U.S. patent application number 09/752680 was filed with the patent office on 2001-07-12 for fluorescence-reddening membrane and red-emitting device using same.
This patent application is currently assigned to Idemitsu Kosan Co., Ltd.. Invention is credited to Eida, Mitsuru, Ikeda, Hidetsugu, Tsuchiya, Jun.
Application Number | 20010007412 09/752680 |
Document ID | / |
Family ID | 12115363 |
Filed Date | 2001-07-12 |
United States Patent
Application |
20010007412 |
Kind Code |
A1 |
Eida, Mitsuru ; et
al. |
July 12, 2001 |
Fluorescence-reddening membrane and red-emitting device using
same
Abstract
The present invention relates to a fluorescence-reddening
membrane which comprises a light-transmittable medium and,
dispersed therein, (a) a rhodamine base fluorescence pigment and
(b) a fluorescence pigment which has absorptions in the blue region
and induces energy transfer to and reabsorptions from the rhodamine
base fluorescence pigment. According to the present invention, it
is made possible to provide the fluorescence-reddening membrane
capable of converting the color of light emitted by a blue-emitting
organic electroluminescence device to a red light at a high
conversion efficiency and also to provide an inexpensive
red-emitting device capable of reducing the size and thickness of
itself.
Inventors: |
Eida, Mitsuru; (Chiba-ken,
JP) ; Ikeda, Hidetsugu; (Chiba-ken, JP) ;
Tsuchiya, Jun; (Chiba-ken, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
Idemitsu Kosan Co., Ltd.
1-1, Marunouchi 3-chome
Tokyo
JP
|
Family ID: |
12115363 |
Appl. No.: |
09/752680 |
Filed: |
January 3, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09752680 |
Jan 3, 2001 |
|
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09117547 |
Aug 7, 1998 |
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6221517 |
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Current U.S.
Class: |
315/169.3 |
Current CPC
Class: |
Y10S 428/917 20130101;
C09K 11/06 20130101; F21V 9/00 20130101 |
Class at
Publication: |
315/169.3 |
International
Class: |
G09G 003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 9, 1996 |
JP |
23609/1996 |
Oct 14, 1996 |
JP |
PCT/JP96/02966 |
Claims
1. A fluorescence-reddening membrane which comprises a
light-transmittable medium and, dispersed therein, (a) a rhodamine
base fluorescence pigment and (b) a fluorescence pigment which has
absorptions in the blue region and induces energy transfer to and
reabsorptions from the rhodamine base fluorescence pigment.
2. The fluorescence-reddening membrane according to claim 1,
wherein the (b) fluorescence pigment has absorptions in the blue
region at a wavelengh of 520 nm or less and has an optical density
OD of 1.0 or more at a wavelengh in the range of 420 to 490 nm.
3. The fluorescence-reddening membrane according to claim 1,
wherein a rhodamine base fluorescence pigment and a naphthalimide
base fluorescence pigment are dispersed in a light-transmittable
medium.
4. The fluorescence-reddening membrane according to claim 2,
wherein a rhodamine base fluorescence pigment and a naphthalimide
base fluorescence pigment are dispersed in a light-transmittable
medium.
5. The fluorescence-reddening membrane according to claim 1,
wherein a rhodamine base fluorescence pigment and a coumarin base
fluorescence pigment are dispersed in a light-transmittable
medium.
6. The fluorescence-reddening membrane according to claim 2,
wherein a rhodamine base fluorescence pigment and a coumarin base
fluorescence pigment are dispersed in a light-transmittable
medium.
7. The fluorescence-reddening membrane according to claim 1,
wherein the (a) rhodamine base fluorescence pigment comprises the
mixture of basic violet 11 and rhodamine 6 G.
8. The fluorescence-reddening membrane according to claim 2,
wherein the (a) rhodamine base fluorescence pigment comprises the
mixture of basic violet 11 and rhodamine 6 G.
9. The fluorescence-reddening membrane according to claim 3,
wherein the (a) rhodamine base fluorescence pigment comprises the
mixture of basic violet 11 and rhodamine 6 G.
10. The fluorescence-reddening membrane according to claim 4,
wherein the (a) rhodamine base fluorescence pigment comprises the
mixture of basic violet 11 and rhodamine 6 G.
11. The fluorescence-reddening membrane according to claim 5,
wherein the (a) rhodamine base fluorescence pigment comprises the
mixture of basic violet 11 and rhodamine 6 G.
12. The fluorescence-reddening membrane according to claim wherein
the (a) rhodamine base fluorescence pigment comprises the mixture
of basic violet 11 and rhodamine 6 G.
13. The fluorescence-reddening membrane according to claim 3,
wherein the naphthalimide base fluorescence pigment comprises the
mixture of solvent yellow 116 and solvent low 44.
14. The fluorescence-reddening membrane according to claim 4,
wherein the naphthalimide base fluorescence pigment comprises the
mixture of solvent yellow 116 and solvent low 44.
15. The fluorescence-reddening membrane according to claim 5,
wherein the coumarin base fluorescence pigment comprises coumarin 6
or coumarin 7.
16. The fluorescence-reddening membrane according to claim 6,
wherein the coumarin base fluorescence pigment comprises coumarin 6
or coumarin 7.
17. The fluorescence-reddening membrane according to claim 1,
wherein the light-transmittable medium is a printing medium.
18. The fluorescence-reddening membrane according to claim 2,
wherein the light-transmittable medium is a printing medium.
19. The fluorescence-reddening membrane according to claim 3,
wherein the light-transmittable medium is a printing medium.
20. The fluorescence-reddening membrane according to claim 4,
wherein the light-transmittable medium is a printing medium.
21. The fluorescence-reddening membrane according to claim 5,
wherein the light-transmittable medium is a printing medium.
22. The fluorescence-reddening membrane according to claim 6,
wherein the light-transmittable medium is a printing medium.
23. A red-emitting device which comprises the
fluorescence-reddening membrane as set forth in any of claims 1 to
22 and a light-emitting device.
24. The red-emitting device according to claim 23, wherein the
light-emitting device is a blue-emitting organic
electroluminescence device.
Description
TECHNICAL FIELD
[0001] The present invention relates to a fluorescence-reddening
membrane and a red-emitting device using the same. More
particularly, the present invention is concerned with a
fluorescence-reddening membrane capable of converting the color of
the light emitted by a blue-emitting organic
electroluminescence(hereinafter abbreviated to "EL") device to a
red light at a high conversion efficiency as high as at least 33% ,
and with an inexpensive red-emitting device which comprises the
aforesaid fluorescence-reddening membrane and a light-emitting
device, emits a red light at a high conversion efficiency, and also
enables reduction in size and thickness.
BACKGROUND ART
[0002] An EL device which utilizes the electroluminescence is
characterized by a high visual-distinguishability because of its
self-emission, excellent impact resistance because of its being in
the form of a complete solid device, and the like properties.
Therefore, the EL device has been attracting attention for
utilization as a light-emitting device in a variety of types of
display apparatuses.
[0003] The EL device is divided into an inorganic EL device in
which an inorganic compound is used as the light-emitting material,
and an organic EL device in which an organic compound is used as
the light-emitting material. Of these, the organic EL device, which
can decrease the applied voltage to a great extent, has been
positively studied for practical application as a display device of
the next generation.
[0004] It is evident that multi-coloring is required for a display
device in order to expand the use and application field of the
above-mentioned organic EL device, as can be seen from the examples
of the cathode-ray tube (CRT) and the liquid crystal display
(LCD)
[0005] Several methods have heretofore been known as a method for
preparing a multi-colored display apparatus by the use of an EL
device, including, for example, (1) a method in which EL materials
which emit light with three primary colors of red (R), green (G)
and blue (B), respectively are each arranged in the form of matrix
[refer to Japanese Patent Application Laid-Open Nos. 157487/1982
(Sho-57), 147989/1983 (Sho-58), 214593/1991 (Hei-3), etc.; (2) a
method in which an EL material which emits light with white color
is combined with a color filter to take out the three primary
colors of R,G,B [refer to Japanese Patent Application Laid-Open
Nos. 315988/1989 (Hei-1), 273496/1990 (Hei-2), 194895/1991 (Hei-3),
etc.; (3) a method in which an EL material which emits light with
blue color is combined with a fluorescence conversion membrane to
take out the three primary colors of R,G,B [refer to Japanese
Patent Application Laid-Open No. 152897/1991 (Hei-3), etc. and the
like methods. However, the above-mentioned method (1) suffers the
disadvantages that the three kinds of light-emitting materials must
be arranged in the form of matrix in high precision and fineness,
thereby causing technological difficulty and failure in its
production at a low cost, and further that the three kinds of
light-emitting materials usually have each a service life
frequently different from one another, whereby the chromaticity of
the emitted light deviates from the normal value with the lapse of
time. On the other hand, the method (2) suffers the drawback that
the utilization efficiency of the EL light is lowered, that is, the
conversion efficiency is lowered, since part of the output light
from the EL device which emits light with white color is taken out
with a color filter to utilize the light. For example, when red
color is taken out by the use of a color filter from a white El
color consisting simply of the three primary colors each having a
same intensity, the maximum obtainable conversion efficiency is
only 33%. In practical application, however, the conversion
efficiency is much lower than 33% taking into consideration the
emission spectrum and visibility. The method (3) is made superior
to the aforesaid method (2), if the three primary colors of R,G,B
are each obtained at a conversion efficiency of at least 33% in the
method (3).
[0006] There is well known a method in which fluorescence
conversion membranes are arranged on an EL device to versatilely
vary the color tone of the EL-emitting light color [refer to
Japanese Patent Application Laid-Open Nos. 18319/1988 (Sho-63) and
152897/1991 (Hei-3)]. The blue color among R,G,B is emitted from
the organic EL device itself, and thus may be utilized as such. In
this case if the conversion efficiency is forced to be stated, it
is 100%. With regard to green, a conversion efficiency of 80% is
obtained by the use of coumarin 153 as is disclosed in Japanese
Patent Application Laid-Open No. 152897/1991 (Hei-3)]. Nevertheless
there is not yet known so far a method for converting the blue
light of an EL device to red light at a conversion efficiency of at
least 33%. For example, as is disclosed in Japanese Patent
Publication Nos. 32879/1993 (Hei-5) and 33514/1993 (Hei-5), the
light-emitting layer of the blue/green light-emitting inorganic EL
device in which layer is dispersed rhodamine, that is, a red
fluorescent coloring material, emits white light, thus failing to
emit objective red light. likewise, white light instead of the
objective red light is emitted from the blue/green light-emitting
inorganic EL device the outside of which is fitted with the
fluorescence conversion membrane composed of rhodamine B [Japanese
Utility Model Application Laid-Open No. 77299/1988 (Sho-63)], also
from the blue/green light-emitting inorganic EL device the outside
of which is fitted with the fluorescence conversion membrane
composed of the pink base fluorescent coloring material (produced
by Sinloihi Co. Ltd. under the trade name "FA001") [Japanese Patent
Application Laid-Open No. 163159/1994 (Hei-6)]. In the case where
the blue light-emitting organic EL device is fitted with the
fluorescence conversion membrane composed of phenoxazone 9 and a
color filter for regulating chromaticity [Japanese Patent
Application Laid-Open No. 152897/1991 (Hei-3)], there is obtained a
red light having a chromaticity, x of 0.62 and y of 0.33, but the
converted light thus obtained is so faint as is visible only in a
bright place with an extremely low conversion efficiency.
[0007] Japanese Patent Application Laid-Open No. 158091/1990
(Hei-2) describes the fluorescent substance having a main
stimulating wavelength in the range of 440 to 560 nm and a main
light-emitting wavelength in the range of 510 to 650 nm. However,
the above-mentioned fluorescent substance is devoid of an
absorption capable of interrupting at least blue color and
consequently, can emit white color only.
[0008] Further, Japanese Patent Application Laid-Open No.
2205971/1985 (Sho-60) describes the wavelength-conversion
fluorescent substance which absorbs the light having a peak
wavelength in the range of 460 to 520 nm and emits light having a
peak wavelength in the range of 590 to 610 nm. However, the
aforesaid wavelength-conversion fluorescent substance fails to
employ a fluorescent substance capable of selectively interrupting
the blue color having a wavelength in the range of 460 to 520 nm
and therefore, the fluorescent substance is incapable of
selectively emitting red color.
[0009] As described hereinbefore, since the red fluorescence
pigment which is typified by rhodamine base fluorescence pigment
and phenoxazone base fluorescence pigment is usually devoid of an
absorption in the blue color region, the independent use of the
aforesaid red fluorescence pigment in a fluorescence-reddening
membrane leads to failure to sufficiently interrupt the original
blue light and as a result, to selectively obtain the objective red
color by reason of the mixing of the blue light with the conversion
light of red color. In the case where a color filter for
chromaticity regulation is superimposed on the
fluorescence-reddening membrane in order to interrupt the original
blue light, the red conversion efficiency is inevitably
lowered.
DISCLOSURE OF THE INVENTION
[0010] It is an object of the present invention to overcome the
disadvantages inherent in the prior arts as described hereinbefore
and at the same time, to provide a fluorescence-reddening membrane
capable of converting the color of the light emitted by a
blue-emitting organic electroluminescence (EL) device to a red
light at a high conversion efficiency as high as at least 33%, and
also to provide an inexpensive red-emitting device which utilizes
the said fluorescence-reddening membrane and also enables reduction
in size and thickness of itself.
[0011] Under such circumstances intensive research and ivestigation
were accumulated by the present inventors in order to achieve the
above-mentioned object. As a result, it has been found that a blue
color of the light emitted by an organic electroluminescence (EL)
device can be converted to a red light at a high conversion
efficiency as high as at least 33% by a fluorescence conversion
membrane which comprises a rhodamine base fluorescence pigment and,
mixed therein a specific fluorescence pigment that has absorptions
in the blue region so as to sufficiently interrupt blue lights and
induces effective energy transfer to or reabsorption from said
rhodamine base fluorescence pigment, said resultant mixture being
dispersed in a light-transmittable medium; and that the device
which comprises the fluorescence-reddening membrane and a
light-emitting device is capable of emitting a red light at a high
conversion efficiency and reducing the size and thickness of the
device itself. The present invention has been accomplished by the
foregoing findings and information.
[0012] That is to say, the present invention provides a
fluorescence-reddening membrane which comprises a
light-transmittable medium and dispersed therein, (a) a rhodamine
base fluorescence pigment and (b) a fluorescence pigment that has
absorptions in the blue region and induces energy transfer to or
reabsorption from said rhodamine base fluorescence pigment. The
present invention further provides an inexpensive red
light-emitting device which comprises said fluorescence-reddening
membrane and a light-emitting device.
[0013] It is preferable that the above-mentioned
fluorescence-reddening membrane according to the present invention
comprise a light-transmittable medium and, dispersed therein, a
rhodamine base fluorescence pigment and a fluorescence pigment of
naphthalimide base, coumarin base or the like. On the other hand, a
fluorescence conversion membrane which comprises a
light-transmittable medium and, dispersed therein, a rhodamine base
fluorescence pigment alone, or a fluorescence pigment of
naphthalimide base or coumarin base alone fails to attain a high
conversion efficiency as high as at least 33%, or fails to emit a
red light.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a chromaticity coordinate showing the region of
each of the colors and the conversion efficiency thereof,
[0015] FIG. 2 is an emission spectrum of an organic EL device,
[0016] FIG. 3 is a transmission spectrum of a color filter for
chromaticity regulation,
[0017] FIG. 4 is a red emission spectrum in Example 1,
[0018] FIG. 5 is an absorption spectrum of a fluorescence-reddening
membrane in Example 1,
[0019] FIG. 6 is an absorption spectrum of a fluorescence-reddening
membrane in Example 3,
[0020] FIG. 7 is an absorption spectrum of a fluorescence
conversion membrane (comprising rhodamine base fluorescence pigment
alone) in Comparative Example 1,
[0021] FIG. 8 is an absorption spectrum corresponding to a
naphthalimide base fluorescence pigment in Example 1,
[0022] FIG. 9 is an absorption spectrum corresponding to a coumarin
base fluorescence pigment in Example 3,
[0023] FIG. 10 is an emission (fluorescence) spectrum of a
naphthalimide base fluorescence pigment (excitation at 460 nm),
and
[0024] FIG. 11 is an emission (fluorescence) spectrum of a cumarin
base fluorescence pigment (excitation at 460 nm).
THE MOST PREFERRED EMBODIMENTS TO CARRY OUT THE INVENTION
[0025] Specifically, the fluorescence pigment which has absorptions
in the blue region so that a blue light can be interrupted and at
the same time, induces energy transfer to and reabsorption from the
rhodamine base fluorescence pigment, preferably has absorptions in
the blue region at a wavelength of 520 nm or less and an absorbance
(optical density OD) of at least 1.0 at a wavelength of 420 to 490
nm. The aforesaid fluorescence pigment can sufficiently interrupt a
blue light.
[0026] In addition, the rhodamine base fluorescence pigment as the
component (a) has absorptions at 450 to 610 nm. Thus the absorption
region of the rhodamine base fluorescence pigment as the component
(a) partially overlaps the absorption region of the fluorescence
pigment as the component (b), thus enabling effective energy
transfer from the component (b) to the component (a). It is also
possible that the rhodamine base fluorescence pigment as the
component (a) reabsorbs the fluorescence from the fluorescence
pigment as the component (b). In this case the rhodamine base
fluorescence pigment as the component (a) emits red fluorescence of
600 nm or more.
[0027] As described hereinbefore, the fluorescence conversion
membrane comprising the combination of both the fluorescence
pigments dispersed in a light-transmittable medium, converts the
blue light emitted by the organic EL device to a red light at a
high conversion efficiency as high as 33%. Consequently, a red
light emission can be assured.
[0028] On the contrary, the fluorescence conversion membrane
comprising the rhodamine base fluorescence pigment alone dispersed
in a light-transmittable medium can not fully interrupt a blue
light emitted by the organic EL device, thereby causing a blue
light to mix in the converted red light, and leading to failure to
selectively assure a red light. In the case where a color filter
for chromaticity regulation is superimposed thereon in order to
interrupt the blue color, a converted red color is obtained, but
the conversion efficiency is as low as less than 33% (refer to
Comparativie Examples 1 & 2). On the other hand, the
fluorescence conversion membrane comprising the naphthalimide base
fluorescence pigment or the coumarin base fluorescence pigment
alone dispersed in a light-transmittable medium results in failure
to emit a red light, since the membrane does not contain a red base
fluorescence pigment, causing it to emit a green light. The
chromaticity of the red color on the CIE coordiate as used in the
present invention is given in FIG. 1.
[0029] By the term (a) a rhodamine base fluorescence pigment as
mentioned herein is meant a fluorescence pigment which contains at
least one kind of rhodamine coloring matter. Specific examples
thereof include rhodamine 6 G, rhodamine B, rhodamine 3 B,
rhodamine 101, rhodamine 110, basic violet 11 and sulforhodamine.
The ratio of said coloring matter in each of the pigments is in the
range of preferably 0.1 to 10%, more preferably 1.0 to 7.0% each by
weight. The ratio of said coloring matter therein of less than 0.1%
by weight unfavorably leads to insufficient color emission, whereas
that of more than 10% by weight unfavorably brings about
deterioration of fluorescence properties due to concentration
quenching by the interaction of each of the coloring matters.
[0030] Examples of pigment components other than the coloring
matters include polymethacrylic acid esters, polyvinyl chloride,
poly(vinyl chloride/vinyl acetate) copolymer, alkyd resin, aromatic
sulfonamide resins, urea resin, melamine resin and benzoguanamine
resin. However, any of the foregoing coloring matters may be
dispersed alone in a light-transmittable medium within the
foregoing proportion.
[0031] Examples of the chemical constitutions of the rhodamine
coloring matters are given in the following. 1
[0032] Any of the naphthalimide coloring matters may be used alone
or in combination with at least one other, and the mixture of
solvent yellow 116 and solvent yellow 44 is preferable.
[0033] In addition, by the term a coumarin base fluorescence
pigment as mentioned herein is meant a fluorescence pigment which
contains at least one kind of the coumarin coloring matter.
Specific examples thereof include coumarin 153, coumarin 6,
coumarin 7, coumarin 30 and basic yellow 51. The ratio of the
coumarin coloring matter in each of the pigment is in the range of
preferably 0.1 to 10%, more preferably 1.0 to 7.0% each by weight.
The ratio of the coloring matter therein of less than 0.1% by
weight unfavorably leads to insufficient color emission, whereas
that of more than 10% by weight unfavorably brings about
deterioration of fluorescence properties due to concentration
quenching by the ieteraction of each of the coloring matters.
[0034] Examples of pigment components other than the coloring
matters include polymethacrylic acid esters, polyvinyl chloride,
poly(vinyl chloride/vinyl acetate) copolymer, alkyd resin, aromatic
sulfonamide resins, urea resin, melamine resin and benzoguanamine
resin. However, any of the foregoing coloring matters may be
dispersed alone in a light-transmittable medium.
[0035] Examples of the chemical constitutions of the coumarin
coloring matters are given in the following. 2
[0036] Any of the above-described coumarin coloring matters may be
used alone or in combination with at least one other, among which
are preferable coumarin 6 and coumarin 7 each having a high
absorption coefficient.
[0037] The light-transmittable medium to be used for the
fluorescence-reddening membrane according to the present invention
needs only to have light-transmittability and membrane formability
without specific limitation, and is exemplified for use by high
molecular compounds, inorganic glass and a medium for printing.
Examples of the high molecular compounds include
polyvinylpyrrodinone, polymethyl methacrylate, polymethyl acrylate,
polystyrene, polycarbonate, polyvinyl acetate, polyvinyl chloride,
polybutene, polyethylene glycol, a copolymer thereof, and further,
photosensitive resins such as photoresists, thermosetting resins
such as epoxy resins. Moreover, a pigment component other than the
above-exemplified coloring matters may be used as a
light-transmittable medium.
[0038] Examples of the inorganic glass include borate glass and
silica glass. Any of the above-exemplified light-transmittable
media may be used alone or in combination with at least one other
species. Of these is preferable a medium for printing, especially a
light-transmittable medium for printing in which polyvinyl chloride
resin or polyester resin is dissolved in a solvent.
[0039] The fluorescence-reddening membrane according to the present
invention comprises, as principal components, the component (a),
the fluorescence pigment as the component (b) and the
light-transmittable medium. The fluorescence pigment as the
component (b) is preferably contained in the aforesaid
fluorescence-reddening membrane in an amount of 5 to 80% by weight.
The amount thereof less than 5% by weight leads to insufficiency in
coloring matter density required to assure the desired red light
and to the necessity for extremely thickening the membrane (as
thick as 100 .mu.m or more), whereby uniform membrane thickness is
made difficult to assure; whereas the amount thereof more than 80%
by weight brings about poor membrane-formability as well as
mechanically brittle membrane.
[0040] From the viewpoints of red color purity and membrane
formability, the above-mentioned fluorescence pigment is dispersed
in the light-transmittable medium preferably in an amount in the
range of 10 to 60% by weight.
[0041] The ratio of the (a) rhodamine base fluorescence pigment to
the (b) naphthalimide-base or coumarin-base fluorescence pigment,
each being used in the invention depends upon the species of the
fluorescence pigments, and it is preferably in the range of 20:1 to
1:20 by weight.
[0042] In the case where the ratio by weight of the (a) rhodamine
base fluorescence pigment exceeds the aforestated range, the amount
of the (b) naphthalimide-base or coumarin-base fluorescence pigment
decreases, whereby the blue light emitted by the organic EL, device
is made impossible to be sufficiently interrupted and accordingly
the desired red light is made unobtainable. Specifically, when the
optical density (OD) at a wavelength in the range of 420 to 490 nm,
that is, the blue light region, is made to less than 1.0, the blue
light leaks remarkably, thereby making the desired red light
difficult to obtain.
[0043] On the other hand, in the case where the ratio by weight of
the (a) rhodamine base fluorescence pigment is made lower than the
aforestated range, the energy transfer from the (b) fluorescence
pigment to the (a) rhodamine base fluorescence pigment, or
reabsorption from the pigment (a) decreases, thereby making the
desired red light difficult to obtain in high efficiency.
[0044] Therefore, from the aspect of the red light purity and the
conversion efficiency, the ratio by weight of the (a) rhodamine
base fluorescence pigment to the (b) fluorescence pigment which has
absorptions in the blue region and thus interrupts blue light is
more preferably in the range of 10:1 to 1:10.
[0045] The process for producing the fluorescence-reddening
membrane according to the present invention is not specifically
limited, but can be selected for use from a variety of processes.
For example, the objective fluorescence-reddening membrane is
obtained by a process in which the (a) rhodamine base fluorescence
pigment and the (b) naphthalimide-base or coumarin-base
fluorescence pigment are mixed with and dispersed in the
light-transmittable medium, and the mixture thus produced is formed
into a membrane by any of the methods including casting, spin
coating, printing, bar coating, extrusion molding, roll molding,
pressing, spraying and roll coating. An organic solvent, when used
for the membrane forming, is exemplified by dichloromethane;
1,2-dichloroethane; chloroform; acetone; cyclohexanone; toluene;
benzene; xylene; N,N-dimethylformamide; dimethylsulfoxide;
1,2-dimethoxyethane; diethylene glycol dimethyl ether;
N-methylpyrrolidone; ethylene glycol monomethyl ether(methyl
Cellosolve); ethylene glycol monoethyl ether(ethyl Cellosolve);
ethylene glycol monoethyl ether acetate(ethyl Cellosolve acetate).
Any of these solvents may be used alone or in combination with at
least one other. In the case of casting, for example, a
fluorescence-reddening membrane in the form of thin film can be
produced by dissolving the fluorescence pigments and the
light-transmittable medium in a suitable solvent selected from
among the aforestated solvents, and gradually adding dropwise the
solution thus obtained onto a substrate such as a glass substrate
to evaporate the solvent.
[0046] Aside from the foregoing, the red-emitting device according
to the present invention comprises the above-described
fluorescence-reddening membrane and a light emitting device. A blue
light-emitting organic EL device is preferably usable for said
light emitting device, and can be prepared by any of the methods
disclosed in Japanese Patent Application Laid-Open Nos. 47890/1991
(Hei-3), 231970/1991 (Hei-3), 17765/1993 (Hei-5), 135878/1993
(Hei-5), 140145/1993 (Hei-5), 247458/1993 (Hei-5), 247459/1993
(Hei-5), 100857/1994 (Hei-6), 132080/1994 (Hei-6), etc. For
example, the blue light-emitting organic EL device can be fabricaed
by a method wherein a transparent supporting substrate composed of
a glass substrate on which a membrane of indium/tin oxide
(hereinafter abbreviated to "ITO") is formed as an electrode is
subjected to vacuum deposition successively by the use of
4,4'-bis[N-phenyl-N-(3-methylphenyl- )amino]biphenyl (TPD);
4,4'-bis(2,2-diphenyl-vinyl)-biphenyl (DPVBi);
tris(8-quinolinol)aluminum (Alq); and magnesium/silver electrode to
form a multi-layer structure (refer to Preparation Example 1).
[0047] In the case where a light-emitting device and a
fluorescence-reddening membrane are brought into tight contact with
each other in the preparation of the red-emitting device according
to the present invention, there is preferably used, between each of
the membranes, a material having a high refractive index as
compared with air such as the above-described light-transmittable
medium in order to prevent light scattering and alao enhance the
conversion efficiency. For the sake of simplicity, it is possible
to insert an inert liquid such as a fluorohydrocarbon therebetween
(refer to Examples 1& 2).
[0048] In the following, the present invention will be described in
more detail with reference to working examples, which however shall
not limit the present invention thereto.
PREPARATION EXAMPLE 1
[0049] (Preparation of Blue-Emitting Organic EL Device)
[0050] A transparent supporting substrate was prepared by forming a
membrane of an ITO electrode in a thickness of 100 nm on a glass
substrate sized 25 mm.times.75 mm.times.1.1 mm, and was subjected
to ultrasonic cleaning by the use of isopropyl alcohol for 5
minutes, to cleaning with pure water for 5 minutes, and further to
ultrasonic cleaning by the use of isopropyl alcohol for 5 minutes.
Thereafter, the resultant transparent supporting substrate was
fixed to a substrate holder of a vacuum deposition apparatus. In a
molybdenum-made boat for resistance heating was placed 200 mg of
4,4'-bis[N-phenyl-N-(3-methylphen- yl)amino]biphenyl (TPD).
Further, in another molybdenum-made boat for resistance heating was
placed 200 mg of 4,4'-bis(2,2-diphenylvinyl)biphen- yl (DPVBi); and
also 200 mg of tris(8-quinolinol)-aluminum (Alq). Then, the inside
of a vacuum chamber was depressurized to 1.times.10.sup.-4 Pa. The
boat containing the TPD was heated to 215 to 220.degree. C. to
deposit the TPD onto the substrate at a deposition rate of 0.1 to
0.3 nm/second to form a membrane in a film thickness of 60 nm as a
hole injection layer, while the substrate maintained room
temperature. In addition, without taking the resultant hole
injection layer out of the vacuum chamber, the DPVBi was deposited
onto the substrate at a boat temperature of 250.degree. C. at a
deposition rate of 0.1 to 0.2 nm/second to form a membrane in a
film thickness of 40 nm as an light-emitting layer.
[0051] Subsequently, the Alq was further deposited onto the
substrate at a boat temperature of 250.degree. C. at a deposition
rate of 0.1 to 0.3 nm/second to form a membrane in a film thickness
of 20 nm as an electron-transporting layer. The substrate thus
treated was taken out of the vacuum chamber, equipped with a
stainless-steel made mask on the side of the electron-transporting
layer, and again fixed to the substrate holder. Then 0.5 g of
silver wire was put in a tungsten-made basket; 1 g of magnesium
ribbon was put in a boat made of molybdenum; the inside of the
vacuum chamber was depressurized to 1.times.10.sup.-4 Pa; and
silver(deposition rate of 0.1 nm/sec) and magnesium (deposition
rate of 0.8 nm/sec) were simultaneously doposited to form a cathode
membrane, thereby preparing a blue-emitting organic EL device.
[0052] The emission spectrum of the resultant organic EL device is
given in FIG. 2.
PREPARATION EXAMPLE 2
[0053] (Preparation of Color Filter for Chromaticity
Regulation)
[0054] A color filter for chromaticity regulation was prepared by
subjecting a red color resist (produced by Fuji Hunt Electronics
Technology Co., Ltd. under the trade name CR-2000) to spin coating
on a glass substrate, drying it at 80.degree. C. in an oven and
curing at 200.degree. C. in the oven. The resultant membrane had a
thickness of 2.3 .mu.m as measured with a surface roughness meter.
The transmission spectrum of the membrane is given in FIG. 3.
EXAMPLE 1
[0055] A bar coated membrane to be used as a fluorescence-reddening
membrane was prepared by dissolving 0.12 g of a naphthalimide base
fluorescence pigment containing benzoguanamine resin, 2% by weight
based on the same of solvent yellow 116 and 6% by weight based on
the same of solvent yellow 44; and 0.2 g of a rhodamine base
fluorescence pigment containing benzoguanamine resin, 2% by weight
based on the same of basic violet 11 and 2% by weight based on the
same of rhodamine 6G, in 2 g of an ink (solid content of 40% by
weight) in which polyvinyl chloride resin (molecular weight of
20,000) as a printing medium was dissolved in cyclohexanone. The
resultant membrane had a thickness of 36 .mu.m as measured with a
micrometer.
[0056] Subsequently, the blue-emitting organic EL device
(chromaticity x of 0.16 and y of 0.15, blue) which had been
obtained in Preparation Example 1 was allowed to emit light under
the conditions including a voltage of 7V and a current density of
4.2 mA/cm.sup.2, and was brought into tight contact with the
fluorescence-reddening membrane thus prepared, while a
fluorohydrocarbon liquid (produced by 3 M Co., Ltd. under the trade
name FC-70) was inserted therebetween so as to form a red-emitting
deveice. A measurement was made of the brightness of the output
light by means of a brightness meter (produced by Minolta Co., Ltd.
under the trade name CS-100). As a result, the blue light prior to
the superimposition of the fluorescence-reddening membrane had a
brightness of 100 cd/m.sup.2, whereas the red light after the
superimposition thereof having a chromaticity x of 0.59 and y of
0.34 was obtained with a brightness of 34 cd/m.sup.2 and with a
conversion efficiency of 34%. The emission spectrum of the red
light thus obtained is given in FIG. 4.
[0057] Moreover, the absorption spectrum of the
fluorescence-reddening membrane thus prepared is given in FIG. 5.
It can be seen that in the absorption spectrum corresponding to the
naphthalimide base fluorescence pigment (FIG. 8), said pigment has
absorptions in the blue region, that is, at 520 nm and less and
also has an optical density OD of 1.0 or more at a wavelength in
the range of 420 to 490 nm, thereby sufficiently interrupting the
blue light from the organic EL device. It is further understood
that effective energy transfer is made to the rhodamine base
fluorescence pigment by partially overlapping the absorption
spectrum corresponding thereto (FIG. 7), or the rhodamine base
fluorescence pigment is allowed to reabsorb the light emitted by
the naphthalimide base fluorescence pigment (FIG. 10).
EXAMPLE 2
[0058] A bar coated membrane to be used as a fluorescence-reddening
membrane was prepared by dissolving 1.3 g of a rhodamine base
fluorescence pigment containing benzoguanamine resin, 1% by weight
based on the same of basic violet 11 and 1% by weight based on the
same of basic red 1; 20 mg of solvent yellow 116; and 20 mg of
solvent yellow 44, in 2.3 g of an ink (solid content of 40% by
weight) in which polyvinyl chloride resin (molecular weight of
20,000) as a printing medium was dissolved in cyclohexanone. The
resultant membrane had a thickness of 60 .mu.m as measured with a
micrometer.
[0059] Subsequently, the blue-emitting organic EL device
(chromaticity x of 0.16 and y of 0.15, blue) which had been
prepared in Preparation Example 1 was allowed to emit light under
the conditions including a voltage of 7V and a current density of
4.2 mA/cm.sup.2, and was brought into tight contact with the
fluorescence-reddening membrane thus prepared, while a
fluorohydrocarbon liquid (FC-70) was inserted therebetween so as to
form a red-emitting deveice. A measurement was made of the
brightness of the output light by means of a brightness meter
(produced by Minolta Co., Ltd. under the trade name CS-100). As a
result, the blue light prior to the superimposition of the
fluorescence-reddening membrane had a brightness of 100 cd/m.sup.2,
whereas the red light after the superimposition thereof having a
chromaticity x of 0.60 and y of 0.33 was obtained with a brightness
of 33 cd/m.sup.2 and with a conversion efficiency of 33%.
[0060] The absorption spectrum of the fluorescence-reddening
membrane thus prepared was the same as that in FIG. 5.
EXAMPLE 3
[0061] A bar coated membrane to be used as a fluorescence-reddening
membrane was prepared by dissolving 0.12 g of a coumarin base
fluorescence pigment containing benzoguanamine resin and 5% by
weight based on the same of coumarin 6; and 0.2 g of a rhodamine
base fluorescence pigment same as that used in Example 1, in 2 g of
an ink (solid content of 40% by weight) in which polyester resin
(molecular weight of 40,000) as a printing medium was dissolved in
ethyl cellosolve acetate. The resultant membrane had a thickness of
35 .mu.m as measured with a micrometer.
[0062] Subsequently, the blue-emitting organic EL device
(chromaticity x of 0.16 and y of 0.15, blue) which had been
obtained in Preparation Example 1 was allowed to emit light under
the conditions including a voltage of 7V and a current density of
4.2 mA/cm.sup.2, and was brought into tight contact with the
fluorescence-reddening membrane thus prepared, while a
fluorohydrocarbon liquid (FC-70)was inserted therebetween so as to
form a red-emitting deveice. A measurement was made of the
brightness of the output light by means of a brightness meter
(produced by Minolta Co., Ltd. under the trade name CS-100). As a
result, the blue light prior to the superim-position of the
fluorescence-reddening membrane had a brightness of 100 cd/m.sup.2,
whereas the red light after the superimposition thereof having a
chromaticity x of 0.60 and y of 0.32 was obtained with a brightness
of 33 cd/m.sup.2 and with a conversion efficiency of 33%.
[0063] Moreover, the absorption spectrum of the
fluorescence-reddening membrane thus prepared is given in FIG. 6.
It can be seen that in the absorption spectrum corresponding to the
coumarin base fluorescence pigment (FIG. 9), said pigment has
absorptions in the blue region, that is, at a wavelength of 520 nm
and less and also has an optical density OD of 1.0 or more at a
wavelength in the range of 420 to 490 nm, thereby sufficiently
interrupting the blue light from the organic EL device. It is
further understood that effective energy transfer is made to the
rhodamine base fluorescence pigment by partially overlapping the
absorption spectrum corresponding thereto (FIG. 7), or the
rhodamine base fluorescence pigment is allowed to reabsorb the
light emitted by the coumarin base fluorescence pigment (FIG.
11).
EXAMPLE 4
[0064] A bar coated membrane to be used as a fluorescence-reddening
membrane was prepared by dissolving 1.3 g of the rhodamine base
fluorescence pigment same as that used in Example 2 and 30 mg of
coumarin 7, in 2.3 g of an ink (solid content of 40% by weight) in
which polyester resin (molecular weight of 40,000) as a printing
medium was dissolved in ethyl cellosolve acetate. The resultant
membrane had a thickness of 45 .mu.m as measured with a
micrometer.
[0065] Subsequently, the blue-emitting organic EL device
(chromaticity x of 0.16 and y of 0.15, blue) which had been
prepared in Preparation Example 1 was allowed to emit light under
the conditions including a voltage of 7V and a current density of
4.2 mA/cm.sup.2, and was brought into tight contact with the
fluorescence-reddening membrane thus prepared, while a
fluorohydrocarbon (FC-70) liquid was inserted therebetween so as to
form a red-emitting deveice. A measurement was made of the
brightness of the output light by means of a brightness meter
(produced by Minolta Co., Ltd. under the trade name CS-100). As a
result, the blue light prior to the superimposition of the
fluorescence-reddening membrane had a brightness of 100 cd/m.sup.2,
whereas the red light after the superimposition thereof having a
chromaticity x of 0.59 and y of 0.33 was obtained with a brightness
of 33 cd/m.sup.2 and with a conversion efficiency of 33%.
[0066] Moreover, the absorption spectrum of the
fluorescence-reddening membrane thus prepared was same as that in
FIG. 6.
COMPARATIVE EXAMPLE 1
[0067] A bar coated membrane to be used as a fluorescence-reddening
membrane was prepared by dissolving 0.2 g of the rhodamine base
fluorescence pigment same as that used in Example 1, in 2.0 g of an
ink (solid content of 40% by weight) in which polyvinyl chloride
resin (molecular weight of 20,000) as a printing medium was
dissolved in cyclohexanone. The resultant membrane had a thickness
of 40 .mu.m as measured with a micrometer.
[0068] Subsequently, the blue-emitting organic EL device
(chromaticity x of 0.16 and y of 0.15, blue) which had been
prepared in Preparation Example 1 was allowed to emit light under
the conditions including a voltage of 7V and a current density of
4.2 mA/cm.sup.2, and was brought into tight contact with the
fluorescence-reddening membrane thus prepared, while a
fluorohydrocarbon liquid (FC-70) was inserted therebetween so as to
form a red-emitting deveice. A measurement was made of the
brightness of the output light by means of a brightness meter
(produced by Minolta Co., Ltd. under the trade name CS-100). As a
result, the blue light prior to the superimposition of the
fluorescence-reddening membrane had a brightness of 100 cd/m.sup.2,
whereas a light after the superimposition thereof having a
chromaticity x of 0.56 and y of 0.23 other than the red light was
obtained with a low brightness of only 10 cd/m.sup.2 and with a low
conversion efficiency of only 10%.
[0069] Moreover, the absorption spectrum of the
fluorescence-reddening membrane thus prepared is given in FIG. 7.
It can be seen therefrom that the absorption spectrum is devoid of
absorptions in the blue region, that is, devoid of optical density
OD of 1.0 or more at a wavelength in the range of 420 to 490 nm,
thereby failing to sufficiently interrupt the blue light from the
organic El device. It is further understood that effective energy
transfer is never made to the rhodamine base fluorescence pigment
from the blue light of the organic EL device, and that the
fluorescence pigment is not allowed to reabsorb the emitted light,
thus causing failure to generate red light and bringing about a low
conversion efficiency.
COMPARATIVE EXAMPLE 2
[0070] A cast membrane to be used as a fluorescence-reddening
membrane was prepared by dissolving 4.2 mg of rhodamine B and 1.8 g
of polyvinyl pyrrolidinone (PVP, molecular weight of 360,000,
approx.) in dichloromethane. The resultant membrane had a thickness
of 50 .mu.m as measured with a micrometer.
[0071] Subsequently, the blue-emitting organic EL device
(chromaticity x of 0.16 and y of 0.15, blue) which had been
obtained in Preparation Example 1 was allowed to emit light under
the conditions including a voltage of 7V and a current density of
4.2 mA/cm.sup.2. The blue-emitting organic EL device, the
fluorescence-reddening membrane thus prepared and the membrane of
the color filter for chromaticity regulation which had been
obtained in Preparation Example 2 were brought into tight contact
with one another in this order, while a fluorohydrocarbon liquid
(FC-70) was inserted therebetween so as to form a red-emitting
deveice. A measurement was made of the brightness of the output
light by means of a brightness meter (produced by Minolta Co., Ltd.
under the trade name CS-100). As a result, the blue light prior to
the superimposition of the fluorescence-reddening membrane and the
membrane of the color filter for chromaticity regulation had a
brightness of 100 cd/m.sup.2, whereas a red light after the
superimposition thereof having a chromaticity x of 0.56 and y of
0.28 was obtained with a low brightness of only 5 cd/m.sup.2 and
with a low conversion efficiency of only 5%.
[0072] It can be seen therefrom that, although the red light was
obtained by the location of the color fiter interrupting the blue
color, effective energy transfer is never made to the rhodamine
base fluorescence pigment from the blue light of the organic EL
device, and that the fluorescence pigment is not allowed to
reabsorb the emitted light, thus bringing about a low conversion
efficiency.
[0073] As a conclusion, the fluorescence-reddening membrane
according to the present invention can convert the light emitted by
a blue-emitting organic EL device at a high conversion efficiency
as high as 33% or more, and besides the red-emitting device which
comprises the fluorescence-reddening membrane and a light-emitting
device can emit a red light at a high conversion efficiency,
curtail its production cost, and reduce the size and thickness of
itself.
INDUSTRIAL APPLICABILITY
[0074] The red-emitting device according to the present invention
is favorably used for a back light of office automation (OA)
equipment, a clock and watch, a back light of each of various
displays and a self-emitting multi-color or full-color display.
* * * * *