U.S. patent application number 11/392508 was filed with the patent office on 2006-10-19 for organic electroluminescent element.
Invention is credited to Hideki Ookawa, Junichi Tonotani.
Application Number | 20060232200 11/392508 |
Document ID | / |
Family ID | 37030709 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060232200 |
Kind Code |
A1 |
Ookawa; Hideki ; et
al. |
October 19, 2006 |
Organic electroluminescent element
Abstract
Disclosed is an organic electroluminescent element, comprising
an anode and a cathode, an organic light-emitting layer interposed
between the anode and the cathode, and an organic hole transfer
layer containing a polymer type organic hole transfer material
having a metal oxide of a semiconductor material or a conductive
material added thereto.
Inventors: |
Ookawa; Hideki;
(Yokohama-shi, JP) ; Tonotani; Junichi;
(Yokohama-shi, JP) |
Correspondence
Address: |
C. IRVIN MCCLELLAND;OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
37030709 |
Appl. No.: |
11/392508 |
Filed: |
March 30, 2006 |
Current U.S.
Class: |
313/504 |
Current CPC
Class: |
H01L 51/0037 20130101;
H01L 51/506 20130101; H01L 51/5048 20130101 |
Class at
Publication: |
313/504 |
International
Class: |
H05B 33/00 20060101
H05B033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2005 |
JP |
2005-100328 |
Claims
1. An organic electroluminescent element, comprising: an anode and
a cathode; an organic light-emitting layer interposed between the
anode and the cathode; and an organic hole transfer layer
containing a polymer type organic hole transfer material having a
metal oxide of a semiconductor material or a conductive material
added thereto.
2. The organic electroluminescent element according to claim 1,
wherein both the metal oxide and the polymer type organic hole
transfer material are soluble in water.
3. The organic electroluminescent element according to claim 1,
wherein the polymer type organic hole transfer material is
poly(ethylenedioxy)thiophene doped with polystyrenesulphonic acid
or a salt thereof (PSS).
4. The organic electroluminescent element according to claim 3,
wherein the poly(ethylenedioxy)thiophene is poly(2,3-ethylenedioxy)
thiophene (PEDOT).
5. The organic electroluminescent element according to claim 1,
wherein the metal oxide is at least one oxide selected from the
group consisting of MoO.sub.x, where x denotes an integer of 2 to 3
and VO.sub.x, where x is an integer of 1 to 2.5.
6. The organic electroluminescent element according to claim 1,
wherein the polymer type organic hole transfer material is a
water-soluble PEDOT:PSS prepared by having
poly(3,4-ethylenedioxy)thiophene doped with polystyrenesulphonic
acid or a salt thereof, the metal oxide is a water-soluble
MoO.sub.x (where x is an integer of 2 to 3), the water-soluble
PEDOT:PSS is mixed with the water-soluble MoO.sub.x in the presence
of water, and the mixed aqueous solution is used for forming the
polymer type organic hole transfer layer on the anode by the ink
jet method or a printing method.
7. The organic electroluminescent element according to claim 1,
wherein the organic light-emitting layer is formed of a material
having a polymer type polyparaphenylene vinylene skeleton or a
polyfluorene skeleton.
8. The organic electroluminescent element according to claim 1,
wherein the cathode has a stacked structure comprising a layer of
an alkali metal or an alkaline earth metal such as Li, Ca, Ba and
an Al layer stacked on the layer of the alkali metal or the
alkaline earth metal.
9. The organic electroluminescent element according to claim 1,
further comprising a cathode buffer layer formed of CsF, which is
interposed between the cathode and the organic light-emitting
layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from prior Japanese Patent Application No. 2005-100328,
filed Mar. 31, 2005, the entire contents of which are incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an organic
electroluminescent element, particularly, to an organic
electroluminescent element comprising an improved organic hole
transfer layer containing a polymer type organic hole transfer
material.
[0004] 2. Description of the Related Art
[0005] The organic electroluminescent element comprises an anode, a
cathode and an organic light-emitting layer interposed between the
anode and the cathode and exhibiting an electric conductivity and a
light-emitting capability. If voltage is applied to the organic
electroluminescent element in the forward direction, holes are
injected from the anode into the organic light-emitting layer and
electrons are injected from the cathode into the organic
light-emitting layer. The organic electroluminescent element is a
spontaneous light-emitting element, in which these holes and
electrons are recombined within the organic light-emitting layer so
as to form excitons, and then light is emitted when the excitons
are relaxed. Since the organic electroluminescent element permits
the process for forming a large display area having a low
temperature, the organic electroluminescent element is expected to
be applied to a thin film display of the next era.
[0006] In such an organic electroluminescent element, the injection
of electrons and holes from the electrodes and the mobility of the
electrons and holes are controlled by arranging a plurality of
layers such as a hole injection layer, a hole transfer layer, an
electron transfer layer and an electron injection layer between the
anode and the cathode in addition to the organic light-emitting
layer. Japanese Patent Application Kokai Publication No. 2004-63408
discloses a hole transfer layer formed of a polymer type hole
transfer material such as a so-called "PEDOT:PSS", which is
prepared by having poly(ethylenedioxy)thiophene (DEDOT) doped with
polystyrenesulphonic acid (PSS). If the hole transfer layer is
interposed between the anode and the organic light-emitting layer,
it is possible to lower the barrier wall for the hole injection,
compared with the case where the holes are injected from the anode
formed of a transparent electrode such as ITO directly into the
organic light-emitting layer. For example, the barrier wall for the
hole injection can be lowered from about 1.0 V to about 0.5 V in
the case of forming a hole transfer layer between the anode and the
organic light-emitting layer.
[0007] However, the organic electroluminescent element comprising
the hole transfer layer is defective in that the organic
light-emitting layer is deteriorated by the hole transfer layer in
the current driving stage so as to shorten the life of the organic
electroluminescent element.
BRIEF SUMMARY OF THE INVENTION
[0008] According to an aspect of the present invention, there is
provided an organic electroluminescent element, comprising:
[0009] an anode and a cathode;
[0010] an organic light-emitting layer arranged between the anode
and the cathode; and
[0011] an organic hole transfer layer arranged between the anode
and the organic light-emitting layer and containing a polymer type
organic hole transfer material having a metal oxide of a
semiconductor material or a conductive material added thereto.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0012] FIG. 1 is a cross sectional view showing the construction of
an organic electroluminescent diode for Example 1 of the present
invention;
[0013] FIGS. 2A and 2B are graphs each showing the current-voltage
characteristics of the organic electroluminescent diode for Example
1 of the present invention;
[0014] FIGS. 3A and 3B are graphs each showing the current-voltage
characteristics of the organic electroluminescent diode for
Comparative Example 1;
[0015] FIG. 4 is a graph showing the changes with the driving time
in the brightness and the voltage of the organic electroluminescent
diode for Example 1 of the present invention; and
[0016] FIG. 5 is a graph showing the changes with the driving time
in the brightness and the voltage of the organic electroluminescent
diode for Comparative Example 1.
DETAILED DESCRIPTION OF THE INVENTION
[0017] An embodiment of the present invention will now be described
in detail.
[0018] The organic electroluminescent element for this embodiment
is constructed such that a stacked body consisting of an organic
light-emitting layer and an organic hole transfer layer is arranged
between the anode and the cathode such that the organic hole
transfer layer is positioned on the side of the anode. The organic
hole transfer layer contains a polymer type organic hole transfer
material having a metal oxide of a semiconductor material or a
conductive material added thereto.
[0019] The anode is formed of a transparent conductive material
such as indium oxide doped with tin (ITO).
[0020] The cathode has a stacked structure comprising a layer of an
alkali metal or an alkaline earth metal having a small work
function, e.g., a layer of Li, Ca or Ba, and an Al layer stacked on
the layer of the alkali metal or the alkaline earth metal. In the
cathode of the stacked structure, the Al layer also acts as a
protective film of the alkali metal or the alkaline earth metal
that tends to be oxidized easily. Incidentally, in the present
invention, it is acceptable for a cathode buffer layer made of, for
example, CsF to be interposed between the cathode and the organic
light-emitting layer.
[0021] The organic light-emitting layer is formed of a material
having a polymer type polyparaphenylene vinylene skeleton or a
polymer type polyfluorene skeleton.
[0022] It is possible for the polymer type organic hole transfer
material to be formed of, for example, PEDOT:PSS, which is prepared
by having poly (3,4-ethylenedioxy) thiophene (PEDOT) doped with
polystyrenesulphonic acid or a salt thereof (PSS). The hole
transfer material of PEDOT:PSS is soluble in water and, thus, it is
possible to employ an ink jet method or a printing technology for
forming a film of the organic hole transfer layer under a liquid
state.
[0023] A metal oxide of a semiconductor material or a conductive
material is added to the polymer type organic hole transfer
material. It is desirable to use at least one oxide selected from
the group consisting of MoO.sub.x, where x denotes an integer of 2
to 3 and VO.sub.x, where x denotes an integer of 1 to 2.5 as the
metal oxide that is added to the polymer type organic hole transfer
material. In particular, MoO.sub.x, which is excellent in electric
conductivity, has a work function of about -5 V, with the result
that the hole injection from the anode made of, for example, ITO
into the light-emitting layer (HOMO level of about 5.5 to 6 V) can
be facilitated. Also, in the case of using a water-soluble
MoO.sub.x, the organic hole transfer layer can be formed by the ink
jet method or a printing technology by adding MoO.sub.x in advance
to the water-soluble PEDOT:PSS.
[0024] In operating the organic electroluminescent element, voltage
is applied between the anode and the cathode so as to permit the
holes to be injected from the anode into the organic light-emitting
layer through the organic hole transfer layer and to permit the
electrons to be injected from the cathode into the organic
light-emitting layer. As a result, the holes and electrons are
recombined within the organic light-emitting layer so as to cause
the light-emitting layer to emit light spontaneously. According to
the embodiment of the present invention, the organic hole transfer
layer contains a polymer type organic hole transfer material having
a metal oxide of a semiconductor material or a conductive material
added thereto, as described above. Because of the particular
construction, it is possible to suppress or prevent the
deterioration of the organic light-emitting layer, which is brought
about by the presence of the organic hole transfer layer when the
holes and electrons are recombined within the organic
light-emitting layer so as to emit light spontaneously.
[0025] Particularly, where the organic hole transfer layer contains
polymer type organic hole transfer material prepared by adding at
least one oxide, e.g., MoO.sub.x, selected from the group
consisting of MoO.sub.x, where x denotes an integer of 2 to 3 and
VO.sub.x, where x denotes an integer of 1 to 2.5 to PEDOT:PSS, it
is possible to suppress or prevent the deterioration of the organic
light-emitting layer during the voltage application between the
anode and the cathode. It is considered reasonable to understand
that the particular effect of the present invention is produced by
the mechanism given below.
[0026] Specifically, the deterioration of the organic
light-emitting layer during the voltage application (during the
driving) is considered to be derived from the diffusion of SO3
separated from PSS in PEDOT:PSS contained in the organic hole
transfer layer into the organic light-emitting layer. The added
MoO.sub.x enters the polymer chain of PEDOT:PSS so as to suppress
the separation of SO.sub.3 from PSS and also suppress the diffusion
of the polymer chain itself containing SO.sub.3 into the organic
light-emitting layer. In particular, it is possible that the water
soluble MoO.sub.x to enter the polymer chain of PEDOT:PSS so as to
fix the polymer chain.
[0027] As described above, it is possible in the present invention
to suppress or prevent the deterioration of the organic
light-emitting layer during the voltage application (during the
driving of the organic electroluminescent element) so as to make it
possible to obtain an organic electroluminescent element having a
long life.
[0028] The present invention will now be described more in detail
with reference to an Example of the present invention.
EXAMPLE 1
[0029] In the first step, an anode was prepared by depositing ITO
by a sputtering method in a thickness of, for example, 150 nm on
the surface of a glass substrate, sized, for example, 24 mm square
and 0.7 mm thick, followed by pattering the deposited ITO layer in
the shape of a stripe. Then, prepared was an aqueous solution
containing about 2% by weight of PEDOT:PSS. Also prepared was an
aqueous solution containing 0.049% of MoO.sub.3. The aqueous
solution of MoO.sub.3 was prepared by dissolving a white powdery
molybdenum oxide (MoO.sub.3) in a pure water at 28.degree. C.
Further, the PEDOT:PSS aqueous solution was mixed with the
MoO.sub.3 aqueous solution at a mixing ratio (PEDOT:PSS aqueous
solution:MoO.sub.3 aqueous solution) of 10:2 so as to obtain a
mixed aqueous solution. The surface of the glass substrate
including the anode described above was coated with the mixed
aqueous solution by a spin coating method, with the rotating speed
of the glass substrate set at about 3,000 to 4,000 rpm, followed by
removing the film in the region other than the light-emitting
region and subsequently baking the glass substrate at 200.degree.
C. so as to form a hole transfer layer.
[0030] In the next step, the surface of the hole transfer layer was
coated with an ink prepared by dissolving a polyfluorene series
blue light-emitting polymer in a tetralin solvent in a
concentration of about 2%. The coating was performed by a spin
coating method, in which the rotating speed of the glass substrate
was set at 2,000 to 3,000 rpm. Further, the resultant ink layer was
baked at 150.degree. C. so as to form an organic light-emitting
layer. Incidentally, the coating process and the baking process of
the light-emitting polymer ink were carried out under a nitrogen
gas atmosphere within a glove box. Then, a cathode buffer layer
having a thickness of 0.5 nm and formed of CsF was formed by a
resistance heating type vacuum deposition, followed by forming a Mg
film 10 nm thick and an Al film 150 nm thick by the same vacuum
deposition, thereby obtaining a cathode. Further, a cover glass 1.6
mm thick, in which a CaO series getter material (drying agent) was
mounted to a groove 0.5 mm deep, was arranged to face the glass
substrate such that the drying agent was positioned on the side of
the cathode, followed by performing the sealing by using a
UV-curing resin so as to manufacture an organic electroluminescent
diode constructed as shown in FIG. 1.
[0031] The organic electroluminescent diode thus manufactured
included the glass substrate 1 having the anode 2 formed on the
surface in the shape of a stripe. The anode 2 was formed of an ITO
layer having a thickness of, for example, 150 nm. The hole transfer
layer 3 prepared by adding 0.49% by weight of molybdenum oxide
(MoO.sub.3) to PEDOT:PSS was formed on that surface of the glass
substrate 1 on which was formed the anode 2. The organic
light-emitting layer 4 was formed on the surface of the hole
transfer layer 3. The cathode buffer layer 5 was formed on the
substrate 1 including the organic light-emitting layer 4. The
cathode 6 was formed on the surface of the cathode buffer layer 5.
Further, the cover glass 8 having the drying agent 7 mounted
thereto was arranged to face and to be fixed to the glass substrate
1 such that the frame-like sealing material 9 formed of a UV-curing
resin was interposed between the cover glass 8 and the glass
substrate 1. To be more specific, the region of the organic
light-emitting layer 4 was sealed by the glass substrate 1, the
cover glass 8 having the drying agent mounted thereto, and the
frame-like sealing material 9.
COMPARATIVE EXAMPLE 1
[0032] An organic electroluminescent diode constructed as shown in
FIG. 1 was manufactured as in Example 1, except that PEDOT:PSS
alone was used for forming the hole transfer layer. In other words,
a metal oxide such as MoO.sub.x was not contained in the hole
transfer layer.
[0033] The current-voltage characteristics were examined in respect
of the diode manufactured in each of Example 1 and Comparative
Example 1. FIGS. 2A and 2B are graphs each showing the
current-voltage characteristics of the diode for Example 1
comprising the hole transfer layer formed of PEDOT:PSS+MoO.sub.3.
On the other hand, FIGS. 3A and 3B are graphs each showing the
current-voltage characteristics of the diode for Comparative
Example 1 comprising the hole transfer layer formed of PEDOT:PSS
alone.
[0034] As apparent from FIGS. 2A, 2B, 3A and 3B, the diodes
manufactured in Example 1 and Comparative Example 1 were found to
be substantially equal to each other in the carrier current
(transfer of hole in this case).
[0035] Also, the change with time in the brightness of the front
surface was examined in respect of the diode manufactured in each
of Example 1 and Comparative Example 1 under the condition that the
diode was driven under a low current that permitted the initial
brightness of about 500 Cd/m.sup.2. FIG. 4 is a graph showing the
changes with time in the brightness and the voltage during the
driving of the diode for Example 1 comprising the hole transfer
layer formed of PEDOT:PSS+MoO.sub.3. On the other hand, FIG. 5 is a
graph showing the changes with time in the brightness and the
voltage during the driving of the diode for Comparative Example 1
comprising the hole transfer layer formed of PEDOT:PSS alone.
[0036] As apparent from FIGS. 4 and 5, it was possible for the
diode for Example 1 to suppress the deterioration of the
brightness, compared with the diode for Comparative Example 1, so
as to improve the life of the diode. To be more specific, the
half-life of the brightness, i.e., the time required for the
brightness to be lowered to half the initial brightness, for the
diode for Example 1 was about 1.8 times as much as that for the
diode for Comparative Example 1.
* * * * *