HK1218962B - Color display device - Google Patents

Description

Color display devices

Technical Field

The present invention relates to a color display device capable of displaying high-quality color states, and an electrophoretic fluid for use in such an electrophoretic display.

Background Art

To achieve color display, color filters are often used. The most common approach is to add color filters on top of the black/white subpixels of a pixelated display to display red, green, and blue. When red is desired, the green and blue subpixels are switched to a black state so that the only color displayed is red. When a black state is desired, all three subpixels are switched to a black state. When a white state is desired, the three subpixels are switched to red, green, and blue, respectively, and as a result, the observer sees a white state.

The biggest drawback of this technique is that, because each subpixel has a reflectivity of about one-third (1/3) the desired white state, the white state is quite dim. To compensate for this, a fourth subpixel can be added that is capable of displaying only the black state and the white state, so that the white level is doubled at the expense of the red, green, or blue level (where each subpixel is only one-quarter (1/4) the area of the pixel). Brighter colors can be achieved by increasing the light from the white pixel, but this is achieved at the expense of the color gamut, resulting in colors that are very light and unsaturated. A similar result can be achieved by reducing the color saturation of three subpixels. Even using these approaches, the white level is typically substantially less than half that of a black and white display, making it an unacceptable choice for display devices such as e-readers or displays that require well-readable black and white brightness and contrast.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 depicts an electrophoretic display device of the present invention.

FIG2 illustrates an example of the present invention.

FIG. 3 shows two options in which the display cells are aligned or not aligned with the pixel electrodes, respectively.

Summary of the Invention

The present invention not only provides a practical solution for a color display device that can display highly saturated color states, but also eliminates the need for color filters.

One aspect of the present invention relates to a display layer comprising an electrophoretic medium and having a first surface and a second surface on opposite sides thereof, the electrophoretic medium comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles that are additive particles, all dispersed in a solvent or a solvent mixture, the first, second, and third types of particles having first, second, and third optical properties, respectively, that are different from one another, the first type of particles having an electric charge of one polarity, and the second, third, and fourth types of particles having an electric charge of an opposite polarity, and the second type of particles having an electric field threshold such that:

(a) application of an electric field greater than the electric field threshold and having the same polarity as the second type of particles will cause the second optical characteristic to be displayed at the first surface;

(b) application of an electric field greater than the electric field threshold and having the same polarity as the first type of particles will cause the first optical characteristic to be displayed at the first surface; and

(c) Once the first optical characteristic is displayed at the first surface, application of an electric field below the electric field threshold and having the same polarity as the third type of particles will cause the third optical characteristic to be displayed at the first surface.

In one embodiment, the first type of particles and the second type of particles have a white color and a black color, respectively. In one embodiment, the third type of particles are non-white and non-black. In one embodiment, the third type of particles have a color selected from the group consisting of red, green and blue, magenta, yellow and cyan. In one embodiment, the third type of particles are larger than the first or second type of particles. In one embodiment, the size of the third type of particles is about 2 times to about 50 times the size of the first or second type of particles. In one embodiment, the fourth type of particles are white. In one embodiment, the optical property is a color state.

In one embodiment, the electrophoretic medium is filled in the display cell and is sandwiched between layers of a common electrode and a pixel electrode. In one embodiment, the display cell is a microcup. In one embodiment, the display cell is a microcapsule. In one embodiment, the display cell is aligned with the pixel electrode. In one embodiment, the display cell is not aligned with the pixel electrode. In one embodiment, the third type of particles has the same color in all display cells. In one embodiment, the third type of particles has different colors in the display cells.

In one embodiment, the pixels are driven by an electric field between a common electrode and a corresponding pixel electrode.

Another aspect of the present invention relates to a method for driving a display layer, the display layer comprising an electrophoretic medium and having a first surface and a second surface on opposite sides thereof, the electrophoretic medium comprising a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles which are additive particles, all of which are dispersed in a solvent or a solvent mixture, the first, second, and third types of particles having first, second, and third optical properties, respectively, different from each other, the first type of particles having an electric charge of one polarity, the second, third, and fourth types of particles having an electric charge of opposite polarities, and the second type of particles having an electric field threshold, the method comprising:

(a) applying an electric field greater than the electric field threshold and having the same polarity as the second type of particles so that the second optical characteristic is displayed at the first surface;

(b) applying an electric field greater than the electric field threshold and having the same polarity as the first type of particles so that the first optical characteristic is displayed at the first surface; and

(c) once the first optical characteristic is displayed at the first surface, applying an electric field below the electric field threshold and having the same polarity as the third type of particles so that the third optical characteristic is displayed at the first surface.

In one embodiment, in step (c), the electric field is applied for no longer than 30 seconds. In one embodiment, in step (c), the electric field is applied for no longer than 15 seconds.

Another aspect of the present invention relates to a method for driving a display layer, wherein the display layer includes an electrophoretic medium and has a first surface and a second surface on opposite sides thereof, the electrophoretic medium including a first type of particles, a second type of particles, a third type of particles and a fourth type of particles which are additive particles, all of which are dispersed in a solvent or a solvent mixture, the first, second and third types of particles having first, second and third optical properties different from each other, respectively, the first type of particles having an electric charge of one polarity, and the second, third and fourth types of particles having an electric charge of opposite polarity, and the second type of particles having an electric field threshold, the method including driving the pixel from the color state of the first type of particles to the color state of the third type of particles by applying an electric field weaker than the electric field threshold of the second type of particles.

In one embodiment, when the color of the third type of particles is seen at a viewing side, the first and second types of particles are concentrated at a side opposite the viewing side, resulting in an intermediate color between the colors of the first and second types of particles.

DETAILED DESCRIPTION

The electrophoretic fluid of the present invention comprises four types of particles dispersed in a dielectric solvent or solvent mixture. For ease of description, these four types of particles may be referred to as first type particles, second type particles, third type particles, and fourth type particles. The fourth type of particles are additive particles. The term "electrophoretic fluid" may also be referred to as "electrophoretic medium."

As shown in the example of FIG1 , the first type of particles are white particles (11), the second type of particles are black particles (12), the third type of particles are colored particles (13), and the fourth type of particles are additive particles (14). The colored particles (13) are particles that are neither white nor black.

It is understood that the scope of the present invention broadly encompasses fluids including particles of any color, as long as, among the four types of particles, three types (ie, the first type of particles, the second type of particles, and the third type of particles) have visually distinguishable colors.

As for white particles, they may be formed _of an inorganic pigment such as _TiO2 , _ZrO2 , ZnO, _Al2O3 , _Sb2O3 , _BaSO4 , _PbSO4 , or _the like.

As for the black particles, they may be formed from CI Pigment Black 26 or 28 or the like (eg, iron manganese black or copper chrome black) or carbon black.

The third type of particles may have a color such as red, green, blue, magenta, cyan, or yellow. Pigments for this type of particles may include, but are not limited to, CI Pigments PR 254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY138, PY150, PY155, and PY20. These are commonly used organic pigments described in the Color Index Handbooks "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, BASF Irgazine Red L 3630, Cinquasia Red L4100HD, and Irgazin Red L 3660HD; Sun Chemical Phthalocyanine Blue, Phthalocyanine Green, Aniline Yellow, or Diarylide AAOT Yellow.

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

The fourth type of particles (ie, additive particles) have color blocking and/or color enhancing properties, and therefore they may also be referred to as "color enhancing" particles. These additive particles may be of any color, and they serve only to enhance the color of other particles.

These additive particles are generally white. These white additive particles can be formed by inorganic pigments (such as TiO ₂ , ZrO ₂ , ZnO, Al ₂ O ₃ , Sb ₂ O ₃ , BaSO ₄ , PbSO ₄ or the like). These pigments are suitable because they have a high refractive index and a light scattering effect. In one embodiment of the present invention, after surface treatment, these additive particles will have a charge polarity and mobility similar or identical to the third type of particles (i.e., colored particles). Therefore, these additive particles will move together with the colored particles under an electric field, or follow the colored particles very closely. As a result, these additive particles can help block the color of the first type of particles and the second type of particles from being seen from the viewing side. This improves the hiding power of the third type of particles (i.e., colored particles) and also enhances the brightness of the color state displayed by the colored particles.

These additive particles are particularly useful when the pigmented particles are formed from organic pigments that have relatively poor hiding power and tinting strength. An example of this would be yellow pigment PY154, which has weak hiding power and when this yellow pigment is used in the pigmented particles, the white additive particles can provide better hiding power and higher brightness to the yellow pigment.

The display layer using the display liquid of the present invention has two surfaces, a first surface (17) located on the viewing side and a second surface (18) located on the opposite side of the first surface (17). The display liquid is sandwiched between the two surfaces. On one side of the first surface (17), there is a common electrode (15) extending over the entire top of the display layer. The common electrode (15) is a transparent electrode layer (e.g., ITO). On one side of the second surface (18), there is an electrode layer (16), which includes a plurality of pixel electrodes (16a).

Pixel electrodes are described in U.S. Patent No. 7,046,228, the contents of which are incorporated herein by reference in their entirety. Note that while active matrix drive with a thin film transistor (TFT) backplane is mentioned for the pixel electrode layer, the scope of the present invention encompasses other types of electrode addressing, as long as these electrodes are used for the desired function.

Each space between two dotted vertical lines in FIG1 represents a pixel (10). As shown in the figure, each pixel has a corresponding pixel electrode. An electric field is generated for the pixel by the potential difference between the voltage applied to the common electrode and the voltage applied to the corresponding pixel electrode.

The percentages of the four types of particles in the fluid can vary. As an example, in a black/white/colored/additive particle fluid, the black particles can occupy approximately 0.1%-10% of the electrophoretic fluid volume, preferably 0.5%-5%; the white particles can occupy approximately 1%-50% of the fluid volume, preferably 5%-15%; and the colored particles can occupy approximately 2%-20% of the fluid volume, preferably 4%-10%. The percentage of additive particles (14) can be 0.1%-5% of the electrophoretic fluid volume, preferably 0.5%-3%.

The solvent in which the three types of particles are dispersed is clear and colorless. 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, for high particle mobility. Examples of suitable dielectric solvents include hydrocarbons such as isopar, decaline, 5-ethylidene-2-norbornene, fatty oils, paraffin oils, silicone fluids, aromatic hydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzene or alkylnaphthalene, halogenated hydrocarbon solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluorobenzene, dichlorononane or pentachlorobenzene, and perfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company, St. Paul, MN, low molecular weight halogenated polymers such as TCI Americas, Inc., Portland, Oregon. Examples of the present invention include poly(perfluoropropylene oxide) from DuPont de Nemours and Company of America, poly(chlorotrifluoroethylene), such as polychlorotrifluoroethylene (Halocarbon Oils) from Halocarbon Products of River Edge, New Jersey, perfluoropolyethers, such as Galden from Ausimont or Krytox Oils, and Greases K-Fluid Series from E.I. DuPont de Nemours and Company of Delaware, and polydimethylsiloxane-based silicone oils (DC-200) from Dow Corning.

The first and second types of particles carry opposite charge polarities. The third and fourth types of particles have the same charge polarity as one of the first and second types of particles. In a fluid of black/white/coloring/additive particles, if the black particles are positively charged and the white particles are negatively charged, then both the coloring particles and the additive particles can be either positively or negatively charged.

Additionally, the charge carried by the colored and additive particles can be weaker than the charge carried by the black and white particles. The term "weaker charge" is intended to refer to particles having a charge less than about 50%, preferably about 5% to about 30%, of the charge of the more strongly charged particles.

The particle of these four types also can have the size of variation.In one embodiment, a type or two types in the particle of these four types can be bigger than other types.It is noted that, among these four types of particle, colored particles preferably have larger size.For example, black and white particles are relatively small, and their size (tested by dynamic light scattering) can be from the scope of about 50nm to about 800nm, and more preferably from the scope of about 200nm to about 700nm, and the colored particles with weaker electric charge are preferably about 2 to about 50 times of the mean size of black particles or white particles, and more preferably about 2 to about 10 times of the mean size of black particles or white particles.The particle of the fourth type (that is, additive particles) can have any size.

In the present invention, at least one type of particle can exhibit an electric field threshold. In one embodiment, one type of higher charged particle has an electric field threshold.

In the context of the present invention, the term "electric field threshold" is defined as the maximum electric field that can be applied to a set of particles for a period of time (typically not longer than 30 seconds, preferably not longer than 15 seconds) without causing these particles to appear at the viewing side of a pixel when the pixel is driven in a color state different from the color state of the set of particles. In this application, the term "viewing side" refers to the first surface of the display layer where the image is seen by a viewer.

The electric field threshold is an intrinsic characteristic or an additive-induced property of charged particles.

In the former case, the electric field threshold is generated by relying on specific attractive forces between oppositely charged particles or between particles and a specific substrate surface.

In the case of an additional induced electric field threshold, a threshold agent that induces or enhances the threshold characteristics of the electrophoretic fluid can be added. The threshold agent can be any material that is soluble or dispersible in the solvent or solvent mixture of the electrophoretic fluid and carries or induces a charge opposite to the charge of the charged particles. The threshold agent can be sensitive or insensitive to changes in the applied voltage. The term "threshold agent" can broadly include dyes or pigments, electrolytes or polyelectrolytes, polymers, oligomers, surfactants, charge control agents and the like.

Additional information regarding threshold agents can be found in US Pat. No. 8,115,729, the contents of which are incorporated herein by reference in their entirety.

The following is an example illustrating the present invention.

Example

This example is shown in Figure 2. The black particles (22) are assumed to have an electric field threshold. Therefore, if the applied electric field is lower than the electric field threshold, the black particles (22) will not move to the viewing side.

White particles (21) are negatively charged, while black particles (22) are positively charged, and both types of particles are smaller than colored particles (23). For illustration purposes, it is assumed that colored particles (23) have a yellow color and additive particles (24) have a white color.

The yellow particles (23) and the white additive particles (24) carry the same charge polarity as the black particles having the electric field threshold, but they carry a weaker charge than the black particles. As a result, when the applied electric field is greater than the electric field threshold of the black particles, the black particles move faster than the yellow particles (23) and the white additive particles (24) due to the stronger charge carried by the black particles.

In FIG2a , the applied voltage potential difference is +15 V. In this case, the electric field generated by the applied drive voltage is greater than the electric field threshold, and thus it causes the white particles (21) to move near or at the pixel electrode (26) and causes the black particles (22), yellow particles (23), and white additive particles (24) to move near or at the common electrode (25). As a result, black is seen on the viewing side. The yellow particles (23) and white additive particles (24) move toward the common electrode (25); however, because they carry a weaker charge, they move slower than the black particles.

In FIG2b , when a voltage potential difference of -15V is applied, the generated electric field has opposite polarity and is greater than the electric field threshold. As a result, the white particles (21) move to the vicinity of or at the common electrode (25) and the black particles (22), yellow particles (23) and white additive particles (24) move to the vicinity of or at the pixel electrode (26). Therefore, white is seen on the viewing side.

The yellow particles (23) and white additive particles (24) move towards the pixel electrode because they are also positively charged. However, because they carry a weaker charge, they move slower than the black particles.

In FIG2c, the applied voltage potential difference is changed to +5V. In this case, the generated electric field is less than the electric field threshold, and therefore, the negatively charged white particles (21) in FIG2(b) are moved toward the pixel electrode (26). The black particles (22) move a little due to their electric field threshold. Due to the fact that the yellow particles (23) and the white additive particles (24) do not have a significant electric field threshold, they move to the vicinity of or at the common electrode (25), and as a result, the color of the yellow particles is seen on the viewing side, while the white additive particles (24) block the black particles and the white particles from being seen on the viewing side, thereby enhancing the yellow state.

As also shown in FIG. 2( c ), when the yellow particles and the white additive particles are located on the common electrode side (i.e., the viewing side), the black particles and the white particles are mixed on the non-viewing side, thereby forming an intermediate color state (i.e., gray) between the white particles and the black particles.

The electrophoretic fluid in an electrophoretic display device is filled within display cells. These display cells can be microcups, as described in U.S. Patent No. 6,930,818, the contents of which are incorporated herein by reference in their entirety. These display cells can also be other types of microcontainers, such as microcapsules, microchannels, or equivalents, regardless of their shape or size. All of these fall within the scope of the present application.

In one embodiment of the present invention, the display device utilizing the present electrophoretic fluid is a high-light display device, and in this embodiment, the colored particles have the same color in all display cells. In this case, as shown in FIG3 , the display cell (31) may be aligned with the pixel electrode (32) (see FIG3 a ) or may not be aligned with the pixel electrode (see FIG3 b ).

In another embodiment, a display device using the present electrophoretic fluid may be a multi-color display device. In this embodiment, the colored particles have different colors in the display cells. In this embodiment, the display cells are aligned with the pixel electrodes.

Although the present invention has been described with reference to specific embodiments of the present invention, it will be understood by those skilled in the art that various changes may be made and substituted with equivalents without departing from the scope of the present invention. In addition, many modifications may be made to adapt specific circumstances, materials, components, processes, process steps or multiple steps to the purpose, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.

Claims (9)

1. A display layer comprising an electrophoretic medium and having a first surface on a viewing side and a second surface on the opposite side of the first surface; the electrophoretic medium, sandwiched between a common electrode and a pixel electrode, comprises particles of a first type, particles of a second type, particles of a third type, and particles of a fourth type as additive particles, all particles being dispersed in a solvent or solvent mixture; the first, second, and third types of particles each having first, second, and third optical properties different from each other; the first type of particles having a polar charge, and the second, third, and fourth types of particles having opposite polar charges, and the second type of particles having an electric field threshold; thereby such that: (a) Applying a voltage potential difference between the common electrode and a pixel electrode to generate an electric field stronger than the electric field threshold and the voltage potential difference having the same polarity as the second type of particle will cause the pixel corresponding to the pixel electrode to display the second optical property at the first surface. (b) Applying a voltage potential difference between the common electrode and a pixel electrode to generate an electric field stronger than the electric field threshold, and the voltage potential difference having the same polarity as the first type of particle, will cause the pixel corresponding to the pixel electrode to display the first optical property at the first surface; and (c) Once the first optical characteristic is displayed on the first surface, the application of a voltage potential difference between the common electrode and a pixel electrode to generate an electric field weaker than the electric field threshold and the voltage potential difference having the same polarity as the third type of particle will cause the pixel corresponding to the pixel electrode to display the third optical characteristic on the first surface. 2. The display layer according to claim 1, wherein the first type of particles and the second type of particles are respectively white and black. 3. The display layer according to claim 1, wherein the third type of particles is neither white nor black. 4. The display layer of claim 3, wherein the third type of particles has a color selected from the group consisting of red, green and blue, magenta, yellow and cyan. 5. The display layer according to claim 1, wherein the third type of particles is larger than the first or second type of particles. 6. The display layer according to claim 5, wherein the size of the third type of particles is 2 to 50 times the size of the first or second type of particles. 7. The display layer according to claim 1, wherein the fourth type of particles is white. 8. The display layer according to claim 1, wherein the optical property is a color state. 9. A driving method for driving a display layer, the display layer comprising an electrophoretic medium and having a first surface on a viewing side and a second surface on the opposite side of the first surface, the electrophoretic medium sandwiched between a common electrode and a pixel electrode comprising first type particles, second type particles, third type particles, and a fourth type of particles as additive particles, all particles being dispersed in a solvent or solvent mixture; the first, second, and third type particles each having first, second, and third optical properties different from each other; the first type particles having a polar charge, and the second, third, and fourth type particles having opposite polar charges; and the second type particles having an electric field threshold; the method comprising: (a) A voltage potential difference is applied between the common electrode and a pixel electrode to generate an electric field stronger than the electric field threshold and the voltage potential difference has the same polarity as the particles of the second type, such that the pixel corresponding to the pixel electrode displays the second optical property at the first surface. (b) Applying a voltage potential difference between the common electrode and a pixel electrode to generate an electric field stronger than the electric field threshold, wherein the voltage potential difference has the same polarity as the particles of the first type, such that the pixel corresponding to the pixel electrode displays the first optical property at the first surface; and (c) Once the first optical property is displayed on the first surface, a voltage potential difference is applied between the common electrode and a pixel electrode to generate an electric field weaker than the electric field threshold and the voltage potential difference has the same polarity as the third type of particle so that the pixel corresponding to the pixel electrode displays the third optical property on the first surface.