U.S. patent application number 15/203901 was filed with the patent office on 2016-10-27 for heterocyclic compound, light-emitting element, light-emitting device, electronic device, and lighting device.
The applicant listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Naoaki HASHIMOTO, Takahiro ISHISONE, Miyako MORIKUBO, Hiromi SEO, Satoshi SEO, Satoko SHITAGAKI, Kyoko TAKEDA.
Application Number | 20160315270 15/203901 |
Document ID | / |
Family ID | 47991906 |
Filed Date | 2016-10-27 |
United States Patent
Application |
20160315270 |
Kind Code |
A1 |
SEO; Satoshi ; et
al. |
October 27, 2016 |
HETEROCYCLIC COMPOUND, LIGHT-EMITTING ELEMENT, LIGHT-EMITTING
DEVICE, ELECTRONIC DEVICE, AND LIGHTING DEVICE
Abstract
To provide a novel heterocyclic compound that can be used as a
host material in which a light-emitting substance of a
light-emitting layer is dispersed. A heterocyclic compound
comprising a dibenzo[f,h]quinoxaline ring and two hole-transport
skeletons, where the dibenzo[f,h]quinoxaline ring and the two
hole-transport skeletons are bonded to an aromatic hydrocarbon
group. A heterocyclic compound represented by the following general
formula (G1) is provided. ##STR00001## Note that in the formula,
A.sup.1 and A.sup.2 each independently represent any of a
substituted or unsubstituted carbazole skeleton, a substituted or
unsubstituted dibenzofuran skeleton, and a substituted or
unsubstituted dibenzothiophen skeleton; B represents a substituted
or unsubstituted dibenzo[f,h]quinoxaline skeleton; and Ar
represents an arene skeleton having 6 to 13 carbon atoms. A
light-emitting element including the heterocyclic compound is
provided.
Inventors: |
SEO; Satoshi; (Kanagawa,
JP) ; SHITAGAKI; Satoko; (Kanagawa, JP) ;
MORIKUBO; Miyako; (Kanagawa, JP) ; HASHIMOTO;
Naoaki; (Kanagawa, JP) ; ISHISONE; Takahiro;
(Kanagawa, JP) ; TAKEDA; Kyoko; (Kanagawa, JP)
; SEO; Hiromi; (Kanagawa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
47991906 |
Appl. No.: |
15/203901 |
Filed: |
July 7, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13598253 |
Aug 29, 2012 |
9394280 |
|
|
15203901 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 51/0071 20130101;
H05B 33/14 20130101; H01L 51/0085 20130101; H01L 51/5016 20130101;
H01L 51/0074 20130101; H01L 51/006 20130101; C09K 2211/185
20130101; C09K 2211/1044 20130101; H01L 51/5024 20130101; C07D
403/14 20130101; C09K 2211/1007 20130101; H01L 51/5012 20130101;
H01L 51/5076 20130101; H01L 51/0072 20130101; C09K 11/025 20130101;
C09K 11/06 20130101; C07D 409/14 20130101 |
International
Class: |
H01L 51/00 20060101
H01L051/00; C07D 409/14 20060101 C07D409/14; C07D 403/14 20060101
C07D403/14; C09K 11/06 20060101 C09K011/06; C09K 11/02 20060101
C09K011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2011 |
JP |
2011-187669 |
Claims
1. A heterocyclic compound comprising: a dibenzo[f,h]quinoxaline
ring; a first hole-transport skeleton; a second hole-transport
skeleton; and an aromatic hydrocarbon group, wherein the
dibenzo[f,h]quinoxaline ring, the first hole-transport skeleton and
the second hole-transport skeleton are bonded to the aromatic
hydrocarbon group.
2. The heterocyclic compound according to claim 1, wherein the
aromatic hydrocarbon group is bonded to a 2-position of the
dibenzo[f,h]quinoxaline ring.
3. The heterocyclic compound according to claim 1, wherein the
first hole-transport skeleton includes a first it-electron rich
heteroaromatic ring, and wherein the second hole-transport skeleton
includes a second .pi.-electron rich heteroaromatic ring.
4. The heterocyclic compound according to claim 3, wherein the
first .pi.-electron rich heteroaromatic ring and the second
.pi.-electron rich heteroaromatic ring are each independently
selected from any one of a carbazole ring, a dibenzofuran ring, and
a dibenzothiophene ring.
5. A light-emitting element comprising: a pair of electrodes; and a
layer containing an organic compound between the pair of
electrodes, wherein the layer containing the organic compound
includes the heterocyclic compound according to claim 1.
6. A light-emitting element according to claim 5, further
comprising a phosphorescent compound as a guest material in the
layer.
7. A light-emitting element according to claim 6, wherein the
phosphorescent compound have a diazine skeleton.
8. A light-emitting device comprising the light-emitting element
according to claim 5.
9. A lighting device comprising the light-emitting element
according to claim 5.
10. An electronic device comprising the light-emitting element
according to claim 5.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 13/598,253, filed Aug. 29, 2012, now allowed, which claims the
benefit of a foreign priority application filed in Japan as Serial
No. 2011-187669 on Aug. 30, 2011, all of which are incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a heterocyclic compound, a
light-emitting element, a light-emitting device, an electronic
device, and a lighting device.
[0004] 2. Description of the Related Art
[0005] As next generation lighting devices or display devices,
display devices using light-emitting elements (organic EL elements)
in which organic compounds are used as light-emitting substances
have been developed at an accelerated pace because of their
advantages of thinness, lightweightness, quick response to input
signals, low power consumption, etc.
[0006] In an organic EL element, voltage application between
electrodes, between which a light-emitting layer is interposed,
causes recombination of electrons and holes injected from the
electrodes. The recombination brings a light-emitting substance
into an excited state, and the return from the excited state to the
ground state is accompanied by light emission. Since the wavelength
of light emitted from a light-emitting substance is peculiar to the
light-emitting substance, use of different types of organic
compounds as light-emitting substances makes it possible to obtain
light-emitting elements which exhibit various wavelengths, i.e.,
various colors.
[0007] In the case of display devices which are expected to display
images, such as displays, at least three-color light, i.e., red
light, green light, and blue light is necessary for reproduction of
full-color images.
[0008] Further, in application to lighting devices, light having
wavelength components uniformly in the visible light region is
ideal for obtaining a high color rendering property, but in
reality, light obtained by mixing two or more kinds of light having
different wavelengths is used for lighting application in many
cases. It is known that, with a mixture of three-color light, i.e.,
red light, green light, and blue light, white light having a high
color rendering property can be obtained.
[0009] Light emitted from a light-emitting substance is peculiar to
the substance, as described above. However, important performances
as a light-emitting element, such as lifetime, power consumption,
and even emission efficiency, are not only dependent on a
light-emitting substance but also greatly dependent on layers other
than a light-emitting layer, an element structure, properties of an
emission center substance and a host material, compatibility
between them, carrier balance, and the like. Therefore, it is true
that many kinds of light-emitting element materials are necessary
for the growth of this field. For the above-described reasons,
light-emitting element materials with a variety of molecular
structures have been proposed.
[0010] As is generally known, the generation ratio of a singlet
excited state to a triplet excited state in a light-emitting
element using electroluminescence is 1:3. Therefore, a
light-emitting element in which a phosphorescent material capable
of converting the triplet excited state to light emission is used
as an emission center substance can theoretically realize higher
emission efficiency than a light-emitting element in which a
fluorescent material capable of converting the singlet excited
state to light emission is used as an emission center
substance.
[0011] However, since the triplet excited state of a substance is
at a lower energy level than the singlet excited state of the
substance, a substance that emits phosphorescence has a larger band
gap than a substance that emits fluorescence when the emissions are
at the same wavelength.
[0012] As a substance serving as a host material in a host-guest
type light-emitting layer or a substance contained in each
transport layer in contact with a light-emitting layer, a substance
having a larger band gap or higher triplet excitation energy
(energy difference between a triplet excited state and a singlet
ground state) than an emission center substance is used for
efficient conversion of excitation energy to light emission from
the emission center substance.
[0013] Therefore, a host material and a carrier-transport material
each having an extremely large band gap are necessary in order to
obtain fluorescence efficiently. There are however not many
variations of materials that have a sufficiently large band gap in
addition to good characteristics as a light-emitting element
material, and as described above, the performance of a
light-emitting element depends also on the compatibility between
substances. In consideration of the above, it is difficult to say
that there are sufficient variations of materials with which
light-emitting elements having good characteristics can be
manufactured.
[0014] Furthermore, since singlet excitation energy (an energy
difference between a ground state and a singlet excited state) is
higher than triplet excitation energy, a substance that has high
triplet excitation energy also has high singlet excitation energy.
Therefore, the above substance that has high triplet excitation
energy is also effective in a light-emitting element using a
fluorescent compound as a light-emitting substance.
[0015] Studies have been conducted on a compound having a
dibenzo[f,h]quinoxaline ring, which are examples of the host
material used when a phosphorescent compound is a guest material
(e.g., see Patent Documents 1 and 2).
REFERENCE
Patent Document
[0016] [Patent Document 1]: PCT International Publication No.
03/058667
[0017] [Patent Document 2]: Japanese Published Patent Application
No. 2007-189001
SUMMARY OF THE INVENTION
[0018] However, the above compound having a dibenzo[f,h]quinoxaline
ring has a planar structure, and accordingly, the compound is
easily crystallized. A light-emitting element including a compound
that is easy to crystallize has a short lifetime. Further, in order
that the compound having dibenzo[h]quinoxaline ring has a
sterically bulky structure for the purpose of preventing the
crystallization, another skeleton may be bonded to the
dibenzo[f,h]quinoxaline ring; however, if the skeleton is directly
bonded to the dibenzo[f,h]quinoxaline ring, the conjugated system
could possibly extend to cause a decrease in triplet excitation
energy. It is difficult to obtain high emission efficiency from a
phosphorescent light-emitting element including a compound with
small triplet excitation energy.
[0019] Furthermore, a quinoxaline skeleton is poor at accepting
holes. When a compound that cannot easily accept holes is used as a
host material of a light-emitting layer, the region of
electron-hole recombination concentrates on an interface of the
light-emitting layer, leading to a reduction in lifetime of the
light-emitting element or a reduction in emission efficiency.
[0020] In view of the above, an object of one embodiment of the
present invention is to provide a novel heterocyclic compound which
can be used as a host material in which a light-emitting substance
of a light-emitting layer in a light-emitting element is dispersed.
In particular, the object is to provide a novel heterocyclic
compound which can be suitably used as a host material in the case
where the light-emitting element is a phosphorescent compound.
[0021] Further, in order to realize a light-emitting device, an
electronic device, and a lighting device each having reduced power
consumption and high reliability, a light-emitting element having
low driving voltage, a light-emitting element having high current
efficiency, or a light-emitting element having a long lifetime has
been expected.
[0022] Another object of one embodiment of the present invention is
to provide a light-emitting element having low driving voltage.
Another object of one embodiment of the present invention is to
provide a light-emitting element having high current efficiency.
Another object of one embodiment of the present invention is to
provide a light-emitting element having a long lifetime. Another
object of one embodiment of the present invention is to provide a
light-emitting device, an electronic device, and a lighting device
each having reduced power consumption by using the above
light-emitting element.
[0023] In the present invention, it is only necessary that at least
one of the above-described objects should be achieved.
[0024] A compound having a quinoxaline skeleton has a high
electron-transport property, and use of such a compound for a
light-emitting element enables the element to have low driving
voltage. On the other hand, a quinoxaline skeleton has a planar
structure and is easily crystallized; therefore, there is a problem
in that it is difficult to obtain a light-emitting element having a
long lifetime by the use of the compound. Furthermore, a
quinoxaline skeleton is poor at accepting holes. When a compound
that cannot easily accept holes is used as a host material of a
light-emitting layer, the region of electron-hole recombination
concentrates on an interface of the light-emitting layer, leading
to a more reduction in lifetime of the light-emitting element or a
reduction in emission efficiency. These problems will be solved by
introduction of a hole-transport skeleton into the molecule.
However, if a hole-transport skeleton is directly bonded to a
quinoxaline skeleton, the conjugated system extends to cause a
decrease in band gap and a decrease in triplet excitation energy.
It is difficult to efficiently obtain light emission at a short
wavelength from a light-emitting element in which a compound with a
small band gap or small triplet excitation energy is used as a host
material.
[0025] Nevertheless, the present inventors have found that the
above problems can be solved by using, for a light-emitting
element, a compound comprising a dibenzo[f,h]quinoxaline ring and
two hole-transport skeletons, where the dibenzo[h]quinoxaline ring
and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group.
[0026] Accordingly, one embodiment of the present invention is a
light-emitting element including a compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group.
[0027] A compound applied to one embodiment of the present
invention has two hole-transport skeletons in addition to a
dibenzo[f,h]quinoxaline ring, making it easy to accept holes.
Accordingly, by the use of the compound as a host material in a
light-emitting layer, electrons and holes recombine in the
light-emitting layer, so that it is possible to suppress the
decrease in the lifetime of the light-emitting element.
Furthermore, the introduction of two hole-transport skeletons
enables the compound to have a sterically bulky structure, and the
compound is difficult to crystallize when formed into a film. By
the use of the compound for a light-emitting element, the element
can have a long lifetime. Moreover, in this compound, since a
dibenzo[f,h]quinoxaline ring and a hole-transport skeleton are
bonded through an aromatic hydrocarbon skeleton, decreases in band
gap and triplet excitation energy can be prevented as compared with
a compound in which a dibenzo[f,h]quinoxaline ring and
hole-transport skeletons are directly bonded. By the use of the
compound for a light-emitting element, the element can have high
current efficiency.
[0028] Thus, the compound described above can be suitably used as a
material for an organic device such as a light-emitting element or
an organic transistor.
[0029] The aromatic hydrocarbon group to which two hole-transport
skeletons are bonded is preferably bonded to the 2-position of the
dibenzo[f,h]quinoxaline ring.
[0030] As the hole-transport skeleton, a t-electron rich
heteroaromatic ring is preferable. As the .pi.-electron rich
heteroaromatic ring, a carbazole ring, a dibenzofuran ring, or a
dibenzothiophene ring is preferable.
[0031] As the aromatic hydrocarbon group, a substituted or
unsubstituted phenyl group is preferable.
[0032] Since a light-emitting element of one embodiment of the
present invention which is obtained as above has low driving
voltage, high current efficiency, and a long lifetime, a
light-emitting device (such as an image display device) using this
light-emitting element can have reduced power consumption. Thus,
one embodiment of the present invention is a light-emitting device
including any of the above light-emitting elements. One embodiment
of the present invention also includes an electronic device using
the light-emitting device in its display portion and a lighting
device using the light-emitting device in its light-emitting
portion.
[0033] Note that the light-emitting device in this specification
includes an image display device using a light-emitting element.
Further, the category of the light-emitting device in this
specification includes a module in which a light-emitting element
is provided with a connector such as an anisotropic conductive
film, TAB (tape automated bonding) tape, or a TCP (tape carrier
package); a module in which the top of the TAB tape or the TCP is
provided with a printed wiring board; and a module in which an IC
(integrated circuit) is directly mounted on a light-emitting
element by a COG (chip on glass) method. Furthermore, the category
includes light-emitting devices that are used in lighting
equipment.
[0034] As examples of the compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group, a
heterocyclic compounds are given below.
[0035] One embodiment of the present invention is a heterocyclic
compound represented by the following general formula (G1).
##STR00002##
[0036] In the general formula (G1), A.sup.1 and A.sup.2 each
independently represent any of a substituted or unsubstituted
carbazolyl group, a substituted or unsubstituted dibenzofuranyl
group, and a substituted or unsubstituted dibenzothiophenyl group;
B represents a substituted or unsubstituted
dibenzo[f,h]quinoxalinyl group; Ar represents an aromatic
hydrocarbon group having 6 to 13 carbon atoms; the aromatic
hydrocarbon group may have a substituent; and substituents of the
aromatic hydrocarbon group may be bonded to each other to form a
ring.
[0037] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G2).
##STR00003##
[0038] In the general formula (G2), A.sup.1 and A.sup.2 each
independently represent any of a substituted or unsubstituted
carbazolyl group, a substituted or unsubstituted dibenzofuranyl
group, and a substituted or unsubstituted dibenzothiophenyl group;
Ar represents an aromatic hydrocarbon group having 6 to 13 carbon
atoms; the aromatic hydrocarbon group may have a substituent;
substituents of the aromatic hydrocarbon group may be bonded to
each other to form a ring; and R.sup.1 to R.sup.9 each
independently represent any of hydrogen, an alkyl group having 1 to
4 carbon atoms, and a substituted or unsubstituted aryl group
having 6 to 13 carbon atoms.
[0039] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G3-1).
##STR00004##
[0040] In the general formula (G3-1), Ar represents an aromatic
hydrocarbon group having 6 to 13 carbon atoms; the aromatic
hydrocarbon group may have a substituent; substituents of the
aromatic hydrocarbon group may be bonded to each other to form a
ring; and R.sup.1 to R.sup.9, R.sup.10 to R.sup.17, and R.sup.20 to
R.sup.27 each independently represent any of hydrogen, an alkyl
group having 1 to 4 carbon atoms, and a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms.
[0041] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G3-2).
##STR00005##
[0042] In the general formula (G3-2), Ar represents an aromatic
hydrocarbon group having 6 to 13 carbon atoms; the aromatic
hydrocarbon group may have a substituent; substituents of the
aromatic hydrocarbon group may be bonded to each other to form a
ring; R.sup.1 to R.sup.9, R.sup.30 to R.sup.36, and R.sup.40 to
R.sup.46 each independently represent any of hydrogen, an alkyl
group having 1 to 4 carbon atoms, and a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms; and Z
represents oxygen or sulfur.
[0043] In the general formulae (G3-1) and (G3-2), Ar is preferably
either a substituted or unsubstituted benzenetriyl group or a
substituted or unsubstituted biphenyltriyl group. In particular, Ar
is preferably a substituted or unsubstituted benzenetriyl
group.
[0044] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G4-1).
##STR00006##
[0045] In the general formula (G4-1), R.sup.1 to R.sup.9, R.sup.10
to R.sup.17, and R.sup.20 to R.sup.27 each independently represent
any of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms.
[0046] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G4-2).
##STR00007##
[0047] In the general formula (G4-2), R.sup.1 to R.sup.9, R.sup.30
to R.sup.36, and R.sup.40 to R.sup.46 each independently represent
any of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms; and Z represents oxygen or sulfur.
[0048] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G5-1).
##STR00008##
[0049] In the general formula (G5-1), R.sup.1, R.sup.10 to
R.sup.17, and R.sup.20 to R.sup.27 each independently represent any
of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms.
[0050] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G5-2).
##STR00009##
[0051] In the general formula (G5-2), R.sup.1, R.sup.30 to
R.sup.36, and R.sup.40 to R.sup.46 each independently represent any
of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms; and Z represents oxygen or sulfur.
[0052] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G6).
##STR00010##
[0053] In the general formula (G6), R.sup.12, R.sup.15, R.sup.22,
and R.sup.25 each independently represent any of hydrogen, an alkyl
group having 1 to 4 carbon atoms, and a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms.
[0054] Another embodiment of the present invention is a
heterocyclic compound represented by the following structural
formula (100).
##STR00011##
[0055] The heterocyclic compound can be expressed as a compound
comprising a dibenzo[f,h]quinoxaline ring and two hole-transport
skeletons, where the dibenzo[f,h]quinoxaline ring and the two
hole-transport skeletons are bonded to an aromatic hydrocarbon
group; therefore, one embodiment of the present invention includes
a light-emitting element including the heterocyclic compound. One
embodiment of the present invention also includes a light-emitting
device, an electronic device, and a lighting device each including
the light-emitting element.
[0056] One embodiment of the present invention is a novel
heterocyclic compound which can be suitably used as a host material
in which a light-emitting substance of a light-emitting layer in a
light-emitting element is dispersed. One embodiment of the present
invention enables to provide a light-emitting element having low
driving voltage. One embodiment of the present invention enables to
provide a light-emitting element having high current efficiency.
One embodiment of the present invention enables to provide a
light-emitting element having a long lifetime. One embodiment of
the present invention enables to provide a light-emitting device,
an electronic device, and a lighting device each having reduced
power consumption.
BRIEF DESCRIPTION OF THE DRAWINGS
[0057] In the accompanying drawings:
[0058] FIGS. 1A and 1B each illustrate a light emitting element
according to one embodiment of the present invention;
[0059] FIGS. 2A and 2B illustrate a light-emitting device according
to one embodiment of the present invention;
[0060] FIGS. 3A and 3B illustrate a light-emitting device according
to one embodiment of the present invention;
[0061] FIGS. 4A to 4D each illustrate an electronic device
according to one embodiment of the present invention;
[0062] FIG. 5 illustrates a liquid crystal display device according
to one embodiment of the present invention;
[0063] FIG. 6 illustrates a lighting device according to one
embodiment of the present invention;
[0064] FIG. 7 illustrates a lighting device according to one
embodiment of the present invention;
[0065] FIG. 8 illustrates an electronic device according to one
embodiment of the present invention;
[0066] FIGS. 9A and 9B illustrate a lighting device according to
one embodiment of the present invention;
[0067] FIGS. 10A and 10B show .sup.1H NMR charts of 2Cz2PDBq;
[0068] FIGS. 11A and 11B show an absorption and emission spectra of
2Cz2PDBq;
[0069] FIG. 12 shows the current density-luminance characteristics
of Light-emitting Element 1;
[0070] FIG. 13 shows the voltage-luminance characteristics of
Light-emitting Element 1;
[0071] FIG. 14 shows the luminance-current efficiency
characteristics of Light-emitting Element 1;
[0072] FIGS. 15A and 15B show NMR charts of
3,5-bis(dibenzothiophen-4-yl)phenylboronic acid;
[0073] FIGS. 16A and 16B show NMR charts of 2DBT2PDBq-II;
[0074] FIGS. 17A and 17B show an absorption and emission spectra of
2DBT2PDBq-II;
[0075] FIGS. 18A and 18B show LC/MS measurement results of
2DBT2PDBq-II (50 eV);
[0076] FIGS. 19A and 19B show LC/MS measurement results of
2DBT2PDBq-II (70 eV);
[0077] FIG. 20 shows the current density-luminance characteristics
of Light-emitting Element 2;
[0078] FIG. 21 shows the voltage-luminance characteristics of
Light-emitting Element 2;
[0079] FIG. 22 shows the luminance-current efficiency
characteristics of Light-emitting Element 2; and
[0080] FIG. 23 shows an emission spectrum of Light-emitting Element
2.
DETAILED DESCRIPTION OF THE INVENTION
[0081] Hereinafter, embodiments and examples of the present
invention will be described with reference to the drawings. Note
that the invention is not limited to the following description, and
it is easily understood by those skilled in the art that various
changes and modifications can be made without departing from the
spirit and scope of the invention. Therefore, the present invention
should not be construed as being limited to the description in the
following embodiments and examples.
Embodiment 1
[0082] In this embodiment, a heterocyclic compound of one
embodiment of the present invention is described. One embodiment of
the present invention is a heterocyclic compound represented by the
following general formula (G1).
##STR00012##
[0083] In the general formula (G1), A.sup.1 and A.sup.2 each
independently represent any of a substituted or unsubstituted
carbazolyl group, a substituted or unsubstituted dibenzofuranyl
group, and a substituted or unsubstituted dibenzothiophenyl group;
B represents a substituted or unsubstituted
dibenzo[f,h]quinoxalinyl group; Ar represents an aromatic
hydrocarbon group having 6 to 13 carbon atoms; the aromatic
hydrocarbon group may have a substituent; and substituents of the
aromatic hydrocarbon group may be bonded to each other to form a
ring.
[0084] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G2).
##STR00013##
[0085] In the general formula (G2), A.sup.1 and A.sup.2 each
independently represent any of a substituted or unsubstituted
carbazolyl group, a substituted or unsubstituted dibenzofuranyl
group, and a substituted or unsubstituted dibenzothiophenyl group;
Ar represents an aromatic hydrocarbon group having 6 to 13 carbon
atoms; the aromatic hydrocarbon group may have a substituent;
substituents of the aromatic hydrocarbon group may be bonded to
each other to form a ring; and R.sup.1 to R.sup.9 each
independently represent any of hydrogen, an alkyl group having 1 to
4 carbon atoms, and a substituted or unsubstituted aryl group
having 6 to 13 carbon atoms.
[0086] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G3-1).
##STR00014##
[0087] In the general formula (G3-1), Ar represents an aromatic
hydrocarbon group having 6 to 13 carbon atoms; the aromatic
hydrocarbon group may have a substituent; substituents of the
aromatic hydrocarbon group may be bonded to each other to form a
ring; and R.sup.1 to R.sup.9, R.sup.10 to R.sup.17, and R.sup.20 to
R.sup.27 each independently represent any of hydrogen, an alkyl
group having 1 to 4 carbon atoms, and a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms.
[0088] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G3-2).
##STR00015##
[0089] In the general formula (G3-2), Ar represents an aromatic
hydrocarbon group having 6 to 13 carbon atoms; the aromatic
hydrocarbon group may have a substituent; substituents of the
aromatic hydrocarbon group may be bonded to each other to form a
ring; R.sup.1 to R.sup.9, R.sup.30 to R.sup.36, and R.sup.40 to
R.sup.46 each independently represent any of hydrogen, an alkyl
group having 1 to 4 carbon atoms, and a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms; and Z
represents oxygen or sulfur.
[0090] In the general formulae (G3-1) and (G3-2), Ar is preferably
either a substituted or unsubstituted benzenetriyl group or a
substituted or unsubstituted biphenyltriyl group. In particular, Ar
is preferably a substituted or unsubstituted benzenetriyl group.
Furthermore, Ar is preferably a 1,3,5-benzenetriyl group.
[0091] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G4-1).
##STR00016##
[0092] In the general formula (G4-1), R.sup.1 to R.sup.9, R.sup.10
to R.sup.17, and R.sup.20 to R.sup.27 each independently represent
any of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms.
[0093] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G4-2).
##STR00017##
[0094] In the general formula (G4-2), R.sup.1 to R.sup.9, R.sup.30
to R.sup.36, and R.sup.40 to R.sup.46 each independently represent
any of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms; and Z represents oxygen or sulfur.
[0095] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G5-1).
##STR00018##
[0096] In the general formula (G5-1), R.sup.1, R.sup.10 to
R.sup.17, and R.sup.20 to R.sup.27 each independently represent any
of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms.
[0097] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G5-2).
##STR00019##
[0098] In the general formula (G5-2), R.sup.1, R.sup.30 to
R.sup.36, and R.sup.40 to R.sup.46 each independently represent any
of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms; and Z represents oxygen or sulfur.
[0099] Another embodiment of the present invention is a
heterocyclic compound represented by the following general formula
(G6).
##STR00020##
[0100] In the general formula (G6), R.sup.12, R.sup.15, R.sup.22,
and R.sup.25 each independently represent any of hydrogen, an alkyl
group having 1 to 4 carbon atoms, and a substituted or
unsubstituted aryl group having 6 to 13 carbon atoms.
[0101] The heterocyclic compound can be expressed as a compound
comprising a dibenzo[f,h]quinoxaline ring and two hole-transport
skeletons, where the dibenzo[f,h]quinoxaline ring and the two
hole-transport skeletons are bonded to an aromatic hydrocarbon
group; therefore, one embodiment of the present invention includes
a light-emitting element including the heterocyclic compound. One
embodiment of the present invention also includes a light-emitting
device, an electronic device, and a lighting device each including
the light-emitting element.
[0102] As specific structures of Ar in the general formulae (G1),
(G2), (G3-1), and (G3-2), for example, substituents shown in the
following structural formulae (1-1) to (1-6) are given.
##STR00021##
[0103] As specific structures of R.sup.1 to R.sup.9, R.sup.10 to
R.sup.17, R.sup.20 to R.sup.27, R.sup.30 to R.sup.36, and R.sup.40
to R.sup.46 in the general formulae (G1), (G2), (G3-1), (G3-2),
(G4-1), (G4-2), (G5-1), (G5-2), and (G6), for example, substituents
shown in the following structural formulae (2-1) to (2-23) are
given.
##STR00022## ##STR00023## ##STR00024## ##STR00025##
[0104] Specific examples of the heterocyclic compound represented
by the general formula (G1) include heterocyclic compounds
represented by the structural formulae (100) to (155), (200) to
(253), and (300) to (353). Note that the present invention is not
limited thereto.
##STR00026## ##STR00027## ##STR00028## ##STR00029## ##STR00030##
##STR00031## ##STR00032## ##STR00033## ##STR00034## ##STR00035##
##STR00036## ##STR00037## ##STR00038## ##STR00039## ##STR00040##
##STR00041## ##STR00042## ##STR00043## ##STR00044## ##STR00045##
##STR00046## ##STR00047## ##STR00048## ##STR00049## ##STR00050##
##STR00051## ##STR00052## ##STR00053##
[0105] A variety of reactions can be applied to the method of
synthesizing the heterocyclic compounds. For example, through the
following synthesis reactions, the heterocyclic compounds described
in this embodiment can be synthesized. Note that the methods of
synthesizing the heterocyclic compounds, which are each one
embodiment of the present invention, are not limited to the
synthesis methods below. Although description of the synthesis
method is made using the general formula (G2), the heterocyclic
compounds represented by the other general formulae in this
embodiment can be synthesized in the similar manner.
<<Synthesis Method 1>>
[0106] First, the synthesis scheme (A-1) is shown below.
##STR00054##
[0107] The heterocyclic compound (G2) of one embodiment of the
present invention can be synthesized according to the synthesis
scheme (A-1). Specifically, a halide of a dibenzo[f,h]quinoxaline
derivative (Compound 1) is coupled with an organoboron compound or
boronic acid of a carbazole derivative, a dibenzofuran derivative,
or a dibenzothiophene derivative (Compound 2) by a Suzuki-Miyaura
reaction, whereby the heterocyclic compound (G2) described in this
embodiment can be obtained.
[0108] In the synthesis scheme (A-1), A.sup.1 and A.sup.2 each
independently represent any of a substituted or unsubstituted
carbazolyl group, a substituted or unsubstituted dibenzothiophenyl
group, and a substituted or unsubstituted dibenzofuranyl group; Ar
represents an aromatic hydrocarbon group having 6 to 13 carbon
atoms; the aromatic hydrocarbon group may have a substituent;
substituents of the aromatic hydrocarbon group may be bonded to
each other to form a ring; R.sup.1 to R.sup.9 each independently
represent any of hydrogen, an alkyl group having 1 to 4 carbon
atoms, and a substituted or unsubstituted aryl group having 6 to 13
carbon atoms; R.sup.50 and R.sup.51 each independently represent
hydrogen or an alkyl group having 1 to 6 carbon atoms; R.sup.50 and
R.sup.51 may be bonded to each other to form a ring; and X.sup.1
represents a halogen.
[0109] Examples of the palladium catalyst that can be used in the
synthesis scheme (A-1) include, but are not limited to,
palladium(II) acetate, tetrakis(triphenylphosphine)palladium(0),
and bis(triphenylphosphine)palladium(II) dichloride.
[0110] Examples of the ligand of the palladium catalyst that can be
used in the synthesis scheme (A-1) include, but are not limited to,
tri(o-tolyl)phosphine, triphenylphosphine, and
tricyclohexylphosphine.
[0111] Examples of the base that can be used in the synthesis
scheme (A-1) include, but are not limited to, an organic base such
as sodium tert-butoxide, inorganic bases such as potassium
carbonate and sodium carbonate.
[0112] Examples of the solvent that can be used in the synthesis
scheme (A-1) include, but are not limited to, a mixed solvent of
toluene and water; a mixed solvent of toluene, alcohol such as
ethanol, and water; a mixed solvent of xylene and water; a mixed
solvent of xylene, alcohol such as ethanol, and water; a mixed
solvent of benzene and water; a mixed solvent of benzene, alcohol
such as ethanol, and water; and a mixed solvent of an ether such as
ethylene glycol dimethyl ether and water. A mixed solvent of
toluene and water; a mixed solvent of toluene, ethanol, and water;
or a mixed solvent of ether such as ethylene glycol dimethyl ether
and water is preferable.
[0113] As a coupling reaction shown in the synthesis scheme (A-1),
the Suzuki-Miyaura Coupling Reaction using the organoboron compound
or boronic acid represented by Compound 2 may be replaced with a
cross coupling reaction using an organoaluminum compound, an
organozirconium compound, an organozinc compound, an organotin
compound, or the like. Note that the present invention is not
limited thereto.
[0114] In the reaction shown in the synthesis scheme (A-1), an
organoboron compound or boronic acid of a dibenzo[f,h]quinoxaline
derivative may be coupled with a halide of a carbazole derivative,
a dibenzofuran derivative, or a dibenzothiophene derivative or with
a carbazole derivative, a dibenzofuran derivative, or a
dibenzothiophene derivative which has a triflate group as a
substituent, by the Suzuki-Miyaura reaction.
[0115] Thus, the heterocyclic compound of this embodiment can be
synthesized.
<<Synthesis Method 2>>
[0116] A method of synthesizing a heterocyclic compound of this
embodiment, which is different from Synthesis Method 1, is
described below. First, the synthesis scheme (B-1) in which a boron
compound of A.sup.1 and A.sup.2 is used as a material is shown
below.
##STR00055##
[0117] As shown in the synthesis scheme (B-1), a dihalide of a
dibenzo[f,h]quinoxaline derivative (Compound 3) is coupled with an
organoboron compound or boronic acid of a carbazole derivative, a
dibenzofuran derivative, or a dibenzothiophene derivative (Compound
4-1 and Compound 4-2) by a Suzuki-Miyaura reaction, whereby the
heterocyclic compound (G2) described in this embodiment can be
obtained.
[0118] In the synthesis scheme (B-1), A.sup.1 and A.sup.2 each
independently represent any of a substituted or unsubstituted
carbazolyl group, a substituted or unsubstituted dibenzothiophenyl
group, and a substituted or unsubstituted dibenzofuranyl group; Ar
represents an aromatic hydrocarbon group having 6 to 13 carbon
atoms; the aromatic hydrocarbon group may have a substituent;
substituents of the aromatic hydrocarbon group may be bonded to
each other to form a ring; R.sup.1 to R.sup.9 each independently
represent any of hydrogen, an alkyl group having 1 to 4 carbon
atoms, and a substituted or unsubstituted aryl group having 6 to 13
carbon atoms; R.sup.52 and R.sup.53 each independently represent
hydrogen or an alkyl group having 1 to 6 carbon atoms; R.sup.52 and
R.sup.53 may be bonded to each other to form a ring; X.sup.2-1 and
X.sup.2-2 each independently represent a halogen or a triflate
group; and the halogen is preferably iodine or bromine.
[0119] Examples of the palladium catalyst that can be used in the
synthesis scheme (B-1) include, but are not limited to,
palladium(II) acetate, tetrakis(triphenylphosphine)palladium(0),
and bis(triphenylphosphine)palladium(II) dichloride.
[0120] Examples of the ligand of the palladium catalyst that can be
used in the synthesis scheme (B-1) include, but are not limited to,
tri(o-tolyl)phosphine, triphenylphosphine, and
tricyclohexylphosphine.
[0121] Examples of the base that can be used in the synthesis
scheme (B-1) include, but are not limited to, an organic base such
as sodium tert-butoxide, inorganic bases such as potassium
carbonate and sodium carbonate.
[0122] Examples of the solvent that can be used in the synthesis
scheme (B-1) include, but are not limited to, a mixed solvent of
toluene and water; a mixed solvent of toluene, alcohol such as
ethanol, and water; a mixed solvent of xylene and water; a mixed
solvent of xylene, alcohol such as ethanol, and water; a mixed
solvent of benzene and water; a mixed solvent of benzene, alcohol
such as ethanol, and water; and a mixed solvent of an ether such as
ethylene glycol dimethyl ether and water. A mixed solvent of
toluene and water; a mixed solvent of toluene, ethanol, and water;
or a mixed solvent of ether such as ethylene glycol dimethyl ether
and water is preferable.
[0123] As a coupling reaction shown in the synthesis scheme (B-1),
the Suzuki-Miyaura Coupling Reaction using the organoboron compound
or boronic acid represented by Compound 4 may be replaced with a
cross coupling reaction using an organoaluminum compound, an
organozirconium compound, an organozinc compound, an organotin
compound, or the like. Note that the present invention is not
limited thereto. In this coupling, a triflate group or the like may
be used other than halogen; however, the present invention is not
limited thereto.
[0124] In the reaction shown in the synthesis scheme (B-1), an
organoboron compound or boronic acid of a dibenzo[f,h]quinoxaline
derivative may be coupled with a halide of a carbazole derivative,
a dibenzofuran derivative, or a dibenzothiophene derivative or with
a carbazole derivative, a dibenzofuran derivative, or a
dibenzothiophene derivative which has a triflate group as a
substituent, by the Suzuki-Miyaura reaction.
[0125] Note that in the synthesis scheme (B-1), X.sup.2-1 and
X.sup.2-2 may be different substituents, and Compound 4-1 and
Compound 4-2 may be introduced one by one in two steps.
[0126] To synthesize the heterocyclic compound represented by the
general formula (G2) in which A is a substituted or unsubstituted
N-carbazolyl group, the following synthesis scheme (B-2) is
employed, thereby obtaining the heterocyclic compound represented
by the general formula (G3-1).
##STR00056##
[0127] As shown in the synthesis scheme (B-2), a dihalide of a
dibenzo[f,h]quinoxaline derivative (Compound 3) is coupled with a
9H-carbazole derivative (Compound 5-1 and Compound 5-2) using a
metal catalyst, metal, or a metal compound in the presence of a
base, whereby the heterocyclic compound (G3-1) described in this
embodiment can be obtained.
[0128] In the synthesis scheme (B-2), R.sup.1 to R.sup.9 each
independently represent any of hydrogen, an alkyl group having 1 to
4 carbon atoms, and a substituted or unsubstituted aryl group
having 6 to 13 carbon atoms; Ar represents an aromatic hydrocarbon
group having 6 to 13 carbon atoms; the aromatic hydrocarbon group
may have a substituent; substituents of the aromatic hydrocarbon
group may be bonded to each other to form a ring; R.sup.10 to
R.sup.17 and R.sup.20 to R.sup.27 each independently represent any
of hydrogen, an alkyl group having 1 to 4 carbon atoms, and a
substituted or unsubstituted aryl group having 6 to 13 carbon
atoms; X.sup.3-1 and X.sup.3-2 each independently represent a
halogen or a triflate group; and the halogen is preferably iodine
or bromine.
[0129] In the case where the Hartwig-Buchwald reaction is performed
in the synthesis scheme (B-2),
bis(dibenzylideneacetone)palladium(0), palladium(II) acetate, or
the like is given as the palladium catalyst that can be used.
[0130] Examples of the ligand of the palladium catalyst that can be
used in the synthesis scheme (B-2) include
tri(tert-butyl)phosphine, tri(n-hexyl)phosphine, and
tricyclohexylphosphine.
[0131] Examples of the base that can be used in the synthesis
scheme (B-2) include an organic base such as sodium tert-butoxide
and an inorganic base such as potassium carbonate.
[0132] Examples of the solvent that can be used in the synthesis
scheme (B-2) include toluene, xylene, benzene, and
tetrahydrofuran.
[0133] In order to synthesize the heterocyclic compound of this
embodiment, the Ullmann reaction or the like may be employed
instead of the Hartwig-Buchwald reaction, but the present invention
is not limited to these.
[0134] Note that in the synthesis scheme (B-1), X.sup.3-1 and
X.sup.3-2 may be different substituents, and Compound 5-1 and
Compound 5-2 may be introduced one by one in two steps.
[0135] Thus, the heterocyclic compound of this embodiment can be
synthesized.
[0136] The heterocyclic compound of this embodiment has a wide band
gap. Accordingly, by the use of such a heterocyclic compound as a
host material, in which a light-emitting substance is dispersed, of
a light-emitting layer in a light-emitting element, the
light-emitting element can have high current efficiency. In
particular, the heterocyclic compound of this embodiment is
suitably used as a host material in which a phosphorescent compound
is dispersed. Further, since the heterocyclic compound of this
embodiment has a high electron-transport property, it can be
suitably used as a material for an electron-transport layer in a
light-emitting element. By the use of the heterocyclic compound of
this embodiment, a light-emitting element having low driving
voltage can be achieved. A light-emitting element having high
current efficiency can be achieved. A light-emitting element having
a long lifetime can be achieved. Furthermore, by the use of this
light-emitting element, a light-emitting device, an electronic
device, and a lighting device each having reduced power consumption
can be obtained.
Embodiment 2
[0137] In this embodiment, a light-emitting element of one
embodiment of the present invention is described with reference to
FIGS. 1A and 1B.
[0138] Accordingly, one embodiment of the present invention is a
light-emitting element including a compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group.
[0139] A compound with a quinoxaline skeleton has a high
electron-transport property, and use of such a compound for a
light-emitting element enables the element to have low driving
voltage. However, a quinoxaline skeleton has a planar structure.
Since a compound having a planar structure is easily crystallized
when formed into a film, use of such a compound for a
light-emitting element causes the element to have a short lifetime.
Furthermore, a quinoxaline skeleton is poor at accepting holes.
When a compound that cannot easily accept holes is used as a host
material of a light-emitting layer, the region of electron-hole
recombination concentrates on an interface on the anode side of the
light-emitting layer, leading to a reduction in the lifetime of the
light-emitting element. It is likely that these problems will be
solved by introduction of a hole-transport skeleton into the
molecule. However, if a hole-transport skeleton is directly bonded
to a quinoxaline skeleton, the conjugated system extends to cause a
decrease in band gap and a decrease in triplet excitation
energy.
[0140] Nevertheless, the present inventors have found that the
above problems can be solved by using, for a light-emitting
element, a compound comprising a dibenzo[f,h]quinoxaline ring and
two hole-transport skeletons, where the dibenzo[f,h]quinoxaline
ring and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group.
[0141] The compound includes two hole-transport skeletons in
addition to a dibenzo[f,h]quinoxaline ring, making it easy to
accept holes. Accordingly, by use of the compound as a host
material of a light-emitting layer, electrons and holes recombine
in the light-emitting layer, so that the reduction in the lifetime
of the light-emitting element, which is caused by concentration on
the interface on the anode side in the light-emitting region, can
be prevented. Furthermore, the introduction of two hole-transport
skeletons enables the compound to have a sterically bulky
structure, and the compound is difficult to crystallize when formed
into a film. By the use of the compound for a light-emitting
element, the element can have a long lifetime. Moreover, in this
compound, since an aromatic hydrocarbon group exists between a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons, the
decrease in band gap and the decrease in triplet excitation energy
can be prevented as compared with a compound in which a
dibenzo[f,h]quinoxaline ring and a hole-transport skeleton are
directly bonded. By the use of the compound for a light-emitting
element, the element can have high current efficiency.
[0142] Thus, the compound described above can be suitably used as a
material for an organic device such as a light-emitting element or
an organic transistor.
[0143] Note that the aromatic hydrocarbon group to which two
hole-transport skeletons are bonded is preferably bonded to the
2-position of the dibenzo[f,h]quinoxaline ring.
[0144] As the hole-transport skeleton, a .pi.-electron rich
heteroaromatic ring is preferable. As the .pi.-electron rich
heteroaromatic ring, a carbazole ring, a dibenzofuran ring, or a
dibenzothiophene ring is preferable. As the aromatic hydrocarbon
group, any of a substituted or unsubstituted benzene skeleton and a
substituted or unsubstituted biphenyl skeleton is preferable.
[0145] The heterocyclic compound described in Embodiment 1 is also
an example of the compound comprising a dibenzo[f,h]quinoxaline
ring and two hole-transport skeletons, where the
dibenzo[f,h]quinoxaline ring and the two hole-transport skeletons
are bonded to an aromatic hydrocarbon group.
[0146] Next, a light-emitting element of this embodiment is
described in detail with reference to FIGS. 1A and 1B. The
light-emitting element of this embodiment includes a plurality of
layers between a pair of electrodes. In this embodiment, the
light-emitting element includes a first electrode 101, a second
electrode 102, and an EL layer 103 provided between the first
electrode 101 and the second electrode 102. Note that in this
embodiment, the first electrode 101 functions as an anode and the
second electrode 102 functions as a cathode. In other words, when a
voltage is applied between the first electrode 101 and the second
electrode 102 so that the voltage of the first electrode 101 is
higher than that of the second electrode 102, light emission can be
obtained.
[0147] A substrate is used as a support of the light-emitting
element. As the substrate, glass, plastic or the like can be used,
for example. Note that a material other than glass or plastic can
be used as far as it can function as a support of the
light-emitting element.
[0148] For the first electrode 101, any of metal, an alloy, an
electrically conductive compound, which have a high work function
(specifically, a work function of 4.0 eV or more), and a mixture
thereof or the like is preferably used. Specifically, for example,
indium oxide-tin oxide (ITO: indium tin oxide), indium oxide-tin
oxide containing silicon or silicon oxide, indium oxide-zinc oxide,
and indium oxide containing tungsten oxide and zinc oxide are
given. Films of these electrically conductive metal oxides are
usually formed by sputtering but may be formed by application of a
sol-gel method or the like. For example, indium oxide-zinc oxide
can be formed by a sputtering method using a target in which zinc
oxide is added to indium oxide at 1 wt % to 20 wt %. Moreover,
indium oxide containing tungsten oxide and zinc oxide can be formed
by a sputtering method using a target in which tungsten oxide is
added to indium oxide at 0.5 wt % to 5 wt % and zinc oxide is added
to indium oxide at 0.1 wt % to 1 wt %. Besides, gold (Au), platinum
(Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo),
iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), nitrides of
metal materials (e.g., titanium nitride), and the like are given.
Furthermore, graphene may be used.
[0149] There is no particular limitation on a stacked structure of
the EL layer 103. The EL layer 103 can be formed by combining a
layer that contains a substance having a high electron-transport
property, a layer that contains a substance having a high
hole-transport property, a layer that contains a substance having a
high electron-injection property, a layer that contains a substance
having a high hole-injection property, a layer that contains a
bipolar substance (a substance having a high electron-transport
property and a hole-transport property), and the like as
appropriate. For example, the EL layer 103 can be formed by
combining a hole-injection layer, a hole-transport layer, a
light-emitting layer, an electron-transport layer, an
electron-injection layer, and the like as appropriate. In this
embodiment, a structure in which the EL layer 103 includes a
hole-injection layer 111, a hole-transport layer 112, a
light-emitting layer 113, an electron-transport layer 114, and an
electron-injection layer 115 stacked in this order from the first
electrode 101 functioning as an anode is described. Note that in
the case where the second electrode 102 is an electrode functioning
as an anode, in a layer containing an organic compound having a
structure similar to the above, the hole-injection layer 111, the
hole-transport layer 112, the light-emitting layer 113, the
electron-transport layer 114, and the electron-injection layer 115
are stacked in this order from the second electrode 102. Materials
included in the layers are specifically given below.
[0150] The hole-injection layer 111 is a layer containing a
substance having a high hole-injection property. Molybdenum oxide,
vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide,
or the like can be used. Alternatively, the hole-injection layer
111 can be formed with a phthalocyanine-based compound such as
phthalocyanine (abbreviation: H.sub.2Pc) or copper phthalocyanine
(abbreviation: CuPc), an aromatic amine compound such as
4,4'-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl
(abbreviation: DPAB) or
N,N'-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N'-diphenyl-[1,1'-
-biphenyl]-4,4'-diamine (abbreviation: DNTPD), a high molecule such
as poly(ethylenedioxythiophene)/poly(styrenesulfonic acid)
(PEDOT/PSS), or the like.
[0151] Alternatively, a composite material in which a substance
having a high hole-transport property contains an acceptor
substance can be used for the hole-injection layer 111. In this
specification, the composite material refers to not a material in
which two materials are simply mixed but a material in the state
where charge transfer between the materials can be caused by a
mixture of a plurality of materials. This charge transfer includes
the charge transfer that is realized only when an electric field
exists.
[0152] Note that the use of such a substance having a high
hole-transport property which contains an acceptor substance
enables selection of a material for an electrode regardless of its
work function. In other words, besides a material having a high
work function, a material having a low work function can also be
used for the first electrode 101.
[0153] As the substance having an acceptor property,
7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:
F.sub.4-TCNQ), chloranil, and the like are given. In addition,
transition metal oxides are given. Oxides of the metals that belong
to Group 4 to Group 8 of the periodic table are given.
Specifically, vanadium oxide, niobium oxide, tantalum oxide,
chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide,
and rhenium oxide are preferable because their electron-accepting
property is high. Among these, molybdenum oxide is especially
preferable because it is stable in the air and its hygroscopic
property is low and is easily treated.
[0154] As the substance having a high hole-transport property used
for the composite material, any of a variety of organic compounds
such as aromatic amine compounds, carbazole compounds, aromatic
hydrocarbons, and high molecular compounds (e.g., oligomers,
dendrimers, or polymers) can be used. Note that the organic
compound used for the composite material is preferably an organic
compound having a high hole-transport property. Specifically, a
substance having a hole mobility of 10.sup.-6 cm.sup.2/Vs or more
is preferably used. Further, other than these substances, any
substance that has a property of transporting more holes than
electrons may be used. Organic compounds that can be used as the
substance having a high hole-transport property in the composite
material are specifically given below.
[0155] Examples of the aromatic amine compounds include
N,N'-di(p-tolyl)-N,N'-diphenyl-p-phenylenediamine (abbreviation:
DTDPPA), 4,4'-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl
(abbreviation: DPAB),
N,N'-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N'-diphenyl-[1,1'-b-
iphenyl]-4,4'-diamine (abbreviation: DNTPD), and
1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene
(abbreviation: DPA3B).
[0156] Examples of the carbazole compounds that can be used for the
composite material specifically include
3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole
(abbreviation: PCzPCA1),
3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole
(abbreviation: PCzPCA2), and
3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole
(abbreviation: PCzPCN1).
[0157] Examples of the carbazole compounds that can be used for the
composite material also include 4,4'-di(N-carbazolyl)biphenyl
(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene
(abbreviation: TCPB),
9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:
CzPA), and
1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.
[0158] Examples of the aromatic hydrocarbons that can be used for
the composite material include
2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),
2-tert-butyl-9,10-di(1-naphthyl)anthracene,
9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),
2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation:
t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),
9,10-diphenylanthracene (abbreviation: DPAnth),
2-tert-butylanthracene (abbreviation: t-BuAnth),
9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA),
2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,
9,10-bis[2-(1-naphthyl)phenyl]anthracene,
2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,
2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9'-bianthryl,
10,10'-diphenyl-9,9'-bianthryl,
10,10'-bis(2-phenylphenyl)-9,9'-bianthryl,
10,10'-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9'-bianthryl,
anthracene, tetracene, rubrene, perylene, and
2,5,8,11-tetra(tert-butyl)perylene. Besides, pentacene, coronene,
or the like can also be used. Thus, an aromatic hydrocarbon having
14 to 42 carbon atoms and having a hole mobility of
1.times.10.sup.-6 cm.sup.2/Vs or more is preferably used.
[0159] Note that the aromatic hydrocarbons that can be used for the
composite material may have a vinyl skeleton. Examples of the
aromatic hydrocarbons having a vinyl group include
4,4'-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi), and
9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation:
DPVPA).
[0160] Moreover, a high molecular compound such as
poly(N-vinylcarbazole) (abbreviation: PVK),
poly(4-vinyltriphenylamine) (abbreviation: PVTPA),
poly[N-(4-{N'-[4-(4-diphenylamino)phenyl]phenyl-N-phenylamino}phenyl)meth-
acrylamide] (abbreviation: PTPDMA), or
poly[N,N-bis(4-butylphenyl)-N,N-bis(phenyl)benzidine]
(abbreviation: poly-TPD) can also be used.
[0161] The hole-transport layer 112 is a layer that contains a
substance having a high hole-transport property. As the substance
having a high hole-transport property, the substances given as the
substances having a high hole-transport property which can be used
for the above composite material can also be used. Note that a
detailed explanation is omitted to avoid repetition. Refer to the
explanation of the composite material.
[0162] The light-emitting layer 113 is a layer containing a
light-emitting substance. The light-emitting layer 113 may be
formed with a film containing only a light-emitting substance or a
film in which an emission center substance is dispersed into a host
material.
[0163] There is no particular limitation on a material that can be
used as the light-emitting substance or the emission center
substance in the light-emitting layer 113, and light emitted from
the material may be either fluorescence or phosphorescence.
Examples of the above light-emitting substance or emission center
substance include the following substances: fluorescent substances
such as
N,N'-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N-diphenyl-pyrene-1,6-diam-
ine (abbreviation: 1,6-FLPAPm),
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-butylperylene
(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-phenyiene)bis[N,N,A-triphen-
yl-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-phenylenediamin-
e (abbreviation: 2DPAPPA),
N,N,N',N',N'',N'',N''',N'''-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetr-
aamine (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
545T, N,N'-diphenylquinacridone (abbreviation: DPQd), 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[ij]quinolizin-9-yl)ethen-
yl]-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)ethenyl]-4H-pyran-4-ylidene}propanedinitrile
(abbreviation: DCJTB), 2-(2,6-bis
{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile
(abbreviation: BisDCM), and
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 phosphorescent substances such as
bis[2-(3',5'-bistrifluoromethylphenyl)pyridinato-N,C.sup.2']iridium(III)
picolinate (abbreviation: Ir(CF.sub.3ppy).sub.2(pic)),
bis[2-(4',6'-difluorophenyl)pyridinato-N,C.sup.2']iridium(III)
acetylacetonate (abbreviation: FIracac),
tris(2-phenylpyridinato-N,C.sup.2')iridium(III) (abbreviation:
Ir(ppy).sub.3),
bis(2-phenylpyridinato-N,C.sup.2')iridium(III)acetylacetonate
(abbreviation: Ir(ppy).sub.2acac),
bis(1,2-diphenyl-1H-benzimidazolato)iridium(II) acetylacetonate
(abbreviation: Ir(pbi).sub.2(acac)),
bis(benzo[h]quinolinato)iridium(III) acetylacetonate (abbreviation:
Ir(bzq).sub.2(acac)), tris(benzo[h]quinolinato)iridium(III)
(abbreviation: Ir(bzq).sub.3),
bis(2,4-diphenyl-1,3-oxazolato-N,C.sup.2')iridium(III)
acetylacetonate (abbreviation: Ir(dpo).sub.2(acac)),
bis[2-(4'-perfluorophenylphenyl)pyridinato]iridium(III)
acetylacetonate (abbreviation: Ir(p-PF-ph).sub.2(acac)),
bis(2-phenylbenzothiazolato-N,C.sup.2')iridium(III) acetylacetonate
(abbreviation: Ir(bt).sub.2(acac)),
(acetylacetonato)bis[2,3-bis(4-fluorophenyl)-5-methylpyrazinato]iridium(I-
II) (abbreviation: Ir(Fdppr-Me).sub.2(acac)),
(acetylacetonato)bis[2-(4-methoxyphenyl)-3,5-dimethylpyrazinato]iridium(I-
II) (abbreviation: Ir(dmmoppr).sub.2(acac)),
tris(2-phenylquinolinato-N,C.sup.2')iridium(III) (abbreviation:
Ir(pq).sub.3), bis(2-phenylquinolinato-N,C.sup.2')iridium(III)
(acetylacetonate) (abbreviation: Ir(pq).sub.2(acac)),
(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)
(abbreviation: Ir(mppr-Me).sub.2(acac)),
(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)
(abbreviation: Ir(mppr-iPr).sub.2(acac)),
bis[2-(2'-benzo[4,5-c]thienyl)pyridinato-N,C.sup.3']iridium(III)(acetylac-
etonate) (abbreviation: Ir(btp).sub.2(acac)),
bis(1-phenylisoquinolinato-N,C.sup.2')iridium(III) acetylacetonate
(abbreviation: [Ir(piq).sub.2(acac)]),
(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)
(abbreviation: [Ir(Fdpq).sub.2(acac)]),
(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)
(abbreviation: [Ir(tppr).sub.2(acac)]),
bis(2,3,5-triphenylpyrazinato) (dipivaloylmethanato)iridium(III)
(abbreviation: [Ir(tppr).sub.2(dpm)]),
(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)
(abbreviation: [Ir(tBuppm).sub.2(acac)]),
(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)
(abbreviation: [Ir(dppm).sub.2(acac)]),
2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II)
(abbreviation: PtOEP),
tris(acetylacetonato)(monophenantbroline)terbium (III)
(abbreviation: [Tb(acac).sub.3(Phen)]),
tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)
(abbreviation: [Eu(DBM).sub.3(Phen)]), and
tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(-
III) (abbreviation: [Eu(TTA).sub.3(Phen)]). Note that the carbazole
compounds according to the present invention, typical examples of
which include the carbazole compound represented by the general
formula (G1) described in Embodiment 1, emit light in the blue to
ultraviolet region, and therefore can also be used as an emission
center substance. Note that the heterocyclic compound described in
Embodiment 1 can be also used as an emission center material.
[0164] The compound comprising a dibenzo[f,h]quinoxaline ring and
two hole-transport skeletons, where the dibenzo[f,h]quinoxaline
ring and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group can be suitably used as a host material in which
the emission center substance is dispersed. The compound may be
used as a material for a carrier-transport layer that is adjacent
to the light-emitting layer. Note that in the case where the
compound is used as a host material, the emission center material
is preferably a substance having a smaller band gap than the
carbazole compound when the emission center material is a
fluorescent light-emitting material, and the emission center
material is preferably a substance having a smaller triplet
excitation energy than the carbazole compound when the emission
center material is a phosphorescent light-emitting material;
however, the material is not limited thereto.
[0165] The above-described compound has two hole-transport
skeletons in addition to a dibenzo[f,h]quinoxaline ring, making it
easy to accept holes. Accordingly, by use of the compound as a host
material of a light-emitting layer, electrons and holes recombine
in the light-emitting layer, so that it is possible to suppress the
decrease in the lifetime of the light-emitting element.
Furthermore, the introduction of two hole-transport skeletons
enables the compound to have a sterically bulky structure, and the
compound is difficult to crystallize when formed into a film. By
the use of the compound for a light-emitting element, the element
can have a long lifetime. Moreover, in this compound, since an
aromatic hydrocarbon group exists between a dibenzo[f,h]quinoxaline
ring and two hole-transport skeletons, decreases in band gap and
triplet excitation energy can be prevented as compared with a
compound in which a dibenzo[f,h]quinoxaline ring and a
hole-transport skeleton are directly bonded. By the use of the
compound for a light-emitting element, the element can have high
current efficiency.
[0166] A compound in which an aryl group to which two
hole-transport skeletons are bonded is bonded to a
dibenzo[f,h]quinoxaline ring is considered as a skeleton where the
LUMO level is predominantly located. Further, the compound has a
deep LUMO level of at least -2.8 eV or less, specifically -2.9 eV
or less on the basis of cyclic voltammetry (CV) measurements. For
example, the LUMO level of 2mDBTPDBq-II is found to be--2.96 eV by
CV measurements. Furthermore, the LUMO level of a phosphorescent
compound having a diazine skeleton, which is typified by the
above-described phosphorescent compound having a pyrazine skeleton,
such as [Ir(mppr-Me).sub.2(acac)], [Ir(mppr-iPr).sub.2(acac)],
[Ir(tppr).sub.2(acac)], or [Ir(tppr).sub.2(dpm)] or the
above-described phosphorescent compound having a pyrimidine
skeleton such as [Ir(tBuppm).sub.2(acac)] or
[Ir(dppm).sub.2(acac)], is substantially as deep as the LUMO level
of the compound in which an aryl group to which two hole-transport
skeletons are bonded is bonded to a dibenzo[f,h]quinoxaline ring.
Therefore, when a light-emitting layer includes a compound of one
embodiment of the present invention as a host material, and a
phosphorescent compound having a diazine skeleton (especially, a
pyrazine skeleton or a pyrimidine skeleton) as a guest material, a
light-emitting element in which traps for electrons in the
light-emitting layer can be reduced to a minimum and driving
voltage is extremely low can be realized.
[0167] When the compound comprising a dibenzo[f,h]quinoxaline ring
and two hole-transport skeletons, where the dibenzo[f,h]quinoxaline
ring and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group is not used as the above host material, any of
the following substances can be used for the host material: metal
complex--such as tris(8-quinolinolato)aluminum(III) (abbreviation:
Alq), tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation:
Almq.sub.3), 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), and bis[2-(2-benzothiazolyl)phenolato]zinc(II)
(abbreviation: ZnBTZ); heterocyclic compounds such as
2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(abbreviation: PBD),
1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene
(abbreviation: OXD-7),
3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole
(abbreviation: TAZ),
2,2',2''-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)
(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen),
bathocuproine (abbreviation: BCP), and
9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole
(abbreviation: CO11); and aromatic amine compounds such as
4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB
or .alpha.-NPD),
N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine
(abbreviation: TPD), and
4,4'-bis[N-(spiro-9,9'-bifluoren-2-yl)-N-phenylamino]biphenyl
(abbreviation: BSPB). In addition, condensed polycyclic aromatic
compounds such as anthracene derivatives, phenanthrene derivatives,
pyrene derivatives, chrysene derivatives, and dibenzo[g,p]chrysene
derivatives are given, and specific examples are
9,10-diphenylanthracene (abbreviation: DPAnth),
N,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine
(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine
(abbreviation: DPhPA),
4-(9H-carbazol-9-yl)-4'-(10-phenyl-9-anthryl)triphenylamine
(abbreviation: YGAPA),
N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine
(abbreviation: PCAPA),
N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-am-
ine (abbreviation: PCAPBA),
N,9-diphenyl-N-(9,10-diphenyl-2-anthryl)-9H-carbazol-3-amine
(abbreviation: 2PCAPA), 6,12-dimethoxy-5,11-diphenylchrysene,
N,N,N',N',N'',N'',N''',N'''-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetr-
amine (abbreviation: DBC1),
9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:
CzPA), 3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole
(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene
(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation:
DNA), 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:
t-BuDNA), 9,9'-bianthryl (abbreviation: BANT),
9,9'-(stilbene-3,3'-diyl)diphenanthrene (abbreviation: DPNS),
9,9'-(stilbene-4,4'-diyl)diphenanthrene (abbreviation: DPNS2),
3,3',3''-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3), and
the like. Other than these, known materials can be used.
[0168] Note that the light-emitting layer 113 can also be a stack
of two or more layers. For example, in the case where the
light-emitting layer 113 is formed by stacking a first
light-emitting layer and a second light-emitting layer in that
order over the hole-transport layer, for example, a substance
having a hole-transport property is used for the host material of
the first light-emitting layer and a substance having an
electron-transport property is used for the host material of the
second light-emitting layer.
[0169] In the case where the light-emitting layer having the
above-described structure includes a plurality of materials,
co-evaporation by a vacuum evaporation method can be used, or
alternatively an inkjet method, a spin coating method, a dip
coating method, or the like with a solution of the materials can be
used.
[0170] The electron-transport layer 114 is a layer containing a
substance having a high electron-transport property: for example, a
layer containing a metal complex having a quinoline skeleton or a
benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum
(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum
(abbreviation: Almq.sub.3),
bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation:
BeBq.sub.2), or
bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum
(abbreviation: BAlq). Alternatively, a metal complex having an
oxazole-based or thiazole-based ligand, such as
bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation:
Zn(BOX).sub.2) or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc
(abbreviation: Zn(BTZ).sub.2), or the like can be used. Besides the
metal complexes,
2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(abbreviation: PBD),
1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene
(abbreviation: OXD-7),
3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole
(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),
bathocuproine (abbreviation: BCP), or the like can also be used.
The substances mentioned here mainly have an electron mobility of
10.sup.-6 cm.sup.2/Vs or more. Note that other than these
substances, any substance that has a property of transporting more
electrons than holes may be used.
[0171] The compound comprising a dibenzo[f,h]quinoxaline ring and
two hole-transport skeletons, where the dibenzo[f,h]quinoxaline
ring and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group has an excellent electron-transport property, and
can be suitably used as a material for the electron-transport layer
114. The introduction of two hole-transport skeletons enables the
compound to have a bulky structure, and the compound is difficult
to crystallize when formed into a film. By the use of the compound
for a light-emitting element, the element can have a long lifetime.
Moreover, in this compound, since an aromatic hydrocarbon group
exists between a dibenzo[f,h]quinoxaline ring and two
hole-transport skeletons, decreases in band gap and triplet
excitation energy can be prevented as compared with a compound in
which a dibenzo[f,h]quinoxaline ring and a hole-transport skeleton
are directly bonded. By the use of the compound for a
light-emitting element, the element can have high current
efficiency.
[0172] Furthermore, the electron-transport layer 114 is not limited
to a single layer and may be a stack of two or more layers
containing any of the above substances.
[0173] Between the electron-transport layer 114 and the
light-emitting layer 113, a layer that controls transport of
electron carriers may be provided. This is a layer formed by
addition of a small amount of a substance having a high
electron-trapping property to a material having a high
electron-transport property as described above, and capable of
adjusting carrier balance by suppressing transport of electron
carriers. Such a structure is very effective in preventing a
problem (such as a reduction in element lifetime) caused when
electrons pass through the light-emitting layer.
[0174] In addition, an electron-injection layer may be provided in
contact with the second electrode 102 between the
electron-transport layer 114 and the second electrode 102. For the
electron-injection layer, an alkali metal, an alkaline earth metal,
or a compound thereof such as lithium fluoride (LiF), cesium
fluoride (CsF), or calcium fluoride (CaF.sub.2) can be used. For
example, a layer that is formed with a substance having an
electron-transport property and contains an alkali metal, an
alkaline earth metal, magnesium (Mg), or a compound thereof, such
as an Alq layer containing magnesium (Mg), can be used. Note that
electron injection from the second electrode 102 is efficiently
performed by the use of a layer that is formed with a substance
having an electron-transport property and contains an alkali metal
or an alkaline earth metal as the electron-injection layer, which
is preferable.
[0175] For the second electrode 102, any of metals, alloys,
electrically conductive compounds, and mixtures thereof which have
a low work function (specifically, a work function of 3.8 eV or
less) or the like can be used. Specific examples of such a cathode
material include elements that belong to Groups 1 and 2 in the
periodic table, i.e., alkali metals such as lithium (Li) and cesium
(Cs), alkaline earth metals such as calcium (Ca) and strontium
(Sr), magnesium (Mg), alloys thereof (e.g., MgAg or AlLi), rare
earth metals such as europium (Eu) and ytterbium (Yb), alloys
thereof, and the like. However, when the electron-injection layer
is provided between the second electrode 102 and the
electron-transport layer, for the second electrode 102, any of a
variety of conductive materials such as Al, Ag, ITO, or indium
oxide-tin oxide containing silicon or silicon oxide can be used
regardless of the work function. Films of these electrically
conductive materials can be formed by a sputtering method, an
inkjet method, a spin coating method, or the like.
[0176] Further, any of a variety of methods can be used to form the
EL layer 103 regardless whether it is a dry process or a wet
process. For example, a vacuum evaporation method, an inkjet
method, a spin coating method or the like may be used. Different
formation methods may be used for the electrodes or the layers.
[0177] In addition, the electrode may be formed by a wet method
using a sol-gel method, or by a wet method using paste of a metal
material. Alternatively, the electrode may be formed by a dry
method such as a sputtering method or a vacuum evaporation
method.
[0178] In the light-emitting element having the above-described
structure, a current flows due to a potential difference between
the first electrode 101 and the second electrode 102, and a hole
and an electron recombine in the light-emitting layer 113 which
contains a substance having a high light-emitting property, so that
light is emitted. That is, a light-emitting region is formed in the
light-emitting layer 113.
[0179] Light emission is extracted out through one or both of the
first electrode 101 and the second electrode 102. Therefore, one or
both of the first electrode 101 and the second electrode 102 are
light-transmitting electrodes. In the case where only the first
electrode 101 is a light-transmitting electrode, light emission is
extracted from the substrate side through the first electrode 101.
In the case where only the second electrode 102 is a
light-transmitting electrode, light emission is extracted from the
side opposite to the substrate side through the second electrode
102. In the case where each of the first electrode 101 and the
second electrode 102 is a light-transmitting electrode, light
emission is extracted from both the substrate side and the side
opposite to the substrate through the first electrode 101 and the
second electrode 102.
[0180] The structure of the layers provided between the first
electrode 101 and the second electrode 102 is not limited to the
above-described structure. Preferably, a light-emitting region
where holes and electrons recombine is positioned away from the
first electrode 101 and the second electrode 102 so that quenching
due to the proximity of the light-emitting region and a metal used
for electrodes and carrier-injection layers can be prevented. The
order of stacking the layers is not limited to the above structure
and may be the following order obtained by reversing the order
shown in FIG. 1A: the second electrode, the electron-injection
layer, the electron-transport layer, the light-emitting layer, the
hole-transport layer, the hole-injection layer, and the first
electrode from the substrate side.
[0181] Further, in order that transfer of energy from an exciton
generated in the light-emitting layer is prevented, it is
preferable that the hole-transport layer 112 and the
electron-transport layer 114 which are directly in contact with the
light-emitting layer, particularly a carrier-transport layer in
contact with a side closer to the light-emitting region in the
light-emitting layer 113 be formed with a substance having a larger
energy gap than the light-emitting substance of the light-emitting
layer or the emission center substance included in the
light-emitting layer.
[0182] Any layer of the light-emitting element of this embodiment
includes a compound comprising a dibenzo[f,h]quinoxaline ring and
two hole-transport skeletons, where the dibenzo[f,h]quinoxaline
ring and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group. The compound has two hole-transport skeletons in
addition to a dibenzo[f,h]quinoxaline ring, making it easy to
accept holes. Accordingly, by use of the compound as a host
material of a light-emitting layer, electrons and holes recombine
in the light-emitting layer, so that the decrease in the lifetime
of the light-emitting element can be prevented. Furthermore, the
introduction of two hole-transport skeletons enables the compound
to have a sterically bulky structure, and the compound is difficult
to crystallize when formed into a film. By the use of the compound
for a host material or an electron-transport material in the
light-emitting layer, the element can have a long lifetime.
Moreover, in this compound, since an aromatic hydrocarbon group
exists between a dibenzo[f,h]quinoxaline ring and two
hole-transport skeletons, decreases in band gap and triplet
excitation energy can be prevented as compared with a compound in
which a dibenzo[f,h]quinoxaline ring and a hole-transport skeleton
are directly bonded. By the use of the compound for a host material
or an electron-transport material in the light-emitting layer, the
element can have high current efficiency.
[0183] In this embodiment, the light-emitting element is formed
over a substrate formed of glass, plastic, or the like. With a
plurality of such light-emitting elements over one substrate, a
passive matrix light-emitting device can be fabricated. In
addition, a light-emitting element may be formed over an electrode
electrically connected to a thin film transistor (TFT) which is
formed over a substrate formed of glass, plastic, or the like;
thus, an active matrix light-emitting device in which the TFT
controls the drive of the light-emitting element can be fabricated.
Note that there is no particular limitation on the structure of the
TFT, which may be a staggered TFT or an inverted staggered TFT. In
addition, crystallinity of a semiconductor used for the TFT is not
particularly limited either; an amorphous semiconductor or a
crystalline semiconductor may be used. In addition, a driver
circuit formed in a TFT substrate may be formed with an n-type TFT
and a p-type TFT, or with either an n-type TFT or a p-type TFT.
Embodiment 3
[0184] In this embodiment, one mode of a light-emitting element
(hereinafter, also referred to as a stacked-type element) having a
structure in which a plurality of light-emitting units is stacked
is described with reference to FIG. 1B. This light-emitting element
is a light-emitting element including a plurality of light-emitting
units between a first electrode and a second electrode. Each
light-emitting unit can have the same structure as the EL layer 103
described in Embodiment 2. In other words, it can be said that the
light-emitting element described in Embodiment 2 is a
light-emitting element having one light-emitting unit and the
light-emitting element of this embodiment is a light-emitting
element having a plurality of light-emitting units.
[0185] In FIG. 1B, a first light-emitting unit 511 and a second
light-emitting unit 512 are stacked between a first electrode 501
and a second electrode 502, and a charge generation layer 513 is
provided between the first light-emitting unit 511 and the second
light-emitting unit 512. The first electrode 501 and the second
electrode 502 correspond, respectively, to the first electrode 101
and the second electrode 102 according to Embodiment 2, and
materials described in Embodiment 2 can be used. Further, the
structures of the first light-emitting unit 511 and the second
light-emitting unit 512 may be the same or different.
[0186] The charge generation layer 513 contains a composite
material of an organic compound and a metal oxide. This composite
material of an organic compound and a metal oxide is the composite
material described in Embodiment 2, and contains an organic
compound and a metal oxide such as vanadium oxide, molybdenum
oxide, or tungsten oxide. As the organic compound, any of a variety
of compounds such as aromatic amine compounds, carbazole compounds,
aromatic hydrocarbons, and high molecular compounds (oligomers,
dendrimers, polymers, or the like) can be used. Note that as the
organic compound, the one having a hole mobility of 10.sup.-6
cm.sup.2/Vs or more as an organic compound having a hole-transport
property is preferably used. Further, other than these substances,
any substance that has a property of transporting more holes than
electrons may be used. Since a composite of an organic compound and
a metal oxide is excellent in carrier-injection property and
carrier-transport property, low voltage driving and low current
driving can be realized.
[0187] The charge generation layer 513 may be formed in such a way
that a layer containing the composite material of an organic
compound and a metal oxide is combined with a layer containing
another material, for example, with a layer that contains a
compound selected from substances having an electron-donating
property and a compound having a high electron-transport property.
The charge generation layer 513 may be formed in such a way that a
layer containing the composite material of an organic compound and
a metal oxide is combined with a transparent conductive film.
[0188] The charge generation layer 513 interposed between the first
light-emitting unit 511 and the second light-emitting unit 512 may
have any structure as far as electrons can be injected to a
light-emitting unit on one side and holes can be injected to a
light-emitting unit on the other side when a voltage is applied
between the first electrode 501 and the second electrode 502. For
example, in FIG. 1B, any layer can be used as the charge generation
layer 513 as far as the layer injects electrons into the first
light-emitting unit 511 and holes into the second light-emitting
unit 512 when a voltage is applied such that the voltage of the
first electrode is higher than that of the second electrode.
[0189] Although the light-emitting element having two
light-emitting units is described in this embodiment, the present
invention can be similarly applied to a light-emitting element in
which three or more light-emitting units are stacked. By
arrangement of a plurality of light-emitting units, which are
partitioned by the charge-generation layer between a pair of
electrodes, as in the light-emitting element of this embodiment,
light emission in a high luminance region can be realized with
current density kept low, thus a light-emitting element having a
long lifetime can be realized. Further, in application to lighting
devices, since a voltage drop due to resistance of an electrode
material can be reduced, light emission in a large area is
possible. Moreover, a light-emitting device having low driving
voltage and having lower power consumption can be realized.
[0190] By making emission colors of the light-emitting units
different from each other, light emission with a desired color can
be obtained from the light-emitting element as a whole. For
example, in a light-emitting element including two light-emitting
units, the emission colors of the first light-emitting unit and the
second light-emitting unit are made complementary, so that the
light-emitting element which emits white light as the whole element
can be obtained. Note that the term "complementary" means color
relationship in which an achromatic color is obtained when colors
are mixed. That is, a mixture of light emissions with complementary
colors gives white light emission. The same can be applied to a
light-emitting element including three light-emitting units. For
example, the light-emitting element as a whole can emit white light
when the emission color of the first light-emitting unit is red,
the emission color of the second light-emitting unit is green, and
the emission color of the third light-emitting unit is blue.
[0191] Since the light-emitting element of this embodiment includes
a compound comprising a dibenzo[f,h]quinoxaline ring and two
hole-transport skeletons, where the dibenzo[f,h]quinoxaline ring
and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group, the light-emitting element can have high
emission efficiency, a long lifetime, and/or low driving
voltage.
[0192] Note that this embodiment can be combined with any of the
other embodiments as appropriate.
Embodiment 4
[0193] In this embodiment, description is given of a light-emitting
device in which a light-emitting element including a compound
comprising a dibenzo[f,h]quinoxaline ring and two hole-transport
skeletons, where the dibenzo[f,h]quinoxaline ring and the two
hole-transport skeletons are bonded to an aromatic hydrocarbon
group is used.
[0194] In this embodiment, an example of the light-emitting device
fabricated using a light-emitting element including the compound
comprising a dibenzo[f,h]quinoxaline ring and two hole-transport
skeletons, where the dibenzo[f,h]quinoxaline ring and the two
hole-transport skeletons are bonded to an aromatic hydrocarbon
group is described with reference to FIGS. 2A and 2B. Note that
FIG. 2A is a top view illustrating the light-emitting device and
FIG. 2B is a cross-sectional view of FIG. 2A taken along lines A-B
and C-D. This light-emitting device includes a driver circuit
portion (source driver circuit) 601, a pixel portion 602, and a
driver circuit portion (gate driver circuit) 603, which are to
control light emission of the light-emitting element and
illustrated with dotted lines. Moreover, a reference numeral 604
denotes a sealing substrate; 625, a desiccant; 605, a sealing
material; and 607, a space surrounded by the sealing material
605.
[0195] Reference numeral 608 denotes a wiring for transmitting
signals to be inputted into the source driver circuit 601 and the
gate driver circuit 603 and receiving signals such as a video
signal, a clock signal, a start signal, and a reset signal from an
FPC (flexible printed circuit) 609 serving as an external input
terminal. Although only the FPC is illustrated here, a printed
wiring board (PWB) may be attached to the FPC. The light-emitting
device in the present specification includes, in its category, not
only the light-emitting device itself but also the light-emitting
device provided with the FPC or the PWB.
[0196] Next, a cross-sectional structure is described with
reference to FIG. 2B. The driver circuit portion and the pixel
portion are formed over an element substrate 610; the source driver
circuit 601, which is a driver circuit portion, and one of the
pixels in the pixel portion 602 are illustrated here.
[0197] As the source driver circuit 601, a CMOS circuit in which an
n-channel TFT 623 and a p-channel TFT 624 are combined is formed.
In addition, the driver circuit may be formed with any of a variety
of circuits such as a CMOS circuit, a PMOS circuit, or an NMOS
circuit. Although a driver integrated type in which the driver
circuit is formed over the substrate is illustrated in this
embodiment, the driver circuit may not necessarily be formed over
the substrate, and the driver circuit can be formed outside, not
over the substrate.
[0198] The pixel portion 602 includes a plurality of pixels
including a switching TFT 611, a current controlling TFT 612, and a
first electrode 613 electrically connected to a drain of the
current controlling TFT. Note that to cover an end portion of the
first electrode 613, an insulator 614 is formed, for which a
positive type photosensitive acrylic resin film is used here.
[0199] In order to improve coverage, the insulator 614 is formed to
have a curved surface with curvature at its upper or lower end
portion. For example, in the case where positive photosensitive
acrylic is used for a material of the insulator 614, only the upper
end portion of the insulator 614 preferably has a curved surface
with a curvature radius (0.2 .mu.m to 3 .mu.m). As the insulator
614, either a negative type photosensitive resin or a positive type
photosensitive resin can be used.
[0200] An EL layer 616 and a second electrode 617 are formed over
the first electrode 613. Here, as a material used for the first
electrode 613 functioning as an anode, a material having a high
work function is preferably used. For example, a single-layer film
of an ITO film, an indium tin oxide film containing silicon, an
indium oxide film containing zinc oxide at 2 wt % to 20 wt %, a
titanium nitride film, a chromium film, a tungsten film, a Zn film,
a Pt film, or the like, a stack of a titanium nitride film and a
film containing aluminum as its main component, a stack of three
layers of a titanium nitride film, a film containing aluminum as
its main component, and a titanium nitride film, or the like can be
used. Note that when the stacked structure is used, the first
electrode 613 has low resistance as a wiring, forms a favorable
ohmic contact, and can function as an anode.
[0201] In addition, the EL layer 616 containing an organic compound
is formed by any of a variety of methods such as an evaporation
method using a shadow mask, an inkjet method, and a spin coating
method. The EL layer 616 includes the compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group. Further,
another material included in the EL layer 616 is not limited, and
may be a low molecular compound or a high molecular compound (which
may be an oligomer and a dendrimer).
[0202] Furthermore, a material having a small work function
described in Embodiment 2 is preferably used as a material used for
the second electrode 617 which is formed over the EL layer 616 and
functions as a cathode. In the case where light generated in the EL
layer 616 passes through the second electrode 617, the second
electrode 617 is preferably a stack of a thin metal film and a
transparent conductive film (such as ITO, indium oxide containing
zinc oxide at 2 wt % to 20 wt %, indium tin oxide containing
silicon, or zinc oxide (ZnO)).
[0203] Note that the light-emitting element is formed with the
first electrode 613, the EL layer 616, and the second electrode
617. The light-emitting element has any of the structures described
in Embodiment 2 or 3. In the light-emitting device of this
embodiment, the pixel portion, which includes a plurality of
light-emitting elements, may include both the light-emitting
element with any of the structures described in Embodiments 2 and 3
and a light-emitting element with a structure other than those.
[0204] Further, the sealing substrate 604 is attached to the
element substrate 610 with the sealing material 605, so that a
light-emitting element 618 is provided in the space 607 surrounded
by the element substrate 610, the sealing substrate 604, and the
sealing material 605. The space 607 may be filled with filler, and
may be filled with an inert gas (such as nitrogen or argon), or the
sealing material 605.
[0205] Note that an epoxy-based resin is preferably used for the
sealing material 605. It is desirable that such a material do not
transmit moisture or oxygen as much as possible. As a material for
the sealing substrate 604, a plastic substrate formed of FRP
(fiberglass-reinforced plastics), PVF (polyvinyl fluoride),
polyester, acrylic, or the like can be used besides a glass
substrate or a quartz substrate.
[0206] As described above, the light-emitting device fabricated
using the light-emitting element including the compound comprising
a dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group can be
obtained.
[0207] The light-emitting element including the compound comprising
a dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group is used in
the light-emitting device of this embodiment, and thus the
light-emitting device can have favorable characteristics.
Specifically, in the compound comprising a dibenzo[f,h]quinoxaline
ring and two hole-transport skeletons, where the
dibenzo[f,h]quinoxaline ring and the two hole-transport skeletons
are bonded to an aromatic hydrocarbon group, a decrease in band gap
and triplet excitation energy is prevented; accordingly, energy
transfer from a light-emitting substance to a host material or a
transport layer can be prevented. Therefore, a light-emitting
element having high emission efficiency can be provided, so that a
light-emitting device having reduced power consumption can be
provided. In addition, a light-emitting element having low driving
voltage can be provided, so that a light-emitting device having low
driving voltage can be provided. Furthermore, a light-emitting
element having a long lifetime can be provided, so that a highly
reliable light-emitting device can be provided.
[0208] Although an active matrix light-emitting device is thus
described above, a passive matrix light-emitting device is
described below. FIGS. 3A and 3B illustrate a passive matrix
light-emitting device fabricated according to the present
invention. FIG. 3A is a perspective view of the light-emitting
device, and FIG. 3B is a cross-sectional view taken along line X-Y
in FIG. 3A. In FIGS. 3A and 3B, over a substrate 951, an EL layer
955 is provided between an electrode 952 and an electrode 956. An
end portion of the electrode 952 is covered with an insulating
layer 953. In addition, a partition layer 954 is provided over the
insulating layer 953. The sidewalls of the partition layer 954 are
aslope such that the distance between both sidewalls is gradually
narrowed toward the surface of the substrate. In other words, a
cross section taken along the direction of the short side of the
partition wall layer 954 is trapezoidal, and the lower side (a side
which is in the same direction as a plane direction of the
insulating layer 953 and in contact with the insulating layer 953)
is shorter than the upper side (a side which is in the same
direction as the plane direction of the insulating layer 953 and
not in contact with the insulating layer 953). The partition layer
954 thus provided can prevent a defect in the light-emitting
element due to static charge or the like. The passive matrix
light-emitting device can also be driven with low power consumption
by including the light-emitting element described in Embodiment 2
or 3 capable of operating at low voltage. In addition, the
light-emitting device can be driven with low power consumption by
including the light-emitting element described in Embodiment 2 or 3
having high emission efficiency. Furthermore, the light-emitting
device having high reliability can be provided by including the
light-emitting element described in Embodiment 2 or 3 having a long
lifetime.
[0209] Since many minute light-emitting elements arranged in a
matrix in the light-emitting device described above can each be
controlled, the light-emitting device can be suitably used as a
display device for displaying images.
Embodiment 5
[0210] In this embodiment, electronic devices each including, as a
part thereof, the light-emitting element described in Embodiment 2
or 3. The light-emitting element described in Embodiment 2 or 3 is
a light-emitting element having reduced power consumption because
an EL layer thereof includes a compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group. Therefore,
the electronic devices of this embodiment can have a light-emitting
portion or display portion having reduced power consumption. In
addition, the electronic devices can be driven at a low driving
voltage because the light-emitting element described in Embodiment
2 or 3 is a light-emitting element driven at a low driving voltage.
The light-emitting element described in Embodiment 2 or 3 is also a
light-emitting element having a long lifetime, whereby an
electronic device having high reliability can be achieved.
[0211] Examples of the electronic devices to which the above
light-emitting element is applied are television devices (also
referred to as TV or television receivers), monitors for computers
and the like, cameras such as digital cameras and digital video
cameras, digital photo frames, cellular phones (also referred to as
portable telephone devices), portable game machines, portable
information terminals, audio playback devices, large game machines
such as pin-ball machines, and the like. Specific examples of these
electronic devices are described below.
[0212] FIG. 4A illustrates an example of a television device. In
the television device, a display portion 7103 is incorporated in a
housing 7101. In addition, here, the housing 7101 is supported by a
stand 7105. The display portion 7103 enables display of images and
includes light-emitting elements which are the same as that
described in Embodiment 2 or 3 and arranged in a matrix. Since each
light-emitting element includes a compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group, the
light-emitting elements can be light-emitting elements having high
emission efficiency, low driving voltage, and a long lifetime.
Accordingly, the television device that has the display portion
7103 including the light-emitting elements can be a television
device having reduced power consumption, low driving voltage, and
high reliability.
[0213] The television device can be operated by an operation switch
of the housing 7101 or a separate remote controller 7110. With
operation keys 7109 of the remote controller 7110, channels and
volume can be controlled and images displayed on the display
portion 7103 can be controlled. Furthermore, the remote controller
7110 may be provided with a display portion 7107 for displaying
data output from the remote controller 7110.
[0214] Note that the television device is provided with a receiver,
a modem, and the like. With the receiver, general television
broadcasting can be received. Furthermore, when the television
device is connected to a communication network by wired or wireless
connection via the modem, one-way (from a transmitter to a
receiver) or two-way (between a transmitter and a receiver, between
receivers, or the like) data communication can be performed.
[0215] FIG. 4B1 illustrates a computer, which includes a main body
7201, a housing 7202, a display portion 7203, a keyboard 7204, an
external connection port 7205, a pointing device 7206, and the
like. Note that this computer is manufactured by using
light-emitting elements arranged in a matrix in the display portion
7203, which are the same as that described in Embodiment 2 or 3.
The computer illustrated in FIG. 4B1 may have a structure
illustrated in FIG. 4B2. The computer illustrated in FIG. 4B2 is
provided with a second display portion 7210 instead of the keyboard
7204 and the pointing device 7206. The second display portion 7210
is a touch screen, and input can be performed by operation of
display for input on the second display portion 7210 with a finger
or a dedicated pen. The second display portion 7210 can also
display images other than the display for input. The display
portion 7203 may be also a touch screen. Connecting the two screens
with a hinge can prevent troubles; for example, the screens can be
prevented from being cracked or broken while the computer is being
stored or carried.
[0216] Since the light-emitting element in such a computer includes
a compound comprising a dibenzo[f,h]quinoxaline ring and two
hole-transport skeletons, where the dibenzo[f,h]quinoxaline ring
and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group, the light-emitting elements can be
light-emitting elements having high emission efficiency, low
driving voltage, and a long lifetime. Accordingly, the computer
that has the display portion 7203 including the light-emitting
elements can have reduced power consumption, low driving voltage,
and/or high reliability.
[0217] FIG. 4C illustrates a portable game machine having two
housings, a housing 7301 and a housing 7302, which are connected
with a joint portion 7303 so that the portable game machine can be
opened or folded. A display portion 7304 including light-emitting
elements which are the same as that described in Embodiment 2 or 3
and arranged in a matrix is incorporated in the housing 7301, and a
display portion 7305 is incorporated in the housing 7302. In
addition, the portable game machine illustrated in FIG. 4C includes
a speaker portion 7306, a recording medium insertion portion 7307,
an LED lamp 7308, an input unit (an operation key 7309, a
connection terminal 7310, a sensor 7311 (a sensor having a function
of measuring 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 rays), or
a microphone 7312), and the like. It is needless to say that the
structure of the portable game machine is not limited to the above
as far as the display portion including light-emitting elements
which are the same as that described in Embodiment 2 or 3 and
arranged in a matrix is used as at least either the display portion
7304 or the display portion 7305, or both, and the structure can
include other accessories as appropriate. The portable game machine
illustrated in FIG. 4C has a function of reading out a program or
data stored in a storage medium to display it on the display
portion, and a function of sharing information with another
portable game machine by wireless communication. The portable game
machine illustrated in FIG. 4C can have a variety of functions
without limitation to the above. The light-emitting element used in
the display portion 7304 can have high emission efficiency, low
driving voltage, and/or a long lifetime by including the compound
comprising a dibenzo[f,h]quinoxaline ring and two hole-transport
skeletons, where the dibenzo[f,h]quinoxaline ring and the two
hole-transport skeletons are bonded to an aromatic hydrocarbon
group. The portable game machine including such a light-emitting
element in the display portion 7304 can have reduced power
consumption, low driving voltage, and/or high reliability.
[0218] FIG. 4D illustrates an example of a cellular phone. The
cellular phone is provided with operation buttons 7403, an external
connection port 7404, a speaker 7405, a microphone 7406, and the
like, in addition to a display portion 7402 incorporated in a
housing 7401. Note that the cellular phone 7400 has the display
portion 7402 including light-emitting elements which are the same
as that described in Embodiment 2 or 3 and arranged in a matrix.
Since each light-emitting element includes the compound comprising
a dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group, the
light-emitting element can have high emission efficiency, low
driving voltage, and/or a long lifetime. Accordingly, the cellular
phone that has the display portion 7402 including the
light-emitting elements can be a cellular phone having reduced
power consumption, low driving voltage, and long lifetime.
[0219] When the display portion 7402 of the cellular phone
illustrated in FIG. 4D is touched with a finger or the like, data
can be input into the cellular phone. In this case, operations such
as making a call and creating e-mail can be performed by touch on
the display portion 7402 with a finger or the like.
[0220] There are mainly three screen modes of the display portion
7402. The first mode is a display mode mainly for displaying an
image. The second mode is an input mode mainly for inputting
information such as characters. The third mode is a
display-and-input mode in which two modes of the display mode and
the input mode are mixed.
[0221] For example, in the case of making a call or creating
e-mail, a text input mode mainly for inputting text is selected for
the display portion 7402 so that text displayed on a screen can be
input. In this case, it is preferable to display a keyboard or
number buttons on almost the entire screen of the display portion
7402.
[0222] When a detection device including a sensor for detecting
inclination, such as a gyroscope or an acceleration sensor, is
provided inside the cellular phone, display on the screen of the
display portion 7402 can be automatically changed by determining
the orientation of the cellular phone (whether the cellular phone
is placed horizontally or vertically for a landscape mode or a
portrait mode).
[0223] The screen modes are switched by touch on the display
portion 7402 or operation with the operation buttons 7403 of the
housing 7401. Alternatively, the screen modes can be switched
depending on the kinds of images displayed on the display portion
7402. For example, when a signal for an image displayed on the
display portion is data of moving images, the screen mode is
switched to the display mode. When the signal is text data, the
screen mode is switched to the input mode.
[0224] Moreover, in the input mode, if a signal detected by an
optical sensor in the display portion 7402 is detected and the
input by touch on the display portion 7402 is not performed during
a certain period, the screen mode may be controlled so as to be
switched from the input mode to the display mode.
[0225] The display portion 7402 can function as an image sensor.
For example, an image of a palm print, a fingerprint, or the like
is taken by touch on the display portion 7402 with the palm or the
finger, so that personal authentication can be performed.
Furthermore, by use of a backlight or a sensing light source that
emits a near-infrared light for the display portion, an image of a
finger vein, a palm vein, or the like can also be taken.
[0226] Note that the structure described in this embodiment can be
combined with any of the structures described in Embodiments 2 to 4
as appropriate. Furthermore, the heterocyclic compound described in
Embodiment 1 shows part of a compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group in
detail.
[0227] As described above, the application range of the
light-emitting device having the light-emitting element according
to Embodiment 2 or 3 which includes any of the carbazole compounds
described in Embodiment 1 is wide so that this light-emitting
device can be applied to electronic devices in a variety of fields.
By use of the compound comprising a dibenzo[f,h]quinoxaline ring
and two hole-transport skeletons, where the dibenzo[f,h]quinoxaline
ring and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group, an electronic device having reduced power
consumption, low driving voltage, and high reliability can be
obtained.
[0228] The light-emitting element including a compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group can also be
used for a lighting device. One mode of application of the
light-emitting element to a lighting device is described with
reference to FIGS. 5 to 7. Note that the lighting device includes
the light-emitting element as a light irradiation unit and at least
includes an input-output terminal portion that supplies a current
to the light-emitting element. Further, the light-emitting element
is preferably shielded from the outside atmosphere (especially
water) by sealing.
[0229] FIG. 5 illustrates an example of a liquid crystal display
device using the light-emitting element including a compound
comprising a dibenzo[f,h]quinoxaline ring and two hole-transport
skeletons, where the dibenzo[f,h]quinoxaline ring and the two
hole-transport skeletons are bonded to an aromatic hydrocarbon
group for a backlight. The liquid crystal display device
illustrated in FIG. 5 includes a housing 901, a liquid crystal
layer 902, a backlight 903, and a housing 904. The liquid crystal
layer 902 is connected to a driver IC 905. The light-emitting
element including a carbazole compound described in Embodiment 1 is
used in the backlight 903, to which a current is supplied through a
terminal 906.
[0230] A light-emitting element including the compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group is used for
the backlight of the liquid crystal display device, and thus a
backlight having reduced power consumption can be obtained. In
addition, use of the compound comprising a dibenzo[f,h]quinoxaline
ring and two hole-transport skeletons, where the
dibenzo[f,h]quinoxaline ring and the two hole-transport skeletons
are bonded to an aromatic hydrocarbon group enables manufacture of
a planar-emission lighting device and further a larger-area
planar-emission lighting device; therefore, the backlight can be a
larger-area backlight, and the liquid crystal display device can
also be a larger-area device. In addition, the backlight using the
light-emitting element can be thinner than a conventional one;
accordingly, the display device can also be thinner. Furthermore, a
backlight including the light-emitting element can have high
reliability.
[0231] FIG. 6 illustrates an example in which a lighting device
including a compound comprising a dibenzo[f,h]quinoxaline ring and
two hole-transport skeletons, where the dibenzo[f,h]quinoxaline
ring and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group is used for a desk lamp that is a lighting
device. The desk lamp illustrated in FIG. 6 includes a housing 2001
and a light source 2002, and the light-emitting element is used for
the light source 2002.
[0232] FIG. 7 illustrates an example in which a light-emitting
element including a compound comprising a dibenzo[f,h]quinoxaline
ring and two hole-transport skeletons, where the
dibenzo[f,h]quinoxaline ring and the two hole-transport skeletons
are bonded to an aromatic hydrocarbon group is used for indoor
lighting devices 3001 and 3002. Since the light-emitting element
has reduced power consumption, a lighting device that has reduced
power consumption can be obtained. Further, since the
light-emitting element can have a large area, the light-emitting
element can be used for a large-area lighting device. Furthermore,
since the light-emitting element is thin, a lighting device having
a reduced thickness can be fabricated. Still furthermore, since the
light-emitting element is driven at low voltage, a lighting device
having low driving voltage can be fabricated.
[0233] A light-emitting element including the compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group can also be
used for an automobile windshield or dashboard. One mode in which
the light-emitting element is used for an automobile windshield and
an automobile dashboard is illustrated in FIG. 8. Displays 5000 to
5005 each include the light-emitting element.
[0234] The display 5000 and the display 5001 are display devices
which are provided in the automobile windshield and in which the
light-emitting element including compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group is
incorporated. The light-emitting element can be formed into
so-called see-through display devices, through which the opposite
side can be seen, by including a first electrode and a second
electrode formed with electrodes having a light-transmitting
property. Such see-through display devices can be provided even in
the automobile windshield, without hindering the vision. Note in
the case where a transistor for driving the light-emitting element
is provided, a transistor having a light-transmitting property,
such as an organic transistor using an organic semiconductor
material or a transistor using an oxide semiconductor, is
preferably used.
[0235] The display 5002 is a display device which is provided in a
pillar portion and in which the light-emitting element including
the compound comprising a dibenzo[f,h]quinoxaline ring and two
hole-transport skeletons, where the dibenzo[f,h]quinoxaline ring
and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group is incorporated. The display 5002 can compensate
for the view hindered by the pillar portion by showing an image
taken by an imaging unit provided in the automobile body.
Similarly, the display 5003 provided in the dashboard can
compensate for the view hindered by the automobile body by showing
an image taken by an imaging unit provided in the outside of the
automobile body, which leads to elimination of blind areas and
enhancement of safety. Showing an image so as to compensate for the
area which a driver cannot see, makes it possible for the driver to
confirm safety easily and comfortably.
[0236] The display 5004 and the display 5005 can provide a variety
of kinds of information such as information of navigation,
speedometer, tachometer, mileage (travel distance), fuel meter,
gearshift indicator, and air condition. The content or layout of
the display can be changed freely by a user as appropriate.
Further, such information can also be shown in the displays 5000 to
5003. Note that the displays 5000 to 5005 can also be used as
lighting devices.
[0237] By including the compound comprising a
dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group, the
light-emitting element can have low driving voltage. In addition, a
light-emitting element having lower power consumption can be
provided. When a number of large screens such as displays 5000 to
5005 are provided, load on a battery can be reduced, which provides
comfortable use. The light-emitting device and the lighting device
each using the light-emitting element can be suitably used as an
in-vehicle light-emitting device or lighting device.
Embodiment 6
[0238] This embodiment shows an example in which the light-emitting
element described in Embodiment 2 or 3 is used for a lighting
device with reference to FIGS. 9A and 9B. FIG. 9B is a top view of
the lighting device, and FIG. 9A is a cross-sectional view taken
along line e-f in FIG. 9B.
[0239] In the lighting device of this embodiment, a first electrode
401 is formed over a substrate 400 which is a support and has a
light-transmitting property. The first electrode 401 corresponds to
the first electrode 101 in Embodiment 2.
[0240] An auxiliary electrode 402 is provided over the first
electrode 401. Since this embodiment shows an example in which
light emission is extracted through the first electrode 401 side,
the first electrode 401 is formed with a material having a
light-transmitting property. The auxiliary electrode 402 is
provided in order to compensate for low conductivity of the
material having a light-transmitting property, and has a function
of suppressing luminance unevenness in a light emission surface due
to voltage drop caused by high resistance of the first electrode
401. The auxiliary electrode 402 is formed with a material having
higher conductivity at least than the material of the first
electrode 401, and is preferably formed using a material having
high conductivity such as aluminum. Note that surfaces of the
auxiliary electrode 402 other than a portion thereof in contact
with the first electrode 401 are preferably covered with an
insulating layer. This is for suppressing light emission over the
upper portion of the auxiliary electrode 402, which cannot be
extracted, for reducing a reactive current, and for suppressing a
reduction in power efficiency. Note that a pad 412 for applying
voltage to a second electrode 404 may be formed concurrently with
the formation of the auxiliary electrode 402.
[0241] An EL layer 403 is formed over the first electrode 401 and
the auxiliary electrode 402. The EL layer 403 has a structure
corresponding to the structure of the EL layer 103 in Embodiment 2
or a structure in which the light-emitting units 511 and 512 and
the charge generation layer 513 in Embodiment 3 are combined. The
description in Embodiments 2 and 3 can be referred to for these
structures. Note that the EL layer 403 is preferably formed to be
slightly larger than the first electrode 401 when seen from above
so as to also serve as an insulating layer that suppresses a short
circuit between the first electrode 401 and the second electrode
404.
[0242] The second electrode 404 is formed to cover the EL layer
403. The second electrode 404 corresponds to the second electrode
102 in Embodiment 2 and has a structure similar to the second
electrode 102. In this embodiment, the second electrode 404 is
preferably formed using a material having high reflectance because
light is extracted through the first electrode 401 side. In this
embodiment, the second electrode 404 is connected to the pad 412,
whereby voltage is applied thereto.
[0243] As described above, the lighting device described in this
embodiment includes a light-emitting element including the first
electrode 401, the EL layer 403, and the second electrode 404 (and
the auxiliary electrode 402). Since the light-emitting element has
high emission efficiency, the lighting device of this embodiment
can have low power consumption. In addition, since the
light-emitting element has high reliability, the lighting device of
this embodiment can be a lighting device having high
reliability.
[0244] The light-emitting element having the above structure is
fixed to a sealing substrate 407 with sealing materials 405 and 406
to seal the light-emitting element, whereby the lighting device is
completed. It is possible to use only either the sealing material
405 or the sealing material 406. In addition, the inner sealing
material 406 can be mixed with a desiccant, whereby moisture is
adsorbed and the reliability is increased.
[0245] When parts of the pad 412, the first electrode 401, and the
auxiliary electrode 402 are extended to the outside of the sealing
materials 405 and 406, the extended parts can serve as external
input terminals. An IC chip 420 provided with a converter or the
like may be provided over the external input terminals.
[0246] In the above manner, the lighting device described in this
embodiment includes an EL element including the light-emitting
element described in Embodiment 2 or 3; thus, the lighting device
can have low power consumption, low driving voltage, and/or high
reliability.
Example 1
Synthesis Example 1
[0247] In this example, a method of synthesizing the heterocyclic
compound described in Embodiment 1,
2-[3,5-bis(9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2Cz2PDBq) represented by the following structural
formula (100) is described. The heterocyclic compound can be
expressed as a compound comprising a dibenzo[f,h]quinoxaline ring
and two hole-transport skeletons, where the dibenzo[f,h]quinoxaline
ring and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group.
##STR00057##
Step 1: Synthesis of
9,9'-(5-bromo-1,3-phenylene)-bis-9H-carbazole
[0248] Into a 300-mL three-neck flask were put 9.4 g (30 mmol) of
1,3,5-tribromobenzene, 10 g (60 mmol) of 9H-carbazole, 1.1 g (6.0
mmol) of copper(I) iodide, 16 g (0.12 mol) of potassium carbonate,
and 0.79 g (3.0 mol) of 18-crown-6-ether, and the air in the flask
was replaced with nitrogen. This mixture was stirred under a
nitrogen stream at 90.degree. C. for 15 minutes. After the
stirring, to this mixture was added 9.0 mL of
1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)pyrimidinone (abbreviation:
DMPU), and the mixture in this flask was stirred at 185.degree. C.
for three hours. After the stirring, the mixture in this flask was
cooled to 85.degree. C. and 70 mL of toluene was added thereto, and
then stirred at 120.degree. C. for four hours. After the stirring,
this mixture was cooled to 80.degree. C. and 200 mL of toluene was
added thereto, and subjected to suction filtration to obtain a
filtrate. The filtrate was washed with saturated saline. Water was
removed from an organic layer using magnesium sulfate, and then the
mixture was gravity-filtered. The obtained filtrate was
concentrated to obtain a solid, and 85 mL of chloroform was added
to the solid. This mixture was subjected to suction filtration to
obtain a filtrate. The filtrate was concentrated to obtain a solid.
The solid was purified by silica gel column chromatography (a
developing solvent: a mixed solvent of toluene: hexane=1:100) to
obtain a solid. This solid was dried under reduced pressure, so
that 2.8 g of white powder, which was the object of the synthesis,
was obtained in a yield of 19%. The synthesis scheme of Step 1 is
shown in (a-1).
##STR00058##
[0249] Into 100-mL three-neck flask was put 2.8 g (5.7 mmol) of
9,9'-(5-bromo-1,3-phenylene)-bis-9H-carbazole, and the air in the
flask was replaced with nitrogen. Into this flask was added 25 mL
of tetrahydrofuran (THF), and the solution was cooled to
-80.degree. C. Into this solution was dripped 3.9 mL (6.3 mmol) of
n-butyllithium (a 1.6 mol/L hexane solution) with a syringe. After
the dripping, this solution was stirred for 90 minutes without
changing the temperature. After the stirring, 0.80 mL (7.4 mmol) of
trimethyl borate was added to this solution, and the mixture was
stirred for one hour while being returned to room temperature.
After the stirring, about 10 mL of dilute hydrochloric acid (1.0
mol/L) was added to this solution. Then, the aqueous layer of this
mixture was extracted with ethyl acetate, and the extracted
solution and the organic layer were combined and washed with a
saturated aqueous solution of sodium hydrogen carbonate and
saturated saline. Water was removed from the organic layer using
magnesium sulfate, and then the mixture was gravity-filtered. The
obtained filtrate was concentrated to obtain a solid. A mixed
solvent of ethyl acetate and hexane was added to the obtained
solid, the mixture was irradiated with ultrasonic waves, and the
solid was collected by suction filtration, so that 2.1 g of orange
powder, which was the object of the synthesis, was obtained in a
yield of 83%. The synthesis scheme of Step 2 is shown in (a-2).
##STR00059##
Step 3: Synthesis of
2-[3,5-bis(9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2Cz2PDBq)
[0250] Into a 100-mL three-neck flask were put 1.2 g (4.8 mmol) of
2-chlorodibenzo[f,h]quinoxaline, 2.1 g (4.7 mmol) of
3,5-bis(9H-carbazol-9-yl)phenylboronic acid, and the air in the
flask was replaced with nitrogen. To this mixture were added 18 mL
of toluene, 6.0 mL of ethanol, and 6.0 mL of an aqueous solution of
potassium carbonate (2.0 mol/L). This mixture was stirred to be
degassed while the pressure was reduced. To this mixture was added
0.11 g (0.096 mmol) of tetrakis(triphenylphosphine)palladium(0),
and the mixture was stirred at 80.degree. C. for six hours under a
nitrogen stream, so that a solid was precipitated. The precipitated
solid was collected by suction filtration. To the obtained solid
was added about 100 mL of water, washed by ultrasonic wave
irradiation, and subjected to suction filtration to obtain a solid.
To the obtained solid was added about 100 mL of ethanol, washed by
ultrasonic wave irradiation, and subjected to suction filtration to
obtain a solid. The obtained solid was dissolved in about 1.5 L of
hot toluene, and this solution was suction-filtered through Celite
and Florisil. A solid obtained by concentration of the filtrate was
washed with toluene, so that 2.8 g of a yellow solid, which was the
object of the synthesis, was obtained in a yield of 94%. The
synthesis scheme of Step 3 is shown in (a-3).
##STR00060##
[0251] By a train sublimation method, 2.7 g of the obtained yellow
solid was purified. In the purification by sublimation, 2Cz2PDBq
was heated at 320.degree. C. under a pressure of 2.7 Pa with a flow
rate of argon gas of 5.0 mL/min. After the purification by
sublimation, 1.6 g of a white solid was obtained in a yield of
60%.
[0252] By analysis of the obtained white solid by .sup.1H NMR, the
solid was identified as 2Cz2PDBq. The results of the analysis are
shown below and FIGS. 10A and 10B are charts thereof. Note that
FIG. 10B is a chart where the range of from 7 ppm to 9.6 ppm in
FIG. 10A is enlarged.
[0253] .sup.1H NMR (CDCl.sub.3, 300 MHz): .delta.=7.36 (t, J=7.5
Hz, 4H), 7.50 (ddd, J=7.2, 0.9 Hz, 4H), 7.67-7.85 (m, 8H), 7.99 (t,
J=1.8 Hz, 1H), 8.21 (d, J=7.2 Hz, 4H), 8.64 (dd, J=2.4 Hz, 2H),
8.69 (d, J=1.8 Hz, 2H), 9.25 (dd, J=1.5, 7.8 Hz, 1H), 9.30 (dd,
J=1.5, 7.8 Hz, 1H), 9.48 (s, 1H)
[0254] Next, absorption and emission spectra of 2Cz2PDBq in a
toluene solution of 2Cz2PDBq are shown in FIG. 11A, and absorption
and emission spectra of a thin film of 2Cz2PDBq are shown in FIG.
11B. The absorption spectrum was measured with a UV-visible
spectrophotometer (V-550, manufactured by JASCO Corporation). A
toluene solution of 2Cz2PDBq was put in a quartz cell and an
absorption spectrum of 2Cz2PDBq in the toluene solution was
measured. From this absorption spectrum, an absorption spectrum of
the toluene solution measured with the quartz cell was subtracted,
and the resultant value was shown in the drawing. In addition, as
for the absorption spectrum of the thin film, a sample was prepared
by evaporation of 2Cz2PDBq on a quartz substrate, and the
absorption spectrum obtained by subtraction of an absorption
spectrum of quartz from the absorption spectrum of this sample is
shown in the drawing. The emission spectrum was measured with a
UV-visible spectrophotometer (V-550, manufactured by JASCO
Corporation) which was used for the measurement of the absorption
spectrum. The emission spectrum of 2Cz2PDBq in a toluene solution
of 2Cz2PDBq was measured with the toluene solution of 2Cz2PDBq put
in a quartz cell, and the emission spectrum of the thin film was
measured with a sample prepared by evaporation of 2Cz2PDBq on a
quartz substrate. These show that the maximum absorption
wavelengths of 2Cz2PDBq in the toluene solution of 2Cz2PDBq were
around 376 nm, around 338 nm, and around 292 nm and that the
maximum emission wavelengths thereof were around 427 nm (an
excitation wavelength of 376 nm). These also show that the maximum
absorption wavelengths of the thin film were around 384 nm, around
372 nm, around 341 nm, around 325 nm, around 311 nm, around 293 nm,
around 262 nm, around 243 nm, and 210 nm and that the largest
maximum emission wavelength thereof was around 442 nm (an
excitation wavelength of 383 nm).
[0255] Further, the ionization potential of a thin film of 2Cz2PDBq
was measured by a photoelectron spectrometer (AC-2, produced by
Riken Keiki, Co., Ltd.) in the air. The obtained value of the
ionization potential was converted to a negative value, so that the
HOMO level of 2Cz2PDBq was -5.89 eV. From the data of the
absorption spectra of the thin film in FIGS. 11A and 11B, the
absorption edge of 2Cz2PDBq, which was obtained from a Tauc plot
with an assumption of direct transition, was 3.06 eV. Therefore,
the optical energy gap of 2Cz2PDBq in the solid state was estimated
at 3.06 eV; from the values of the HOMO level obtained above and
this energy gap, the LUMO level of 2Cz2PDBq was able to be
estimated at -2.83 eV. It was thus found that 2Cz2PDBq had a wide
energy gap of 3.06 eV in the solid state. Accordingly, it can be
said that the band gap and the triplet excitation energy were
decreased because of the heterocyclic compound including an
aromatic hydrocarbon group between a dibenzo[f,h]quinoxaline ring
and two hole-transport skeletons.
Example 2
[0256] In this example, a light-emitting element in which the
heterocyclic compound described in Embodiment 1,
2-[3,5-bis(9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2Cz2PDBq) (the structural formula 100) is used as a
host material and an electron-transport material in a
light-emitting layer including an emission center substance
emitting orange phosphorescence. The heterocyclic compound is a
compound comprising a dibenzo[f,h]quinoxaline ring and two
hole-transport skeletons, where the dibenzo[f,h]quinoxaline ring
and the two hole-transport skeletons are bonded to an aromatic
hydrocarbon group.
[0257] The molecular structures of organic compounds used in this
example are shown in the following structural formulae (i) to (v)
and (100). An element structure is the same as that of FIG. 1A.
##STR00061## ##STR00062##
<<Fabrication of Light-Emitting Element 1>>
[0258] First, a glass substrate over which a film of indium tin
oxide containing silicon (ITSO) was formed to a thickness of 110 nm
was prepared as the first electrode 101. A surface of the ITSO film
was covered with a polyimide film such that an area of 2 mm.times.2
mm of the surface was exposed, which corresponded to the electrode
area. In pretreatment for forming the light-emitting elements over
the substrate, the surface of the substrate was washed with water
and baked at 200.degree. C. for one hour, and then a UV ozone
treatment was performed for 370 seconds. Then, the substrate was
transferred into a vacuum evaporation apparatus where the pressure
was reduced to about 10.sup.-4 Pa, vacuum baking at 170.degree. C.
for 30 minutes was performed in a heating chamber of the vacuum
evaporation apparatus, and then the substrate was cooled down for
about 30 minutes.
[0259] Then, the substrate was fixed to a holder provided in the
vacuum evaporation apparatus such that the surface of the substrate
over which the ITSO film was formed faced downward.
[0260] After the pressure in the vacuum evaporation apparatus was
reduced to 10.sup.-4 Pa, 1,3,5-tri(dibenzothiophen-4-yl)-benzene
(abbreviation: DBT3P-II) represented by the above structural
formula (i) and molybdenum(VI) oxide were co-evaporated so that the
weight ratio of DBT3P-II: molybdenum oxide was 2:1; thus, the
hole-injection layer 111 was formed. The thickness thereof was set
to 40 nm. Note that the co-evaporation is an evaporation method in
which a plurality of different substances is concurrently vaporized
from the respective different evaporation sources.
[0261] Next, 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine
(abbreviation: BPAFLP) represented by the above structural formula
(ii) was evaporated to a thickness of 20 nm, thereby forming the
hole-transport layer 112.
[0262] Over the hole-transport layer 112,
2-[3,5-bis(9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2Cz2PDBq) that is the heterocyclic compound
represented by the structural formula (100) and described in
Embodiment 1, 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl
(abbreviation: NPB) represented by the structural formula (iii),
and (Acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)
(abbreviation: [Ir(dppm).sub.2(acac)] represented by the structural
formula (iv) were evaporated so that the weight ratio of 2Cz2PDBq
to NPB and [Ir(nbppm).sub.2(acac)] was 0.8:0.2:0.05 to form the
light-emitting layer 113 with a thickness of 40 nm.
[0263] Next, 2Cz2PDBq was evaporated to a thickness of 15 nm, and
then bathophenanthroline (abbreviation: BPhen) represented by the
above structural formula (v) was evaporated to a thickness of 15
nm, so that the electron-transport layer 114 was formed.
[0264] Further, lithium fluoride was evaporated to a thickness of 1
nm over the electron-transport layer 114, so that the
electron-injection layer was formed. Lastly, an aluminum film was
formed to a thickness of 200 nm as the second electrode 102
functioning as a cathode. Accordingly, Light-emitting Element 1 was
completed. Note that in the above evaporation processes,
evaporation was all performed by a resistance heating method.
Light-emitting Element 1 corresponds to the light-emitting element
described in detail in Embodiment 2.
<<Operation Characteristics of Light-Emitting Element
1>>
[0265] Light-emitting Element 1 thus obtained was sealed in a glove
box under a nitrogen atmosphere without being exposed to the air.
Then, the operation characteristics of the light-emitting element
were measured. Note that the measurement was carried out at room
temperature (in an atmosphere kept at 25.degree. C.).
[0266] FIG. 12 shows current density-luminance characteristics of
Light-emitting Element 1, FIG. 13 shows voltage-luminance
characteristics thereof, and FIG. 14 shows luminance-current
efficiency characteristics thereof. In FIG. 12, the vertical axis
represents luminance (cd/m.sup.2), and the horizontal axis
represents current density (mA/cm.sup.2). In FIG. 13, the vertical
axis represents luminance (cd/m.sup.2), and the horizontal axis
represents voltage (V). In FIG. 14, the vertical axis represents
current efficiency (cd/A), and the horizontal axis represents
luminance (cd/m.sup.2).
[0267] FIG. 12 shows that Light-emitting Element 1 (the
light-emitting element described in detail in Embodiment 2) which
emits orange phosphorescence, in which 2Cz2PDBq that is the
heterocyclic compound described in Embodiment 1 is used as a host
material in a light-emitting layer, exhibits a faborable current
efficiency-luminance characteristics; therefore, Light-emitting
Element 1 is a light-emitting element having a high current
efficiency. Moreover, FIG. 13 shows that Light-emitting Element 1
exhibits favorable luminance-voltage characteristics and is a
light-emitting element having low driving voltage. This indicates
that the heterocyclic compound described in Embodiment 1 has an
excellent carrier-transport property. Therefore, a light-emitting
element including a compound comprising a dibenzo[f,h]quinoxaline
ring and two hole-transport skeletons, where the
dibenzo[f,h]quinoxaline ring and the two hole-transport skeletons
are bonded to an aromatic hydrocarbon group is a light-emitting
element having high emission efficiency. In addition, the
light-emitting element is a light-emitting element having low
driving voltage.
Example 3
Synthesis Example 2
[0268] In this example, a method of synthesizing the heterocyclic
compound described in Embodiment 1,
2-[3,5-bis(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2DBT2PDBq-II) represented by the following
structural formula (200) is described. The heterocyclic compound
can be expressed as a compound in which an aromatic hydrocarbon
group to which two hole-transport skeletons (dibenzothiophen
skeletons) are bonded is bonded to a dibenzo[f,h]quinoxaline
ring.
##STR00063##
Step 1: Synthesis of 3,5-bis(dibenzothiophen-4-yl)bromobenzene
[0269] Into a 50-L reactor vessel were put 1.33 kg (4.23 mol) of
tribromobenzene, 3.04 g (13.3 mol) of dibenzothiophene-4-boronic
acid, 38.6 g (127 mmol) of tris(2-methylphenyl)phosphine, 3.68 kg
(26.6 mol) of potassium carbonate, and 14.2 g (63.4 mmol) of
palladium(II) acetate. To the mixture were added 13.3 L of water,
17.5 L of ethanol, and 39.5 L of toluene, and this mixture was
degassed by being stirred under reduced pressure. After the
degassing, the mixture was stirred under a nitrogen stream at
105.degree. C. for three hours. After the stirring, the obtained
mixture was filtrated to remove a solid. The obtained filtrate was
concentrated to give a solid. This solid was washed with toluene
and then with ethanol, and recrystallized from toluene, so that
11.7 g of a light red-brown solid was obtained in a yield of 0.53%.
The synthesis scheme of Step 1 is shown in (b-1).
##STR00064##
[0270] By analysis of the obtained light red-brown solid by .sup.1H
NMR, the solid was identified as
3,5-bis(dibenzothiophen-4-yl)bromobenzene. The results of the
analysis are shown below.
[0271] .sup.1H NMR (CDCl.sub.3, 500 MHz): .delta.=7.47-7.51 (m,
4H), 7.57-7.62 (m, 4H), 7.85-7.88 (m, 2H), 7.97 (d, J1=1.5 Hz, 2H),
8.07 (t, J1=1.5 Hz, 1H), 8.19-8.23 (m, 4H)
Step 2: Synthesis of 3,5-bis(dibenzothiophen-4-yl)phenylboronic
acid
[0272] Into a 500-mL three-neck flask was put 5.2 g (10 mmol) of
3,5-bis(dibenzothiophen-4-yl)bromobenzene, and the air in the flask
was replaced with nitrogen. Into this flask was added 250 mL of
THF, and the obtained solution was cooled to -80.degree. C. To this
solution was dripped 8.0 mL (13 mmol) of n-butyllithium (1.6 mol/L
hexane solution). After completion of dripping, this solution was
stirred for three hours. After a predetermined time, 4.0 mL (6.5
mmol) of n-butyllithium (1.6 mol/L hexane solution) was dripped.
After completion of dripping, this solution was stirred for five
hours. After the stirring, to this solution was added 2.8 mL (25
mmol) of trimethyl borate, and the mixture was stirred overnight
while being returned to room temperature. After that, about 30 mL
of diluted hydrochloric acid (1.0 mol/L) was added to this
solution, followed by stirring for 30 minutes. Then, the aqueous
layer of this mixture was extracted with ethyl acetate, and the
extracted solution and an organic layer were combined and washed
with a saturated aqueous solution of sodium hydrogen carbonate and
then with saturated saline. The organic layer was dried with
magnesium sulfate, and then the mixture was gravity filtered. The
obtained filtrate was concentrated to give a solid. The obtained
solid was reprecipitated from an ethyl acetate/THF/hexane solution,
whereby 2.3 g of a white solid, which was the object of the
synthesis, was obtained in a yield of 47%. A synthetic scheme of
Step 2 is shown in (b-2).
##STR00065##
[0273] By analysis of the obtained white solid by .sup.1H NMR, the
solid was identified as 3,5-bis(dibenzothiophen-4-yl)phenylboronic
acid. The results of the analysis are shown below and FIGS. 15A and
15B are charts thereof. Note that FIG. 15B is a chart where the
range of from 4.5 ppm to 8.5 ppm in FIG. 15A is enlarged.
[0274] .sup.1H NMR (CDCl.sub.3, 500 MHz): .delta.=4.86 (s, 2H),
7.46-7.50 (m, 4H), 7.59-7.63 (m, 4H), 7.85-7.87 (m, 2H), 8.19-8.24
(m, 7H)
Step 3: Synthesis of
2-[3,5-bis(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2DBT2PDBq-II)
[0275] Into a 200-mL three-neck flask were put 1.2 g (4.5 mmol) of
2-chlorodibenzo[f,h]quinoxaline, 2.2 g (4.5 mmol) of
3,5-bis(dibenzothiophen-4-yl)phenylboronic acid, and 1.2 g (9.0
mmol) of potassium carbonate. To this mixture were added 23 mL of
toluene, 4.5 mL of water, and 5 mL of ethanol, and this mixture was
degassed by being stirred under reduced pressure. To the mixture
was added 104 mg (0.09 mmol) of
tetrakis(triphenylphosphine)palladium(0), and this mixture was
stirred under a nitrogen stream at 100.degree. C. for 3.5 hours.
After the stirring, the obtained mixture was filtrated, and the
solid was washed with water and then with ethanol. To the obtained
solid was added toluene, and the mixture was suction-filtered
through Celite, alumina, and Florisil. The filtrate was
concentrated to give a solid. This solid was recrystallized from
toluene, so that 1.4 g of a white solid was obtained in a yield of
46%. By a train sublimation method, 1.3 g of the obtained solid was
purified. In the purification by sublimation, 2DBT2PDBq-II was
heated at 320.degree. C. under a pressure of 2.8 Pa with a flow
rate of argon gas of 5 mL/min. After the purification by
sublimation, 0.94 g of a white solid was obtained at a collection
rate of 72%. The synthesis scheme of Step 3 is shown in (b-3).
##STR00066##
[0276] By analysis of the obtained white solid by .sup.1H NMR, the
solid was identified as 2DBT2PDBq-II. The results of the analysis
are shown below and FIGS. 16A and 16B are NMR charts thereof. Note
that FIG. 16B is a chart where the range of from 7 ppm to 10 ppm in
FIG. 16A is enlarged.
[0277] .sup.1H NMR (CDCl.sub.3, 500 MHz): .delta.=7.49-7.54 (m,
4H), 7.69 (t, J1=7.5 Hz, 2H), 7.73-7.84 (m, 6H), 7.88-7.92 (m, 2H),
8.25-8.28 (m, 5H), 8.67 (dd, J1=7.5 Hz, J2=3.5 Hz, 2H), 8.88 (d,
J1=1.5 Hz, 2H), 9.28 (dd, J1=1.0 Hz, J2=8.0 Hz, 1H), 9.47 (dd,
J1=1.0 Hz, J2=8.0 Hz, 1H), 9.60 (s, 1H)
[0278] Next, absorption and emission spectra of 2DBT2PDBq-II in a
toluene solution of 2DBT2PDBq-II are shown in FIG. 17A, and
absorption and emission spectra of a thin film of 2DBT2PDBq-II are
shown in FIG. 17B. The absorption spectrum was measured with a
UV-visible spectrophotometer (V-550, manufactured by JASCO
Corporation). A toluene solution of 2DBT2PDBq-II was put in a
quartz cell and an absorption spectrum of 2DBT2PDBq-II in the
toluene solution was measured. From this absorption spectrum, an
absorption spectrum of the toluene solution measured with the
quartz cell was subtracted, and the resultant value was shown in
the drawing. In addition, as for the absorption spectrum of the
thin film, a sample was prepared by evaporation of 2DBT2PDBq-II on
a quartz substrate, and the absorption spectrum obtained by
subtraction of an absorption spectrum of quartz from the absorption
spectrum of this sample is shown in the drawing. The emission
spectrum was measured with a UV-visible spectrophotometer (V-550,
manufactured by JASCO Corporation) which was used for the
measurement of the absorption spectrum. The emission spectrum of
2DBT2PDBq-II in a toluene solution of 2DBT2PDBq-II was measured
with the toluene solution of 2DBT2PDBq-II put in a quartz cell, and
the emission spectrum of the thin film was measured with a sample
prepared by evaporation of 2DBT2PDBq-II on a quartz substrate.
These show that the maximum absorption wavelengths of 2DBT2PDBq-II
in the toluene solution of 2DBT2PDBq-II were around 373 nm and
around 281 nm and that the maximum emission wavelengths thereof
were around 389 nm and around 410 nm (an excitation wavelength of
363 nm). These also show that the maximum absorption wavelengths of
the thin film were around 386 nm, around 370 nm, around 339 nm,
around 316 nm, around 292 nm, around 265 nm, and around 246 nm and
that the largest maximum emission wavelength thereof was around 428
nm (an excitation wavelength of 382 nm).
[0279] Further, the ionization potential of a thin film of
2DBT2PDBq-II was measured by a photoelectron spectrometer (AC-2,
produced by Riken Keiki, Co., Ltd.) in the air. The obtained value
of the ionization potential was converted to a negative value, so
that the HOMO level of 2DBT2PDBq-II was -6.37 eV. From the data of
the absorption spectra of the thin film in FIG. 17B, the absorption
edge of 2DBT2PDBq-II, which was obtained from a Tauc plot with an
assumption of direct transition, was 3.07 eV. Therefore, the
optical energy gap of 2DBT2PDBq-II in the solid state was estimated
at 3.07 eV; from the values of the HOMO level obtained above and
this energy gap, the LUMO level of 2DBT2PDBq-II was able to be
estimated at -3.30 eV. It was thus found that 2DBT2PDBq-II had a
wide energy gap of 3.07 eV in the solid state. Accordingly, it can
be said that the band gap and the triplet excitation energy were
decreased because of the heterocyclic compound including an
aromatic hydrocarbon group between a dibenzo[f,h]quinoxaline ring
and two hole-transport skeletons.
[0280] Further, 2DBT2PDBq-II was analyzed by liquid chromatography
mass spectrometry (abbreviation: LC/MS).
[0281] In the analysis by LC/MS, liquid chromatography (LC)
separation was carried out with Acquity UPLC (manufactured by
Waters Corporation), and mass spectrometry (MS) analysis was
carried out with Xevo G2 Tof MS (manufactured by Waters
Corporation). An Acquity UPLC BEH C8 column (2.1 mm.times.100 mm,
1.7 m) was used in the LC separation, where a column temperature
was 40.degree. C. Acetonitrile was used for Mobile Phase A and 0.1%
of a formic acid solution was used for Mobile Phase B. A sample was
prepared in such a manner that 2DBT2PDBq-II was dissolved in
chloroform at a given concentration and the mixture was diluted
with acetonitrile. An injection amount was 5.0 L.
[0282] In the LC separation, a gradient method, in which
composition of mobile phases is changed, was employed; the
composition ratio of Mobile Phase A to Mobile Phase B was 75 to 25
for 0 minutes to 1 minute after the measurement started, and then
the composition was changed, so that the ratio of Mobile Phase A to
Mobile Phase B in the 10th minute was 95 to 5. The composition
ratio was changed linearly.
[0283] In the MS analysis, ionization was carried out by an
electrospray ionization (ESI) method. Capillary voltage and sample
cone voltage were set to 3.0 kV and 30 V, respectively. Detection
was performed in a positive mode. A mass range for the measurement
was m/z=100 to 1200.
[0284] A component with m/z of 670.15 which underwent the
separation and the ionization under the above-mentioned conditions
was collided with an argon gas in a collision cell to dissociate
into product ions. Energy (collision energy) for the collision with
argon was 50 eV and 70 eV. The results of MS analysis of the
dissociated product ions by time-of-flight (TOF) mass spectrometry
are shown in FIGS. 18A and 18B and FIGS. 19A and 19B. FIGS. 18A and
18B show the results when the collision energy is 50 eV. FIGS. 19A
and 19B show the results when the collision energy is 70 eV. Note
that FIG. 18B is a graph where the m/z range in FIG. 18A is
enlarged and FIG. 19B is a graph where the m/z range in FIG. 19A is
enlarged.
[0285] The results in FIGS. 18A and 18B show that product ions of
partial skeletons of 2DBT2PDBq-II are detected around m/z=644.
Furthermore, the results in FIGS. 19A and 19B show that product
ions of partial skeletons of 2DBT2PDBq-II are detected around
m/z=229, m/z=202, m/z=177, and m/z=165.
[0286] The peak around m/z=644 is assumed to be a peak of a cation
in a state where one C atom and one N atom are detached from
2DBT2PDBq-II. This peak is characteristic of a compound including a
dibenzo[f,h]quinoxaline skeleton having a substituent at the
2-position, which is one feature of the heterocyclic compound of
one embodiment of the present invention.
[0287] The peak around m/z=229 is assumed to be a peak of a cation
of a diazatriphenylenyl group. Moreover, the peaks around m/z=202,
m/z=177, and m/z=165 are also detected; therefore, the
diazatriphenylenyl group is assumed to be a product ion derived
from dibenzo[f,h]quinoxaline. These peaks are characteristic of a
compound including a dibenzo[f,h]quinoxaline skeleton.
[0288] The above results indicate that 2DBT2PDBq-II, which is the
heterocyclic compound of one embodiment of the present invention,
includes a dibenzo[f,h]quinoxaline ring having a substituent at the
2-position.
[0289] In FIG. 19B, peaks relatively markedly appears around
m/z=453 and around m/z=442. The peak around m/z=453 corresponds to
a peak of a product ion composed of two dibenzothiophenyl groups, a
benzene skeleton, and a carbon. The product ion is probably a
phenyl group having two dibenzothiophenyl groups which removes from
2DBT2PDBq-II accompanied by a carbon at the 2-position. The peak
around m/z=442 corresponds to a peak of a product ion composed of
two dibenzothiophenyl groups and a benzene skeleton.
[0290] The peak around m/z=220 corresponds to a peak of a product
ion of dibenzo[f,h]quinoxaline from which a carbon is removed. The
product ion is probably dibenzo[f,h]quinoxaline skeleton of
2DBT2PDBq-II from which a phenyl group having two dibenzothiophenyl
groups accompanied with a carbon at the 2-position is removed.
Example 4
[0291] In this example, a light-emitting element (Light-emitting
Element 2) in which
2-[3,5-bis(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline
(abbreviation: 2DBT2PDBq-II) (the structural formula 200) which is
the heterocyclic compound described in Embodiment 1 and synthesized
in Example 3, is used as a host material and an electron-transport
material in a light-emitting layer including an emission center
substance emitting yellow-green phosphorescence. The heterocyclic
compound is a compound in which an aromatic hydrocarbon group to
which two hole-transport skeletons (dibenzothiophenyl groups) are
bonded is bonded to a dibenzo[f,h]quinoxaline ring.
[0292] The molecular structures of organic compounds used in this
example are shown in the following structural formulae (i), (ii),
(v) to (vii) and (200). An element structure is the same as that of
FIG. 1A.
##STR00067## ##STR00068##
<<Fabrication of Light-Emitting Element 2>>
[0293] First, a glass substrate over which a film of indium tin
oxide containing silicon (ITSO) was formed to a thickness of 110 nm
was prepared as the first electrode 101. A surface of the ITSO film
was covered with a polyimide film such that an area of 2 mm.times.2
mm of the surface was exposed, which corresponded to the electrode
area. In pretreatment for forming the light-emitting elements over
the substrate, the surface of the substrate was washed with water
and baked at 200.degree. C. for one hour, and then a UV ozone
treatment was performed for 370 seconds. Then, the substrate was
transferred into a vacuum evaporation apparatus where the pressure
was reduced to about 10.sup.-4 Pa, vacuum baking at 170.degree. C.
for 30 minutes was performed in a heating chamber of the vacuum
evaporation apparatus, and then the substrate was cooled down for
about 30 minutes.
[0294] Then, the substrate was fixed to a holder provided in the
vacuum evaporation apparatus such that the surface of the substrate
over which the ITSO film was formed faced downward.
[0295] After the pressure in the vacuum evaporation apparatus was
reduced to 10.sup.-4 Pa, 1,3,5-tri(dibenzothiophen-4-yl)-benzene
(abbreviation: DBT3P-II) represented by the above structural
formula (i) and molybdenum(VI) oxide were co-evaporated so that the
weight ratio of DBT3P-II: molybdenum oxide was 2:1; thus, the
hole-injection layer 111 was formed. The thickness thereof was set
to 20 nm. Note that the co-evaporation is an evaporation method in
which a plurality of different substances is concurrently vaporized
from the respective different evaporation sources.
[0296] Next, 4-phenyl-4'-(9-phenylfluoren-9-yl)triphenylamine
(abbreviation: BPAFLP) represented by the above structural formula
(ii) was evaporated to a thickness of 20 nm, thereby forming the
hole-transport layer 112.
[0297] Furthermore, 2DBT2PDBq-II represented by the structural
formula (200),
4,4'-di(1-naphthyl)-4''-(9-phenyl-9H-carbazol-3-yl)-triphenylamine
(abbreviation: PCBNBB) represented by the structural formula (vi),
and
bis[2-(6-tert-butyl-4-pyrimidinyl-.kappa.N3)phenyl-.kappa.C](2,4-pentaned-
ionato-.kappa..sup.2O,O')iridium(III) (abbreviation:
[Ir(tBuppm).sub.2(acac)]) represented by the structural formula
(vii) are deposited over the hole-transport layer 112 to a
thickness of 20 nm so that the weight ratio of 2DBT2PDBq-II to
PCBNBB and [Ir(tBuppm)2(acac)] is 0.7:0.3:0.05, and then
evaporation was performed to a thickness of 20 nm so that the
weight ratio of 2DBT2PDBq-II to PCBNBB and [Ir(tBuppm)2(acac)] is
0.8:0.2:0.05, thereby forming the light-emitting layer 113.
[0298] Next, 2DBT2PDBq-II was evaporated to a thickness of 5 nm,
and then bathophenanthroline (abbreviation: BPhen) represented by
the above structural formula (v) was evaporated to a thickness of
20 nm, so that the electron-transport layer 114 was formed.
[0299] Further, lithium fluoride was evaporated to a thickness of 1
nm over the electron-transport layer 114, so that the
electron-injection layer was formed. Lastly, an aluminum film was
formed to a thickness of 200 nm as the second electrode 102
functioning as a cathode. Accordingly, Light-emitting Element 2 was
completed. Note that in the above evaporation processes,
evaporation was all performed by a resistance heating method.
Light-emitting Element 2 corresponds to the light-emitting element
described in detail in Embodiment 2.
<<Operation Characteristics of Light-Emitting Element
2>>
[0300] Light-emitting Element 2 thus obtained was sealed in a glove
box under a nitrogen atmosphere without being exposed to the air
(specifically, a sealing material was applied onto an outer edge
and heat treatment was performed at 80.degree. C. for one hour at
the time of sealing). Then, the operation characteristics of the
light-emitting element were measured. Note that the measurement was
carried out at room temperature (in an atmosphere kept at
25.degree. C.).
[0301] FIG. 20 shows current density-luminance characteristics of
Light-emitting Element 2, FIG. 21 shows voltage-luminance
characteristics thereof, FIG. 22 shows luminance-current efficiency
characteristics thereof, and FIG. 23 shows an emission spectrum
thereof.
[0302] FIG. 22 shows that Light-emitting Element 2 exhibits
favorable luminance-current efficiency characteristics; thus, the
element has high emission efficiency. Moreover, FIG. 21 shows that
Light-emitting Element 2 exhibits favorable voltage-luminance
characteristics and is a light-emitting element having low driving
voltage. This indicates that the heterocyclic compound described in
Embodiment 1 has an excellent carrier-transport property.
Therefore, a light-emitting element including a compound comprising
a dibenzo[f,h]quinoxaline ring and two hole-transport skeletons,
where the dibenzo[f,h]quinoxaline ring and the two hole-transport
skeletons are bonded to an aromatic hydrocarbon group is a
light-emitting element having high emission efficiency. In
addition, the light-emitting element is a light-emitting element
having low driving voltage.
Reference Example 1
[0303] An example of synthesizing an organometallic complex,
bis[2-(6-tert-butyl-4-pyrimidinyl-N3)phenyl-.kappa.C](2,4-pentanedionato--
.kappa..sup.2O,O')iridium(III) (abbreviation:
[Ir(tBuppm).sub.2(acac)]), which is used in the above Example. A
structure of [Ir(tBuppm).sub.2(acac)] is shown below.
##STR00069##
Step 1: Synthesis of 4-tert-butyl-6-phenylpyrimidine (abbreviation:
HtBuppm)
[0304] First, into a recovery flask equipped with a reflux pipe
were put 22.5 g of 4,4-dimethyl-1-phenylpentane-1,3-dione and 50 g
of formamide, and the air in the flask was replaced with nitrogen.
This reaction container was heated, so that the reacted solution
was refluxed for five hours. After that, this solution was poured
into aqueous sodium hydroxide, and an organic layer was extracted
with dichloromethane. The obtained organic layer was washed with
water and saturated saline, and dried with magnesium sulfate. The
solution which had been dried was filtered. The solvent of this
solution was distilled off, and then the obtained residue was
purified by silica gel column chromatography using hexane and ethyl
acetate as a developing solvent in a volume ratio of 10:1, so that
a pyrimidine derivative HtBuppm (colorless oily substance, yield of
14%) was obtained. A scheme of the synthesis of Step 1 is shown
below.
##STR00070##
(Step 2: Synthesis of
di-.mu.-chloro-bis[bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)]
(abbreviation: [Ir(tBuppm).sub.2Cl].sub.2))
[0305] Next, into a recovery flask equipped with a reflux pipe were
put 15 mL of 2-ethoxyethanol, 5 mL of water, 1.49 g of HtBuppm
obtained in Step 1, and 1.04 g of iridium chloride hydrate
(IrCl.sub.3.times.H.sub.2O), and the air in the flask was replaced
with argon. After that, irradiation with microwaves (2.45 GHz, 100
W) was performed for one hour to cause a reaction. The solvent was
distilled off, and then the obtained residue was suction-filtered
with ethanol and washed to give a dinuclear complex
[Ir(tBuppm).sub.2Cl].sub.2 (yellow-green powder, yield of 73%). A
synthesis scheme of Step 2 is shown below.
##STR00071##
Step 3: Synthesis of
bis[2-(6-tert-butyl-4-pyrimidinyl-.kappa.N3)phenyl-.kappa.C](2,4-pentaned-
ionato-.kappa..sup.2O,O')iridium(III) (abbreviation:
[Ir(tBuppm).sub.2(acac)])
[0306] Furthermore, into a recovery flask equipped with a reflux
pipe were put 40 mL of 2-ethoxyethanol, 1.61 g of the dinuclear
complex [Ir(tBuppm).sub.2Cl].sub.2 obtained in Step 2, 0.36 g of
acetylacetone, and 1.27 g of sodium carbonate, and the air in the
flask was replaced with argon. After that, irradiation with
microwaves (2.45 GHz, 120 W) was performed for one hour to cause a
reaction. The solvent was distilled off, and the obtained residue
was suction-filtered with ethanol and washed with water and then
with ethanol. This solid was dissolved in dichloromethane, and the
mixture was filtered through a filter aid in which Celite (produced
by Wako Pure Chemical Industries, Ltd., Catalog No. 531-16855),
alumina, and Celite were stacked in this order. The solvent was
distilled off, and the obtained solid was recrystallized from a
mixed solvent of dichloromethane and hexane, so that yellow powder
was obtained as a target substance (yield of 68%). A synthesis
scheme of Step 3 is shown below.
##STR00072##
[0307] The yellow powder obtained in Step 3 was measured by nuclear
magnetic resonance spectrometry (.sup.1H-NMR). The measurement data
are shown below. These results revealed that the organometallic
complex [Ir(tBuppm).sub.2(acac)] was obtained. These results show
that the organometallic complex [Ir(tBuppm).sub.2(acac)] was
obtained.
[0308] .sup.1H NMR. .delta. (CDCl.sub.3): 1.50 (s, 18H), 1.79 (s,
6H), 5.26 (s, 1H), 6.33 (d, 2H), 6.77 (t, 2H), 6.85 (t, 2H), 7.70
(d, 2H), 7.76 (s, 2H), 9.02 (s, 2H)
[0309] This application is based on Japanese Patent Application
serial no. 2011-187669 filed with Japan Patent Office on Aug. 30,
2011, the entire contents of which are hereby incorporated by
reference.
* * * * *