Driving method of color display device

Abstract

The present invention relates to a driving method for driving a color display device capable of displaying a high-quality color state. The display device uses an electrophoretic fluid including three types of pigment particles having different optical characteristics.

Description

[0001] This application is a divisional application of the Chinese invention patent application with application number 201680018641.7, application date April 1, 2016, and name “Driving method of color display device”. Technical Field

[0002] The present invention relates to a driving method of a color display device for displaying a high-quality color state. Background Art

[0003] In order to achieve a color display, color filters are usually used. The most common method is to attach color filters on top of the black / white sub-pixels of the pixelated display to display red, green and blue. When red is required, the green and blue sub-pixels are changed to a black state so that only red is displayed. When blue is required, the green and red sub-pixels are changed to a black state so that only blue is displayed. When green is required, the red and blue sub-pixels are changed to a black state so that only green is displayed. When a black state is required, all three sub-pixels are changed to a black state. When a white state is required, the three sub-pixels are changed to red, green and blue respectively, so that the viewer sees a white state.

[0004] The biggest drawback of this technique is that, since each sub-pixel has a reflectivity of approximately one-third (1 / 3) of the desired white state, the white state is quite dark. To compensate for this, a fourth sub-pixel that can only display black and white states can be added so that the white level is twice the red, green, or blue level cost (where each sub-pixel only occupies one-quarter of the pixel area). Brighter colors can be achieved by increasing the light from the white pixel, but this is achieved at the expense of the color gamut, making the colors very light and unsaturated. Similar results can be achieved by reducing the color saturation of the three sub-pixels. Even using these methods, the white level is typically substantially less than half of the white level of a black and white display, making it an unacceptable choice for display devices such as e-readers or displays that require good readable black and white brightness and contrast. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figure 1 An electrophoretic display fluid suitable for use in the present invention is shown.

[0006] Figure 2 is a simplified diagram showing an example of a driving scheme.

[0007] Figure 3 The driving method of the present invention is shown.

[0008] Figure 4 An alternative drive method of the present invention is shown. DETAILED DESCRIPTION

[0009] The present invention relates to a driving method for a color display device.

[0010] Figure 1 An apparatus using an electrophoretic fluid is shown. The fluid contains three types of pigment particles dispersed in a dielectric solvent or solvent mixture. For ease of illustration, the three types of pigment particles may be referred to as white particles (11), black particles (12), and colored particles (13). The colored particles are non-white and non-black.

[0011] However, it should be understood that the scope of the present invention broadly covers pigment particles of any color, as long as the three types of pigment particles are visually distinguishable. Therefore, the three types of pigment particles may also be referred to as first type of pigment particles, second type of pigment particles and third type of pigment particles.

[0012] As for the white particles (11), they may be formed of inorganic pigments such as TiO2, ZrO2, ZnO, Al2O3, Sb2O3, BaSO4, PbSO4, etc.

[0013] As for the black particles (12), they may be formed of CI Pigment Black 26 or 28 or the like (eg, manganese ferrite black spinel or copper chrome black spinel) or carbon black.

[0014] The third type of particles can have a color such as red, green, blue, magenta, cyan or yellow. Pigments for this type of particles can include, but are not limited to, CI pigments PR254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY138, PY150, PY155 or PY20. These are commonly used organic pigments described in the color index manuals "New Pigment Application Technology" (CMC Publishing Co., Ltd., 1986) and "Printing Ink Technology" (CMC Publishing Co., Ltd., 1984). Specific examples include Clariant 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, Hostaperm Green GNX, BASFIrgazine red L 3630, Cinquasia Red L 4100HD and Irgazin Red L 3660HD; Sun Chemical Phthalocyanine Blue, Phthalocyanine Green, Aniline Yellow or Aniline AAOT Yellow.

[0015] In addition to color, the first, second and third types of particles may also have other different optical properties, such as light transmission, reflectivity, luminescence, or in the case of displays for machine reading, false color in the sense of changes in reflectivity for electromagnetic wavelengths outside the visible range.

[0016] The solvent in which the three types of pigment particles are dispersed is transparent and colorless. For high particle mobility, it preferably has a dielectric constant in the range of about 2 to about 30, more preferably about 2 to about 15. Examples of suitable dielectric solvents include hydrocarbons such as Isopar, decalin, 5-ethylidene-2-norbornene, fatty oils, paraffin oils, silicone fluids, aromatic hydrocarbons such as toluene, xylene, phenylxylene ethane, dodecylbenzene or alkylnaphthalene; halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorotrifluorotoluene, 3,4,5-trichlorotrifluorotoluene, chloropentafluorobenzene, dichlorononane or pentachlorobenzene; and perfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company of St. Paul, Minnesota, low molecular weight halogens containing polymers such as poly(perfluoropropylene oxide) from TCI Americ of Portland, Oregon, USA, such as Halocarbon from River Edge, New Jersey. Product company's halocarbon oils, such as Galden from Ausimont or Krytox oils and greases from DuPont of Delaware, K-Fluid series of perfluoropolyalkyl ethers, polydimethylsiloxane-based silicone oils (DC-200) from Dow-corning.

[0017] The display layer using the display fluid of the present invention has two surfaces, a first surface (16) on the viewing side and a second surface (17) on the opposite side of the first surface (16). Therefore, the second surface is on the non-viewing side. The term "viewing side" refers to the side where the image is viewed.

[0018] The display fluid is sandwiched between the two surfaces. On one side of the first surface (16) there is a common electrode (14), which is a transparent electrode layer (e.g., ITO) extending over the entire top of the display layer. On one side of the second surface (17) there is an electrode layer (15) including a plurality of pixel electrodes (15a).

[0019] The display fluid is filled in the display cell. The display cell may or may not be aligned with the pixel electrode. The term "display cell" refers to a microcontainer filled with an electrophoretic fluid. Examples of a "display cell" may include a cup-shaped microcell as described in U.S. Patent No. 6,930,818 and a microcapsule as described in U.S. Patent No. 5,930,026. The microcontainer may have any shape or size, all of which are within the scope of the present application.

[0020] The area corresponding to the pixel electrode may be referred to as a pixel (or sub-pixel). The area corresponding to the pixel electrode is driven by applying a potential difference (or referred to as a driving voltage or an electric field) between the common electrode and the pixel electrode.

[0021] The pixel electrodes may belong to an active matrix drive system with a thin film transistor (TFT) backplane, or other type of electrode addressing, as long as the electrodes are used for the required function.

[0022] The space between the two vertical dashed lines represents a pixel (or sub-pixel). For simplicity, when referring to a "pixel" in the driving method, the term also includes a "sub-pixel".

[0023] Two of the three types of pigment particles have opposite charge polarities, and the third type of pigment particles is slightly charged. The term "slightly charged" or "lower charge intensity" is intended to mean that the charge level of the particles is about 50% lower than the charge level of the more strongly charged particles, preferably about 5% to about 30%. In one embodiment, the charge intensity can be measured according to the Zeta potential. In one embodiment, the Zeta potential is determined by a Colloid Dynamics Acoustic Filter IIM (ESA EN#Attn flow cell (K: 127)) with a CSPU-100 signal processing unit. Instrument constants 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 input before testing, all of which are obtained at the test temperature (25°C). 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 control agent (Solsperse Available from Lubrizol, Berkshire Hathaway, "Solsperse" is a registered trademark), the weight ratio of charge control agent to particles is 1: 10. The mass of the diluted sample is measured and then the sample is loaded into the flow cell to measure the zeta potential.

[0024] For example, if the black particles are positively charged and the white particles are negatively charged, the colored pigment particles can be slightly charged. In other words, in this example, the charge level carried by the black and white particles is higher than the charge level carried by the colored particles.

[0025] Furthermore, the slightly charged colored particles have the same charge polarity as the charge carried by either of the other two types of more strongly charged particles.

[0026] It should be noted that among the three types of pigment particles, the slightly charged particle type may preferably have a larger size.

[0027] Furthermore, in the context of this application, a high drive voltage (V H1 or V H2 ) is defined as a driving voltage sufficient to drive a pixel from one extreme color state to another extreme color state. If the first and second types of pigment particles are highly charged particles, then the high driving voltage (V H1 or V H2 ) refers to a driving voltage sufficient to drive a pixel from a color state of a first type of pigment particle to a color state of a second type of pigment particle, and vice versa. For example, a high driving voltage V H1 refers to a drive voltage that is sufficient to drive a pixel from a color state of a first type of pigment particle to a color state of a second type of pigment particle when applied for an appropriate period of time, and V H2 Refers to a drive voltage that is sufficient to drive a pixel from the color state of the second type of pigment particles to the color state of the first type of pigment particles when applied for an appropriate period of time.

[0028] The following is an example of a drive scheme illustrating how different color states may be displayed by an electrophoretic fluid as described above.

[0029] Example

[0030] This example is in Figure 2 The white pigment particles (21) are negatively charged, while the black pigment particles (22) are positively charged, and both types of pigment particles are smaller than the colored particles (23).

[0031] The colored particles (23) have the same charge polarity as the black particles, but are slightly charged. As a result, at certain drive voltages, the black particles move faster than the colored particles (23).

[0032] exist Figure 2 In a, the driving voltage applied is +15V (i.e. V H1 ). In this case, the white particles (21) move to the pixel electrode (25) or near the pixel electrode (25), and the black particles (22) and the colored particles (23) move to the common electrode (24) or near the common electrode (24). As a result, a black color is seen on the viewing side. The colored particles (23) move toward the common electrode (24) on the viewing side; however, due to their lower charge strength and larger size, they move slower than the black particles.

[0033] exist Figure 2 b, when -15V (i.e. V H2 ), the white particles (21) move to the common electrode (24) or near the common electrode (24) on the viewing side, and the black particles and the colored particles move to the pixel electrode (25) or near the pixel electrode (25). As a result, white color is seen on the viewing side.

[0034] It should be noted that V H1 and V H2 have opposite polarity and either have the same magnitude or different magnitudes. Figure 2 In the example shown, V H1 is positive (same polarity as the black particles), V H2 is negative (same polarity as the white particles).

[0035] exist Figure 2 In Fig. 3, when a low voltage (e.g., +5V) sufficient to drive the colored particles to the viewing side and having the same polarity as the colored particles is applied, the white particles are pushed downward and the colored particles move upward toward the common electrode (24) to reach the viewing side. The black particles cannot move to the viewing side due to the low driving voltage because when two stronger and oppositely charged particles (i.e., black particles and white particles) meet, the low driving voltage is not enough to separate the two types of pigment particles from each other.

[0036] There are two issues that may affect the quality of each of the three color states.

[0037] One of the issues is the color tint of the black and white states. If the colored particles are red, then the white state may suffer from having a red tint (i.e., a high a* value) that originates from the red particles not being well separated from the white particles. Although the white and red particles carry opposite charge polarities, a small amount of red particles displayed on the viewing side in the white state may cause a red tint that is offensive to the viewer.

[0038] The black state is also affected by the red tint. Black and red particles carry the same charge polarity, but have different levels of charge strength. The higher charged black particles are expected to move faster than the lower charged red particles to show a good black state without a red tint; but in reality, the red tint is difficult to avoid.

[0039] The second problem is ghosting, which is caused by pixels being driven from a different color state to the same color state, with the resulting color state typically exhibiting a difference in L* (i.e., ΔL*) and / or a* (i.e., Δa*) because the previous state had a different color.

[0040] In one example, two groups of pixels are driven to the black state at the same time. The first group of pixels driven from the white state to the black state may show an L* of 15, while the other group of pixels driven from the black state to the final black state may show an L* of 10. In this case, the ΔL* of the final black state is 5.

[0041] In such Figure 2 In another example of a three-color system shown, three groups of pixels are driven to a black state simultaneously. The first group of pixels driven from red to a black state may exhibit an L* of 17 and an a* value of 7 (a high a* value here also indicates color tinting). The second group of pixels driven from a black state to a final black state may exhibit an L* of 10 and an a* value of 1. The third group of pixels driven from a white state to a final black state may exhibit an L* of 15 and an a* of 3. In this case, the most severe ghosting is caused by a ΔL* of 7 and a Δa* of 6.

[0042] The inventors have now discovered an improved driving method that can provide both problems. In other words, the driving method of the present invention can not only reduce / eliminate color tinting (i.e., reduce the a* value of the black and / or white state), but also reduce / eliminate ghosting (i.e., reduce ΔL* and Δa*).

[0043] Figure 3 and Figure 4 The driving methods of the present invention are shown. Each method can also be considered as "resetting" or "pre-processing" before driving the pixel to the desired color state.

[0044] Figure 3 The waveform in the figure consists of three parts: (i) driving to white, (ii) applying a driving voltage with the same polarity as the black particles (V H1 , e.g. +15V) a shorter time period t1 which is not long enough to drive from the white state to the black state, thus resulting in a grey state, (iii) dithering.

[0045] Figure 4 The waveform in the figure consists of three parts: (i) driving to black, (ii) applying a driving voltage with the same polarity as the white particles (V H2 , e.g. -15V) a shorter time period t2 which is not long enough to drive from the black state to the white state, thus resulting in a grey state, (iii) jitter.

[0046] The length of t1 or t2 depends not only on Figure 3 or Figure 4The final color state to which the color is driven (after the reset and preprocessing waveform) depends also on the desired optical properties of the final color state (e.g., a*, ΔL*, and Δa*). Figure 3 When t1 in the waveform is 40msec, there is minimal ghosting, and the pixel is driven to the red state regardless of whether it is driven from red, black or white. Similarly, when t1 is 60msec, there is minimal ghosting, and the pixel is driven to the black state regardless of whether it is driven from red, black or white.

[0047] The symbol "msec" means milliseconds.

[0048] The jitter waveform includes a pair of opposite drive pulses repeated for multiple cycles. For example, the jitter waveform may include a +15V pulse of 20msec and a -15V pulse of 20msec, and such a pair of pulses is repeated 50 times. The total time of such a jitter waveform will be 2000msec.

[0049] Each drive pulse in the dithering waveform is applied for no more than half the drive time required to drive from a fully black state to a fully white state, or vice versa. For example, if it takes 300 msec to drive a pixel from a fully black state to a fully white state, or vice versa, then the dithering waveform may include a positive pulse and a negative pulse, each pulse being applied for no more than 150 msec. In practice, it is preferred that the pulses be shorter.

[0050] It should be noted that Figure 3 and 4 In the example above, the jitter waveform is shortened (i.e., the number of pulses is less than the actual number).

[0051] After the dithering is completed, the three types of particles are in a mixed state in the display fluid.

[0052] exist Figure 3 or Figure 4 After the "reset" or "pre-processing" is completed, the pixel is driven to the desired color state (e.g., black, red, or white). For example, a positive pulse can be applied to drive the pixel to black; a negative pulse can be applied to drive the pixel to white; or a negative pulse can be applied followed by a lower amplitude positive pulse to drive the pixel to red.

[0053] When comparing drive methods with and without the "resetting" or "preprocessing" of the present invention, the method with the "resetting" or "preprocessing" of the present invention has the additional advantage that shorter waveform times are required to achieve the same level of optical performance (including ghosting).

[0054] The driving method of the present invention can be summarized as follows:

[0055] A driving method of an electrophoretic display, the electrophoretic display comprising a first surface on a viewing side, a second surface on a non-viewing side, and an electrophoretic fluid, the electrophoretic fluid being sandwiched between a common electrode and a pixel electrode layer and comprising a first type of pigment particles, a second type of pigment particles, and a third type of pigment particles, all of the three types of pigment particles being dispersed in a solvent or a solvent mixture, wherein:

[0056] (a) The three types of pigment particles have different optical properties from each other;

[0057] (b) the first type of pigment particles and the second type of pigment particles carry opposite charge polarities; and

[0058] (c) the third type of pigment particles have the same charge polarity as the second type of pigment particles, but have a lower charge intensity,

[0059] The method comprises the following steps:

[0060] (i) driving pixels in the electrophoretic display to a color state of the first type of pigment particles or to a color state of the second type of pigment particles;

[0061] (ii) applying a first drive voltage having the same polarity as the second type of pigment particles to the pixel in the color state of the first type of pigment particles for a first time period, and the first time period is not long enough to drive the pixel to the color state of the second type of pigment particles, or

[0062] applying a second drive voltage to the pixel in the color state of the second type of pigment particles for a second time period, the drive voltage having the same polarity as the first type of pigment particles, and the second time period is not long enough to drive the pixel to the color state of the first type of pigment particles; and

[0063] (iii) Apply a dither waveform.

[0064] In one embodiment, the first type of pigment particles are negatively charged and the second type of pigment particles are positively charged.

[0065] In one embodiment, the first type of pigment particles are white and the second type of pigment particles are black.

[0066] In one embodiment, the third type of pigment particles is red.

[0067] Although the present invention has been described with reference to specific embodiments of the present invention, 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, and process steps to the purpose, spirit, and scope of the present invention. All such modifications are intended to be included within the scope of the appended claims.

Claims

1. A driving method for driving an electro-optical display having a plurality of display pixels and an electrophoretic fluid, the electrophoretic fluid comprising a first type of pigment particles, a second type of pigment particles and a third type of pigment particles, wherein the three types of pigment particles have different optical properties from each other, and wherein, The first type of pigment particles and the second type of pigment particles carry opposite charge polarities, The method comprises the following steps, in the following order: (i) driving a display pixel to a color state of pigment particles of the first type or to a color state of pigment particles of the second type; (ii) immediately upon completion of step (i), driving the pixel to a grey state between the colour state of the first type of pigment particles and the colour state of the second type of pigment particles; (iii) immediately after step (ii) is completed, applying a shaking waveform comprising at least one pair of opposite driving pulses, wherein three types of pigment particles are mixed in the electrophoretic fluid at the end of the shaking waveform; as well as (iv) applying a driving voltage having the same polarity as the first type of pigment particles to drive the pixel to the color state of the first type of pigment particles on the viewing side, or applying a driving voltage having the same polarity as the second type of pigment particles to drive the pixel to the color state of the second type of pigment particles on the viewing side, or applying a driving voltage having the same polarity as the third type of pigment particles to drive the pixel to the color state of the third type of pigment particles on the viewing side.

2. The method according to claim 1, wherein: The first type of pigment particles are negatively charged and the second type of pigment particles are positively charged.

3. The method according to claim 1, wherein: The three types of pigment particles have different colors.

4. The method according to claim 1, wherein: The first type of pigment particles are white and the second type of pigment particles are black.

5. The method according to claim 1, wherein: The third type of pigment particles are non-white and non-black.

6. The method according to claim 1, wherein: The third type of pigment particles are red.

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