Method for achieving a color state of less charged particles in an electrophoretic medium comprising at least four types of particles

Abstract

A method for driving an electrophoretic medium comprising two pairs of oppositely charged particles. The first pair comprises positive particles of a first type and negative particles of a first type, and the second pair consists of positive particles of a second type and negative particles of a second type, wherein the first pair of particles and the second pair of particles have different amounts of charge (identifiable as electromotive force). In particular, when the intermediate drive voltage is modified, these drive methods produce a purer light column of the less charged particles, with less contamination from other particles and more consistent electro-optical performance.

Description

[0001] Related Applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 035,088, filed on June 5, 2020, the entire contents of which are incorporated herein by reference. All patents and publications disclosed herein are incorporated herein by reference in their entirety. Technical Field

[0003] The present invention relates to a driving method for a color display device including an electrophoretic medium, the electrophoretic medium having at least four different particle groups, each particle group having a charge polarity and a charge amount, and no particle group having the same charge polarity and charge amount. Using the method described herein, each pixel can display a high-quality color state of particles with less charge. Background Art

[0004] In order to achieve color display, color filters are often used. The most common method is to add color filters on top of the black / white sub-pixels of a pixelated display to display red, green and blue. When red is required, the green and blue sub-pixels change to a black state, so the only color displayed is red. When blue is required, the green and red sub-pixels change to a black state, so the only color displayed is blue. When green is required, the red and blue sub-pixels change to a black state, so the only color displayed is green. When a black state is required, all three sub-pixels change to a black state. When a white state is required, the three sub-pixels change to red, green and blue respectively, and as a result, the observer sees a white state.

[0005] The biggest drawback of this technique is that the white state is quite dim, since each subpixel has a reflectivity of about one-third of the desired white state. To compensate for this, a fourth subpixel can be added that can only display black and white states, thereby doubling the white level at the expense of the red, green, or blue levels (where each subpixel has only one-quarter the area of ​​a pixel). Even with this approach, the white level is typically well below half that of a black-and-white display, making it an unacceptable choice for display devices such as e-readers or monitors that require readable black-and-white brightness and contrast. Summary of the invention

[0006] A first aspect of the present invention relates to a driving method for driving pixels of an electrophoretic display, the electrophoretic display comprising a first surface on an observation side, a second surface on a non-observation side, and an electrophoretic fluid disposed between a first light-transmitting electrode and a second electrode, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles all dispersed in a solvent, wherein

[0007] (a) The four types of pigment particles have different optical properties;

[0008] (b) the first type of particles and the third type of particles are positively charged, wherein the first type of particles have a greater positive charge than the third type of particles; and

[0009] (c) the second type of particles and the fourth type of particles are negatively charged, wherein the second type of particles have a greater negative charge than the fourth type of particles,

[0010] The method comprises the following steps:

[0011] (i) applying a first drive voltage to a pixel of the electrophoretic display at a first amplitude for a first time period to drive the pixel to a color state of the first or second type of particles at a viewing side;

[0012] (ii) applying a second drive voltage to the pixel of the electrophoretic display for a second time period, wherein the second drive voltage has a polarity opposite to that of the first drive voltage and a second amplitude less than the first amplitude to drive the pixel from the color state of the first type of particles to the color state of the fourth type of particles or from the color state of the second type of particles to the color state of the third type of particles on the viewing side, and repeating steps (i)-(ii);

[0013] (iii) not applying a driving voltage to the pixel during a third time period;

[0014] (iv) applying a second drive voltage to the pixels of the electrophoretic display during a fourth time period to drive the pixels on the observation side from the color state of the first type of particles to the color state of the fourth type of particles or from the color state of the second type of particles to the color state of the third type of particles, and repeating steps (iii)-(iv), wherein a drive voltage having the same polarity as the first drive voltage is not applied between steps (iii) and (iv).

[0015] In some embodiments, the second time period in step (ii) is longer than the first time period in step (i). In some embodiments, steps (i) and (ii) are repeated at least 8 times. In some embodiments, steps (iii) and (iv) are repeated at least 8 times. In some embodiments, the amplitude of the second driving voltage is less than 50% of the amplitude of the first driving voltage. In some embodiments, the positive charge of the third particle is less than 50% of the positive charge of the first particle. In some embodiments, the negative charge of the fourth particle is less than 75% of the negative charge of the second particle. In some embodiments, a voltage having a vibration waveform is applied to the pixel before step (i). In some embodiments, the fourth time period in step (iv) is shorter than the second time period in step (ii). In some embodiments, a third driving voltage is applied to the pixel of the electrophoretic display in a fifth time period between steps (ii) and (iii), wherein the third driving voltage has the same polarity as the second driving voltage and the same amplitude as the first amplitude.

[0016] A second aspect of the present invention relates to a driving method for driving a pixel of an electrophoretic display, the electrophoretic display comprising a first surface on an observation side, a second surface on a non-observation side, and an electrophoretic fluid disposed between a first light-transmitting electrode and a second electrode, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles all dispersed in a solvent, wherein

[0017] (a) The four types of pigment particles have different optical properties;

[0018] (b) the first type of particles and the third type of particles are positively charged, wherein the first type of particles have a greater positive charge than the third type of particles; and

[0019] (c) the second type of particles and the fourth type of particles are negatively charged, wherein the second type of particles have a greater negative charge than the fourth type of particles,

[0020] The method comprises the following steps:

[0021] (i) applying a first drive voltage to a pixel of the electrophoretic display at a first amplitude for a first time period to drive the pixel to a color state of the first or second type of particles at a viewing side;

[0022] (ii) applying a second drive voltage to a pixel of the electrophoretic display during a second time period, wherein the second drive voltage has a polarity opposite to that of the first drive voltage and a second amplitude less than the first amplitude to drive the pixel from a color state of the first type of particles to a color state of the fourth type of particles or from a color state of the second type of particles to a color state of the third type of particles at a viewing side;

[0023] (iii) not applying a driving voltage to the pixel during a third time period, and repeating steps (i) to (iii);

[0024] (iv) not applying a driving voltage to the pixel during a fourth time period;

[0025] (v) applying a second drive voltage to the pixels of the electrophoretic display during a fifth time period to drive the pixels on the observation side from the color state of the first type of particles to the color state of the fourth type of particles or from the color state of the second type of particles to the color state of the third type of particles, and repeating steps (iv)-(v), wherein a drive voltage having the same polarity as the first drive voltage is not applied between steps (iv) and (v).

[0026] In some embodiments, the second time period in step (ii) is longer than the first time period in step (i). In some embodiments, steps (i)-(iii) are repeated at least 8 times. In some embodiments, steps (iv) and (v) are repeated at least 8 times. In some embodiments, the amplitude of the second driving voltage is less than 50% of the amplitude of the first driving voltage. In some embodiments, the positive charge of the third particle is less than 50% of the positive charge of the first particle. In some embodiments, the negative charge of the fourth particle is less than 75% of the negative charge of the second particle. In some embodiments, a voltage having a vibration waveform is applied to the pixel before step (i). In some embodiments, the fifth time period in step (v) is shorter than the second time period in step (ii). In some embodiments, a third driving voltage is applied to the pixel of the electrophoretic display in a sixth time period between steps (iii) and (iv), wherein the third driving voltage has the same polarity as the second driving voltage and the same amplitude as the first amplitude.

[0027] A third aspect of the present invention relates to a driving method for driving pixels of an electrophoretic display, the electrophoretic display comprising a first surface on an observation side, a second surface on a non-observation side, and an electrophoretic fluid disposed between a first light-transmitting electrode and a second electrode, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles all dispersed in a solvent, wherein

[0028] (a) The four types of pigment particles have different optical properties;

[0029] (b) the first type of particles and the third type of particles are positively charged, wherein the first type of particles have a greater positive charge than the third type of particles; and

[0030] (c) the second type of particles and the fourth type of particles are negatively charged, wherein the second type of particles have a greater negative charge than the fourth type of particles,

[0031] The method comprises the following steps:

[0032] (i) applying a first drive voltage to a pixel of the electrophoretic display at a first amplitude for a first time period to drive the pixel to a color state of the first or second type of particles at a viewing side;

[0033] (ii) not applying a driving voltage to the pixel during a second time period;

[0034] (iii) applying a second drive voltage to a pixel of the electrophoretic display during a third time period, wherein the second drive voltage has a polarity opposite to that of the first drive voltage and a second amplitude less than the first amplitude to drive the pixel from a color state of the first type of particles to a color state of the fourth type of particles or from a color state of the second type of particles to a color state of the third type of particles at a viewing side;

[0035] (iv) not applying a driving voltage to the pixel during a fourth time period, and repeating steps (i) to (iv);

[0036] (v) not applying a driving voltage to the pixel during a fifth time period;

[0037] (vi) applying a second drive voltage to the pixels of the electrophoretic display during a sixth time period to drive the pixels on the observation side from the color state of the first type of particles to the color state of the fourth type of particles or from the color state of the second type of particles to the color state of the third type of particles, and repeating steps (v)-(vi), wherein a drive voltage having the same polarity as the first drive voltage is not applied between steps (v) and (vi).

[0038] In some embodiments, the third time period in step (iii) is longer than the first time period in step (i). In some embodiments, steps (i)-(iv) are repeated at least 8 times. In some embodiments, steps (v) and (vi) are repeated at least 8 times. In some embodiments, the amplitude of the second drive voltage is less than 50% of the amplitude of the first drive voltage. In some embodiments, the positive charge of the third particle is less than 50% of the positive charge of the first particle. In some embodiments, the negative charge of the fourth particle is less than 75% of the negative charge of the second particle. In some embodiments, a voltage having a vibration waveform is applied to the pixel before step (i). In some embodiments, the sixth time period in step (vi) is shorter than the third time period in step (iii). In some embodiments, a third drive voltage is applied to the pixel of the electrophoretic display in a seventh time period between steps (iv) and (v), wherein the third drive voltage has the same polarity as the second drive voltage and the same amplitude as the first amplitude. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 A display layer comprising an electrophoretic medium is depicted, the electrophoretic medium comprising four particle groups, each particle group having a charge polarity and a charge magnitude, and no particle groups having the same charge polarity and charge magnitude. The display layer is capable of displaying at least four different color states.

[0040] Figures 2A-2F An exemplary electrophoretic medium is shown that includes four particle groups, each particle group having a charge polarity and a charge amount, and no particle group having the same charge polarity and charge amount. Figures 2A-2F In this system, the yellow and black particles are oppositely charged, and the white and red particles are oppositely charged. The yellow and black particles have a higher charge than the white and red particles. The color groups are arbitrary, and any specific combination of the four particles can be used with this system.

[0041] Figure 3 Vibration waveforms that can be incorporated into the driving method are shown.

[0042] Figure 4 and 5 A first driving method of the present invention is shown.

[0043] Figure 6 and 9 A second driving method of the present invention is shown.

[0044] Figure 7 , 8 , 10 and 11 show a driving sequence using the second driving method of the present invention.

[0045] Fig.12 and 15 A third driving method of the present invention is shown.

[0046] Fig.13 , 14 , 16 and 17 show a driving sequence using the third driving method of the present invention.

[0047] Fig.18 and 21 A fourth driving method of the present invention is shown.

[0048] Fig.19 , 20 , 22 and 23 show a driving sequence using the fourth driving method of the present invention.

[0049] Fig.24 An increased waveform that can be used to improve the color state of the less charged particle group is shown.

[0050] Fig.25 A driving method is shown to achieve high quality color states for less charged particles.

[0051] Fig.26 A driving method is shown to achieve high quality color states for less charged particles.

[0052] Fig. 27 A driving method is shown to achieve high quality color states for less charged particles.

[0053] Fig.28 An increased waveform that can be used to improve the color state of the less charged particle group is shown.

[0054] Fig.29 A driving method is shown to achieve high quality color states for less charged particles.

[0055] Fig.30 A driving method is shown to achieve high quality color states for less charged particles.

[0056] Fig.31 A driving method is shown to achieve high quality color states for less charged particles.

[0057] Fig.32 An improved driving method is shown to achieve high quality color states for less charged particles.

[0058] Fig.33 An improved driving method is shown to achieve high quality color states for less charged particles.

[0059] Fig.34 Shown is the measured change in electro-optical (EO) performance as a function of voltage for the lower voltage waveform. Fig.29 The waveform (original WF) is Fig.33 The waveform (improved WF) was compared. DETAILED DESCRIPTION

[0060] The electrophoretic fluid related to the present invention comprises two pairs of oppositely charged particles, the first pair consisting of positive particles of a first type and negative particles of a first type, and the second pair consisting of positive particles of a second type and negative particles of a second type.

[0061] In two pairs of oppositely charged particles, one pair carries a stronger charge than the other pair. Therefore, the four types of particles can also be called high positive particles, high negative particles, low positive particles, and low negative particles.

[0062] like Figure 1In the example shown in FIG, black particles (K) and yellow particles (Y) are a first pair of oppositely charged particles, in which the black particles are high positive particles and the yellow particles are high negative particles. Red particles (R) and white particles (W) are a second pair of oppositely charged particles, in which the red particles are low positive particles and the white particles are low negative particles.

[0063] In another example not shown, black particles may be high positive particles; yellow particles may be low positive particles; white particles may be low negative particles, and red particles may be high negative particles.

[0064] In addition, the color states of the four types of particles can be intentionally mixed. For example, since yellow pigments usually have a green tint in nature, if a better yellow state is needed, yellow particles and red particles can be used, where the two types of particles have the same charge polarity and the yellow particles have a higher charge than the red particles. Therefore, in the yellow state, there will be a small amount of red particles mixed with a little green yellow particles, so that the yellow state has better color purity.

[0065] It should be understood that the scope of the present invention broadly includes particles of any color, as long as the four types of particles have visually distinguishable colors.

[0066] As for white particles, they can be formed of inorganic pigments such as TiO2, ZrO2, ZnO, Al2O3, Sb2O3, BaSO4, PbSO4, etc.

[0067] As for the black particles, they may be formed of Cl Pigment Black 26 or 28 or the like (eg, manganese iron black spinel or copper chrome black spinel) or carbon black.

[0068] Non-white and non-black particles are independent colors such as red, green, blue, magenta, cyan or yellow. Pigments for color particles can include but are not limited to CI pigments PR254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY83, PY138, PY150, PY155 or PY20. These are commonly used organic pigments described in color index handbook " New Pigment Application Technology " (CMC Publishing Co, Ltd, 1986) and " Printing Ink Technology " (CMC Publishing Co, Ltd, 1984). Specific examples include Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS, NovopermYellow HR-70-EDS, Hostaperm Green GNX, BASF Irgazine red L 3630, Cinquasia Red L4100HD, and Irgazin Red L 3660HD; Phthalocyanine Blue, Phthalocyanine Green, Aniline Yellow or Aniline AAOT Yellow from Sun Chemical.

[0069] The color particles may also be inorganic pigments, such as red, green, blue and yellow. Examples may include, but are not limited to, CI Pigment Blue 28, CI Pigment Green 50 and CI Pigment Yellow 227.

[0070] In addition to color, the four types of particles can have other different optical properties, such as light transmission, reflection and luminescence, or in the case of displays for machine reading, false colors in the sense of changes in reflectivity for electromagnetic wavelengths outside the visible range.

[0071] The display layer using the display fluid of the present invention has two surfaces, a first surface (13) on the observation side and a second surface (14) on the opposite side to the first surface (13). The display fluid is sandwiched between the two surfaces. On one side of the first surface (13), there is a common electrode (11), which is a transparent electrode layer (e.g., ITO) and spreads over the entire top of the display layer. On one side of the second surface (14), there is an electrode layer (12), and the electrode layer (12) includes a plurality of pixel electrodes (12a).

[0072] Pixel electrodes are described in U.S. Patent No. 7,046,228, the contents of which are incorporated herein by reference in their entirety. It is noteworthy that while active matrix drive using a thin film transistor (TFT) backplane is mentioned for the pixel electrode layer, the scope of the present invention includes other types of electrode addressing, as long as the electrodes provide the desired functionality.

[0073] Figure 1 Each space between two vertical dashed lines in the diagram represents a pixel. As shown, each pixel has a corresponding pixel electrode. An electric field is created for the pixel by the potential difference between the voltage applied to the common electrode and the voltage applied to the corresponding pixel electrode.

[0074] The solvent for dispersing the four types of particles is colorless and transparent. It preferably has a low viscosity and a dielectric constant in the range of about 2 to about 30, preferably about 2 to about 15, to obtain high particle mobility. Examples of suitable dielectric solvents include, for example, Decalin, 5-ethylidene-2-norbornene, hydrocarbons of fatty oils, paraffin oils, silicone fluids, aromatic hydrocarbons such as toluene, xylene, diarylethane, dodecylbenzene or alkylnaphthalene, halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, monochloropentafluorobenzene, dichlorononane or pentachlorobenzene, and perfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company, St. Paul, Minnesota, low molecular weight halogen polymers such as poly(perfluoropropylene oxide) from TCI America, Portland, Oregon, poly(chlorotrifluoroethylene) such as halocarbon oils from Halocarbon Products, River Edge, New Jersey, such as from Ausimont or Krytox Oils from Galden and Grease's K-Fluid series of perfluoropolyalkyl ethers from DuPont of Delaware, and polydimethylsiloxane-based silicone oils (DC-200) from Dow Corning.

[0075] In one embodiment, the charge carried by the "low charge" particles may be less than about 50%, preferably about 5% to about 30%, of the charge carried by the "high charge" particles. In another embodiment, the "low charge" particles may be less than about 75%, or about 15% to about 55% of the charge carried by the "high charge" particles. In another embodiment, the comparison of charge levels shown applies to two types of particles with the same charge polarity.

[0076] The charge intensity can be measured based on the zeta potential. In one embodiment, the zeta potential is determined by a Colloidal Dynamics AcoustoSizer IIM, ESA EN#Attn flow-through electrolytic cell (K:127) with a CSPU-100 signal processing unit. Instrument constants at the test temperature (25°C), such as the density of the solvent used in the sample, the dielectric constant of the solvent, the speed of sound in the solvent, and the viscosity of the solvent are all entered before the test. The pigment sample is dispersed in a solvent (which is typically a hydrocarbon fluid with less than 12 carbon atoms) and diluted to 5-10% by weight. The sample also contains a charge modifier (Solsperse Available from Lubrizol Corporation, a Berkshire Hathaway company; "Solsperse" is a registered trademark) with a 1:10 weight ratio of charge modifier to particle. The mass of the diluted sample is determined, and the sample is then loaded into a flow-through electrolytic cell to determine the zeta potential.

[0077] The magnitudes of a "high positive" particle and a "high negative" particle can be the same or different. Likewise, the magnitudes of a "low positive" particle and a "low negative" particle can be the same or different. However, the zeta potential of a "high positive" or positive particle with a greater charge strength or greater charge magnitude is greater than the zeta potential of a "low positive" or positive particle with a lesser charge strength or lesser charge magnitude, and the same logic applies to high negative and low negative particles. In the same medium, in the same field, a higher charged particle has a greater electrophoretic mobility, that is, a higher charged particle will travel the same distance in a shorter time than a lower charged particle.

[0078] It should also be noted that in the same fluid, two pairs of high and low charged particles may have different degrees of charge difference. For example, in one pair, the charge strength of the low positively charged particles may be 30% of the charge strength of the high positively charged particles, while in another pair, the charge strength of the low negatively charged particles may be 50% of the charge strength of the high negatively charged particles.

[0079] The following examples illustrate display devices using such display fluids.

[0080] Exemplary drive schemes

[0081] Figures 2A-2F An exemplary driving scheme using an exemplary four-particle system is shown in . High positive particles are black (K); high negative particles are yellow (Y); low positive particles are red (R); and low negative particles are white (W). Figure 2AIn , when a high negative voltage potential difference (e.g., -15V) is applied to the pixel for a sufficiently long period of time, an electric field is generated so that the yellow particles (Y) are pushed to the common electrode (21) side and the black particles (K) are pulled to the pixel electrode (22a) side. The red (R) and white (W) particles, because they carry a weaker charge, move slower than the higher charged black and yellow particles, so they stay in the middle of the pixel with the white particles above the red particles. In this case, yellow is seen on the viewing side. Figure 2B In the case of a pixel, when a high positive voltage potential difference (e.g. +15V) is applied to the pixel for a sufficiently long period of time, an electric field of opposite polarity is generated, which causes the particle distribution to Figure 2A The opposite is shown, so black is seen on the viewing side.

[0082] exist Figure 2C and 2D When a lower positive voltage potential difference (e.g. +3V) is applied to Figure 2C When the pixel is driven from the yellow state (i.e., driven from the yellow state) for a sufficiently long period of time, an electric field is generated to cause the yellow particles (Y) to move toward the pixel electrode (22a), and the black particles (K) to move toward the common electrode (21). However, when they meet in the middle of the pixel, they are significantly decelerated and stay there because the electric field strength generated by the low driving voltage is not strong enough to overcome the strong attraction between them. Figure 2D As shown, the electric field generated by the low driving voltage is sufficient to separate the white and red particles with weaker charges (less charge), thereby allowing the low positive red particles (R) to move all the way to the common electrode (21) side (i.e., the observation side) and the low negative (less charged) white particles (W) to move to the pixel electrode (22a) side. As a result, red is observed. It should also be noted that in this figure, there is also an attraction between the particles with weaker charges (such as R) and the particles with stronger charges of the opposite polarity (such as Y). However, these attractions are not as strong as the attraction between the two types of particles with stronger charges (K and Y), so they can be overcome by the electric field generated by the low driving voltage. Importantly, this system allows the separation of particles with weaker charges from particles with stronger charges of the opposite polarity.

[0083] exist Figure 2E and 2F When a lower negative voltage potential difference (e.g., -3V) is applied to Figure 2E When the pixel is driven from the yellow state (i.e., from the yellow state) for a sufficiently long period of time, an electric field is generated to cause the black particles (K) to move toward the pixel electrode (22a) and the white particles (W) to move toward the common electrode (21). When the black and yellow particles meet in the middle of the pixel, they are significantly decelerated and remain there because the electric field generated by the low drive voltage is not sufficient to overcome the strong attraction between them. Figure 2FAs shown, the electric field generated by the low driving voltage is sufficient to separate the white and red particles, so that the low negative white particles (W) move all the way to the common electrode side (i.e., the observation side) and the low positive red particles (R) move to the pixel electrode side. As a result, white is observed. It should also be noted that in this figure, there is also an attraction between the weaker charged particles (such as W) and the stronger charged particles (such as K) of the opposite polarity. However, these attractions are not as strong as the attraction between the two types of particles with stronger charges (K and Y), so they can be overcome by the electric field generated by the low driving voltage. In other words, the weaker charged particles can be separated from the stronger charged particles of the opposite polarity.

[0084] Although in this example, black particles (K) carry high positive charge, yellow particles (Y) carry high negative charge, red (R) particles carry low positive charge, and white particles (W) carry low negative charge, in fact, the four groups of particles in the electrophoretic medium of the present invention can have high positive charge, high negative charge, low positive charge, and low negative charge of any color. All these changes are expected to be within the scope of the present application.

[0085] It should also be noted that in order to achieve Figure 2D and 2F The lower voltage potential difference applied to drive the pixel from a color state of high positive particles to a color state of high negative particles (or vice versa) can be from about 5% to about 50% of the full drive voltage potential difference required to drive the pixel from a color state of high positive particles to a color state of high negative particles (or vice versa).

[0086] The electrophoretic fluid is filled in a display unit. The display unit may be a cup-shaped microunit as described in U.S. Patent No. 6,930,818, the entire contents of which are incorporated herein by reference. The display unit may also be other types of microcontainers, such as microcapsules, microchannels, or equivalents, regardless of their shape or size. All of these are within the scope of the present application.

[0087] To ensure color brightness and color purity, a vibration waveform can be used before driving from one color state to another. The vibration waveform consists of a pair of opposite drive pulses repeated for many cycles. For example, the vibration waveform can consist of a +15V pulse lasting 20 milliseconds and a -15V pulse lasting 20 milliseconds, and such a pair of pulses is repeated 50 times. The total time of this vibration waveform is 2000 milliseconds (see Figure 3 ). In practice, one shaking pulse may have at least 10 repetitions (i.e., ten pairs of positive and negative pulses). A drive sequence may include more than one shaking pulse. Before applying the drive voltage, the shaking waveform can be applied regardless of the optical state (black, white, red, or yellow). After applying the shaking waveform, the optical state will not be pure white, pure black, pure yellow, or pure red. Instead, the color state will come from a mixture of the four types of pigment particles.

[0088] In an example, each drive pulse in the vibration waveform is applied for no more than 50% (or no more than 30%, 10% or 5%) of the drive time required to drive from a fully black state to a fully yellow state (or vice versa). For example, if it takes 300 milliseconds to drive the display device from a fully black state to a fully yellow state (or vice versa), the vibration waveform can be composed of positive and negative pulses, each of which is applied for no more than 150 milliseconds. In practice, shorter pulses are preferred. The described vibration waveform can be used in the driving method of the present invention. [Please note that in all figures throughout this application, the vibration waveform is simplified (i.e., the number of pulses is less than the actual number).]

[0089] Furthermore, in the context of this application, a high drive voltage (VH1 or VH2) is defined as a drive voltage sufficient to drive a pixel from a color state of high positive particles to a color state of high negative particles, or vice versa (see Figure 2A and 2B In the described case, the low drive voltage (VL1 or VL2) is defined as a drive voltage sufficient to drive the pixel from the color state of the more highly charged particles to the color state of the less highly charged particles (see Figure 2D and 2F ). Typically, the magnitude of VL (eg, VL1 or VL2) is less than 50%, or preferably less than 40%, of the magnitude of VH (eg, VH1 or VH2).

[0090] First drive method:

[0091] Part A:

[0092] Figure 4 A driving method for driving a pixel from a yellow state (high negative) to a red state (low positive) is shown. In this method, a high negative driving voltage (VH2, e.g. -15V) is applied during a time period of t2 to drive the pixel towards the yellow state following the vibration waveform. From the yellow state, the pixel can be driven towards the red state (i.e., moving the pixel from the yellow state to the red state) by applying a low positive voltage (VL1, e.g. +5V) during a time period of t3. Figure 2C Drive to Figure 2D ). The driving period t2 is a period sufficient to drive the pixel to the yellow state when VH2 is applied, and the driving period t3 is a period sufficient to drive the pixel from the yellow state to the red state when VL1 is applied. It is preferred that the driving voltage is applied for a period t1 before the oscillation waveform to ensure DC balance. Figure 4The entire waveform is DC balanced. Throughout this application, the term "DC balance" is intended to mean that when integrated over a time period (e.g., the time period of the entire waveform), the drive voltage applied to the pixel is substantially zero. DC balance can be achieved by balancing each stage of the waveform, that is, the first positive voltage will be selected so that the integration over the subsequent negative voltage results in zero or substantially zero. Later, if the stage is repeated, the integrated voltage over a series of repetitions will also be zero or substantially zero, i.e., DC balance. Alternatively, one (or more) stages of the waveform may be unbalanced because the integration over this stage will result in a positive (or negative) DC offset. However, the subsequent stages can be designed to be unbalanced in the opposite direction so that the total waveform is DC balanced.

[0093] Part B:

[0094] Figure 5 A driving method for driving a pixel from a black state (high positive) to a white state (low negative) is shown. In this method, a high positive driving voltage (VH1, e.g., +15V) is applied during a time period of t5 to drive the pixel toward the black state following the vibration waveform. From the black state, the pixel can be driven toward the white state (i.e., the pixel is driven from the black state to the white state) by applying a low negative voltage (VL2, e.g., -5V) during a time period of t6. Figure 2E Drive to Figure 2F ). The driving time period t5 is a time period sufficient to drive the pixel to a black state when VH1 is applied, and the driving time period t6 is a time period sufficient to drive the pixel from a black state to a white state when VL2 is applied. It is preferred that the driving voltage is applied for a time period t4 before the oscillating waveform to ensure DC balance. In one embodiment, Figure 5 The entire waveform is DC balanced.

[0095] Generally speaking, Figure 4 and Figure 5 The driving method can be summarized as follows:

[0096] A driving method for an electrophoretic display, the electrophoretic display comprising a first surface on an observation side, a second surface on a non-observation side, and an electrophoretic fluid sandwiched between a common electrode and a pixel electrode layer, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles all dispersed in a solvent or a solvent mixture, wherein

[0097] (a) The four types of pigment particles have different optical properties from each other;

[0098] (b) the first type of particles carries a high positive charge and the second type of particles carries a high negative charge; and

[0099] (c) the third type of particles carries a low positive charge, and the fourth type of particles carries a low negative charge,

[0100] The method comprises the following steps:

[0101] (i) applying a first drive voltage to a pixel in the electrophoretic display for a first time period to drive the pixel towards a color state of the first or second type of particles at a viewing side; and

[0102] (ii) applying a second drive voltage to the pixel during a second time period, wherein the second drive voltage has a polarity opposite to that of the first drive voltage and has a magnitude lower than that of the first drive voltage, so as to drive the pixel from a color state of the first type of particles to a color state of the fourth type of particles or from a color state of the second type of particles to a color state of the third type of particles on the observation side.

[0103] Second drive method:

[0104] Part A:

[0105] The second driving method of the present invention is Figure 6 It involves a driving waveform that is used to replace Figure 4 The driving time period of t3.

[0106] In an initial step, a high negative drive voltage (VH2, e.g., -15V) is applied during a time period of t7 to push the yellow particles toward the viewing side, followed by a positive drive voltage (+V') during a time period of t8, which pulls the yellow particles down and pushes the red particles toward the viewing side. The amplitude of +V' is lower than the amplitude of VH (e.g., VH1 or VH2). In one embodiment, the amplitude of +V' is less than 50% of the amplitude of VH (e.g., VH1 or VH2). In one embodiment, t8 is greater than t7. In one embodiment, t7 can be in the range of 20-400 milliseconds and t8 can be ≥ 200 milliseconds.

[0107] Figure 6 The waveform is repeated for at least 2 cycles (N≥2), preferably at least 4 cycles, and more preferably at least 8 cycles. As measured using a handheld spectrophotometer, the red color becomes more intense after each driving cycle. As described above, Figure 6 The drive waveforms shown can be used instead of Figure 4 The driving time period of t3 (see Figure 7 ). In other words, the driving sequence can be: vibrate the waveform, then drive to the yellow state for a period of time t2, and then apply Figure 6 In another embodiment, the step of driving to the yellow state during time period t2 can be completely eliminated, in which case, before applying Figure 6 The vibration waveform is applied before the waveform of Figure 8 In one embodiment, Figure 7 The entire waveform is DC balanced. In another embodiment, Figure 8 The entire waveform is DC balanced.

[0108] Part B:

[0109] In a similar way, Fig. 9 Shown is an alternative Figure 5 In the driving waveform of the driving time period of t6 in . In the initial step, a high positive driving voltage (VH1, such as +15V) is applied during the time period of t9 to push the black particles to the observation side, and then a negative driving voltage (-V') is applied during the time period of t10, which pulls the black particles down and pushes the white particles to the observation side. The amplitude of -V' is lower than the amplitude of VH (e.g., VH1 or VH2). In one embodiment, the amplitude of -V' is less than 50% of the amplitude of VH (e.g., VH1 or VH2). In one embodiment, t10 is greater than t9. In one embodiment, t9 can be in the range of 20-400 milliseconds and t10 can be ≥200 milliseconds. Fig. 9 The waveform is repeated for at least 2 cycles (N≥2), preferably at least 4 cycles, and more preferably at least 8 cycles. After each driving cycle, the white becomes more intense. As mentioned above, Fig. 9 The drive waveforms shown can be used instead of Figure 5 The driving time period of t6 (see Fig.10 ). In other words, the driving sequence may be: vibrate the waveform, then drive to the black state for a period of time t5, and then apply Fig. 9 In another embodiment, the step of driving to the black state during time period t5 can be eliminated, in which case, before applying Fig. 9 The vibration waveform is applied before the waveform of Fig.11 In one embodiment, Fig.10 The entire waveform is DC balanced. In another embodiment, Fig.11 The entire waveform is DC balanced.

[0110] Figure 6-11 The second driving method shown can be summarized as follows:

[0111] A driving method for an electrophoretic display, the electrophoretic display comprising a first surface on an observation side, a second surface on a non-observation side, and an electrophoretic fluid sandwiched between a common electrode and a pixel electrode layer, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles all dispersed in a solvent or a solvent mixture, wherein

[0112] (a) The four types of pigment particles have different optical properties from each other;

[0113] (b) the first type of particles carries a high positive charge and the second type of particles carries a high negative charge; and

[0114] (c) the third type of particles carries a low positive charge, and the fourth type of particles carries a low negative charge,

[0115] The method comprises the following steps:

[0116] (i) applying a first drive voltage to a pixel in the electrophoretic display for a first time period to drive the pixel towards a color state of the first or second type of particles at a viewing side;

[0117] (ii) applying a second drive voltage to the pixel for a second time period, wherein the second time period is greater than the first time period, the second drive voltage having a polarity opposite to the polarity of the first drive voltage and a magnitude lower than the magnitude of the first drive voltage, to drive the pixel from a color state of the first type of particles to a color state of the fourth type of particles or from a color state of the second type of particles to a color state of the third type of particles on the viewing side; and

[0118] Repeat steps (i) and (ii).

[0119] In one embodiment, the amplitude of the second drive voltage is less than 50% of the amplitude of the first drive voltage. In one embodiment, steps (i) and (ii) are repeated at least 2 times, preferably at least 4 times, and more preferably at least 8 times. In one embodiment, the method further comprises a vibration waveform before step (i). In one embodiment, the method further comprises driving the pixel to the color state of the first or second type of particles after the vibration waveform but before step (i).

[0120] The third driving method:

[0121] Part A:

[0122] The third driving method of the present invention is Fig.12 It involves Figure 6 An alternative to the drive waveform, which can also be used to replace Figure 4 In this alternative waveform, a waiting time t13 is added. During the waiting time, the driving voltage is not applied. Fig.12 The entire waveform is also repeated at least 2 times (N≥2), preferably at least 4 times, and more preferably at least 8 times. Fig.12The waveform is designed to release charge imbalances stored in dielectric layers and / or interfaces between different material layers in an electrophoretic display device, especially when the resistance of the dielectric layer is high, for example at low temperatures. (This charge accumulation is also referred to as residual voltage.) In the context of this application, the term "low temperature" refers to temperatures below about 10°C, such as 0°C or colder, such as -5°C or colder, such as -10°C or colder, such as -20°C or colder.

[0123] The waiting time allows the unwanted charge stored in the dielectric layer to dissipate and results in a more efficient short pulse (t11) for driving the pixel towards the yellow state and a longer pulse (t12) for driving the pixel towards the red state. Thus, this alternative driving method will better separate the low-charge pigment particles from the higher-charge pigment particles. In addition, because the charge stored in the dielectric layer has more time to dissipate, the final optical state of the display drifts less.

[0124] The time periods t11 and t12 are similar to Figure 6 In other words, t12 is greater than t11. The waiting time (t13) can be in the range of 5-5,000 milliseconds, depending on the resistance of the dielectric layer. As mentioned earlier, Fig.12 The driving waveform shown can also be used instead of Figure 4 The driving time period of t3 (see Fig.13 ). In other words, the driving sequence may be: vibrate the waveform, then drive to the yellow state for a period of time t2, then apply Fig.12 In another embodiment, the step of driving to the yellow state during time period t2 can be eliminated, in which case, before applying Fig.12 The vibration waveform is applied before the waveform of Fig.14 In one embodiment, Fig.13 The entire waveform is DC balanced. In another embodiment, Fig.14 The entire waveform is DC balanced.

[0125] Part B:

[0126] Fig.15 Shows Fig. 9 An alternative to the drive waveform, which can also be used to replace Figure 5 In the alternative waveform, a waiting time t16 is added. During the waiting time, no driving voltage is applied. Fig.15 The entire waveform is also repeated at least 2 times (N ≥ 2), preferably at least 4 times, and more preferably at least 8 times. Fig.12 The waveform is the same as Fig.15The waveform of is also designed to release the charge imbalance stored in the dielectric layer and / or the interface of different material layers in the electrophoretic display device. As described above, the waiting time may dissipate the unwanted charge stored in the dielectric layer and make the short pulse (t14) for driving the pixel towards the black state and the longer pulse (t15) for driving the pixel towards the white state more efficient. The time periods t14 and t15 are similar to Fig. 9 In other words, t15 is greater than t14. The waiting time (t16) can also be in the range of 5-5,000 milliseconds, depending on the resistance of the dielectric layer. As mentioned above, Fig.15 The driving waveform shown can also be used instead of Figure 5 The driving time period of t6 (see Fig.16 ). In other words, the driving sequence may be: vibrate the waveform, then drive to the black state for a period of time t5, and then apply Fig.15 In another embodiment, the step of driving to the black state during time period t5 can be eliminated, in which case, before applying Fig.15 The vibration waveform is applied before the waveform of Fig.17 In one embodiment, Fig.16 The entire waveform is DC balanced. In another embodiment, Fig.17 The entire waveform is DC balanced.

[0127] The third driving method, such as Figure 12-17 As shown, it can be summarized as follows:

[0128] A driving method for an electrophoretic display, the electrophoretic display comprising a first surface on an observation side, a second surface on a non-observation side, and an electrophoretic fluid sandwiched between a common electrode and a pixel electrode layer, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles all dispersed in a solvent or a solvent mixture, wherein

[0129] (a) The four types of pigment particles have different optical properties from each other;

[0130] (b) the first type of particles carries a high positive charge and the second type of particles carries a high negative charge; and

[0131] (c) the third type of particles carries a low positive charge, and the fourth type of particles carries a low negative charge,

[0132] The method comprises the following steps:

[0133] (i) applying a first drive voltage to a pixel in the electrophoretic display for a first time period to drive the pixel towards a color state of the first or second type of particles at a viewing side;

[0134] (ii) applying a second drive voltage to the pixel during a second time period, wherein the second time period is greater than the first time period, the second drive voltage has a polarity opposite to the polarity of the first drive voltage, and has a magnitude lower than the magnitude of the first drive voltage, to drive the pixel from a color state of the first type of particles to a color state of the fourth type of particles or from a color state of the second type of particles to a color state of the third type of particles on the viewing side;

[0135] (iii) not applying a driving voltage to the pixel during a third time period; and

[0136] Repeat steps (i)-(iii).

[0137] In one embodiment, the amplitude of the second driving voltage is less than 50% of the amplitude of the first driving voltage. In one embodiment, steps (i), (ii) and (iii) are repeated at least 2 times, preferably at least 4 times, more preferably at least 8 times. In one embodiment, the method also includes a vibration waveform before step (i). In one embodiment, the method also includes a driving step of reaching a full color state of the first or second type of particles after the vibration waveform but before step (i). It should be noted that the length of any driving time period mentioned in the application may be related to temperature.

[0138] Fourth drive method:

[0139] Part A:

[0140] The fourth driving method of the present invention is Fig.18 It involves a driving waveform that can also be used instead of Figure 4 In the initial step, a high negative driving voltage (VH2, for example, -15V) is applied to the pixel during a time period of t17, followed by a waiting time of t18. After the waiting time, a positive driving voltage (+V', for example, less than 50% of VH1 or VH2) is applied to the pixel during a time period of t19, followed by a second waiting time t20. Fig.18 The waveform is repeated at least 2 times, preferably at least 4 times, and more preferably at least 8 times. As mentioned above, the term "waiting time" refers to a period of time during which no driving voltage is applied. Fig.18 In the waveform of , the first waiting time t18 is very short, while the second waiting time t20 is relatively long. The time period of t17 is also shorter than the time period of t19. For example, t17 may be in the range of 20-200 milliseconds; t18 may be less than 100 milliseconds; t19 may be in the range of 100-200 milliseconds; and t20 may be less than 1000 milliseconds. Fig.19 yes Figure 4 and Fig.18 In Figure 4In the example, the yellow state is displayed during the time period t2. Generally speaking, the better the yellow state in this time period, the better the red state that will be displayed in the end. In one embodiment, the step of driving to the yellow state during the time period t2 can be eliminated, and in this case, the yellow state is displayed during the time period t2. Fig.18 The vibration waveform is applied before the waveform of Fig. 20 In one embodiment, Fig.19 The entire waveform is DC balanced. In another embodiment, Fig. 20 The entire waveform is DC balanced.

[0141] Part B:

[0142] Fig.21 shows a driving waveform, which can also be used instead of Figure 5 In the initial step, a high positive driving voltage (VH1, for example, +15V) is applied to the pixel during a time period of t21, followed by a waiting time of t22. After the waiting time, a negative driving voltage (-V', for example, less than 50% of VH1 or VH2) is applied to the pixel during a time period of t23, followed by a second waiting time t24. Fig.21 The waveform may also be repeated at least 2 times, preferably at least 4 times, and more preferably at least 8 times. Fig.21 In the waveform of , the first waiting time t22 is very short, while the second waiting time t24 is relatively long. The time period of t21 is also shorter than the time period of t23. For example, t21 may be in the range of 20-200 milliseconds; t22 may be less than 100 milliseconds; t23 may be in the range of 100-200 milliseconds; and t24 may be less than 1000 milliseconds. Fig. 22 yes Figure 5 and Fig.21 In Figure 5 In the example, the black state is displayed during the time period t5. Generally speaking, the better the black state in this time period, the better the white state displayed at the end. In one embodiment, the step of driving to the black state during the time period t5 can be eliminated, and in this case, the step of driving to the black state during the time period t5 can be eliminated. Fig.21 The vibration waveform is applied before the waveform of Fig.23 In one embodiment, Fig. 22 The entire waveform is DC balanced. In another embodiment, Fig.23 The entire waveform is DC balanced.

[0143] The fourth driving method, such as Figure 18-23 As shown, it can be summarized as follows:

[0144] A driving method for an electrophoretic display, the electrophoretic display comprising a first surface on an observation side, a second surface on a non-observation side, and an electrophoretic fluid sandwiched between a common electrode and a pixel electrode layer, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles all dispersed in a solvent or a solvent mixture, wherein

[0145] (a) The four types of pigment particles have different optical properties from each other;

[0146] (b) the first type of particles carries a high positive charge and the second type of particles carries a high negative charge; and

[0147] (c) the third type of particles carries a low positive charge, and the fourth type of particles carries a low negative charge,

[0148] The method comprises the following steps:

[0149] (i) applying a first drive voltage to a pixel in the electrophoretic display for a first time period to drive the pixel towards a color state of the first or second type of particles at a viewing side;

[0150] (ii) not applying a driving voltage to the pixel during a second time period;

[0151] (iii) applying a second drive voltage to the pixel during a third time period, wherein the third time period is greater than the first time period, the second drive voltage has a polarity opposite to the polarity of the first drive voltage and a magnitude lower than the magnitude of the first drive voltage, so as to drive the pixel from the color state of the first type of particles to the color state of the fourth type of particles or from the color state of the second type of particles to the color state of the third type of particles on the viewing side;

[0152] (iv) not applying a driving voltage to the pixel during a fourth time period; and

[0153] Repeat steps (i)-(iv).

[0154] In one embodiment, the amplitude of the second driving voltage is less than 50% of the amplitude of the first driving voltage. In one embodiment, steps (i)-(iv) are repeated at least 2 times, preferably at least 4 times, and more preferably at least 8 times. In one embodiment, the method also includes a vibration waveform before step (i). In one embodiment, the method also includes driving the pixel to the color state of the first or second type of particles after the vibration waveform but before step (i). This driving method is not only particularly effective at low temperatures, but also can provide a display device with better tolerance to structural changes caused during the manufacture of the display device. Therefore, its usefulness is not limited to low temperature driving.

[0155] Suffix pulse for less charged particle states

[0156] Various push-pull waveforms in the above drive schemes can be used to achieve good red and white states, such as the less charged particle optical state. Typically, these waveforms provide high brightness and are robust to environmental changes (such as temperature changes) and the spectrum of the incident light. However, in some applications, such as digital signage, color variations in the final image are unacceptable to consumers. For example, Fig.10 The white waveform may leave a slight yellowish tint in the white state, which consumers find objectionable, especially when the display is adjacent to a light-colored or white border.

[0157] To some extent, the color of the final state of the less charged particles can be improved by slightly increasing the magnitude of the voltage (V'), e.g. Fig.10 In the case of a white state, a larger V' will raise L* and make the final state appear whiter. However, an increase in V' will also increase the amount of yellow remaining, which translates into an increase in b*.

[0158] The inventors have discovered that by adding a series of pulses after the push-pull waveform, a lower voltage V" can be used to process the less charged particles than the voltage V' that would achieve the highest L*. These pulses can be considered "wait-pull" or "suffix" pulses. The end result is a combination of the push-pull waveform and the suffix waveform, but a higher L* value (in the white state) is achieved without a full increase in b*. Because this final state is more "pure" in the color of the less charged particles, it is generally more pleasing to consumers.

[0159] Specifically, a series of suffix pulses ("wait-pull" pulses), such as the overall Fig.24 and 28 As described in, it can be used to improve the final state of the less charged particle state by providing a less charged color state that is less contaminated by the more highly charged particles. Likewise, although these less charged particle states are described as red and white, respectively, it should be understood that the color states are arbitrary and the less charged particles can be any color, such as red, orange, yellow, green, blue, purple, brown, black, white, magenta or cyan. In addition, the less charged particles can be reflective, absorbing, scattering or partially transparent.

[0160] The red suffix pulse train is Fig.24, including a waiting period t25, followed by a driving impulse with a voltage -V' of a period t26, after which the sequence is repeated. The period t25 is longer than the period t26. The typical range of the waiting period t25 is between 20ms and 5000ms, while the driving period t26 is between 20ms and 3000ms. Such a waveform can be repeated at least 2 times (N'≥2), preferably at least 4 times, and more preferably at least 8 times.

[0161] The corresponding white suffix pulse train is Fig.28 As shown in , it includes a waiting period t27, followed by a driving impulse with a voltage +V' for a period t28, after which the sequence is repeated. The time period t27 is longer than the time period t28. The typical range of the waiting period t27 is between 20ms and 5000ms, while the driving period t28 is between 20ms and 3000ms. Such a waveform can be repeated at least 2 times (N'≥2), preferably at least 4 times, and more preferably at least 8 times. As previously described, the amplitudes of the driving voltages -V' and +V" can be 50% or less of the amplitude of VH (e.g., VH1 or VH2). It should also be noted that the amplitude of -V' can be the same as or different from the amplitude of +V'.

[0162] The suffix pulse is the same as described above (e.g. Figure 4-23 ) push-pull waveform combination. The resulting red state waveform is as follows Figure 25-27 As shown, they correspond to Fig.24 Towards Figure 8 , 14 and 20 additions, despite Fig.24 A suffix pulse may also be added to any of the red state waveforms described herein, including but not limited to Figure 7 , 13 and 19. In the same way, Fig.28 The white state suffix pulse is added to Fig.11 , 17 and 23 in the white state waveform, thereby generating Figure 29-31 The new white state waveform. Similarly, Fig.28 A suffix pulse may also be added to any white state waveform described herein, including but not limited to Fig.10 , 16 and 22. In one embodiment, Fig.24 and 28 The waveform is DC balanced. In another embodiment, Fig.24 and 28 The waveform is DC unbalanced, but it is different from the previous waveform (such as Figure 4-23 ) coordination, so that Figure 25-27The complete waveforms of and 29-31 are DC balanced. It should be understood that V' and V" are somewhat arbitrary. Both V' and V" are less than VH1 or VH2, typically less than 50% of VH1 or VH2. V" is typically less than V', however, V' and V" can be the same, depending on the final color state (e.g., red vs. white) and the final application.

[0163] Experiments show that the new waveform, including the suffix pulse, can drive the final optical state of the less charged particles to a more saturated color state and less contaminated by the more highly charged particles. For example, when driven to the white state, the L* of the final state is the same as the push-pull waveform alone (indicating the same brightness), but when used together with e.g. Fig.11 , 17 Compared with the waveform of 23, the b* value is smaller. In other words, the same white brightness can be achieved using the waveform with the suffix pulse while reducing the yellow pigment contamination. Figure 25-27 The same results were found for the red state achieved with a combination of push-pull and suffix waveforms. In the case of the red state, the push-pull / suffix red waveform resulted in a higher L* while maintaining the same b*, indicating less black pigment in the resulting red state. In both cases, the improvement in the final color state using the modified waveform (i.e., including the suffix pulse) compared to the waveform without the suffix pulse (e.g., the push-pull waveform alone) was visible to the naked eye.

[0164] Back-push pulse for improved particle separation

[0165] Although the above Figure 24-31 The described suffix pulses improve the electro-optical properties of the optical state of the smaller charged particles, but it has been observed that, for example, when the suffix pulses are added to the waveform, the overall electro-optical performance, particularly the L* value, shifts more, while the drive voltage changes less, than when the suffix pulses are not included. This is particularly evident when observing the white state when the white particles are less charged and negatively charged (see discussion below). Fig.34 ). While the mechanism responsible for this drift is not fully understood, it is hypothesized that some of the desired lower charge particles are complexed with particles of opposite charge. The amount of complexation is highly voltage dependent, so, for example, the L* of the white state decreases as more white particles are complexed with red or black particles. Drift can be problematic in situations where the drive voltage for the lower charge particles must be increased due to changes in the surrounding operating environment. For example, in cooler conditions, it may be necessary to increase the drive voltage (V' and V") for the lower charge pulses. However, when dithering is used to achieve intermediate colors, which may be a combination of, for example, white at one pixel and red at an adjacent pixel, drift in the optical state may result in unexpected colors.

[0166] It has been found that the variability of the measured electro-optical state can be improved by adding a "back-push" pulse between the addressing push-pull pulse and the suffix pulse. It is speculated, but not yet experimentally proven, that such a sharp pulse helps to break up the complex so that the suffix pulse can bring clean, lower-charged particles to the viewing surface. These pulses are called back-push because they have a similar shape to the initial push-pull drive pulse but with opposite polarity. Such back-push pulses (e.g., for the red state) are used in Fig.32 , located between the last of the addressing push-pull waveform and the beginning of the suffix voltage. Width t30 is usually similar to t7, but it can be longer or shorter. The height of the pulse is the highest drive voltage of the same polarity as the pull pulse, i.e. Fig.32 The waiting times t29 and t31 between the last address pulse, the back-push pulse, and the suffix pulse are somewhat arbitrary and can be adjusted to (for example) coordinate the suffix pulse with other pulses on nearby pixels.

[0167] Fig.33 The corresponding reverse push pulse (width t33, drive voltage VH2) for another low-charge particle (e.g., for the white state) is shown in FIG. Again, width t33 is generally similar to t9, but it can be longer or shorter. The height of the pulse is the highest drive voltage of the same polarity as the pull pulse, i.e. Fig.33 The waiting times t32 and t34 between the last address pulse, the reverse push pulse, and the suffix pulse are somewhat arbitrary.

[0168] Example

[0169] Above about Figures 2A-2F A four-particle electrophoretic medium of the type described was prepared and placed in a microcell such as described in U.S. Patent No. 6,930,818. The top electrode was a light-transmissive film of PET coated with ITO and the bottom electrode was a simple carbon electrode. The resulting display was attached to a variable voltage driver. Using Fig.29 and Fig.33 The waveforms were measured using an electro-optical measurement bench including a spectrophotometer to evaluate the changes in L* and b*. See D. Hertel, “Optical measurement standards for reflective e-paper to predict colors displayed in ambient illumination environments,” Color Research & Application, 43, 6, (907-921), (2018). All measurements were performed at room temperature.

[0170] Fig.34The measured values ​​of L* and b* of the white state test pattern on the display are shown as the V" ranges from -4V to -13V. Fig.34 As shown, Fig.29 The waveform of the original WF-dark line results in significant changes in L* and b* values ​​over the “typical” V” voltage range (shown as the dashed box). In particular, the difference between 64L* and 67L* is obvious even to an untrained observer. It is worth noting that the preferred white state has a b* value of approximately 0.5, Fig.29 The waveform is far from the desired b* result at -9.5V.

[0171] Instead, by including a reverse push pulse, e.g. Fig.33 As shown (Improved WF - grey line), the variations in L* and b* are clearly flattened out over the typical operating range (dashed box). In particular, the b* value is around 0.5 throughout the range, while L* is 66-67, which is less noticeable to the observer. Therefore, Fig.33 The improved waveform improves optical state consistency over the typical voltage range for lower voltage pulses.

[0172] Although the present invention has been described with reference to its specific embodiments, it will be appreciated by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt specific circumstances, materials, compositions, processes, process steps or steps to the purpose and scope of the present invention. All of these modifications are intended to fall within the scope of the appended claims.

Claims

1. A driving method for driving a pixel of an electrophoretic display, the electrophoretic display comprising a first surface on an observation side, a second surface on a non-observation side, and an electrophoretic fluid disposed between a first light-transmitting electrode and a second electrode, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles all dispersed in a solvent, wherein (a) The four types of pigment particles have different optical properties; (b) the first type of particles and the third type of particles are positively charged, wherein the first type of particles have a greater positive charge than the third type of particles; and (c) the second type of particles and the fourth type of particles are negatively charged, wherein the second type of particles have a greater negative charge than the fourth type of particles, The method comprises the following steps: (i) applying a first drive voltage to a pixel of the electrophoretic display at a first amplitude for a first time period to drive the pixel to a color state of the first type of particles or the second type of particles at the viewing side; (ii) applying a second drive voltage to a pixel of the electrophoretic display for a second time period, wherein the second drive voltage has a polarity opposite to that of the first drive voltage and a second amplitude less than the first amplitude to drive the pixel at the viewing side from the color state of the first type of particles to the color state of the fourth type of particles or from the color state of the second type of particles to the color state of the third type of particles, and repeating steps (i)-(ii); (iii) not applying a driving voltage to the pixel during a third time period; (iv) applying the second drive voltage to a pixel of the electrophoretic display for a fourth time period to drive the pixel at the viewing side from the color state of the first type of particles to the color state of the fourth type of particles or from the color state of the second type of particles to the color state of the third type of particles, and repeating steps (iii)-(iv), wherein no drive voltage having the same polarity as the first drive voltage is applied between steps (iii) and (iv); The method further comprises applying a third driving voltage to the pixels of the electrophoretic display during a fifth time period between steps (ii) and (iii), wherein the third driving voltage has the same polarity as the second driving voltage and the same amplitude as the first amplitude.

2. The driving method according to claim 1, wherein: The second period of time in step (ii) is longer than the first period of time in step (i).

3. The driving method according to claim 1, wherein: Steps (i) and (ii) were repeated at least 8 times.

4. The driving method according to claim 1, wherein: Steps (iii) and (iv) were repeated at least 8 times.

5. The driving method according to claim 1, wherein: The magnitude of the second driving voltage is less than 50% of the magnitude of the first driving voltage.

6. The driving method according to claim 1, wherein: The positive charge amount of the third particles is less than 50% of the positive charge amount of the first particles.

7. The driving method according to claim 1, wherein: The negative charge amount of the fourth particles is less than 75% of the negative charge amount of the second particles. 8 . The driving method according to claim 1 , further comprising applying a voltage having a vibration waveform to the pixel before step (i).

9. The driving method according to claim 1, wherein: The fourth time period in step (iv) is shorter than the second time period in step (ii).

10. A driving method for driving a pixel of an electrophoretic display, the electrophoretic display comprising a first surface on an observation side, a second surface on a non-observation side, and an electrophoretic fluid disposed between a first light-transmitting electrode and a second electrode, the electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles all dispersed in a solvent, wherein (a) The four types of pigment particles have different optical properties; (b) the first type of particles and the third type of particles are positively charged, wherein the first type of particles have a greater positive charge than the third type of particles; and (c) the second type of particles and the fourth type of particles are negatively charged, wherein the second type of particles have a greater negative charge than the fourth type of particles, The method comprises the following steps: (i) applying a first drive voltage to a pixel of the electrophoretic display at a first amplitude for a first time period to drive the pixel to a color state of the first type of particles or the second type of particles at the viewing side; (ii) applying a second drive voltage to a pixel of the electrophoretic display during a second time period, wherein the second drive voltage has a polarity opposite to that of the first drive voltage and a second amplitude less than the first amplitude to drive the pixel at the viewing side from a color state of the first type of particles to a color state of the fourth type of particles or from a color state of the second type of particles to a color state of the third type of particles; (iii) not applying a driving voltage to the pixel during a third time period, and repeating steps (i) to (iii); (iv) not applying a driving voltage to the pixel during a fourth time period; (v) applying the second drive voltage to a pixel of the electrophoretic display during a fifth time period to drive the pixel at the viewing side from the color state of the first type of particles to the color state of the fourth type of particles or from the color state of the second type of particles to the color state of the third type of particles, and repeating steps (iv)-(v), wherein no drive voltage having the same polarity as the first drive voltage is applied between steps (iv) and (v); The method further comprises applying a third driving voltage to the pixels of the electrophoretic display during a sixth time period between steps (iii) and (iv), wherein the third driving voltage has the same polarity as the second driving voltage and the same amplitude as the first amplitude.

11. The driving method according to claim 10, wherein: The second period of time in step (ii) is longer than the first period of time in step (i).

12. The driving method according to claim 10, wherein: Steps (i)-(iii) were repeated at least 8 times.

13. The driving method according to claim 10, wherein: Steps (iv) and (v) were repeated at least 8 times.

14. The driving method according to claim 10, wherein: The magnitude of the second driving voltage is less than 50% of the magnitude of the first driving voltage.

15. The driving method according to claim 10, wherein: The positive charge amount of the third particles is less than 50% of the positive charge amount of the first particles.

16. The driving method according to claim 10, wherein: The negative charge amount of the fourth particles is less than 75% of the negative charge amount of the second particles.

17. The driving method according to claim 10, further comprising applying a voltage having a vibration waveform to the pixel before step (i).

18. The driving method according to claim 10, wherein: The fifth time period in step (v) is shorter than the second time period in step (ii).

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