U.S. patent application number 09/275409 was filed with the patent office on 2002-12-05 for organic electroluminescent device with a defraction grading and luminescent layer.
Invention is credited to ISHIKAWA, HITOSHI, ODA, ATSUSHI, TOGUCHI, SATORU.
Application Number | 20020180348 09/275409 |
Document ID | / |
Family ID | 13758248 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020180348 |
Kind Code |
A1 |
ODA, ATSUSHI ; et
al. |
December 5, 2002 |
ORGANIC ELECTROLUMINESCENT DEVICE WITH A DEFRACTION GRADING AND
LUMINESCENT LAYER
Abstract
This invention provides organic electroluminescent devices in
which a diffraction grating is formed as a constituent element
thereof on the reflecting surface of the cathode or on the light
output side, resulting in an improvement in light output
efficiency.
Inventors: |
ODA, ATSUSHI; (TOKYO,
JP) ; ISHIKAWA, HITOSHI; (TOKYO, JP) ;
TOGUCHI, SATORU; (TOKYO, JP) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Family ID: |
13758248 |
Appl. No.: |
09/275409 |
Filed: |
March 24, 1999 |
Current U.S.
Class: |
313/506 |
Current CPC
Class: |
B32B 7/023 20190101;
H01L 51/5262 20130101; H01L 51/5225 20130101 |
Class at
Publication: |
313/506 |
International
Class: |
H05B 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 1998 |
JP |
10-081860 |
Claims
What is claimed is:
1. An organic electroluminescent device having one or more organic
layers including a luminescent layer between an anode and a
cathode, wherein said device additionally includes a diffraction
grating or zone plate as a constituent element.
2. An organic electroluminescent device as claimed in claim 1
wherein the anode and the cathode form the same picture elements,
one of these electrodes is an electrode reflecting visible light,
and the diffraction grating or zone plate is formed in this
reflecting electrode.
3. An organic electroluminescent device as claimed in claim 2
wherein said device has a structure in which the diffraction
grating or zone plate, the reflecting electrode, the organic layers
and the transparent electrode are formed on a substrate in the
order mentioned.
4. An organic electroluminescent device as claimed in claim 1
wherein the anode and the cathode form the same picture elements,
one of these electrodes is an electrode reflecting visible light,
and the diffraction grating or zone plate is formed in the
electrode opposite to the reflecting electrode.
5. An organic electroluminescent device as claimed in claim 4
wherein said device has a structure in which the diffraction
grating or zone plate, the transparent electrode, the organic
layers and the reflecting electrode are formed on a transparent
substrate in the order mentioned.
6. An organic electroluminescent device as claimed in claim 4
wherein the diffraction grating or zone plate has no
light-intercepting part.
7. An organic electroluminescent device as claimed in claim 5
wherein the diffraction grating or zone plate has no
light-intercepting part.
8. An organic electroluminescent device as claimed in claim 1
wherein the diffraction grating has a two-dimensional periodic
configuration.
9. An organic electroluminescent device as claimed in claim 2
wherein the diffraction grating has a two-dimensional periodic
configuration.
10. An organic electroluminescent device as claimed in claim 3
wherein the diffraction grating has a two-dimensional periodic
configuration.
11. An organic electroluminescent device as claimed in claim 4
wherein the diffraction grating has a two-dimensional periodic
configuration.
12. An organic electroluminescent device as claimed in claim 5
wherein the diffraction grating has a two-dimensional periodic
configuration.
13. An organic electroluminescent device as claimed in claim 6
wherein the diffraction grating has a two-dimensional periodic
configuration.
14. An organic electroluminescent device as claimed in claim 7
wherein the diffraction grating has a two-dimensional periodic
configuration.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to organic electroluminescent devices
having high luminous efficiency.
[0003] 2. Description of the Prior Art
[0004] Organic electroluminescent devices are self-luminous devices
based on the principle that, when an electric field is applied, a
fluorescent material emits light owing to the energy of the
recombination of positive holes injected from an anode and
electrons injected from a cathode. Since low-voltage driven organic
electroluminescent devices of the laminated structure type were
reported by C. W. Tang et al. (e.g., C. W. Tang and S. A. VanSlyke,
Applied Physics Letters, Vol. 51, p. 913, 1987), active
investigations on organic electroluminescent devices using organic
materials as components have been carried on. Tang et al. used
tris(8-quinolinol)-aluminum for the luminescent layer and a
triphenyldiamine derivative for the hole transport layer.
Advantages of the laminated structure are such that the efficiency
of the injection of positive holes into the luminescent layer can
be enhanced, the efficiency of the formation of excitons by
recombination can be enhanced by blocking electrons injecting from
the cathode, and the excitons formed in the luminescent layer can
be confined. As can be seen from these examples, the well-known
structures of organic electroluminescent devices include, for
example, a two-layer type consisting of a hole transport (or
injection) layer and an electron-transporting luminescent layer,
and a triple-layered type consisting of a hole transport (or
injection) layer, a luminescent layer and an electron transport (or
injection) layer. In these devices of the laminated structure type,
various attempts have been made to modify the device structure or
their fabrication method and thereby enhance the efficiency of the
recombination of injected positive holes and electrons.
[0005] However, in organic electroluminescent devices, the
probability of singlet formation during carrier recombination is
limited owing to its dependence on spin statistics. Consequently,
there is an upper limit to the probability of light emission. This
upper limit is known to have a value of about 25%. Moreover, in
organic electroluminescent devices, light having an exit angle
greater than the critical angle undergoes total reflection owing to
the influence of the refractive index of the luminescent material,
and cannot be taken out of the device as shown in FIG. 1.
Consequently, on the assumption that the luminescent material has a
refractive index of 1.6, only 20% of the total light produced can
be effectively utilized. When the probability of singlet formation
is also taken into consideration, energy conversion efficiency is
inevitably limited to as low as about 5% (Tetsuo Tsutsui, "Present
State and Trend of Organic Electroluminescent", The Display
Monthly, Vol. 1, No. 3, p. 11, September, 1995). In organic
electroluminescent devices in which the probability of light
emission is severely limited, low light output efficiency would
cause a fatal reduction in efficiency.
[0006] The method of improving light output efficiency has
conventionally been investigated in light-emitting devices having a
similar structure, such as inorganic electroluminescent devices.
For example, there have been proposed a method for enhancing
efficiency by imparting light-condensing properties to the
substrate (Japanese Patent Laid-Open No. 314795/'88) and a method
for enhancing efficiency by forming reflecting surfaces on the
sides or other parts of the device (Japanese Patent Laid-Open No.
220394/'89). These methods are effective for devices having a large
light emission area. However, for devices having a minute picture
element area, such as dot matrix displays, it is difficult to
fabricate lenses for providing light-condensing properties or form
lateral reflecting surfaces or the like. Moreover, since the
luminescent layer of an organic electroluminescent device has a
thickness of several micrometers or less, it is difficult to make
the device tapered and form reflecting mirrors on the sides thereof
according to current fine machining techniques. Even if it is
possible, a considerable increase in cost will be caused.
Furthermore, a method for forming an antireflection film by
interposing a flat layer having an intermediate refractive index
between the glass substrate and the luminescent layer is also known
(Japanese Patent Laid-Open No. 172691/'87). This method is
effective in improving light output efficiency in the forward
direction, but cannot prevent total reflection. Consequently, this
method is effective for inorganic electroluminescent devices having
a high refractive index, but fails to produce a remarkable
efficiency-improving effect on organic electroluminescent devices
using a luminescent material having a relatively low refractive
index.
[0007] Accordingly, the conventional light output method used for
organic electroluminescent devices is still unsatisfactory, and the
development of a new light output method is essential for the
purpose of enhancing the efficiency of organic electroluminescent
devices.
[0008] Japanese Patent Laid-open No. 83688/96 discloses an organic
EL device having a light scattering part on an outside surface of
the element. Japanese Patent Laid-open No. 115667/97 discloses an
EL device having a light reflecting structure which reflects light
from the light emitting surface. Japanese Utility-model Laid-open
No. 54184/88 discloses an EL device having micro lense film on the
EL element.
[0009] These three publications neither teach nor suggest the
present organic EL device having a diffraction grating or zone
plate as a constituent element.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to improve light
output efficiency in organic electroluminescent devices and thereby
provide organic electroluminescent devices having higher
efficiency.
[0011] In order to accomplish the above objects, the present
invention provides a EL device which has the following feature.
[0012] (1) In an organic electroluminescent device having one or
more organic layers including a luminescent layer between an anode
and a cathode, the device additionally includes a diffraction
grating or zone plate as a constituent element.
[0013] In preferred embodiments, the present invention also has the
following features.
[0014] (2) In the device described above in (1), the anode and the
cathode form the same picture elements, one of these electrodes is
an electrode reflecting visible light, and the diffraction grating
or zone plate is formed in this reflecting electrode.
[0015] (3) In the device described above in (2), the device has a
structure in which the diffraction grating or zone plate, the
reflecting electrode, the organic layers and the transparent
electrode are formed on a substrate in the order mentioned.
[0016] (4) In the device described above in (1), the anode and the
cathode form the same picture elements, one of these electrodes is
an electrode reflecting visible light, and the diffraction grating
or zone plate is formed in the electrode opposite to the reflecting
electrode.
[0017] (5) In the device described above in (4), the device has a
structure in which the diffraction grating or zone plate, the
transparent electrode, the organic layers and the reflecting
electrode are formed on a transparent substrate in the order
mentioned.
[0018] (6) In the device described above in (4) or (5), the
diffraction grating or zone plate has no light-intercepting
part.
[0019] (7) In the device described above in any of (1) to (6), the
diffraction grating has a two-dimensional periodic
configuration.
[0020] As described above, the present invention relates to an
organic electroluminescent device having one or more organic
thin-film layers including a luminescent layer between an anode and
a cathode, the device additionally includes a diffraction grating
or zone plate as a constituent element. This diffraction grating or
zone plate may be either of the reflection type or the transmission
type. In the case of a diffraction grating or zone plate of the
transmission type, not only an amplitude grating formed by
providing it with light-intercepting parts can be used, but also a
phase grating formed by modulating the thickness of a layer having
a different refractive index may be used to further enhance light
output efficiency. Moreover, in the case of a diffraction grating,
a grating having a two-dimensional periodic configuration may be
used. Thus, as compared with a conventional diffraction grating
consisting of a plurality of stripes, light output in a direction
parallel to the stripes can be improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a sectional view for explaining the structure of a
device having a reflection type diffraction grating according to
the present invention;
[0022] FIG. 2 is a sectional view for explaining the structure of a
device having a transmission type diffraction grating according to
the present invention;
[0023] FIG. 3 is a schematic view for explaining the reflection of
light on a diffraction grating;
[0024] FIG. 4 is a graph for explaining the relationship between
incidence angle and exit angle for a diffraction grating having a
grating interval of 1 mm, a wavelength of 500 nm, and a refractive
index of 1.7;
[0025] FIG. 5 is a graph showing the dependence of incidence angle
and exit angle on the grating interval/optical wavelength ratio for
first-order diffraction by a diffraction grating;
[0026] FIG. 6 is a plan view for explaining a zone plate;
[0027] FIG. 7 is a plan view of a two-dimensional grating pattern
used in Examples 4 and 5; and
[0028] FIG. 8 is a sectional view for explaining a conventional
organic electroluminescent device.
DETAILED DESCRIPTION OF THE INVENTION
[0029] First of all, the principle of the present invention is
explained below.
[0030] In an organic electroluminescent device, the refractive
index of the organic layer including the luminescent layer is
higher than that of the substrate material (e.g., glass), so that
all of the light produced therein cannot be taken out owing to the
occurrence of total reflection at the interface between the organic
layer and the substrate. Even where the light is taken out from the
side opposite to the substrate, total reflection also occurs at the
interface between the device and air owing to the difference in
refractive index between them. The principle of the present
invention is that, in order to suppress such total reflection, a
diffraction grating is formed in the substrate interface or the
reflecting surface so as to alter the incidence angle of light with
respect to the light output surface and thereby enhance light
output efficiency.
[0031] It is known that, when light strikes on a diffraction
grating at an incidence angle .alpha. as shown in FIG. 3, the
relationship among exit angle .beta., grating interval d, light
wavelength .lambda., refractive index n, and order of diffraction
k. is expressed by the following equation (A). 1 d ( sin - sin ) =
k n ( A )
[0032] Accordingly, for light having an incidence angle greater
than the critical angle for total reflection, its incidence angle
can be reduced to a value smaller than the critical angle by
controlling the grating interval properly. For example, on the
assumption that an organic material having a refractive index of
1.7 is used, the critical angle for total reflection is 36.0
degrees. The exit angle observed when light having a wavelength of
500 nm is incident on a reflection type diffraction grating having
a grating interval of 2 .mu.m is shown in FIG. 4. It can be seen
from this figure that, in order to give an exit angle within 36
degrees, the incidence angle must be less than 46 degrees for
first-order diffraction, must be less than 60 degrees for
second-order diffraction, and may have any desired value for
third-order diffraction.
[0033] In the case of a device structure as shown in FIG. 1, i.e.,
a structure obtained by forming a reflection type diffraction
grating 5 on a surface of a substrate 1 so as to serve as a cathode
4, too, and depositing thereon an organic layer 3 and an anode 2
comprising a transparent electrode, the diffraction grating serves
as a reflecting surface. Consequently, most of the light having an
incidence angle greater than 36 degrees and having undergone total
reflection at the interface between the transparent electrode 2 and
the ambient medium of the device has an exit angle less than 36
degrees. Thus, this light reaches again the interface between the
transparent electrode and the ambient medium of the device, and
leaves the device without undergoing total reflection. The
component obtained by first-order diffraction and reflected at an
exit angle greater than 36 degrees undergoes total reflection at
the interface between the transparent electrode and the ambient
medium of the device, and strikes again on the diffraction grating.
After this process is repeated, almost all of the light is
eventually taken out of the device.
[0034] The reflection type diffraction grating used in this case
may have any desired shape, so long as it can function as a
diffraction grating. For example, a laminary grating having a
rectangular cross section or an echelette grating having a tapered
cross section may be formed on the substrate, and the cathode may
be deposited thereon so as to serve as a reflecting surface.
Alternatively, the cathode may be deposited in the form of
alternating stripes by using two cathode materials having different
reflection coefficients, or the cathode itself may be formed in a
striped pattern to make a diffraction grating.
[0035] Where it is desired to use a transmission type diffraction
grating, a device may be fabricated by forming a diffraction
grating 5 on a substrate 1 and then depositing thereon an anode 2,
an organic layer 3 and a cathode 4 in that order, as shown in FIG.
2. In this case, the transmission type diffraction grating may
comprise either an amplitude grating or a phase grating, and may
have any desired shape. For example, a phase grating may be made by
forming grooves in the substrate surface, depositing thereon a
layer of a transparent material having a different refractive
index, planarizing it, and then depositing an anode, an organic
layer and a cathode successively in the usual manner. In the case
of an amplitude grating, a material opaque to light may be
deposited on the substrate surface in the form of stripes, or the
anode itself may be formed in a striped pattern. In the latter
case, the anode material may be either transparent or opaque. For
example, a device may be fabricated by forming a gold electrode
having a striped pattern as the anode, and then depositing thereon
an organic layer and a cathode.
[0036] When a transmission type diffraction grating is used, the
light incident on the diffraction grating is divided into
transmitted light and reflected light. However, since the reflected
light has a smaller exit angle, it strikes on the diffraction
grating again at a smaller incidence angle after being reflected by
the cathode. Thus, similarly to a device using a reflection type
diffraction grating, almost all of the light can be taken out of
the device.
[0037] The dimensions of the diffraction grating should be
determined so that the light output efficiency is enhanced for the
desired wavelength region of the electroluminescent device.
Specifically, when the wavelength of the electroluminescent device
is in the region of visible light (i.e., in the wavelength region
of 350 to 800 nm), the effect of the ratio (R) of the grating
interval to the optical wavelength for the desired wavelength
(i.e., the value obtained by dividing the wavelength by the
refractive index) is shown in FIG. 5. Specifically, if the ratio is
unduly large, the diffraction grating is less effective in reducing
the exit angle, so that reflection at a mirror surface is repeated
many times to cause a considerable loss. If the ratio is unduly
small, light having a large incidence angle gives reflected light
having a large exit angle, so that the proportion of light taken
out in the forward direction is decreased. Thus, unduly large and
unduly small ratios both reduce the light output efficiency.
Accordingly, it is desirable that the ratio is in the range of 0.1
to 10.
[0038] In the case of an ordinary diffraction grating, no
diffraction effect is produced in a direction parallel to the
stripes, so that the light output efficiency in this direction
cannot be enhanced. This disadvantage can be overcome by using a
two-dimensional diffraction grating. Alternatively, a diffraction
grating made by forming grooves in a concentric pattern may also be
used. In this case, the intervals of the concentric grooves may be
periodic or, as shown in FIG. 6, may be determined according to the
interval rule for the formation of a zone plate. Similarly to the
above-described diffraction gratings, these diffraction gratings
may also be made by forming grooves in the substrate or by forming
an electrode itself in a grating pattern. Moreover, the groove may
have any desired cross-sectional shape.
[0039] Next, the various constituent elements of the device are
explained below. With respect to the electrodes of an organic
electroluminescent device, the anode functions to inject positive
holes into a hole transport layer, and it is effective that the
anode has a work function of not less than 4.5 eV. Specific
examples of the anode materials which can be used in the present
invention include indium-tin oxide alloy (ITO); tin oxide (NESA);
metals such as gold, silver, platinum and copper, and their oxides;
and mixtures thereof. On the other hand, the cathode serves to
inject electrons into an electron transport layer or a luminescent
layer, and it is preferable to use a material having a small work
function. Although no particular limitation is placed on the type
of the cathode material, specific example of usable cathode
materials include indium, aluminum, magnesium, magnesium-indium
alloy, magnesium-aluminum alloy, aluminum-lithium alloy,
aluminum-scandium-lithi- um alloy, magnesium-silver alloy, and
mixtures thereof.
[0040] With respect to these electrodes, one of the anode and the
cathode is transparent in the region of visible light, and the
other has high reflectivity. No particular limitation is placed on
the thicknesses of these electrodes, so long as they can perform
their proper functions. However, their thicknesses are preferably
in the range of 0.02 to 2 .mu.m.
[0041] The organic electroluminescent devices of the present
invention have a structure in which one or more organic layers are
disposed between the aforesaid electrodes, and no additional
restriction is imposed on their structure. Examples thereof are
those consisting of (1) an anode, a luminescent layer and a
cathode, (2) an anode, a hole transport layer, a luminescent layer,
an electron transport layer and a cathode, (3) an anode, a hole
transport layer, a luminescent layer and a cathode, and (4) an
anode, a luminescent layer, an electron transport layer and a
cathode. Moreover, in order to improve charge injection
characteristics, suppress dielectric breakdown, or enhance luminous
efficiency, a thin-film layer formed of an inorganic dielectric or
insulator (e.g., lithium fluoride, magnesium fluoride, silicon
oxide, silicon dioxide or silicon nitride), a layer formed of a
mixture of an organic material and an electrode material or metal,
or a thin film of an organic polymer (e.g., polyaniline, a
polyacetylene derivative, a polydiacetylene derivative, a polyvinyl
carbazole derivative or a poly(p-phenylene-vinylene) derivative)
may be interposed between adjacent organic layers and/or between an
organic layer and an electrode.
[0042] No particular limitation is placed on the type of the
luminescent material used in the present invention, and there may
be used any compound that is commonly used as a luminescent
material. As given below, examples thereof include
tris(8-quinolinol)-aluminum complex (Alq3) [1],
bis(diphenylvinyl)biphenyl (BDPVBi) [2],
1,3-bis(p-t-butylphenyl-1,3,4-ox- adiazolyl)phenyl (OXD-7) [3],
N,N'-bis(2,5-di-t-butylphenyl)perylenetetrac- arboxylic acid
diimide (BPPC) [4] and 1,4-bis(p-tolyl-p-methylstyrylphenyl-
amino)naphthalene [5]. 1
[0043] Alternatively, a layer of a charge transport material doped
with a fluorescent material may be used as a luminescent material.
Examples thereof include a layer of a quinolinol-metal complex such
as the aforesaid Alq3[1], doped with
4-dicyanomethylene-2-methyl-6-(p-dimethylam- inostyryl)-4H-pyran
(DCM) [6], a quinacridone derivative such as 2,3-quinacridone [7]
or a coumarin derivative such as
3-(2'-benzothiazole)-7-diethylaminocoumarin [8]; a layer of the
electron transport material
bis(2-methyl-8-hydroxyquinoline)-4-phenylphenol-alumin- um complex
[9] doped with a fused polycyclic aromatic compound such as
perylene [10]; or a layer of the hole transport material
4,4'-bis(m-tolylphenylamino)biphenyl (TPD) [11] doped with rubrene
[12]. 2
[0044] No particular limitation is placed on the type of the hole
transport material used in the present invention, and there may be
used any compound that is commonly used as a hole transport
material. Examples thereof include triphenyldiamines such as
bis[di(p-tolyl)aminophenyl]-1,1- -cyclohexane [13], TPD [11] and
N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-bi- phenyl-4,4'-diamine
(NPB) [14]; and starburst type molecules (e.g., [15] to [17]).
3
[0045] No particular limitation is placed on the type of the
electron transport material used in the present invention, and
there may be used any compound that is commonly used as an electron
transport material. Examples thereof include oxadiazole derivatives
such as 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole
(Bu-PBD) [18] and OXD-7[3]; triazole derivatives (e.g., [19] and
[20]); and quinolinol-metal complexes (e.g., [1], [9], and [21] to
[24]). 4
[0046] No particular limitation is placed on the method for forming
the various layers constituting the organic electroluminescent
device of the present invention. Any conventionally known methods
such as vacuum evaporation and spin coating may be employed. The
organic thin-film layers each containing a compound as described
above, which is used in the organic electroluminescent device of
the present invention, can be formed according to any well-known
method such as vapor evaporation, molecular beam evaporation (MBE),
or coating method such as dipping (in a solution prepared by
dissolving the compound in a solvent), spin coating, casting, bar
coating or roll coating.
[0047] No particular limitation is placed on the thickness of each
organic layer used in the organic electroluminescent device of the
present invention. However, it is usually preferable that each
organic layer have a thickness ranging from several tens of
nanometers to 1 micrometer.
EXAMPLES
[0048] The present invention is further illustrated by the
following examples. However, these examples are not to be construed
to limit the scope of the invention.
Comparative Example 1
[0049] The procedure for the fabrication of an organic thin-film
electroluminescent device serving as a comparative example is
described below. As illustrated in FIG. 8, this device comprises a
substrate 1 having thereon an anode 2, an organic layer 3 (composed
of a hole injection layer, a luminescent layer and an electron
transport layer) and a cathode 4. An ITO layer having a thickness
of 100 nm was deposited on a 50 mm.times.25 mm glass substrate (a
thickness of 1.1 mm; NA 45 manufactured by Hoya Corp.) by
sputtering. In this step, a metal mask was used to deposit the ITO
layer in the form of stripes measuring 2 mm.times.50 mm. Its sheet
resistance was 20 .OMEGA./.quadrature..
[0050] Then, an organic luminescent layer was deposited by means of
a resistance heating type vapor evaporator. While the substrate was
mounted in the upper part of a vacuum chamber, a molybdenum boat
was placed at a position 250 mm below the substrate. The substrate
was arranged so as to give an incidence angle of 38.degree. and
rotated at a speed of 30 rotation per minutes. As soon as a
pressure of 5.times.10.sup.-7 Torr was reached, evaporation was
started, and the deposition rate was controlled by means of a
crystal oscillator type film thickness controller mounted beside
the substrate. The deposition rate was preset at 0.15 nm per
second. Under the above-described conditions, a hole injection
layer comprising compound [15] was deposited to a thickness of 40
nm. Thereafter, a 70 nm thick luminescent layer comprising compound
[5] and a 40 nm thick electron transport layer comprising compound
[19] were successively evaporated under the same conditions as
described above.
[0051] Subsequently, a cathode comprising a magnesium-silver alloy
was deposited by evaporating magnesium and silver simultaneously
from separate boats. Using the aforesaid film thickness controller,
the deposition rates of magnesium and silver were adjusted to 1.0
and 0.2 nm per second, respectively, and the film thickness was
preset at 200 nm. During this evaporation, a metal mask was used to
deposit the cathode in such a way that it consisted of 12 stripes
measuring 25 mm.times.2 mm which were arranged at intervals of 1 mm
and in a direction orthogonal to the stripes of ITO. When a voltage
of 10 V was applied, this device exhibited a current density of 50
mA/cm.sup.2 and a luminance of 1,950 cd/m.sup.2. Consequently, its
efficiency was 3.9 cd/A or 1.22 lm/W.
Example 1
[0052] On a glass substrate similar to that used in Comparative
Example 1, a grating pattern having a line width of 1 .mu.m and an
interval of 1 .mu.m was formed according to a photolithographic
process. Specifically, a 2 .mu.m thick layer of an i-line resist
(THMR-iP1700; manufactured by Tokyo Ohka Kogyo Co., Ltd.) was
formed on the substrate by spin coating, and patterned by means of
an i-line stepper. Then, this substrate was soaked in a
hydrofluoric acid solution to form grooves having a depth of 200
nm, and the remaining resist was removed by use of an exclusive
stripping fluid. After a cathode comprising a 200 nm thick layer of
a magnesium-silver alloy was evaporated thereon under the same
conditions as described in Comparative Example 1, organic layers
with reverse order of Comparative Example 1 and an ITO layer were
successively deposited.
[0053] When a voltage of 10 V was applied, this device exhibited a
current density of 55 mA/cm.sup.2 and a luminance of 3,265
cd/m.sup.2. Consequently, its efficiency was 5.94 cd/A or 1.86
lm/W.
Example 2
[0054] A device was fabricated in exactly the same manner as in
Example 1, except that the grating pattern had a line width of 0.40
.mu.m and an interval of 0.40 .mu.m.
[0055] When a voltage of 10 V was applied, this device exhibited a
current density of 58 mA/cm.sup.2 and a luminance of 4,028
cd/m.sup.2. Consequently, its efficiency was 6.94 cd/A or 2.18
lm/W.
Example 3
[0056] In order to make a diffraction grating, grooves were formed
in a substrate according to the same procedure as described in
Example 1. Thereafter, a 500 nm thick layer having a high
refractive index was deposited over the grooves according to a
sputtering process using titanium oxide as the target, and its
surface was planarized by ordinary optical polishing. Subsequently,
an ITO layer, an organic layer and a cathode were deposited thereon
in exactly the same manner as in Comparative Example 1 to fabricate
a device.
[0057] When a voltage of 10 V was applied, this device exhibited a
current density of 50 mA/cm.sup.2 and a luminance of 2,623
cd/m.sup.2. Consequently, its efficiency was 5.246 cd/A or 1.647
lm/W.
Example 4
[0058] A device was fabricated in exactly the same manner as in
Example 1, except that the two-dimensional grating pattern shown in
FIG. 7 was used. When a voltage of 10 V was applied, this device
exhibited a current density of 52 mA/cm.sup.2 and a luminance of
3,733 cd/m.sup.2. Consequently, its efficiency was 7.17 cd/A or
2.25 lm/W.
Example 5
[0059] A device was fabricated in exactly the same manner as in
Example 3, except that the two-dimensional grating pattern shown in
FIG. 7 was used. When a voltage of 10 V was applied, this device
exhibited a current density of 58 mA/cm.sup.2 and a luminance of
3,210 cd/m.sup.2. Consequently, its efficiency was 5.53 cd/A or
1.73 lm/W.
Example 6
[0060] The two-dimensional grating pattern shown in FIG. 6 was
used. According to Fresnel's method for the formation of annular
zones, the widths and intervals of zones were determined on the
basis of the radius r from the center as expressed by the following
equation (B). 2 r = r 0 sin [ cos - 1 ( n1 n0 ) ] ( B )
[0061] In this equation, 1 is 0.08 .mu.m, r.sub.0 is 3 .mu.m, and n
is an integer ranging from 1 to 100. Grooves were formed in the
zones where n changes from an even number to an odd number. After
this zone pattern was formed so as to cover a 5 mm.times.5 mm area
of the substrate surface, the treatment (e.g., planarization) and
the formation of several layers were carried out in the same manner
as in Example 3.
[0062] When a voltage of 10 V was applied, this device exhibited a
current density of 50 mA/cm.sup.2 and a luminance of 3,640
cd/m.sup.2. Consequently, its efficiency was 7.28 cd/A or 2.28
lm/W.
* * * * *