U.S. patent application number 09/257778 was filed with the patent office on 2001-11-29 for organic electroluminescent devices.
Invention is credited to ENDOH, JUN, KIDO, JUNJI, MIZUKAMI, TOKIO.
Application Number | 20010046611 09/257778 |
Document ID | / |
Family ID | 12840441 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010046611 |
Kind Code |
A1 |
KIDO, JUNJI ; et
al. |
November 29, 2001 |
ORGANIC ELECTROLUMINESCENT DEVICES
Abstract
An organic electroluminescent device includes at least one light
emission layer from an organic compound, the light emission layer
being positioned between an anode electrode and a cathode electrode
opposed to the anode electrode, in which an organic layer
positioned adjacent to the anode electrode is from an organic
compound which includes, as an electron-accepting dopant, an
electron-accepting compound having a property of oxidizing the
organic compound of said organic layer, said electron-accepting
compound being doped to said organic layer in vacuum with a
simultaneous evaporation method.
Inventors: |
KIDO, JUNJI; (NARA-KEN,
JP) ; MIZUKAMI, TOKIO; (KANAGAWA-KEN, JP) ;
ENDOH, JUN; (KANAGAWA-KEN, JP) |
Correspondence
Address: |
DONALD K HUBER
MCCORMICK PAULDING & HUBER
CITYPLACE II
185 ASYLUM STREET
HARTFORD
CT
061034102
|
Family ID: |
12840441 |
Appl. No.: |
09/257778 |
Filed: |
February 25, 1999 |
Current U.S.
Class: |
428/690 ;
313/504; 313/506; 428/917 |
Current CPC
Class: |
H01L 51/506 20130101;
H01L 51/5088 20130101; H01L 51/0059 20130101; H01L 51/0081
20130101 |
Class at
Publication: |
428/690 ;
428/917; 313/504; 313/506 |
International
Class: |
H05B 033/12 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 2, 1998 |
JP |
10-49771(P) |
Claims
What is claimed is:
1. An organic electroluminescent device comprising at least one
light emission layer from an organic compound, the light emission
layer being positioned between an anode electrode and a cathode
electrode opposed to the anode electrode, in which an organic layer
positioned adjacent to the anode electrode is from an organic
compound comprising, as an electron-accepting dopant, an
electron-accepting compound having a property of oxidizing the
organic compound of said organic layer, said electron-accepting
compound being doped to said organic layer in vacuum with a
simultaneous evaporation method.
2. The organic electroluminescent device according to claim 1, in
which said electron-accepting dopant comprises an inorganic
compound.
3. The organic electroluminescent device according to claim 1 or 2,
in which said electron-accepting dopant comprises at least one
Lewis acid compound selected from the group consisting of ferric
chloride, aluminum chloride, gallium chloride, indium chloride and
antimony pentachloride.
4. The organic electroluminescent device according to claim 1, in
which said electron-accepting dopant comprises an organic
compound.
5. The organic electroluminescent device according to claim 1, in
which a molar ratio of the electron-accepting compound to the
organic compound of said organic layer is in the range of 0.1 to
10.
6. The organic electroluminescent device according to claim 1, in
which a total thickness of said organic layer is more than 2000
.ANG..
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an organic
electroluminescent device or element (hereinafter, referred also to
as an "organic EL device") which is utilized as a planar light
source or utilized in display devices.
[0003] 2. Description of the Related Art
[0004] Attention has been made to an organic EL device in which a
luminescent layer, i.e., light emission layer is formed from a
specific organic compound, because it ensures a large area display
device which can be operated at a low voltage. To obtain a highly
efficient EL device, Tang et al., as is reported in Appl. Phys.
Lett., 51, 913 (1987), have succeeded in attaining an EL device
having a structure in which organic compound layers having
different carrier transporting properties are laminated to thereby
introduce holes and electrons with a good balance from an anode,
and a cathode. In addition, since the thickness of the organic
compound layers is not more than about 2,000 .ANG., the EL device
can exhibit a high luminance and efficiency sufficient for
practical use; i.e., a luminance of about 1,000 cd/m.sup.2 and an
external quantum efficiency of about 1% at an applied voltage of
not more than about 10 volts.
[0005] In this highly efficient EL device, Tang et al. have used
magnesium (Mg) having a low work function in combination with an
organic compound which is essentially considered to be an
electrically insulating material, in order to reduce an energy
barrier which can cause a problem during injection of electrons
from a metal-made electrode. However, since the magnesium is liable
to be oxidized and is unstable, and also exhibits only a poor
adhesion to a surface of the organic layers, magnesium was used
after alloying. Alloying is carried out by vapor co-deposition or
simultaneous vapor evaporation of magnesium and silver (Ag) which
is relatively stable and exhibits good adhesion to a surface of the
organic layers.
[0006] Further, in the EL device developed by Tang et al., an
indium-tin-oxide (ITO) is coated as an anode electrode over a glass
substrate. However, the use of the ITO anode electrode device in
the Tang et al. to obtain good contact (near to ohmic contact) is
considered to be made due to an unexpected and fortunate
occurrence; namely, the ITO electrode is frequently used as a
transparent anode electrode made of metal oxide in the hole
injection of the organic compound to satisfy the requirement for
the omission of light in the planar area, and the ITO electrode can
exhibit a relatively large work function of a maximum of 5.0
eV.
[0007] Furthermore, in their EL device, Tang et al. have inserted a
layer of copper phthalocyanine (hereinafter termed as `CuPc`)
having a thickness of not more than 200 .ANG. between the anode and
the hole-transporting organic compound layer to further improve
contact efficiency of the anode interface region, thereby achieving
the operation of the device at a low voltage.
[0008] Similar effects have been also confirmed from the starburst
type arylamine compounds, proposed by Shirota et al. of Osaka
University, by the researchers of Pioneer Co., Ltd. Both the CuPc
compounds and the starburst type arylamine compounds have
characteristics that show a work function smaller than that of ITO,
and a relatively high mobility of hole charge, and thus improving
the stability of the EL devices during the continuous usage
thereof, facilitating low-voltage consumption and an improved
interfacial contact.
[0009] In addition to the above-described devices having the vacuum
evaporated layers, there are also known EL devices having the
layers formed from a coating solution of a film-forming polymeric
material by a coating method such as spin coating. In such EL
devices, the coating solution is prepared by previously dispersing
an electron-accepting compound in a hole-transporting polymeric
material. For example, Partridge, as is reported in POLYMER,
Vol.24, June 1983, has confirmed that an ohmic current can be
obtained if an antimony pentachloride (hereinafter, SbCl.sub.5) as
an electron-accepting compound is dispersed in dichroromethane
solution of polyvinyl carbazole (hereinafter, PVK), whereas such
ohmic current could not been realized with the sole use of PVK in
the layer formation. In this layer formation, it is understood that
SbCl.sub.5 can act as a Lewis acid so that a carbazole pendant
group of PVK is oxidized to produce radical cations. SbCl.sub.5
used by Partridge in the layer formation is in a liquid state at a
room temperature, and is a Lewis acid compound having a remarkably
high reactivity so that fumes can be produced upon reaction with
water in atmospheric air. However, contrary to this, if it is
reacted with PVK in a glove chamber under an inert atmosphere,
SbCl.sub.5 can form a stable complex compound, thereby enabling to
form a layer of the complex compound under relatively stable
conditions of atmospheric air. Thus, the above layer formation
method is considered to be a rational one if it is intended to form
a hole injection layer from a side of the ITO electrode. However,
in the recent organic EL devices, a highly increased efficiency of
the device has been achieved largely relying upon the high purity
layer formation process which is based on vacuum evaporation and
does not cause cross-contamination. In this production of EL
devices, assuming that the above-described method by Partridge is
directly applied without any modification, some questions arise
because stable driving of the EL devices can be adversely affected
by any residues of the solvent used in the coating solution and any
impurities in the layer-forming materials.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to solve the
above-described problems in the EL devices of the prior art. An
object of the present invention is to reduce the energy barrier in
the hole injection from a transparent ITO anode electrode to a
hole-transporting organic layer, and to achieve low-voltage
consumption regardless of the work function of the anode
material.
[0011] To accomplish the above object, the inventors have
researched extensively and have now discovered that in the hole
injection from an anode electrode to an organic layer adjacent to
the anode electrode, an injection barrier (and thus, the voltage)
can be reduced if the organic layer is doped with a compound
capable of acting as an electron-accepting dopant by a
co-deposition or simultaneous evaporation method.
[0012] According to the present invention, there is provided an
organic electroluminescent (EL) device including at least one light
emission layer from an organic compound, the light emission layer
being positioned between an anode electrode and a cathode electrode
opposed to the anode electrode, in which an organic layer
positioned adjacent to the anode electrode is from an organic
compound and includes, as an electron-accepting dopant, an
electron-accepting compound having a property of oxidizing the
organic compound of said organic layer, said electron-accepting
compound being doped to said organic layer in vacuum by a
simultaneous evaporation method.
[0013] In the organic EL devices, the hole injection process from
an anode to an organic layer which is basically constituted from an
electrically insulating organic compound is intended to carry out
oxidation of the organic compound on a surface region of the
organic layer, i.e., formation of a radical cation state thereof
(cf., Phys. Rev. Lett., 14, 229 (1965)). In the organic EL device,
an electron-accepting dopant substance or compound which can act as
an oxidizing agent for the organic compound is previously doped in
an organic layer in contact with the anode electrode, and thus the
energy barrier of the hole injection from the anode electrode can
be lowered as a result of such provisional doping of the dopant
compound in the organic layer. Since the molecules in the oxidized
state (oxidized by the dopant), i.e., in the electron-donated
state, are already included in the doped organic layer, a barrier
of the hole injection energy is low in the EL device, and therefore
a driving voltage of the device can be lowered in comparison with
the EL devices of the prior art.
[0014] In practice, the electron-accepting dopant used in the
formation of the organic layer in contact with the anode electrode
may be either an inorganic compound or an organic compound, as long
as they have an electron-accepting property and can oxidize an
organic compound in the organic layer. Particularly, suitable
electron-accepting dopant compounds in the form of an inorganic
compound include a Lewis acid compound such as ferric chloride,
aluminum chloride, gallium chloride, indium chloride, antimony
pentachloride. Further, if an organic dopant compound is used,
suitable dopant compounds include an organic electron-accepting
compound such as trinitrofluorenone. These dopant compounds may be
used alone or in combination.
[0015] When the above-described dopant compounds are doped in the
organic layer by the co-deposition method, the compounds having a
relatively low saturated vapor pressure such as ferric chloride and
indium chloride can be contained in a crucible, followed by
depositing using a conventional resistance heating method.
Alternatively, if the dopant compounds used have a high vapor
pressure at an ordinary temperature and therefore the pressure in
the vacuum deposition apparatus can not be maintained at a level
below the predetermined degree of vacuum, the vapor pressure may be
controlled by using an orifice (opening size)-controlling means
such as a needle valve or mass flow controller or by using a
susceptor or sample-supporting means having a separate
temperature-controlling system to cool the dopant compounds.
[0016] A thickness of the produced doped organic layer is not
restricted to the specific thickness range, however, it is
generally preferred that the thickness is not less than 5 .ANG.
[0017] In the organic layer, the organic compounds contained
therein can be present in a state of radical cations in the absence
of electric field, and therefore they can act as an internal
charge. Namely, no specific requirement of the layer thickness is
given to the organic layer, and therefore a layer thickness of the
organic layer can be increased without causing an undesirable
increase of the voltage of the device. Therefore, in the EL device,
the organic layer can be also utilized as a means for remarkably
diminishing the possibility of short-circuiting, if a distance
between the opposed electrodes of the device has increased more
than conventional EL devices. Accordingly, it becomes possible to
increase a total thickness of the organic layer(s) between the
electrodes to 2,000 .ANG. or more.
[0018] In the doped organic layer, the concentration of the dopant
compound is not restricted to a specific range; however, it is
generally preferred that a molar ratio of the organic compound or
molecule to the dopant compound or molecule (i.e., organic molecule
: dopant molecule) is in the range of about 1:0.1 to about 1:10.
The molar ratio of the dopant molecule of less than 0.1 will result
in only a poor doping effect, because a concentration of the
molecules oxidized with the dopant (hereinafter "oxidized
molecules") is an excessively low level. Similarly, if the molar
ratio of the dopant molecule is above 10 times, only a poor and
reduced doping effect will be obtained, because the dopant
concentration is remarkably increased beyond that of the organic
molecule, thereby causing an excessive reduction of the
concentration of the oxidized molecules in the organic layer.
[0019] The present disclosure relates to subject matter contained
in Japanese Patent Application No.10-49771 (filed on Mar. 2, 1998)
which is expressly incorporated herein by reference in its
entirety.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The present invention will be more clearly understood from
the description as set forth below with reference to the
accompanying drawings, wherein:
[0021] FIG. 1 is a cross-sectional view illustrating a lamination
structure of the organic EL device according to an embodiment of
the present invention;
[0022] FIG. 2 is a graph showing the relationship between the bias
voltage and the luminance for the organic EL device according the
present invention, and a comparative organic EL device;
[0023] FIG. 3 is a graph showing the relationship between the bias
voltage and the current density for the organic EL device according
the present invention, and a comparative organic EL device;
[0024] FIG. 4 is a graph showing the relationship between the bias
voltage and the luminance for the organic EL device according the
present invention, and a comparative organic EL device; and
[0025] FIG. 5 is a graph showing the relationship between the bias
voltage and the current density for the organic EL device according
the present invention, and a comparative organic EL device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] The present invention will be further described with
reference to the preferred embodiments thereof.
[0027] FIG. 1 is a simplified cross-sectional view illustrating the
organic EL device according to an embodiment of the present
invention. In the illustrated EL device, a glass substrate
(transparent substrate) 1 includes (laminated in the following
order on a surface thereof): a transparent electrode 2 constituting
an anode electrode, a hole injection layer (organic layer) 3 doped
with an electron-accepting compound, a hole transportation layer 4
having hole-transporting properties, a luminescent or light
emission layer 5, and a back electrode 6 constituting a cathode
electrode. Among the above elements (layers) of the device, the
glass substrate (transparent substrate) 1, the transparent
electrode 2, the hole transportation layer 4, the light emission
layer 5 and the back electrode 6 are the well-known elements.
However, the hole injection layer 3 includes specific features of
the present invention.
[0028] In addition to the illustrated lamination structure of the
layers, the organic EL device of the present invention may include
other lamination structures, for example: an anode, a hole
injection layer, a hole transportation layer, a light emission
layer, an electron transportation layer, and a cathode; an anode, a
hole injection layer, a light emission layer, an electron injection
layer, and a cathode; an anode, a hole injection layer, a hole
transportation layer, a light emission layer, an electron
transportation layer, an electron injection layer, and a cathode.
The organic EL device may have any desired lamination structure, as
long as a hole injection layer 3 doped with the electron-accepting
compound is positioned in an interfacial region with the anode
electrode 2.
[0029] In the production of the organic EL device, the organic
compounds which can be used in the formation of the light emission
layer and the electron transportation layer are not restricted to
specific compounds. Typical examples of suitable organic compounds
include polycyclic compounds such as p-terphenyl and quaterphenyl,
as well as derivatives thereof; condensed polycyclic hydrocarbon
compounds such as naphthalene, tetracene, pyrene, coronene,
chrysene, anthracene, diphenylanthracene, naphthacene and
phenanthrene, as well as derivatives thereof; condensed
heterocyclic compounds such as phenanthroline, bathophenanthroline,
phenanthridine, acridine, quinoline, quinoxaline and phenazine, as
well as derivatives thereof; and fluoresceine, perylene,
phthaloperylene, naphthaloperylene, perynone, phthaloperynone,
naphthaloperynone, diphenylbutadiene, tetraphenylbutadiene,
oxadiazole, aldazine, bisbenzoxazoline, bisstyryl, pyrazine,
cyclopentadiene, oxine, aminoquinoline, imine, diphenylethylene,
vinylanthracene, diaminocarbazole, pyrane, thiopyrane, polymethine,
merocyanine, quinacridone and rubrene, as well as derivatives
thereof.
[0030] In addition to these organic compounds, metal-chelated
complex compounds disclosed in Japanese Unexamined Patent
Publication (Kokai) Nos. 63-295695, 8-22557, 8-81472, 5-9470 and
5-17764 can be suitably used as the organic compounds in the light
emission layer and the electron transportation layer. Among these
metal-chelated complex compounds, the metal-chelated oxanoide
compounds, for example, metal complexes which contain (as a ligand
thereof) at least one member selected from 8-quinolinolato as well
as derivatives thereof, such as tris (8-quinolinolato) aluminum,
bis (8-quinolinolato) magnesium, bis[benzo(f)-8-quinolinolato]
zinc, bis (2-methyl-8-quinolinolato) aluminum, tri
(8-quinolinolato) indium, tris (5-methyl-8-quinolinolato) aluminum,
8-quinolinolatolithium, tris (5-chloro-8-quinolinolato) gallium and
bis (5-chloro-8-quinolinolato) calcium can be particularly suitably
used.
[0031] It is also preferred that oxadiazoles disclosed in Japanese
Patent Kokai Nos. 5-202011, 7-179394, 7-278124 and 7-228579,
triazines disclosed in Japanese Patent Kokai No. 7-157473, stilbene
derivatives and distyrylarylene derivatives disclosed in Japanese
Patent Kokai No. 6-203963, styryl derivatives disclosed in Japanese
Patent Kokai Nos. 6-132080 and 6-88072, and diolefin derivatives
disclosed in Japanese Patent Kokai Nos.6-100857 and 6-207170 are
used as the organic compounds in the formation of the light
emission layer and the electron transportation layer.
[0032] Further, a fluorescent whitening agent such as benzoxazoles,
benzothiazoles and benzoimidazoles may be used as the organic
compounds, and it includes, for example, those disclosed in
Japanese Patent Kokai No. 59-194393. Typical examples of the
fluorescent whitening agent include the fluorescent whitening
agents classified under the group of benzoxazoles such as 2,5-bis
(5,7-di-t-pentyl-2-benzoxazolyl)-1,3,4-thiad- iazole, 4,4'-bis
(5,7-t-pentyl-2-benzoxazolyl)stilbene,
4,4'-bis[5,7-di(2-methyl-2-butyl)-2-benzoxazolyl]stilbene,
2,5-bis(5,7-di-t-pentyl-2-benzoxazolyl)thiophene,
2,5-bis[5-(.alpha.,
.alpha.-dimethylbenzyl)-2-benzoxazolyl]thiophene,
2,5-bis[5,7-di(2-methyl-
-2-butyl)-2-benzoxazolyl]-3,4-diphenylthiophene,
2,5-bis(5-methyl-2-benzox- azolyl)thiophene,
4,4'-bis(2-benzoxazolyl)biphenyl, 5-methyl-2-
{2-[4-(5-methyl-2-benzoxazolyl)phenyl]vinyl} benzoxazole and
2-[2-(4-chlorophenyl)vinyl]naphtho(1,2-d)oxazole; under the group
of benzothiazoles such as
2,2'-(p-phenylenedipynylene)-bisbenzothiazole; and under the group
of benzoimidazoles such as 2-{2-[4-(2-benzoimidazolyl)
phenyl]vinyl} benzoimidazole and
2-[2-(4-carboxyphenyl)vinyl]benzoimidazo- le.
[0033] As the distyrylbenzene compound, the compounds disclosed in
European Patent No. 373,582 may be used. Typical examples of the
distyrylbenzene compound include 1,4-bis(2-methylstyryl)benzene,
1,4-bis(3-methylstyryl)benzene, 1,4-bis(4-methylstyryl)benzene,
distyrylbenzene, 1,4-bis(2-ethylstyryl)benzene,
1,4-bis(3-ethylstyryl)ben- zene,
1,4-bis(2-methylstyryl)-2-methylbenzene and
1,4-bis(2-methylstyryl)-- 2-ethylbenzene.
[0034] Furthermore, distyrylpyrazine derivatives disclosed in
Japanese Patent Kokai No. 2-252793 may also be used as the organic
compounds in the formation of the light emission layer and the
electron transportation layer. Typical examples of the
distyrylpyrazine derivatives include
2,5-bis(4-methylstyryl)pyrazine, 2,5-bis(4-ethylstyryl)pyrazine,
2,5-bis[2-(1-naphthyl)vinyl]pyrazine,
2,5-bis(4-methoxystyryl)pyrazine,
2,5-bis[2-(4-biphenyl)vinyl]pyrazine and
2,5-bis[2-(1-pyrenyl)vinyl]pyraz- ine.
[0035] In addition, dimethylidine derivatives disclosed in European
Patent No. 388,768 and Japanese Patent Kokai No. 3-231970 may also
be used as the material of the light emission layer and the
electron transportation layer. Typical examples of the
dimethylidine derivatives include 1,4-phenylenedimethylidine,
4,4'-phenylenedimethylidine, 2,5-xylylenedimethylidine,
2,6-naphthylenedimethylidine, 1,4-biphenylenedimethylidine,
1,4-p-terephenylenedimethylidine, 9,10-anthracenediyldimethylidine,
4,4'-(2,2-di-t-butylphenylvinyl)bipheny- l and
4,4'-(2,2-diphenylvinyl)biphenyl as well as derivatives thereof;
silanamine derivatives disclosed in Japanese Patent Kokai Nos.
6-49079 and 6-293778; polyfunctional styryl compounds disclosed in
Japanese Patent Kokai Nos. 6-279322 and 6-279323; oxadiazole
derivatives disclosed in Japanese Patent Kokai Nos. 6-107648 and
6-92947; anthracene compounds disclosed in Japanese Patent Kokai
No. 6-206865; oxynate derivatives disclosed in Japanese Patent
Kokai No. 6-145146; tetraphenylbutadiene compounds disclosed in
Japanese Patent Kokai No. 4-96990; and organic trifunctional
compounds disclosed in Japanese Patent Kokai No. 3-296595; as well
as coumarin derivatives disclosed in Japanese Patent Kokai No.
2-191694; perylene derivatives disclosed in Japanese Patent Kokai
No. 2-196885; naphthalene derivatives disclosed in Japanese Patent
Kokai No. 2-255789; phthaloperynone derivatives disclosed in
Japanese Patent Kokai Nos. 2-289676 and 2-88689; and styrylamine
derivatives disclosed in Japanese Patent Kokai No. 2-250292.
[0036] Moreover, any other well-known organic compounds which are
conventional in the production of the organic EL devices may be
suitably used.
[0037] The arylamine compounds used in the formation of the hole
injection layer (doping layer), the hole transportation layer and
the hole-transporting light emission layer, although they are not
restricted to the following, preferably include those disclosed in
Japanese Patent Kokai Nos. 6-25659, 6-203963, 6-215874, 7-145116,
7-224012, 7-157473, 8-48656, 7-126226, 7-188130, 8-40995, 8-40996,
8-40997, 7-126225, 7-101911 and 7-97355. Typical examples of
suitable arylamine compounds include, for example,
N,N,N',N'-tetraphenyl-4,4'-diaminophenyl,
N,N'-diphenyl-N,N'-di(3-methylphenyl)-4,4'-diaminobiphenyl,
2,2-bis(4-di p-tolylaminophenyl)propane,
N,N,N',N'-tetra-p-tolyl-4,4'-diaminobiphenyl,
bis(4-di-p-tolylaminophenyl)phenylmethane,
N,N'-diphenyl-N,N'-di(4-methox- yphenyl)-4,4'-diaminobiphenyl,
N,N,N',N'-tetraphenyl-4,4'-diaminodiphenyle- ther,
4,4'-bis(diphenylamino)quadriphenyl,
4-N,N-diphenylamino-(2-diphenyl- vinyl) benzene,
3-methoxy-4.sup.1-N,N-diphenylaminostilbenzene, N-phenylcarbazole,
1,1-bis(4-di-p-triaminophenyl)cyclohexane,
1,1-bis(4-di-p-triaminophenyl)-4-phenylcyclohexane,
bis(4-dimethylamino-2-methylphenyl) phenylmethane,
N,N,N-tri(p-tolyl)amine,
4-(di-p-tolylamino)-4'-[4-(di-p-tolylamino)styry- l]stilbene,
N,N,N',N'-tetrapheny 1-4,4'-diaminobiphenyl N-phenylcarbazole,
4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl,
4,4"-bis[N-(1-naphthyl)-N- -phenylamino] p-terphenyl,
4,4'-bis[N-(2-naphthyl)-N-phenylamino]biphenyl,
4,4'-bis[N-(3-acenaphthenyl)-N-phenylamino]biphenyl,
1,5-bis[N-(1-naphthyl)-N-phenylamino]naphthalene,
4,4'-bis[N-(9-anthryl)-- N-phenylamino]biphenyl,
4,4"-bis[N-(1-anthryl)-N-phenylamino] p-terphenyl,
4,4'-bis[N-(2-phenanthryl)-N-phenylamino]biphenyl,
4,4'-bis[N-(8-fluoranthenyl)-N-phenylamino]biphenyl,
4,4'-bis[N-(2-pyrenyl)-N-phenylamino]biphenyl,
4,4'-bis[N-(2-perylenyl)-N- -phenylamino]biphenyl,
4,4'-bis[N-(1-coronenyl)-N-phenylamino]biphenyl,
2,6-bis(di-p-tolylamino)naphthalene, 2,6-bis[di-(1-naphthyl)
amino]naphthalene,
2,6-bis[N-(1-naphthyl)-N-(2-naphthyl)amino]naphthalene- ,
4,4"-bis[N,N-di(2-naphthyl)amino]terphenyl, 4,4'-bis
{N-phenyl-N-[4-(1-naphthyl)phenyl]amino} biphenyl, 4,4'-bis
[N-phenyl-N-(2-pyrenyl)amino]biphenyl, 2,6-bis[N,N-di(2-naphthyl)
amino]fluorene, 4,4"-bis(N,N-di-p-tolylamino)terphenyl and
bis(N-1-naphthyl)(N-2-naphthyl)amine. Also, any other well-known
arylamine compounds which are conventional in the production of the
organic EL devices may be suitably used.
[0038] The cathode electrode used in the EL device of the present
invention is not particularly restricted, as long as it is formed
from a metal which can be stably used in air. Aluminum, which is
generally and widely used as the wiring electrode, is preferably
used as the anode material of the present device.
[0039] As can be appreciated from the above detailed descriptions
and the appended working examples, since an organic compound layer
doped with an electron-accepting compound is disposed in an
interfacial region with the anode electrode, it becomes possible to
provide an organic EL device capable of operating at a low voltage.
Accordingly, the EL device of the present invention can show a high
utility in practical use, and ensures their effective utilization
as display devices, light sources and others.
EXAMPLES
[0040] The present invention will be further described with
reference to the following examples; however, it should be noted
that the present invention is not restricted by these examples.
[0041] In the following examples, a vapor deposition of the organic
compound and that of the metal each was carried out by the vapor
deposition apparatus "VPC-400" commercially available from Shinku
Kikou Co.,Ltd. The thickness of the deposited layers was determined
by the profilometer "DekTak3ST" commercially available from Sloan
Co.,Ltd.
[0042] Further, the characteristics of the organic EL device were
determined by the source meter "2400" commercially available from
Keithley & Co. and the luminance meter "BM-8" commercially
available from Topcon Co.,Ltd. A DC voltage was applied in steps at
an increasing rate of one volt per 2 seconds to the EL device
having an ITO anode and an aluminum (Al) cathode, and the luminance
and the electric current were determined after one second had
passed from the completion of each increase of the voltage. The EL
spectrum was determined by the optical multichannel analyzer
"PMA-10", commercially available from Hamamatsu Photonics Co.,
Ltd., driven at a constant electric current.
Example 1
[0043] The organic EL device having the lamination structure
illustrated in FIG. 1 was produced according to the present
invention.
[0044] A glass substrate 1 was coated with an ITO
(indium-tin-oxide) layer having a sheet resistance of 25
.OMEGA./.quadrature., commercially available as a sputtered product
from Sanyo Shinku Co.,Ltd., to form a transparent anode electrode
2. Then, alpha (.alpha.)-NPD having a hole transporting property,
represented by the following formula (1), and ferric chloride
(FeCl.sub.3) were co-depositioned at a molar ratio of 1:2 onto the
ITO-coated glass substrate 1 under the pressure of 10.sup.-6 Torr
and an evaporation rate of 3 .ANG./sec to form a hole injection
layer 3 having a thickness of 100 .ANG.. 1
[0045] Under the same vacuum evaporation conditions, .alpha.-NPD
was deposited onto the hole injection layer 3 to form a hole
transportation layer 4 having a thickness of 500 .ANG..
[0046] Next, an aluminum complex of tris (8-quinolinolato)
(hereinafter, referred to as "Alq") represented by the following
formula (2) was deposited onto the hole transportation layer 4
under the same vacuum evaporation conditions as in the
above-described deposition of the hole transportation layer 4 to
form a light emission layer 5 having a thickness of 700 .ANG..
2
[0047] After the formation of the light emission layer 5, aluminum
(Al) was deposited at the evaporation rate of 15 .ANG./sec onto the
light emission layer 5 to form a back electrode 6, acting as a
cathode, having a thickness of 1,000 .ANG.. The organic EL device
having a square luminescent area of 0.5 cm (length) by 0.5 cm
(width) was thus obtained.
[0048] In the thus produced organic EL device, a DC voltage was
applied between the anode electrode (ITO) 2 and the cathode
electrode (Al) 6, and a luminance of the green luminescence from
the light emission layer (Alq) 5 was determined to obtain the
results plotted with circles in FIG. 2 showing the relationship
between the bias voltage and the luminance of the EL device, and in
FIG. 3 showing the relationship between the bias voltage and the
current density of the EL device. These results indicate that a
luminance of at most 4,700 cd/M.sup.2 could be obtained at the
applied bias voltage of 12 volts. The current density was
determined to be 650 mA/cm.sup.2 at the same bias voltage. Further,
it was determined that the light emission was started at the
applied bias voltage of 3 volts.
Comparative Example 1
[0049] The procedure of Example 1 was repeated with the proviso
that, for the purpose of comparison, a doped hole injection layer
was omitted from the organic EL device. Namely, .alpha.-NPD was
first deposited onto the ITO-coated glass substrate to form a hole
transportation layer having a thickness of 500 .ANG., and then Alq
was deposited under the same vacuum evaporation conditions as in
the deposition of the hole transportation layer to form a light
emission layer having a thickness of 700 .ANG.. Thereafter,
aluminum (Al) was deposited at a thickness of 1,000 .ANG. over the
light emission layer to form a cathode electrode.
[0050] In the produced organic EL device, the luminance was
determined as in Example 1 to obtain the results plotted with
triangular marks in each of FIG. 2 and FIG. 3. These results
indicate that a luminance of at most 2,400 cd/m.sup.2 could be
obtained at the applied bias voltage of 16 volts. The current
density was determined to be 110 mA/cm.sup.2 at the same bias
voltage, and the voltage required to start the light emission was 7
volts. It is appreciated from these results that the presence of
the hole injection layer 3 which is essential to the organic EL
device of the present invention is effective to reduce the driving
voltage of the EL device.
Example 2
[0051] The procedure of Example 1 was repeated with the proviso
that in this example, a thickness of the hole injection layer was
increased to 4,000 .ANG..
[0052] In the produced organic EL device, the luminance was
determined as in Example 1 to obtain the results plotted with
circles in FIG. 4 showing the relationship between the bias voltage
and the luminance of the EL device, and in FIG. 5 showing the
relationship between the bias voltage and the current density of
the EL device. These results indicate that a luminance of at most
4,500cd/m.sup.2 could be obtained at the applied bias voltage of 12
volts which is the same as that of Example 1. The current density
was determined to be 610 mA/cm.sup.2 at the same bias voltage.
Further, it was determined that the light emission was started at
the applied bias voltage of 3 volts. Furthermore, the result
obtained in the Comparative Example 1 is plotted with triangular
marks in FIG. 4 and FIG. 5. It was found from these results, the
results of Example 1 and the results of Comparative Example 1 that
an increase of the thickness of the doped hole injection layer does
not cause an increase of the driving voltage of the EL device.
[0053] Although the invention has been described with reference to
particular means, materials and embodiment, it is to be understood
that the invention is not limited to the particulars disclosed and
extends to all equivalents within the scope of the claims.
* * * * *