U.S. patent application number 11/050082 was filed with the patent office on 2005-12-22 for visible light emitting diodes fabricated from soluble semiconducting polymers.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Braun, David, Heeger, Alan J..
Application Number | 20050280020 11/050082 |
Document ID | / |
Family ID | 24657148 |
Filed Date | 2005-12-22 |
United States Patent
Application |
20050280020 |
Kind Code |
A1 |
Heeger, Alan J. ; et
al. |
December 22, 2005 |
Visible light emitting diodes fabricated from soluble
semiconducting polymers
Abstract
Visible light LEDs are produced having a layer of conjugated
polymer which is cast directly from solution or formed as a
gel-processed admixture with a carrier polymer. The LEDs can be
formed so as to emit polarized light.
Inventors: |
Heeger, Alan J.; (Santa
Barbara, CA) ; Braun, David; (San Luis Obispo,
CA) |
Correspondence
Address: |
MORRISON & FOERSTER LLP
755 PAGE MILL RD
PALO ALTO
CA
94304-1018
US
|
Assignee: |
The Regents of the University of
California
Oakland
CA
|
Family ID: |
24657148 |
Appl. No.: |
11/050082 |
Filed: |
February 2, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11050082 |
Feb 2, 2005 |
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10223917 |
Aug 20, 2002 |
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6878974 |
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10223917 |
Aug 20, 2002 |
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09243173 |
Feb 2, 1999 |
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6534329 |
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09243173 |
Feb 2, 1999 |
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08359883 |
Dec 20, 1994 |
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5869350 |
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08359883 |
Dec 20, 1994 |
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07662290 |
Feb 27, 1991 |
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5408109 |
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Current U.S.
Class: |
257/103 |
Current CPC
Class: |
H01L 51/56 20130101;
H01L 51/0012 20130101; H01L 51/5012 20130101; H01L 51/5203
20130101; H01L 51/5221 20130101; H01L 51/5206 20130101; H01L
51/5293 20130101; H01L 2251/5338 20130101; H01L 51/0038
20130101 |
Class at
Publication: |
257/103 |
International
Class: |
H01L 029/24 |
Claims
1-47. (canceled)
48. A light-emitting diode which emits visible light and which has
a quantum efficiency of at least 1%, comprising: i) a substrate;
ii) a transparent conducting first layer coated onto said
substrate, said first layer having high work function and serving
as a hole-injecting electrode; iii) a transparent layer of a
soluble semiconducting luminescent conjugated polymer fabricated
onto the transparent conducting first layer; and iv) an
electron-injecting contact fabricated from a low work function
metal onto the semiconducting conjugated polymer layer.
49. The light-emitting diode of claim 48, wherein the substrate is
a transparent, inorganic substrate.
50. The light-emitting diode of claim 48, wherein the substrate is
a transparent, organic polymer substrate.
51. The light-emitting diode of claim 48, wherein the conducting
first layer is an electronegative metal.
52. The light-emitting diode of claim 48, wherein the conducting
first layer is a conductive metal-metal oxide mixture.
53. The light-emitting diode of claim 48, wherein the
semiconducting conjugated polymer layer comprises
poly(2-methoxy,5-(2'-ethylhexyloxy)-1,- 4-phenylenevinylene).
54. The light-emitting diode of claim 48, wherein the low work
function metal is calcium.
55. The light-emitting diode of claim 48, wherein the
semiconducting conjugated polymer layer comprises a semiconducting
conjugated polymer selected from the group consisting of soluble
alkoxy derivatives of poly(phenylenevinylene).
56. The light-emitting diode of claim 48, wherein the conducting
first layer comprises gold or silver.
57. The light-emitting diode of claim 48, wherein the conducting
first layer comprises indium-tin oxide.
58. The light-emitting diode of claim 48, wherein the substrate
comprises an organic polymer selected from the group consisting of
polyesters, polycarbonates, polyacrylates, and polystyrenes.
59. The light-emitting diode of claim 48, wherein the low work
function metal is barium.
60. A light-emitting diode which emits visible light and which has
a quantum efficiency of at least 1%, comprising: i) a substrate;
ii) a transparent conducting first layer coated onto said
substrate, said first layer having high work function and serving
as a hole-injecting electrode; iii) a transparent layer of a
soluble semiconducting luminescent conjugated polymer fabricated
onto the transparent conducting first layer; and iv) an
electron-injecting contact fabricated from calcium or a lower work
function alkaline earth metal onto the semiconducting conjugated
polymer layer.
61. The light-emitting diode of claim 60, wherein the substrate is
a transparent, inorganic substrate.
62. The light-emitting diode of claim 60, wherein the substrate is
a transparent, organic polymer substrate.
63. The light-emitting diode of claim 60, wherein the substrate
comprises an organic polymer selected from the group consisting of
polyesters, polycarbonates, polyacrylates, and polystyrenes.
64. The light-emitting diode of claim 60, wherein the conducting
first layer is a conductive metal-metal oxide mixture.
65. The light-emitting diode of claim 60, wherein the conducting
first layer comprises indium-tin oxide.
66. The light-emitting diode of claim 60, wherein the conducting
first layer is an electronegative metal.
67. The light-emitting diode of claim 60, wherein the conducting
first layer comprises gold or silver.
68. The light-emitting diode of claim 60, wherein the
semiconducting conjugated polymer layer comprises a semiconducting
conjugated polymer selected from the group consisting of soluble
alkoxy derivatives of poly(phenylenevinylene).
69. The light-emitting diode of claim 60, wherein the
semiconducting conjugated polymer layer comprises
poly(2-methoxy,5-(2'-ethylhexyloxy)-1,- 4-phenylenevinylene).
70. The light-emitting diode of claim 60, wherein the
electron-injecting contact is calcium.
71. The light-emitting diode of claim 60, wherein the
electron-injecting contact is barium.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to light-emitting diodes
and their fabrication. More particularly, it concerns
light-emitting diodes fabricated from semiconducting (conjugated)
polymers which are soluble in common organic solvents, and yet more
particularly to the fabrication of such diodes on flexible polymer
substrates.
BACKGROUND OF THE INVENTION
[0002] Solid-state light-emitting diodes (LEDs) have found
widespread application in displays, as well as in a variety of less
common applications. Currently, LEDs are fabricated from
conventional semiconductors; for example, gallium arsenide (GaAs),
typically doped with aluminum, indium, or phosphorus. Using this
technology, it is very difficult to make large area displays. In
addition, the LEDs made of these materials are typically limited to
the emission of light at the long wavelength end of the visible
spectrum. For these reasons, there has been considerable interest
for many years in the development of suitable organic materials for
use as the active (light-emitting) components of LEDs. (See
references 1-6). The need for relatively high voltages (i.e.,
voltages incompatible with digital electronics) for the onset of
light emission has been a hindrance to the commercialization of
LEDs fabricated from organic materials.
[0003] The utilization of semiconducting organic polymers (i.e.,
conjugated polymers) in the fabrication of LEDs expands the use of
organic materials in electroluminescent devices and expands the
possible applications for conducting polymers into the area of
active light sources, (see Reference 7) with the possibility of
significant advantages over existing LED technology. Controlling
the energy gap of the polymer, either through the judicious choice
of the conjugated backbone structure or through side-chain
functionalization, should make possible the emission of a variety
of colors throughout the visible spectrum.
[0004] In the prior art, Tomozawa et al (see Reference 8) disclosed
diodes fabricated by casting semiconducting polymers from
solution.
[0005] Also in the art, Burroughs et al (see Reference 7) disclosed
a multi-step process in the fabrication of LED structures
characterized as follows:
[0006] 1) A glass substrate is utilized. The substrate is
pre-coated with a transparent conducting layer of indium/tin oxide
(ITO). This ITO coating, having high work function serves as the
ohmic hole-injecting electrode.
[0007] 2) A soluble precursor polymer to the conjugated polymer,
poly(phenylene vinylene), PPV, is cast from solution onto the
substrate as a thin, semitransparent layer (approximately 100-200
nm).
[0008] 3) The precursor polymer is converted to the final
conjugated PPV by heat treating the precursor polymer (already
formed as a thin film on the substrate) to temperatures in excess
of 200.degree. C. while pumping in vacuum.
[0009] 4) The negative, electron-injecting contact is fabricated
from a low work function metal such as aluminum, or
magnesium-silver alloy; said negative electrode acting as the
rectifying contact in the diode structure.
[0010] The resulting devices showed asymmetric current versus
voltage curves indicative of the formation of a diode, and the
diodes were observed to emit visible light under conditions of
forward bias at bias voltages in excess of about 14 V with quantum
efficiencies up to 0.05%.
[0011] The methods of Burroughs et al, therefore, suffer a number
of specific disadvantages. Because of the use of a rigid glass
substrate, the resulting LED structures are rigid and inflexible.
The need for heating to temperatures in excess of 200.degree. C. to
convert the precursor polymer to the final conjugated polymer
precludes the use of flexible transparent polymer substrates, such
as, for example, polyethyleneterephthalate, polystyrene,
polycarbonate and the like, for the fabrication of flexible LED
structures with novel shapes and forms. The need for heating to
temperatures in excess of 200.degree. C. to convert the precursor
polymer to the final conjugated polymer has the added disadvantage
of possibly creating defects in the conjugated polymer and in
particular at the upper surface of the conjugated polymer which
forms the rectifying contact with the low work function metal.
[0012] Thus, the ability to fabricate light-emitting diodes from
organic materials and in particular from polymers, remains
seriously limited.
[0013] References
[0014] 1. P. S. Vincent, W. A. Barlow, R. A. Hann and G. G.
Roberts, Thin Solid Films, 94, 476 (1982).
[0015] 2. C. W. Tang, S. A. Van Syke, Appl. Phys. Lett. 51, 913
(1987).
[0016] 3. C. W. Tang, S. A. Van Syke and C. H. Chen, J. Appl. Phys.
65, 3610 (1989).
[0017] 4. C. Adachi, S. Tokito, T. Tsutsui, and S. Saito, Appl.
Phys. Lett. 55, 1489 (1989).
[0018] 5. C. Adachi, S. Tokito, T. Tsutsui, and S. Saito, Appl.
Phys. Lett. 56, 799 (1989).
[0019] 6. M. Nohara, M. Hasegawa, C. Hosohawa, H. Tokailin, T.
Kusomoto, Chem. Lett. 189 (1990).
[0020] 7. J. H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N.
Marks, K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes,
Nature 347, 539 (1990).
[0021] 8. H. Tomozawa, D. Braun, S. D. Phillips, R. Worland, A. J.
Heeger, and H. Kroemer, Synth. Met. 28, C687 (1989).
[0022] 9. F. Wudl, P.-M. Allemand, G. Srdanov, Z. Ni, and D.
McBranch, in Materials for Non-linear Optics: Chemical Perspectives
(to be published in 1991).
[0023] 10. S. M. Sze, Physics of Semiconductor Devices (John Wiley
& Sons, New York, 1981).
[0024] 11. a. T. W. Hagler, K. Pakbaz, J. Moulton, F. Wudl, P.
Smith, and A. J. Heeger, Polym. Commun. (in press). b. T. W.
Hagler, K. Pakbaz, K. Voss and A. J. Heeger, Phys. Rev. B. (in
press).
SUMMARY OF THE INVENTION
[0025] It is accordingly an object of the present invention to
overcome the aforementioned disadvantages of the prior art and,
primarily, to provide light-emitting diodes fabricated from
semiconducting polymers which are soluble in the conjugated form
and therefore require no subsequent heat treatment at elevated
temperatures
[0026] It is additionally an object of the present invention to
utilize the processing advantages associated with the fabrication
of diode structures from soluble semiconductor polymers cast from
solution to enable the fabrication of large active areas.
[0027] It is additionally an object of the present invention to
provide light-emitting diodes fabricated from semiconducting
polymers using flexible organic polymer substrates.
[0028] It is additionally an object of the present invention to
provide methods for the fabrication of light-emitting diodes
fabricated from semiconducting polymers which turn on at bias
voltages compatible with digital electronics (i.e., at voltages
less than 5 volts).
[0029] Additional objects, advantages and novel features of the
invention will be set forth in part in the description which
follows, and in part will become apparent to those skilled in the
art on examination of the following, or may be learned by practice
of the invention. The objects and advantages of the invention may
be realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
[0030] In one aspect this invention provides a process for
fabricating light-emitting diodes (LEDs). In this embodiment the
process involves a process for fabrication of light-emitting diodes
which emit visible light. This process comprises the steps of:
[0031] i) precoating a substrate with a transparent conducting
first layer having high work function and serving as an ohmic
hole-injecting electrode;
[0032] ii) casting upon the first layer directly from solution, a
thin transparent layer of a soluble conjugated polymer; and
[0033] iii) fabricating a negative, electron-injecting contact onto
the conjugated polymer film. This contact is formed from a low work
function metal and acts as the rectifying contact in the diode
structure.
[0034] In another embodiment, an alternative process for
fabricating light-emitting diodes which emit visible light is
provided. This process comprises the steps of:
[0035] i) casting a free-standing, semi-transparent film of a
soluble conjugated polymer from solution, said film serving as a
luminescent, semiconducting polymer and simultaneously as a
substrate;
[0036] ii) coating the free-standing, conjugated polymer film on
one side with a transparent conducting first layer having a high
work function and serving as the ohmic hole-injecting electrode;
and
[0037] iii) fabricating a negative electron-injecting contact onto
the other side of the conjugated polymer film. This contact is made
of a low work function metal and acts as the rectifying contact in
the diode structure.
[0038] In yet an additional embodiment this invention provides a
process for making oriented polymer-based LEDs which emit polarized
visible light. This process includes the steps of:
[0039] i) gel-processing a soluble conjugated polymer as a member
of an admixture with ultra-high molecular weight carrier polymer.
The gel-processed mixture is formed into an oriented, free-standing
film in which the conjugated polymer is chain-aligned. This
chain-aligning resulting in polarized luminescence for the
polymer.
[0040] ii) coating the free-standing, oriented polymer film on one
side with a transparent, conducting first layer having high work
function and serving as an ohmic hole-injecting electrode, and
[0041] iii) fabricating a negative, electron-injecting contact onto
the other side of the conjugated polymer film. This contact is
fabricated from a low work function metal and acts as the
rectifying contact in the diode structure.
[0042] In another general aspect this invention provides the LEDs
fabricated by any of these processes. In a more particular aspect
of this invetion, the LED devices employ
poly(2-methoxy,5-(2-ethyl-hexyloxy)-1,4-- phenylene vinylene),
MEH-PPV, as the conjugated polymer. MEH-PPV offers the advantage of
being a conjugated polymer which is soluble in organic solvents.
LED device fabrication is simplified because of the direct casting
of the conjugated polymer from solution.
[0043] Surprisingly, it was found that by using calcium as the low
work function rectifying contact, and by using ITO coated PET films
as the substrate, flexible LED structures are fabricated which
benefit from the excellent mechanical properties of both the
polymer substrate and the conjugated polymer semiconducting layer
and which exhibit the advantageous characteristics of a turn-on
voltage reduced to 3-4 volts (i.e TTL compatible), and a quantum
efficiency which is improved by more than an order of magnitude to
values of approximately 1%.
[0044] Specific advantages of this invention over the prior art
include the following:
[0045] (i) Because the luminescent semiconducting polymer is
soluble in its final conjugated form, there is no need for heat
treatment at elevated temperatures. This greatly simplifies the
fabrication procedure and enables a continuous manufacturing
process,
[0046] (ii) Since the luminescent semiconducting polymer layer can
be cast onto the substrate directly from solution at room
temperature, the LED structure can be fabricated on a flexible
transparent polymer substrate. These polymer films are manufactured
as large area continuous films. Thus, the use of flexible polymer
films as substrate enables the fabrication of large area polymer
LEDs using either a batch process or a continuous process.
[0047] (iii) The use of Calcium as the low work function contact
onto MEH-PPV as the luminescent polymer leads to unexpected
improvements in the efficiency of the device and in the
compatibility of the device with modern digital electronic
circuitry.
DETAILED DESCRIPTION OF THE INVENTION
[0048] The Substrates
[0049] In some embodiments, the conjugated polymer-based LEDs are
prepared on a substrate. The substrate should be transparent and
nonconducting. It can be a rigid material such as a rigid plastic
including rigid acrylates, carbonates, and the like, rigid
inorganic oxides such as glass, quartz, sapphire, and the like. It
can also be a flexible transparent organic polymer such as
polyester--for example polyethyleneterephthalate, flexible
polycarbonate, poly (methyl methacrylate), poly(styrene) and the
like.
[0050] The thickness of this substrate is not critical.
[0051] The Conjugated Polymer
[0052] The invention provides LEDs based on conjugated
polymers.
[0053] In one embodiment the conjugated polymer is cast directly
from a solution onto a precooled substrate to form a
substrate-supported film.
[0054] In another, the conjugated polymer is present as a
free-standing film.
[0055] In a third embodiment, the conjugated polymer is present as
a component of a gel-processed admixture with a carrier polymer and
the film is formed from this admixture. This embodiment offers an
easy way to obtain aligned conjugated polymer structures which lead
to LEDs which can emit polarized light.
[0056] The conjugated polymers used herein include soluble
conjugated polymers known in the art. These include, for example,
poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene) or
"MEH-PPV", P3ATs, poly(3-alkylthiophenes) (where alkyl is from 6 to
16 carbons), such as poly(2,5-dimethoxy-p-phenylene
vinylene)-"PDMPV", and poly(2,5-thienylenevinylene);
poly(phenylenevinylene) or "PPV" and alkoxy derivatives thereof;
and polyanilines. Of these, the MEH-PPV materials are preferred.
The preparation of MEH-PPV is given in the examples herein.
[0057] The conjugated polymer can be deposited or cast directly
from solution. The solvent employed is one which will dissolve the
polymer and not interfere with its subsequent deposition.
[0058] Typically, organic solvents are used. These can include
halohydrocarbons such as methylene chloride, chloroform, and carbon
tetrachloride, aromatic hydrocarbons such as xylene, benzene,
toluene, other hydrocarbons such as decaline, and the like. Mixed
solvents can be used, as well. Polar solvents such as water,
acetone, acids and the like may be suitable. These are merely a
representative exemplification and the solvent can be selected
broadly from materials meeting the criteria set forth above.
[0059] When depositing the conjugated polymer on a substrate, the
solution can be relatively delete, such as from 0.1 to 20% w in
concentration, especially 0.2 to 5% w. Film thicknesses of 50-400
and 100-200 nm are used.
[0060] The Carrier Polymer.
[0061] In some embodiments, the conjugated polymer is present in
admixture with a carrier polymer.
[0062] The criteria for the selection of the carrier polymer are as
follows. The material should allow for the formation of
mechanically coherent films, at low concentrations, and remain
stable in solvents that are capable of dispersing, or dissolving
the conjugated polymers for forming the final film. Low
concentrations of carrier polymer are preferred in order to
minimize processing difficulties, i.e., excessively high viscosity
or the formation of gross inhomogeneities; however the
concentration of the carrier should be high enough to allow for
formation of coherent structures. Preferred carrier polymers are
high molecular weight (M.W.>100,000) flexible chain polymers,
such as polyethylene, isotactic polypropylene, polyethylene oxide,
polystyrene, and the like. Under appropriate conditions, which can
be readily determined by those skilled in the art, these
macromolecular materials enable the formation of coherent
structures from a wide variety of liquids, including water, acids,
and numerous polar and nonpolar organic solvents. Films or sheets
manufactured using these carrier polymers have sufficient
mechanical strength at polymer concentrations as low as 1%, even as
low as 0.1%, by volume to enable the coating and subsequent
processing as desired.
[0063] Mechanically coherent films can also be prepared from lower
molecular weight flexible chain polymers, but generally, higher
concentrations of these carrier polymers are required.
[0064] Selection of the carrier polymer is made primarily on the
basis of compatibility of the conjugated polymer, as well as with
the solvent or solvents used. For example, blending of polar
conducting polymers generally requires carrier structures that are
capable of co-dissolving with or absorbing polar reactants.
Examples of such coherent structures are those comprised of
poly(vinyl alcohol), poly(ethylene oxide), poly-para(phenylene
terephthalate), poly-para-benzamide, etc., and suitable liquids. On
the other hand, if the blending of the final polymer cannot proceed
in a polar environment, nonpolar carrier structures are selected,
such as those containing polyethylene, polypropylene,
poly(butadiene), and the like.
[0065] Turning now to the issue of concentration, it is of crucial
importance that the carrier structure formed have sufficient
mechanical coherence for further handling during the formation of
the final polymer blend. Therefore, the initial concentration of
the carrier polymer generally is selected above 0.1% by volume, and
more preferably above about 0.75% by volume. On the other hand, it
is not desirable to select carrier polymer concentrations exceeding
90% by volume, because this has a diluting effect on the final
conjugated polymer composite product. More preferably, the
concentration of the carrier polymer in the solution is below 50%
by volume, and still more preferably below 25% by volume.
[0066] Thus, solution is provided by dissolving a selected carrier
polymer and conjugated polymer in a compatible solvent (or mixed
solvents) to a predetermined concentration (using the
aforementioned guidelines). In the present process the "compatible
solvent" is a solvent system into which a desired quantity of
soluble conjugated polymer (or soluble precursor polymer) can be
dissolved. The solvent system is also one in which the carrier
polymer is substantially soluble, and will not interfere with the
subsequent structure formation process. The carrier solution is
formed into selected shape, e.g. a fiber, film or the like, by
extrusion or by any other suitable method. The solvent is then
removed (through evaporation, extraction, or any other convenient
method).
[0067] Gels can be formed from the carrier conjugate solution in
various ways, e.g., through chemical crosslinking of the
macromolecules in solution, swelling of cross-linked
macromolecules, thermoreversible gelation, and coagulation of
polymer solutions. In the present invention, the two latter types
of gel formation are preferred, although under certain experimental
conditions, chemically crosslinked gels may be preferred.
[0068] Thermoreversible gelation refers to the physical
transformation of polymer solution to polymer gel upon lower of the
temperature of a homogeneous polymer solution (although in
exceptional cases a temperature elevation may be required). This
mode of polymer gelation requires the preparation of a homogeneous
solution of the selected carrier polymer in an appropriate solvent
according to standard techniques known to those skilled in the art.
The polymer solution is cast or extruded into a fiber, rod or film
form, and the temperature is lowered to below the gelation
temperature of the polymer in order to form coherent gels. This
procedure is well known and is commercially employed, e.g., for the
formation of gels of high molecular weight polyethylene in decalin,
paraffin oil, oligomeric polyolefins, xylene, etc., as precursors
for high strength polyolefin fibers and films.
[0069] "Coagulation" of a polymer solution involves contacting the
solution with a nonsolvent for the dissolved polymer, thus causing
the polymer to precipitate. This process is well known, and is
commercially employed, for example, in the formation of rayon
fibers and films, and spinning of high-performance aramid fibers,
etc.
[0070] Frequently, it is desirable to subject the carrier
polymer/conducting polymer composite to mechanical deformation,
typically by stretching, during or after the initial forming step.
Deformation of polymeric materials is carried out in order to
orient the macromolecules in the direction of draw, which results
in improved mechanical properties. Maximum deformations of
thermoreversible gels are substantially greater than melt processed
materials. (P. Smith and P. J. Lemstra, Colloid and Polym. Sci.,
258, 891, (1980).) The large draw ratios possible with
thermoreversible gels are also advantageous if composite materials
may be prepared with materials limited in their drawability due to
low molecular weights. In the case of conducting polymers, not only
do the mechanical properties improve, but, more importantly, the
electrical conductivity also often displays drastic enhancement by
tensile drawing and the orientation of the conjugated polymer gives
rise to LEDs which will emit polarized light because of the
orientation.
[0071] The Transparent Conducting First Layer
[0072] The conjugated polymer layer of the LEDs of this invention
is bounded on one surface by a transparent conducting first
layer.
[0073] When a substrate is present, this layer is between the
substrate and the conjugated polymer layer. This first layer is a
transparent conductive layer made of a high work function material
that is a material with a work function above 4.5 eV. This layer
can be a film of an electronegative metal such as gold or silver,
with gold being the preferred member of that group. It can also be
formed of a conductive metal-metal oxide mixture such as indium-tin
oxide.
[0074] These layers are commonly deposited by vacuum sputtering (RF
or Magnetron) electron beam evaporation, thermal vapor deposition,
chemical deposition and the like.
[0075] The ohmic contact layer should be low resistance: preferably
less than 300 ohms/square and more preferably less than 100
ohms/square.
[0076] The Electron Injecting Contact
[0077] On the other side of the conjugated polymer film an
electron-injecting contact is present. This is fabricated from a
low work function metal or alloy (a low work function material has
a work function below 4.3. Typical materials include indium,
calcium, barium and magnesium, with calcium being a particularly
good material. These electrodes are applied by using methods
well-known to the art (e.g. evaporated, sputtered, or electron-beam
evaporation) and acting as the rectifying contact in the diode
structure.
EXAMPLES
[0078] This invention will be further described by the following
examples. These are intended to embody the invention but not to
limit its scope.
Example 1
[0079] This example involves the preparation of
poly(2-methoxy,5-(2'-ethyl- hexyloxy)-p-phenylenevinylene)
"MEH-PPV".
[0080] Monomer Synthesis
[0081] 1. Preparation of 1-Methoxy-4-(2-Ethyl-Hexyloxy)Benzene
[0082] A solution of 24.8 g (0.2 mole) of 4-methoxy phenol in 150
ml dry methanol was mixed under nitrogen with 2.5 M solution of
sodium methoxide (1.1 equivalent) and refluxed for 20 min. After
cooling the reaction mixture to room temperature, a solution of
2-ethylbromohexane (42.5 ml, 1.1 equivalent) in 150 ml methanol was
added dropwise. After refluxing for 16 h, the brownish solution
turned light yellow. The methanol was evaporated and the remaining
mixture of the white solid and yellow oil was combined with 200 ml
of ether, washed several times with 10% aqueous sodium hydroxide,
H.sub.2O and dried over MgSO.sub.4. After the solvent was
evaporated, 40 g (85%) of yellow oil was obtained. The crude
material was distilled under vacuum (2.2 mm Hg, b.p.
148-149.degree. C.), to give a clear, viscous liquid. .sup.1H NMR
(CDCl.sub.3) .delta. 6.98 (4H, s, aromatics), 3.8 (5H, t,
O-CH.sub.2, O-CH.sub.3), 0.7-1.7 (15 H, m, C.sub.7H.sub.15. IR
(NaCl plate) 750, 790, 825, 925, 1045, 1105, 1180, 1235, 1290,
1385, 1445, 1470, 1510, 1595, 1615, 1850, 2030, 2870, 2920, 2960,
3040. MS. Anal. Calc. for C.sub.15H.sub.24O.sub.2: C, 76.23; H,
10.23; O, 13.54. Found: C, 76.38; H, 10.21; O, 13.45.
[0083] 2. Preparation of
2,5-bis(Chloromethyl)-1-Methoxy-4-(2-Ethyl-Hexylo- xy)Benzene
[0084] To the solution of 4.9 g (20.7 mmoles) of compound (1) in
100 ml p-dioxane cooled down to 0-5.degree. C., 18 ml of conc. HCl,
and 10 ml of 37% aqueous formalin solution was added. Anhydrous HCl
was bubbled for 30 min, the reaction mixture warmed up to R.T. and
stirred for 1.5-2 h. Another 10 ml of formalin solution was added
and HCl gas bubbled for 5-10 min at 0-5.degree. C. After stirring
at R.T. for 16 h, and then refluxed for 3-4 h. After cooling and
removing the solvents, an off-white "greasy" solid was obtained.
The material was dissolved in a minimum amount of hexane and
precipitated by adding methanol until the solution became cloudy.
After cooling, filtering and washing with cold methanol, 3.4 g
(52%) of white crystalline material (mp 52-54.degree. C.) was
obtained. .sup.1H NMR (CDCl.sub.3) .delta. 6.98 (2H, s, aromatics),
4.65 (4H, s, CH.sub.2-Cl), 3.86 (5H, t, O-CH.sub.3, O-CH.sub.2),
0.9-1.5 (15H, m, C.sub.7H.sub.15), IR (KBr) 610, 700, 740, 875,
915, 1045, 1140, 1185, 1230, 1265, 1320, 1420, 1470, 1520, 1620,
1730, 2880, 2930, 2960, 3050. MS. Anal. Calc. for
C.sub.17H.sub.26O.sub.2Cl.sub.2: C, 61.26; H, 7.86; 0, 9.60; Cl,
21.27. Found: C, 61.31; h, 7.74; O, 9.72; Cl, 21.39.
[0085] Polymerization
[0086] Preparation of
Poly(1-Methoxy-4-(2-Ethylhexyloxy-2,5-Phenylenevinyl- ene)
MEH-MPV
[0087] To a solution of 1.0 g (3 mmol) of
2,5-bis(chloromethyl)-methoxy-4-- (2-ethylhexyloxy)benzene in 20 ml
of anhydrous THF was added dropwise a solution of 2.12 g (18 mmol)
of 95% potassium tert-butoxide in 80 ml of anhydrous THF at R.T.
with stirring. The reaction mixture was stirred at ambient
temperature for 24 h and poured into 500 ml of methanol with
stirring. The resulting red precipitate was washed with distilled
water and reprecipitated from THF/methanol and dried under vacuum
to afford 0.35 g (45% yield). UV (CHCl.sub.3) 500. IR (film) 695,
850, 960, 1035, 1200, 1250, 1350, 1410, 1460, 1500, 2840, 2900,
2940, 3040. Anal. Calc. for C.sub.17H.sub.24O.sub.2: C, 78.46; H,
9.23. Found: C, 78.34; H, 9.26.
[0088] Molecular weight (GPC vs. polystyrene) 3.times.10.sup.5.
Inherent viscosity.about.5 dl/g (but time dependent due to the
tendency to form aggregates). As is the case with a few other stiff
chain polymers, the viscosity increases with standing, particularly
in benzene. The resulting solution is therefore thixotropic.
[0089] The conjugated polymer is highly colored (bright
red-orange).
Example 2
Preparation of MEH-PPV via a Precursor Polymer Route
[0090] Monomer Synthesis
[0091] The monomer synthesis is exactly the same as in Example
1.
[0092] Polymerization of the Precursor Polymer and Conversion to
MEH-PPV
[0093] A solution of 200 mg (0.39 mmol) of the monomer salt of
Example 1 in 1.2 ml dry methanbl was cooled to 0.degree. C. for 10
min and a cold degassed solution of 28 mg (1.7 equivalents) of
sodium hydroxide in 0.7 ml methanol was added slowly. After 10 min
the reaction mixture became yellow and viscous. The above mixture
was maintained at 0.degree. C. for another 2-3 h and then the
solution was neutralized. A very thick, gum-like material was
transferred into a Spectrapore membrane (MW cutoff 12,000-14,000)
and dialyzed in degassed methanol containing 1% water for 3 days.
After drying in vacuo, 70 mg (47%) of "plastic" yellow precursor
polymer material was obtained. UV (CHCl.sub.3) 365. IR (film) 740,
805, 870, 1045, 1075, 1100, 1125, 1210, 1270, 1420, 1470, 1510,
2930, 2970, 3020. Soluble in C.sub.6H.sub.5Cl,
C.sub.6H.sub.3Cl.sub.3, CH.sub.2C.sub.2, CHCl.sub.3, Et.sub.2O,
THF. Insoluble in MeOH.
[0094] The precursor polymer was converted to the conjugated
MEH-PPV by heating to reflux (approx. 214.degree. C.) in
1,2,4-trichlorobenzene solvent. The product was identical with the
material obtained in Example 1.
Example 3
[0095] Light-emitting diodes (LEDs) were fabricated consisting of a
rectifying indium (work function=4.2 eV, Reference 10) contact on
the front surface of an MEH-PPV film which is deposited by
spin-casting from dilute tetrahydrofuran solution containing 1%
MEH-PPV by weight onto a glass substrate. The resulting MEH-PPV
films have uniform surfaces with thicknesses near 1200 .ANG.. The
glass substrate had been previously coated with a layer of
indium/tin-oxide to form an ohmic contact. The Indium contact is
deposited on top of the MEH-PPV polymer film by vacuum evaporation
at pressures below 4.times.10.sup.-7 Torr yielding active areas of
0.04 cm.sup.2.
[0096] While ramping the applied bias, yellow-orange light becomes
visible to the eye just below 9 V forward bias (no light is
observed under reversed bias). Above 15 V, the rectification ratio
of the diode exceeds 10.sup.4.
[0097] The EL spectra, obtained with 3 V AC superposed (at 681 Hz)
on 13V forward bias, showed characteristic spectral features
similar to those observed in the photoluminescence of
MEH-PPV..sup.11 The room temperature electroluminescence peaks near
2.1 eV with a hint of a second peak above 1.9 eV. At 90K, the
intensity increases and shifts to the red, and the two peaks become
clearly resolved.
[0098] The electroluminecence intensity was measured as a function
of current flow under increasing forward bias. The quantum
efficiency was determined with a calibrated Silicon photodiode and
corrected for the spectral response and the solid angle of the
collecting optics. The measured quantum efficiency at 0.8 mA is
.apprxeq.5.times.10.sup.-4 photons per electron for Indium
electrodes.
Example 4
[0099] Light-emitting diodes (LEDs) were fabricated consisting of a
rectifying calcium (work function=3 eV, Reference 10) contact on
the front surface of an MEH-PPV film which is deposited by
spin-casting from dilute solution onto a glass substrate. The
resulting MEH-PPV films have uniform surfaces with thicknesses near
1200 .ANG.. The glass substrate has been partially coated with a
layer of indium/tin-oxide to form an "ohmic" contact. The calcium
contact is deposited on top of the MEH-PPV polymer film by vacuum
evaporation at pressures below 4.times.10.sup.-7 Torr yielding
active areas of 0.04 cm.sup.2.
[0100] For the calcium/MEH-PPV diodes, rectification ratios as high
as 10.sup.5 are achieved.
[0101] While ramping the applied bias, yellow-orange light becomes
visible to the eye just above 3 V forward bias (no light is
observed under reversed bias). The quantum efficiency was
determined with a calibrated Silicon photodiode and corrected for
the spectral response and the solid angle of the collecting optics.
The measured quantum efficiency at 0.8 mA is
.apprxeq.7.times.10.sup.-3 photons per electron for calcium
electrodes (i.e., nearly 1%!!). The emission from the
Calcium/MEH-PPV LEDs is bright and easily seen in a lighted room at
4V forward bias.
Example 5
[0102] Light-emitting diodes (LEDs) were fabricated consisting of a
rectifying calcium (work function=3 eV, Reference 10) contact on
the front surface of an MEH-PPV film which is deposited by
spin-casting from dilute solution onto a flexible transparent
polyethyleneterephthalate (PET) film (7 mils thickness) as
substrate. The resulting MEH-PPV films on PET have uniform surfaces
with thicknesses near 1200 .ANG.. The PET substrate is pre-coated
with a layer of indium/tin-oxide to form an "ohmic" contact. The
calcium rectifying contact is deposited on top of the MEH-PPV
polymer film by vacuum evaporation at pressures below
4.times.10.sup.-7 Torr yielding active areas of 0.04 cm.sup.2.
[0103] For the calcium/MEH-PPV diodes, rectification ratios as high
as 10.sup.3 are achieved.
[0104] While ramping the applied bias, yellow-orange light becomes
visible to the eye just above 9 V forward bias (no light is
observed under reversed bias). The quantum efficiency was
determined with a calibrated Silicon photodiode and corrected for
the spectral response and the solid angle of the collecting optics.
The measured quantum efficiency at 5.5 .mu.A is 4.times.10.sup.-3
photons per electron for calcium electrodes. The emission from the
calcium/MEH-PPV LEDs is bright and easily seen in a lighted room at
4V forward bias.
Example 6
[0105] MEH-PPV is cast onto a film of pure UHMW-PE which has been
stretched to a moderate draw ratio (e.g. draw ratio >20,
Reference 11). The MEH-PPV is observed to orient spontaneously
along the draw direction; both the photo-absorption and the
photoluminescence spectra are highly anisotropic. Since the
luminescence spectrum is polarized with electric vector along the
chain alignment direction, light-emitting diodes can be fabricated
which emit polarized light.
Example 7
[0106] MEH-PPV was gel-processed and chain oriented as a guest in
UHMW-PE. The gel-processing of conjugated polymer as a guest in a
gel-processed blend involves three steps:
[0107] A. Co-solution with a suitable carrier polymer
[0108] B. Carrier Structure Formation
[0109] C. Drawing of the Carrier/Polymer blend.
[0110] Carrier Solution Preparation, Film Formation, Gelation, and
Drawing.
[0111] PE-MEH-PPV blends are prepared by mixing MEH-PPV
(M.sub.w=450,000) in xylene with UHMW polyethylene (Hostalen GUR
415; M.sub.w=4.times.10.sup.6) in xylene such that the PE to
solvent ratio was 0.75% by weight. This solution is thoroughly
mixed and allowed to equilibrate in a hot oil bath at 126.degree.
C. for one hour. The solution is then poured onto a glass surface
to cool, forming a gel which was allowed to dry (into a film).
Films were then cut into strips and tensile-drawn over a hot pin at
110-120.degree. C. Once processed in this manner, the films are
oriented. The high work function and low work function electrodes
are offered as in Examples 4 and 5, and LEDs result.
* * * * *