Abstract:
An EL display comprises a transparent substrate and a first electrode layer made of a transparent material disposed on the transparent substrate. A luminescent layer, a first insulation layer and a second electrode layer are disposed successively on a surface of the first electrode layer except for a preselected surface portion thereof defining an exposed portion. A second insulation layer is disposed on a surface of the second electrode layer except for a preselected surface portion thereof defining an exposed portion. A first contact layer is disposed on the exposed portion of the second electrode layer for electrical connection to an external driving control circuit. A second contact layer is disposed on the second insulation layer for electrical connection to the exposed portion of the first electrode layer and to the external driving control circuit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electroluminescent (hereinafter referred to as &#34;EL&#34;) display used as an EL face of a watch or other device. 
     BACKGROUND OF THE INVENTION 
     An EL element in a prior art EL display has been formed by laminating a transparent electrode layer, a luminescent layer, an insulation layer and a back electrode layer, and attaching electrode connecting portions which project respectively from the transparent electrode layer and the back electrode layer for applying an AC electric field between the transparent electrode layer and the back electrode layer. Flat cables and the like are connected to the electrode connecting portions to electrically connect the EL display with an external driving control circuit. 
     Further, as a prior art technology pertaining to wristwatches, there has been known an EL display in which a transparent electrode layer is connected with an electrode by an electrode connecting portion formed by extending the transparent electrode layer, and a back electrode layer is connected with an electrode by an electrode contact layer provided on the back of the back electrode layer, such as shown in U.S. Pat. No. 5,265,071. In this technology, the transparent electrode layer is rendered conductive with the electrode by winding the electrode connecting portion of the transparent electrode layer around the side of a frame and by screwing to the electrode of a circuit board provided on the back of the frame. 
     However, if the electrode connecting portion is formed so as to project toward the outside like the above-mentioned prior art technology, the luminescent area of the EL display is reduced because the electrode connecting portion is a non-luminescent area. Further, a cavity portion has to be provided on the side of a case corresponding to the projecting electrode connecting portion, thus requiring a complicated mold and increasing the cost. Moreover, because the electrode connecting portion is provided so as to project within a small area at an extremely limited specific position in order to minimize the non-luminescent area, there have been such problems that the connecting position at which the electrode connecting portion is connected with the driving circuit is naturally limited into a narrow range and a high precision positioning is required, thus increasing the cost of the assembly process. 
     U.S. Pat. No. 5,265,071 has had another problem, beside the above-mentioned problems, in that it requires high precision processing in narrow sections to provide screw holes, for example, on the electrode connecting portion, the frame and the circuit board to screw the electrode connecting portion of the transparent electrode layer. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an EL display which overcomes the aforementioned problems associated with the prior art EL displays. 
     According to the present invention, a pair of contact layers for electrical connection to a driving control circuit are disposed on the back of a back electrode which is superimposed on a luminescent area of an EL element by making the most use of the screen printing technology without extending a non-luminescent area toward the outside, in order to relatively increase a luminescent area and to facilitate the connection with the driving circuit. 
     An EL display of the present invention has a transparent electrode layer formed on the back of a transparent substrate, a part of the peripheral portion of the face of the transparent electrode layer being left as an exposed portion. A luminescent layer, a first insulation layer and a back electrode layer are coated successively on the remaining face of the transparent electrode layer, a part of the back electrode layer being left as an exposed portion, and a second insulation layer is coated on the remaining face of the back electrode layer. A contact layer of the back electrode layer is formed on the exposed portion of the back electrode layer, and a contact layer of the transparent electrode layer which electrically connects with the exposed portion of the transparent electrode layer is formed on the back of the second insulation layer. Contact terminal portions of an external driving control circuit are connected with the respective contact layers of the transparent electrode layer and the back electrode layer. 
     The respective contact layers of the transparent electrode layer and the back electrode layer may be formed in the shape of concentric arcs. 
     Alternatively, the contact layer of the back electrode layer may be formed in a circle at the center part and the contact layer of the transparent electrode layer may be formed in a ring-like shape at the peripheral portion of the circle. 
     As another alternative, the contact layer of the back electrode layer may be formed in a semi-circle and the contact layer of the transparent electrode layer may be formed in a complementary semi-circle. 
     Preferably, a back protection layer made of an insulating material is formed over the second insulation layer such that the respective part of each contact layer is exposed through apertures in the back protection layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing a main part of an EL display according to the present invention; 
     FIG. 2 is a rear view thereof in a state before a back protection layer is formed; 
     FIG. 3 is a rear view of another embodiment showing other contact layers in the same state as that in FIG. 2; 
     FIG. 4 is a rear view of another embodiment showing other contact layers in the same state as that in FIG. 2; 
     FIG. 5 is a rear view of a further embodiment showing other contact layers in the same state as that in FIG. 2; 
     FIG. 6 is a rear view of another embodiment showing other contact layers in the same state as that in FIG. 2; 
     FIG. 7 is a rear view of a further embodiment having an octagonal outside shape and showing a state before the back protection layer is formed; 
     FIG. 8 is a rear view of another embodiment showing other contact layers in the same state as that in FIG. 7; 
     FIG. 9 is a rear view of a further embodiment showing other contact layers in the same state as that in FIG. 7; and 
     FIG. 10 is a rear view of another embodiment showing other contact layers in the same state as that in FIG. 7. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a cross section of an inventive EL display wherein a transparent electrode layer 2 is formed by evaporating ITO on a transparent substrate 1 which is made of a synthetic resin, such as a surface of a PET film. Then, leaving a part of the peripheral portion of the transparent electrode layer 2 as an exposed portion 2a, a luminescent layer 3, a first insulation layer 4 and a back electrode layer 5 are formed sequentially on the remaining portion of the face of the transparent electrode layer 2 by means of screen printing and the like. 
     For the luminescent layer 3, ink produced by mixing and agitating a luminescent substance in which copper is doped in zinc sulfide, i.e., a luminescent material, with a fluororesin binder may be used. For the first insulation layer 4, ink produced by mixing and agitating barium titanate, i.e., a high dielectric substance, with fluorine binder may be used. The back electrode layer 5 is made from carbon paste. 
     A take-out electrode 2b of the transparent electrode layer 2 is formed by carbon paste on the exposed portion 2a at a part of the peripheral portion of the transparent electrode layer 2 which is insulated from the back electrode layer 5. The take-out electrode 2b is formed by printing at the same time that the back electrode layer 5 is printed. 
     A second insulation layer 6 is formed on the back electrode layer 5. While the same ink as that used for the first insulation layer 4 may be used for the second insulation layer 6, ink produced by mixing and agitating a low dielectric substance with fluorine binder may be also used. As shown in FIG. 2, a C-shaped arcuate aperture 6a is formed in the second insulation layer 6, and the back electrode layer 5 is exposed in the aperture as an exposed portion 5a of the back electrode layer. A contact layer 5b of the back electrode layer 5 is formed on the exposed portion 5a by using silver paste. 
     A C-shaped arcuate contact layer 2c of the transparent electrode layer 2 is formed on a surface of the second insulation layer 6 and on the take-out electrode 2b by using silver paste or the like so as to electrically connect the contact layer 2c with the take-out electrode 2b of the transparent electrode layer 2. The contact layer 5b of the back electrode layer 5 and the contact layer 2c of the transparent electrode layer 2 are formed in the shape of concentric arcs with the contact layer 2c extending to the peripheral side of the EL display. 
     A back protection layer 7 is formed on the second insulation layer 6 such that part of the contact layers 2c and 5b of the transparent electrode layer and the back electrode layer, respectively, are exposed. The back protection layer 7 is formed of an insulating material such as silicon and vinyl chloride. The contact layer 5b of the back electrode layer 5 is exposed through an aperture 7a of the back protection layer 7 and the contact layer 2c of the transparent electrode layer 2 is exposed through an aperture 7b. 
     By constructing the EL display as described above, contact terminal portions 8a and 8b of a driving control circuit 10 may be connected selectively to arbitrary points on the C-shaped arc contact layers 2c and 5b, thus allowing the connection to be made readily and conveniently. Further, because these connection points are located on the back or rear of the luminescent area, they do not reduce the luminescent area, and the non-luminescent area is limited only to a range of the exposed portion 2a which is limited to a very small localized peripheral portion of the transparent electrode layer 2. For example, the contact terminal portion 8a of the driving control circuit may be connected selectively to an arbitrary point so long as it is on the circular arc of the contact layer 5b, and the contact terminal portion 8b may be connected selectively to an arbitrary point so long as it is on the circular arc of the contact layer 2c. 
     It is noted that because the positions of the contact terminal portions 8a and 8b for the driving control circuit 10 are determined in advance, it is possible to open the apertures on the back protection layer 7 at positions corresponding to the contact terminal portions 8a and 8b and to otherwise cover and protect all of the other parts except the apertures. That is, if the arcuate contact layers 2c and 5b are formed in advance as described above, it is possible to make the printing screens for each of the other layers, from the transparent electrode layer 2 to the second insulation layer 6 as well as the contact layer 2c, of common, standard construction and to accommodate different models in which the contact terminal portions are positioned in a variety of positions simply by changing the printing screen of the back projection layer 7 so that the apertures 7a and 7b are formed at the positions corresponding to the positions of the contact terminal portions 8a and 8b of the driving control circuit. 
     FIG. 3 shows an embodiment in which a contact layer 12c of the transparent electrode layer and a contact layer 15b of the back electrode layer are formed in the shape of concentric rings, wherein a back protection layer (not shown) is formed in the same manner as that described above after positioning the contact layer 15b within a ringed aperture 16a of a second insulation layer 16. Contact terminal portions 8a and 8b of the driving control circuit are shown by two-dot chain lines, respectively. 
     FIG. 4 shows an embodiment in which an arc aperture 26a is opened at the peripheral portion of a second insulation layer 26 and a contact layer 25b of the back electrode layer is provided within the aperture 26a when the positions of the contact terminal portions 8a and 8b (not shown) for the driving control circuit are limited further. A contact layer 22c of the transparent electrode layer is also formed in an arc at another part of the peripheral portion of the second insulation layer 26. 
     FIG. 5 shows an embodiment devised so that the contact terminal portions 8a and 8b of the driving control circuit may be connected selectively in a wider range, wherein a circular aperture 36a is opened at the center part of a second insulation layer 36, and a circular contact layer 35b of the back electrode layer is provided within the aperture 36a. A contact layer 32c of the transparent electrode layer is formed in a ring-like or annular shape around the peripheral portion of the aperture 36a of the second insulation layer 36. The remaining structure is the same as that described above. This structure allows the contact terminal portion 8a of the driving control circuit to be connected at any position on the circular contact layer 35b within the aperture 36a and the contact terminal portion 8b to be connected at any position on the peripheral ring-like contact layer 32c, thus widening the selective range remarkably. 
     FIG. 6 shows an embodiment in which a second insulation layer 46 is formed on a left half part of the back electrode layer and a contact layer 45b of the back electrode layer is formed on the right half part. A contact layer 42c of the transparent electrode layer is formed on the second insulation layer 46. This structure allows the contact terminal portion 8a of the driving control circuit to be connected at any position on the right semi-circular contact layer 45b and the contact terminal portion 8b to be connected at any position on the left semi-circular contact layer 42c, thus widening a selective range remarkably. 
     FIGS. 7 to 10 show embodiments in which the outer shape of the EL display is octagonal. FIGS. 7 and 8 show embodiments in which the same contact layers 2c, 5b and 12c, 15b as those described in FIGS. 2 and 3 are provided, respectively. FIGS. 9 and 10 show embodiments in which contact layers are formed with a predetermined width in parallel with each side of the octagonal outer shape. FIG. 9 shows contact layers 52c and 55b having an octagonal shape except that one leg or side of the octagon is cut out, and FIG. 10 shows contact layers 62c and 65b having a complete octagonal shape. All of the other structure of all the embodiments shown in FIGS. 3-10 is substantially the same as that shown in FIGS. 1 and 2. 
     As described above, the embodiments in which the contact layers are formed in the shapes of concentric arcs or rings accommodate a wide degree of angular or rotational movement of the contact layers of the EL display relative to the contact terminal portions of the driving control circuit during mounting and assembly. Further, the embodiments in which the contact layers are formed of linear portions, such as an octagon, accommodate a wide degree of relative linear movement during mounting and assembly. 
     As described above, the respective contact layers of the transparent electrode layer and the back electrode layer of the EL display which are to be connected with the contact terminal portions of the driving control circuit are formed on the back of the back electrode layer which is superimposed on the luminescent region or area, so that the non-luminescent area does not extend toward the outside and almost the whole area may be used as the luminescent area. Further, because the positions at which the contact layers of the EL display which are to be connected with the contact terminal portions of the driving control circuit are not limited to a narrow range and no precision positioning is required, it becomes easy to assemble. The extent to which the contact layers need be exposed may be minimized and damage which might be otherwise caused by contact with the contact layers and the like may be prevented by providing the back protection layer.