U.S. patent application number 13/603964 was filed with the patent office on 2012-12-27 for full color reflective display with multichromatic sub pixels.
This patent application is currently assigned to E INK CORPORATION. Invention is credited to Paul Drzaic, Russell J. Wilcox.
Application Number | 20120326957 13/603964 |
Document ID | / |
Family ID | 33543695 |
Filed Date | 2012-12-27 |
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United States Patent
Application |
20120326957 |
Kind Code |
A1 |
Drzaic; Paul ; et
al. |
December 27, 2012 |
FULL COLOR REFLECTIVE DISPLAY WITH MULTICHROMATIC SUB PIXELS
Abstract
A full color, reflective display having superior saturation and
brightness is achieved with a novel display element comprising
multichromatic elements. In one embodiment a capsule includes more
than three species of particles which differ visually. One
embodiment of the invention employs three sub-pixels, each
sub-pixel comprising capsules including three species of particles
which differ visually. Another embodiment of the invention employs
color filters to provide different visual states to the user. The
display element provides a visual display in response to the
application of an electrical signal to at least one of the
capsules.
Inventors: |
Drzaic; Paul; (Morgan Hill,
CA) ; Wilcox; Russell J.; (Natick, MA) |
Assignee: |
E INK CORPORATION
Cambridge
MA
|
Family ID: |
33543695 |
Appl. No.: |
13/603964 |
Filed: |
September 5, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11926201 |
Oct 29, 2007 |
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13603964 |
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10827745 |
Apr 20, 2004 |
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11926201 |
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09289507 |
Apr 9, 1999 |
7075502 |
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10827745 |
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60081362 |
Apr 10, 1998 |
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Current U.S.
Class: |
345/107 |
Current CPC
Class: |
H01L 51/0512 20130101;
G09G 3/2003 20130101; H01L 27/28 20130101; B41J 3/4076 20130101;
G09G 3/344 20130101; H01L 51/0077 20130101; G02F 1/1334 20130101;
G02F 1/167 20130101; G09F 9/372 20130101; G09F 9/302 20130101; H01L
51/005 20130101; G02F 2001/1678 20130101; G02F 2202/28 20130101;
G02B 26/026 20130101; G02F 1/133305 20130101; G09G 3/2074
20130101 |
Class at
Publication: |
345/107 |
International
Class: |
G09G 3/34 20060101
G09G003/34 |
Claims
1. A color reflective electronic display comprising at least two
microcells each containing at least two particles and not having
any suspending fluid, the microcells being disposed adjacent a
color filter.
2. A display according to claim 1 in combination with at least one
electrode.
3. A display according to claim 2 having a plurality of electrodes
arranged to drive the display using a passive matrix addressing
scheme.
4. A display according to claim 2 further comprising at least one
thin film transistor arranged to control the potential of the at
least one electrode.
5. A display according to claim 4 further comprising a flexible
substrate on which the thin film transistor is disposed.
6. A display according to claim 1 wherein the at least two
microcells are formed by photolithography.
7. A display according to claim 1 wherein the at least two
microcells are formed by embossing a plastic substrate.
8. A color reflective electronic display comprising at least one
pixel having a plurality of sub-pixels, wherein at least one of the
sub-pixels is capable of displaying three colors.
9. A display according to claim 8 wherein said at least one
sub-pixel comprises an electrophoretic medium comprising first and
second types of electrically charged particles bearing charges of
opposite polarity and differing in at least one optical
characteristic.
10. A display according to claim 9 wherein said at least one
sub-pixel has a viewing surface and is capable of being driven to a
first optical state, in which the first type of particles are
disposed adjacent the viewing surface and the second type of
particles are disposed spaced from the viewing surface, a second
optical state, in which the second type of particles are disposed
adjacent the viewing surface and the first type of particles are
disposed spaced from the viewing surface, and a third optical state
in which both the first and second types of particles are disposed
spaced from the viewing surface.
11. A display according to claim 10 wherein the first and second
types of particles are disposed in a colored fluid, and the third
optical state displays the color of the colored fluid.
12. A display according to claim 10 wherein said at least one
sub-pixel further comprises a third type of particles, the third
type of particles being substantially uncharged and differing from
the first and second types of particles in at least one optical
characteristic, and wherein the third optical state displays the
optical characteristic of the third type of particles.
13. A display according to claim 10 wherein said at least one
sub-pixel further comprises a colored member disposed on the
opposed side of the sub-pixel from the viewing surface, and wherein
the third optical state displays the optical characteristic of the
colored member.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of copending application
Ser. No. 11/926,201, filed Oct. 29, 2007 (Publication No.
2008/0048970), which is itself a continuation of application Ser.
No. 10/877,745, filed Apr. 20, 2004 (Publication No. 2004/0263947),
which is itself a continuation of application Ser. No. 09/289,507,
filed Apr. 9, 1999 (now U.S. Pat. No. 7,075,502, issued Jul. 11,
2006), which itself claims benefit of Application Ser. No.
60/081,362 filed Apr. 10, 1998. The entire disclosures of all the
aforementioned applications are incorporated herein by
reference.
FIELD OF INVENTION
[0002] The present invention relates to electronic displays and, in
particular, to full color electrophoretic displays and methods of
creating full-color microencapsulated electrophoretic displays.
BACKGROUND OF INVENTION
[0003] There are a number of enhanced reflective display media
which offer numerous benefits such as enhanced optical appearance,
the ability to be constructed in large form factors, capable of
being formed using flexible substrates, characterized by easy
manufacturability and manufactured at low cost. Such reflective
display media include microencapsulated electrophoretic displays,
rotating ball displays, suspended particle displays, and composites
of liquid crystals with polymers (known by many names including but
not limited to polymer dispersed liquid crystals, polymer
stabilized liquid crystals, and liquid crystal gels).
Electrophoretic display media, generally characterized by the
movement of particles through an applied electric field, are highly
reflective, can be made bistable, and consume very little power.
Further, encapsulated electrophoretic displays also may be printed.
These properties allow encapsulated electrophoretic display media
to be used in many applications for which traditional electronic
displays are not suitable, such as flexible, printed displays.
[0004] While bichromatic electrophoretic displays have been
demonstrated in a limited range of colors (e.g. black/white or
yellow/red), to date there has not been successful
commercialization of a full-color electrophoretic display. Indeed,
no reflective display technology to date has shown itself capable
of satisfactory color. Full-color reflective displays typically are
deficient when compared to emissive displays in at least two
important areas: brightness and color saturation.
[0005] One traditional technique for achieving a bright, full-color
display which is known in the art of emissive displays is to create
sub-pixels that are red, green and blue. In this system, each pixel
has two states: on, or the emission of color; and off. Since light
blends from these sub-pixels, the overall pixel can take on a
variety of colors and color combinations. In an emissive display,
the visual result is the summation of the wavelengths emitted by
the sub-pixels at selected intensities, white is seen when red,
green and blue are all active in balanced proportion or full
intensity. The brightness of the white image is controlled by the
intensities of emission of light by the sub-pixels. Black is seen
when none are active or, equivalently, when all are emitting at
zero intensity. As an additional example, a red visual display
appears when the red sub-pixel is active while the green and blue
are inactive, and thus only red light is emitted.
[0006] It is known that this method can be applied to bichromatic
reflective displays, typically using the cyan-magenta-yellow
subtractive color system. In this system, the reflective sub-pixels
absorb characteristic portions of the optical spectrum, rather than
generating characteristic portions of the spectrum as do the pixels
in an emissive display. White reflects everything, or equivalently
absorbs nothing. A colored reflective material reflects light
corresponding in wavelength to the color seen, and absorbs the
remainder of the wavelengths in the visible spectrum. To achieve a
black display, all three sub-pixels are turned on, and they absorb
complementary portions of the spectrum.
[0007] However, the colors displayed by a full-color display as
described above are sub-optimal. For example, to display red, one
pixel displays magenta, one displays yellow, and one displays
white. The white sub-pixel reduces the saturation of red in the
image and reduces display contrast. The overall effect is a washed
out red. This further illustrates why no method to date has been
capable of generating a high-contrast, high-brightness full color
reflective display with good color saturation.
SUMMARY OF INVENTION
[0008] This invention teaches practical ways to achieve brighter,
more saturated, reflective full-color displays than previously
known, particularly full-color encapsulated, electrophoretic
displays.
[0009] An object of the invention is to provide a brighter, more
saturated, reflective full-color display. In some embodiments, the
displays are highly flexible, can be manufactured easily, consume
little power, and can, therefore, be incorporated into a variety of
applications. The invention features a printable display comprising
an encapsulated electrophoretic display medium. In an embodiment
the display media can be printed and, therefore the display itself
can be made inexpensively.
[0010] An encapsulated electrophoretic display can be constructed
so that the optical state of the display is stable for some length
of time. When the display has two states which are stable in this
manner, the display is said to be bistable. If more than two states
of the display are stable, then the display can be said to be
multistable. For the purpose of this invention, the terms bistable
and multistable, or generally, stable, will be used to indicate a
display in which any optical state remains fixed once the
addressing voltage is removed. The definition of a stable state
depends on the application for the display. A slowly-decaying
optical state can be effectively stable if the optical state is
substantially unchanged over the required viewing time. For
example, in a display which is updated every few minutes, a display
image which is stable for hours or days is effectively bistable or
multistable, as the case may be, for that application. In this
invention, the terms bistable and multistable also indicate a
display with an optical state sufficiently long-lived as to be
effectively stable for the application in mind. Alternatively, it
is possible to construct encapsulated electrophoretic displays in
which the image decays quickly once the addressing voltage to the
display is removed (i.e., the display is not bistable or
multistable). As will be described, in some applications it is
advantageous to use an encapsulated electrophoretic display which
is not bistable or multistable. Whether or not an encapsulated
electrophoretic display is stable, and its degree of stability, can
be controlled through appropriate chemical modification of the
electrophoretic particles, the suspending fluid, the capsule, and
binder materials.
[0011] An encapsulated electrophoretic display may take many forms.
The display may comprise capsules dispersed in a binder. The
capsules may be of any size or shape. The capsules may, for
example, be spherical and may have diameters in the millimeter
range or the micron range, but is preferably from ten to a few
hundred microns. The capsules may be formed by an encapsulation
technique, as described below. Particles may be encapsulated in the
capsules. The particles may be two or more different types of
particles. The particles may be colored, luminescent,
light-absorbing or transparent, for example. The particles may
include neat pigments, dyed (laked) pigments or pigment/polymer
composites, for example. The display may further comprise a
suspending fluid in which the particles are dispersed.
[0012] The successful construction of an encapsulated
electrophoretic display requires the proper interaction of several
different types of materials and processes, such as a polymeric
binder and, optionally, a capsule membrane. These materials must be
chemically compatible with the electrophoretic particles and fluid,
as well as with each other. The capsule materials may engage in
useful surface interactions with the electrophoretic particles, or
may act as a chemical or physical boundary between the fluid and
the binder. Various materials and combinations of materials useful
in constructing encapsulated electrophoretic displays are described
in co-pending application Ser. No. 09/140,861, the contents of
which are incorporated by reference herein.
[0013] In some cases, the encapsulation step of the process is not
necessary, and the electrophoretic fluid may be directly dispersed
or emulsified into the binder (or a precursor to the binder
materials) and an effective "polymer-dispersed electrophoretic
display" constructed. In such displays, voids created in the binder
may be referred to as capsules or microcapsules even though no
capsule membrane is present. The binder dispersed electrophoretic
display may be of the emulsion or phase separation type.
[0014] Throughout the specification, reference will be made to
printing or printed. As used throughout the specification, printing
is intended to include all forms of printing and coating,
including: premetered coatings such as patch die coating, slot or
extrusion coating, slide or cascade coating, and curtain coating;
roll coating such as knife over roll coating, forward and reverse
roll coating; gravure coating; dip coating; spray coating; meniscus
coating; spin coating; brush coating; air knife coating; silk
screen printing processes; electrostatic printing processes;
thermal printing processes; and other similar techniques. A
"printed element" refers to an element formed using any one of the
above techniques.
[0015] As noted above, electrophoretic display elements can be
encapsulated. Throughout the Specification, reference will be made
to "capsules," "pixels," and "sub-pixels." A pixel display element
can be formed by one or more capsules or sub-pixels. A sub-pixel
may itself comprise one or more capsules or other structures.
[0016] A full color, reflective display having superior saturation
and brightness is achieved with a novel display element comprising
multichromatic sub-elements. One embodiment of the display employs
three sub-pixels, each sub-pixel comprising a capsule including
three species of particles which differ visually. Another
embodiment of the display employs color filters combined with an
encapsulated electrophoretic display to provide different visual
states. In still another embodiment, the display employs display
elements capable of more than three visual states. In yet another
embodiment, the visual display states are selected from specific
colors, for example, the primary colors red, green and blue, or
their complements, and white and/or black. The display element
presents a visual display in response to the application of an
electrical signal to at least one of the capsules.
[0017] In one aspect, the present invention relates to an
electrophoretic display element. The display element comprises a
first capsule including a first species of particles having a first
optical property and a second species of particles having a second
optical property visually different from the first optical
property. The display element further comprises a second capsule
including a third species of particles having a third optical
property and a fourth species of particles having a fourth optical
property visually different from the third optical property. The
display element presents a visual display in response to the
application of an electrical signal to at least one of the first
and second capsules. The first optical property and the third
optical property can be, but are not required to be, substantially
similar in appearance.
[0018] The electrophoretic display element can further comprise a
fifth species of particles having a fifth optical property visually
different from the first and second optical properties in the first
capsule. It can also comprise a sixth species of particles having a
sixth optical property visually different from the third and fourth
optical properties in the second capsule. It can also include a
third capsule having a seventh species of particles having a
seventh optical property, an eighth species of particles having a
eighth optical property, and a ninth species of particles having a
ninth optical property.
[0019] The electrophoretic display element can include particles
such that the first, third and seventh optical properties have a
white visual appearance. The electrophoretic display element can
include particles such that the second, fourth and eighth optical
properties have a black visual appearance. The electrophoretic
display element can have at least one of the optical properties be
red, green, blue, yellow, cyan, or magenta in visual appearance.
The electrophoretic display element can have at least one of the
optical properties comprising color, brightness, or
reflectivity.
[0020] The electrophoretic display element can have capsules which
include a suspending fluid. The suspending fluid can be
substantially clear, or it can be dyed or otherwise colored.
[0021] In another aspect, the present invention relates to a
display apparatus comprising at least one display element which
includes at least two capsules such as are described above and at
least one electrode adjacent to the display element, wherein the
apparatus presents a visual display in response to the application
of an electrical signal via the electrode to the display
element.
[0022] The display apparatus can include a plurality of electrodes
adjacent the display element. The plurality of electrodes can
include at least one which has a size different from others of the
plurality of electrodes, and can include at least one which has a
color different from others of the plurality of electrodes.
[0023] In another aspect, the present invention relates to an
electrophoretic display element comprising a capsule containing a
first species of particles having a first optical property, a
second species of particles having a second optical property
visually different from the first optical property, a third species
of particles having a third optical property visually different
from the first and second optical properties and a fourth species
of particles having a fourth optical property visually different
from the first, second, and third optical properties such that the
element presents a visual display in response to the application of
an electrical signal to the capsule. The electrophoretic display
element can also include a suspending fluid within the capsule.
[0024] In yet another aspect, the present invention relates to an
electrophoretic display element comprising a capsule containing a
first species of particles having a first optical property, a
second species of particles having a second optical property
visually different from the first optical property, a third species
of particles having a third optical property visually different
from the first and second optical properties, a fourth species of
particles having a fourth optical property visually different from
the first, second, and third optical properties, and a fifth
species of particles having a fifth optical property visually
different from the first, second, third, and fourth optical
properties such that the element presents a visual display in
response to the application of an electrical signal to said
capsule. The electrophoretic display element can also include a
suspending fluid within the capsule.
[0025] In still another aspect, the present invention relates to a
method of manufacturing an electrophoretic display. The
manufacturing method comprises the steps of providing a first
capsule containing a first species of particles having a first
optical property and a second species of particles having a second
optical property visually different from the first optical
property, and providing a second capsule containing a third species
of particles having a third optical property and a fourth species
of particles having a fourth optical property visually different
from the third optical property, such that when an electrical
signal is applied to at least one of the first and second capsules
the element presents a visual display in response to the signal. In
this method of manufacture, the first optical property and the
third optical property can be substantially similar in
appearance.
[0026] In still a further aspect, the present invention relates to
a method of manufacturing an electrophoretic display. This
manufacturing method comprises the steps of providing a first
capsule containing a first species of particles having a first
optical property, a second species of particles having a second
optical property visually different from the first optical property
and containing a third species of particles having a third optical
property visually different from the first and second optical
properties, providing a second capsule containing a fourth species
of particles having a fourth optical property, a fifth species of
particles having a fifth optical property visually different from
the fourth optical property and a sixth species of particles having
a sixth optical property visually different from the fourth and
fifth optical properties, and providing a third capsule containing
a seventh species of particles having a seventh optical property,
an eighth species of particles having a eighth optical property
visually different from the seventh optical property, and a ninth
species of particles having a ninth optical property visually
different from the seventh and eighth optical properties, such that
when an electrical signal is applied to at least one of the first,
second and third capsules, the element presents a visual display in
response to the signal.
[0027] The manufacturing method can include the step of providing a
first capsule wherein the third optical property is red visual
appearance, or is yellow visual appearance. The manufacturing
method can include the step of providing a second capsule wherein
the sixth optical property is green visual appearance, or is cyan
visual appearance. The manufacturing method can include the step of
providing a third capsule wherein the ninth optical property is
blue visual appearance, or is magenta visual appearance. The
manufacturing method can include the step of providing capsules
wherein the first, fourth and seventh optical properties are white
visual appearance, or wherein the second, fifth and eighth optical
properties are black visual appearance.
BRIEF DESCRIPTION OF DRAWINGS
[0028] The invention is pointed out with particularity in the
appended claims. The advantages of the invention described above,
together with further advantages, may be better understood by
referring to the following description taken in conjunction with
the accompanying drawings. In the drawings, like reference
characters generally refer to the same parts throughout the
different views. Also, the drawings are not necessarily to scale,
emphasis instead generally being placed upon illustrating the
principles of the invention.
[0029] FIG. 1A is a diagrammatic side view of an embodiment of a
rear-addressing electrode structure for a particle-based display in
which a smaller electrode has been placed at a voltage relative to
the large electrode causing the particles to migrate to the smaller
electrode.
[0030] FIG. 1B is a diagrammatic side view of an embodiment of a
rear-addressing electrode structure for a particle-based display in
which the larger electrode has been placed at a voltage relative to
the smaller electrode causing the particles to migrate to the
larger electrode.
[0031] FIG. 1C is a diagrammatic top-down view of one embodiment of
a rear-addressing electrode structure.
[0032] FIG. 1D is a diagrammatic perspective view of one embodiment
of a display element having three sub-pixels, each sub-pixel
comprising a relatively larger rear electrode and a relatively
smaller rear electrode.
[0033] FIG. 2A is a diagrammatic side view of an embodiment of a
rear-addressing electrode structure having a retroreflective layer
associated with the larger electrode in which the smaller electrode
has been placed at a voltage relative to the large electrode
causing the particles to migrate to the smaller electrode.
[0034] FIG. 2B is a diagrammatic side view of an embodiment of a
rear-addressing electrode structure having a retroreflective layer
associated with the larger electrode in which the larger electrode
has been placed at a voltage relative to the smaller electrode
causing the particles to migrate to the larger electrode.
[0035] FIG. 2C is a diagrammatic side view of an embodiment of a
rear-addressing electrode structure having a retroreflective layer
disposed below the larger electrode in which the smaller electrode
has been placed at a voltage relative to the large electrode
causing the particles to migrate to the smaller electrode.
[0036] FIG. 2D is a diagrammatic side view of an embodiment of a
rear-addressing electrode structure having a retroreflective layer
disposed below the larger electrode in which the larger electrode
has been placed at a voltage relative to the smaller electrode
causing the particles to migrate to the larger electrode.
[0037] FIG. 3A is a diagrammatic side view of an embodiment of an
addressing structure in which a direct-current electric field has
been applied to the capsule causing the particles to migrate to the
smaller electrode.
[0038] FIG. 3B is a diagrammatic side view of an embodiment of an
addressing structure in which an alternating-current electric field
has been applied to the capsule causing the particles to disperse
into the capsule, obscuring a rear substrate.
[0039] FIG. 3C is a diagrammatic side view of an embodiment of an
addressing structure having transparent electrodes, in which a
direct-current electric field has been applied to the capsule
causing the particles to migrate to the smaller electrode,
revealing a rear substrate.
[0040] FIG. 3D is a diagrammatic side view of an embodiment of an
addressing structure having transparent electrodes, in which an
alternating-current electric field has been applied to the capsule
causing the particles to disperse into the capsule.
[0041] FIG. 3E is a diagrammatic side view of an embodiment of an
addressing structure for a display element having three
sub-pixels.
[0042] FIG. 3F is a diagrammatic side view of an embodiment of a
dual particle curtain mode addressing structure addressing a
display element to appear white.
[0043] FIG. 3G is a diagrammatic side view of an embodiment of a
dual particle curtain mode addressing structure addressing a
display element to appear red.
[0044] FIG. 3H is a diagrammatic side view of an embodiment of a
dual particle curtain mode addressing structure addressing a
display element to absorb red light.
[0045] FIG. 3I is a diagrammatic side view of an embodiment of a
dual particle curtain mode addressing structure for a display
element having three sub-pixels, in which the display is addressed
to appear red.
[0046] FIG. 3J is a diagrammatic side view of another embodiment of
a dual particle curtain mode addressing structure for a display
element.
[0047] FIG. 3K is a diagrammatic plan view of an embodiment of an
interdigitated electrode structure.
[0048] FIG. 3L is a diagrammatic side view of another embodiment of
a dual particle curtain mode display structure having a dyed fluid
and two species of particles, addressed to absorb red.
[0049] FIG. 3M is a diagrammatic side view of another embodiment of
a dual particle curtain mode display structure having clear fluid
and three species of particles, addressed to absorb red.
[0050] FIG. 4A is a diagrammatic side view of an embodiment of a
rear-addressing electrode structure for a particle-based display
having colored electrodes and a white electrode, in which the
colored electrodes have been placed at a voltage relative to the
white electrode causing the particles to migrate to the colored
electrodes.
[0051] FIG. 4B is a diagrammatic side view of an embodiment of a
rear-addressing electrode structure for a particle-based display
having colored electrodes and a white electrode, in which the white
electrode has been placed at a voltage relative to the colored
electrodes causing the particles to migrate to the white
electrode.
[0052] FIG. 5 is a diagrammatic side view of an embodiment of a
color display element having red, green, and blue particles of
different electrophoretic mobilities.
[0053] FIGS. 6A-6B depict the steps taken to address the display of
FIG. 5 to display red.
[0054] FIGS. 7A-7D depict the steps taken to address the display of
FIG. 5 to display blue.
[0055] FIGS. 8A-8C depict the steps taken to address the display of
FIG. 5 to display green.
[0056] FIG. 9 is a cross-sectional view of a rear electrode
addressing structure that is formed by printing.
[0057] FIG. 10 is a perspective view of an embodiment of a control
grid addressing structure.
DETAILED DESCRIPTION
[0058] An electronic ink is an optoelectronically active material
that comprises at least two phases: an electrophoretic contrast
media phase and a coating/binding phase. The electrophoretic phase
comprises, in some embodiments, a single species of electrophoretic
particles dispersed in a clear or dyed medium, or more than one
species of electrophoretic particles having distinct physical and
electrical characteristics dispersed in a clear or dyed medium. In
some embodiments the electrophoretic phase is encapsulated, that
is, there is a capsule wall phase between the two phases. The
coating/binding phase includes, in one embodiment, a polymer matrix
that surrounds the electrophoretic phase. In this embodiment, the
polymer in the polymeric binder is capable of being dried,
crosslinked, or otherwise cured as in traditional inks, and
therefore a printing process can be used to deposit the electronic
ink onto a substrate.
[0059] In one embodiment, the ink may comprise sub-pixels capable
of displaying different colors. Sub-pixels may be grouped to form
pixels. In one particular embodiment, each sub-pixel contains red
particles, green particles, and blue particles, respectively. In
another particular embodiment, each sub-pixel contains cyan
particles, yellow particles, and magenta particles, respectively.
In each example, each sub-pixel can additionally include particles
which are white and particles which are black. By addressing each
sub-pixel to display some fraction of its colored particles, and
some portion of the white and black particles, a pixel can be
caused to give an appearance corresponding to a selected color at a
selected brightness level.
[0060] An electronic ink is capable of being printed by several
different processes, depending on the mechanical properties of the
specific ink employed. For example, the fragility or viscosity of a
particular ink may result in a different process selection. A very
viscous ink would not be well-suited to deposition by an inkjet
printing process, while a fragile ink might not be used in a knife
over roll coating process.
[0061] The optical quality of an electronic ink is quite distinct
from other electronic display materials. The most notable
difference is that the electronic ink provides a high degree of
both reflectance and contrast because it is pigment based (as are
ordinary printing inks). The light scattered from the electronic
ink comes from a very thin layer of pigment close to the top of the
viewing surface. In this respect it resembles an ordinary, printed
image. Also, electronic ink is easily viewed from a wide range of
viewing angles in the same manner as a printed page, and such ink
approximates a Lambertian contrast curve more closely than any
other electronic display material. Since electronic ink can be
printed, it can be included on the same surface with any other
printed material, including traditional inks Electronic ink can be
made optically stable in all display configurations, that is, the
ink can be set to a persistent optical state. Fabrication of a
display by printing an electronic ink is particularly useful in low
power applications because of this stability.
[0062] Electronic ink displays are novel in that they can be
addressed by DC voltages and draw very little current. As such, the
conductive leads and electrodes used to deliver the voltage to
electronic ink displays can be of relatively high resistivity. The
ability to use resistive conductors substantially widens the number
and type of materials that can be used as conductors in electronic
ink displays. In particular, the use of costly vacuum-sputtered
indium tin oxide (ITO) conductors, a standard material in liquid
crystal devices, is not required. Aside from cost savings, the
replacement of ITO with other materials can provide benefits in
appearance, processing capabilities (printed conductors),
flexibility, and durability. Additionally, the printed electrodes
are in contact only with a solid binder, not with a fluid layer
(like liquid crystals). This means that some conductive materials,
which would otherwise dissolve or be degraded by contact with
liquid crystals, can be used in an electronic ink application.
These include opaque metallic inks for the rear electrode (e.g.,
silver and graphite inks), as well as conductive transparent inks
for either substrate. These conductive coatings include
semiconducting colloids, examples of which are indium tin oxide and
antimony-doped tin oxide. Organic conductors (polymeric conductors
and molecular organic conductors) also may be used. Polymers
include, but are not limited to, polyaniline and derivatives,
polythiophene and derivatives, poly3,4-ethylenedioxythiophene
(PEDOT) and derivatives, polypyrrole and derivatives, and
polyphenylenevinylene (PPV) and derivatives. Organic molecular
conductors include, but are not limited to, derivatives of
naphthalene, phthalocyanine, and pentacene. Polymer layers can be
made thinner and more transparent than with traditional displays
because conductivity requirements are not as stringent.
[0063] As an example, there are a class of materials called
electroconductive powders which are also useful as coatable
transparent conductors in electronic ink displays. One example is
Zelec ECP electroconductive powders from DuPont Chemical Co. of
Wilmington, Del.
[0064] It is possible to produce any selected color from the
superposition of suitable proportions of three properly chosen
colors. In one embodiment, the colors red, green, and blue can be
combined in various proportions to produce an image which is
perceived as a selected color. Emissive or transmissive displays
operate according to additive rules, where the perceived color is
created by summing the emission wavelengths of a plurality of
emitting or transmitting objects. For an emissive or transmissive
display which includes three sub-pixels, one of which can produce
red light, one green light, and one blue light, respectively, one
can generate all colors, as well as white and black. At one
extreme, the combination of all three at full intensity is
perceived as white, and at the other, the combination of all three
at zero intensity is perceived as black. Specific combinations of
controlled proportions of these three colors can be used to
represent other colors.
[0065] In a reflective display, the light which a viewer perceives
is the portion of the spectrum which is not absorbed when the light
to be reflected falls on the reflector surface. One may thus
consider a reflecting system as a subtractive system, that is, that
each reflective surface "subtracts" from the light that portion
which the reflector absorbs. The color of a reflector represents
the wavelengths of light the reflector absorbs. A yellow reflector
absorbs substantially blue light. A magenta reflector absorbs
substantially green light. A cyan reflector absorbs substantially
red light. Thus, in an alternative embodiment employing reflectors,
nearly the same results as an emissive system can be obtained by
use of the three colors cyan, yellow, and magenta as the primary
colors, from which all other colors, including black but not white,
can be derived. To obtain white from such a display, one must
further introduce a third state per sub-pixel, namely white.
[0066] One approach which may be taken to overcome the shortcomings
inherent in two state displays is to create a display comprising
individual pixels or pixels comprising sub-pixels that can support
multiple color states. The use of multiple color states permits
more robust color rendition and provides better contrast than is
possible with two color states per pixel or per sub-pixel. For
example, using a microencapsulated electrophoretic display, a
single microcapsule with five kinds of particles could display
white, cyan, magenta, yellow, or black all with excellent
saturation. By foregoing black and using cyan/magenta/yellow to
combine to black, a similar effect can be achieved with a display
element capable of four color states.
[0067] The invention can also utilize any reflective display
element that can create three color states within a single
sub-pixel, where sub-pixels are combined to generate a variety of
overall pixel colors. Such a display is capable of greatly improved
appearance yet relies on only three color states per sub-pixel
instead of four or five or more. A sub-pixel having only three
color states can have advantages with regard to the operation of
the display. Fewer and simpler applied voltage signals are needed
to operate each sub-pixel of the display element, A sub-pixel
having fewer stable states may be capable of being addressed more
quickly than one with more stable states.
[0068] Various methods are possible by which three color states
could be achieved within a single addressable region, which can be
a display element sub-pixel. For example, a microencapsulated
electrophoretic display element sub-pixel may contain particles in
a clear suspension medium. A simple addressing method is to provide
white particles having a positive charge, cyan particles having a
negative charge, and red particles having no charge. In this
example, white is achieved when the top electrode is negative and
the bottom electrodes are both positive. Cyan is achieved when the
top electrode is positive and the bottom electrodes are both
negative. Red is achieved when the top electrode is set to ground,
one bottom electrode is positive and attracts the cyan particles,
and the other bottom electrode is negative and attracts the white
particles, so that the red particles are uppermost and are
seen.
[0069] Another example combines top and bottom motion with a
sideways or so-called in-plane switching, control grid or
shutter-effect method. In one example, red particles have strong
positive charge, black particles have lesser positive charge, and
the sub-pixel of the display incorporates a white sheet behind a
clear bottom electrode. The clear bottom electrode comprises a
larger sub-electrode and a smaller sub-electrode. In this example,
using a shutter effect, red is achieved when the top electrode has
a negative voltage and the bottom electrode, including both
subelectrodes, has a positive voltage. Black is achieved when the
top electrode has a positive voltage and the bottom electrode,
including both subelectrodes, has a negative voltage. White is
achieved when the smaller subelectrode of the bottom electrode is
switched to a negative voltage but the top electrode and the larger
subelectrode of the bottom electrode is switched to a less negative
voltage. Thus the red and black particles are attracted to cluster
at the smaller sub-electrode, with the slower black particles
clustering on top and blocking from sight the red particles, and
the bulk of the microcapsule is clear, allowing the white sheet to
be visible. The top electrode may be masked so that the clustered
particles are not visible. Additionally, the backing sheet could be
replaced with a backlight or color filter and backlight. In another
embodiment, a brief alternating voltage signal may be used prior to
addressing methods described above to mix the particles into a
random order.
[0070] While the methods described discuss particles, any
combination of dyes, liquids droplets and transparent regions that
respond to electrophoretic effects could also be used. Particles of
various optical effects may be combined in any suitable proportion.
For example, certain colors may be over- or under-populated in the
electrophoretic suspension to account for the sensitivities of the
human eye and to thereby achieve a more pleasing or uniform effect.
Similarly, the sizes of the sub-pixels may also be disproportionate
to achieve various optical effects.
[0071] Although these examples describe microencapsulated
electrophoretic displays, the invention can be utilized across
other reflective displays including liquid crystal,
polymer-dispersed liquid crystal, rotating ball, suspended particle
and any other reflective display capable of imaging three colors.
For example, a bichromal rotating ball (or pyramid, cube, etc.)
could be split into regions of multiple colors. One way to address
such a display element would be to provide differing charge
characteristics (such as charged vertices in the case of the
pyramid) and to use various combinations and sequences of electrode
voltage potentials across the surrounding top, bottom, or side
electrodes to rotate the shape in a desired manner. In short, many
addressing schemes are possible by which a sub-pixel in a direct
color reflective display could be switched among three colors. Such
switching mechanism will vary by the nature of the display and any
suitable means may be used.
[0072] One embodiment of the invention is to separate each pixel
into three sub-pixels, each sub-pixel being capable of displaying
three color states, and to choose as the color state combinations a
first sub-pixel being capable of displaying white, cyan or red, a
second sub-pixel being capable of displaying white, magenta or
green, and a third sub-pixel being capable of displaying white,
yellow or blue. As has already been explained, for a reflective
display, black can be displayed with the three sub-pixels turned to
red, green and blue, respectively. This display achieves a more
saturated black than is achieved under the two-state system.
Alternatively, red is displayed with the sub-pixels turned to red,
magenta and yellow, respectively, which offers a more saturated red
than is obtained with a two-state system. Other colors may be
obtained by suitable choices of the individual states of the
sub-pixels.
[0073] Another embodiment of the invention is to separate each
pixel into three sub-pixels, each sub-pixel being capable of
displaying three color states, and to choose as the color state
combinations a first sub-pixel being capable of displaying white,
cyan or black, a second sub-pixel being capable of displaying
white, magenta or black, and a third sub-pixel being capable of
displaying white, yellow or black. In this embodiment, black and
white are displayed directly with high saturation. For example, to
display red, the first (cyan) sub-pixel is set to either white or
black to achieve a dimmer or brighter color, respectively, the
second sub-pixel is set to magenta, and the last sub-pixel is set
to yellow.
[0074] Another embodiment of the invention is to separate each
pixel into three sub-pixels, each sub-pixel being capable of
displaying three color states, and to choose as the color state
combinations a first sub-pixel being capable of displaying white,
red or black, a second sub-pixel being capable of displaying white,
green or black, and a third sub-pixel being capable of displaying
white, blue or black. In this embodiment, black and white are
displayed directly with high saturation. For example, to display
red, the first sub-pixel is set to red, and the second and the
third sub-pixels are set to either white or black to achieve a
dimmer or brighter color, respectively.
[0075] While the embodiments above describe a pixel of three
sub-pixels, each sub-pixel having three possible color states, the
invention is embodied by any pixel containing two or more
sub-pixels, where at least one sub-pixel can achieve three or more
colors. In this manner a better effect can be achieved for
reflective displays than can be achieved by adopting the simple
two-state sub-pixel color change technique that is common to
emissive displays.
[0076] Additionally, the invention can be extended to four or more
color states to permit full color displays without the need for
sub-pixels, and illustrates addressing mechanisms that work for
three states and which can be extended or combined to achieve a
display with four or more states.
[0077] Another means of generating color in a microencapsulated
display medium is the use of color filters in conjunction with a
contrast-generating optical element. One manifestation of this
technique is to use a pixel element which switches between white
and black. This, in conjunction with the color filter, allows for
switching between a light and dark colored state to occur. However,
it is known to those skilled in the art that different numbers of
color filters (ranging from one to three) can be used in a
sub-pixel, depending on how many colors are desired. Also, the
microencapsulated particle display can switch between colors other
than white and black. In this case, it is advantageous to use a
color filter which is opposed (in a color sense) to the color of
the pixel. For example, a yellow color filter used with a blue or
white electrophoretic display would result in a green or yellow
color to that element.
[0078] Additionally, there is an electrophoretic device known as a
"shutter mode" display, in which particles are switched
electrically between a widely-dispersed state on one electrode and
a narrow band on the other electrode. Such a device can act as a
transmissive light valve or reflective display. Color filters can
be used with such a device.
[0079] Referring now to FIGS. 1A and 1B, an addressing scheme for
controlling particle-based displays is shown in which electrodes
are disposed on only one side of a display, allowing the display to
be rear-addressed. Utilizing only one side of the display for
electrodes simplifies fabrication of displays. For example, if the
electrodes are disposed on only the rear side of a display, both of
the electrodes can be fabricated using opaque materials, which may
be colored, because the electrodes do not need to be
transparent.
[0080] FIG. 1A depicts a single capsule 20 of an encapsulated
display media. In brief overview, the embodiment depicted in FIG.
1A includes a capsule 20 containing at least one particle 50
dispersed in a suspending fluid 25. The capsule 20 is addressed by
a first electrode 30 and a second electrode 40. The first electrode
30 is smaller than the second electrode 40. The first electrode 30
and the second electrode 40 may be set to voltage potentials which
affect the position of the particles 50 in the capsule 20.
[0081] The particles 50 represent 0.1% to 20% of the volume
enclosed by the capsule 20. In some embodiments the particles 50
represent 2.5% to 17.5% of the volume enclosed by capsule 20. In
preferred embodiments, the particles 50 represent 5% to 15% of the
volume enclosed by the capsule 20. In more preferred embodiments
the particles 50 represent 9% to 11% of the volume defined by the
capsule 20. In general, the volume percentage of the capsule 20
that the particles 50 represent should be selected so that the
particles 50 expose most of the second, larger electrode 40 when
positioned over the first, smaller electrode 30. As described in
detail below, the particles 50 may be colored any one of a number
of colors. The particles 50 may be either positively charged or
negatively charged.
[0082] The particles 50 are dispersed in a dispersing fluid 25. The
dispersing fluid 25 should have a low dielectric constant. The
fluid 25 may be clear, or substantially clear, so that the fluid 25
does not inhibit viewing the particles 50 and the electrodes 30, 40
from position 10. In other embodiments, the fluid 25 is dyed. In
some embodiments the dispersing fluid 25 has a specific gravity
matched to the density of the particles 50. These embodiments can
provide a bistable display media, because the particles 50 do not
tend to move in certain compositions absent an electric field
applied via the electrodes 30, 40.
[0083] The electrodes 30, 40 should be sized and positioned
appropriately so that together they address the entire capsule 20.
There may be exactly one pair of electrodes 30, 40 per capsule 20,
multiple pairs of electrodes 30, 40 per capsule 20, or a single
pair of electrodes 30, 40 may span multiple capsules 20. In the
embodiment shown in FIGS. 1A and 1B, the capsule 20 has a
flattened, rectangular shape. In these embodiments, the electrodes
30, 40 should address most, or all, of the flattened surface area
adjacent the electrodes 30, 40. The smaller electrode 30 is at most
one-half the size of the larger electrode 40. In preferred
embodiments the smaller electrode is one-quarter the size of the
larger electrode 40; in more preferred embodiments the smaller
electrode 30 is one-eighth the size of the larger electrode 40. In
even more preferred embodiments, the smaller electrode 30 is
one-sixteenth the size of the larger electrode 40. It should be
noted that reference to "smaller" in connection with the electrode
30 means that the electrode 30 addresses a smaller amount of the
surface area of the capsule 20, not necessarily that the electrode
30 is physically smaller than the larger electrode 40. For example,
multiple capsules 20 may be positioned such that less of each
capsule 20 is addressed by the "smaller" electrode 30, even though
both electrodes 30, 40 are equal in size. It should also be noted
that, as shown in FIG. 1C, electrode 30 may address only a small
corner of a rectangular capsule 20 (shown in phantom view in FIG.
1C), requiring the larger electrode 40 to surround the smaller
electrode 30 on two sides in order to properly address the capsule
20. Selection of the percentage volume of the particles 50 and the
electrodes 30, 40 in this manner allow the encapsulated display
media to be addressed as described below.
[0084] Electrodes may be fabricated from any material capable of
conducting electricity so that electrode 30, 40 may apply an
electric field to the capsule 20. As noted above, the
rear-addressed embodiments depicted in FIGS. 1A and 1B allow the
electrodes 30, 40 to be fabricated from opaque materials such as
solder paste, copper, copper-clad polyimide, graphite inks, silver
inks and other metal-containing conductive inks. Alternatively,
electrodes may be fabricated using transparent materials such as
indium tin oxide and conductive polymers such as polyaniline or
polythiophenes. Electrodes 30, 40 may be provided with contrasting
optical properties. In some embodiments, one of the electrodes has
an optical property complementary to optical properties of the
particles 50. Alternatively, since the electrodes need not be
transparent, an electrode can be constructed so as to display a
selected color.
[0085] In one embodiment, the capsule 20 contains positively
charged black particles 50, and a substantially clear suspending
fluid 25. The first, smaller electrode 30 is colored black, and is
smaller than the second electrode 40, which is colored white or is
highly reflective. When the smaller, black electrode 30 is placed
at a negative voltage potential relative to larger, white electrode
40, the positively-charged particles 50 migrate to the smaller,
black electrode 30. The effect to a viewer of the capsule 20
located at position 10 is a mixture of the larger, white electrode
40 and the smaller, black electrode 30, creating an effect which is
largely white. Referring to FIG. 1B, when the smaller, black
electrode 30 is placed at a positive voltage potential relative to
the larger, white electrode 40, particles 50 migrate to the larger,
white electrode 40 and the viewer is presented a mixture of the
black particles 50 covering the larger, white electrode 40 and the
smaller, black electrode 30, creating an effect which is largely
black. In this manner the capsule 20 may be addressed to display
either a white visual state or a black visual state.
[0086] Other two-color schemes are easily provided by varying the
color of the smaller electrode 30 and the particles 50 or by
varying the color of the larger electrode 40. For example, varying
the color of the larger electrode 40 allows fabrication of a
rear-addressed, two-color display having black as one of the
colors. Alternatively, varying the color of the smaller electrode
30 and the particles 50 allow a rear-addressed two-color system to
be fabricated having white as one of the colors. Further, it is
contemplated that the particles 50 and the smaller electrode 30 can
be different colors. In these embodiments, a two-color display may
be fabricated having a second color that is different from the
color of the smaller electrode 30 and the particles 50. For
example, a rear-addressed, orange-white display may be fabricated
by providing blue particles 50, a red, smaller electrode 30, and a
white (or highly reflective) larger electrode 40. In general, the
optical properties of the electrodes 30, 40 and the particles 50
can be independently selected to provide desired display
characteristics. In some embodiments the optical properties of the
dispersing fluid 25 may also be varied, e.g. the fluid 25 may be
dyed.
[0087] In another embodiment, this technique may be used to provide
a full color display. Referring now to FIG. 1D, a pixel embodiment
is depicted that comprises three sub-pixels. It should be
understood that although FIG. 1D depicts a hexagonal pixel having
equally-sized sub-pixels, a pixel may have any shape and may be
comprised of unequal sub-pixels. The sub-pixels may each be
contained in a single large capsule, or each may be distributed
across any number of small microcapsules or microcells. For the
purposed of illustration, the simpler case of a single large
sub-cell for each sub-pixel is shown. In both cases we refer to the
regions, 20, 20', 20'', as capsules. Thus, a first capsule 20
contains positively charged black particles 50 and a substantially
clear suspending fluid 25. A first, smaller electrode 30 is colored
black, and is smaller than the second electrode 40, which is
colored red. When the smaller, black electrode 30 is placed at a
negative voltage potential relative to larger, red electrode 40,
the positively-charged particles 50 migrate to the smaller, black
electrode 30. The effect to a viewer of the capsule 20 located at
position 10 is a mixture of the larger, red electrode 40 and the
smaller, black electrode 30, creating an effect which is largely
red. When the smaller, black electrode 30 is placed at a positive
voltage potential relative to the larger, red electrode 40,
particles 50 migrate to the larger, red electrode 40 and the viewer
is presented a mixture of the black particles 50 covering the
larger, red electrode 40 and the smaller, black electrode 30,
creating an effect which is largely black. In this manner the first
capsule 20 may be addressed to display either a red visual state or
a black visual state. One can equally have a second capsule 20'
wherein the larger electrode 40' is green, and a third capsule 20''
wherein the larger electrode 40'' is blue. A second capsule 20'
contains positively charged black particles 50' and a substantially
clear suspending fluid 25'. A first, smaller electrode 30' is
colored black, and is smaller than the second electrode 40', which
is colored green. When the smaller, black electrode 30' is placed
at a negative voltage potential relative to larger, green electrode
40', the positively-charged particles 50' migrate to the smaller,
black electrode 30'. The effect to a viewer of the capsule 20'
located at position 10' is a mixture of the larger, green electrode
40' and the smaller, black electrode 30', creating an effect which
is largely green. When the smaller, black electrode 30' is placed
at a positive voltage potential relative to the larger, green
electrode 40', particles 50' migrate to the larger, green electrode
40' and the viewer is presented a mixture of the black particles
50' covering the larger, green electrode 40' and the smaller, black
electrode 30', creating an effect which is largely black.
Similarly, a third capsule 20'' contains positively charged black
particles 50'' and a substantially clear suspending fluid 25''. A
first, smaller electrode 30'' is colored black, and is smaller than
the second electrode 40'', which is colored blue. When the smaller,
black electrode 30'' is placed at a negative voltage potential
relative to larger, blue electrode 40'', the positively-charged
particles 50'' migrate to the smaller, black electrode 30''. The
effect to a viewer of the capsule 20'' located at position 10'' is
a mixture of the larger, blue electrode 40'' and the smaller, black
electrode 30'', creating an effect which is largely blue. When the
smaller, black electrode 30'' is placed at a positive voltage
potential relative to the larger, blue electrode 40'', particles
50'' migrate to the larger, blue electrode 40'' and the viewer is
presented a mixture of the black particles 50'' covering the
larger, blue electrode 40'' and the smaller, black electrode 30'',
creating an effect which is largely black. Further, the relative
intensities of these colors can be controlled by the actual voltage
potentials applied to the electrodes. By choosing appropriate
combinations of the three colors, one may create a visual display
which appears as the effective combination of the selected colors
as an additive process. As an alternative embodiment, the first,
second and third capsules can have larger electrodes 40, 40', 40''
which are respectively colored cyan, yellow, and magenta. Operation
of the alternative cyan, yellow, and magenta embodiment is
analogous to that of the red, green, and blue embodiment, with the
feature that the color to be displayed is selected by a subtractive
process.
[0088] In other embodiments the larger electrode 40 may be
reflective instead of white. In these embodiments, when the
particles 50 are moved to the smaller electrode 30, light reflects
off the reflective surface 60 associated with the larger electrode
40 and the capsule 20 appears light in color, e.g. white (see FIG.
2A). When the particles 50 are moved to the larger electrode 40,
the reflecting surface 60 is obscured and the capsule 20 appears
dark (see FIG. 2B) because light is absorbed by the particles 50
before reaching the reflecting surface 60. The reflecting surface
60 for the larger electrode 40 may possess retroreflective
properties, specular reflection properties, diffuse reflective
properties or gain reflection properties. In certain embodiments,
the reflective surface 60 reflects light with a Lambertian
distribution The surface 60 may be provided as a plurality of glass
spheres disposed on the electrode 40, a diffractive reflecting
layer such as a holographically formed reflector, a surface
patterned to totally internally reflect incident light, a
brightness-enhancing film, a diffuse reflecting layer, an embossed
plastic or metal film, or any other known reflecting surface. The
reflecting surface 60 may be provided as a separate layer laminated
onto the larger electrode 40 or the reflecting surface 60 may be
provided as a unitary part of the larger electrode 40. In the
embodiments depicted by FIGS. 2C and 2D, the reflecting surface may
be disposed below the electrodes 30, 40 vis-a-vis the viewpoint 10.
In these embodiments, electrode 30 should be transparent so that
light may be reflected by surface 60. In other embodiments, proper
switching of the particles may be accomplished with a combination
of alternating-current (AC) and direct-current (DC) electric fields
and described below in connection with FIGS. 3A-3D.
[0089] In still other embodiments, the rear-addressed display
previously discussed can be configured to transition between
largely transmissive and largely opaque modes of operation
(referred to hereafter as "shutter mode"). Referring back to FIGS.
1A and 1B, in these embodiments the capsule 20 contains at least
one positively-charged particle 50 dispersed in a substantially
clear dispersing fluid 25. The larger electrode 40 is transparent
and the smaller electrode 30 is opaque. When the smaller, opaque
electrode 30 is placed at a negative voltage potential relative to
the larger, transmissive electrode 40, the particles 50 migrate to
the smaller, opaque electrode 30. The effect to a viewer of the
capsule 20 located at position 10 is a mixture of the larger,
transparent electrode 40 and the smaller, opaque electrode 30,
creating an effect which is largely transparent. Referring to FIG.
1B, when the smaller, opaque electrode 30 is placed at a positive
voltage potential relative to the larger, transparent electrode 40,
particles 50 migrate to the second electrode 40 and the viewer is
presented a mixture of the opaque particles 50 covering the larger,
transparent electrode 40 and the smaller, opaque electrode 30,
creating an effect which is largely opaque. In this manner, a
display formed using the capsules depicted in FIGS. 1A and 1B may
be switched between transmissive and opaque modes. Such a display
can be used to construct a window that can be rendered opaque.
Although FIGS. 1A-2D depict a pair of electrodes associated with
each capsule 20, it should be understood that each pair of
electrodes may be associated with more than one capsule 20.
[0090] A similar technique may be used in connection with the
embodiment of FIGS. 3A, 3B, 3C, and 3D. Referring to FIG. 3A, a
capsule 20 contains at least one dark or black particle 50
dispersed in a substantially clear dispersing fluid 25. A smaller,
opaque electrode 30 and a larger, transparent electrode 40 apply
both direct-current (DC) electric fields and alternating-current
(AC) fields to the capsule 20. A DC field can be applied to the
capsule 20 to cause the particles 50 to migrate towards the smaller
electrode 30. For example, if the particles 50 are positively
charged, the smaller electrode is placed a voltage that is more
negative than the larger electrode 40. Although FIGS. 3A-3D depict
only one capsule per electrode pair, multiple capsules may be
addressed using the same electrode pair.
[0091] The smaller electrode 30 is at most one-half the size of the
larger electrode 40. In preferred embodiments the smaller electrode
is one-quarter the size of the larger electrode 40; in more
preferred embodiments the smaller electrode 30 is one-eighth the
size of the larger electrode 40. In even more preferred
embodiments, the smaller electrode 30 is one-sixteenth the size of
the larger electrode 40.
[0092] Causing the particles 50 to migrate to the smaller electrode
30, as depicted in FIG. 3A, allows incident light to pass through
the larger, transparent electrode 40 and be reflected by a
reflecting surface 60. In shutter mode, the reflecting surface 60
is replaced by a translucent layer, a transparent layer, or a layer
is not provided at all, and incident light is allowed to pass
through the capsule 20, i.e. the capsule 20 is transmissive. If the
translucent layer or the transparent layer comprises a color, such
as a color filter, the light which is transmitted will be those
wavelengths that the filter passes, and the reflected light will
consist of those wavelengths that the filter reflects, while the
wavelengths that the filter absorbs will be lost. The visual
appearance of a shutter mode display may thus depend on whether the
display is in a transmissive or reflective condition, on the
characteristics of the filter, and on the position of the
viewer.
[0093] Referring now to FIG. 3B, the particles 50 are dispersed
into the capsule 20 by applying an AC field to the capsule 20 via
the electrodes 30, 40. The particles 50, dispersed into the capsule
20 by the AC field, block incident light from passing through the
capsule 20, causing it to appear dark at the viewpoint 10. The
embodiment depicted in FIGS. 3A-3B may be used in shutter mode by
not providing the reflecting surface 60 and instead providing a
translucent layer, a transparent layer, a color filter layer, or no
layer at all. In shutter mode, application of an AC electric field
causes the capsule 20 to appear opaque. The transparency of a
shutter mode display formed by the apparatus depicted in FIGS.
3A-3D may be controlled by the number of capsules addressed using
DC fields and AC fields. For example, a display in which every
other capsule 20 is addressed using an AC field would appear fifty
percent transmissive.
[0094] FIGS. 3C and 3D depict an embodiment of the electrode
structure described above in which electrodes 30, 40 are on "top"
of the capsule 20, that is, the electrodes 30, 40 are between the
viewpoint 10 and the capsule 20. In these embodiments, both
electrodes 30, 40 should be transparent. Transparent polymers can
be fabricated using conductive polymers, such as polyaniline,
polythiophenes, or indium tin oxide. These materials may be made
soluble so that electrodes can be fabricated using coating
techniques such as spin coating, spray coating, meniscus coating,
printing techniques, forward and reverse roll coating and the like.
In these embodiments, light passes through the electrodes 30, 40
and is either absorbed by the particles 50, reflected by
retroreflecting layer 60 (when provided), transmitted throughout
the capsule 20 (when retroreflecting layer 60 is not provided), or
partially transmitted and/or reflected if a color filter is present
in place of retroreflecting layer 60.
[0095] Referring to FIG. 3E, three sub-pixel capsules 22, 22' and
22'' each contain at least one white particle 50 dispersed in a
substantially clear dispersing fluid 25. In one embodiment, each
sub-pixel capsule 22, 22' and 22'' has a transparent electrode 42,
42', and 42'' disposed above it and a colored filter 60, 60' and
60'' disposed below it. A common reflective surface 70 may be
shared behind the color filter layer. In an alternative embodiment,
the display includes an emissive light source 70.
[0096] Smaller, opaque electrodes 30, 30' and 30'' and Larger,
transparent electrodes 40, 40' and 40'' may apply direct-current
(DC) electric fields and alternating-current (AC) fields to the
capsules 20, 20' and 20''. A DC field can be applied to the
capsules 20, 20' and 20'' to cause the particles 50, 50' 50'' to
migrate towards the smaller electrodes 30, 30' and 30''. For
example, if the particles 50, 50' and 50'' are positively charged,
the smaller electrodes 30, 30' and 30'' are placed a voltage that
is more negative than the larger electrodes 40, 40' and 40''.
[0097] The smaller electrode 30 is at most one-half the size of the
larger electrode 40. In preferred embodiments the smaller electrode
30 is one-quarter the size of the larger electrode 40; in more
preferred embodiments the smaller electrode 30 is one-eighth the
size of the larger electrode 40. In even more preferred
embodiments, the smaller electrode 30 is one-sixteenth the size of
the larger electrode 40.
[0098] Causing the particles 50 to migrate to the smaller electrode
30, as depicted in the first two capsules of FIG. 3E, allows
incident light to pass through the larger, transparent electrode 40
filter 60 and reflect off substrate 70. If the first, second and
third filters 60, 60' and 60'' are colored cyan, magenta, and
yellow respectively, and the particles 50 are white, this system
can display full color in a standard two-color fashion.
[0099] The filter layer 60 may be a translucent layer, a
transparent layer, a color filter layer, or a layer is not provided
at all, and further substrate 70 may be reflective, emissive,
translucent or not provided at all. If the layer 60 comprises a
color, such as a color filter, the light which is transmitted will
be those wavelengths that the filter passes, and the reflected
light will consist of those wavelengths that the filter reflects,
while the wavelengths that the filter absorbs will be lost. The
visual appearance of a the display element in 3E may thus depend on
whether the display is in a transmissive or reflective condition,
on the characteristics of the filter, and on the position of the
viewer. In an alternative embodiment layer 60 may be provided on
top of the capsule adjacent to electrode 42.
[0100] Referring now to FIGS. 3F-3K, one embodiment of a tri-color
pixel is described. Clear electrode 42 allows light to pass into
capsule 22 and to strike either white particles W, red particles R,
or a colored substrate 60. The substrate 60 can be a combination of
color filter and non-colored substrate or it can be provided as a
unitary colored substrate. Capsule 22 also includes a suspending
fluid that can be dye-colored (possibly eliminating the need for a
separate color filter 60) or substantially clear. Electrodes 45 and
35 are transparent and may be equally sized or sized in any
suitable manner taking into account the relative particles sizes
and mobilities of particles W and R. A gap exists between 45 and
35. Assume that particles W are negatively charged and particles R
are positively charged. In FIG. 3F, top electrode 42 is set at a
positive voltage potential relative to bottom electrodes 35 and 45,
moving particles W to the top and particles R to the bottom and
thus white is displayed. In FIG. 3G by reversing the polarity of
the electrodes, red is displayed. In both FIGS. 3F and 3G the
particles obscure substrate 60. In FIG. 3H electrode 45 is at a
negative voltage potential relative to electrode 35, while
electrode 42 is at a voltage potential between the potentials of 45
and 35, such as zero. Alternatively, electrode 42 switches between
the potentials of 45 and 35 so that over time the effective voltage
of 42 is again between the potentials of 45 and 35. In this state,
the particles R move toward electrode 45 and the particles W move
toward electrode 35 and both particles R and W move away from the
gap in the center of the capsule 22. This reveals substrate 60,
permitting a third color such as cyan to be imaged. In alternate
embodiments the color combinations can differ. The specific colors
of the filters and particles need not differ. This system, called
"dual particle curtain mode," can image three arbitrary colors. In
a preferred embodiment the colors are as described wherein one
color is white and the other two colors are complements. In this
manner, referring again to FIG. 3H, if a small portion of red is
visible it absorbs part of the light reflected from the cyan
substrate and the net result is black, which may be offset by a
small portion of visible white. Thus, the pixel in FIG. 3H may
appear to be cyan even if some red and white is visible. As
mentioned above, the edges of the pixel may be masked to hide
particles R and W when in the mode shown in FIG. 3H.
[0101] Referring now to FIG. 3I, a full-color pixel is shown
comprising three sub-pixels, each operating in the manner taught by
FIGS. 3F-3H wherein the colored particles are positively charged,
and the white particles are negatively charged. The system may
still function with top electrode 42 extended as a common top
electrode as shown in FIG. 3I. For example, to achieve the state
shown, electrodes 42, 45, 35, 45', 35', 45'', 35'' may be set to
voltage potentials -30V, 60V, 60V, -60V, +60V, -60V, +60V
respectively.
[0102] Referring now to FIGS. 3J-3K, an electrode scheme is shown
whereby a cluster of microcapsules may be addressed for an entire
sub-pixel in a manner similar to those described above. Clear
electrode 42 allows light to pass into microcapsules 27 and to
strike either white particles W, red particles R, or colored
substrate 60. As above, colored substrate 60 may be a combination
of color filter and non-colored substrate 60 or colored substrate
60 may be provided as a unitary colored substrate. Capsules 27
include a suspending fluid that may be dye-colored (possibly
eliminating the need for a separate color filter 60) or
substantially clear. Electrodes 45 and 35 are transparent and may
be equally sized or sized in any suitable manner taking into
account the relative particle sizes and mobilities of particles W
and R. A gap exists between 45 and 35. Assume that particles W are
negatively charged and particles R are positively charged. The
system operates in the manner described in FIGS. 3F-3K, although
for any given microcapsule 27 there may be multiple gaps. FIG. 3K
illustrates an embodiment of a suitable electrode pattern in which
45 and 35 are interdigitated.
[0103] Referring now to 3L-3M, an alternate embodiment is shown.
Again clear electrode 42 allows light to pass into capsule 22 and
to strike white particles W or red particles R. In the embodiment
shown in FIG. 3L, capsule 22 includes a suspending fluid 62 that is
dyed cyan. When electrodes 45 and 35 are set at appropriate
voltages particles, R and W move down to electrodes 45 and 35
respectively, where they are obscured by light-absorbing suspending
fluid 62. Alternatively, as shown in FIG. 3M, suspending fluid 62
is substantially clear and a third species of cyan particles C is
included in capsules 22. The cyan particles have a relatively
neutral charge. When electrodes 45 and 35 are set at appropriate
voltages particles R and W move down to electrodes 45 and 35
respectively, revealing the cyan particles.
[0104] The addressing structure depicted in FIGS. 3A-3M may be used
with electrophoretic display media and encapsulated electrophoretic
display media. FIGS. 3A-3M depict embodiments in which electrode
30, 40 are statically attached to the display media. In certain
embodiments, the particles 50 exhibit bistability, that is, they
are substantially motionless in the absence of a electric
field.
[0105] While various of the substrates described above are
reflective, an analogous technique may be employed wherein the
substrates emit light, with the particles again acting in a
"shutter mode" to reveal or obscure light. A preferred substrate
for this use is an electroluminescent (EL) backlight. Such a
backlight can be reflective when inactive, often with a
whitish-green color, yet emit lights in various wavelengths when
active. By using whitish EL substrates in place of static white
reflective substrates, it is possible to construct a full-color
reflective display that can also switch its mode of operation to
display a range of colors in an emissive state, permitting
operation in low ambient light conditions.
[0106] FIGS. 4A and 4B depict an embodiment of a rear-addressing
electrode structure that creates a reflective color display in a
manner similar to half-toning or pointillism. The capsule 20
contains white particles 55 dispersed in a clear suspending fluid
25. Electrodes 42, 44, 46, 48 are colored cyan, magenta, yellow,
and white respectively. Referring to FIG. 4A, when the colored
electrodes 42, 44, 46 are placed at a positive potential relative
to the white electrode 48, negatively-charged particles 55 migrate
to these three electrodes, causing the capsule 20 to present to the
viewpoint 10 a mix of the white particles 55 and the white
electrode 48, creating an effect which is largely white. Referring
to FIG. 4B, when electrodes 42, 44, 46 are placed at a negative
potential relative to electrode 48, particles 55 migrate to the
white electrode 48, and the eye 10 sees a mix of the white
particles 55, the cyan electrode 42, the magenta electrode 44, and
the yellow electrode 46, creating an effect which is largely black
or gray. By addressing the electrodes, any color can be produced
that is possible with a subtractive color process. For example, to
cause the capsule 20 to display a red color to the viewpoint 10,
the yellow electrode 46 and the magenta electrode 42 are set to a
voltage potential that is more positive than the voltage potential
applied by the cyan electrode 42 and the white electrode 48.
Further, the relative intensities of these colors can be controlled
by the actual voltage potentials applied to the electrodes. Again,
AC current may be used appropriately to randomize the position of
the particles as a step in this process.
[0107] The technique used in FIGS. 4A and 4B could be used in a
similar manner with fewer electrodes and controlling fewer colors.
For example, if electrode 42 were not present, the pixel could
still display three colors. If electrodes 44 and 46 were colored
red and cyan respectively, the capsule could display red, cyan and
white. This construction could be used then employed as a
sub-pixel, to be matched with similar sub-pixels displaying other
trios of colors thus achieving a full-color display as described
above.
[0108] In another embodiment, depicted in FIG. 5, a color display
is provided by a capsule 20 of size d containing multiple species
of particles in a clear, dispersing fluid 25. Each species of
particles has different optical properties and possess different
electrophoretic mobilities (.mu.) from the other species. In the
embodiment depicted in FIG. 5, the capsule 20 contains red
particles 52, blue particles 54, and green particles 56, and [0109]
|.mu..sub.R||.mu..sub.B||.mu..sub.G| That is, the magnitude of the
electrophoretic mobility of the red particles 52, on average,
exceeds the electrophoretic mobility of the blue particles 54, on
average, and the electrophoretic mobility of the blue particles 54,
on average, exceeds the average electrophoretic mobility of the
green particles 56. As an example, there may be a species of red
particle with a zeta potential of 100 millivolts (mV), a blue
particle with a zeta potential of 60 mV, and a green particle with
a zeta potential of 20 mV. The capsule 20 is placed between two
electrodes 32, 42 that apply an electric field to the capsule. By
addressing the capsule 20 with positive and negative voltage fields
of varying time durations, it is possible to move any of the
various particle species to the top of the capsule to present a
certain color.
[0110] FIGS. 6A-6B depict the steps to be taken to address the
display shown in FIG. 5 to display a red color to a viewpoint 10.
Referring to FIG. 6A, all the particles 52, 54, 56 are attracted to
one side of the capsule 20 by applying an electric field in one
direction. The electric field should be applied to the capsule 20
long enough to attract even the more slowly moving green particles
56 to the electrode 34. Referring to FIG. 6B, the electric field is
reversed just long enough to allow the red particles 52 to migrate
towards the electrode 32. The blue particles 54 and green particles
56 will also move in the reversed electric field, but they will not
move as fast as the red particles 52 and thus will be obscured by
the red particles 52. The amount of time for which the applied
electric field must be reversed can be determined from the relative
electrophoretic mobilities of the particles, the strength of the
applied electric field, and the size of the capsule.
[0111] FIGS. 7A-7D depict addressing the display element to a blue
state. As shown in FIG. 7A, the particles 52, 54, 56 are initially
randomly dispersed in the capsule 20. All the particles 52, 54, 56
are attracted to one side of the capsule 20 by applying an electric
field in one direction (shown in FIG. 7B). Referring to FIG. 7C,
the electric field is reversed just long enough to allow the red
particles 52 and blue particles 54 to migrate towards the electrode
32. The amount of time for which the applied electric field must be
reversed can be determined from the relative electrophoretic
mobilities of the particles, the strength of the applied electric
field, and the size of the capsule. Referring to FIG. 7D, the
electric field is then reversed a second time and the red particles
52, moving faster than the blue particles 54, leave the blue
particles 54 exposed to the viewpoint 10. The amount of time for
which the applied electric field must be reversed can be determined
from the relative electrophoretic mobilities of the particles, the
strength of the applied electric field, and the size of the
capsule.
[0112] FIGS. 8A-8C depict the steps to be taken to present a green
display to the viewpoint 10. As shown in FIG. 8A, the particles 52,
54, 56 are initially distributed randomly in the capsule 20. All
the particles 52, 54, 56 are attracted to the side of the capsule
20 proximal the viewpoint 10 by applying an electric field in one
direction. The electric field should be applied to the capsule 20
long enough to attract even the more slowly moving green particles
56 to the electrode 32. As shown in FIG. 8C, the electric field is
reversed just long enough to allow the red particles 52 and the
blue particles 54 to migrate towards the electrode 54, leaving the
slowly-moving green particles 56 displayed to the viewpoint. The
amount of time for which the applied electric field must be
reversed can be determined from the relative electrophoretic
mobilities of the particles, the strength of the applied electric
field, and the size of the capsule.
[0113] In other embodiments, the capsule contains multiple species
of particles and a dyed dispersing fluid that acts as one of the
colors. In still other embodiments, more than three species of
particles may be provided having additional colors. In one of these
embodiments, the capsule contains white particles which have a
strong positive charge, cyan particles which have a weakly positive
charge, and red particles having a negative charge. Since the
electrophoretic mobilities of these types of particles will be
proportional to charge and of a direction related to the sign or
polarity of the charge, these three types of particles will have
different mobilities in the same voltage field. In this example,
white is achieved when the top electrode is negative and the bottom
electrode is positive. Red is achieved when the top electrode is
positive and the bottom electrode is negative. Cyan is achieved by
first setting the sub-pixel to white and then briefly reversing the
voltage field so that the higher mobility white particles migrate
past the cyan particles and the lower mobility, or slower, cyan
particles remain topmost and visible. Although FIGS. 6-8C depict
two electrodes associated with a single capsule, the electrodes may
address multiple capsules or less than a full capsule.
[0114] The addressing structures described in FIGS. 1-8 typically
comprise a top electrode controlled by display driver circuitry. It
may be seen that if the top electrode is absent, the display may be
imaged by an externally applied voltage source, such as a passing
stylus or electrostatic print head. The means that techniques
applied above to generate a full-color electrophoretic display
could also be applied for a full-color electrophoretic media.
[0115] In FIG. 9, the rear electrode structure can be made entirely
of printed layers. A conductive layer 166 can be printed onto the
back of a display comprised of a clear, front electrode 168 and a
printable display material 170. A clear electrode may be fabricated
from indium tin oxide or conductive polymers such as polyanilines
and polythiophenes. A dielectric coating 176 can be printed leaving
areas for vias. Then, the back layer of conductive ink 178 can be
printed. If necessary, an additional layer of conductive ink can be
used before the final ink structure is printed to fill in the
holes.
[0116] This technique for printing displays can be used to build
the rear electrode structure on a display or to construct two
separate layers that are laminated together to form the display.
For example an electronically active ink may be printed on an
indium tin oxide electrode. Separately, a rear electrode structure
as described above can be printed on a suitable substrate, such as
plastic, polymer films, or glass. The electrode structure and the
display element can be laminated to form a display.
[0117] Referring now to FIG. 10, a threshold may be introduced into
an electrophoretic display cell by the introduction of a third
electrode. One side of the cell is a continuous, transparent
electrode 200 (anode). On the other side of the cell, the
transparent electrode is patterned into a set of isolated column
electrode strips 210. An insulator 212 covers the column electrodes
210, and an electrode layer on top of the insulator is divided into
a set of isolated row electrode strips 230, which are oriented
orthogonal to the column electrodes 210. The row electrodes 230 are
patterned into a dense array of holes, or a grid, beneath which the
exposed insulator 212 has been removed, forming a multiplicity of
physical and potential wells.
[0118] A positively charged particle 50 is loaded into the
potential wells by applying a positive potential (e.g. 30V) to all
the column electrodes 210 while keeping the row electrodes 230 at a
less positive potential (e.g. 15V) and the anode 200 at zero volts.
The particle 50 may be a conformable capsule that situates itself
into the physical wells of the control grid. The control grid
itself may have a rectangular cross-section, or the grid structure
may be triangular in profile. It can also be a different shape
which encourages the microcapsules to situate in the grid, for
example, hemispherical.
[0119] The anode 200 is then reset to a positive potential (e.g.
50V). The particle will remain in the potential wells due to the
potential difference in the potential wells: this is called the
Hold condition. To address a display element the potential on the
column electrode associated with that element is reduced, e.g. by a
factor of two, and the potential on the row electrode associated
with that element is made equal to or greater than the potential on
the column electrode. The particles in this element will then be
transported by the electric field due to the positive voltage on
the anode 200. The potential difference between row and column
electrodes for the remaining display elements is now less than half
of that in the normal Hold condition. The geometry of the potential
well structure and voltage levels are chosen such that this also
constitutes a Hold condition, i.e., no particles will leave these
other display elements and hence there will be no half-select
problems. This addressing method can select and write any desired
element in a matrix without affecting the pigment in any other
display element. A control electrode device can be operated such
that the anode electrode side of the cell is viewed.
[0120] The control grid may be manufactured through any of the
processes known in the art, or by several novel processes described
herein. That is, according to traditional practices, the control
grid may be constructed with one or more steps of photolithography
and subsequent etching, or the control grid may be fabricated with
a mask and a "sandblasting" technique.
[0121] In another embodiment, the control grid is fabricated by an
embossing technique on a plastic substrate. The grid electrodes may
be deposited by vacuum deposition or sputtering, either before or
after the embossing step. In another embodiment, the electrodes are
printed onto the grid structure after it is formed, the electrodes
consisting of some kind of printable conductive material which need
not be clear (e.g. a metal or carbon-doped polymer, an
intrinsically conducting polymer, etc.).
[0122] In a preferred embodiment, the control grid is fabricated
with a series of printing steps. The grid structure is built up in
a series of one or more printed layers after the cathode has been
deposited, and the grid electrode is printed onto the grid
structure. There may be additional insulator on top of the grid
electrode, and there may be multiple grid electrodes separated by
insulator in the grid structure. The grid electrode may not occupy
the entire width of the grid structure, and may only occupy a
central region of the structure, in order to stay within
reproducible tolerances. In another embodiment, the control grid is
fabricated by photoetching away a glass, such as a photostructural
glass.
[0123] In an encapsulated electrophoretic image display, an
electrophoretic suspension, such as the ones described previously,
is placed inside discrete compartments that are dispersed in a
polymer matrix. This resulting material is highly susceptible to an
electric field across the thickness of the film. Such a field is
normally applied using electrodes attached to either side of the
material. However, as described above in connection with FIGS.
3A-3F, some display media may be addressed by writing electrostatic
charge onto one side of the display material. The other side
normally has a clear or opaque electrode. For example, a sheet of
encapsulated electrophoretic display media can be addressed with a
head providing DC voltages.
[0124] In another embodiment, the encapsulated electrophoretic
suspension can be printed onto an area of a conductive material
such as a printed silver or graphite ink, aluminized Mylar, or any
other conductive surface. This surface which constitutes one
electrode of the display can be set at ground or high voltage. An
electrostatic head consisting of many electrodes can be passed over
the capsules to addressing them. Alternatively, a stylus can be
used to address the encapsulated electrophoretic suspension.
[0125] In another embodiment, an electrostatic write head is passed
over the surface of the material. This allows very high resolution
addressing. Since encapsulated electrophoretic material can be
placed on plastic, it is flexible. This allows the material to be
passed through normal paper handling equipment. Such a system works
much like a photocopier, but with no consumables. The sheet of
display material passes through the machine and an electrostatic or
electrophotographic head addresses the sheet of material.
[0126] In another embodiment, electrical charge is built up on the
surface of the encapsulated display material or on a dielectric
sheet through frictional or triboelectric charging. The charge can
built up using an electrode that is later removed. In another
embodiment, charge is built up on the surface of the encapsulated
display by using a sheet of piezoelectric material.
[0127] Microencapsulated displays offer a useful means of creating
electronic displays, many of which can be coated or printed. There
are many versions of microencapsulated displays, including
microencapsulated electrophoretic displays. These displays can be
made to be highly reflective, bistable, and low power.
[0128] To obtain high resolution displays, it is useful to use some
external addressing means with the microencapsulated material. This
invention describes useful combinations of addressing means with
microencapsulated electrophoretic materials in order to obtain high
resolution displays.
[0129] One method of addressing liquid crystal displays is the use
of silicon-based thin film transistors to form an addressing
backplane for the liquid crystal. For liquid crystal displays,
these thin film transistors are typically deposited on glass, and
are typically made from amorphous silicon or polysilicon. Other
electronic circuits (such as drive electronics or logic) are
sometimes integrated into the periphery of the display. An emerging
field is the deposition of amorphous or polysilicon devices onto
flexible substrates such as metal foils or plastic films.
[0130] The addressing electronic backplane could incorporate diodes
as the nonlinear element, rather than transistors. Diode-based
active matrix arrays have been demonstrated as being compatible
with liquid crystal displays to form high resolution devices.
[0131] There are also examples of crystalline silicon transistors
being used on glass substrates. Crystalline silicon possesses very
high mobilities, and thus can be used to make high performance
devices. Presently, the most straightforward way of constructing
crystalline silicon devices is on a silicon wafer. For use in many
types of liquid crystal displays, the crystalline silicon circuit
is constructed on a silicon wafer, and then transferred to a glass
substrate by a "liftoff" process. Alternatively, the silicon
transistors can be formed on a silicon wafer, removed via a liftoff
process, and then deposited on a flexible substrate such as
plastic, metal foil, or paper. As another embodiment, the silicon
could be formed on a different substrate that is able to tolerate
high temperatures (such as glass or metal foils), lifted off, and
transferred to a flexible substrate. As yet another embodiment, the
silicon transistors are formed on a silicon wafer, which is then
used in whole or in part as one of the substrates for the
display.
[0132] The use of silicon-based circuits with liquid crystals is
the basis of a large industry. Nevertheless, these display possess
serious drawbacks. Liquid crystal displays are inefficient with
light, so that most liquid crystal displays require some sort of
backlighting. Reflective liquid crystal displays can be
constructed, but are typically very dim, due to the presence of
polarizers. Most liquid crystal devices require precise spacing of
the cell gap, so that they are not very compatible with flexible
substrates. Most liquid crystal displays require a "rubbing"
process to align the liquid crystals, which is both difficult to
control and has the potential for damaging the TFT array.
[0133] The combination of these thin film transistors with
microencapsulated electrophoretic displays should be even more
advantageous than with liquid crystal displays. Thin film
transistor arrays similar to those used with liquid crystals could
also be used with the microencapsulated display medium. As noted
above, liquid crystal arrays typically requires a "rubbing" process
to align the liquid crystals, which can cause either mechanical or
static electrical damage to the transistor array. No such rubbing
is needed for microencapsulated displays, improving yields and
simplifying the construction process.
[0134] Microencapsulated electrophoretic displays can be highly
reflective. This provides an advantage in high-resolution displays,
as a backlight is not required for good visibility. Also, a
high-resolution display can be built on opaque substrates, which
opens up a range of new materials for the deposition of thin film
transistor arrays.
[0135] Moreover, the encapsulated electrophoretic display is highly
compatible with flexible substrates. This enables high-resolution
TFT displays in which the transistors are deposited on flexible
substrates like flexible glass, plastics, or metal foils. The
flexible substrate used with any type of thin film transistor or
other nonlinear element need not be a single sheet of glass,
plastic, metal foil, though. Instead, it could be constructed of
paper. Alternatively, it could be constructed of a woven material.
Alternatively, it could be a composite or layered combination of
these materials.
[0136] As in liquid crystal displays, external logic or drive
circuitry can be built on the same substrate as the thin film
transistor switches.
[0137] In another embodiment, the addressing electronic backplane
could incorporate diodes as the nonlinear element, rather than
transistors.
[0138] In another embodiment, it is possible to form transistors on
a silicon wafer, dice the transistors, and place them in a large
area array to form a large, TFT-addressed display medium. One
example of this concept is to form mechanical impressions in the
receiving substrate, and then cover the substrate with a slurry or
other form of the transistors. With agitation, the transistors will
fall into the impressions, where they can be bonded and
incorporated into the device circuitry. The receiving substrate
could be glass, plastic, or other nonconductive material. In this
way, the economy of creating transistors using standard processing
methods can be used to create large-area displays without the need
for large area silicon processing equipment.
[0139] While the examples described here are listed using
encapsulated electrophoretic displays, there are other
particle-based display media which should also work as well,
including encapsulated suspended particles and rotating ball
displays.
[0140] While the invention has been particularly shown and
described with reference to specific preferred embodiments, it
should be understood by those skilled in the art that various
changes in form and detail may be made therein without departing
from the spirit and scope of the invention as defined by the
appended claims.
* * * * *