U.S. patent application number 10/347378 was filed with the patent office on 2003-09-11 for luminescent device.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Hashimoto, Yuichi, Senoo, Akihiro, Toshida, Yomishi, Ueno, Kazunori.
Application Number | 20030168971 10/347378 |
Document ID | / |
Family ID | 15327671 |
Filed Date | 2003-09-11 |
United States Patent
Application |
20030168971 |
Kind Code |
A1 |
Hashimoto, Yuichi ; et
al. |
September 11, 2003 |
Luminescent device
Abstract
A luminescent device in accordance with the present invention
includes an anode, a cathode, and an organic layer containing at
least one organic compound provided therebetween. The work function
of the anode (Wf(anode)) and the Fermi level of the organic layer
(Ef(anode)) being in contact with the anode satisfies the following
equation (I):
Ef(anode)-0.2.ltoreq.Wf(anode).ltoreq.Ef(anode)+0.2[eV] (I)
Inventors: |
Hashimoto, Yuichi; (Tokyo,
JP) ; Senoo, Akihiro; (Kanagawa, JP) ;
Toshida, Yomishi; (Kanagawa, JP) ; Ueno,
Kazunori; (Kanagawa, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
CANON KABUSHIKI KAISHA
TOKYO
JP
|
Family ID: |
15327671 |
Appl. No.: |
10/347378 |
Filed: |
January 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10347378 |
Jan 21, 2003 |
|
|
|
09080356 |
May 18, 1998 |
|
|
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Current U.S.
Class: |
313/504 |
Current CPC
Class: |
H01L 51/5206 20130101;
H01L 51/5004 20130101; Y10T 428/24942 20150115; Y10S 428/917
20130101; H05B 33/12 20130101; H05B 33/26 20130101; H05B 33/14
20130101; H01L 51/5221 20130101 |
Class at
Publication: |
313/504 |
International
Class: |
H05B 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 19, 1997 |
JP |
1997/142960 |
Claims
What is claimed is:
1. A luminescent device comprising an anode, a cathode, and an
organic layer containing at least one organic compound provided
therebetween, wherein the work function of the anode (Wf(anode))
and the Fermi level of the organic layer (Ef(anode)) being in
contact with the anode satisfies the following equation (I):
Ef(anode)-0.2.ltoreq.Wf(anode).ltoreq.Ef(anod- e)+0.2[eV] (I)
2. A luminescent device according to claim 1, wherein the work
function of the cathode (Wf(cathode)) and the Fermi level of the
organic layer (Ef(cathode)) being in contact with the cathode
satisfies the following equation (II):
Wf(cathode).ltoreq.Ef(cathode)[eV] (II)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to luminescent devices which
have a luminescent layer containing a luminescent material and
directly convert applied electrical energy into optical energy. In
particular, the present invention relates to a thin, light and
solid luminescent device with a large luminescent area and high
resolution, enabling high-speed operation. Such a luminescent
device is quite different from conventional incandescent lamps,
fluorescent lamps, and light emitting diodes (LEDs), and can be
used for electroluminescence panels which are expected as devices
satisfying advanced needs in industrial fields.
[0003] 2. Description of the Related Art
[0004] Pope et al., first discovered an electroluminescence (EL) of
an organic material, that is, single-crystal anthracene in 1963 (J.
Chem. Phys., 38, 2042 (1963)). Helfinch and Schneider succeeded
with observation of relatively strong EL in an injection EL
material containing a solution system having a high injection
efficiency in 1965 (Phys. Rev. Lett., 14, 229 (1965)). Many studies
of organic luminescent materials containing conjugated organic
hosts and conjugated organic activators having condensed benzene
rings have been disclosed in U.S. Pat. Nos. 3,172,862, 3,173,050,
and 3,710,167; J. Chem. Phys., 44, 2902 (1966); J. Chem. Phys., 58,
1542 (1973); and Chem. Phys. Lett., 36, 345 (1975). Examples of
disclosed organic hosts include naphthalene, anthracene,
phenanthrene, tetracene, pyrene, benzpyrene, chrysene, picene,
carbazole, fluorene, biphenyl, terphenyl, triphenylene oxide,
dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene. Examples
of disclosed activators include anthracene, tetracene and
pentacene. Since these organic luminescent materials are provided
as single layers having a thickness of more than 1 .mu.m, a high
electric field is required for luminescence. Under in the
circumferences, thin film devices formed by a vacuum deposition
process have been proposed (for example, "Thin Solid Films" p. 94
(1982); Polymer, 24, 748 (1983); and J. Appl. Phys., 25, L773
(1986)). Although the thin film devices are effective for reducing
the driving voltage, their luminance is far from a level for
practical use.
[0005] In recent years, Tang et al. has developed an EL device
having a high luminance for a low driving voltage (Appl. Phys.
Lett., 51, 913 (1987) and U.S. Pat. No. 4,356,429). The EL device
is fabricated by depositing two significantly thin layers, that is,
a charge transport layer and a luminescent layer, between the anode
and the cathode by a vacuum deposition process. Such layered
organic EL devices are disclosed in, for example, Japanese Patent
Laid-Open Nos. 59-194393, 3-264692, and 3-163188, U.S. Pat. Nos.
4,539,507 and 4,720,432, and Appl. Phys. Lett., 55, 1467
(1989).
[0006] Also, an EL device of a triple-layered-structure having
independently a carrier transport function and a luminescent
ability was disclosed in Jpn. J. Apply. Phys., 27, L269 and L713
(1988). Since the carrier transportability is improved in such an
EL device, the versatility of possible dyes in the luminescent
layer is considerably increased. Further, the device configuration
suggests feasibility of improved luminescence by effectively
trapping holes and electrons (or excimers) in the central
luminescent layer.
[0007] Layered organic EL devices are generally formed by vacuum
deposition processes. EL devices having considerable luminance are
also formed by casting processes (as described in, for example,
Extended Abstracts (The 50th Autumn Meeting (1989), p. 1006 and The
51st Autumn Meeting (1990), p. 1041; The Japan Society of Applied
Physics). Considerably high luminance is also achieved by a
single-layered mixture-type EL device, in which the layer is formed
by immersion-coating a solution containing polyvinyl carbazole as a
hole transport compound, an oxadiazole derivative as an electron
transport compound and coumarin-6 as a luminescent material (as
described in Extended Abstracts (The 38th Spring Meeting (1991), p.
1086; The Japan Society of Applied Physics and Related
Societies).
[0008] As described above, the organic EL devices have been
significantly improved and have suggested feasibility of a wide
variety of applications; however, these EL devices have some
problems for practical use, for example, insufficient luminance, a
change in luminance during use for a long period, and deterioration
by atmospheric gas containing oxygen and humidity. Further, it is
hard to say that the EL devices sufficiently satisfy needs of
diverse wavelengths of luminescent light for precisely determining
luminescent hues of blue, green and red colors in full-color
displays etc.
SUMMARY OF THE INVENTION
[0009] It is an object of the present invention to provide a
luminescent device having high output luminance for solving the
above-mentioned problems.
[0010] It is another object of the present invention to provide a
luminescent device having excellent durability.
[0011] It is a further object of the present invention to provide a
luminescent device easily produced at relatively low production
costs.
[0012] A luminescent device in accordance with the present
invention includes an anode, a cathode, and an organic layer
containing at least one organic compound provided therebetween,
wherein the work function of the anode Wf(anode) and the Fermi
level of the organic layer (Ef(anode)) being in contact with the
anode satisfies the following equation (I):
Ef(anode)-0.2.ltoreq.Wf(anode).ltoreq.Ef(anode)+0.2[eV] (I)
[0013] Preferably, the work function of the cathode (Wf(cathode))
and the Fermi level of the organic layer (Ef(cathode)) being in
contact with the cathode satisfy the following equation (II):
Wf(cathode).ltoreq.Ef(cathode)[eV] (II)
[0014] The luminescent device satisfying the equation (I) or (II)
can emerge light with significantly high luminance by a low applied
voltage and has excellent durability.
[0015] A luminescent device with a large area can be easily formed
by a vacuum deposition or casting process with low production
costs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a cross-sectional view of an embodiment of a
luminescent device in accordance with the present invention;
[0017] FIG. 2 is a cross-sectional view of another embodiment of a
luminescent device in accordance with the present invention;
[0018] FIG. 3 is a cross-sectional view of a further embodiment of
a luminescent device in accordance with the present invention;
[0019] FIG. 4 is a cross-sectional view of still another embodiment
of a luminescent device in accordance with the present invention;
and
[0020] FIG. 5 is a graph illustrating changes in accumulated charge
and intensity of liminescent light with the work function of the
metal in the anode in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] The present inventors have studied intensively towards the
resolution of the above-mentioned problems, and have discovered a
preferable luminescent device including an anode, a cathode, and an
organic layer containing at least one organic compound provided
therebetween, the work function of the anode (Wf(anode)) and the
Fermi level of the organic layer (Ef(anode)) being in contact with
the anode satisfying the following equation (I) and more preferably
the equation (II):
Ef(anode)-0.2.ltoreq.Wf(anode).ltoreq.Ef(anode)+0.2[eV] (I)
Wf(cathode).ltoreq.Ef(cathode)[eV] (II)
[0022] The present invention is completed under such findings.
[0023] In the present invention, the work functions of the anode
and the cathode, Wf (anode) and Wf(cathode), respectively, are
determined from thresholds of photoelectron emission with a surface
analyzer AC-1 made by Riken Keiki Co., Ltd. The Fermi levels of the
organic layer Ef(anode) and Ef(cathode) are determined by a contact
potential method (Kelvin method) with a Fermi level meter FAC-1
made by Riken Keiki Co., Ltd. These measurements are performed in
an atmospheric condition at a temperature of 20.degree. C. and a
humidity of 40%.
[0024] The luminescent device in accordance with the present
invention will now be described in detail with reference to the
drawings.
[0025] FIG. 1 is a schematic cross-sectional view of an embodiment
of the luminescent device in accordance with the present invention.
An anode 2, a luminescent layer 3 and a cathode 4 are formed on a
substrate 1 in that order. In such a configuration, a usable
luminescent layer 3 is generally composed of a single compound
having hole transportability, electron transportability and
luminescence, or a mixture of compounds each having one of these
properties.
[0026] FIG. 2 is a schematic cross-sectional view of another
embodiment of the luminescent device in accordance with the present
invention. An anode 2, a hole transport layer 5, an electron
transport layer 6 and a cathode 4 are formed on a substrate 1 in
that order. The hole transport layer 5 and the electron transport
layer 6 function as a luminescent layer 3. In such a configuration,
a usable hole transport layer 5 is generally composed of a
luminescent material having hole transportability or a mixture
including such a material and a non-luminescent material having
hole transportability. The luminescent and non-luminescent
materials may also have electron transportability. The electron
transport layer 6 may be composed of a luminescent material having
electron transportability or a mixture including such a material
and a non-luminescent material having electron transportability.
The luminescent and non-luminescent materials may also have hole
transportability.
[0027] FIG. 3 is a schematic cross-sectional view of a further
embodiment of the luminescent device in accordance with the present
invention. An anode 2, a hole transport layer 5, a luminescent
layer 3, an electron transport layer 6 and a cathode 4 are formed
on a substrate 1 in that order. FIG. 4 is a schematic
cross-sectional view of a still further embodiment of the
luminescent device in accordance with the present invention. An
anode 2, a luminescent layer 3, an electron transport layer 6, and
a cathode 4 are formed on a substrate 1 in that order. In these
configurations, carrier transport and luminescence are performed in
the individual layers. Such configurations permit a wide variety of
combinations of a material having excellent hole transportability,
a material having excellent electron transportability and a
material having excellent luminescence. Further, the configurations
permit the use of various compounds emitting light with different
wavelengths; hence the hue of the luminescent light can be
controlled within a wide range. Trapping effectively holes and
electrons (or excimers) in the central luminescent layer will
increase the luminescent efficiency.
[0028] The luminescent device in accordance with the present
invention has excellent hole injection and electron injection
characteristics compared with conventional luminescent devices, and
can have the configurations shown in FIGS. 1 to 4.
[0029] Organic EL devices generally belong to carrier injection
luminescent devices and their luminance greatly depends on the
number of carriers, that is holes or electrons, injected from the
relevant electrode. It is preferable that the whole number of the
carriers injected from the anode or cathode be constant in use for
a long time. The existing devices, however, deteriorate and cause
imperfect electrical or physical matching between the electrodes
and the relevant layer(s) in use for a long time. As a result, the
output luminance significantly decreases because of a reduced
number of injected carriers in the device.
[0030] The luminescent device satisfying the equation (I) and
preferably also satisfying the equation (II) in accordance with the
present invention shows an optimized electrical matching between
the electrodes and the adjoining organic layer. A significantly
large number of carriers are therefore injected through the
electrodes even when in use for a long time, and a reduction in
current flow through the device can be suppressed as much as
possible after long-term use.
[0031] The number of carriers injected from the electrodes of a
series of luminescent devices in accordance with the present
invention is observed as follows. These luminescent devices have
the same layer configuration as in FIG. 4. Each cathode is composed
of aluminum. The luminescent layer is composed of Alq.sub.3
represented by the following formula: 1
[0032] Each hole transport layer is composed of the compound
represented by the following formula (a) and a polycarbonate resin
(1:1 mixture by weight ratio): 2
[0033] Each anode is composed of any one of Al, Fe, Pd, Ag, Au, Ni
and Cu, which have different work functions. The anode was formed
so that it was translucent. The number of holes injected from the
anode to the hole transfer layer and the intensity of the
luminescent light were determined. The results are shown in the
graph of FIG. 5. The horizontal axis of the graph represents the
work function of the metal used in the anode. The work function was
determined using a surface analyzer AC-1 made by Riken Keiki Co.,
Ltd. The accumulated charge by injected carriers at the vertical
axis was determined with a spatial charge distribution analyzer
PEANUTS made by Five Labs. Inc. The Fermi level of the hole
transport layer determined with a Fermi level meter FAC-1 made by
Riken Keiki Co., Ltd. was 4.52 eV.
[0034] FIG. 5 demonstrates that an anode having a work function
within a range of .+-.0.2 eV from the Fermi level (4.52 eV) of the
hole transport layer has an accumulated charge (a number of the
injected holes) which is approximately 70% of the maximum value.
The intensity of the luminescent light depends on the number of the
injected holes. It is preferable that the anode has a work function
within a range of .+-.0.1 eV from the Fermi level of the hole
transport layer in order to achieve a larger intensity of the
luminescent light. Since in such a luminescent device a large
number of carriers are injected, the luminescent light has a large
intensity and the decrease in current flow in the device is not
remarkable after long-term use.
[0035] A slightly different phenomenon was observed between the
electron transport layer and the cathode. A window accepting
electrons from the cathode widely spreads near the Fermi level of
the organic compound having electron transportability, hence the
number of the injected carriers (electrons) and thus the
luminescent intensity increase as the work function of the cathode
becomes lower than the Fermi level of the electron transport
layer.
[0036] As components of the luminescent layer in the luminescent
device in accordance with the present invention, hole transport
materials studied in the field of electrophotographic
photosensitive members and known luminescent hole transport
materials as shown in Tables 1 to 4 or electron transport materials
and known luminescent electron transport materials as shown in
Table 5 to 6 can be used. These materials are used alone or in
combination.
1TABLE 1 Hole Transport Compounds 3 4 5 6
[0037]
2TABLE 2 Hole Transport Compounds 7 8 9 10
[0038]
3TABLE 3 Hole Transport Compounds 11 12
[0039]
4TABLE 4 Luminescent Hole Transparent Compounds 13 14 15 16
[0040]
5TABLE 5 Electron Transport Compounds 17 18 19
[0041]
6TABLE 6 Luminescent Electron Transport Compounds 20 21 22 23 24 25
26 27
[0042] In the luminescent device in accordance with the present
invention, the luminescent layer is generally formed by a vacuum
deposition process or using a binding resin.
[0043] Non-limiting examples of the binding resins include
polyvinyl carbazole resins, polycarbonate resins, polyester resins,
polyarylate resins, butyral resins, polystyrene resins, polyvinyl
acetal resins, diallyl phthalate resins, acrylate resins,
methacrylate resins, phenol resins, epoxy resins, silicon resins,
polysulfone resins, and urea resins. These binding resins can be
used alone or in combination.
[0044] Preferable anode materials have large work functions.
Examples of such materials include nickel, gold, platinum,
palladium, selenium, rhenium, and iridium; alloys thereof; and tin
oxide, indium tin oxide, and copper iodide. Conductive polymers,
such as poly(3-methylthiophene), polyphenylene sulfide and
polypyrrole are also usable.
[0045] In contrast, preferable cathode materials have small work
functions. Examples of such materials include silver, lead, tin,
magnesium, aluminum, calcium, manganese, indium and chromium, and
alloys thereof.
[0046] It is preferable that at least one constituent of the anode
and cathode transmits 50% or more of incident light over the
wavelength region of the luminescent light.
[0047] As the transparent substrate, glass and plastic films are
used in the present invention.
[0048] The luminescent device in accordance with the present
invention is a thin, light and solid device with a large
luminescent area and high resolution, enabling high-speed
operation. Such a luminescent device is quite different from
conventional incandescent lamps, fluorescent lamps, and light
emitting diodes, and can be used for electroluminescence panels
which are expected as devices satisfying advanced needs in
industrial fields.
EXAMPLES
[0049] The present invention is described in further detail with
reference to the following examples.
[0050] Examples 1 to 3 Comparative Examples 1 and 2
[0051] Luminescent devices of Examples 1, 2 and 3 were prepared as
follows. Translucent or transparent anodes with a thickness of 50
nm composed of iron, silver and indium tin oxide (ITO),
respectively were formed on glass plates by a deposition process.
On each anode, a hole transport layer with a thickness 100 nm of
the following compound (b), an electron transport layer with a
thickness of 75 nm composed of Alq.sub.3, and a cathode with a
thickness of 120 nm composed of aluminum were formed in that order
by a vacuum deposition process. The electron transport layer also
functions as a luminescent layer. 28
[0052] Luminescent devices of Comparative Examples 1 and 2 were
also prepared as in Example 1, but the anode materials were
zirconium and copper, respectively, and the cathode was composed of
gold and had a thickness of 100 nm.
[0053] Table 7 shows the results of the work functions Wf (eV) of
the anodes and the cathodes determined with the above-mentioned
surface analyzer AC-1.
[0054] The Fermi levels of the films composed of the compound (b)
and Alq.sub.3 determined with the above-mentioned Fermi level meter
FAC-1 were 4.63 eV and 4.70 eV, respectively.
[0055] A current flow of a current density of 20 mA/cm.sup.2 was
applied to the luminescent devices for 100 hours. The results are
also shown in Table 7.
7TABLE 7 Initial After 100 hours Cathode Anode Applied Luminescent
Applied Luminescent Wf Wf Voltage Output Voltage Output Sample
Metal (eV) Metal (eV) (V) (.mu.W/cm.sup.2) (V) (.mu.W/cm.sup.2)
Example 1 Al 4.08 Fe 4.43 19 30 32 30 Example 2 Al 4.08 Ag 4.71 13
40 17 37 Example 3 Al 4.08 ITO 4.66 10 44 13 41 Comp. Ex 1 Au 4.71
Zn 4.26 50 None 64 None Luminescence Luminescence Comp. Ex. 2 Au
4.71 Cu 5.08 55 None 77 None Luminescence Luminescence
[0056] Example 4 and 5, and Comparative Example 3 and 4
[0057] Luminescent devices of Examples 4 and 5 were prepared as
follows. Translucent or transparent anodes with a thickness of 50
nm composed of ITO and molybdenum, respectively were formed on
glass plates by a deposition process. On each anode, a hole
transport layer with a thickness 130 nm of the following compound
(c), an electron transport layer with a thickness of 100 nm
composed of titanyloxyphthalocyanine, and a cathode with a
thickness of 120 nm composed of aluminum were formed in that order
by a vacuum deposition process. The electron transport layer also
functions as a luminescent layer. 29
[0058] Luminescent devices of Comparative Examples 3 and 4 were
also prepared as in Example 4, but the anode materials were zinc
and nickel, respectively, and the cathode was composed of platinum
and had a thickness of 125 nm.
[0059] Table 8 shows the results of the work functions Wf (eV) of
the anodes and the cathodes determined with the above-mentioned
surface analyzer AC-1.
[0060] The Fermi levels of the films composed of the compound (c)
and titanyloxyphthalocyanine determined with the above-mentioned
Fermi level meter FAC-1 were 4.56 eV and 4.94 eV, respectively.
[0061] A current flow of a current density of 60 mA/cm.sup.2 was
applied to the luminescent devices for 100 hours. The results are
also shown in Table 8.
8TABLE 8 Initial After 100 hours Cathode Anode Applied Luminescent
Applied Luminescent Wf Wf Voltage Output Voltage Output Sample
Metal (eV) Metal (eV) (V) (.mu.W/cm.sup.2) (V) (.mu.W/cm.sup.2)
Example 4 Al 4.08 ITO 4.66 30 4.0 35 3.3 Example 5 Al 4.08 Mo 4.36
38 3.0 48 1.9 Comp. Ex 3 Pt 5.20 Zn 4.13 88 None 108 None
Luminescence Luminescence Comp. Ex. 4 Pt 5.20 Ni 4.84 80 None 98
None Luminescence Luminescence
[0062] While the present invention has been described with
reference to what are presently considered that the invention is
not limited to the disclosed embodiments. To the contrary, the
invention is intended to cover various modifications and equivalent
arrangements, included within the spirit and scope of the appended
claims. The scope of the following claims is to be accorded the
broadest interpretation so as to encompass all such modifications
and equivalent structures and functions.
* * * * *