U.S. patent application number 10/992624 was filed with the patent office on 2005-11-24 for organic light - emitting diodes and methods for assembly and enhanced charge injection.
Invention is credited to Cui, Ji, Huang, Qinglan, Marks, Tobin J., Veinot, Jonathan.
Application Number | 20050260443 10/992624 |
Document ID | / |
Family ID | 28039524 |
Filed Date | 2005-11-24 |
United States Patent
Application |
20050260443 |
Kind Code |
A1 |
Marks, Tobin J. ; et
al. |
November 24, 2005 |
Organic light - emitting diodes and methods for assembly and
enhanced charge injection
Abstract
New organic light-emitting diodes and related electroluminescent
devices and methods for fabrication, using siloxane self-assembly
techniques.
Inventors: |
Marks, Tobin J.; (Evanston,
IL) ; Huang, Qinglan; (Skokie, IL) ; Cui,
Ji; (Acton, MA) ; Veinot, Jonathan; (Palatine,
IL) |
Correspondence
Address: |
REINHART BOERNER VAN DEUREN S.C.
ATTN: LINDA GABRIEL, DOCKET COORDINATOR
1000 NORTH WATER STREET
SUITE 2100
MILWAUKEE
WI
53202
US
|
Family ID: |
28039524 |
Appl. No.: |
10/992624 |
Filed: |
November 18, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10992624 |
Nov 18, 2004 |
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10099131 |
Mar 15, 2002 |
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6939625 |
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10099131 |
Mar 15, 2002 |
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09187891 |
Nov 6, 1998 |
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6399221 |
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09187891 |
Nov 6, 1998 |
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08673600 |
Jun 25, 1996 |
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5834100 |
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Current U.S.
Class: |
428/690 ;
313/504; 313/506; 427/66; 428/917 |
Current CPC
Class: |
H01L 51/0059 20130101;
H01L 51/5012 20130101; H01L 51/007 20130101; H01L 51/5048 20130101;
B82Y 30/00 20130101; H01L 51/0012 20130101; H01L 51/0043 20130101;
H01L 51/0078 20130101; H01L 51/0069 20130101; H01L 51/0081
20130101; H01L 51/0077 20130101; H01L 2251/308 20130101; H01L
51/0075 20130101; Y10T 428/31663 20150401; H01L 51/0036 20130101;
H01L 51/5088 20130101; H01L 51/0595 20130101; H01L 51/0034
20130101; Y10S 428/917 20130101; B82Y 10/00 20130101; H01L 51/0039
20130101; H01L 51/0037 20130101; H01L 51/0035 20130101; H01L
51/0094 20130101 |
Class at
Publication: |
428/690 ;
428/917; 313/504; 313/506; 427/066 |
International
Class: |
H05B 033/12 |
Goverment Interests
[0002] The United States Government has certain rights to this
invention pursuant to Grant Nos. N0014-95-1-1319 and DMR-00769097
from the Office of Naval Research and National Science Foundation,
respectively, to Northwestern University.
Claims
1. A method of using an amine molecular component to enhance hole
injection across the electrode-organic interface of a light
emitting diode device, said method comprising: providing an anode;
and incorporating an electroluminescent medium adjacent said anode,
said medium comprising an amine molecular layer, coupled to said
anode, said molecular layer having at the least one of an arylamine
molecular component and an arylalkylamine molecular component, each
said component substituted with at least one silyl group, and on
said molecular layer a hole transport layer of molecular components
having said amine structure.
2. The method of claim 1 wherein said molecular layer components
are selected from the group consisting of alkylsilyl-substituted
compounds of FIGS. 2A and 2C.
3. The method of claim 2 wherein said molecular layer components
are selected from the group consisting of alkylsilyl-substituted
TAA and alkylsilyl-substituted TPD.
4. The method of claim 3 wherein said molecular layer is
spin-coated on said anode.
5. The method of claim 3 wherein said anode is immersed in a
solution of said molecular layer components.
6. The method of claim 1 wherein a plurality of molecular layers
are coupled to said anode.
7. The method of claim 1 wherein said hole transport layer is
TPD.
8. The method of claim 7 wherein said hole transport layer is
spin-coated on said anode.
9. An electroluminescent device for generating light upon
application of an electrical potential across two electrodes, said
device comprising: an anode; at least one amine molecular layer,
coupled to said anode, said molecular layer having at least one of
an arylamine molecular component and an arylalkylamine molecular
component, each said component substituted with at least one silyl
group; a conductive layer of molecular components having said amine
structure; and a cathode in electrical contact with said anode
layer.
10. The device of claim 9 wherein said molecular layer components
are selected from the group consisting of alkylsilyl-substituted
compounds of FIGS. 2A and 2C.
11. The device of claim 10 wherein said molecular layer components
are selected from the group consisting of alkylsilyl-substituted
TAA and alkylsilyl-substituted TPD.
12. The device of claim 11 wherein said conductive layer is a hole
transport layer of TPD.
13. The device of claim 9 wherein a plurality of molecular layers
are coupled to said anode.
14. An electroluminescent device for generating light upon
application of an electrical potential across two electrodes, said
device comprising: an anode; at least one molecular layer, coupled
to said anode, of arylamine molecular components substituted with
at least two silyl groups; a hole transport layer of TPD molecular
components, said hole transport layer substantially without
crystallization upon annealing; and a cathode in electrical contact
with said anode.
15. The device of claim 14 wherein said molecular layer components
are selected from the group consisting of alkylsilyl-substituted
TAA and alkylsilyl-substituted TPD.
16. The device of claim 14 wherein a plurality of molecular layers
are coupled to said anode.
17. The device of claim 14 further including an electron transport
layer.
18. An electroluminescent device for generating light upon
application of an electrical potential across two electrodes, said
device comprising; an anode; at least one molecular layer, coupled
to said anode, of alkylsilyl-substituted TPD molecular components;
a hole transport layer of TPD molecular components and a cathode in
electrical contact with said anode.
19. The device of claim 18 further including, an electron transport
layer.
20. The device of claim 18 wherein a plurality of molecular layers
are coupled to said anode.
Description
[0001] This application is a continuation-in-part of and claims
priority benefit from application Ser. No. 09/187,891, filed on
Nov. 6, 1998, which is a continuation-in-part of application Ser.
No. 08/673,600, filed on Jun. 25, 1996, and issued as U.S. Pat. No.
5,834,100, each of which are incorporated herein by reference in
their entirety.
BACKGROUND OF THE INVENTION
[0003] This invention relates generally to organic
electroluminescent devices with organic films between anodic and
cathodic electrodes, and more particularly to such devices and
methods for their assembly using the condensation of various
silicon moieties.
[0004] Organic electroluminescent devices have been known, in
various degrees of sophistication, since the early 1970's.
Throughout their development and consistent with their function and
mode of operation, they can be described generally by way of their
physical construction. Such devices are characterized generally by
two electrodes which are separated by a series of layered organic
films that emit light when an electric potential is applied across
the two electrodes. A typical device can consist, in sequence, of
an anode, an organic hole injection layer, an organic hole
transport layer, an organic electron transport layer, and a
cathode. Holes are generated at a transparent electrode, such as
one constructed of indium-tin-oxide, and transported through a
hole-injecting or hole-transporting layer to an interface with an
electron-transporting or electron-injecting layer which transports
electrons from a metal electrode. An emissive layer can also be
incorporated at the interface between the hole-transporting layer
and the electron-transporting layer to improve emission efficiency
and to modify the color of the emitted light.
[0005] Significant progress has been made in the design and
construction of polymer- and molecule-based electroluminescent
devices, for light-emitting diodes, displays and the like. Other
structures have been explored and include the designated "DH"
structure which does not include the hole injection layer, the
"SH-A" structure which does not include the hole injection layer or
the electron transport layer, and the "SH-B" structure which does
not include the hole injection layer or the hole transport layer.
See, U.S. Pat. No. 5,457,357 and in particular col. 1 thereof,
which is incorporated herein by reference in its entirety.
[0006] The search for an efficient, effective electroluminescent
device and/or method for its production has been an ongoing
concern. Several approaches have been used with certain success.
However, the prior art has associated with it a number of
significant problems and deficiencies. Most are related to the
devices and the methods by which they are constructed, and result
from the polymeric and/or molecular components and assembly
techniques used therewith.
[0007] The fabrication of polymer-based electroluminescent devices
employs spin coating techniques to apply the layers used for the
device. This approach is limited by the inherently poor control of
the layer thickness in polymer spin coating, diffusion between the
layers, pinholes in the layers, and inability to produce thin
layers which leads to poor light collection efficiency and the
necessity of high D.C. driving voltages. The types of useful
polymers, typically poly(phenylenevinylenes), are greatly limited
and most are environmentally unstable over prolonged use
periods.
[0008] The molecule-based approach uses vapor deposition techniques
to put down thin films of volatile molecules. It offers the
potential of a wide choice of possible building blocks, for
tailoring emissive and other characteristics, and reasonably
precise layer thickness control. Impressive advances have recently
been achieved in molecular building blocks--especially in electron
transporters and emitters, layer structure design (three versus two
layers), and light collection/transmission structures
(microcavities).
[0009] Nevertheless, further advances must be made before these
devices are optimum. Component layers which are thinner than
achievable by organic vapor deposition techniques would allow lower
DC driving voltages and better light transmission collection
characteristics. Many of the desirable component molecules are
nonvolatile or poorly volatile, with the latter requiring
expensive, high vacuum or MBE growth equipment. Such line-of-site
growth techniques also have limitation in terms of conformal
coverage. Furthermore, many of the desirable molecular components
do not form smooth, pinhole-free, transparent films under these
conditions nor do they form epitaxial/quasiepitaxial multilayers
having abrupt interfaces. Finally, the mechanical stability of
molecule-based films can be problematic, especially for large-area
applications or on flexible backings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIGS. 1A and 1B show structural formulae for porphyrinic
compounds which are illustrative examples of compounds of the type
which can be used as hole injection components/agents in the
preparation of the molecular conductive or hole injection layers
and electroluminescent media of this invention. In FIG. 1, M is Cu,
Zn, SiCl.sub.2, or 2H; Q is N or C(X), where X is a substituted or
unsubstituted alkyl or aryl group; and R is H, trichlorosilyl,
trialkoxysilyl, or a moiety having 1 to 6 carbon atoms which can
include trichlorosilyl or trialkoxysilyl groups, substituted on the
C.sub.1-C.sub.4, C.sub.8-C.sub.11, C.sub.15- C.sub.18 and/or
C.sub.22-C.sub.25 positions. In FIG. B, M is Cu, Zn, SiCl.sub.2, or
2H; Q is N or C(X), where X is a substituted or unsubstituted alkyl
or aryl group; and T.sub.1/T.sub.2 is H, trichlorosilyl,
trialkoxysilyl, or a moiety having 1 to 6 carbon atoms which can
include trichlorosilyl or trialkoxysilyl groups.
[0011] FIGS. 2A-2C show structural formulae for arylamine compounds
which are illustrative examples of compounds of the type which can
be used as hole transport compounds/agents in the preparation of
the molecular conductive or hole transport layers and
electroluminescent media of this invention. In FIG. 2A, R.sub.2,
R.sub.3 and/or R.sub.4 can be H, trihalosilyl, trialkoxysilyl,
dihalosilyl, dialkoxysilyl, or a moiety having 1 to 6 carbon atoms
which can include dialkyldichlorosilyl, dialkyldialkoxysilyl,
trichlorosilyl or trialkoxysilyl groups substituted anywhere on the
aryl positions. In FIG. 2B, Q.sub.1 and Q.sub.2 can be substituted
or unsubstituted tertiary aryl amines, such as those described with
FIG. 2A; and G is a linking group to include but not limited to an
alkyl, aryl, cylcohexyl or heteroatom group. In FIG. 2C, Ar is an
arylene group; n is the number of arylene groups from 1-4; and
R.sub.5, R.sub.6, R.sub.7, and/or R.sub.8 can be H, trihalosilyl,
trialkoxysilyl, dihalosilyl, dialkoxysilyl or a moiety having 1 to
6 carbon atoms which can include dialkyldichlorosilyl,
dialkyldialkoxysilyl, trichlorosilyl or trialkoxysilyl groups
substituted anywhere on the aryl positions.
[0012] FIGS. 3A-3C show structural formulae for aryl compounds
which are illustrative of examples of compounds of the type which
can be used as emissive compounds/agents in the preparation of the
molecular conductive layers and electroluminescent media of this
invention. In FIG. 3A, R.sub.9 and R.sub.10 can be H, trihalosilyl,
trialkoxysilyl, dihalosilyl, dialkoxysilyl, or a moiety having 1 to
6 carbon atoms which can include dialkyldichlorosilyl,
dialkyldialkoxysilyl, trichlorosilyl or trialkoxysilyl groups
substituted anywhere on the aryl positions. In FIG. 3B, M is Al or
Ga; and R.sub.11-R.sub.14 can be H, trihalosilyl, trialkoxysilyl,
dihalosilyl, dialkoxysilyl, or a moiety having 1 to 6 carbon atoms
which can include dialkyldichlorosilyl, dialkyldialkoxysilyl,
trichlorosilyl or trialkoxysilyl groups substituted anywhere on the
aryl positions. In FIG. 3C, Ar is arylene; and R.sub.15-R.sub.18
can be H, trihalosilyl, trialkoxysilyl, dihalosilyl, dialkoxysilyl,
or a moiety having 1 to 6 carbon atoms which can include
dialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilyl or
trialkoxysilyl groups substituted anywhere on the aryl
positions.
[0013] FIGS. 4A-4C show structural formulae for heterocyclic
compounds which are illustrative examples of compounds of the type
which can be used as electron transport components/agents in the
preparation of the molecular conductive or electron transport
layers and in electroluminescent media of this invention. In FIGS.
4A-4C, X is O or S; and R.sub.19-R.sub.24 can be aryl groups
substituted with the following substituents anywhere on the aryl
ring: trihalosilyl, trialkoxysilyl, dihalosilyl, dialkoxysilyl, or
a moiety having 1 to 6 carbon atoms which can contain
dialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilyl or
trialkoxysilyl groups.
[0014] FIGS. 5A and 5B (ITO is indium-tin-oxide; HTL is hole
transport layer and ETL is electron transport layer) show,
schematically and in a step-wise manner by way of illustrating the
present invention, use of the components/agents of Examples 1-5 and
FIGS. 1-4 in the self-assembly and preparation of an organic
light-emitting diode device. In particular, the molecular
representation FIG. 5A illustrates the hydrolysis of an assembled
silicon/silane component/agent to provide an Si--OH functionality
reactive toward a silicon/silane moiety of another component, agent
or conductive layer. The block and molecular representations of
FIG. 5B illustrate a completed assembly.
[0015] FIG. 6 shows an alternative synthetic sequence enroute to
several arylamine components/agents, also in accordance with the
present invention.
[0016] FIG. 7 shows, schematically and by way of illustrating an
alternative embodiment of the present invention, use of the
components/agents of FIG. 6 in the preparation of another
representative electroluminescent device.
[0017] FIG. 8 graphically correlates x-ray reflectivity
measurements of film thickness with the number of capping layers
applied to a substrate. As calculated from the slope of the line
(y=8.3184x), each layer is about 7.84 .ANG. in dimensional
thickness.
[0018] FIG. 9 graphically shows cyclic voltametry measurements,
using 10.sup.-3M ferrocene in acetonitrile, taken after successive
layer (c-e) deposition and as compared to a bare ITO electrode (a).
Even one capping layer (b), in accordance with this invention,
effectively blocks the electrode surface. Complete blocking is
observed after deposition of three or four layers. The sweep rate
was 100 mV/sec, and the electrode area was about 0.7 cm.sup.2.
[0019] FIGS. 10A-C graphically illustrate various utilities and/or
performance characteristics (current density, quantum efficiency
and forward light output, respectively, versus voltage) achievable
through use of the present invention, as a function of the number
of capping layers on an electrode surface. 0 layers, bare ITO
(<>), 1 layer, 8 .ANG. (.quadrature.), 2 layers, 17 .ANG.
(.circle-solid.), 3 layers, 25 .ANG. (.tangle-solidup.) and 4
layers, 33 .ANG. (.gradient.). Reference is made to example 10.
[0020] FIG. 11 A shows molecular structures of hole
adhesion/injection molecular components: TAA (shown after
crosslinking), TPD-Si.sub.2 (shown after crosslinking), and prior
art copper phthalocyanine, Cu(Pc). FIG. 11B illustrates one
possible scheme for the synthesis of a preferred TPD-Si.sub.2
adhesion/injection interlayer molecular precursor. Reference is
also made to the procedures described in Example 2.
[0021] FIGS. 12A-D provide optical microscopic images of
vapor-deposited TPD film (100 nm) morphology after annealing at
80.degree. C. for 1.0 h on ITO substrates coated with a cured 40 nm
thick TPD-Si.sub.2 film (12A) and on bare ITO (12B); polarized
optical image of TPD film (100 nm) morphology before (12C) and
after (12D) annealing the bilayer structure: ITO/CuPc(10 nm)/TPD
(100 nm) at 80.degree. C. for 0.50 h.
[0022] FIGS. 13A-C show, in turn, (A) light output, (B) external
quantum efficiency, and (C) current-voltage characteristics as a
function of operating voltage for OLED devices having the
structure: ITO/(adhesion/injection/molecular component
interlayer)/TPD hole transport layer (50 nm)/Alq (60 nm)/Al, where
the injection/adhesion component interlayer is prior art Cu(Pc) (10
nm), TAA (15 nm), and TPD-Si.sub.2 (40 nm).
[0023] FIG. 14 compares injection characteristics of hole-only
devices having the structure ITO/molecular component interlayer/TPD
(250 nm)/Au(5 nm)/Al (180 nm) for various anode functionalization
layers.
[0024] FIG. 15 graphically illustrates by comparison the effect of
thermal stressing (90.degree. C. under vacuum) on device
characteristics of ITO/(injection/adhesion interlayer)/TPD(50
nm)/Alq(60 nm)/Al (100 nm) devices, where the molecular component
interlayer is TPD-Si.sub.2 (40 nm), TAA (15 nm), and prior art CuPc
(10 nm).
SUMMARY OF THE INVENTION
[0025] In light of the foregoing, it is an object of the present
invention to provide electroluminescent articles and/or devices and
method(s) for their production and/or assembly, thereby overcoming
various deficiencies and shortcomings of the prior art, including
those outlined above. It will be understood by those skilled in the
art that one or more aspects of this invention can meet certain
objectives, while one or more other aspects can meet certain other
objectives. Each objective may not apply equally, in all its
respects, to every aspect of this invention. As such, the following
objects can be viewed the alternative with respect to any one
aspect of this invention.
[0026] It is an object of the present invention to provide control
over the thickness dimension of a luminescent medium and/or the
conductive layers of such a medium, to control the wavelength of
light emitted from any electroluminescent device and enhance the
efficiency of such emission.
[0027] It can be another object of the present invention to provide
molecular components for the construction and/or modification of an
electroluminescent medium and/or the conductive layers thereof,
which will allow lower driving and/or turn-on voltages than are
available through use of conventional materials.
[0028] It can also be an object of the present invention to provide
component molecules which can be used effectively in liquid media
without resort to high vacuum or MBE growth equipment.
[0029] It can also be an object of the present invention to provide
conformal conductive layers and the molecular components thereof
which allows for the smooth, uniform deposition on an electrode,
substrate surface and/or previously-deposited layers.
[0030] It can also be an object of this invention to provide an
electroluminescent medium having a hybrid structure and where one
or more of the layers is applied by a spin-coat or vapor deposition
technique to one or more self-assembled conductive layers.
[0031] Other objects, features and advantages of the present
invention will be apparent from this summary of the invention and
its descriptions of various preferred embodiments, and will be
readily apparent to those skilled in the art having knowledge of
various electroluminescent devices and assembly/production
techniques. Such objects, features, benefits and advantages will be
apparent from the above as taken into conjunction with the
accompanying examples, data, figures and all reasonable inferences
to be drawn therefrom, alone or with consideration of the
references incorporated herein.
[0032] This invention describes, in part, a new route to the
fabrication of light-emitting organic multilayer heterojunction
devices, useful for both large and small, multicolored display
applications. As described more fully below, electron and hole
transporting layers, as well as the emissive layer, as well as any
other additional layers, are applied, developed and/or modified by
molecular self-assembly techniques. As such, the invention can
provide precise control over the thickness of a luminescent medium
or the conductive layers which make up such a medium, as well as
provide maximum light generation efficiency. Use of the present
invention provides strong covalent bonds between the constituent
molecular components, such that the mechanical, thermal, chemical
and/or photochemical stability of such media and/or conductive
layers, as can be used with an electroluminescent device, are
enhanced. The use of such components also promotes conformal
surface coverage to prevent cracks and pinhole deformities.
[0033] More specifically, the siloxane self-assembly techniques
described herein allow for the construction of molecule-based
electroluminescent media and devices. As described more fully
below, various molecular components can be utilized to control the
thickness dimension of the luminescent media and/or conductive
layers. Nanometer dimensions can be obtained, with self-sealing,
conformal coverage. The resulting covalent, hydrophobic siloxane
network imparts considerable mechanical strength, as well as
enhancing the resistance of such media and/or devices to dielectric
breakdown, moisture intrusion, and other degradative processes.
[0034] In part, the present invention is an electroluminescent
article or device which includes (1) an anode, (2) a plurality of
molecular conductive layers where one of the layers is coupled to
the anode with silicon-oxygen bonds and each of the layers is
coupled one to another with silicon-oxygen bonds, and (3) a cathode
in the electrical contact with the conductive layers. More
generally and within the scope of this invention, an anode is
separated from a cathode by an organic luminescent medium. The
anode and the cathode are connected to an external power source by
conductors. The power source can be a continuous direct,
alternating or an intermittent current voltage source. A convenient
conventional power source, including any desired switching
circuitry, which is capable of positively biasing the anode with
respect to the cathode, can be employed. Either the anode or
cathode can be at ground potential.
[0035] The conductive layers can include but are limited to a hole
transport layer, a hole injection layer, an electron transport
layer and an emissive layer. Under forward biasing conditions, the
anode is at a higher potential than the cathode, and the anode
injects holes (positive charge carriers) into the conductive layers
and/or luminescent medium while the cathode injects electrons
therein. The portion of the layers/medium adjacent to the anode
forms a hole injecting and/or transporting zone while the portion
of the layers/medium adjacent to the cathode forms an electron
injecting and/or transporting zone. The injected holes and
electrons each migrate toward the oppositely charged electrode,
resulting in hole-electron interaction within the organic
luminescent medium of conductive layers. A migrating electron drops
from its conduction potential to a valence band in filling a hole
to release energy as light. In such a manner, the organic
luminescent layers/medium between the electrodes performs as a
luminescent zone receiving mobile charge carriers from each
electrode. Depending upon the construction of the article/device,
the released light can be emitted from the luminescent conductive
layers/medium through one or more of edges separating the
electrodes, through the anode, through the cathode, or through any
combination thereof. See, U.S. Pat. No. 5,409,783 and, in
particular cols. 4-6 and FIG. 1 thereof, which is incorporated
herein by reference in its entirety. As would be understood by
those skilled in the art, reverse biasing of the electrodes will
reverse the direction of mobile charge migration, interrupt charge
injection, and terminate light emission. Consistent with the prior
art, the present invention contemplates a forward biasing DC power
source and reliance on external current interruption or modulation
to regulate light emission.
[0036] As demonstrated and explained below, it is possible to
maintain a current density compatible with efficient light emission
while employing a relatively low voltage across the electrodes by
limiting the total thickness of the organic luminescent medium to
nanometer dimensions. At the molecular dimensions possible through
use of this invention, an applied voltage of less than about 10
volts is sufficient for efficient light emission. As discussed more
thoroughly herein, the thickness of the organic luminescent
conductive layers/medium can be designed to control and/or
determine the wavelength of emitted light, as well as reduce the
applied voltage and/or increase in the field potential.
[0037] Given the nanometer dimensions of the organic luminescent
layers/medium, light is usually emitted through one of the two
electrodes. The electrode can be formed as a translucent or
transparent coating, either on the organic layer/medium or on a
separate translucent or transparent support. The layer/medium
thickness is constructed to balance light transmission (or
extinction) and electrical conductance (or resistance). Other
considerations relating to the design, construction and/or
structure of such articles or devices are as provided in the above
referenced U.S. Pat. No. 5,409,783, such considerations as would be
modified in accordance with the molecular conductive layers and
assembly methods of the present invention.
[0038] In preferred embodiments, the conductive layers have
molecular components, and each molecular component has at least two
silicon moieties. In highly preferred embodiments, each silicon
moiety is a halogenated or alkoxylated silane and silicon-oxygen
bonds are obtainable from the condensation of the silane moieties
with hydroxy functionalities. In preferred embodiments, the present
invention employs an anode with a substrate having a hydroxylated
surface portion. The surface portion is transparent to near-IR and
visible wavelengths of light. In such highly preferred embodiments
the hydroxylated surface portions include SiO.sub.2,
In.sub.2.xSnO.sub.2, Ge and Si, among other such materials.
[0039] In conjunction with anodes and the hydroxylated surface
portions thereof, the conductive layers include molecular
components, and each molecular component has at least two silicon
moieties. As discussed above, in such embodiments, each silicon
moiety is a halogenated or alkoxylated silane, and silicon-oxygen
bonds are obtainable from the condensation of the silane moieties
with hydroxy functionalities which can be on a surface portion of
an anode. Consistent with such preferred embodiments, a cathode is
in electrical contact with the conductive layers. In highly
preferred embodiments, the cathode is vapor deposited on the
conductive layers, and constructed of a material including Al, Mg,
Ag, Au, In, Ca and alloys thereof.
[0040] In part, the present invention is a method of producing a
light-emitting diode having enhanced stability and light generation
efficiency. The method includes (1) providing an anode with a
hydroxylated surface; (2) coupling the surface to a hole transport
layer having a plurality of molecular components, with each
component having at least two silicon moieties reactive with the
surface, with coupling of one of the silicon moieties to form
silicon-oxygen bonds between the surface and the hole transport
layer; (3) coupling the hole transport layer to an electron
transport layer, the electron transport layer having a plurality of
molecular components with each of the components having at least
two silicon moieties reactive with the hole transport layer, with
the coupling of one of the silicon moieties to form silicon-oxygen
bonds between the hole and electron transport layers; and (4)
contacting the electron transport layer with a cathode
material.
[0041] In preferred embodiments of this method, the hole transport
layer includes a hole injecting zone of molecular components and a
hole transporting zone of molecular components. Likewise, in
preferred embodiments, each silicon moiety is a halogenated or
alkoxylated silane such that, with respect to this embodiment,
coupling the hole transport layer to the electron transport layer
further includes hydrolyzing the halogenated or alkoxylated silane.
Likewise, with respect to a halogenated or alkoxylated silane
embodiment, contacting the electron transport layer with the
cathode further includes hydrolyzing the silane.
[0042] In part, the present invention is a method of controlling
the wavelength of light emitted from an electroluminescent device.
The inventive method includes (1) providing in sequence a hole
transport layer, an emissive layer and an electron transport layer
to form a medium of organic luminescent layers; and (2) modifying
the thickness dimension of at least one of the layers, each of the
layers including molecular components corresponding to the layer
and having at least two silicon moieties reactive to a hydroxy
functionality and the layers coupled one to another by Si--O bonds,
the modification by reaction of the corresponding molecular
components one to another to form Si--O bonds between the molecular
components, and the modification in sequence of the provision of
the layers.
[0043] In preferred embodiments of this inventive method, at least
one silicon moiety is unreacted after reaction with a hydroxy
functionality. In highly preferred embodiments, modification then
includes hydrolyzing the unreacted silicon moiety of one of the
molecular components to form a hydroxysilyl functionality and
condensing the hydroxysilyl functionality with a silicon moiety of
another molecular component to form a siloxane bond sequence
between the molecular components.
[0044] In highly preferred embodiments, the silicon moieties are
halogenated or alkoxylated silane moieties. Such embodiments
include modifying the thickness dimension by hydrolyzing the
unreacted silane moiety of one of the molecular components to form
a hydroxysilyl functionality and condensing the hydroxysilyl
functionality with a silane moiety of another molecular component
to form a siloxane bond sequence between the molecular
components.
[0045] While the organic luminescent conductive layers/medium of
this invention can be described as having a single organic hole
injecting or transporting layer and a single electron injecting or
transporting layer, modification of each of these layers with
respect to dimensional thickness or into multiple layers, as more
specifically described below, can result in further refinement or
enhancement of device performance by way of the light emitted
therefrom. When multiple electron injecting and transporting layers
are present, the layer receiving holes is the layer in which
hole-electron interaction occurs, thereby forming the luminescent
or emissive layer of the device.
[0046] The articles/devices of this invention can emit light
through either the cathode or the anode. Where emission is through
the cathode, the anode need not be light transmissive. Transparent
anodes can be formed of selected metal oxides or a combination of
metal oxides having a suitably high work function. Preferred metal
oxides have a work function of greater than 4 electron volts (eV).
Suitable anode metal oxides can be chosen from among the high
(>4 eV) work function materials. A transparent anode can also be
formed of a transparent metal oxide layer on a support or as a
separate foil or sheet.
[0047] The devices/articles of this invention can employ a cathode
constructed of any metal, including any high or low work function
metal, heretofore taught to be useful for this purpose and as
further elaborated in that portion of the incorporated patent
referenced in the preceding paragraph. As mentioned therein,
fabrication, performance, and stability advantages can be realized
by forming the cathode of a combination of a low work function
(<4 eV) metal and at least one other metal. Available low work
function metal choices for the cathode are listed in cols. 19-20 of
the aforementioned incorporated patent, by periods of the Periodic
Table of Elements and categorized into 0.5 eV work function groups.
All work functions provided therein are from Sze, Physics of
Semiconductor Devices, Wiley, New York, 1969, p. 366.
[0048] A second metal can be included in the cathode to increase
storage and operational stability. The second metal can be chosen
from among any metal other than an alkali metal. The second metal
can itself be a low work function metal and thus be chosen from the
above-referenced list and having a work function of less than 4 eV.
To the extent that the second metal exhibits a low work function it
can, of course, supplement the first metal in facilitating electron
injection.
[0049] Alternatively, the second metal can be chosen from any of
the various metals having a work function greater than 4 eV. These
metals include elements resistant to oxidation and, therefore,
those more commonly fabricated as metallic elements. To the extent
the second metal remains invariant in the article or device, it can
contribute to the stability. Available higher work function (4 eV
or greater) metal choices for the cathode are listed in lines 50-69
of col. 20 and lines 1-15 of col. 21 of the aforementioned
incorporated patent, by periods of the Periodic Table of Elements
and categorized into 0.5 eV work function groups.
[0050] As described more fully in U.S. Pat. No. 5,156,918 which is
incorporated herein by reference in its entirety, the electrodes
and/or substrates of this invention have, preferably, a surface
with polar reactive groups, such as a hydroxyl (--OH) group.
Materials suitable for use with or as electrodes and/or substrates
for anchoring the conductive layers and luminescent media of this
invention should conform to the following requirements: any solid
material exposing a high energy (polar) surface to which
layer-forming molecules can bind. These may include: metals, metal
oxides such as SiO.sub.2, TiO.sub.2, MgO, and Al.sub.2O.sub.3
(sapphire), semiconductors, glasses, silica, quartz, salts, organic
and inorganic polymers, organic and inorganic crystals and the
like.
[0051] Inorganic oxides (in the form of crystals or thin films) are
especially preferred because oxides yield satisfactory hydrophilic
metal hydroxyl groups on the surface upon proper treatment. These
hydroxyl groups react readily with a variety of silyl coupling
reagents to introduce desired coupling functionalities that can in
turn facilitate the introduction of other organic components.
[0052] The physical and chemical nature of the anode materials
dictates specific cleaning procedures to improve the utility of
this invention. Alkaline processes (NaOH aq.) are generally used.
This process will generate a fresh hydroxylated surface layer on
the substrates while the metal oxide bond on the surface is cleaved
to form vicinal hydroxyl groups. High surface hydroxyl densities on
the anode surface can be obtained by sonicating the substrates in
an aqueous base bath. The hydroxyl groups on the surface will
anchor and orient any of the molecular components/agents described
herein. As described more fully below, molecules such as
organosilanes with hydrophilic functional groups can orient to form
the conductive layers.
[0053] Other considerations relating to the design, material choice
and construction of electrodes and/or substrates useful with this
invention are as provided in the above referenced and incorporated
U.S. Pat. No. 5,409,783 and in particular cols. 21-23 thereof, such
considerations as would be modified by those skilled in the art in
accordance with the molecular conductive layers, and assembly
methods and objects of the present invention.
[0054] The conductive layers and/or organic luminescent medium of
the devices/articles of this invention preferably contain at least
two separate layers, at least one layer for transporting electrons
injected from the cathode and at least one layer for transporting
holes injected from the anode. As is more specifically taught in
U.S. Pat. No. 4,720,432, incorporated herein by reference in its
entirety, the latter is in turn preferably at least two layers, one
in contact with the anode, providing a hole injecting zone and a
layer between the hole injecting zone and the electron transport
layer, providing a hole transporting zone. While several preferred
embodiments of this invention are described as employing at least
three separate organic layers, it will be appreciated that either
the layer forming the hole injecting zone or the layer forming the
hole transporting zone can be omitted and the remaining layer will
perform both functions. However, enhanced initial and sustained
performance levels of the articles or devices of this invention can
be realized when separate hole injecting and hole transporting
layers are used in combination.
[0055] Porphyrinic and phthalocyanic compounds of the type
described in cols. 11-15 of the referenced/incorporated U.S. Pat.
No. 5,409,783 can be used to form the hole injecting zone. In
particular, the phthalocyanine structure shown in column 11 is
representative, particularly where X can be, but is not limited to,
an alkyltrichlorosilane, alkyltrialkoxysilane,
dialkyldialkoxysilane, or dialkyldichlorosilane functionality and
where the alkyl and alkoxy groups can contain 1-6 carbon atoms or
is hydrogen. Preferred porphyrinic compounds are represented by the
structure shown in col. 14 and where R, T.sup.1 and T.sup.2 can be
but are not limited to an alkyltrichlorosilane,
alkyltrialkoxysilane, dialkyldialkoxysilane, or
dialkyldichlorosilane functionality and where the alkyl and alkoxy
groups contain 1-6 carbon atoms or is hydrogen. (See, also, FIGS.
1A and 1B, herein.) Preferred phthalocyanine- and porphyrin-based
hole injection agents include silicon phthalocyanine dichloride and
5,10,15,20-tetraphenyl-21H,23H-porphine silicon (IV) dichloride,
respectively.
[0056] The hole transporting layer is preferably one which contains
at least one tertiary aromatic amine, examples of which are as
described in FIGS. 2A-2C and Examples 1 and 2. Other exemplary
arylamine core structures are illustrated in U.S. Pat. No.
3,180,730, which is incorporated herein by reference in its
entirety, where the core structures are modified as described
herein. Other suitable triarylamines substituted with a vinyl or
vinylene radical and/or containing at least one active hydrogen
containing group are disclosed in U.S. Pat. Nos. 5,409,783,
3,567,450 and 3,658,520. These patents are incorporated herein by
reference in their entirety and the core structures disclosed are
modified as described herein. In particular, with respect to the
arylamines represented by structural formulas XXI and XXIII in
cols. 15-16 of U.S. Pat. No. 5,409,703, R.sup.24, R.sup.25,
R.sup.26, R.sup.27, R.sup.30, R.sup.31 and R.sup.32 can be an
alkyltrichlorosilane, alkyltrialkoxysilane, dialkyldialkoxysilane,
or dialkyldichlorosilane functionality where the alkyl and alkoxy
groups can contain 1-6 carbon atoms or is hydrogen.
[0057] Molecular components of this invention comprising emissive
agents and/or the emissive layer include those described herein in
FIGS. 3A-3C and Example 5. Other such components/agents include
various metal chelated oxinoid compounds, including chelates of
oxine (also commonly referred to as 8-quinolinol or
8-hydroxyquinoline), such as those represented by structure III in
col. 8 of the referenced and incorporated U.S. Pat. No. 5,409,783,
and where Z.sup.2 can be but is not limited to an
alkyltrichlorosilane, alkyltrialkoxysilane, dialkyldialkoxysilane,
or dialkyldichlorosilane functionality and where the alkyl and
alkoxy groups can contain 1-6 carbon atoms or is hydrogen. Other
such molecular components/emissive agents include the quinolinolato
compounds represented in cols. 7-8 of U.S. Pat. No. 5,151,629, also
incorporated herein by reference in its entirety, where a ring
substituent can be but is not limited to an alkyltrichlorosilane,
alkyltrialkoxysilane, dialkyldialkoxysilane, or
dialkyldichlorosilane functionality and where the alkyl and alkoxy
groups can contain 1-6 carbon atoms or is hydrogen. In a similar
fashion, the dimethylidene compounds of U.S. Pat. No. 5,130,603,
also incorporated herein by reference in its entirety, can be used,
as modified in accordance with this invention such that the aryl
substituents can include an alkyltrichlorosilane,
alkyltrialkoxysilane, dialkyldialkoxysilane, or
dialkyldichlorosilane functionality and where the alkyl and alkoxy
groups can contain 1-6 carbon atoms or is hydrogen.
[0058] Other components which can be used as emissive agents
include without limitation anthracene, naphthalene, phenanthrene,
pyrene, chrysene, perylene and other fused ring compounds, or as
provided in col. 17 of the previously referenced and incorporated
U.S. Pat. No. 5,409,783, such compounds as modified in accordance
with this invention and as more fully described above. Modifiable
components also include those described in U.S. Pat. Nos.
3,172,862, 3,173,050 and 3,710,167--all of which are incorporated
herein by reference in their entirety.
[0059] Molecular components which can be utilized as electron
injecting or electron transport agents and/or in conjunction with
an electron injection or electron transport layer are as described
in FIGS. 4A-4C and Examples 3(a)-(d) and 4. Other such components
include oxadiazole compounds such as those shown in cols. 12-13 of
U.S. Pat. No. 5,276,381, also incorporated herein by reference in
its entirety, as such compounds would be modified in accordance
with this invention such that the phenyl substituents thereof each
include an alkyltrichlorosilane, alkyltrialkoxysilane,
dialkyldialkoxysilane, or dialkyldichlorosilane functionality and
where the alkyl and alkoxy groups can contain 1-6 carbon atoms or
is hydrogen. Likewise, such components can be derived from the
thiadiazole compounds described in U.S. Pat. No. 5,336,546 which is
incorporated herein by reference in its entirety.
[0060] As described above, inorganic silicon moieties can be used
in conjunction with the various molecular components, agents,
conductive layers and/or capping layers. In particular, silane
moieties can be used with good effect to impart mechanical,
thermal, chemical and/or photochemical stability to the luminescent
medium and/or device. Such moieties are especially useful in
conjunction with the methodology described herein. Degradation is
minimized until further synthetic modification is desired.
Hydrolysis of an unreacted silicon/silane moiety provides an Si--OH
functionality reactive with a silicon/silane moiety of another
component, agent and/or conductive layer. Hydrolysis proceeds
quickly in quantitative yield, as does a subsequent condensation
reaction with an unreacted silicon/silane moiety of another
component to provide a siloxane bond sequence between components,
agents and/or conductive layers.
[0061] In general, the molecular agents/components in FIGS. 1-4 can
be prepared with a lithium or Grignard reagent using synthetic
techniques known to one skilled in the art and subsequent reaction
with halosilane or alkoxysilane reagents. Alternatively,
unsaturated olefinic or acetylenic groups can be appended from the
core structures using known synthetic techniques. Subsequently,
halosilane or alkoxysilane functional groups can be introduced
using hydrosilation techniques, also known to one skilled in the
art. Purification is carried out using procedures appropriate for
the specific target molecule.
[0062] It has been observed previously that the performance
characteristics of electroluminescent articles of the type
described herein can be enhanced by the incorporation of a layer
having a dielectric function between the anode and, for instance, a
hole transport layer. Previous studies show that the vapor
deposition of thin layers of LiF onto an anode before deposition of
the other layer(s) improves performance in the areas of
luminescence and quantum efficiency. However, this technique is
limited in that the deposited LiF films are rough, degrade in air
and do not form comformal, pinhole-free coatings.
[0063] The present invention is also directed to the application of
self-assembly techniques to form layers which cap an electrode,
provide dielectric and other functions and/or enhance performance
relative to the prior art. Such capping layers are self-assembled
films which are conformal in their coverage, can have dimensions
less than one nanometer and can be deposited with a great deal of
control over the total layer thickness. Accordingly, the present
invention also includes an electroluminescent article or device
which includes (1) an anode, (2) at least one molecular capping
layer coupled to the anode with silicon-oxygen bonds, with each
capping layer coupled one to another with silicon-oxygen bonds, (3)
a plurality of molecular conductive layers, with one of the layers
coupled to the capping layer with silicon-oxygen bonds and each
conductive layer coupled one to another with silicon-oxygen bonds,
and (4) a cathode in electric contact with a conductive layer.
Likewise, and in accordance with this invention, the capping layer
can be deposited on a conductive layer and/or otherwise introduced
so as to be adjacent to a cathode, to enhance overall
performance.
[0064] More generally and within the scope of this invention, the
anode is separated from the cathode by an organic luminescent
medium. The anode and cathode are connected to an external power
source by conductors. The power source can be a continuous direct,
alternating or intermittent current voltage source. A convenient
conventional power source, including any desired switching
circuitry, which is capable of positively biasing the anode with
respect to the cathode, can be employed. Either the anode or
cathode can be at ground potential.
[0065] In preferred embodiments, each conductive and/or capping
layer has molecular components, and each molecular component has at
least two silicon moieties. In highly preferred embodiments, each
such conductive and/or capping component is a halogenated or
alkoxylated silane, and silicon-oxygen bonds are obtainable from
the condensation of the silane moieties with hydroxy
functionalities. Without limitation, a preferred capping material
is octachlorotrisiloxane. The anode and cathode can be chosen
and/or constructed as otherwise described herein.
[0066] In part, the present invention is a method of using
molecular dimension to control the forward light output of an
electroluminescent device. The inventive method includes (1)
providing an electrode and a molecular layer thereon, the layer
coupled to the electrode with first molecular components having at
least two silicon moieties reactive to a hydroxy functionality; and
(2) modifying the thickness of the layer by reacting the molecular
components with second components to form a siloxane bond sequence
between the first and second molecular components, the second
molecular components having at least two silicon moieties also
reactive to a hydroxy functionality.
[0067] In preferred embodiments of this inventive method, at least
one silicon moiety is unreacted after reaction with a hydroxy
functionality. In highly preferred embodiments, the modification
further includes hydrolyzing an unreacted silicon moiety of one of
the molecular components to form a hydroxysilyl functionality and
condensing the hydroxysilyl functionality with a silicon moiety of
a third molecular component to form a siloxane bond sequence
between the second and third molecular components. In highly
preferred embodiments, the silicon moieties are halogenated or
alkoxylated silane moieties.
[0068] In part, the present invention also includes any
electroluminescent article for generating light upon application of
an electrical potential across two electrodes. Such an article
includes an electrode having a surface portion and a molecular
layer coupled and/or capped thereon. The layer includes molecular
components, and each component has at least two silicon moieties.
The layer is coupled to the electrode with silicon-oxygen bonds. In
preferred embodiments, each silicon moiety is a halogenated silane,
and silicon-oxygen bonds are obtained from a condensation reaction.
Likewise, and without limitation, the electrode has a substrate
with a hydroxylated surface portion transparent to near-IR and
visible wavelengths of light. Such a layer can be utilized to cap
the electrode and/or enhance performance as otherwise described
herein. More generally, in such an article or any other described
herein, the luminescent medium can be constructed using either the
self-assembly techniques described herein or the materials and
techniques of the prior art.
[0069] The electroluminescent devices and related methods of this
invention can demonstrate various interlayer/interfacial phenomena
through choice of layer/molecular components and design of the
resulting electroluminescent medium. As a point of reference, a
number of cathode and anode interfacial structures can enhance
charge injection, hence device performance. For instance, with
vapor-deposited, anode/TPD
(N--N'-diphenyl-N--N'-bis(3-methylphenyl)-(1-1'-biphenyl)-4-4'-diamine)/A-
lq (tris(quinoxalinato)Al(III))/cathode devices of the prior art, a
dramatic increase in light output and quantum efficiency occurs
when .ANG.-scale LiF or CsF layers are interposed between the
cathode and electron transport layer (ETL). Such thin dielectric
layers are thought to lower the Al work function, thus reducing the
effective electron injection barrier (energy level offset between
the Alq LUMO and the Al Fermi level).
[0070] In contrast, modification of the ITO anode--hole transport
layer (HTL) interface is somewhat more controllable, although
similar mechanistic uncertainties pertain. Thus, a variety of ITO
functionalization approaches produce phenomenologically similar
effects, although less dramatic than those observed for the
interposition of alkali fluoride at Al cathodes. These approaches
include deposition onto ITO of nanoscale layers of various organic
acids, copper phthalocyanine, or thicker (30-100 nm) layers of
polyaniline or polythiophene (PEDOT), all resulting in somewhat
enhanced luminous performance. Explanations for these phenomena are
diverse, ranging from altering interfacial electric fields,
balancing electron/hole injection fluence, confining electrons in
the emissive layer, reducing injected charge back-scattering, and
moderating anode Fermi level-HTL HOMO energetic discontinuities.
This diversity of proposed mechanisms accurately reflects the
complexity of interactions at OLED interfaces and, in many cases,
the lack of necessary microstructural information.
[0071] In a departure from the prior art, the present invention can
be considered in the context of one or more structural
relationships between an OLED anode and/or its associated organic
layers. Without restriction to any one theory or mode of operation,
moderation of the surface energy mismatch can be effected at a
hydrophilic oxide anode-hydrophobic HTL interface, as demonstrated
below using nanoscopic self-assembled silyl group functionalized
amine components (see, for example, FIG. 11A and 1-4 TAA layers; 11
.ANG./layer). Promoting anode/ITO-HTL physical cohesion
significantly enhances luminous performance and durability.
Furthermore, as relates to another aspect of this invention, a
silyl-group functionalized, crosslinkable amine layer having the
core amine structure of the HTL component, TPD, (TPD-Si.sub.2, FIG.
11 A) significantly improves ITO/TPD/Alq/Al device performance and
thermal durability (one metric of device stability) to an extent
surprising, unexpected and unattainable with other anode
functionalization structures. In contrast thereto, the commonly
used copper phthalocyanine (Cu(Pc); FIG. 11A) anode
functionalization layer actually templates crystallization of
overlying TPD films at modest temperatures (Example 13 and FIG.
12D), consistent with the thermal instability of many
Cu(Pc)-buffered OLED devices of the prior art.
[0072] Accordingly, in its broader respects, the present invention
contemplates a method of using an amine molecular component to
enhance hole injection across the electrode-organic interface of a
light emitting diode device. The inventive method includes (1)
providing an anode; and (2) incorporating an electroluminescent
medium adjacent the anode, the medium including but not limited to
a molecular layer, coupled to the anode, of amine molecular
components substituted with at least one silyl group, and thereon a
hole transport layer of molecular components having the amine
structure of the aforementioned molecular layer components. The
molecular layer can have at least one of an arylamine component and
an arylalkylamine component, including but not limited to those
monoarylamine, diarylamine and triarylamine components described in
the aforementioned and incorporated U.S. Pat. No. 5,409,783,
modified and/or silyl-functionalized as provided herein. Other
suitable arylamine and/or arylalkylamine structures are disclosed
in U.S. Pat. Nos. 3,180,730, 3,567,450 and 3,658,520, each of which
is incorporated herein in its entirety, such structures as can also
be modified to provide silyl-functionality in accordance herewith.
Likewise, a combination of such silyl-substituted components can be
employed with beneficial effect.
[0073] In preferred embodiments of this inventive method, the
aforementioned amine molecular layer components are
alkylsilyl-substituted compounds of the type illustrated in FIGS.
2A and 2C. In highly preferred embodiments, such components include
the alkylsilyl-substituted TAA and alkylsilyl substituted TPD
compounds prepared as described herein. Regardless, such a
molecular layer can be spin-coated on the anode surface or
self-assembled, as described more fully above, to provide
silicon-oxygen bonds therewith. A plurality of such molecular
layers can be coupled successively on an anode surface--each layer
coupled one to another with silicon-oxygen bonds--to improve
structural stability and enhance device performance. As described
herein and with reference to several of the following examples,
hole injection can be enhanced by choice of a molecular layer with
components having a structural relationship with those arylamine or
arylalkylamine components of the hole transport layer. In preferred
embodiments, such enhancement can be achieved through use of a
silyl-functionalized TPD layer in conjunction with a TPD hole
transport layer.
[0074] As such, the present invention also includes an organic
electroluminescent device for generating light upon application of
an electrical potential cross to electrodes. Such a device includes
(1) an anode; (2) at least one molecular layer, coupled to the
anode, of one or more of the aforementioned amine molecular
components substituted with at least one silyl group; (3) a
conductive layer of molecular components having the amine
structure; and (4) a cathode in electrical contact with the anode.
A preferred conductive layer includes a hole transport layer
comprising components of the prior art incorporated herein by
reference, or modified as described above. Preferred molecular
layer components are alkylsilyl-substituted compounds of the type
illustrated in FIGS. 2A and 2C, in particular silyl-functionalized
TAA and TPD. In light of the aforementioned structural
relationships and associated methodologies, a preferred conductive
layer of such a device is a TPD hole transport layer, such a layer
substantially without crystallization upon annealing and/or at
device operation temperatures when used in conjunction with a
molecular layer of components having the same or a structurally
similar amine structure.
[0075] As demonstrated herein, hydrophobic amine HTL--hydrophilic
anode integrity is a factor in OLED performance; poor physical
cohesion contributes to inefficient hole injection and ultimately,
device failure. Enhanced performance can be achieved through use of
molecular layer structures which maximize interfacial cohesion and
charge transport. With reference to one preferred embodiment, a
conveniently applied, spincoated silyl (Si) functionalized TPD
analogue, TPD-Si.sub.2 (FIGS. 11A-B), structurally similar to the
overlying HTL, hence well-suited to stabilizing the interface,
undergoes rapid crosslinking upon spincoating from solution and
subsequent thermal curing to form a dense, robust siloxane matrix
with imbedded TPD hole-transport components. The thickness of these
layers (.about.40 nm) was determined by specular X-ray reflectivity
on samples deposited via identical techniques on single-crystal
silicon. The RMS roughness of the TPD-Si.sub.2 molecular layer
films on ITO substrates of 30 .ANG. RMS roughness is 8-12 .ANG. by
contact mode AFM. Crosslinked TPD-Si.sub.2 films exhibit high
thermal stability, with only 5% weight loss observed up to
400.degree. C. by TGA, indicating substantial resistance to thermal
degradation. Furthermore, cyclic voltammetry of 40 nm TPD-Si.sub.2
films on ITO electrodes indicates that they support facile hole
transport and are electrochemically stable.
[0076] The densely crosslinked nature of TPD-Si.sub.2 molecular
layer films is evident in the relatively large separation of
oxidative and reductive peaks (200 mV), suggesting kinetically
hindered oxidation/reduction processes with retarded counterion
mobility. P. E. Smolenyak, E. J. Osbum, S.-Y. Chen, L.-K. Chau, D.
F. O'Brian, N. R. Armstrong, Langmuir 1997, 21, 6568. That
TPD-Si.sub.2 film coverage on ITO is conformal and largely
pinhole-free is supported by studies using a previously described
ferrocene probe technique. W. Li, Q. Wang, J. Cui, H. Chou, T. J.
Marks, G. E. Jabbour, S. E. Shaheen, B. Kippelen, N. Pegyhambarian,
P. Dutta, A. J. Richter, J. Anderson, P. Lee, N. Armstrong, Adv.
Mater. 1999, 11, 730. The lack of significant current flow near the
formal potential for ferrocene oxidation at a TPD-Si.sub.2-coated
ITO working electrode indicates suppression of ferrocene oxidation,
consistent with largely pinhole-free surface coverage. G. Inzelt,
Electroanalytical Chemistry, Vol, 18, Marcel Dekker, New York,
1994, p. 89.
EXAMPLES OF THE INVENTION
[0077] The following non-limiting examples and data illustrate
various aspects and features relating to the articles/devices
and/or methods of the present invention, including the assembly of
a luminescent medium having various molecular components/agents
and/or conductive layers, as are available through the synthetic
methodology described herein. In comparison with the prior art, the
present methods and articles/devices provide results and data which
are surprising, unexpected and contrary to the prior art. While the
utility of this invention is illustrated through the use of several
articles/devices and molecular components/agents/layer- s which can
be used therewith, it will be understood by those skilled in the
art that comparable results are obtainable with various other
articles/devices and components/agents/layers, as are commensurate
with the scope of this invention.
Example 1
[0078] 1
[0079] Synthesis of a Silanated Hole Transport Agent [1]. With
reference to reaction scheme, above, hole transport components,
agents and/or layers can be prepared, in accordance with this
invention and/or for use in conjunction with light-emitting diodes
and other similar electroluminescent devices. Accordingly, 500 mg.
(1.0 mmole) of trisbromophenylamine (Aldrich Chemical Company) was
dissolved in 30 ml of dry dimethoxyethane (DME). This solution was
cooled to -45.degree. C. and 1.2 ml (3.3 mmole) of a 2.5 M solution
of n-butyl lithium in hexane was added to the reaction mixture. The
entire mixture was then slowly warmed to 20.degree. C. After
stirring at 20.degree. C. for an additional hour, the solvent was
removed in vacuo. The resulting white precipitate was washed
(3.times.20 ml) with dry pentane and redissolved in 30 ml dry DME.
This solution was subsequently poured into 10 ml (87 mmole) of
silicon tetrachloride at a rate of 1 m/min. The entire reaction
mixture was then refluxed for two hours. The resulting supernatent
was separated from the precipitate, and the solvent again removed
in vacuo yielding a green-brown residue. A white solid was obtained
from this residue upon sublimation at 10.sup.-6 torr.
Characterization: .sup.1H NMR (600 MHz, C.sub.6D.sub.6, 20.degree.
C.): .delta.7.07 (d, 6H, Ar--H); .delta.7.05 (d, 6H, Ar--H); EI-MS
(m/z): 645 (M+).
Example 2
[0080] With reference to FIGS. 2A-2C and the representative
arylamines provided therein, other hole transport agents and/or
layers of this invention can be obtained by straightforward
application of the silanation procedure described above in Example
1, with routine synthetic modification(s) and optimization of
reaction conditions as would be well-known to those skilled in the
art and as required by the particular arylamine. Likewise,
preliminary halogenation/bromination can be effected using known
synthetic procedures. Alternatively, the arylamines of FIGS. 2A-2C
and other suitable substrates can be prepared using other available
synthetic procedures to provide multiple silane reaction centers
for use with the self-assembly methods and light-emitting diodes of
this invention. Core molecular substrates of the type from which
the arylamines of FIGS. 2A-2C can be prepared are described by
Strukelji et al. in Science, 267, 1969 (1995), which is
incorporated herein by reference in its entirety.
Example 3
[0081] Synthesis of a Silanated Electron Transport Agent. With
reference to Examples 3(a)-(d) and corresponding reaction schemes,
below, electron transport agents and/or layers can be prepared, in
accordance with this invention and/or for use in conjunction with
light-emitting diodes and other similar electroluminescent devices.
2
Example 3a
[0082] Synthesis of 4'-Bromo-2-(4-bromobenzoyl)acetophenone [2]. In
a 1-liter three neck round bottom flask, 43 g (0.2 mol) methyl
4-bromobenzoic acid and 17.6 g (0.4 mol) sodium hydride were
dissolved in 200 ml dried benzene and heated to 60.degree. C. Next,
39.8 g (0.2 mol) 4-bromoacetophenone in 100 ml dry benzene was
slowly-added through a dropping funnel, and 1 ml methanol was added
to the flask to initiate the reaction. After the mixture was
refluxed overnight, the reaction was quenched by adding methanol
and pouring it into ice water. The pH of the mixture was brought
down to 7.0 using 5 N sulfuric acid. A solid was collected, washed
with water, and recrystallized from benzene to give a light yellow
product. Characterization. Yield: 30.3 g (40%). .sup.1H NMR (300
MHz, CDCl.sub.3, 20.degree. C., .delta.): 7.84 (d, 4H, ArH); 7.62
(d, 4H, ArH); 6.77 (s, 2H, CH.sub.2). EI-MS: 382(M+), 301, 225,
183, 157.
Example 3b
[0083] Synthesis of 3,5-Bis(4-bromophenyl)isoxazole [3]. In a 250
ml round bottom flask, 4 g (10.4 mmol) of [2] was dissolved in 100
ml dry dioxane and heated to reflux, then 3.0 g (43.2 mmol)
hydroxylamine hydrogen chloride in 10 ml water and 5 ml (25 mmol) 5
N NaOH was then dropped into the refluxing mixture. After 12 hours,
the reaction mixture was cooled down to room temperature, and the
solvent was removed in vacuo. The product was recrystallized from
ethanol. Characterization. Yield: 3.41 g (85%). M.P.
218.5-219.5.degree. C. .sup.1H NMR (300 MHz, CDCl.sub.3, 20.degree.
C., .delta.): 7.78 (d, 2H, ArH), 7.74 (d, 2H, Ar'H), 7.66 (d, 2H,
ArH), 7.62 (d, 2H, Ar'H), 6.82 (s, 1H, isoxazole proton). EI-MS:
379(M+), 224, 183, 155.
Example 3c
[0084] Synthesis of 3,5-Bis(4-allylphenyl)isoxazole [4]. In a 250
ml three-neck round bottom flask, 3.77 g (10 mmol) of [3], 460 mg.
(0.4 mmol) tetrakis-(triphenylphosphine) palladium, and 7.28 g (22
mmol) tributylallyltin were dissolved in 100 ml. dried toluene and
degassed with nitrogen for 30 min. The mixture was heated to
100.degree. C. for 10 h, then cooled down to room temperature.
Next, 50 ml. of a saturated aqueous ammonium fluoride solution was
subsequently added to the mixture. The mixture was extracted with
ether, and the combined organic layer was washed by water, then
brine, and finally dried over sodium sulfate. The solvent was
removed in vacuo. The residue was purified by column
chromatography. (first, 100% hexanes, then chloroform:hexanes
[80:20]). Characterization. Yield: 1.55 g (57%). .sup.1H NMR (300
MHz, CDCl.sub.3, 20.degree. C., .delta.): 7.78 (d, 2H, ArH), 7.74
(d, 2H, Ar'H), 7.34 (d, 2H, ArH), 7.30 (d, 2H, Ar'H), 6.78 (s, 1H,
isoxazole proton), 5.96 (m, 2H, alkene H), 5.14 (d, 4H, terminal
alkene H), 3.44 (d, 4H, methylene group). EI-MS: 299(M+), 258,
217.
Example 3d
[0085] Synthesis of
3,5-Bis(4-(N-trichlorosilyl)propylphenyl)isoxazole [5]. To 2 ml of
THF was added 5 mg of [4], 3.4 .mu.l of HSiCl.sub.3 and 0.8 mg. of
H.sub.2PtCl.sub.6 were added to 2 ml of THF. The reaction was
heated at 50.degree. C. for 14 h. The solvent was then removed in
vacuo. A white solid was obtained from this residue upon
sublimation at 10.sup.-6 torr. Characterization. .sup.1H NMR (300
MHz, d.sup.8-THF, 20.degree. C., .delta.): 7.72 (d, 2H, ArH), 7.68
(d, 2H, Ar'H), 7.36 (d, 2H, ArH), 7.32 (d, 2H, Ar'H), 6.30 (s, 1H,
isoxazole); 2.52 (t, 2H, CH); 1.55 (m, 4H, CH.sub.2); 0.85 (t, 6H,
CH.sub.3).
Example 4
[0086] With reference to FIGS. 4A-4C and the representative
heterocycles provided therein, other electron transport agents
and/or layers of this invention can be obtained by straight-forward
application of the silanation procedure described above in Example
3, with routine synthetic modification(s) and optimization of
reaction conditions as would be well-known to those skilled in the
art and as required by the particular heterocyclic substrate.
Preliminary halogenation/bromination can be effected using known
synthetic procedures or through choice of starting materials
enroute to a given heterocycle. Alternatively, the heterocycles of
FIGS. 4A-4C and other suitable substrates can be prepared using
other available synthetic procedures to provide multiple silane
reaction centers for use with the self-assembly methods and
light-emitting diodes of this invention. Core molecular substrates
of the type from which the heterocycles of FIGS. 4A-4C can be
prepared are also described by Strukelji et al. in Science, 267,
1969 (1995).
Example 5
[0087] With reference to FIGS. 3A-3C and the representative
chromophores provided therein, emissive agents and/or layers, in
accordance with this invention, can be obtained by appropriate
choice of starting materials and using halogenation and silanation
procedures of the type described in Examples 1-4, above.
Alternatively, other chromophores can be silanated using other
available synthetic procedures to provide multiple silane reaction
centers for use with the self-assembly methods and light-emitting
diodes of this invention. Regardless, in accordance with this
invention, such emissive agents or chromophores can be used for
emission of light at wavelengths heretofore unpractical or
unavailable. Likewise, the present invention allows for the use of
multiple agents or chromophores and construction of an emissive
layer or layers such that a combination of wavelengths and/or white
light can be emitted.
Example 6
[0088] Examples 6(a)-6(c) together with FIG. 6 illustrate the
preparation of other molecular components which can be used in
accordance with this invention.
Example 6a
[0089] Synthesis of Tertiary Arylamine [6]. Together, 14.46 g (20
mmole) of tris(4-bromophenyl)amine and 500 ml of dry diethyl ether
were stirred at -78.degree. C. under a nitrogen atmosphere. Next,
112.5 ml of a 1.6 M n-butyllithium solution in hexanes was slowly
added to the reaction mixture over 1.5 hours. The reaction was then
warmed to -10.degree. C. and stirred for an additional 30 minutes.
The reaction was then cooled down again to -78.degree. C. before
the addition of 22 g (0.5 mole) of ethylene oxide. The mixture was
stirred and slowly warmed to room temperature over 12 hours. Next,
2 ml of a dilute NH.sub.4Cl solution was then added to the reaction
mixture. The solvent was evaporated under vacuum yielding a light
green solid. The product was purified using column chromatography.
The column was first eluted with chloroform and then with
MeOH:CH.sub.2Cl.sub.2 (5:95 v/v). The resulting light gray solid
was recrystallized using chloroform to give 1.89 g. Yield: 25%.
.sup.1H NMR (.delta., 20.degree. C., DMSO): 2.65 (t, 6H), 3.57 (q,
6H), 4.64 (t, 3H), 6.45 (d, 6H), 7.09 (d, 6H). EI-MS: 377
(M.sup.+), 346 (M.sup.+-31), 315 (M.sup.+-62). HRMS: 377.2002.
calcd; 377.1991. Anal. Calculated for C.sub.24H.sub.27NO.sub.3; C,
76.36; H, 7.21; N, 3.71. Found: C, 76.55; H, 7.01; N, 3.52.
Example 6b
[0090] Synthesis of Tosylated Arylamine [7]. A pyridine solution of
tosyl chloride (380 mg in 5 ml) was added over 5 minutes to a
pyridine solution of [6] (500 mg in 10 ml, from Example 6a) cooled
to 0.degree. C. The mixture was stirred for 12 hours, then quenched
with water and extracted with chloroform. The organic extract was
washed with water, 5% sodium bicarbonate, and dried with magnesium
sulfate. After filtration, the chloroform solution was then
evaporated to dryness under vacuum and purified using column
chromatography. The column was first eluted with hexane:CHCl.sub.3
(1:2 v/v) yielding [7]. .sup.1H NMR (300 MHz, .delta., 20.degree.
C., CDCl.sub.3): 2.45 (s, 3H), 2.90 (t, 6H), 3.02 (t, 3H), 3.70 (t,
3H), 4.19 (t, 6H), 6.92 (d, 2H), 6.98 (d, 4H), 7.00 (d, 4H), 711
(d, 2H), 7.32 (d, 2H), 7.77 (d, 2H).
Example 6c
[0091] Synthesis of Tosylated Arylamine [8]. Continuing the
chromatographic procedure similar for 2 (from Example 6b)but
changing the eluting solvent to 100% CHCl.sub.3 yielded [8].
.sup.1H NMR (300 MHz, .delta., 20.degree. C., CDCl.sub.3): 2.44 (s,
6H), 2.91 (t, 3H), 3.02 (t, 6H), 3.70 (t, 6H), 4.19 (t, 3H), 6.92
(d, 4H), 6.98 (d, 2H), 7.00 (d, 2H), 7.11 (d, 4H), 7.32 (d, 4H),
7.77 (d, 4H).
Example 7
[0092] Using the arylamines of Examples 6 and with reference to
FIG. 7, an electroluminescent article/device also in accordance
with this invention is prepared as described, below. It is
understood that the arylamine component can undergo another or a
series of reactions with a silicon/silane moiety of another
molecular component/agent to provide a siloxane bond sequence
between components, agents and/or conductive layers. Similar
electroluminescent articles/devices and conductive layers/media can
be prepared utilizing the various other molecular components/agents
and/or layers described above, such as in Examples 1-5 and FIGS.
1-4, in conjunction with the synthetic modifications of this
invention and as required to provide the components with the
appropriate reactivity and functionality necessary for the assembly
method(s) described herein.
Example 7a
[0093] This example of the invention shows how slides can be
prepared/cleaned prior to use as or with electrode materials. An
indium-tin-oxide (ITO)-coated soda lime glass (Delta Technologies)
was boiled in a 20% aqueous solution of ethanolamine for 5 minutes,
rinsed with copious amounts of distilled water and dried for 1 hour
at 120.degree. C.; alternatively and with equal effect, an
ITO-coated soda lime glass (Delta Technologies) was sonicated in
0.5M KOH for 20 minutes, rinsed with copious amounts of distilled
water and then ethanol, and dried for 1 hour at 120.degree. C.
Example 7b
[0094] Electroluminescent Article Fabrication and Use. The freshly
cleaned ITO-coated slides were placed in a 1% aqueous solution of
3-aminopropyltrimethoxysilane and then agitated for 5 minutes.
These coated slides were then rinsed with distilled water and cured
for 1 hour at 120.degree. C. The slides were subsequently placed in
a 1% toluene solution of [7] (or [8] from Example 6) and stirred
for 18 hours under ambient conditions. Afterwards, the slides were
washed with toluene and cured for 15 minutes at 120.degree. C.
AlQ.sub.3 (or GaQ.sub.3; Q=quinoxalate) was vapor deposited on top
of the amine-coated slides. Finally, 750-1000 .ANG. of aluminum was
vapor deposited over the metal quinolate layer. Wires were attached
to the Al and ITO layers using silver conducting epoxy
(CircuitWorks.TM.), and when a potential (<7V) was applied, red,
orange, and/or green light was emitted from the device.
Example 8
[0095] One or more capping layers comprising
Cl.sub.3SiOSiCl.sub.2OSiCl.su- b.3 are successively deposited onto
clean ITO-coated glass where hydrolysis of the deposited material
followed by thermal curing/crosslinking in air at 125.degree. C.
yields a thin (.about.7.8 .ANG.) layer of material on the ITO
surface. X-ray reflectivity measurements indicate that the total
film thickness increases linearly with repeated layer deposition,
as seen in FIG. 8. Other molecular components can be used with
similar effect. Such components include, without limitation, the
bifunctional silicon compounds described in U.S. Pat. No.
5,156,918, at column 7 and elsewhere therein, incorporated by
reference herein in its entirety. Other useful components, in
accordance with this invention include those trifuctional compounds
which cross-link upon curing. As would be well known to those
skilled in the art and made aware of this invention, such
components include those compounds chemically reactive with both
the electrode capped and an adjacent conductive layer.
Example 9
[0096] Cyclic voltametry measurements shown in FIG. 9 using aqueous
ferri/ferrocyanide show that there is considerable blocking of the
electrode after the deposition of just one layer of the
self-assembled capping material specified in Example 8. Other
molecular components described, above, show similar utility. Almost
complete blocking, as manifested by the absence of pinholes, is
observed after application of three layers of capping material.
Example 10
[0097] Conventional organic electroluminescent devices consisting
of TPD (600 .ANG.)/Alq (600 .ANG.)/Mg (2000 .ANG.) were
vapor-deposited on ITO substrates modified with the capping
material specified in Example 8. FIGS. 10A-C show the behavior of
these devices with varying thickness of the self-assembled capping
material. These results show that such a material can be used to
modify forward light output and device quantum efficiency. For a
device with two capping layers, higher current densities and
increased forward light output are achieved at lower voltages,
suggesting an optimum thickness of capping material can be used to
maximize performance of an electroluminescent article.
Example 11
[0098] This example illustrates how a capping material can be
introduced to and/or used in the construction of an
electroluminescent article. ITO-coated glass substrates were
cleaned by sonication in acetone for 1 hour followed by sonication
in methanol for 1 hour. The dried substrates were then reactively
ion etched in an oxygen plasma for 30 seconds. Cleaned substrates
were placed in a reaction vessel and purged with nitrogen. A
suitable silane, for instance a 24 mM solution of
octachlorotrisiloxane in heptane, was added to the reaction vessel
in a quantity sufficient to totally immerse the substrates. (Other
such compounds include those described in Example 8). Substrates
were allowed to soak in the solution under nitrogen for 30 minutes.
Following removal of the siloxane solution the substrates were
washed and sonicated in freshly distilled pentane followed by a
second pentane wash under nitrogen. Substrates were then removed
from the reaction vessel washed and sonicated in acetone.
Substrates were dried in air at 125.degree. C. for 15 minutes. This
process can be repeated to form a capping layer of precisely
controlled thickness.
Example 12
[0099] The stability of device-type TPD-Si.sub.2 molecular
layer/TPD hole transport layer interfaces under thermal stress (one
measure of durability) was investigated by annealing
ITO/TPD-Si.sub.2 (40 nm)/TPD (100 nm) bilayers at 80.degree. C. for
1.0 h. The optical image of the annealed TPD film shows no evidence
of TPD de-wetting/decohesion (FIG. 12A), indicating that the
ITO-TPD surface energy mismatch is effectively moderated by the
interfacial TPD-Si.sub.2 molecular layer. In contrast, the bare
ITO/TPD interface exhibits catastrophic de-wetting/de-cohesion
under identical thermal cycling (FIG. 12B), visible even under a
layer of Alq). Despite seemingly similar cohesive effects for both
TAA and TPD-Si.sub.2 as interfacial buffer layers, it is reasonable
to suggest that the interfacial cohesion between TPD-Si.sub.2 and
TPD is greater, given closer structural similarity, evidenced by
comparing advancing aqueous contact angles: values for bare ITO,
silyl-functionalized TAA, silyl (Si.sub.2) functionalized TPD and
TPD film surfaces are 0.degree., 45.degree., 70.degree., and
85.degree. respectively, indicating a closer surface energy match
at TPD-TPD-Si.sub.2 interfaces.
Example 13
[0100] Speculation that one role of Cu(Pc) in enhancing OLED
performance might be via the above adhesion mechanism led to
parallel thermal studies. In contrast to a preferred
alkylsilyl-substituted arylamine TPD-Si.sub.2, Cu(Pc)-buffered ITO
does not prevent TPD de-cohesion upon heating to temperatures
near/above the TPD glass transition temperature (T.sub.g). FIG. 12D
illustrates the morphology of a 100 nm TPD film on 10 nm Cu(Pc)
following heating at 80.degree. C. It is clearly seen that thermal
annealing induces TPD crystallization on the Cu(Pc) film surface
(visible even under a layer of Alq), yielding star-shaped dendritic
crystallites (as-deposited TPD films on Cu(Pc) are smooth and
featureless, FIG. 12C). It is likely that such Cu(Pc)-nucleated
crystallization occurs during localized heating in operating OLEDs
and contributes to observed device instability.
Example 14
[0101] Device characteristics employing spincoated TAA,
TPD-Si.sub.2, and vapor-deposited Cu(Pc) hole injection
layer/adhesion layers in OLEDs having the structure
ITO/interlayer/TPD/Alq/Al are compared in FIG. 13. Versus the bare
ITO system, all of the molecular/buffer layer-incorporated devices
exhibit higher light output, enhanced quantum efficiencies, and
lower turn-on voltages. Note that a preferred TPD-Si.sub.2
component layer affords .about.15,000 cd/m.sup.2 of maximum light
output, which is 10-100.times. greater than the bare ITO-based
device. Similar increases in ITO/TPD/Alq/AI-type device performance
with electrode functionalization have only been reported previously
for LiF or CsF-modified Al cathodes, via what remains an unresolved
mechanism. It is widely accepted that conventional ITO/TPD/Alq/AI
heterostructures are electron-limited due to the low Alq electron
mobility and Alq LUMO-Al Fermi level energetic mismatch, thus
raising the question of why TPD-Si.sub.2 anode modification
produces similar effects. It is suggested that under conditions of
anode-HTL surface energy mismatch and poor anode-HTL cohesion (an
unmodified device of prior art), non-ohmic contacts dominate device
behavior, resulting in significant hole injection barriers typical
of poor electrode-organic contact.
Example 15
[0102] With reference to FIG. 14, TAA and TPD-Si.sub.2
interfacial/molecular component layers are significantly more
effective injection structures than conventional Cu(Pc) layers. The
maximum light output for a Cu(Pc)-based device is .about.1500
cd/m.sup.2 at 25 V, while that of a TAA-based device is 2600
cd/m.sup.2, and at a much lower bias voltage (16 V). The external
quantum efficiency of the Cu(Pc)-based device also falls well below
those based on TAA and TPD-Si.sub.2 anode layers: the maximum
quantum efficiency for Cu(Pc) is .about.0.3%, in contrast to those
of TPD-Si.sub.2 (.about.1.2%) and TAA (.about.0.9%). The
TPD-Si.sub.2-functionalized device is most efficient, producing a
maximum light output .about.10.times. greater than the
Cu(Pc)-buffered device, and .about.100.times. greater than the bare
ITO-based device. Improvements observed and differences between
TPD-Si.sub.2 and silyl-functionalized TAA molecular layers can be
explained, without limitation, in relation to: (1) closer aromatic
structural similarity of TPD-Si.sub.2 to TPD, producing a stronger
interlayer affinity and presumably greater .pi.-.pi. interfacial
overlap, and (2) the higher triarylamine: siloxane linker ratio in
TPD-Si.sub.2, consistent with more facile hole hopping via denser
triarylamine packing.
Example 16
[0103] These findings argue that promotion of ITO-TPD interfacial
contact/adhesion leads to more efficient hole injection due to
reduced interfacial contact resistance. This hypothesis was tested
by examining characteristics of hole-only devices having the
structure ITO/molecular interlayer/TPD(250 nm)/Au(6 nm)/Al(80 nm),
in which electron injection at the cathode is blocked. Here the Au
layer is deposited by rf-sputtering to avoid excessive heating of
the TPD underlayer. The hole injection capacity falls in the order
TPD-Si.sub.2.gtoreq.TAA>Cu(Pc)>bare ITO (FIG. 14). Compared
to bare ITO, the silyl-functionalized TAA- and
TPD-Si.sub.2-modified anodes enhance the hole current density by
10-100.times. for the same field strength, with ITO/TPD-Si.sub.2
being most effective. Thus, when contact resistance at the OLED
anode side is reduced and hole injection increased, the greater
electric field induced across the Alq layer enhances electron
injection and transport, affording higher light output and
comparable or, in the cases where recombination is more probable,
enhanced quantum efficiency. In contrast, the present and related
data for Cu(Pc) devices show that the Cu(Pc) significantly
suppresses hole injection. E. W. Forsythe, M. A. Abkowitz, Y. Gao,
J. Phys. Chem. B 2000, 104, 3948; S. C. Kim, G. B. Lee, M. Choi, Y.
Roh, C. N. Whang, K. Jeong, Appl. Phys. Lett. 2001, 78, 1445. It is
believed, in light of these results, that Cu(Pc) enhances quantum
efficiency via better balancing hole and electron injection
fluences, rather than by facilitating hole injection or interfacial
stability.
Example 17
[0104] To examine cohesion and crystallization effects on device
durability, thermal stress tests were carried out on devices based
on bare ITO, having a 10 nm Cu(Pc) interlayer, and having a 40 nm
TPD-Si.sub.2 interlayer. These were subjected to heating at
95.degree. C. for 0.5 h in vacuum and subsequently examined for
changes in luminous response. The irreversible degradation of the
bare ITO and Cu(Pc)-based devices upon heating at 95.degree. C. for
0.5 h (FIG. 15) is reasonably ascribed to TPD de-wetting and
Cu(Pc)-nucleated TPD crystallization, respectively. Both processes
would disrupt the multilayer structure, leading to direct hole
injection into, and consequent degradation of, the emissive Alq
layer, and possible amplification of pinholes and defects. In
contrast, TPD-Si.sub.2-buffered molecular layer devices exhibit
enhanced performance after heating, which is presumably a
consequence of interfacial reconstruction that promotes charge
injection. These experiments unambiguously demonstrate that
covalently interlinked alkylsilyl-substituted compounds such as
TPD-Si.sub.2 and TAA, when used as described herein, offer
significant improvements in stabilizing the anode-HTL interface and
promoting hole injection.
[0105] The results of this and several preceding examples,
demonstrate that a spincoated, hole injecting TPD-Si.sub.2 layer
can significantly increase maximum OLED device luminence
(.about.100.times.) and quantum efficiency (.about.6.times.) by
promoting ITO-TPD interfacial cohesion, hence promoting more
efficient hole injection. Devices having a TPD-Si.sub.2 anode
adhesion layer afford a maximum luminance level of 15,000
cd/m.sup.2 in absence of dopants or low work function cathodes,
while exhibiting excellent thermal stability. In addition, the same
results demonstrate that Cu(Pc) interlayers nucleate TPD
crystallization upon heating above the T.sub.g of TPD
Example 18
[0106] The synthesis of alkylsilyl-functionalized TAA is as was
previously described, both herein and in the literature. W. Li, Q.
Wang, J. Cui, H. Chou, T. J. Marks, G. E. Jabbour, S. E. Shaheen,
B. Kippelen, N. Pegyhambarian, P. Dutta, A. J. Richter, J.
Anderson, P. Lee, N. Armstrong, Adv. Mater. 1999, 11, 730. TPD,
Alq, and Cu(Pc) were obtained from Aldrich and purified by gradient
sublimation. All other reagents were used as received unless
otherwise indicated. TPD-Si.sub.2 can be synthesized as provided
elsewhere, herein (see Example 2) or according to FIG. 11B and
Examples 19-21, and further characterized by .sup.1H NMR
spectroscopy and elemental analysis.
Example 19
[0107] With reference to FIG. 11B enroute to TPD-Si.sub.2, the
synthesis of 4,4'-bis[(p-bromophenyl)phenylamino)biphenyl (9). To a
solution of tris(dibenzyldeneacetone)dipalladium (0.55 g, 0.6
mmol), and bis-(diphenylphosphino)ferrocene; (0.50 g, 0.9 mmol) in
toluene (50 mL), was added 1,4-dibromobenzene (18.9 g, 0.0800 mol)
at 25.degree. C., and the solution stirred under N.sub.2 for 10
min. Subsequently, sodium tert-butoxide (4.8 g, 0.050 mol) and
N,N'-diphenylbenzidine (6.8 g, 0.020 mol) were added, and the
reaction mixture stirred at 90.degree. C. for 12 h. The reaction
mixture was subsequently cooled to 25.degree. C. and poured into
water. The organic layer was separated, and the aqueous layer was
extracted with toluene (3.times.100 mL). The extract was combined
with the original organic layer, and the solvent was removed in
vacuo to give the crude product. This was purified by
chromatography on silica gel using hexane: ethylene chloride (6:1)
as the eluant. A white solid (6.9 g) was obtained in 50% yield.
.sup.1H NMR (CDCl.sub.3): .delta. 6.99 (d, J=8.8 Hz, 4H), 7.02-7.16
(m, 10H), 7.28 (t, J=7.6 Hz, 4H), 7.34 (d, J=8.8 Hz, 4H), 7.45 (d,
J=8.4 Hz, 4H).
Example 20
[0108] With further reference to FIG. 11B enroute to TPD-Si.sub.2,
the synthesis of 4,4'-bis[(p-allylphenyl)'phenylamino]biphenyl
(10). To a stirring, anhydrous ether solution (10 mL) of 1(1.02 g,
1.58 mmol) under N.sub.2 was added dropwise at 25.degree. C. 1.6 mL
(3.5 mmol) n-butyl lithium(2.5 M in hexanes), and the mixture
stirred for 2 h. CuI (0.76 g, 4.0 mmol) was then added, the
reaction mixture cooled to 0.degree. C., and allyl bromide (0.60 g,
5.0 mmol) added in one portion. The solution was stirred for 14 h,
after which time it was quenched with 100 mL saturated aqueous
NH.sub.4.sup.+Cl.sup.- solution, followed by extraction with ether
(3.times.100 mL). The combined ether extracts were washed with
water (2.times.100 mL) and brine (2.times.100 mL), and dried over
anhydrous Na.sub.2SO.sub.4. Following filtration, solvent was
removed in vacuo to yield a yellow oil. Chromatography on silica
gel with hexane: ethylene chloride (4:1) afforded 0.63 g white
solid. Yield, 70%. .sup.1H NMR (CDCl.sub.3) .delta. 3.40 (d, J=10
Hz, 4H), 5.10-5.20 (m, 4H), 6.02 (m, 2H), 6.99-7.10 (m, 2H),
7.10-7.20 (m, 16H), 7.28 (t, J=7.6 Hz, 4H), 7.46 (d, J=8.8 Hz, 4H).
Anal. Calcd for C.sub.42H.sub.36N.sub.2: C 88.68; H 6.39; N 5.23
Found, C 87.50; H 6.35, N 4.93.
Example 21
[0109] With reference to FIG. 11B enroute to TPD-Si.sub.2, the
synthesis of
4,4'-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (11).
To a solution of 2 (0.32 g, 0.55 mol) in 30 mL CH.sub.2Cl.sub.2 at
25.degree. C. was added a grain of H.sub.2PtCl.sub.6.xH.sub.2O,
followed by HSiCl.sub.3 (0.73 g, 5.5 mmol). The reaction solution
was warmed to 30.degree. C. and stirred for 4 h. Removal of the
solvent in vacuum yielded a dark-yellow oil. A mixture of 50 mL
pentane and 10 mL toluene was then added. The resulting solid was
filtered off, and the filtrate was concentrated under vacuum to a
viscous, pale-yellow oil. Yield, 98%. .sup.1H NMR (CDCl.sub.3):
.delta.1.45 (t, J=7 Hz, 4H), 1.90 (t, J=7 Hz, 4H), 2.70 (brs, 4H),
6.80-7.80 (m, 26H). Anal. Calcd for
C.sub.42H.sub.38Cl.sub.6N.sub.2Si.sub.2: C 60.07, H 4.57; Found, C
60.52, H 4.87.
Example 22
[0110] With reference to Examples 19-21, a wide variety of
arylalkylamine molecular components and their silyl-functionalized
analogs can be prepared using straight-forward modifications of the
synthetic techniques described herein. For instance, with reference
to Example 19, diphenylbenzidene can be mono- or dialkylated with
the appropriate haloalkyl reagent to provide the desired
arylalkylamine hole transport layer component. As would also be
well known to those skilled in the art made aware of this
invention, the corresponding silyl-functionalized molecular layer
component can be prepared via mono- or dialkylation with the
appropriate dihaloalkyl reagent followed by subsequent silation,
adopting the procedures illustrated in Examples 20 and 21.
Accordingly, by way of further example, the alkylafed mono- and
diarylamine components, discussed above, and their
silyl-functionalized analogs can be prepared to provide the
structurally-related molecular and hole transport layers of this
invention, and the enhanced performance and/or hole injection
resulting therefrom.
Example 23
[0111] TPD-Si.sub.2 and TAA Thin Film Deposition and
Characterization. Indium tin oxide (ITO) glass sheets with a
resistance of 20 .OMEGA./.quadrature. from Donnelly Corp. were
subjected to a standard literature cleaning procedure. TAA and
TPD-Si.sub.2-based buffer layers were spincoated onto cleaned ITO
surfaces from their respective toluene solutions (10 mg/mL) at 2
Krpm, followed by curing in moist air at 110.degree. C. for 15 min.
Cyclic voltammetry of spincoated TPD-Si.sub.2 films on ITO was
performed with a BAS 100 electrochemical workstation (scan rate,
100 mV/s; Ag wire pseudo-reference electrode, Pt wire counter
electrode, supporting electrolyte, 0.1 M TBAHFP in anhydrous MeCN).
For TPD-Si.sub.2 film contiguity assessment, 1.0 mM ferrocene in
0.1 M TBAHFP/MeCN was used as the probe. Thermogravimetric analysis
(TGA) was carried out on an SDT 2960 DTA-TGA instrument with a scan
rate of 10.degree. C./min under N.sub.2. TGA sample preparation
involved drop-coating a TPD-Si.sub.2 solution in toluene (10 mM)
onto clean glass substrates under ambient conditions. Following
solvent evaporation, the TPD-Si.sub.2-coated slides were cured at
120.degree. C. for 1 h. Upon cooling, the films were detached from
the glass substrates using a razor blade and collected as powders
for TGA characterization.
Example 24
[0112] ITO/Buffer Layer/TPD Interfacial Stability Studies. TPD
de-cohesion analysis of the interfacial structures ITO/buffer
layer/TPD (100 nm) (spincoated TPD-Si.sub.2, spincoated TAA,
vapor-deposited Cu(Pc)) were carried out in the following manner.
Following vapor deposition of 50-100 nm TPD films onto the
respective buffer layer-coated ITO substrates, the samples were
annealed at 80-100.degree. C. under N.sub.2 for 1.0 h, and the film
morphology subsequently imaged by optical microscopy and AFM.
Example 25
[0113] OLED Device Fabrication. OLED devices of the structure:
ITO/interlayer/TPD(50 nm)/Alq(60 nm)/Al(100 nm) were fabricated
using standard vacuum deposition procedures (twin evaporators
interfaced to a <1 ppm O.sub.2 glove box facility). Deposition
rates for organic and metal were 2-4 .ANG./sec and 1-2 .ANG./sec
respectively, at 1.times.10.sup.-6 Torr. The OLED devices were
characterized inside a sealed aluminum sample container under
N.sub.2 using instrumentation described elsewhere. J. Cui, Q.
Huang, Q. Wang, T. J. Marks, Langmuir 2001, 17, 2051; W. Li, Q.
Wang, J. Cui, H. Chou, T. J. Marks, G. E. Jabbour, S. E. Shaheen,
B. Kippelen, N. Pegyhambarian, P. Dutta, A. J. Richter, J.
Anderson, P. Lee, N. Armstrong, Adv. Mater. 1999, 11, 730.
Example 26
[0114] Device Thermal Stability Evaluations. OLED devices were
subjected to heating under vacuum at 95.degree. C. for 0.5 h, and
were subsequently evaluated for I-V and L-V characteristics as
described above.
[0115] While the principles of this invention have been described
in connection with specific embodiments, it should be understood
clearly that these descriptions are added only by way of example
and are not intended to limit, in any way, the scope of this
invention. For instance, the present invention can be applied more
specifically to the construction of second-order nonlinear optical
materials as have been described in U.S. Pat. No. 5,156,918 which
is incorporated herein by reference in its entirety. Likewise, the
present invention can be used in conjunction with the preparation
of optical waveguides. Another advantages and features will become
apparent from the claims hereinafter, with the scope of the claims
determined by the reasonable equivalents, as understood by those
skilled in the art.
* * * * *