U.S. patent application number 09/754959 was filed with the patent office on 2001-05-24 for patterned light emitting diode devices.
Invention is credited to Bao, Zhenan, Rogers, John A..
Application Number | 20010001485 09/754959 |
Document ID | / |
Family ID | 22250852 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001485 |
Kind Code |
A1 |
Bao, Zhenan ; et
al. |
May 24, 2001 |
Patterned light emitting diode devices
Abstract
An LED device that emits light in a pattern is disclosed. The
LED device is a layer of active material that is sandwiched between
a transparent substrate with an anode formed thereon and a cathode.
The active material has a layer of light emitting material that
emits light when electron/hole recombination is induced in the
material. The patterned emission is defined by a patterned layer in
the active material of the LED device. The patterned layer has at
least a first thickness and a second thickness. When the device is
on, the portion of the device associated with the first thickness
of the patterned layer is visually distinct from the portion of the
device that is associated with the second thickness of the
patterned layer.
Inventors: |
Bao, Zhenan; (North
Plainfield, NJ) ; Rogers, John A.; (New Providence,
NJ) |
Correspondence
Address: |
Docket Administrator (Room 3C-512)
Lucent Technologies Inc.
600 Mountain Avenue
P.O. Box 636
Murray Hill
NJ
07974-0636
US
|
Family ID: |
22250852 |
Appl. No.: |
09/754959 |
Filed: |
January 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09754959 |
Jan 5, 2001 |
|
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09095236 |
Jun 10, 1998 |
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Current U.S.
Class: |
257/89 |
Current CPC
Class: |
H01L 51/5036 20130101;
H01L 51/5281 20130101; H01L 51/5225 20130101; H01L 51/0003
20130101; H01L 51/56 20130101; H01L 27/3239 20130101 |
Class at
Publication: |
257/89 |
International
Class: |
H01L 033/00 |
Claims
What is claimed is:
1. An LED device comprising: a substrate; an anode; a cathode; and
an active material sandwiched between the anode and cathode wherein
the active material comprises at least one material layer and
wherein the at least one material layer is a layer of organic
light-emitting material and wherein at least one layer of the
active material is a patterned layer having a first thickness and a
second thickness and wherein the layer of organic light-emitting
material and the patterned layer are the same layer or different
layers and wherein when a voltage that is sufficient to cause light
to emit from at least a portion of the organic light-emitting
material, a first portion of the LED device associated with the
first thickness of the patterned layer is visually distinct from a
second portion of the LED device associated with the second
thickness of the patterned layer.
2. The LED device of claim 1 wherein the active material has a
first material layer and a second material layer and wherein the
first material is a light-emitting material with uniform thickness
and the second material layer is a material selected from the group
consisting of hole transport materials and electron transport
materials that is patterned to have a first thickness and a second
thickness.
3. The LED device of claim 1 wherein the active material has a
first material layer and a second material layer and wherein the
first material is a light-emitting material that is patterned to
have a first thickness and a second thickness and the second
material is selected from the group consisting of hole transport
materials and electron transport materials that is patterned to
have a first thickness and a second thickness.
4. The LED of claim 1 wherein the first portion of the LED emits
light and the second portion does not emit light.
5. The LED of claim 1 wherein the first portion of the LED emits
light of a first color and the second portion of the LED emits
light of a second color.
6. The LED of claim 1 wherein the first portion of the LED emits
light of a first intensity and the second portion of the LED emits
light of a second intensity.
7. The LED of claim 1 wherein the LED emits light from boundaries
between the first portion and the second portion.
8. A process for fabricating a patterned LED device comprising:
forming a layer of an active material over one of either an anode
or a cathode; placing a mold in contact with the layer of active
material, wherein the mold is a surface with a patterned recessed
portion therein; causing the active material to flow into the
recessed pattern thereby forming a layer of patterned active
material with a first portion having a first thickness and a second
portion having a second thickness, the portion having the second
thickness corresponding to the pattern defined by the patterned
recessed portion in the mold surface; removing the mold from
contact with the active material; and forming the other of an an
anode or a cathode over the layer of active material such that the
active material is between one anode and one cathode.
9. The process of claim 7 wherein the first thickness of the active
layer is selected to provide a first area of the light emitting
diodes associated with the first thickness that is visually
distinct from a second area of the light emitting diode.
10. The process of claim 9 wherein the patterned active material is
selected from the group consisting of a light-emitting material, a
hole transport material, an electron transport material, and
combinations thereof.
11. The process of claim 10 wherein the anode is formed on a
substrate.
12. The process of claim 10 wherein the anode and the substrate on
which the anode is formed are transparent to the light emitted by
the light emitting material.
13. The process of claim 8 wherein the mold is made of an
elastomeric material.
14. The process of claim 10 wherein the patterned active material
is a light-emitting material that is soluble in an organic solvent
and wherein the process further comprises wetting the mold surface
having the recessed portion with an organic solvent prior to
contact between the mold surface and the light-emitting material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The invention is directed to Light Emitting Diodes (LEDs)
and, in particular, to LEDs that emit light in a pattern.
[0003] 2. Art Background
[0004] Flat panel displays containing light emitting diodes are
ubiquitous features of many products. Because of the need to
minimize the manufacturing cost of most products, inexpensive ways
to manufacture flat panel displays are of considerable interest. As
noted in Lidzey, D. G., et al., "Photoprocessed and micropatterned
conjugated polymer LEDs," Synthetic Metals, Vol. 82, pp. 141-148
(1996), organic materials have been investigated for use as the
emissive layers in LEDs because large-area devices can be made
cheaply and easily using such materials. Also, a greater variety of
emission colors is obtained when organic emissive layers are used
instead of inorganic emissive layers. LEDs with organic emissive
layers have a greater electrical efficiency than comparable LEDs
with an inorganic emissive layer.
[0005] LEDs are generally formed on transparent substrates such as
glass or plastic. A light emitting material is sandwiched between
an anode formed on the substrate and a cathode. When current is
supplied to the anode, electrons and holes recombine in the
light-emitting material sandwiched between the anode and the
cathode. As a result of this recombination, light emits from the
light-emitting material and through the transparent substrate.
[0006] One use for LEDs is in displays having a fixed pattern. In
such displays, there are at least two areas of contrast when the
display is on. The areas of contrast (e.g. light and dark) provide
a desired picture (e.g., a logo) or message (e.g. an "EXIT" sign).
Such a patterned array of LEDs is described in the previously
mentioned Lidzey et al. reference which was mentioned previously. A
patterned cathode is formed over an emissive layer
(poly(2,5-dialkoxy-p-phenylenevinylene). When a voltage is applied
to the ITO anode, light is emitted in a pattern that corresponds to
the cathode pattern, because light is only emitted from those
portions of the emissive layer sandwiched between the anode and the
cathode. A patterned display is also obtained by patterning the
anode instead of the cathode.
[0007] However, there are certain limitations on the patterns that
can be obtained by patterning the anode or the cathode. For
example, a simple pattern such as the letter "O" is not easily
obtained by patterning the cathode. This is because the mask used
to form the pattern must be one integral unit. The letter "O"
requires complete physical separation between the portion of the
mask inside the "O" from the portion outside the "O." Such a
complete physical separation cannot be obtained in a single unit
mask. There must be some physical connection between the portion of
the mask inside the "O" and the portion of the mask outside the
"O." Furthermore, the expedients used to pattern the cathode in the
manner described in Lidzey et al. degrade the organic emissive
layer underlying the cathode.
[0008] Different restrictions are placed on a patterned anode such
as indium tin oxide. For example, the conductivity of ITO is
reduced when patterned into narrow lines. Therefore, the brightness
of the display is not evenly distributed if a narrow portion of ITO
is required by the pattern. Furthermore, the ITO must be
electrically interconnected and therefore a pattern that is not
continuous is not practicable.
[0009] In response to the limitations imposed by patterning anodes
and cathodes, Renak, M., et al., Microlithographic Process for
Patterning Conjugated Emissive Polymers," Advanced Materials, Vol.
9, No. 5, pp. 392-395 (1995) describes a patterned LED display in
which the electron emissive layer (poly(p-phenylenevinylene)) is
patterned. Renak et al. describes a device in which the patterned
layer of poly(p-phenylenevinylene) (PPV) is formed over an ITO
layer. An electron transport layer was cast over the PPV layer. A
cathode was formed over the electron transport layer. The electron
transport layer is present to prevent direct electrical contact
between the ITO anode and the cathode.
[0010] When a voltage is applied to the ITO of the device described
in Renak et al., light is emitted from the patterned PPV layer in
the pattern of the PPV layer. However, the approach does not afford
much flexibility, as the only contrast provided by such a display
is the contrast between the PPV area of the display (which emits
light when the device is on) and the non-PPV area of the display
(which does not emit light even when the device is on). Thus the
basis for contrast in such a display is basically either on or off.
Furthermore, Renak et al requires the use of light-emitting
polymers that are also photosensitive in order to pattern the
light-emitting layer. Thus, the choices for the light-emitting
material for the Renak et al. device are extremely limited.
[0011] A display that provides the potential for a greater variety
of visual contrast, yet does not require that either the anode or
the cathode be patterned, is desired.
SUMMARY OF THE INVENTION
[0012] LED devices have a layer or layers of active material
sandwiched between an anode and a cathode. Active layers, as used
herein are layers of material in which either electron transport,
hole transport, light emission, or some combination thereof, occur.
The present invention is directed to an LED device in which at
least one of the active layers is patterned to have at least a
first thickness and a second thickness. The patterned organic layer
is sandwiched between an anode and a cathode. When the LED device
is on (i.e. when sufficient current is provided to the anode to
induce electron/hole recombination in the light emitting layer)
there is a visually perceivable contrast between the portion of the
LED device that corresponds to the active layer of the first
thickness and the portion of the LED device that corresponds to the
active layer having the second thickness.
[0013] The active layer is one or more layers of organic material.
In one embodiment, the active layer is a patterned layer of a
material in which electron/hole recombination and, thus, light
emission occurs. In a second embodiment, the active layer is a
combination of two layers: a layer of material in which light
emission occurs coupled with a hole transport or electron transport
layer. The hole transport layer, if present, is in contact with the
anode. The electron transport layer, if present, is in contact with
the cathode. In the second embodiment, the aggregate thickness of
the active layer (i.e. the combined thickness of the light emitting
layer and the hole transport or electron transport layer) is not
uniform because one of either the light emitting layer and the
electron transport layer or the hole transport layer is patterned.
An active layer consisting of a patterned electron transport layer
formed on a layer of light emitting material of uniform thickness
is one example.
[0014] In a third embodiment both the light emitting layer and the
hole transport or electron transport layer are patterned. However,
the patterns are complimentary (the thinner portion of one layer is
aligned with the thicker portion of the other layer and vice-versa)
so that the aggregate thickness of the two layers is uniform.
[0015] As a result of the one or more patterned layers in the
active layer, the LED device emits light through one portion
associated with a first layer thickness that is visually distinct
from a second portion of the LED device associated with a second
layer thickness. In the context of the present invention, the
thickness that is referred to is the thickness of the patterned
layer and not the aggregate thickness of the active layers. In one
embodiment of the present invention, when the LED is on, the LED
device only emits light through the portion associated with the
thinner portion of the patterned light-emitting layer and not
through the second portion associated with the thicker portion of
the light-emitting layer. In an alternate embodiment, the LED
device emits light of a first color through the first portion and
light of a second color through the second portion. In yet another
embodiment, the LED device emits light of a first intensity through
the first portion and light of a second intensity through the
second portion. In this embodiment, the difference between the
first intensity and the second intensity is visually
perceivable.
[0016] It is also contemplated that the patterned layer has more
than two thicknesses. When the LED device is on, the region of the
LED device associated with a particular thickness of the patterned
layer is visually distinct from the other regions associated with
the other thicknesses. The LED device of the present invention
provides the flexibility to produce LED devices in a variety of
patterns. The LED devices of the present invention are produced at
low cost because of the ease in which a patterned layer is
formed.
[0017] Since the light-emitting material of the LED of the present
invention is sandwiched between an anode and a cathode, one of
either the anode or the cathode is transparent to the emitted light
so that the light emission is observable. Typically one of either
the anode or the cathode is formed on a substrate. If the
transparent anode is formed on a substrate, then the substrate on
which the anode is formed is also transparent. Similarly, if the
transparent cathode is formed on a substrate, then the substrate on
which the cathode is formed is also transparent. For convenience,
in the embodiments described herein, the anode is formed on the
substrate. However, the present invention contemplates that the
patterned active layer of the device of the present invention can
also be placed between a cathode formed on a substrate and an
anode. Furthermore, in certain embodiments the anode or the cathode
is the substrate.
[0018] In one embodiment of the present invention, the LED device
is formed by spinning a precursor of the organic light emitting
material on a transparent substrate with an anode formed thereon.
It is advantageous if the precursor is soluble in an organic
solvent such as methanol. A mold is used to form the layer of
precursor into a desired pattern. An elastomeric mold that has a
first surface with a recessed portion in the desired pattern is one
example of a suitable mold. The recessed portion functions as a
channel for the precursor when the mold surface is placed in
contact with the layer of organic light emitting material.
[0019] The mold surface is wetted with an organic solvent. When the
wetted surface contacts the precursor, the precursor is partially
dissolved and the dissolved precursor fills the channels in the
mold. The solvent is evaporated and the mold is removed. After the
mold is removed, the precursor layer has a surface relief pattern
that corresponds to the pattern in the mold. In order to form a
light emitting layer with three or more thicknesses, a mold that
has channels with more than one depth is contemplated as
suitable.
[0020] In the context of the present invention, any light emitting
material that can be formed on a substrate in a desired pattern is
contemplated as suitable. A precursor of poly(p-phenylene vinylene)
is one example of a suitable material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The advantages, nature and various additional features of
the invention will appear more fully upon consideration of the
illustrative embodiments now to be described in detail. In the
accompanying drawings:
[0022] FIG. 1 is a schematic side view of an LED device of the
present invention;
[0023] FIG. 2 is a perspective view of a relief pattern used to
form a mold which is then used to form a patterned layer of
light-emitting material.
[0024] FIG. 3 is a perspective view of an elastomeric material in
contact with the relief pattern of FIG. 2.
[0025] FIG. 4 is a perspective view of a mold in contact with a
layer of light-emitting material.
[0026] FIG. 5 is a perspective view of a layer of light-emitting
material with a relief pattern formed therein.
[0027] FIG. 6 is a top view of a patterned LED device of the
present invention in its on state.
[0028] FIG. 7A is a schematic side view of a second embodiment of
an LED of the present invention.
[0029] FIG. 7B is a top view of the device of FIG. 7A in its on
state.
DETAILED DESCRIPTION
[0030] The present invention is directed to an LED device that has
an active layer that is sandwiched between an anode and a cathode.
The active layer at least contains a layer of light emitting
material, and optionally contains either an electron transport
layer, a hole transport layer, or both. At least one of the light
emitting layer, electron transport layer or hole transport layer
has at least a first thickness and a second thickness. When the LED
device is on, a first portion of the LED device associated with the
portion of the layer having the first thickness is visually
distinct from a second portion of the LED device associated with
the portion of the layer having the second thickness. In one
embodiment of the present invention, the first portion of the LED
emits light and the second portion does not emit light. In a second
embodiment of the present invention, the first portion emits light
of a first color and the second portion emits light of a second
color. In a third embodiment of the present invention, the first
portion emits light of a first intensity and the second portion
emits light of a second intensity.
[0031] In an alternate embodiment of the present invention, there
are more than two portions of the LED device, each associated with
a different layer thickness. Each portion is visually distinct from
the other portions when the LED device is on (i.e. the first
portion is off, the second portion is a first color, the third
portion is a second color, etc.).
[0032] One example of an LED device of the present invention is
illustrated in FIG. 1. The LED device 10 is formed on a transparent
substrate 15. Examples of suitable substrates include glass and
transparent plastic substrates. The use of plastic substrates is
limited because the substrate cannot be exposed to temperatures
that exceed the melting point of the substrate during subsequent
processing. However, plastic substrates are attractive alternatives
when suitable because they are lightweight, inexpensive, and
flexible, among other advantages.
[0033] An anode 20 is formed on the substrate 15. The anode is a
conventional material such as indium tin oxide (ITO). A layer of a
light emitting material 25 is formed on the anode 20. The light
emitting material is an organic material such as poly(p-phenylene
vinylene) (PPV). Organic light emitting materials are well known to
one skilled in the art.
[0034] The layer of light emitting material has two thicknesses 26
and 27. The voltage required to induce light emission from an
organic light emitting films is dependent on film thickness. For
example, a voltage of 6 V must be supplied to the anode to induce
light emission in an 80 nm thick PPV film. A voltage of 12 V must
be supplied to the anode to induce light emission in a 200 nm thick
PPV film. Therefore, if, for example, a PPV film has a first
thickness of 80 nm and a second thickness of 200 nm, the portion of
the film that is 80 nm thick will emit light when 6 volts is
applied to the anode. However, the portion of the film that is 200
nm thick will not emit light when 6 volts is applied to the anode.
In the LED device depicted in FIG. 1, the first thickness 26 is
selected so that, when the device is on, the light emitting layer
does not emit light. The second thickness 27 is chosen so that,
when the device is on, the light emitting layer emits light.
[0035] A cathode 30 is formed over the layer of light emitting
material. Conventional cathode materials are well known to one
skilled in the art. All of these cathode materials are contemplated
as suitable. Examples of suitable cathode materials are calcium,
magnesium and aluminum.
[0036] In the embodiment of the present invention wherein light is
emitted through the cathode, the cathode is transparent. One
example of a transparent cathode is a layer of aluminum that is
sufficiently thin so that light emitted by the organic
light-emitting layer passes through it. In this embodiment, the
substrate is not transparent, nor is the anode required to be
transparent.
[0037] The thickness of the light emitting layer is controlled to
provide an LED device that emits light in a desired pattern. In the
embodiment of the present invention wherein light is emitted by the
first portion of the LED device and not emitted by the second
portion of the device, the light emitting layer is patterned so
that the thinner portion corresponds to the desired pattern and the
thicker portion corresponds to the inverse of the desired pattern.
A number of techniques for forming the layer of light emitting
material with two thicknesses, the first thickness forming a
desired pattern in the layer and the second thickness forming the
inverse of that pattern, are contemplated as suitable.
[0038] One example of a suitable technique involves the use of a
mold. In this technique, a mold is formed by casting an elastomeric
material against a topographic patterned surface. The patterned
surface is the desired pattern to be formed in the light emitting
layer. An example of a topographic surface is illustrated in FIG.
2. The surface 100 has raised portions 110 formed thereon. The
raised portions are formed used conventional lithographic
techniques in which a layer of energy-definable material is formed
on a planar substrate. A pattern is introduced into the layer of
energy-definable material by introducing an image of that pattern
into the layer of energy-definable material. That image is
introduced by exposing the energy-definable material to patterned
radiation. The radiation introduces a chemical contrast between the
exposed and unexposed portions of the energy-definable material.
The pattern is developed by exploiting the chemical contrast
between the unexposed portion and the exposed portion of the
energy-definable material. That is, the portion of the
energy-definable material that does not correspond to raised
portion 110 in FIG. 2 is removed from the substrate.
[0039] The height of the raised portions 110 corresponds to the
difference between the first thickness and the second thickness in
the light emitting layer. Referring to FIG. 3, a layer of a liquid
precursor of an elastomeric material 200 is formed over topographic
surface 110. The topographic surface forms an impression in the
elastomeric material precursor 200. The elastomeric material 200 is
solidified and separated from the topographic surface 110.
[0040] Referring to FIG. 4, the solidified, elastomeric material
200 with impressions 210 formed therein is placed in contact with
an organic light emitting material 215 formed on a substrate 220
with an anode 225 (ITO) formed thereon. The light emitting material
215 has a viscosity that permits it to flow into the impressions
210, which serve as capillary channels for the light emitting
material 215. One way to induce the light emitting material to flow
into the impressions is to select a material such as a precursor of
poly(p-phenylene vinylene) which can be spin cast onto the
ITO-coated substrate. Since the precursor is soluble in organic
solvent, the surface 216 of the elastomeric material is wetted with
an organic solvent such as methanol. Bringing the precursor into
contact with the solvent-coated elastomeric mold partially
dissolves the precursor and causes it to flow into the
channels.
[0041] After the precursor has been molded into the desired
pattern, the elastomeric material is removed from contact with the
light emitting material. If a precursor of the light emitting
material is used, that precursor is then converted to the light
emitting material using the requisite expedient (e.g. heating). The
resulting structure 300 is illustrated in FIG. 5. In FIG. 5 the
layer of light emitting material 215 is formed on the glass
substrate 220 with the layer of ITO 225 formed thereon. The layer
of light emitting material 215 has the relief pattern formed
therein which consists of a pattern of raised portions 310 with a
desired orientation and thickness. The thickness of the raised
portions 310 is selected so that light will not emit from these
portions of the layer 215 when the LED device is on. The thickness
of the thinner portion 350 of layer 215 is selected so that light
will emit from this portion of the layer when the LED device is on.
The emission pattern of the LED device formed from the structure
illustrated in FIG. 5 is illustrated in FIG. 6. The light portion
410 of the LED device 400 corresponds to the pattern of the thinner
portion of 350 of layer 215 (FIG. 5) from which light does emit.
The dark portion 420 of the LED device 400 corresponds to the
pattern of the thicker portion 310 of layer 215 (FIG. 5).
EXAMPLE 1
[0042] Films of a precursor of poly(p-phenylene vinylene)
synthesized from p-xylenebis(tetrahydrothiophenium chloride) via
the Wessling route were spin cast onto an ITO-coated glass
substrate. The precursor was obtained from Lark Enterprises of
Webster, Mass. The films was cast onto the substrate at a speed of
500 to 1000 rpm for 45 seconds. The thickness of the resulting
films were between 50 nm and 150 nm.
[0043] An elastomeric mold having a series of impressions formed in
one surface thereof was used to mold the precursor films. The
impressions in the surface of the elastomeric mold had a width of
about 2 .mu.m to about 15 .mu.m and a depth of 300 nm. The
impressions were introduced into the surface of the elastomeric
mold by forming the elastomeric material over a topographic
surface. The raised portions of that surface formed the impressions
in the elastomeric material. The raised portions were prepared by
patterning a layer of energy sensitive material on a substrate.
Conventional lithographic techniques were used to pattern the
energy sensitive material.
[0044] The surface of the elastomeric mold was wetted with
methanol. The surface of the elastomeric mold with the impressions
therein was brought into contact with the PPV precursor. After the
solvent was evaporated, the mold was removed from contact with the
precursor. The precursor was then baked at a temperature of
260.degree. C. in vacuum (about 10.sup.-6 Torr) for about 10 hours.
Atomic force microscopy confirmed that the baking step did not
significantly alter the surface relief imparted by the mold. A dual
layer cathode was formed over the topographic PPV layer by vacuum
deposition of a layer of calcium (40 nm thick) followed by a layer
of aluminum (200 nm thick).
[0045] Light was observed to emit from the thinner (about 100 nm
thick) portion of the PPV film when at least about 8 volts was
applied to the resulting device. At this voltage light did not emit
from the thicker (about 400 nm thick) portion of the PPV film. The
device was observed to have an external quantum efficiency of about
10.sup.-3% photons/electron. The efficiency could be improved by
forming a layer of electron transport material over the PPV layer,
choosing polymers with greater efficiency, or doping the polymer
with a dye.
[0046] In order to avoid a change in brightness at the portion of
the LED device that corresponds to a change a thickness of the
light emitting film in the device, one skilled in the art will
appreciate that a steep profile is advantageous. The more gradual
the transition from first thickness to second thickness in the
light emitting film thickness, the greater the blur between the
light emission effect of the first thickness and the light emission
effect of the second thickness. Such a steep profile is obtained by
ensuring that the indentations in the mold surface have the desired
profile. The precision of these indentations is in turn dependent
upon the accuracy of the method used to form the indentations. In
the embodiment of the present invention wherein the indentations
are formed by casting an elastomeric film over a topographic
surface, the lithographic technique employed to form topographic
surface must be sufficiently precise to form features with the
requisite profile.
[0047] In an alternate embodiment, two or more different organic
light emitting materials are used to form an LED device that emits
light in a desired pattern. For example, a first material emits
green light when a first threshold voltage is applied to an LED
device with the first organic light emitting material. A second
material emits red light when a second threshold voltage is applied
to an LED device with the second material. The threshold voltage is
the voltage that is required for light emission to occur. If a
voltage that is less than the threshold voltage is applied to the
device, no light emits from the organic light emitting
material.
[0048] An LED device that emits light in a desired pattern is
formed using such materials. An example of such a device is
illustrated in FIG. 7A. The LED device 500 has a light emitting
layer 510 which is sandwiched between a transparent substrate 505
with an anode 506 formed thereon and a cathode 507. The light
emitting layer 510 has a first continuous layer of the first
material 515 that has a patterned layer of the second material 520
formed thereon. Referring to FIG. 7B, which is a top view of the
device 500 in its on state, the portion of the LED device 500
associated with the single layer of the first material is 525. The
portion of the LED device associated with the dual layer of first
material 515 and second material 520 is 530.
[0049] If the first threshold voltage is greater than the second
threshold voltage, then if a voltage is applied to the device 500
that is greater than the first threshold voltage but less than the
second threshold voltage, portion 530 emits light and portion 525
does not. If a voltage is applied to the LED device that is greater
than the threshold voltages of both the first material and the
second material, then portion 530 emits light in one color and
portion 525 emits in another color.
[0050] For example, a device which, when a certain threshold
voltage is applied, emits red from one portion and green from
another portion of the device is prepared by forming a layer of PPV
on an ITO coated transparent substrate. A 100 nm thick layer of PPV
emits green light when 8 volts are applied to an LED device with
such a layer. A layer of ALQ (aluminum tris(8-hydroxyquinoline) is
formed over the PPV layer. Since ALQ over PPV also emits green
light, the ALQ layer is doped to change the color of emission from
green to a color that contrasts with the green light emitted by the
single layer of PPV. The color of emission can be changed by doping
the ALQ layer with a fluorescent dye such as
4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran
(DCM) or 5,10,15,20-tetraphenyl-21H,23H-porphine. The concentration
of dye in the ALQ layer is typically about 1 to about 5 weight
percent. In some instances even higher concentrations of dye are
used.
[0051] The ALQ layer is patterned. The ALQ layer emits at a lower
threshold voltage than the PPV layer. Therefore, when a voltage is
applied to the LED device that is greater than the threshold
voltage of the doped ALQ layer but less than the threshold voltage
of the PPV layer, light emits from the device in the pattern
defined by the dual ALQ/PPV layer. No light emits from the pattern
defined by the single PPV layer. When a voltage is applied to the
LED device that is greater than the threshold voltages of both
layers, red or red orange light (depending upon the choice of
dopants described above) emits from the dual layer of doped ALQ and
PPV. Green light emits from the single layer of PPV.
[0052] The patterned ALQ layer is formed on the PPV layer by vacuum
deposition using a shadow mask or by printing a solution soluble
red-emitting material on the PPV in the desired pattern. Examples
of suitable printing techniques include screen printing, inkjet
printing or spraying. In one embodiment of the invention, a layer
of undoped ALQ is formed over the patterned, doped layer of ALQ. In
this embodiment, there is only one threshold voltage for emission.
When the LED device is on, red or red orange light (depending upon
the choice of dopants described above) emits from the tri layer of
undoped ALQ, doped ALQ and PPV. Green light emits from the dual
layer of undoped ALQ and PPV.
[0053] In another embodiment of the present invention, the voltage
applied to the device is selected to have a particular effect. In
the device of the present invention, the voltage is selected so
that light emits only from the portion of the patterned layer that
transitions from the first thickness to the second thickness.
Although applicants do not wish to be held to a particular theory,
it is believed that the selective emission from the transition
region is due to the steep slope of the material as it transitions
from the first thickness to the second thickness. Light emits from
this transition region at a voltage that is less than the voltage
required for the first portion of the LED to emit light
preferentially over the second portion of the device. An LED in
which light emits from ordered arrays of small sources is useful in
a variety of applications that use patterned light such as
near-field photolithography, near-field microscopy, spectroscopy,
and high density optical data storage.
[0054] It is to be understood that the above-described embodiments
are illustrative of only a few of the many possible specific
embodiments which can represent applications of the principles of
the invention. Numerous and varied other arrangements can be
readily devised in accordance with these principles by those
skilled in the art without departing from the spirit and scope of
the invention.
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