U.S. patent application number 09/111178 was filed with the patent office on 2001-10-04 for organic el device having a hole injecting electrode including a transparent electrode and a metal electrode.
Invention is credited to CODAMA, MITSUFUMI, NAKAYA, KENJI.
Application Number | 20010026126 09/111178 |
Document ID | / |
Family ID | 16404769 |
Filed Date | 2001-10-04 |
United States Patent
Application |
20010026126 |
Kind Code |
A1 |
NAKAYA, KENJI ; et
al. |
October 4, 2001 |
ORGANIC EL DEVICE HAVING A HOLE INJECTING ELECTRODE INCLUDING A
TRANSPARENT ELECTRODE AND A METAL ELECTRODE
Abstract
An organic EL device comprises a hole injecting electrode, an
electron injecting electrode, and at least one organic layer
located between both the electrodes. The hole injecting electrode
comprises a transparent electrode at a light emitting area, and a
metal electrode located at a portion other than the light emitting
area and having a sheet resistance of 1 .OMEGA./.quadrature. or
lower.
Inventors: |
NAKAYA, KENJI; (IBARAKI,
JP) ; CODAMA, MITSUFUMI; (IBARAKI, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & FLOOR
FOUTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
|
Family ID: |
16404769 |
Appl. No.: |
09/111178 |
Filed: |
July 8, 1998 |
Current U.S.
Class: |
313/506 |
Current CPC
Class: |
H01L 51/5212 20130101;
H01L 27/329 20130101; H01L 27/3237 20130101; H05B 33/26 20130101;
H05B 33/28 20130101 |
Class at
Publication: |
313/506 |
International
Class: |
H01J 001/62; H01J
063/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 9, 1997 |
JP |
9-199256 |
Claims
What we claim is:
1. An organic electroluminescent device comprising a hole injecting
electrode, an electron injecting electrode, and at least one
organic layer interleaved between both said electrodes, wherein:
said hole injecting electrode comprises a transparent electrode at
a light emitting area, and a metal electrode located at a portion
other than said light emitting area and connected to said
transparent electrode, and having a sheet resistance of 1
.OMEGA./.quadrature. or lower.
2. The organic electroluminescent device of claim 1, wherein said
transparent electrode has a thickness of 100 mn or less.
3. The organic electroluminescent device of claim 1, wherein said
transparent electrode is an ITO transparent electrode.
4. The organic electroluminescent device of claim 1, wherein said
transparent electrode is an IZO transparent electrode.
5. The organic electroluminescent device of claim 1, which further
includes a barrier layer between said transparent electrode and
said metal electrode.
6. The organic electroluminescent device of claim 1, wherein said
metal electrode is connected to all the peripheral area of the
transparent electrode.
7. The organic electroluminescent device of claim 6 forming a
segment type display.
8. The organic electroluminescent device of claim 6 forming a
simple matrix type display.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to an organic EL
(electroluminescent) device, and specifically to an improvement in
or relating to a hole injecting electrode for feeding holes
(charges) to a light emitting layer.
[0002] In recent years, organic EL devices have been under
intensive investigation. One such device is basically built up of a
thin film form of hole transporting material such as
triphenyldiamine (TPD) deposited by evaporation on a hole injecting
electrode, a light emitting layer of fluorescent material such as
an aluminum quinolinol complex (Alq.sup.3) laminated thereon, and a
metal (electron injecting) electrode of a metal having a low work
function such as Mg and formed on the light emitting layer. This
organic EL device now attracts attentions because a very high
luminance ranging from several hundreds to tens of thousands
cd/m.sup.2 can be achieved with a voltage of approximately 10
V.
[0003] A hole injecting electrode considered to be effective for
such organic EL devices is made up of a material capable of
injecting more electrons into a light emitting layer or a hole
injecting and transporting layer. One requirement for the material
is that it be transparent and electrically conductive because, in
most cases, an organic EL device is usually designed to take out
the emitted light from a substrate side thereof.
[0004] ITO (tin-doped indium oxide), IZO (Zinc-doped indite oxide),
ZnO, SnO.sub.2, In.sub.2O.sub.3, etc. have been known for such
transparent electrodes. Among these, ITO electrodes have a visible
light transmittance of 90% or higher and a sheet resistance of 10
.OMEGA./.quadrature. or lower, and so find wide applications as
transparent electrodes for liquid crystal displays (LCDs), dimmer
glasses, and solar batteries. The ITO electrodes are also
considered to be promising for hole injecting electrodes in the
organic EL devices.
[0005] An organic EL device requires a given current for light
emission, and the light emission luminance increases in proportion
to an applied current density. Consequently, when sophisticated
display patterns or large-screen displays are driven with high
luminance or at a high duty ratio, the resistance of
interconnecting lines in the hole injecting electrode is found to
give rise to a voltage drop problem., although this resistance is
negligible when the length of interconnecting lines is short. If,
for example, a display of 256 dots.times.64 dots is driven at a
light emission luminance of 150 cd/m.sup.2, a {fraction (1/64)}
second-light emission will occur at a light emission luminance of
150.times.64 cd/m.sup.2. When the sheet resistance of the hole
injecting electrode is sufficiently reduced, the device may be
driven at a voltage value approximate to an effective applied
voltage shows in FIG. 15 for instance. When the resistance of the
hole injecting electrode is about 260 .OMEGA., however, a voltage
increase of as high as 2 V is needed, as can be seen from the
applied voltage shown in FIG. 15. Even with an electrode having a
sheet resistance of the order of 7 to 8 .OMEGA./.quadrature. such
as a low resistance ITO electrode, a voltage drop across the hole
injecting electrode becomes a grave problem because a resistance of
64.times.7=448 .OMEGA. exists even in a narrow pixel area of 64
dots.
SUMMARY OF THE INVENTION
[0006] An object of the invention is to provide an organic EL
device which can lower the interconnecting resistance of a hole
injecting electrode, and so easily accommodate itself to a
sophisticated display pattern or large-screen display designed to
be driven with high luminance or at a high duty ratio.
[0007] The aforesaid object is achieved by the inventions defined
below as (1) to (8).
[0008] (1) An organic electroluminescent device comprising a hole
injecting electrode, an electron injecting electrode, and at least
one organic layer interleaved between both said electrodes,
wherein:
[0009] said hole injecting electrode comprises a
transparent-electrode at a light emitting area, and a metal
electrode located at a portion other than said light emitting area
and connected to said transparent electrode, and having a sheet
resistance of 1 .OMEGA./.quadrature. or lower.
[0010] (2) The organic electroluminescent device according to (1),
wherein said transparent electrode has a thickness of 100 nm or
less.
[0011] (3) The organic electroluminescent device according to (1)
or (2), wherein said transparent electrode is an ITO transparent
electrode.
[0012] (4) The organic electroluminescent device according to (1)
or (2), wherein said transparent electrode is an IZO transparent
electrode.
[0013] (5) The organic electroluminescent device according to any
one of (1) to (4), which further includes a barrier layer between
said transparent electrode and said metal electrode.
[0014] (6) The organic electroluminescent device of (1), wherein
said metal electrode is connected to all the peripheral area of the
transparent electrode.
[0015] (7) The organic electroluminescent device of (6) forming a
segment type display.
[0016] (8) The organic electroluminescent device of (6) forming a
simple matrix type display.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The above and other objects, features, and advantages of the
invention will be better understood from the following description
taken in conjunction with the accompanying drawings.
[0018] FIG. 1 is a general representation illustrative of a
7-segment type hole injecting electrode that is a first embodiment
of the organic EL device according to the invention.
[0019] FIG. 2 is a sectional view of FIG. 1 as taken along the A-A'
line.
[0020] FIG. 3 is a general representation illustrative of the
process of forming a simple matrix type hole injecting electrode
that is a second embodiment of the organic EL device according to
the invention.
[0021] FIG. 4 is a sectional view of FIG. 3 as taken along the B-B'
line.
[0022] FIG. 5 is a general representation illustrative of the
second embodiment of the organic EL device according to the
invention, in which the process of forming the metal electrode is
illustrated.
[0023] FIG. 6 is a sectional view of FIG. 5 as taken along the B-B'
line.
[0024] FIG. 7 is a general representation illustrative of the
second embodiment of the organic EL device according to the
invention, in which the process of forming the insulating film is
illustrated.
[0025] FIG. 8 is a sectional view of FIG. 7 as taken along the B-B'
line.
[0026] FIG. 9 is a general representation illustrative of the
second embodiment of the organic EL device according to the
invention, in which the process of forming the element-isolating
structure is illustrated.
[0027] FIG. 10 is a sectional view of FIG. 9 as taken along the
C-C' line.
[0028] FIG. 11 is a general representation illustrative of the
second embodiment of the organic EL device according to the
invention, in which the process of mask lamination is
illustrated.
[0029] FIG. 12 is a sectional view of FIG. 11 as taken along the
C-C' line.
[0030] FIG. 13 is a partial plan view illustrative of one exemplary
pattern of the ITO transparent electrode and metal electrode
according to the invention.
[0031] FIG. 14 is a sectional view of FIG. 13 as taken along the
D-D' line.
[0032] FIG. 15 is a graph illustrative of a specific applied
voltage vs. light emission luminance relation of the organic EL
device.
DETAILED DESCRIPTION OF THE INVENTION
[0033] Some illustrative embodiments of the invention will now be
explained at great length.
[0034] The organic EL device of the invention comprises a hole
injecting electrode, an electron injecting electrode, and at least
one organic layer interleaved between both the electrodes. The hole
injecting electrode comprises a transparent electrode at a light
emitting area, and a metal electrode located at a portion other
than the light emitting area and electrically connected to the
transparent electrode, and having a sheet resistance of 1
.OMEGA./.quadrature. or lower. Thus, the transparent electrode is
located at the light emitting area out of which light should be
taken, and the metal electrode having a low sheet resistance is
located at the non-light emitting area out of which light may not
be taken, so that the overall resistance value of the hole
injecting electrode can remain low. The metal electrode is
connected to all the peripheral or circumferential area of the
transparent electrode.
[0035] The transparent electrode is located at least at the light
emitting area (pixel area). By the term "light emitting area" used
herein is intended by an area at which a light emitting layer can
give out light, and out of which the emitted light can be taken for
use. While the transparent electrode area may be larger than the
light emitting area, for instance, the transparent electrode may
extend slightly from the periphery of the light emitting area, it
is to be understood that as the transparent electrode area becomes
larger than required, the resistance of the hole injecting
electrode becomes high. Thus, it is preferred that the transparent
electrode area is equal to, or slightly larger than, the light
emitting area.
[0036] For the transparent electrode, it is preferable to use ITO
(tin-doped indium oxide), IZO (zinc-doped indium oxide), ZnO,
SnO.sub.2, In.sub.2O.sub.3 or the like, and especially ITO
(tin-doped indium oxide), and IZO (zinc-doped indium oxide). It is
desired that the mixing ratio of SnO.sub.2 with respect to
In.sub.2O.sub.3 be in the range of 1 to 20 wt %, and especially 5
to 12 wt %. Again, the mixing ratio of ZnO with respect to
In.sub.2O.sub.3 is preferably in the range of 1 to 20 wt %, and
especially 5 to 12 wt %. Besides, ITO, and IZO may contain an oxide
form of Sn, Ti, Pb, etc. in an amount of up to 1 wt % calculated as
oxide.
[0037] The transparent electrode may have at least a certain
thickness enough to inject holes, and is usually in the range of 10
to 100 nm. However, a thickness of 30 to 100 nm, and especially 50
to 90 nm is preferred. At a film thickness of up to 100 nm, the
transparent electrode has so low surface roughness that its surface
properties can be improved, resulting in improvements in light
emission properties, light emission life, etc. Too large a
thickness offers problems in conjunction with film thickness upon
film formation, and the ability of the electrode to transport
holes.
[0038] The metal electrode has a sheet resistance of preferably 1
.OMEGA./.quadrature. or lower, and especially 0.5
.OMEGA./.quadrature. or lower. Although there is no lower limit on
sheet resistance, a sheet resistance of about 0.1
.OMEGA./.quadrature. is generally preferred. Preferably, the metal
electrode has a thickness of the order of preferably 10 to 2,000
nm, especially 20 to 1,000 nm, and more especially 100 to 500
nm.
[0039] The metal electrode may be made up of metals such as Al, Cu,
Au and Ag, or alloys of Al with transition elements such as Sc, Nb,
Zr, Hf, Nd, Ta, Cu, Si, Cr, Mo, Mn, Ni, Pd, Pt, and W. However,
preference is given to Al, and Al alloys, among which an alloy of
Al with at least one transition element is most preferred with an
Al content of 90 at % or higher, and preferably 95 at % or
higher.
[0040] Preferably in the hole injecting electrode of the invention,
a barrier metal layer is provided between the transparent electrode
and the metal electrode. By the provision of the barrier metal
layer, the interface between the transparent electrode and the
metal electrode is stabilized with stabilized contact resistance.
The barrier metal layer may be made up of metals such as Cr and Ti,
and nitrides such as titanium nitride (TiN). However, particular
preference is given to Cr, and TiN, because they can be wet etched
with a specific reagent having no attack on the transparent
electrode such as an ITO electrode. Preferably, the barrier metal
layer has a thickness of 10 to 200 nm, and especially 30 to 100
nm.
[0041] The transparent electrode, metal electrode, and barrier
metal layer may be formed as by evaporation, but should preferably
be formed by a sputtering technique. When a sputtering process is
applied to the formation of an ITO or IZO transparent electrode, it
is preferable to use a target comprising In.sub.2O.sub.3 doped with
SnO.sub.2 or ZnO. When the metal electrode or barrier electrode in
a film form is provided, it is preferable to form a sintered
product of the aforesaid starting metal material or its alloy by
means of a DC or RF sputtering process. An ITO transparent
electrode, when formed by the sputtering technique, suffers a
lesser light emission luminance change as compared with an
electrode formed by evaporation. Power input is preferably in the
range of 0.1 to 4 W/cm.sup.2. Power input for a DC sputtering
system in particular is preferably in the range of 0.1 to 10
W/cm.sup.2, and especially 0.2 to 5 W/cm.sup.2. The film forming
rate is preferably in the range of 2 to 100 nm/min., and especially
5 to 50 nm/min.
[0042] Preferably but no exclusively, an inert gas such as Ar, He,
Ne, Kr, and Xe or a mixture of such inert gases is used as the
sputtering gas. The sputtering pressure of such gases may usually
be of the order of 0.1 to 20 Pa.
[0043] In the practice of the invention, no particular limitation
is imposed on how to provide the transparent electrode, metal
electrode, and barrier layer on each of the light emitting area and
the non-light emitting area. However, it is preferable to rely upon
a patterning technique using an ordinarily available resist
material. Etching may be done either wet or dry. An etching
solution used for wet etching is preferably an aqueous system for
an ITO transparent electrode, and a mixed solution system of
phosphoric acid, nitric acid and acetic acid is preferable for a
specific metal electrode, e.g., an Al electrode. In the latter
case, it is preferable to use an etching solution that enables a
specific metal to be selectively etched.
[0044] Specific embodiments of the hole injecting electrode
according to the invention are now explained with reference to the
drawings.
[0045] FIG. 1 is a general representation of one embodiment of the
hole injecting electrode according to the invention, and FIG. 2 is
a sectional view of FIG. 1 as taken along the A-A' line. The hole
injecting electrode illustrated has an electrode structure for
constructing a so-called 7-segment type display, wherein a
transparent electrode 2 and a metal electrode 3 are provided on a
substrate 1. The transparent electrode 2 is located at a light
emitting (pixel) area of each segment, while the metal electrode is
located around the light emitting portion and an interconnection
portion extending to a terminal electrode. By locating the
transparent electrode at the light emitting area and the metal
electrode at a portion other than the light emitting area, it is
possible to make resistance low because the resistance of the hole
injecting electrode, for the most part, results from the sheet
resistance component of the metal electrode.
[0046] After the formation of the hole injecting electrode, an
insulating layer is provided at a portion other than the light
emitting area, optionally or if required, followed by the provision
of an element isolating structure. Then, organic layers such as a
hole injecting and transporting layer, a light emitting layer and
an electron injecting and transporting layer are laminated.
Finally, an electron injecting (common) electrode is provided, if
required, followed by the lamination of a protective film, etc.,
thereby assembling a segment type of organic EL display.
[0047] FIGS. 3 to 12 are general representations of another
embodiment of the hole injecting electrode according to the
invention. In other word, the steps of constructing a so-called
simple matrix type display are illustrated in order. FIGS. 3, 5, 7,
9, and 11 are plan views, and FIGS. 4, 6, 8, 10, and 12 are
sectional views of FIGS. 3, 5, 7, 9, and 11 as taken along the
lines B-B', and C-C', respectively.
[0048] As can be seen from FIGS. 3 and 4, a transparent electrode
(ITO) 2 is first provided on a substrate 1 (not shown in the plan
view for simplification) according to such a given pattern that
non-exclusive scanning lines are formed at a light emitting area,
i.e., a pixel area. As shown in FIGS. 5 and 6, then, a metal
electrode 3 is formed at a portion other than the light emitting
area while the transparent electrode is covered by a part thereof.
Subsequently, an insulating layer 4 is provided for the insulation
of the portion other than the light emitting area, i.e., the pixel
area, as shown in FIGS. 7 and 8.
[0049] In the embodiment shown in FIGS. 9 and 8, an element
isolating structure 5 is provided to form organic layers such as a
light emitting layer and an electron injecting electrode in such a
manner that non-exclusive scanning lines are formed. This element
isolating structure is a solid structure including an overhang
portion as shown in FIG. 10. During the formation of the organic
layers and electron injecting and transporting layer, evaporated or
sputtered particles are deposited at an area except the element
isolating structure 5 and its shadows, thereby isolating structural
films for each line. For details, see Japanese Patent Application
Kokai No. 9-330792. While masked as shown at 6 in FIGS. 11 and 12,
the organic layers, electron injecting and transporting layer, etc.
are formed to assemble a matrix type of organic EL display. The
organic EL device of the invention is not limited to the
illustrated embodiment, and so may have various structures; the
number of segments and pixels, the pattern thereof, etc. may be
properly determined as occasion demands.
[0050] The organic layer comprises at least one hole transporting
layer and at least one light emitting layer, and include an
electron injecting electrode thereon. The organic layer may be
provided with a protective electrode in the form of the uppermost
layer. It is here to be noted that the hole transporting layer may
be dispensed with. The electron injecting electrode is then made up
of a metal compound or alloy material having a low work function by
means of evaporation or sputtering, and preferably sputtering.
[0051] For the material that forms a film form of electron
injecting electrode, it is preferable to use a material effective
for injection of electrons and having a low work function, e.g.,
any one of metal elements K, Li, Na, Mg, La, Ce, Ca, Sr, Ba, Al,
Ag, In, Sn, Zn, Zr, Cs, Er, Eu, Ga, Hf, Nd, Rb, Sc, Sm, Ta, Y, and
Yb, or compounds such as BaO, BaS, CaO, HfC, LbB.sub.6, MgO, MoC,
NbC, PbS, SrO, TaC, ThC, ThO.sub.2, ThS, TiC, TiN, UC, UN,
UO.sub.2, W.sub.2C, Y.sub.2O.sub.3, ZrC, ZrN, and ZrO.sub.2. To
improve the stability of the electrode, it is also preferable to
use binary or ternary alloy systems containing metal elements.
Preferred alloy systems, for instance, are aluminum alloy systems
such as Al.Ca (Ca: 5 to 20 at %), Al.In (In: 1 to 10 at %), Al.Li
(0.1 at %.ltoreq.Li<20 at %), and Al.R where R stands for a rare
earth element including Y, and Sc, and In.Mg systems (Mg: 50 to 80
at %). Particular preference is given to pure Al, and aluminum
alloy systems such as Al.Li (0.4 at %.ltoreq.Li<6.5 at % or 6.5
at %.ltoreq.Li.ltoreq.14 at %), and Al.R (R: 0.1 to 25 at %,
especially 0.5 to 20 at %) because they are unlikely to produce
compression stress. Thus, such electron injecting electrode-forming
metals or alloys are usually employed as sputtering targets. These
metals or alloys have a work function of 4.5 eV or lower. In the
practice of the invention, it is particularly preferable to use
metals or alloys having a work function of 4.0 eV or lower.
[0052] In the electron injecting electrode film formed by the
sputtering technique, the atoms or atom groups upon sputtering have
a kinetic energy relatively higher than would be obtained with the
evaporation technique, so that the adhesion of the electron
injecting electrode film to the organic layer at their interface is
improved due to a surface migration effect. In addition, an oxide
layer is removed in vacuum from the surface of the electrode by
pre-sputtering or moisture or oxygen is removed from the organic
layer interface, on which they are absorbed, by reverse sputtering
to form a clean electrode-organic layer interface or a clean
electrode, so that consistent organic EL displays of high quality
can be produced. For the target, the alloys having the aforesaid
composition range, and pure metals may be used alone or in
combination with an additional target comprising a subordinate
component or components or with the addition of a subordinate
component or components thereto. It is also acceptable to use a
mixture of materials having largely varying vapor pressures as the
target, because there is only slight a deviation of the composition
of the resultant film from the target composition. There is thus no
limitation on the material used with the sputtering technique,
whereas there are some limitations such as vapor pressure on the
evaporation technique. The sputtering technique is additionally
advantageous over the evaporation technique in terms of consistent
film thickness and quality as well as productivity, because it is
unnecessary to feed the raw material over an extended period of
time.
[0053] The electron injecting electrode formed by the sputtering
technique is a film so very dense that the penetration of moisture
into the film is much more reduced as compared with a coarse film
prepared by evaporation, and so the chemical stability of the film
is much more increased. This ensures the production of organic EL
displays having an ever longer service life.
[0054] Preferably, the sputtering gas pressure during sputtering is
in the range of 0.1 to 5 Pa. By regulating the sputtering gas
pressure within this range, it is possible to easily obtain an AlLi
alloy having an Li concentration in the aforesaid range. By
altering the sputtering gas pressure in the aforesaid range during
film formation, it is also possible to easily obtain an electron
injecting electrode having such an Li concentration gradient as
defined above. It is also preferable to meet a particular condition
that the product of the film forming gas pressure and the
substrate-to-target distance must be 20 to 65 Pa.cm.
[0055] For the sputtering gas, use is made of inert gases employed
with ordinary sputtering systems. For reactive sputtering, reactive
gases such as N.sub.2, H.sub.2, O.sub.2, C.sub.2H.sub.4, and
NH.sub.3 may be used in addition to these gases.
[0056] In the practice of the invention, it is possible to use an
RF sputtering process using an RF power source or the like as the
sputtering technique. In view of the ease with which the film
forming rate is controlled, and less damage to an organic EL device
structure, however, it is preferable to use a DC sputtering
process. Power for operating a CD sputtering system is in the range
of preferably 0.1 to 10 W/cm.sup.2, and especially 0.5 to 7
W/cm.sup.2. The film forming rate is preferably in the range of 5
to 100 nm/min., and especially 10 to 50 nm/min.
[0057] The thin film form of electron injecting electrode may have
at least a certain thickness enough for the injection of electrons,
e.g., of at least 1 nm, and preferably at least 3 nm. Thus, a film
thickness of the order of 3 to 500 nm is usually preferable
although there is no upper limit thereon.
[0058] The organic EL display of the invention may preferably have
a protective electrode on the electron injecting electrode, i.e.,
on the side of the electron injecting electrode that is not
opposite to the organic layer. By the provision of the protective
electrode, the electron injecting electrode is protected against
the air, moisture, etc., so that the degradation of the
constituting thin film can be prevented, resulting in the
stabilization of electron injection efficiency and an ever greater
increase in the service life of the device. The protective
electrode has a very low resistance, and so may also function as an
interconnecting electrode when the electron injecting electrode has
a high resistance. The protective electrode may contain at least
one of Al; Al and a transition metal except Ti; Ti; and titanium
nitride (TiN). When these are used alone, the protective electrode
preferably contains Al in an amount of about 90 to 100 at %, Ti in
an amount of about 90 to 100 at %, and TiN in an amount of about 90
to 100 mol %. Two or more of Al, Ti and TiN may be used at any
desired mixing ratio. For instance, a mixture of Al and Ti
preferably contains Ti in an amount of up to 10 at %.
Alternatively, it is acceptable to laminate together laminae each
containing a single species. In particular, favorable results are
obtained when Al or Al plus transition metal are used as the
interconnecting electrode to be described later. TiN, on the other
hand, provides a film having a striking sealing effect because of
its good corrosion resistance. For TiN, an about 10% deviation from
its stoichiometric composition is acceptable. In addition, Al
alloys, and transition metal alloys may contain transition metals,
especially Sc, Nb, Zr, Hf, Nd, Ta, Cu, Si, Cr, Mo, Mn, Ni, Pd, Pt
and W in the total amount of up to 10 at %, especially up to 5 at
%, and more especially up to 2 at %. When the protective electrode
functions as the interconnecting material, the thin film resistance
becomes lower with a decrease in the content of the transition
metal.
[0059] The protective electrode may have at least a certain
thickness enough to make sure of electron injection efficiency and
prevent penetration of moisture, oxygen or organic solvents, for
instance, of at least 50 nm, preferably at least 100 nm, and
especially 100 to 1,000 nm. With too thin a protective electrode
layer, neither are the advantages of the invention obtainable, nor
is sufficient connection with terminal electrodes obtainable
because the ability of the protective electrode layer to cover
steps becomes low. When the protective electrode layer is too
thick, on the other hand, the growth rate of dark spots becomes
high because of an increase in the stress of the protective
electrode layer. It is here to be noted that when the protective
electrode functions as an interconnecting electrode, its thickness
may be usually of the order of 100 to 500 nm so as to make up for
the high film resistance of the electron injecting electrode due to
its thinness, and that when the protective electrode functions as
other interconnecting electrode, its thickness may be of the order
of 100 to 300 nm.
[0060] Preferably but not exclusively, the total thickness of the
electron injecting electrode plus the protective electrode is
usually of the order of 100 to 1,000 nm.
[0061] In addition to the aforesaid protective electrode, an
additional protective film may be formed after the formation of the
electrode. The protective film may be formed of either an inorganic
material such as SiOx or an organic material such as Teflon, and a
chlorine-containing carbon fluoride polymer. The protective film
may be either transparent or opaque, and has a thickness of the
order of 50 to 1,200 nm. The protective film may be formed either
by the aforesaid reactive sputtering process or conventional
processes such as general sputtering, evaporation or PECVD.
[0062] In the practice of the invention, it is preferred to form a
sealing layer on the device in order to prevent oxidation of the
organic layers and electrodes. The sealing layer for preventing
generation of moisture may be formed by bonding sealing plates such
as glass plates with adhesive resin layers of low hygroscopicity
such as commercially available sheets of photo-curable adhesives,
epoxy adhesives, silicone adhesives, and crosslinking
ethylene-vinyl acetate copolymer adhesives. Instead of the glass
plates, metal or plastic plates may also be used.
[0063] Next, the organic material layers provided in the organic EL
device of the invention are explained.
[0064] The light emitting layer has functions of injecting holes
and electrons, transporting them, and recombining holes and
electrons to create excitons. For the light emitting layer, it is
preferable to use a relatively electronically neutral compound.
[0065] The hole injecting and transporting layer has functions of
facilitating injection of holes from the anode, providing stable
transportation of holes and blocking electrons, and the electron
injecting and transporting layer has functions of facilitating
injection of electrons from the cathode, providing stable
transportation of electrons and blocking holes. These layers are
effective for increasing the number of holes and electrons injected
into the light emitting layer and confining holes and electrons
therein for optimizing the recombination region to improve light
emission efficiency.
[0066] The thickness of the light emitting layer, the hole
injecting and transporting layer, and the electron injecting and
transporting layer is not critical and varies with a particular
formation technique although it is usually of the order of
preferably 5 to 500 nm, and especially 10 to 300 nm.
[0067] The thickness of the hole injecting and transporting layers,
and the electron injecting and transporting layer is equal to, or
about {fraction (1/10)} times to about 10 times as large as, the
thickness of the light emitting layer although it depends on the
design of the recombination/light emitting region. When the
electron or hole injecting and transporting layer is separated into
an injecting layer and a transporting layer, it is preferable that
the injecting layer is at least 1 nm thick and the transporting
layer is at least 1 nm thick. The upper limit on thickness is
usually about 500 nm for the injecting layer and about 500 nm for
the transporting layer. The same film thickness applies when two
injecting and transporting layers are provided.
[0068] In the organic EL device according to the invention, the
light emitting layer contains a fluorescent material that is a
compound capable of emitting light. The fluorescent material used
herein, for instance, may be at least one compound selected from
compounds such as those disclosed in JP-A 63-264692, e.g.,
quinacridone, rubrene, and styryl dyes. Use may also be made of
quinoline derivatives such as metal complex dyes containing
8-quinolinol or its derivative as ligands, for instance,
tris(8-quinolinolato)aluminum, tetraphenylbutadiene, anthracene,
perylene, coronene, and 12-phthaloperinone derivatives. Use may
further be made of phenylanthracene derivatives disclosed in
Japanese Patent Application Kokai No. 8-12600, and tetraarylethene
derivatives disclosed in Japanese Patent Application Kokai No.
8-12969.
[0069] Preferably, the fluorescent compound is used in combination
with a host substance capable of emitting light by itself; that is,
it is preferable that the fluorescent compound is used as a dopant.
In such a case, the content of the fluorescent compound in the
light emitting layer is in the range of preferably 0.01 to 10% by
weight, and especially 0.1 to 5% by weight. By using the
fluorescent compound in combination with the host substance, it is
possible to vary the wavelength performance of light emission,
thereby making E light emission possible on a longer wavelength
side and, hence, improving the light emission efficiency and
stability of the device.
[0070] Quinolinolato complexes, and aluminum complexes containing
8-quinolinol or its derivatives as ligands are preferred for the
host substance. Such aluminum complexes are typically disclosed in
JP-A's 63-264692, 3-255190, 5-70733, 5-258859, 6-215874, etc.
[0071] Exemplary aluminum complexes include
tris(8-quinolinolato)aluminum, bis(8-quinolinolato)magnesium,
bis(benzo{f}-8-quinolinolato)zinc,
bis(2-methyl-8-quinolinolato)aluminum oxide,
tris(8-quinolinolato)indium,
tris(5-methyl-8-quinolinolato)aluminum, 8-quinolinolatolithium,
tris(5-chloro-8-quinolinolato)gallium,
bis(5-chloro8-quinolinolato)calciu- m,
5,7-dichloro-8-quinolinolatoaluminum,
tris(5,7-dibromo-8-hydroxyquinoli- nolato)aluminum, and
poly[zinc(II)-bis(8-hydroxy-5-quinolinyl)methane].
[0072] Use may also be made of aluminum complexes containing other
ligands in addition to 8-quinolinol or its derivatives, for
instance, bis(2-methyl-8-quinolinolato)(phenolato) aluminum (III),
bis(2-methyl-8-quinolinolato)(o-cresolato) aluminum (III),
bis(2-methyl-8-quinolinolato)(m-cresolato) aluminum (III),
bis(2-methyl-8-quinolinolato)(p-cresolato) aluminum (III),
bis(2-methyl-8-quinolinolato)(o-phenylphenolato)aluminum (III),
bis(2-methyl-8-quinolinolato)(m-phenylphenolato)aluminum (III),
bis(2-methyl-8-quinolinolato) (p-phenylphenolato)aluminum (III),
bis(2-methyl-8-quinolinolato)(2,3-dimethylphenolato)aluminum (III),
bis(2-methyl-8-quinolinolato)(2,6-dimethyl-phenolato)aluminum
(III),
bis(2-methyl-8-quinolinolato)(3,4-dimethyl-phenolato)aluminum
(III), bis(2-methyl-8-quinolinolato)(3,5-dimethyl-phenolato)
aluminum (III),
bis(2-methyl-8-quinolinolato)(3,5-di-tert-butylphenolato)aluminum
(III), bis(2-methyl-8-quinolinolato)(2,6-diphenylphenolato)aluminum
(III),
bis(2-methyl-8-quinolinolato)(2,4,6-triphenylphenolato)aluminum
(III),
bis(2-methyl-8-quinolinolato)(2,3,6-trimethyl-phenolato)aluminum
(III),
bis(2-methyl-8-quinolinolato)(2,3,5,6-tetramethyl-phenolato)aluminum
(III), bis(2-methyl-8-quinolinolato)(1-naphtholato)aluminum (III),
bis(2-methyl-8-quinolinolato)(2-naphtholato)aluminum (III),
bis(2,4-dimethyl-8-quinolinolato)(o-phenylphenolato)aluminum (III),
bis (2,4-dimethyl-8-quinolinolato) (p-phenylphenolato) aluminum
(III), bis(2,4-dimethyl-8-quinolinolato)(m-phenylphenolato)
aluminum (III),
bis(2,4-dimethyl-8-quinolinolato)(3,5-dimethylphenolato)aluminum
(III),
bis(2,4-dimethyl-8-quinolinolato)(3,5-di-tert-butylphenolato)aluminum
(III), bis(2-methyl-4-ethyl-8-quinolinolato)(p-cresolato)aluminum
(III),
bis(2-methyl-4-methoxy-8-quinolinolato)(p-phenyl-phenolato)aluminum
(III), bis(2-methyl-5-cyano-8-quinolinolato)(o-cresolato)aluminum
(III), and
bis(2-methyl-6-trifluoromethyl-8-quinolinolato)(2-naphtholato)aluminu-
m (III).
[0073] Besides, use may be made of
bis(2-methyl-8-quinolinolato)aluminum
(III)-.mu.-oxo-bis(2-methyl-8-quinolinolato) aluminum (III),
bis(2,4-dimethyl-8-quinolinolato)aluminum
(III)-.mu.-oxo-bis(2,4-dimethyl- -8-quinolinolato)aluminum (III),
bis(4-ethyl-2-methyl-8-quinolinolato)alum- inum
(III)-.mu.-oxo-bis(4-ethyl-2-methyl-8-quinolinolato)aluminum (III),
bis(2-methyl-4-methoxyquinolinolato)aluminum
(III)-.mu.-oxo-bis(2-methyl-- 4-methoxyquinolinolato)aluminum
(III), bis(5-cyano-2-methyl-8-quinolinolat- o)aluminum
(III)-.mu.-oxo-bis(5-cyano-2-methyl-8-quinolinolato)aluminum (III),
bis(2-methyl-5-trifluoromethyl-8-quinolinolato)aluminum
(III)-.mu.-oxo-bis(2-methyl-5-trifluoromethyl-8-quinolinolato)aluminum
(III), etc.
[0074] Other preferable host substances include phenyl-anthracene
derivatives disclosed in Japanese Patent Application Kokai No.
8-12600, tetraarylethene derivatives disclosed in Japanese Patent
Application Kokai No. 8-12969, etc.
[0075] In the practice of the invention, the light emitting layer
may also serve as an electron injecting and transporting layer. In
this case, it is preferable to use a fluorescent material, e.g.,
tris(8-quinolinolato)aluminum or the like, which may be provided by
evaporation.
[0076] If necessary or preferably, the light emitting layer is
formed of a mixed layer of at least one compound capable of
injecting and transporting holes with at least one compound capable
of injecting and transporting electrons. Preferably in this case, a
dopant is incorporated in the mixed layer. The content of the
dopant compound in the mixed layer is in the range of preferably
0.01 to 20% by weight, and especially 0.1 to 15% by weight.
[0077] In the mixed layer with a hopping conduction path available
for carriers, each carrier migrates in the polarly prevailing
substance, so making the injection of carriers having an opposite
polarity unlikely to occur. This leads to an increase in the
service life of the device due to less damage to the organic
compound. By incorporating the aforesaid dopant in such a mixed
layer, it is possible to vary the wavelength performance of light
emission that the mixed layer itself possesses, thereby shifting
the wavelength of light emission to a longer wavelength side and
improving the intensity of light emission, and the stability of the
device as well.
[0078] The compound capable of injecting and transporting holes and
the compound capable of injecting and transporting electrons, both
used to form the mixed layer, may be selected from compounds for
the injection and transportation of holes and compounds for the
injection and transportation of electrons, as will be described
later. Especially for the compounds for the injection and
transportation of holes, it is preferable to use amine derivatives
having strong fluorescence, for instance, hole transporting
materials such as triphenyldiamine derivatives, styrylamine
derivatives, and amine derivatives having an aromatic fused
ring.
[0079] For the compounds capable of injecting and transporting
electrons, it is preferable to use metal complexes containing
quinoline derivatives, especially 8-quinolinol or its derivatives
as ligands, in particular, tris(8-quinolinolato) aluminum
(Alq.sup.3). It is also preferable to use the aforesaid
phenylanthracene derivatives, and tetraarylethene derivatives.
[0080] For the compounds for the injection and transportation of
holes, it is preferable to use amine derivatives having strong
fluorescence, for instance, hole transporting materials such as
triphenyldiamine derivatives, styrylamine derivatives, and amine
derivatives having an aromatic fused ring.
[0081] In this case, the ratio of mixing the compound capable of
injecting and transporting holes with the compound capable of
injecting and transporting electrons is determined while the
carrier mobility and carrier density are taken into consideration.
In general, however, it is preferred that the weight ratio between
the compound capable of injecting and transporting holes and the
compound capable of injecting and transporting electrons is of the
order of 1/99 to 99/1, particularly 10/90 to 90/10, and more
particularly 20/80 to 80/20.
[0082] The thickness of the mixed layer must correspond to the
thickness of a single molecular layer, and so is preferably less
than the thickness of the organic compound layer. More
specifically, the mixed layer has a thickness of preferably 1 to 85
nm, especially 5 to 60 nm, and more especially 5 to 50 nm.
[0083] Preferably, the mixed layer is formed by co-evaporation
where the selected compounds are evaporated from different
evaporation sources. When the compounds to be mixed have identical
or slightly different vapor pressures (evaporation temperatures),
however, they may have previously been mixed together in the same
evaporation boat for the subsequent evaporation. Preferably, the
compounds are uniformly mixed together in the mixed layer. However,
the compounds in an archipelagic form may be present in the mixed
layer. The light emitting layer may generally be formed at a given
thickness by the evaporation of the organic fluorescent substance
or coating a dispersion of the organic fluorescent substance in a
resin binder.
[0084] For the hole injecting and transporting layer, use may be
made of various organic compounds as disclosed in JP-A's 63-295695,
2-191694, 3-792, 5-234681, 5-239455, 5-299174, 7-126225, 7-126226
and 8-100172 and EP 0650955A1. Examples are tetraarylbenzidine
compounds (triaryldiamine or triphenyldiamine (TPD)), aromatic
tertiary amines, hydrazone derivatives, carbazole derivatives,
triazole derivatives, imidazole derivatives, oxadiazole derivatives
having an amino group, and polythiophenes. Where these compounds
are used in combination of two or more, they may be stacked as
separate layers, or otherwise mixed.
[0085] When the hole injecting and transporting layer is provided
as a separate hole injecting layer and a separate hole transporting
layer, two or more compounds are selected in a preferable
combination from the compounds already mentioned for the hole
injecting and transporting layer. In this regard, it is preferred
to laminate layers in such an order that a compound layer having a
lower ionization potential is disposed nearest to the hole
injecting electrode (ITO, etc.). It is also preferred to use a
compound having good thin film forming ability at the anode
surface. This order of lamination holds for the provision of two or
more hole injecting and transporting layers, and is effective as
well for lowering driving voltage and preventing the occurrence of
current leakage and the appearance and growth of dark spots. Since
evaporation is utilized in the manufacture of devices, films as
thin as about 1 to 10 nm can be formed in a uniform and
pinhole-free state, which restrains any change in color tone of
light emission and a drop of efficiency by re-absorption even if a
compound having a low ionization potential and absorption in the
visible range is used in the hole injecting layer. Like the light
emitting layer and so on, the hole injecting and transporting layer
or layers may be formed by evaporating the aforesaid compounds.
[0086] For the electron injecting and transporting layer which is
provided if necessary, there may be used quinoline derivatives such
as organic metal complexes containing 8-quinolinol or its
derivatives as ligands, for instance, tris(8-quinolinolato)aluminum
(Alq.sup.3), oxadiazole derivatives, perylene derivatives, pyridine
derivatives, pyrimidine derivatives, quinoxaline derivative,
diphenylquinone derivatives, and nitro-substituted fluorene
derivatives. The electron injecting and transporting layer may also
serve as a light emitting layer. In this case, it is preferable to
use tris(8-quinolilato)aluminum, etc. As is the case with the light
emitting layer, the electron injecting and transporting layer may
then be formed by evaporation or the like.
[0087] Where the electron injecting and transporting layer is a
double-layered structure comprising an electron injecting layer and
an electron transporting layer, two or more compounds are selected
in a preferably combination from the compounds commonly used for
electron injecting and transporting layers. In this regard, it is
preferred to laminate layers in such an order that a compound layer
having a greater electron affinity is disposed nearest to the
cathode. This order of lamination also applies where a plurality of
electron injecting and transporting layers are provided.
[0088] For the substrate material, transparent or translucent
materials such as glass, quartz and resins are used. The substrate
may be provided with a color filter film, fluorescent
material-containing color conversion film or dielectric reflecting
film for controlling the color of light emission.
[0089] For the color filter film, a color filter employed with
liquid crystal display devices may be used. However, it is
preferable to control the properties of the color filter in
conformity to the light emitted from the organic EL device, thereby
optimizing the efficiency of taking out light emission and color
purity.
[0090] By using a color filter capable of cutting off extraneous
light of such short wavelength as absorbed by the EL device
material or the fluorescent conversion layer, it is possible to
improve the light resistance of the device and the contrast of what
is displayed on the device.
[0091] Instead of the color filter, an optical thin film such as a
dielectric multilayer film may be used.
[0092] The fluorescent color conversion film absorbs light emitted
from an EL device and gives out light from the phosphors contained
therein for the color conversion of light emission, and is composed
of three components, a binder, a fluorescent material and a light
absorbing material.
[0093] In the practice of the invention, it is basically preferable
to use a fluorescent material having high fluorescent quantum
efficiency, and especially a fluorescent material having strong
absorption in an EL light emission wavelength region. Laser dyes
are suitable for the practice of the invention. To this end, for
instance, it is preferable to use rohodamine compounds, perylene
compounds, cyanine compounds, phthalocyanine compounds (including
subphthalocyanine compounds, etc.), naphthaloimide compounds, fused
cyclic hydrocarbon compounds, fused heterocyclic compounds, styryl
compounds, and coumarin compounds.
[0094] For the binder, it is basically preferable to make an
appropriate selection from materials that do not extinguish
fluorescence. It is particularly preferable to use a material that
can be finely patterned by photolithography, printing or the like.
It is also preferable to use a material that is not damaged during
ITO or IZO film formation.
[0095] The light absorbing material is used when light is not fully
absorbed by the fluorescent material, and so may be dispensed with,
if not required. For the light absorbing material, it is preferable
to make a selection from materials that do not extinguish
fluorescence.
[0096] To form the hole injecting and transporting layer, the light
emitting layer and the electron injecting and transporting layer,
it is preferable to use a vacuum evaporation technique which
enables a homogeneous thin film to be obtained. According to the
vacuum evaporation process, it is possible to obtain homogeneous
thin films in an amorphous state or with a crystal grain diameter
of at most 0.1 .mu.m. The use of a thin film having a crystal grain
diameter exceeding 0.1 .mu.m results in non-uniform light emission.
To avoid this, it is required to increase the driving voltage of
the device; however, there is a striking drop of charge injection
efficiency.
[0097] No particular limitation is imposed on vacuum evaporation
conditions. However, an evaporation rate of the order of 0.01 to 1
nm/sec. is preferably applied at a degree of vacuum of 10.sup.-4 Pa
or lower. It is also preferable to form the layers continuously in
vacuum. If the layers are continuously formed in vacuum, high
properties are then obtained because the adsorption of impurities
on the interface between the adjacent layers can be avoided.
Furthermore, the driving voltage of the device can be lowered while
the growth and occurrence of dark spots are inhibited.
[0098] When the vacuum evaporation process is used to form the
layers, each containing a plurality of compounds, It is preferable
to carry out co-evaporation while boats charged with the compounds
are individually placed under temperature control.
[0099] The organic EL device of the invention is generally of the
DC drive type while it may be of the AC or pulse drive type. The
applied voltage is generally of the order of 2 to 20 volts.
EXAMPLE
[0100] The present invention are explained more specifically with
reference to some illustrative examples.
Example 1
[0101] A glass substrate was provided thereon with an 85 nm-thick
ITO transparent electrode (hole injecting electrode), and pixels of
280.times.280 .mu.m.sup.2 size were patterned on the ITO
transparent electrode at an interval of 20 .mu.m and according to
an array of 64 dots.times.7 lines. Then, an about 1.2 .mu.m-thick
Al-W (W: 3.0 at %) alloy electrode (having a sheet resistance of
0.4 .OMEGA./.quadrature.) was patterned around the ITO transparent
electrode. Used for metal electrodes between pixels, and lines was
a common electrode. The resistance of the obtained hole injecting
electrode was measured. As a result, it was found that the obtained
resistance value is {fraction (1/50)} or lower of that of a hole
injecting electrode made up of ITO alone, as can be seen from a
resistance of 64 .OMEGA. obtained for each line.
Example 2
[0102] A hole injecting electrode comprising a metal electrode and
an ITO transparent electrode prepared as in Example 1 with the
exception that the film thickness was changed to 120 nm was formed
on a substrate, which was in turn ultrasonically washed with
neutral detergent, acetone, and ethanol, and then pulled up from
boiling ethanol, followed by drying. This substrate was cleaned on
its surface with UV/O.sub.3, and fixed to a substrate holder in a
vacuum evaporation system, which was evacuated to a vacuum of
1.times.10.sup.-4 Pa or lower. 4,4', 4"-tris
(-N-methylphenyl)-N-phenylamino) triphenylamine (hereinafter
m-MTDATA for short) was deposited by evaporation at a deposition
rate of 0.2 nm/sec. to a thickness of 40 nm, thereby forming a hole
injecting layer. With the vacuum still maintained,
N,N'-diphenyl-N,N'-m-tolyl-4,4'-diamino-1,1'-bip- henyl
(hereinafter TPD for short) was then deposited by evaporation at a
deposition rate of 0.2 nm/sec. to a thickness of 35 nm, thereby
forming a hole transporting layer. With the vacuum still kept,
tris(8-quinolinolato)aluminum (hereinafter Alq.sup.3 for short) was
deposited by evaporation at a deposition rate of 0.2 nm/sec. to a
thickness of 50 nm, thereby forming a combined electron
injecting/transporting and light emitting layer. With the vacuum
still kept, MgAg was deposited by co-evaporation at an evaporation
rate ratio of Mg:Ag=10:1 to a thickness of 200 nm, thereby forming
an electron injecting layer.
[0103] The thus assembled organic EL device was driven in a dry air
atmosphere in such a way that a luminance of 100 cd/M.sup.2 was
obtained. By comparison, it was found that there is no difference
between the light emission luminance at 7 dots.times.7 lines on the
current feed (terminal electrode) side of the organic EL device and
that on the opposite (end) side thereof. This showed that there is
no influence due to a voltage drop.
Example 3
[0104] An organic EL device was obtained as in Example 2 with the
exception that the thickness of the ITO transparent electrode was
changed to 50 nm. This device was driven for 5,000 hours. The
density of dark spots found decreased to {fraction (1/2)} or lower.
From this, it was found that the reliability of the device is much
more improved.
[0105] The obtained organic EL device was driven as in Example 2
for estimation purposes. Again, substantially similar results as in
Example 2 were obtained with less luminance variations.
Example 4
[0106] Organic EL devices were obtained as in Example 2 with the
exception that the metal electrode-forming material was changed
from Al to Al--Si--Cu (Si: 0.97 at %, Cu:0.53 at %), Al--Nd (Nd:
2.1 at %), Al--Ta (Ta: 2.1 at %), Al--Sc (Sc: 0.13 at %), and Al--W
(W: 2.0 at %), respectively.
[0107] As a result of estimation of the obtained organic EL devices
made as in Example 2, substantially similar results as in Example 2
were obtained.
Example 5
[0108] An organic EL device was prepared as in Example 2. However,
a Cr barrier layer of 15 .mu.m in width and 60 an in thickness was
formed at an interface portion between the ITO transparent
electrode provided and the surrounding metal electrode. The metal
electrode was formed as in Example 2.
[0109] As a result of estimation of the obtained organic EL device
made as in Example 2, substantially similar results as in Example 2
were obtained.
Example 6
[0110] Organic EL devices were obtained as in Example 5 with the
exception that the barrier layer-forming material was changed from
Cr to Ti, and TiN (N: 52 at %), respectively. As a result of
estimation of these devices made as in Example 5, much the same
results as in Example 5 were obtained.
Example 7
[0111] An organic EL device was obtained as in Example 2 with the
exception that an IZO transparent prepared as in Example 1 was
used.
[0112] The obtained EL device was driven and estimated as in
Example 2. As a result, much the same results as in Example 2 were
obtained.
Example 8
[0113] As shown in FIGS. 13 and 14, a glass substrate was provided
thereon with a 95 nm-thick ITO transparent electrode (hole
injecting electrode) 2, and pixels of 280.times.280 .mu.m.sup.2
size were patterned on the ITO transparent electrode at an interval
of 20 .mu.m and according to an array of 64 dots.times.256 lines.
FIG. 13 is a partial plan view illustrative of a pattern of the ITO
transparent electrode and metal electrode, and FIG. 14 is a
perspective view of FIG. 13 as taken along the D-D' line. Then, a
Cr barrier layer 7 of 100 nm in thickness and an Al metal electrode
3 of 300 nm in thickness (and having a sheet resistance of 0.1
.OMEGA./.quadrature.) were continuously formed and patterned at a
total width of 30 .mu.m around the ITO transparent electrode.
[0114] The hole injecting electrode comprising the ITO transparent
electrode and metal electrode was formed on a substrate, which was
in turn ultrasonically washed with neutral detergent, acetone, and
ethanol, and then pulled up from boiling ethanol, followed by
drying. This substrate was then cleaned on its surface with
UV/O.sub.3, and fixed to a substrate holder in a vacuum evaporation
system, which was evacuated to a vacuum of 1.times.10.sup.-4 Pa or
lower. 4,4',4'-tris(-N-methylphenyl)-N-- phenylamino)triphenylamine
(m-MTDATA) was deposited by evaporation at a deposition rate of 0.2
nm/sec. to a thickness of 40 nm, thereby forming a hole injecting
layer. With the vacuum still maintained,
N,N'-diphenyl-N,N'-m-tolyl-4,4'-diamino-1,1'-biphenyl (TPD) was
then deposited by evaporation at a deposition rate of 0.2 nm/sec.
to a thickness of 35 nm, thereby forming a hole transporting layer.
With the vacuum still kept, tris(8-quinolinolato)aluminum
(Alq.sup.3) was deposited by evaporation at a deposition rate of
0.2 nm/sec. to a thickness of 50 nm, thereby forming a combined
electron injecting/transporting and light emitting layer. With the
vacuum still kept, MgAg was deposited by co-evaporation at an
evaporation rate ratio of Mg:Ag=10:1 to a thickness of 200 nm,
thereby forming an electron injecting layer.
[0115] The thus assembled organic EL device was driven in a dry air
atmosphere in such a way that a luminance of 100 cd/m.sup.2 was
obtained. The luminance of the organic EL device from the current
feed side to the opposite (end) side thereof was measured with the
range of 7 dots.times.7 lines on the current feed (terminal
electrode) side defined as one measurement unit. Consequently, it
was found that the light emission luminance change across the
device was within -5% with respect to the time-average light
emission luminance of 100 cd/m.sup.2 on the current feed side of
the device. From this, it was found that even a display having a
large display area is little affected by a voltage drop.
Comparative Example 1
[0116] An organic EL device was obtained as in Example 1 with the
exception that a hole injecting electrode was formed only by use of
an ITO transparent electrode. In this case, the resistance of the
hole injecting electrode per line was 3.2 k.OMEGA..
[0117] The obtained organic EL device was driven and estimated as
in Example 1. Consequently, it was found that the luminance change
shows a high of -20% with respect to the time-average light
emission luminance of 100 cd/m.sup.2 on the current feed side of
the device.
[0118] According to the present invention, it is possible to
achieve an organic EL device which can reduce the interconnecting
resistance of a hole injecting electrode, and so accommodate itself
to sophisticated display patterns or large-screen displays designed
to be driven with high E luminance or at a high duty ratio, as can
be understood from the foregoing.
[0119] Japanese Patent Application No. 199256/1997 is herein
incorporated by reference.
[0120] While the invention has been described with reference to
preferred embodiments, it will be obvious to those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiments disclosed as the best mode contemplated for
carrying out the invention, but that the invention will include all
embodiments falling within the scope of the appended claims.
* * * * *