Title: Electroluminescent display device with substrate having regions with different refractive indexes
Abstract: The present invention provides an electroluminescent display which can efficiently radiate light emitted from an electroluminescent layer to a viewer side and, at the same time, can reduce a deterioration in contrast of a displayed image caused by external light. The electroluminescent display includes a light transparent substrate and an electroluminescent layer provided on the light-transparent substrate. The electroluminescent layer includes at least a transparent electrode, an organic luminescent layer, and a cathode. Two or more regions with different refractive indexes are provided within said light-transparent substrate.
Patent Number: 6,906,452 Issued on 06/14/2005 to Ichikawa
| Inventors:
|
Ichikawa; Nobuhiko (Shinjuku-Ku, JP)
|
| Assignee:
|
Dai Nippon Printing Co., Ltd. (JP)
|
| Appl. No.:
|
394652 |
| Filed:
|
March 24, 2003 |
Foreign Application Priority Data
| Mar 26, 2002[JP] | 2002-084933 |
| Current U.S. Class: |
313/110; 313/512 |
| Intern'l Class: |
H01J 005/16; H01J001/62 |
| Field of Search: |
313/505,506,512,110,112,117
|
References Cited [Referenced By]
U.S. Patent Documents
| 4670690 | Jun., 1987 | Ketchpel.
| |
| 4810931 | Mar., 1989 | McKenna et al.
| |
| 5557295 | Sep., 1996 | Miyashita et al.
| |
| 5745199 | Apr., 1998 | Suzuki et al.
| |
| 6069443 | May., 2000 | Jones et al.
| |
| 6075317 | Jun., 2000 | Keyser et al.
| |
| 6091384 | Jul., 2000 | Kubota et al.
| |
| 6166489 | Dec., 2000 | Thompson et al.
| |
| 6198220 | Mar., 2001 | Jones et al.
| |
| 6414439 | Jul., 2002 | Tuenge et al.
| |
Primary Examiner: Patel; Vip
Assistant Examiner: Perry; Anthony
Attorney, Agent or Firm: Parkhurst & Wendel, L.L.P.
Claims
1. An electroluminescent display comprising: a light transparent substrate; and
an electroluminescent layer provided on said light transparent substrate, said
electroluminescent layer comprising at least a transparent electrode, an organic
luminescent layer, and a cathode, wherein
(1) two or more regions with different refractive indexes are provided within
said light transparent substrate and (2), as viewed in cross section of the substrate,
a boundary line between the higher refractive index region and the lower refractive
index region is a tapered line satisfying a requirement represented by the formula
θ
2≧θ
1-sin
-1 (n
2/n
1) wherein
θ
2≧
0, θ
1 represents the angle of light
radiated from the electroluminescent layer to a direction normal to the substrate;
θ
2 represents a taper angle at a point where, when θ
1
has been increased, the light is applied to the interface between the lower refractive
index region and the higher refractive index region; n
2 represents the refractive
index of the lower refractive index region; and n
1 represents the refractive
index of the higher refractive index region.
2. The electroluminescent display according to claim 1, wherein the regions with
different refractive indexes have been patterned in the direction of the surface
of the substrate.
3. The electroluminescent display according to claim 2, wherein the regions with
different refractive indexes are two regions, one of which has a relatively higher
refractive index and the other region has a relatively lower refractive index,
and, in the substrate, the area ratio of the higher refractive index region to
the whole region on the light outgoing surface side is higher than that on the
electroluminescent layer side.
4. The electroluminescent display according to claim 3, wherein, as viewed in
cross section of the substrate, the boundary line between the higher refractive
index region and the lower refractive index region is not linear.
5. The electroluminescent display according to claim 3, wherein the electroluminescent
layer in its luminescent sites correspond respectively to positions in tbe higher
refractive index region on its side in contact with the electroluminescent layer
in the substrate.
6. The electroluminescent display according to claim 5, wherein luminescence
emitted from the electroluminescent layer is passed through the higher refractive
index region and is radiated toward a viewer side.
7. The electroluminescent display according to claim 3, wherein the lower refractive
index region is a region of gas.
8. The electrolumiflescent display according to claim 7, wherein the gas is nitrogen.
9. The electroluminescent display according to claim 3, wherein at least one
member selected from the group consisting of light absorbing materials and getter
materials, for adsorbing oxygen or water has been filled into the lower refractive
index region.
Description
TECHNICAL FIELD
The present invention relates to an organic electroluminescent display including
an organic electroluminescent device as a luminescent source. More particularly,
the present invention relates to an electroluminescent display which can improve
the efficiency of taking out light, emitted from an electroluminescent device,
to the outside of the electroluminescent display (light take-out efficiency) and,
at the same time, can suppress a deterioration in contrast of a displayed image
caused by external light reflection.
BACKGROUND ART
Electroluminescent devices have features including wide angle
of visibility by virtue of selfluminous nature and lower power consumption. Because
of these features, up to now, various inorganic electroluminescent devices using
inorganic compounds as luminescent materials and various organic electroluminescent
devices using organic compounds as luminescent materials (these organic compounds
will be hereinafter referred to as "organic luminescent materials") have been proposed,
and an attempt has been made to put the electroluminescent devices into practical use.
Among others, organic electroluminescent devices can significantly reduce necessary
application voltage, as compared with inorganic electroluminescent devices. This
has led to active studies on the development of organic electroluminescent devices
having higher performance through the development of and an improvement in materials
for the organic electroluminescent devices. Studies on the utilization of organic
electroluminescent devices as a surface light source has been put forward. At the
same time, the development of organic electroluminescent devices capable of emitting
various colors has led to studies on the utilization of organic electroluminescent
devices as pixels of displays. In a display using organic electroluminescent devices
as pixels, a plurality of organic electroluminescent devices are two-dimensionally
arranged on an identical plane to form a panel (a display panel), and these devices
are driven independently of each other or one another to display a desired image.
FIG. 1 is a schematic diagram illustrating a basic construction of an organic
electroluminescent display. As shown in FIG. 1, the organic electroluminescent
display includes a light transparent substrate 1 and, stacked on the substrate
1 in the following order, a transparent anode 2, an organic luminescent
layer (hereinafter simply referred to as "luminescent layer") 3, and a cathode
4. A display comprising the transparent anode 2, the luminescent
layer 3, and the cathode 4 stacked in that order on a specific substrate
1 is an organic electroluminescent display referred to in the present invention.
The position of the anode and the position of the cathode are sometimes reversed.
Further, in order to improve the performance, the interposition of a hole transport
layer between the anode and the luminescent layer, the interposition of an electron
injection layer between the cathode and the luminescent layer, or the interposition
of an adhesive layer between the cathode and the luminescent layer or between the
electron injection layer and the luminescent layer is sometimes adopted. The luminescent
layer is generally formed of one or a plurality of organic luminescent materials.
In some cases, however, the luminescent layer is formed of, for example, a mixture
composed of an organic luminescent material and a hole transport material and/or
an electron injection material.
Further, in the organic electroluminescent display, in general, a surface
in a substantially parallel positional relationship with the main surface of the
luminescent layer serves as a light outgoing surface, and, in the pair of electrodes
(anode and cathode) constituting the organic electroluminescent device, the electrode
(=anode) located on the light outgoing surface side is formed of a transparent
or translucent thin film (hereinafter often referred to as "transparent electrode")
for light take-out efficiency improvement purposes or for reasons of construction
of a surface emitting device. On the other hand, the electrode (=cathode) located
opposite to the light outgoing surface is formed of a specific metallic thin film
(a thin film of a metal, an alloy, a mixed metal or the like).
The above organic electroluminescent display also involves several problems to
be solved. One of the problems is low effective light take-out efficiency. Even
in the case of an organic luminescent material which exhibits a considerably high
internal quantum efficiency in the luminescence, since the refractive index of
the substrate is high, the critical angle at which the light emitted from the electroluminescent
layer can be radiated to the outside of the electroluminescent display is small.
Therefore, as shown in FIG. 1, a considerably large proportion of light emitted
from the electroluminescent layer cannot be radiated from within the substrate
to the outside of the electroluminescent display and is propagated in the facial
direction while undergoing multiple reflection before radiation to the outside
of the electroluminescent display. Further, in many cases, the wavelength of light
emitted from the electroluminescent layer is likely to be absorbed in the substrate.
Therefore, multiple reflection of the light within the substrate is disadvantageous
from the viewpoint of the utilization of luminescence and thus has become a serious
problem. The light take-out efficiency of the organic electroluminescent display
is generally as low as about 20%.
In order to improve the light take-out efficiency, the provision of a lower refractive
index layer on the electroluminescent layer in its light outgoing side has also
been proposed, for example, in Advanced Materials 2001, 13, No. 15, August, P.
1149-1152, "Doubling Coupling-Out Efficiency in Organic Light-Emitting Devices
Using a Thin Silica Aerogel Layer." The claimed advantage of this construction
is as follows. Light emitted from the luminescent layer is first radiated to the
lower refractive index layer having a refractive index substantially equal to the
refractive index of an air layer. By virtue of this, even after subsequent passage
through a higher refractive index layer, total reflection does not take place,
and, theoretically, the light can be entirely radiated to the air layer on the
viewer side. In the above technique, a silica aerogel having a refractive index
n of 1.01 to 1.10 is provided between the luminescent layer and the substrate,
and the whole light emitted from the electroluminescent layer except for a single
mode light constituting a part of the light emitted from the electroluminescent
layer which is propagated through the electroluminescent layer, once enters the
lower refractive index layer. Therefore, the occurrence of the total reflection
in a subsequent stage at the interface between the higher refractive index layer
and the lower refractive index layer can be prevented. The use of the silica aerogel,
however, necessitates the provision of the step of dipping in water. Since water
is a vital unfavorable factor in the organic electroluminescent layer which shortens
the service life of the device, when commercialization is taken into consideration,
the adoption of this technique using silica aerogel is difficult.
Further, Japanese Patent Laid-Open No. 74072/1999 discloses a method wherein
a micro lens array is formed on a substrate to enhance light take-out efficiency.
Even in this method, however, sealing of an insulating liquid having a lower refractive
index into between a luminescent layer and a lens array poses problems of service
life of an electroluminescent display, a complicate process and the like.
Japanese Patent Laid-Open No. 283751/1999 (Japanese Patent No. 2991183)
discloses a method wherein the direction of advance of light, which has been radiated
from the electroluminescent layer and causes total reflection, is deviated by a
diffraction grating from the total reflection angle to improve light take-out efficiency.
In this method, however, since wavelength dispersion occurs, particularly for short-wavelength
display colors in a full-color display, a diffraction angle necessary for deviating
the direction of advance of the light from the total reflection angle cannot be
sometimes provided. Further, the dispersion of light caused by diffraction is likely
to cause color blurring.
Japanese Patent Laid-Open No. 260559/2000 discloses a method wherein an
assembly of optical fibers made of quartz glass or a polymer is cut into thin pieces
to prepare a substrate, the whole cut surface thereof is covered with an organic
material or an inorganic material, the surface roughness thereof is finished into
300 nm or less, and pixels are formed thereon. The claimed advantage of this device
is that luminescence is incident to the inside of each fiber of the group of optical
fibers, is then easily transmitted while being confined in each fiber by total
reflection, and can be efficiently taken out, as transmitted light, to the outside
of the system.
In the light propagated in this way, however, the angle of light outgoing surface
to a normal is originally less than the total reflection angle. Therefore, in this
method, to begin with, light rays in the total reflection region are leaked without
being guided through the group of fibers. In the above publication, there is also
a description on the use of tapered fibers. The intention of using the tapered
fibers, however, is merely to perform magnified display of the pixels. Further,
in the preparation of the substrate, bundling of a fiber array is carried out,
and the bundle is then sliced to prepare the substrate. Therefore, difficulties
are encountered in the preparation of the substrate.
Another problem involved in the organic electroluminescent device is a deterioration
in visibility and contrast of a displayed image derived from external light. The
deteriorated visibility and contrast of the displayed image are attributable to
such a phenomenon that a large part of light introduced into the device from the
outside thereof is reflected from a cathode formed of a metallic thin film, which
reflects about 70% of visible light, and is radiated from the light outgoing surface
of the device. A conventional method for solving this problem is to adopt such
a construction that a polarizing plate and a quarter-wavelength plate are disposed
in the front of an electroluminescent layer. In this construction, external light
is first attenuated by the polarizing plate to not more than ½ and is further
converted to a circularly polarized light by the quarter-wavelength plate. The
circularly polarized light is converted by an electrode to a circularly polarized
light in the opposite direction which is again converted by the quarter-wavelength
plate to a linearly polarized light in a direction orthogonal to the incident light.
The linearly polarized light is substantially entirely absorbed in the polarizing
plate. In this method, however, disadvantageously, luminescence per se is also
attenuated to about ½. Therefore, the efficiency of the display is sacrificed.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide an electroluminescent display
which can efficiently radiate light emitted from an electroluminescent layer to
a viewer side and, at the same time, can reduce a deterioration in contrast of
a displayed image caused by external light.
The above object can be attained by an electroluminescent display comprising:
a light transparent substrate; and an electroluminescent layer provided on said
light transparent substrate, said electroluminescent layer comprising at least
a transparent electrode, an organic luminescent layer, and a cathode, wherein two
or more regions with different refractive indexes are provided within said light-transparent substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory view of a basic construction of a conventional electroluminescent display;
FIG. 2 is a cross-sectional view of one embodiment of the electroluminescent
display according to the present invention;
FIG. 3 is a cross-sectional view of another embodiment of the electroluminescent
display according to the present invention;
FIG. 4 is a cross-sectional view of a further embodiment of the electroluminescent
display according to the present invention;
FIG. 5 is an explanatory view of one embodiment of the arrangement of a higher
refractive index region and a lower refractive index region according to the present
invention; and
FIG. 6 is an explanatory view of another embodiment of the arrangement of a
higher refractive index region and a lower refractive index region according to
the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention will be described in more detail with reference to the
following preferred embodiments. Embodiments of the construction of the electroluminescent
display according to the present invention will be described with reference to
FIGS. 2 to
6. The construction of the electroluminescent display according
to the present invention may be the same as that of the conventional electroluminescent
display shown in FIG. 1, except that the construction of a light transparent substrate
1 is different from that of the conventional electroluminescent display.
Specifically, as shown in FIGS. 2 to
4, in the electroluminescent display,
an electroluminescent layer comprising at least a transparent electrode
2,
a luminescent layer
3, and a cathode
4 is provided on the light transparent
substrate
1. The characteristic feature of this electroluminescent display
according to the present invention is that two regions with different refractive
indexes, that is, a region
5 with a relatively higher refractive index and
a region
6 with a relatively lower refractive index are provided within
the substrate.
In the embodiment shown in FIG. 2, a higher refractive index region
5
and
a lower refractive index region
6 are provided within a light transparent
substrate
1. Both the higher refractive index region
5 and the lower
refractive index region
6 have been patterned in the surface direction of
the light transparent substrate
1. In this embodiment, as indicated by arrows
in FIG. 2, light rays emitted, from the surface of a transparent anode
2,
at an angle which causes total reflection from the light outgoing surface in the
upper part in the drawing, are reflected from the interface between the higher
refractive index region
5 and the lower refractive index region
6,
and the reflected light rays reach the light outgoing surface and are observed
as outgoing light by a viewer. Therefore, a significantly improved light take-out
efficiency can be realized.
In the present invention, preferably, for example, the regions with different
refractive indexes are two regions, one of which has a relatively higher refractive
index and the other region has a relatively lower refractive index, and, in the
substrate, the area ratio of the higher refractive index region to the whole region
(sum of the higher refractive index region and the lower refractive index region)
on the light outgoing surface side is higher than that on the electroluminescent
layer side. This construction can improve the light take-out efficiency.
In FIG. 2, the area ratio on the light outgoing surface side of the substrate
is a/(a+0) wherein a represents the area of the higher refractive index region
5 on the light outgoing surface side, and the area of the lower refractive
index region
6 on the light outgoing surface side is 0 (zero). That is,
100% of the area of the light outgoing surface side of the substrate is accounted
for by the higher refractive index region
5. On the other hand, the area
ratio on the electroluminescent layer side of the substrate is c/(b+c) wherein
c represents the area of the higher refractive index region
5 on the electroluminescent
layer side and b represents the area of the lower refractive index region
6
on the electroluminescent layer side. That is, the area ratio of the higher refractive
index region
5 is higher on the light outgoing surface side.
The surface of the boundary between the higher refractive index region
5
and the lower refractive index region
6 may be linear as shown in FIG.
2.
For example, the lower refractive index region
6 may be in a cone, taper,
or wedge form. As shown in FIGS. 3 and 4, preferably, as viewed in cross section
of the substrate, the boundary line between the higher refractive index region
and the lower refractive index region is partially or entirely concaved in a parabolic
or elliptically arc form. According to this construction, as compared with the
case where the above construction is not adopted, the proportion of light rays,
which advance at such an angle that causes the confinement of the light rays within
the light transparent substrate, can be lowered, contributing to improved light
take-out efficiency.
When the form of the higher refractive index region
5 is such that the
sectional area of the higher refractive index region
5 is changed in the
thickness-wise direction as in this embodiment, a large part of light rays emitted
from the electroluminescent layer is guided toward the viewer side while undergoing
total reflection along the inner wall of the higher refractive index region
5.
At that time, since the lower refractive index region
6 is in the above
cone or other form, while undergoing total reflection, the light rays are subjected
to a change in the direction of advance in a direction perpendicular to the substrate.
Therefore, the proportion of light rays, which are totally reflected at the interface
between the air layer and the higher refractive index region
5 on the viewer
side, can be reduced, and a larger proportion of light rays can be taken out to
the air.
In this case, the change in sectional area of the lower refractive index region
6, for example, in a cone form is not limited to the linear change. For
example, the form of the lower refractive index region
6 may be such that
the skirt of the lower refractive index region
6 on the electroluminescent
layer side is narrowed so that light rays emitted from the electroluminescent layer
strike against the light outgoing surface at a larger angle than the critical angle
of the total reflection caused by the difference in refractive index between the
substrate and the air. This construction can further increase the quantity of light
which advances through the higher refractive index region
5 while undergoing
total reflection from the lower refractive index region
6.
Publication No. 509327/1995 of the Japanese Translation of International
Patent Application discloses the adoption of a conical region in a display. Unlike
the electroluminescent display according to the present invention, however, the
display disclosed in this publication adopts the conical region for widening the
visual field of display light from a liquid crystal display illuminated mainly
with highly parallel special backlight.
In the present invention, light emitted from the electroluminescent layer is
guided
through the higher refractive index region
5 toward the surface of the substrate
1 while undergoing a change in the direction of advance to a direction perpendicular
to the surface of the substrate
1. On the other hand, external light is
guided through the higher refractive index region
5 while undergoing a change
in the direction of advance to a direction parallel to the surface. Therefore,
on the electroluminescent layer side, the angle of advance of a large part of the
external light exceeds the critical angle of the total reflection, and the external
light is passed into the lower refractive index region
6. When a light absorbing
layer is provided in the lower refractive index region
6, the external light
rays are mostly absorbed in the light absorbing layer.
Preferably, the position of the pattern of the electroluminescent layer
(pattern of the transparent anode
2 shown in FIGS. 2 to
6) is coincident
with the position of the pattern of the higher refractive index region
5
in its portion which constitutes the boundary between the higher refractive index
region
5 and the electroluminescent layer and thus is in contact with the
electroluminescent layer. According to this construction, light emitted from the
electroluminescent layer is propagated through the higher refractive index region
and is taken out to the viewer side.
The arrangement of the higher refractive index region
5 and the lower
refractive index region
6 in FIGS. 2 to
4 may be as shown in FIG.
5, which is a cross-sectional view taken on line A—A of FIG. 4, that is,
shows the substrate on its electroluminescent layer side, and FIG. 6 which likewise
shows the substrate on its electroluminescent layer side. That is, as shown in
FIGS. 5 and 6, higher refractive index regions
5 and lower refractive index
regions
6 are alternately arranged in the direction of the surface of the
substrate. The arrangement may be in a one-dimensional form (stripe form) as shown
in FIG. 5 or alternatively may be a two-dimensional form (dot matrix form) as shown
in FIG.
6. In this case, in the thickness-wise direction of the substrate,
the sectional area of the higher refractive index region on its electroluminescent
layer side may be smaller than that of the higher refractive index region on its
light outgoing surface (viewer) side. The design of the pitch, size, and number
of the higher refractive index regions
5 in the direction of the surface
of the substrate may vary depending upon the opening pattern of the electroluminescent
layer (pattern of the transparent anode
2). More specifically, the opening
of the electroluminescent layer and the cross-section of the higher refractive
index region on its electroluminescent layer side may be substantially identical
to each other in its position and area. In FIGS. 5 and 6, the area ratio between
the higher refractive index region
5 and the lower refractive index region
6 is preferably higher refractive index region
5: lower refractive
index region
6=approximately 1:2 to 2:1.
The material for the higher refractive index region
5 of the light transparent
substrate
1 in the present invention is not particularly limited. The material
for the higher refractive index region
5 is preferably formed of a conventional
transparent solid polymer material from the viewpoint of moldability. An example
of the material is a polymer which has been produced by photopolymerization of
polymethyl methacrylate, polycarbonate, polyester, polystyrene, or acrylic monomer
and has a refractive index of about 1.45 to 1.65.
The thickness of the substrate formed of the above material is preferably about
0.5 to 1.1 mm. Voids corresponding to the lower refractive index region
6
are formed within the substrate
1, for example, by pressing a heating mold
having convexes conforming to the shape of the lower refractive index region
6
in its lowermost part against the light transparent substrate
1. The voids
are filled with at least one member selected from gas, a liquid and a solid to
form a lower refractive index region
6. In some cases, voids as such may
be used as the lower refractive index region
6. The voids may be filled
with light absorbing particles. The filling of these materials can enhance the
absorbance of external light. The voids may be filled with a getter material (mainly
in a powder form) for adsorbing oxygen and water, such as barium oxide (BaO), either
alone or in combination with the above light absorbing material. According to this
construction, the prolongation of the service life of the electroluminescent device
can be expected.
After the filling of the voids with the getter material or the like, a thin-film
layer having a refractive index substantially equal to the higher refractive index
region
5, which does not greatly influence the optical characteristics,
may be formed on the electroluminescent layer side of the substrate
1, from
the viewpoint of enhancing the smoothness of the higher refractive index region
5 and the lower refractive index region
6 to facilitate the practice
of the subsequent step. For example, a very thin glass plate (for example, AF 45,
thickness 50 μm, manufactured by Schott) may be applied with the aid of an
optical adhesive (for example, NOA 61, manufactured by Norland Corp.). Thus, preferably,
for example, the gas, such as nitrogen, or the light absorbing material, which
has been filled into the lower refractive index region
6, is hermetically
sealed to avoid adverse effect on the electroluminescent layer.
According to the above construction, the light take-out efficiency of the
electroluminescent display of the present invention can be improved. Further, as
shown in FIG. 2, as with the above embodiment, external light incident to the higher
refractive index region
5 in the substrate
1 is reflected from or
absorbed in the interface between the higher refractive index region
5 and
the lower refractive index region
6. Therefore, excellent external light
reflection preventive effect can be realized. Thus, according to the present invention,
an electroluminescent display can be provided in which light take-out efficiency
can be improved and, at the same time, the incidence of external light can be suppressed
to significantly improve the contrast of the displayed image.
EXAMPLES
The following examples and comparative examples further illustrate the present invention.
Example 1
The present invention will be described in more detail with reference to FIG.
2. FIG. 2 is a partially enlarged cross-sectional view of an electroluminescent
display in one embodiment of the present invention. An acrylic substrate (Sumipex,
refractive index 1.49, manufactured by Sumitomo Chemical Co., Ltd.) was provided
as a light transparent substrate. A photopolymerizable acrylic monomer was slit
coated onto the acrylic substrate to a predetermined thickness. In this example,
the total thickness of the substrate after curing was brought to 0.5 mm. Next,
a lower refractive index region 6 (void part in this example) was provided
as follows. A mold, which had been subjected to release treatment and can perform
embossing, was pressed against the above substrate, and a predetermined quantity
of ultraviolet light was applied from the acrylic substrate side. After the completion
of curing, the mold was separated. Thus, a substrate with voids having a desired
shape could be formed. In this case, the substrate was designed and prepared to
have the following dimension.
Total thickness of substrate=0.5 mm
Pitch of concaves/convexes (a part corresponding to a in FIG. 2)=0.3 mm
Width of convex part (a part corresponding to c in FIG. 2)=0.15 mm
Taper angle of lower refractive index region (angle of interfacial boundary
to normal of substrate)=8.5 degrees
Pixel forming region=diagonally 4 in. (3:4)
Next, a light absorbing material (carbon black) and a getter material (BaO
powder) were sealed into the voids. In this case, since carbon black is electrically
conductive, in a dot matrix display, there is a fear of causing shortcircuiting
of the inter-pixel electrode. In this example, shortcircuiting was prevented by
sealing the above two types of powders, then adding an acrylic monomer dropwise
to the inside of the voids, spin coating the acrylic monomer over the whole surface
to form a thin film, and then applying ultraviolet light to the acrylic monomer
for curing. The thin film layer formation has also the effect of facilitating the
formation of an electroluminescent layer.
Subsequently, an electroluminescent layer was formed by the following
method on the substrate in its region where the lower refractive index region was
not formed. Specifically, at the outset, an ITO transparent electrode, which had
been patterned in a pixel form, was sputtered on the substrate in its convex part
through a passivation layer. For the formation of an organic electroluminescent
device, a luminescent organic material Alq
3 [tris(8-hydroxyquinoline)aluminium]
and a hole injection layer TPD [N,N′-diphenyl-N,N′-bis(3-methyl-phenyl)-1,1-diphenyl-4,4′-diamine]were
stacked to constitute a luminescent layer. ITO was used as the transparent anode,
and a Mg—Ag (magnesium-silver) alloy was used as a cathode. In this case,
TPD was provided so as to come into contact with ITO.
More specifically, ITO was first formed to a thickness of 150 nm. Next, TPD,
which had been purified in a satisfactorily preheated sublimation purification
apparatus under high vacuum, was loaded in a tungsten board, and a 50 nm-thick
TPD layer was formed by electric resistance heating. Thereafter, Alq
3,
which had been subjected to sublimation purification, was loaded in a quartz board,
and a 30 nm-thick Alq
3 layer was formed on the TPD layer by electric
resistance heating. A Mg—Ag alloy (Mg:Ag=10:1) was vapor deposited to a
thickness of 150 nm. Further, a 200 nm-thick silver layer was formed as a protective
layer on the alloy layer by vapor deposition. Finally, the assembly was sealed
with separately provided glass plate and UV curing seal material. Thus, a panel
part of an organic electroluminescent display was prepared. A controller and a
power supply circuit were connected to this panel part to complete an electroluminescent
display according to the present invention.
Subsequently, the power supply circuit of the electroluminescent display
was operated for lighting. The electroluminescent display was examined for an improvement
in brightness with a brightness measuring device (BM-7, manufactured by Topcon
Corp.). The results of the measurement have revealed that an about 20% improvement
in light take-out efficiency in electroluminescence could be achieved over a display
in which the same electroluminescent layer pattern as used above was formed on
a conventional substrate with a single refractive index layer. The possible reason
for this improvement in light take-out efficiency is as follows. When a conventional
substrate is used, the angle of advance of electroluminescence radiated within
the substrate exceeds the critical angle, 41.8 degrees to the vertical direction.
As a result, the electroluminescence is totally reflected and thus cannot be taken
out. On the other hand, in this example of the present invention, the lower refractive
index region has a taper angle of 8.5 degrees. Therefore, the critical angle of
the total reflection is increased by this taper angle, and, thus, the increased
critical angle could increase the proportion of emitted light which can be taken
out to the outside of the electroluminescent display without causing total reflection.
In fact, in the above case, the light take-out efficiency improvement was limited
to 20% probably due to the occurrence of boundary reflection at the interface between
the substrate and the air on the viewer side. When the design is taken into consideration,
an about 45% improvement in light take-out efficient can be theoretically expected.
Therefore, the provision of an antireflection layer at the interface between the
substrate and the air on the viewer side can realize a light take-out efficiency
improvement close to the designed value.
On the other hand, the electroluminescent display was evaluated for a reduction
in incidence of external light to the electroluminescent display. In order to examine
a change in incident light intensity depending upon the incident angle, collimated
light from a xenon light source was applied so as to be incident to the electroluminescent
display while successively changing the incident angle. In this case, a brightness
measuring device was installed in a regular reflection direction of the substrate
for measurement of the brightness. In the measurement, a surface reflection component
on light outgoing side face of the substrate was previously measured and was subtracted
from the measured values. The results of the evaluation showed that a significant
reduction in light could be provided for light rays falling within the range of
an angle of not less than about 31 degrees in terms of angle of incident to the
substrate. About 40% reduction in light could be provided in an integral value
of full angle range of incident light as converted from the measured values, indicating
that the contrast of the display image could be significantly improved.
Example 2
In Example 2 of the present invention, an electroluminescent display according
to the present invention was prepared in the same manner as in Example 1, except
that three colors, i.e., the green light emitting material Alq
3 used
in Example 1 and, in addition, DPVBi (1,4-bis(2,2-diphonylivinyl)biphenyl) as a
blue light emitting material and a mixture, composed of Alq
3 and 1.0%
by weight of DCM (dicyanomethylenepyran derivative) added to Alq
3, as
a red light emitting material were vapor deposited using a mask so as to be juxtaposed
to one another to constitute subpixels, whereby a full-color display was prepared.
The electroluminescent display thus prepared was measured in the same manner as
in Example 1. As a result, also for the electroluminescent display using the three
primary colors, R (red), G (green), and B (blue), in this example, the same effect
as attained in Example 1 could be confirmed.
Example 3
In Example 3 of the present invention, the procedure of Examples 1 and 2 was
repeated,
except that high-molecular weight organic electroluminescent materials were used
instead of the low-molecular weight organic electroluminescent materials used in
Examples 1 and 2. The hole injection layer was formed by spin coating PEDOT (polythiophene:
CH 8000, manufactured by Bayer) to a thickness of 80 nm and baking the coating
at 160° C. The following high-molecular weight organic electroluminescent
materials were dissolved in a solvent to prepare coating liquids. The coating liquids
were deposited by ink jet recording on PEDOT so that the three colors were juxtaposed
to one another to constitute subpixels, whereby a full-color display was prepared.
| (Composition of coating liquid for formation |
| of organic electroluminescent layer) |
| |
| |
Polyvinylcarbazole |
70 |
pts.wt. |
| |
Oxadiazole compound |
30 |
pts.wt. |
| |
(*Fluorescent dye) coumarin 6 |
1 |
pt.wt. |
| |
(Solvent) monochlorobenzene |
4,900 |
pts.wt. |
| |
| |
*When the fluorescent dye was coumarin 6, green light with a peak wavelength
of 501 nm was emitted; when the fluorescent dye was perylene, blue light with a
peak |
| |
# wavelength of 460 to 470 nm was emitted; and when the fluorescent dye was
DCM, red light with a peak wavelength of 570 nm was emitted. |
In order to protect the organic electroluminescent material against water- and
oxygen-derived deterioration, all steps from baking of PEDOT to sealing were carried
out in a glove box in which the air had been replaced by nitrogen. For the electroluminescent
display prepared in this example, as with the electroluminescent displays prepared
in Examples 1 and 2, an improvement in light take-out efficiency and an improvement
in contrast by the prevention of incidence of external light could be achieved.
The improvement level was the same as that in Examples 1 and 2.
Example 4
In Example 4, with a view to further improving the take-out efficiency of radiated
light rather than a reduction in incidence of external light, a substrate having
the following construction was prepared and evaluated. The substrate was prepared
in the same manner as in Example 1. The total thickness of the substrate, the pitch,
the width of convex, and the specifications of the pixel forming region were the
same as in Example 1, and what is different from Example 1 was the shape of the
lower refractive index region.
As shown in FIG. 3, the shape of the taper was such that the sectional area was
decreased on the electroluminescent layer side. In this example, a curve satisfying
a requirement represented by the following formula was designed: θ2≧θ1-sin
-1
(n2/n1) wherein θ2≧0. In the formula,
θ1 represents the angle of light radiated from the electroluminescent
layer to the direction of normal to the substrate; θ2 represents the
taper angle at a point where, when θ1 has been increased, the light
is applied to the interface between the lower refractive index region and the higher
refractive index region; n2 represents the refractive index of the lower
refractive index region; and n1 represents the refractive index of the higher
refractive index region. The curve is as shown in FIG. 3.
When the angle of the radiated light to the normal direction at which the light
is applied to the interface between the higher refractive index region and the
lower refractive index region increases and, in this case, when the angle exceeds
a certain critical angle, the interface is escaped from total reflection conditions
and, consequently, the light is transmitted from the higher refractive index region
into the lower refractive index region. On the other hand, when the sectional area
of the higher refractive index region is changed to form a shape satisfying the
above formula, radiated light rays at any angle can be brought to totally reflection
light rays and can be guided to the light outgoing side. The electroluminescent
layer was formed in the same manner as in Example 3.
The electroluminescent display thus prepared was measured for brightness. As
a result, it was found that an about 70% improvement in brightness was achieved.
In the above design, theoretically, by virtue of the adoption of the curve represented
by the above formula, even at angles above the conventional total reflection angle,
all light rays can be guided to the viewer side. In the above calculation, however,
only light rays radiated from the center of the electroluminescent layer were traced,
and, for radiated light rays from the other regions was somewhat lost by total
reflection. Further, also for light rays bent to an angle at which the light rays
can be taken out, in fact, boundary reflection at the interface between the substrate
and the air on the viewer side causes some loss of light rays. Therefore, it is
considered that the above factors limited the brightness improvement to 70%. As
with Example 1, the formation of an antireflection layer can realize a further
improvement in light take-out efficiency.
A variant of this example is shown in FIG. 4. In this variant, the thickness
of the lower refractive index region is substantially equal to the thickness of
the substrate. The curve shown in FIG. 4 is a parabolic curve in which the center
of the electroluminescent layer constitutes a focal plane. According to this construction,
radiated light rays in all directions can be reflected from this curve, and substantially
all light rays are arrayed parallel to the direction of normal to the substrate.
This provides such a light quantity distribution that the light quantity is further
concentrated on the viewer side.
The present invention is not limited to the above examples, and various modifications
and alterations are possible. For example, the preparation of the substrate is
not limited to the method wherein a metallic mold is pressed against an acrylic
monomer for embossing, and, for example, shape formation by photolithography or
shape formation by sandblasting can be adopted. Further, for the material for the
substrate, glass may be used instead of the acrylic material. In this case, shape
formation by patterning using photolithography, by etching with fluoric acid or
the like, or by sandblasting is also possible.
Further, for the formation of the additional thin-film layer on the electroluminescent
layer formation side, for example, the application of a thin glass layer (for example,
AF 45, thickness 50 μm, manufactured by Schott) through an adhesive layer
may be used instead of the spin coating of the acrylic resin.
As described above, unlike the prior art method wherein a large part of light
rays radiated from the electroluminescent layer are guided in the facial direction
of the substrate resulting in significantly deteriorated light take-out efficiency,
in the present invention, the take-out efficiency of electroluminescence can be
significantly improved through the utilization of simple process and materials.
At the same time, the present invention can reduce another problem of the prior
art, that is, a problem of lowered contrast caused by the incidence of external light.
*
