U.S. patent application number 09/162569 was filed with the patent office on 2002-04-11 for organic electroluminescent element.
Invention is credited to FURUKAWA, KEIICHI, TERASAKA, YOSHIHISA, UEDA, HIDEAKI.
Application Number | 20020041975 09/162569 |
Document ID | / |
Family ID | 17439165 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020041975 |
Kind Code |
A1 |
UEDA, HIDEAKI ; et
al. |
April 11, 2002 |
ORGANIC ELECTROLUMINESCENT ELEMENT
Abstract
The object of the invention is to provide an organic
electroluminescent element which reduces the luminescence starting
voltage, increases the luminescence brightness, and has excellent
stability with repeated use. The invention achieves these objects
by providing an organic electroluminescent element comprising at
least a positive electrode, luminescing layer, and negative
electrode, wherein said negative electrode is a compound layer of
magnesium and a metal having a higher work function than magnesium,
and the exterior surface side of said compound layer has a higher
percentage of metal having a high work function.
Inventors: |
UEDA, HIDEAKI;
(KISHIWADA-SHI, JP) ; FURUKAWA, KEIICHI;
(SUITA-SHI, JP) ; TERASAKA, YOSHIHISA; (SUITA-SHI,
JP) |
Correspondence
Address: |
BARRY E BRETSCHNEIDER
MORRISON AND FOERSTER
2000 PENNSYLVANIA AVENUE N W
WASHINGTON
DC
200061888
|
Family ID: |
17439165 |
Appl. No.: |
09/162569 |
Filed: |
September 29, 1998 |
Current U.S.
Class: |
428/690 ;
313/503; 313/504; 313/505; 428/212; 428/457; 428/917 |
Current CPC
Class: |
H01L 51/5221 20130101;
Y10T 428/31678 20150401; Y10T 428/24942 20150115; H01L 2251/5346
20130101 |
Class at
Publication: |
428/690 ;
428/917; 428/212; 428/457; 313/503; 313/504; 313/505 |
International
Class: |
H05B 033/26 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 1997 |
JP |
09-267036 |
Claims
What is claimed is:
1. An organic electroluminescent element comprising: a positive
electrode; a luminescing layer; and a negative electrode comprising
a compound layer of magnesium and a metal having a work function
higher than magnesium, said compound layer having a higher
percentage of metal having a high work function on the exterior
side of the element.
2. The organic electroluminescent element claimed in claim 1,
wherein the metal having a high work function is at least one metal
selected from among groups comprising aluminum, indium, silver,
gold, nickel, and tin.
3. The organic electroluminescent element claimed in claim 2,
wherein the metal having a high work function is silver or
indium.
4. The organic electroluminescent element claimed in claim 1,
wherein the thickness of said negative electrode is 5.about.500
nm.
5. The organic electroluminescent element claimed in claim 4,
wherein the thickness of said negative electrode is 10.about.300
nm.
6. The organic electroluminescent element claimed in claim 1,
wherein said negative electrode is formed by resistance heating,
spattering, EB vacuum deposition, ion plating, or ionization
deposition.
7. The organic electroluminescent element claimed in claim 1,
wherein said negative electrode comprises a plurality of layers
including a layer having a high percentage of metal of high work
function, and a layer having a low percentage of metal having a
high work function.
8. The organic electroluminescent element claimed in claim 7,
wherein said negative electrode is formed of a plurality of layers,
such that percentage of metal having a high work function is
highest in the layer on the exterior side of the element.
9. The plurality of layers of claim 7 include both magnesium and a
metal having a high work function.
10. The organic electroluminescent element claimed in claim 1,
wherein the percentage of metal having a high work function changes
sequentially in the depth direction, and the highest percentage of
metal having a high work function is on the exterior surface side
of the negative electrode.
11. The organic electroluminescent element claimed in claim 1,
wherein the percentage of metal having a high work function on the
side of the negative electrode nearest the organic luminescing
layer is less than {fraction (1/10)} the total content of
magnesium.
12. The organic electroluminescent element claimed in claim 10,
wherein the percentage of metal having a high work function on the
side of the negative electrode nearest the organic luminescing
layer is less than {fraction (1/20)} the total content of
magnesium.
13. The organic electroluminescent element claimed in claim 1,
wherein the percentage of metal having a high work function on the
side of the negative electrode nearest the organic luminescing
layer is two times or more the total content of magnesium.
14. The organic electroluminescent element claimed in claim 13,
wherein the percentage of metal having a high work function on the
interior surface side of said negative electrode is 5 times or more
the content of magnesium.
15. The organic electroluminescent element claimed in claim 14,
wherein the percentage of metal having a high work function on the
interior surface side of said negative electrode is 100 times or
more the content of magnesium.
16. The organic electroluminescent element claimed in claim 15,
wherein the percentage of metal having a high work function on the
interior surface side of said negative electrode is actually
100%.
17. The organic electroluminescent element claimed in claim 1,
further comprising an electron injecting layer interposed between
said negative electrode and said organic luminescing layer.
18. The organic electroluminescent element claimed in claim 11
further comprising an electron transporting layer interposed
between said electron injecting layer and said organic luminescing
layer.
19. The organic electroluminescing layer claimed in claim 1 further
comprising an electron injecting/transporting layer interposed
between said negative electrode and said organic luminescing
layer.
20. The organic electroluminescing element claimed in claim 1
further comprising a sealing layer on the exterior surface of said
negative electrode.
Description
RELATED APPLICATIONS
[0001] This application is based on Application No. HEI 9-267036
filed in Japan, the content of which is hereby incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an organic
electroluminescent element.
[0004] 2. Description of the Related Art
[0005] Organic electroluminescent elements are elements constructed
using organic compounds as luminescent materials which emit light
in response to electric signals.
[0006] Organic electroluminescent elements have a basic structure
of an organic luminescent layer interposed between a pair of
opposing electrodes.
[0007] Electroluminescence is a phenomenon wherein electrons are
injected from one electrode and holes are injected from another
electrode so as to excite an illuminant within the luminescent
layer to a higher energy level, and excess energy is discharged as
light when the illuminant returns to its original base state.
[0008] In addition to the aforesaid basic structure, a hole
injecting layer is added to the electrode which injects holes and
an electron transporting layer is added to the electrode injecting
electrons so as to improve luminance efficiency.
[0009] An example of an organic electoluminescent element is
disclosed in U.S. Pat. No. 3,530,325, which describes an
electroluminescent element using a monocrystal anthracene as a
luminant.
[0010] U.S. Pat. No. 4,539,507 discloses an organic
electroluminescent element combining a hole injecting layer and an
organic electroluminescent layer.
[0011] U.S. Pat. No. 4,720,432 discloses an organic
electroluminescent element combining an organic hole injecting
layer and an organic electron injecting layer.
[0012] These electroluminescent elements with multi-layer
structures comprise multiple layers of an organic fluorescent body,
charge-transporting organic material (charge-transporting member),
and electrodes, wherein luminescence is accomplished by holes and
electrons injected by said respective electrodes moving through
said charge-transporting member and again coupling. Examples of
organic fluorescent bodies include organic colorants which
fluoresce such as 8-quinolinol aluminum complex. Examples of
charge-transporting materials include
N,N'-di(m-tolyl)N,N'-diphenylbenzidene, diamino compounds such as
1,1-bis[N,N-di(p-tolyl)aminophenyl]cyclohexane and the like, and
4-(N,N-diphenyl)aminobenzaldehyde-N,N-diphenylhydrazone compounds
and the like. Porphyrin compounds such as copper phthalocyanine
have also been proposed.
[0013] Although organic electroluminescent elements have high
luminance characteristics, they also are unstable when luminescing
and have poor stability during storage so as to be impractical for
use. One disadvantage of the aforesaid elements regarding storage
stability and stability during luminescence pertains to the
stability of the charge-transporting material. The layers of the
electroluminescent element formed of organic material are quite
thin at 100 to several hundred nanometers, and an extremely high
voltage is applied to the layer per unit thickness. Heat is
generated by luminescence and current flow, such that electrical,
thermal, and chemical stability is required by the
charge-transporting material.
[0014] Japanese Laid-Open Patent Nos. HEI 2-15595, 3-37994,
4-132191, and 5-121172 disclose elements which replace the
conventionally used aluminum with a negative electrode to reduce
the luminescence starting voltage of the organic electroluminescent
element.
[0015] Further disadvantages arise, however when metals other than
aluminum are used, inasmuch as the layer formation conditions
become more difficult, oxidation may occur during layer formation,
black spots become prevalent when luminescing.
OBJECTS AND SUMMARY
[0016] In light of the aforesaid information, an object of the
present invention is to provide an organic electroluminescent
element which possesses increased luminescent intensity and
exhibits stable characteristics even with repeated use.
[0017] The present invention relates to an organic
electroluminescent element having at least a positive electrode,
luminescent layer, and negative electrode, wherein said negative
electrode is a mixed layer of a plurality of metals having
different work functions, and a higher percentage of metals having
high work function is greater on the exterior side of said mixed
layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These and other objects and features of the present
invention will become apparent from the following description of
the preferred embodiments thereof taken in conjunction with the
accompanying drawings, in which:
[0019] FIG. 1 is a brief section view showing an example of the
structure of an organic electroluminescent element of the present
invention;
[0020] FIG. 2 is a brief section view showing an example of the
structure of an organic electroluminescent element of the present
invention;
[0021] FIG. 3 is a brief section view showing an example of the
structure of an organic electroluminescent element of the present
invention; and
[0022] FIG. 4 is a brief section view showing an example of the
structure of an organic electroluminescent element of the present
invention.
[0023] In the following description, like parts are designated by
like reference numbers throughout the several drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] The present invention relates to an organic
electroluminescent element having at least a positive electrode,
luminescent layer, and negative electrode, wherein said negative
electrode is a mixed layer of magnesium and a metal having a work
function higher than magnesium, and wherein the percentage of metal
having a high work function is greater on the exterior side of said
negative electrode.
[0025] The organic electroluminescent element of the present
invention is provided with at least an organic luminescing layer
interposed between a positive electrode and negative electrode.
[0026] The present invention is basically characterized by a
negative electrode of an organic electroluminescent element which
has a mixed layer of magnesium and metal having a work function
greater than magnesium, said mixed layer having different mix
ratios in the depth direction and a greater percentage of metal
with a high work function disposed on the exterior side, i.e., the
side farthest from the luminescing layer.
[0027] It is believed possible to improve electron injectability by
using a mixed layer of magnesium and a metal having a work function
greater than magnesium on the negative electrode, and providing a
different mixture ratio in the depth direction so as to have an
increasing the percentage of metal having a high work function on
the exterior side of the negative electrode (i.e., on the opposite
side relative to the luminescing layer) produces an extremely
smooth electron flow in conjunction with a strong electric field,
such that the luminescence starting voltage required to produce
luminescence of the organic electroluminescent element of the
present invention is adequately reduced and the work function of
the negative electrode surface is increased so as to allow stable
long-term luminescence by preventing the generation of black spots
as well as deterioration caused by oxidation of the negative
electrode surface. It is further believed that the negative
electrode has superior layer formability due to the excellent layer
forming characteristics of magnesium as an alloy, and that the
formed element possesses luminescence characteristics of excellent
stability due to the relatively excellent stability of the alloy
compared to other metals. The present invention is based on the
aforesaid knowledge.
[0028] A combination of magnesium having a work function less than
4 eV and metal having a higher work function is used as the metal
forming the negative electrode of the present invention.
[0029] It is desirable to use a combination of magnesium, and least
one metal from among aluminum, indium, silver, gold, nickel, and
tin.
[0030] The negative electrode can be constructed of a plurality of
layers such that the layers nearer the exterior surface have a
larger percentage of metals with a work function greater than the
interior layers, so as to increase the percentage of metal having a
large work function on the exterior surface within the layers of
the negative electrode. For example, when the negative electrode is
formed of magnesium and silver, the structural ratio of the two
layers can be inclined such that the layer formed on the outermost
exterior surface has, for example, an Mg:Ag ratio of 1:2 and the
interior layer has an Mg:Ag ratio of, for example, 10:1. Similarly,
when a negative electrode is formed by a structure of three or more
layers, a plurality of layers may be superimposed so that the
layers on the exterior side has a higher percentage of metal having
a large work function. The structural ratio may also be
consecutively changed by increasing the vacuum deposition rate of
the metal having a high work function, or by gradually decreasing
the vacuum deposition rate f the metal having a smaller work
function, when depositing the layers on the exterior surface when
forming the negative electrode. The concentration of specific metal
types also may be zero on the exterior side or interior side.
[0031] The ratio of magnesium and metal having a higher work
function than magnesium may be set at an optional ratio insofar as
the ratio of the metal having the higher work function is greater
on the exterior side of the negative electrode element. That is,
the concentration of magnesium actually may be zero on the exterior
side of the negative electrode element. Conversely, the
concentration of the metal having a larger work function actually
may be zero on the interior side of the negative electrode
element.
[0032] The percentage of metal having a higher work function should
selectively be at least twice the concentration of magnesium on the
exterior side of the negative electrode element, and desirably 5
times greater or more, and more desirably 10 times greater or more.
The percentage of metal having higher work function on the interior
side of the negative electrode element is desirably at most less
than {fraction (1/100)}, and preferably less than {fraction (1/20)}
of the magnesium concentration.
[0033] The negative electrode may be formed by a variety of
well-known vacuum deposition methods such as normal resistance
heating, spattering, EB vacuum deposition, ion plating, ionization
vacuum deposition and the like of a mixture of magnesium and a
metal having a high work function.
[0034] The thickness of the negative electrode is desirably
5.about.500 nm, and more desirably 10.about.300 nm. In the case of
multiple layers, the total layer thickness is set within the
aforesaid range. Since the resistance of the layer itself increases
the thicker the layer, the applied voltage must be somewhat higher,
whereas a uniform layer is difficult to form as the layer is made
thinner, such that defects are likely to form which adversely
affect luminous efficiency and shorten the service life of the
organic electroluminescent element.
[0035] Materials having conductivity characteristics with a work
function greater than 4 eV are useful as the positive electrode of
the organic electroluminescent element including, for example,
carbon, aluminum, vanadium, iron, cobalt, nickel, copper, zinc,
tungsten, silver, tin, gold and the like as well as alloys thereof,
as well as conductive metal compounds such as tin oxide, indium
oxide, antimony oxide, zinc oxide, zirconium oxide and the
like.
[0036] At least the positive electrode or negative electrode in the
organic electroluminescent element must be transparent for the
luminescence to be visible. In this instance, it is desirable that
the positive electrode is transparent inasmuch as a negative
electrode is subject to rapid loss of transparency.
[0037] When forming a transparent electrode, a conductive material
such as the aforesaid metals is deposited on a substrate via a
means such as vacuum deposition, spattering or the like, or means
for dispersing and applying a resin containing said conductive
material, or sol-gel method, so as to maintain desired transparency
and conductivity.
[0038] The material used for the transparent substrate is not
specifically limited insofar as said substrate is not adversely
affected by heat during vacuum deposition or during manufacture of
the organic electroluminescent element, it is possible to use a
glass substrate, or transparent resin such as, for example,
polyethylene, polypropylene, polyethersulfone, polyether ether
ketone and the like. Well-known commercial products such as ITO,
NESA and the like used to form a transparent electrode on a glass
substrate may also be used.
[0039] The organic electroluminescent element of the present
embodiment comprises, for example, the aforesaid positive electrode
1, negative electrode 4, and at least a hole injecting/transporting
layer 2, and organic luminescing layer 3 interposed between said
electrodes.
[0040] The hole injecting/transporting layer 2 formed on positive
electrode 1 is desirably formed by vacuum deposition of a chemical
compound, said chemical compound being dissolved in solvent or
fluid in which a suitable resin is dissolved, and applied by dip
coating or spin coating.
[0041] When forming the hole injecting/transporting layer by vacuum
deposition, the thickness of said layer is normally 1.about.200 nm,
and desirably 5.about.100 nm; when said layer is formed by an
application method, the thickness of said layer is about
5.about.500 nm.
[0042] A thicker layer requires a higher application voltage to
achieve luminescence, thereby adversely affecting luminous
efficiency and causing deterioration of the organic
electroluminescent element. Although luminous efficiency improves
with a thinner layer, the layer readily breaks down and shortens
the service life of the organic electroluminescent element.
[0043] Well-known material may be used as the hole
injecting/transporting material in the hole injecting/transporting
layer, for example,
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine,
N,N'-diphenyl-N,N'-bis(4-methylphenyl)-1,1'-diphenyl-4,4'-diamine,
N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-diphenyl-4,4'-diamine,
N,N'-diphenylN,N'-bis(2-naphthyl)-1,1'-diphenyl-4,4'-diamine,
N,N'-tetra(4-methylphenyl)-1,1'-diphenyl-4,4'-diamine,
N,N'-tetra(methylphenyl)-1,1'-bis(3-methylphenyl)-4,4'-diamine,
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-bis(3-methylphenyl)-4,4'-diam-
ine, N,N'-bis(N-carbazolyl)-1,1'-diphenyl-4,4'-diamine,
4,4',4"-tris(N-carbazolyl)triphenylamine,
N,N',N"-triphenyl-N,N',N"-tris(-
3-methylphenyl)-1,3,5-tri(4-aminophenyl)benzene,
4,4',4"-tris([N,N',N"-tri-
phenyl-N,N',N"-tris(3-methylphenyl)]triphenylamine and the like.
These materials may be used in combinations of two or more.
[0044] Next, an organic luminescing layer 3 is formed over the hole
injecting/transporting layer 2. Well-known organic luminants may be
used for the organic luminescing layer 3, for example,
2,5-bis[5,7-di-t-pentyl- -2-benzooxazolyl]thiophene,
2,2'-(1,4-phenylenedivenylene)bisbenzothiazole- ,
2,2'-(4,4'-biphenylene)bisbisbenzothiazole,
5-methyl-2-{2-[4-(5-methyl-2-
-benzooxazolyl)phenyl]vinyl}benzooxazole,
2,5-bis(5-methyl-2-benzooxazolyl- )thiophene, anthracene,
naphthalene, phenanthrene, pyrene, chrysene, perylene, perynone,
1,4-diphenylbutadiene, tetraphenylbutadiene, cumarin, acridine,
stilbene, 2-(4-biphenyl)-6-phenylbenzooxazole, aluminum trisoxine,
magnesium trisoxine, bis(benzo-8-quinolinole)zinc,
bis(2-methyl-8-quinolinolaurate)aluminum oxide, indium trisoxine,
aluminum tris(5-methyloxine), lithium oxine, galliumtrisoxine,
calcium bis(5-chlorooxine),
polyzinc-bis(8-hydroxy-5-quinolinolyl)methane, dilithium, zinc
bisoxine, 1,2-phthaloperinone, 1,2-naphthaloperinone and the
like.
[0045] Typical fluorescent dyes may also be used, including, for
example, fluorescent cumarin dye, fluorescent perylene dye,
fluorescent pyran dye, fluorescent thiopyran dye, fluorescent dye,
fluorescent dye, fluorescent imidazole dye and the like. Among the
aforesaid, chelated oxynoid compounds are particularly
desirable.
[0046] Organic luminescing layer 3 may have a monolayer structure
of the aforesaid luminescent material, or may have a multi-layer
structure to regulate characteristics such as color of
luminescence, intensity of luminescence and the like. Furthermore,
luminescent material such as rubrene, pyrin and the like may be
doped, or the aforesaid materials may be used in mixtures of two or
more types.
[0047] When forming the organic luminescing layer via vacuum
deposition, the thickness of said layer is normally 1.about.200 nm,
and desirably 1.about.100 nm, whereas when said layer is formed by
an application method, the thickness of said layer may be
5.about.500 nm. A thicker layer requires a higher application
voltage to achieve luminescence, thereby adversely affecting
luminous efficiency and causing deterioration of the organic
electroluminescent element. Although luminous efficiency improves
with a thinner layer, the layer readily breaks down and shortens
the service life of the organic electroluminescent element.
[0048] Then, the aforesaid negative electrode 4 is formed over the
organic luminescing layer 3. The transparent electrodes negative
electrode 4 and positive electrode 1 are connected by a suitable
lead wire 11 of nickel-chrome wire, gold wire, copper wire,
platinum wire or the like, such that the organic electroluminescent
element luminesces by the application of a suitable voltage Vs to
both electrodes.
[0049] Another construction of the organic electroluminescent
element is shown in FIGS. 2-4. In FIG. 2, reference number 1 refers
to a positive electrode, over which are sequentially superimposed a
hole injecting/transporting layer 2, organic luminescing layer 3,
electron injecting/transporting layer 5, and negative electrode 4,
said negative electrode 4 being a mixed layer of two types of
alloys having different work functions with the mixture ratio
differing in the depth direction such that the percentage of metal
having a higher work function is greater on the exterior surface
side.
[0050] In FIG. 3, reference number 1 refers to a positive
electrode, over which is sequentially superimposed a hole injecting
layer 6, hole transporting layer 7, organic luminescing layer 3,
electron transporting layer 8, electron injecting layer 9, and
negative electrode 4, said negative electrode 4 being a mixed layer
of two types of alloys having different work functions with the
mixture ratio differing in the depth direction such that the
percentage of metal having a higher work function is greater on the
exterior surface side.
[0051] In FIG. 4, reference number 1 refers to a positive electrode
over which is sequentially superimposed a hole injecting layer 6,
hole transporting layer 7, organic luminescing layer 3, electron
injecting/transporting layer 5, and negative electrode 4, and
sealing layer 10, said negative electrode 4 being a mixed layer of
two types of alloys having different work functions with the
mixture ratio differing in the depth direction such that the
percentage of metal having a higher work function is greater on the
exterior surface side.
[0052] The electroluminescent element of the construction shown in
FIG. 2 differs from the construction of the electroluminescent
element of FIG. 1 in that it is provided with an electron
injecting/transporting layer 5 interposed between negative
electrode 4 and organic luminescing layer 3. The electron
injecting/transporting layer is provided to accelerate electron
injection and transport.
[0053] The electron injecting/transporting layer may be formed
using electron transporting material, for example, oxadiazole
derivative, thiadiazole derivative, chelated oxynoid compound,
benzothiazole complex, benzooxazole complex, and mixtures
thereof.
[0054] The electron injecting/transporting layer may be formed by
well-known conventional methods such as vacuum deposition and
application methods; when forming the layer by vacuum deposition
the layer thickness may be 1.about.500 nm, and when forming the
layer by application methods the layer thickness may be
5.about.1,000 nm.
[0055] The structure shown in FIG. 3 provides, in comparison to the
structure of FIG. 1, a function-separated structure wherein the
hole injecting-transporting layer of FIG. 1 is function-separated
into two layers of hole injecting layer 6 and hole transporting
layer 7. Hole injecting layer 6 may be formed using well-known
materials, e.g., phthalocyanine compound, conductive high polymers,
arylamine compounds and the like formed to a layer thickness of
about 1.about.30 nm via means such as vacuum deposition and like
methods. Hole transporting layer 6 may be formed of well-known
materials, e.g., benzidine compound, arylaminine compound, styryl
compound and the like formed to a layer thickness of about
10.about.200 nm via means such as vacuum deposition and the
like.
[0056] As shown in FIG. 3, the hole injecting/transporting layer
may be formed in a function-separated structure comprising a hole
injecting layer and hole transporting layer particularly by forming
a layer of a material having high hole injecting characteristics on
the negative electrode side. Such a hole injecting layer may be
formed via vacuum deposition and the like using a mixture of the
aforesaid electron transporting material and a metal having a work
function of less than 4 eV. Examples of usable metals include
magnesium, calcium, titanium, yttrium, lithium, gadolinium,
ytterbium, ruthenium, manganese, and alloys thereof. The thickness
of the electron transporting layer may be about 1.about.200 nm, and
the thickness of the electron injecting layer may be about
0.1.about.30 nm.
[0057] When the sealing layer 10 is formed as shown in FIG. 4, the
sealing layer is formed using compounds such as silicone oxide,
zinc oxide, manganese fluoride, magnesium oxide and the like to
form a thin layer about 5.about.1,000 nm in thickness via vacuum
deposition.
[0058] The organic electroluminescent element of the present
embodiment is suitable for various types of display devices.
[0059] The present invention is described hereinafter by way of
examples.
EXAMPLE 1
[0060] A thin hole injecting/transporting layer was formed on a
glass substrate coated with indium-tin oxide via vacuum deposition
using
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine
to form a layer 60 nm in thickness.
[0061] A thin layer of aluminum trisoxine 60 nm in thickness was
superimposed over the aforesaid hole injecting/transporting layer
via vacuum deposition to form an organic luminescing layer.
[0062] Then, magnesium and silver were vacuum deposited together to
form a thin layer 100 nm in thickness with an atomic ratio of 10:1
to form a negative electrode. A thin layer of magnesium and silver
about 100 nm in thickness was then co-deposited over the aforesaid
layer via resistance heating at an atomic ratio of 1:2.
[0063] The organic electroluminescent element was produced in this
manner.
[0064] The magnesium used has a work function of 3.66 eV.
[0065] The silver used had a work function of 4.26 eV.
[0066] The work function values are data recorded in The Journal of
Applied Physics, 4th Ed. (1977), p.4729. Data appearing the
following examples and reference examples are from the same source.
The work function of indium is recorded in the Chemical
Handbook.
EXAMPLE 2
[0067] A thin hole injecting/transporting layer was formed on a
glass substrate coated with indium-tin oxide via vacuum deposition
using
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine
to form a layer 60 nm in thickness.
[0068] A thin layer of aluminum trisoxine 60 nm in thickness was
superimposed over the aforesaid hole injecting/transporting layer
via vacuum deposition to form an organic luminescing layer.
[0069] Then, magnesium and silver were co-deposited via vacuum
deposition to form a thin layer 50 nm in thickness with an atomic
ratio of 10:1 to form a negative electrode. A thin layer of
magnesium and silver about 100 nm in thickness was then
co-deposited over the aforesaid layer via resistance heating at an
atomic ratio of 1:5.
[0070] The organic electroluminescent element was produced in this
manner.
[0071] The magnesium used has a work function of 3.66 eV.
[0072] The silver used had a work function of 4.26 eV.
EXAMPLE 3
[0073] A thin hole injecting/transporting layer was formed on a
glass substrate coated with indium-tin oxide via vacuum deposition
using
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine
to form a layer 60 nm in thickness.
[0074] A thin layer of aluminum trisoxine 60 nm in thickness was
superimposed over the aforesaid hole injecting/transporting layer
via vacuum deposition to form an organic luminescing layer.
[0075] Then, magnesium and indium were vacuum deposited together to
form a thin layer 60 nm in thickness with an atomic ratio of 10:1
to form a negative electrode. A thin layer of magnesium and indium
about 140 nm in thickness was then co-deposited over the aforesaid
layer via resistance heating at an atomic ratio of 1:5.
[0076] The organic electroluminescent element was produced in this
manner.
[0077] The magnesium used has a work function of 3.66 eV.
[0078] The indium used had a work function of 4.09 eV.
REFERENCE EXAMPLE 1
[0079] An organic electroluminescent element was prepared in the
same manner as in Example 1 with the exception that the negative
electrode was formed by co-depositing magnesium and silver via
resistance heating at an atomic ratio of 10:1 and layer thickness
of 100 nm.
[0080] The magnesium used has a work function of 3.66 eV.
[0081] The silver used had a work function of 4.26 eV.
REFERENCE EXAMPLE 2
[0082] A thin hole injecting/transporting layer was formed on a
glass substrate coated with indium-tin oxide via vacuum deposition
using N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-diphenyl-4,4'-diamine
to form a layer 55 nm in thickness.
[0083] A thin, 10 nm thick layer of 5 wt % doped rubrene in
aluminum trisoxine was superimposed over the aforesaid hole
injecting/transporting layer via vacuum deposition, and over this
was superimposed a thin, 45 nm thick layer of aluminum trisoxine
via vacuum deposition to form an organic luminescing layer.
[0084] Then, aluminum and lithium were co-deposited by resistance
heating to form a thin layer 150 nm in thickness with an atomic
ratio of 20:1 to form a negative electrode.
[0085] The organic electroluminescent element was produced in this
manner.
[0086] The aluminum used has a work function of 4.28 eV.
[0087] The lithium used had a work function of 2.9 eV.
REFERENCE EXAMPLE 4
[0088] A thin hole injecting/transporting layer was formed on a
glass substrate coated with indium-tin oxide via vacuum deposition
using N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-diphenyl-4,4'-diamine
to form a layer 55 nm in thickness.
[0089] A thin, 10 nm thick layer of 5 wt % doped rubrene in
aluminum trisoxine was superimposed over the aforesaid hole
injecting/transporting layer via vacuum deposition, and over this
was superimposed a thin, 45 nm thick layer of aluminum trisoxine
via vacuum deposition to form an organic luminescing layer.
[0090] Then, magnesium and indium were co-deposited by resistance
heating to form a thin layer with an initial atomic ratio of 20:1,
and the deposition rate of the indium was gradually accelerated
while the deposition rate of the magnesium was gradually reduced to
ultimately form a negative electrode 200 nm in thickness.
[0091] The organic electroluminescent element was produced in this
manner.
[0092] The magnesium used has a work function of 3.66 eV.
[0093] The indium used had a work function of 4.09 eV.
REFERENCE EXAMPLE 3
[0094] An organic electroluminescent element was prepared in the
same manner as in Example 4 with the exception that the negative
electrode was formed by depositing magnesium via resistance heating
to achieve a layer thickness of 200 nm.
[0095] The magnesium used has a work function of 3.66 eV.
[0096] Evaluations
[0097] The organic electroluminescent elements prepared in Examples
1.about.4 and Reference Examples 1.about.3 were evaluated by
measuring the voltage V required to start luminescence when a DC
voltage was gradually applied, luminance brightness (cd/m2) when a
5 V DC voltage was applied, and luminance brightness (cd/m2) when a
10 V DC voltage was applied.
[0098] The loss rate (%) of initial output when operated for 5 hr
at a current density of 5 mA/cm2 was determined (i.e., [output
after 5 hr (mW/cm2)/initial output (mW/cm2).times.100]).
[0099] Measurement results are shown in Table 1.
1 TABLE 1 Brightness Brightness Luminescence at 5 V at 10 V Drop in
initial Starting Voltage (cd/m2) (cd/m2) output (%) Ex. 1 3.5 30
4650 92 Ex. 2 3.5 27 4570 93 Ex. 3 3.5 32 5120 93 Ref Ex. 1 3.5 30
3920 86 Ref Ex. 2 3.0 75 8942 89 Ex. 4 3.5 35 5640 93 Ref Ex. 3 3.5
37 4025 85
[0100] As can be understood from Table 1, the organic
electroluminescent element of the present invention starts
luminescing at a low potential and exhibit excellent luminance
brightness.
[0101] The organic electroluminescent element of the present
invention exhibits only slight output reduction, and stable
luminance over a long period was observed.
[0102] The organic electroluminescent element of the present
invention achieves superior luminance efficiency and luminance
brightness, and improved durability, and is not restricted as to
luminescent material, luminescence-enhancing material,
charge-transporting material, sensitizers, resins, positive
electrode materials, nor in the method of manufacturing the
element.
[0103] The present invention provides an organic electroluminescent
element which increases luminance intensity via a negative
electrode of novel construction, and achieves excellent durability
by reducing the luminance starting voltage.
[0104] Although the present invention has been fully described by
way of examples with reference to the accompanying drawings, it is
to be noted that various changes and modification will be apparent
to those skilled in the art. Therefore, unless otherwise such
changes and modifications depart from the scope of the present
invention, they should be construed as being included therein.
* * * * *