U.S. patent application number 14/709008 was filed with the patent office on 2015-11-19 for light-emitting element, light-emitting device, display device, electronic device, and lighting device.
The applicant listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Takahiro ISHISONE, Yusuke NONAKA, Nobuharu OHSAWA, Satoshi SEO.
Application Number | 20150333283 14/709008 |
Document ID | / |
Family ID | 54361887 |
Filed Date | 2015-11-19 |
United States Patent
Application |
20150333283 |
Kind Code |
A1 |
ISHISONE; Takahiro ; et
al. |
November 19, 2015 |
LIGHT-EMITTING ELEMENT, LIGHT-EMITTING DEVICE, DISPLAY DEVICE,
ELECTRONIC DEVICE, AND LIGHTING DEVICE
Abstract
Provided is a light-emitting element which includes a first
electrode, a second electrode over the first electrode, and first
and second light-emitting layers therebetween. The first
light-emitting layer contains a first host material and a first
light-emitting material, and the second light-emitting layer
contains a second host material and a second light-emitting
material. The first light-emitting material is a fluorescent
material, and the second light-emitting material is a
phosphorescent material. The level of the lowest triplet excited
state (T.sub.1 level) of the first light-emitting material is
higher than the T.sub.1 level of the first host material. A
light-emitting device, an electronic device, and a lighting device
including the light-emitting element are further provided.
Inventors: |
ISHISONE; Takahiro; (Atsugi,
JP) ; SEO; Satoshi; (Sagamihara, JP) ; NONAKA;
Yusuke; (Atsugi, JP) ; OHSAWA; Nobuharu;
(Zama, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
54361887 |
Appl. No.: |
14/709008 |
Filed: |
May 11, 2015 |
Current U.S.
Class: |
257/89 |
Current CPC
Class: |
H01L 51/0085 20130101;
H01L 51/5016 20130101; H01L 2251/5361 20130101; H01L 2251/5376
20130101; H01L 51/5004 20130101; H01L 51/5044 20130101; H01L 51/504
20130101; H01L 51/006 20130101 |
International
Class: |
H01L 51/50 20060101
H01L051/50 |
Foreign Application Data
Date |
Code |
Application Number |
May 13, 2014 |
JP |
2014-099560 |
Nov 28, 2014 |
JP |
2014-241575 |
Claims
1. A light-emitting element comprising: a first electrode; a second
electrode over the first electrode; and a first light-emitting
layer and a second light-emitting layer between the first electrode
and the second electrode, wherein the first light-emitting layer
comprises a first host material and a first light-emitting
material, wherein the second light-emitting layer comprises a
second host material and a second light-emitting material, wherein
the first light-emitting material is a fluorescent material, and
the second light-emitting material is a phosphorescent material,
and wherein a T.sub.1 level of the first light-emitting material is
higher than a T.sub.1 level of the first host material.
2. The light-emitting element according to claim 1, wherein the
first light-emitting layer and the second light-emitting layer are
in contact with each other.
3. The light-emitting element according to claim 1, wherein the
first light-emitting layer and the second light-emitting layer are
separated from each other.
4. The light-emitting element according to claim 1, further
comprising a layer between the first light-emitting layer and the
second light-emitting layer, wherein the layer comprises a
hole-transport material and an electron-transport material.
5. The light-emitting element according to claim 1, wherein the
second light-emitting layer is located over the first
light-emitting layer.
6. A light-emitting device comprising the light-emitting element
according to claim 1.
7. A lighting device comprising the light-emitting element
according to claim 1.
8. A light-emitting element comprising: a first electrode; a second
electrode over the first electrode; a first light-emitting unit and
a second light-emitting unit between the first electrode and the
second electrode; and an interlayer between the first
light-emitting unit and the second light-emitting unit, wherein the
first light-emitting unit comprises a first light-emitting layer
and a second light-emitting layer, wherein the first light-emitting
layer comprises a first host material and a first light-emitting
material, wherein the second light-emitting layer comprises a
second host material and a second light-emitting material, wherein
the first light-emitting material is a fluorescent material and the
second light-emitting material is a phosphorescent material, and
wherein a T.sub.1 level of the first light-emitting material is
higher than a T.sub.1 level of the first host material.
9. The light-emitting element according to claim 8, wherein the
first light-emitting layer and the second light-emitting layer are
in contact with each other.
10. The light-emitting element according to claim 8, wherein the
first light-emitting layer and the second light-emitting layer are
separated from each other.
11. The light-emitting element according to claim 8, further
comprising a layer between the first light-emitting layer and the
second light-emitting layer, wherein the layer comprises a
hole-transport material and an electron-transport material.
12. The light-emitting element according to claim 8, wherein the
second light-emitting layer is located over the first
light-emitting layer.
13. The light-emitting element according to claim 8, wherein the
second light-emitting unit is located over the first light-emitting
unit.
14. The light-emitting element according to claim 8, wherein the
second light-emitting unit comprises a third light-emitting layer
which comprises a third host material and a third light-emitting
material.
15. A light-emitting device comprising the light-emitting element
according to claim 8.
16. A lighting device comprising the light-emitting element
according to claim 8.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] One embodiment of the present invention relates to a
light-emitting element, and a light-emitting device, a display
device, an electronic device, and a lighting device each including
a light-emitting element. The technical field of one embodiment of
the present invention also includes a semiconductor device
including the light-emitting element and its manufacturing
method.
[0003] 2. Description of the Related Art
[0004] A light-emitting element in which a layer containing an
organic compound is provided between a pair of electrodes and a
light-emitting device including the light-emitting element are
called an organic electroluminescent element and an organic
electroluminescent device, respectively. Organic electroluminescent
devices can be used for display devices, lighting devices, and the
like (see Patent Document 1, for example).
REFERENCE
Patent Document
[0005] [Patent Document 1] United States Patent Application
Publication No. 2012/0205632
SUMMARY OF THE INVENTION
[0006] An object of one embodiment of the present invention is to
improve the emission efficiency of a light-emitting element.
Another object of one embodiment of the present invention is to
provide a light-emitting element and a semiconductor device
including the light-emitting element. Note that the description of
these objects does not disturb the existence of other objects. In
one embodiment of the present invention, there is no need to
achieve all the objects. Other objects will be apparent from and
can be derived from the description of the specification, the
drawings, the claims, and the like.
[0007] One embodiment of the present invention is a light-emitting
element which includes a first electrode, a second electrode over
the first electrode, a first light-emitting layer, and a second
light-emitting layer. The first light-emitting layer and the second
light-emitting layer are both provided between the first electrode
and the second electrode and have regions which overlap with each
other. The first light-emitting layer contains a first host
material and a first light-emitting material, and the second
light-emitting layer contains a second host material and a second
light-emitting material. The first light-emitting material is a
fluorescent material, and the second light-emitting material is a
phosphorescent material. The level of the lowest triplet excited
state (T.sub.1 level) of the first light-emitting material is
higher than the T.sub.1 level of the first host material.
[0008] Another embodiment of the present invention is a
light-emitting element which includes a first electrode, a second
electrode over the first electrode, a first light-emitting unit,
and a second light-emitting unit. The first light-emitting unit and
the second light-emitting unit are both provided between the first
electrode and the second electrode and have regions which overlap
with each other. An interlayer is provided between the first
light-emitting unit and the second light-emitting unit. The first
light-emitting unit includes a first light-emitting layer and a
second light-emitting layer which overlap with each other, and the
second light-emitting unit includes a third light-emitting layer.
The first light-emitting layer contains a first host material and a
first light-emitting material, the second light-emitting layer
contains a second host material and a second light-emitting
material, and the third light-emitting layer contains a third host
material and a third light-emitting material. The first
light-emitting material is a fluorescent material, the second
light-emitting material is a phosphorescent material, and the third
light-emitting material is a fluorescent material or a
phosphorescent material. The T.sub.1 level of the first
light-emitting material is higher than the T.sub.1 level of the
first host material.
[0009] In this specification and the claims, a fluorescent material
refers to a material that emits light in the visible light region
when the level of the lowest singlet excited state (S.sub.1 level)
relaxes to the ground state. A phosphorescent material refers to a
material that emits light in the visible light region at room
temperature when the T.sub.1 level relaxes to the ground state.
That is, a phosphorescent material refers to a material that can
convert triplet excitation energy into visible light.
[0010] In the first light-emitting layer, the first host material
is present in the highest proportion by weight; in the second
light-emitting layer, the second host material; and in the third
light-emitting layer, the third host material.
[0011] The T.sub.1 level of the second host material is preferably
higher than that of the first host material.
[0012] A region of the first light-emitting layer and a region of
the second light-emitting layer may be in contact with each
other.
[0013] The first light-emitting layer and the second light-emitting
layer may be separated from each other. In this case, a layer in
which a hole-transport material and an electron-transport material
are mixed or a layer containing a bipolar material may be provided
between the first light-emitting layer and the second
light-emitting layer. The hole-transport material or the
electron-transport material may be the same as the second host
material. The bipolar material may be the same as the second host
material.
[0014] The second light-emitting layer may be provided over the
first light-emitting layer; alternatively, the first light-emitting
layer may be provided over the second light-emitting layer.
[0015] The second light-emitting unit may be provided over the
first light-emitting unit; alternatively, the first light-emitting
unit may be provided over the second light-emitting unit.
[0016] One embodiment of the present invention is a light-emitting
device which includes a plurality of light-emitting elements having
the above structure and a transistor or a substrate.
[0017] One embodiment of the present invention is an electronic
device which includes the light-emitting device having the above
structure.
[0018] One embodiment of the present invention is a lighting device
which includes the light-emitting device having the above structure
and a housing or a support.
[0019] In this specification and the claims, a light-emitting
device refers to an image display device or a light source used for
an image display device. Furthermore, the category of the
light-emitting device includes a module in which a connector such
as a flexible printed circuit (FPC) or a tape carrier package (TCP)
is attached to a light-emitting device, a module in which a printed
wiring board is provided on the tip of a TCP, and a module in which
an integrated circuit (IC) is directly mounted on a light-emitting
device by a chip on glass (COG) method.
[0020] According to one embodiment of the present invention, a
light-emitting element, a light-emitting device, an electronic
device, or a lighting device having high efficiency can be
provided. Note that the description of these effects does not
disturb the existence of other effects. One embodiment of the
present invention does not necessarily achieve all the effects.
Other effects will be apparent from and can be derived from the
description of the specification, the drawings, the claims, and the
like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1A to 1D illustrate structural examples of a
light-emitting element of one embodiment of the present
invention.
[0022] FIG. 2 illustrates a light emission mechanism of a
light-emitting element of one embodiment of the present
invention.
[0023] FIGS. 3A to 3C illustrate structural examples of a
light-emitting element of one embodiment of the present
invention.
[0024] FIGS. 4A and 4B illustrate a structural example of a
light-emitting device of one embodiment of the present
invention.
[0025] FIGS. 5A and 5B illustrate a structural example of a
light-emitting device of one embodiment of the present
invention.
[0026] FIGS. 6A to 6D illustrate examples of an electronic device
of one embodiment of the present invention.
[0027] FIGS. 7A and 7B illustrate an example of an electronic
device of one embodiment of the present invention.
[0028] FIG. 8 illustrates examples of a lighting device of one
embodiment of the present invention.
[0029] FIGS. 9A and 9B are schematic views of a light-emitting
element 1 and a light-emitting element 2 in Example 1 and 2.
[0030] FIG. 10 shows a voltage--luminance curve of the
light-emitting element 1 in Example 1.
[0031] FIG. 11 shows a luminance--current efficiency curve and a
luminance--external quantum efficiency curve of the light-emitting
element 1 in Example 1.
[0032] FIG. 12 shows a luminance--power efficiency curve of the
light-emitting element 1 in Example 1.
[0033] FIG. 13 shows an electroluminescence spectrum of the
light-emitting element 1 in Example 1.
[0034] FIG. 14 shows a voltage--luminance curve of the
light-emitting element 2 in Example 2.
[0035] FIG. 15 shows a luminance--current efficiency curve and a
luminance--external quantum efficiency curve of the light-emitting
element 2 in Example 2.
[0036] FIG. 16 shows a luminance--power efficiency curve of the
light-emitting element 2 in Example 2.
[0037] FIG. 17 shows an electroluminescence spectrum of the
light-emitting element 2 in Example 2.
[0038] FIGS. 18A and 18B are schematic views of light-emitting
elements 3 to 6 (LEEs 3 to 6) in Reference Example 1.
[0039] FIG. 19 shows luminance--current efficiency curves of the
light-emitting elements 3 to 6 in Reference Example 1.
[0040] FIG. 20 shows voltage--luminance curves of the
light-emitting elements 3 to 6 in Reference Example 1.
[0041] FIG. 21 shows luminance--external quantum efficiency curves
of the light-emitting elements 3 to 6 in Reference Example 1.
[0042] FIG. 22 shows electroluminescence spectra of the
light-emitting elements 3 to 6 in Reference Example 1.
[0043] FIG. 23 shows results of reliability tests of the
light-emitting elements 3 to 6 in Reference Example 1.
DETAILED DESCRIPTION OF THE INVENTION
[0044] Hereinafter, embodiments of the present invention will be
described in detail with reference to the accompanying drawings.
Note that the present invention is not limited to the description
below, and modes and details of thereof can be modified in various
ways without departing from the spirit and the scope the present
invention. Therefore, the present invention should not be construed
as being limited to the description in the following
embodiments.
Embodiment 1
1. Structural Example of Light-Emitting Element
[0045] A structural example of a light-emitting element of one
embodiment of the present invention will be described with
reference to FIG. 1A. The light-emitting element includes a first
electrode 100, a second electrode 102, and a first light-emitting
layer 120 and a second light-emitting layer 122 provided
therebetween. The first light-emitting layer 120 and the second
light-emitting layer 122 overlap with each other. In the following
description, the first electrode 100 serves as an anode and the
second electrode 102 serves as a cathode.
1-1. Electrode
[0046] The first electrode 100 has a function of injecting holes
into the first light-emitting layer 120 and the second
light-emitting layer 122, and the second electrode 102 has a
function of injecting electrons into the first light-emitting layer
120 and the second light-emitting layer 122. These electrodes can
be formed using a metal, an alloy, a conductive compound, a mixture
or a stack of such materials, or the like. Typical examples of the
metal are aluminum (Al) and silver (Ag); besides, a transition
metal such as tungsten, chromium, molybdenum, copper, or titanium,
an alkali metal such as lithium (Li) or cesium, or a Group 2 metal
such as calcium or magnesium (Mg) can be used. As the transition
metal, a rare earth metal may be used. An alloy containing any of
the above metals can be used, and MgAg and AlLi can be given as
examples. As the conductive compound, a metal oxide such as indium
oxide-tin oxide (indium tin oxide) can be given. It is also
possible to use an inorganic carbon-based material such as graphene
as the conductive compound. As described above, the first electrode
100 and/or the second electrode 102 may be formed by stacking two
or more of these materials.
[0047] Light emitted from the first light-emitting layer 120 and
the second light-emitting layer 122 is extracted through the first
electrode 100 and/or the second electrode 102. Therefore, at least
one of the electrodes transmits visible light. In the case where
the electrode through which light is extracted is formed using a
material with low light transmittance, such as metal or alloy, the
first electrode 100, the second electrode 102, or part thereof is
formed to a thickness that is thin enough to transmit visible
light. In this case, the specific thickness is 1 nm or more and 10
nm or less.
1-2. First Light-Emitting Layer
[0048] The first light-emitting layer 120 contains a first host
material and a first light-emitting material, and the first
light-emitting material is a fluorescent material. In the first
light-emitting layer 120, the first host material is present in the
highest proportion by weight, and the first light-emitting material
is dispersed in the first host material. The T.sub.1 level of the
first light-emitting material is higher than the T.sub.1 level of
the first host material. The S.sub.1 level of the first host
material is preferably higher than the S.sub.1 level of the first
light-emitting material. The light emission mechanism of the first
light-emitting layer 120 will be described later.
[0049] An anthracene derivative or a tetracene derivative is
preferably used as the first host material. This is because these
derivatives each have a high S.sub.1 level and a low T.sub.1 level.
Specific examples include
9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (PCzPA),
3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (PCPN)
9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (CzPA),
7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole
(cgDBCzPA),
6-[3-(9,10-diphenyl-2-anthryl)phenyl]-benzo[b]naphtho[1,2-d]furan
(2mBnfPPA), and
9-phenyl-10-{4-(9-phenyl-9H-fluoren-9-yl)biphenyl-4'-yl}anthracene
(FLPPA). Besides, 5,12-diphenyltetracene,
5,12-bis(biphenyl-2-yl)tetracene, and the like can be given.
[0050] Examples of the first light-emitting material include a
pyrene derivative, an anthracene derivative, a triphenylene
derivative, a fluorene derivative, a carbazole derivative, a
dibenzothiophene derivative, a dibenzofuran derivative, a
dibenzoquinoxaline derivative, a quinoxaline derivative, a pyridine
derivative, a pyrimidine derivative, a phenanthrene derivative, and
a naphthalene derivative. A pyrene derivative is particularly
preferable because it has a high emission quantum yield. Specific
examples of the pyrene derivative include
N,N'-bis(3-methylphenyl)-N,N'-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyre-
ne-1,6-diamine (1,6mMemFLPAPrn),
N,N'-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N'-diphenylpyrene-1,6-diam-
ine (1,6FLPAPrn),
N,N'-bis(dibenzofuran-2-yl)-N,N'-diphenylpyrene-1,6-diamine
(1,6FrAPrn), and
N,N'-bis(dibenzothiophen-2-yl)-N,N'-diphenylpyrene-1,6-diamine
(1,6ThAPrn).
1-3. Second Light-Emitting Layer
[0051] The second light-emitting layer 122 contains a second host
material and a second light-emitting material, and the second
light-emitting material is a phosphorescent material. In the second
light-emitting layer 122, the second host material is present in
the highest proportion by weight, and the second light-emitting
material is dispersed in the second host material. The T.sub.1
level of the second host material is preferably higher than the
T.sub.1 level of the second light-emitting material.
[0052] As the second light-emitting material, an iridium-,
rhodium-, or platinum-based organometallic complex or metal complex
can be used; in particular, an organoiridium complex such as an
iridium-based ortho-metalated complex is preferable. As an
ortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand,
an imidazole ligand, a pyridine ligand, a pyrimidine ligand, a
pyrazine ligand, an isoquinoline ligand, or the like can be given.
As the metal complex, a platinum complex having a porphyrin ligand
or the like can be given.
[0053] Examples of the second host material include a zinc- or
aluminum-based metal complex, an oxadiazole derivative, a triazole
derivative, a benzimidazole derivative, a quinoxaline derivative, a
dibenzoquinoxaline derivative, a dibenzothiophene derivative, a
dibenzofuran derivative, a pyrimidine derivative, a triazine
derivative, a pyridine derivative, a bipyridine derivative, and a
phenanthroline derivative. Other examples are an aromatic amine and
a carbazole derivative.
[0054] The second light-emitting layer 122 may further contain an
additive which can form an exciplex (i.e., a heteroexcimer)
together with the second host material. In this case, it is
preferable that the second host material, the additive, and the
second light-emitting material be selected so that the emission
peak of the exciplex overlaps with an adsorption band, specifically
an adsorption band on the longest wavelength side, of a triplet
metal-to-ligand charge transfer (MLCT) transition of the second
light-emitting material. This makes it possible to provide a
light-emitting element with drastically improved emission
efficiency.
[0055] There is no limitation on the emission colors of the first
light-emitting material and the second light-emitting material, and
they may be the same or different. Light emitted from the
light-emitting materials is mixed and extracted out of the element;
therefore, for example, in the case where their emission colors are
complementary colors, the light-emitting element can emit white
light. In consideration of the reliability of the light-emitting
element, the emission peak wavelength of the first light-emitting
material is preferably shorter than that of the second
light-emitting material. For example, it is preferable that the
first light-emitting material emit blue light and that the second
light-emitting material emit green, yellow, or red light.
[0056] The second light-emitting layer 122 may have a structure in
which a plurality of layers is stacked. In this case, different
structures or different materials may be used for the plurality of
layers.
[0057] Note that the first light-emitting layer 120 and the second
light-emitting layer 122 can be formed by an evaporation method
(including a vacuum evaporation method), an inkjet method, a
coating method, gravure printing, or the like.
1-4. Other Layers
[0058] As illustrated in FIG. 1A, the light-emitting element of one
embodiment of the present invention may include another layer
besides the first light-emitting layer 120 and the second
light-emitting layer 122. For example, the light-emitting element
may include a hole-injection layer, a hole-transport layer, an
electron-blocking layer, a hole-blocking layer, an
electron-transport layer, or an electron-injection layer.
Furthermore, each of these layers may be formed of a plurality of
layers. These layers can reduce a carrier injection barrier,
improve the carrier transport property, or suppress a quenching
phenomenon by an electrode, thereby contributing to an improvement
in emission efficiency or a reduction in drive voltage. The
light-emitting element in FIG. 1A includes a hole-injection layer
124, a hole-transport layer 126, an electron-transport layer 128,
and an electron-injection layer 130 besides the first
light-emitting layer 120 and the second light-emitting layer 122.
In this specification and the claims, all layers provided between
the first electrode 100 and the second electrode 102 is
collectively defined as an EL layer. For example, in FIG. 1A, a
stack including the hole-injection layer 124, the hole-transport
layer 126, the first light-emitting layer 120, the second
light-emitting layer 122, the electron-transport layer 128, and the
electron-injection layer 130 corresponds to an EL layer.
1-4-1. Hole-Injection Layer
[0059] The hole-injection layer 124 has a function of reducing a
barrier for hole injection from the first electrode 100 to promote
hole injection and is formed using a transition metal oxide, a
phthalocyanine derivative, or an aromatic amine, for example. As
the transition metal oxide, molybdenum oxide, vanadium oxide,
ruthenium oxide, tungsten oxide, manganese oxide, or the like can
be given. As the phthalocyanine derivative, phthalocyanine, a metal
phthalocyanine, or the like can be given. As the aromatic amine, a
benzidine derivative, a phenylenediamine derivative, or the like
can be given. It is also possible to use a high molecular compound
such as polythiophene or polyaniline; a typical example thereof is
poly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which is a
doped polythiophene.
[0060] As the hole-injection layer 124, a mixed layer containing a
hole-transport material and a material having a property of
accepting electrons from the hole-transport material can also be
used. Alternatively, a stack of a layer containing a material
having an electron accepting property and a layer containing a
hole-transport material may also be used. Electric charge can be
transferred between these materials in the presence or absence of
an electric field. As examples of the material having an electron
accepting property, organic acceptors such as a quinodimethane
derivative, a chloranil derivative, and a hexaazatriphenylene
derivative can be given. A specific example is a material having an
electron-withdrawing group (a halogen group or a cyano group), such
as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane
(abbreviation: F.sub.4-TCNQ), chloranil, or
2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene
(abbreviation: HAT-CN). Alternatively, a transition metal oxide
such as an oxide of a metal from Group 4 to Group 8 can also be
used. Specifically, vanadium oxide, niobium oxide, tantalum oxide,
chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide,
rhenium oxide, or the like can be used. In particular, molybdenum
oxide is preferable because it is stable in the air, has a low
hygroscopic property, and is easily handled.
[0061] A material having a property of transporting more holes than
electrons can be used as the hole-transport material, and a
material having a hole mobility of 1.times.10.sup.-6 cm.sup.2/Vs or
higher is preferable. Specifically, an aromatic amine, a carbazole
derivative, an aromatic hydrocarbon, a stilbene derivative, or the
like can be used. Furthermore, the hole-transport material may be a
high molecular compound.
1-4-2. Hole-Transport Layer
[0062] The hole-transport layer 126 is a layer containing a
hole-transport material and can be formed using any of the
materials given as examples of the material of the hole-injection
layer 124. In order that the hole-transport layer 126 has a
function of transporting holes injected into the hole-injection
layer 124 to the first light-emitting layer 120, the highest
occupied molecular orbital (HOMO) level of the hole-transport layer
126 is preferably equal or close to the HOMO level of the
hole-injection layer 124.
1-4-3. Electron-Injection Layer
[0063] The electron-injection layer 130 has a function of reducing
a barrier for electron injection from the second electrode 102 to
promote electron injection and can be formed using a Group 1 metal
or a Group 2 metal, or an oxide, a halide, or a carbonate of the
metal, for example. Alternatively, a mixed layer containing an
electron-transport material (described later) and a material having
a property of donating electrons to the electron-transport material
can also be used. As the material having an electron donating
property, a Group 1 metal, a Group 2 metal, an oxide of the metal,
or the like can be given.
1-4-4. Electron-Transport Layer
[0064] The electron-transport layer 128 has a function of
transporting, to the second light-emitting layer 122, electrons
injected from the second electrode 102 through the
electron-injection layer 130. A material having a property of
transporting more electrons than holes can be used as an
electron-transport material, and a material having an electron
mobility of 1.times.10.sup.-6 cm.sup.2/Vs or higher is preferable.
Specific examples include a metal complex having a quinoline
ligand, a benzoquinoline ligand, an oxazole ligand, or a thiazole
ligand; an oxadiazole derivative; a triazole derivative; a
phenanthroline derivative; a pyridine derivative; and a bipyridine
derivative.
[0065] Note that the hole-injection layer 124, the hole-transport
layer 126, the electron-injection layer 130, and the
electron-transport layer 128 described above can each be formed by
an evaporation method (including a vacuum evaporation method), an
inkjet method, a coating method, a gravure printing method, or the
like.
[0066] Besides the above-mentioned materials, an inorganic compound
or a high molecular compound (e.g., an oligomer, a dendrimer, or a
polymer) may be used for the hole-injection layer 124, the
hole-transport layer 126, the electron-injection layer 130, and the
electron-transport layer 128.
2. Another Structural Example of Light-Emitting Element
[0067] In the light-emitting element in FIG. 1A, the second
light-emitting layer 122 is provided over the first light-emitting
layer 120; however, one embodiment of the present invention is not
limited to this structure. As illustrated in FIG. 1B, the first
light-emitting layer 120 may be positioned over the second
light-emitting layer 122.
3. Light Emission Mechanism of First Light-Emitting Layer 120
[0068] FIG. 2 illustrates a correlation between energy levels of
the first host material and the first light-emitting material. The
symbols in FIG. 2 denote as follows:
[0069] S.sub.0(h): the level of the ground state of the first host
material;
[0070] S.sub.0(g): the level of the ground state of the first
light-emitting material;
[0071] S.sub.1(h): the level of the lowest singlet excited state of
the first host material;
[0072] S.sub.1(g): the level of the lowest singlet excited state of
the first light-emitting material;
[0073] T.sub.1(h): the level of the lowest triplet excited state of
the first host material; and
[0074] T.sub.1(g): the level of the lowest triplet excited state of
the first light-emitting material.
[0075] As described above, the first light-emitting layer 120
contains the first host material and the first light-emitting
material whose T.sub.1 level is higher than that of the first host
material. That is, T.sub.1(g) is higher than T.sub.1(h).
Furthermore, in the first light-emitting layer 120, the first host
material is present in a larger amount than the first
light-emitting material. FIG. 2 shows the energy levels of two
molecules of the first host material and one molecule of the first
light-emitting material.
[0076] In the first light-emitting layer 120, excited states are
formed by carrier recombination. Since the first host material is
present in a larger amount than the first light-emitting material,
most of the excited states are excited states of the first host
material. Here, the ratio of the singlet excited state to the
triplet excited state produced by carrier recombination
(hereinafter, exciton generation probability) is approximately 1:3.
That is, the singlet excited state with S.sub.1(h) and the triplet
excited state with T.sub.1(h) are generated in the proportion of
approximately 1 to 3.
[0077] In the case where S.sub.1(g) is lower than S.sub.1(h), light
emission can be obtained in the following manner: energy is rapidly
transferred from the first host material in the singlet excited
state to the first light-emitting material (singlet energy
transfer: Process (a)), a singlet excited state of the first
light-emitting material is produced, and the singlet excited state
relaxes to the ground state through a radiative process (Process
(b)). Here, if T.sub.1(h) is higher than T.sub.1(g), energy is
rapidly transferred from the first host material in the triplet
excited state to the first light-emitting material (triplet energy
transfer), so that a triplet excited state of the first
light-emitting material is formed. However, since the first
light-emitting material is a fluorescent material, its triplet
excited state does not provide light emission in the visible light
region. Consequently, the triplet excited state of the first host
material cannot be utilized for light emission. Thus, if T.sub.1(h)
is higher than T.sub.1(g), only the light emission through Process
(a) can be used; as a result, no more than approximately 25% of
injected carriers can be used for light emission.
[0078] On the other hand, in the light-emitting element of one
embodiment of the present invention, T.sub.1(g) is higher than
T.sub.1(h) as shown in FIG. 2. Therefore, triplet energy transfer
(Process (c)) from the first host material to the first
light-emitting material does not occur or is negligible. In this
case, as for the triplet state of the first host material, a
relaxation process (Process (d)) to the ground state through a
non-radiative process competes against a process through which a
singlet excited state of the first host material is generated by
triplet-triplet annihilation (TTA). From the singlet excited state
of the first host material generated by TTA, energy is transferred
to the lower level S.sub.1(g) (Process (e)), whereby a singlet
excited state of the first light-emitting material can be
produced.
[0079] In summary, in the first light-emitting layer 120, the
singlet excited state of the first light-emitting material is
formed through the following two processes: (1) Process (a) through
which energy is transferred from the singlet excited state of the
first host material generated directly by carrier recombination and
(2) Process (e) through which energy is transferred from the
singlet excited state of the first host material generated by TTA.
As described above, if T.sub.1(g) is lower than T.sub.1(h), only
the former process can be utilized, and thus, the efficiency of the
light-emitting element is limited by the exciton generation
probability. In contrast, in the case where T.sub.1(g) is higher
than T.sub.1(h) as in the light-emitting element of one embodiment
of the present invention, both the processes can be utilized;
therefore, an emission efficiency exceeding the exciton generation
probability can be achieved, and a light-emitting element with high
efficiency can be provided.
[0080] The structures described in this embodiment can be used in
appropriate combination with any of the structures described in the
other embodiments.
Embodiment 2
[0081] In this embodiment, a light-emitting element of one
embodiment of the present invention will be described with
reference to FIG. 1C. The light-emitting element in this embodiment
is different from the light-emitting element in Embodiment 1 in
that a separation layer 135 is provided between the first
light-emitting layer 120 and the second light-emitting layer 122.
The separation layer 135 is in contact with the first
light-emitting layer 120 and the second light-emitting layer 122.
The structures of the other layers are similar to those in
Embodiment 1; therefore, description thereof is omitted.
[0082] The separation layer 135 is provided to prevent energy
transfer by the Dexter mechanism (particularly triplet energy
transfer) from the second host material in an excited state or the
second light-emitting material in an excited state which is
generated in the second light-emitting layer 122 to the first host
material or the first light-emitting material in the first
light-emitting layer 120. Therefore, the thickness of the
separation layer may be approximately several nanometers,
specifically 0.1 nm or more and 20 nm or less, 1 nm or more and 10
nm or less, or 1 nm or more and 5 nm or less.
[0083] The separation layer 135 may contain a single material or
both a hole-transport material and an electron-transport material.
In the case of a single material, a bipolar material may be used.
The bipolar material here refers to a material in which the ratio
between the electron mobility and the hole mobility is 100 or less.
As a material contained in the separation layer 135, the
hole-transport material, the electron-transport material, or the
like given as an example in Embodiment 1 can be used. Furthermore,
at least one of materials contained in the separation layer 135 may
be the same as the second host material. This facilitates the
manufacture of the light-emitting element and reduces the drive
voltage.
[0084] Alternatively, at least one of materials contained in the
separation layer 135 may have a higher T.sub.1 level than the
second host material.
[0085] The recombination region can be adjusted by adjusting the
mixed ratio of the hole-transport material and the
electron-transport material in the separation layer 135, whereby
the emission color can be controlled. For example, in the case
where the first electrode 100 and the second electrode 102 serve as
an anode and a cathode, respectively, the recombination region can
be shifted from the first electrode 100 side to the second
electrode 102 side by increasing the proportion of the
hole-transport material in the separation layer 135. As a result,
the contribution of the second light-emitting layer 122 to light
emission can be increased. In contrast, by increasing the
proportion of the electron-transport material in the separation
layer 135, the recombination region can be shifted from the second
electrode 102 side to the first electrode 100 side, so that the
contribution of the first light-emitting layer 120 to light
emission can be increased. In the case where the first
light-emitting layer 120 and the second light-emitting layer 122
have different emission colors, the emission color of the
light-emitting element as a whole can be changed by adjusting the
recombination region.
[0086] The hole-transport material and the electron-transport
material may form an exciplex in the separation layer 135, which
effectively prevents exciton diffusion. Specifically, energy
transfer from the second host material in an excited state or the
second light-emitting material in an excited state to the first
host material or the first light-emitting material can be
prevented.
[0087] As in the light-emitting element described in Embodiment 1,
the first light-emitting layer 120 may be positioned over the
second light-emitting layer 122 as illustrated in FIG. 1D. In this
case, the second light-emitting layer 122 is provided over the
hole-transport layer 126, and the first light-emitting layer 120 is
provided over the second light-emitting layer 122 with the
separation layer 135 interposed therebetween.
[0088] The structures described in this embodiment can be used in
appropriate combination with any of the structures described in the
other embodiments.
Embodiment 3
[0089] In this embodiment, a light-emitting element of one
embodiment of the present invention will be described with
reference to FIG. 3A.
1. Structural Example of Light-Emitting Element
[0090] As illustrated in FIG. 3A, the light-emitting element in
this embodiment includes the first electrode 100, the second
electrode 102, and a first light-emitting unit 140-1 and a second
light-emitting unit 140-2 provided therebetween. The first
light-emitting unit 140-1 and the second light-emitting unit 140-2
overlap with each other with an interlayer 150 provided
therebetween. In the following description, the first electrode 100
serves as an anode and the second electrode 102 serves as a
cathode. Components denoted by the same reference numerals or names
as those in Embodiments 1 and 2, such as the first electrode 100
and the second electrode 102, are similar to those in Embodiments 1
and 2; therefore, detailed description thereof is omitted.
1-1. First Light-Emitting Unit
[0091] The first light-emitting unit 140-1 includes the first
light-emitting layer 120 and the second light-emitting layer 122.
The structures and materials of these layers are similar to those
in Embodiment 1. Therefore, although the second light-emitting
layer 122 is provided over the first light-emitting layer 120 in
the light-emitting element in the FIG. 3A, the first light-emitting
layer 120 may be provided over the second light-emitting layer 122.
As illustrated in FIG. 3A, the hole-injection layer 124, a
hole-transport layer 126-1, and an electron-transport layer 128-1
may be further provided. As these layers, layers similar to the
hole-injection layer 124, the hole-transport layer 126, and the
electron-transport layer 128 described in Embodiment 1 can be used.
Although not illustrated, the separation layer 135 may be provided
between the first light-emitting layer 120 and the second
light-emitting layer 122 as described in Embodiment 2.
1-2. Interlayer
[0092] The interlayer 150 has a function of injecting electrons
into the first light-emitting unit 140-1 and injecting holes into
the second light-emitting unit 140-2 when a voltage is applied
between the first electrode 100 and the second electrode 102. In
addition, it is preferable that the interlayer 150 be capable of
transmitting visible light and have a visible light transmittance
of 40% or higher. Here, the interlayer 150 includes a first layer
150-1 and a second layer 150-2. The first layer 150-1 is provided
on the first light-emitting unit 140-1 side, and the second layer
150-2 is provided on the second light-emitting unit 140-2 side.
[0093] The first layer 150-1 can be formed using a Group 1 metal or
a Group 2 metal, or a compound thereof (e.g., an oxide, a halide,
or a carbonate), for example. Alternatively, a mixed layer
containing the electron-transport material described in Embodiment
1 and a material having a property of donating electrons to the
electron-transport material can also be used.
[0094] As the second layer 150-2, a layer containing the transition
metal oxide described in Embodiment 1 can be used. It is also
possible to use a mixed layer containing a hole-transport material
and a material having a property of accepting electrons from the
hole-transport material or a stack of a layer containing a material
having an electron accepting property and a layer containing a
hole-transport material. Specifically, the mixed layer or the stack
which is described in Embodiment 1 and can be used as the
hole-injection layer 124 can be used.
[0095] Although not illustrated, a buffer layer may be provided
between the first layer 150-1 and the second layer 150-2. The
buffer layer can prevent a material of the first layer 150-1 and a
material of the second layer 150-2 from reacting with each other at
the interface. The buffer layer contains an electron-transport
material, examples of which include a perylene derivative and a
nitrogen-containing condensed aromatic compound.
[0096] The interlayer 150 can be formed by an evaporation method
(including a vacuum evaporation method), an inkjet method, a
coating method, a gravure printing method, or the like.
1-3. Second Light-Emitting Unit
[0097] The second light-emitting unit 140-2 includes a third
light-emitting layer 132. The third light-emitting layer 132
contains a third host material and a third light-emitting material,
and the third light-emitting material is a fluorescent material or
a phosphorescent material. In the third light-emitting layer 132,
the third host material is present in the highest proportion by
weight, and the third light-emitting material is dispersed in the
third host material. As the third host material, a material similar
to the first host material or the second host material described in
Embodiment 1 can be used. The third host material may be the same
as or different from the first host material or the second host
material. In the case where a fluorescent material is used as the
third light-emitting material, the S.sub.1 level of the third host
material is preferably higher than that of the third light-emitting
material. In the case where a phosphorescent material is used as
the third light-emitting material, on the other hand, the T.sub.1
level of the third host material is preferably higher than that of
the third light-emitting material. As the third light-emitting
material, a material similar to the first light-emitting material
or the second light-emitting material described in Embodiment 1 can
be used.
[0098] The third light-emitting material may be the same as or
different from the first light-emitting material or the second
light-emitting material. For example, the first light-emitting
material, the second light-emitting material, and the third
light-emitting material are used to provide light in the three
primary colors of red, blue, and green, whereby white light with
high color rendering properties can be extracted from the
light-emitting element.
[0099] In the light-emitting element in FIG. 3A, the second
light-emitting unit 140-2 further includes a hole-transport layer
126-2, an electron-transport layer 128-2, and the
electron-injection layer 130. Layers similar to those in Embodiment
1 can be used as these layers.
[0100] Although having a higher drive voltage than the
light-emitting elements described in Embodiments 1 and 2, the
light-emitting element described in this embodiment can have a
current efficiency which is twice or more that of the
light-emitting elements described in Embodiments 1 and 2 at
substantially the same current density; thus, a light-emitting
element with high efficiency can be achieved.
2. Another Structural Example of Light-Emitting Element
[0101] In the light-emitting element in FIG. 3A, the light-emitting
unit (the first light-emitting unit 140-1) including the first
light-emitting layer 120 and the second light-emitting layer 122 is
formed on the first electrode 100 side; however, as illustrated in
FIG. 3B, the first light-emitting unit 140-1 may be formed on the
second electrode 102 side. Also in this case, the separation layer
135 may be provided between the first light-emitting layer 120 and
the second light-emitting layer 122 as in Embodiment 2.
[0102] Although the light-emitting elements each including two
light-emitting units are described so far with reference to FIGS.
3A and 3B, embodiments of the present invention also include a
light-emitting element illustrated in FIG. 3C in which n (n is an
integer of 3 or more) light-emitting units (140-1 to 140-n) are
stacked. In this case, interlayers (150(1) to 150(n-1)) are
provided between the respective adjacent light-emitting units. In
addition, at least one of the n light-emitting units has a
structure similar to that of the first light-emitting unit, and at
least another one of the n light-emitting units has a structure
similar to that of the second light-emitting unit.
[0103] The structures described in this embodiment can be used in
appropriate combination with any of the structures described in the
other embodiments.
Embodiment 4
[0104] In this embodiment, as an example of a light-emitting device
including the light-emitting element of one embodiment of the
present invention, an active matrix light-emitting device will be
described with reference to FIGS. 4A and 4B. FIG. 4A is a top view
of the light-emitting device, and FIG. 4B is a cross-sectional view
taken along line A-A' in FIG. 4A.
[0105] As illustrated in FIGS. 4A and 4B, the light-emitting device
includes a source side driver circuit 403, a pixel portion 402, and
gate side driver circuits 404a and 404b over an element substrate
401. Reference numeral 406 denotes a sealing substrate, and
reference numeral 405 denotes a sealant. A region 418 is surrounded
by the sealant 405. As the element substrate 401 and the sealing
substrate 406, a glass substrate, a quartz substrate, or a flexible
substrate formed of fiber reinforced plastic (FRP), poly(vinyl
fluoride) (PVF), a polyester, an acrylic resin, or the like can be
used. A wiring 407 is a lead wiring for receiving a variety of
signals from an FPC 408 and transmitting them to the source side
driver circuit 403 and the gate side driver circuits 404a and 404b.
A printed wiring board (PWB) may be attached to the FPC.
[0106] or simplicity, FIG. 4B illustrates part of the source side
driver circuit 403 and one pixel in the pixel portion 402. As
illustrated in FIG. 4B, in the source side driver circuit 403, a
CMOS circuit in which an n-channel transistor 409 and a p-channel
transistor 410 are combined is formed; however, a circuit different
from the CMOS circuit, such as a PMOS circuit or an NMOS circuit,
may be provided. Furthermore, the source side driver circuit 403
and the gate side driver circuits 404a and 404b may be partly or
entirely formed not over the substrate but outside the substrate.
The transistors may be staggered transistors or inverted staggered
transistors. A semiconductor layer for forming the transistors may
be formed using any material as long as it exhibits semiconductor
characteristics; for example, a Group 14 element such as silicon or
germanium, a compound such as gallium arsenide or indium phosphide,
or an oxide such as zinc oxide or tin oxide can be used. As the
oxide exhibiting semiconductor characteristics (oxide
semiconductor), a composite oxide of elements selected from indium,
gallium, aluminum, zinc, and tin, or the like can be used. The
semiconductor layer may be crystalline or amorphous. Specific
examples of a crystalline semiconductor include a single crystal
semiconductor, a polycrystalline semiconductor, and a
microcrystalline semiconductor.
[0107] The pixel portion 402 includes a plurality of pixels each
including a switching transistor 411, a current controlling
transistor 412, and a first electrode 413 electrically connected to
the current controlling transistor 412. An insulator 414 is formed
to cover an end portion of the first electrode 413.
[0108] A light-emitting element 417 which has the structure of the
light-emitting element described in Embodiment 1, 2, or 3 is
provided in an opening portion of the insulator 414. That is, the
light-emitting element 417 includes the first electrode 413, an EL
layer 415, and a second electrode 416; the EL layer 415 includes at
least a first light-emitting layer and a second light-emitting
layer and may further include a third light-emitting layer. Note
that a plurality of light-emitting elements is formed in the pixel
portion 402; some of them may have a structure different from the
structures of the light-emitting elements described in Embodiments
1 to 3.
[0109] The sealing substrate 406 and the element substrate 401 are
bonded to each other by the sealant 405, and the light-emitting
element 417 is provided in the region 418. The region 418 is filled
with an inert gas or a resin and/or a drying agent. An epoxy-based
resin or glass frit is preferably used as the sealant 405.
[0110] The structures described in this embodiment can be used in
appropriate combination with any of the structures described in the
other embodiments.
Embodiment 5
[0111] In this embodiment, as an example of a light-emitting device
including the light-emitting element of one embodiment of the
present invention, a passive matrix light-emitting device will be
described with reference to FIGS. 5A and 5B. FIG. 5A is a
perspective view of the light-emitting device, and FIG. 5B is a
cross-sectional view taken along line X-Y in FIG. 5A.
[0112] The light-emitting device includes a substrate 551, a first
electrode 552, a second electrode 556, and an EL layer 555, and the
EL layer 555 includes the first light-emitting layer 120 and the
second light-emitting layer 122 described in Embodiment 1, 2, or 3.
Part of the first electrode 552 is covered with an insulating layer
553, and a partition layer 554 is provided over the insulating
layer 553. The width of the partition layer 554 increases with
distance from the substrate 551. In other words, a cross section of
the partition layer 554 in the short side direction is trapezoidal,
and the base in contact with the insulating layer 553 is shorter
than the upper side. Accordingly, a defect of the light-emitting
element due to crosstalk can be prevented.
[0113] The structures described in this embodiment can be used in
appropriate combination with any of the structures described in the
other embodiments.
Embodiment 6
[0114] In this embodiment, examples of an electronic device which
includes a light-emitting device including the light-emitting
element of one embodiment of the present invention will be
described with reference to FIGS. 6A to 6D and FIGS. 7A and 7B.
[0115] Examples of the electronic device are a television device, a
computer, a camera (a digital camera or a digital video camera), a
digital photo frame, a mobile phone, a portable information
terminal, a game machine, and an audio reproducing device. Specific
examples of these electronic devices are illustrated in FIGS. 6A to
6D.
[0116] FIG. 6A illustrates an example of a television device. In a
television device 6100, a display portion 6103 is incorporated in a
housing 6101. In the display portion 6103, a light-emitting device
including the light-emitting element described in Embodiment 1, 2,
or 3 is provided.
[0117] FIG. 6B illustrates an example of a computer. The computer
includes a main body 6201, a housing 6202, a display portion 6203,
a keyboard 6204, an external connection port 6205, a pointing
device 6206, and the like. In the display portion 6203, a
light-emitting device including the light-emitting element
described in Embodiment 1, 2, or 3 is provided.
[0118] FIG. 6C illustrates an example of a smart watch. The smart
watch includes a housing 6302, a display panel 6304, operation
buttons 6311 and 6312, a connection terminal 6313, a band 6321, a
clasp 6322, and the like. In the display panel 6304, a
light-emitting device including the light-emitting element
described in Embodiment 1, 2, or 3 is provided. Furthermore, the
display panel 6304 has a non-rectangular display region and can
display an icon 6305 indicating time, another icon 6306, and the
like.
[0119] FIG. 6D illustrates an example of a mobile phone. A mobile
phone 6400 is provided with a display portion 6402 incorporated in
a housing 6401, an operation button 6403, an external connection
port 6404, a speaker 6405, a microphone 6406, and the like. In the
display portion 6402, a light-emitting device including the
light-emitting element described in Embodiment 1, 2, or 3 is
provided. Although not illustrated, the display portion 6402 is
provided with a touch panel; when a user touches the display
portion 6402 with his or her finger or the like, the user can
operate the mobile phone 6400 or input data to the mobile phone
6400. An image sensor may be mounted on the display portion 6402 to
provide an imaging function.
[0120] FIGS. 7A and 7B illustrate an example of a foldable tablet
terminal. In FIG. 7A, the tablet terminal is open (unfolded). The
tablet terminal includes a housing 730, a display portion 731a, a
display portion 731b, a display mode switch 734, a power switch
735, a power-saving mode switch 736, a clasp 733, an operation
switch 738, and the like. In the display portion 731a and/or the
display portion 731b, a light-emitting device including the
light-emitting element described in Embodiment 1, 2, or 3 is
provided. By closing the housing 730 of the foldable tablet
terminal when not in use, the display portion 731a and the display
portion 731b can be protected.
[0121] The display portion 731a and the display portion 731b can be
partly or entirely a touch panel region 732a and a touch panel
region 732b, respectively, and a variety of operations such as data
input may be performed by touching an operation key 737 or an
operation switch 739 displayed thereon.
[0122] With the display mode switch 734, the display can be
switched between a portrait mode, a landscape mode, and the like,
and between monochrome display and color display, for example. With
the power-saving mode switch 736, the luminance of display can be
optimized in accordance with the amount of external light detected
by an optical sensor incorporated in the tablet terminal.
[0123] Although having the same display area in the example in FIG.
7A, the display portion 731a and the display portion 731b may have
different areas. In addition, the display portion 731a and the
display portion 731b may have different display specifications; for
example, one may have higher resolution than the other.
[0124] In FIG. 7B, the tablet terminal is closed (folded), and a
solar cell 750, a charge and discharge control circuit 752, a
battery 754, a DCDC converter 756, and the like are illustrated.
The solar cell 750 can supply power to the tablet terminal. Note
that the solar cell 750 may be provided on one side or both sides
of the housing 730.
[0125] In the above-described manner, electronic devices can be
obtained by the use of the light-emitting device of one embodiment
of the present invention. The light-emitting device has a
considerably wide application range and can be used for electronic
devices in a variety of fields.
[0126] The structures described in this embodiment can be used in
appropriate combination with any of the structures described in the
other embodiments.
Embodiment 7
[0127] In this embodiment, examples of a lighting device which
includes a light-emitting device including the light-emitting
element of one embodiment of the present invention will be
described with reference to FIG. 8.
[0128] FIG. 8 illustrates a lighting device 801 on the ceiling, a
lighting device 803 on the wall, a lighting device 802 on a curved
surface, and a lighting device 804 on furniture such as a table.
The lighting device 801 includes a housing 821 and a light-emitting
device 811 provided in the housing 821. The lighting device 802
includes a support 822 and a light-emitting device 812 on the
support 822. As the lighting device 803, a light-emitting device
813 is provided on the wall. The lighting device 804 includes a
support 824 and a light-emitting device 814 on the support 824. The
light-emitting element described Embodiment 1, 2, or 3 can be used
for the light-emitting devices included in these lighting
devices.
[0129] The structures described in this embodiment can be used in
appropriate combination with any of the structures described in the
other embodiments.
EXAMPLE 1
[0130] In this example, an example of fabricating a light-emitting
element of one embodiment of the present invention will be
described. FIG. 9A is a schematic view of a light-emitting element
(light-emitting element 1) fabricated in this example, Table 1
shows the detailed structure of the element, and structures and
abbreviations of compounds used here are given below.
##STR00001## ##STR00002## ##STR00003##
TABLE-US-00001 TABLE 1 Structure of Light-emitting Element 1
Reference Thickness Layer numeral (nm) Material Weight ratio Second
electrode 102 200 Al -- Electron-injection layer 130 1 LiF --
Electron-transport layer 128(2) 15 Bphen -- 128(1) 10 2mDBTBPDBq II
-- Second light-emitting layer 122(2) 20
2mDBTBPDBq-II:PCBBiF:Ir(tBuppm).sub.2(acac) 0.7:0.3:0.05 122(1) 10
2mDBTBPDBq-II:PCBBiF:Ir(tppr).sub.2(dpm) 0.5:0.5:0.05 First
light-emitting layer 120 20 cgDBCzPA:PCzPA:1,6mMemFLPAPrn
0.3:0.7:0.05 Hole-transport layer 126 20 PCPPn -- Hole-injection
layer 124 30 DBT3P-II:MoO.sub.3 2:1 First electrode 100 110 ITSO
--
1. Fabrication of Light-Emitting Element 1
[0131] Indium tin oxide containing silicon oxide (indium tin oxide
doped with SiO.sub.2: ITSO) which was formed over a glass substrate
to have a thickness of 110 nm and an area of 4 mm.sup.2 (2
mm.times.2 mm) was used as the first electrode 100. On the first
electrode 100, 1,3,5-tri(dibenzothiophen-4-yl)benzene (DBT3P-II)
and molybdenum oxide were deposited by co-evaporation in a weight
ratio of DBT3P-II:MoO.sub.3=2:1 to a thickness of 30 nm, so that
the hole-injection layer 124 was formed.
[0132] On the hole-injection layer 124,
3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (PCPPn) was
deposited by evaporation to a thickness of 20 nm, so that the
hole-transport layer 126 was formed.
[0133] On the hole-transport layer 126,
7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole
(cgDBCzPA), 9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole
(PCzPA), and
N,N'-bis(3-methylphenyl)-N,N'-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyre-
ne-1,6-diamine (1,6mMemFLPAPrn) were deposited by co-evaporation in
a weight ratio of cgDBCzPA:PCzPA:1,6mMemFLPAPrn=0.3:0.7:0.05 to a
thickness of 20 nm, so that the first light-emitting layer 120 was
formed.
[0134] On the first light-emitting layer 120,
2-[3'-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[fh]quinoxaline
(2mDBTBPDBq-II),
N-(1,1'-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)pheny-
l]-9H-fluoren-2-amine (PCBBiF), and
bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)
(Ir(tppr).sub.2(dpm)) were deposited by co-evaporation in a weight
ratio of 2mDBTBPDBq-II:PCBBiF:Ir(tppr).sub.2(dpm)=0.5:0.5:0.05 to a
thickness of 10 nm, so that a first layer 122(1) of the second
light-emitting layer 122 was formed.
[0135] On the first layer 122(1), 2mDBTBPDBq-II, PCBBiF, and
bis[2-(6-tert-butyl-4-pyrimidinyl-.kappa.N.sub.3)phenyl-.kappa.C](2,4-pen-
tanedionato-.kappa..sup.2O,O')iridium(III) (Ir(tBuppm).sub.2(acac))
were deposited by co-evaporation in a weight ratio of
2mDBTBPDBq-II:PCBBiF:Ir(tBuppm).sub.2(acac)=0.7:0.3:0.05 to a
thickness of 20 nm, so that a second layer 122(2) of the second
light-emitting layer 122 was formed.
[0136] On the second layer 122(2), 2mDBTBPDBq-II and
bathophenanthroline (Bphen) were sequentially deposited by
evaporation to a thickness of 10 nm and 15 nm, respectively, so
that electron-transport layers 128(1) and 128(2) were formed. On
the electron-transport layers 128(1) and 128(2), lithium fluoride
(LiF) was deposited by evaporation to a thickness of 1 nm to form
the electron-injection layer 130. Furthermore, aluminum (Al) was
deposited by evaporation to a thickness of 200 nm to form the
second electrode 102. For sealing, a counter glass substrate was
fixed to the glass substrate using a sealant in a nitrogen
atmosphere. In this manner, the light-emitting element 1 was
obtained.
2. Characteristics of Light-Emitting Element 1
[0137] FIG. 10 to FIG. 13 show initial characteristics of the
light-emitting element 1. As shown in FIG. 10, the light-emitting
element 1 starts emitting light at around 2.4 V and its luminance
exceeds 8000 cd/m.sup.2 at a voltage of 5.0 V, which indicates that
it can be driven at a low voltage. At 1000 cd/m.sup.2, the current
efficiency and the external quantum efficiency are 20.4 cd/A and
10.7%, respectively (see FIG. 11), and the power efficiency is 16.9
lm/W (see FIG. 12). These results reveal that the light-emitting
element 1 has high efficiency. The light-emitting element 1
contains a blue-emitting fluorescent material (1,6mMemFLPAPrn) in
the first light-emitting layer 120, a red-emitting phosphorescent
material (Ir(tppr).sub.2(dpm)) in the first layer 122(1) of the
second light-emitting layer 122, and a green-emitting
phosphorescent material (Ir(tBuppm).sub.2(acac)) in the second
layer 122(2) of the second light-emitting layer 122. FIG. 13 shows
an electroluminescence spectrum of the light-emitting element at
500 cd/m.sup.2. A peak is observed in each of the red, blue, and
green wavelength regions, which indicates that light is
concurrently emitted from these three light-emitting materials. The
chromaticity of the light emission at 1000 cd/m.sup.2 was (x,
y)=(0.38, 0.36), which suggests white light emission.
EXAMPLE 2
[0138] In this example, an example of fabricating a light-emitting
element of one embodiment of the present invention will be
described. FIG. 9B is a schematic view of a light-emitting element
(light-emitting element 2) fabricated in this example, Table 2
shows the detailed structure of the element, and structures and
abbreviations of compounds used here are given below. Note that the
structures and abbreviations of the compounds used for the
light-emitting element 1 described in Example 1 are omitted.
##STR00004##
TABLE-US-00002 TABLE 2 Structure of Light-emitting Element 2
Reference Thickness Layer numeral (nm) Material Weight ratio Second
electrode 102 200 Ag -- 1 Ag:Mg .sup. 0.6:0.2 .sup.a
Electron-injection layer 130 1 LiF -- Electron-transport layer
128(2) 15 Bphen -- 128(1) 10 2mDBTBPDBq II -- Second light-emitting
layer 122 20 2mDBTBPDBq-II:PCBBiF:Ir(ppm-dmp).sub.2(acac)
0.8:0.2:0.05 Separation layer 135 2 2mDBTBPDBq-II:PCBBiF 0.6:0.4
First light-emitting layer 120 10 PCzPA:1,6mMemFLPAPrn 1:0.05
Hole-transport layer 126 20 PCPPn -- Hole-injection layer 124 30
DBT3P-II:MoO.sub.3 2:1 First electrode 100 110 IT SO -- .sup.a
Volume ratio.
1. Fabrication of Light-Emitting Element 2
[0139] In a manner similar to that of the light-emitting element 1,
the hole-injection layer 124 and the hole-transport layer 126 were
formed over the first electrode 100, and then, PCzPA and
1,6mMemFLPAPrn were deposited by co-evaporation in a weight ratio
of PCzPA:1,6mMemFLPAPrn=1:0.05 to a thickness of 10 nm, so that the
first light-emitting layer 120 was formed.
[0140] On the first light-emitting layer 120, 2mDBTBPDBq-II and
PCBBiF were deposited by co-evaporation in a weight ratio of
2mDBTBPDBq-II:PCBBiF=0.6:0.4 to a thickness of 2 nm, so that the
separation layer 135 was formed.
[0141] On the separation layer 135, 2mDBTBPDBq-II, PCBBiF, and
bis{2-[6-(2,6-dimethylphenyl)-4-pyrimidinyl-.kappa.N3]phenyl-.kappa.C}
(2,4-pentanedionato-.kappa.O, O')iridium(III)
(Ir(ppm-dmp).sub.2(acac)) were deposited by co-evaporation in a
weight ratio of 2mDBTBPDBq-II:PCBBiF:
Ir(ppm-dmp).sub.2(acac)=0.8:0.2:0.05 to a thickness of 20 nm, so
that the second light-emitting layer 122 was formed.
[0142] On the second light-emitting layer 122, 2mDBTBPDBq-II and
Bphen were sequentially deposited by evaporation to a thickness of
10 nm and 15 nm, respectively, so that the electron-transport
layers 128(1) and 128(2) were formed. On the electron-transport
layers 128(1) and 128(2), lithium fluoride was deposited by
evaporation to a thickness of 1 nm to form the electron-injection
layer 130. Furthermore, an alloy of silver and magnesium was
deposited by co-evaporation in a volume ratio of Ag:Mg=0.6:0.2 to a
thickness of 1 nm, and silver was deposited thereon by evaporation
to a thickness of 200 nm; thus, the second electrode 102 was
formed. Lastly, sealing was performed in a manner similar to that
of the light-emitting element 1. In this manner, the light-emitting
element 2 was obtained.
2. Characteristics of Light-Emitting Element 2
[0143] FIG. 14 to FIG. 17 show initial characteristics of the
light-emitting element 2. As shown in FIG. 14, the light-emitting
element 2 starts to emit light at around 2.6 V and its luminance
exceeds 10000 cd/m.sup.2 at a voltage of 4.3 V, which indicates
that it can be driven at a low voltage. At 1000 cd/m.sup.2, the
current efficiency and the external quantum efficiency are 55.3
cd/A and 17.2%, respectively (see FIG. 15), and the power
efficiency is 52.6 lm/W (see FIG. 16). These results reveal that
the light-emitting element 2 has high efficiency. The
light-emitting element 2 contains a blue-emitting fluorescent
material (1,6mMemFLPAPrn) in the first light-emitting layer 120 and
a yellow-emitting phosphorescent material (Ir(ppm-dmp).sub.2(acac))
in the second light-emitting layer 122. FIG. 17 shows an
electroluminescence spectrum of the light-emitting element 2 at
1000 cd/m.sup.2. A peak is observed in each of the blue and yellow
wavelength regions, which indicates that light is concurrently
emitted from these two light-emitting materials. The chromaticity
of the light emission at 1000 cd/m.sup.2 was (x, y)=(0.41,
0.49).
[0144] As demonstrated above, the light-emitting elements of this
example show a high current efficiency and can be operated at a low
drive voltage. The structures described in this example can be used
in appropriate combination with any of the embodiments and the
other example.
REFERENCE EXAMPLE 1
[0145] As described in Embodiment 1, the addition of the additive
to the second light-emitting layer 122 to form an exciplex allows a
drastic increase in the emission efficiency. Examples are shown by
using the following light-emitting elements (light-emitting
elements 3 to 6 (LEEs 3 to 6)). Table 3 shows the detailed
structures of the elements. Structures and abbreviations of
compounds used here are given below. FIG. 18A is a schematic
cross-sectional view of the light-emitting elements 3 and 5 and
FIG. 18B is a schematic cross-sectional view of the light-emitting
elements 4 and 6.
##STR00005##
TABLE-US-00003 TABLE 3 Structures of LEEs 3 to 6. Reference
Thickness LEE Layer numeral (nm) Material Weight ratio 3 Second
electrode 1102 200 Al -- Electron-injection layer 1130 1 LiF --
1128(2) 10 Bphen -- Electron-transport layer 1128(1) 20
2mDBTBPDBq-II -- Light-emitting layer 1122(2) 20
2mDBTBPDBq-II:PCBBiF:Ir(tBuppm).sub.2(acac) 0.8:0.2:0.05 1122(1) 20
2mDBTBPDBq-II:PCBBiF:Ir(tBuppm).sub.2(acac) 0.7:0.3:0.05
Hole-transport layer 1126 20 BPAFLP -- Hole-injection layer 1124 50
DBT3P-II:MoO.sub.3 1:0.5 First electrode 1100 110 ITSO -- 4 Second
electrode 1102 130 Al -- Electron-injection layer 1130 1 LiF --
Electron-transport layer 1128(2) 20 Bphen -- 1128(1) 15
2mDBTBPDBq-II -- Light-emitting layer 1122 40
2mDBTBPDBq-II:PCBBiF:Ir(mpmppm).sub.2(acac) 0.8:0.2:0.06
Hole-transport layer 1126 20 BPAFLP -- Hole-injection layer 1124 10
DBT3P-II:MoO.sub.3 1:0.5 First electrode 1100 110 ITSO -- 5 Second
electrode 1102 200 Al -- Electron-injection layer 1130 1 LiF --
Electron-transport layer 1128(2) 20 Bphen -- 1128(1) 20
2mDBTBPDBq-II -- Light-emitting layer 1122(2) 20
2mDBTBPDBq-II:PCBBiF:Ir(dppm).sub.2(acac) 0.8:0.2:0.05 1122(1) 20
2mDBTBPDBq-II:PCBBiF:Ir(dppm).sub.2(acac) 0.7:0.3:0.05
Hole-transport layer 1126 20 BPAFLP -- Hole-injection layer 1124 20
DBT3P-II:MoO.sub.3 1:0.5 First electrode 1100 110 ITSO -- 6 Second
electrode 1102 200 Al -- Electron-injection layer 1130 1 LiF --
Electron-transport layer 1128(2) 15 Bphen -- 1128(1) 20
2mDBTBPDBq-II -- Light-emitting layer 1122 40
2mDBTBPDBq-II:PCBBiF:Ir(tppr).sub.2(dpm) 0.8:0.2:0.05
Hole-transport layer 1126 20 BPAFLP -- Hole-injection layer 1124 60
BPAFLP:MoO.sub.3 1:0.5 First electrode 1100 70 ITSO --
1. Fabrication of Light-Emitting Element 3
[0146] As a first electrode 1100, an ITSO film was formed over a
glass substrate to a thickness of 110 nm. The electrode area of the
first electrode 1100 was 4 mm.sup.2 (2 mm.times.2 mm).
[0147] Next, as a hole-injection layer 1124, DBT3P-II and MoO.sub.3
were deposited on the first electrode 1100 by co-evaporation in a
weight ratio of DBT3P-II:MoO.sub.3=1:0.5 to a thickness of 50
nm.
[0148] Then, as a hole-transport layer 1126,
4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:
BPAFLP) was deposited on the hole-injection layer 1124 by
evaporation to a thickness of 20 nm.
[0149] Subsequently, as light-emitting layers 1122(1) and 1122(2),
2mDBTBPDBq-II, PCBBiF, and Ir(tBuppm).sub.2(acac) were deposited on
the hole-transport layer 1126 by co-evaporation in a weight ratio
of 2mDBTBPDBq-II:PCBBiF:Ir(tBuppm).sub.2(acac)=0.7:0.3:0.05 to a
thickness of 20 nm, and further deposited by co-evaporation in a
weight ratio of
2mDBTBPDBq-II:PCBBiF:Ir(tBuppm).sub.2(acac)=0.8:0.2:0.05 to a
thickness of 20 nm. In a light-emitting layer 1122, 2mDBTBPDBq-II
is a host material, PCBBiF is an additive which can form an
exciplex together with the host material, and
Ir(tBuppm).sub.2(acac) is a light-emitting material.
[0150] Next, as an electron-transport layer 1128(1), an
electron-transport layer 1128(2), and an electron-injection layer
1130, 2mDBTBPDBq-II, Bphen, and LiF were sequentially deposited on
the light-emitting layer 1122(2) by evaporation to a thickness of
20 nm, 10 nm, and 1 nm, respectively.
[0151] Subsequently, as a second electrode 1102, aluminum (Al) was
formed on the electron-injection layer 1130 to a thickness of 200
nm.
[0152] Through the above steps, the light-emitting element 3 was
fabricated over the glass substrate. Note that in the above
deposition process, evaporation was all performed by a resistance
heating method.
[0153] Next, the light-emitting element 3 was sealed by fixing a
sealing substrate to the glass substrate using a sealant for an
organic EL device in a glove box containing a nitrogen atmosphere.
Specifically, the sealant was applied to surround the
light-emitting element, the glass substrate and the sealing
substrate were bonded to each other, irradiation with 365-nm
ultraviolet light at 6 J/cm.sup.2 was performed, and heat treatment
was performed at 80.degree. C. for 1 hour. Through the above steps,
the light-emitting element 3 was obtained.
2. Fabrication of Light-Emitting Element 4
[0154] The light-emitting element 4 was fabricated by the same
method as the light-emitting element 3, except for the following
steps.
[0155] As the hole-injection layer 1124 on the first electrode
1100, DBT3P-II and MoO.sub.3 were deposited by co-evaporation in a
weight ratio of DBT3P-II:MoO.sub.3=1:0.5 to a thickness of 10
nm.
[0156] As the light-emitting layer 1122, 2mDBTBPDBq-II, PCBBiF, and
bis{2-[5-methyl-6-(2-methylphenyl)-4-pyrimidinyl-.kappa.N3]phenyl-.kappa.-
C} (2,4-pentanedionato-.kappa..sup.2O,O')iridium(III))
(Ir(mpmppm).sub.2(acac) were deposited on the hole-transport layer
1126 by co-evaporation in a weight ratio of
2mDBTBPDBq-II:PCBBiF:Ir(mpmppm).sub.2(acac)=0.8:0.2:0.06 to a
thickness of 40 nm. In the light-emitting layer 1122, 2mDBTBPDBq-II
is a host material, PCBBiF is an additive which can form an
exciplex together with the host material, and
Ir(mpmppm).sub.2(acac) is a light-emitting material.
[0157] Next, as the electron-transport layers 1128(1) and 1128(2),
2mDBTBPDBq-II and Bphen were sequentially deposited on the
light-emitting layer 1122 by evaporation to a thickness of 15 nm
and 20 nm, respectively.
3. Fabrication of Light-Emitting Element 5
[0158] The light-emitting element 5 was fabricated by the same
method as the light-emitting element 3, except for the following
steps.
[0159] As the hole-injection layer 1124 on the first electrode
1100, DBT3P-II and MoO.sub.3 were deposited by co-evaporation in a
weight ratio of DBT3P-II:MoO.sub.3=1:0.5 to a thickness of 20
nm.
[0160] As the light-emitting layers 1122(1) and 1122(2),
2mDBTBPDBq-II, PCBBiF, and
(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)
(abbreviation: Ir(dppm).sub.2(acac)) were deposited on the
hole-transport layer 1126 by co-evaporation in a weight ratio of
2mDBTBPDBq-II:PCBBiF:Ir(dppm).sub.2(acac)=0.7:0.3:0.05 to a
thickness of 20 nm, and further deposited by co-evaporation in a
weight ratio of
2mDBTBPDBq-II:PCBBiF:Ir(dppm).sub.2(acac)=0.8:0.2:0.05 to a
thickness of 20 nm. In the light-emitting layer 1122, 2mDBTBPDBq-II
is a host material, PCBBiF is an additive which can form an
exciplex together with the host material, and Ir(dppm).sub.2(acac)
is a light-emitting material.
[0161] Next, as the electron-transport layers 1128(1) and 1128(2),
2mDBTBPDBq-II and Bphen were sequentially deposited on the
light-emitting layer 1122(2) by evaporation each to a thickness of
20 nm.
4. Fabrication of Light-Emitting Element 6
[0162] The light-emitting element 6 was fabricated by the same
method as the light-emitting element 3, except for the following
steps.
[0163] As the first electrode 1100, an ITSO film was formed over a
glass substrate to a thickness of 70 nm.
[0164] Next, as the hole-injection layer 1124 on the first
electrode 1100, BPAFLP and MoO.sub.3 were deposited by
co-evaporation in a weight ratio of BPAFLP:MoO.sub.3=1:0.5 to a
thickness of 60 nm.
[0165] As the light-emitting layer 1122, 2mDBTBPDBq-II, PCBBiF, and
Ir(tppr).sub.2(dpm) were deposited on the hole-transport layer 1126
by co-evaporation in a weight ratio of
2mDBTBPDBq-II:PCBBiF:Ir(tppr).sub.2(dpm)=0.8:0.2:0.05 to a
thickness of 40 nm. In the light-emitting layer 1122, 2mDBTBPDBq-II
is a host material, PCBBiF is an additive which can form an
exciplex together with the host material, and Ir(tppr).sub.2(dpm)
is a light-emitting material.
[0166] Next, as the electron-transport layers 1128(1) and 1128(2),
2mDBTBPDBq-II and Bphen were sequentially deposited on the
light-emitting layer 1122 by evaporation to a thickness of 20 nm
and 15 nm, respectively.
5. Characteristics of Light-Emitting Elements 3 to 6
[0167] FIG. 19 shows luminance--current efficiency curves of the
fabricated light-emitting elements 3 to 6. FIG. 20 shows their
voltage--luminance curves. FIG. 21 shows their luminance--external
quantum efficiency curves. The measurements of the light-emitting
elements were performed at room temperature (in an atmosphere kept
at 23.degree. C.).
[0168] Table 4 shows the device characteristics of the
light-emitting elements 3 to 6 at around 1000 cd/m.sup.2. Note that
the external quantum efficiency in FIG. 21 and Table 4 was
calculated in consideration of light distribution
characteristics.
TABLE-US-00004 TABLE 4 Device characteristics of the LEEs 3 to 6.
Current External density CIE Lumi- Current quantum Voltage (mA/
chromaticity nance efficiency efficiency LEE (V) cm.sup.2) (x, y)
(cd/m.sup.2) (cd/A) (%) 3 2.9 0.76 (0.41, 0.58) 920 120 31 4 2.8
0.88 (0.49, 0.50) 1000 120 32 5 2.8 1.1 (0.56, 0.44) 960 85 29 6
3.4 2.2 (0.66, 0.34) 800 36 24
[0169] FIG. 22 shows electroluminescence spectra obtained by
supplying the light-emitting elements 3 to 6 with a current at a
current density of 2.5 mA/cm.sup.2. The light-emitting element 3
emits green light, the light-emitting element 4 emits yellow light,
the light-emitting element 5 emits orange light, and the
light-emitting element 6 emits red light.
[0170] As shown in FIG. 19 to FIG. 22 and Table 4, the
light-emitting elements 3 to 6 emit light with high current
efficiency and high external quantum efficiency at a low drive
voltage. Furthermore, the efficiency of the light-emitting elements
3 to 6 decreases only slightly even in a high luminance region, and
the high emission efficiency is maintained.
[0171] Results of reliability tests of the light-emitting elements
3 to 6 are shown in FIG. 23. In the reliability tests, the
light-emitting elements 3 to 6 were driven under the conditions
where the initial luminance of the light-emitting elements was set
to 5000 cd/m.sup.2 and the current density was constant.
[0172] From the results, the time (LT90) taken for the luminance of
the light-emitting elements 3 to 6 to decrease to 90% of the
initial luminance was estimated: the light-emitting element 3, 1000
hours; the light-emitting element 4, 1300 hours; the light-emitting
element 5, 2800 hours; and the light-emitting element 6, 560 hours.
The above results prove the high reliability of the elements.
[0173] As described above, when the light-emitting layer 1122
contains an additive capable of forming an exciplex together with a
host material and the exciplex is utilized as an energy transfer
medium, regardless of emission color, green- to red-emissive
light-emitting elements with high current efficiency, high external
quantum efficiency, and a low drive voltage are obtained. Moreover,
highly reliable light-emitting elements are obtained.
REFERENCE EXAMPLE 2
[0174] A synthesis method of Ir(ppm-dmp).sub.2(acac) used in
Example 2 will be described. The synthesis scheme is as
follows.
##STR00006##
1. Synthesis of 4-Chloro-6-phenylpyrimidine
[0175] A mixture of 5.0 g of 4,6-dichloropyrimidine, 4.9 g of
phenylboronic acid, 7.1 g of sodium carbonate, 0.34 g of
bis(triphenylphosphine)palladium(II)dichloride
(PdCl.sub.2(PPh.sub.3).sub.2), 20 mL of acetonitrile, and 20 mL of
water was heated to reflux by irradiation with microwaves (2.45
GHz, 100 W) under an argon stream for 1 hour. The obtained mixture
was subjected to extraction with dichloromethane and purified by
silica gel column chromatography (developing solvent:
dichloromethane), whereby 1.6 g of 4-chloro-6-phenylpyrimidine were
obtained (yield: 23%, a pale yellow solid). Note that the microwave
irradiation in this reference example was performed using a
microwave synthesis system (Discover, manufactured by CEM
Corporation).
2. Synthesis of 4-Phenyl-6-(2,6-dimethylphenyl)pyrimidine
(Hppm-dmp)
[0176] A mixture of 1.6 g of 4-chloro-6-phenylpyrimidine, 1.5 g of
2,6-dimethylphenylboronic acid, 1.8 g of sodium carbonate, 59 mg of
PdCl.sub.2(PPh.sub.3).sub.2, 20 mL of N,N-dimethylformamide, and 20
mL of water was heated to reflux by irradiation with microwaves
(2.45 GHz, 100 W) under an argon stream for 2 hours. The obtained
mixture was subjected to extraction with dichloromethane and
purified by silica gel column chromatography (developing
solvent:ethyl acetate and hexane in a ratio of 1:5), whereby 0.50 g
of Hppm-dmp were obtained (yield: 23%, a pale yellow oily
substance).
3. Synthesis of
Di-.mu.-chloro-tetrakis{2-[6-(2,6-dimethylphenyl)-4-pyrimidinyl-.kappa.N3-
]phenyl-.kappa.C}diiridium(III) ([Ir(ppm-dmp).sub.2Cl].sub.2)
[0177] A mixture of 1.0 g of Hppm-dmp, 0.57 g of iridium(III)
chloride hydrate, 20 mL of 2-ethoxyethanol, and 20 mL of water was
heated to reflux by irradiation with microwaves (2.45 GHz, 100 W)
under an argon stream for 3 hours. The obtained mixture was
filtrated and the resulting solid was washed with methanol, whereby
1.1 g of [Ir(ppm-dmp).sub.2Cl].sub.2 were obtained (yield: 74%, an
orange solid).
4. Synthesis of Ir(ppm-dmp).sub.2(acac)
[0178] A mixture of 1.1 g of [Ir(ppm-dmp).sub.2Cl].sub.2, 0.77 g of
sodium carbonate, 0.23 g of acetylacetone (Hacac), and 30 mL of
2-ethoxyethanol was heated to reflux by irradiation with microwaves
(2.45 GHz, 120 W) under an argon stream for 2 hours. The obtained
mixture was filtrated, and an insoluble part was washed with
methanol. The obtained filtrate was condensed, a residue was
purified by silica gel column chromatography (developing
solvent:ethyl acetate and hexane in a ratio of 1:5), and the
obtained solid was recrystallized from hexane, whereby
Ir(ppm-dmp).sub.2(acac) was obtained (yield: 59%, an orange
powdered solid). By a train sublimation method, 0.21 g of the
obtained orange powdered solid were purified, whereby the objective
orange solid was collected in a yield of 48%. The conditions of the
purification by sublimation were as follows: the pressure was 2.7
Pa; the flow rate of an argon gas was 5.0 mL/min; and the
temperature was 240.degree. C. .sup.1H-NMR (nuclear magnetic
resonance) spectrum data of the obtained Ir(ppm-dmp).sub.2(acac)
are shown below.
[0179] .sup.1H-NMR. .delta. (CDCl.sub.3): 1.85 (s, 6H), 2.26 (s,
12H), 5.35 (s, 1H), 6.46-6.48 (dd, 2H), 6.83-6.90 (dm, 4H),
7.20-7.22 (d, 4H), 7.29-7.32 (t, 2H), 7.63-7.65 (dd, 2H), 7.72 (ds,
2H), 9.24 (ds, 2H).
[0180] This application is based on Japanese Patent Application
serial no. 2014-099560 filed with Japan Patent Office on May 13,
2014 and Japanese Patent Application serial no. 2014-241575 filed
with Japan Patent Office on Nov. 28, 2014, the entire contents of
which are hereby incorporated by reference.
* * * * *