HK1222713B - Full color display device - Google Patents

Description

Full-color display device

Technical Field

The present invention relates to full color display devices in which each pixel can display multiple high quality color states, and to electrophoretic fluids for use in such electrophoretic displays.

Background Art

To achieve a color display, color filters are typically used. The most common approach 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 desired, the green and blue sub-pixels are transformed into a black state so that the only color displayed is red. When green is desired, the red and blue sub-pixels are transformed into a black state so that the only color displayed is green. When blue is desired, the green and red sub-pixels are transformed into a black state so that the only color displayed is blue. When a black state is desired, all three sub-pixels are transformed into a black state. When a white state is desired, the three sub-pixels are transformed into red, green, and blue, respectively, so that the observer sees a white state.

The biggest drawback of this technique is that, since each of the sub-pixels has about one-third the reflectivity of the desired white state, the white state is relatively dark. To compensate for this, a fourth sub-pixel can be added that can only display black and white states, doubling the white level at the expense of the red, green, or blue level (where each sub-pixel is only one-quarter the area of the pixel). Brighter colors can be achieved by adding light from the white pixels, but this is achieved at the expense of the color gamut, making the colors very light and unsaturated. A similar result can be achieved by reducing the color saturation of three of the sub-pixels. Even with this approach, the white level is typically much 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 good, readable black-white brightness and contrast.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrophoretic display device according to the present invention.

2-1 to 2-4 show examples of the present invention.

FIG3 shows the vibration waveform.

4 and 5 illustrate how the yellow, magenta and cyan states may be displayed by the display device of FIG. 2 .

6-1 to 6-4 show another example of the present invention.

7 and 8 illustrate how the purple, orange, and green states may be displayed by the display device of FIG. 6 .

9A and 9B illustrate display cells aligned and misaligned with pixel electrodes, respectively.

Summary of the Invention

One aspect of the present invention relates to an electrophoretic display comprising:

(a) a plurality of pixels; and

(b) an electrophoretic fluid in which particles of a first type, particles of a second type, particles of a third type, and particles of a fourth type are dispersed in a solvent or a solvent mixture, and the particles of the first and second types carry a high level of charge and are electrically opposite, and the particles of the third and fourth types carry a low level of charge and are electrically opposite,

Each of the pixels is capable of displaying at least five different color states.

In one embodiment, the first and second types of particles are white and red, respectively. In one embodiment, the third and fourth types of particles are blue and green, respectively. In one embodiment, each of the pixels can display white, red, green, blue, and black states. In another embodiment, each of the pixels can display yellow, magenta, and cyan states.

In one embodiment, the third and fourth types of particles are blue and yellow, respectively. In one embodiment, each of the pixels can display white, red, yellow, blue, and black states. In one embodiment, each of the pixels can display green, orange, and purple states.

In one embodiment, the low level of charge is less than approximately 50% of the high level of charge. In another embodiment, the low level of charge is less than approximately 75% of the high level of charge.

In one embodiment, the electrophoretic fluid further comprises substantially uncharged neutral suspended particles. In another embodiment, the substantially uncharged neutral suspended particles are free of charge.

Another aspect of the present invention relates to a display layer comprising an electrophoretic fluid and having first and second surfaces on opposite sides of the display layer, the electrophoretic fluid comprising highly positive particles, highly negative particles, low positive particles, and low negative particles, all dispersed in a solvent or solvent mixture, the four types of particles each having optical properties that differ from one another such that:

(a) application of an electric field having the same polarity as the highly positive particles will cause the optical properties of the highly positive particles to be displayed at the first surface; or

(b) application of an electric field having the same polarity as the highly negative particles will cause the optical properties of the highly negative particles to be displayed at the first surface; or

(c) once the optical property of the highly positive particles is manifested at the first surface, application of an electric field having the same polarity as the low negative particles that is not strong enough to overcome the attractive force between the highly positive particles and the highly negative particles, but sufficient to overcome the attractive force between other oppositely charged particles, will cause the optical property of the low negative particles to be manifested at the first surface; or

(d) once the optical property of the high negative particle is manifested at the first surface, application of an electric field having the same polarity as the low positive particle, which is not strong enough to overcome the attractive force between the high positive particle and the high negative particle, but is sufficient to overcome the attractive force between other oppositely charged particles, will cause the optical property of the low positive particle to be manifested at the first surface; or

(e) Application of the vibration waveform will cause a fifth optical property to be displayed at the first surface.

In one embodiment of this aspect of the invention, the four types of particles are red, green, blue, and white. In another embodiment, the four types of particles are red, yellow, blue, and white. In another embodiment, the four types of particles are cyan, magenta, yellow, and white.

In one embodiment, none of the four types of particles are black particles, and the fifth optical property is a black state.

DETAILED DESCRIPTION

The electrophoretic fluid of the present invention includes four types of particles dispersed in a dielectric solvent or solvent mixture. For ease of illustration, the four types of pigment particles may be referred to as particles of a first type (11), a second type (12), a third type (13), and a fourth type (14), as shown in FIG1 . However, using only four types of pigment particles, a display device using the electrophoretic fluid can display at least five different color states, resulting in a full-color display.

Typically, the four types of particles are divided into two groups: a high-charge group and a low-charge group. Within the two groups of oppositely charged particles, one carries a stronger charge than the other. Therefore, the four types of pigment particles can also be referred to as high-positive particles, high-negative particles, low-positive particles, and low-negative particles.

As an example, red particles (R) and white particles (W) can be a first group of oppositely charged particles, and in this group, red particles are highly positive particles, and white particles are highly negative particles. Blue particles (B) and green particles (G) can be a second group of oppositely charged particles, and in this group, blue particles are low positive particles, and green particles are low negative particles.

In another example, red particles may be high positive particles; white particles may be high negative particles; blue particles may be low positive particles; and yellow particles may be low negative particles.

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

As for white particles, they may be formed of inorganic pigments such as TiO ₂ , ZrO ₂ , ZnO, Al ₂ O _3, Sb ₂ O _3, BaSO ₄ or PbSO ₄ , etc.

As for the black particles, if present, 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.

Particles of other colors (non-white and non-black) are independent of colors such as red, green, blue, magenta, cyan or yellow. Pigments used for colored particles can include, but are not limited to, CI pigments PR 254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY83, 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 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; and Sun Chemical's Phthalocyanine Blue, Phthalocyanine Green, Aniline Yellow, or Aniline AAOT Yellow.

Non-black and non-white colored particles may also be inorganic pigments such as red, green, blue and yellow pigments. Examples may include, but are not limited to, CI Pigment Blue 28, CI Pigment Green 50 and CI Pigment Yellow 227.

In addition to color, the four types of particles may have other different optical properties, such as light transmission, reflection, luminescence, or, in the case of displays intended for machine reading, false color in the sense of variations in reflectivity at electromagnetic wavelengths outside the visible range.

As also shown in FIG1 , the display layer of the present invention using a display fluid has two surfaces, a first surface (17) on the viewing side and a second surface (18) on the opposite side of the first surface (17). The display fluid is sandwiched between the two surfaces. On the side of the first surface (17), there is a common electrode (17) distributed across the entire top of the display layer, which is a transparent electrode layer (e.g., ITO). On the side of the second surface (18), there is an electrode layer (16) including 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 although active matrix drive utilizing a thin film transistor (TFT) backplane is mentioned for the layer of pixel electrodes, the scope of the present invention includes other types of electrode addressing, so long as the electrodes are used for the desired function.

Each space between two vertical dashed lines in FIG1 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.

The percentages of the four types of particles in the fluid may vary. For example, one type of particle may occupy 0.1% to 50%, preferably 0.5% to 15% of the volume of the electrophoretic fluid.

The solvent in which the four types of particles are dispersed is clear and colorless. For high particle mobility, 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. Examples of suitable dielectric solvents include hydrocarbons such as isopar, decaline (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils, silicone fluids, aromatic hydrocarbons such as toluene, xylene, diarylethane, dodecylbenzene, or alkylnaphthalenes, 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, MN, low molecular weight halogen containing polymers such as polyperfluoropropylene oxide from TCI America, Portland, Oregon, polychlorotrifluoroethylene such as halocarbon oils from Halocarbon Product Corp., River Edge, NJ, Galden from Ausimont or Krytox Oils, and Greases from DuPont, Delaware. Perfluoropolyether from the K-Fluid Series, polydimethylsiloxane-based silicone oil (DC-200) from Dow-corning.

In one embodiment, the charge carried by the "low-charge" particles can be less than about 50%, or about 5% to about 30%, of the charge carried by the "high-charge" particles. In another embodiment, the "low-charge" particles can 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 having the same charge polarity.

Charge intensity can be measured based on zeta potential. In one embodiment, the zeta potential is determined using a Colloidal Dynamics AcoustoSizer IIM, ESA EN#Attn flow-through electrolysis cell (K:127) with a CSPU-100 signal processing unit. Instrument constants at the test temperature (25°C) are input prior to testing, 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. The pigment sample is dispersed in a solvent (typically a hydrocarbon fluid with fewer than 12 carbon atoms) and diluted to between 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 charge modifier to particle weight ratio of 1:10. The mass of the diluted sample is determined, and the sample is then loaded into the flow-through electrolysis cell to determine the zeta potential.

The magnitude of "high positive" particles and "high negative" particles can be the same or different. Similarly, the magnitude of "low positive" particles and "low negative" particles can be the same or different.

It should also be noted that in the same fluid, two groups of high-low charged particles can have different levels of charge difference. For example, in one group, the low positively charged particles can have a charge strength that is 30% of the charge strength of the high positively charged particles, and in another group, the low negatively charged particles can have a charge strength that is 50% of the charge strength of the high negatively charged particles.

The charge polarity and level of the particles' charge can be adjusted according to the methods described in US Publication No. 2014-0011913, which is incorporated herein by reference in its entirety.

It is also noted that the four types of particles can have different particle sizes. For example, the smaller particles can have a size ranging from about 50 nm to about 800 nm. The larger particles can have a size of about 2 to about 50 times the size of the smaller particles, and more preferably about 2 to about 10 times the size of the smaller particles.

Example 1:

This example is shown in Figure 2. The fluid in this example has red, green, blue, and white pigment particles. The red particles (R) carry a high positive charge, the white particles (W) carry a high negative charge, the blue (B) particles carry a low positive charge, and the green particles (G) carry a low negative charge.

In Figure 2(a), when a high negative voltage potential difference (e.g., -15V) is applied to a pixel, white particles (W) are pushed toward the common electrode (21) side, and red particles (R) are pulled toward the pixel electrode (22a) side. Blue (B) and green (G) particles, due to their lower charge levels, move slower than the more highly charged white and red particles, so they stay in the middle of the pixel, with the green particles above the blue particles. As a result, a white color is seen on the viewing side.

In FIG2( b ), when a high positive voltage potential difference (eg, +15 V) is applied to the pixel, the particle distribution is opposite to that shown in FIG2( a ), and as a result, a red color is seen on the viewing side.

In FIG2(c), when a relatively low positive voltage potential difference (e.g., +3V) is applied to the pixel of FIG2(a) (i.e., driven from the white state), the white particles (W) move toward the pixel electrode (22a), while the red particles (R) move toward the common electrode (21). When they meet while moving, they stop moving and remain in the middle of the pixel due to their strong attraction to each other. In other words, the electric field generated by the low positive voltage potential difference is not strong enough to separate the white and red particles.

However, the electric field is strong enough to separate the lower-charged blue and green particles, and is also strong enough to overcome the attraction between the oppositely charged high-low particle pairs (white/blue and red/green). As a result, the lower-charged (positive) blue particles (B) always move to the common electrode (21) side (i.e., the viewing side), and the lower-charged (negative) green particles (G) move to the pixel electrode (22a) side. Therefore, a blue color is seen on the viewing side.

In Figure 2(d), when a relatively low negative voltage potential difference (e.g., -3V) is applied to the pixel of Figure 2(b) (i.e., driven from the red state), the red particles (R) move toward the pixel electrode (22a), while the white particles (W) move toward the common electrode (21). When the white and red particles meet, they stop moving and remain in the middle of the pixel due to their strong attraction to each other. In other words, the electric field generated by the low negative voltage potential difference is not strong enough to separate the white and red particles.

However, the electric field is strong enough to separate the lower-charged blue and green particles, and is also strong enough to overcome the attraction between the oppositely charged high-low particle pairs (white/blue and red/green). As a result, the lower-charged (negative) green particles (G) always move to the common electrode side (i.e., the viewing side), and the lower-charged (positive) blue particles (B) move to the pixel electrode side. Therefore, the green color is seen on the viewing side.

In Figure 2(e), black is seen from the viewing side. This can be achieved by applying a vibration waveform when the pixel is in the red state (seen in Figure 2b) to cause the red, green, and blue particles to mix in the upper part of the pixel, resulting in a black state seen from the viewing side.

The vibration waveform consists of a pair of opposite drive pulses repeated for many cycles. For example, the vibration waveform can include a 20 millisecond +15V pulse and a 20 millisecond -15V pulse, and such a pair of pulses is repeated 50 times. The total duration of such a vibration waveform will be 2000 milliseconds (see Figure 3).

In practice, there may be at least 10 repetitions (ie, ten pairs of positive and negative pulses).

After applying the vibration waveform, the optical state will be from the mixture of particles, seen as black in this example.

In an example, each of the drive pulses in the dithering 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 full white state to a full red state. For example, if it takes 300 milliseconds to drive a pixel from a full white state to a red-yellow state or vice versa, the dithering waveform may consist of positive and negative pulses, each applied for no more than 150 milliseconds. In practice, it is preferred that the pulses be shorter.

Note also that the lower voltage potential difference applied to achieve the color states in Figures 2(c) and 2(d) can be from about 5% to about 50% of the total drive voltage potential difference required to drive the pixel from the red state to the white state or from the white state to the red state.

While Example 2 shows the possibility of pixels assuming a black, white, red, green or blue state, the present invention also provides the possibility for pixels to assume a yellow, magenta or cyan state.

In FIG4 , each pixel has two sub-pixels. In FIG4( a), a yellow state is displayed when one sub-pixel displays red and the other sub-pixel displays green. In FIG4( b), a magenta state is displayed when one sub-pixel displays red and the other sub-pixel displays blue. In FIG4( c), a cyan state is displayed when one sub-pixel displays blue and the other sub-pixel displays green.

In order to display a brighter yellow, magenta or cyan state, a pixel can be made up of three sub-pixels. This is shown in Figure 5, where a third sub-pixel is added that only displays a white state.

Example 2:

This example is shown in Figure 6. The fluid in this example has red, yellow, blue, and white pigment particles. The red particles (R) carry a high positive charge, the white particles (W) carry a high negative charge, the blue (B) particles carry a low positive charge, and the yellow particles (Y) carry a low negative charge.

In Figure 6(a), when a high negative voltage potential difference (e.g., -15V) is applied to the pixel, the white particles (W) are pushed to the common electrode (61) side, and the red particles (R) are pulled to the pixel electrode (62a) side. The blue (B) and yellow (Y) particles, due to their lower charge levels, move slower than the more highly charged white and red particles, so they stay in the middle of the pixel, with the yellow particles above the blue particles. As a result, white is seen on the viewing side.

In FIG6( b ), when a high positive voltage potential difference (eg, +15 V) is applied to the pixel, the particle distribution is reversed from that shown in FIG6( a ), with the result that a red color is seen on the viewing side.

In FIG6(c), when a relatively low positive voltage potential difference (e.g., +3V) is applied to the pixel of FIG6(a) (i.e., driven from the white state), the white particles (W) move toward the pixel electrode (62a), while the red particles (R) move toward the common electrode (61). When they meet while moving, they stop moving and remain in the middle of the pixel due to their strong attraction to each other. In other words, the electric field generated by the low positive voltage potential difference is not strong enough to separate the white and red particles.

However, the electric field is strong enough to separate the lower-charged blue and yellow particles, and is also strong enough to overcome the attraction between the oppositely charged high-low particle pairs (white/blue and red/yellow). As a result, the lower-charged (positive) blue particles (B) always move to the common electrode (61) side (i.e., the viewing side), and the lower-charged (negative) yellow particles (Y) move to the pixel electrode (62a) side. Therefore, the blue color is seen on the viewing side.

In FIG6( d ), when a relatively low negative voltage potential difference (e.g., -3 V) is applied to the pixel of FIG6( b ) (i.e., driven from the red state), the red particles (R) move toward the pixel electrode (62 a), while the white particles (W) move toward the common electrode (61 ). When the white and red particles meet, they stop moving and remain in the middle of the pixel due to their strong attraction to each other. In other words, the electric field generated by the low negative voltage potential difference is not strong enough to separate the white and red particles.

However, the electric field is strong enough to separate the lower-charged blue and yellow particles, and is also strong enough to overcome the attraction between the oppositely charged high-low particle pairs (white/blue and red/yellow). As a result, the lower-charged (negative) yellow particles (Y) always move to the common electrode side (i.e., the viewing side), and the lower-charged (positive) blue particles (B) move to the pixel electrode side. Therefore, a yellow color is seen on the viewing side.

In Figure 6(e), black is seen from the viewing side. This can be achieved by applying a vibration waveform when the pixel is in the red state (seen in Figure 6b), resulting in a black state being seen from the viewing side.

Similarly, as described in Example 2, the lower voltage potential difference applied to achieve the color states in Figures 6(c) and 6(d) can be from about 5% to about 50% of the total drive voltage potential difference required to drive the pixel from the red state to the white state or from the white state to the red state.

While Example 2 shows the possibility of pixels assuming a black, white, red, yellow or blue state, the present invention also provides the possibility for pixels to assume a purple, orange or green state.

In FIG7 , each pixel has two sub-pixels. In FIG7( a ), a purple state is displayed when one sub-pixel displays red and the other sub-pixel displays blue. Similarly, in FIG7( b ), one sub-pixel displays red and the other sub-pixel displays yellow, resulting in the pixel displaying an orange state. In FIG7( c ), a green state is displayed when one sub-pixel displays blue and the other sub-pixel displays yellow.

To display a brighter purple, orange or green state, a pixel can be made up of three sub-pixels. This is shown in Figure 8, where a third sub-pixel is added that only displays a white state.

Although in these two examples, particles of specific colors are demonstrated for use, in reality, as described above, particles carrying high positive charge, high negative charge, low positive charge, or low negative charge can be of any color. All of these variations are intended to be within the scope of this application. For example, the four types of particles can be cyan, magenta, yellow, and white.

In another aspect of the present invention, the fluid may further comprise substantially uncharged neutral suspended particles.

The term "substantially uncharged" refers to particles that have no charge or carry a charge that is less than 5% of the average charge carried by charged particles. In one embodiment, the neutral suspension particles are uncharged.

The term "neutral suspension" refers to particles that do not rise or fall with gravity. In other words, the particles will float in the fluid between the two electrode plates. In one embodiment, the density of the neutrally suspended particles can be the same as the density of the solvent or solvent mixture in which they are dispersed.

The concentration of substantially uncharged neutral suspended particles in the display fluid is preferably in the range of about 0.1 to about 10% by volume, more preferably in the range of about 0.1 to about 5% by volume.

The substantially uncharged neutral suspended particles may be formed from a polymeric material. The polymeric material may be a copolymer or a homopolymer.

Examples of polymer materials for substantially uncharged neutral suspended particles may include, but are not limited to, polyacrylates, polymethacrylates, polystyrenes, polyanilines, polypyrroles, polyphenols, and polysiloxanes. Specific examples of polymer materials may include, but are not limited to, poly(pentabromophenyl methacrylate), poly(2-vinylnaphthalene), poly(naphthyl methacrylate), poly(α-methylstyrene), poly(N-benzylmethacrylamide), and poly(benzyl methacrylate).

More preferably, the substantially uncharged neutral suspended particles are formed from a polymer that is insoluble in the solvent of the display fluid and also have a high refractive index. In one embodiment, the substantially uncharged neutral suspended particles have a refractive index that is different from the refractive index of the solvent or solvent mixture in which the particles are dispersed. However, the refractive index of the substantially uncharged neutral suspended particles is typically higher than the refractive index of the solvent or solvent mixture. In some cases, the refractive index of the substantially uncharged neutral suspended particles can be greater than 1.45.

In one embodiment, the material used for the substantially uncharged neutral suspension particles may include aromatic groups.

Substantially uncharged neutral suspended particles can be prepared from monomers by polymerization techniques such as suspension polymerization, dispersion polymerization, seed polymerization, soap-free polymerization, and emulsion polymerization, or physical methods including an inverse emulsification-evaporation process. The monomers are polymerized in the presence of a dispersant. The presence of a dispersant allows the polymer particles to be formed in a desired size range, and the dispersant can also form a layer that physically or chemically binds to the surface of the polymer particles to prevent particle agglomeration.

The dispersant preferably has a long chain (of at least eight atoms) that can stabilize the polymer particles in the hydrocarbon solvent. Such dispersants can be acrylate-terminated or vinyl-terminated polymers, which are suitable because acrylates or vinyl groups can copolymerize with the monomers in the reaction medium.

One specific example of a dispersant is an acrylate-terminated polysiloxane (Gelest, MCR-M17, MCR-M22).

Another type of suitable dispersant is polyethylene macromonomers, as shown below:

CH ₃ -[-CH ₂ -] _n -CH ₂ OC(=O)-C(CH ₃ )=CH ₂

The backbone of the macromonomer may be a polyethylene chain, and the integer "n" may be from 30 to 200. The synthesis of this type of macromonomer may be found in Seigou Kawaguchi et al., Designed Monomers and Polymers, 2000, 3, 263.

If the fluid system is fluorinated, the dispersant is preferably also fluorinated.

Alternatively, the substantially uncharged neutral suspended particles may also be formed from core particles coated with a polymer shell, and the shell may, for example, be formed from any of the polymer materials described above.

The core particles can be inorganic pigments such as _TiO2 , _ZrO2 , ZnO, _Al2O3 , CI Pigment Black 26 or 28 (for example, iron manganese black or _copper chrome black), or organic pigments such as phthalocyanine blue, phthalocyanine green, aniline yellow, aniline AAOT yellow, and quinacridone, azo, rhodamine, and dinaphthalene pigment series from Sun Chemical Company, Hansa Yellow G particles from Kanto Chemical, and Carbon Lampblack from Fisher.

In the case of core-shell, essentially uncharged, neutral suspended particles, they can be formed by microencapsulation methods such as coacervation, interfacial polycondensation, interfacial cross-linking, in situ polymerization, or matrix polymerization.

The size of the substantially uncharged neutral suspended particles is preferably in the range of about 100 nanometers to about 5 microns.

In one embodiment of this aspect of the invention, the substantially uncharged neutral suspended particles added to the fluid can have a color that is visually substantially the same as the color of one of the four types of charged particles. For example, in a display fluid, there can be charged red, green, blue, and white particles, as well as substantially uncharged neutral suspended particles, and in this case, the substantially uncharged neutral suspended particles can be red, green, blue, or white.

In another embodiment, the substantially uncharged neutral suspended particles may have a color that is substantially different from the color of any of the four types of charged particles.

The presence of substantially uncharged neutral suspended particles in the fluid increases the reflection of incident light and thereby also improves the contrast, particularly if they are formed from a reflective material.

Image stability can also be improved by adding substantially uncharged neutral suspended particles to the four-particle fluid system. The substantially uncharged neutral suspended particles can fill the gaps created by excessive accumulation of charged particles on the surface of the electrode under the electric field, thereby preventing the charged particles from settling due to gravity.

Additionally, if the substantially uncharged neutral suspended particles are white, they can increase the reflectivity of the display. If they are black, they can increase the black level of the display.

In any case, the essentially uncharged neutral suspended particles do not affect the driving behavior of the four types of charged particles in the fluid.

The electrophoretic fluid described above is filled in a display unit. The display unit can 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 can 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 this application.

As shown in Figures 9A and 9B, the display unit (90) and the pixel electrode (92a) in the present invention can be aligned or unaligned.

The term "about" in this application is intended to mean ±5% of the indicated value.

Although the present invention has been described with reference to specific embodiments, it will be understood 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 processing steps to the goals, spirit, and scope of the present invention. All such modifications are intended to be within the scope of the appended claims.

Claims (7)

1. A display layer comprising an electrophoretic fluid and having first and second surfaces on opposite sides of the display layer, said electrophoretic fluid comprising high-positive particles, high-negative particles, low-positive particles, and low-negative particles, all dispersed in a solvent or solvent mixture, the four types of particles having different optical properties from each other, such that: (a) The application of an electric field with the same polarity as the highly positive particles will cause the optical properties of the highly positive particles to be displayed at the first surface; or (b) The application of an electric field with the same polarity as the high negative particle will cause the optical properties of the high negative particle to be displayed at the first surface; or (c) Once the optical properties of the highly positive particles are manifested at the first surface, the application of an electric field having the same polarity as the low-negative particles, insufficient to overcome the attraction between the highly positive and highly negative particles, but sufficient to overcome the attraction between other particles with opposite charges, will cause the optical properties of the low-negative particles to be manifested at the first surface; or (d) Once the optical properties of the high-negative particles are manifested at the first surface, the application of an electric field having the same polarity as the low-positive particles, insufficient to overcome the attraction between the high-positive and high-negative particles, but sufficient to overcome the attraction between other particles with opposite charges, will cause the optical properties of the low-positive particles to be manifested at the first surface; or (e) The application of the vibration waveform will cause the fifth optical property to be displayed at the first surface. 2. The layer according to claim 1, wherein the four types of particles are red, green, blue and white particles. 3. The layer according to claim 1, wherein the four types of particles are red, yellow, blue, and white particles. 4. The layer according to claim 1, wherein the four types of particles are cyan, magenta, yellow, and white particles. 5. The layer according to claim 1, wherein the charge of the lower charged particles is less than 50% of the charge of the higher charged particles. 6. The layer according to claim 1, wherein the charge of the lower charged particles is less than 75% of the charge of the higher charged particles. 7. The layer according to claim 1, wherein none of the four types of particles are black particles, and the fifth optical property is a black state.