U.S. patent application number 14/868724 was filed with the patent office on 2016-03-31 for light-emitting element, display device, electronic device, and lighting device.
The applicant listed for this patent is Semiconductor Energy Laboratory Co., LTD.. Invention is credited to Nobuharu Ohsawa, Satoshi Seo, Takeyoshi Watabe.
Application Number | 20160093823 14/868724 |
Document ID | / |
Family ID | 55585395 |
Filed Date | 2016-03-31 |
United States Patent
Application |
20160093823 |
Kind Code |
A1 |
Seo; Satoshi ; et
al. |
March 31, 2016 |
LIGHT-EMITTING ELEMENT, DISPLAY DEVICE, ELECTRONIC DEVICE, AND
LIGHTING DEVICE
Abstract
Provided is a light-emitting element including a
fluorescence-emitting material with high emission efficiency. The
light-emitting element includes a pair of electrodes and an EL
layer between the pair of electrodes. The EL layer includes a first
organic compound, a second organic compound, and a guest material.
The first organic compound has a function of emitting a thermally
activated delayed fluorescence at room temperature. The guest
material has a function of emitting fluorescence. A HOMO level of
the first organic compound higher than or equal to a HOMO level of
the second organic compound. A LUMO level of the first organic
compound is lower than or equal to a LUMO level of the second
organic compound.
Inventors: |
Seo; Satoshi; (Sagamihara,
JP) ; Ohsawa; Nobuharu; (Zama, JP) ; Watabe;
Takeyoshi; (Atsugi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., LTD. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
55585395 |
Appl. No.: |
14/868724 |
Filed: |
September 29, 2015 |
Current U.S.
Class: |
257/40 |
Current CPC
Class: |
H01L 51/0067 20130101;
H01L 27/3209 20130101; H01L 51/5004 20130101; H01L 2251/5384
20130101; H01L 51/0054 20130101; H01L 51/006 20130101; H01L 51/5028
20130101; H01L 51/0074 20130101; H01L 51/5016 20130101; H01L
51/5012 20130101; H01L 51/5278 20130101; H01L 2251/5376 20130101;
H01L 51/0072 20130101; H01L 51/0058 20130101 |
International
Class: |
H01L 51/50 20060101
H01L051/50; H01L 51/52 20060101 H01L051/52 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2014 |
JP |
2014-200355 |
Claims
1. A light-emitting element comprising: a pair of electrodes; and
an EL layer between the pair of electrodes, wherein the EL layer
comprises a first organic compound, a second organic compound, and
a guest material, wherein the first organic compound has a function
of emitting a thermally activated delayed fluorescence at room
temperature, wherein the guest material has a function of emitting
fluorescence, wherein a HOMO level of the first organic compound is
higher than or equal to a HOMO level of the second organic
compound, and wherein a LUMO level of the first organic compound is
lower than or equal to a LUMO level of the second organic
compound.
2. The light-emitting element according to claim 1, wherein a
difference between a singlet excitation energy level of the first
organic compound and a triplet excitation energy level of the first
organic compound is larger than 0 eV and smaller than or equal to
0.2 eV.
3. The light-emitting element according to claim 1, wherein the
guest material emits light.
4. The light-emitting element according to claim 1, wherein the
first organic compound comprises a first .pi.-electron deficient
heteroaromatic skeleton and a first .pi.-electron rich
heteroaromatic skeleton, and wherein the second organic compound
comprises a second .pi.-electron deficient heteroaromatic skeleton
and a second .pi.-electron rich heteroaromatic skeleton.
5. The light-emitting element according to claim 4, wherein the
first .pi.-electron deficient heteroaromatic skeleton comprises a
diazine skeleton or a triazine skeleton, wherein the first
.pi.-electron rich heteroaromatic skeleton comprises any one or
more of an acridine skeleton, a phenoxazine skeleton, or a
3-(9-phenyl-9H-carbazole-3-yl)-9H-carbazole skeleton, wherein the
second .pi.-electron deficient heteroaromatic skeleton comprises a
pyridine skeleton or a diazine skeleton, and wherein the second
.pi.-electron rich heteroaromatic skeleton comprises any one or
more of a furan skeleton, a thiophene skeleton, a fluorine
skeleton, and a pyrrole skeleton.
6. The light-emitting element according to claim 1, wherein a
weight ratio of the second organic compound to the first organic
compound is from 1:0.05 to 1:0.5, and wherein a weight ratio of the
second organic compound to the guest material is from 1:0.001 to
1:0.01.
7. A display device comprising: the light-emitting element
according to claim 1; and a color filter, a sealant, or a
transistor.
8. An electronic device comprising: the display device according to
claim 7, and a housing or a function of a touch sensor.
9. A lighting device comprising: the light-emitting element
according to claim 1, and a housing or a function of a touch
sensor.
10. A light-emitting element comprising: a pair of electrodes; and
an EL layer between the pair of electrodes, wherein the EL layer
comprises a first organic compound, a second organic compound, and
a guest material, wherein the first organic compound has a function
of emitting a thermally activated delayed fluorescence at room
temperature, wherein the guest material has a function of emitting
fluorescence, wherein an oxidation potential of the first organic
compound is lower than or equal to an oxidation potential of the
second organic compound, and wherein a reduction potential of the
first organic compound is higher than or equal to a reduction of
the second organic compound.
11. The light-emitting element according to claim 10, wherein a
difference between a singlet excitation energy level of the first
organic compound and a triplet excitation energy level of the first
organic compound is larger than 0 eV and smaller than or equal to
0.2 eV.
12. The light-emitting element according to claim 10, wherein the
guest material emits light.
13. The light-emitting element according to claim 10, wherein the
first organic compound comprises a first .pi.-electron deficient
heteroaromatic skeleton and a first .pi.-electron rich
heteroaromatic skeleton, and wherein the second organic compound
comprises a second .pi.-electron deficient heteroaromatic skeleton
and a second .pi.-electron rich heteroaromatic skeleton.
14. The light-emitting element according to claim 13, wherein the
first .pi.-electron deficient heteroaromatic skeleton comprises a
diazine skeleton or a triazine skeleton, wherein the first
.pi.-electron rich heteroaromatic skeleton comprises any one or
more of an acridine skeleton, a phenoxazine skeleton, or a
3-(9-phenyl-9H-carbazole-3-yl)-9H-carbazole skeleton, wherein the
second .pi.-electron deficient heteroaromatic skeleton comprises a
pyridine skeleton or a diazine skeleton, and wherein the second
.pi.-electron rich heteroaromatic skeleton comprises any one or
more of a furan skeleton, a thiophene skeleton, a fluorine
skeleton, and a pyrrole skeleton.
15. The light-emitting element according to claim 10, wherein a
weight ratio of the second organic compound to the first organic
compound is from 1:0.05 to 1:0.5, and wherein a weight ratio of the
second organic compound to the guest material is from 1:0.001 to
1:0.01.
16. A display device comprising: the light-emitting element
according to claim 10; and a color filter, a sealant, or a
transistor.
17. An electronic device comprising: the display device according
to claim 16, and a housing or a function of a touch sensor.
18. A lighting device comprising: the light-emitting element
according to claim 10, and a housing or a function of a touch
sensor.
Description
TECHNICAL FIELD
[0001] One embodiment of the present invention relates to a
light-emitting element in which a light-emitting layer capable of
providing light emission by application of an electric field is
provided between a pair of electrodes, and also relates to a
display device, an electronic device, and a lighting device
including the light-emitting element.
[0002] Note that one embodiment of the present invention is not
limited to the above technical field. The technical field of one
embodiment of the invention disclosed in this specification and the
like relates to an object, a method, or a manufacturing method. One
embodiment of the present invention relates to a process, a
machine, manufacture, or a composition of matter.
[0003] Specifically, examples of the technical field of one
embodiment of the present invention disclosed in this specification
include a semiconductor device, a display device, a liquid crystal
display device, a light-emitting device, a lighting device, a power
storage device, a storage device, a method for driving any of them,
and a method for manufacturing any of them.
BACKGROUND ART
[0004] In recent years, research and development have been
extensively conducted on light-emitting elements utilizing
electroluminescence (EL). In a basic structure of such a
light-emitting element, a layer containing a light-emitting
substance (an EL layer) is interposed between a pair of electrodes.
By application of a voltage between the electrodes of this element,
light emission from the light-emitting substance can be
obtained.
[0005] Since the above light-emitting element is a self-luminous
type, a display device using this light-emitting element has
advantages such as high visibility, no necessity of a backlight,
and low power consumption. Furthermore, such a light-emitting
element also has advantages in that the element can be formed to be
thin and lightweight, and that response time is high.
[0006] In the case of a light-emitting element in which an EL layer
containing an organic material as the light-emitting material is
provided between a pair of electrodes (e.g., an organic EL
element), application of a voltage between the pair of electrodes
causes injection of electrons from the cathode and holes from the
anode into the EL layer having a light-emitting property, and thus
a current flows. By recombination of the injected electrons and
holes, the organic material having a light-emitting property is put
in an excited state, whereby light emission is obtained from the
excited organic compound having a light-emitting property.
[0007] The excited state of an organic material can be a singlet
excited state or a triplet excited state, and light emission from
the singlet excited state (S.sub.1) is referred to as fluorescence,
and light emission from the triplet excited state (T.sub.1) is
referred to as phosphorescence. The statistical generation ratio of
the excited states in the light-emitting element is considered to
be S.sub.1:T.sub.1=1:3. In other words, a light-emitting element
containing a phosphorescent material has higher emission efficiency
than a light-emitting element containing a fluorescent material.
Therefore, a light-emitting element containing a phosphorescent
material capable of converting the triplet excited state into light
emission has been actively developed in recent years.
[0008] As one of materials capable of partly converting the triplet
excited state into light emission, a thermally activated delayed
fluorescence (TADF) substance has been known. In a thermally
activated delayed fluorescence substance, a singlet excited state
is generated from a triplet excited state by reverse intersystem
crossing, and the singlet excited state is converted into light
emission. Patent Document 1 and Patent Document 2 each disclose a
thermally activated delayed fluorescence substance.
[0009] In order to increase emission efficiency of a light-emitting
element using a thermally activated delayed fluorescence substance,
not only efficient generation of a singlet excited state from a
triplet excited state but also efficient emission from a singlet
excited state, that is, high fluorescence quantum yield are
important in a thermally activated delayed fluorescence substance.
It is, however, difficult to design a light-emitting material that
meets these two.
[0010] Patent Document 3 discloses a method: in a light-emitting
element containing a thermally activated delayed fluorescence
substance and a material emitting fluorescence, singlet excitation
energy of the thermally activated delayed fluorescence substance is
transferred to the material emitting fluorescence and light
emission is obtained from the material emitting fluorescence.
REFERENCE
Patent Document
[0011] [Patent Document 1] Japanese Published Patent Application
No. 2004-241374 [0012] [Patent Document 2] Japanese Published
Patent Application No. 2006-24830 [0013] [Patent Document 3]
Japanese Published Patent Application No. 2014-45179
DISCLOSURE OF INVENTION
[0014] In order to increase emission efficiency of a light-emitting
element containing a thermally activated delayed fluorescence
substance and a material emitting fluorescence, efficient
generation of a singlet excited state from a triplet excited state
is important. In addition, efficient energy transfer from an
excited state of the thermally activated delayed fluorescence
substance to an excited state of the material emitting fluorescence
is important.
[0015] An object of one embodiment of the present invention is to
provide a light-emitting element having high emission efficiency
which contains a fluorescent material as a light-emitting material.
Another object of one embodiment of the present invention is to
provide a light-emitting element with high reliability. Another
object of one embodiment of the present invention is to provide a
light-emitting element with high emission efficiency and high
reliability. Another object of one embodiment of the present
invention is to provide a novel light-emitting element. Another
object of one embodiment of the present invention is to provide a
novel light-emitting element with high emission efficiency and low
power consumption.
[0016] Note that the description of the above object does not
disturb the existence of other objects. In one embodiment of the
present invention, there is no need to achieve all the objects.
Objects other than the above objects will be apparent from and can
be derived from the description of the specification and the
like.
MEANS FOR SOLVING THE PROBLEMS
[0017] One embodiment of the present invention is a light-emitting
element including a pair of electrodes and an EL layer between the
pair of electrodes. The EL layer includes a first organic compound,
a second organic compound, and a guest material. The first organic
compound has a function of emitting a thermally activated delayed
fluorescence at room temperature. The guest material has a function
of emitting fluorescence. A HOMO of the first organic compound has
an energy level higher than or equal to an energy level of a HOMO
of the second organic compound. A LUMO of the first organic
compound has an energy level lower than or equal to an energy level
of a LUMO of the second organic compound.
[0018] Another embodiment of the present invention is a
light-emitting element including a pair of electrodes and an EL
layer between the pair of electrodes. The EL layer includes a first
organic compound, a second organic compound, and a guest material.
The first organic compound has a function of emitting a thermally
activated delayed fluorescence at room temperature. The guest
material has a function of emitting fluorescence. An oxidation
potential of the first organic compound is lower than or equal to
an oxidation potential of the second organic compound. A reduction
potential of the first organic compound is higher than or equal to
a reduction of the second organic compound.
[0019] In the above structure, a difference between a singlet
excitation energy level of the first organic compound and a triplet
excitation energy level of the first organic compound is preferably
larger than 0 eV and smaller than or equal to 0.2 eV.
[0020] In the above structure, the guest material preferably emits
light.
[0021] In the above structure, it is preferable that the first
organic compound include a first .pi.-electron deficient
heteroaromatic skeleton and a first .pi.-electron rich
heteroaromatic skeleton and the second organic compound include a
second .pi.-electron deficient heteroaromatic skeleton and a second
.pi.-electron rich heteroaromatic skeleton.
[0022] In the above structure, it is preferable that the first
.pi.-electron deficient heteroaromatic skeleton include a diazine
skeleton or a triazine skeleton, the first .pi.-electron rich
heteroaromatic skeleton include any one or more of an acridine
skeleton, a phenoxazine skeleton, or a
3-(9-phenyl-9H-carbazole-3-yl)-9H-carbazole skeleton, the second
.pi.-electron deficient heteroaromatic skeleton include a pyridine
skeleton or a diazine skeleton, and the second .pi.-electron rich
heteroaromatic skeleton include any one or more of a furan
skeleton, a thiophene skeleton, a fluorine skeleton, and a pyrrole
skeleton.
[0023] In the above structure, it is preferable that a weight ratio
of the second organic compound to the first organic compound be
from 1:0.05 to 1:0.5 (the second organic compound: the first
organic compound) and a weight ratio of the second organic compound
to the guest material be from 1:0.001 to 1:0.01 (the second organic
compound: the guest material).
[0024] Another embodiment of the present invention is a display
device which includes the light-emitting element and a color
filter, a sealant, or a transistor. Another embodiment of the
present invention is an electronic device which includes the
display device and a housing or a function of a touch sensor.
Another embodiment of the present invention is a lighting device
which includes the light-emitting element in the above embodiment
and a housing or a touch sensor.
[0025] One embodiment of the present invention makes it possible to
provide a light-emitting element having high emission efficiency
which contains a fluorescent material as a light-emitting material.
One embodiment of the present invention makes it possible to
provide a light-emitting element with high reliability. One
embodiment of the present invention makes it possible to provide a
light-emitting element with high emission efficiency and high
reliability. One embodiment of the present invention makes it
possible to provide a novel light-emitting element. One embodiment
of the present invention makes it possible to provide a novel
light-emitting element with high emission efficiency and low power
consumption.
[0026] 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 objects listed
above. 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 DRAWINGS
[0027] FIGS. 1A and 1B are schematic cross-sectional views
illustrating a light-emitting element of one embodiment of the
present invention.
[0028] FIGS. 2A to 2C show correlations of energy levels in a
light-emitting element of one embodiment of the present
invention.
[0029] FIGS. 3A and 3B are a schematic cross-sectional view of a
light-emitting element of one embodiment of the present invention
and a diagram illustrating the correlation of energy levels in a
light-emitting layer.
[0030] FIGS. 4A and 4B are a schematic cross-sectional view of a
light-emitting element of one embodiment of the present invention
and a diagram illustrating the correlation of energy levels in a
light-emitting layer.
[0031] FIGS. 5A and 5B are a block diagram and a circuit diagram
illustrating a display device of one embodiment of the present
invention.
[0032] FIGS. 6A and 6B are perspective views of an example of a
touch panel of one embodiment of the present invention.
[0033] FIGS. 7A to 7C are cross-sectional views of examples of a
display device and a touch sensor of one embodiment of the present
invention.
[0034] FIGS. 8A and 8B illustrate examples of a touch panel of one
embodiment of the present invention.
[0035] FIGS. 9A and 9B are a block diagram and a timing chart of a
touch sensor of one embodiment of the present invention.
[0036] FIG. 10 is a circuit diagram illustrating a touch sensor of
one embodiment of the present invention.
[0037] FIG. 11 is a perspective view illustrating a display module
of one embodiment of the present invention.
[0038] FIGS. 12A to 12G illustrate electronic devices of one
embodiment of the present invention.
[0039] FIG. 13 illustrates a lighting device of one embodiment of
the present invention.
[0040] FIG. 14 is a schematic cross-sectional view illustrating
light-emitting elements of Examples 1 and 2.
[0041] FIG. 15 shows transient fluorescence characteristics of a
host material of Example 1.
[0042] FIG. 16 shows current efficiency-luminance characteristics
of light-emitting elements of Example 1.
[0043] FIG. 17 shows current-voltage characteristics of
light-emitting elements of Example 1.
[0044] FIG. 18 shows external quantum efficiency-luminance
characteristics of light-emitting elements of Example 1.
[0045] FIG. 19 shows electroluminescence spectra of light emitted
from light-emitting elements of Example 1.
[0046] FIG. 20 shows current efficiency-luminance characteristics
of light-emitting elements of Example 2.
[0047] FIG. 21 shows current-voltage characteristics of
light-emitting elements Example 2.
[0048] FIG. 22 shows external quantum efficiency-luminance
characteristics of light-emitting elements Example 2.
[0049] FIG. 23 shows electroluminescence spectra of light-emitting
elements of Example 2.
[0050] FIG. 24 shows current efficiency-luminance characteristics
of light-emitting elements of Example 2.
[0051] FIG. 25 shows current-voltage characteristics of
light-emitting elements of Example 2.
[0052] FIG. 26 shows external quantum efficiency-luminance
characteristics of light-emitting elements of Example 2.
[0053] FIG. 27 shows electroluminescence spectra of light-emitting
elements of Example 2.
BEST MODE FOR CARRYING OUT THE INVENTION
[0054] Embodiments of the present invention will be explained below
with reference to the drawings. However, the present invention is
not limited to description to be given below, and it is to be
easily understood that modes and details thereof can be variously
modified without departing from the purpose and the scope of the
present invention. Accordingly, the present invention should not be
interpreted as being limited to the content of the embodiments
below.
[0055] Note that the position, the size, the range, or the like of
each structure illustrated in drawings and the like is not
accurately represented in some cases for simplification. Therefore,
the disclosed invention is not necessarily limited to the position,
the size, the range, or the like disclosed in the drawings and the
like.
[0056] Ordinal numbers such as "first" and "second" in this
specification and the like are used for convenience and do not
denote the order of steps or the stacking order of layers in some
cases. Therefore, for example, description can be made even when
"first" is replaced with "second" or "third", as appropriate. In
addition, the ordinal numbers in this specification and the like
are not necessarily the same as those which specify one embodiment
of the present invention.
[0057] In the description of modes of the present invention in this
specification and the like with reference to the drawings, the same
components in different diagrams are commonly denoted by the same
reference numeral in some cases.
[0058] In this specification and the like, the terms "film" and
"layer" can be interchanged with each other depending on the case
or circumstances. For example, the term "conductive layer" can be
changed into the term "conductive film" in some cases. Also, the
term "insulating film" can be changed into the term "insulating
layer" in some cases.
[0059] Note that in this specification and the like, a singlet
excited state means a singlet state with excited energy. An S.sub.1
level means the lowest level of the singlet excitation energy, that
is, the lowest level of excited energy in a singlet excited state.
A triplet excited state means a triplet state with excited energy.
A T.sub.1 level means the lowest level of the triplet excitation
energy, that is, the lowest level of excited energy in a triplet
excited state. Note that in this specification and the like, a
singlet excited state and a singlet excitation energy level mean
the lowest singlet excited state and the S.sub.1 level,
respectively, in some cases. A triplet excited state and a triplet
excitation energy level mean the lowest singlet excited state and
the T.sub.1 level, respectively, in some cases.
[0060] In this specification and the like, 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 level of the lowest triplet excited state
(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.
[0061] In this specification and the like, a thermally activated
delayed fluorescent substance is a material which can generate a
singlet excited state from a triplet excited state by reverse
intersystem crossing by thermal activation. The thermally activated
delayed fluorescent substance may include a material which can
generate a singlet excited state by itself from a triplet excited
state by reverse intersystem crossing, for example, a material
which emits TADF. Alternatively, the thermally activated delayed
fluorescent substance may include a combination of two kinds of
materials which form exciplexes.
[0062] It also can be said that the thermally activated delayed
fluorescent substance is a material of which a triplet excited
state is close to a singlet excited state. Specifically, a material
in which the difference between the energy levels of the triplet
excited state and the singlet excited state is larger than 0 eV and
smaller than or equal to 0.2 eV is preferably used. That is, it is
preferable that the difference between the energy levels of the
triplet excited state and the singlet excited state be larger than
0 eV and smaller than or equal to 0.2 eV in a material which can
generate a singlet excited state by itself from a triplet excited
state by reverse intersystem crossing, for example, a material
which emits TADF, or it is preferable that the difference between
the levels of the triplet excited state and the singlet excited
state be larger than 0 eV and smaller than or equal to 0.2 eV in
exciplexes.
[0063] In this specification and the like, thermally activated
delayed fluorescence emission energy refers to an emission peak
(including a shoulder) on the shortest wavelength side of thermally
activated delayed fluorescence. In this specification and the like,
phosphorescence emission energy or a triplet excitation energy
refers to a phosphorescence emission peak (including a shoulder) on
the shortest wavelength side of phosphorescence emission. Note that
the phosphorescence emission can be observed by time-resolved
photoluminescence in a low-temperature (e.g., 10 K)
environment.
[0064] Note that in this specification and the like, "room
temperature" refers to a temperature in a range of 0.degree. C. to
40.degree. C.
Embodiment 1
[0065] In this embodiment, a light-emitting element according to
one embodiment of the present invention will be described with
reference to FIGS. 1A and 1B and FIGS. 2A to 2C.
1. Structure Example of Light-Emitting Element
[0066] First, a structure of a light-emitting element of one
embodiment of the present invention will be described with
reference to FIGS. 1A and 1B.
[0067] A light-emitting element 150 includes an EL layer 100
between a pair of electrodes (an electrode 101 and an electrode
102). The EL layer 100 includes at least a light-emitting layer
120. Although the electrode 101 is an anode and the electrode 102
is a cathode in this embodiment, they can be interchanged for the
structure of the light-emitting element 150.
[0068] The EL layer 100 in FIG. 1A includes a functional layer in
addition to the light-emitting layer 120. The functional layer is
composed of a hole-injection layer 111, a hole-transport layer 112,
an electron-transport layer 118, and an electron-injection layer
119. Note that the structure of the EL layer 100 is not limited to
the structure illustrated in FIG. 1A, and at least one selected
from the hole-injection layer 111, the hole-transport layer 112,
the electron-transport layer 118, and the electron-injection layer
119 is included. The EL layer 100 may include another functional
layer which can reduce a barrier to hole or electron injection,
enhance a hole/electron-transport property, inhibit a hole/electron
transport property, suppress a quenching phenomenon due to an
electrode, and the like.
[0069] FIG. 1B is a schematic cross-sectional view of an example of
the light-emitting layer 120 in FIG. 1A. The light-emitting layer
120 in FIG. 1B includes an organic compound 131, an organic
compound 132, and a guest material 133.
[0070] A thermally activated delayed fluorescence substance is
preferably used for the organic compound 131. A thermally activated
delayed fluorescence substance can convert triplet excitation
energy into singlet excitation energy by reverse intersystem
crossing. At least part of the triplet excitation energy generated
in the light-emitting layer 120 is thus converted into singlet
excitation energy by the organic compound 131. The singlet
excitation energy is transferred to the guest material 133 and then
is extracted as fluorescent emission. For this reason, a difference
in energy level between singlet excitation energy and triplet
excitation energy of the organic compound 131 is preferably larger
than 0 eV and smaller than or equal to 0.2 eV. In addition, the
singlet excitation energy level of the organic compound 131 is
preferably higher than the singlet excitation energy level of the
guest material 133, and the triplet excitation energy level of the
organic compound 131 is preferably higher than the singlet
excitation energy level of the guest material 133, in which case
the triplet excitation energy level of the organic compound 131 can
be closer to the singlet excitation energy level.
[0071] A wide-bandgap material is preferably used for the organic
compound 132 to prevent deactivation of the organic compound 131
and the guest material 133. In other words, the singlet excitation
energy level of the organic compound 132 is preferably higher than
the singlet excitation energy level of the organic compound 131 and
that of the guest material 133, and the triplet excitation energy
level of the organic compound 132 is preferably higher than the
triplet excitation energy level of the organic compound 131 and
that of the guest material 133. The light-emitting layer 120 may
contain other compounds having a function similar to the organic
compound 132.
[0072] The guest material 133 may be a light-emitting organic
material, which preferably is capable of emitting fluorescence
(hereinafter also referred to as a fluorescent material). An
example in which a fluorescent material is used as the guest
material 133 will be described. Note that the guest material 133
may be referred to as the fluorescent material.
2. Emission Mechanism of Light-Emitting Element
[0073] First, an emission mechanism of the light-emitting element
150 will be described.
[0074] In the light-emitting element 150 of one embodiment of the
present invention, voltage application between the pair of
electrodes (the electrodes 101 and 102) allows electrons and holes
to be injected from the cathode and the anode, respectively, into
the EL layer 100 and thus current flows. By recombination of the
injected electrons and holes, the guest material 133 in the
light-emitting layer 120 of the EL layer 100 is brought into an
excited state to provide light emission.
[0075] Note that light emission from the guest material 133 can be
obtained through the following three processes:
(.alpha.) direct recombination process in a guest material;
(.beta.) energy transfer process from a thermally activated delayed
fluorescence substance; and (.gamma.) energy transfer process from
a host material.
<<(.alpha.) Direct Recombination Process in a Guest
Material>>
[0076] First, the direct recombination process in the guest
material 133 is explained with reference to FIG. 2A, which is a
schematic diagram showing the correlation between energy levels.
Note that the following explains what terms and signs in FIG. 2A
represent:
[0077] Host1 (131): the organic compound 131;
[0078] Host2 (132): the organic compound 132;
[0079] Guest (133): the guest material 133 (fluorescent
material);
[0080] S.sub.A: the level of the lowest singlet excitation energy
of the organic compound 131;
[0081] T.sub.A: the level of the lowest triplet excitation energy
of the organic compound 131;
[0082] S.sub.H: the level of the lowest singlet excitation energy
of the organic compound 132;
[0083] T.sub.H: the level of the lowest triplet excitation energy
of the organic compound 132;
[0084] S.sub.G: the level of the lowest singlet excitation energy
of the guest material 133 (fluorescent material); and
[0085] T.sub.G: the level of the lowest triplet excitation energy
of the guest material 133 (fluorescent material).
[0086] As shown in FIG. 2A, carriers (electrons and holes) are
recombined in the guest material 133, and the guest material 133 is
brought into an excited state. In the case where the excited state
of the guest material 133 is a singlet excited state, fluorescence
is obtained. In contrast, in the case where the excited state of
the guest material 133 is a triplet excited state, thermal
deactivation occurs.
[0087] In the (.alpha.) direct recombination process in a guest
material, high emission efficiency can be obtained from the singlet
excited state of the guest material 133 when the fluorescence
quantum efficiency of the guest material 133 is high. However, the
triplet excited state of the guest material 133 does not contribute
to light emission.
<<(.beta.) Energy Transfer Process from a Thermally Activated
Delayed Fluorescence Substance>>
[0088] Next, the energy transfer process of the organic compound
131 and the guest material 133 is described with reference to FIG.
2B, which is a schematic diagram showing the correlation between
energy levels. Note that indication and numerals in FIG. 2B are
similar to those in FIG. 2A.
[0089] Carriers are recombined in the organic compound 131, and the
organic compound 131 is brought into an excited state. In the case
where the excited state of the organic compound 131 is a single
excited state and S.sub.A of the organic compound 131 is higher
than S.sub.G of the guest material 133, the singlet excitation
energy of the organic compound 131 is transferred from S.sub.A of
the organic compound 131 to S.sub.G of the guest material 133 as
shown by a route E.sub.1 in FIG. 2B, whereby the guest material 133
is brought into the singlet excited state. Fluorescence is obtained
from the guest material 133 in the singlet excited state.
[0090] Note that since direct transition of the guest material 133
from a singlet ground state to a triplet excited state is
forbidden, energy transfer from the organic compound 131 in the
singlet excited state to the guest material 133 in the triplet
excited state is unlikely to be a main energy transfer process;
therefore, the description is omitted. In other words, energy
transfer from the organic compound 131 in the singlet excited state
to the guest material 133 in the singlet excited state as shown in
the following general formula (G1) is important.
.sup.1A*+.sup.1G.fwdarw..sup.1A+.sup.1G (G1)
[0091] Note that in the general formula (G1), .sup.1A* and .sup.1G*
represent the singlet excitation states of the organic compound 131
and the guest material 133, respectively, and .sup.1A and .sup.1G
represent the singlet ground states of the organic compound 131 and
the guest material 133, respectively.
[0092] In the case where the organic compound 131 is brought into a
triplet excitation state, fluorescence is obtained through the
following two processes.
[0093] Since the organic compound 131 is a thermally activated
delayed fluorescence substance, excitation energy is transferred
from T.sub.A to S.sub.A of the organic compound 131 by reverse
intersystem crossing (upconversion) as shown by a route A.sub.1 in
FIG. 2B. This is the first process.
[0094] Subsequently, in the case where S.sub.A of the organic
compound 131 is higher than S.sub.G of the guest material 133,
excitation energy is transferred from S.sub.A of the organic
compound 131 to S.sub.G of the guest material 133 as shown by the
route E.sub.1 in FIG. 2B, whereby the guest material 133 is brought
into the singlet excited state. This is the second process.
Fluorescence is obtained from the guest material 133 in the singlet
excited state.
[0095] The first and second processes are represented by the
following general formula (G2).
.sup.3A*+.sup.1G.fwdarw.(reverse intersystem
crossing).fwdarw..sup.1A*+.sup.1G.fwdarw..sup.1A+.sup.1G* (G2)
[0096] Note that in the general formula (G2), .sup.1A* represents
the triplet excitation state of the organic compound 131; .sup.1A*
and .sup.1G* represent the singlet excitation states of the organic
compound 131 and the guest material 133, respectively; and .sup.1A
and .sup.1G represent the singlet ground states of the organic
compound 131 and the guest material 133, respectively.
[0097] As represented by the general formula (G2), the singlet
excited state (.sup.1A*) of the organic compound 131 is generated
from the triplet excited state (.sup.3A*) of the organic compound
131, which is a thermally activated delayed fluorescence substance,
by reverse intersystem crossing. Then, excitation energy is
transferred to the singlet excited state (.sup.1G*) of the guest
material 133.
[0098] When all the energy transfer processes described in the
(.beta.) Energy transfer process from a thermally activated delayed
fluorescence substance occur efficiently, both the triplet
excitation energy and the singlet excitation energy of the organic
compound 131 are efficiently converted into the singlet excited
state (.sup.1G*) of the guest material 133, leading to
high-efficiency light emission.
[0099] However, if the organic compound 131 releases excitation
energy as light or heat and is deactivated before the excitation
energy is transferred from the singlet excited state of the organic
compound 131 to the singlet excited state of the guest material
133, the emission efficiency of the light-emitting element is
decreased. In addition, the emission efficiency is also decreased
by a decrease in efficiency of A.sub.1, which is the previous
process where the organic compound 131 is transferred from a
triplet excited state to a singlet excited state by reverse
intersystem crossing. The energy difference between T.sub.A and
S.sub.A is large particularly when T.sub.A of the organic compound
131 is lower than T.sub.G of the guest material 133 and
S.sub.A.gtoreq.S.sub.G>T.sub.G>T.sub.A is satisfied. As a
result, the reverse intersystem crossing shown by the route A.sub.1
in FIG. 2B is unlikely to occur; accordingly, the efficiency of the
subsequent energy transfer process shown by the route E.sub.1 is
decreased to lower efficiency for generating a singlet excited
state of the guest material 133. Thus, T.sub.A is preferably higher
than T.sub.G, that is, emission energy of the organic compound 131,
which is a thermally activated delayed fluorescence substance, is
preferably higher than phosphorescence emission energy of the guest
material 133.
[0100] The excitation energy is thermally deactivated also when
excitation energy is transferred from T.sub.A of the organic
compound 131 to T.sub.G of the guest material 133 as shown by a
route E.sub.2 in FIG. 2B. It is thus preferable that the energy
transfer process shown by the route E.sub.2 in FIG. 2B be less
likely to occur because the generation efficiency of the triplet
excited state of the guest material 133 can be decreased and the
occurrence of thermal deactivation of excitation energy can be
reduced. Thus, the weight percentage of the guest material 133 is
preferably smaller than that of the organic compound 131.
Specifically, their weight ratio (the organic compound 131:the
guest material 133) is preferably from 1:0.001 to 1:0.05, further
preferably from 1:0.001 to 1:0.01.
[0101] Note that when the direct recombination process in the guest
material 133 becomes dominant, the triplet excited state of the
guest material 133 very likely to occur in the light-emitting layer
to cause thermal deactivation of excited energy, resulting in a
decreased emission efficiency. That is, it is preferable that the
probability of the (.beta.) energy transfer process from a
thermally activated delayed fluorescence substance be higher than
that of the (or) direct recombination process in a guest material
because the generation efficiency of the triplet excited state of
the guest material 133 can be decreased and the occurrence of
thermal deactivation of excited energy when the excited state of
the guest material 133 is a triplet excited state can be reduced.
Thus, as mentioned above, it is preferable that the weight
percentage of the guest material 133 be smaller than that of the
organic compound 131. Specifically, their weight ratio (the organic
compound 131:the guest material 133) is preferably from 1:0.001 to
1:0.05, further preferably from 1:0.001 to 1:0.01.
<<(.gamma.) Energy Transfer Process from a Host
Material>>
[0102] Next, the energy transfer process from the organic compound
132 to the organic compound 131 or the guest material 133 is
described with reference to FIG. 2C, which is a schematic diagram
showing the correlation between energy levels. Note that indication
and numerals in FIG. 2C are similar to those in FIG. 2A.
[0103] Carriers are recombined in the organic compound 132, and the
organic compound 132 is brought into an excited state. In the case
where the excited state of the organic compound 132 is a single
excited state and S.sub.H of the organic compound 132 is higher
than S.sub.A of the organic compound 131 and S.sub.G of the guest
material 133, the singlet excitation energy is transferred from
S.sub.H of the organic compound 132 to S.sub.G of the guest
material 133, whereby the guest material 133 is brought into the
singlet excited state. Alternatively, the singlet excitation energy
transferred from S.sub.H of the organic compound 132 to S.sub.A of
the organic compound 131 is transferred to S.sub.G of the guest
material 133 through the above-described (.beta.) energy transfer
process from a thermally activated delayed fluorescence substance.
Fluorescence is obtained from the guest material 133 in the singlet
excited state. Note that the organic compound 132 in this
embodiment is a host material.
[0104] Note that since direct transition of the guest material 133
from a singlet ground state to a triplet excited state is
forbidden, energy transfer from the organic compound 132 in the
singlet excited state to the guest material 133 in the triplet
excited state is unlikely to be a main energy transfer process;
therefore, a description thereof is omitted. In other words, energy
transfer from the organic compound 132 in the singlet excited state
to the guest material 133 in the singlet excited state as shown in
the following general formula (G3) or (G4) is possible.
.sup.1H*+.sup.1A+.sup.1G.fwdarw..sup.1H+.sup.1A+.sup.1G* (G3)
.sup.1H*+.sup.1A+.sup.1G.fwdarw.H+.sup.1A*+.sup.1G.fwdarw.H+.sup.1A+.sup-
.1G* (G4)
[0105] Note that in the general formula (G3) or (G4), .sup.1H*,
.sup.1A*, and .sup.1G* represent the singlet excitation states of
the organic compound 132, the organic compound 131, and the guest
material 133, respectively; and .sup.1H, .sup.1A, and .sup.1G
represent the singlet ground states of the organic compound 132,
the organic compound 131, and the guest material 133,
respectively.
[0106] In the case where the exited state of the organic compound
132 is the triplet excited state, when T.sub.H of the organic
compound 132 is higher than T.sub.A of the organic compound 131 and
S.sub.A of the organic compound 131 is higher than S.sub.G of the
guest material 133, fluorescence is obtained through the following
process.
[0107] First, energy is transferred from T.sub.H of the organic
compound 132 to T.sub.A of the organic compound 131.
[0108] Subsequently, as described in the (.beta.) energy transfer
process from a thermally activated delayed fluorescence substance,
energy is transferred from S.sub.A of the organic compound 131 to
S.sub.G of the guest material 133 through the reverse intersystem
crossing (the route A.sub.1) in the organic compound 131, which is
a thermally activated delayed fluorescence substance, so that
fluorescence is obtained from the guest material 133 in the singlet
excited state.
[0109] The energy transfer process is expressed by the following
general formula (G5).
.sup.3H*+.sup.1A+.sup.1G.fwdarw..sup.1H+.sup.3A*+.sup.1G.fwdarw.(reverse
intersystem
crossing).fwdarw..sup.1H+.sup.1A*+.sup.1G.fwdarw..sup.1H+.sup.1A+.sup.1G*
(G5)
[0110] Note that in the general formula (G5), .sup.3H* and .sup.3A*
represent the triplet excitation states of the organic compounds
132 and 131, respectively; .sup.1A*, and .sup.1G* represent the
singlet excitation states of the organic compound 131 and the guest
material 133, respectively; and .sup.1H, .sup.1A, and .sup.1G
represent the singlet ground states of the organic compound 132,
the organic compound 131, and the guest material 133,
respectively.
[0111] As represented by the general formula (G5), the triplet
excited state (.sup.3A*) of the organic compound 131 is generated
from the triplet excited state (.sup.3H*) of the organic compound
132. Immediately after that, the singlet excited state (.sup.1A*)
of the organic compound 131 is generated by reverse intersystem
crossing, and then, energy is transferred to the singlet excited
state (.sup.1G*) of the guest material 133.
[0112] When all the energy transfer processes described above in
the (.gamma.) energy transfer process from a host material occur
efficiently, both the triplet excitation energy and the singlet
excitation energy of the organic compound 132 are efficiently
converted into the singlet excited state (.sup.1G*) of the guest
material 133, and emission from the guest material 133 is
possible.
[0113] However, if the organic compound 132 releases excitation
energy as light or heat and is deactivated before the excitation
energy is transferred from the singlet excited state of the organic
compound 132 to the singlet excited state of the guest material
133, the emission efficiency of the light-emitting element is
decreased. In addition, the emission efficiency is also decreased
by a decrease in efficiency of the route A.sub.1, which is the
previous process where the organic compound 131 is transferred from
a triplet excited state to a singlet excited state by reverse
intersystem crossing. Particularly when T.sub.H of the organic
compound 132 is lower than T.sub.A of the organic compound 131,
energy transfer process from T.sub.H of the organic compound 132 to
T.sub.A of the organic compound 131 is unlikely to occur and
reverse intersystem crossing in the organic compound 131 does not
occur, leading to a decrease in generation efficiency of a singlet
excitation state of the guest material 133. Thus, T.sub.H of the
organic compound 132 is preferably higher than T.sub.A of the
organic compound 131.
[0114] In the case where excitation energy is transferred from the
T.sub.H of the organic compound 132 to the T.sub.G of the guest
material 133 as shown by a route E.sub.3 in FIG. 2C, the excitation
energy is also thermally deactivated. Therefore, it is preferable
that the energy transfer process shown by the route E.sub.3 in FIG.
2C be less likely to occur because the generation efficiency of the
triplet excited state of the guest material 133 can be decreased
and the occurrence of thermal deactivation can be reduced. Thus,
the weight percentage of the guest material 133 is preferably
smaller than that of the organic compound 132. Specifically, their
weight ratio (the organic compound 132:the guest material 133) is
preferably from 1:0.001 to 1:0.05, further preferably from 1:0.001
to 1:0.01.
[0115] As described above, although part of excitation energy is
converted into fluorescence of the guest material 133 in the
(.gamma.) energy transfer process from a host material, there is a
possibility of thermal deactivation in the routes E.sub.2 and
E.sub.3 in FIG. 2C. Thus, it is preferable that the probability of
the (0) energy transfer process from a thermally activated delayed
fluorescence substance be higher than those of the (.gamma.) energy
transfer process from a host material and the (.alpha.) direct
recombination process in a guest material because the generation
efficiency of the triplet excited state in the light-emitting layer
120 can be decreased, that is, the occurrence of thermal
deactivation can be reduced and the emission efficiency of the
light-emitting element 150 can be increased. Carrier recombination
in the organic compound 131, which is a thermally activated delayed
fluorescence substance, is important for increasing the probability
of the (.beta.) energy transfer process from a thermally activated
delayed fluorescence substance.
<<Carrier Recombination>>
[0116] The relationship of energy level between the organic
compound 131 and the organic compound 132 is important to generate
carrier recombination in the organic compound 131. In particular,
the relationship of energy level between a highest occupied
molecular orbital (HOMO) and a lowest unoccupied molecular orbital
(LUMO), or the relationship between an oxidation potential and a
reduction potential is important.
[0117] Carriers injected from the pair of electrodes into the EL
layer 100 reach the light-emitting layer 120, whereby they are
injected into a substance included in the light-emitting layer 120.
At this time, holes and electrons tend to enter more stable HOMO
and LUMO, respectively. Thus, what is important for carrier
recombination in the organic compound 131, which is a thermally
activated delayed fluorescence substance, is that the HOMO level of
the organic compound 131 is higher than or equal to the HOMO level
of the organic compound 132 and that the LUMO level of the organic
compound 131 is lower than or equal to the LUMO level of the
organic compound 132. It is also important that the oxidation
potential of the organic compound 131 is lower than or equal to the
oxidation potential of the organic compound 132 and the reduction
potential of the organic compound 131 is higher than or equal to
the reduction potential of the organic compound 132.
[0118] In such a structure, exciplexes are unlikely to be formed
between the organic compounds 131 and 132.
[0119] Although carriers can easily transfer between adjacent
molecules of the organic compound 131 in the light-emitting layer
120, carriers easily transfer also to a functional layer (e.g., the
hole-transport layer 112 and the electron-transport layer 118)
other than the light-emitting layer 120. It is thus preferable that
the weight percentage of the organic compound 131 is smaller than
that of the organic compound 132 for carrier recombination in the
organic compound 131 in the light-emitting layer 120. In addition,
in order to suppress energy transfer between excited-state
molecules and ground-state molecules of the organic compound 131,
the weight percentage of the organic compound 131 is preferably
smaller than that of the organic compound 132. In the case where a
molecule of the organic compound 131 is adjacent to a molecule of
the guest material 133, there is a possibility of energy transfer
from a triplet excitation state of the organic compound 131 to a
triplet excitation state of the guest material 133. Thus, the
weight percentage of the organic compound 131 is preferably smaller
than that of the organic compound 132 in order to suppress the
energy transfer. Specifically, their weight ratio (the organic
compound 132:the organic compound 131) is preferably from 1:0.05 to
1:0.5.
3. Energy Transfer Mechanism
[0120] Next, factors controlling the above-described processes of
intermolecular energy transfer between the organic compound 131 or
132 and the guest material 133 are described. As mechanisms of the
intermolecular energy transfer, two mechanisms, i.e., Forster
mechanism (dipole-dipole interaction) and Dexter mechanism
(electron exchange interaction), have been proposed. Although
intermolecular energy transfer between the organic compound 131 and
the guest material 133 is described here, the same is applied to
intermolecular energy transfer between the organic compound 132 and
the guest material 133.
<<Forster Mechanism>>
[0121] In Forster mechanism, energy transfer does not require
direct contact between molecules and energy is transferred through
a resonant phenomenon of dipolar oscillation between the organic
compound 131 and the guest material 133. By the resonant phenomenon
of dipolar oscillation, the organic compound 131 provides energy to
the guest material 133, and thus, the organic compound 131 in an
excited state is put in a ground state and the guest material 133
in a ground state is put in an excited state. Note that the rate
constant k.sub.h*.fwdarw.g of Forster mechanism is expressed by
Formula 1.
k h * -> g = 9000 c 4 K 2 .phi. ln 10 128 .pi. 5 n 4 N .tau. R 6
.intg. f h ' ( v ) g ( v ) v 4 v [ Formula 1 ] ##EQU00001##
[0122] In Formula 1, .nu. denotes a frequency, f'.sub.h(.nu.)
denotes a normalized emission spectrum of the organic compound 131
(a fluorescent spectrum in energy transfer from a singlet excited
state, and a phosphorescent spectrum in energy transfer from a
triplet excited state), .epsilon..sub.g(.nu.) denotes a molar
absorption coefficient of the guest material 133, N denotes
Avogadro's number, n denotes a refractive index of a medium, R
denotes an intermolecular distance between the organic compound 131
and the guest material 133, .tau. denotes a measured lifetime of an
excited state (fluorescence lifetime or phosphorescence lifetime),
c denotes the speed of light, .phi. denotes a luminescence quantum
yield (a fluorescence quantum yield in energy transfer from a
singlet excited state, and a phosphorescence quantum yield in
energy transfer from a triplet excited state), and K.sup.2 denotes
a coefficient (0 to 4) of orientation of a transition dipole moment
between the organic compound 131 and the guest material 133. Note
that K.sup.2=2/3 in random orientation.
<<Dexter Mechanism>>
[0123] In Dexter mechanism, the organic compound 131 and the guest
material 133 are close to a contact effective range where their
orbitals overlap, and the organic compound 131 in an excited state
and the guest material 133 in a ground state exchange their
electrons, which leads to energy transfer. Note that the rate
constant k.sub.h*.fwdarw.g of Dexter mechanism is expressed by
Formula 2.
k h * -> g = ( 2 .pi. h ) K 2 exp ( - 2 R L ) .intg. f h ' ( v )
g ' ( v ) v [ Formula 2 ] ##EQU00002##
[0124] In Formula 2, h denotes a Planck constant, K denotes a
constant having an energy dimension, .nu. denotes a frequency,
f'.sub.h(.nu.) denotes a normalized emission spectrum of the
organic compound 131 (a fluorescent spectrum in energy transfer
from a singlet excited state, and a phosphorescent spectrum in
energy transfer from a triplet excited state),
.epsilon.'.sub.g(.nu.) denotes a normalized absorption spectrum of
the guest material 133, L denotes an effective molecular radius,
and R denotes an intermolecular distance between the organic
compound 131 and the guest material 133.
[0125] Here, the efficiency of energy transfer from the organic
compound 131 to the guest material 133 (energy transfer efficiency
.phi..sub.ET) is expressed by Formula 3. In Formula 3, k.sub.r
denotes a rate constant of a light-emission process (fluorescence
in energy transfer from a singlet excited state, and
phosphorescence in energy transfer from a triplet excited state) of
the organic compound 131, k.sub.n denotes a rate constant of a
non-light-emission process (thermal deactivation or intersystem
crossing) of the organic compound 131, and r denotes a measured
lifetime of an excited state of the organic compound 131.
.phi. ET = k h * -> g k r + k n + k h * -> g = k h * -> g
( 1 .tau. ) + k h * -> g [ Formula 3 ] ##EQU00003##
[0126] According to Formula 3, it is found that the energy transfer
efficiency .phi..sub.ET can be increased by increasing the rate
constant k.sub.h*.fwdarw.g of energy transfer so that another
competing rate constant k.sub.r+k.sub.n(=1/.tau.) becomes
relatively small.
<<Concept for Promoting Energy Transfer>>
[0127] In both the energy transfer processes of the general
formulae (G1) and (G2), since energy is transferred from the
singlet excited state (.sup.1A*) of the organic compound 131 to the
singlet excited state (.sup.1G*) of the guest material 133, energy
transfers by both Forster mechanism (Formula 1) and Dexter
mechanism (Formula 2) occur.
[0128] First, an energy transfer by Forster mechanism is
considered. When .tau. is eliminated from Formula 1 and Formula 3,
it can be said that the energy transfer efficiency .phi..sub.ET is
higher when the quantum yield .phi. (here, a fluorescence quantum
yield because energy transfer from a singlet excited state is
discussed) is higher. However, in practice, a more important factor
is that the emission spectrum of the organic compound 131 (here, a
fluorescent spectrum because energy transfer from a singlet excited
state is discussed) largely overlaps with the absorption spectrum
of the guest material 133 (absorption corresponding to the
transition from the singlet ground state to the singlet excited
state). Note that it is preferable that the molar absorption
coefficient of the guest material 133 be also high. This means that
the emission spectrum of the organic compound 131 overlaps with the
absorption band of the guest material 133 which is on the longest
wavelength side.
[0129] Next, an energy transfer by Dexter mechanism is considered.
According to Formula 2, in order to increase the rate constant
k.sub.h*.fwdarw.g, it is preferable that an emission spectrum of
the organic compound 131 (here, a fluorescent spectrum because
energy transfer from a singlet excited state is discussed) largely
overlap with an absorption spectrum of the guest material 133
(absorption corresponding to transition from a singlet ground state
to a singlet excited state).
[0130] The above description suggests that the energy transfer
efficiency can be optimized by making the emission spectrum of the
organic compound 131 overlap with the absorption band of the guest
material 133 which is on the longest wavelength side.
[0131] In view of this, one embodiment of the present invention
provides a light-emitting element which includes the organic
compound 131 having a function as an energy donor capable of
efficiently transferring energy to the guest material 133. The
organic compound 131 is a thermally activated delayed fluorescence
substance and thus has a feature that the singlet excitation energy
level and the triplet excitation energy level are close to each
other. Specifically, it is preferable that the organic compound 131
have a difference of larger than 0 eV and smaller than or equal to
0.2 eV between the singlet excitation energy level and the triplet
excitation energy level. This enables transition (reverse
intersystem crossing) of the organic compound 131 from the triplet
excited state to the singlet excited state to be likely to occur.
Therefore, the generation efficiency of the singlet excited state
of the organic compound 131 can be increased. Furthermore, in order
to facilitate energy transfer from the singlet excited state of the
organic compound 131 to the singlet excited state of the guest
material 133 having a function as an energy acceptor, it is
preferable that the emission spectrum of the organic compound 131
overlap with the absorption band of the guest material 133 which is
on the longest wavelength side. Thus, the generation efficiency of
the singlet excited state of the guest material 133 can be
increased.
[0132] In addition, in the light-emitting element 150 of one
embodiment of the present invention, the HOMO level of the organic
compound 131 is higher than or equal to the HOMO level of the
organic compound 132, and the LUMO level of the organic compound
131 is lower than or equal to the LUMO level of the organic
compound 132; or the oxidation potential of the organic compound
131 is lower than or equal to the oxidation potential of the
organic compound 132 and the reduction potential of the organic
compound 131 is higher than or equal to the reduction potential of
the organic compound 132, which allows recombination of carriers
injected into the EL layer 100 to be performed efficiently in the
organic compound 131. Thus, the occurrence of thermal deactivation
can be reduced and the emission efficiency can be increased.
4. Materials
[0133] Next, components of a light-emitting element of one
embodiment of the present invention are described in detail.
<<Light-Emitting Layer>>
[0134] The organic compound 131 in the light-emitting layer 120 is
composed of one kind of material. Note that another compound having
a function similar to the organic compound 131 may be included in
the light-emitting layer 120. For example, in the case where the
organic compound 131 is composed of one kind of material, any of
the following materials can be used.
[0135] First, a fullerene, a derivative thereof, an acridine
derivative such as proflavine, eosin, or the like can be given.
Furthermore, a metal-containing porphyrin, such as a porphyrin
containing magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn),
platinum (Pt), indium (In), or palladium (Pd), can be given.
Examples of the metal-containing porphyrin include a
protoporphyrin-tin fluoride complex (SnF.sub.2(Proto IX)), a
mesoporphyrin-tin fluoride complex (SnF.sub.2(Meso IX)), a
hematoporphyrin-tin fluoride complex (SnF.sub.2(Hemato IX)), a
coproporphyrin tetramethyl ester-tin fluoride complex
(SnF.sub.2(Copro III-4Me)), an octaethylporphyrin-tin fluoride
complex (SnF.sub.2(OEP)), an etioporphyrin-tin fluoride complex
(SnF.sub.2(Etio I)), and an octaethylporphyrin-platinum chloride
complex (PtCl.sub.2(OEP)), which are shown in the following
structural formulae.
##STR00001## ##STR00002## ##STR00003##
[0136] Alternatively, a heterocyclic compound having a
.pi.-electron rich heteroaromatic ring and a .pi.-electron
deficient heteroaromatic ring, such as
2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1-
,3,5-triazine (abbreviation: PIC-TRZ), 2-{4-[3
(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-t-
riazine (abbreviation: PCCzPTzn),
2-[4-(10H-phenoxazine-0-yl)phenyl]-4,6-diphenyl-1,3,5-triazine
(abbreviation: PXZ-TRZ),
3-[4-(5-phenyl-5,10-dihydrophenazine-10-yl)phenyl]-4,5-diphenyl-1,2,4-tri-
azole (abbreviation: PPZ-3TPT),
3-(9,9-dimethyl-9H-acridin-10-yl)-9H-xanthen-9-one (abbreviation:
ACRXTN), bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone
(abbreviation: DMAC-DPS), or 10-phenyl-10H,
10'H-spiro[acridin-9,9'-anthracen]-10'-one (abbreviation: ACRSA)
shown in the following structural formulae, can be used as the
organic compound 131. The heterocyclic compound is preferably used
because of the .pi.-electron rich heteroaromatic ring and the
.pi.-electron deficient heteroaromatic ring, for which the
electron-transport property and the hole-transport property are
high. Note that a substance in which the .pi.-electron rich
heteroaromatic ring is directly bonded to the .pi.-electron
deficient heteroaromatic ring is particularly preferably used
because the donor property of the .pi.-electron rich heteroaromatic
ring and the acceptor property of the .pi.-electron deficient
heteroaromatic ring are both increased and the difference between
the level of the singlet excited state and the level of the triplet
excited state becomes small. The heterocyclic compound is
preferably used because of the .pi.-electron rich heteroaromatic
ring and the .pi.-electron deficient heteroaromatic ring, for which
the electron-transport property and the hole-transport property are
high. Among skeletons having the .pi.-electron deficient
heteroaromatic ring, a diazine skeleton (a pyrimidine skeleton, a
pyrazine skeleton, or a pyridazine skeleton) and a triazine
skeleton have favorable stability and reliability and are
particularly preferable. Among skeletons having the .pi.-electron
rich heteroaromatic ring, an acridine skeleton, a phenoxazine
skeleton, or a 3-(9-phenyl-9H-carbazole-3-yl)-9H-carbazole skeleton
has favorable stability and reliability; thus, any of the skeletons
is particularly preferable. Note that a substance in which the
.pi.-electron rich heteroaromatic ring is directly bonded to the
.pi.-electron deficient heteroaromatic ring is particularly
preferably used because the donor property of the .pi.-electron
rich heteroaromatic ring and the acceptor property of the
.pi.-electron deficient heteroaromatic ring are both increased and
the difference between the level of the singlet excited state and
the level of the triplet excited state becomes small.
##STR00004## ##STR00005##
[0137] The following compounds can be used as the organic compound
132 in the light-emitting layer 120. Because the organic compound
132 functions as a host material in the light-emitting layer 120,
it preferably contains a skeleton which easily receives electrons
(a skeleton having an electron-transport property) and/or a
skeleton which easily receives holes (a skeleton having an
hole-transport property).
[0138] As the compound containing a skeleton which easily accepts
electrons (a skeleton having an electron-transport property), a
compound, a metal complex, or the like including a .pi.-electron
deficient heteroaromatic skeleton can be used. Specific examples
include a metal complex such as
bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation:
BeBq.sub.2), bis(2-methyl-8-quinolinolato)
(4-phenylphenolato)aluminum(III) (abbreviation: BAlq),
bis(8-quinolinolato)zinc(II) (abbreviation: Znq),
bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), or
bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); a
heterocyclic compound having an azole skeleton such as
2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(abbreviation: PBD),
3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole
(abbreviation: TAZ),
9-[4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl]-9H-carbazole
(abbreviation: CzTAZ1),
1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene
(abbreviation: OXD-7),
9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole
(abbreviation: CO11),
2,2',2''-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)
(abbreviation: TPBI), or
2-[3-(dibenzothiophene-4-yl)phenyl]-1-phenyl-1H-benzimidazole
(abbreviation: mDBTBIm-II); a heterocyclic compound having a
diazine skeleton such as
2-[3-(dibenzothiophene-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2mDBTPDBq-II),
2-[3'-(dibenzothiophene-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline
(abbreviation: 2mDBTBPDBq-II),
2-[3'-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline
(abbreviation: 2mCzBPDBq),
2-[3-(3,9'-bi-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2mCzCzPDBq),
4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation;
4,6mCzP2Pm), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine
(abbreviation: 4,6mPnP2Pm), or
4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:
4,6mDBTP2Pm-II); a heterocyclic compound having a triazine skeleton
such as
2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphe-
nyl-1,3,5-triazine (abbreviation: PCCzPTzn); and a heterocyclic
compound having a pyridine skeleton such as
3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:
35DCzPPy) or 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation:
TmPyPB). Among the heterocyclic compounds, the heterocyclic
compounds having diazine skeletons (pyrimidine, pyrazine,
pyridazine) or having a pyridine skeleton are highly reliable and
stable and is thus preferably used. In addition, the heterocyclic
compounds having the skeletons have a high electron-transport
property to contribute to a reduction in drive voltage.
[0139] As the compound having a skeleton which easily accepts holes
(a skeleton having a hole-transport property), a compound having a
.pi.-electron rich heteroaromatic skeleton, an aromatic amine
skeleton, or the like can be favorably used. Specific examples
include a compound having an aromatic amine skeleton such as
2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9'-bifluorene
(abbreviation: PCASF),
4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),
N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine
(abbreviation: TPD),
4,4'-bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl
(abbreviation: BSPB),
4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:
BPAFLP), 4-phenyl-3'-(9-phenylfluoren-9-yl)triphenylamine
(abbreviation: mBPAFLP),
4-phenyl-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBA1BP),
4,4'-diphenyl-4''-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBBi1BP),
4-(1-naphthyl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBANB),
4,4'-di(1-naphthyl)-4''-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBNBB),
9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-am-
ine (abbreviation: PCBAF), or
N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9'-bifluoren-2-am-
ine (abbreviation: PCBASF);
[0140]
N-(1,1'-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl-
)phenyl]-9H-fluoren-2-amine (abbreviation: PCBBiF), a compound
having a carbazole skeleton such as 1,3-bis(N-carbazolyl)benzene
(abbreviation: mCP), 4,4'-di(N-carbazolyl)biphenyl (abbreviation:
CBP), 3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation:
CzTP), 3,6-di(9H-carbazol-9-yl)-9-phenyl-9H-carbazole
(abbreviation: PhCzGI), 2,8-di(9H-carbazol-9-yl)-dibenzothiophene
(abbreviation: Cz2DBT), or
9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation:
PCCP); a compound having a thiophene skeleton such as
4,4',4''-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:
DBT3P-II),
2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene
(abbreviation: DBTFLP-III), or
4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene
(abbreviation: DBTFLP-IV); and a compound having a furan skeleton
such as 4,4',4''-(benzene-1,3,5-triyl)tri(dibenzofuran)
(abbreviation: DBF3P-II) or
4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran
(abbreviation: mmDBFFLBi-II). Among the above-described compounds,
a compound including any one or more of a furan skeleton, a
thiophene skeleton, a fluorine skeleton, and a pyrrole skeleton is
preferable because it is stable and reliable and has a high
hole-transport property to contribute to a reduction in driving
voltage.
[0141] In addition, among the above-described compounds, a compound
including a pyridine skeleton or a diazine skeleton (a pyrimidine
skeleton, a pyrazine skeleton, and a pyridazine skeleton) as a
.pi.-electron deficient heteroaromatic skeleton and including any
one or more of a furan skeleton, a thiophene skeleton, a fluorine
skeleton, and a pyrrole skeleton as a .pi.-electron rich
heteroaromatic skeleton has a high carrier-transport property and
thus contributes to a reduction in driving voltage. In addition, a
compound including any of the skeletons has favorable reliability
and thus is preferable. Note that an indole skeleton, a carbazole
skeleton, or the 3-(9H-carbazol-9-yl)-9H-carbazole skeleton is
particularly preferable as a pyrrole skeleton.
[0142] Note that the above-described compounds are non-limiting
examples of the organic compounds 131 and 132, and other materials
may be used as long as they can transport carriers and they satisfy
the following conditions: the HOMO level of the organic compound
131 is higher than or equal to the HOMO level of the organic
compound 132, and the LUMO level of the organic compound 131 is
lower than or equal to the LUMO level of the organic compound 132;
or the oxidation potential of the organic compound 131 is lower
than or equal to the oxidation potential of the organic compound
132 and the reduction potential of the organic compound 131 is
higher than or equal to the reduction potential of the organic
compound 132. In addition, a thermally activated delayed
fluorescence substance may be used for the organic compound
132.
[0143] Table 1 shows measurement results of HOMO and LUMO levels of
the above-described compounds in the thin-film state, which are
non-limiting examples of the organic compounds 131 and 132. Table 2
shows measurement results of the oxidation potentials and the
reduction potentials of the compounds in the solution state and the
HOMO and LUMO levels estimated from the results. Table 3 shows
measurement results of the triplet excitation energy levels.
[0144] The structures and abbreviations of these compounds are
shown below.
##STR00006## ##STR00007##
TABLE-US-00001 TABLE 1 HOMO(eV) LUMO(eV) in thin- in thin-
Abbreviation film state film state Organic compound PCCzPTzn -5.86
-3.01 131 (First organic PXZ-TRZ -5.63 -3.13 compound)) Organic
compound 2mDBTBPDBq-II -6.17 -3.07 132 (Second organic compound)
2mCzBPDBq -5.78 -2.67 2mCzCzPDBq -5.95 -2.88 4,6mDBTP2Pm-II -6.36
-2.87 4,6mCzP2Pm -6.23 -2.77 35DCzPPy -6.21 -2.73
TABLE-US-00002 TABLE 2 HOMO(eV) LUMO(eV) Oxidation Reduction
estimated from estimated from potential(V) potential(V) oxidation
potential oxidation potential Abbreviation in solution state in
solution state in solution state in solution state Organic compound
PCCzPTzn 0.70 -1.97 -5.64 -2.97 131 (First organic PXZ-TRZ 0.39
-1.95 -5.33 -2.99 compound) Organic compound 2mDBTBPDBq-II 1.28
-2.00 -6.22 -2.94 132 (Second organic 2mCzBPDBq 0.97 -1.99 -5.91
-2.95 compound) 2mCzCzPDBq 0.80 -1.97 -5.74 -2.97 4,6mDBTP2Pm-II
1.28 -2.12 -6.22 -2.83 4,6mCzP2Pm 0.95 -2.06 -5.89 -2.88 35DCzPPy
0.96 -2.56 -5.90 -2.39
TABLE-US-00003 TABLE 3 Triplet excitation Abbreviation energy
level(T.sub.1) (eV) Organic compound PCCzPTzn 2.53 131 (First
organic compound) Organic compound 2mDBTBPDBq-II 2.41 132 (Second
organic 2mCzCzPDBq 2.42 compound) 4,6mDBTP2Pm-II 2.62 4,6mCzP2Pm
2.70 35DCzPPy 2.75
[0145] To obtain the HOMO level of each compound in the thin-film
state, the ionization potential of each compound was measured by a
photoelectron spectrometer (AC-3, manufactured by Riken Keiki, Co.,
Ltd.) in the air, and the measured ionization potentials were
converted into negative values. In addition, to estimate the
optical bandgap of each compound in the solid state, an absorption
spectrum of each compound in the thin-film state was measured and
the absorption edge was obtained from Tauc plot with an assumption
of direct transition. The LUMO energy in the thin-film state was
calculated from the energy of the estimated bandgaps and the HOMO
levels, which have been obtained.
[0146] Electrochemical characteristics (oxidation and reduction
characteristics) of each compound in the solution state were
measured by cyclic voltammetry (CV). Note that an electrochemical
analyzer (ALS model 600A or 600C, product of BAS Inc.) was used for
the measurement. In the measurements, the potential of a working
electrode with respect to the reference electrode was changed
within an appropriate range, whereby the oxidation peak potential
and the reduction peak potential were obtained. In addition, the
HOMO and LUMO levels of each compound were calculated from the
estimated redox potential of the reference electrode of -4.94 eV
and the obtained peak potentials.
[0147] The triplet excitation energy levels were measured by
phosphorescence measurement of the compounds. The measurement was
performed by using a PL microscope, LabRAM HR-PL, produced by
HORIBA, Ltd., a He--Cd laser (325 nm) as excitation light, and a
CCD detector at a measurement temperature of 10 K. The triplet
excitation energy levels were calculated from a peak on the
shortest wavelength side of the phosphorescent spectrum obtained by
the measurement.
[0148] With the use of compounds which satisfy the following
condition: the HOMO level of the organic compound 131 is higher
than or equal to the HOMO level of the organic compound 132, and
the LUMO level of the organic compound 131 is lower than or equal
to the LUMO level of the organic compound 132; or the oxidation
potential of the organic compound 131 is lower than or equal to the
oxidation potential of the organic compound 132 and the reduction
potential of the organic compound 131 is higher than or equal to
the reduction potential of the organic compound 132, which are
shown in Table 1 and Table 2 as an example, recombination of
carriers injected into the EL layer 100 can be efficiently
performed in the organic compound 131 and thus a light-emitting
element with high emission efficiency can be provided.
[0149] In addition, with the use of compounds such that the triplet
excitation energy level of the organic compound 132 is higher than
that of the organic compound 131, which are shown in Table 3 as an
example, energy can be easily transferred from the triplet
excitation energy level of the organic compound 132 to that of the
organic compound 131. Thus, the (.gamma.) energy transfer process
from a host material becomes easily occur and a light-emitting
element with high emission efficiency can be provided.
[0150] In the light-emitting layer 120, the guest material 133
(fluorescent material) is preferably, but not particularly limited
to, an anthracene derivative, a tetracene derivative, a chrysene
derivative, a phenanthrene derivative, a pyrene derivative, a
perylene derivative, a stilbene derivative, an acridone derivative,
a coumarin derivative, a phenoxazine derivative, a phenothiazine
derivative, or the like, and for example, any of the following
materials can be used.
[0151] The examples include
5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2'-bipyridine
(abbreviation: PAP2BPy),
5,6-bis[4'-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2'-bipyridine
(abbreviation: PAPP2BPy),
N,N'-diphenyl-N,N'-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diam-
ine (abbreviation: 1,6FLPAPrn),
N,N'-bis(3-methylphenyl)-N,N-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyren-
e-1,6-diamine (abbreviation: 1,6mMemFLPAPrn),
N,N'-bis[4-(9H-carbazol-9-yl)phenyl]-N,N'-diphenylstilbene-4,4'-diamine
(abbreviation: YGA2S),
4-(9H-carbazol-9-yl)-4'-(10-phenyl-9-anthryl)triphenylamine
(abbreviation: YGAPA),
4-(9H-carbazol-9-yl)-4'-(9,10-diphenyl-2-anthryl)triphenylamine
(abbreviation: 2YGAPPA),
N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine
(abbreviation: PCAPA), perylene, 2,5,8,11-tetra(tert-butyl)perylene
(abbreviation: TBP),
4-(10-phenyl-9-anthryl)-4'-(9-phenyl-9H-carbazol-3-yl)triphenylamine
(abbreviation: PCBAPA),
N,N''-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N',N'-triph-
enyl-1,4-phenylenediamine] (abbreviation: DPABPA),
N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine
(abbreviation: 2PCAPPA),
N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N',N-triphenyl-1,4-phenylenediami-
ne (abbreviation: 2DPAPPA),
N,N,N,N',N'',N'',N'',N''-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraam-
ine (abbreviation: DBC1), coumarin 30,
N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine
(abbreviation: 2PCAPA),
N-[9,10-bis(1,1'-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-ami-
ne (abbreviation: 2PCABPhA),
N-(9,10-diphenyl-2-anthryl)-N,N',N-triphenyl-1,4-phenylenediamine
(abbreviation: 2DPAPA),
N-[9,10-bis(1,1'-biphenyl-2-yl)-2-anthryl]-N,N',N'-triphenyl-1,4-phenylen-
ediamine (abbreviation: 2DPABPhA),
9,10-bis(1,1'-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthr-
acen-2-amine (abbreviation: 2YGABPhA),
N,N,9-triphenylanthracen-9-amine (abbreviation: DPhAPhA), coumarin
6, coumarin 545T, N,N'-diphenylquinacridone (abbreviation: DPQd),
5,6,11,12-tetraphenylnaphthacene (common name: rubrene),
5,12-bis(1,1'-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation:
BPT),
2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)pro-
panedinitrile (abbreviation: DCM1),
2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[i]quinolizin-9-yl)etheny-
l]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCM2),
N,N,N',N'-tetrakis(4-methylphenyl)tetracene-5,11-diamine
(abbreviation: p-mPhTD),
7,14-diphenyl-N,N,N',N'-tetrakis(4-methylphenyl)acenaphtho[1,2--
a]fluoranthene-3,10-diamine (abbreviation: p-mPhAFD),
2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[i-
j]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile
(abbreviation: DCJTI),
2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[-
ij]quinolizin-9-yl)ethe nyl]-4H-pyran-4-ylidene}propanedinitrile
(abbreviation: DCJTB), 2-(2,6-bis
{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile
(abbreviation: BisDCM),
2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benz-
o[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile
(abbreviation: BisDCJTM), and
5,10,15,20-tetraphenylbisbenzo[5,6]indeno[1,2,3-cd:
1',2',3'-lm]perylene.
[0152] Because the above-described materials of the guest material
133 are non-limiting examples, other materials may be used as long
as light emission (thermally activated delayed fluorescence) of the
organic compound 131 which is an energy donor overlaps with an
absorption band (absorption corresponding to the transition of the
guest material 133 from the singlet ground state to the singlet
excited state) on the longest wavelength in an absorption spectrum
of the guest material 133 which is an energy accepter.
[0153] Note that the light-emitting layer 120 can be formed by an
evaporation method (including a vacuum evaporation method), an
inkjet method, a coating method, gravure printing, or the like.
[0154] Next, details of other components of the light-emitting
element 150 in FIG. 1A are described.
<<Pair of Electrodes>>
[0155] The electrode 101 and the electrode 102 have functions of
injecting holes and electrons into the light-emitting layer 120.
The electrodes 101 and 102 can be formed using a metal, an alloy,
or a conductive compound, or a mixture or a stack thereof, for
example. A typical example of the metal is aluminum; besides, a
transition metal such as silver, tungsten, chromium, molybdenum,
copper, or titanium, an alkali metal such as lithium, sodium, or
cesium, or a Group 2 metal such as calcium or magnesium can be
used. As the transition metal, a rare earth metal such as ytterbium
(Yb) may be used. An alloy containing any of the above metals can
be used as the alloy, 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 electrode 101 and/or
the electrode 102 may be formed by stacking two or more of these
materials.
[0156] Light emitted from the light-emitting layer 120 is extracted
through the electrode 101 and/or the electrode 102. Therefore, at
least one of the electrodes 101 and 102 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 electrode 101 and/or the electrode 102 is formed to a
thickness that is thin enough to transmit visible light (e.g., a
thickness of 1 nm to 10 nm).
<<Hole-Injection Layer>>
[0157] The hole-injection layer 111 has a function of reducing a
barrier for hole injection from the electrode 101 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, 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
self-doped polythiophene.
[0158] As the hole-injection layer 111, a layer containing a
composite material of 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. In a
steady state or in the presence of an electric field, electric
charge can be transferred between these materials. 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 compound 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 more preferable because of its stability in the
atmosphere, low hygroscopic property, and easiness of handling.
[0159] 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. The compound including a skeleton that easily
accepts holes which is described as an example of the organic
compound 132 can be used. Furthermore, the hole-transport material
may be a high molecular compound.
<<Hole-Transport Layer>>
[0160] The hole-transport layer 112 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 111. In order that the hole-transport layer 112 has a
function of transporting holes injected into the hole-injection
layer 111 to the light-emitting layer 120, the HOMO level of the
hole-transport layer 112 is preferably equal or close to the HOMO
energy level of the hole-injection layer 111.
<<Electron-Transport Layer>>
[0161] The electron-transport layer 118 has a function of
transporting, to the light-emitting layer 120, electrons injected
from the electrode 102 through the electron-injection layer 119. 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. The compound having a skeleton that easily
accepts electrons which is described as an example of the organic
compound 132 can be used.
<<Electron-Injection Layer>>
[0162] The electron-injection layer 119 has a function of reducing
a barrier for electron injection from the 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 any of the
metals, for example. Alternatively, a composite material containing
an electron-transport material (described above) 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 any of the metals, or the like can be given.
[0163] Note that the hole-injection layer 111, the hole-transport
layer 112, the electron-transport layer 118, and the
electron-injection layer 119 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.
[0164] 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 111, the
hole-transport layer 112, the light-emitting layer 120, the
electron-transport layer 118, and the electron-injection layer
119.
<<Substrate>>
[0165] The light-emitting element 150 is fabricated over a
substrate of glass, plastic, or the like. As the way of stacking
layers over the substrate, layers may be sequentially stacked from
the electrode 101 side or sequentially stacked from the electrode
102 side.
[0166] Note that, for example, glass, quartz, plastic, or the like
can be used for the substrate over which the light-emitting element
150 can be formed. Alternatively, a flexible substrate can be used.
The flexible substrate is a substrate that can be bent, such as a
plastic substrate made of polycarbonate or polyarylate, for
example. A film, an inorganic film formed by evaporation, or the
like can also be used. Note that materials other than these can be
used as long as they can function as a support in a manufacturing
process of the light-emitting element and an optical element or as
long as they have a function of protecting the light-emitting
element and the optical element.
[0167] The light-emitting element 150 can be formed using a variety
of substrates, for example. The type of substrate is not limited to
a certain type. As the substrate, a semiconductor substrate (e.g.,
a single crystal substrate or a silicon substrate), an SOI
substrate, a glass substrate, a quartz substrate, a plastic
substrate, a metal substrate, a stainless steel substrate, a
substrate including stainless steel foil, a tungsten substrate, a
substrate including tungsten foil, a flexible substrate, an
attachment film, paper including a fibrous material, a base
material film, or the like can be used, for example. Examples of
the glass substrate include a barium borosilicate glass substrate,
an aluminoborosilicate glass substrate, and a soda lime glass
substrate. Examples of the flexible substrate, the attachment film,
the base film, and the like are substrates of plastics typified by
polyethylene terephthalate (PET), polyethylene naphthalate (PEN),
polyether sulfone (PES), and polytetrafluoroethylene (PTFE).
Another example is a resin such as acrylic. Other examples are
polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride,
and the like. Other examples are polyamide, polyimide, aramid,
epoxy, an inorganic film formed by evaporation, paper, and the
like.
[0168] Alternatively, a flexible substrate may be used as the
substrate, and the light-emitting element may be provided directly
on the flexible substrate. Alternatively, a separation layer may be
provided between the substrate and the light-emitting element. The
separation layer can be used when part or the whole of the
light-emitting element formed over the separation layer is
completed, separated from the substrate, and transferred to another
substrate. In such a case, the light-emitting element can be
transferred to a substrate having low heat resistance or a flexible
substrate as well. For the above separation layer, a stack
including inorganic films, which are a tungsten film and a silicon
oxide film, or a resin film of polyimide or the like formed over a
substrate can be used, for example.
[0169] In other words, after the light-emitting element is formed
using a substrate, the light-emitting element may be transferred to
another substrate. Examples of a substrate to which the
light-emitting element is transferred include, in addition to the
above-described substrates, a cellophane substrate, a stone
substrate, a wood substrate, a cloth substrate (including a natural
fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g.,
nylon, polyurethane, or polyester), a regenerated fiber (e.g.,
acetate, cupra, rayon, or regenerated polyester, or the like), a
leather substrate, and a rubber substrate. By using such a
substrate, a light-emitting element with high durability, a
light-emitting element with high heat resistance, a lightweight
light-emitting element, or a thin light-emitting element can be
obtained.
[0170] The light-emitting element 150 may be formed over an
electrode electrically connected to a field-effect transistor
(FET), for example, which is formed over the above-mentioned
substrate, so that an active matrix display device in which the FET
controls the drive of the light-emitting element 150 can be
manufactured.
[0171] In this embodiment, one embodiment of the present invention
has been described. Embodiments of the present invention are
described in other embodiments. Note that one embodiment of the
present invention is not limited to the above examples. For
example, one embodiment of the present invention is not limited to
the above-described example in which the HOMO level of the organic
compound 131 is higher than or equal to the HOMO level of the
organic compound 132 and the LUMO level of the organic compound 131
is lower than or equal to the LUMO level of the organic compound
132, and the example in which the oxidation potential of the
organic compound 131 is lower than or equal to the oxidation
potential of the organic compound 132 and the reduction potential
of the organic compound 131 is higher than or equal to the
reduction potential of the organic compound 132. Depending on
circumstances or conditions, in one embodiment of the present
invention, the HOMO level of the organic compound 131 is not
necessarily higher than or equal to the HOMO level of the organic
compound 132, and the LUMO level of the organic compound 131 is not
necessarily lower than or equal to the LUMO level of the organic
compound 132. The oxidation potential of the organic compound 131
is not necessarily lower than or equal to the oxidation potential
of the organic compound 132, and the reduction potential of the
organic compound 131 is not necessarily higher than or equal to the
reduction potential of the organic compound 132. Alternatively, one
embodiment of the present invention is not limited to the
above-described example in which the organic compound 131 is a
substance which exhibits thermally activated delayed fluorescence
at room temperature. Depending on circumstances or conditions, the
organic compound 131 in one embodiment of the present invention may
contain a substance other than the substance which exhibits
thermally activated delayed fluorescence at room temperature, for
example. Alternatively, depending on circumstances or conditions,
the organic compound 131 in one embodiment of the present invention
does not necessarily contain the substance which exhibits thermally
activated delayed fluorescence at room temperature, for example.
Alternatively, one embodiment of the present invention is not
limited to the above example in which the weight percentage of the
organic compound 131 is smaller than that of the organic compound
132. Depending on circumstances or conditions, the weight
percentage of the organic compound 131 is not limited to be smaller
than that of the organic compound 132.
[0172] The structure described above in this embodiment can be
combined with any of the structures described in the other
embodiments as appropriate.
Embodiment 2
[0173] In this embodiment, a light-emitting element having a
structure different from that described in Embodiment 1 and an
emission mechanism of the light-emitting element will be described
below with reference to FIGS. 3A and 3B.
<Structure Example of Light-Emitting Element>
[0174] FIG. 3A is a schematic cross-sectional view of a
light-emitting element 450.
[0175] The light-emitting element 450 illustrated in FIG. 3A
includes a plurality of light-emitting units (in FIG. 3A, a
light-emitting unit 441 and a light-emitting unit 442) between a
pair of electrodes (an electrode 401 and an electrode 402). One
light-emitting unit has the same structure as the EL layer 100
illustrated in FIG. 1A. That is, the light-emitting element 150 in
FIG. 1A includes one light-emitting unit, while the light-emitting
element 450 includes the plurality of light-emitting units. Note
that the electrode 401 functions as an anode and the electrode 402
functions as a cathode in the following description of the
light-emitting element 450; however, the functions may be
interchanged in the light-emitting element 450.
[0176] In the light-emitting element 450 illustrated in FIG. 3A,
the light-emitting unit 441 and the light-emitting unit 442 are
stacked, and a charge-generation layer 445 is provided between the
light-emitting unit 441 and the light-emitting unit 442. Note that
the light-emitting unit 441 and the light-emitting unit 442 may
have the same structure or different structures. For example, it is
preferable that the EL layer 100 illustrated in FIGS. 1A and 1B be
used in the light-emitting unit 441 and that a light-emitting layer
containing a phosphorescent material as a light-emitting material
be used in the light-emitting unit 442.
[0177] That is, the light-emitting element 450 includes a
light-emitting layer 443 and a light-emitting layer 444. The
light-emitting unit 441 includes a hole-injection layer 411, a
hole-transport layer 412, an electron-transport layer 413, and an
electron-injection layer 414 in addition to the light-emitting
layer 443. The light-emitting unit 442 includes a hole-injection
layer 415, a hole-transport layer 416, an electron-transport layer
417, and an electron-injection layer 418 in addition to the
light-emitting layer 444.
[0178] The charge-generation layer 445 preferably contains a
composite material of an organic material and a material having an
electron accepting property. For the composite material, the
composite material that can be used for the hole-injection layer
111 described in Embodiment 1 may be used. As the organic material,
a variety of compounds such as an aromatic amine compound, a
carbazole compound, an aromatic hydrocarbon, and a high molecular
compound (such as an oligomer, a dendrimer, or a polymer) can be
used. An organic material having a hole mobility of
1.times.10.sup.-6 cm.sup.2/Vs or higher is preferably used. Note
that any other material may be used as long as it has a property of
transporting more holes than electrons. Since the composite
material of an organic material and a material having an electron
accepting property has excellent carrier-injection and
carrier-transport properties, low-voltage driving or low-current
driving can be realized. Note that when a surface of a
light-emitting unit on the anode side is in contact with the
charge-generation layer 445 as that of the light-emitting unit 442,
the charge-generation layer 445 can also serve as a hole-injection
layer or a hole-transport layer of the light-emitting unit; thus, a
hole-injection layer or a hole-transport layer does not need to be
included in the light-emitting unit.
[0179] The charge-generation layer 445 may have a stacked-layer
structure of a layer containing the composite material of an
organic material and a material having an electron accepting
property and a layer containing another material. For example, the
charge-generation layer 445 may be formed using a combination of a
layer containing the composite material of an organic material and
a material having an electron accepting property with a layer
containing one material selected from among materials having an
electron donating and a compound having a high electron-transport
property. Furthermore, the charge-generation layer 445 may be
formed using a combination of a layer containing the composite
material of an organic material and a material having an electron
accepting property with a layer including a transparent conductive
film.
[0180] The charge-generation layer 445 provided between the
light-emitting unit 441 and the light-emitting unit 442 may have
any structure as long as electrons can be injected to the
light-emitting unit on one side and holes can be injected into the
light-emitting unit on the other side when a voltage is applied
between the electrode 401 and the electrode 402. For example, in
FIG. 3A, the charge-generation layer 445 injects electrons into the
light-emitting unit 441 and holes into the light-emitting unit 442
when a voltage is applied such that the potential of the electrode
401 is higher than that of the electrode 402.
[0181] The light-emitting element having two light-emitting units
is described with reference to FIG. 3A; however, a similar
structure can be applied to a light-emitting element in which three
or more light-emitting units are stacked. With a plurality of
light-emitting units partitioned by the charge-generation layer
between a pair of electrodes as in the light-emitting element 450,
it is possible to provide a light-emitting element which can emit
light with high luminance with the current density kept low and has
a long lifetime. A light-emitting element with low power
consumption can be provided.
[0182] When the structure of the EL layer 100 shown in FIGS. 1A and
1B is applied to at least one of the plurality of units, a
light-emitting element with high emission efficiency can be
provided.
[0183] The light-emitting layer 443 contains an organic compound
421, an organic compound 422, and a guest material 423. The
light-emitting layer 444 contains an organic compound 431, an
organic compound 432, and a guest material 433.
[0184] In this embodiment, the light-emitting layer 443 has a
structure similar to that of the light-emitting layer 120 in FIGS.
1A and 1B. That is, the organic compound 421, the organic compound
422, and the guest material 423 in the light-emitting layer 443
correspond to the organic compound 131, the organic compound 132,
and the guest material 133 in the light-emitting layer 120,
respectively. In the following description, the guest material 433
contained in the light-emitting layer 444 is a phosphorescent
material. Note that the electrode 401, the electrode 402, the
hole-injection layers 411 and 415, the hole-transport layers 412
and 416, the electron-transport layers 413 and 417, and the
electron-injection layers 414 and 418 correspond to the electrode
101, the electrode 102, the hole-injection layer 111, the
hole-transport layer 112, the electron-transport layer 118, and the
electron-injection layer 119 in Embodiment 1, respectively.
Therefore, detailed description thereof is omitted in this
embodiment.
<Emission Mechanism of Light-Emitting Layer 443>
[0185] An emission mechanism of the light-emitting layer 443 is
similar to that of the light-emitting layer 120 in FIGS. 2A to
2C.
<Emission Mechanism of Light-Emitting Layer 444>
[0186] Next, an emission mechanism of the light-emitting layer 444
will be described.
[0187] The organic compound 431 and the organic compound 432 which
are contained in the light-emitting layer 444 form exciplexes. The
organic compound 431 serves as a host material and the organic
compound 432 serves as an assist material in the description
here.
[0188] Although it is acceptable as long as the combination of the
organic compound 431 and the organic compound 431 can form
exciplexes in the light-emitting layer 444, it is preferred that
one organic compound be a material having a hole-transport property
and the other organic compound be a material having an
electron-transport property.
[0189] FIG. 3B illustrates the correlation of energy levels of the
organic compound 431, the organic compound 432, and the guest
material 433 in the light-emitting layer 444. The following
explains what terms and signs in FIG. 3B represent:
[0190] Host (431): the host material (organic compound 431);
[0191] Assist (432): the assist material (organic compound
432);
[0192] Guest (433): the guest material 433 (phosphorescent
material);
[0193] S.sub.PH: the level of the lowest singlet excited state of
the host material (organic compound 431);
[0194] T.sub.PH: the level of the lowest triplet excited state of
the host material (organic compound 431);
[0195] T.sub.PG: the level of the lowest triplet excited state of
the guest material 433 (the phosphorescent material);
[0196] S.sub.PE: the level of the lowest singlet excited state of
exciplexes; and
[0197] T.sub.PE: the level of the lowest triplet excited state of
exciplexes.
[0198] The level (S.sub.PE) of the lowest singlet excited state of
exciplexes, which is formed by the organic compound 432 and the
organic compound 431 and the level (T.sub.PE) of the lowest triplet
excited state of exciplexes are close to each other (see E.sub.7 in
FIG. 3B).
[0199] Both energies of S.sub.PE and T.sub.PE of exciplexes are
then transferred to the level (T.sub.PG) of the lowest triplet
excited state of the guest material 433 (the phosphorescent
material); thus, light emission is obtained (see E.sub.8 in FIG.
3B).
[0200] The above-described processes through a route E.sub.7 and a
route E.sub.8 may be referred to as exciplex-triplet energy
transfer (ExTET) in this specification and the like.
[0201] When one of the organic compounds 431 and 432 receiving
holes and the other receiving electrons come close to each other,
exciplexes are formed at once. Alternatively, when one compound is
brought into an excited state, the one immediately interacts with
the other compound to form exciplexes. Therefore, most excitons in
the light-emitting layer 444 exist as exciplexes. The band gap of
the exciplex is narrower than that of each of the organic compounds
431 and 432; therefore, the driving voltage of the light-emitting
element can be lowered when exciplexes are formed.
[0202] When the light-emitting layer 444 has the above structure,
light emission from the guest material 433 (the phosphorescent
material) of the light-emitting layer 444 can be efficiently
obtained.
[0203] Note that light emitted from the light-emitting layer 443
preferably has a peak on the shorter wavelength side than light
emitted from the light-emitting layer 444. The luminance of a
light-emitting element using the phosphorescent material emitting
light with a short wavelength tends to degrade quickly. In view of
the above, fluorescence is used for light emission with a short
wavelength, so that a light-emitting element with less degradation
of luminance can be provided.
[0204] Furthermore, the light-emitting layer 443 and the
light-emitting layer 444 may be made to emit light with different
emission wavelengths, so that the light-emitting element can be a
multicolor light-emitting element. In that case, the emission
spectrum of the light-emitting element is formed by combining light
having different emission peaks, and thus has at least two
peaks.
[0205] The above structure is also suitable for obtaining white
light emission. When the light-emitting layer 443 and the
light-emitting layer 444 emit light of complementary colors, white
light emission can be obtained.
[0206] In addition, white light emission with a high color
rendering property that is formed of three primary colors or four
or more colors can be obtained by using a plurality of
light-emitting materials emitting light with different wavelengths
for one of the light-emitting layers 443 and 444 or both. In that
case, one of the light-emitting layers 443 and 444 or both may be
divided into layers and each of the divided layers may contain a
different light-emitting material from the others.
[0207] Next, materials that can be used for the light-emitting
layers 443 and 444 will be described.
<Material that can be Used for Light-Emitting Layer 443>
[0208] A material that can be used for the light-emitting layer 120
described in Embodiment 1 may be used as a material that can be
used for the light-emitting layer 443.
<Material that can be Used for Light-Emitting Layer 444>
[0209] In the light-emitting layer 444, the organic compound 431
(the host material) exists in the highest proportion in weight
ratio, and the guest material 433 (the phosphorescent material) is
dispersed in the organic compound 431 (the host material).
[0210] Examples of the organic compound 431 (the 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, a phenanthroline derivative,
and the like. Other examples are an aromatic amine, a carbazole
derivative, and the like. In addition, the compound having a
skeleton which easily accepts electrons and the compound having a
skeleton which easily accepts holes, which are described in
Embodiment 1, can be used.
[0211] As the guest material 433 (the phosphorescent 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, and the like can
be given. As the metal complex, a platinum complex having a
porphyrin ligand or the like can be given.
[0212] As the organic compound 432 (the assist material), a
substance which can form exciplexes together with the organic
compound 431 is used. In that case, it is preferable that the
organic compound 431, the organic compound 432, and the guest
material 433 (the phosphorescent material) be selected such that
the emission peak of the exciplexes 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 phosphorescent material. This makes it possible
to provide a light-emitting element with drastically improved
emission efficiency. Note that in the case where a thermally
activated delayed fluorescence material is used instead of the
phosphorescent material, it is preferable that the adsorption band
on the longest wavelength side be a singlet absorption band.
Specifically, the compound having a skeleton which easily accepts
electrons or the compound having a skeleton which easily accepts
holes, which are described in Embodiment 1, can be used as the
organic compound 432.
[0213] As the light-emitting material contained in the
light-emitting layer 444, any material can be used as long as the
material can convert triplet excitation energy into light emission.
As an example of the material that can convert triplet excitation
energy into light emission, a thermally activated delayed
fluorescence material can be given in addition to the
phosphorescent material. Therefore, the term "phosphorescent
material" in the description can be replaced with the term
"thermally activated delayed fluorescence material". Note that the
thermally activated delayed fluorescence material is a material
that can up-convert a triplet excited state into a singlet excited
state (i.e., reverse intersystem crossing is possible) using a
little thermal energy and efficiently exhibits light emission
(fluorescence) from the singlet excited state. Thermally activated
delayed fluorescence is efficiently obtained under the condition
where the difference between the triplet excitation energy level
and the singlet excitation energy level is larger than 0 eV and
smaller than or equal to 0.2 eV, preferably larger than 0 eV and
smaller than or equal to 0.1 eV.
[0214] There is no limitation on the emission colors of the
light-emitting material included in the light-emitting layer 443
and the light-emitting material included in the light-emitting
layer 444, 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
light-emitting material contained in the light-emitting layer 443
is preferably shorter than that of the light-emitting material
contained in the light-emitting layer 444.
[0215] Note that the light-emitting layers 443 and 444 can be
formed by an evaporation method (including a vacuum evaporation
method), an inkjet method, a coating method, gravure printing, or
the like.
[0216] Note that the structure described above in this embodiment
can be combined with any of the structures described in the other
embodiments as appropriate.
Embodiment 3
[0217] In this embodiment, a light-emitting element having a
structure different from those described in Embodiment 1 and
Embodiment 2 will be described below with reference to FIGS. 4A and
4B.
<Structure Example of Light-Emitting Element>
[0218] FIG. 4A is a schematic cross-sectional view of a
light-emitting element 452 of one embodiment of the present
invention.
[0219] The light-emitting element 452 includes a plurality of
light-emitting units (in FIG. 4A, a light-emitting unit 446 and a
light-emitting unit 447) between an electrode 401 and an electrode
402. One light-emitting unit has the same structure as the EL layer
100 illustrated in FIG. 1A. That is, the light-emitting element 150
in FIG. 1A includes one light-emitting unit, while the
light-emitting element 452 includes the plurality of light-emitting
units. Note that the electrode 401 functions as an anode and the
electrode 402 functions as a cathode in the following description
of this embodiment; however, the functions may be interchanged in
the light-emitting element 452.
[0220] In the light-emitting element 452 illustrated in FIG. 4A,
the light-emitting unit 446 and the light-emitting unit 447 are
stacked, and a charge-generation layer 445 is provided between the
light-emitting unit 446 and the light-emitting unit 447. Note that
the light-emitting unit 446 and the light-emitting unit 447 may
have the same structure or different structures. For example, it is
preferable that a light-emitting layer containing a fluorescent
material as a light-emitting material be used in the light-emitting
unit 446 and that the EL layer 100 illustrated in FIG. 1A be used
in the light-emitting unit 447.
[0221] That is, the light-emitting element 452 includes a
light-emitting layer 448 and a light-emitting layer 449. The
light-emitting unit 446 includes a hole-injection layer 411, a
hole-transport layer 412, an electron-transport layer 413, and an
electron-injection layer 414 in addition to the light-emitting
layer 448. The light-emitting unit 447 includes a hole-injection
layer 415, a hole-transport layer 416, an electron-transport layer
417, and an electron-injection layer 418 in addition to the
light-emitting layer 449.
[0222] The light-emitting element having two light-emitting units
is described with reference to FIG. 4A; however, a similar
structure can be applied to a light-emitting element in which three
or more light-emitting units are stacked. With a plurality of
light-emitting units partitioned by the charge-generation layer
between a pair of electrodes as in the light-emitting element 452,
it is possible to provide a light-emitting element which can emit
light with high luminance with the current density kept low and has
a long lifetime. A display device with low power consumption can be
provided.
[0223] When the structure of the EL layer 100 shown in FIG. 1A is
applied to at least one of the plurality of units, a light-emitting
element with high emission efficiency can be provided.
[0224] The light-emitting layer 448 contains a host material 461
and a guest material 462. The light-emitting layer 449 contains an
organic compound 471, an organic compound 472, and a guest material
473.
[0225] In this embodiment, the light-emitting layer 449 has a
structure similar to that of the light-emitting layer 120 in FIGS.
1A and 1B. That is, the organic compound 471, the organic compound
472, and the guest material 473 in the light-emitting layer 449
correspond to the organic compound 131, the organic compound 132,
and the guest material 133 in the light-emitting layer 120,
respectively. In the following description, the guest material 462
contained in the light-emitting layer 448 is a fluorescent
material.
<Emission Mechanism of Light-Emitting Layer 448>
[0226] First, an emission mechanism of the light-emitting layer 448
will be described.
[0227] In the light-emitting layer 448, an excited state is
generated by recombination of carriers. Because the amount of the
host material 461 is large as compared to the guest material 462,
the host material 461 is brought into an excited state by the
exciton generation. The ratio of singlet excitons to triplet
excitons generated by carrier recombination (hereinafter referred
to as exciton generation probability) is approximately 1:3.
[0228] First, a case where the triplet excitation energy level of
the host material 461 is higher than the triplet excitation energy
level of the guest material 462 will be described below.
[0229] The triplet excitation energy level of the host material 461
is transferred to the triplet excitation energy level of the guest
material 462 (triplet energy transfer). However, the guest material
462 in the triplet excitation energy state does not provide light
emission in a visible light region because the guest material 462
is the fluorescent material. Thus, it is difficult to use the
triplet excitation energy of the host material 461 for light
emission. Therefore, when the triplet excitation energy level of
the host material 461 is higher than the triplet excitation energy
level of the guest material 462, it is difficult to use more than
approximately 25% of injected carriers for light emission.
[0230] FIG. 4B illustrates the correlation of energy levels of the
host material 461 and the guest material 462 in the light-emitting
layer 448 of one embodiment of the present invention. The following
explains what terms and signs in FIG. 4B represent:
[0231] Host (461): the host material 461;
[0232] Guest (462): the guest material 462 (fluorescent
material);
[0233] S.sub.FH: the level of the lowest singlet excited state of
the host material 461;
[0234] T.sub.FH: the level of the lowest triplet excited state of
the host material 461;
[0235] S.sub.FG: the level of the lowest singlet excited state of
the guest material 462 (fluorescent material); and
[0236] T.sub.FG: the level of the lowest triplet excited state of
the guest material 462 (fluorescent material).
[0237] As illustrated in FIG. 4B, the triplet excitation energy
level of the guest material 462 (T.sub.FG in FIG. 4B) is higher
than the triplet excitation energy level of the host material 461
(T.sub.FH in FIG. 4B).
[0238] In addition, as illustrated in FIG. 4B, triplet excitons
collide with each other by triplet-triplet annihilation (TTA) (see
a route E.sub.9 in FIG. 4B), and their excitation energy are partly
converted into singlet excitons having an energy at the level of
the lowest singlet excited state of the host material 461
(S.sub.FH). The singlet excitation energy of the host material 461
is transferred from the level of the lowest singlet excited state
of the host material 461 (S.sub.FH) to the level of the lowest
singlet excited state of the guest material 462 (the fluorescent
material) (S.sub.FG) that is a level lower than S.sub.FH (see a
route E.sub.10 in FIG. 4B). Thus, the guest material 462 (the
fluorescent material) is brought into the singlet excited state and
accordingly emits light.
[0239] Because the triplet excitation energy level of the host
material 462 is lower than the triplet excitation energy level of
the guest material, excitation energy at T.sub.FG is transferred to
T.sub.FH without deactivation (see a route E.sub.11 in FIG. 4B),
which is utilized for TTA.
[0240] When the light-emitting layer 448 has the above structure,
light emission from the guest material 462 of the light-emitting
layer 448 can be efficiently obtained.
[0241] Note that the light-emitting layer 448 and the
light-emitting layer 449 may be made to emit light with different
emission wavelengths, so that the light-emitting element can be a
multicolor light-emitting element. In that case, the emission
spectrum of the light-emitting element is formed by combining light
having different emission peaks, and thus has at least two
peaks.
[0242] The above structure is also suitable for obtaining white
light emission. When the light-emitting layer 448 and the
light-emitting layer 449 emit light of complementary colors, white
light emission can be obtained.
[0243] In addition, white light emission with a high color
rendering property that is formed of three primary colors or four
or more colors can be obtained by using a plurality of
light-emitting materials emitting light with different wavelengths
for one of the light-emitting layers 448 and 449 or both. In that
case, one of the light-emitting layers 448 and 449 or both may be
divided into layers and each of the divided layers may contain a
different light-emitting material from the others.
<Emission Mechanism of Light-Emitting Layer 449>
[0244] An emission mechanism of the light-emitting layer 449 is
similar to that of the light-emitting layer 120 in FIGS. 2A to
2C.
[0245] Next, materials that can be used for the light-emitting
layers 448 and 449 will be described.
<Material that can be Used for Light-Emitting Layer 448>
[0246] In the light-emitting layer 448, the host material 461 is
present in the highest proportion in weight ratio, and the guest
material 462 (the fluorescent material) is dispersed in the host
material 461. The singlet excitation energy level of the host
material 461 is preferably higher than the singlet excitation
energy level of the guest material 462 (the fluorescent material),
while the triplet excitation energy level of the host material 461
is preferably lower than the triplet excitation energy level of the
guest material 462 (the fluorescent material).
[0247] An anthracene derivative or a tetracene derivative is
preferably used as the host material 461. This is because these
derivatives each have a high singlet excitation energy level and a
low triplet excitation energy level. Specific examples include
9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole
(abbreviation: PCzPA),
3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation:
PCPN), 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole
(abbreviation: CzPA),
7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole
(abbreviation: cgDBCzPA),
6-[3-(9,10-diphenyl-2-anthryl)phenyl]-benzo[b]naphtho[1,2-d]furan
(abbreviation: 2mBnfPPA), and
9-phenyl-10-{4-(9-phenyl-9H-fluoren-9-yl)biphenyl-4'-yl}anthracene
(abbreviation: FLPPA). Besides, 5,12-diphenyltetracene,
5,12-bis(biphenyl-2-yl)tetracene, and the like can be given.
[0248] Examples of the guest material 462 (the fluorescent
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, a naphthalene derivative, and the like. 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 (abbreviation: 1,6mMemFLPAPrn),
N,N'-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N'-diphenylpyrene-1,6-diam-
ine (abbreviation: 1,6FLPAPrn),
N,N'-bis(dibenzofuran-2-yl)-N,N'-diphenylpyrene-1,6-diamine
(abbreviation: 1,6FrAPrn),
N,N'-bis(dibenzothiophene-2-yl)-N,N-diphenylpyrene-1,6-diamine
(abbreviation: 1,6ThAPrn), and the like. Any of the fluorescent
materials described in Embodiment 1 can be used.
<Material that can be Used for Light-Emitting Layer 449>
[0249] A material that can be used for the light-emitting layer 120
described in Embodiment 1 may be used as a material that can be
used for the light-emitting layer 449.
[0250] There is no limitation on the emission colors of the
light-emitting material included in the light-emitting layer 448
and the light-emitting material included in the light-emitting
layer 449, 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
light-emitting material contained in the light-emitting layer 448
is preferably shorter than that of the light-emitting material
contained in the light-emitting layer 449.
[0251] Note that the light-emitting layers 448 and 449 can be
formed by an evaporation method (including a vacuum evaporation
method), an inkjet method, a coating method, gravure printing, or
the like.
[0252] Note that the above-described structure can be combined with
any of the structures in this embodiment and the other
embodiments.
Embodiment 4
[0253] In this embodiment, a display device including a
light-emitting element of one embodiment of the present invention
will be described with reference to FIGS. 5A and 5B.
[0254] FIG. 5A is a block diagram illustrating the display device
of one embodiment of the present invention, and FIG. 5B is a
circuit diagram illustrating a pixel circuit of the display device
of one embodiment of the present invention.
<Display Device>
[0255] The display device illustrated in FIG. 5A includes a region
including pixels of display elements (the region is hereinafter
referred to as a pixel portion 802), a circuit portion provided
outside the pixel portion 802 and including circuits for driving
the pixels (the portion is hereinafter referred to as a driver
circuit portion 804), circuits having a function of protecting
elements (the circuits are hereinafter referred to as protection
circuits 806), and a terminal portion 807. Note that the protection
circuits 806 are not necessarily provided.
[0256] A part or the whole of the driver circuit portion 804 is
preferably formed over a substrate over which the pixel portion 802
is formed, in which case the number of components and the number of
terminals can be reduced. When a part or the whole of the driver
circuit portion 804 is not formed over the substrate over which the
pixel portion 802 is formed, the part or the whole of the driver
circuit portion 804 can be mounted by chip-on-glass (COG) or tape
automated bonding (TAB).
[0257] The pixel portion 802 includes a plurality of circuits for
driving display elements arranged in X rows (X is a natural number
of 2 or more) and Y columns (Y is a natural number of 2 or more)
(such circuits are hereinafter referred to as pixel circuits 801).
The driver circuit portion 804 includes driver circuits such as a
circuit for supplying a signal (scan signal) to select a pixel (the
circuit is hereinafter referred to as a scan line driver circuit
804a) and a circuit for supplying a signal (data signal) to drive a
display element in a pixel (the circuit is hereinafter referred to
as a signal line driver circuit 804b).
[0258] The scan line driver circuit 804a includes a shift register
or the like. Through the terminal portion 807, the scan line driver
circuit 804a receives a signal for driving the shift register and
outputs a signal. For example, the scan line driver circuit 804a
receives a start pulse signal, a clock signal, or the like and
outputs a pulse signal. The scan line driver circuit 804a has a
function of controlling the potentials of wirings supplied with
scan signals (such wirings are hereinafter referred to as scan
lines GL_1 to GL_X). Note that a plurality of scan line driver
circuits 804a may be provided to control the scan lines GL_1 to
GL_X separately. Alternatively, the scan line driver circuit 804a
has a function of supplying an initialization signal. Without being
limited thereto, the scan line driver circuit 804a can supply
another signal.
[0259] The signal line driver circuit 804b includes a shift
register or the like. The signal line driver circuit 804b receives
a signal (video signal) from which a data signal is derived, as
well as a signal for driving the shift register, through the
terminal portion 807. The signal line driver circuit 804b has a
function of generating a data signal to be written to the pixel
circuit 801 which is based on the video signal. In addition, the
signal line driver circuit 804b has a function of controlling
output of a data signal in response to a pulse signal produced by
input of a start pulse signal, a clock signal, or the like.
Furthermore, the signal line driver circuit 804b has a function of
controlling the potentials of wirings supplied with data signals
(such wirings are hereinafter referred to as data lines DL_1 to
DL_Y). Alternatively, the signal line driver circuit 804b has a
function of supplying an initialization signal. Without being
limited thereto, the signal line driver circuit 804b can supply
another signal.
[0260] The signal line driver circuit 804b includes a plurality of
analog switches or the like, for example. The signal line driver
circuit 804b can output, as the data signals, signals obtained by
time-dividing the video signal by sequentially turning on the
plurality of analog switches. The signal line driver circuit 804b
may include a shift register or the like.
[0261] A pulse signal and a data signal are input to each of the
plurality of pixel circuits 801 through one of the plurality of
scan lines GL supplied with scan signals and one of the plurality
of data lines DL supplied with data signals, respectively. Writing
and holding of the data signal to and in each of the plurality of
pixel circuits 801 are controlled by the scan line driver circuit
804a. For example, to the pixel circuit 801 in the m-th row and the
n-th column (m is a natural number of less than or equal to X, and
n is a natural number of less than or equal to Y), a pulse signal
is input from the scan line driver circuit 804a through the scan
line GL_m, and a data signal is input from the signal line driver
circuit 804b through the data line DL_n in accordance with the
potential of the scan line GL_m.
[0262] The protection circuit 806 shown in FIG. 5A is connected to,
for example, the scan line GL between the scan line driver circuit
804a and the pixel circuit 801. Alternatively, the protection
circuit 806 is connected to the data line DL between the signal
line driver circuit 804b and the pixel circuit 801. Alternatively,
the protection circuit 806 can be connected to a wiring between the
scan line driver circuit 804a and the terminal portion 807.
Alternatively, the protection circuit 806 can be connected to a
wiring between the signal line driver circuit 804b and the terminal
portion 807. Note that the terminal portion 807 means a portion
having terminals for inputting power, control signals, and video
signals to the display device from external circuits.
[0263] The protection circuit 806 is a circuit that electrically
connects a wiring connected to the protection circuit to another
wiring when a potential out of a certain range is applied to the
wiring connected to the protection circuit.
[0264] As illustrated in FIG. 5A, the protection circuits 806 are
provided for the pixel portion 802 and the driver circuit portion
804, so that the resistance of the display device to overcurrent
generated by electrostatic discharge (ESD) or the like can be
improved. Note that the configuration of the protection circuits
806 is not limited to that, and for example, a configuration in
which the protection circuits 806 are connected to the scan line
driver circuit 804a or a configuration in which the protection
circuits 806 are connected to the signal line driver circuit 804b
may be employed. Alternatively, the protection circuits 806 may be
configured to be connected to the terminal portion 807.
[0265] In FIG. 5A, an example in which the driver circuit portion
804 includes the scan line driver circuit 804a and the signal line
driver circuit 804b is shown; however, the structure is not limited
thereto. For example, only the scan line driver circuit 804a may be
formed and a separately prepared substrate where a signal line
driver circuit is formed (e.g., a driver circuit substrate formed
with a single crystal semiconductor film or a polycrystalline
semiconductor film) may be mounted.
<Structural Example of Pixel Circuit>
[0266] Each of the plurality of pixel circuits 801 in FIG. 5A can
have a structure illustrated in FIG. 5B, for example.
[0267] The pixel circuit 801 illustrated in FIG. 5B includes
transistors 852 and 854, a capacitor 862, and a light-emitting
element 872.
[0268] One of a source electrode and a drain electrode of the
transistor 852 is electrically connected to a wiring to which a
data signal is supplied (hereinafter referred to as a data line
DL_n). A gate electrode of the transistor 852 is electrically
connected to a wiring to which a gate signal is supplied
(hereinafter referred to as a scan line GL_m).
[0269] The transistor 852 has a function of controlling whether to
write a data signal.
[0270] One of a pair of electrodes of the capacitor 862 is
electrically connected to a wiring to which a potential is supplied
(hereinafter referred to as a potential supply line VL_a), and the
other is electrically connected to the other of the source
electrode and the drain electrode of the transistor 852.
[0271] The capacitor 862 functions as a storage capacitor for
storing written data.
[0272] One of a source electrode and a drain electrode of the
transistor 854 is electrically connected to the potential supply
line VL_a. Furthermore, a gate electrode of the transistor 854 is
electrically connected to the other of the source electrode and the
drain electrode of the transistor 852.
[0273] One of an anode and a cathode of the light-emitting element
872 is electrically connected to a potential supply line VL_b, and
the other is electrically connected to the other of the source
electrode and the drain electrode of the transistor 854.
[0274] As the light-emitting element 872, any of the light-emitting
elements described in Embodiments 1 to 3 can be used.
[0275] Note that a high power supply potential VDD is supplied to
one of the potential supply line VL_a and the potential supply line
VL_b, and a low power supply potential VSS is supplied to the
other.
[0276] In the display device including the pixel circuits 801 in
FIG. 5B, the pixel circuits 801 are sequentially selected row by
row by the scan line driver circuit 804a in FIG. 5A, for example,
whereby the transistors 852 are turned on and a data signal is
written.
[0277] When the transistors 852 are turned off, the pixel circuits
801 in which the data has been written are brought into a holding
state. Furthermore, the amount of current flowing between the
source electrode and the drain electrode of the transistor 854 is
controlled in accordance with the potential of the written data
signal. The light-emitting element 872 emits light with a luminance
corresponding to the amount of flowing current. This operation is
sequentially performed row by row; thus, an image is displayed.
[0278] A light-emitting element of one embodiment of the present
invention can be used for an active matrix method in which an
active element is included in a pixel of a display device or a
passive matrix method in which an active element is not included in
a pixel of a display device.
[0279] In the active matrix method, as an active element (a
non-linear element), not only a transistor but also a variety of
active elements (non-linear elements) can be used. For example, a
metal insulator metal (MIM), a thin film diode (TFD), or the like
can also be used. Since these elements can be formed with a smaller
number of manufacturing steps, manufacturing cost can be reduced or
yield can be improved. Alternatively, since the size of these
elements is small, the aperture ratio can be improved, so that
power consumption can be reduced or higher luminance can be
achieved.
[0280] As a method other than the active matrix method, the passive
matrix method in which an active element (a non-linear element) is
not used can also be used. Since an active element (a non-linear
element) is not used, the number of manufacturing steps is small,
so that manufacturing cost can be reduced or yield can be improved.
Alternatively, since an active element (a non-linear element) is
not used, the aperture ratio can be improved, so that power
consumption can be reduced or higher luminance can be achieved, for
example.
[0281] The structure described in this embodiment can be combined
with any of the structures described in the other embodiments or
examples as appropriate.
Embodiment 5
[0282] In this embodiment, a display device including a
light-emitting element of one embodiment of the present invention
and an electronic device in which the display device is provided
with an input device will be described with reference to FIGS. 6A
and 6B, FIGS. 7A to 7C, FIGS. 8A and 8B, FIGS. 9A and 9B, and FIG.
10.
<Description 1 of Touch Panel>
[0283] In this embodiment, a touch panel 2000 including a display
device and an input device will be described as an example of an
electronic device. In addition, an example in which a touch sensor
is used as an input device will be described.
[0284] FIGS. 6A and 6B are perspective views of the touch panel
2000. Note that FIGS. 6A and 6B illustrate only main components of
the touch panel 2000 for simplicity.
[0285] The touch panel 2000 includes a display device 2501 and a
touch sensor 2595 (see FIG. 6B). The touch panel 2000 also includes
a substrate 2510, a substrate 2570, and a substrate 2590. The
substrate 2510, the substrate 2570, and the substrate 2590 each
have flexibility. Note that one or all of the substrates 2510,
2570, and 2590 may be inflexible.
[0286] The display device 2501 includes a plurality of pixels over
the substrate 2510 and a plurality of wirings 2511 through which
signals are supplied to the pixels. The plurality of wirings 2511
are led to a peripheral portion of the substrate 2510, and parts of
the plurality of wirings 2511 form a terminal 2519. The terminal
2519 is electrically connected to an FPC 2509(1).
[0287] The substrate 2590 includes the touch sensor 2595 and a
plurality of wirings 2598 electrically connected to the touch
sensor 2595. The plurality of wirings 2598 are led to a peripheral
portion of the substrate 2590, and parts of the plurality of
wirings 2598 form a terminal. The terminal is electrically
connected to an FPC 2509(2). Note that in FIG. 6B, electrodes,
wirings, and the like of the touch sensor 2595 provided on the back
side of the substrate 2590 (the side facing the substrate 2510) are
indicated by solid lines for clarity.
[0288] As the touch sensor 2595, a capacitive touch sensor can be
used. Examples of the capacitive touch sensor are a surface
capacitive touch sensor and a projected capacitive touch
sensor.
[0289] Examples of the projected capacitive touch sensor are a self
capacitive touch sensor and a mutual capacitive touch sensor, which
differ mainly in the driving method. The use of a mutual capacitive
type is preferable because multiple points can be sensed
simultaneously.
[0290] Note that the touch sensor 2595 illustrated in FIG. 6B is an
example of using a projected capacitive touch sensor.
[0291] Note that a variety of sensors that can sense proximity or
touch of a sensing target such as a finger can be used as the touch
sensor 2595.
[0292] The projected capacitive touch sensor 2595 includes
electrodes 2591 and electrodes 2592. The electrodes 2591 are
electrically connected to any of the plurality of wirings 2598, and
the electrodes 2592 are electrically connected to any of the other
wirings 2598.
[0293] The electrodes 2592 each have a shape of a plurality of
quadrangles arranged in one direction with one corner of a
quadrangle connected to one corner of another quadrangle as
illustrated in FIGS. 6A and 6B.
[0294] The electrodes 2591 each have a quadrangular shape and are
arranged in a direction intersecting with the direction in which
the electrodes 2592 extend.
[0295] A wiring 2594 electrically connects two electrodes 2591
between which the electrode 2592 is positioned. The intersecting
area of the electrode 2592 and the wiring 2594 is preferably as
small as possible. Such a structure allows a reduction in the area
of a region where the electrodes are not provided, reducing
variation in transmittance. As a result, variation in luminance of
light passing through the touch sensor 2595 can be reduced.
[0296] Note that the shapes of the electrodes 2591 and the
electrodes 2592 are not limited thereto and can be any of a variety
of shapes. For example, a structure may be employed in which the
plurality of electrodes 2591 are arranged so that gaps between the
electrodes 2591 are reduced as much as possible, and the electrodes
2592 are spaced apart from the electrodes 2591 with an insulating
layer interposed therebetween to have regions not overlapping with
the electrodes 2591. In this case, it is preferable to provide,
between two adjacent electrodes 2592, a dummy electrode
electrically insulated from these electrodes because the area of
regions having different transmittances can be reduced.
<Display Device>
[0297] Next, the display device 2501 will be described in detail
with reference to FIG. 7A. FIG. 7A corresponds to a cross-sectional
view taken along dashed-dotted line X1-X2 in FIG. 6B.
[0298] The display device 2501 includes a plurality of pixels
arranged in a matrix. Each of the pixels includes a display element
and a pixel circuit for driving the display element.
[0299] In the following description, an example of using a
light-emitting element that emits white light as a display element
will be described; however, the display element is not limited to
such an element. For example, light-emitting elements that emit
light of different colors may be included so that the light of
different colors can be emitted from adjacent pixels.
[0300] For the substrate 2510 and the substrate 2570, for example,
a flexible material with a vapor permeability of lower than or
equal to 1.times.10.sup.-5 gm.sup.-2day.sup.-1, preferably lower
than or equal to 1.times.10.sup.-6 gm.sup.-2day.sup.-1 can be
favorably used. Alternatively, materials whose thermal expansion
coefficients are substantially equal to each other are preferably
used for the substrate 2510 and the substrate 2570. For example,
the coefficients of linear expansion of the materials are
preferably lower than or equal to 1.times.10.sup.-3/K, further
preferably lower than or equal to 5.times.10.sup.-5/K, and still
further preferably lower than or equal to 1.times.10.sup.-5/K.
[0301] Note that the substrate 2510 is a stacked body including an
insulating layer 2510a for preventing impurity diffusion into the
light-emitting element, a flexible substrate 2510b, and an adhesive
layer 2510c for attaching the insulating layer 2510a and the
flexible substrate 2510b to each other. The substrate 2570 is a
stacked body including an insulating layer 2570a for preventing
impurity diffusion into the light-emitting element, a flexible
substrate 2570b, and an adhesive layer 2570c for attaching the
insulating layer 2570a and the flexible substrate 2570b to each
other.
[0302] For the adhesive layer 2510c and the adhesive layer 2570c,
for example, materials that include polyester, polyolefin,
polyamide (e.g., nylon, aramid), polyimide, polycarbonate, or a
resin having an acrylic resin, polyurethane, an epoxy resin, or a
resin having a siloxane bond such as silicone can be used.
[0303] A sealing layer 2560 is provided between the substrate 2510
and the substrate 2570. The sealing layer 2560 preferably has a
refractive index higher than that of air. In the case where light
is extracted to the sealing layer 2560 side as illustrated in FIG.
7A, the sealing layer 2560 can also serve as an adhesive layer.
[0304] A sealant may be formed in the peripheral portion of the
sealing layer 2560. With the use of the sealant, a light-emitting
element 2550R can be provided in a region surrounded by the
substrate 2510, the substrate 2570, the sealing layer 2560, and the
sealant. Note that an inert gas (such as nitrogen or argon) may be
used instead of the sealing layer 2560. A drying agent may be
provided in the inert gas so as to adsorb moisture or the like. An
ultraviolet curable resin or a heat curable resin may be used; for
example, a polyvinyl chloride (PVC) based resin, an acrylic resin,
a polyimide-based resin, an epoxy-based resin, a silicone-based
resin, a polyvinyl butyral (PVB) based resin, or an ethylene vinyl
acetate (EVA) based resin can be used. For example, an epoxy-based
resin or a glass frit is preferably used as the sealant. As a
material used for the sealant, a material which is impermeable to
moisture or oxygen is preferably used.
[0305] The display device 2501 includes a pixel 2502R. The pixel
2502R includes a light-emitting module 2580R.
[0306] The pixel 2502R includes the light-emitting element 2550R
and a transistor 2502t that can supply electric power to the
light-emitting element 2550R. Note that the transistor 2502t
functions as part of the pixel circuit. The light-emitting module
2580R includes the light-emitting element 2550R and a coloring
layer 2567R.
[0307] The light-emitting element 2550R includes a lower electrode,
an upper electrode, and an EL layer between the lower electrode and
the upper electrode. As the light-emitting element 2550R, any of
the light-emitting elements described in Embodiments 1 to 3 can be
used, for example.
[0308] A microcavity structure may be employed between the lower
electrode and the upper electrode so as to increase the intensity
of light having a specific wavelength.
[0309] In the case where the sealing layer 2560 is provided on the
light extraction side, the sealing layer 2560 is in contact with
the light-emitting element 2550R and the coloring layer 2567R.
[0310] The coloring layer 2567R is positioned in a region
overlapping with the light-emitting element 2550R. Accordingly,
part of light emitted from the light-emitting element 2550R passes
through the coloring layer 2567R and is emitted to the outside of
the light-emitting module 2580R as indicated by an arrow in FIG.
7A.
[0311] The display device 2501 includes a light-blocking layer
2567BM on the light extraction side. The light-blocking layer
2567BM is provided so as to surround the coloring layer 2567R.
[0312] The coloring layer 2567R is a coloring layer having a
function of transmitting light in a particular wavelength region.
For example, a color filter for transmitting light in a red
wavelength range, a color filter for transmitting light in a green
wavelength range, a color filter for transmitting light in a blue
wavelength range, a color filter for transmitting light in a yellow
wavelength range, or the like can be used. Each color filter can be
formed with any of various materials by a printing method, an
inkjet method, an etching method using a photolithography
technique, or the like.
[0313] An insulating layer 2521 is provided in the display device
2501. The insulating layer 2521 covers the transistor 2502t. Note
that the insulating layer 2521 has a function of planarizing
unevenness caused by the pixel circuit. The insulating layer 2521
may have a function of suppressing impurity diffusion. This can
prevent the reliability of the transistor 2502t or the like from
being lowered by impurity diffusion.
[0314] The light-emitting element 2550R is formed over the
insulating layer 2521. A partition 2528 is provided so as to
overlap with an end portion of the lower electrode of the
light-emitting element 2550R. Note that a spacer for controlling
the distance between the substrate 2510 and the substrate 2570 may
be formed over the partition 2528.
[0315] A scan line driver circuit 2503g(1) includes a transistor
2503t and a capacitor 2503c. Note that the driver circuit can be
formed in the same process and over the same substrate as those of
the pixel circuits.
[0316] The wirings 2511 through which signals can be supplied are
provided over the substrate 2510. The terminal 2519 is provided
over the wirings 2511. The FPC 2509(1) is electrically connected to
the terminal 2519. The FPC 2509(1) has a function of supplying a
video signal, a clock signal, a start signal, a reset signal, or
the like. Note that the FPC 2509(1) may be provided with a printed
wiring board (PWB).
[0317] In the display device 2501, transistors with any of a
variety of structures can be used. FIG. 7A illustrates an example
of using bottom-gate transistors; however, the present invention is
not limited to this example, and top-gate transistors may be used
in the display device 2501 as illustrated in FIG. 7B.
[0318] In addition, there is no particular limitation on the
polarity of the transistor 2502t and the transistor 2503t. For
these transistors, n-channel and p-channel transistors may be used,
or either n-channel transistors or p-channel transistors may be
used, for example. Furthermore, there is no particular limitation
on the crystallinity of a semiconductor film used for the
transistors 2502t and 2503t. For example, an amorphous
semiconductor film or a crystalline semiconductor film may be used.
Examples of semiconductor materials include Group 13 semiconductors
(e.g., a semiconductor including gallium), Group 14 semiconductors
(e.g., a semiconductor including silicon), compound semiconductors
(including oxide semiconductors), organic semiconductors, and the
like. An oxide semiconductor that has an energy gap of 2 eV or
more, preferably 2.5 eV or more, further preferably 3 eV or more is
preferably used for one of the transistors 2502t and 2503t or both,
so that the off-state current of the transistors can be reduced.
Examples of the oxide semiconductors include an In--Ga oxide, an
In-M-Zn oxide (M represents aluminum (Al), gallium (Ga), yttrium
(Y), zirconium (Zr), lanthanum (La), cerium (Ce), tin (Sn), hafnium
(Hf), or neodymium (Nd)), and the like.
<Touch Sensor>
[0319] Next, the touch sensor 2595 will be described in detail with
reference to FIG. 7C. FIG. 7C corresponds to a cross-sectional view
taken along dashed-dotted line X3-X4 in FIG. 6B.
[0320] The touch sensor 2595 includes the electrodes 2591 and the
electrodes 2592 provided in a staggered arrangement on the
substrate 2590, an insulating layer 2593 covering the electrodes
2591 and the electrodes 2592, and the wiring 2594 that electrically
connects the adjacent electrodes 2591 to each other.
[0321] The electrodes 2591 and the electrodes 2592 are formed using
a light-transmitting conductive material. As a light-transmitting
conductive material, a conductive oxide such as indium oxide,
indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to
which gallium is added can be used. Note that a film including
graphene may be used as well. The film including graphene can be
formed, for example, by reducing a film containing graphene oxide.
As a reducing method, a method with application of heat or the like
can be employed.
[0322] The electrodes 2591 and the electrodes 2592 may be formed
by, for example, depositing a light-transmitting conductive
material on the substrate 2590 by a sputtering method and then
removing an unnecessary portion by any of various pattern forming
techniques such as photolithography.
[0323] Examples of a material for the insulating layer 2593 are a
resin such as an acrylic resin or an epoxy resin, a resin having a
siloxane bond, and an inorganic insulating material such as silicon
oxide, silicon oxynitride, or aluminum oxide.
[0324] Openings reaching the electrodes 2591 are formed in the
insulating layer 2593, and the wiring 2594 electrically connects
the adjacent electrodes 2591. A light-transmitting conductive
material can be favorably used as the wiring 2594 because the
aperture ratio of the touch panel can be increased. Moreover, a
material with higher conductivity than the conductivities of the
electrodes 2591 and 2592 can be favorably used for the wiring 2594
because electric resistance can be reduced.
[0325] One electrode 2592 extends in one direction, and a plurality
of electrodes 2592 are provided in the form of stripes. The wiring
2594 intersects with the electrode 2592.
[0326] Adjacent electrodes 2591 are provided with one electrode
2592 provided therebetween. The wiring 2594 electrically connects
the adjacent electrodes 2591.
[0327] Note that the plurality of electrodes 2591 are not
necessarily arranged in the direction orthogonal to one electrode
2592 and may be arranged to intersect with one electrode 2592 at an
angle of more than 0 degrees and less than 90 degrees.
[0328] The wiring 2598 is electrically connected to any of the
electrodes 2591 and 2592. Part of the wiring 2598 functions as a
terminal. For the wiring 2598, a metal material such as aluminum,
gold, platinum, silver, nickel, titanium, tungsten, chromium,
molybdenum, iron, cobalt, copper, or palladium or an alloy material
containing any of these metal materials can be used.
[0329] Note that an insulating layer that covers the insulating
layer 2593 and the wiring 2594 may be provided to protect the touch
sensor 2595.
[0330] A connection layer 2599 electrically connects the wiring
2598 to the FPC 2509(2).
[0331] As the connection layer 2599, any of various anisotropic
conductive films (ACF), anisotropic conductive pastes (ACP), or the
like can be used.
<Description 2 of Touch Panel>
[0332] Next, the touch panel 2000 will be described in detail with
reference to FIG. 8A. FIG. 8A corresponds to a cross-sectional view
taken along dashed-dotted line X5-X6 in FIG. 6A.
[0333] In the touch panel 2000 illustrated in FIG. 8A, the display
device 2501 described with reference to FIG. 7A and the touch
sensor 2595 described with reference to FIG. 7C are attached to
each other.
[0334] The touch panel 2000 illustrated in FIG. 8A includes an
adhesive layer 2597 and an anti-reflective layer 2567p in addition
to the components described with reference to FIGS. 7A and 7C.
[0335] The adhesive layer 2597 is provided in contact with the
wiring 2594. Note that the adhesive layer 2597 attaches the
substrate 2590 to the substrate 2570 so that the touch sensor 2595
overlaps with the display device 2501. The adhesive layer 2597
preferably has a light-transmitting property. A heat curable resin
or an ultraviolet curable resin can be used for the adhesive layer
2597. For example, an acrylic resin, an urethane-based resin, an
epoxy-based resin, or a siloxane-based resin can be used.
[0336] The anti-reflective layer 2567p is positioned in a region
overlapping with pixels. As the anti-reflective layer 2567p, a
circularly polarizing plate can be used, for example.
[0337] Next, a touch panel having a structure different from that
illustrated in FIG. 8A will be described with reference to FIG.
8B.
[0338] FIG. 8B is a cross-sectional view of a touch panel 2001. The
touch panel 2001 illustrated in FIG. 8B differs from the touch
panel 2000 illustrated in FIG. 8A in the position of the touch
sensor 2595 relative to the display device 2501. Different parts
are described in detail below, and the above description of the
touch panel 2000 is referred to for the other similar parts.
[0339] The coloring layer 2567R is positioned in a region
overlapping with the light-emitting element 2550R. The
light-emitting element 2550R illustrated in FIG. 8B emits light to
the side where the transistor 2502t is provided. Accordingly, part
of light emitted from the light-emitting element 2550R passes
through the coloring layer 2567R and is emitted to the outside of
the light-emitting module 2580R as indicated by an arrow in FIG.
8B.
[0340] The touch sensor 2595 is provided on the substrate 2510 side
of the display device 2501.
[0341] The adhesive layer 2597 is provided between the substrate
2510 and the substrate 2590 and attaches the touch sensor 2595 to
the display device 2501.
[0342] As illustrated in FIG. 8A or 8B, light may be emitted from
the light-emitting element to one of upper and lower sides, or
both, of the substrate.
<Method for Driving Touch Panel>
[0343] Next, an example of a method for driving a touch panel will
be described with reference to FIGS. 9A and 9B.
[0344] FIG. 9A is a block diagram illustrating the structure of a
mutual capacitive touch sensor. FIG. 9A illustrates a pulse voltage
output circuit 2601 and a current sensing circuit 2602. Note that
in FIG. 9A, six wirings X1 to X6 represent the electrodes 2621 to
which a pulse voltage is applied, and six wirings Y1 to Y6
represent the electrodes 2622 that detect changes in current. FIG.
9A also illustrates capacitors 2603 that are each formed in a
region where the electrodes 2621 and 2622 overlap with each other.
Note that functional replacement between the electrodes 2621 and
2622 is possible.
[0345] The pulse voltage output circuit 2601 is a circuit for
sequentially applying a pulse voltage to the wirings X1 to X6. By
application of a pulse voltage to the wirings X1 to X6, an electric
field is generated between the electrodes 2621 and 2622 of the
capacitor 2603. When the electric field between the electrodes is
shielded, for example, a change occurs in the capacitor 2603
(mutual capacitance). The approach or contact of a sensing target
can be sensed by utilizing this change.
[0346] The current sensing circuit 2602 is a circuit for detecting
changes in current flowing through the wirings Y1 to Y6 that are
caused by the change in mutual capacitance in the capacitor 2603.
No change in current value is detected in the wirings Y1 to Y6 when
there is no approach or contact of a sensing target, whereas a
decrease in current value is detected when mutual capacitance is
decreased owing to the approach or contact of a sensing target.
Note that an integrator circuit or the like is used for sensing of
current values.
[0347] FIG. 9B is a timing chart showing input and output waveforms
in the mutual capacitive touch sensor illustrated in FIG. 9A. In
FIG. 9B, sensing of a sensing target is performed in all the rows
and columns in one frame period. FIG. 9B shows a period when a
sensing target is not sensed (not touched) and a period when a
sensing target is sensed (touched). Sensed current values of the
wirings Y1 to Y6 are shown as the waveforms of voltage values.
[0348] A pulse voltage is sequentially applied to the wirings X1 to
X6, and the waveforms of the wirings Y1 to Y6 change in accordance
with the pulse voltage. When there is no approach or contact of a
sensing target, the waveforms of the wirings Y1 to Y6 change in
accordance with changes in the voltages of the wirings X1 to X6.
The current value is decreased at the point of approach or contact
of a sensing target and accordingly the waveform of the voltage
value changes.
[0349] By detecting a change in mutual capacitance in this manner,
the approach or contact of a sensing target can be sensed.
<Sensor Circuit>
[0350] Although FIG. 9A illustrates a passive matrix type touch
sensor in which only the capacitor 2603 is provided at the
intersection of wirings as a touch sensor, an active matrix type
touch sensor including a transistor and a capacitor may be used.
FIG. 10 illustrates an example of a sensor circuit included in an
active matrix type touch sensor.
[0351] The sensor circuit in FIG. 10 includes the capacitor 2603
and transistors 2611, 2612, and 2613.
[0352] A signal G2 is input to a gate of the transistor 2613. A
voltage VRES is applied to one of a source and a drain of the
transistor 2613, and one electrode of the capacitor 2603 and a gate
of the transistor 2611 are electrically connected to the other of
the source and the drain of the transistor 2613. One of a source
and a drain of the transistor 2611 is electrically connected to one
of a source and a drain of the transistor 2612, and a voltage VSS
is applied to the other of the source and the drain of the
transistor 2611. A signal G1 is input to a gate of the transistor
2612, and a wiring ML is electrically connected to the other of the
source and the drain of the transistor 2612. The voltage VSS is
applied to the other electrode of the capacitor 2603.
[0353] Next, the operation of the sensor circuit in FIG. 10 will be
described. First, a potential for turning on the transistor 2613 is
supplied as the signal G2, and a potential with respect to the
voltage VRES is thus applied to the node n connected to the gate of
the transistor 2611. Then, a potential for turning off the
transistor 2613 is applied as the signal G2, whereby the potential
of the node n is maintained.
[0354] Then, mutual capacitance of the capacitor 2603 changes owing
to the approach or contact of a sensing target such as a finger,
and accordingly the potential of the node n is changed from
VRES.
[0355] In reading operation, a potential for turning on the
transistor 2612 is supplied as the signal G1. A current flowing
through the transistor 2611, that is, a current flowing through the
wiring ML is changed in accordance with the potential of the node
n. By sensing this current, the approach or contact of a sensing
target can be sensed.
[0356] In each of the transistors 2611, 2612, and 2613, an oxide
semiconductor layer is preferably used as a semiconductor layer in
which a channel region is formed. In particular, such a transistor
is preferably used as the transistor 2613 so that the potential of
the node n can be held for a long time and the frequency of
operation of resupplying VRES to the node n (refresh operation) can
be reduced.
[0357] The structure described in this embodiment can be combined
with any of the structures described in the other embodiments or
examples as appropriate.
Embodiment 6
[0358] In this embodiment, a display module and electronic devices
including a light-emitting element of one embodiment of the present
invention will be described with reference to FIG. 11 and FIGS. 12A
to 12G.
<Display Module>
[0359] In a display module 8000 in FIG. 11, a touch sensor 8004
connected to an FPC 8003, a display device 8006 connected to an FPC
8005, a frame 8009, a printed board 8010, and a battery 8011 are
provided between an upper cover 8001 and a lower cover 8002.
[0360] The light-emitting element of one embodiment of the present
invention can be used for the display device 8006, for example.
[0361] The shapes and sizes of the upper cover 8001 and the lower
cover 8002 can be changed as appropriate in accordance with the
sizes of the touch sensor 8004 and the display device 8006.
[0362] The touch sensor 8004 can be a resistive touch sensor or a
capacitive touch sensor and may be formed to overlap with the
display device 8006. A counter substrate (sealing substrate) of the
display device 8006 can have a touch sensor function. A photosensor
may be provided in each pixel of the display device 8006 so that an
optical touch sensor is obtained.
[0363] The frame 8009 protects the display device 8006 and also
serves as an electromagnetic shield for blocking electromagnetic
waves generated by the operation of the printed board 8010. The
frame 8009 may serve as a radiator plate.
[0364] The printed board 8010 has a power supply circuit and a
signal processing circuit for outputting a video signal and a clock
signal. As a power source for supplying power to the power supply
circuit, an external commercial power source or the battery 8011
provided separately may be used. The battery 8011 can be omitted in
the case of using a commercial power source.
[0365] The display module 8000 can be additionally provided with a
member such as a polarizing plate, a retardation plate, or a prism
sheet.
<Electronic Device>
[0366] FIGS. 12A to 12G illustrate electronic devices. These
electronic devices can include a housing 9000, a display portion
9001, a speaker 9003, operation keys 9005 (including a power switch
or an operation switch), a connection terminal 9006, a sensor 9007
(a sensor having a function of measuring or sensing force,
displacement, position, speed, acceleration, angular velocity,
rotational frequency, distance, light, liquid, magnetism,
temperature, chemical substance, sound, time, hardness, electric
field, current, voltage, electric power, radiation, flow rate,
humidity, gradient, oscillation, odor, or infrared ray), a
microphone 9008, and the like.
[0367] The electronic devices illustrated in FIGS. 12A to 12G can
have a variety of functions, for example, a function of displaying
a variety of data (a still image, a moving image, a text image, and
the like) on the display portion, a touch sensor function, a
function of displaying a calendar, date, time, and the like, a
function of controlling a process with a variety of software
(programs), a wireless communication function, a function of being
connected to a variety of computer networks with a wireless
communication function, a function of transmitting and receiving a
variety of data with a wireless communication function, a function
of reading a program or data stored in a memory medium and
displaying the program or data on the display portion, and the
like. Note that functions that can be provided for the electronic
devices illustrated in FIGS. 12A to 12G are not limited to those
described above, and the electronic devices can have a variety of
functions. Although not illustrated in FIGS. 12A to 12G, the
electronic devices may include a plurality of display portions. The
electronic devices may have a camera or the like and a function of
taking a still image, a function of taking a moving image, a
function of storing the taken image in a memory medium (an external
memory medium or a memory medium incorporated in the camera), a
function of displaying the taken image on the display portion, or
the like.
[0368] The electronic devices illustrated in FIGS. 12A to 12G will
be described in detail below.
[0369] FIG. 12A is a perspective view of a portable information
terminal 9100. The display portion 9001 of the portable information
terminal 9100 is flexible. Therefore, the display portion 9001 can
be incorporated along a bent surface of a bent housing 9000. In
addition, the display portion 9001 includes a touch sensor, and
operation can be performed by touching the screen with a finger, a
stylus, or the like. For example, when an icon displayed on the
display portion 9001 is touched, an application can be started.
[0370] FIG. 12B is a perspective view of a portable information
terminal 9101. The portable information terminal 9101 functions as,
for example, one or more of a telephone set, a notebook, and an
information browsing system. Specifically, the portable information
terminal can be used as a smartphone. Note that the speaker 9003,
the connection terminal 9006, the sensor 9007, and the like, which
are not shown in FIG. 12B, can be positioned in the portable
information terminal 9101 as in the portable information terminal
9100 shown in FIG. 12A. The portable information terminal 9101 can
display characters and image information on its plurality of
surfaces. For example, three operation buttons 9050 (also referred
to as operation icons, or simply, icons) can be displayed on one
surface of the display portion 9001. Furthermore, information 9051
indicated by dashed rectangles can be displayed on another surface
of the display portion 9001. Examples of the information 9051
include display indicating reception of an incoming email, social
networking service (SNS) message, call, and the like; the title and
sender of an email and SNS message; the date; the time; remaining
battery; and the reception strength of an antenna. Instead of the
information 9051, the operation buttons 9050 or the like may be
displayed on the position where the information 9051 is
displayed.
[0371] FIG. 12C is a perspective view of a portable information
terminal 9102. The portable information terminal 9102 has a
function of displaying information on three or more surfaces of the
display portion 9001. Here, information 9052, information 9053, and
information 9054 are displayed on different surfaces. For example,
a user of the portable information terminal 9102 can see the
display (here, the information 9053) with the portable information
terminal 9102 put in a breast pocket of his/her clothes.
Specifically, a caller's phone number, name, or the like of an
incoming call is displayed in a position that can be seen from
above the portable information terminal 9102. Thus, the user can
see the display without taking out the portable information
terminal 9102 from the pocket and decide whether to answer the
call.
[0372] FIG. 12D is a perspective view of a watch-type portable
information terminal 9200. The portable information terminal 9200
is capable of executing a variety of applications such as mobile
phone calls, e-mailing, viewing and editing texts, music
reproduction, Internet communication, and computer games. The
display surface of the display portion 9001 is bent, and images can
be displayed on the bent display surface. The portable information
terminal 9200 can employ near field communication that is a
communication method based on an existing communication standard.
In that case, for example, mutual communication between the
portable information terminal 9200 and a headset capable of
wireless communication can be performed, and thus hands-free
calling is possible. The portable information terminal 9200
includes the connection terminal 9006, and data can be directly
transmitted to and received from another information terminal via a
connector. Power charging through the connection terminal 9006 is
possible. Note that the charging operation may be performed by
wireless power feeding without using the connection terminal
9006.
[0373] FIGS. 12E, 12F, and 12G are perspective views of a foldable
portable information terminal 9201. FIG. 12E is a perspective view
illustrating the portable information terminal 9201 that is opened.
FIG. 12F is a perspective view illustrating the portable
information terminal 9201 that is being opened or being folded.
FIG. 12G is a perspective view illustrating the portable
information terminal 9201 that is folded. The portable information
terminal 9201 is highly portable when folded. When the portable
information terminal 9201 is opened, a seamless large display
region is highly browsable. The display portion 9001 of the
portable information terminal 9201 is supported by three housings
9000 joined together by hinges 9055. By folding the portable
information terminal 9201 at a connection portion between two
housings 9000 with the hinges 9055, the portable information
terminal 9201 can be reversibly changed in shape from an opened
state to a folded state. For example, the portable information
terminal 9201 can be bent with a radius of curvature of greater
than or equal to 1 mm and less than or equal to 150 mm.
[0374] The electronic devices described in this embodiment each
include the display portion for displaying some sort of data. Note
that the light-emitting element of one embodiment of the present
invention can also be used for an electronic device which does not
have a display portion. The structure in which the display portion
of the electronic device described in this embodiment is flexible
and display can be performed on the bent display surface or the
structure in which the display portion of the electronic device is
foldable is described as an example; however, the structure is not
limited thereto and a structure in which the display portion of the
electronic device is not flexible and display is performed on a
plane portion may be employed.
[0375] The structure described in this embodiment can be combined
with any of the structures described in the other embodiments or
examples as appropriate.
Embodiment 7
[0376] In this embodiment, examples of lighting devices in which
the light-emitting element of one embodiment of the present
invention is used will be described with reference to FIG. 13.
[0377] FIG. 13 illustrates an example in which the light-emitting
element is used for an indoor lighting device 8501. Since the
light-emitting element can have a larger area, a lighting device
having a large area can also be formed. In addition, a lighting
device 8502 in which a light-emitting region has a curved surface
can also be formed with the use of a housing with a curved surface.
A light-emitting element described in this embodiment is in the
form of a thin film, which allows the housing to be designed more
freely. Therefore, the lighting device can be elaborately designed
in a variety of ways. Furthermore, a wall of the room may be
provided with a large-sized lighting device 8503. Touch sensors may
be provided in the lighting devices 8501, 8502, and 8503 to control
the power on/off of the lighting devices.
[0378] Moreover, when the light-emitting element is used on the
surface side of a table, a lighting device 8504 which has a
function as a table can be obtained. When the light-emitting
element is used as part of other furniture, a lighting device which
has a function as the furniture can be obtained.
[0379] In this manner, a variety of lighting devices to which the
light-emitting element is applied can be obtained. Note that such
lighting devices are also embodiments of the present invention.
[0380] The structure described in this embodiment can be combined
with any of the structures described in the other embodiments as
appropriate.
Example 1
[0381] This example shows a fabrication example of light-emitting
elements (light-emitting elements 1 and 2) in which a mixture of a
thermally activated delayed fluorescence substance (a first organic
compound), a host material (a second organic compound), and a guest
material emitting fluorescence are used for a light-emitting layer.
FIG. 14 is a schematic cross-sectional view of the light-emitting
element fabricated in this example. Table 4 shows a detailed
structure of the element. In addition, structures and abbreviations
of compounds used here are shown below. Note that Embodiment 1 can
be referred to for other compounds.
##STR00008## ##STR00009##
TABLE-US-00004 TABLE 4 Reference Thickness Layer numeral (nm)
Material Weight ratio Light-emitting Electrode 502 200 Al --
element 1 Electron- 534 1 LiF -- injection layer Electron- 533b 15
Bphen -- transport layer 533a 15 4,6mCzP2Pm -- Light-emitting 521
30 PCCzPTzn:4,6mCzP2Pm:1,6mMemFLPAPrn 0.3:0.7:0.0025 layer
Hole-transport 532 20 PhCzGI -- layer Hole-injection 531 70
DBT3P-II:MoO.sub.3 1:0.5 layer Electrode 501 110 ITSO --
Light-emitting Electrode 502 200 Al -- element 2 Electron- 534 1
LiF -- injection layer Electron- 533b 15 Bphen -- transport layer
533a 15 4,6mCzP2Pm -- Light-emitting 521 30 PCCzPTzn:4,6mCzP2Pm:TBP
0.3:0.7:0.0025 layer Hole-transport 532 20 PhCzGI -- layer
Hole-injection 531 70 DBT3P-II:MoO.sub.3 1:0.5 layer Electrode 501
110 ITSO --
[0382] Fabrication methods of the light-emitting elements 1 and 2
are described.
<Fabrication of Light-Emitting Element 1>
[0383] A film of indium tin oxide containing silicon oxide (ITSO)
was formed to a thickness of 110 nm over a substrate 520 by a
sputtering method to form an electrode 501. Note that the area of
the electrode 501 was 4 mm.sup.2 (2 mm.times.2 mm).
[0384] Then, as pretreatment for forming the light-emitting element
over the substrate 520, UV ozone treatment was performed for 370
seconds after washing of a surface of the substrate with water and
heat treatment that was performed at 200.degree. C. for 1 hour.
[0385] Next, the substrate 520 was fixed to a substrate holder
inside a vacuum evaporation apparatus reduced to approximately
1.times.10.sup.-4 Pa with the electrode 501 side down. Then, as a
hole-injection layer 531, DBT3P-II and molybdenum oxide (MoO.sub.3)
were deposited on the electrode 501 by co-evaporation in a weight
ratio of DBT3P-II:MoO.sub.3=1:0.5 to a thickness of 70 nm.
[0386] As a hole-transport layer 532, PhCzGI was deposited to a
thickness of 20 nm over the hole-injection layer 531.
[0387] As a light-emitting layer 521, PCCzPTzn, 4,6mCzP2Pm, and
1.6mMemFLPAPrn were deposited over the hole-transport layer 532 by
co-evaporation such that the deposited layer has a weight ratio of
PCCzPTzn:4,6mCzP2Pm:1,6mMemFLPAPrn=0.3:0.7:0.0025 and a thickness
of 30 nm. Note that in the light-emitting layer 521, PCCzPTzn is a
thermally activated delayed fluorescence substance (a first organic
compound), 4,6mCzP2Pm is a host material (a second organic
compound), and 1,6mMemFLPAPm is a guest material.
[0388] Next, 4,6mCzP2Pm and bathophenanthroline (abbreviation:
Bphen) were sequentially deposited to thicknesses of 15 nm each, as
electron-transport layers 533a and 533b over the light-emitting
layer 521.
[0389] Then, as the electron-injection layer 534, lithium fluoride
(abbreviation: LiF) was deposited to a thickness of 1 nm over the
electron-transport layer 533b.
[0390] As an electrode 502, aluminum (Al) was deposited on the
electron-injection layer 534 to a thickness of 200 nm.
[0391] Through the above steps, the components over the substrate
520 were formed. Note that a resistance heating method was used for
the above deposition process.
[0392] Next, a light-emitting element was sealed by fixing a
sealing substrate to the substrate 520 using a sealant for an
organic EL device in a glove box under a nitrogen atmosphere.
[0393] Specifically, the sealant was applied to surround the
light-emitting element, the substrate 520 and the sealing substrate
were bonded to each other, irradiation with ultraviolet light a
wavelength of 365 nm at 6 J/cm.sup.2 was performed, and heat
treatment was performed at 80.degree. C. for 1 hour.
[0394] Through the above steps, the light-emitting element 1 was
obtained.
<Fabrication of Light-Emitting Element 2>
[0395] The light-emitting element 2 was fabricated through the same
steps as those for the light-emitting element 1 except that TBP was
used as a guest material for the light-emitting element 2.
[0396] In other words, as the light-emitting layer 521 of the
light-emitting element 2, PCCzPTzn, 4,6mCzP2Pm, and TBP were
deposited by co-evaporation such that the deposited layer has a
weight ratio of PCCzPTzn:4,6mCzP2Pm:TBP=0.3:0.7:0.0025 and a
thickness of 30 nm. Note that in the light-emitting layer 521,
PCCzPTzn is a thermally activated delayed fluorescence substance (a
first organic compound), 4,6mCzP2Pm is a host material (a second
organic compound), and TBP is a guest material.
<Measurement of Transient Fluorescent Characteristics>
[0397] Transient fluorescent characteristics of PCCzPTzn which was
the host material of the light-emitting elements in this example
(the light-emitting elements 1 and 2) were measured using
time-resolved emission measurement.
[0398] The time-resolved emission measurement was performed on a
thin-film sample in which PCCzPTzn was deposited over a quartz
substrate to a thickness of 50 nm. The thin-film sample was sealed
by fixing a sealing substrate to the quartz substrate over which
the thin-film sample was deposited using a sealant for an organic
EL device in a glove box under a nitrogen atmosphere. Specifically,
after a sealant was applied to surround the thin-film over the
quartz substrate and the quartz substrate was bonded to the sealing
substrate, irradiation with ultraviolet light having a wavelength
of 365 nm at 6 J/cm.sup.2 and heat treatment at 80.degree. C. for
one hour were performed.
[0399] A picosecond fluorescence lifetime measurement system
(manufactured by Hamamatsu Photonics K.K.) was used for the
measurement. In this measurement, the thin film was irradiated with
pulsed laser, and emission of the thin film which was attenuated
from the laser irradiation underwent time-resolved measurement
using a streak camera to measure the lifetime of fluorescent
emission of the thin film. A nitrogen gas laser with a wavelength
of 337 nm was used as the pulsed laser. The thin film was
irradiated with pulsed laser with a pulse width of 500 ps at a
repetition rate of 10 Hz. By integrating data obtained by the
repeated measurement, data with a high S/N ratio was obtained. The
measurement was performed at room temperature (in an atmosphere
kept at 23.degree. C.).
[0400] FIG. 15 shows transient fluorescent characteristics of
PCCzPTzn obtained by the measurement.
[0401] The attenuation curve shown in FIG. 15 was fitted with
Formula 4.
L = n = 1 A n exp ( - t a n ) [ Formula 4 ] ##EQU00004##
[0402] In Formula 4, L and t represent normalized emission
intensity and elapsed time, respectively. The attenuation curve was
able to be fitted when n was 1 to 3. This fitting results show that
the emission component of the PCCzPTzn thin-film sample contains a
fluorescent component having an emission lifetime of 0.015 ps and a
delayed fluorescence component having an emission lifetime of 1.5
is. In other words, it is found that PCCzPTzn is a thermally
activated delayed fluorescence substance exhibiting delayed
fluorescent at room temperature.
<Characteristics of Light-Emitting Elements>
[0403] FIGS. 16, 17, and 18 show luminance-current density
characteristics, luminance-voltage characteristics, and external
quantum efficiency-luminance characteristics, respectively, of the
light-emitting elements 1 and 2. The measurement of the
light-emitting elements was performed at room temperature (in an
atmosphere kept at 23.degree. C.).
[0404] Table 5 shows element characteristics of the light-emitting
elements 1 and 2 at around 1000 cd/m.sup.2.
TABLE-US-00005 TABLE 5 CIE Current Power External Voltage Current
density chromaticity Luminance efficiency efficiency quantum (V)
(mA/cm.sup.2) (x, y) (cd/m.sup.2) (cd/A) (lm/W) efficiency (%)
Light-emitting 3.5 8.49 (0.18, 0.27) 995 11.7 10.5 6.09 element 1
Light-emitting 3.4 7.10 (0.18, 0.29) 924 13.0 12.0 6.70 element
2
[0405] FIG. 19 shows the electroluminescence spectra of the
light-emitting elements 1 and 2 through which current flows at a
current density of 2.5 mA/cm.sup.2. It is found from FIG. 19 that
blue light emission originating from the guest material is obtained
from the light-emitting elements 1 and 2.
[0406] In addition, the light-emitting elements 1 and 2 show
element characteristics of low driving voltage and high emission
efficiency as shown in FIGS. 16, 17, and 18. In particular, the
light-emitting element 2 shows high external quantum efficiency
exceeding 10% at a maximum. In the case where a fluorescent
substance is used as a guest material and energy only from a
singlet excited state is used for emission, the maximum external
quantum efficiency of a light-emitting element is approximately 6%
on the assumption that the light extraction efficiency from the
light-emitting element to the outside is 25%. However, the
light-emitting elements 1 and 2 using one embodiment of the present
invention exhibited higher external quantum efficiency. This is
because a triplet excited state generated by recombined carriers in
a thermally activated delayed fluorescence substance was converted
into a single excited state by reverse intersystem crossing.
[0407] In addition, in the light-emitting elements 1 and 2, the
HOMO level of a thermally activated delayed fluorescence substance
is higher than or equal to the HOMO level of a host material, and
the LUMO level of the thermally activated delayed fluorescence
substance is lower than or equal to the LUMO level of the host
material, which are shown in Table 1 in Embodiment 1. In addition,
the oxidation potential and the reduction potential of the
thermally activated delayed fluorescence substance are lower than
or equal to the oxidation potential of the host material and higher
than or equal to the reduction potential of the host material,
respectively, as shown in Table 2 in Embodiment 1. Thus, the HOMO
level and the LUMO level of the thermally activated delayed
fluorescence substance which are estimated from the oxidation
potential and the reduction potential are higher than or equal to
the HOMO level of the host material and lower than or equal to the
LUMO level of the host material, respectively.
[0408] The triplet excited energy level of 1,6mMemFLPAPrn was 1.84
eV, which was measured by the method similar to that in Embodiment
1. Therefore, as shown in Table 3 in Embodiment 1, the triplet
excited energy level of the thermally activated delayed
fluorescence substance (PCCzPTzn) and that of the host material
(4,6mCzP2Pm) are each higher than that of the guest material.
[0409] Therefore, in the light-emitting elements 1 and 2, both the
singlet excited state and the triplet excited state which are
efficiently formed by carrier recombination in the thermally
activated delayed fluorescence substance can be transferred
efficiently to the guest material. As a result, the light-emitting
elements 1 and 2 show high emission efficiency.
[0410] The high emission efficiency of the light-emitting elements
1 and 2 means that the weight ratio of the host material to the
thermally activated delayed fluorescence substance is preferably
from 1:0.05 to 1:0.5 (host material: thermally activated delayed
fluorescence substance) and the weight ratio of the host material
to the guest material is preferably from 1:0.001 to 1:0.01 (host
material:guest material).
[0411] As described above, the use of a structure of one embodiment
of the present invention can provide a light-emitting element with
high emission efficiency.
Example 2
[0412] In this example, light-emitting elements including and not
including a thermally activated delayed fluorescence substance, and
light-emitting elements with different weight ratios of a host
material and a guest material (light-emitting elements 3 to 5 and
comparative light-emitting element 1 to 4) were fabricated. A
schematic cross-sectional view of the light-emitting elements
fabricated in this example is similar to FIG. 14 in Example 1.
Details of the light-emitting elements fabricated in this example
are shown in Table 6 and Table 7. In addition, structures and
abbreviations of compounds used here are given below. Embodiment 1
or Example 1 may be referred to for other compounds.
##STR00010##
TABLE-US-00006 TABLE 6 Reference Thickness Layer numeral (nm)
Material Weight ratio Light-emitting Electrode 502 200 Al --
element 3 Electron- 534 1 LiF -- injection layer Electron- 533 30
Bphen -- transport layer Light-emitting 521 30
PCCzPTzn:Cz2DBT:1,6mMemFLPAPrn 0.1:0.9:0.005 layer Hole-transport
532 20 Cz2DBT -- layer Hole-injection 531 70 DBT3P-II:MoO.sub.3
1:0.5 layer Electrode 501 110 ITSO -- Comparative Electrode 502 200
Al -- light-emitting Electron- 534 1 LiF -- element 1 injection
layer Electron- 533 30 Bphen -- transport layer Light-emitting 521
30 Cz2DBT:1,6mMemFLPAPrn 1:0.05 layer Hole-transport 532 20 Cz2DBT
-- layer Hole-injection 531 70 DBT3P-II:MoO.sub.3 1:0.5 layer
Electrode 501 110 ITSO -- Comparative Electrode 502 200 Al --
light-emitting Electron- 534 1 LiF -- element 2 injection layer
Electron- 533 30 Bphen -- transport layer Light-emitting 521 30
PCCzPTzn:Cz2DBT:1,6mMemFLPAPrn 0.1:0.9:0.05 layer Hole-transport
532 20 Cz2DBT -- layer Hole-injection 531 70 DBT3P-II:MoO.sub.3
1:0.5 layer Electrode 501 110 ITSO --
TABLE-US-00007 TABLE 7 Reference Thickness Layer numeral (nm)
Material Weight ratio Light-emitting Electrode 502 200 Al --
element 4 Electron- 534 1 LiF -- injection layer Electron- 533 30
Bphen -- transport layer Light-emitting 521 30 PCCzPTzn:Cz2DBT:TBP
0.1:0.9:0.005 layer Hole-transport 532 20 Cz2DBT -- layer
Hole-injection 531 70 DBT3P-II:MoO.sub.3 1:0.5 layer Electrode 501
110 ITSO -- Light-emitting Electrode 502 200 Al -- element 5
Electron- 534 1 LiF -- injection layer Electron- 533 30 Bphen --
transport layer Light-emitting 521 30 PCCzPTzn:CzTAZ1:TBP
0.1:0.9:0.005 layer Hole-transport 532 20 Cz2DBT -- layer
Hole-injection 531 70 DBT3P-II:MoO.sub.3 1:0.5 layer Electrode 501
110 ITSO -- Comparative Electrode 502 200 Al -- light-emitting
Electron- 534 1 LiF -- element 3 injection layer Electron- 533 30
Bphen -- transport layer Light-emitting 521 30 PCCzPTzn:Cz2DBT:TBP
0.1:0.9:0.05 layer Hole-transport 532 20 Cz2DBT -- layer
Hole-injection 531 70 DBT3P-II:MoO.sub.3 1:0.5 layer Electrode 501
110 ITSO -- Comparative Electrode 502 200 Al -- light-emitting
Electron- 534 1 LiF -- element 4 injection layer Electron- 533 30
Bphen -- transport layer Light-emitting 521 30 PCCzPTznCzTAZ1:TBP
0.1:0.9:0.05 layer Hole-transport 532 20 Cz2DBT -- layer
Hole-injection 531 70 DBT3P-II:MoO.sub.3 1:0.5 layer Electrode 501
110 ITSO --
[0413] A method for fabricating the light-emitting elements 3 to 5
and the comparative light-emitting elements 1 to 4 will be
described.
<Fabrication of Light-Emitting Element 3>
[0414] An ITSO film was formed to a thickness of 110 nm over the
substrate 520 by a sputtering method to form the electrode 501.
Note that the area of the electrode 501 was 4 mm.sup.2 (2
mm.times.2 mm).
[0415] Then, as pretreatment for forming the light-emitting element
over the substrate 520, UV ozone treatment was performed for 370
seconds after washing of a surface of the substrate with water and
heat treatment that was performed at 200.degree. C. for 1 hour.
[0416] Next, the substrate 520 was fixed to a substrate holder
inside a vacuum evaporation apparatus reduced to approximately
1.times.10.sup.-4 Pa with the electrode 501 side down. Then, as a
hole-injection layer 531, DBT3P-II and MoO.sub.3 were deposited on
the electrode 501 by co-evaporation in a weight ratio of
DBT3P-II:MoO.sub.3=1:0.5 to a thickness of 70 nm.
[0417] As a hole-transport layer 532, Cz2DBT was deposited to a
thickness of 20 nm over the hole-injection layer 531.
[0418] As a light-emitting layer 521, PCCzPTzn, Cz2DBT, and
1.6mMemFLPAPrn were deposited over the hole-transport layer 532 by
co-evaporation such that the deposited layer has a weight ratio of
PCCzPTzn:Cz2DBT: 1,6mMemFLPAPrn=0.1:0.9:0.005 and a thickness of 30
nm. Note that in the light-emitting layer 521, PCCzPTzn is a
thermally activated delayed fluorescence substance (a first organic
compound), Cz2DBT is a host material (a second organic compound),
and 1,6mMemFLPAPrn is a guest material.
[0419] Next, Bphen were sequentially deposited to a thickness of 30
nm each, as electron-transport layer 533 over the light-emitting
layer 521.
[0420] Then, as the electron-injection layer 534, LiF was deposited
to a thickness of 1 nm over the electron-transport layer 533.
[0421] As an electrode 502, Al was deposited on the
electron-injection layer 534 to a thickness of 200 nm.
[0422] Through the above steps, the components over the substrate
520 were formed. Note that a resistance heating method was used for
the above deposition process.
[0423] Next, a light-emitting element was sealed by fixing a
sealing substrate to the substrate 520 using a sealant for an
organic EL device in a glove box under a nitrogen atmosphere.
[0424] Specifically, the sealant was applied to surround the
light-emitting element, the substrate 520 and the sealing substrate
were bonded to each other, irradiation with ultraviolet light
having a wavelength of 365 nm 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.
<Fabrication of Light-Emitting Elements 4 and 5 and Comparative
Light-Emitting Elements 1 to 4>
[0425] The light-emitting elements 4 and 5 and the comparative
light-emitting elements 1 to 4 were fabricated through the same
steps as those for the above-mentioned light-emitting element 3
except that structures of their light-emitting layers were
different from the structure of the light-emitting layer of the
light-emitting element 3.
[0426] As the light-emitting layer 521 of the light-emitting
element 4, PCCzPTzn, Cz2DBT, and TBP were deposited by
co-evaporation such that the deposited layer has a weight ratio of
PCCzPTzn:Cz2DBT:TBP=0.1:0.9:0.005 and a thickness of 30 nm. Note
that in the light-emitting layer 521, PCCzPTzn is a thermally
activated delayed fluorescence substance (a first organic
compound), Cz2DBT is a host material (a second organic compound),
and TBP is a guest material. In other words, the light-emitting
element 4 has a structure similar to that of the light-emitting
element 3 except for the guest material.
[0427] As the light-emitting layer 521 of the light-emitting
element 5, PCCzPTzn, CzTAZ1, and TBP were deposited by
co-evaporation such that the deposited layer has a weight ratio of
PCCzPTzn:CzTAZ1:TBP=0.1:0.9:0.005 and a thickness of 30 nm. Note
that in the light-emitting layer 521, PCCzPTzn is a thermally
activated delayed fluorescence substance (a first organic
compound), CzTAZ1 is a host material (a second organic compound),
and TBP is a guest material. In other words, the light-emitting
element 5 has a structure similar to that of the light-emitting
element 4 except for the host material.
[0428] As the light-emitting layer 521 of the comparative
light-emitting element 1, Cz2DBT and 1.6mMemFLPAPrn were deposited
by co-evaporation such that the deposited layer has a weight ratio
of Cz2DBT:1,6mMemFLPAPrn=1:0.05 and a thickness of 30 nm. Note that
in the light-emitting layer 521, Cz2DBT is a host material (a
second organic compound) and 1,6mMemFLPAPrn is a guest material. In
other words, a thermally activated delayed fluorescence substance
(a first organic compound) is not used for the comparative
light-emitting element 1.
[0429] As the light-emitting layer 521 of the comparative
light-emitting element 2, PCCzPTzn, Cz2DBT, and 1.6mMemFLPAPrn were
deposited by co-evaporation such that the deposited layer has a
weight ratio of PCCzPTzn:Cz2DBT:1,6mMemFLPAPrn=0.1:0.9:0.05 and a
thickness of 30 nm. Note that in the light-emitting layer 521,
PCCzPTzn is a thermally activated delayed fluorescence substance (a
first organic compound), Cz2DBT is a host material (a second
organic compound), and 1,6mMemFLPAPrn is a guest material. In other
words, the comparative light-emitting element 2 has a structure
similar to that of the light-emitting element 3 except for the
concentration of the guest material.
[0430] As the light-emitting layer 521 of the comparative
light-emitting element 3, PCCzPTzn, Cz2DBT, and TBP were deposited
by co-evaporation such that the deposited layer has a weight ratio
of PCCzPTzn:Cz2DBT:TBP=0.1:0.9:0.05 and a thickness of 30 nm. Note
that in the light-emitting layer 521, PCCzPTzn is a thermally
activated delayed fluorescence substance (a first organic
compound), Cz2DBT is a host material (a second organic compound),
and TBP is a guest material. In other words, the comparative
light-emitting element 3 has a structure similar to that of the
light-emitting element 4 except for the concentration of the guest
material.
[0431] As the light-emitting layer 521 of the comparative
light-emitting element 4, PCCzPTzn, CzTAZ1, and TBP were deposited
by co-evaporation such that the deposited layer has a weight ratio
of PCCzPTzn:CzTAZ1: TBP=0.1:0.9:0.05 and a thickness of 30 nm. Note
that in the light-emitting layer 521, PCCzPTzn is a thermally
activated delayed fluorescence substance (a first organic
compound), CzTAZ1 is a host material (a second organic compound),
and TBP is a guest material. In other words, the comparative
light-emitting element 4 has a structure similar to that of the
light-emitting element 5 except for the concentration of the guest
material.
<Characteristics of Light-Emitting Elements>
[0432] FIGS. 20, 21, and 22 show current efficiency-luminance
characteristics, current-voltage characteristics, and external
quantum efficiency-luminance characteristics, respectively, of the
light-emitting element 3 and the comparative light-emitting
elements 1 and 2. FIG. 23 shows electroluminescence spectra when a
current at a current density of 2.5 mA/cm.sup.2 was supplied to the
light-emitting element 3 and the comparative light-emitting
elements 1 and 2. FIGS. 24, 25, and 26 show current
efficiency-luminance characteristics, current-voltage
characteristics, and external quantum efficiency-luminance
characteristics, respectively, of the light-emitting elements 4 and
5 and the comparative light-emitting elements 3 and 4. FIG. 27
shows s electroluminescence spectra when a current at a current
density of 2.5 mA/cm.sup.2 was supplied to the light-emitting
elements 4 and 5 and the comparative light-emitting elements 3 and
4. The measurements of the light-emitting elements were performed
at room temperature (in an atmosphere kept at 23.degree. C.).
[0433] Table 8 shows the element characteristics of the
light-emitting elements 3 to 5 and the comparative light-emitting
element 1 to 4 at around 1000 cd/m.sup.2.
TABLE-US-00008 TABLE 8 Current CIE Current Power External Voltage
density chromaticity Luminance efficiency efficiency quantum (V)
(mA/cm.sup.2) (x, y) (cd/m.sup.2) (cd/A) (lm/W) efficiency (%)
Light-emitting 4.2 1.05 (0.16, 0.19) 81 7.7 5.8 5.52 element 3
Light-emitting 4.0 1.05 (0.16, 0.23) 103 9.8 7.7 6.22 element 4
Light-emitting 3.8 0.83 (0.17, 0.26) 89 10.7 8.9 6.17 element 5
Comparative 4.4 2.35 (0.14, 0.17) 94 4.0 2.9 3.19 light-emitting
element 1 Comparative 4.8 2.16 (0.14, 0.19) 104 4.8 3.2 3.56
light-emitting element 2 Comparative 4.2 1.77 (0.15, 0.25) 87 4.9
3.7 2.98 light-emitting element 3 Comparative 4.2 2.00 (0.16, 0.26)
75 3.7 2.8 2.19 light-emitting element 4
[0434] It is found from the emission spectra in FIG. 23 and FIG. 27
that blue light emission originating from the guest material is
obtained from the light-emitting elements 3 to 5 and the
comparative light-emitting element 1 to 4.
[0435] As shown in FIGS. 20 to 22 and FIGS. 24 to 26, the
light-emitting elements 3 to 5 show high emission efficiency. In
contrast, the comparative light-emitting elements 1 to 4 do not
show sufficient emission efficiency.
[0436] Table 9 shows measurement results of the oxidation
potentials and the reduction potentials of the thermally activated
delayed fluorescence material (PCCzPTzn) and the host material
(Cz2DBT or CzTAZ1) in the solution state and the HOMO and LUMO
levels estimated from the results. Note that the measurement method
is similar to that described in Embodiment 1.
TABLE-US-00009 TABLE 9 HOMO(eV) LUMO(eV) Oxidation Reduction
estimated from estimated from potential(V) potential(V) oxidation
potential oxidation potential Abbreviation in solution state in
solution state in solution state in solution state Thermally
activated PCCzPTzn 0.70 -1.97 -5.64 -2.97 delayed fluorescence
substance Host material Cz2DBT 0.92 -2.62 -5.86 -2.33 CzTAZ1 1.00
-2.70 -5.94 -2.24
[0437] As shown in Table 9, in the light-emitting elements 3 to 5,
the oxidation potential and the reduction potential of the
thermally activated delayed fluorescence substance are lower than
or equal to the oxidation potential of the host material and higher
than or equal to the reduction potential of the host material,
respectively. Thus, the HOMO level and the LUMO level of the
thermally activated delayed fluorescence substance which are
estimated from the oxidation potential and the reduction potential
are higher than or equal to the HOMO level of the host material and
lower than or equal to the LUMO level of the host material,
respectively. Therefore, both the singlet excited state and the
triplet excited state which are efficiently formed by carrier
recombination in the thermally activated delayed fluorescence
substance can be transferred efficiently to the guest material. As
a result, the light-emitting elements 3 to 5 show high emission
efficiency.
[0438] In addition, the results in which the emission efficiency of
the light-emitting element 3 is higher than that of the comparative
light-emitting element 2, and the emission efficiency of the
comparative light-emitting element 2 is higher than that of the
comparative light-emitting element 1 show that the use of PCCzPTzn
as the thermally activated delayed fluorescence substance for the
light-emitting layer 521 can increase the emission efficiency. This
is because a triplet excited state generated by recombined carriers
in PCCzPTzn, which was the thermally activated delayed fluorescence
substance, was converted into a single excited state by reverse
intersystem crossing.
[0439] The emission efficiency of the light-emitting element 3
higher than that of the comparative light-emitting element 2 means
that the weight ratio of the host material (Cz2DBT) to the guest
material (1,6mMemFLPAPrn) in the light-emitting layer 521 is
preferably from 1:0.001 to 1:0.01 (host material:guest material).
This is because the concentration of a guest material that is
sufficiently lower than that of a host material can suppress
generation of a triplet excited state of the guest material.
[0440] The emission efficiency of the light-emitting element 4 is
higher than that of the comparative light-emitting element 3 and
the emission efficiency of the light-emitting element 5 is higher
than that of the comparative light-emitting element 4, which means
that the weight ratio of the host material (Cz2DBT or CzTAZ1) to
the guest material (TBP) in the light-emitting layer 521 is
preferably from 1:0.001 to 1:0.01 (host material:guest
material).
[0441] Thus, the weight ratio of the host material to the thermally
activated delayed fluorescence substance is preferably from 1:0.05
to 1:0.5 (host material:thermally activated delayed fluorescence
substance) and the weight ratio of the host material to the guest
material is preferably from 1:0.001 to 1:0.01 (host material:guest
material).
[0442] As described above, the use of a structure of one embodiment
of the present invention can provide a light-emitting element with
high emission efficiency.
EXPLANATION OF REFERENCE
[0443] 100: EL layer, 101: electrode, 102: electrode, 111:
hole-injection layer, 112: hole-transport layer, 118:
electron-transport layer, 119: electron-injection layer, 120:
light-emitting layer, 131: organic compound, 132: organic compound,
133: guest material, 150: light-emitting element, 401: electrode,
402: electrode, 411: hole-injection layer, 412: hole-transport
layer, 413: electron-transport layer, 414: electron-injection
layer, 415: hole-injection layer, 416: hole-transport layer, 417:
electron-transport layer, 418: electron-injection layer, 421:
organic compound, 422: organic compound, 423: guest material, 431:
organic compound, 432: organic compound, 433: guest material, 441:
light-emitting unit, 442: light-emitting unit, 443: light-emitting
layer, 444: light-emitting layer, 445: charge-generation layer,
446: light-emitting unit, 447: light-emitting unit, 448:
light-emitting layer, 449: light-emitting layer, 450:
light-emitting element, 452: light-emitting element, 461: host
material, 462: guest material, 471: organic compound, 472: organic
compound, 473: guest material, 501: electrode, 502: electrode, 520:
substrate, 521: light-emitting layer, 531: hole-injection layer,
532: hole-transport layer, 533: electron-transport layer, 533a:
electron-transport layer, 533b: electron-transport layer, 534:
electron-injection layer, 801: pixel circuit, 802: pixel portion,
804: driver circuit portion, 804a: scan line driver circuit, 804b:
signal line driver circuit, 806: protective circuit, 807: terminal
portion, 852: transistor, 854: transistor, 862: capacitor, 872:
light-emitting element, 2000: touch panel, 2001: touch panel, 2501:
display device, 2502R: pixel, 2502t: transistor, 2503c: capacitor,
2503g(1): scan line driver circuit, 2503t: transistor, 2509: FPC,
2510: substrate, 2510a: insulating layer, 2510b: flexible
substrate, 2510c: adhesive layer, 2511: wiring, 2519: terminal,
2521: insulating layer, 2528: partition, 2550R: light-emitting
element, 2560: sealing layer, 2567BM: light-blocking layer, 2567p:
anti-reflective layer, 2567R: coloring layer, 2570: substrate,
2570a: insulating layer, 2570b: flexible substrate, 2570c: adhesive
layer, 2580R: light-emitting module, 2590: substrate, 2591:
electrode, 2592: electrode, 2593: insulating layer, 2594: wiring,
2595: touch sensor, 2597: adhesive layer, 2598: wiring, 2599:
connection layer, 2601: pulse voltage output circuit, 2602: current
sensing circuit, 2603: capacitor, 2611: transistor, 2612:
transistor, 2613: transistor, 2621: electrode, 2622: electrode,
8000: display module, 8001: upper cover, 8002: lower cover, 8003:
FPC, 8004: touch sensor, 8005: FPC, 8006: display device, 8009:
frame, 8010: printed board, 8011: battery, 8501: lighting device,
8502: lighting device, 8503: lighting device, 8504: lighting
device, 9000: housing, 9001: display portion, 9003: speaker, 9005:
operation key, 9006: connection terminal, 9007: sensor, 9008:
microphone, 9050: operation button, 9051: information, 9052:
information, 9053: information, 9054: information, 9055: hinge,
9100: portable information terminal, 9101: portable information
terminal, 9102: portable information terminal, 9200: portable
information terminal, 9201: portable information terminal.
[0444] This application is based on Japanese Patent Application
serial no. 2014-200355 filed with Japan Patent Office on Sep. 30,
2014, the entire contents of which are hereby incorporated by
reference.
* * * * *