U.S. patent application number 11/538018 was filed with the patent office on 2008-03-13 for screen printable electroluminescent polymer ink.
Invention is credited to Jane Breeden, Susan A. Carter, Sara Tuttle, John G. Victor.
Application Number | 20080061682 11/538018 |
Document ID | / |
Family ID | 23342432 |
Filed Date | 2008-03-13 |
United States Patent
Application |
20080061682 |
Kind Code |
A1 |
Carter; Susan A. ; et
al. |
March 13, 2008 |
Screen Printable Electroluminescent Polymer Ink
Abstract
The addition of a variety of additives to a soluble
electroluminescent polymer in solution is used to improve the
printability and performance of screen printed light-emitting
polymer-based devices. Examples of such additives include
transparent polymers, gel-retarders, high viscosity liquids,
organic and inorganic salts, and oxide nanoparticles. The additives
are used to control the viscosity of the electroluminescent polymer
ink, to decrease the solvent evaporation rate, and to improve the
ink consistency and working time. In addition, these additives can
improve the charge injection and power efficiency of light emitting
devices manufactured from the screen printable electroluminescent
polymer ink.
Inventors: |
Carter; Susan A.; (Santa
Cruz, CA) ; Victor; John G.; (Chicago, IL) ;
Tuttle; Sara; (Santa Cruz, CA) ; Breeden; Jane;
(Felton, CA) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Family ID: |
23342432 |
Appl. No.: |
11/538018 |
Filed: |
October 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10327628 |
Dec 20, 2002 |
7115216 |
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11538018 |
Oct 2, 2006 |
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60342580 |
Dec 20, 2001 |
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Current U.S.
Class: |
313/504 |
Current CPC
Class: |
H01L 51/0007 20130101;
H01L 51/0038 20130101; C09D 11/50 20130101; H01L 51/5012 20130101;
H01L 51/0004 20130101 |
Class at
Publication: |
313/504 |
International
Class: |
H01L 51/54 20060101
H01L051/54 |
Claims
1. An electroluminescent device comprising an electroluminescent
film, the electroluminescent film including a screen printed
light-emitting polymer-based ink, wherein the light-emitting
polymer-based ink includes: a soluble electroluminescent organic
material; a first additive, the first additive being an organic
solvent having a boiling point between about 120 and 200 degrees
Celsius; and a second additive, the second additive being a
viscosity enhancer added to maintain a viscosity of above about 50
centipoises.
Description
PRIORITY CLAIM
[0001] This is a divisional of U.S. application Ser. No. 10/327,628
filed Dec. 20, 2002 and claims priority benefit from U.S.
Provisional Patent Application No. 60/342,580 filed Dec. 20, 2001
and entitled "Screen Printable Electroluminescent Polymer Ink", the
contents of which are incorporated herein by reference.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] The present application is related to U.S. application Ser.
No. 09/844,703, filed Apr. 27, 2001, entitled "Screen Printing
Light-Emitting Polymer Pattern Devices", now U.S. Pat. No.
6,605,483 issued Aug. 12, 2003, which is commonly owned by the
present assignee, and the contents of which are incorporated herein
by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to relates to the
manufacturing method, namely screen printing, used to produce
polymer light emitting devices. More specifically, this invention
relates to electrically active polymer-containing compositions and
their use in producing light emitting displays.
BACKGROUND OF THE INVENTION
[0004] Electroluminescent polymers are materials that emit light
when sandwiched between two suitable electrodes and when a
sufficient voltage is applied. A number of electroluminescent
devices have been disclosed which use organic materials as an
active light-emitting layer sandwiched between two electrodes. For
example, U.S. Pat. No. 4,539,507 to VanSlyke et al. discloses a
device having a bi-layer of two vacuum-sublimed films of small
organic molecules sandwiched between two contacts. The small
organic molecules however are not printable using a solution-based
process. In a related patent, U.S. Pat. No. 5,247,190 to Friend et
al. discloses a device having a thin dense polymer film made up of
at least one conjugated polymer sandwiched between two electrodes.
Additionally, U.S. Pat. No. 5,408,109 to Braun at al. shows that
high brightness light emitting devices can be made using soluble
electroluminescent polymers.
[0005] The results of these patents indicate that it might be
possible to make light emitting displays using inexpensive
solution-based atmospheric processing techniques, such as ink-jet
printing, reel-to-reel or screen printing. However, obtaining
efficient device operation requires the use of low work-function
metals, such as Calcium, that are not stable under atmospheric
processing (i.e. printing) conditions.
[0006] U.S. Pat. No. 5,682,043 to Pei et al. describes a polymer
light-emitting electrochemical cell that contains a solid state
electrolyte and salt that is used to electrochemically dope an
organic electroluminescent layer, such as a conjugated polymer, via
ionic transport. This system allows the ability to achieve
efficient device operation without relying on the use of low
work-function metals. Following this work, U.S. Pat. No. 6,284,435
to Yang Cao shows in that organic anionic surfactants cause a
similar effect without needing ionic transport through the polymer
film.
[0007] In theory, electrochemical doping or anionic surfactants
could be used to make a electroluminescent polymer device that
would be fully compatible with liquid-based processing under
atmospheric conditions. Nonetheless, the electroluminescent polymer
solutions and electrodes mentioned in these patents are not easy
applicable to many fully liquid-based manufacturing process, such
as screen printing.
[0008] Screen printing is one of the most promising methods to
inexpensively manufacture large-area electroluminescent displays.
Screen printing has been successfully applied to manufacturing
large area inorganic phosphor-based electroluminescent displays,
for example see U.S. Pat. No. 4,665,342 to by Topp et al. More
recently, commonly owned U.S. patent application Ser. No.
09/844,703 to Victor et al. shows screen printing can also be used
to manufacture polymer-based electroluminescent displays. This
application describes a method to make a screen printable
electroluminescent ink that substantially improves the screen
printability and performance of electroluminescent polymer
solutions through the use of soluble or dispersible additives,
including transparent polymers, gel-retarders (i.e., viscous
solvents), high boiling point solvents, oxide nanoparticles, and
ionic dopants.
SUMMARY OF THE INVENTION
[0009] The addition of a variety of additives to a soluble
electroluminescent polymer in solution is used to improve the
printability and performance of screen printed light-emitting
polymer-based devices. Examples of such additives include
transparent and/or charge transporting polymers, gel-retarders,
high viscosity liquids, organic and inorganic salts, and oxide
nanoparticles. The additives are used to control the viscosity of
the electroluminescent polymer ink, to decrease the solvent
evaporation rate, and to improve the ink consistency and working
time. In addition, these additives can improve the charge injection
and power efficiency of light emitting devices manufactured from
the screen printable electroluminescent polymer inks.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] These and other aspects and features of the present
invention will become apparent to those ordinarily skilled in the
art upon review of the following description of specific
embodiments of the invention in conjunction with the accompanying
figures, wherein:
[0011] FIG. 1 is a diagram of a simplified polymer
electroluminescent device, and
[0012] FIG. 2 shows the device performance of a fully screen
printed light emitting polymer (LEP) device.
DETAILED DESCRIPTION
[0013] The present invention will now be described in detail with
reference to the drawings, which are provided as illustrative
examples of the invention so as to enable those skilled in the art
to practice the invention. Notably, the figures and examples below
are not meant to limit the scope of the present invention.
Moreover, where certain elements of the present invention can be
partially or fully implemented using known components, only those
portions of such known components that are necessary for an
understanding of the present invention will be described, and
detailed descriptions of other portions of such known components
will be omitted so as not to obscure the invention. Further, the
present invention encompasses present and future known equivalents
to the known components referred to herein by way of
illustration.
[0014] In an embodiment of the present invention, an
electroluminescent polymer ink that can be readily screen printed
in a manufacturable process has been developed. For example, by
adding transparent polymers (i.e., ones that do not significantly
absorb the light emission from the light emitting polymer, or LEP,
layer), gel-retarder (viscous solvent), and high-boiling point
solvents to an electroluminescent polymer solution, control over
ink viscosity, solvent evaporation rate, and print definition can
be obtained to enable the ability to screen print uniform
electroluminescent polymer films. Furthermore, the addition of
electrochemical dopants, anionic surfactants, and oxide
nanoparticles to the electroluminescent polymer solution can be
used to further improve power efficiency and brightness in a
electroluminescent device structure consisting of the screen
printed electroluminescent polymer ink sandwiched between air
stable electrodes. In this exemplary embodiment, these additions
allow the formulation of a screen printable electroluminescent ink
that can be used to make fully screen printed electroluminescent
displays. These exemplary screen printed films are typically
between about 100 nm and 1 micron thick.
[0015] Typically, an electroluminescent polymer solution is defined
to include a soluble electroluminescent polymer material that is
mixed at 0.3% to 5% by weight into solution with an appropriate
solvent. An example of this would involve mixing 1% by weight of an
electroluminescent polymer, such as MEH-PPV, into an organic
solvent, such as p-xylene or chlorobenzene, to form the
electroluminescent polymer solution. Another example of this would
involve mixing an organic dye, such as rhodantine, into a charge
transporting polymer, such as PVK, at a ratio of 1:20 (dye to
polymer) and than mixing 1% by weight of this composite material
into an organic solvent, such as p-xylene, to form the
electroluminescent polymer solution. Another example is where the
soluble electroluminescent organic material is a conjugated
polymer.
[0016] A screen printable electroluminescent polymer ink according
to an embodiment of the present invention includes a mixture of the
electroluminescent polymer solution, as described above, with one
or more additional non-emissive polymers. The additional
non-emissive polymers can include: high-boiling point solvents;
gel-retarders; electrochemical dopants; anionic surfactants; and
oxide nanoparticles. As used herein, a conjugated polymer is a
material with alternating single and double bonds between carbon
atoms along the polymer backbone, and an organic chromophore is a
material that emits light when excited. Examples of screen
printable electroluminescent polymer inks according to the present
invention, and the resulting device properties the are given
below.
[0017] According to an embodiment of the present invention,
high-boiling point organic solvents can be added to the
electroluminescent polymer solution to decrease solvent evaporating
rate. This additive improves the ability of the screen printed
domains to flow together to produce a film of uniform
thickness.
[0018] In one aspect of this embodiment, the organic solvent chosen
should have a boiling point above about 130.degree. C. Solvents
with too high a boiling point lead to greater film non-uniformity
and are more difficult to remove from the film after deposition.
Therefore, solvents with a boiling point between about 120 and 200
degrees Celsius are desired, with about 130 degrees Celsius being
preferred.
[0019] In another aspect of this embodiment, the organic solvent
chosen should have minimal reactions with the electroluminescent
polymer solution and other additives, and allow the
electroluminescent polymer and any other additives to go into
solution. Solvents having a solubility parameter in the region of
8.8-10.0 (cal/cm.sup.3).sup.1/2, and preferably in the 9.4-9.9
(cal/cm.sup.3).sup.1/2 range, satisfy these requirements. Examples
of high-boiling point solvent additives that meet the criteria of
this aspect of the present invention include, but are not limited
to, chlorobenzene, p-xylene, diethylbenzene, and cyclohexanone.
[0020] In this embodiment of the present invention, the
high-boiling point solvent is added to the electroluminescent
polymer solution so that the electroluminescent polymer solution is
between about 0.3% to 5% by weight of the total solution. According
to this embodiment, the high-boiling point solvent is removed from
the screen printed electroluminescent polymer film by heating, by
applying a vacuum to the film, or by both heating and applying a
vacuum.
[0021] According to another embodiment of the present invention, a
gel-retarder, such as Coates Screen VPK Retarder Paste and Gel-100
Retarder Base, can be added to the electroluminescent polymer
solution to decrease solvent evaporation as well as to improve ink
stability and workability. Solvent evaporation that occurs too
quickly can result in the screen mesh pattern being transferred to
the substrate through improper coalescing of ink droplets. Fast
evaporation can also cause the electroluminescent ink to dry in the
screen, making multiple runs (without cleaning) difficult. In
addition, the gel-retarder can be used to extend the working
lifetime of the electroluminescent polymer ink and to decrease
cob-webbing.
[0022] According to the present invention, the gel-retarder is
chosen to have high viscosity, increasing the viscosity of the
electroluminescent polymer solution as needed, yet remaining
chemically inert in the electroluminescent polymer solution. The
gel-retarder of this embodiment is added at a ratio of about 1% to
20% by weight of the solvent to the electroluminescent polymer ink,
preferably, the minimum amount of gel-retarder is added to obtain
printability. The addition of the gel-retarder is controlled to
obtain a viscosity, preferably, of above about 50 centipoises.
[0023] The gel-retarder can be removed with the solvent from the
electroluminescent polymer by heating, by applying a vacuum to the
film, or by both heating and applying a vacuum. The polymer-based
electroluminescent ink is screen printed using multiple passes to
result in a dry film thickness between about 100 nm and 1
micron.
[0024] According to another embodiment of the present invention,
non-electroluminescent polymer additives of various molecular
weights can be added to the electroluminescent polymer solution to
increase the viscosity of the polymer solution since typically
electroluminescent polymer solutions have too low of a viscosity
for effective screen printing. Solutions that have too low of
viscosity can run, or bleed, through the screen, resulting in
blurred edges, loss of patterning, and sticking between the screen
and substrate. In this embodiment, the viscosity can be increased
and controlled to improve printability through the use of polymer
additives of various molecular weights.
[0025] The polymer additives of the present invention should meet
several conditions: they should be soluble in a similar solvent as
the electroluminescent polymer; they should be electrochemically
inert in the chosen medium and operating conditions; they should
have a certain electronic structure so that no significant charge
transfer occurs from the electroluminescent polymer to the polymer
additive (although charge transfer can occur from the polymer
additive to the electroluminescent polymer); and they should have a
sufficiently large band-gap so that the polymer additives do not
significantly absorb the light emission from the electroluminescent
polymer. Finally, the polymer additives should have a sufficiently
high decomposition temperature so that they remain solid in the
electroluminescent polymer film after the solvent is removed by
heating or applying a vacuum to the film.
[0026] Acceptable polymer additives according to the criteria of
this embodiment include, but are not limited to: aromatic polymers,
such as polystyrene; poly(methyl methacrylates); and polymer
electrolytes, such as polyethylene oxide (PEO). In the case of a
PEO, the polymer electrolyte can also serve as an ion transporter
for the ionic dopants. In another aspect of this embodiment, charge
transporting polymers, such as polyvinylcarbozol, can be used to
facilitate charge injection into the electroluminescent
polymer.
[0027] In a typical solution of the present invention, the
non-electroluminescent polymer with a molecular weight between
about 300,000 and 20,000,000 will be added to the
electroluminescent polymer solution at between about 2% to 100% by
weight of the electroluminescent polymer depending on the relative
solubility and molecular weights. The addition of the polymer is
controlled to obtain a viscosity, preferably, of above about 50
centipoises.
[0028] The polymer-based electroluminescent ink is then screen
printed using multiple passes to result in a dry film thickness
between about 100 nm and 1 micron.
[0029] According to another embodiment of the present invention,
ionic-dopants and surfactants can be added as additional additives
to the electroluminescent polymer solution, for example, similar to
U.S. Pat. Nos. 5,682,043, 5,895,717 and 6,284,435, to increase the
device efficiency and brightness in electroluminescent device
structures that consist of air-stable electrodes. The ionic dopants
and surfactants are chosen in this embodiment so that they do not
cause significant irreversible electrochemical reactions under
operating conditions, they enable efficient device operation
(quantum efficiency above 0.5%) at low voltages (below 15V) with
air stable contacts, they have reasonable switching speeds for the
display application, they are stable to the solvent removal
process, and they are stable to the encapsulation process. In this
embodiment, the ionic dopants and surfactants are added in ratios
of about 1% to 10% by weight of the ionic dopant and surfactant to
the electroluminescent polymer solution.
[0030] The ionic dopants of this embodiment include, but are not
limited to, those having: a cation hat is an ion of a metal, such
as calcium, barium and aluminum; a cation that is a singly ionized
alkali metal, such as lithium, sodium, potassium or cesium; an
organic cation, such as tetrabutyl ammonium, tetraethyl ammonium,
tetrapropyl ammonium, tetramethyl ammonium, and phenyl ammonium; an
inorganic ion that includes singly inonized halogens, such as
fluorine, chlorine, bromine and iodine; an inorganic anion such as
sulfate, tetrafluoroborate, hexafluorophosphate, and aluminum
tetrachlorate; and an organic anion, such a trifluormethane
sulfonate, trifluroacetate, tetraphenylborate, and toluene
sulfonate.
[0031] According to another embodiment of the present invention,
oxide nanoparticles, such as silicon dioxide, can be added to the
electroluminescent polymer solution to increase polymer viscosity
and to improve device power efficiency and brightness. The oxide
nanoparticles according to this embodiment are preferably chosen so
that they are transparent, they are in the size range of between
about 5 nm and 500 nm in diameter (depending on the desired
thickness of the electroluminescent film), they readily disperse in
the electroluminescent polymer solution, they do not detrimentally
effect film morphology by inducing shorts, and they do not
detrimentally affect device performance. Large oxide nanoparticle
aggregates are removed by filtering. Suitable oxide nanoparticles
for use in the present invention include, but are not limited to,
silicon dioxide (SiO.sub.x), titanium dioxide (TiO.sub.x),
zirconium dioxide (ZrO.sub.x), or aluminum oxide
(Al.sub.2O.sub.x+1), where 1.5<x<2.5.
[0032] In this embodiment, the oxide nanoparticle is added at a
ratio of about 5% to 70% by weight of the electroluminescent
polymer solution. The addition of the oxide nanoparticle is
controlled to obtain a viscosity, preferably, of above about 50
centipoises. The polymer-based electroluminescent ink is then
screen printed using multiple passes to result in a dry film
thickness between about 100 nm and 1 micron.
[0033] An example of the present invention in use is now provided,
and consists of a fully screen printed device using a screen
printed polymer electroluminescent ink. The screen printed ink of
this example consists of 360 mg of MEH-PPV electroluminescent
polymer, 120 mg of PEO with a molecular weight of 9,000,000, 40 mg
of tetrabutylammonium-tetrafluoroborate
(C.sub.16H.sub.36BFN.sub.4), and 30 grams of chlorobenzene
(C.sub.6H.sub.5Cl).
[0034] FIG. 1 illustrates the LEP devise of this example. As shown
in FIG. 1, this exemplary device consists of four layers:
substrate; transparent electrode; polymer emissive ink; and top
electrode. The basic LEP construction is illustrated in commonly
owned U.S. patent application Ser. No. 09/844,703, filed Apr. 27,
2001, entitled "Screen Printing Light-Emitting Polymer Pattern
Devices", to Victor et al., the details of which will not be herein
repeated.
[0035] The polymer emissive layer in this example of the present
invention is printed through a 305 mesh pain-weave polyester cloth
using 3 wet passes, a drying step, and an additional 3 wet passes.
A commercially available screen printable silver conductive flake
paste from Conductive Compounds is printed onto the LEP emissive
layer through a 230 mesh plain-weave polyester cloth using 1 pass.
After drying at 125.degree. C. for 5 minutes, the silver conductive
flake paste forms a highly conductive top electrode capable of
supplying current to the LEP device over areas as large as several
square inches, without hard shorts.
[0036] FIG. 2 illustrates the performance of the FIG. 1 device. As
shown in FIG. 2, the solid-circle marked trace represents current
density, whereas the hollow-circle marked trace represents radiance
(i.e., light output).
[0037] Although the present invention has been particularly
described with reference to the preferred embodiments thereof, it
should be readily apparent to those of ordinary skill in the art
that changes and modifications in the form and details thereof may
be made without departing from the spirit and scope of the
invention. For example, those skilled in the art will understand
that variations can be made in the number and arrangement of
components illustrated in the above block diagrams. It is intended
that the appended claims include such changes and
modifications.
* * * * *