U.S. patent application number 11/206754 was filed with the patent office on 2009-03-05 for organic light-emitting device comprising buffer layer and method for fabricating the same.
Invention is credited to Yun Hye Hahm, Min Soo Kang, Young Chul Lee, Jeoung Kwen Noh, Se Hwan Son.
Application Number | 20090058260 11/206754 |
Document ID | / |
Family ID | 35907633 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090058260 |
Kind Code |
A9 |
Noh; Jeoung Kwen ; et
al. |
March 5, 2009 |
Organic light-emitting device comprising buffer layer and method
for fabricating the same
Abstract
Disclosed herein are an organic light-emitting device having a
structure formed by the sequential deposition of a substrate, a
first electrode, at least two organic layers and a second
electrode, in which the organic layers include a light-emitting
layer, and one of the organic layers, which is in contact with the
second electrode, is a buffer layer comprising a compound
represented by the following formula 1, as well as a fabrication
method thereof: ##STR1## wherein R.sup.1 to R.sup.6 have the same
meanings as defined in the specification. The buffer layer makes it
possible to minimize or prevent damage to the organic layer, which
can occur when forming the second electrode on the organic
layer.
Inventors: |
Noh; Jeoung Kwen; (Daejeon
Metropolitan City, KR) ; Son; Se Hwan; (Daejeon
Metropolitan City, KR) ; Lee; Young Chul; (Daejeon
Metropolitan City, KR) ; Hahm; Yun Hye; (Daejeon
Metropolitan City, KR) ; Kang; Min Soo; (Daejeon
Metropolitan City, KR) |
Correspondence
Address: |
MCKENNA LONG & ALDRIDGE LLP
1900 K STREET, NW
WASHINGTON
DC
20006
US
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20060038484 A1 |
February 23, 2006 |
|
|
Family ID: |
35907633 |
Appl. No.: |
11/206754 |
Filed: |
August 19, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10798584 |
Mar 10, 2004 |
|
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11206754 |
Aug 19, 2005 |
|
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09914731 |
Aug 30, 2001 |
6720573 |
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PCT/KR00/01537 |
Dec 27, 2000 |
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10798584 |
Mar 10, 2004 |
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Current U.S.
Class: |
313/499 ;
313/506 |
Current CPC
Class: |
H01L 2251/5323 20130101;
H01L 51/0072 20130101; H05B 33/14 20130101; H01L 51/5092 20130101;
C09K 11/06 20130101; H01L 51/5088 20130101; C09K 2211/1074
20130101; H01L 51/0059 20130101; H01L 51/0058 20130101; H01L
51/0081 20130101; C07D 487/16 20130101; H01L 51/0078 20130101 |
Class at
Publication: |
313/499 ;
313/506 |
International
Class: |
H01J 1/62 20060101
H01J001/62 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 19, 2004 |
KR |
10-2004-0065517 |
Dec 26, 2000 |
KR |
2000-82085 |
Dec 31, 1999 |
KR |
1999-037746 |
Claims
1. An organic light-emitting device comprising a substrate, a first
electrode, at least two organic layers and a second electrode in
the sequentially laminated form, in which the organic layers
include a light-emitting layer, and one of the organic layers,
which is in contact with the second electrode, is a buffer layer
comprising a compound represented by the following formula 1:
##STR10## wherein R.sup.1 to R.sup.6 are each independently
selected from the group consisting of hydrogen, halogen atoms,
nitrile (--CN), nitro (--NO.sub.2), sulfonyl (--SO.sub.2R),
sulfoxide (--SOR), sulfonamide (--SO.sub.2NR), sulfonate
(--SO.sub.3R), trifluoromethyl (--CF.sub.3), ester (--COOR), amide
(--CONHR or --CONRR'), substituted or unsubstituted straight or
branched chain C.sub.1-C.sub.12 alkoxy, substituted or
unsubstituted straight or branched C.sub.1-C.sub.12 alkyl,
substituted or unsubstituted aromatic or non-aromatic heterocyclic
rings, substituted or unsubstituted aryl, substituted or
unsubstituted mono- or di-arylamine, and substituted or
unsubstituted aralkylamine, and R and R' are each independently
selected from the group consisting of substituted or unsubstituted
C.sub.1-C.sub.60 alkyl, substituted or unsubstituted aryl, and
substituted or unsubstituted 5-7 membered heterocyclic rings.
2. The organic light-emitting device of claim 1, wherein the
compound of formula 1 is selected from compounds represented by the
following formulas 1-1 to 1-6: ##STR11## ##STR12##
3. The organic light-emitting device of claim 1, wherein the
organic light-emitting device is a top emission type or both-side
emission type light-emission device.
4. The organic light-emitting device of claim 1, wherein the second
electrode is formed by thin-film formation technology capable of
causing damage to the organic layer in the absence of the buffer
layer comprising the compound of formula 1 by involving charges or
particles with high kinetic energy.
5. The organic light-emitting device of claim 4, wherein the
thin-film formation technology is selected from the group
consisting of sputtering, physical vapor deposition (PVD) using a
laser, and ion-beam assisted deposition.
6. The organic light-emitting device of claim 1, wherein the first
electrode is a cathode, the second electrode is an anode, and the
device is fabricated by forming the cathode on the substrate and
then sequentially forming the organic layers and the anode on the
cathode.
7. The organic light-emitting device of claim 1, wherein the second
electrode is made of a conductive oxide film or metal having work
function of 2-6 eV.
8. The organic light-emitting device of claim 7, wherein the second
electrode is made of ITO (indium tin oxide).
9. The organic light-emitting device of claim 7, wherein the second
electrode is made of IZO (indium zinc oxide).
10. The organic light-emitting device of claim 1, wherein an thin
oxide film having an insulating property is additionally formed
between the second electrode and the buffer layer.
11. The organic light-emitting device of claim 1, wherein the
buffer layer also serves as a hole injection layer.
12. The organic light-emitting device of claim 1, wherein the
buffer layer comprising the compound of formula 1 has a thickness
of equal to or more than 20 nm.
13. The organic light-emitting device of claim 1, wherein the
organic layers include an electron transport layer and the electron
transport layer comprising a material having a group selected from
the group consisting of imidazole, oxazole and thiazole.
14. The organic light-emitting device of claim 13, wherein the
electron transport layer comprising a compound selected from the
group consisting a compound of the formula 2 below and a compound
of the formula 3 below: ##STR13## wherein, R.sup.7 and R.sup.8 are
each independently selected from the group consisting of hydrogen,
aliphatic hydrocarbons of 1-20 carbon atoms, and aromatic
heterocyclic rings or aromatic rings, provided that R.sup.7 and
R.sup.8 is not hydrogen concurrently; Ar is selected from the group
consisting of aromatic heterocyclic rings or aromatic rings;
R.sup.9 is selected from the group consisting of hydrogen,
aliphatic hydrocarbons having 1-6 carbon atoms, and aromatic
heterocyclic rings or aromatic rings; and X is selected from the
group consisting of O, S and NR.sup.10 wherein R.sup.10 is selected
from the group consisting of hydrogen, aliphatic hydrocarbons of
1-7 carbon atoms, and aromatic heterocyclic rings or aromatic
rings. ##STR14## wherein n is an integer of from 3 to 8; Z is O, S
or NR; R and R' are individually hydrogen; alkyl of 1-24 carbon
atoms; aryl or hetero-atom substituted aryl of 5-20 carbon atoms;
or halo; or atoms necessary to complete a fused aromatic ring; B is
a linkage unit consisting of alkyl, aryl, substituted alkyl, or
substituted aryl, which conjugatedly or unconjugately connects the
multiple benzazoles together.
15. The organic light-emitting device of claim 13, wherein an
electron injection layer is formed between the first electrode and
the electron transport layer.
16. The organic light-emitting device of claim 15, wherein the
electron injection layer is a LiF layer.
17. A method for fabricating an organic light-emitting device,
comprising the step of sequentially laminating a first electrode,
at least two organic layers and a second electrode on a substrate,
in which one of the organic layers is formed as a light-emitting
layer, and one of the organic layers, which is in contact with the
second electrode, is formed from a compound represented by the
following formula 1: ##STR15## wherein R.sup.1 to R.sup.6 are each
independently selected from the group consisting of hydrogen,
halogen atoms, nitrile (--CN), nitro (--NO.sub.2), sulfonyl
(--SO.sub.2R), sulfoxide (--SOR), sulfonamide (--SO.sub.2NR),
sulfonate (--SO.sub.3R), trifluoromethyl (--CF.sub.3), ester
(--COOR), amide (--CONHR or --CONRR'), substituted or unsubstituted
straight or branched chain C.sub.1-C.sub.12 alkoxy, substituted or
unsubstituted straight or branched C.sub.1-C.sub.12 alkyl,
substituted or unsubstituted aromatic or non-aromatic heterocyclic
rings, substituted or unsubstituted aryl, substituted or
unsubstituted mono- or di-arylamine, and substituted or
unsubstituted aralkylamine, and R and R' are each independently
selected from the group consisting of substituted or unsubstituted
C.sub.1-C.sub.60 alkyl, substituted or unsubstituted aryl, and
substituted or unsubstituted 5-7 membered heterocyclic rings.
18. The method of claim 17, wherein the second electrode is formed
by thin-film formation technology capable of causing damage to the
organic layer in no presence of the buffer layer comprising the
compound of formula I by involving charges or particles having high
kinetic energy.
Description
TECHNICAL FIELD
[0001] The present invention relates to an organic light-emitting
device and a method for fabricating the same. More particularly,
the present invention relates to an organic light-emitting device
including a layer for preventing an organic layer from being
damaged when forming an electrode on the organic layer in a process
of fabricating the organic light-emitting device, and a method for
fabricating the same.
BACKGROUND ART
[0002] Organic light-emitting devices (OLED) are generally composed
of two electrodes (an anode and a cathode) and at least one organic
layer located between these electrodes. When voltage is applied
between the two electrodes of the organic light-emitting device,
holes and electrons are injected into the organic layer from the
anode and cathode, respectively, and are recombined in the organic
layer to form excitons. In turn, when these excitons decay to their
ground state, photons corresponding to the energy difference are
emitted. By this principle, the organic light-emitting devices
generate visible ray, and they are used in the fabrication of
information display devices and illumination devices.
[0003] The organic light-emitting devices are classified into three
types: a bottom emission type in which light produced in the
organic layer is emitted in the direction of a substrate; a top
emission type in which the light is emitted in direction opposite
the substrate; and a both-side emission type in which the light is
emitted in both the direction of the substrate and the direction
opposite the substrate.
[0004] In passive matrix organic light-emitting device (PMOLED)
displays, an anode and a cathode perpendicularly cross each other,
and the area of the crossing point acts as a pixel. Thus, the
bottom emission and top emission types have no great difference in
effective display area ratios (aperture ratios).
[0005] However, active matrix organic light-emitting device
(AMOLED) displays include thin-film transistors (TFTs) as switching
devices for driving the respective pixels. Because the fabrication
of these TFTs generally requires a high-temperature process (at
least several hundred .degree. C.), a TFT array required for the
driving of organic light-emitting devices is formed on a glass
substrate before the deposition of electrodes and organic layers.
In this regard, the glass substrate having the TFT array formed
thereon is defined as a backplane. When the active matrix organic
light-emitting device displays having this backplane are fabricated
to have the bottom emission structure, a portion of light emitted
toward the substrate is blocked by the TFT array, resulting in a
reduction in the effective display aperture ratio. This problem
becomes more severe when pluralities of TFTs are given to one pixel
in order to fabricate more elaborate displays. For this reason, the
active matrix organic light-emitting devices need to be fabricated
to have the top emission structure.
[0006] In the top emission type or both-side emission type organic
light-emitting devices, an electrode located on the opposite side
of the substrate without making contact with the substrate must be
transparent in the visible ray region. In the organic
light-emitting devices, a conductive oxide film made of, for
example, indium zinc oxide (IZO) or indium tin oxide (ITO), is used
as the transparent electrode. However, this conductive oxide film
has a very high work function of generally more than 4.5 eV. For
this reason, if the cathode is made of this oxide film, the
injection of electrons from the cathode into the organic layer
becomes difficult, resulting in a great increase in the operating
voltage of the organic light-emitting devices and deteriorations in
important device characteristics, such as light emission
efficiency. The top emission or both-side emission type organic
light-emitting devices need to be fabricated to have the so-called
"inverted structure" formed by the sequential lamination of the
substrate, the cathode, the organic layer and the anode.
[0007] Furthermore, if an a-Si thin-film transistor is used in the
active matrix organic light-emitting device, the a-Si TFT has a
structure where source and drain junctions are doped with n-type
impurities because the a-Si TFT has a physical property such that
the main charge carriers are electrons. Thus, in the case of
fabricating the active matrix organic light-emitting device with
the a-Si TFT, it is preferable in terms of charge injection and
process simplification that the active matrix organic
light-emitting device is fabricated to have the so-called "inverted
structure" by forming the cathode of the organic light-emitting
device on the source junction or drain junction of the a-Si TFT
formed on the substrate, and then, sequentially forming the organic
layer and the anode made of conductive oxide, such as ITO or
IZO.
[0008] In a process of fabricating the organic light-emitting
device with the above-described inverted structure, if the
electrode located on the organic layer is formed of a transparent
conductive oxide film, such as IZO or ITO, by the use of resistive
heating evaporation, the resistive heating evaporation will cause
the collapse of the inherent chemical composition ratio of the
oxide due to, for example, thermal decomposition during a thermal
evaporation procedure. This will result in the loss of
characteristics, such as electrical conductivity and visible ray
permeability. For this reason, the resistive heating evaporation
cannot be used in the deposition of the conductive oxide film, and
in most cases, techniques, such as plasma sputtering, are now
used.
[0009] However, if the electrode is formed on the organic layer by
techniques such as sputtering, the organic layer can be damaged due
to, for example, electrically charged particles present in plasma
used in the sputtering process. Furthermore, the kinetic energy of
atoms, which reach the organic layer and form an electrode on the
organic layer in the sputtering process, is several tens to several
thousands of eV, which is much higher than the kinetic energy of
atoms (generally less than 1 eV) in the resistive heating
evaporation. Thus, the physical properties of the organic layer can
be deteriorated by particle bombardment on the organic layer,
resulting in deterioration of electron or hole injection and
transport characteristics and light emission characteristics.
Particularly, organic materials consisting mainly of covalent bonds
of C and H, and thin films made of these materials, are generally
very weak against plasma during a sputtering process, compared to
inorganic semiconductor materials (e.g., Si, Ge, GaAs, etc.) and,
once damaged, the organic materials cannot be returned to their
original state.
[0010] Thus, in order to fabricate good organic light-emitting
devices, damage to the organic layer, which can occur when forming
an electrode on the organic layer by a technique, such as
sputtering must be minimized or eliminated.
[0011] To avoid damage to the organic layer, which can occur when
forming an electrode on the organic layer, for example, by
sputtering, methods for controlling the rate of thin-film formation
are used. For instance, in one method, RF power or DC voltage in an
RF or DC sputtering process can be lowered to reduce the number and
mean kinetic energy of atoms incident from a sputtering target onto
the substrate of the organic light-emitting device, thus reducing
sputtering damage to the organic layer.
[0012] In another method for preventing sputtering damage to the
organic layer, the distance between the sputtering target and the
substrate of the organic light-emitting device can be increased to
enhance the opportunity of the collisions between atoms, incident
to the substrate of the organic light-emitting device from a
sputtering target, and sputtering gases (e.g., Ar), thus
intentionally reducing the kinetic energy of the atoms.
[0013] However, as most of the above-described methods result in a
very low deposition rate, the processing time of the sputtering
step becomes very long, resulting in a significant reduction in
productivity throughout a batch process for fabricating the organic
light-emitting device. Furthermore, even in an instance when the
sputtering process has a low deposition rate as described above,
the possibility of particles having high kinetic energy reaching
the surface of the organic layer still exists, and thus, it is
difficult to effectively prevent sputtering damage to the organic
layer.
[0014] "Transparent organic light emitting devices," Applied
Physics Letters, May 1996, Volume 68, p. 2606, describes a method
of forming an anode and organic layers on a substrate, and then
forming a thin layer of mixed metal film of Mg:Ag having excellent
electron injection performance thereon, and lastly, forming a
cathode using ITO by sputtering deposition thereon. The structure
of the organic light-emitting device described in this article is
illustrated in FIG. 1. However, the Mg:Ag metal film has
shortcomings in that the metal film is lower in visible ray
permeability than ITO or IZO and also its process control is
somewhat complicated.
[0015] "A metal-free cathode for organic semiconductor devices,"
Applied Physics Letters, Volume 72, April 1998, p. 2138, describes
an organic light-emitting device having a structure formed by the
sequential lamination of a substrate, a anode, an organic layer and
a cathode, where a CuPc layer, relatively resistant to sputtering,
is deposited between the organic layer and the cathode in order to
prevent sputtering damage to the organic layer, which is caused by
the deposition of the cathode. FIG. 2 illustrates the structure of
the organic light-emitting device described in the article.
[0016] However, while CuPc is generally used to form a hole
injection layer, in the above literature, CuPc serves as an
electron injection layer in a state damaged by sputtering, between
the organic layer and the cathode in the organic light-emitting
device with a structure formed by the sequential lamination of the
substrate, the anode, the organic layer and the cathode. This
deteriorates device characteristics, such as the charge injection
characteristic and electric current efficiency of the organic
light-emitting device. Furthermore, CuPc has large light absorption
in the visible ray region, and thus, increasing the thickness of
the CuPc film leads to rapid deterioration of the device
performance.
[0017] "Interface engineering in preparation of organic surface
emitting diodes", Applied Physics Letters, Volume 74, May 1999, p.
3209, describes an attempt to improve the low electron injection
characteristic of the CuPc layer by depositing a second electron
transport layer (e.g., Li thin film) between an electron transport
layer and the CuPc layer. FIG. 3 illustrates the structure of the
organic light-emitting device described in this literature.
However, this method for preventing sputtering damage has problems
in that an additional thin metallic film is required and process
control also becomes difficult.
[0018] Accordingly, there is a need for the development of
technology to prevent the organic layer from being damaged when
forming the anode in the organic light-emitting device with the
above-described inverted structure.
[0019] Meanwhile, an electron injection characteristic from a
cathode to an electron transport layer in a regular organic
light-emitting device, is improved by depositing a thin LiF layer,
which helps the injection of electrons, between the electron
transport layer and the cathode. However, the electron injection
characteristic is improved only when the method is used in a device
in which the cathode is used as a top contact electrode, while the
electron injection characteristic is very poor when the method is
used in a device having an inverted structure in which the cathode
is used as a bottom contact electrode.
[0020] "An effective cathode structure for inverted top-emitting
organic light-emitting device," Applied Physics Letters, Volume 85,
September 2004, p. 2469, describes an attempt to improve the
electron injection characteristic through a structure having a very
thin Alq3-LiF--Al layer between a cathode and an electron transport
layer. However, the structure has a disadvantage that the
fabricating process is very complicated. In addition, "Efficient
bottom cathodes for organic light-emitting device," Applied Physics
Letters, Volume 85, August 2004, p. 837, describes an attempt to
improve the electron injection characteristic by depositing a thin
Al layer between a metal-halide layer (NaF, CsF, KF) and an
electron transport layer. However, the method also has a problem in
the process because a new layer must be used.
[0021] Accordingly, in an organic light-emitting device having an
inverted structure, a method to improve the electron injection
characteristic and to simplify the process for fabricating a device
is required.
[Disclosure]
[Technical Problem]
[0022] The present inventors have conducted studies on an organic
light-emitting device with a structure formed by the sequential
lamination of a substrate, a first electrode, at least two organic
layers, and a second electrode, and consequently, found that if one
of the organic layers, which is in contact with the second
electrode, is formed of an organic material discovered by the
present inventors, it is possible to minimize damage to the organic
layer, which can occur during the formation of the second
electrode. By this, a top emission type or both-side emission type
organic light-emitting device having an inverted structure formed
by the sequential lamination of a substrate, a cathode, organic
layers and an anode can be fabricated without adversely affecting
the device characteristics. Moreover, the present inventors have
found an electron transport material appropriate to the above
organic light-emitting device having an inverted structure and the
use of such an electron transport material can simplify the
fabricating process of the device and improve the electron
injection characteristic.
[0023] Therefore, it is an objective of the present invention to
provide an organic light-emitting device including a buffer layer
capable of preventing an organic layer from being damaged when
forming an electrode in the organic light-emitting device and
having improved electron injection characteristic, as well as a
fabrication method thereof.
[Technical Solution]
[0024] In one embodiment, the present invention provides an organic
light-emitting device comprising a substrate, a first electrode, at
least two organic layers and a second electrode in the sequentially
laminated form, in which the organic layers include a
light-emitting layer, and one of the organic layers, which is in
contact with the second electrode, is a buffer layer comprising a
compound represented by the following formula 1: ##STR2## wherein
R.sup.1 to R.sup.6 are each independently selected from the group
consisting of hydrogen, halogen atoms, nitrile (--CN), nitro
(--NO.sub.2), sulfonyl (--SO.sub.2R), sulfoxide (--SOR),
sulfonamide (--SO.sub.2NR), sulfonate (--SO.sub.3R),
trifluoromethyl (--CF.sub.3), ester (--COOR), amide (--CONHR or
--CONRR'), substituted or unsubstituted straight or branched chain
C.sub.1-C.sub.12 alkoxy, substituted or unsubstituted straight or
branched C.sub.1-C.sub.12 alkyl, substituted or unsubstituted
aromatic or non-aromatic heterocyclic rings, substituted or
unsubstituted aryl, substituted or unsubstituted mono- or
di-arylamine, and substituted or unsubstituted aralkylamine, and R
and R' are each independently selected from the group consisting of
substituted or unsubstituted C.sub.1-C.sub.60 alkyl, substituted or
unsubstituted aryl, and substituted or unsubstituted 5-7 membered
heterocyclic rings.
[0025] In another embodiment of the present invention, the
inventive organic light-emitting device is a top emission type or
both-side emission type device.
[0026] In another embodiment of the present invention, the second
electrode in the inventive organic light-emitting device is formed
by thin-film formation technology capable of causing damage to the
organic layer in the absence of the buffer comprising the compound
of formula 1 by involving charges or particles with high kinetic
energy.
[0027] In another embodiment of the present invention, the second
electrode in the inventive organic light-emitting device is formed
of a conductive oxide film or metal having work function of 2-6
eV.
[0028] In another embodiment, the first electrode in the inventive
organic light-emitting device is a cathode, and the second
electrode is an anode.
[0029] In another embodiment of the present invention, the organic
layers in the inventive organic light-emitting device include an
electron transport layer and the electron transport layer comprises
a material having a group selected from the group consisting of
imidazole, oxazole and thiazole.
[0030] In yet another embodiment, the present invention provides a
method for fabricating an organic light-emitting device, comprising
the step of sequentially laminating a first electrode, at least two
organic layers and a second electrode on a substrate, in which one
of the organic layers is formed as a light-emitting layer, and one
of the organic layers, which is in contact with the second
electrode, is formed of the compound represented by formula 1.
[Advantageous Effects]
[0031] According to the present invention, damage to the organic
layer, which can occur when forming an electrode on the organic
layer, can be prevented by the buffer layer comprising the compound
of formula 1 below. By this, an organic light-emitting device
having a structure formed by the sequential lamination of a
substrate, a cathode, organic layers and an anode can be fabricated
without damage to the organic layer, which can occur when forming
the electrode on the organic layer.
DESCRIPTION OF DRAWINGS
[0032] The above and other objects, features and advantages of the
present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0033] FIG. 1 illustrates the structure of the prior organic
light-emitting device formed by sequentially laminating a
substrate, an anode, organic layers and a cathode (ITO), in which
an Mg:Ag layer is applied between one of the organic layers and the
ITO cathode;
[0034] FIG. 2 illustrates the structure of the prior organic
light-emitting device formed by sequentially laminating a
substrate, an anode, organic layers and a cathode (ITO), in which a
CuPc layer is applied between one of the organic layers and the ITO
cathode;
[0035] FIG. 3 illustrates the structure of the prior organic
light-emitting device shown in FIG. 2, in which a Li thin film
(electron injection layer) is laminated as an organic layer in
contact with the CuPc layer in the light-emitting device;
[0036] FIG. 4 illustrates the structure of a top emission type
organic light-emitting device according to the present
invention;
[0037] FIG. 5 illustrates the structure of a both-side emission
type organic light-emitting device according to the present
invention;
[0038] FIG. 6 is a graphic diagram showing a change in the reverse
voltage-current (leakage current) characteristic of an organic
light-emitting device as a function of the thickness of the
inventive buffer layer;
[0039] FIG. 7 is a graphic diagram showing a change in the forward
voltage-current characteristic of an organic light-emitting device
as a function of the thickness of the inventive buffer layer;
[0040] FIG. 8 is a graphic diagram showing the luminous
intensity-current density characteristic of an organic
light-emitting device as a function of the thickness of the
inventive buffer layer;
[0041] FIG. 9 is a graphic diagram showing the luminance
efficiency-current density characteristic of an organic
light-emitting device as a function of the thickness of the
inventive buffer layer; and
[0042] FIG. 10 is a graphic diagram showing visible ray
permeability as a function of the deposition thickness of the
inventive buffer layer consisting of the compound represented by
formula 1.
[0043] FIG. 11 shows the crystal structure in c-axial of the
compound of formula 1-1.
[0044] FIG. 12 is SEM image showing the surface of a film
consisting of the compound of formula 1-1.
[0045] FIG. 13 illustrates a structure of a device having a
symmetrical structure consisting of Al--LiF-electron transport
layer-LiF--Al fabricated in Example 7.
[0046] FIG. 14 is a graphic diagram showing a forward
voltage-current characteristic and reverse voltage-current
characteristic by electrons in the device having a symmetrical
structure fabricated in Example 7.
MODE FOR INVENTION
[0047] Hereinafter, the present invention will be described in
detail.
[0048] An organic light-emitting device according to the present
invention has a structure formed by sequentially laminating a
substrate, a first electrode, at least two organic layers and a
second electrode, in which the organic layers include a
light-emitting layer, and one of the organic layers, which is in
contact with the second electrode, is a buffer layer comprising a
compound represented by the following formula 1: ##STR3## wherein,
R.sup.1 to R.sup.6 are each independently selected from the group
consisting of hydrogen, halogen atoms, nitrile (--CN), nitro
(--NO.sub.2), sulfonyl (--SO.sub.2R), sulfoxide (--SOR),
sulfonamide (--SO.sub.2NR), sulfonate (--SO.sub.3R),
trifluoromethyl (--CF.sub.3), ester (--COOR), amide (--CONHR or
--CONRR'), substituted or unsubstituted straight or branched chain
C.sub.1-C.sub.12 alkoxy, substituted or unsubstituted straight or
branched C.sub.1-C.sub.12 alkyl, substituted or unsubstituted
aromatic or non-aromatic heterocyclic rings, substituted or
unsubstituted aryl, substituted or unsubstituted mono- or
di-arylamine, and substituted or unsubstituted aralkylamine, and R
and R' are each independently selected from the group consisting of
substituted or unsubstituted C.sub.1-C.sub.60 alkyl, substituted or
unsubstituted aryl, and substituted or unsubstituted 5- to
7-membered heterocyclic rings.
[0049] In the inventive organic light-emitting device, the buffer
layer comprising the compound of formula 1 is an organic layer in
contact with the second electrode, and can prevent the organic
layer from being damaged when forming the second electrode on the
organic layer during the process of fabricating the organic
light-emitting device. For example, if a technique, such as
sputtering, is used for the formation of the second electrode,
particularly a second transparent electrode, on the organic layer,
electrical or physical damage to the organic layer can occur due to
electrically charged particles or atoms having high kinetic energy,
which are generated in plasma during a sputtering process. This
damage to the organic layer can likewise occur when forming an
electrode on the organic layer not only by sputtering but also by
thin-film formation technology capable of causing damage to the
organic layer by involving charges or particles having high kinetic
energy. However, when the second electrode is formed on the buffer
layer comprising the compound of formula 1 using the
above-described method, electrical or physical damage to the
organic layer can be minimized or prevented. This can be attributed
to the fact that the compound of formula 1 has a higher
crystallinity than that of organic materials used in the prior
organic light-emitting devices, so that the organic layer
comprising the compound has a higher density. FIG. 11 shows the
crystal structure in c-axial of the compound of formula 1-1, which
is an example of the compound of formula 1. FIG. 12 is SEM image
showing the surface of a film formed by the compound of formula
1-1. As shown in FIGS. 11 and 12, the compound of formula 1 is
confirmed to have a high crystallinity.
[0050] In the present invention, electrical or physical damage to
the organic layer can be minimized or prevented as described above,
so that the light-emitting characteristics of the device can be
prevented from being deteriorated by damage to the organic layer.
Also, because it is possible to prevent damage to the organic layer
in a process of forming the second electrode, the control of
process parameters and the optimization of a process apparatus
during the formation of the second electrode becomes easier, so
that process productivity throughout can also be improved. Also,
the material and deposition method of the second electrode can be
selected from a wide range thereof. For example, in addition to a
transparent electrode, a thin film made of metal, such as Al, Ag,
Mo, Ni, etc. can also be formed by sputtering or by physical vapor
deposition (PVD) using laser, ion-beam assisted deposition or
similar technologies which can cause damage to the organic layer in
the absence of the buffer comprising the compound of formula 1 by
involving charges or particles having high kinetic energy.
[0051] By the function of the buffer layer comprising the compound
of formula 1 in the inventive organic light-emitting device, the
material and deposition method of the second electrode can be
selected from a wide range thereof. Thus, a top emission type or
both-side emission type light-emitting device or an active matrix
organic light-emitting device having a-Si TFTs, where a cathode,
organic layers and an anode are sequentially laminated on a
substrate, can be fabricated without causing damage to the organic
layer. Up to now, there has been no disclosure showing the organic
light-emitting device having the above-described inverted
structure, which has been fabricated without the problem of damage
to the organic layer.
[0052] In the present invention, the electrical properties of the
organic light-emitting device can be improved by the use of a
buffer layer comprising the compound of formula 1. For example, the
inventive organic light-emitting device shows a reduction in
leakage current in a reverse bias state, leading to a remarkable
improvement in current-voltage characteristics, and thus, a very
clear rectification characteristic. As used herein, the term
"rectification characteristic," which is a general characteristic
of diodes means that the magnitude of current in a region applied
with reverse voltage is much lower than the magnitude of current in
a region applied with forward voltage. The compound of formula 1
has excellent crystallinity compared to organic materials, which
have been used in the prior organic light-emitting devices as
described above so that a layer made of the compound of formula 1
has a high density. Thus, the compound of formula 1 effectively
prevents structural defects of molecules or defects to interfacial
characteristics, which occur when particles having high kinetic
energy are implanted into the inside or interlayer interface of the
organic layer by a sputtering process or the like. For this reason,
the electrical characteristics, such as rectification
characteristic, of the device seem to be maintained.
[0053] Also, the buffer layer comprising the compound of formula 1
has higher visible ray permeability than an inorganic layer used in
the prior buffer layer that are made of, for example, metal or
CuPc, so that its thickness is controlled more variably than the
prior buffer layer. FIG. 10 shows permeability in the visible ray
region as a function of the thickness of a thin film made of the
compound of formula 1. When the inorganic layer which has been used
as the buffer layer in the prior art is generally formed to a
thickness of 200 nm, it has very low visible ray permeability,
however, the layer comprising the compound of formula 1 did not
show a reduction in visible ray permeability even when its
thickness was 200 nm.
[0054] Furthermore, if the second electrode in the inventive
organic light-emitting device is an anode, the buffer layer
comprising the compound of formula 1 not only functions to prevent
sputtering damage but also acts as a hole injection layer for
injecting holes from the anode into a hole transport layer or a
light-emitting layer or as a charge generation layer for forming
hole-electron pairs. Accordingly, the inventive organic
light-emitting device can become more efficient without requiring a
separate hole injection layer or hole transport layer.
[0055] Concrete examples of the compound of formula 1 include
compounds represented by the following formulas: ##STR4##
##STR5##
[0056] Other examples, synthetic methods and various features of
the compound of formula 1 are described in the U.S. patent
application No. 2002-0158242, U.S. Pat. No. 6,436,559 and U.S. Pat.
No. 4,780,536, the disclosures of which are all incorporated herein
by reference.
[0057] In the present invention, the effect of the buffer layer
comprising the compound of formula 1 can be enhanced by increasing
its thickness. This is proven by an improvement in the leakage
current resulting from an increase in the thickness of the buffer
layer. FIG. 6 shows leakage current as a function of the thickness
of the buffer layer in contact with the anode in the organic
light-emitting device having a structure formed by the sequential
lamination of the substrate, the cathode, the organic layers and
the anode. As can be seen in FIG. 6, as the thickness of the layer
comprising the compound of formula 1 increases from 5-10 nm to 50
nm, the leakage current is rapidly reduced, leading to a remarkable
improvement in voltage-current characteristics.
[0058] In the present invention, the optimum thickness of the
buffer layer comprising the compound of formula 1 may vary
depending on sputtering process factors, such as, deposition rate,
RF power, DC voltage and the like, used in the formation of the
second electrode. For example, in the case of a sputtering process
using high voltage and power for rapid deposition, the optimum
thickness of the buffer layer increases. In the present invention,
the thickness of the buffer layer comprising the compound of
formula 1 is preferably equal to or more than 20 nm, and more
preferably equal to or more than 50 nm. If the thickness of the
buffer layer is less than 20 nm, the layer can function as a hole
injection or transport layer, but cannot sufficiently function as
the buffer layer. Meanwhile, the thickness of the buffer layer is
preferred to be equal to or less than 250 nm. If the thickness of
the buffer layer is more than 250 nm, the process time required for
the fabrication of the device will become long and the surface
shape of the layer comprising the compound of formula 1 will become
rough, thus adversely affecting the other characteristics of the
device.
[0059] In the present invention, the buffer layer comprising the
compound of formula 1 can be formed between the anode and the
cathode by vacuum deposition or solution application techniques.
Examples of the solution application techniques include, but are
not limited to, spin coating, dip coating, doctor blading, inkjet
printing, thermal transfer techniques, etc. The buffer layer
comprising the compound of formula 1 may also additionally comprise
other materials, if necessary, and the buffer layer may be formed
of a thin film made of a mixture of organic and inorganic
materials.
[0060] In the present invention, a thin oxide film having an
insulating property may be additionally formed between the second
electrode and the buffer layer.
[0061] Meanwhile, in the inventive organic light-emitting device,
the organic layers may include an electron transport layer and the
electron transport layer can be formed by the co-deposition of an
organic material with a metal having a low work function, such as,
Li, Cs, Na, Mg, Sc, Ca, K, Ce, Eu or a thin metal film containing
at least one of these metals. However, the electron transport layer
in the inventive organic light-emitting device is preferred to
comprise a material having a group selected from the group
consisting of imidazole, oxazole and thiazole, more preferably
imidazole group. Examples of the material include the compound of
the following formula 2 having imidazole group, as described in
Korean Paten Laid-open Publication 2003-0067773 and the compound of
the following formula 3, as described in U.S. Pat. No. 5,645,948,
etc. The materials can be co-deposited with a metal having a low
work function, such as, Li, Cs, Na, Mg, Sc, Ca, K, Ce, Eu, etc.
Korean Paten Laid-open Publication 2003-0067773 and U.S. Pat. No.
5,645,948 entirely are incorporated into this specification.
##STR6##
[0062] wherein, R.sup.7 and R.sup.8 are each independently selected
from the group consisting of hydrogen, aliphatic hydrocarbons of
1-20 carbon atoms, and aromatic heterocyclic rings or aromatic
rings, such as benzene, naphthalene, biphenyl and anthracene,
provided that R.sup.7 and R.sup.8 is not hydrogen concurrently;
[0063] Ar is selected from the group consisting of aromatic
heterocyclic rings or aromatic rings, such as, benzene,
naphthalene, biphenyl and anthracene;
[0064] R.sup.9 is selected from the group consisting of hydrogen,
aliphatic hydrocarbons having 1-6 carbon atoms, and aromatic
heterocyclic rings or aromatic rings, such as, substituted benzene,
naphthalene, biphenyl and anthracene; and
[0065] X is selected from the group consisting of O, S and
NR.sup.10 wherein R.sup.10 is selected from the group consisting of
hydrogen, aliphatic hydrocarbons of 1-7 carbon atoms, and aromatic
heterocyclic rings or aromatic rings, such as, benzene,
naphthalene, biphenyl and anthracene. ##STR7##
[0066] wherein n is an integer of from 3 to 8;
[0067] Z is O, S or NR;
[0068] R and R' are individually hydrogen; alkyl of 1-24 carbon
atoms, for example, propyl, t-butyl, heptyl, and the like; aryl or
hetero-atom substituted aryl of 5-20 carbon atoms, for example,
phenyl and naphthyl, furyl, thienyl, pyridyl, quinolinyl and other
heterocyclic systems; or halo such as chloro, fluoro; or atoms
necessary to complete a fused aromatic ring;
[0069] B is a linkage unit consisting of alkyl, aryl, substituted
alkyl, or substituted aryl, which conjugatedly or unconjugately
connects the multiple benzazoles together.
[0070] In the present invention, if the electron transport layer is
formed to comprise the above material, the inventive device is
preferred to include an electron injection layer. The electron
injection layer is preferred to a LiF layer.
[0071] The inventive organic light-emitting device has a structure
formed by the sequential lamination of a substrate, a first
electrode, at least two organic layers and a second electrode, and
can be fabricated by the use of the same materials and methods as
known in the art except that one of the organic layers, which is in
contact with the second electrode, is formed as the buffer layer
comprising the compound of formula 1.
[0072] As described above, in the present invention, there is no
specific limitation on methods of forming the second electrode on
the buffer layer, and thus, the material and formation process of
the second electrode can be selected from a wider range thereof
than that of the prior art.
[0073] For example, the second electrode in the present invention
can be formed by thin-film formation technology capable of causing
damage to the organic layer in the absence of the buffer layer
comprising the compound of formula 1 by involving charges or
particles with high kinetic energy, such as sputtering, physical
vapor deposition (PVD) using a laser, ion-beam-assisted deposition
or technology similar thereto. Thus, electrode materials, which can
be formed into a film only by these techniques may also be used.
For example, the second electrode may be formed of a conductive
oxide transparent in the visible ray region, such as indium-doped
zinc oxide (IZO) or indium-doped tin oxide (ITO), or Al, Ag, Au,
Ni, Pd, Ti, Mo, Mg, Ca, Zn, Te, Pt, Ir or an alloy material
containing at least one of these metals.
[0074] Examples of the organic light-emitting device according to
the present invention are shown in FIGS. 4 and 5. FIG. 4
illustrates a top emission type light-emitting device, and FIG. 5
illustrates a both-side emission type light-emitting device.
However, it will be understood that the structure of the inventive
organic light-emitting device is not limited only to these
structures.
[0075] The organic layers in the inventive organic light-emitting
device may consist not only of a monolayer structure but also of a
multilayer structure formed by the lamination of at least two
organic layers. For example, the inventive organic light-emitting
device may have a structure comprising a hole injection layer, a
hole transport layer, a light-emitting layer, an electron transport
layer, an electron injection layer, a buffer layer formed between
an anode and the hole injection layer, and the like as organic
layers. However, the structure of the organic light-emitting device
is not limited only to this structure and may comprise a smaller
number of organic layers.
[0076] Hereinafter, the present invention will be described in
detail using examples. It is to be understood, however, that these
examples are given for illustrative purpose only and are not to be
construed to limit the scope of the present invention.
EXAMPLES
Examples 1-5
[0077] On a glass substrate, a cathode (Al) having a thickness of
150 nm and an electron injection layer (LiF) having a thickness of
1.5 nm were sequentially formed by a thermal evaporation process.
Then, on the electron injection layer, an electron transport layer
consisting of a thin film made of a material represented by the
following formula 2-1 comprising imidazole group was formed to a
thickness of 20 nm. ##STR8##
[0078] Then, on the electron transport layer, an Alq.sub.3
light-emitting host was co-deposited with C545T
(10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahyro-1H,5H,11H-1)-
benzopyrano[6,7,8-ij]quinolizin-11-one) to form a light-emitting
layer having a thickness of 30 nm. On the light-emitting layer, a
hole transport layer consisting of a thin film made of NPB
(4,4'-bis[N-(1-napthyl)-N-phenylamino]biphenyl) was deposited to a
thickness of 40 nm. On the hole transport layer, a hole
injection/buffer layer made of a compound represented by the
following formula 1-1 was formed to a thickness of 5 nm (Example
1), 10 nm (Example 2), 20 nm (Example 3), 50 nm (Example 4) or 70
nm (Example 5): ##STR9##
[0079] On the buffer layer, an IZO anode having a thickness of 150
nm was formed by a sputtering process at a rate of 1.3 .ANG./sec,
thus fabricating a top emission type organic light-emitting
device.
Example 6
[0080] A both-side emission type organic light-emitting device was
fabricated in the same manner as described in Examples 1-5 except
that a cathode consisting of an thin Al film having a very small
thickness of 5 nm formed on an ITO film having a thickness of 150
nm is used in place of the cathode consisting of the thin Al film
having a thickness of 150 nm.
[0081] [Measurement of Current-Voltage Characteristics and Light
Emission Characteristics of Device]
[0082] To the organic light-emitting device fabricated in Example
1, each of reverse and forward electric fields was applied at a
voltage increasing at increments of 0.2 volts while current at each
voltage value was measured. The measurement results are shown in
FIGS. 6 and 7, respectively. Also, to the organic light-emitting
device fabricated in Example 1, current was applied while gradually
increasing current density from 10 mA/cm.sup.2 to 100 mA/cm.sup.2,
and at the same time, the luminous intensity of the device was
measured using photometry. The measurement results are shown in
FIGS. 8 and 9.
[0083] In organic light-emitting devices, damage to an organic
layer occurring in the formation of an electrode leads to
deterioration in current-voltage characteristics and light emission
characteristics. Thus, the current-voltage characteristics and
light emission characteristics shown in FIGS. 6 to 9 indicate that
the compound of formula 1 has the effect of preventing damage to
the organic layer.
[0084] FIGS. 6 and 7 show the current-voltage characteristics of
the organic light-emitting device as a function of the thickness of
the inventive buffer layer. It is known that when an organic layer
in contact with the second electrode located opposite the substrate
is made of an organic material, which has been generally used in
the prior organic light-emitting device, an organic light-emitting
device comprising this organic layer will not show normal
rectification and light emission characteristics due to the damage
to the organic layer, which occurs when forming the second layer on
the organic layer by sputtering. However, as shown in FIGS. 6 and
7, the inherent characteristics (e.g., rectification
characteristic) of the organic light-emitting device were clearly
shown as the thickness of the buffer layer made of the compound of
formula 1 increased.
[0085] Regarding a reverse current-voltage characteristic shown in
FIG. 6, the case of forming the buffer layer to a thickness of
about 5-10 nm showed little improvement in the leakage current of
the device, and the case of forming the buffer layer to a thickness
of more than 50 nm showed a remarkable improvement in the leakage
current of the device, indicating a very clear rectification
characteristic. Regarding a forward current-voltage characteristic
shown in FIG. 7, when the thickness of the layer made of the
compound of formula 1 was increased from 10 nm to 50 nm, current
was consequently increased rapidly.
[0086] Furthermore, as shown in FIG. 8, a light emission
characteristic was also improved in proportion to an increase in
the current as described above. Regarding luminance efficiency
shown in FIG. 9, an increase in the thickness of the buffer layer
comprising the compound of formula 1 showed a remarkable increase
in luminance efficiency. This is attributable to the effect of the
buffer layer of preventing sputtering damage.
Example 7
[0087] On a glass substrate, a cathode (Al) having a thickness of
150 nm and an electron injection layer (LiF) having a thickness of
1.5 nm were sequentially formed by a thermal evaporation process.
Then, on the electron injection layer, an electron transport layer
consisting of a thin film made of the material comprising imidazole
group represented by the above formula 2-1 was formed to a
thickness of 150 nm. On the electron transport layer, an electron
injection layer (LiF) having a thickness of 1.5 nm and Al layer
having a thickness of 150 nm were formed sequentially to fabricate
a symmetrical-type device as shown in FIG. 13 in which electric
current runs only through electrons.
Comparative Example 1
[0088] A symmetrical-type device, as shown in FIG. 13 in which
electric current runs only through electrons, was fabricated in the
same manner as described in Example 7, except that Alq3 in place of
the compound of formula 2-1 was used in forming an electron
transport layer.
[0089] [Measurement of Current-Voltage Characteristic of the
Device]
[0090] Example 7 and Comparative Example 1 were symmetrical-type
devices having the structure of Al--LiF-electron transport
material-LiF--Al, in which the electric current running through the
electron transport material is generated only by electrons.
[0091] FIG. 14 shows current-voltage characteristic in Example 7
and Comparative Example 1. In FIG. 14, the positive voltage shows
electron injection from top Al electrode to the electron transport
layer and the negative voltage shows electron injection from bottom
Al electrode to the electron transport layer. In Comparative
Example 1 that used Alq3 which is frequently used in organic
light-emitting device as an electron transport material, electron
injection from top Al electrode took place very well while electron
injection from bottom Al electrode did not take place very well in
spite of a symmetrical-type device. On the other hand, in Example 7
that used the compound of formula 2-1 as an electron transport
material, current voltage characteristic is symmetrical and this
means that electron injection from both of top Al electrode and
bottom Al electrode to the electron transport layer took place very
well.
[0092] The reason that the electron injection from the bottom
electrode to the electron transport layer took place more
effectively through the compound of formula 2-1 than Alq3 is
considered as the reactivity of imidazole group in the compound of
formula 2-1 to Li ion in Li-fluoride (LiF) is larger than that of
Alq3. Accordingly, when a material having a group of a large
reactivity to Li ion, such as, the imidazole group, is used as an
electron transport material, electron injection characteristic from
bottom electrode to electron transport layer can be improved.
[0093] An organic light-emitting device having an inverted
structure requires electron injection from bottom electrode to
electron transport layer. Accordingly, if an electron transport
material comprising imidazole, or, oxazole or thiazole having
similar properties to imidazole, such as the compound of formula 2
or 3, as described above, is used, an organic light-emitting device
having improved electron injection characteristic can be
provided.
INDUSTRIAL APPLICABILITY
[0094] According to the present invention, damage to the organic
layer, which can occur when forming an electrode on the organic
layer, can be prevented by the buffer layer comprising the compound
of formula 1. By this, an organic light-emitting device having a
structure formed by the sequential lamination of a substrate, a
cathode, organic layers and an anode can be fabricated without
damage to the organic layer, which can occur when forming the
electrode on the organic layer. In addition, in the organic
light-emitting device having an inverted structure, if an electron
transport material comprising imidazole, oxazole or thiazole, such
as, the compound of formula 2 or 3, is used, electron injection
characteristic from the bottom cathode to the electron transport
layer is improved and an organic light-emitting device of an
inverted structure operating at a low voltage can be provided.
[0095] Although a preferred embodiment of the present invention has
been described for illustrative purposes, those skilled in the art
will appreciate that various modifications, additions and
substitutions are possible without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
* * * * *