U.S. patent application number 12/465038 was filed with the patent office on 2009-11-19 for organic electroluminescence element and manufacturing method thereof.
This patent application is currently assigned to Panasonic Corporation. Invention is credited to Kenji HARADA, Takahiro KOMATSU, Kei SAKANOUE, Takayuki TAKEUCHI.
Application Number | 20090284141 12/465038 |
Document ID | / |
Family ID | 41315524 |
Filed Date | 2009-11-19 |
United States Patent
Application |
20090284141 |
Kind Code |
A1 |
SAKANOUE; Kei ; et
al. |
November 19, 2009 |
ORGANIC ELECTROLUMINESCENCE ELEMENT AND MANUFACTURING METHOD
THEREOF
Abstract
To improve the light emission characteristics of a device when a
transition metal oxide is used for the hole injection layer, in
particular, to enhance the electron blocking characteristics of a
transition metal oxide. An organic electroluminescence element
comprising an anode, a cathode and a plurality of functional layers
formed between the anode and the cathode, the functional layer
containing a layer with a light-emitting function composed of at
least one kind of an organic semiconductor and, between the anode
and the layer with a light-emitting function, a charge injection
layer composed of at least one kind of a transition metal oxide,
wherein the ratio of the metal to oxygen at the anode side of the
transition metal oxide layer is smaller than the stoichiometric
ratio and at the same time, the ratio of the metal to oxygen at the
layer with a light-emitting function side is greater than that at
the anode side.
Inventors: |
SAKANOUE; Kei; (Fukuoka,
JP) ; KOMATSU; Takahiro; (Fukuoka, JP) ;
TAKEUCHI; Takayuki; (Osaka, JP) ; HARADA; Kenji;
(Osaka, JP) |
Correspondence
Address: |
GREENBLUM & BERNSTEIN, P.L.C.
1950 ROLAND CLARKE PLACE
RESTON
VA
20191
US
|
Assignee: |
Panasonic Corporation
Osaka
JP
|
Family ID: |
41315524 |
Appl. No.: |
12/465038 |
Filed: |
May 13, 2009 |
Current U.S.
Class: |
313/504 ;
257/E21.002; 438/22 |
Current CPC
Class: |
H01L 51/5096 20130101;
H01L 51/0037 20130101; H01L 27/3244 20130101; H01L 51/5088
20130101; H01L 2251/308 20130101; H01L 51/0039 20130101; H01L
51/0043 20130101 |
Class at
Publication: |
313/504 ; 438/22;
257/E21.002 |
International
Class: |
H01J 1/63 20060101
H01J001/63; H01L 21/00 20060101 H01L021/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 13, 2008 |
JP |
P2008-126112 |
Claims
1-15. (canceled)
16. An organic electroluminescence element comprising: an anode; a
cathode; a plurality of functional layers formed between the anode
and the cathode, the functional layer including a layer with a
light-emitting function formed from at least one kind of an organic
semiconductor, a charge injection layer formed between the anode
and the layer with a light-emitting function and formed of at least
one kind of a transition metal oxide, wherein a ratio of the
transition metal to oxygen at the anode side in the transition
metal oxide layer is smaller than a stoichiometric ratio and a
ratio of the transition metal to oxygen at the layer with a
light-emitting function side is greater than that at the anode
side.
17. The organic electroluminescence element according to claim 16,
wherein the transition metal oxide layer is a transition metal
oxide layer formed by performing a surface oxidation treatment
after film formation.
18. The organic electroluminescence element according to claim 17,
wherein the transition metal oxide layer contains a transition
metal oxide layer which surface is oxidized by a heat
treatment.
19. The organic electroluminescence element according to claim 17,
wherein said transition metal oxide layer contains a transition
metal oxide layer which surface is oxidized by an ultraviolet
treatment.
20. The organic electroluminescence element according to claim 17,
wherein the transition metal oxide layer contains a transition
metal oxide layer which surface is oxidized with an
oxygen-containing plasma.
21. The organic electroluminescence element according to claim 16,
wherein the transition metal oxide layer is formed by a dry
process.
22. The organic electroluminescence element according to claim 17,
wherein the transition metal oxide layer is formed by a dry
process.
23. The organic electroluminescence element according to claim 18,
wherein the transition metal oxide layer is formed by a dry
process.
24. The organic electroluminescence element according to claim 19,
wherein the transition metal oxide layer is formed by a dry
process.
25. The organic electroluminescence element according to claim 20,
wherein the transition metal oxide layer is formed by a dry
process.
26. The organic electroluminescence element according to claim 16,
wherein out of the transition metal oxide layer, the transition
metal oxide layer positioned on the anode side has a specific
resistance of 1.times.10 exp(5).OMEGA.cm or more.
27. The organic electroluminescence element according to claim 17,
wherein out of the transition metal oxide layer, the transition
metal oxide layer positioned on the anode side has a specific
resistance of 1.times.10 exp(5).OMEGA.cm or more.
28. The organic electroluminescence element according to claim 18,
wherein out of the transition metal oxide layer, the transition
metal oxide layer positioned on the anode side has a specific
resistance of 1.times.10 exp(5) .OMEGA.cm or more.
29. The organic electroluminescence element according to claim 19,
wherein out of the transition metal oxide layer, the transition
metal oxide layer positioned on the anode side has a specific
resistance of 1.times.10 exp(5).OMEGA.cm or more.
30. The organic electroluminescence element according to claim 20,
wherein out of the transition metal oxide layer, the transition
metal oxide layer positioned on the anode side has a specific
resistance of 1.times.10 exp(5).OMEGA.cm or more.
31. The organic electroluminescence element according to claim 16,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
32. The organic electroluminescence element according to claim 17,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
33. The organic electroluminescence element according to claim 18,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
34. The organic electroluminescence element according to claim 19,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
35. The organic electroluminescence element according to claim 20,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
36. The organic electroluminescence element according to claim 21,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
37. The organic electroluminescence element according to claim 22,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
38. The organic electroluminescence element according to claim 23,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
39. The organic electroluminescence element according to claim 24,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
40. The organic electroluminescence element according to claim 25,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
41. The organic electroluminescence element according to claim 26,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
42. The organic electroluminescence element according to claim 27,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
43. The organic electroluminescence element according to claim 28,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
44. The organic electroluminescence element according to claim 29,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
45. The organic electroluminescence element according to claim 30,
wherein the transition metal oxide layer is integrally formed
across a plurality of picture elements.
46. The organic electroluminescence element according to claim 16,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
47. The organic electroluminescence element according to claim 17,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
48. The organic electroluminescence element according to claim 18,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
49. The organic electroluminescence element according to claim 19,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
50. The organic electroluminescence element according to claim 20,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
51. The organic electroluminescence element according to claim 21,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
52. The organic electroluminescence element according to claim 22,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
53. The organic electroluminescence element according to claim 23,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
54. The organic electroluminescence element according to claim 24,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
55. The organic electroluminescence element according to claim 25,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
56. The organic electroluminescence element according to claim 26,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
57. The organic electroluminescence element according to claim 27,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
58. The organic electroluminescence element according to claim 28,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
59. The organic electroluminescence element according to claim 29,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
60. The organic electroluminescence element according to claim 30,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
61. The organic electroluminescence element according to claim 31,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
62. The organic electroluminescence element according to claim 32,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
63. The organic electroluminescence element according to claim 33,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
64. The organic electroluminescence element according to claim 34,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
65. The organic electroluminescence element according to claim 35,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
66. The organic electroluminescence element according to claim 36,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
67. The organic electroluminescence element according to claim 37,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
68. The organic electroluminescence element according to claim 38,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
69. The organic electroluminescence element according to claim 39,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
70. The organic electroluminescence element according to claim 40,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
71. The organic electroluminescence element according to claim 41,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
72. The organic electroluminescence element according to claim 42,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
73. The organic electroluminescence element according to claim 43,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
74. The organic electroluminescence element according to claim 44,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
75. The organic electroluminescence element according to claim 45,
wherein the visible light transmittance of the metal oxide layer is
70% or more.
76. A method for manufacturing an electroluminescence element which
comprises an anode; a cathode; a plurality of functional layers
formed between the anode and the cathode, the functional layer
including a layer with a light-emitting function formed of at least
one kind of an organic semiconductor; and a charge injection layer
formed between the anode and the layer with a light-emitting
function and formed from at least one kind of a transition metal
oxide layer, the method comprising: the step of forming the
transition metal oxide layer is a step of forming the transition
metal oxide layer such that the ratio of the metal to oxygen at the
anode side in the transition metal oxide layer is smaller than the
stoichiometric ratio and the ratio of the metal to oxygen at the
layer with a light-emitting function side is greater than that at
the anode side.
77. The method for manufacturing an organic electroluminescence
element according to claim 76, wherein the step of forming said
transition metal oxide layer contains a step of film-forming a
transition metal oxide layer and a surface oxidation treatment step
of oxidizing the surface of said transition metal oxide layer.
78. The method for manufacturing an organic electroluminescence
element according to claim 77, wherein the surface oxidation
treatment step contains a step of oxidatively treating the surface
by a heat treatment.
79. The method for manufacturing an organic electroluminescence
element according to claim 77, wherein the surface oxidation
treatment step contains a step of oxidatively treating the surface
by an ultraviolet treatment.
80. The method for manufacturing an organic electroluminescence
element according to claim 77, wherein the surface oxidation
treatment step contains a step of oxidatively treating the surface
with an oxygen-containing plasma.
81. The method for manufacturing an organic electroluminescence
element according to claim 76, wherein the step of forming said
transition metal oxide layer is a dry process.
82. The method for manufacturing an organic electroluminescence
element according to claim 77, wherein the step of forming said
transition metal oxide layer is a dry process.
83. The method for manufacturing an organic electroluminescence
element according to claim 78, wherein the step of forming said
transition metal oxide layer is a dry process.
84. The method for manufacturing an organic electroluminescence
element according to claim 79, wherein the step of forming said
transition metal oxide layer is a dry process.
85. The method for manufacturing an organic electroluminescence
element according to claim 80, wherein the step of forming said
transition metal oxide layer is a dry process.
Description
BACKGROUND
[0001] The present invention relates to an organic
electroluminescence element and a manufacturing method thereof.
More specifically, the present invention relates to an organic
electroluminescence element which is used, for example, in a
display or display element for cellular phones or in various light
sources and which is an electroluminescence element driven over a
wide brightness range from low brightness to high brightness in
usage for a light source or the like.
[0002] The organic electroluminescence element is a light-emitting
device utilizing an electroluminescence phenomenon of a solid
fluorescent substance and is partially put into practical use as a
small display.
[0003] The organic electroluminescence element can be classified
into several groups by the material used for the light-emitting
layer. One representative example is a low-molecular organic
electroluminescence element using an organic compound with a low
molecular weight for the light-emitting layer, which is
manufactured mainly using vacuum vapor deposition. Another example
is a polymer organic electroluminescence element using a polymer
compound for the light-emitting layer.
[0004] In the case of a polymer organic electroluminescence
element, use of a solution prepared by dissolving materials
constituting each functional layer enables film formation by a wet
coating method such as spin coating, inkjet coating, nozzle
coating, cap coating, spraying and printing. Thanks to this simple
process, the wet coating method is attracting attention as a
technique that can be expected to realize cost reduction and large
screen area.
[0005] A typical polymer organic electroluminescence element is
fabricated by stacking a plurality of functional layers such as
charge injection layer and light-emitting layer. The construction
and fabrication procedure of a representative polymer organic
electroluminescence element are described below.
[0006] For example, as shown in FIG. 8, on a glass substrate 100
where an ITO (indium tin oxide) film is formed as the anode 1122, a
PEDOT:PSS (a mixture of polythiophene and polystyrenesulfonic acid;
hereinafter referred to as PEDT) thin film is formed as the
electron (hole) injection layer 1123 by a spin coating method or
the like. PEDT is a material that is a de-facto standard as the
charge injection layer, and functions as the hole injection layer
when disposed on the anode side.
[0007] On the PEDT layer, an interlayer 1124 composed of an organic
polymer material is provided. The interlayer is, for example, a
copolymer of a triphenylamine derivative and polyfluorene. For
example,
poly-(2,7(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-sec-butylphenyl)imin-
o)-1,4-phenylene)) indicated by TFB is used. This compound is
excellent in the hole injection property and at the same time, has
an electron blocking function. Therefore use of the compound brings
about elevation of the light emission efficiency and improvement of
the driving lifetime. Next, polyphenylene vinylene (hereinafter,
indicated by PPV) and a derivative thereof, or polyfluorene and a
derivative thereof, is film-formed as the light-emitting layer 1125
by a spin coating method or the like. On the light-emitting layer,
injection of an organic material into the lowest unoccupied
molecular orbital (LUMO) is efficiently performed using an electron
injecting material 1126 of a material with a small work function
such as an alkali metal or alkaline earth metal (e.g., Ba, Ca, Mg,
Li, Cs) or a fluoride, oxide or carbonate of the metal above, e.g.,
LiF, BaO, CsCO.sub.3. Thereafter, a metal electrode 1127 such as Al
or Ag is provided as the cathode. The film of these metals is
formed by a vacuum vapor deposition method, a sputtering method or
a wet coating method.
[0008] In this way, the polymer organic electroluminescence element
can be fabricated by a simple and easy process and because of this
excellent property, its application to various uses is expected.
However, PEDT:PSS used as the hole injection layer is an acidic
water-soluble chemical compound, and this incurs the following
problems: first, a problem in view of apparatus, that is, the
compound corrodes metal portions of the coating apparatus used;
secondly, a problem ascribable to bad wettability to a partition
wall formed mainly of a resist material and provided to divide a
picture element; and thirdly, a serious problem that because the
compound has bad wettability to a material which has a
light-emitting function and is dissolved in an organic solvent,
when the picture element is divided into fine pixels of a display
or the like, the uniformity of the coated film within a pixel
becomes insufficient to impair the light emission uniformity or
allow easy occurrence of short circuiting. Also, it is known that
chemical deterioration is caused by the injection of an electric
charge and adversely affects the lifetime.
[0009] Reduction of the light emission intensity, that is,
deterioration, of the polymer organic electroluminescence element
proceeds in proportion to the product of the electric energization
time and the current flowing in the element, but its details are
not yet elucidated and intensive studies thereon are being
made.
[0010] Reduction of the light emission intensity is presumed to be
brought about by various causes, but the cause is considered to be
a combination of various factors such as stability of the
light-emitting material itself or a functional layer (e.g., hole
injection layer, electron injection layer) against an electron or a
hole, side reaction from an exciton, thermal stability, stability
of interface between layers, diffusion of a material due to heat,
and oxidation of a cathode material.
[0011] In the polymer electroluminescence element, as described
above, deterioration of PEDT is considered to be one of main causes
of the reduction of the light emission intensity. As previously
indicated, PEDT is a mixture of two polymer substances, that is,
polystyrenesulfonic acid and polythiophene. The former is ionic and
the latter has local polarity in the polymer chain. These two
polymer substances are loosely bound through a Coulomb interaction
ascribable to the electric charge anisotropy, and excellent charge
injection characteristics are thereby exerted.
[0012] In order for PEDT to exert excellent characteristics, an
intimate interaction between those two substances is indispensable,
but in general, a mixture of polymer substances is likely to cause
phase separation due to a subtle difference of solubility in a
solvent. This is no exception to PEDT. Occurrence of phase
separation indicates relatively easy breaking of the loose binding
between two polymers. The phase separation suggests that when
driven in an organic electroluminescence element PEDT may be
unstable. Also, as a result of phase separation, a component not
contributing to the binding, in particular, an ionic component, may
diffuse due to an electric field associated with electric
energization and adversely affect other functional layers. In this
way, despite excellent charge injection characteristics, PEDT is
not a stable substance by any means.
[0013] Against the above-described concern about PEDT, the present
inventors have made various experiments and, based on the
experiment results, proposed to form a transition metal oxide such
as molybdenum oxide MoO.sub.3 between an anode and a light-emitting
layer instead of PEDT, and good injection characteristics can be
thereby obtained (see, JP-2005-203340).
[0014] The problem relevant to the hole injection layer is greatly
improved by the above invention, but from the standpoint of light
emission efficiency, more improvements are being demanded, because
the light emission efficiency sometimes decreases depending on the
material used.
[0015] There have been also proposed a light-emitting diode having
a laminated film of MoS.sub.2 and MoO.sub.3 formed by a coating
method and a light-emitting element containing an electrode having
a structure of ITO/MoS.sub.2/MoO.sub.3/polymer organic
semiconductor layer with MoS.sub.2 being annealed (see, Journal of
Applied Physics, Vol. 92, 7556-7563 (2002) and Advanced Materials
2002, Vol. 14, 265-268). In both of these, MoS.sub.2 is formed by a
coating method and therefore, there is a problem that not only
formation with a uniform thickness is difficult due to surface
bulging in the pattern edge but also MoS.sub.2 allows for a large
leakage current to increase the leakage current with an adjacent
pixel and is hard to integrally form particularly when achieving
microfabrication and high integration.
[0016] Also, it is reported that when tungsten oxide is evaporated
on ITO by an electron beam method and heat-treated at 450.degree.
C. to vapor-deposit a low molecular-type organic EL material, the
light emission efficiency is enhanced (see, Synthetic metals, 151,
141-146 (2005)). But this technique cannot be used because the
annealing temperature is as high as 450.degree. C. and the high
temperature adversely affects other constituent members such as
insulating film or partition wall for the separation of a picture
element in fabricating a display or the like. Furthermore, because
the optimal film thickness is as very thin as 1.5 nm and the film
thickness dependency is also large, there is a serious drawback of
variation when a large-size substrate of second or greater
generation is used.
[0017] A case of using nickel oxide is also known (see, Thin Solid
Films, 515, 5099-5102 (2007)). This is a method of vapor-depositing
a 10 nm-thick Ni metal and then heat-treating it at 500.degree. C.
to effect conversion into nickel oxide. The publication above
indicates that emission efficiency is enhanced by performing a heat
treatment and the optimal condition is 4 hours. However, also in
the technique of this publication, the annealing temperature is
high and, since metallic Ni is underlying, there is a problem that
cross-talk occurs if the underlayer is entirely oxidized. In
addition, it is not indicated that high efficiency can be achieved
compared with the conventionally employed hole injection layer such
as starburst amine or copper phthalocyanine.
[0018] In the case of using a large-size substrate, the large film
thickness dependency greatly affects the yield and leads to
incapability of stable mass production.
[0019] In this way, in the structure above, a hole injection layer
composed of a transition metal oxide having a film thickness of
approximately from 10 to 100 nm is used on an anode, and a
functional layer such as light-emitting layer is formed thereon.
The functional layer is mainly formed from an interlayer and a
light-emitting layer or an electron transport layer and since the
interlayer used here is a thin film having a thickness of around
about 20 nm and contains almost the same organic solvent as the
light-emitting material, intermixing between layers often occurs.
Furthermore, the interlayer is required to have an electron
blocking function so as to cause an electron injected from a
cathode to stay in the light-emitting layer but cannot completely
block an electron due to an intermixing problem between layers or a
problem in view of chemical structure and a part of electrons are
allowed to pass into an anode without being used for recombination,
as a result, there arises a problem such as failure in obtaining
sufficient emission efficiency.
[0020] Under these circumstances, the present invention has been
made and an object of the present invention is to improve the light
emission characteristics of a device when a transition metal oxide
is used for the hole injection layer.
[0021] In particular, an object of the present invention is to
enhance the electron blocking characteristics of a transition metal
oxide.
SUMMARY
[0022] The present invention is an electroluminescence element
comprising an anode; a cathode; a plurality of functional layers
formed between the anode and the cathode, the functional layer
including a layer with a light-emitting function formed from at
least one kind of an organic semiconductor; a charge injection
layer formed between the anode and the layer with a light-emitting
function and formed of at least one kind of a transition metal
oxide. A ratio of the transition metal to oxygen at the anode side
in the transition metal oxide layer is smaller than a
stoichiometric ratio and a ratio of the transition metal to oxygen
at the layer with a light-emitting function side is greater than
that at the anode side.
[0023] Usually, a transition metal oxide such as molybdenum oxide
film-formed in a reducing atmosphere is oxygen-deficient based on
the stoichiometric ratio and as compared with those where the ratio
of the metal to oxygen is the stoichiometric ratio, the specific
resistance is small enough to allow for hole transport. On the
other hand, molybdenum oxide at the stoichiometric ratio is known
to be an insulator. Accordingly, when a hole injection layer formed
in this way is subjected to a surface oxidation treatment such as
heat treatment, UV treatment or oxygen plasma treatment in the
atmosphere, oxidation proceeds only in the surface to relatively
increase the proportion of oxygen and bring about approximation to
the theoretical ratio of the compound, and the insulating property
is thereby enhanced, as a result, an electron blocking function is
exerted. Also, in the case of forming a hole injection layer across
a plurality of picture elements, when the specific resistance is
small, cross-talk readily occurs to decrease the image contrast and
therefore, although depending on the required specification of
contrast, the original specific resistance at the film formation is
preferably above a certain resistance. Furthermore, it is
experimentally known that if oxidation further proceeds to
establish the stoichiometric ratio in the entire layer, conversely,
the injection efficiency is greatly impaired. For this reason, an
oxygen-deficient region and a region having a value close to the
stoichiometric ratio need to be present together. As described
above, a layer having a composition nearly in the stoichiometric
ratio is an insulator and therefore, in this case, the mechanism of
bringing about injection of a hole is considered to utilize a
tunnel current. Accordingly, this layer can be imparted with an
electron blocking function, enabling omission of an electron
blocking layer formed of an organic material, and thanks to more
reduction in the film thickness, an organic electroluminescence
element that is driven at a low voltage and has high emission
efficiency can be provided.
[0024] The present invention includes the organic
electroluminescence element according to aforementioned one that
the transition metal oxide layer is a transition metal oxide layer
formed by performing a surface oxidation treatment after film
formation.
[0025] By virtue of this construction, a state where an
oxygen-deficient region and a region having a value close to the
stoichiometric ratio are present together can be easily obtained
after film formation of the transition metal oxide Layer. In the
oxidation treatment, oxidation is considered to proceed in the
thickness direction of a thin film according to the time, power,
temperature and the like of the oxidation treatment, but if
oxidation proceeds and the stoichiometric ratio is established in
the entire layer, conversely, the injection efficiency is greatly
impaired. Accordingly, in order to maximally bring out the effect
of enhancing the light emission efficiency of the present
invention, it is considered to be necessary that on the side in
contact with the anode, oxygen deficiency is present to form a
defect level on HOMO (Highest Occupied Molecular Orbital) and on
the side in contact with a material having a light-emitting
function, only the extreme surface is subjected to an oxidation
treatment.
[0026] The present invention also includes the organic
electroluminescence element according to aforementioned one that
the transition metal oxide layer contains a transition metal oxide
layer with the surface which surface is oxidized by a heat
treatment.
[0027] According to this construction, the transition metal oxide
layer can be formed in a short time by performing an oxidation
treatment with good workability.
[0028] The present invention also includes the organic
electroluminescence element according to aforementioned one that
the transition metal oxide layer contains a transition metal oxide
layer which surface is oxidized by an ultraviolet treatment.
[0029] According to this construction, the ultraviolet irradiation
time is easy to control, so that the oxide depth can be controlled
with high precision.
[0030] The present invention also includes the organic
electroluminescence element according to aforementioned one that
the transition metal oxide layer contains a transition metal oxide
layer which surface is oxidized with an oxygen-containing
plasma.
[0031] Accordingly to this construction, the oxide depth can be
controlled with higher precision by controlling the plasma
intensity, plasma density and accelerating time.
[0032] The present invention also includes the organic
electroluminescence element according to aforementioned one that
the transition metal oxide layer is formed by a dry process.
[0033] By virtue of this construction, a transition metal oxide
that is stable and excellent in the film quality can be formed.
[0034] In organic electroluminescence element above of the present
invention out of the transition metal oxide layers, the transition
metal oxide layer positioned on the anode side preferably has a
specific resistance of 1.times.10 exp(5).OMEGA.cm or more.
[0035] By virtue of this construction, cross-talk can be reduced
even when the transition metal oxide layer is integrally formed
across a plurality of picture elements. Incidentally, in the case
where the hole injection layer is not formed across a plurality of
picture elements but is independently formed for each picture
element, the cross-talk does not substantially occur as a problem
and therefore, in order to reduce the voltage drop ascribable to
the transition metal oxide layer itself and enable high brightness
emission, the transition metal oxide layer above preferably has a
lower specific resistance of 10,000 .OMEGA.cm or less.
[0036] The present invention also includes the organic
electroluminescence element according to aforementioned one that
the transition metal-oxide layer is integrally formed across a
plurality of picture elements.
[0037] According to this construction, even when the transition
oxide layer is integrally formed across a plurality of picture
elements, the problem of cross-talk does not arise by virtue of the
large specific resistance and the layer with a light-emitting
function formed thereon, particularly the coating-type layer, can
easily have a uniform film thickness without causing variation of
the contact angle, because the underlying layer is entirely formed
of the same material. In this case, a picture element regulating
layer is preferably formed below the transition metal oxide
layer.
[0038] In the organic electroluminescence element above of the
present invention, the visible light transmittance of the metal
oxide layer is preferably 70% or more.
[0039] By virtue of this construction, a sufficiently large
quantity of emitted light can be maintained.
[0040] Also, the present invention is a method for manufacturing an
electroluminescence element comprising an anode; a cathode; a
plurality of functional layers formed between the anode and the
cathode, the functional layer including a layer with a
light-emitting function formed from at least one kind of an organic
semiconductor; and a charge injection layer formed between the
anode and the layer with a light-emitting function and formed of at
least one kind of a transition metal oxide layer. The method
comprises the step of forming the transition metal oxide layer is a
step of forming the transition metal oxide layer such that the
ratio of the metal to oxygen at the anode side in the transition
metal oxide layer is smaller than the stoichiometric ratio and the
ratio of the metal to oxygen at the layer with a light-emitting
function side is greater than that at the anode side
[0041] As described above, according to the present invention, the
transition metal oxide thin film formed to have oxygen deficiency
is in an oxygen-deficient state and allows for injection of a hole,
despite high specific resistance, but the light emission efficiency
sometimes slightly decreases according to the light-emitting
material used. The reason therefor is considered because the
electron blocking ability is insufficient. In order to compensate
for this reduction of light emission efficiency, it may be
effective to shift the recombination region of a hole and an
electron to the side closer to the cathode without locating it at
the interface between the interlayer and the light-emitting layer,
but the energy is sometimes transferred out to the cathode side
depending on the diffusion distance of an exciton produced and a
sufficiently high effect may not be obtained. In the present
invention, after the formation of a transition metal oxide thin
film having oxygen deficiency, only the extreme surface layer is
oxidized by surface oxidation or the like to relatively increase
the proportion of oxygen and bring about approximation to the
theoretical ratio of the compound, and an electron blocking
function is thereby exerted in the surface, so that an organic
electro-luminescence element with high emission efficient can be
manufactured.
BRIEF DESCRIPTION OF DRAWINGS
[0042] FIG. 1 A schematic explanatory view showing the structure of
the organic electroluminescence element according to embodiment 1
of the present invention.
[0043] FIG. 2 A schematic explanatory view showing the structure of
the organic electroluminescence element according to embodiment 2
of the present invention.
[0044] FIG. 3 A schematic explanatory view showing the structure of
the organic electroluminescence element in Example 3 of the present
invention.
[0045] FIG. 4 An equivalent circuit diagram of the display device
according to embodiment 3 of the present invention.
[0046] FIG. 5 A layout explanatory diagram of the display device
according to embodiment 3 of the present invention.
[0047] FIG. 6 A cross-sectional view of the display device
according to embodiment 3 of the present invention.
[0048] FIG. 7 A top surface explanatory diagram of the display
device according to embodiment 3 of the present invention.
[0049] FIG. 8 An explanatory view showing the organic
electroluminescence element of a conventional example.
EMBODIMENTS
[0050] Embodiments of the present invention are described in detail
below by referring to the drawings.
Embodiment 1
[0051] The fundamental structure of the polymer organic
electroluminescence element according to embodiment 1 of the
present invention illustrated in FIG. 1 is described.
[0052] As shown in FIG. 1 illustrating a schematic explanatory view
of the structure, this embodiment is characterized in that by
oxidizing the surface of a molybdenum oxide layer (MoO.sub.x) as
the transition metal oxide layer formed between an anode and a
layer with a light-emitting function, the ratio of the molybdenum
to oxygen on the anode side (MoO.sub.x1) of the molybdenum oxide
layer is made smaller in terms of the oxygen content than the
stoichiometric ratio and the ratio of the molybdenum to oxygen at
the layer with a light-emitting function side (MoO.sub.x2) is made
greater than that at the anode side. In the formulae, X2<3 and
X1<X2. In other words, a bottom emission-type organic
electroluminescence element is fabricated, where a surface oxidized
molybdenum oxide thin film as the hole injection layer 3 and a
polymer material as the light-emitting layer 4 are sequentially
stacked on an anode 2 composed of an indium tin oxide (ITO) layer
formed on a light-transmitting glass substrate 1, an electron
injection layer 5 composed of an alkaline earth metal is further
formed thereon, and a cathode 6 composed of an aluminum layer is
sequentially stacked as an upper layer.
[0053] More specifically, the organic electroluminescence element
of this embodiment comprises, as shown in FIG. 1, a substrate 1
composed of a light-transmitting glass material, an ITO thin film
as the anode 2 formed on the substrate 1, and layers further formed
thereon, that is, a transition metal oxide thin film as the charge
injection layer 3, a light-emitting layer 4 composed of a polymer
material, an electron injection layer 5 composed of a barium layer,
and a cathode 6 composed of an aluminum layer.
[0054] When a DC voltage or a DC current is applied using the anode
2 of the organic electroluminescence element above as a plus
electrode and the cathode 6 as a minus electrode, a hole is
injected into the light-emitting layer 4 composed of a polymer film
from the anode 2 through the hole injection layer 3 and at the same
time, an electron is injected thereinto from the cathode 6. In the
light-emitting layer 4, recombination of the thus-injected hole and
electron takes place and when an exciton generated by the
recombination undergoes a transition from an excited state to a
ground state, a luminescence phenomenon occurs.
[0055] According to the organic electroluminescence element of this
embodiment, the hole injection layer is composed of a
surface-oxidized molybdenum oxide and therefore, stabilization and
enhancement of injection characteristics can be achieved, which
enables enhancing the light emission characteristics and prolonging
the lifetime, so that a bottom emission-type organic
electroluminescence element with high reliability can be
fabricated.
[0056] In embodiment 1, it is more effective that not only the
ratio of molybdenum to oxygen at the layer with a light-emitting
function side (MoO.sub.x2) is greater than that at the anode side
but also the proportion of oxygen is greater than the
stoichiometric ratio (X2>3).
Embodiment 2
[0057] The fundamental structure of the polymer organic
electroluminescence element according to embodiment 2 of the
present invention illustrated in FIG. 2 is described.
[0058] The difference of this embodiment from the organic
electroluminescence element of embodiment 1 is that, as shown in
FIG. 2, an interlayer (electron blocking layer) 7 having a film
thickness of about 20 nm and being composed of TFB is caused to
intervene between the light-emitting layer 4 and the molybdenum
oxide 3 as the hole injection layer of the organic
electroluminescence element of embodiment 1 shown in FIG. 1. This
interlayer has a LUMO level at a position shallower than the
light-emitting layer and can be designed such that substantially no
electron transfer occurs by forming a barrier to electron injection
from the light-emitting layer into the interlayer or making the
electron mobility smaller than the hole mobility. Other parts are
formed similarly to the organic electroluminescence element of
embodiment 1.
[0059] More specifically, the organic electroluminescence element
of this embodiment comprises, as shown in FIG. 2, a substrate 1
composed of a light-transmitting glass material, an indium tin
oxide (ITO) thin film as the anode 2 formed on the substrate 1, and
layers further formed thereon, that is, a surface-oxidized
transition metal oxide thin film as the hole injection layer 3, an
interlayer 7, a light-emitting layer 4 composed of a polymer
material, an electron injection layer 5, and a cathode 6 composed
of an aluminum layer.
[0060] Also in this case, when a DC voltage or a DC current is
applied using the anode 2 of the organic electroluminescence
element above as a plus electrode and the cathode 6 as a minus
electrode, a hole is injected into the light-emitting layer 4 that
is composed of a polymer film and formed by a coating method, from
the anode 2 through the hole injection layer 3 and at the same
time, an electron is injected thereinto from the cathode 6. Here,
the interlayer 7 acts as the electron blocking layer. In the
light-emitting layer 4, recombination of the thus-injected hole and
electron takes place and when an exciton generated by the
recombination undergoes a transition from an excited state to a
ground state, a luminescence phenomenon occurs.
[0061] According to the organic electroluminescence element of this
embodiment, in addition to the operation and effect of embodiment
1, owing to intervention of the interlayer 7, the electron blocking
function is more enhanced and the probability of recombination of
an electron and a hole can be raised, which enables enhancing the
light emission characteristics and prolonging the lifetime, so that
an organic electroluminescence element with high reliability can be
fabricated.
[0062] In advance of description of the following Examples,
respective components constituting the organic electroluminescence
element of the present invention are described.
(Electron Injection Layer)
[0063] The electron injection layer of Examples can be composed of
an alkali metal or alkaline earth metal having a small work
function. Specific examples thereof include, but are not limited
to, Ca, Ba, Li and Cs. Other than these metals, their oxides (e.g.,
CaO, BaO, Li.sub.2O, Cs.sub.2O.sub.3, MgO) and halides (e.g., LiF)
are included.
[0064] It is also preferred to use titanium oxide, zinc oxide or
the like having a defect level. In the case of using such a
material, even when the element is left standing in the atmosphere,
the reaction with moisture or oxygen is decreased in comparison
with the case of using an alkali metal and/or an oxide, halide or
carbonate thereof and driving in the atmosphere, which is supposed
to be a drawback of the organic EL, becomes possible.
[0065] In the case where such titanium oxide or zinc oxide is used
as the electron injection layer and combined with the hole
injection layer of the present invention, since the hole injection
layer of the present invention has an electron blocking function,
even when an interlayer is not used, an exciton produced undergoes
recombination without allowing energy transfer to an electrode and
therefore, the light emission region in the light-emitting layer
can be shifted to the hole side. In turn, the layer construction
becomes simple, which contributes to enhancement of the yield or
reduction of the cost. In this case, the layer construction is
anode/transition metal oxide layer/organic light-emitting
layer/transition metal oxide layer/cathode and is a very simple
construction. The transition metal oxide layer sandwiched between
the cathode and the organic light-emitting layer preferably
contains, as a dopant, an alkali metal or alkaline earth metal such
as CaO, BaO, Li.sub.2O, Cs.sub.2O.sub.3 and MgO. These are unstable
in the atmosphere but when doped in molybdenum oxide or the like
having a defect level, the instability is decreased and at the same
time, the electron injection property can be improved. Moreover,
since the periphery thereof is covered with a transition metal
oxide matrix, diffusion into other layers less occurs and an
adverse effect is hardly given on the light emission efficiency or
driving lifetime. In general, release of an electron is expected to
become difficult, but in the case of the present invention, as a
result of various experiments, good electron releasability is
exhibited with little occurrence of such a side effect.
Accordingly, by the use of the element structure of the present
invention, an electron transport layer composed of an organic
material or a layer composed of an alkali metal or the like having
a low work function, which has been conventionally required, need
not be disposed and a robust element can be obtained.
[0066] As for the impurity-containing transition metal oxide layer
used as the electron injection layer in the present invention,
those having a work function of 4 to 6 eV are preferably used, but
the present invention is not limited thereto.
[0067] Incidentally, zinc is recently sometimes classified into the
representative element but is dealt with here as an element
included in the transition metal element of the present
invention.
(Hole Injection Layer)
[0068] For the hole injection layer used in the present invention,
a transition metal oxide is preferably used.
[0069] When a transition metal oxide is used, an electrode material
even containing an ionic substance acts as a barrier layer, and a
reactive substance is not allowed to diffuse from the electrode
side and incur deterioration of the light-emitting layer. Also, in
the case of forming a thin film of a transition metal oxide such as
molybdenum oxide, nickel oxide and vanadium oxide by a process of
creating a defect level or producing a difference in the oxidation
number, efficient charge injection of the organic
electroluminescence element is attained. Furthermore, when the
specific resistance is small, in use as a thin film of 1 .mu.m or
less, the voltage drop is reduced and an electric field applied
between two electrodes is directly applied to the light-emitting
layer, which enables driving at a low voltage. In addition, the
transition metal oxide thin film of the present invention has
multiple functions such as electron injection property, electron
transport property and electron blocking property and therefore,
high functionality can be obtained by a single layer, making it
possible to simplify the layer construction of the element and
realize a low-cost device.
[0070] The thickness of the surface-oxidized transition metal oxide
layer is preferably from 1 nm to 1 .mu.m.
[0071] If the thickness exceeds 1 .mu.m, high transmittance can be
hardly ensured. Considering the film-forming time, the thickness is
more preferably 500 nm or less. Also, in the case of a thin
thickness, even when the layer is not in a film state but in an
island state, as long as the average thickness is about 1 nm, the
same effects as above can be obtained. If the thickness is less
than 1 nm, sufficient hole injection characteristics cannot be
obtained.
[0072] Specifically, other than molybdenum oxide, for example,
tungsten oxide, nickel oxide, vanadium oxide and ruthenium oxide
may be used, but the present invention is not limited thereto. Such
a compound takes a plurality of oxidation states and becomes an
insulator when the ratio of metal to oxygen is the stoichiometric
ratio or exhibits electrical conductivity when having an oxygen
deficiency, and an electron blocking agility can be imparted to the
compound by controlling the oxidation state in the film thickness
direction as in the present invention. The oxidation number or
composition can be confirmed by XPS (X-ray photoelectron
spectroscopy) analysis.
[0073] As for the surface oxidation method, oxidation after film
formation is simple and easy. For example, various methods such as
UV irradiation or heat treatment in an oxygen-present atmosphere,
oxygen plasma irradiation, and oxidation treatment by solution may
be applied, and the method is not limited. It is also possible to
form a film while varying the conditions such that the ratio of
metal to oxygen reaches a desired value during, the film formation
described later.
(Light-Emitting Layer-Interlayer)
[0074] On the hole injection layer, a light-emitting layer is
formed by coating an organic semiconductor material. At this time,
in view of light emission efficiency, an interlayer is preferably
provided as the hole blocking layer between the light-emitting
layer and the hole injection layer. For the hole blocking layer, a
polyfluorene-based polymer material having a LUMO level higher than
the material used for the light-emitting layer or having low
electron mobility, such as TFB having a triphenylamine skeleton, is
used, but the present invention is not limited thereto. As regards
the light-emitting layer, in the case of a polymer type, in
addition to the polyfluorene-based and polyphenylene vinylene-based
copolymers, as long as a thin film can be formed by dissolving the
polymer in a solvent and coating the solution, the polymer is not
limited in its kind, including a pendant type, a dendrimer type,
and a type that is coated after doping a low-molecular
light-emitting material into a coating-type low molecular or
polymer host capable of dissolving in a solution and exhibiting
good thin-film performance without causing crystallization or the
like.
[0075] In the organic electroluminescence element of the present
invention, the layer with a light-emitting function is not limited
to a polymer compound, and any of a low molecular compound, an
oligomer and the like may be used. As for these materials,
conventionally known materials may be used.
[0076] A representative structure of the low molecular
electroluminescence device includes a layer structure of
substrate/anode/hole injection layer/hole transport layer/electron
blocking layer/light-emitting layer (including a doping
material)/hole blocking layer/electron transport layer/electron
injection layer/cathode, but other than this, the layer structure
has various variations. Like this, the structure is a multilayer
structure compared with the polymer-type electroluminescence
device, which is a factor of rising cost. By using the hole
injection layer of the present invention, a hole injection layer, a
hole transport layer and an electron blocking layer can be
integrated, and this is effective in reducing the cost.
(Layer Construction)
[0077] In the example above, a so-called bottom emission type of
extracting light from the substrate side is described, but the top
emission type of extracting light from the direction opposite the
substrate (in this case, a high-reflectance silver alloy or
aluminum alloy is preferably used as the anode) includes a reverse
structure type where the anode and the cathode are reversely
disposed, a top emission type thereof, and the like and, in terms
of the material, is applicable when using various compounds such as
fluorescent material and phosphorescent material. In this case, by
providing the hole injection layer of the present invention,
generation of a so-called hillock that is readily produced when
heating a reflective anode can be prevented.
(Cathode)
[0078] As for the cathode, a material capable of establishing ohmic
contact with the electron injection layer of the present invention
is preferred. A general metal typified by Al, Ag or Au, a
transparent electrically conductive oxide typified by ITO and IZO,
and the like are preferably used.
(Encapsulation)
[0079] The device of the present invention is preferably subjected
to encapsulation. In the case of a conventional electroluminescence
device, an enormous cost is required for ensuring the reliability,
for example, use of an encapsulating resin having as small moisture
permeability as possible, film encapsulation by a thin film layer
formed on the element, or sealing of a desiccant is employed.
According to the present invention, simple encapsulation of the
device is necessary, but the cost can be reduced by a conventional
encapsulation method. The simple encapsulating material can be
widely selected from existing materials.
(Film Formation Method)
[0080] Out of the functional layers constituting the organic
electroluminescence element of the present invention, the
transition metal oxide layer configured such that the ratio of the
transition metal to oxygen on the anode side is small in terms of
the oxygen content than the stoichiometric ratio and at the same
time, the ratio of the transition metal to oxygen at the layer with
a light-emitting function side is greater than that at the anode
side, is preferably formed by a dry process such as vacuum vapor
deposition, electron beam vapor deposition, molecular beam epitaxy,
sputtering, reactive sputtering, ion plating, laser ablation,
thermal CVD, plasma CVD and MOCVD.
[0081] It is known from experimental results that in such a dry
process, the substrate temperature is preferably controlled.
Incidentally, in the case of a vapor deposition method, enhancement
of brightness and reduction of light emission initiating voltage
can be achieved by setting the substrate temperature to from 60 to
100.degree. C. Also in the case of a sputtering method, despite a
rise of the substrate temperature during sputtering, in view of
introducing necessary oxygen defects, it is preferred to raise the
substrate temperature from the starting point of sputtering.
[0082] The film may also be formed while changing the introduced
amount of oxygen in the course of sputtering a metal target by
using a reactive sputtering method. That is, a continuous formation
method of film-forming a first layer having an oxygen defect and
then film-forming a transition metal oxide layer while increasing
the oxygen content is also effective.
[0083] Furthermore, a composition gradient film may also be formed
by vapor co-deposition while changing the amount of evaporation
from the target.
[0084] It is also effective to form a film by a co-sputtering
method using as a target an alloy obtained by mixing transition
metal oxides differing in the composition or using a plurality of
targets containing a plurality of kinds of transition metals such
as molybdenum and tungsten. Also in this case, a transition metal
oxide film having a desired oxygen content can be obtained by
performing the sputtering while changing the amount of oxygen or
while switching the target between those differing in the oxygen
content.
[0085] The film thickness is preferably in the range not impairing
the hole injection property and is preferably from several nm to
500 nm. The film thickness is preferably thin because the
transmittance loss increases when the film thickness becomes thick,
but the film thickness may be determined by taking into
consideration the variation and the like at the mass
production.
[0086] A nanoparticle or the like of an oxide is also applicable.
In this case, the film formation method may be appropriately
selected also from wet processes such as sol-gel process,
Langmuir-Blodgett method (LB method), layer-by-layer method, spin
coating, inkjet coating, dip coating and spraying, and as long as
the film can be formed to finally provide the effects of the
present invention, any method may be used.
[0087] In the case of forming a functional layer (a light-emitting
layer or a hole or electron injection layer that is formed, if
desired) of the present invention from a polymer material, a spin
coating method, a casting method, a dipping method, a bar coating
method, or a wet process such as roll coating, inkjet coating,
nozzle coating and spraying, is used. In this case, a large-scale
vacuum apparatus is not necessary, enabling film formation using
inexpensive equipment, and at the same time, a large-area organic
electroluminescence element can be easily manufactured.
[0088] Incidentally, the glass substrate 100 is one sheet of a
colorless transparent glass. Examples of the glass substrate 100
which can be used include a transition metal oxide glass such as
transparent or semi-transparent soda lime glass,
barium/strontium-containing glass, lead glass, aluminosilicate
glass, borosilicate glass, barium borosilicate glass and quartz
glass, and an inorganic glass such as inorganic fluoride glass.
Embodiment 3
[0089] The display device using the organic electroluminescence
element in an embodiment of the present invention is described
below. This embodiment is one example of the top emission-type
polymer organic electroluminescence display device.
[0090] First a display device using an organic electroluminescence
element is described. The display device of this embodiment is
fundamentally manufactured as follows: an insulating film is
provided on a driving substrate comprising a glass substrate having
produced and provided thereon a transistor composed of polysilicon;
an aluminum alloy is patterned thereon as the anode; and a
sputtering film of molybdenum oxide and tungsten is formed thereon
as the hole injection layer to extend across a plurality of picture
elements. By forming the hole injection layer in this way without
separating between picture elements, the process can be simplified.
Subsequently, the surface is oxidized by an annealing treatment in
the atmosphere; a partition wall is provided to separate respective
picture elements of RGB; an interlayer and a light-emitting layer
are coated by an inkjet method; a Ba-doped low-molecular electron
transport material is vapor co-deposited as the electron transport
layer on the entire surface of RGB picture elements; ITO is then
sputtered as the cathode; and the stack is subjected to
encapsulation to manufacture a device. Thanks to the annealing
treatment in the atmosphere performed here, the light emission
efficiency is enhanced. In this embodiment, the surface is annealed
to relatively increase the proportion of oxygen and bring about
approximation to the theoretical ratio of the compound, whereby an
electron blocking function is caused to be exerted in the surface.
Furthermore, in this embodiment, a barium oxide-containing
molybdenum oxide layer (electron injection layer) 5 is provided as
a functional layer to intervene on the cathode 6 side, and an
active matrix-type display device is fabricated using the same
light-emitting device as the organic electroluminescence element of
embodiment 1 shown in FIG. 1. It is considered that by virtue of
such a construction, although the transition metal oxide is
oxygen-deficient and has a small specific resistance, when
molybdenum oxide (transition metal oxide) is oxidized by surface
oxidation or the like, this allows increasing the proportion of
oxygen on the light-emitting layer side and bringing about
approximation to the theoretical ratio of the compound, as a
result, an electron blocking function is exerted in the
surface.
[0091] As shown in FIG. 4 illustrating an equivalent circuit
diagram of this matrix-type display device, in FIG. 5 illustrating
an layout explanatory diagram, in FIG. 6 illustrating a
cross-sectional view and in FIG. 7 illustrating a top surface
explanatory diagram, the display device above constitutes an active
matrix-type display device where a driving circuit is formed for
each picture element.
[0092] This display device 140 is fabricated, as shown in FIG. 4
illustrating an equivalent circuit diagram and in FIG. 5
illustrating an layout explanatory diagram, such that a plurality
of driving circuits each consisting of an organic
electroluminescence element (electroluminescence) 110 forming a
picture element, two thin-film transistors (TFT: T1, T2) composed
of a switching transistor 130 and a current transistor 120 as the
photodetection element, and a capacitor C are arrayed vertically
and horizontally, a gate electrode of a first TFT (T1) in each of
the driving circuits arranged in a horizontal row is connected to a
scanning line 143 to give a scanning signal, and a drain electrode
of a first TFT in each of the driving circuits arranged in a
vertical row is connected to a data line to supply a light emission
signal. To one end of the electroluminescence element
(electroluminescence), a driving power source (not shown) is
connected, and one end of the capacitor C is grounded. Reference
numeral 143 denotes a scanning line, 144 denotes a signal line, 145
denotes a common power feeder cable, 147 denotes a scanning line
driver, 148 denotes a signal line driver and 149 denotes a common
power feeder driver.
[0093] FIG. 6 is a cross-sectional explanatory view of the organic
electroluminescence element (FIG. 6 is an A-A cross-sectional view
of FIG. 5), and FIG. 7 is a top surface explanatory diagram of the
display device, where on a glass substrate 400 having formed
thereon driving TFTs (not shown), an anode (Al) 112, a
surface-oxidized molybdenum oxide layer (transition metal oxide
layer) 113, an organic interlayer (charge blocking layer) (not
shown), a light-emitting layer 114 (a red light-emitting layer R, a
green light-emitting layer G and a blue light-emitting layer B), a
barium oxide-containing molybdenum oxide layer 115 and a cathode
116 are formed to fabricate a top emission-type organic
electroluminescence element. As for the structure, the anode and
the charge injection layer are individually formed, the
light-emitting layer has an opening area defined by a protrusion
composed of a silicon oxide layer as the picture element regulating
layer 117, and the cathode 116 is formed like a stripe running in a
direction orthogonal to the anode.
[0094] The driving TFT is formed such that, for example, after an
organic semiconductor layer (polymer layer) is formed on a glass
substrate 100 and covered with a gate insulating film, a gate
electrode is formed thereon and at the same time, a source/drain
electrode is formed through a through-hole formed in the gate
insulating film. On this transistor, a polyimide film or the like
is coated to form an insulating layer (flat layer) and furthermore,
an anode (ITO) 112, a surface-oxidized molybdenum oxide layer 113,
an electron blocking layer, an organic semiconductor layer 114 such
as light-emitting layer, a barium oxide-containing molybdenum oxide
layer 115 and a cathode 116 (Al ultra-thin film and ITO) are formed
to manufacture an organic electroluminescence element. In FIG. 7,
the capacitor and wiring are omitted, but these are formed on the
same glass substrate. A plurality of picture elements each composed
such TFT and organic electroluminescence element are formed in a
matrix manner on 11e same substrate to constitute an active
matrix-type display device.
[0095] At the manufacturing, as shown in FIG. 5, a picture element
regulating layer 117 is formed, for example, on a scanning line
143, a signal line 144, a switching TFT 130 and an electrode 112
composed of a pattern of aluminum constituting a picture element
electrode, which are formed on a glass substrate 100, and an
opening is then provided.
[0096] As an upper layer thereof, a transition metal oxide layer
113 is formed over the entire surface by a sputtering method, and
the surface is oxidized with an ultraviolet ray.
[0097] Thereafter, if desired, TFB as an interlayer is coated by an
inkjet method. This TFB layer may be coated over the entire surface
similarly to the transition metal oxide layer or may be coated only
on a portion corresponding to the opening.
[0098] After passing through a drying step, a polymer organic
electroluminescence material for a desired color (any one of RGB)
is coated by an inkjet method on a position corresponding to the
opening to form a light-emitting layer 114.
[0099] Furthermore, a barium oxide-containing molybdenum layer 115
is film-formed by vapor co-deposition or the like, and finally, a
cathode 116 is formed in a region where a display picture element
141 is disposed.
[0100] According to this construction, a display device capable of
high-speed driving and assured of high reliability can be provided.
Since a molybdenum oxide layer that is integrally formed and is an
oxide of a transition metal intervenes between the light-emitting
layer and the anode in the form of being surface-oxidized, no
cross-talk occurs and the light-emitting layer is filled in a
recess part smoothed by the molybdenum oxide layer and controlled
in size with high precision. Therefore, the light-emitting layer
can be unfailingly formed by an inkjet method without causing
position slippage, and a light-emitting layer controlled in the
film thickness and size with high precision can be obtained. Also
on the light-emitting layer, an integrally-formed molybdenum oxide
layer is formed and therefore, the light-emitting layer is free
from a sputtering damage when forming the cathode or a plasma
damage in the patterning step.
[0101] In this way, the light-emitting layer can be formed on a
uniformly-formed surface and the surface can be kept in a smooth
state, so that the light-emitting layer can be uniformly formed and
an electric field applied by the anode and cathode can be uniformly
imparted to the light-emitting layer without occurrence of electric
field concentration, succeeding in obtaining good light emission
characteristics. Also, each light-emitting layer is uniformly
formed, so that good light emission characteristics can be obtained
without variation in the light emission characteristics.
[0102] An example of the lighting device using a light-emitting
device having two-dimensionally disposed therein a plurality of
electroluminescence elements is described below by referring to
FIG. 5. As regards the two-dimensionally disposed
electroluminescence elements 110, for example, such a construction
as concurrently lighting on/off all electroluminescence elements
can be quite easily realized. However, even in the case of such a
construction as concurrently lighting on/off the
electroluminescence elements, it is preferred to take a
construction where at least one electrode (for example, a picture
element electrode composed of Al (see, the anode 112 in FIG. 6)) is
separated for individual electroluminescence elements. This is
because even when a display picture element 141 is found to have a
defect by some factors, the defect remains in the display picture
element 141 and therefore, the production yield of the lighting
device as a whole can be enhanced. The lighting device having the
above-described construction is applicable, for example, to
domestic lighting equipment in general. In this application, since
the lighting device can be constructed extremely thin, the lighting
device can be easily installed not only on a ceiling but also on a
wall.
[0103] Furthermore, the light emission pattern of the
two-dimensionally disposed electroluminescence elements can be
easily controlled by supplying arbitrary data, and the
electroluminescence element of the present invention can be
constructed to give a light emitting region in a size of, for
example, 40 .mu.m-square, so that an application allowing the
lighting device to serve also as a panel-type display device by
supplying data can be constructed. Of course, in this case, the
display picture elements 141 need to be color-coded red, green or
blue depending on the position, but multiple coloration can be very
easily realized by using an inkjet method.
[0104] Conventionally, when a lighting device is compared with a
display device, the light emission brightness is higher in the
lighting device. However, the electro-luminescence element 110 of
the present invention can take a sufficiently large area and has
very high light emission brightness and therefore, this element can
be used as both a lighting device and a display device. In this
case, a mechanism for adjusting the light emission brightness is
needed due to difference in the function (that is, the use mode)
between the lighting device and the display device, and the
mechanism therefor can be realized, for example, by employing the
construction of embodiment 2 above and controlling the drive
current, thereby adjusting the light emission brightness of each
electroluminescence element. More specifically, the light emission
brightness can be adjusted by, in use as a lighting device, driving
all electroluminescence elements with a larger current, and in use
as a display device, driving each electroluminescence element with
a small current at a current value controlled according to the
gradation (that is, according to the image data). In such an
application, a single power source may be used for the power source
when functioning as a lighting device and when functioning as a
display device, but in the case where a drive current is
controlled, for example, where the dynamic range of a
digital-to-analog converter is large and the number of gradations
becomes insufficient in use as a display device, it is preferred to
take a construction of switching the power source between those
(not illustrated) connected to a common power feeder cable 145
shown in FIGS. 4 and 5 according to the use mode. Of course, even
in the use mode as a lighting device, in the case of an embodiment
where brightness needs to be controlled (that is, a lighting device
having a light control function), this can be easily coped with by
the above-described control of a current value according to the
gradation. Furthermore, the electroluminescence element of the
present invention can be formed not only on a glass substrate 100
but also on a resin substrate such as PET and therefore, can be
applied as a lighting device for various illuminations.
[0105] Incidentally, the thin film transistor may be composed of an
organic transistor. Also, a structure where an organic
electroluminescence element is stacked on a thin film transistor,
or a structure where a thin film transistor is stacked on an
organic electroluminescence element, is also effective.
[0106] In addition, in order to obtain a high-image quality
electroluminescence display device, an electroluminescence
substrate having formed thereon an organic electro-luminescence
element may be laminated together with a TFT substrate having
formed thereon TFT, a capacitor, a wiring and the like, such that
an electrode of the electro-luminescence substrate and an electrode
of the TFT substrate are connected using a connection bank.
[0107] As a modified example of embodiment 3, the method for color
coding in a color display device is described below.
[0108] In this example, RGB coding is performed using a substrate
having formed thereon a thin film transistor. A flattening film is
formed of an insulating organic material on a TFT substrate, a
transparent electrode is formed on the substrate by sputtering ITO,
image regulating layers in respective thicknesses are formed of SiN
similarly to embodiment 4, and dry etching is applied to give a
desired light emission region.
[0109] Using this TFT substrate, RGB coating is performed. A
flattening film is formed of an insulating organic material on a
TFT substrate, a transparent electrode is formed on the substrate
by sputtering ITO, image regulating layers in respective
thicknesses are formed of SiON similarly to embodiment 1, and dry
etching is applied to give a desired light emission region.
Thereafter, as the hole injection layer of the present invention,
sputtering is performed while flowing oxygen and argon by using an
alloy of tungsten and molybdenum as the target to form a hole
injection layer composed of an oxide. Subsequently, the substrate
is introduced into an oxygen plasma apparatus and irradiated 200 W
for 30 seconds to oxidize the outermost surface.
[0110] Furthermore, a bank composed of polyimide is formed for each
row of RGB picture elements, whereby a substrate divided in a
stripe manner into respective rows of elements by a bank is
obtained. An electroluminescence element is formed using the
resulting substrate. This substrate is characterized by high
resistance compared with PEDT and no occurrence of cross-talk and
therefore can be used in such a way. Thereafter, TFB as an
interlayer is coated to a thickness of 20 nm by an inkjet method
for each of rows divided by a bank. After drying and baking, an ink
prepared from a red light-emitting material, a green light-emitting
material or a blue light-emitting material is coated as the
light-emitting layer by using a dispenser to an average thickness
of 80 nm on each of rows divided by a bank. Furthermore, a layer
composed of a Ba-doped low-molecular electron transport material is
formed as the electron injection layer by a resistance heating
vapor deposition method, and aluminum is then vacuum
vapor-deposited to a thickness of 100 nm as the cathode. The
electron injection layer and cathode are formed to cover all
picture elements.
[0111] TFT in a part of the obtained sample is operated by an
external circuit and evaluated for the light emission state and
lifetime. As a result, sufficient light emission state and lifetime
are obtained.
Embodiment 5
Lighting Device
[0112] As an example of the lighting device, molybdenum oxide is
sputtered to a thickness of 100 nm on a 30 cm-square glass
substrate having provided thereon ITO, and a polymer-type white
light-emitting material is then spin-coated to a thickness of 100
nm. Subsequently, sodium fluoride is 10% vapor co-deposited with
zinc oxide of the present invention as the electron injection
layer, and Ag is further formed to a thickness of 100 nm as the
cathode.
[0113] DC of 10 V is applied to the thus-obtained sample, as a
result, uniform light emission is obtained and even in an
unencapsulated state, the device is stably driven for 1 hour
without causing a short circuit or producing a dark spot.
Example 1
[0114] The present invention is described below by referring to
Examples. The organic electroluminescence light-emitting device of
the present invention is an example corresponding to embodiment 1
and is described by referring to FIG. 1.
[0115] On a glass substrate 1 with a 0.7 mm-thick patterned ITO(2),
a molybdenum oxide layer was film-formed as the hole injection
layer to a thickness of 10 nm by a sputtering method and irradiated
for 3 minutes at an oscillation wavelength of 172 nm by using an
excimer UV exposure apparatus, SNA/14, manufactured by Ushio Inc.
to oxidize the surface. The sample was then placed in a glove box
filled with nitrogen, and TFB
(poly-2,7-9,9-di-n-octylfluorene-alt-1,4-phenylene-4-sec-butylphenylimino-
-1,4-phenylene), which is a copolymerization polymer of fluorene
and triphenylamine, was spin-coated as the interlayer to a
thickness of 20 nm and baked at 180.degree. C. in nitrogen.
Subsequently, a green light-emitting material (produced by Sumation
Co., Ltd.) was spin-coated to 80 nm and similarly baked in nitrogen
to form a light-emitting layer. The element was then transferred to
a vacuum vapor deposition apparatus, and Ba of 5 nm in thickness as
the electron injection layer and Al of 100 nm in thickness as the
cathode were vapor-deposited. After sealing a getter agent for an
organic EL in a nitrogen atmosphere, the periphery of the obtained
element was sealed with a UV-sensitive encapsulating resin. This
was designated as Sample 103. Sample 104 was manufactured in the
same manner as Sample 102 except that in Sample 102, after forming
the molybdenum oxide film, the substrate was introduced into an
oxygen plasma apparatus and irradiated with a plasma of 200 W for 1
minute.
[0116] Here, at the surface oxidation, even when the treatment is
performed at 250.degree. C. for 45 minutes in the atmosphere, the
same results were obtained.
[0117] As regards Comparative Sample 102 for comparison, the sample
obtained by not performing UV irradiation or oxygen plasma
irradiation in Sample 103 or 104 was used directly. Also, Sample
101 was manufactured in the same manner as Samples 102 to 104
except that in the manufacture of Comparative Sample 102, instead
of vacuum vapor-depositing molybdenum oxide to a thickness of 10
nm, PEDT produced by H. C. Starck was spin-coated as the hole
injection layer on a glass substrate 1 with a 0.7 mm-thick
patterned ITO(2) in the atmosphere to a thickness of 60 nm and
dried by baking at 200.degree. C. for 10 minutes and after
transferring the element into a glove box, an interlayer and layers
therebelow were coated.
[0118] As described above, Sample 103 was manufactured in the same
manner as Sample 102 except that in Sample 102, UV irradiation was
performed after forming the molybdenum oxide layer.
[0119] Sample 104 was manufactured in the same manner as Sample 102
except that in Sample 102, after forming the molybdenum oxide film,
the substrate was introduced into an oxygen plasma apparatus and
irradiated with a plasma of 200 W for 1 minute. Samples 101 to 104
obtained were evaluated for IV characteristics and light emission
brightness characteristics by using ITO and Al as the anode and the
cathode, respectively.
[0120] As a result, the voltage for driving Samples 102 to 104 was
decreased by about 0.5 V as compared with Sample 101. The light
emission characteristics are shown in the Table below.
TABLE-US-00001 TABLE 1 Driving Light Emission Lifetime Efficiency
(Cd/A) at 12,000 Sample Hole Injection Layer at 20 mA/cm.sup.2
cd/m.sup.2 101 (Comparative PEDT 10.2 120 hr Example) 102
(Comparative MoO.sub.x (untreated) 8.4 230 hr Example) 103
(Iinvention) MoO.sub.x (UV treated) 10.5 310 hr 104 (Invention)
MoO.sub.x 11.1 320 hr (O.sub.2 plasma treated)
[0121] As apparent from Table 1, in the case of using untreated
MoO.sub.x, the driving lifetime is improved but the light emission
efficiency is worse than in Comparative Example using PEDT, whereas
in the sample of the present invention subjected to an oxidation
treatment, the light emission efficiency is equal to or higher than
that when using PEDT and the driving lifetime is also enhanced.
[0122] Next, a pattern having a plurality of picture elements as
shown FIG. 5 was produced. ITO as the anode was divided for
individual picture elements, and the pattern was produced to make
it possible to externally drive individual elements. On this
substrate, a photosensitive resist was coated as an insulating
film, exposed and developed to form a picture element with a
desired size. Furthermore, a material used in Samples 101 to 104
was coated or vapor-deposited on the entire surface thereof. In the
samples obtained, the hole injection layer, interlayer,
light-emitting layer and anode each was formed across a plurality
of picture elements. In these samples, a voltage was applied to
only one picture element to emit light at about 1,000 cd/m.sup.2,
as a result, in Sample 101, light emission at a level enabling an
adjacent picture element to be viewed with an eye was observed. In
Samples 102 to 104, light emission was not confirmed.
[0123] In general, an interlayer and a light-emitting layer are
formed of an organic material and therefore, have a high resistance
and even when such a layer is integrally formed across picture
elements, cross-talk does not occur. On the other hand, PEDT is
originally a solution obtained by dispersing the mixture in water
and even when dried, is small in the specific resistance.
Accordingly, in the case of forming a film thereof across a
plurality of picture elements, there raises a serious problem that
when a common cathode is used, an adjacent picture element not
applied with a voltage also emits light. Molybdenum oxide for use
in the present invention has a high specific resistance in the
transverse direction and advantageously causes no cross-talk but is
deficient in that when molybdenum oxide as-deposited is used, light
emission efficiency equal to or greater than that in using PEDT is
not obtained depending on the light-emitting material. However,
when the oxidation treatment of the present invention is performed,
the light emission efficiency becomes equal to or greater than that
in using PEDT, and IV characteristics at an equal level are
obtained. At the same time, a merit can be found in that thanks to
enhancement of the light emission efficiency, the drive current
value is decreased and the lifetime is also improved.
[0124] In Example 1, the ratio of molybdenum to oxygen was examined
by analyzing the surface composition of the molybdenum oxide thin
film used in Samples 102 to 104 by the use of an X-ray
photoelectron spectroscopy. As a result, in Sample 102, the ratio
of Mo to oxygen was determined as 2.7. This is a value for a ratio
between 3p orbital signal of Mo and 2p orbital signal of oxygen.
The ratio was determined as 2.9 and 3.0 in Sample 103 and Sample
104, respectively. This apparently reveals that molybdenum oxide is
oxidized by the surface oxidation treatment indicated in Example 1
and the proportion of oxygen is relatively increased in comparison
with molybdenum. That is, by virtue of the oxidation treatment of
the present invention, the oxygen-deficient portion is oxidized by
the oxidation treatment, as a result, the surface of
oxygen-deficient molybdenum oxide comes close to the theoretical
ratio of the compound. In such an oxidation treatment, oxidation is
considered to proceed in the thickness direction of a thin film
according to the time, power, temperature or the like of the
oxidation treatment, but if oxidation proceeds and the
stoichiometric ratio is established in the entire layer,
conversely, the injection efficiency is greatly impaired.
Accordingly, in order to maximally bring out the effect of
enhancing the light emission efficiency of the present invention,
it is considered to be necessary that on the side in contact with
the anode, oxygen deficiency is present to form a defect level on
HOMO and on the side in contact with a material having a
light-emitting function, only the extreme surface is subjected to
an oxidation treatment.
[0125] Here, it is more effective that not only the ratio of
molybdenum to oxygen at the layer with a light-emitting function
side is greater than that at the anode side but also the proportion
of oxygen is greater than the stoichiometric ratio (X>3).
Example 2
[0126] Samples 202 to 204 were manufactured in the same manner by
forming a tungsten oxide-sputtered film to a thickness of 20 nm in
place of molybdenum oxide of Samples 102 to 104 of Example 1. As
for the tungsten oxide, a tungsten oxide film was formed by
so-called reactive sputtering of introducing oxygen by using a
metal target. Other steps are the same as in Example 1. When the
same evaluations as in Example 1 were performed, almost the same
results were obtained.
Example 3
[0127] Samples 302 to 304 were manufactured and evaluated in the
same manner by preparing an alloy target of molybdenum and tungsten
in an element ratio of 30:70 and using the target in the samples of
Example 2. The results are shown in Table 2.
TABLE-US-00002 TABLE 2 Driving Light Emission Lifetime Efficiency
(Cd/A) at 12,000 Sample Hole Injection Layer at 20 mA/cm.sup.2
cd/m.sup.2 101 (Comparative PEDT 10.2 120 hr Example) 302
(Comparative MoWO.sub.x (untreated) 8.2 190 hr Example) 303
(Invention) MoWO.sub.x (UV treated) 11.5 350 hr 304 (Invention)
MoWO.sub.x 12.6 380 hr (O.sub.2 plasma treated)
[0128] As apparent from these results, in the samples using an
alloy target of tungsten and molybdenum and being subjected to the
surface treatment of the present invention, the increase of light
emission efficiency is larger than in the results when
manufacturing the sample by using molybdenum alone or tungsten
alone and performing the surface treatment. Detailed reasons
therefor are not clearly known but are considered that the defect
level formed in the extreme surface becomes a level suitable for an
electron blocking layer.
[0129] Also, in the case where a large number of organic
electroluminescence element cells are arrayed and formed and where
a resinous partition wall is provided and a layer with a
light-emitting function is formed in the region surrounded by the
partition wall, a resin film needs to be patterned on the electron
injection layer and in this patterning step, molybdenum oxide may
dissolve out. However, by virtue of containing tungsten, molybdenum
oxide can be prevented from dissolving out and also in this case,
not only a smooth surface can be maintained without losing surface
smoothness but also characteristic deterioration can be
prevented.
Example 4
[0130] Using the hole injection layer used in Samples 302 to 304 of
Example 3, samples were manufactured without use of an interlayer
by coating a green light-emitting material and forming a cathode in
the same manner, and designated as Samples 402 to 404. These
samples were evaluated for the characteristics, as a result, the
drive voltage was further decreased by about 0.5 V. This is
considered to occur because the entire film thickness of the
organic semiconductor layer is decreased due to removal of an
interlayer. The light emission efficiency was greatly reduced in
the case of untreated molybdenum oxide, whereas in samples of the
present invention subjected to an oxidation treatment, reduction of
the light emission efficiency does not occur even when an
interlayer is removed. This is considered to be ascribable to the
fact that by applying an oxidation treatment, only the surface
layer in which the surface of molybdenum oxide is oxidized comes to
exhibit the property as an insulator and an effective depletion
layer is not allowed to be present at the energy level of an
electron transported by hopping conduction on LUMO of the
light-emitting layer. As for the hole injection, it can be
understood that tunnel injection is possible with a thickness of
several nm and the hole injection efficiency is not decreased.
TABLE-US-00003 TABLE 3 Driving Light Emission Lifetime Efficiency
(Cd/A) at 12,000 Sample Hole Injection Layer at 20 mA/cm.sup.2
cd/m.sup.2 101 (Comparative PEDT 10.2 120 hr Example) 402
(Comparative MoO.sub.x (untreated) 6.1 240 hr Example) 403
(Invention) MoO.sub.x (UV treated) 10.5 420 hr 404 (Invention)
MoO.sub.x 10.1 440 hr (O.sub.2 plasma treated)
[0131] It is clearly seen from these Examples that the oxidation
treatment of the present invention can enhance the light emission
efficiency without adversely affecting the IV characteristics and
at the same time, the items required in terms of cross-talk are
satisfied.
[0132] According to the organic electroluminescence element of the
present invention, an organic electroluminescence element ensuring
particularly enhanced electron blocking characteristics as well as
long lifetime can be provided, and this organic electroluminescence
element can be applied not only to an application requiring
multicolor emission, such as television and display, but also to a
device utilizing monochromatic emission, such as exposure device,
printer and facsimile.
* * * * *