DRIVING SEQUENCES TO REMOVE PRIOR STATE INFORMATION FROM COLOR ELECTROPHORETIC DISPLAYS

Abstract

Methods for efficiently clearing previous state information when driving a multi-particle color electrophoretic medium, for example, wherein at least two of the particles are colored and subtractive and at least one of the particles is scattering. Typically, such a system includes a white particle and cyan, yellow, and magenta subtractive primary colored particles. The clearing pulse may include two different portions of alternating impulses and the overall waveform may be DC balanced.

Claims

1. A method of driving an electrophoretic display including an electrophoretic medium having four different types of electrophoretic particles, each type of electrophoretic particle having a distinct color and a distinct combination of charge polarity and charge magnitude, the method comprising: providing a clearing pulse comprising a first set of impulses of magnitude V.sub.1 and length t.sub.1 that alternate with impulses of magnitude V.sub.2 and length t.sub.2, where V.sub.1 is a positive voltage and V.sub.2 is a negative voltage of lower magnitude than V.sub.1, and a second set of impulses of magnitude V.sub.3 and length t.sub.3 that alternate with impulses of length V.sub.4 and length t.sub.4, wherein the ratio V.sub.1.Math.t.sub.1/V.sub.2.Math.t.sub.2 is greater than the ratio V.sub.3.Math.t.sub.3/V.sub.4.Math.t.sub.4; and providing a push-pull color addressing pulse having a push impulse of magnitude V.sub.5 and length t.sub.5 and a pull impulse of magnitude V.sub.6 and length t.sub.6, wherein V.sub.5 and V.sub.6 have opposite polarities.

2. The method of claim 1, wherein the electrophoretic display includes two types of positively-charged electrophoretic particles with different charge magnitudes, and two types of negatively-charged electrophoretic particles with different charge magnitudes.

3. The method of claim 2, wherein the two types of positively-charged particles are cyan and magenta in color and the two types of negatively-charged particles are white and yellow in color.

4. The method of claim 1, further comprising providing at least three impulses of magnitude V.sub.1 and length t.sub.1, at least three impulses of magnitude V.sub.2 and length t.sub.2, at least three impulses of magnitude V.sub.3 and length t.sub.3, and at least three impulses of length V.sub.4 and length t.sub.4.

5. The method of claim 4, wherein at least two impulses of magnitude V.sub.1 and length t.sub.1 are interspersed with an impulse of magnitude V.sub.2 and length t.sub.2, and at least two impulse of magnitude V.sub.3 and length t.sub.3 are interspersed with an impulse of magnitude V.sub.4 and length t.sub.4.

6. The method of claim 4, further comprising providing a DC balancing pulse, including a first DC balance impulse of magnitude V.sub.7 and length t.sub.7 and a second DC balance impulse of magnitude V.sub.8 and length t.sub.8, wherein V.sub.7 and V.sub.8 have opposite polarities, and wherein the sum of the voltage-time areas of all of the positive voltage pulses is equal to the sum of the voltage-time areas of all of the negative voltage pulses.

7. The method of claim 6, wherein
Σ.sub.i=1.sup.nV.sub.i.Math.t.sub.i=0 n=1-8, and V.sub.n and t.sub.n are as defined above.

8. The method of claim 6, wherein the DC balancing pulse precedes the clearing pulse and the push-pull color addressing pulse.

9. The method of claim 6, wherein the DC balancing pulse is between the clearing pulse and the push-pull color addressing pulse.

10. The method of claim 1, wherein the electrophoretic display comprises a first light-transmissive electrode layer, a second electrode layer comprising a plurality of pixel electrodes, and an electrophoretic layer comprising the electrophoretic medium disposed between the first light-transmissive electrode layer and the second electrode layer.

11. The method of claim 10, wherein the electrophoretic layer comprises a plurality of microcells containing the electrophoretic medium.

12. The method of claim 10, wherein the electrophoretic layer comprises a plurality of microcapsules containing the electrophoretic medium.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0041] FIG. 1 shows a prior art clearing waveform for an ACeP-type electrophoretic display. FIG. 1 is equivalent to FIG. 6 of U.S. Pat. No. 10,593,572.

[0042] FIG. 2 is a schematic cross-section showing the positions of the various colored particles in an electrophoretic medium of the present invention when displaying black, white, the three subtractive primary and the three additive primary colors.

[0043] FIG. 3A shows in schematic form four types of different pigment particles used in a multi-particle electrophoretic medium.

[0044] FIG. 3B shows in schematic form four types of different pigment particles used in a multi-particle electrophoretic medium.

[0045] FIG. 3C shows in schematic form four types of different pigment particles used in a multi-particle electrophoretic medium.

[0046] FIG. 4 illustrates an exemplary equivalent circuit of a single pixel of an electrophoretic display.

[0047] FIG. 5 shows the layers of an exemplary electrophoretic color display.

[0048] FIG. 6 shows the simple push pull waveforms that can be used to achieve a set of primary colors in an optimized system including one reflective (white) particle, and three subtractive (cyan, yellow, magenta) particles, wherein two particles are negatively charged, but have different magnitudes, and two particles are positively charged, but have different magnitudes.

[0049] FIG. 7A is a generalized clearing pulse, having alternating pulses of V.sub.1, t.sub.1 and V.sub.2, t.sub.2. V.sub.1 is greater in magnitude than V.sub.2, and t.sub.1 is shorter than t.sub.2.

[0050] FIG. 7B is a generalized clearing pulse, including a first portion having alternating pulses of V.sub.1, t.sub.1 and V.sub.2, t.sub.2, and a second portion having alternating pulses of V.sub.3,t.sub.3, and V.sub.4; t.sub.4.

[0051] FIG. 8 is a specific example of a clearing pulse, including a first portion that drives an ACeP-type medium toward a magenta color state, and a second portion that drives the ACeP-type medium toward a neutral white color state.

[0052] FIG. 9 shows an exemplary clearing pulse of the invention coupled to a push-pull waveform suitable to achieve a desired color in an ACeP-type electrophoretic medium, when incorporated into a display having a metal oxide backplane and using a seven-level driver.

DETAILED DESCRIPTION

[0053] The invention details methods for efficiently clearing previous state information when driving a multi-particle color electrophoretic medium, for example, wherein at least two of the particles are colored and subtractive and at least one of the particles is scattering. Typically, such a system includes a white particle and cyan, yellow, and magenta subtractive primary colored particles. Such a system is shown schematically in FIG. 2, and it can provide white, yellow, red, magenta, blue, cyan, green, and black at every pixel.

[0054] In the instance of ACeP, each of the eight principal colors (red, green, blue, cyan magenta, yellow, black and white) corresponds to a different arrangement of the four pigments, such that the viewer only sees those colored pigments that are on the viewing side of the white pigment (i.e., the only pigment that scatters light). It has been found that waveforms to sort the four pigments into appropriate configurations to make these colors need at least five voltage levels (high positive, low positive, zero, low negative, high negative). See FIG. 2. To achieve the wider range of colors, additional voltage levels must be used for finer control of the pigments, e.g., seven voltage levels, e.g., nine voltage levels. However, as discussed in the Background, it has been found that not all second color states are available from all first color states, which results in more complicated (and longer) driving waveforms. The invention described herein overcomes this difficulty by providing a push-pull clearing waveform of unmatched voltages. Clearing waveforms of this type allow fast transitions between most color states without noticeable flashing.

[0055] The term color as used herein includes black and white. White particles are often of the light scattering type. The term gray state is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the E Ink patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate gray state would actually be pale blue. Indeed, as already mentioned, the change in optical state may not be a color change at all. The terms black and white may be used hereinafter to refer to the two extreme optical states of a display, and should be understood as normally including extreme optical states which are not strictly black and white, for example the aforementioned white and dark blue states.

[0056] The terms bistable and bistability are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called multi-stable rather than bistable, although for convenience the term bistable may be used herein to cover both bistable and multi-stable displays.

[0057] The term impulse, when used to refer to driving an electrophoretic display, is used herein to refer to the integral of the applied voltage with respect to time during the period in which the display is driven.

[0058] A particle that absorbs, scatters, or reflects light, either in a broad band or at selected wavelengths, is referred to herein as a colored or pigment particle. Various materials other than pigments (in the strict sense of that term as meaning insoluble colored materials) that absorb or reflect light, such as dyes or photonic crystals, etc., may also be used in the electrophoretic media and displays of the present invention.

[0059] The three particles providing the three subtractive primary colors, e.g., for an ACeP system, may be substantially non-light-scattering (“SNLS”). The use of SNLS particles allows mixing of colors and provides for more color outcomes than can be achieved with the same number of scattering particles. These thresholds must be sufficiently separated relative to the voltage driving levels for avoidance of cross-talk between particles, and this separation necessitates the use of high addressing voltages for some colors. In addition, addressing the colored particle with the highest threshold also moves all the other colored particles, and these other particles must subsequently be switched to their desired positions at lower voltages. Such a step-wise color-addressing scheme produces flashing of unwanted colors and a long transition time.

[0060] FIG. 2 of the accompanying drawings is a schematic cross-section showing the positions of the various particles in an ACeP-type electrophoretic medium when displaying black, white, the three subtractive primary colors and the three additive primary colors. In FIG. 2, it is assumed that the viewing surface of the display is at the top (as illustrated), i.e., a user views the display from this direction, and light is incident from this direction. As already noted, in preferred embodiments only one of the four particles used in the electrophoretic medium of the present invention substantially scatters light, and in FIG. 2 this particle is assumed to be the white pigment. This light-scattering white particle forms a white reflector against which any particles above the white particles (as illustrated in FIG. 2) are viewed. Light entering the viewing surface of the display passes through these particles, is reflected from the white particles, passes back through these particles and emerges from the display. Thus, the particles above the white particles may absorb various colors and the color appearing to the user is that resulting from the combination of particles above the white particles. Any particles disposed below (behind from the user's point of view) the white particles are masked by the white particles and do not affect the color displayed. Because the second, third and fourth particles are substantially non-light-scattering, their order or arrangement relative to each other is unimportant, but for reasons already stated, their order or arrangement with respect to the white (light-scattering) particles is critical.

[0061] More specifically, when the cyan, magenta and yellow particles lie below the white particles (Situation [A] in FIG. 2), there are no particles above the white particles and the pixel simply displays a white color. When a single particle is above the white particles, the color of that single particle is displayed, yellow, magenta and cyan in Situations [B], [D] and [F] respectively in FIG. 2. When two particles lie above the white particles, the color displayed is a combination of those of these two particles; in FIG. 2, in Situation [C], magenta and yellow particles display a red color, in Situation [E], cyan and magenta particles display a blue color, and in Situation [G], yellow and cyan particles display a green color. Finally, when all three colored particles lie above the white particles (Situation [H] in FIG. 2), all the incoming light is absorbed by the three subtractive primary colored particles and the pixel displays a black color.

[0062] It is possible that one subtractive primary color could be rendered by a particle that scatters light, so that the display would comprise two types of light-scattering particle, one of which would be white and another colored. In this case, however, the position of the light-scattering colored particle with respect to the other colored particles overlying the white particle would be important. For example, in rendering the color black (when all three colored particles lie over the white particles) the scattering colored particle cannot lie over the non-scattering colored particles (otherwise they will be partially or completely hidden behind the scattering particle and the color rendered will be that of the scattering colored particle, not black).

[0063] It would not be easy to render the color black if more than one type of colored particle scattered light.

[0064] FIG. 2 shows an idealized situation in which the colors are uncontaminated (i.e., the light-scattering white particles completely mask any particles lying behind the white particles). In practice, the masking by the white particles may be imperfect so that there may be some small absorption of light by a particle that ideally would be completely masked. Such contamination typically reduces both the lightness and the chroma of the color being rendered. In the electrophoretic medium of the present invention, such color contamination should be minimized to the point that the colors formed are commensurate with an industry standard for color rendition. A particularly favored standard is SNAP (the standard for newspaper advertising production), which specifies L*, a* and b* values for each of the eight primary colors referred to above. (Hereinafter, “primary colors” will be used to refer to the eight colors, black, white, the three subtractive primaries and the three additive primaries as shown in FIG. 2.)

[0065] FIGS. 3A and 3B show schematic cross-sectional representations of the four pigment types (1-4; 5-8) used in an ACeP-type electrophoretic display. In FIG. 3A, the polymer shell adsorbed to the core pigment is indicated by the dark shading, while the core pigment itself is shown as unshaded. A wide variety of forms may be used for the core pigment: spherical, acicular or otherwise anisometric, aggregates of smaller particles (i.e., “grape clusters”), composite particles comprising small pigment particles or dyes dispersed in a binder, and so on as is well known in the art. The polymer shell may be a covalently-bonded polymer made by grafting processes or chemisorption as is well known in the art, or may be physisorbed onto the particle surface. For example, the polymer may be a block copolymer comprising insoluble and soluble segments.

[0066] In the embodiment of FIG. 3A, first and second particle types preferably have a more substantial polymer shell than third and fourth particle types. The light-scattering white particle is of the first or second type (either negatively or positively charged). In the discussion that follows it is assumed that the white particle bears a negative charge (i.e., is of Type 1), but it will be clear to those skilled in the art that the general principles described will apply to a set of particles in which the white particles are positively charged.

[0067] Additionally, as depicted in FIG. 3B, it is not required that the first and second particle types have differential polymer shells as compared to the third and fourth particle types. As shown in FIG. 3B, sufficient differential charge on the four particles will allow for electrophoretic control of the particles and creation of the desired color at the viewing surface. For example, particle 5 may have a negative charge of greater magnitude than particle 7, while particle 6 has a greater magnitude positive charge as compared to particle 8. It is also possible that other combinations of polymer functionality and charge (or particle size) can be used; however, it must be the case that all four particles can be separated from each other in the presence of suitable electric fields, e.g., lower voltage electric fields that can be produced with commercial digital electronics.

[0068] In a system of FIG. 3A, the present invention the electric field required to separate an aggregate formed from mixtures of particles of types 3 and 4 in the suspending solvent containing a charge control agent is greater than that required to separate aggregates formed from any other combination of two types of particle. The electric field required to separate aggregates formed between the first and second types of particle is, on the other hand, less than that required to separate aggregates formed between the first and fourth particles or the second and third particles (and of course less than that required to separate the third and fourth particles).

[0069] In FIG. 3A the core pigments comprising the particles are shown as having approximately the same size, and the zeta potential of each particle, although not shown, is assumed to be approximately the same. What varies is the thickness of the polymer shell surrounding each core pigment. As shown in FIG. 3A, this polymer shell is thicker for particles of types 1 and 2 than for particles of types 3 and 4.

[0070] It is not necessary in the present invention that all the colored pigments behave as described above with reference to FIGS. 3A and 3B. As shown in FIG. 3C, the third particle may have a substantial polymer shell and may have a wide range of charge, including weakly positive. In this case the surface chemistry of the third particle must be different from that of the first particle. For example, the first particle my bear a covalently-attached silane shell to which is grafted a polymer that may be comprised of acrylic or styrenic monomers that are preferably hydrophobic. The third particle may comprise a polymer shell that is not covalently attached, but is deposited onto the surface of the core particle by dispersion polymerization. In such cases the invention is not limited to the mechanism described above with reference to FIGS. 3A and 3B.

[0071] To obtain a high-resolution display, individual pixels of a display must be addressable without interference from adjacent pixels. One way to achieve this objective is to provide an array of non-linear elements, such as transistors or diodes, with at least one non-linear element associated with each pixel, to produce an “active matrix” display. An addressing or pixel electrode, which addresses one pixel, is connected to an appropriate voltage source through the associated non-linear element. Typically, when the non-linear element is a transistor, the pixel electrode is connected to the drain of the transistor, and this arrangement will be assumed in the following description, although it is essentially arbitrary and the pixel electrode could be connected to the source of the transistor. Conventionally, in high resolution arrays, the pixels are arranged in a two-dimensional array of rows and columns, such that any specific pixel is uniquely defined by the intersection of one specified row and one specified column. The sources of all the transistors in each column are connected to a single column electrode, while the gates of all the transistors in each row are connected to a single row electrode; again the assignment of sources to rows and gates to columns is conventional but essentially arbitrary, and could be reversed if desired. The row electrodes are connected to a row driver, which essentially ensures that at any given moment only one row is selected, i.e., that there is applied to the selected row electrode a select voltage such as to ensure that all the transistors in the selected row are conductive, while there is applied to all other rows a non-select voltage such as to ensure that all the transistors in these non-selected rows remain non-conductive. The column electrodes are connected to column drivers, which place upon the various column electrodes voltages selected to drive the pixels in the selected row to their desired optical states. (The aforementioned voltages are relative to a common front electrode which is conventionally provided on the opposed side of the electro-optic medium from the non-linear array and extends across the whole display.) After a pre-selected interval known as the “line address time” the selected row is deselected, the next row is selected, and the voltages on the column drivers are changed so that the next line of the display is written. This process is repeated so that the entire display is written in a row-by-row manner. The time between addressing in the display is known as a “frame.” Thus, a display that is updated at 60 Hz has frames that are 16 msec.

[0072] Conventionally, each pixel electrode has associated therewith a capacitor electrode such that the pixel electrode and the capacitor electrode form a capacitor; see, for example, International Patent Application WO 01/07961. In some embodiments, N-type semiconductor (e.g., amorphous silicon) may be used to from the transistors and the “select” and “non-select” voltages applied to the gate electrodes can be positive and negative, respectively.

[0073] FIG. 4 of the accompanying drawings depicts an exemplary equivalent circuit of a single pixel of an electrophoretic display. As illustrated, the circuit includes a capacitor 10 formed between a pixel electrode and a capacitor electrode. The electrophoretic medium 20 is represented as a capacitor and a resistor in parallel. In some instances, direct or indirect coupling capacitance 30 between the gate electrode of the transistor associated with the pixel and the pixel electrode (usually referred to a as a “parasitic capacitance”) may create unwanted noise to the display. Usually, the parasitic capacitance 30 is much smaller than that of the storage capacitor 10, and when the pixel rows of a display is being selected or deselected, the parasitic capacitance 30 may result in a small negative offset voltage to the pixel electrode, also known as a “kickback voltage”, which is usually less than 2 volts. In some embodiments, to compensate for the unwanted “kickback voltage”, a common potential V.sub.com, may be supplied to the top plane electrode and the capacitor electrode associated with each pixel, such that, when V.sub.com is set to a value equal to the kickback voltage (V.sub.KB), every voltage supplied to the display may be offset by the same amount, and no net DC-imbalance experienced.

[0074] A set of waveforms for driving a color electrophoretic display having four particles is described in U.S. Pat. No. 9,921,451, incorporated by reference herein. In U.S. Pat. No. 9,921,451, seven different voltages are applied to the pixel electrodes: three positive, three negative, and zero. However, in some embodiments, the maximum voltages used in these waveforms are higher than that can be handled by amorphous silicon thin-film transistors. In such instances, suitable high voltages can be obtained by the use of top plane switching. It is costly and inconvenient, however, to use as many separate power supplies as there are V.sub.com settings when top plane switching is used. Furthermore, top plane switching is known to increase kickback, thereby degrading the stability of the color states.

[0075] Methods for fabricating an ACeP-type electrophoretic display have been discussed in the prior art. The electrophoretic fluid may be encapsulated in microcapsules or incorporated into microcell structures that are thereafter sealed with a polymeric layer. The microcapsule or microcell layers may be coated or embossed onto a plastic substrate or film bearing a transparent coating of an electrically conductive material. This assembly may be laminated to a backplane bearing pixel electrodes using an electrically conductive adhesive. Alternatively, the electrophoretic fluid may be dispensed directly on a thin open-cell grid that has been arranged on a backplane including an active matrix of pixel electrodes. The filled grid can then be top-sealed with an integrated protective sheet/light-transmissive electrode.

[0076] FIG. 5 shows a schematic, cross-sectional drawing (not to scale) of a display structure 200 of an ACeP-type electrophoretic display. In display 200 the electrophoretic fluid is illustrated as being confined to microcups, although equivalent structures incorporating microcapsules may also be used. Substrate 202, which may be glass or plastic, bears pixel electrodes 204 that are either individually addressed segments or associated with thin film transistors in an active matrix arrangement. (The combination of substrate 202 and electrodes 204 is conventionally referred to as the back plane of the display.) Layer 206 is an optional dielectric layer according to the invention applied to the backplane. (Methods for depositing a suitable dielectric layer are described in U.S. patent application Ser. No. 16/862,750, incorporated by reference.) The front plane of the display comprises transparent substrate 222 that bears a transparent, electrically conductive coating 220. Overlying electrode layer 220 is an optional dielectric layer 218. Layer (or layers) 216 are polymeric layer(s) that may comprise a primer layer for adhesion of microcups to transparent electrode layer 220 and some residual polymer comprising the bottom of the microcups. The walls of the microcups 212 are used to contain the electrophoretic fluid 214. The microcups are sealed with layer 210 and the whole front plane structure is adhered to the backplane using electrically-conductive adhesive layer 208. Processes for forming the microcups are described in the prior ar, e.g., in U.S. Pat. No. 6,930,818. In some instance, the microcups are less than 20 μm in depth, e.g., less than 15 μm in depth, e.g., less than 12 μm in depth, e.g., about 10 μm in depth, e.g., about 8 μm in depth.

[0077] Most commercial electrophoretic displays use amorphous silicon based thin-film transistors (TFTs) in the construction of active matrix backplanes (202/024) because of the wider availability of fabrication facilities and the costs of the various starting materials. Unfortunately, amorphous silicon thin-film transistors become unstable when supplied gate voltages that would allow switching of voltages higher than about +/−15V. Nonetheless, as described below, the performance of ACeP is improved when the magnitudes of the high positive and negative voltages are allowed to exceed +/−15V. Accordingly, as described in previous disclosures, improved performance is achieved by additionally changing the bias of the top light-transmissive electrode with respect to the bias on the backplane pixel electrodes, also known as top-plane switching. Thus, if a voltage of +30V (relative to the backplane) is needed, the top plane may be switched to −15V while the appropriate backplane pixel is switched to +15V. Methods for driving a four-particle electrophoretic system with top-plane switching are described in greater detail in, for example, U.S. Pat. No. 9,921,451.

[0078] There are several disadvantages to the top-plane switching approach. Firstly, when (as is typical) the top plane is not pixelated, but is a single electrode extending over the whole surface of the display, its electrical potential affects every pixel in the display. If it is set to match one of the voltages of the largest magnitude available from the backplane (for example, the largest positive voltage) when this voltage is asserted on the backplane there will be no net voltage across the ink. When any other available voltage is supplied to a backplane, there will always be a voltage of negative polarity supplied to any pixel in the display. Thus, if a waveform requires a positive voltage this cannot be supplied to any pixel until the top plane voltage is changed. Atypical waveform for use in a multicolor display of the third embodiment uses multiple pulses of both positive and negative polarity, and the lengths of these pulses are not of the same length in waveforms used for making different colors. In addition, the phase of the waveform may be different for different colors: in other words, a positive pulse may precede a negative pulse for some colors, whereas a negative pulse may precede a positive pulse for others. To accommodate such cases, “rests” (i.e., pauses) must be built into the waveforms. In practice, this results in waveforms being much longer (by as much as a factor of two) than they ideally need to be.

[0079] Secondly, in top plane switching there are limits to the voltage levels that may be chosen. If the voltages applied to the top plane are denoted V.sub.t+ and V.sub.t−, respectively, and those applied to the back plane V.sub.b+ and V.sub.b−, respectively, in order to achieve a zero volt condition across the electrophoretic fluid it must be true that |V.sub.t+|=|V.sub.b+| and |V.sub.t−|=|V.sub.b−|. However, it is not necessary for the magnitudes of the positive and negative voltages to be the same.

[0080] In prior embodiments of the Advanced Color electronic Paper (ACeP®), the waveform (voltage against time curve) applied to the pixel electrode of the backplane of a display of the invention is described and plotted, while the front electrode is assumed to be grounded (i.e., at zero potential). The electric field experienced by the electrophoretic medium is of course determined by the difference in potential between the backplane and the front electrode and the distance separating them. The display is typically viewed through its front electrode, so that it is the particles adjacent the front electrode which control the color displayed by the pixel, and if it is sometimes easier to understand the optical transitions involved if the potential of the front electrode relative to the backplane is considered; this can be done simply by inverting the waveforms discussed below.

[0081] FIG. 6 shows typical waveforms (in simplified form) used to drive a four-particle color electrophoretic display system described above. Such waveforms have a simple “push-pull” structure: i.e., they consist of a dipole comprising two pulses of opposite polarity. The magnitudes and lengths of these pulses determine the color obtained. At a minimum, there should be five such voltage levels. FIG. 6 shows high and low positive and negative voltages, as well as zero volts. Typically, “low” (L) refers to a range of about five-15V, while “high” (H) refers to a range of about 15-30V. In general, the higher the magnitude of the “high” voltages, the better the color gamut achieved by the display. The “medium” (M) level is typically around 15V; however, the value for M will depend somewhat on the composition of the particles, as well as the environment of the electrophoretic medium. If only three voltages are available (i.e., +V.sub.high, 0, and −V.sub.high) it may be possible to achieve the same result as addressing at a lower voltage (say, V.sub.high/n where n is a positive integer >1) by addressing with pulses of voltage V.sub.high but with a duty cycle of 1/n. An electrophoretic particle set useful with the push-pull waveforms of FIG. 6 may include a negatively-charged white particle, a negatively-charged yellow particle, a positively-charged magenta particle and a positively-charged cyan particle.

[0082] In alternate embodiments, seven level drivers may be used to directly address each pixel without the need for top plane switching. Implementing seven-level drivers with sufficient voltage amplitude is difficult with standard amorphous silicon backplanes, however. It has been found that using control transistors from less-common materials, which have a higher electron mobility, allow the transistors to switch larger control voltages, for example +/−30V, as needed to implement seven-level driving. Newly-developed active matrix backplanes may include thin film transistors incorporating metal oxide materials, such as tungsten oxide, tin oxide, indium oxide, and zinc oxide. In these applications, a channel formation region is formed for each transistor using such metal oxide materials, allowing faster switching of higher voltages, e.g., within the range of about −27V to +27V. Such transistors typically include a gate electrode, a gate-insulating film (typically SiO.sub.2), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film over the gate-insulating film, at least partially overlapping the gate electrode, source electrode, and drain electrode. Such backplanes are available from manufacturers such as Sharp/Foxconn, LG, and BOE. One preferred metal oxide material for such applications is indium gallium zinc oxide (IGZO). IGZO-TFT has 20-50 times the electron mobility of amorphous silicon. By using IGZO TFTs in an active matrix backplane, it is possible to provide voltages of greater than 30V via a suitable display driver.

[0083] Herein the term “frame” refers to a single update of all the rows in the display. It will be clear to one of ordinary skill in the art that in a display of the invention driven using a thin-film transistor (TFT) array the available time increments on the abscissa of FIG. 6 will typically be quantized by the frame rate of the display. Likewise, it will be clear that the display is addressed by changing the potential of the pixel electrodes relative to the front electrode and that this may be accomplished by changing the potential of either the pixel electrodes or the front electrode, or both. In the present state of the art, typically a matrix of pixel electrodes is present on the backplane, whereas the front electrode is common to all pixels. Therefore, when the potential of the front electrode is changed, the addressing of all pixels is affected. The basic structure of the waveform described above with reference to FIG. 6 is the same whether or not varying voltages are applied to the front electrode.

[0084] The reset of the display (i.e., clearing pulse) ideally erases any memory of a previous state, including remnant voltages and pigment configurations specific to previously-displayed colors, and allows a multi-particle electrophoretic display to reliably achieve the same color state with the same drive scheme. A generalized clearing pulse is shown in FIG. 7A. In general, the inventive reset pulses includes a high positive voltage (V.sub.H) followed by a lower negative voltage (V.sub.L), and the length of the positive voltage pulse (t) must be shorter than that of the negative voltage pulse (t′). Each period of voltage as a function of time may be referred to as an “impulse”, and each impulse i may be described as having a voltage V.sub.i and a time t.sub.i. The waveform may be DC-balanced or DC-imbalanced. The waveform may be DC balanced by adding additional impulses that result in the sum of the voltage-time areas of all of the positive voltage pulses is equal to the sum of the voltage-time areas of all of the negative voltage pulses. Mathematically, DC balancing can be represented as

Σ.sub.i=1.sup.nV.sub.i.Math.t.sub.i=0

where n=1-8, and V.sub.n and t.sub.n are as defined above. Furthermore, the magnitude and duration of each impulse in a push-pull sequence may be varied, and optionally “rests” (i.e., periods during which no voltage is applied) may be inserted between the impulses. In some embodiments,

[0085] In other instances, the clearing waveform includes two parts, each specifically designed for a particular purpose. For example, as shown in FIG. 7B, a first part of the prior-state clearing phase implements a push-pull sequence to achieve a universal color initiation state, e.g., magenta, to achieve the best possible clearing. The second part comprises a white-like neutral state, which is the state from which all subsequent colors can be made with simplified push pull color transitions. In some embodiments, the first part includes pulses of magnitude V.sub.1 and length t.sub.1 that alternate with pulses of magnitude V.sub.2 and length t.sub.2, where V.sub.1 is a positive voltage and V.sub.2 a negative voltage of lower magnitude than V.sub.1. In such embodiments, the second part includes pulses of magnitude V.sub.3 and length t.sub.3 that alternate with pulses of length V.sub.4 and length t.sub.4. The ratio V.sub.1.Math.t.sub.1/V.sub.2.Math.t.sub.2 is greater than the ratio V.sub.3.Math.t.sub.3/V.sub.4.Math.t.sub.4.

[0086] An exemplary waveform may drive the electrophoretic medium from an unknown state to a magenta optical state, and then a neutral white state, for example, as shown in FIG. 8. As described above, in FIG. 8, the impulse difference, i.e., |V.sub.1.Math.t.sub.1|−|V.sub.2.Math.t.sub.2| is greater for a sequence rendering a magenta state than for a sequence rendering a white state.

[0087] The prior state clearing pulses of the invention can be combined with all color waveforms, e.g., as shown in FIG. 6. For example, in some of the waveforms described in the aforementioned U.S. Pat. No. 9,921,451, seven different voltages can be applied to the pixel electrodes: three positive, three negative, and zero. FIG. 9 depicts schematically one such waveform used to clear and then display a desired color at a pixel. As shown in FIG. 9, the waveforms for every color have the same basic form: (A) a preliminary series of frames that is used to provide a “reset” of the display to a state from which any color may reproducibly be obtained and during which a DC imbalance equal and opposite to the DC imbalance of the remainder of the waveform is provided, and (B) a series of frames that is particular to the color that is to be rendered. If a DC balance pulse is included in the waveform, the DC balance pulse may come before the clearing pulse, or after the clearing pulse, but before the push-pull color address pulse. It should be realized that FIG. 9 is a generalization of a clearing pulse combined with a color addressing pulse. A two-part clearing pulse of the type shown in FIG. 7B may be used as an alternative to the one part clearing pulse shown in FIG. 9. As such, and the impulses V.sub.1.Math.t.sub.1 and V.sub.2.Math.t.sub.2 would precede impulses V.sub.3.Math.t.sub.3 and V.sub.4.Math.t.sub.4 in FIG. 9.

[0088] For comparison, regard FIG. 1 [Prior Art]. In phase A (the reset phase) it is seen that this phase is divided into two sections of equal duration (illustrated by the dotted lines). When top plane switching is used, the top plane will be held at one potential in the first of these sections, and at a potential of the opposite polarity in the second section. In the particular case of FIG. 1, during the first such section the top plane would have been held at V.sub.pH, and the backplane at V.sub.nH, to achieve a potential drop across the electrophoretic fluid of V.sub.nH−V.sub.pH (where the convention is used of referencing the backplane potential relative to that of the top plane). During the second section, the top plane would have been held at Val, and the backplane at V.sub.pH. As shown, during the second section the electrophoretic fluid would have been subjected to a potential of V.sub.pH−V.sub.nH, the highest potential available. For rendition of certain colors, however, exposure to this high voltage might result in an initial pigment arrangement from which an ideal final configuration would be difficult to achieve. For example, as noted in the prior art, in order to render the color cyan, it is necessary for the magenta pigment (which has the same charge polarity as the cyan pigment) to be tied up in an aggregate with the yellow pigment. Such an aggregate would be split by a high applied potential, and thus the magenta would not be controlled and would contaminate the cyan.

[0089] The foregoing discussion of the waveforms, and specifically the discussion of DC balance, ignores the question of kickback voltage. In practice, as previously, every backplane voltage is offset from the voltage supplied by the power supply by an amounts equal to the kickback voltage V.sub.KB. Thus, if the power supply used provides the three voltages +V, 0, and −V, the backplane would actually receive voltages V+V.sub.KB, V.sub.KB, and −V+V.sub.KB (note that V.sub.KB, in the case of amorphous silicon TFTs, is usually a negative number). The same power supply would, however, supply +V, 0, and −V to the front electrode without any kickback voltage offset. Therefore, for example, when the front electrode is supplied with −V the display would experience a maximum voltage of 2V+V.sub.KB and a minimum of V.sub.KB. Instead of using a separate power supply to supply V.sub.KB to the front electrode, which can be costly and inconvenient, a waveform may be divided into sections where the front electrode is supplied with a positive voltage, a negative voltage, and V.sub.KB.

[0090] Thus, the invention provides for clearing waveforms for multi-particle electrophoretic displays. Having thus described several aspects and embodiments of the technology of this application, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those of ordinary skill in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described in the application. For example, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the embodiments described herein. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described. In addition, any combination of two or more features, systems, articles, materials, kits, and/or methods described herein, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the scope of the present disclosure.