall vary with the material and with the encapsulation process variables (especially the mixing rate). It is desirable that the capsule wall should be sufficiently elastic so that the overall capsule height to diameter ratio can be between 0.33 and 0.5. However, the capsule wall also ideally allows for local variations (bend radius that bends close to 90 degrees at the acute angle in the hexagonal close-packed structure of the capsule on the substrate on which the capsule is coated). Should. This is described, for example, in US Pat. Nos. 6,067,185 and 6,392,785.

  It is believed that the elasticity of the capsule wall can be influenced by the strength of the cross-linking of the capsule wall material (the weaker the cross-linking, the more flexible the capsule wall) and the thickness of the capsule wall (but , I'm never caught by this idea). Wall thickness is affected by inner phase formation, gelatin / gum arabic levels and process parameters. In any capsule dense pattern, reducing the wall thickness is the “aperture ratio” of the medium (ie, the portion of the region of the electrophoretic medium where the change in optical state occurs; in the region occupied by the capsule wall, such as The area where no change can occur can be improved. However, too thin walls can easily burst.

  Some process parameters that have been found to have a significant effect on wall thickness are shown in the table below. The results shown in the table were obtained by encapsulation experiments performed on a 4L scale. The table also shows the relative qualitative ranking for wall thickness compared to the standard operating procedure for encapsulation. For 4L encapsulation, the standard process conditions are: gum arabic level (standard level is 100% index), pH (4.95), emulsification processing temperature (40 ° C.), cooling rate (3 hours), and inner phase Speed. In the table, rank 3 means a standard thickness wall, 1 means a very thin wall, and 5 means a very thick wall.

  The pH is an important parameter for the wall properties. This is not only because of the wall thickness, but because the solid content and viscosity of the coacervate vary greatly with pH level. Finally, the type of gelatin and gum arabic used can also have a dramatic effect on wall properties.

(B: Binder evaporation as a mechanism to change dry capsule shape)
The effect of binder evaporation varies depending on how close the capsules are packed while the coated slurry is still wet. Even with the same capsule diameter and the same binder ratio, it can be flat (oblong ellipsoid) or high (substantially prismatic) depending on the proximity of the wet capsule.

FIG. 2 represents the situation where the capsule / binder slurry is coated on the substrate 110. The capsule 112 is sparsely coated. That is, they are separated from each other by an interval corresponding to the diameter of the capsule 112. As shown in FIG. 2, in this situation, a portion of the capsule 112 that is remote from the substrate 110 is only partially buried in the uncured binder 114. In this way, the capsule 112 protrudes from the layer of the binder 114. The binder and the boundaries of each capsule, a circle of radius r _c of the capsule are each substantially spherical.
During drying, the surface tension causes a downward force on the capsule,
F = 2πσr _c sinψ _c
It is understood that here,
F is the downward force acting on the capsule,
σ is the surface tension of the liquid around the capsule,
r _c, as shown in FIG. 2, the diameter of the contact circle liquid makes with the capsule,
[psi _c is complementary angle of the contact angle of the liquid around the capsule (i.e., 90 ° - contact angle) is.

From Figure 2, as the size of the capsule is increased, as the ambient level of the liquid drops, the r _c increases are evident.

  The case where the influence of this downward force is extreme is shown in FIGS. 3A to 3B and FIGS. 4A to 4B of the accompanying drawings. 3A and 4A, arrow A indicates the evaporation of water from the wet binder, and arrow B indicates the force acting on the capsule due to the influence of the surface tension. In FIG. 3B and FIG. 4B, “C” indicates a dried binder. (Note that in FIGS. 3A-4B, the presence of a binder outside the two illustrated capsules is ignored.) FIGS. 3A and 3B show the downward force of a sparsely coated wet capsule. Shows the effect of. The capsules are coated to leave a spacing between adjacent capsules that is substantially a small length relative to the capsule diameter. From FIGS. 3A and 3B, under the influence of the downward force, the original spherical capsules are flattened and become spheroids, typically in contact with each other in the final dry layer, however, these spheroids It becomes clear that there is little or no distortion due to contact between adjacent capsules. In contrast, FIGS. 4A and 4B show the effect of downward force on a densely coated wet capsule. The wet capsules are in contact with each other while still wet. From FIGS. 4A and 4B, the downward force effect causes the capsules to gradually contact each other over a larger area. In the final dry layer, the capsules are in the form of polygonal prisms that are substantially higher in height than width. This is most ideal if the wet capsules are hexagonal dense, but the dry capsules exhibit a substantially hexagonal prismatic shape. It should be noted that if the capsules are too loosely packed, voids can be left between the capsules as they dry and can be incorporated into the clusters.

(C: Effect of slurry preparation on dry capsule shape)
The pH level of the capsule can affect the shape of the dry capsule. As the pH increases, the charge amount of the gelatin changes and affects the attractive force from the gelatin to the substrate (typically the ITO surface). The capsule is coated on this substrate, making it difficult or easy to shift the capsule position. Changing the substrate and selecting the surface energy of the substrate can affect this relationship.

  The level of surfactant affects the adhesion of the capsules to each other (“stickiness”) and possibly the adhesion of the capsules to the binder (“stickiness”). A more active surface active agent should reduce the surface tension and reduce the surface tension acting on the capsule during drying. If the surface has a more inert configuration, it can contribute to the flattening of the capsule.

  The binder ratio is a major factor affecting the dry capsule shape. As the binder ratio decreases, the capsule becomes more rounded. A 2: 1 binder ratio (ie, 2 parts capsule weight to 1 part binder weight) is sufficient to completely surround each capsule as a complete sphere when dried. The result is therefore the least flat capsule. When the binder ratio is low, the binder is sufficient to fill the gaps between the capsules when dried. A binder ratio of 8: 1 is sufficient to form both flat and high (polygonal) capsules, depending on the coating conditions.

(D: Effect of coating parameters on dry capsule shape)
As described above, the coating process should optimally deposit wet capsules at a preset distance. Parameters important to achieve such a desired spacing include coating speed, die type, die height, and slurry flow rate.

  Experimentally, increasing the slurry flow rate to the coater while keeping other parameters constant will tend to increase the coating weight. As a result, the wet capsules are placed closer together. This results in a less flattened / higher dry capsule as some capsules can result in very close spacing.

  In related coating experiments, reducing the gap in the die to a low value (eg, 40-50 μm) resulted in a value comparable to the size of the wet capsule used. At this die height, a single layer of capsules is virtually ensured, but the packing is usually very tight. Coating with a lower die height tends to result in higher dry capsules. Ideally, the capsules are “almost touched together” and coated, but when wet, they are not packed together.

(E: Effect of drying parameters on dry capsule shape)
Experimentally, drying at 60 ° C. for 2 minutes in a conveyor oven can produce a film that is flattened, raised, and contains spherical dry capsules. Attempting to dry the capsule very quickly can result in a “skin” that forms on top of the binder. This skin traps moisture in the binder, causing the membrane to dry very slowly.

  Ventilation speed during drying affects the evaporation rate and affects whether evaporative gas is trapped between the capsules. Without adequate ventilation, it is difficult to achieve success in drying. Ventilation throughout the binder is effective.

  The above description has emphasized the application of the present invention to electrophoretic displays. Such an electrophoretic display can be of any type and still provide benefits from at least some aspects of the present invention. As described above, the present display can include a microcavity electrophoretic display such as an encapsulation, a microcell, a microcup, and a polymer dispersed display, an electrophoretic display using one or more kinds of particles (of course, according to the present invention). (Except for this aspect of electrophoretic displays with two specific types of particles), electrophoretic displays with clear or dyed suspension fluids, electrophoretic displays with oily and gaseous suspension media, Flexible and robust electrophoretic display, non-linear device (eg thin film transistor), passive means (eg control grid) and electrophoretic display addressed by direct drive, lateral or in-plane movement of electrophoretic medium, vertical movement or Move from electrode to electrode Electrophoretic display operating any combination thereof, full-color, and the like spot color and monochrome electrophoretic display.

  Finally, although the present invention has been primarily described for application to electrophoretic displays, many aspects of this invention apply to any electro-optic display or medium with residual voltage, and most importantly, Emphasis once again applied to bistable electro-optic displays

FIG. 3 is a graph showing a typical curve of residual voltage decay over time in an electrophoretic medium. FIG. FIG. 6 is a schematic side view showing a contact circle between a capsule and a surrounding liquid during coating of an encapsulated electrophoretic medium. FIG. 6 is a schematic side view showing forces acting on a loosely coated capsule during coating of an encapsulated electrophoretic medium. FIG. 3B is a schematic side view similar to FIG. 3A but showing the capsule shape of the final dry capsule layer as a result of the force shown in FIG. 3A acting. FIG. 3B is a schematic side view similar to FIG. 3A but showing the forces acting on a densely coated capsule. FIG. 4B is a schematic side view similar to FIG. 3B but showing the capsule shape of the final dry capsule layer as a result of the force shown in FIG. 4A acting.

Claims (25)

A method of driving a bistable electro-optic display having a plurality of pixels capable of displaying at least two gray levels,
Applying to each pixel of the display a waveform determined by the initial gray level and the final gray level of the pixel, the method comprising:
For at least one change from a particular initial gray level to a particular final gray level, if the pixel remains at that initial gray level for less than a predetermined interval, a first waveform is used and the pixel A method wherein a second waveform different from the first waveform is used if is kept at its initial gray level for longer than the predetermined interval. A different first waveform, second waveform and third waveform are used for at least one change,
If the pixel remains at its initial gray level for less than a first predetermined interval, the first waveform is used,
If the pixel is longer than the first predetermined interval but remains at its initial gray level for less than the second predetermined interval, the second waveform is used,
The method of claim 1, wherein the third waveform is used if the pixel remains at its initial gray level for longer than the second predetermined interval.   The method of claim 2, wherein the first predetermined interval ranges from 0.3 seconds to 3 seconds, and the second predetermined interval ranges from 1.5 seconds to 15 seconds. Storing, for each possible change between gray levels of pixels, a look-up table containing data representing the one or more waveforms used for the change;
Storing initial state data representing at least the initial state of each pixel;
Storing dwell time data representing the period during which each pixel remains in its initial state;
Receiving an input signal representative of a desired final state of at least one pixel of the display;
An output signal representing a waveform required to convert the initial state of the one pixel to the desired final state of the one pixel, the output depending on the initial state data, the dwell time data, and the input signal Generating the signal as determined from the look-up table. Further comprising storing data representing at least one previous state of each pixel prior to the initial state of each pixel;
The method according to claim 4, wherein the output signal is generated depending on both the at least one previous state and the initial state of the one pixel.   The method of claim 4, comprising receiving a temperature signal representative of a temperature of at least one pixel of the display and generating the output signal dependent on the temperature signal.   The method of claim 4, comprising generating a lifetime signal representative of an operating time of the pixel and generating the output signal dependent on the lifetime signal. A device controller for controlling a bistable electro-optic display having a plurality of pixels capable of displaying at least two gray levels,
Input means for receiving an input signal representative of a desired final state of at least one pixel of the display;
Storage means arranged to store, for each possible change between the gray levels of a pixel, look-up table data representing the waveform used for the change, comprising at least an initial state of each pixel; Storage means arranged to store initial state data representing;
Calculating means for determining from the input signal, the initial state data and the lookup table a waveform required to change the initial state of the one pixel to the desired final state;
A control device comprising output means for generating an output signal representing the waveform,
For at least one change, the storage means stores at least two different associated waveforms and dwell time data representing a period during which each pixel remains in its initial state, and the calculation means comprises the at least A control device, characterized in that which of two different waveforms is used for a change that depends in part on the dwell time.   The storage means is also arranged to store data of a previous state representing at least one previous state of each pixel prior to the initial state of each pixel, and the calculating means includes the input signal, the initial state 9. The controller of claim 8, arranged to determine the waveform depending on data, the dwell time data, the previous state data and the look-up table.   The input means is arranged to receive a temperature signal representative of the temperature of at least one pixel of the display, and the calculation means depends on the input signal, the initial state data, the dwell time data and the temperature signal. 9. The control device according to claim 8, wherein the control device is arranged to determine the waveform.   Life signal generating means arranged to generate a life signal representing the operation time of the pixel is further provided, and the calculation means is configured to generate the waveform from the input signal, the initial state data, the dwell time data, and the life signal. The control device according to claim 8, wherein: An electro-optic display comprising a layer of electro-optic material and voltage supply means for applying a voltage not greater than a predetermined value in any direction across the layer of electro-optic material,
The display, wherein the electro-optic material has a threshold voltage that is greater than zero and less than one third of the predetermined value.   The electro-optic display according to claim 12, wherein the electro-optic material has a threshold voltage that is not less than 1/15 and less than 1/3 of the predetermined value.   The electro-optic material comprises a suspending fluid and a plurality of charged particles held in the suspending fluid and movable through the suspending fluid by applying a voltage across the layer of the electro-optic material The electro-optic display of claim 13 comprising a base electrophoretic material.   The electro-optic display according to claim 14, wherein the electrophoretic material is an encapsulated electrophoretic material, a polymer-dispersed electrophoretic material, or a microcell electrophoretic material.   The electro-optic display of claim 14, wherein the suspending fluid is gaseous. An electrophoretic medium,
Suspension fluid,
A plurality of first type charged particles that are retained in the suspension fluid and are movable through the suspension fluid by application of an electric field to the electrophoretic medium;
A plurality of second type charged particles kept in the suspension fluid and movable through the suspension fluid by applying an electric field to the electrophoretic medium, wherein the second type particles are: A charged particle of the second type having a charge opposite to that of the first type of particle, the medium comprising:
2. The electrophoretic medium according to claim 1, wherein the total charge amount of the second type particles is between one-half and twice the total charge amount of the first type particles.   The electrophoretic medium according to claim 17, wherein the electrophoretic medium is an encapsulated electrophoretic medium, a polymer-dispersed electrophoretic medium, or a microcell electrophoretic medium.   The electrophoretic medium according to claim 17, wherein the suspending fluid is gaseous.   The electrophoretic medium of claim 17, wherein the electrophoretic medium exhibits a residual voltage of less than 1 volt after 1 second of applying a 300 millisecond square wave addressing pulse at 15 volts.   21. The electrophoretic medium of claim 20, wherein the electrophoretic medium exhibits a residual voltage of less than 0.2 volts after 1 second of applying a 300 millisecond square wave addressing pulse at 15 volts. An electrophoretic medium,
Suspension fluid,
A plurality of first type charged particles that are retained in the suspension fluid and are movable through the suspension fluid by application of an electric field to the electrophoretic medium;
A plurality of second type charged particles kept in the suspension fluid and movable through the suspension fluid by applying an electric field to the electrophoretic medium, wherein the second type particles are: A charged particle of the second type having a charge opposite to that of the first type of particle, the medium comprising:
An electrophoretic medium characterized in that the residual voltage is less than 1 volt after 1 second of applying a 300 millisecond square wave addressing pulse at 15 volts.   23. The electrophoretic medium of claim 22, wherein the residual voltage is less than 0.2 volts after 1 second of applying a 300 millisecond square wave addressing pulse at 15 volts. A method of driving a bistable electro-optic display having a plurality of pixels capable of displaying at least two gray levels,
Applying to each pixel of the display a waveform determined by the initial gray level and the final gray level of the pixel, the method comprising:
For at least one change from a specific initial gray level to a specific final gray level, at least a first waveform and a second waveform that are different from each other are available, and the residual voltage of the pixel in which the change proceeds is , Wherein the method is determined before the change, and the first waveform or the second waveform is used for a change depending on the determined residual voltage. An electrophoretic medium comprising a plurality of discrete droplets of a suspending fluid dispersed in a continuous phase,
The droplet further comprises a plurality of charged particles retained in the suspended fluid and movable by application of an electric field to the electrophoretic medium,
The medium is such that the continuous phase has a volume resistivity less than one half of the volume resistivity of the droplets, and both the continuous phase and the droplets have a volume resistivity of less than 10 ¹¹ Ωcm. A medium characterized by that.

Images