An electroluminescent device having improved moisture resistance. The device comprises a transparent substrate having a transparent electrode layer. A luminescent layer and a dielectric layer are interposed between the transparent electrode layer and a back electrode layer. The luminescent layer comprises a resinous binder containing electroluminescent particles. The dielectric layer comprises a resinous binder containing dielectric particles. The back electrode layer comprises a resinous binder containing conductive particles. The resinous binder of at least one of the luminescent layer and the dielectric layer is made from a fluoride resin. A reaction accelerator for promoting polymerization of the fluoride resin is contained in the back electrode layer.

FIELD OF THE INVENTION 
The present invention relates to an electroluminescent device which can be 
used in display devices for various apparatuses, in a backlighting 
arrangement, and in other devices. 
BACKGROUND OF THE INVENTION 
A conventional electroluminescent device is fabricated in the manner 
described now. A transparent electrode layer consisting of indium tin 
oxide (ITO) is deposited on a transparent substrate which is made of a 
sheet of polyethylene terephthalate or the like. A luminescent layer, a 
dielectric layer, and a back electrode layer are laminated on the 
transparent electrode lying on the transparent substrate. These are sealed 
by transparent moisture-proof film, thus completing the electroluminescent 
device. 
In this prior art technique, it is common practice to use a cyanoethylated 
resin as a resinous binder for both luminescent layer and dielectric 
layer. However, this cyanoethylated resin has the disadvantage that it is 
highly hygroscopic. On the other hand, the electroluminescent material in 
the luminescent layer is severely deteriorated by intrusion of moisture. 
Therefore, in order to protect the electroluminescent material against 
moisture and to improve the durability, it is essential that the 
electroluminescent device be sealed by moisture-proof film. Consequently, 
in the prior art technique, the necessity of the moisture-proof film 
increases the thickness of the electroluminescent device itself 
accordingly and decreases its flexibility. Because the moisture-proof film 
must have a mating space along its outer periphery, the luminescent area 
is smaller than the two-dimensional size of the electroluminescent device. 
Furthermore, the moisture-proof film is expensive. Hence, the cost to 
fabricate the electroluminescent device that needs a sealing step is 
increased. 
A first improved technique for dispensing with the moisture-proof film is 
described by Timex Corporation in U.S. Pat. No. 4,775,964 relating to a 
luminescent dial on a watch. In this improved technique, epoxy resin is 
used as the resinous binder for the luminescent layer. This luminescent 
dial is installed in a watch case that is a confined narrow space and so 
this technique can be put into practical use. However, if it is used under 
an exposed state, the moisture resistance and durability are not 
satisfactorily high. Furthermore, there is room for improvement of the 
luminescent brightness. 
Meanwhile, we have already proposed an improved electroluminescent device 
in Japanese patent application No. 231709/1993. In particular, a binder 
consisting of a fluoride resin is used, so that moisture-proof film can be 
dispensed with. In addition, high luminescent brightness can be obtained. 
However, it cannot be said that this second improved technique provides 
complete moisture resistance. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
electroluminescent device using a binder made from a fluoride resin to 
thereby dispense with moisture-proof film and to improve the moisture 
resistance and durability of the device. 
The above object is achieved in accordance with the teachings of the 
invention by an electroluminescent device in which a luminescent layer and 
a dielectric layer are interposed between a transparent electrode layer 
and a back electrode layer. The luminescent layer comprises a resinous 
binder that contains electroluminescent particles. The dielectric layer 
comprises a resinous binder that contains dielectric particles. The back 
electrode layer comprises a resinous binder that contains conductive 
particles. This device is characterized in that the resinous binder of at 
least one of the luminescent layer and the dielectric layer is made from a 
fluoride resin, and that a reaction accelerator for promoting 
polymerization of the fluoride resin binder is contained in the back 
electrode layer. 
In another electroluminescent device according to the invention, a 
luminescent layer and a dielectric layer are interposed between a 
transparent electrode layer and a back electrode layer. The luminescent 
layer comprises a resinous binder that contains electroluminescent 
particles. The dielectric layer comprises a resinous binder that contains 
dielectric particles. This device is characterized in that the resinous 
binder of at least one of the luminescent layer, the dielectric layer, and 
the back electrode layer is made from a fluoride resin, and that a 
protective layer made from an electrically insulating resin is formed on 
an outer surface of the back electrode layer. A reaction accelerator for 
promoting polymerization of the fluoride resin is contained in the 
protective layer. 
Preferably, the reaction accelerator is an organic silicon monomer having 
two or more different reaction groups per molecule. Preferably, the amount 
of the reaction accelerator added to the back electrode layer is more than 
2% by weight. Preferably, the end portion of the back electrode layer is 
retreated slightly from the end portion of the electroluminescent device, 
taking account of deterioration of the end surfaces of the outer periphery 
of the luminescent layer. 
Other objects and features of the invention will appear in the course of 
the description thereof, which follows.

DETAILED DESCRIPTION OF THE INVENTION 
An electroluminescent device according to the invention is now described by 
referring to FIGS. 1-3. A transparent electrode layer 1b is formed on a 
transparent substrate 1a. A luminescent layer 2 is formed on the electrode 
layer 1b. A dielectric layer 3 is formed on the luminescent layer 2. A 
back electrode layer 4 is formed on the dielectric layer 3. 
The transparent substrate 1a is made of a sheet of polyethylene 
terephthalate. ITO is evaporated on this substrate to form the transparent 
electrode layer 1b. 
The luminescent layer 2 is formed by printing luminescent ink on the 
transparent electrode layer 1b. The luminescent ink is made up of 
luminescent particles. One example of the material of these particles is 
zinc sulfide (ZnS) doped with Cu that exhibits fluorescence. Taking 
account of moisture resistance, a fluoride resin binder prepared by 
dissolving 10 g of copolymer of vinylidene fluoride and propylene 
hexafluoride in a solvent, or 25 g of methyl ethyl ketone, is used 
together with 60 g of the luminescent particles or a fluorescent material. 
In use, these two kinds of materials are mixed. The luminescent ink is 
printed on the transparent electrode layer 1b by screen printing or other 
method, and then the ink is heated and dried, thus completing the 
luminescent layer 2. 
Thereafter, dielectric particles of a high dielectric constant are 
dispersed in the fluoride resin binder and they are kneaded together, thus 
forming a dielectric ink. This ink is applied to the surface of the 
luminescent layer 2 to form a dielectric layer 3. The dielectric ink is 
created in the manner described now. First, barium titanate (BaTiO.sub.3) 
having a high dielectric constant is used as dielectric particles. Then, 
60 g of this barium titanate is mixed with the fluoride resin binder and 
they are stirred to thereby form the dielectric ink. As described above, 
the fluoride resin binder has been previously prepared by dissolving 10 g 
of copolymer of vinylidene fluoride and propylene hexafluoride in 25 g of 
methyl ethyl ketone, the vinylidene fluoride having excellent moisture 
resistance. This dielectric ink is printed on the luminescent layer 2, 
heated, and dried, thus forming the dielectric layer 3. The dielectric 
constant of the fluoride binder is low but the dielectric constant of 
barium titanate is very high, or 1800 F/m. Therefore, the whole dielectric 
layer 3 shows a high dielectric constant. Hence, the electroluminescent 
device does not suffer from low brightness. The proper function of this 
dielectric layer 3 is to enhance the electric field acting on the 
luminescent layer 2. In addition, the dielectric layer 3 acts as a barrier 
that prevents moisture from entering the luminescent layer 2. 
If the copolymer (or, fluoride copolymer) of vinylidene fluoride and 
propylene hexafluoride is directly used in the luminescent layer 2 and in 
the dielectric layer 3, the moisture resistance will be improved to some 
extent. To improve the moisture resistance further, the following 
contrivances are made in the present invention. 
The back electrode layer 4 is formed by mixing powdered carbon into 
polyester resin. The powdered carbon is an example of conductive 
particles. More specifically, 10 g of polyester resin is dissolved in 90 g 
of isophorone, or a solvent, to produce a resinous binder. Then, 80 g of 
powdered carbon is added to the resinous binder and they are stirred well. 
In this way, a conductive ink is prepared. A reaction accelerator for 
promoting copolymerization of the fluoride resin binder in the luminescent 
layer 2 and in the dielectric layer 3 is added to the conductive ink. This 
ink is printed on the dielectric layer 3, heated, and dried, thus forming 
the back electrode layer 4. The reaction accelerator added to the 
conductive ink, or the back electrode layer 4, permeates the dielectric 
layer 3 and the luminescent layer 2 from the back electrode layer 4 during 
the heating and drying, so that the copolymerization of the fluoride resin 
in the dielectric layer 3 and in the luminescent layer 2 is accelerated. 
As a result, the density of the fluoride resin is increased. This 
effectively prevents intrusion of moisture. 
One appropriate example of the reaction accelerator is N-.beta.(aminoethyl) 
.gamma.-aminopropyl trimethoxysilane (H.sub.2 NC.sub.2 H.sub.4 NHC.sub.3 
H.sub.6 Si(OCH.sub.3).sub.3), which is an organic silicon monomer having 
two or more different kinds of reaction groups per molecule. 
The organic silicon monomer having two or more different kinds of reaction 
groups per molecule performs other excellent functions. Specifically, one 
of the different reaction groups reacts with the luminescent particles 
which are an inorganic substance. Another reaction group reacts with the 
fluoride resin that is an organic substance, and becomes coupled with the 
resin. In this way, the organic silicon monomer acts as one kind of 
bonding agent and encases the electroluminescent particles in the fluoride 
resin. The fluoride resin prevents moisture from entering the 
electroluminescent particles. Similarly, the dielectric particles of a 
high dielectric constant is encased in the fluoride resin. The fluoride 
resin prevents moisture from entering the dielectric particles. In 
consequence, the moisture resistance of the whole electroluminescent 
device is improved greatly. Data about the amount of the added reaction 
accelerator, or bonding agent, in the back electrode layer 4 are listed in 
Table 1. 
TABLE 1 
______________________________________ 
amount (wt. %) of 
bonding agent added to 
brightness 
sample 
carbon back electrode layer 
(100V .times. 400 Hz (cd/m.sup.2)) 
______________________________________ 
a 0 63.2 
b 1.0 60.5 
c 2.0 58.1 
d 4.0 62.0 
e 10.0 60.6 
f 14.0 61.0 
g 20.0 61.9 
h 40.0 60.7 
______________________________________ 
Another reaction accelerator consisting of an organic silicon monomer 
having two or more different kinds of reaction groups per molecule is 
.gamma.-glycidexypropyltrimethoxysilane given by 
##STR1## 
Anhydrotrimellitate given by. 
##STR2## 
is a reaction accelerator which is neither an organic silicon monomer nor 
has two or more kinds of reaction groups per molecule. 
The electroluminescent device fabricated in this way was operated so as to 
emit light for 200 hours with 100 V.times.400 Hz and with 40.degree. 
C..times.90% RH (relative humidity). Luminescent brightness maintenance 
characteristics as shown in FIG. 2 and loss coefficient tan .delta. 
characteristic (FIG. 3) that is one of electrical characteristics of the 
electroluminescent device were obtained. 
As can be seen from FIG. 2, the addition of the reaction accelerator has 
improved the brightness maintenance characteristics over the whole range 
compared with the case in which no reaction accelerator is added (the 
amount of addition is 0%). If the amount of the reaction accelerator added 
is in excess of 2% by weight, then a substantial improvement arises. 
Especially, if the amount of the-reaction accelerator added is in the 
range from 5 to 14% by weight, then the brightness maintenance 
characteristics are improved greatly. 
It can be seen from FIG. 3 that as the amount of the reaction accelerator 
added to the back electrode layer is increased, the loss coefficient tan 
.delta. decreases. This means that less moisture is absorbed, i.e., the 
moisture resistance is improved. In this way, we have confirmed that the 
addition of the reaction accelerator improves the characteristics greatly. 
It may be contemplated to add a vulcanizing agent as a reaction accelerator 
for the vinylidene fluoride and propylene hexafluoride so as to induce 
vulcanization when the luminescent layer and the dielectric layer are 
formed. The vulcanizing agent is typified by peroxides. If the fluoride 
resin is vulcanized, the moisture resistance is improved somewhat but not 
high enough to make moisture-proof film unnecessary. Also, the luminescent 
brightness of the electroluminescent device is halved by the 
vulcanization. This is a fatal problem. 
In the example described above, a fluoride resin is dissolved in a solvent 
to create a fluoride resin binder. Luminescent particles are added to the 
binder, thus creating a luminescent ink. This luminescent ink is printed 
on the transparent electrode layer 1b, heated, and dried. Thus, the 
luminescent layer is formed. As soon as a reaction accelerator is added to 
the luminescent ink, polymerization of the fluoride resin is started even 
at room temperature. As a result, the luminescent ink cures in a short 
time. It is substantially impossible to print the luminescent ink. A 
similar phenomenon is observed regarding the dielectric ink. 
In the present example, the fluoride resin binder used in the luminescent 
layer and in the dielectric layer may be made from polyvinylidene 
fluoride, i.e., polymer of vinylidene fluoride. Alternatively, the 
fluoride resin binder is made from a copolymer of vinylidene fluoride and 
other copolymerizable fluoride resin (e.g., at least one of ethylene 
fluoride, vinyl fluoride, ethylene trifluoride, ethylene chloride 
trifluoride, ethylene tetrafluoride, and propylene hexafluoride). Zinc 
sulfide doped with Cu has been used as the luminescent particles. These 
particles may be previously coated with a transparent inorganic dielectric 
substance such as SiO.sub.2, TiO.sub.2, and Al.sub.2 O.sub.3. 
Furthermore, in the present example, the back electrode layer may be coated 
with a moisture-proof layer consisting of a fluoride resin. In particular, 
10 g of copolymer of vinylidene fluoride and propylene hexafluoride is 
dissolved in 25 g of methyl ethyl ketone to create a fluoride resin ink. 
This ink is printed on the back electrode layer, heated, and dried. Thus, 
a moisture-proof layer consisting of the fluoride resin is created. This 
further enhances the moisture resistance. Examples of the fluoride resin 
used in the moisture-proof layer formed on the back electrode layer 
include copolymers of two or more of ethylene fluoride, vinyl fluoride, 
vinylidene fluoride, ethylene trifluoride, ethylene chloride trifluoride, 
ethylene tetrafluoride, and propylene hexafluoride and copolymers of these 
monomers. The resinous material of the moisture-proof layer can consist of 
other resins such as polyester resins, acrylic resins, and vinyl resins. 
Additionally, moisture-proof film may be stuck on the outer surface of the 
transparent substrate and on the outer surface of the back electrode 
layer. This further enhances the moisture resistance. 
Another electroluminescent device according to the invention is next 
described by referring to FIGS. 4-7. A transparent substrate 11 has a 
transparent electrode layer 11a on which a luminescent layer 12 is formed. 
A dielectric layer 13 is formed on top of the luminescent layer 12. A back 
electrode layer 14 is formed on the top surface of the dielectric layer 
13. The back electrode layer 14 has an end portion retreated a slight 
distance (e.g., 1 mm) inwardly from the end of the electroluminescent 
device, or the luminescent layer 12. The top surface of the back electrode 
layer 14 is coated with a protective layer 15 having an end portion which 
is formed integrally with the end portion of the electroluminescent 
device. Therefore, the outer peripheral portion of the protective layer 15 
is joined to the outer peripheral portion of the dielectric layer 13. The 
transparent substrate 11, the luminescent layer 12, and the dielectric 
layer 13 are exactly the same as their counterparts 1, 2, and 3, 
respectively, of the example described already in conjunction with FIGS. 
1-3 and so these layers 11-13 are not described here. 
The material of the back electrode layer 14 formed on the dielectric layer 
13 is created by mixing powdered carbon that is conductive particles into 
polyester resin. More specifically, 10 g of polyester resin is dissolved 
in 90 g of isophorone, or a solvent, to produce a resinous binder. Then, 
80 g of powdered carbon is added to the resinous binder and they are 
stirred well. In this way, a conductive ink is prepared. This ink is 
printed on the dielectric layer 13, heated, and dried, thus forming the 
back electrode layer 14. 
The back electrode layer 14 has an end portion retreated a distance of 1 mm 
inwardly from the end of the electroluminescent device, or the ends of the 
luminescent layer 12 and of the dielectric layer 13, for the reason 
described later in connection with FIG. 7. 
The outer surface of the back electrode layer 14 is coated with the 
protective layer 15. For this purpose, a protective ink is created by 
dissolving vinyl chloride resin in a solvent consisting of butyl acetate. 
This protective ink contains 2.0% by weight of the reaction accelerator 
for promoting polymerization of the fluoride resin used in the luminescent 
layer 12 and in the dielectric layer 13. 
This protective ink containing the reaction accelerator is printed on the 
back electrode layer 14, heated, and dried. During the heating and drying 
of the protective layer 15, the reaction accelerator permeates the 
dielectric layer 13 and the luminescent layer 12 so that the 
copolymerization of the fluoride resin in the dielectric layer 13 and in 
the luminescent layer 12 is accelerated. As a result, the density of the 
fluoride resin is increased. This effectively prevents intrusion of 
moisture. Hence, the moisture resistance is improved greatly. 
Since the same reaction accelerator as used in the example described 
already in connection with FIGS. 1-3 is employed, the accelerator is not 
described here. 
The electroluminescent device fabricated in this way was operated so as to 
emit light for 200 hours with 100 V.times.400 Hz and with 40.degree. 
C..times.90% RH (relative humidity). Luminescent brightness maintenance 
characteristics as shown in FIG. 5 and loss coefficient tan .delta. 
characteristic as shown in FIG. 6 were obtained, the tan .delta. 
characteristic being one of electrical characteristics of the 
electroluminescent device. 
As can be seen from FIG. 5, the addition of the reaction accelerator has 
improved the brightness maintenance characteristics over the whole range 
compared with the case in which no reaction accelerator is-added (the 
amount of addition is 0%). If the amount of the reaction accelerator added 
is in excess of 2% by weight, then a substantial improvement arises. 
Especially, if the amount of the reaction accelerator added is in the 
range from 10 to 40% by weight, then the brightness maintenance 
characteristics are improved greatly. 
As can be seen from FIG. 6, as the amount of the reaction accelerator added 
is increased, the loss coefficient tan .delta. decreases. This means that 
less moisture is absorbed, i.e., the moisture resistance is improved. In 
this way, we have confirmed that the addition of the reaction accelerator 
to the protective layer 15 improves the moisture resistance. 
As shown in FIG. 4, the back electrode layer 14 has an end portion that is 
retreated a distance of 1 mm inwardly from the end of the 
electroluminescent device, for the reason described now. Two 
electroluminescent devices A and B were fabricated. These two devices were 
similar except for the following points. In the device A, the distance of 
retreat from the end of the electroluminescent device to the end of the 
back electrode layer was 0 mm, and 10% by weight of a bonding agent was 
added to the back electrode layer. In the device B according to the 
present invention, the distance of retreat from the end of the 
electroluminescent device to the end of the back electrode layer was 1.0 
mm, and 20% by weight of a bonding agent was added to the protective 
layer. These two devices were operated so as to emit light for 200 hours 
with 100 V.times.400 Hz and with 40.degree. C..times.90% RH (relative 
humidity). Deteriorations of the end portions as shown in FIG. 7 were 
observed. As can be seen from FIG. 7 that in the electroluminescent device 
A in which the distance of retreat is 0 mm, the maximum value of the 
deterioration of the end portion of the luminescent region is about 0.7 
mm. In the electroluminescent device B in which the distance of retreat is 
1.0 mm, the end portion of the luminescent region is not deteriorated at 
all. Since the end portions of the luminescent layer 12 and of the 
dielectric layer 13 are not deteriorated, the image displayed is not 
adversely affected. 
In the example described in connection with FIGS. 3-7, the resinous binder 
in the back electrode layer 14 may also consist of a fluoride resin. More 
specifically, 10 g of copolymer of vinylidene fluoride and propylene 
hexafluoride is dissolved in 90 g of isophorone, or a solvent, to produce 
a fluoride resin binder. Then, 80 g of powdered carbon is added to the 
resinous binder and they are stirred well. In this way, a conductive ink 
is prepared. This ink is printed on the dielectric layer 13, heated, and 
dried, thus forming the back electrode layer 14, in the same way as in the 
above-described method. 
Where the back electrode layer 14 contains no fluoride resin, a conductive 
ink containing 2.0% by weight of a reaction accelerator consisting of an 
organic silicon monomer is prepared. The silicon monomer has two or more 
different kinds of reaction groups per molecule. This conductive ink is 
printed on the dielectric layer 13, heated, and dried to form the back 
electrode layer 14, in the same manner as the method described above. 
Moreover, the electroluminescent device may be sealed by moisture-proof 
film. This further enhances the moisture resistance. 
As described thus far, in the present invention, a luminescent layer and a 
dielectric layer are interposed between a transparent electrode layer and 
a back electrode layer. The luminescent layer comprises a resinous binder 
containing electroluminescent particles. The dielectric layer comprises a 
resinous binder containing dielectric particles. The back electrode layer 
comprises a resinous binder containing conductive particles. In this 
electroluminescent device, the resinous binder of at least one of the 
luminescent layer and the dielectric layer is made from a fluoride resin. 
The back electrode layer contains a reaction accelerator for promoting 
polymerization of the fluoride resin binder. The reaction accelerator 
permeates the dielectric layer and the luminescent layer from the back 
electrode layer. This accelerates polymerization of the fluoride resin in 
the dielectric layer and in the luminescent layer, thus increasing the 
density of the fluoride resin. Also, intrusion of moisture is prevented. 
Therefore, even if moisture-proof film is omitted, the electroluminescent 
device can have high moisture resistance. 
In another electroluminescent device according to the present invention, a 
luminescent layer and a dielectric layer are interposed between a 
transparent electrode layer and a back electrode layer. The luminescent 
layer comprises a resinous binder containing electroluminescent particles. 
The dielectric layer comprises a resinous binder containing dielectric 
particles. The back electrode layer comprises a resinous binder containing 
conductive particles. In this electroluminescent device, the resinous 
binder of at least one of the luminescent layer, the dielectric layer, and 
the back electrode layer is made from a fluoride resin. The protective 
layer contains a reaction accelerator for promoting polymerization of the 
fluoride resin. The reaction accelerator permeates the dielectric layer 
and the luminescent layer from the back electrode layer to thereby promote 
polymerization of the fluoride resin, thus increasing the density of the 
fluoride resin. Also, intrusion of moisture is prevented. Therefore, even 
if moisture-proof film is omitted, the electroluminescent device can have 
high moisture resistance. 
Since expensive moisture-proof film can be omitted, the thickness of the 
electroluminescent device can be reduced. Hence, the flexibility of the 
device can be improved. Furthermore, a sealing step can be dispensed with. 
Hence, an inexpensive electroluminescent device can be provided. 
The end portion of the back electrode layer is retreated inwardly from the 
end of the electroluminescent device. Since the end portions of the 
luminescent layer and of the dielectric layer are not deteriorated, the 
image displayed is not adversely affected.