Abstrict In a sealing structure of an electroluminescent display device,
in which a first glass substrate formed with an EL element and a
second glass substrate as a cap are attached to each other, breaking
of the element device is prevented when external force is applied
to the first glass substrate and the second glass substrate. The
sealing structure has the first glass substrate provided with the
EL element on a surface thereof, the second glass substrate attached
to the first glass substrate with a sealing resin, a desiccant layer
formed on a surface of the second glass substrate and a stress buffer
layer covering a surface of the desiccant layer.
Claims What is claimed is:
1. An electroluminescent display device comprising: a first substrate
having an electroluminescent element thereon; a second substrate
attached to the first substrate; a desiccant layer coated on the
second substrate so that the desiccant layer faces the first substrate;
and a stress buffer layer covering the desiccant layer so that a
peripheral portion of the stress buffer layer is attached directly
to the second substrate.
2. The electroluminescent display device of claim 1 wherein the
stress buffer layer is made of a resin.
3. The electroluminescent display device of claim 1 wherein the
stress buffer layer includes a plurality of air vents.
4. The electroluminescent display device of claim 2 wherein the
stress buffer layer includes a plurality of air vents.
5. The electroluminescent display device of claim 1 wherein the
second substrate includes a concave portion and the desiccant layer
is disposed in the concave portion.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an electroluminescent display device,
particularly to a sealing structure of the electroluminescent display
device.
2. Description of the Related Art
In recent years, electroluminescent (hereafter, referred to as
an EL) display devices with EL elements have been receiving an attention
as a display device substituting a CRT and an LCD.
Since the EL element is sensitive to moisture, there has been known
an EL display panel structure in which the EL element is covered
with a metal cap or a glass cap coated with a desiccant. FIG. 9
is a cross-sectional view showing such a conventional structure
of the EL display panel.
A first glass substrate 70 has a display region formed with many
EL elements 71 thereon. The first glass substrate 70 is attached
to a second glass substrate 80 working as a cap with sealing resin
75 made of an epoxy resin. The second glass substrate 80 has a concave
portion 81 (hereafter, referred to as a pocket portion 81) in a
region corresponding to the above display region. The pocket portion
81 is coated with a desiccant layer 82 for absorbing moisture.
Here, the forming of the pocket portion 81 is for securing a space
between the desiccant layer 82 and the EL element 71 thereby preventing
the EL element 71 from being contacted by the desiccant layer 82
which may result in damaging the EL element 71.
As shown in FIG. 10 however, an external force can be applied
to a surface of the first glass substrate 70. This can occur even
in a manufacturing process of the EL display device (for example,
a process of conveying a glass substrate) and also when a panel
surface of the EL display device is touched by a user. This external
force causes flexure in the first glass substrate 70 and the desiccant
layer 82 and the EL element 71 contact each other. With the application
of the further external force, the EL element 71 may be broken by
stress from the desiccant layer 82. Furthermore, the same problems
are caused by applying of the external force to a surface of the
second glass substrate 80.
SUMMARY OF THE INVENTION
The invention provides an electroluminescent display device that
includes a first substrate having an electroluminescent element
thereon, a second substrate attached to the first substrate, a desiccant
layer disposed on the second substrate so that the desiccant layer
faces the first substrate, and a stress buffer layer covering the
desiccant layer.
The invention also provides an electroluminescent display device
that includes a first substrate having an electroluminescent element
thereon, a second substrate attached to the first substrate, and
a desiccant layer disposed on the second substrate so that the desiccant
layer faces the first substrate. The desiccant layer has an elastic
coefficient low enough to absorb mechanical stresses generated by
the electroluminescent element when it contacts the desiccant layer
under an application of an external force to the display device.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of an electroluminescent display device according
to a first embodiment of the invention.
FIG. 2 is a cross-sectional view of the device of FIG. 1 along
line A-A' FIG. 1.
FIG. 3 is a perspective view of a stress buffer layer of the first
embodiment of the invention.
FIG. 4 is a cross-sectional view of the electroluminescent display
device of FIG. 1 under application of an external force.
FIG. 5 is a cross-sectional view of an electroluminescent display
device according to a second embodiment of the invention.
FIG. 6 is a cross-sectional view of the electroluminescent display
device of FIG. 5 under application of an external force.
FIG. 7 is a plan view of a pixel of the display devices of the
first and second embodiments.
FIGS. 8A and 8B are cross-sectional views of the pixel of the organic
EL display device of FIG. 7.
FIG. 9 is a cross-sectional view of a conventional electroluminescent
display device.
FIG. 10 is a cross-sectional view of the electroluminescent display
device of FIG. 9 a cross-sectional view of the electroluminescent
display device of FIG. 1 under application of an external force.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a plan view of an electroluminescent display device according
to a first embodiment of the invention. FIG. 2 is a cross-sectional
view along line A-A' of FIG. 1.
A first glass substrate 100 (a display panel) has a display region
formed with many EL elements 101 on a surface thereof. The thickness
of the first glass substrate 100 is approximately 0.7 mm. In this
display region, a plurality of pixels is disposed in a matrix and
the EL element 101 is disposed in each of those pixels.
A second glass substrate 200 is a glass substrate for sealing the
above mentioned first glass substrate 100 and its thickness is approximately
0.7 mm. This second glass substrate 200 has a concave portion 201
(hereafter, referred to as a pocket portion 201) in a region corresponding
to the display region, which is formed by etching. The depth of
the pocket portion 201 is approximately 0.3 mm.
There is coated in the pocket portion 201 a desiccant layer 210
for absorbing moisture. The desiccant layer 210 is formed, for example,
by coating a solvent dissolved with powdered calcium oxide or barium
oxide and resin as an adhesive on a bottom of the pocket portion
201 and then hardening the solvent by UV irradiation or heating.
The desiccant layer 210 is covered with a stress buffer layer 211.
The stress buffer layer 211 is formed, for example, by coating the
desiccant layer 210 with an epoxy resin or by covering the desiccant
layer 210 with a sheet having elasticity made of, for example, polyethylene
terephthalate (PET) or fluoroplastic.
Furthermore, the stress buffer layer 211 is preferably has many
air vents 212 as shown in FIG. 3. This is for keeping air permeability
of the desiccant layer 210 high to prevent it from losing the function
as a desiccant.
The first glass substrate 100 and the second glass substrate 200
are attached with sealing resin 150 made of an epoxy resin in a
chamber of N.sub.2 gas atmosphere. Thus, N.sub.2 gas fills a space
surrounded by the stress buffer layer 211 the first glass substrate
100 and the sealing resin 150 to form an N.sub.2 gas layer 102.
According to this embodiment, the electroluminescent display device
has a structure in which the stress buffer layer 211 is disposed
between the desiccant layer 210 and the EL element 101. Therefore,
as shown in FIG. 4 when an external force is applied to the first
glass substrate 100 to cause flexure therein, and the EL element
101 and the stress buffer layer 211 contact each other, elastic
deformation occurs in the stress buffer layer 211 so that the stress
applied to the EL element 101 is dispersed and absorbed by this
stress buffer layer 211 thereby preventing the breaking of the
EL element 101.
Second Embodiment
FIG. 5 is a cross-sectional view showing an electroluminescent
display device according to a second embodiment of the invention.
FIG. 5 corresponds to a cross-sectional view along line A-A' of
FIG. 1. Note that the same numerals are given to the same portions
as those of FIG. 2.
While in the first embodiment the desiccant layer 210 is covered
with the stress buffer layer 211 in this embodiment the elasticity
is provided in the desiccant layer 213 itself, thereby relaxing
the stress applied to the EL element 101.
The desiccant layer 213 is formed by coating a solvent dissolved
with powdered calcium oxide or barium oxide and a resin as an adhesive
on a bottom of the pocket portion 201 and then hardening the solvent
by UV irradiation or heating. Here, the amount of the resin in this
desiccant layer 213 is increased to 20 or more weight % for increasing
the elasticity. Epoxy resin or UV resin is appropriate as the resin.
Consequently, as shown in FIG. 6 when an external force is applied
to the first glass substrate 100 to cause flexure therein, and the
EL element 101 and the desiccant layer 213 contact each other, elastic
deformation occurs in the desiccant layer 213 itself so that the
stress applied to the EL element 101 is dispersed and absorbed by
the desiccant layer 213 thereby preventing the breaking of the
EL element 101.
Next, there is described an example of structures of the pixel
of the EL display device applied to the first and second embodiments
described above.
FIG. 7 is a plan view showing a pixel of an organic EL display
device. FIG. 8A is a cross-sectional view along line A--A of FIG.
7 and FIG. 8B is a cross-sectional view along line B--B of FIG.
7.
As shown in FIG. 7 a pixel 115 is formed in a region enclosed
with a gate signal line 51 and a drain signal line 52. A plurality
of the pixels 115 is disposed in a matrix.
There are disposed in the pixel 115 an organic EL element 60 as
a self-emission device, a switching TFT (thin film transistor) 30
for controlling a timing of supplying an electric current to the
organic EL element 60 a driving TFT 40 for supplying an electric
current to the organic EL element 60 and a storage capacitor. The
organic EL element 60 includes an anode 61 an emissive made of
an emission material and a cathode 65.
The switching TFT 30 is provided in a periphery of a point of intersection
of the both signal lines 51 and 52. A source 33s of the switching
TFT 30 serves as a capacitor electrode 55 for forming a capacitor
with a storage capacitor electrode line 54 and is connected to a
gate electrode 41 of the driving TFT 40. A source 43s of the driving
TFT 40 is connected to the anode 61 of the organic EL element 60
while a drain 43d is connected to a driving source line 53 as a
current source to be supplied to the organic EL element 60.
The storage capacitor electrode line 54 is disposed in parallel
with the gate signal line 51. The storage capacitor electrode line
54 is made of Cr (chromium) etc and forms a capacitor by storing
an electric charge with the capacitor electrode 55 connected to
the source 33s of the TFT through a gate insulating film 12. The
storage capacitor 56 is provided for storing voltage applied to
the gate electrode 41 of the driving TFT 40.
As shown in FIGS. 8A and 8B, the organic EL display device is formed
by laminating the TFTs and the organic EL element sequentially on
a substrate 10 such as a substrate made of a glass or a synthetic
resin, a conductive substrate, or a semiconductor substrate. When
using a conductive substrate or a semiconductor substrate as the
substrate 10 however, an insulating film such as SiO.sub.2 or SiN.sub.x
is formed on the substrate 10 and then the switching TFT 30 the
driving TFT 40 and the organic EL element 60 are formed thereon.
Each of the two TFTs has a so-called top gate structure in which
a gate electrode is disposed above an active layer with a gate insulating
film being interposed therebetween.
There will be described the switching TFT 30 first. As shown in
FIG. 8A, an amorphous silicon film (hereafter, referred to as an
a-Si film) is formed on the insulating substrate 10 made of a silica
glass or a non-alkali glass by a CVD method. The a-Si film is irradiated
by laser beams for melting and recrystalizing to form a poly-silicon
film (hereafter, referred to as a p-Si film) as an active layer
33. On the active layer 33 a single-layer or a multi-layer of an
SiO.sub.2 film and an SiN.sub.x film is formed as the gate insulating
film 12. There are disposed on the gate insulating film 12 the gate
signal line 51 made of metal having a high melting point such as
Cr or Mo (molybdenum) and also serving as a gate electrode 31 the
drain signal line 52 made of Al (aluminum), and the driving source
line 53 made of Al and serving as a driving source of the organic
EL element.
An interlayer insulating film 15 laminated with an SiO.sub.2 film,
an SiN.sub.x film and an SiO.sub.2 film sequentially is formed on
the whole surfaces of the gate insulating film 12 and the active
layer 33. There is provided a drain electrode 36 by filling metal
such as Al in a contact hole provided correspondingly to a drain
33d. Furthermore, a planarization insulation film 17 for planarizing
a surface which is made of organic resin is formed on the whole
surface.
Next, there will be described the driving TFT 40 of the organic
EL element. As shown in FIG. 8B, an active layer 43 formed by poly-crystalizing
an a-Si film by irradiating laser beams thereto, the gate insulating
film 12 and the gate electrode 41 made of metal having a high melting
point such as Cr or Mo are formed sequentially on the insulating
substrate 10. There are provided in the active layer 43 a channel
43c, and a source 43s and a drain 43d on both sides of the channel
43c. The interlayer insulating film 15 is formed on the whole surfaces
of the gate insulating film 12 and the active layer 43. There is
disposed the driving source line 53 connected to a driving source
by filling metal such as Al in a contact hole provided correspondingly
to a drain 43d. Furthermore, a planarization insulation film 17
for planarizing the surface, which is made of, for example, an organic
resin is formed on the whole surface. A contact hole is formed in
a position corresponding to a source 43s in the planarization insulation
film 17. There is formed on the planarization insulation film 17
a transparent electrode made of ITO (Indium Tin Oxide) and contacting
to the source 43s through the contact hole, i.e., the anode 61 of
the organic EL element. The anode 61 is formed in each of the pixels,
being isolated as an island.
The organic EL element 60 has a structure of laminating sequentially
the anode 61 made of a transparent electrode such as ITO, a hole
transport layer 62 including a first hole transport layer made of
MTDATA (44-bis(3-methylphenylphenylamino)biphenyl) and a second
hole transport layer made of TPD (444-tris(3-methylphenylphenylamino)
triphenylanine), an emissive 63 made of Bebq.sub.2 (bis(10-hydroxybenzo[h]quinolinato)beryllium)
containing a quinacridone derivative, an electron transport layer
64 made of Bebq.sub.2 and a cathode 65 made of magnesium-indium
alloy, aluminum or aluminum alloy.
Incidentally, the planarization insulation film 17 is formed with
a second planarization insulation film 66 thereon. The second planarization
insulation film 66 is removed on the anode 61.
In the organic EL element 60 a hole injected from the anode 61
and an electron injected from the cathode 65 are recombined in the
emissive and an exciton is formed by exciting an organic module
forming the emissive 63. Light is emitted from the emissive 63 in
a process of radiation of the exciton and then released outside
after going through the transparent anode 61 to the transparent
insulating substrate 10 thereby to complete light-emission.
According to this embodiment, in the sealing structure of the electroluminescent
display device in which the first glass substrate (a display panel)
having the EL element and the second glass substrate for sealing
the EL element are attached together, the stress buffer layer is
disposed between the desiccant layer and the EL element, thereby
preventing the breaking of the EL element 101 when external force
is applied to the first glass substrate or the second glass substrate.
Furthermore, the amount of the resin in the desiccant layer is increased
so that the same effect is obtained without using the stress buffer
layer. |