U.S. patent application number 10/306170 was filed with the patent office on 2003-06-26 for organic electroluminescent element and process for its manufacture.
Invention is credited to Kato, Tetsuya, Ozaki, Masaaki, Tachi, Kojiro.
Application Number | 20030117069 10/306170 |
Document ID | / |
Family ID | 27347902 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030117069 |
Kind Code |
A1 |
Kato, Tetsuya ; et
al. |
June 26, 2003 |
Organic electroluminescent element and process for its
manufacture
Abstract
An organic EL element is formed having a crystalline CuPc film
as a positive hole injection layer and a positive hole transport
layer, a luminescent layer, an electron transport layer and an
electron injection layer, composed of amorphous organic materials,
laminated between a pair of electrodes, wherein the change in the
diffraction peak value of the CuPc film appearing with X-ray
diffraction, which is produced by heating in the utilization
temperature range of the organic EL element, is within .+-.25% of
the diffraction peak value before heating. The organic EL element
comprising a crystalline organic material is resistant to current
shorts and leaks and exhibits satisfactory luminance properties in
the utilization temperature range.
Inventors: |
Kato, Tetsuya; (Anjyo-city,
JP) ; Tachi, Kojiro; (Nagoya-city, JP) ;
Ozaki, Masaaki; (Kariya-city, JP) |
Correspondence
Address: |
POSZ & BETHARDS, PLC
11250 ROGER BACON DRIVE
SUITE 10
RESTON
VA
20190
US
|
Family ID: |
27347902 |
Appl. No.: |
10/306170 |
Filed: |
December 2, 2002 |
Current U.S.
Class: |
313/504 |
Current CPC
Class: |
H01L 51/5088 20130101;
H01L 51/5012 20130101; H01L 51/5206 20130101 |
Class at
Publication: |
313/504 |
International
Class: |
H05B 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 2001 |
JP |
2001-369031 |
Dec 3, 2001 |
JP |
2001-369032 |
Sep 12, 2002 |
JP |
2002-266943 |
Claims
What is claimed is:
1. An organic electroluminescent element comprising a substrate, a
first electrode layer on said substrate, at least one crystalline
organic material layer on said first electrode layer, and a second
electrode layer on said crystalline organic material layer(s),
wherein the change in the diffraction peak value of said
crystalline organic material layer based on X-ray diffraction after
heating at 120.degree. C. is within .+-.25% with respect to said
diffraction peak value before said heating.
2. An organic electroluminescent element according to claim 1,
wherein said first electrode is an ITO layer composed of indium-tin
oxide, and said crystalline organic material layer is formed on
said ITO layer in contact with said ITO layer.
3. An organic electroluminescent element according to claim 2,
wherein said crystalline organic material layer is a copper
phthalocyanine layer, and said diffraction peak value is the
diffraction peak value of the (200) plane of said copper
phthalocyanine layer parallel to said substrate.
4. An organic electroluminescent element according to claim 1,
which further comprises at least one amorphous organic material
layer on said crystalline organic material layer.
5. An organic electroluminescent element comprising a substrate, an
ITO layer composed of indium-tin oxide as a first electrode layer
on said substrate, a crystalline organic material layer on said ITO
layer, and a second electrode layer on said crystalline organic
material layer, wherein said ITO layer has no peak bonded water at
near 330.degree. C. in the moisture-derived spectrum of the surface
as measured by thermal desorption method.
6. An organic electroluminescent element according to claim 5,
wherein said crystalline organic material layer is formed on said
ITO layer in contact with said ITO layer.
7. An organic electroluminescent element according to claim 5,
wherein said crystalline organic material layer is a copper
phthalocyanine layer.
8. An organic electroluminescent element comprising a substrate, a
first electrode layer on said substrate, said first electrode layer
being an ITO layer composed of indium-tin oxide, a crystalline
organic metal complex layer on said first electrode layer, at least
one organic material layer on said crystalline organic metal
complex layer, said organic material layer comprising a luminescent
layer, and a second electrode layer on said at least one organic
material layer, wherein all of the organic materials composing said
organic material layer(s) exhibit volatility during film formation
by vacuum vapor deposition, and the change in the diffraction peak
value of said organic metal complex layer based on X-ray
diffraction after heating at 120.degree. C. is within .+-.25% with
respect to said diffraction peak value before said heating.
9. An organic electroluminescent element according to claim 8,
wherein said organic metal complex layer is a copper phthalocyanine
layer.
10. An organic electroluminescent element according to claim 8 or
9, which is exposed to a temperature of 70.degree. C. or
higher.
11. A process for manufacturing an organic electroluminescent
element, which comprises the steps of providing a substrate,
depositing an ITO layer composed of indium-tin oxide on said
substrate, subjecting the surface of said ITO layer to treatment
for desorption of the bonded water, and depositing a crystalline
organic material layer on said ITO layer after said desorption
treatment.
12. A process for manufacturing an organic electroluminescent
element according to claim 11, which is carried out so that in the
moisture-derived spectrum of said ITO layer surface after said
desorption treatment, measured by thermal desorption, the bonded
water peak value at near 330.degree. C. is within 50% of said
bonded water peak value of said ITO layer surface before desorption
treatment.
13. A process for manufacturing an organic electroluminescent
element according to claim 11, which is carried out so that in the
moisture-derived spectrum of said ITO layer surface after said
desorption treatment, measured by thermal desorption method, the
bonded water peak value at near 330.degree. C. disappears.
14. A process for manufacturing an organic electroluminescent
element according to claim 11, wherein said crystalline organic
material layer is formed on said ITO layer in contact with said ITO
layer.
15. A process for manufacturing an organic electroluminescent
element according to claim 11, wherein said crystalline organic
material layer is a copper phthalocyanine layer.
16. A process for manufacturing an organic electroluminescent
element, which comprises the steps of providing a substrate,
depositing an ITO layer composed of indium-tin oxide on said
substrate, subjecting said ITO layer to heat treatment at a
temperature of 250.degree. C. or higher, and depositing a
crystalline organic material layer on said ITO layer after said
heat treatment.
17. A process for manufacturing an organic electroluminescent
element according to claim 16, wherein said crystalline organic
material layer is formed on said ITO layer in contact with said ITO
layer.
18. A process for manufacturing an organic electroluminescent
element according to claim 16, wherein said crystalline organic
material layer is a copper phthalocyanine layer.
19. A process for manufacturing an organic electroluminescent
element, which comprises the steps of providing a substrate,
depositing an ITO layer composed of indium-tin oxide on said
substrate, and depositing a crystalline organic material layer on
said ITO layer, and then subjecting said crystalline organic
material layer to heat treatment in a vacuum or in an inert gas
atmosphere, to complete deposition of said crystalline organic
material layer.
20. A process for manufacturing an organic electroluminescent
element according to claim 19, wherein said heat treatment
temperature is 70.degree. C. or higher.
21. A process for manufacturing an organic electroluminescent
element according to claim 19, wherein said crystalline organic
material layer is formed on said ITO layer in contact with said ITO
layer.
22. A process for manufacturing an organic electroluminescent
element according to claim 19, wherein said crystalline organic
material layer is a copper phthalocyanine layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an organic
electroluminescent (organic EL) element having an organic material
as the luminescent material, and to a process for its manufacture.
The invention is particularly suitable for vehicle displays and the
like which are exposed to high-temperature environments.
[0003] 2. Description of the Related Art
[0004] Organic EL elements exhibit excellent visibility due to
their self-luminescent nature, and allow weight reduction of
driving circuits as well because of their low driving voltage of
from a few volts to a few dozen volts. They therefore show promise
for applications as thin-film displays, lightings and backlights.
Organic EL elements are also characterized by abundant color
variations.
[0005] The basic structure of an organic EL element has a laminate
of a plurality of organic thin-films formed on an electrode formed
on a substrate, with an electrode formed over the organic thin-film
laminate. The major types of materials used for the organic
thin-films are low molecular types applied by vacuum vapor
deposition and high molecular types applied by coating of solutions
onto substrates.
[0006] The major types of low molecular materials used are
non-crystalline amorphous materials for formation of films by
vacuum vapor deposition. These materials exhibit no diffraction
peak with analysis by X-ray diffraction.
[0007] However, amorphous organic thin-films undergo
crystallization when their glass transition temperature
(hereinafter referred to as "Tg point") is exceeded due to
temperature variation, resulting in inconveniences such as
unevenness of the film producing shorter distances between
electrodes, causing current shorts or leaks, and creating electric
field condensing.
[0008] Techniques aimed at achieving a longer working life for such
low molecular organic thin-film materials by means of a crystalline
structure are disclosed in Japanese Unexamined Patent Publication
HEI No. 3-173095 and Japanese Unexamined Patent Publication HEI No.
5-182764.
[0009] The former publication describes the feature of a positive
hole transport layer and a luminescent layer in an organic compound
thin-film with a crystalline structure, and the examples therein
include using
N,N'-diphenyl-N,N'-di(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine
(hereinafter referred to as TPD) as the positive hole transport
layer and an aluminum complex of 8-hydroxyquinoline (hereinafter
referred to as Alq3) as the luminescent layer, and employing a
substrate temperature of 50.degree. C. during film formation to
form a crystalline organic thin-film.
[0010] The latter publication describes using the same materials
mentioned above, TPD and Alq3, for the positive hole transport
layer and luminescent layer, first forming the positive hole
transport layer and then the luminescent layer, and immediately
following this by heat treatment or performing heat treatment after
formation of each layer, to create a fine crystalline aggregate
structure of Alq3 as the luminescent layer.
[0011] However, when the present inventors used X-ray diffraction
to analyze the crystalline states of simple films of TPD and Alq3
formed on ITO (transparent electrode)-formed glass substrates under
the conditions described in each of the prior art documents
mentioned above, no appearance of crystallinity-indicating
diffraction peaks was found. That is, although the aforementioned
prior art documents teach that the organic thin-films are
crystalline, their crystallinity is such one which does not exhibit
diffraction peaks in X-ray diffraction.
[0012] The present inventors explain these results as follows. This
is illustrated in FIGS. 1A and 1B. FIG. 1A is a structure showing
an amorphous state, and FIG. 1B is the presumed crystalline
structure according to the aforementioned prior art documents.
[0013] Specifically, the lack of diffraction peaks in X-ray
diffraction suggests that the crystalline structure described in
the aforementioned prior art documents is not that of common
crystals with a structure wherein the molecules are arranged in a
regular parallel fashion on the substrate, but rather it is a
structure with fine crystal aggregates or with fine crystals
scattered throughout an amorphous thin-film.
[0014] Further examination by the present inventors revealed that,
even though the organic compound film structures in these prior art
documents are preformed as thin films with a fine crystalline
aggregate structure to inhibit heat-induced alteration, formation
of the fine crystalline aggregate structure reduces the luminescent
efficiency by creating a smaller contact area and lower charge
mobility between each of the layers, thereby resulting in reduced
luminance or luminance irregularities, and producing new problems
of electrode shorts and leaks due to the increased surface
irregularities.
[0015] The luminance reduction is temperature-dependent, becoming
more rapid with higher utilization temperature, while the luminance
irregularities occur as light areas and dark areas are produced due
to uneven luminance in the element.
[0016] According to a first aspect of the present invention, which
has been accomplished in light of the new problems discovered by
the present inventors, it is an object to realize an organic EL
element comprising an organic material which is not amorphous and
hence prone to changes in the film, but which has a crystallinity
that includes fine crystals, the element being resistant to current
shorts or leaks and exhibiting satisfactory luminance
characteristics within the range of utilization temperature.
[0017] Of the materials used for organic thin-films of organic EL
elements, the materials primarily used for low molecular materials,
for film formation by vacuum vapor deposition, may be largely
classified as either evaporating materials which gasify when in a
liquid state, and sublimating materials which gasify when in a
solid state.
[0018] Generally speaking, most tertiary amine compounds used for
positive hole transport layers exhibit volatility, while
8-hydroxyquinoline aluminum complex used in luminescent layers and
electron transport and injection layers are sublimating materials.
That is, most organic EL elements have a structure comprising a
combination of a evaporating material and a sublimating
material.
[0019] Such organic EL elements exhibit notably reduced luminance
at high temperatures and therefore are insufficiently durable as
vehicle displays, which are used in higher temperature environments
than most other commercial products.
[0020] Material manufacturers are therefore developing heat
resistant materials, i.e. materials with high glass transition
temperatures, and device manufacturers commonly employ methods of
film formation using materials with higher glass transition
temperatures than the temperatures to which the elements will be
exposed. For example, Japanese Unexamined Patent Publication HEI
No. 11-3782 teaches that when the environmental temperature exceeds
the Tg of the constituent material, crystallization of the
constituent material progresses leading to current leaks and
shorts.
[0021] Experimental research by the present inventors, however, has
confirmed that the problem of leaks or shorts is not solved even if
the organic thin-film of the element is constructed with a material
having a higher Tg than the temperatures to which the organic EL
element will be exposed.
[0022] In light of these problems, therefore, it is an object
according to a second aspect of the invention to provide an organic
EL element having an organic thin-film including a luminescent
layer between a pair of electrodes, which adequately prevents
current leaks or shorts.
SUMMARY OF THE INVENTION
[0023] The present inventors have conducted much research aimed at
achieving the aforementioned first aspect of the invention. The
results have shown that the aforementioned luminance reduction,
luminance irregularities, shorts and leaks are still produced even
with an organic thin-film composed of an organic material
possessing no glass transition temperature and having crystallinity
which exhibits a diffraction peak in X-ray diffraction.
[0024] In other words, it was found that the cause of the
above-mentioned luminance reduction, luminance irregularities,
shorts and leaks can be traced to alterations in the crystalline
state of the crystalline material, such as copper phthalocyanine
(CuPc) which is often used for the positive hole injection layer.
This cause will now be explained in detail based on data obtained
from experiments conducted by the present inventors.
[0025] An anode ITO (transparent electrode) was formed on a glass
substrate and surface treated with an argon and oxygen mixed
plasma, after which a CuPc film was formed as the positive hole
injection layer to a film thickness of 10 nm at a material heating
temperature of 420.degree. C., and then a triphenylamine tetramer
as the positive hole transport layer, dimethyl
quinacridone-containing Alq3 as the luminescent layer, Alq3 as the
electron transport layer, LiF as the electron injection layer and
Al as the cathode were formed in that order, to fabricate an
organic EL element which was then sealed with a hermetic can. This
fabricated sample will be referred to as the trial product. Of the
organic thin-films, CuPc was crystalline while the other layers
were amorphous.
[0026] FIG. 2 shows the voltage-luminance characteristic (V-I
characteristic) of the trial product after standing at high
temperature of 100.degree. C. for 12 hours. As seen in FIG. 2, the
V-I characteristic shifted toward the about +3V end after standing
at high temperature, as compared to the initial characteristic
before standing at high temperature.
[0027] This increases the burden on the driving circuit and leads
to a higher circuit design cost. Also, since this occurs only
partially in the element, regions of greater and lesser current
flow are formed, being seen as luminance irregularities.
[0028] The material with the lowest Tg among the layers, or organic
thin-films, of the trial product is the triphenylamine tetramer, at
approximately 144.degree. C. Since standing at 100.degree. C. means
standing in an environment of over 40.degree. C. below the Tg point
of this triphenylamine tetramer, the effect of progression of the
fine crystal aggregate structure of the amorphous films other than
CuPc is believed to be less with respect to this shift
phenomenon.
[0029] Attention was therefore directed toward alteration in the
crystalline state of the CuPc film (positive hole injection layer),
which is an organic material with crystallinity. As a result, it
was found that the crystalline state of the CuPc film differs
notably before and after standing in a high temperature
environment. The following are the results of examining the change
in the crystalline state of the CuPc film.
[0030] For efficient confirmation of the crystalline state change,
the evaluation was conducted with the standing environment
temperature accelerated to as high as 120.degree. C., with a
standing time of 2 hours. Standing under these conditions will
hereunder be referred to as accelerated high-temperature
standing.
[0031] A CuPc film was formed on an ITO-formed glass substrate
under the same conditions as the previous trial product which
exhibited a shift of about 3V in the V-I characteristic as
mentioned above. The state of crystallinity of the CuPc film in
this case was analyzed by X-ray diffraction before and after the
above-mentioned accelerated high-temperature standing, and the
results are shown in FIG. 3.
[0032] As seen in FIG. 3, the diffraction peak occurring at
2.theta.=6.68.degree. is due to the crystal structure of CuPc. In
FIG. 3, the solid line represents the peak before accelerated
high-temperature standing, i.e. the initial peak, and the dotted
line represents the peak after accelerated high-temperature
standing, i.e. the peak after 120.degree. C., 2 hours.
[0033] A larger integral value for a peak, i.e. a higher peak
value, indicates greater crystallinity. Here, the accelerated
high-temperature standing at 120.degree. C. for 2 hours resulted in
a peak value (integral value) which was 1.5 times compared to the
peak value before accelerated high-temperature standing.
[0034] Thus, the present inventors surmised that since the change
in the crystalline state occurs in the CuPc film after formation of
the positive hole transport layer, luminescent layer, electron
transport layer, cathode, etc. on the CuPc film as the positive
hole injection layer, that is, after the luminescent element has
been fabricated for this trial product, this is likely a major
factor inducing changes in V-I characteristics in high temperature
environments, thereby resulting in luminance reduction and
luminance irregularities, as well as shorts and leaks.
[0035] This phenomenon is believed to likewise occur for materials
other than low molecular materials, for example, high molecular
materials such as PPV (polyphenylvinylene)-based materials, so long
as they exhibit diffraction peaks in X-ray diffraction.
[0036] The present inventors devised the present invention upon
considering that for an organic EL element comprising a crystalline
organic material, the most effective means against alteration of
the crystalline state of the organic material under high
temperature environments is to form the film of the crystalline
organic material so that it has as high a level of crystallinity as
possible.
[0037] In other words, the first mode of the invention is an
organic EL element comprising at least one crystalline organic
material (that exhibits crystallinity), characterized in that among
the values of the diffraction peaks appearing with X-ray
diffraction of the crystalline organic material, the change in the
diffraction peak value produced by heating in the utilization
temperature range of the organic EL element is within +25% of the
diffraction peak value before heating. Here, the term "crystalline"
also includes fine crystals.
[0038] If, among the values of the diffraction peaks appearing with
X-ray diffraction of the crystalline organic material, the change
in the diffraction peak value produced by heating in the
utilization temperature range of the organic EL element is within
.+-.25% of the diffraction peak value before heating as according
to the invention, it is possible to limit the change in the
crystalline state of the organic material to a level which does not
result in luminance reduction or luminance irregularities, or
shorts and leaks, even with use in high temperature
environments.
[0039] According to the present invention, therefore, it is
possible to prevent current shorts and leaks to produce
satisfactory luminance characteristics in utilization temperature
ranges of organic EL elements comprising crystalline organic
materials.
[0040] When an ITO film composed of indium-tin oxide is formed on
the substrate, the crystalline organic material may be constructed
as an organic film formed on the ITO film.
[0041] The organic material exhibiting crystalline properties may
be a copper phthalocyanine film, in which case the diffraction peak
value is the diffraction peak value of the (200) face parallel to
the substrate in the copper phthalocyanine film.
[0042] According to a third mode of the invention, there is
provided a process for manufacture of an organic EL element wherein
an ITO film composed of indium-tin oxide is formed on a substrate,
and a crystalline organic material is formed as a film on the ITO
film, characterized in that the bonded water on the surface of the
ITO film is subjected to desorption treatment before forming the
organic material film.
[0043] This can reduce adsorption water and bonded water on the
surface of the ITO film underlying the crystalline organic
material. The crystallinity of the organic material formed as a
film on the ITO film with reduced bonded water can thereby be
increased. It will thus be possible to limit the change in the
crystalline state of the organic material to a level which does not
result in luminance reduction or luminance irregularities, or
shorts and leaks, even with use in high temperature
environments.
[0044] According to the invention, therefore, it is possible to
prevent current shorts and leaks to produce satisfactory luminance
characteristics in utilization temperature ranges of organic EL
elements comprising crystalline organic materials.
[0045] The desorption treatment is preferably carried out so that
in the moisture-derived spectrum of the ITO film surface after
desorption treatment, measured by thermal desorption, the bonded
water peak value at near 330.degree. C. is within 50% of the bonded
water peak value of the ITO film surface before desorption
treatment.
[0046] The desorption treatment is more preferably carried out so
that in the moisture-derived spectrum of the ITO film surface after
desorption treatment, measured by thermal desorption, the bonded
water peak value at near 330.degree. C. disappears.
[0047] The organic EL element according to the second mode of the
invention is an organic EL element wherein an ITO film composed of
indium-tin oxide is formed on a substrate, and a crystalline
organic material is formed as a film on the ITO film, characterized
in that the ITO film has no bonded water peak value at near
330.degree. C. in the moisture-derived spectrum of the surface as
measured by thermal desorption.
[0048] This makes it possible to prevent current shorts and leaks
and produce satisfactory luminance characteristics in utilization
temperature ranges of organic EL elements comprising crystalline
organic materials.
[0049] According to the invention it is possible to form a film of
a crystalline organic material directly on an ITO film. That is, it
is possible to obtain a structure wherein a film of a crystalline
organic material is in direct contact with the ITO film.
[0050] According to a fourth mode of the invention, there is
provided a process for manufacture of an organic EL element wherein
an ITO film composed of indium-tin oxide is formed on a substrate,
and an organic material exhibiting crystalline properties is formed
as a film on the ITO film, characterized in that the ITO film is
heat treated at a temperature of 250.degree. C. or higher before
forming the organic material film.
[0051] This can reduce adsorption water and bonded water on the
surface of the ITO film underlying the crystalline organic
material, such that, similar to the third mode of the invention,
the crystallinity of the organic material formed as a film on the
ITO film with reduced bonded water can be increased.
[0052] According to the invention, therefore, it is possible to
prevent current shorts and leaks and produce satisfactory luminance
characteristics in utilization temperature ranges of organic EL
elements comprising crystalline organic materials.
[0053] According to a fifth mode of the invention, there is
provided a process for manufacture of an organic EL element wherein
an ITO film composed of indium-tin oxide is formed on a substrate,
and a crystalline organic material is formed as a film on the ITO
film, characterized in that after the crystalline organic material
is formed as a film on the ITO film, it is heat treated in a vacuum
or in an inert gas atmosphere to complete film formation of the
crystalline organic material.
[0054] By such heat treatment it is possible to increase the
crystallinity of the film of the crystalline organic material which
is formed on the ITO film. For example, the crystallinity increases
up to a small level of change in the diffraction peak value of
within .+-.25% of the diffraction peak value before heating, as
described by the first mode of the invention.
[0055] It is thus possible to limit the change in the crystalline
state of the organic material to a small level which does not
result in luminance reduction or luminance irregularities, or
shorts and leaks, even with use in high temperature environments.
Therefore, according to this mode of the invention as well, it is
possible to prevent current shorts and leaks and produce
satisfactory luminance characteristics in utilization temperature
ranges of organic EL elements comprising crystalline organic
materials.
[0056] Investigation by the present inventors has demonstrated that
it is preferred for the heat treatment temperature to be 70.degree.
C. or above, in order to more reliably exhibit the effect of the
fifth mode of the invention for organic EL elements used in high
temperature environments.
[0057] A film of a crystalline organic material can be formed
directly on the ITO film in all of the aforementioned manufacturing
processes. That is, the ITO film and the film of the crystalline
organic material may be formed in direct contact with each
other.
[0058] The following experimental investigation was conducted in
order to achieve the object of the second aspect of the
invention.
[0059] Specifically, an anode ITO film (transparent electrode) was
formed on a glass substrate and surface treated with an argon and
oxygen mixed plasma, after which a CuPc film was formed as the
positive hole injection layer to a film thickness of 50 nm, and a
triphenylamine tetramer as the positive hole transport layer,
dimethyl quinacridone-containing Alq3 as the luminescent layer,
Alq3 as the electron transport layer, LiF as the electron injection
layer and Al as the cathode were formed in that order, to fabricate
an organic EL element which was then sealed with a hermetic can.
This will be referred to as the trial product.
[0060] The material with the lowest Tg among the constituent
materials of the trial product is the triphenylamine tetramer, at
140.degree. C. When the trial product was operated under various
test temperature conditions, non-luminance due to shorts was
confirmed in the element at 90.degree. C. and above, while the
non-luminance was accelerated in both number and period of
generation as the temperature increased. That is, it was found that
shorts and leaks are still generated even at a temperature of
50.degree. C. below the Tg point 140.degree. C. of the
triphenylamine tetramer.
[0061] The results of the experiment indicated that the cause of
shorts or leaks upon exposure to high temperature environments is
not crystallization of the amorphous materials with a Tg, but
rather they are due mainly to transformation of the organic
thin-film composed of the sublimating material, i.e. the
luminescent layer of the trial product, composed of the dimethyl
quinacridone-added Alq3.
[0062] The experiment conducted to investigate this cause will now
be explained in detail. To complete the experiment in a shorter
time period, the trial product was allowed to stand for 2 hours at
a temperature of 120.degree. C. as an accelerated condition. This
accelerated test will hereinafter be referred to as accelerated
high-temperature standing. The voltage-luminance characteristic
(V-I characteristic) of the trial product was examined before and
after the accelerated high-temperature standing.
[0063] The results are shown in FIG. 4. In FIG. 4, the applied
voltage is shown on the horizontal axis and the current density is
shown on the vertical axis, with the value before accelerated
high-temperature standing indicated as "initial" and the value
after accelerated high-temperature standing indicated as
"120.degree. C., 2 hr".
[0064] FIG. 4 shows the normal V-I characteristic before
accelerated high-temperature standing, whereby the current density
increased as the applied voltage increased, with an applied voltage
of 4V as the threshold voltage. After accelerated high-temperature
standing, however, abnormalities were observed such as occurrence
of leaks or shorts and a large current flow with an applied voltage
below the threshold value.
[0065] For further examination of this phenomenon, the organic
thin-film composed of the sublimating material, i.e. the Alq3
electron transport layer, of the trial product was observed. The
results of observation by microscope are shown in FIGS. 5A to 5C,
as schematic cross-sections of the microscope photographs.
[0066] In FIGS. 5A to 5C, there are successively laminated a CuPc
film 30, a triphenylamine tetramer film 40 and an Alq3 film 50 made
of Alq3 on an ITO film 20, with the state after standing at
85.degree. C. shown in FIG. 5A, the state after standing at
100.degree. C. shown in FIG. 5B and the state after standing at
120.degree. C. shown in FIG. 5C. As seen in FIGS. 5A to 5C, more
voids B are seen in the Alq3 surface with increasing standing
temperature, suggesting a relationship between leaks and formation
of voids B.
[0067] The sizes of the voids B are represented by the void depth D
shown in FIG. 5C, and FIG. 6 shows a graph representing the
temperature dependency of the void depth D. The sizes of the voids
B are seen to have temperature dependency, and the threshold
temperature for formation of voids B is estimated to be 70.degree.
C. by extrapolation.
[0068] This suggests that when the organic EL element is exposed to
a high temperature environment of, for example, 70.degree. C. or
above, voids are produced in the surface of the organic thin-film
composed of the sublimating material in the element, and
irregularities are produced in the organic thin-film with these
voids, thereby resulting in shorts and leaks between the
electrodes.
[0069] Furthermore, since the voids occur in the Alq3 section, it
is surmised that voids occur because the characteristics of the
Alq3 sublimating material causes numerous gaps in the film, i.e.
results in formation of a low density film, and we therefore
concentrated on the difference in the film forming properties of
sublimating materials and evaporating materials. Here, an
evaporating material is a material which first melts to liquid when
slowly heated and then converts from the liquid (melt) to a
gas.
[0070] The difference in film formation between sublimating
materials and evaporating materials will now be explained based on
the experimental data obtained by the present inventors.
[0071] After treating the surface of ITO (transparent electrode)
film formed on a glass substrate with an argon and oxygen mixed
plasma, a CuPc film was formed to a film thickness of 50 nm, and
then a triphenylamine tetramer film was formed to a thickness of 40
nm thereover as a positive hole transport layer.
[0072] Next, Alq3 was formed to a thickness of 40 nm as a
luminescent layer of a sublimating material, on the positive hole
transport layer. This will be referred to as the sublimating
element. An adamantane derivative having the chemical structure
shown below was also formed to a thickness of 40 nm on a separate
positive hole transport layer, as a luminescent layer of a
evaporating material. This will be referred to as the volatile
element. 1
[0073] Also, the surface of the CuPc film on the ITO film and the
surfaces of the luminescent layers on the sublimating element and
the volatile element were observed by microscope. The results are
shown in FIGS. 7A to 7C. FIG. 7A shows the surface of the CuPc film
on the ITO film, FIG. 7B shows the surface of the luminescent layer
of the evaporating material on the volatile element, and FIG. 7C
shows the surface of the luminescent layer of the sublimating
material on the sublimating element, in schematic form based on the
microscope photographs.
[0074] As seen in FIGS. 7A to 7C, the irregularities of the
underlying CuPc film were very clearly reflected with the
luminescent layer composed of the volatile adamantine derivative
material, but absolutely none of the irregularities of the
underlying film were observed with the luminescent layer composed
of the sublimating Alq3 material.
[0075] In other words, it is thought that since the evaporating
material covers the underlying film in a manner which follows the
irregularities, the shapes of the irregularities remain, whereas
the sublimating material does not follow the irregularities of the
underlying film and therefore the shape of the underlying film does
not remain.
[0076] The reason for this is believed to be as follows. the
evaporating material melts and intermolecular interaction is
severed, such that it attaches to the substrate as very small
particles. On the other hand, the sublimating material does not
undergo severing of the intermolecular interaction, and therefore
attaches to the substrate as relatively large clusters.
Consequently, the sublimating material forms voids in the film and
tends to create low-density regions.
[0077] When an element having voids or low-density regions formed
in the film is heated, densification proceeds due to the thermal
activation energy. As a result, it is surmised, the transformation
or void formation shown in FIG. 11 occurs, resulting in a shorter
distance between electrodes at the sections of void formation in
the film, such that the electric field condenses at the irregular
sections of the film created by the voids, eventually producing
leaks and shorts.
[0078] This phenomenon where the sublimating material film does not
follow the irregularities of the underlying film is believed to
occur in the same manner for particle or ITO film irregularities as
well as for CuPc film irregularities. The problem definitely occurs
during the manufacturing the steps in the case of particles.
[0079] The present inventors therefore devised the concept that in
order to prevent leaks and shorts which occur when the element is
exposed to high temperature, for an organic EL element having an
organic thin-film comprising a luminescent layer between a pair of
electrodes, it is necessary to form the organic thin-film using
only a evaporating material and to form the film densely with a
high degree of coverage. The present invention has been
accomplished on the basis of this concept.
[0080] Thus according to a sixth mode of the invention, there is
provided an organic EL element comprising organic thin-films
including a luminescent layer between a pair of electrodes,
characterized in that all of the organic materials composing the
organic thin-films are volatile during formation of the films by
vacuum vapor deposition.
[0081] According to the invention, all of the organic materials
composing the organic thin-films are evaporating materials which
are volatile during film formation by vacuum vapor deposition, and
therefore the aforementioned generation of voids in
high-temperature environments can be prevented to allow suitable
prevention of current leaks and shorts.
[0082] As a result of further investigation, it was found that
although current leaks and shorts are adequately prevented with
accelerated high-temperature standing in the sixth mode of the
invention, a phenomenon occurs whereby the voltage-current
characteristic (V-I characteristic) of the element in a
high-temperature environment such as with accelerated
high-temperature standing shifts toward the high voltage end, as
shown in FIG. 8. This is the same thing as reduced luminance with
operation at the same voltage. This also sometimes occurs partially
in the element, in which case luminance irregularities are
perceived.
[0083] The cause of this shift phenomenon is believed to be as
follows. The film of a sublimating material which has low density
regions has a buffering property against thermal stress, i.e. the
ability to readily absorb thermal stress, which is generated during
heating, but a film composed of a evaporating material with a high
density has a low buffering property and tends to transmit thermal
stress.
[0084] Stated differently, the concept is that a sublimating
material film is relatively soft and resistant to deformation under
thermal stress, whereas a evaporating material film is relatively
hard and tends to deform under thermal stress. It is therefore
thought that the thermal stress is propagated to the entire
element, producing deformation in all of the constituent films,
thereby reducing positive hole and electron transport properties
and injection properties, and resulting in the aforementioned shift
phenomenon.
[0085] Even if all of the organic thin-films of the organic EL
element are formed of evaporating materials, an amine-based
material or the like is used as the evaporating material for the
positive hole transport layer on the anode. This positive hole
transport layer is usually highly amorphous such that no chemical
bond is formed with the electrode composed of the underlying ITO
film, and with the low cohesion between them, the interface between
the ITO film and the positive hole transport layer peels due to the
thermal stress. As a result, it is believed, it becomes difficult
to inject a charge and the aforementioned shift phenomenon
occurs.
[0086] When this shift phenomenon occurs after shipping, it is
perceived in the form of reduced luminance or luminance
irregularities, and it is therefore often effective to perform heat
treatment prior to shipping in order to produce the shift
beforehand.
[0087] As a different strategy against this shift phenomenon, the
present inventors investigated improving the cohesion between the
ITO film and the positive hole transport layer. A seventh mode of
the invention was thus devised with the purpose of suppressing the
shift phenomenon while achieving the objects of the invention
stated above.
[0088] Specifically, according to a seventh mode of the invention,
there is provided an organic EL element comprising an organic
thin-films including a luminescent layer between a pair of
electrodes, characterized in that one of the pair of electrodes is
an ITO film composed of indium-tin oxide as a transparent
electrode, with a crystalline organic metal complex film formed on
the ITO film and one or more organic thin-films formed on the
organic metal complex film, and in that all of the organic
materials composing the organic thin-films are volatile during
formation of the film by vacuum vapor deposition.
[0089] Like the sixth mode of the invention, since all of the
organic materials composing the organic thin-films are evaporating
materials, it is possible to adequately prevent current leaks and
shorts.
[0090] When an organic metal complex film is formed on the ITO film
and an organic thin-film is formed thereover, the organic thin-film
on the ITO film side is the positive hole transport layer. Thus,
the organic metal complex film becomes sandwiched between the ITO
film and the positive hole transport layer.
[0091] When an organic metal complex film is formed on the ITO
film, the film is epitaxially formed by vacuum vapor deposition or
the like, and therefore high cohesion is achieved with the ITO
film. Also, the organic metal complex film has high crystallinity
or molecular polarity, such that the cohesion is also high with the
molecular polar positive hole transport material.
[0092] Thus, sandwiching of an organic metal complex film between
the ITO film and positive hole transport layer is highly effective
for improving the cohesion between the ITO film and the positive
hole transport layer. It is thereby possible to inhibit peeling
between the ITO film and the positive hole transport layer by
heating, and thus maintain charge injection properties between the
two layers.
[0093] According to the invention, therefore, it is possible to
adequately prevent current shorts and leaks while inhibiting shift
of the V-I characteristic of the element toward the high voltage
end in high-temperature environments.
[0094] The organic metal complex film preferably undergoes minimal
change in crystallinity in high-temperature environments. This will
make it possible to reduce irregularities produced in the organic
metal complex film by changes in crystallinity in high-temperature
environments, thereby greatly minimizing changes in the positive
hole injection property from the ITO film into the positive hole
transport layer through the organic metal complex film.
[0095] From this viewpoint, further experimental investigation was
conducted toward minimizing change in the positive hole injection
property from the ITO film into the positive hole transport layer
through the organic metal complex film and reducing to a minimum
the shift in the V-I characteristic of the element toward the high
voltage end in high-temperature environments, in order to determine
the preferred degree of change in crystallinity of the organic
metal complex film.
[0096] The eight mode of the invention was accomplished based on
the aforementioned sixth and seventh modes, and also based on the
results of investigating change in crystallinity of the organic
metal complex film and, like the first aspect of the invention, it
is characterized in that among the values of the diffraction peaks
appearing with X-ray diffraction of the organic metal complex film,
the change in the diffraction peak value produced by heating in the
utilization temperature range of the organic EL element is within
.+-.25% of the diffraction peak value before heating.
[0097] If, among the values of the diffraction peaks appearing with
X-ray diffraction of the crystalline organic metal complex film,
the change in the diffraction peak value produced by heating in the
utilization temperature range of the organic EL element is within
.+-.25% of the diffraction peak value before heating, it is
possible to limit to a higher degree the shift in the V-I
characteristic of the element toward the high-voltage end in
high-temperature environments, in addition to the effect of the
seventh mode of the invention.
[0098] Copper phthalocyanine may be used as the organic complex
film.
[0099] The effect of the organic EL element described above can be
adequately exhibited even when applied for exposure to
high-temperature environments of 70.degree. C. and above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0100] FIGS. 1A and 1B are illustrations showing the assumed
structure of the amorphous state and the assumed structure of the
crystalline state of a conventional organic material.
[0101] FIG. 2 is a graph showing the shift in V-I characteristic
before and after high-temperature standing at 100.degree. C., 12
hr, for the trial product obtained by the present inventors.
[0102] FIG. 3 is a graph showing the X-ray diffraction spectrum of
a CuPc film before and after high-temperature standing at
100.degree. C., 12 hr, for the trial product obtained by the
present inventors.
[0103] FIG. 4 is graph showing V-I characteristics before and after
high-temperature standing at 100.degree. C., 12 hr, for the trial
product obtained by the present inventors.
[0104] FIGS. 5A to 5C are schematic cross-sectional views of the
above-mentioned trial product, based on results of observing the
organic thin-film composed of Alq3 as a sublimating material.
[0105] FIG. 6 is a graph showing the temperature dependency of the
depth of the voids shown in FIG. 5C.
[0106] FIGS. 7A to 7C are illustrations based on the result of
microscope observation of the surface of a CuPc film on ITO film,
the surface of a luminescent layer composed of a sublimating
material and the surface of a luminescent layer composed of a
evaporating material.
[0107] FIG. 8 is a graph showing the shift phenomenon of the
voltage-current characteristic in a high-temperature environment
for an organic EL element according to sixth mode of the
invention.
[0108] FIG. 9 is a schematic cross-sectional view of an organic EL
element according to an mode of the invention.
[0109] FIG. 10 is a graph showing the TDS spectrum directly after
formation of an ITO film on a glass substrate.
[0110] FIG. 11 is a graph showing the TDS spectrum of the surface
of an ITO film upon heat treatment at a temperature of 300.degree.
C. in a nitrogen atmosphere.
[0111] FIG. 12 is a graph showing the TDS spectrum of the surface
of an ITO film upon heat treatment at a temperature of 300.degree.
C. in a vacuum.
[0112] FIG. 13 is a graph showing the relationship between the CuPc
crystalline peak ratio and the V-I characteristic shift before and
after high-temperature standing at 120.degree. C., 2 hr.
[0113] FIG. 14 is a table showing the data for FIG. 13.
[0114] FIG. 15 is a schematic cross-sectional view of an organic EL
element according to the sixth embodiment of the invention.
[0115] FIG. 16 is a schematic cross-sectional view of an organic EL
element according to the seventh embodiment of the invention, which
solves the voltage-current characteristic shift phenomenon shown in
FIG. 13.
DETAILED DESCRIPTION OF THE INVENTION
[0116] The present invention will now be explained with reference
to the attached drawings. FIG. 9 is a schematic view of the
cross-sectional structure of an organic EL element S1 according to
the first aspect of the invention.
[0117] An anode 20 composed of indium-tin oxide (hereinafter
referred to as ITO) is formed on a substrate 10 composed of
transparent glass. On the anode 20 there is formed a positive hole
injection layer 30 composed of CuPc (copper phthalocyanine) as the
crystalline organic material, and on the positive hole injection
layer 30 there is formed a positive hole transport layer 40
composed of a triphenylamine tetramer.
[0118] On the positive hole transport layer 40 there is formed a
luminescent layer 50 composed of dimethylquinacridone-added Alq3,
and on the luminescent layer 50 there is formed an electron
transport layer 60 composed of Alq3 (8-hydroxyquinoline-aluminum
complex). On the electron transport layer 60 there is further
formed an electron injection layer 70 composed of LiF and thereover
a cathode 80 composed of Al.
[0119] Thus, there are laminated between the pair of electrodes 20
and 80, the positive hole injection layer 30 composed of a
crystalline organic material, and the layers composed of amorphous
organic materials, i.e., the positive hole transport layer 40, the
luminescent layer 50, the electron transport layer 60 and the
electron injection layer 70, to form an organic EL element S1.
[0120] In this organic EL element S1, an electric field is applied
between the anode 20 and the cathode 80, such that positive holes
are injected and transported from the anode 20 and electrons from
the cathode 80 into the luminescent layer 50, with rebonding of the
electrons and positive holes occurring in the luminescent layer 50
so that the luminescent layer 50 emits light by the bonding energy.
The light emission is visible from the substrate 10 side, for
example.
[0121] The organic EL element S1 of the invention is employed for a
vehicle display or the like, and may be used at a temperature of
about -40.degree. C. to 120.degree. C.
[0122] When the positive hole injection layer 30 composed of CuPc,
i.e. the CuPc film 30, as the crystalline organic material was
measured by X-ray diffraction, the diffraction peak of the (200)
plane of the CuPc film 30 parallel to the substrate 10 is the
diffraction peak indicating the crystallinity of the CuPc film 30.
This diffraction peak corresponds to the peak generated at
2.theta.=6.68.degree. shown in FIG. 3, and will hereinafter be
referred to as the CuPc crystallinity peak.
[0123] According to this embodiment, the change in the CuPc
crystallinity peak value, or the peak integral value, due to heat
in the range of utilization temperature of the organic EL element
S1 (for example, -40.degree. C. to 120.degree. C.), from the CuPc
crystallinity peak value (2.theta.=6.68.degree.) is within .+-.25%
of the CuPc crystallinity peak value before heating.
[0124] (First Mode)
[0125] By minimizing the change in the CuPc crystallinity peak
value between before and after heating to within .+-.25%, it is
possible to limit the change in the crystalline state of the
organic material to a level which does not result in luminance
reduction or luminance irregularities, or shorts and leaks, even
with use in high-temperature environments. It is preferably kept to
within .+-.15% more preferably within .+-.10% of the peak
value.
[0126] Thus, according to the organic EL element S1 of this
embodiment, it is possible to prevent current leaks and shorts and
thus realize satisfactory luminance properties, in the range of
utilization temperature.
[0127] Incidentally, as mentioned above, an organic EL element with
a conventional crystalline organic material exhibited a significant
change in the CuPc crystalline peak value after heating of 1.5
times the CuPc crystalline peak value before heating, as shown in
FIG. 3, and thus produces shorts and leaks, while exhibiting
reduced luminance or luminance irregularities due to a major shift
in the V-I characteristic, as shown in FIG. 2.
[0128] A specific example of a manufacturing process for an organic
EL element S1 according to this embodiment will now be explained in
a non-limitative manner.
[0129] [First Manufacturing Process] (Third Mode)
[0130] On a glass substrate 10 there is formed an ITO film 20 by
sputtering or the like, as an anode, and the surface of the ITO
film 20 is exposed to ultraviolet irradiation while heating to
300.degree. C. This treatment of the ITO film 20 will be referred
to as UV-300.degree. C. treatment.
[0131] After the UV-300.degree. C. treatment, a CuPc film 30 is
formed to a thickness of 10 nm as a positive hole injection layer
by vapor deposition at a material heating temperature of
420.degree. C. This is followed by successive formation of a
positive hole transport layer 40 composed of a triphenylamine
tetramer film, a luminescent layer 50 composed of Alq3 (host
material)+dimethylquinacridone (guest material), an electron
transport layer 60 composed of Alq3, an electron injection layer 70
composed of LiF and a cathode 80 composed of Al. This completes the
organic EL element S1 shown in FIG. 9.
[0132] The CuPc film 30 formed on this UV-300.degree. C. treated
ITO film 20 was subjected to X-ray diffraction analysis before and
after accelerated high-temperature standing (120.degree. C., 2
hr).
[0133] As a result, the ratio of the CuPc crystallinity peak
produced at 2.theta.=6.680 after standing treatment compared to the
value before standing treatment was confirmed to be a very small
value of 1.02. That is, it is exhibits of that the crystalinity of
the CuPc film 30 as film formed in accordance with the present
invention is very high and stable.
[0134] A sealed element obtained by sealing the prepared organic EL
element in a hermetic can was examined under accelerated
high-temperature standing at 120.degree. C., 2 hr, and this
revealed virtually no shift in the V-I characteristic, or any
luminance reduction or luminance irregularities. Also, no current
shorts or leaks were produced.
[0135] That is, the organic EL element S1 produced by the first
manufacturing process prevented from current shorts and leaks and
exhibited satisfactory luminance properties in the range of
utilization temperature.
[0136] This effect was due to the UV-300.degree. C. treatment in
the manufacturing process. The mechanism of the effect achieved by
this treatment will now be explained in detail.
[0137] It is known that efficient injection of positive holes from
the ITO film 20 as the anode into the CuPc film 30 as the positive
hole injection layer requires cleaning treatment of the ITO film
surface. However, evaluation is generally made based on the
ionization potential (Ip) of the ITO film surface after cleaning,
and the present inventors had judged that an Ip of 5.5 eV or lower
of the ITO film 20 immediately after cleaning treatment of the ITO
film 20 was satisfactory in light of the positive hole injection
property.
[0138] However, according to investigation by the present
inventors, the Ip after 5 minutes of plasma cleaning treatment with
argon and oxygen (1:1 proportion) was 5.45 eV, the Ip after 20
minutes of UV treatment alone was 5.5 eV, and the Ip after 20
minutes of UV-300.degree. C. treatment was 5.46 eV, and therefore
no significant difference was found.
[0139] Nevertheless, only the element subjected to UV-300.degree.
C. treatment exhibited no V-I characteristic shift, i.e. no
luminance reduction, luminance irregularities, shorts or leaks,
after high-temperature standing. That is, the crystallinity of the
crystalline organic material formed as a film on the ITO is not
determined solely by the Ip, but is also dependent on other
factors.
[0140] Since the heat treatment was implicated in the
UV-300.degree. C. treatment, the moisture on the ITO surface was
examined by measuring the moisture release, or moisture desorption,
from the surface of the ITO film 20 at various temperatures,
according to the thermal desorption method (hereinafter referred to
as "TDS method").
[0141] FIG. 10 shows the measurement results by the TDS method
immediately after forming the ITO film 20 on the glass substrate
10. The TDS spectrum was obtained by measuring the spectrum with a
molecular weight, or M/z according to the TDS method, of 18 for
H.sub.2O or 17 for OH.
[0142] The results shown in FIG. 10 show a moisture desorption peak
at 70.degree. C. and 330.degree. C. The former peak is believed to
be the portion of moisture present as adsorption water actually
physically adhering to the surface of the ITO film 20, and the
latter is believed to be the moisture present as bonded water
chemically bonded to ITO on the surface of the ITO film 20. It was
thus conjectured that the bonded water on the surface of the ITO
film 20 might be one factor determining the crystallinity during
formation of the CuPc film 30.
[0143] FIG. 11 shows the TDS spectrum for an actual case with the
ITO film 20 formed on the glass substrate 10, and heat treated at a
temperature of 300.degree. C. in a nitrogen atmosphere. The peak
value at 330.degree. C. is reduced by about 50% with respect to the
TDS spectrum shown in FIG. 10 for a case without heat treatment.
FIG. 12 shows the TDS spectrum for a case with heat treatment at
300.degree. C. in a vacuum. In this case, almost no peak is
discernible at 330.degree. C.
[0144] These organic EL elements SI prepared with heat treatment of
an ITO film at 300.degree. C. either in a nitrogen atmosphere or in
a vacuum, sealed in hermetic cans as sealed elements, were examined
under accelerated high-temperature standing conditions of
120.degree. C., 2 hr, and as a result virtually no V-I
characteristic shift or luminance reduction or irregularities were
found. Also, no current shorts or leaks were produced.
[0145] This indicates that in a process for manufacture of an
organic EL element comprising an ITO film 20 formed on a substrate
10 and a crystalline organic material, such as a CuPc film 30,
formed on the ITO film 20, it is effective to subject the surface
of the ITO film 20 to bonded water desorption treatment before
forming the organic material film.
[0146] Since it is thereby possible to reduce bonding water as well
as adsorption water on the surface of the ITO film 20 underlying
the crystalline organic material, the crystallinity of the organic
material film may be increased, to prevent current shorts and leaks
and realize satisfactory luminance properties in the range of
utilization temperature.
[0147] As shown in FIG. 11, the bonded water peak at near
330.degree. C. in the TDS spectrum derived from moisture on the
surface of the ITO film 20 after desorption treatment is preferably
within 50% of the bonded water peak value of the ITO film 20 before
desorption treatment.
[0148] Also, as shown in FIG. 12, it is more preferred for there to
be no bonded water peak at near 330.degree. C. in the TDS spectrum
derived from moisture on the surface of the ITO film 20 after
desorption treatment. (Second mode)
[0149] In order to reduce the bonded water on the surface of the
ITO film 20 to within about 50%, the heat treatment temperature of
the ITO film 20 is preferably 250.degree. C. or higher before
forming the crystalline organic material of the CuPc film 30 on the
ITO film 20.
[0150] According to this mode, there is provided an organic EL
element S1 having an ITO film 20 composed of indium-tin oxide
formed on a substrate 10, with a crystalline organic material for a
CuPc film 30 formed as a film on the ITO film 20, characterized in
that the ITO film 20 has no bonded water peak at near 330.degree.
C. in the moisture-derived spectrum measured by the TDS method on
the surface.
[0151] This makes it possible to prevent current shorts and leaks
and realize satisfactory luminance properties in the range of
utilization temperature.
[0152] [Second Manufacturing Process]
[0153] In the first manufacturing process described above, heat
treatment of the ITO film 20 underlying the CuPc film 30 and others
made of a crystalline organic material increases the crystallinity
of the CuPc film 30.
[0154] The second manufacturing process focuses on the material
temperature during formation of the CuPc film 30. On a glass
substrate 10 there is formed an ITO film 20 by sputtering or the
like, as an anode. The ITO deposited glass substrate is subjected
to the argon/oxygen plasma cleaning treatment described above, and
then a CuPc film 30 is formed on the ITO film 20 as a positive hole
injection layer by vapor deposition, at a material heating
temperature of 420.degree. C. This will be referred to as the
"420.degree. C. deposited product".
[0155] Separately, a glass substrate with an ITO film is subjected
to the argon/oxygen plasma cleaning treatment described above, and
then a CuPc film 30 is formed on the ITO film 20 as a positive hole
injection layer by vapor deposition, at a material heating
temperature of 520.degree. C. This will be referred to as the
"520.degree. C. deposited product".
[0156] Next, the 420.degree. C. deposited product and the
520.degree. C. deposited product are used to manufacture organic EL
elements S1 by successively forming on each CuPc film 30 a positive
hole transport layer 40, a luminescent layer 50, an electron
transport layer 60, an electron injection layer 70 and a cathode
80, in the same manner as the first manufacturing process.
[0157] The ratio of the CuPc crystalline peak value of the
420.degree. C. deposited product and the 520.degree. C. deposited
product before and after the aforementioned accelerated
high-temperature standing (120.degree. C., 2 hr) was determined by
X-ray diffraction analysis. The V-I characteristic shift before and
after the accelerated high-temperature standing was also examined
after sealing in a hermetic can.
[0158] In the case of the 420.degree. deposited product, the CuPc
crystallinity peak ratio was approximately 1.5 (see FIG. 3) and the
shift was approximately 3 V (see FIG. 2), whereas in the case of
the 520.degree. C. deposited product, the CuPc crystallinity peak
ratio was approximately 1.15 and the shift was satisfactory, at
approximately 1 V.
[0159] Another example of the second manufacturing process will now
be illustrated. On a glass substrate 10 there is formed an ITO film
20 by sputtering or the like, as an anode. The surface of the ITO
film 20 of the ITO deposited glass substrate is exposed to
ultraviolet irradiation while heating at 150.degree. C.
(UV-150.degree. C. treatment).
[0160] Next, a CuPc film 30 is formed on the ITO film 20 as a
positive hole injection layer by vapor deposition, at a material
heating temperature of 520.degree. C. An organic EL element S1 is
then manufactured by successively forming on the CuPc film 30 a
positive hole transport layer 40, a luminescent layer 50, an
electron transport layer 60, an electron injection layer 70 and a
cathode 80, in the same manner as the first manufacturing
process.
[0161] The organic EL element S1 obtained in this manner is sealed
in a hermetic can to obtain a sealed element, and as a result of
analyzing this after accelerated high-temperature standing at
120.degree. C., 2 hr, the V-I characteristic shift was small and no
luminance reduction or luminance irregularities were found. Also,
no current shorts or leaks were produced. The V-I characteristic
shift before and after accelerated high-temperature standing was
1.2 V, and the CuPc crystallinity peak ratio was 1.21.
[0162] As an additional example, the surface of the ITO film 20 of
an ITO deposited glass substrate formed in the same manner as above
was exposed to ultraviolet irradiation at ordinary temperature
(UV-room temperature treatment). A CuPc film 30 was then formed as
a positive hole injection layer on the ITO film 20 by vapor
deposition with a material heating temperature of 520.degree.
C.
[0163] The organic EL element S1 obtained by successively forming
the upper layers 40-80 in the same manner as above was sealed in a
hermetic can to obtain a sealed element and subjected to conditions
of accelerated high-temperature standing at 120.degree. C., 2
hr.
[0164] As a result, the V-I characteristic shift was small and no
luminance reduction or luminance irregularities were found. Also,
no current shorts or leaks were produced. The V-I characteristic
shift before and after accelerated high-temperature standing for
this example was 1.5 V, and the CuPc crystallinity peak ratio was
1.21.
[0165] According to the second manufacturing process in which the
depositing temperature for the CuPc film 30 is 520.degree. C.,
there is obtained an organic EL element which satisfies the
required properties regardless of the surface treatment method for
the ITO film 20 and even without UV-300.degree. C. treatment which
was employed in the aforementioned first manufacturing process.
This indicates that the ITO surface is not the only factor
affecting the CuPc crystallinity, since it also depends on the
depositing method.
[0166] In other words, an organic EL element S1 manufactured by the
second manufacturing process also has very highly stable
crystallinity of the CuPc film 30, and prevents current shorts or
leaks and exhibits satisfactory luminance properties in the range
of utilization temperature.
[0167] [Third Manufacturing Process] (Fifth Mode)
[0168] In this third manufacturing process, a CuPc film 30 as the
crystalline organic material film is formed on an ITO film 20 and
heat treated in a vacuum or in an inert gas atmosphere to complete
formation of the CuPc film 30.
[0169] That is, an ITO film 20 is formed on a glass substrate 10,
and then a CuPc film 30 is formed thereover by vapor deposition or
the like, the CuPc film 30 is heat treated, and then a positive
hole transport layer 40, a luminescent layer 50, an electron
transport layer 60, an electron injection layer 70 and a cathode 80
are successively formed thereover to manufacture an organic EL
element S1. A concrete example of the third manufacturing process
carried out by the present inventors will now be explained.
[0170] (Concrete Example 1 of Third Manufacturing process)
[0171] On a glass substrate 10 there was formed an ITO film 20 by
sputtering, as an anode. The surface of the ITO film 20 of the ITO
deposited glass substrate was subjected to argon/oxygen plasma
cleaning treatment.
[0172] Next, a CuPc film 30 was formed as the positive hole
injection layer on the ITO film 20 to a film thickness of 10 nm at
a material heating temperature of 420.degree. C. by vapor
deposition. It was then heat treated for 20 minutes at a substrate
temperature of 150.degree. C. in a vacuum.
[0173] This was followed by successive formation of a positive hole
transport layer 40 composed of a triphenylamine tetramer film, a
luminescent layer 50 composed of Alq3 (host
material)+dimethylquinacridon- e (guest material), an electron
transport layer 60 composed of Alq3, an electron injection layer 70
composed of LiF and a cathode 80 composed of Al.
[0174] The organic EL element obtained in this manner was then
sealed with a hermetic can to obtain a sealed element, and as a
result of analyzing this under accelerated high-temperature
standing conditions at 120.degree. C., 2 hr, the V-I characteristic
shift was small and no luminance reduction or luminance
irregularities were found. Also, no current shorts or leaks were
produced. The V-I characteristic shift before and after accelerated
high-temperature standing was 1.0 V, and the CuPc crystallinity
peak ratio was 1.13.
[0175] (Concrete Example 2 of Third Manufacturing Process)
[0176] In the same manner as Concrete example 1, an ITO film 20 was
formed on a glass substrate 10 and subjected to argon/oxygen plasma
cleaning treatment, after which a CuPc film 30 was formed to a film
thickness of 10 nm at a material heating temperature of 420.degree.
C. by vapor deposition. For this example, it was then heat treated
for 20 minutes at a substrate temperature of 100.degree. C. in a
vacuum.
[0177] Next, the upper layers 40-80 were successively formed in the
same manner as above to obtain an organic EL element S1 which was
then sealed in a hermetic can to obtain a sealed element and
analyzed under accelerated high-temperature standing conditions at
120.degree. C., 2 hr.
[0178] As a result, the V-I characteristic shift was small and no
luminance reduction or luminance irregularities were found. Also,
no current shorts or leaks were produced. In this example, the V-I
characteristic shift before and after accelerated high-temperature
standing was 1.0 V, and the CuPc crystallinity peak ratio was
1.15.
[0179] (Concrete Example 3 of Third Manufacturing Process)
[0180] In the same manner as Concrete example 1, an ITO film 20 was
formed on a glass substrate 10 and subjected to argon/oxygen plasma
cleaning treatment, after which a CuPc film 30 was formed to a film
thickness of 10 nm at a material heating temperature of 420.degree.
C. by vapor deposition. For this example, it was then heat treated
for 20 minutes at a substrate temperature of 70.degree. C. in a
vacuum.
[0181] Next, the upper layers 40-80 were successively formed in the
same manner as above to obtain an organic EL element S1 which was
then sealed in a hermetic can to obtain a sealed element and
analyzed under accelerated high-temperature standing conditions at
120.degree. C., 2 hr.
[0182] As a result, the V-I characteristic shift was small and no
luminance reduction or luminance irregularities were found. Also,
no current shorts or leaks were produced. In this example, the V-I
characteristic shift before and after accelerated high-temperature
standing was 1.6 V, and the CuPc crystallinity peak ratio was
1.25.
[0183] Thus, according to the third manufacturing process,
formation of the CuPc film 30 as the crystalline organic material
film on the ITO film 20 is followed by heat treatment in a vacuum
or in an inert gas atmosphere, such that the crystallinity of the
formed CuPc film 30 is increased by the heat treatment.
[0184] The reason for this is believed to be that the molecules in
the CuPc film 30 vibrate due to the activation energy by heating,
and move into a solid state with a more stable phase. This is
closely correlated with temperature. Investigation by the present
inventors has confirmed that subsequent heat treatment, rather than
prior heat treatment, results in a larger crystallinity peak of the
CuPc film 30 by X-ray diffraction, and thus improved
crystallinity.
[0185] In Concrete examples 1 to 3 above, a CuPc film 30 heating
temperature of 70.degree. C. or above in the third manufacturing
process can adequately increase the crystallinity of the CuPc film
30 and reduce the change in the crystallinity state of the CuPc
film 30 to a level which produces no luminance reduction, luminance
irregularities, shorts or leaks, even with use in high-temperature
environments.
[0186] Thus, the third manufacturing process also can realize an
organic EL element comprising a crystalline organic material,
wherein current shorts or leaks are prevented and satisfactory
luminance properties are exhibited in the range of utilization
temperature. Incidentally, since the heat treatment is pointless if
the heated film peels back off, the upper limit for the heat
treatment temperature must naturally be no greater than the
volatilization temperature or sublimation temperature of the film
to be heated.
[0187] (Summary of Relationship Between Organic Material Layer
Crystallinity and EL Properties)
[0188] The relationship between the CuPc crystallinity peak ratio
before and after accelerated high-temperature standing (120.degree.
C., 2 hr) and the degree of V-I characteristic shift will now be
summarized. This relationship is shown in the graph in FIG. 13, the
data for which are shown in FIG. 14. In FIGS. 13 and 14, the CuPc
crystallinity peak ratio is indicated by "X-ray diffraction peak
ratio", and the V-I characteristic shift is indicated by "V-I
shift".
[0189] As mentioned above, the CuPc crystallinity peak ratio is the
ratio of the CuPc crystallinity peak integral value after
accelerated high-temperature standing with respect to the CuPc
crystallinity peak integral value before accelerated
high-temperature standing, with a ratio of greater than 1
indicating higher crystallinity of the CuPc film by accelerated
high-temperature standing, and a ratio of less than 1 indicating
lower crystallinity.
[0190] The V-I characteristic shift is the degree of shift, in
volts, of the V-I characteristic after accelerated high-temperature
standing based on the V-I characteristic before accelerated
high-temperature standing. Specifically, since the CuPc
crystallinity peak ratio showed a major increase in crystallinity
of 1.5 in FIG. 3, the V-I characteristic shift was large at about 3
V as shown in FIG. 2.
[0191] In FIG. 14, the "Conditions before cleaning" is the
conditions for cleaning of the ITO film 20 formed on the glass
substrate 10, before forming the CuPc film 30, "Plasma" is the
argon/oxygen plasma cleaning treatment, "UV300.degree. C." and
"UV250.degree. C.", "UV150.degree. C." and "UV room temperature"
respectively indicate treatment carried out with ultraviolet
irradiation at those temperatures. Also, "CuPc layer heating
temperature" in FIG. 14 is the material temperature during
formation of the CuPc film.
[0192] "Deposition method" in FIG. 14 indicates the conditions
employed when conducting the third manufacturing method. "Heating
after deposition (150.degree. C. vacuum)", Heating after deposition
(100.degree. C. vacuum) and "Post-film heating (70.degree. C.
vacuum)" each indicate Concrete example 1, Concrete example 2 and
Concrete example 3 of the third manufacturing method.
[0193] The symbols ".circleincircle. ", ".largecircle." and "X"
represent "very excellent", "excellent" and "bad",
respectively.
[0194] No obvious luminance irregularities could be found in the
organic EL element with an X-ray diffraction peak ratio of 1.25 and
a V-I shift of 1.6 V, shown in the graph in FIG. 13. However,
obvious luminance irregularities were found in the organic EL
element with an X-ray diffraction peak ratio of about 1.3 and a V-I
characteristic shift of about 2.0 V.
[0195] This suggests that, when considering product quality, the
threshold value for the X-ray diffraction peak ratio is
approximately 1.25 and the threshold value for the V-I shift is
approximately 1.6 V.
[0196] In other words, as explained above, if the degree of change
in the CuPc crystallinity peak value due to heating within the
range of utilization temperature of the organic EL element S1 (for
example, -40.degree. C. to 120.degree. C.) is kept to within
.+-.25% of the CuPc crystallinity peak before heating, then it is
possible to prevent current shorts and leaks and realize
satisfactory luminance properties of a practical level in the range
of utilization temperature. (First mode)
[0197] FIG. 14 shows that if the heat treatment temperature for the
ITO film 20 is 250.degree. C. or higher, it is possible to
effectively reduce both adsorption and bonded water on the surface
of the underlying ITO film, to allow formation of an organic
material film with increased crystallinity to achieve the effect
described above. (Fourth mode).
[0198] Furthermore, the change in the CuPc crystallinity peak value
by heating may be in a decreasing instead of an increasing
direction. That is, the change may be no more than +25% or no less
than -25% with respect to the CuPc crystallinity peak before
heating, and as shown in FIG. 13, the X-ray diffraction peak ratio
may even be 0.75 or greater. For example, luminance irregularities
were observed with a V-I shift of 2.8 V, resulting when the X-ray
diffraction peak ratio was 0.68.
[0199] The construction and manufacturing method suitable for the
CuPc film, i.e. the positive hole injection layer 30, may also be
suitably applied for the positive hole transport layer 40,
luminescent layer 50, electron transport layer 60 and electron
injection layer 70, when the organic materials composing them are
also crystalline.
[0200] Embodiments of the second aspect of the invention
illustrated in the drawings will now be explained. FIG. 15 shows
the schematic cross-sectional construction of an organic EL element
S1 according to the second aspect of the invention.
[0201] An anode 20 composed of indium-tin oxide (hereinafter,
"ITO") is formed on a substrate 10 made of transparent glass. A
positive hole transport layer 40 composed of triphenylamine
tetramer is formed on the anode 20.
[0202] On the positive hole transport layer 40 there is formed a
luminescent layer 50 composed of the above-mentioned
dimethylquinacridone-added adamantane derivative, and on the
luminescent layer 50 there is formed an electron transport layer 60
composed of the adamantane derivative. On the electron transport
layer 60 there is further formed an electron injection layer 70
composed of LiF, over which there is formed a cathode 80 composed
of Al.
[0203] In this organic EL element S1, an electric field is applied
between the anode 20 and the cathode 80, such that positive holes
are injected and transported from the anode 20 and electrons from
the cathode 80 into the luminescent layer 50, with rebonding of the
electrons and positive holes occurring in the luminescent layer 50
so that the luminescent layer 50 emits light by the bonding energy.
The light emission is visible from the substrate 10 side, for
example.
[0204] The organic EL element S1 according to this embodiment is
employed for a vehicle display or the like, and may be used at a
temperature of about -40.degree. C. to 120.degree. C.
[0205] In this organic EL element S1, all of the organic materials
composing the organic thin-films 40-60 formed between the pair of
electrodes 20, 80 are evaporating materials, i.e. materials with
volatility during film formation by vacuum vapor deposition.
[0206] Since all of the organic materials composing the organic
thin-films in this organic EL element S1 are evaporating materials
with volatility during film formation by vacuum vapor deposition,
it is possible to prevent the problem of voids produced in
high-temperature environments in conventional EL elements employing
sublimating materials, as described above, to thereby adequately
prevent generation of current leaks or shorts.
[0207] The manufacturing method for this organic EL element S1 will
now be explained. on a glass substrate 10 there is formed an ITO
film 20 by sputtering or the like, as an anode, and the surface of
the ITO film 20 is treated by an argon/oxygen mixed plasma, after
which a triphenylamine tetramer film is formed to a thickness of 40
nm thereover as a positive hole transport layer 40, by vacuum vapor
deposition.
[0208] Next, a luminescent layer 50 comprising dimethylquinacridone
added at 1% to the above-mentioned adamantane derivative is formed
to a thickness of 20 nm by vacuum vapor deposition. On this there
are formed an electron transport layer 60 composed of the
adamantane derivative and an electron injection layer 70 composed
of LiF by vacuum vapor deposition, and then a cathode 80 film
composed of Al is formed. This completes the organic EL element S1
shown in FIG. 15. The organic EL element S1 is then sealed in a
hermetic can (not shown).
[0209] A completed organic EL element S1 was subjected to
accelerated high-temperature standing, i.e. allowed to stand for 2
hours at a temperature of 120.degree. C., voids were prevented in
the organic thin-films 40-60, and current leaks and shorts were
adequately prevented in the element S1.
[0210] When the organic EL element S1 of this embodiment was
examined, it was found to exhibit the phenomenon whereby the
voltage-current characteristic (V-I characteristic) of the element
S1 shifted toward the high voltage end by the accelerated
high-temperature standing, as shown in FIG. 8. In FIG. 8, a high
voltage end shift of about 2 V is shown.
[0211] As a solution to this shift phenomenon, FIG. 16 shows a
schematic cross-sectional structure of an organic EL element S2
according to another embodiment of the invention. In FIG. 16, the
organic EL element S1 shown in FIG. 15 has a CuPc (copper
phthalocyanine) film 30 as a crystalline organic metal complex
film, sandwiched between the ITO film 20 as the anode and the
positive hole transport layer 40. The CuPc film 30 functions as the
positive hole injection layer.
[0212] Similar to the element S1 shown in FIG. 15, in this method
an ITO film 20 is formed on a substrate 10 and the ITO film 20 is
surface treated with argon/oxygen mixed plasma, after which a CuPc
film 30 is formed to a thickness of 50 nm as a positive hole
injection layer by vacuum vapor deposition, and there are
successively formed a positive hole transport layer 40, a
luminescent layer 50, an electron transport layer 60, an electron
injection layer 70 and a cathode 8, in the same manner as
above.
[0213] When the organic EL element S2 according to this embodiment
was subjected to the above-mentioned accelerated high-temperature
standing, voids were prevented in the organic thin-films 40-60 as
with the organic EL element S1 shown in FIG. 15, and no current
leaks and shorts occurred in the element S2. In addition, shift of
the V-I characteristic toward the high voltage end with the organic
EL element S2 of this embodiment was drastically reduced even with
accelerated high-temperature standing.
[0214] [Examining X-Ray Diffraction Peaks of Organic Metal Complex
Film]
[0215] In a preferred mode of the organic EL element S2 of this
embodiment shown in FIG. 16, the change in the diffraction peak
value of the CuPc film 30, as the organic metal complex film, by
X-ray diffraction by heating in the range of utilization
temperature of the organic EL element S2 (for example, -40.degree.
C. to 120.degree. C.) is preferably within .+-.25% of the
diffraction peak value before heating.
[0216] If the degree of change in the diffraction peak value of the
CuPc film 20, as the crystalline organic metal complex film, by
X-ray diffraction due to heating in the range of utilization
temperature of the organic EL element S2 is kept to within .+-.25%
of the diffraction peak value before heating, then it is possible
to inhibit to a higher degree the shift in the V-I characteristic
of the element toward the high voltage end in high-temperature
environments, in addition to the effect of the aforementioned
embodiment.
[0217] Since the support for the preferred mode of the organic EL
element S2 according to this embodiment is the same as explained
for the first aspect of the invention, a detailed description
thereof is not considered necessary.
* * * * *