U.S. patent application number 09/212408 was filed with the patent office on 2001-11-15 for organic electroluminescent device.
Invention is credited to ARAI, MICHIO, YAMAMOTO, HIROSHI.
Application Number | 20010041268 09/212408 |
Document ID | / |
Family ID | 18490806 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010041268 |
Kind Code |
A1 |
ARAI, MICHIO ; et
al. |
November 15, 2001 |
ORGANIC ELECTROLUMINESCENT DEVICE
Abstract
An organic EL device includes a substrate, an organic EL
structure, and a barrier layer therebetween. The substrate is of
alkali glass. The barrier layer is of silicon oxide. The device has
advantages including retarded occurrence of dark spots, improved
storage stability and durability, and reduced expense of
manufacture.
Inventors: |
ARAI, MICHIO; (TOKYO,
JP) ; YAMAMOTO, HIROSHI; (TOKYO, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Family ID: |
18490806 |
Appl. No.: |
09/212408 |
Filed: |
December 16, 1998 |
Current U.S.
Class: |
428/690 ;
313/504; 313/506; 428/428; 428/917 |
Current CPC
Class: |
H01L 51/525 20130101;
Y10S 428/917 20130101; H01L 51/5253 20130101; H01L 51/52
20130101 |
Class at
Publication: |
428/690 ;
428/917; 428/428; 313/504; 313/506 |
International
Class: |
H05B 033/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 1997 |
JP |
9-368031 |
Claims
1. An organic electroluminescent device comprising a substrate of
alkali glass, an organic electroluminescent structure on the
substrate, and a barrier layer between said substrate and said
organic electroluminescent structure, said barrier layer containing
silicon oxide.
2. The organic electroluminescent device of claim 1 wherein said
barrier layer has a refractive index of 1.40 to 1.55 at a
wavelength of 632 nm.
3. The organic electroluminescent device of claim 1 wherein said
silicon oxide is represented by SiOx wherein x is from 1.8 to
2.2.
4. The organic electroluminescent device of claim 1 wherein said
substrate contains 10 to 30% by weight, calculated as Na.sub.2O and
K.sub.2O, of sodium or potassium or a mixture of sodium and
potassium.
5. The organic electroluminescent device of claim 1 wherein said
barrier layer has a thickness of 5 to 50 nm.
6. The organic electroluminescent device of claim 1 wherein said
barrier layer has a mean surface roughness (Ra) of 2 to 50 nm.
7. The organic electroluminescent device of claim 1 wherein said
substrate has a mean surface roughness (Ra) of 2 to 50 nm.
8. The organic electroluminescent device of claim 1 wherein said
barrier layer has been formed by sputtering.
9. The organic electroluminescent device of claim 1 wherein said
substrate has been mirror polished.
Description
[0001] This invention relates to an organic electroluminescent (EL)
device using an organic compound and more particularly, to an
organic EL device having a barrier layer between a substrate and an
organic EL structure.
BACKGROUND OF THE INVENTION
[0002] Recently, active research works have been made on organic EL
devices. As a basic configuration, the organic EL device includes a
hole injecting electrode of tin-doped indium oxide (ITO) etc., a
thin film formed thereon by depositing a hole transporting material
such as triphenyldiamine (TPD), a light emitting layer deposited
thereon of a fluorescent material such as an aluminum quinolinol
complex (Alq3), and a metal electrode or electron injecting
electrode formed thereon from a metal having a low work function
such as magnesium. Such organic EL devices are attractive in that
they can achieve a very high luminance ranging from several 100 to
several 10,000 cd/m.sup.2 with a drive voltage of approximately 10
volts.
[0003] The organic EL devices sometimes suffer from a decline of
luminance with the lapse of driving time, abnormal light emission
due to current leakage, and a phenomenon of generating and
propagating non-luminous regions known as dark spots. As dark spots
propagate and the abnormal light emission worsens, the devices can
deteriorate to a practically unacceptable level. Also, the organic
EL devices are quite vulnerable to moisture. Penetration of
moisture can cause separation between the light emitting layer and
the electrode layer or alter the properties of the constituent
materials, also creating dark spots and failing to maintain light
emission of the desired quality. It is then an important task to
prevent the occurrence of defects and the deterioration of device
characteristics.
[0004] One factor that causes deterioration of the organic EL
device is impurities in substrate glass such as sodium and
potassium which migrate and diffuse from the substrate to the hole
injecting electrode. A common solution to this problem is to use
alkali-free glass, which is expensive. It is commercially
advantageous if inexpensive alkali glass can be used.
SUMMARY OF THE INVENTION
[0005] An object of the present invention is to provide an organic
EL device which has advantages including minimized occurrence of
abnormal light emission and dark spots, improved storage life and
durability and which can be manufactured at a low cost.
[0006] According to the invention, there is provided an organic
electroluminescent (EL) device comprising a substrate of alkali
glass, an organic EL structure on the substrate, and a barrier
layer between the substrate and the organic EL structure, the
barrier layer containing silicon oxide.
[0007] In preferred embodiments, the barrier layer has a refractive
index of 1.40 to 1.55 at a wavelength of 632 nm; the barrier layer
consists essentially of silicon oxide which is represented by SiOx
wherein x is from 1.8 to 2.2; the barrier layer is formed by
sputtering, especially to a thickness of 5 to 50 nm and a mean
surface roughness (Ra) of 2 to 50 nm.
[0008] In further preferred embodiments, the substrate contains 10
to 30% by weight, calculated as Na.sub.2O and K.sub.2O, of sodium
or potassium or a mixture of sodium and potassium; the substrate
has a mean surface roughness (Ra) of 2 to 50 nm; the substrate has
been mirror polished.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The only FIGURE, FIG. 1 is a schematic view illustrating one
exemplary construction of an organic EL device according to the
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0010] The organic EL device of the present invention has a barrier
layer between a glass substrate and an organic EL structure, the
barrier layer containing silicon oxide SiOx.
[0011] Barrier Layer
[0012] The barrier layer containing SiOx is in passivated form
which is effective for preventing migration of impurities in the
substrate glass such as sodium and potassium. By preventing sodium,
potassium and other impurities in the substrate from diffusing into
the hole injecting electrode, the device is improved in storage
life and durability.
[0013] Although organic EL devices are vulnerable to moisture, the
barrier layer containing SiOx is effective for protecting the
organic EL structure from the ambient atmosphere and moisture, also
contributing to the improvements in storage life and
durability.
[0014] The barrier layer preferably has a refractive index of 1.40
to 1.55, more preferably 1.44 to 1.48 at a wavelength of 632 nm.
Outside this range, a layer with a higher refractive index would
become a weak barrier to alkali metals such as sodium and potassium
whereas a layer with a lower refractive index would becomes a weak
barrier to moisture.
[0015] In addition to SiOx, the barrier layer may contain N, C, Ar,
etc. as incidental impurities in an amount of up to 0.5% by
weight.
[0016] When silicon oxide is represented by SiOx, x preferably
ranges from 1.8 to 2.2, more preferably from 1.90 to 2.05. As long
as x has a value in this range as an average throughout the barrier
layer, the value of x may have a gradation in a thickness
direction.
[0017] Further preferably, the barrier layer has a transmittance of
at least 90% of emitted light. A barrier layer with a lower
transmittance would attenuate the light emitted from the light
emitting layer to below the luminance level necessary as a light
emitting device.
[0018] The thickness of the barrier layer is not particularly
limited as long as the above preferred parameters are met.
Preferably the barrier layer is about 5 to 50 nm thick, more
preferably about 10 to 30 nm thick.
[0019] The barrier layer containing SiOx can be formed by plasma
chemical vapor deposition (CVD) and other processes although
sputtering is the most preferred process. In forming a film having
the above-described attributes, RF sputtering using an RF power
supply is preferable. The plasma CVD process has the risk that
hydrogen is introduced from reactive gases into the film,
deteriorating the barrier properties of the film against
moisture.
[0020] When the barrier layer is formed by sputtering, the
sputtering gas may be any of inert gases used in conventional
sputtering equipment. In particular, Ar, Kr, or Xe or a gas mixture
containing at least one of these rare gases is preferred.
[0021] Ar, Kr, and Xe are preferred because they are inert gases
and have a relatively high atomic weight. When Ar, Kr or Xe gas is
used, the sputtered atoms perform repetitive collisions with the
gas and reduce their kinetic energy until they arrive at the
substrate. This restrains grain growth and insures a smooth surface
to the film.
[0022] When Ar, Kr or Xe is used as the main sputtering gas, the
product of the substrate-to-target distance multiplied by the
pressure is preferably in the range of 20 to 60 Pa.multidot.cm,
especially 30 to 50 Pa.multidot.cm. Within this range, better
results are obtained independent of the identity of the sputtering
gas selected from Ar, Kr and Xe. Use of Ar is especially
preferred.
[0023] To compensate for oxygen defects, oxygen gas such as O.sub.2
may be mixed with the sputtering gas. Oxygen gas is preferably
introduced so as to give a partial pressure of about 0.1% to about
100% of the sputtering gas. Even when oxygen gas is introduced
under an equal partial pressure, the quantity of oxygen taken into
the barrier layer differs depending on the substrate temperature,
sputtering gas pressure, target-to-substrate distance, input power
and other conditions. Then the partial pressure of oxygen gas may
be adjusted as appropriate in accordance with these conditions.
[0024] The sputtering process is preferably RF sputtering. The
power of the RF sputtering equipment is preferably in the range of
about 10 to about 100 W/cm.sup.2. The selected frequency is 13.56
MHz. The deposition rate is preferably in the range of about 5 to
about 50 nm/min. An appropriate operating pressure is in the range
of 0.1 to 1 Pa.
[0025] The barrier layer on its surface (or interface with the hole
injecting electrode) preferably has a mean surface roughness (Ra)
of 2 to 50 nm. If the barrier layer surface loses flatness, such an
irregular surface can cause generation of current leakage and dark
spots. On this account, it is preferable to control the mean
surface roughness (Ra) of the barrier layer at the interface with
the hole injecting electrode so as to fall in the above range, by
selecting appropriate deposition conditions for suppressing
abnormal grain growth.
[0026] Substrate
[0027] The substrate is a flat plate of alkali glass having a
certain strength which is easy to handle, readily available, and
inexpensive. Since the SiOx-containing barrier layer prevents
diffusion of Na, K and other undesirable elements from the
substrate into the hole injecting electrode, the substrate glass
may contain about 10% to 30% by weight, calculated as Na.sub.2O and
K.sub.2O, of sodium or potassium or a mixture of sodium and
potassium. Such alkali glass is commercially advantageous since it
is less expensive than alkali-free glass conventionally employed as
the substrate. The content of silicon oxide in glass is preferably
55% to 80% by weight calculated as SiO.sub.2. Additionally, the
glass may contain divalent metal oxides such as calcium oxide and
magnesium oxide, aluminum oxide, boron oxide, and phosphorus oxide
in amounts of up to 35% by weight, calculated as CaO, MgO,
Al.sub.2O.sub.3, B.sub.2O.sub.3, and P.sub.2O.sub.5,
respectively.
[0028] The thickness of the substrate is not critical and may be
determined in accordance with the necessary strength, transparency,
display size, and the presence or absence of a filter. Usually the
substrate is about 0.3 to about 20 mm thick, preferably about 0.5
to about 10 mm thick.
[0029] Preferably, the substrate has a mean surface roughness (Ra)
of 2 to 50 nm. If the substrate surface loses flatness, there is a
likelihood of inducing current leakage and dark spots. Then the
substrate at the surface is mirror polished with abrasives of
diamond or cerium oxide so that the mean surface roughness (Ra) may
fall in the above range.
[0030] Referring to FIG. 1, there is illustrated one exemplary
construction of the organic EL device of the invention. The EL
device shown in FIG. 1 includes a substrate 1 and has a barrier
layer 2, an anode 3, a hole injecting and transporting layer 4, a
light emitting layer 5, an electron injecting and transporting
layer 6, and a cathode 7 arranged on the substrate 1 in the
described order.
[0031] Organic EL Structure
[0032] Now the organic EL structure included in the organic EL
device of the invention is described.
[0033] The organic EL structure is situated on the substrate and
includes a hole injecting electrode, an electron injecting
electrode, and at least one organic layer disposed between these
electrodes. The at least one organic layer includes at least one
hole transporting layer and at least one light emitting layer, on
which the electron injecting electrode is situated. A protective
electrode may be provided as the uppermost layer. The hole
transporting layer may be omitted. The electron injecting electrode
is constructed of a metal, compound or alloy having a low work
function by evaporation or sputtering, preferably by
sputtering.
[0034] A transparent electrode is preferred as the hole injecting
electrode because a structure allowing emitted light to exit from
the substrate side is typical. Useful materials include tin-doped
indium oxide (ITO), zinc-doped indium oxide (IZO), zinc oxide
(ZnO), tin oxide (SnO.sub.2), and indium oxide (In.sub.2O.sub.3),
with ITO and IZO being preferred. For ITO, an appropriate
proportion of SnO.sub.2 mixed with In.sub.2O.sub.3 is about 1 to
20%, more preferably about 5 to 12% by weight. For IZO, an
appropriate proportion of ZnO mixed with In.sub.2O.sub.3 is about 1
to 20%, more preferably about 5 to 12% by weight. Additionally, Sn,
Ti, Pb and so forth may be contained in oxide form in an amount of
up to 1% by weight calculated as oxide.
[0035] Although the hole injecting electrode can be formed by
evaporation and other processes, sputtering is the preferred
process. Where an ITO or IZO electrode is formed by sputtering, a
target of In.sub.2O.sub.3 doped with SnO.sub.2 or ZnO is preferably
used. A transparent ITO electrode formed by sputtering experiences
a less change with time of luminance than an ITO electrode formed
by evaporation. The preferred sputtering process is dc sputtering.
The input power is preferably in the range of about 0.1 to about 4
W/cm.sup.2. In particular, the preferred input power of the dc
sputtering equipment is in the range of about 0.1 to about 10
W/cm.sup.2, more preferably 0.2 to 5 W/cm.sup.2. The deposition
rate is preferably in the range of about 2 to about 100 nm/min.,
especially about 5 to about 50 nm/min.
[0036] The sputtering gas may be any of inert gases used in
conventional sputtering equipment, for example, Ar, He, Ne, Kr, Xe,
and mixtures of such inert gases. The sputtering gas is kept under
a pressure of about 0.1 to about 20 Pa during sputtering.
[0037] The hole injecting electrode should have a sufficient
thickness for hole injection and is preferably about 5 to about 500
nm thick, especially about 10 to 300 nm thick.
[0038] The electron injecting electrode is preferably formed from
materials having a low work function for effective electron
injection. Exemplary materials include metal elements such as 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, and compounds such as BaO,
BaS, CaO, HfC, LaB.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. Binary or ternary alloys
made of two or three metal elements are useful for stability
improvement. Exemplary alloys are aluminum base alloys such as
Al--Ca (Ca: 5 to 20 at %), Al--In (In: 1 to 10 at %), Al--Li (Li:
0.1 to less than 20 at %), and Al--R wherein R is Y, Sc or rare
earth element, and In--Mg (Mg: 50 to 80 at %). Of these, elemental
aluminum and aluminum alloys such as Al--Li (Li: 0.4 to less than
6.5 at % or Li: 6.5 to 14 at %) and Al--R (R: 0.1 to 25 at %,
especially 0.5 to 20 at %) are preferable because they are
relatively free from compression stresses. Then, for sputtering,
targets of such electron injecting electrode-forming metals or
alloys are used. These materials should preferably have a work
function of up to 4.5 eV, with metals and alloys having a work
function of up to 4.0 eV being especially preferred.
[0039] The use of the sputtering process in forming the electron
injecting electrode has several advantages. The electron injecting
electrode film formed by the sputtering process is improved in
adhesion at the interface with the organic layer because as
compared with the evaporation process, the sputtered atoms and
atomic groups acquire relatively high kinetic energy so that the
surface migration effect may occur. Also, since the surface oxide
layer can be removed in vacuum by effecting pre-sputtering or the
moisture and oxygen which have adsorbed on the organic layer
interface can be removed by effecting back sputtering, it is
possible to form a clean electrode-organic layer interface and a
clean electrode and eventually, to fabricate an organic EL device
of quality and stable performance. Alloys within the above-defined
compositional range or elemental metals may be used as the target
while targets of such alloys or metals with any desired component
added are also acceptable. Further, even when a mixture of
materials having significantly different vapor pressures is used as
the target, there occurs only a slight shift in composition between
the target and the film deposited therefrom, which eliminates the
limits that are imposed on the materials by the vapor pressure or
the like in the case of the evaporation process. The sputtering
process is also advantageous in productivity as compared with the
evaporation process, because it is unnecessary to supply the
material for a long period of time and the resulting film is well
uniform in thickness and quality.
[0040] Since the electron injecting electrode formed by the
sputtering process is a dense film which minimizes the ingress of
moisture as compared with an evaporated film of sparse packing,
there can be obtained an organic EL device having high chemical
stability and a long lifetime.
[0041] The pressure of the sputtering gas during sputtering is
preferably in the range of 0.1 to 5 Pa. By controlling the pressure
of the sputtering gas within this range, an Al--Li alloy having a
lithium concentration within the above-defined range can be easily
obtained. Also, by changing the pressure of the sputtering gas
within this range during film deposition, an electron injecting
electrode having a graded lithium concentration as described above
can be easily obtained. Furthermore, deposition conditions are
preferably controlled such that the product of the
substrate-to-target distance multiplied by the pressure of the
sputtering gas may fall in the range of 20 to 65
Pa.multidot.cm.
[0042] The sputtering gas may be an inert gas as used in
conventional sputtering apparatus, and in the case of reactive
sputtering, a reactive gas such as N.sub.2, H.sub.2, O.sub.2,
C.sub.2H.sub.4 or NH.sub.3 may be used in addition to the inert
gas.
[0043] The sputtering process may be a high-frequency sputtering
process using an RF power supply although a dc sputtering process
is preferably used because the rate of film deposition is easily
controllable and for the purpose of minimizing the damage to the
organic EL device structure. The power to the dc sputtering
apparatus is preferably in the range of about 0.1 to 10 W/cm.sup.2,
especially about 0.5 to 7 W/cm.sup.2. Also, the rate of film
deposition is preferably in the range of about 5 to 100 nm/min.,
especially about 10 to 50 nm/min.
[0044] The electron injecting electrode thin film should have a
sufficient thickness for electron injection, typically at least 1
nm, preferably at least 3 nm. The upper limit of thickness is not
critical although the film thickness generally ranges from about 3
nm to about 500 nm.
[0045] The organic EL device of the invention may include a
protective electrode on the electron injecting electrode, that is,
on the surface of the electron injecting electrode which is
situated remote from the organic layer. The provision of the
protective electrode is effective for protecting the electron
injecting electrode from the surrounding atmosphere and moisture,
preventing the thin films from deterioration, stabilizing the
efficiency of electron injection, and remarkably increasing the
lifetime of the device. The protective electrode has a very low
resistivity so that it may serve as a wiring electrode in case the
electron injecting electrode has a high resistivity. This
protective electrode contains at least one of aluminum, a mixture
of aluminum and a transition metal (excluding titanium), titanium
and titanium nitride (TiN). When these components are used alone,
it is preferred that the protective electrode contain about 90 to
100 at % of Al, about 90 to 100 at % of Ti or about 90 to 100 mol %
of TiN. When a mixture of two or more components is used, the mix
ratio of the components is not critical. When a mixture of aluminum
and titanium is used, the mixture preferably contains up to 10 at %
of Ti. Alternatively, layers each composed of a single component
are laid up. In particular, aluminum or a mixture of aluminum and a
transition metal gives better results when used as the wiring
electrode as will be described later; and titanium nitride is well
corrosion resistant and thus effective as a sealing film. Titanium
nitride (TiN) may deviate about 10% from its stoichiometry. The
alloys of aluminum and a transition metal may contain transition
metals such as Sc, Nb, Zr, Hf, Nd, Ta, Cu, Si, Cr, Mo, Mn, Ni, Pd,
Pt and W, preferably in a total amount of up to 10 at %, more
preferably up to 5 at %, and most preferably up to 2 at %. The
lower the content of transition metal, the lower becomes the thin
film resistance when the electrode serves as a wiring
conductor.
[0046] The protective electrode has a thickness sufficient to
ensure efficient electron injection and prevent ingress of
moisture, oxygen and organic solvents, preferably a thickness of at
least 50 nm, more preferably at least 100 nm, and especially 100 to
1,000 nm. Too thin a protective electrode layer would fail to
achieve the above effects or to provide step coverage so that its
connection to a terminal electrode becomes insufficient. Too thick
a protective electrode layer would accumulate more stresses,
resulting in an increased growth rate of dark spots. In the
embodiment wherein the protective electrode serves as the wiring
electrode, the thickness of the protective electrode is usually
about 100 to 500 nm when the electron injecting electrode is thin
and has a high film resistance and the protective electrode must
compensate for that film resistance, and otherwise about 100 to 300
nm.
[0047] The total thickness of the electron injecting electrode and
the protective electrode combined is preferably about 100 to 1,000
nm though not critical.
[0048] Following the electrode formation, a protective film may be
formed in addition to the protective electrode, using an inorganic
material such as SiOx or an organic material such as Teflon and
chlorine-containing fluorinated hydrocarbon polymers. The
protective film may be either transparent or opaque and have a
thickness of about 50 to 1,200 nm. Apart from the reactive
sputtering process mentioned above, the protective film may also be
formed by an ordinary sputtering, evaporation or PECVD process.
[0049] Further preferably, a sealing layer may be provided on the
device in order to prevent the organic layers and electrodes from
oxidation. In order to prevent the ingress of moisture, the sealing
layer is formed by attaching a shield plate to the substrate
through an adhesive resin layer for sealing. The sealing gas is
preferably an inert gas such as argon, helium, and nitrogen. The
sealing gas should preferably have a moisture content of less than
100 ppm, more preferably less than 10 ppm, especially less than 1
ppm. The lower limit of the moisture content is usually about 0.1
ppm though not critical. The containment of the sealing gas is
effective for restraining deterioration of the hole injecting
electrode, organic layers, electron injecting electrode of the
organic EL structure themselves and at their interface through
chemical reaction with moisture and hence, for maintaining the
initial performance.
[0050] The shield plate is selected from plates of transparent or
translucent materials such as glass, quartz and resins, with glass
being especially preferred. Alkali glass is preferred although
other glass compositions such as soda lime glass, lead alkali
glass, borosilicate glass, aluminosilicate glass, and silica glass
are also useful. Such glass plates are preferably prepared by the
roll-out, down-draw, fusion or float method. Glass plates are often
subject to surface treatment, preferably polishing or SiO.sub.2
barrier coating. Of these, plates of soda lime glass prepared by
the float method are inexpensive and useful without surface
treatment. Metal plates and plastic plates may also be used as the
shield plate.
[0051] A spacer is preferably used as means for adjusting the
height of the shield plate although the height adjusting means is
not limited thereto. The spacer is an inexpensive and simple height
adjusting means for holding the shield plate at a desired height
over the layer structure. The spacer may be formed from resin
beads, silica beads, glass beads, and glass fibers, with the glass
beads being especially preferred. Usually the spacer is formed from
particles having a narrow particle size distribution while the
shape of particles is not critical. Particles of any shape which
does not obstruct the spacer function may be used. Preferred
particles have an equivalent circle diameter of about 1 to 20
.mu.m, more preferably about 1 to 10 .mu.m, most preferably about 2
to 8 .mu.m. Particles of such diameter should preferably have a
length of less than about 100 .mu.m, with the lower limit of length
being usually about 1 .mu.m though not critical.
[0052] When a shield plate having a recess is used, the spacer may
be used or not. When used, the spacer should preferably have a
diameter in the above-described range, especially 2 to 8 .mu.m.
[0053] The spacer may be premixed in a sealing adhesive or mixed
with a sealing adhesive at the time of bonding. The content of the
spacer in the sealing adhesive is preferably 0.01 to 30% by weight,
more preferably 0.1 to 5% by weight.
[0054] Any of adhesives which can maintain stable bond strength and
gas tightness may be used although UV curable epoxy resin adhesives
of cation curing type are preferred. Since layer-forming materials
of the organic EL multilayer structure usually have a glass
transition temperature of at most about 140.degree. C., typically
from about 80.degree. C. to about 100.degree. C., the use of
conventional thermosetting adhesives having a curing temperature of
about 140.degree. C. to about 180.degree. C. gives rise to the
problem that upon curing of the adhesive, the organic EL multilayer
structure can soften and deteriorate its characteristics. In
contrast, UV-curing adhesives do not give rise to the problem that
the organic EL multilayer structure can soften. However, the
currently available UV-curing adhesives yet have the problem that
since they are acrylic base adhesives, acrylic monomers can
volatilize upon curing of the adhesive, which adversely affects the
layer-forming materials of the organic EL multilayer structure to
deteriorate their characteristics. Therefore, use of a UV-curing
epoxy resin adhesive of the cationic curing type is recommended
since it eliminates or minimizes the above-described problems.
[0055] It is noted that UV-curing/thermosetting combined type epoxy
resin adhesives are encompassed in commercially available UV-curing
epoxy resin adhesives. Since most of these adhesives are obtained
by the mixing or modification of acrylic resins of the radical
curing type with epoxy resins of the thermosetting type, they have
not solved the acrylic monomer volatilization problem of acrylic
resins and the curing temperature problem of thermosetting epoxy
resins. Then, the UV-curing/thermosetting combined type epoxy resin
adhesives are not desirable as the adhesive for use in the organic
EL devices of the invention.
[0056] The UV -curing epoxy resin adhesive of the cationic curing
type, as used herein, is an adhesive composition comprising an
epoxy resin as a base component and a Lewis acid salt type curing
agent as a main curing agent which upon exposure to light
containing UV radiation, releases a Lewis acid catalyst through
photolysis whereby the epoxy resin polymerizes and cures through a
reaction mechanism of cationic polymerization type. The epoxy
resins used as the base component of the adhesive include
epoxidized olefinic resins, alicyclic epoxy resins, and novolak
epoxy resins. Examples of the curing agent include Lewis acid salts
of aromatic diazonium, Lewis acid salts of diallyl iodonium, Lewis
acid salts of triallyl sulfonium, and Lewis acid salts of triallyl
selenium.
[0057] The amount of the adhesive coated is usually about 1 to 100
mg/cm.sup.2, preferably about 1 to 10 mg/cm.sup.2, though it varies
with the size of spacer used.
[0058] The thickness of the adhesive layer corresponds to the
height where the shield plate is positioned, that is the thickness
of the organic EL structure plus the distance of the desired space
maintained above the organic EL structure. The thickness of the
adhesive layer is usually about 500 to 1 .mu.m, preferably about 20
to 2 .mu.m, though not critical.
[0059] Next, the organic material layers included in the organic EL
device of the invention are described.
[0060] The light emitting layer has the functions of injecting
holes and electrons, transporting them, and recombining holes and
electrons to create excitons. It is preferred that relatively
electronically neutral compounds be used in the light emitting
layer.
[0061] The hole injecting and transporting layer has the functions
of facilitating injection of holes from the hole injecting
electrode, transporting holes stably, and obstructing electron
transportation. The electron injecting and transporting layer has
the functions of facilitating injection of electrons from the
electron injecting electrode, transporting electrons stably, and
obstructing hole transportation. 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.
[0062] The thicknesses of the light emitting layer, the hole
injecting and transporting layer, and the electron injecting and
transporting layer are not critical and vary with a particular
formation technique although their thickness is usually preferred
to range from about 5 nm to about 500 nm, especially about 10 nm to
about 300 nm.
[0063] The thickness of the hole injecting and transporting layer
and the electron injecting and transporting layer is equal to or
ranges from about {fraction (1/10)} times to about 10 times the
thickness of the light emitting layer although it depends on the
design of a recombination/light emitting region. When the electron
or hole injecting and transporting layer is divided into an
injecting layer and a transporting layer, preferably the injecting
layer is at least 1 nm thick and the transporting layer is at least
1 nm thick. The upper limit of 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/transporting layers are provided.
[0064] The light emitting layer of the organic EL device of the
invention contains a fluorescent material that is a compound having
a light emitting capability. The fluorescent material may be at
least one member selected from compounds as disclosed, for example,
in JP-A 264692/1988, such as quinacridone, rubrene, and styryl
dyes. Also, quinoline derivatives such as metal complex dyes having
8-quinolinol or a derivative thereof as the ligand such as
tris(8-quinolinolato)aluminum are included as well as
tetraphenylbutadiene, anthracene, perylene, coronene, and
12-phthaloperinone derivatives. Further useful are phenylanthracene
derivatives described in JP-A 12600/1996 and the tetraarylethene
derivatives described in JP-A 12969/1996.
[0065] It is preferred to use such a compound in combination with a
host material capable of light emission by itself, that is, to use
the compound as a dopant. In this embodiment, the content of the
compound in the light emitting layer is preferably 0.01 to 10% by
weight, especially 0.1 to 5% by weight. By using the compound in
combination with the host material, the light emission wavelength
of the host material can be altered, allowing light emission to be
shifted to a longer wavelength and improving the luminous efficacy
and stability of the device.
[0066] As the host material, quinolinolato complexes are
preferable, with aluminum complexes having 8-quinolinol or a
derivative thereof as the ligand being more preferable. These
aluminum complexes are disclosed in JP-A 264692/1988, 255190/1991,
70733/1993, 258859/1993 and 215874/1994.
[0067] Illustrative examples 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-chloro-8-quinolinolato)calci- um,
5,7-dichloro-8-quinolinolatoaluminum,
tris(5,7-dibromo-8-hydroxyquinol- inolato)aluminum, and
poly[zinc(II)-bis(8-hydroxy-5-quinolinyl)methane].
[0068] Also useful are aluminum complexes having another ligand in
addition to 8-quinolinol or a derivative thereof. Examples include
bis(2-methyl-8-quinolinolato)(phenolato)-aluminum(III),
bis(2-methyl-8-quinolinolato)(ortho-cresolato)aluminum(III),
bis(2-methyl-8-quinolinolato)(meta-cresolato)aluminum(III),
bis(2-methyl-8-quinolinolato)(para-cresolato)aluminum(III),
bis(2-methyl-8-quinolinolato)-(ortho-phenylphenolato)aluminum(III),
bis(2-methyl-8-quinolinolato)(meta-phenylphenolato)aluminum(III),
bis(2-methyl-8-quinolinolato)(para-phenylphenolato)aluminum(III),
bis(2-methyl-8-quinolinolato)(2,3-dimethylphenolato)-aluminum(III),
bis(2-methyl-8-quinolinolato)(2,6-dimethylphenolato)aluminum(III),
bis(2-methyl-8-quinolinolato)(3,4-dimethylphenolato)aluminum(III),
bis(2-methyl-8-quinolinolato)(3,5-dimethylphenolato)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-trimethylphenolato)aluminum(III),
bis(2-methyl-8-quinolinolato)(2,3,5,6-tetramethylphenolato)-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)(ortho-phenylphenolato)aluminum(III),
bis(2,4-dimethyl-8-quinolinolato)(para-phenylphenolato)aluminum(III),
bis(2,4-dimethyl-8-quinolinolato)(meta-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)(para-cresolato)-aluminum(III),
bis(2-methyl-4-methoxy-8-quinolinolato)(para-phenylphenolato)aluminum(III-
),
bis(2-methyl-5-cyano-8-quinolinolato)(ortho-cresolato)aluminum(III),
and
bis(2-methyl-6-trifluoromethyl-8-quinolinolato)(2-naphtholato)-alumin-
um(III).
[0069] Also acceptable are
bis(2-methyl-8-quinolinolato)-aluminum(III)-.mu-
.-oxo-bis(2-methyl-8-quinolinolato)aluminum (III),
bis(2,4-dimethyl-8-quin-
olinolato)aluminum(III)-.mu.-oxo-bis(2,4-dimethyl-8-quinolinolato)aluminum-
(III),
bis(4-ethyl-2-methyl-8-quinolinolato)aluminum(III)-.mu.-oxo-bis(4-e-
thyl-2-methyl-8-quinolinolato)aluminum(III),
bis(2-methyl-4-methoxyquinoli-
nolato)aluminum(III)-.mu.-oxo-bis(2-methyl-4-methoxyquinolinolato)aluminum-
(III),
bis(5-cyano-2-methyl-8-quinolinolato)aluminum(III)-.mu.-oxo-bis(5-c-
yano-2-methyl-8-quinolinolato)aluminum(III), and
bis(2-methyl-5-trifluorom-
ethyl-8-quinolinolato)aluminum(III)-.mu.-oxo-bis(2-methyl-5-trifluoromethy-
l-8-quinolinolato)aluminum(III).
[0070] Other useful host materials are the phenylanthracene
derivatives described in JP-A 12600/1996 and the tetraarylethene
derivatives described in JP-A 12969/1996.
[0071] The light emitting layer may also serve as the electron
injecting and transporting layer. In this case,
tris(8-quinolinolato)aluminum etc. are preferably used. These
fluorescent materials may be evaporated.
[0072] Also, if necessary, the light emitting layer may also be a
layer of a mixture of at least one hole injecting and transporting
compound and at least one electron injecting and transporting
compound, in which a dopant is preferably contained. In such a mix
layer, the content of the dopant is preferably 0.01 to 20% by
weight, especially 0.1 to 15% by weight.
[0073] In the mix layer, carrier hopping conduction paths are
created, allowing carriers to move through a polarly predominant
material while injection of carriers of opposite polarity is rather
inhibited, and the organic compound becomes less susceptible to
damage, resulting in the advantage of an extended device life. By
incorporating the aforementioned dopant in such a mix layer, the
light emission wavelength the mix layer itself possesses can be
altered, allowing light emission to be shifted to a longer
wavelength and improving the luminous intensity and stability of
the device.
[0074] The hole injecting and transporting compound and electron
injecting and transporting compound used in the mix layer may be
selected from compounds for the hole transporting layer and
compounds for the electron injecting and transporting layer to be
described later, respectively. Inter alia, the compound for the
hole transporting layer is preferably selected from amine
derivatives having strong fluorescence, for example,
triphenyldiamine derivatives which are hole transporting materials,
styrylamine derivatives and amine derivatives having an aromatic
fused ring.
[0075] The electron injecting and transporting compound is
preferably selected from quinoline derivatives and metal complexes
having 8-quinolinol or a derivative thereof as a ligand, especially
tris(8-quinolinolato)aluminum (Alq3). The aforementioned
phenylanthracene derivatives and tetraarylethene derivatives are
also preferable.
[0076] For the hole transporting layer, amine derivatives having
intense fluorescence are useful, for example, the triphenyldiamine
derivatives, styrylamine derivatives, and amine derivatives having
an aromatic fused ring, exemplified above as the hole transporting
material.
[0077] The mix ratio is preferably determined in accordance with
the carrier density and carrier mobility. It is usually preferred
that the weight ratio of the hole injecting and transporting
compound to the electron injecting and transporting compound range
from about 1/99 to about 99/1, more preferably from about 10/90 to
about 90/10, especially from about 20/80 to about 80/20.
[0078] Also preferably, the thickness of the mix layer ranges from
the thickness of a mono-molecular layer to less than the thickness
of the organic compound layer, specifically from 1 to 85 nm, more
preferably 5 to 60 nm, especially 5 to 50 nm.
[0079] Preferably the mix layer is formed by a co-deposition
process of evaporating the compounds from distinct sources. If both
the compounds have approximately equal or very close vapor
pressures or evaporation temperatures, they may be pre-mixed in a
common evaporation boat, from which they are evaporated together.
The mix layer is preferably a uniform mixture of both the compounds
although the compounds can be present in island form. The light
emitting layer is generally formed to a predetermined thickness by
evaporating an organic fluorescent material or coating a dispersion
thereof in a resin binder.
[0080] In the hole transporting layer, there may be used various
organic compounds as described, for example, in JP-A 295695/1988,
191694/1990, 792/1991, 234681/1993, 239455/1993, 299174/1993,
126225/1995, 126226/1995, and 100172/1996, and EP 0650955A1.
Exemplary are tetraarylbenzidine compounds (triaryldiamines or
triphenyldiamines: TPD), aromatic tertiary amines, hydrazone
derivatives, carbazole derivatives, triazole derivatives, imidazole
derivatives, oxadiazole derivatives having an amino group, and
polythiophenes. Two or more of these compounds may be used, and on
such combined use, they may be formed as separate layers or
mixed.
[0081] Where the hole injecting and transporting layer is formed
separately as a hole injecting layer and a hole transporting layer,
two or more compounds are selected in a proper combination from the
compounds commonly used in hole injecting and transporting layers.
In this regard, it is preferred to laminate layers in such an order
that a layer of a compound having a lower ionization potential may
be disposed adjacent the hole injecting electrode (ITO). It is also
preferred to use a compound having good thin film forming ability
at the hole injecting electrode surface. The order of lamination
also applies where a plurality of hole injecting and transporting
layers are provided. Such an order of lamination is effective for
lowering the drive voltage and preventing current leakage and the
development 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 uniform and pinhole-free, 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, the hole injecting and
transporting layer may be formed by evaporating the above-mentioned
compounds.
[0082] In the electron injecting and transporting layer which is
optionally provided, there may be used quinoline derivatives
including organic metal complexes having 8-quinolinol or a
derivative thereof as a ligand such as
tris(8-quinolinolato)aluminum (Alq3), oxadiazole derivatives,
perylene derivatives, pyridine derivatives, pyrimidine derivatives,
quinoxaline derivatives, diphenylquinone derivatives, and
nitro-substituted fluorene derivatives. The electron injecting and
transporting layer can also serve as the light emitting layer. In
this case, use of tris(8-quinolinolato)aluminum etc. is preferred.
Like the light emitting layer, the electron injecting and
transporting layer may be formed by evaporation or the like.
[0083] Where the electron injecting and transporting layer is
formed separately as an electron injecting layer and an electron
transporting layer, two or more compounds are selected in a proper
combination from the compounds commonly used in electron injecting
and transporting layers. In this regard, it is preferred to stack
layers in such an order that a layer of a compound having a greater
electron affinity may be disposed adjacent the electron injecting
electrode. The order of stacking also applies where a plurality of
electron injecting and transporting layers are provided.
[0084] The substrate may be provided with a color filter film, a
fluorescent material-containing color conversion film or a
dielectric reflecting film for controlling the color of light
emission.
[0085] The color filter film used herein may be a color filter as
used in liquid crystal displays and the like. The properties of a
color filter may be adjusted in accordance with the light emission
of the organic EL device so as to optimize the extraction
efficiency and color purity. It is also preferred to use a color
filter capable of cutting external light of short wavelength which
is otherwise absorbed by the EL device materials and fluorescence
conversion layer, because the light resistance and display contrast
of the device are improved. An optical thin film such as a
multilayer dielectric film may be used instead of the color
filter.
[0086] The fluorescence conversion filter film is to convert the
color of light emission by absorbing electroluminescence and
allowing the fluorescent material in the film to emit light. It is
formed from three components: a binder, a fluorescent material, and
a light absorbing material. The fluorescent material used may
basically have a high fluorescent quantum yield and desirably
exhibits strong absorption in the electroluminescent wavelength
region. In practice, laser dyes are appropriate. Use may be made of
rhodamine compounds, perylene compounds, cyanine compounds,
phthalocyanine compounds (including sub-phthalocyanines),
naphthalimide compounds, fused ring hydrocarbon compounds, fused
heterocyclic compounds, styryl compounds, and coumarin compounds.
The binder is selected from materials which do not cause extinction
of fluorescence, preferably those materials which can be finely
patterned by photolithography or printing technique. Also, where
the filter film is formed on the substrate so as to be contiguous
to the hole injecting electrode, those materials which are not
damaged during deposition of the hole injecting electrode (such as
ITO or IZO) are preferable. The light absorbing material is used
when the light absorption of the fluorescent material is short and
may be omitted if unnecessary. The light absorbing material may
also be selected from materials which do not cause extinction of
fluorescence of the fluorescent material.
[0087] In forming the hole injecting and transporting layer, the
light emitting layer, and the electron injecting and transporting
layer, vacuum evaporation is preferably used because homogeneous
thin films are available. By utilizing vacuum evaporation, there is
obtained a homogeneous thin film which is amorphous or has a
crystal grain size of less than 0.1 .mu.m. If the grain size is
more than 0.1 .mu.m, uneven light emission would take place and the
drive voltage of the device must be increased with a substantial
drop of hole injection efficiency.
[0088] The conditions for vacuum evaporation are not critical
although a vacuum of 10.sup.-4 Pa or lower and a deposition rate of
about 0.01 to 1 nm/sec. are preferred. It is preferred to
successively form layers in vacuum because the successive formation
in vacuum can avoid adsorption of impurities on the interface
between the layers, thus ensuring better performance. Also, the
drive voltage of a device can be reduced and the development and
growth of dark spots be restrained.
[0089] In the embodiment wherein the respective layers are formed
by vacuum evaporation, where it is desired for a single layer to
contain two or more compounds, boats having the compounds received
therein are individually temperature controlled to achieve
co-deposition.
[0090] The organic EL device of the invention is generally of the
dc drive type while it can be of the ac or pulse drive type. The
applied voltage is generally about 2 to 20 volts.
EXAMPLE
[0091] Examples of the present invention are given below by way of
illustration and not by way of limitation.
Example 1
[0092] A glass substrate was dimensioned 100 mm.times.100
mm.times.0.7 mm thick and composed of, in % by weight, 70% of
SiO.sub.2, 15% of Na.sub.2O, 3% of Al.sub.2O.sub.3, 10% of CaO, and
2% of MgO.
[0093] By RF sputtering using a target of SiO.sub.2, a barrier
layer was deposited on the glass substrate at a deposition rate of
10 nm/min. to a thickness of 30 nm. The sputtering gas used was a
mixture of 100 scam of argon and 10 scam of oxygen (O.sub.2)
Sputtering conditions included room temperature, an operating
pressure of 0.5 Pa, an input power of 500 W at a frequency of 13.56
MHz, and a substrate-to-target distance of 5 cm. The barrier layer
as deposited had a composition of SiO.sub.2.01 and a refractive
index of 1.45 at a wavelength of 632 .mu.m.
[0094] Next, a transparent ITO electrode (or hole injecting
electrode) was deposited to a thickness of 85 nm and patterned so
as to define pixels in a matrix of 64 dots.times.7 lines (each
pixel sized 280.times.280 .mu.m). The substrate having the
patterned hole injecting electrode was subjected to ultrasonic
washing with neutral detergent, acetone, and ethanol, pulled up
from boiling ethanol, and dried. The surface was further cleaned
with UV/ozone.
[0095] Thereafter, the substrate was secured by a holder in a
vacuum evaporation chamber, which was evacuated to a vacuum of
1.times.10.sup.-4 Pa or lower.
4,4',4"-tris(N-(3-methylphenyl)-N-phenylamino)triphenylamine
(m-MTDATA) was evaporated at a deposition rate of 0.2 nm/sec. to a
thickness of 40 nm, forming a hole injecting layer. With the vacuum
kept, N,N'-diphenyl-N,N'-m-tolyl-4,4'-diamino-1,1'-biphenyl (TPD)
was evaporated at a deposition rate of 0.2 nm/sec. to a thickness
of 35 nm, forming a hole transporting layer. With the vacuum kept,
tris(8-quinolinolato)aluminum (Alq3) was evaporated at a deposition
rate of 0.2 nm/sec. to a thickness of 50 nm, forming an electron
injecting and transporting/light emitting layer. These organic
layers had an overall thickness of 130 nm.
[0096] Next, the EL device substrate was transferred from the
vacuum evaporation chamber to a sputtering apparatus. By dc
sputtering using a target of Ag--Mg, a cathode or an electron
injecting electrode was deposited at a deposition rate of 10
nm/min. to a thickness of 150 nm. The sputtering gas used was Ar.
Sputtering conditions included a pressure of 1 Pa, an input power
of 100 W, and a substrate-to-target distance of 80 mm.
[0097] With the vacuum kept, by dc sputtering using an aluminum
target under a pressure of 0.3 Pa, a protective electrode of
aluminum was deposited to a thickness of 200 nm. The sputtering gas
used was Ar. Sputtering conditions included an input power of 500
W, a target diameter of 4 inches, and a substrate-to-target
distance of 90 mm.
[0098] Finally, using an adhesive and spacer of the predetermined
size, a sealing glass plate was joined to the device for
sealing.
[0099] With a dc voltage applied in the ambient atmosphere, the
organic EL device thus obtained was driven at a constant current
density of 10 mA/cm.sup.2. In the initial stage, no dark spots were
observed. The device was stored under accelerating conditions
including a temperature of 60.degree. C. and a humidity of 95%. The
device was evaluated for deterioration by measuring the time taken
until dark spots grew to 100 .mu.m. The results are shown in Table
1.
Example 2
[0100] An organic EL device was prepared as in Example 1 except
that the flow rate of O.sub.2 in the sputtering gas during
deposition of the barrier layer was changed. The resulting barrier
layer had a composition of SiO.sub.1.95 and a refractive index of
1.47 at a wavelength of 632 nm. The device was evaluated as in
Example 1, with the results shown in Table 1.
Comparative Example 1
[0101] An organic EL device was prepared as in Example 1 except
that the barrier layer was not formed. The device was evaluated as
in Example 1, with the results shown in Table 1.
Comparative Example 2
[0102] An organic EL device was prepared as in Example 1 except
that the flow rate of O.sub.2 in the sputtering gas during
deposition of the barrier layer was changed. The resulting barrier
layer had a composition of SiO.sub.2.23 and a refractive index of
1.38 at a wavelength of 632 nm. The device was evaluated as in
Example 1, with the results shown in Table 1.
1 TABLE 1 SiOx Refractive Growth time of 100 .mu.m Example x index
n dark spot (hr) E1 2.01 1.45 >300 E2 1.95 1.47 >300 CE1 --
-- 5 CE2 2.23 1.38 30
[0103] As is evident from Table 1, the organic EL devices within
the scope of the invention are retarded from generating and growing
dark spots and thus improved in storage stability and
durability.
[0104] There has been described an organic EL device having a
silicon oxide base barrier layer between a substrate and an organic
EL structure which despite the use of alkali glass as the
substrate, has advantages including restrained occurrence or growth
of dark spots, improved storage stability and durability, and
reduced expense of manufacture.
[0105] The invention has been described in detail with particular
reference to preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
* * * * *