DRIVING VOLTAGES FOR ADVANCED COLOR ELECTROPHORETIC DISPLAYS AND DISPLAYS WITH IMPROVED DRIVING VOLTAGES

Abstract

Improved methods for driving a four particle electrophoretic medium including a scattering particle and at least two subtractive particles. Such methods allow displays such as a color electrophoretic display including a backplane having an array of thin film transistors, wherein each thin film transistor includes a layer of metal oxide semiconductor. The metal oxide transistors allow faster, higher voltage switching, and thus allow direct color switching of a four-particle electrophoretic medium without a need for top plane switching. As a result, the color electrophoretic display can be updated faster and the colors are reproduced more reliably.

Claims

1. A color electrophoretic display comprising: a controller; a light-transmissive electrode at a viewing surface; a backplane including an array of thin film transistors coupled to pixel electrodes, each thin film transistor comprising a layer of a metal oxide semiconductor; and a color electrophoretic medium disposed between the light-transmissive electrode and the backplane, the color electrophoretic medium comprising: (a) a fluid; (b) a plurality of first and a plurality of second particles dispersed in the fluid, the first and second particles bearing charges of opposite polarity, the first particle being a light-scattering particle and the second particle having one of the subtractive primary colors; and (c) a plurality of third and a plurality of fourth particles dispersed in the fluid, the third and fourth particles bearing charges of opposite polarity, the third and fourth particles each having a subtractive primary color different from each other and from the second particles, wherein the controller is configured to provide a plurality of driving voltages to the pixel electrodes such that white, yellow, red, magenta, blue, cyan, green, and black can be displayed at each pixel electrode while keeping the light-transmissive electrode at a constant voltage.

2. The color electrophoretic display of claim 1, wherein the controller is configured to provide a voltage of greater than 25 Volts and less than −25 Volts to the pixel electrodes.

3. The color electrophoretic display of claim 2, wherein the controller is configured to additionally provide a voltage between 25 V and 0V and a voltage between −25V and 0V.

4. The color electrophoretic display of claim 1, wherein the metal oxide semiconductor is indium gallium zinc oxide (IGZO).

5. A method, comprising: providing a color electrophoretic display comprising a controller; a light-transmissive electrode at a viewing surface; a backplane including an array of thin film transistors coupled to pixel electrodes, each thin film transistor comprising a layer of a metal oxide semiconductor; and a color electrophoretic medium disposed between the light-transmissive electrode and the backplane, wherein the color electrophoretic medium comprises: (a) a fluid; (b) a plurality of first and a plurality of second particles dispersed in the fluid, the first and second particles bearing charges of opposite polarity, the first particles being light-scattering particles and the second particles having one of the subtractive primary colors; and (c) a plurality of third and a plurality of fourth particles dispersed in the fluid, the third and fourth particles bearing charges of opposite polarity, the third and fourth particles each having a subtractive primary color different from each other and from the second particles; and applying a plurality of driving voltages to the pixel electrodes to cause white, yellow, red, magenta, blue, cyan, green, and black to be displayed at each pixel electrode, while keeping the light-transmissive electrode at a constant voltage.

6. The method of claim 5, wherein the driving voltages applied to the pixel electrodes are between 25 Volts and 0 Volts and between −25 Volts and 0 Volts.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0036] FIG. 1 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.

[0037] FIG. 2A shows in schematic form four types of different pigment particles used in a multi-particle electrophoretic medium.

[0038] FIG. 2B shows in schematic form four types of different pigment particles used in a multi-particle electrophoretic medium.

[0039] FIG. 3 illustrates an exemplary equivalent circuit of a single pixel of an electrophoretic display.

[0040] FIG. 4 shows the layers of an exemplary electrophoretic color display.

[0041] FIG. 5 shows exemplary push-pull drive schemes for addressing an electrophoretic medium including three subtractive particles and a scattering (white) particle.

[0042] FIG. 6 is an exemplary waveform from the prior art including a two-part reset phase (A) and a color transition phase (B), which is achieved using top-plane switching.

[0043] FIG. 7 depicts the correlation between available color gamut and the number of dipoles (“flashes”) in an update of a four-particle full color electrophoretic display.

[0044] FIG. 8 depicts simplified top plane driving waveforms for the production of eight colors in an electrophoretic medium including three subtractive particles and a scattering (white) particle.

[0045] FIG. 9 shows the calculated effect of modifying the top plane voltage rails by +2V in an electrophoretic medium including three subtractive particles and a scattering (white) particle.

[0046] FIG. 10 shows the calculated effect of modifying the backplane voltage rails by +2V in an electrophoretic medium including three subtractive particles and a scattering (white) particle.

[0047] FIG. 11 shows the calculated effect of modifying both the top plane voltage rails and the backplane voltage rails by +2V and −2V in an electrophoretic medium including three subtractive particles and a scattering (white) particle.

DETAILED DESCRIPTION

[0048] The invention includes improved methods for driving a four-particle electrophoretic medium 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. 1, and it can provide white, yellow, red, magenta, blue, cyan, green, and black at every pixel.

[0049] 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. 1. To achieve the wider range of colors, additional voltage levels must be used for finer control of the pigments. The invention provides several improved ways to drive such an electrophoretic medium so that they refreshes of pixel colors are faster, less flashy, and result in a color spectrum that is more pleasing to the viewer.

[0050] The three particles providing the three subtractive primary colors 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 for avoidance of cross-talk, 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. The present invention does not require the use of a such a stepwise waveform and addressing to all colors can, as described below, be achieved with only two positive and two negative voltages (i.e., only five different voltages, two positive, two negative and zero are required in a display, although as described below in certain embodiments it may be preferred to use more different voltages to address the display).

[0051] As already mentioned, FIG. 1 of the accompanying drawings is a schematic cross-section showing the positions of the various particles in an electrophoretic medium of the present invention when displaying black, white, the three subtractive primary and the three additive primary colors. In FIG. 1, 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. 1 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. 1) 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.

[0052] More specifically, when the cyan, magenta and yellow particles lie below the white particles (Situation [A] in FIG. 1), 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. 1. When two particles lie above the white particles, the color displayed is a combination of those of these two particles; in FIG. 1, 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. 1), all the incoming light is absorbed by the three subtractive primary colored particles and the pixel displays a black color.

[0053] 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).

[0054] It would not be easy to render the color black if more than one type of colored particle scattered light.

[0055] FIG. 1 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. 1.)

[0056] FIGS. 2A and 2B show schematic cross-sectional representations of the four pigment types (1-4; 5-8) used in preferred embodiments of the invention. In FIG. 2A, 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. Some methods for affixing the polymer shell to the core pigments are described in the Examples below.

[0057] In the embodiment of FIG. 2A, 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.

[0058] Additionally, as depicted in FIG. 2B, 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. 2B, 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.

[0059] In a system of FIG. 2A, 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).

[0060] In FIG. 2A 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. 2A, this polymer shell is thicker for particles of types 1 and 2 than for particles of types 3 and 4.

[0061] 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.

[0062] 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.

[0063] FIG. 3 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.

[0064] Problems may arise, however, when V.sub.com is set to a voltage that is not compensated for the kickback voltage. This may occur when it is desired to apply a higher voltage to the display than is available from the backplane alone. It is well known in the art that, for example, the maximum voltage applied to the display may be doubled if the backplane is supplied with a choice of a nominal +V, 0, or −V, for example, while V.sub.com is supplied with −V.sub.com The maximum voltage experienced in this case is +2V (i.e., at the backplane relative to the top plane), while the minimum is zero. If negative voltages are needed, the V.sub.com potential must be raised at least to zero. Waveforms used to address a display with positive and negative voltages using top plane switching must therefore have particular frames allocated to each of more than one V.sub.com voltage setting.

[0065] A set of waveforms for driving a color electrophoretic display having four particles 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. When (as described above) V.sub.com is deliberately set to V.sub.KB, a separate power supply may be used. 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.

[0066] A display device may be constructed using an electrophoretic fluid of the invention in several ways that are known 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.

[0067] FIG. 4 shows a schematic, cross-sectional drawing (not to scale) of a display structure 200 suitable for use with the invention. 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.

[0068] 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.

[0069] 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. A typical 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.

[0070] 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.

[0071] 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. [Para 62] These waveforms require that each pixel of the display can be driven at five different addressing voltages, designated +V.sub.high, +V.sub.low, 0, −V.sub.low and −V.sub.high, illustrated as 30V, 15V, 0, −15V and −30V. In practice, it may be preferred to use a larger number of addressing voltages. 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.

[0072] FIG. 5 shows typical waveforms (in simplified form) used to drive a four-particle color electrophoretic display system described above. Such waveforms have a “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. 5 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.

[0073] Although FIG. 5 shows the simplest dipoles required to form colors, it will be appreciated that practical waveforms may multiple repetitions of these patterns, or other patterns that are aperiodic and use more than five voltage levels.

[0074] Thus, the generic driving voltage require that the driving electronics provide as many as seven different voltages to the data lines during the update of a selected pixel of the display (+H, +M, +L, 0, −L, −M, −H). While multi-level source drivers capable of delivering seven different voltages are available, most commercially-available source drivers for electrophoretic displays permit only three different voltages to be delivered during a single frame (typically a positive voltage, zero, and a negative voltage). Accordingly, as discussed previously, it is necessary to modify the generic waveforms of FIG. 5 to accommodate a three level source driver architecture provided that the three voltages supplied to the backplane (typically +V, 0 and −V) can be changed from one frame to the next. The remaining voltage levels can be achieved by using a “top plane switching” drive scheme, wherein the light transmissive (top-plane) common electrode is switched between −V, 0 and +V, while the voltages applied to the pixel electrodes can also vary from −V, 0 to +V with pixel transitions in one direction being handled when the common electrode is at 0 and transitions in the other direction being handled when the common electrode is at +V.

[0075] Of course, achieving the desired color with the driving pulses of FIG. 5 is contingent on the particles starting the process from a known state, which is unlikely to be the last color displayed on the pixel. Accordingly, a series of reset pulses precede the driving pulses, which increases the amount of time required to update a pixel from a first color to a second color. The reset pulses are described in greater detail in U.S. Pat. No. 10,593,272, incorporated by reference. The lengths of these pulses (refresh and address) and of any rests (i.e., periods of zero voltage between them may be chosen so that the entire waveform (i.e., the integral of voltage with respect to time over the whole waveform) is DC balanced (i.e., the integral of voltage over time is substantially zero). DC balance can be achieved by adjusting the lengths of the pulses and rests in the reset phase so that the net impulse supplied in the reset phase is equal in magnitude and opposite in sign to the net impulse supplied in the address phase, during which phase the display is switched to a particular desired color.

[0076] In addition, 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. In addition to the kickback

[0077] In commercial embodiments using amorphous silicon transistor control, seven different voltages can be applied to the pixel electrodes: three positive, three negative, and zero. Once the complications of top-plane switching, reset pulses, and DC Balancing are factored in, the resulting waveforms are rather complicated. For example, FIG. 6, which is taken from U.S. Pat. No. 10,593,272, depicts schematically one such waveform used to display a single color. As shown in FIG. 6, the waveforms for every color have the same basic form: i.e., the waveform is intrinsically DC-balanced and can comprise two sections or phases: (1) 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 (2) a series of frames that is particular to the color that is to be rendered; cf. Sections A and B of the waveform shown in FIG. 6.

[0078] During the first “reset” phase, the reset of the display ideally erases any memory of a previous state, including remnant voltages and pigment configurations specific to previously-displayed colors. Such an erasure is most effective when the display is addressed at the maximum possible voltage in the “reset/DC balancing” phase. In addition, sufficient frames may be allocated in this phase to allow for balancing of the most imbalanced color transitions. Because some colors require a positive DC-balance in the second section of the waveform and others a negative balance, in approximately half of the frames of the “reset/DC balancing” phase, the front electrode voltage V.sub.com is set to V.sub.pH (allowing for the maximum possible negative voltage between the backplane and the front electrode), and in the remainder, V.sub.com is set to V.sub.nH (allowing for the maximum possible positive voltage between the backplane and the front electrode). Empirically it has been found preferable to precede the V.sub.com=V.sub.pH frames by the V.sub.com=V.sub.pH frames.

[0079] The “desired” waveform (i.e., the actual voltage against time curve that is desired to apply across the electrophoretic medium) is illustrated at the bottom of FIG. 6, and its implementation with top plane switching is shown above, where the potentials applied to the front electrode (Vcom) and to the backplane (BP) are illustrated. It is assumed that the column driver is used connected to a power supply capable of supplying the following voltages: V.sub.pH, V.sub.nH (the highest positive and negative voltages, typically in the range of ±10-15 V), V.sub.pL, V.sub.nL (lower positive and negative voltages, typically in the range of ±1-10 V), and zero. In addition to these voltages, a kickback voltage V.sub.KB (a small value that is specific to the particular backplane used, measured as described, for example, in U.S. Pat. No. 7,034,783) can be supplied to the front electrode by an additional power supply. As shown in FIG. 6, every backplane voltage is offset by V.sub.KB (shown as a negative number) from the voltage supplied by the power supply while the front electrode voltages are not so offset, except when the front electrode is explicitly set to V.sub.KB, as described above.

[0080] Based upon feedback from potential users, it has been determined that driving pulses (waveforms) such as shown in FIG. 6 are A) too long, and B) too flashy. (“Flashy” refers to an excessive number of end state pigment addresses, known as “dipoles”, during an update. As the number of dipoles per update increases, a viewer is more likely to perceive that that the display is “flashing” even though there is no light emitted from the display.)

Simplified Top-Plane Switching

[0081] To reduce the length of time and flashiness of an update, the complexity of the front-plane switching can be reduced in exchange for a smaller number of available colors. (A calculation of the available color gamut as a function of the number of dipoles (“flashes”) is presented in FIG. 7. Additionally, because the particles have a finite speed within the electrophoretic medium, the amount of time for which the dipole is applied also influences the size of the color gamut.

[0082] FIG. 8 shows such a solution in which a simplified top plane switching pulse sequence is used (top left panel), with simplified backplane pulse sequences (left; below) being matched to the single top-plane sequence, thereby providing at least distinct colors. The top plane is switched between two voltages, one positive and one negative, while the back plane can take three different voltages: positive, negative, and zero. (In FIG. 8, the voltage levels are relative, i.e., 1, 0, −1, but would in many instances actually be 15V, 0, and −15V as is typically with commercial backplanes including amorphous silicon thin film transistors.) Note that by subtracting the pulse sequence of the top-plane from the backplane pulse sequence (FIG. 8 left), the eight color sequences in FIG. 5 are achieved (FIG. 8 right). It is understood that for the pulse sequences in FIG. 5 and FIG. 8, the electrophoretic fluid includes a white pigment that is negatively charged, a magenta pigment and a cyan pigment that are positively charged, and the yellow pigment may be either positively or negatively charged, or essentially neutral. Other color/charge combinations are possible and the waveforms can be adujsted accordingly.

[0083] As discussed previously, in the waveforms of FIG. 8 at least five different voltages are required. In an active matrix driving environment, this may be achieved either (a) by supplying a choice of five different voltages to the columns when a particular row is selected at a particular time, or (b) by providing a choice of fewer (say, three) different voltages to the columns when a particular row is selected at a first time, and a different set of voltages when the same row is selected at a second time, or (c) by providing the same choice of three voltages to the columns at both the first and second times, but changing the potential of the front electrode between the first and second times. Option (c) is particularly helpful when at least one of the voltages required to be supplied is higher than the backplane electronics can support.

[0084] Because, with top plane switching, it is not possible to assert a high positive and a high negative potential simultaneously, it is necessary to offset the +/− dipoles of the top plane with respect to the −/+ dipoles of the backplane. In the waveform shown in FIG. 8, there is only one dipole per transition. This provides the least “flashy” waveform possible, since each dipole results in two visible optical changes to the display. In cases where five different voltage levels can be supplied to the backplane electrodes when each row is selected, and where the backplane electronics can support the highest voltages needed, it is not necessary to offset the dipoles in the manner shown in FIG. 8.

Driving with Modified Rail Voltages

[0085] For the drive sequences of FIG. 8, 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, and |V.sub.t+|=|V.sub.t−|=|V.sub.b+|=|V.sub.b−|=V. Accordingly, when the maximum supply voltage is +/−15 volts, as is typical with commercial backplanes, the voltages across the electrophoretic medium become 30V, 28V, 0V, −28V, and −30V.

[0086] The maximum voltage magnitudes (i.e., “rail”) of the top-plane electrode and the back-plane electrode need not be the same, however. For example, rail voltages offsets can be calculated from some nominal maximum voltage magnitude value, V. The offset for each rail may be denoted w, x, y and z, while it is assumed that the zero voltage rail is kept at zero and not applied to the top plane. (That is, the top plane is only high and low, while the backplane is high, low, and zero, as depicted in FIG. 8.) Thus:

V.sub.t+=V+w

V.sub.t−=−V+x

V.sub.b+=V+y

V.sub.b−=−V+z

V.sub.b0=0

[0087] Referenced to the backplane voltage, three different negative voltages of high, medium and low magnitudes may be applied to the electrophoretic ink when the top plane is set to V.sub.t+, denoted as V.sub.H−, V.sub.M−, and V.sub.L−, (i.e., V.sub.b−V.sub.t, where V.sub.b can take any of the three values shown above).

These voltages are:

V.sub.H−=−2V+z−w

V.sub.M−=−V−w

V.sub.L−=y−w

[0088] The voltages available when the top plane is set to V.sub.t− are:

V.sub.H+=2V+y−x

V.sub.M+=V−x

V.sub.L+=z−x

[0089] It is apparent that when w=x=y=z only the medium voltages V.sub.M+ and V.sub.M− are affected by the offsets. Thus, it is possible to maintain the high voltage magnitudes as 2V, and the zero voltages as zero, while the medium voltages are each decreased by the amount of the offset (assumed to be positive). The difference between the two medium voltages will always be 2V.

[0090] In, for example, a five-level driving system, if the top plane rail(s) are increased (or decreased) by some amount δ, i.e., w=δ or x=δ, while the backplane rails stay the same, the overall magnitude of some drive voltages will increase (or decrease), and the offset will create a new drive level as the V.sub.L+ and V.sub.L− states are different. The effect of such a change can be calculated, when w=δ or x=δ and δ=+2V, and is shown graphically in FIG. 9. In FIG. 9, eight primary colors are shown to vary with modification of the driving voltages. In FIG. 9, the open square represents the “base,” evenly-distributed, driving levels, the filled triangle represents an additional +2V of the highest driving level, while all other levels stay the same, and the filled circle represents an additional +2V for the intermediate top driving voltage, while all other states stay as in the base driving case. As can be seen from FIG. 9, modifying only individual top plane rails by +2V produces only minor effects. (Compare positions of open circles to closed triangles and closed circles.) The greatest change is seen in the white state (center of FIG. 9) when a positive offset is applied to the positive rail, in which case the b* is (undesirably) raised.

[0091] In a similar fashion, backplane rail(s) may be increased (or decreased) by some amount δ, i.e., y=δ or z=δ, while the top plane rails stay the same. The effect of a similar backplane rail adjustment of y=z=δ=+2V is shown in FIG. 10. In FIG. 10, eight primary colors are shown to vary with modification of the driving voltages. In FIG. 10, the open square represents the “base,” evenly-distributed, driving levels, the filled triangle represents an additional +2V of the lowest driving level, while all other levels stay the same, and the filled circle represents an additional +2V for the intermediate lower driving voltage, while all other states stay as in the base driving case. While the change in the performance is not pronounced, when the positive or negative rail are modified by +2V the yellow color becomes increasingly green. This is typically undesirable as it causes, e.g., flesh tones to look green. However, in some digital signage applications, it may be preferred to trade yellow tones for stronger green tones.

[0092] Surprisingly, when the same offset δ=w=x=y=z is applied to all four rail voltages, more pronounced shifts in the electro-optic performance occur, thereby providing opportunities to adjust the color performance of the electrophoretic medium as may be required by the specific application. In FIG. 11, eight primary colors are shown to vary with modification of the driving voltages. In FIG. 11, the open square represents the “base,” evenly-distributed, driving levels, the filled triangle represents an additional +2V for all driving levels, and the filled circle represents an additional −2V for all driving levels. In FIG. 11, the electro-optic performance has been calculated where δ is −2V, 0 and +2V, and the shift in electro-optic response is shown with arrows. It can be seen that some states (yellow, red, cyan) are only very slightly affected by the changes in offset, whereas the white and magenta states are very substantially shifted towards more negative b* as the offset is made more negative. As can be seen from the above equations, making the offset more negative makes V.sub.M− less negative and VM.sub.+ more positive. The most important of these effects is the reduction of b* in the white state.

[0093] Thus, taken together, these results imply that the most useful adjustment is to apply the same offset to all four voltage rails. This results in an ability to change the mid-voltage (V.sub.M) levels without affecting the high voltage magnitudes or the zero. Doing this allows the white state to be adjusted so as to be more neutral, especially in the b* dimension.

Higher Voltage Addressing with Metal Oxide Backplanes

[0094] While modifying the rail voltages provides some flexibility in achieving differing electro-optical performance from a four-particle electrophoretic system, there are many limitations introduced by top-plane switching. For example, it is typically preferred, in order to make a white state with displays of the present invention, that the lower negative voltage V.sub.M− is less than half the maximum negative voltage V.sub.H−. As shown in the equations above, however, top-plane switching requires that the lower positive voltage is always at least half the maximum positive voltage, typically more than half.

[0095] An alternative solution to the complications of top-plane switching can be provided by fabricating the control transistors from less-common materials that have a higher electron mobility, thereby allowing the transistors to switch larger control voltages, for example +/−30V, directly. 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. 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.

[0096] 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. Furthermore, a source driver capable of supplying at least five, and preferably seven levels provides a different driving paradigm for a four-particle electrophoretic display system. In an embodiment, there will be two positive voltages, two negative voltages, and zero volts. In another embodiment, there will be three positive voltages, three negative voltages, and zero volts. In an embodiment, there will be four positive voltages, four negative voltages, and zero volts. These levels may be chosen within the range of about −27V to +27V, without the limitations imposed by top plane switching as described above.

[0097] Using advanced backplanes, such as metal oxide backplanes, it is possible to directly address each pixel with a suitable push-pull waveform, i.e., as described in FIG. 5. This greatly reduces the time required to update each pixel, in some instances transforming a six-second update to less than one second. While, in some cases, it may be necessary to use reset pulses to establish a starting point for addressing, the reset can be done quicker at higher voltages. Additionally, in four-color electrophoretic displays having reduced color sets, it is possible to directly drive from a first color to a second color with a specific waveform that is only slightly longer than the push-pull waveforms shown in FIG. 5.

[0098] Thus, the invention provides for full color electrophoretic displays that are capable of directly addressing the electrophoretic medium without top plane switching, as well as waveforms for such 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.