Moisture proof thin film electroluminescent panel

Abstract: A thin film electroluminescent panel (1) in which a thin film electroluminescent device formed on a transparent substrate (2) is covered with a black silicone resin layer (13) at back side thereof and further covered by a moistureproof sheet (14) the periphery of which is adhered to that of the substrate (2). Desirably, a moisture absorber is arranged between the black silicone resin layer (13) and the moistureproof sheet (14). Further, the moistureproof sheet (14) has at least one small hole (21) pierced therein for dehydrating the inner space (19) of the EL panel (1) which is scaled after dehydration.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a moistureproof thin film electroluminescent panel used for a display device of a personal computer, a wordprocessor or the like. 2. Description of the Prior Art In a conventional manufacturing method of thin film electroluminescent panels, as shown in Fig. 9, lower transparent electrodes 3, a lower insulating layer 4, an electroluminescent layer 5, an upper insulating layer 6 and upper electrodes 7 are deposited on a glass substrate 2 in a stacked manner to form an electroluminescent (hereinafter referred to as EL for brevity) device 1 and, thereafter, a lining backup glass plate 8 is put on said EL device 1 by adhering the periphery thereof to that of the glass substrate 2 with an adhesive agent 9 of epoxy resin in order to prevent moisture from penetrating into the EL device. Further, to guarantee a high credibility against moisture, silicone oil in which silica gel powder and black powder are dispersed to give a moisture absorbing property is filled into a space 11 between the EL device 1 and the backup glass plate 8 through a hole 10 of the same and, thereafter, the hole 10 is sealed tightly by plugging a "TORR SEAL ®" 12 thereinto. However, in such a manufacturing method, the cost of the silicone oil is high and the filling process thereof is very time-consuming and, accordingly, it is very difficult to lower the manufacturing cost thereof. Further, due to the thickness and the weight of the backup glass 8, it is difficult to make the EL device of this type thinner and lighter. A thin film electroluminescent panel similar to the above-described one is known from JP-A-2-75193, in which a black gelatinous or solid insulating material is filled and sealed in a void between a panel substrate and a back substrate. This material is stated to improve the contrast of the display. Also in JP-A-61-290693, there is disclosed a moistureproof structure of the EL device wherein a laminated film is used for preventing moisture from penetrating. In this manufacturing method, no structure for absorbing penetrated moisture is provided and nothing is considered to enhance the contrast of the EL device. EP-A-0 350 907 discloses a thin film electroluminescent panel according to the preamble of claim 1, and further comprising a moisture-absorbing sheet disposed between the moistureproof sheet and the EL device. The moisture-absorbing sheet is coloured black in order to enhance the display contrast. SUMMARY OF THE INVENTION An object of the present invention is to provide a thin film EL panel having a highly moistureproofed structure and offering a high contrast. The present invention, as defined by claim 1, provides a thin film electroluminescent panel comprising a transparent substrate, a thin film electroluminescent device formed on said substrate, and a flexible moistureproof sheet which covers said device and is sealed to said substrate around the periphery of said device, characterised in that a coating of black silicone resin of addition reaction type envelops the outer surface of said device covered by said sheet. According to the present invention, in the thin film EL panel, the back side of the thin film EL device is double-sealed with the coating of black silicone resin of addition reaction type and the moistureproof sheet. Preferably, powder-like or pellet-like moisture absorbing material is arranged for absorbing moisture penetrating through the moistureproof sheet. In one embodiment of the present invention, this powder-like or pellet-like material is arranged by dispersing it in the black silicone resin coating. In another embodiment of the present invention, it is arranged between the black silicone resin coating and the moistureproof sheet, either on the surface of the coating or in a gas-permeable sack. In a further embodiment of the present invention, a sheet-like moisture-absorbing body is arranged between the black silicone resin coating and the moistureproof sheet. The moisture absorbers mentioned above can enhance the moisture resistance of the thin film EL panel. In order to enhance the moistureproof property of the thin film EL panel, it is desirable to pierce at least one hole of a small diameter in the moistureproof sheet through which moisture existing inside of the EL panel can be dehydrated. After dehydration, the hole is sealed by a suitable sealing means. In this case, the moisture introduced into the EL panel during the fabrication process thereof can be dehydrated and, accordingly, the reliability of moisture-proof is enhanced much more. Further, the contrast thereof is highly enhanced since the EL device is lined with the black coating and, thereby, the quality of display is much improved. Also, according to the present invention, since the lining backup glass and the silicone oil become unnecessary, the manufacturing process is simplified and the EL panel can be made thinner and lighter. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 to 5 are respective partial sectional views of a thin film EL panel according to a first through fifth preferred embodiment of the present invention, Fig. 6 is a graph showing a relation between the hardness of the black silicone resin and the number of pixels broken regarding the thin film EL panel, Fig. 7 is a partial sectional view of a thin film EL panel according to a sixth preferred embodiment of the present invention, Fig. 8 is a graph for showing the life-time of sealing according to the present invention, and Fig. 9 is a partial sectional view of a conventional thin film EL panel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figs. 1 to 5 and 7 show partial sectional views of thin film EL panels according to the present invention. Through all these Figs., the same reference numerals denote the same elements as those used in Fig. 9. The thin film EL panels shown in Figs. 1 to 3 have a substantially same fundamental structure except for the manner of arrangement of moisture absorbing means. In Figs. 1 to 3 , the reference numeral 1 denotes a thin film EL device. Thin film EL device 1 comprises lower transparent stripe electrodes 3 formed on a transparent substrate 2 such as a glass substrate in parallel to each other at an equal pitch, a lower insulating layer 4, an EL layer 5, an upper insulating layer 6 and upper stripe electrodes 7 arranged in parallel to each other at an equal pitch and in direction orthogonal to that of the lower transparent electrodes3. These elements 3 to 7 are formed stacked one on the other. The reference numeral 13 denotes a layer of a black silicone resin. This black silicone resin layer 13 is formed by painting the black silicone resin so as to cover the EL device 1. The reference numeral 14 denotes powder like or pellet-like moisture absorber composed of powder or pellets of silica (SiO 2 ), molecular sieve or the like. In the structure shown in Fig. 1, the moisture absorber 14 is dispersed in the black silicone resin 13. In the structure shown in Fig. 2, it is arranged on the black silicone layer 13. Further, in Fig. 3, it is contained in a gas-permeable sack 20 and the sack 20 is arranged on the black silicone layer 13. Further, the reference numeral 18 denotes a moistureproof sheet which is composed of a laminated sheet having such a structure that a metal foil 16 of 5 to 50 µm thickness is sandwiched by organic resin films 15 and 17 of 5 to 50 µm thickness such as polyester films. These organic resin films 15 and 17 may be made of different materials. The moistureproof effect of said sheet 18 is mainly obtained by the metal foil 16 since it is impermeable against moisture. The organic resin films 15 and 17 are mainly for protection of the surface of the metal foil 16 and for insulation from the EL device 1, but are desirably made of a resin material having a low moisture-permeability. The moistureproof sheet 18 covers the EL device 1 so as to define a space 19 therebetween and is bonded to the transparent substrate 2 along the periphery thereof with an epoxy resin adhesive agent 9 to seal the inside of the EL panel completely. Since the moisture absorber 14 absorbs moisture existing inside of the moistureproof sheet 18, the credibility of the EL panel against moisture is highly enhanced. In order to prevent the moistureproof sheet 18 from suffering damage due to sharp edges of the moisture absorber 14, it is desirable to arrange a non-woven fabric, woven fabric, paper sheet, plastic film or the like inside thereof. Next, the fabrication method of the thin film EL panel is explained. After forming the thin film EL device 1 on the transparent substrate 2, gas discharge from the EL device 1 is done by raising the temperature up to 200°C or more in a vacuum to remove residual moisture as much as possible. These processes are common to all the embodiments shown in Fig. 1 to 3. Next, in the case of Fig. 1, the black silicone resin 13 in which powder-like or pellet-like moisture absorber 14 is dispersed is painted or printed on the thin film EL device 1 in dry N 2 gas or dry air at room temperature. In this case, the liquid of black silicone resin is prepared by mixing, at room temperature, powder-like or pellet-like moisture absorber 14 having been vacuum-dehydrated at a temperature of 100°C or more into the liquid of black silicone resin having been dehydrated and deformed in a vacuum. Thereafter, the black silicone resin 13 is hardened by maintaining the same at a constant temperature for a predetermined time according to the property thereof (for instance at 150°C for one hour). After hardening of the black silicone resin 13, the moistureproof sheet 18 is adhered with an epoxy adhesive agent 9 in dry N 2 gas or dry air. Accordingly, dry N 2 gas or dry air is filled in the space 19. It is desirable to arrange a buffer sheet of non-woven fabric, woven fabric, paper, plastic film or the like inside of the moistureproof sheet 18. This is common to all the embodiments shown in Figs. 1 to 3. In the case of Fig. 2 or Fig. 3, after moving the EL panel in dry N 2 gas dry air, the black silicone resin having been dehydrated and deformed in a vacuum beforehand is applied or printed on the surface of the EL panel. Thereafter, it is hardened by heating the same up to a predetermined temperature. In the case of Fig. 2, the predetermined amount of the moisture absorber 14 having been vacuum-dehydrated by heating the same at 100°C or more is arranged on the hardened black silicone resin 13. In the case of Fig. 3, the gas-permeable sack 20 containing a predetermined amount of the moisture absorber 14 is vacuum-dehydrated by heating the same up to 100°C or more beforehand, and it is put on the hardened black silicone resin 13. The use of the sack makes treatment of the moisture absorber easier. After these processes, the moistureproof sheet 18 is adhered to the periphery of the substrate 2 with the epoxy resin adhesive agent 9 in dry N 2 gas or dry air. In the preferred embodiment shown in Fig. 4, the moisture absorber 14 is a sheet made from non-woven fabric or the like which contains silica micro powder, molecular sieve powder or the like therein. With respect to the EL panel having such a structure as shown in Fig. 4, an endurance test for estimating the life thereof was done under heating and humidifying conditions. In this test, two types of the black silicone resin were used. One was of condensation reaction type and the other was of addition reaction type. The life estimated based on results of the test was 16,000 to 24,000 hours with respect to the condensation reaction type and was 50,000 to 70,000 hours with respect to the addition reaction type. In accordance with the present invention, the EL panel uses the latter type of black silicone resin. This black silicone resin 13 enhances the contrast of the EL panel by forming the black background and contributes to relax stresses generated in the EL panel. Fig. 6 shows the number of broken pixels of the EL panel in relation with the hardness of the hardened black silicone resin. The EL panel vibrates with a small amplitude during action thereof and, accordingly, the breaking property of the pixel is affected by stresses exerted thereto from the black silicone resin 13 coated thereon. In other words, it is strongly related to the hardness of the hardened black silicone resin. As is apparent from the result shown in Fig. 6, the hardness of the black silicone resin 13 should be smaller or equal to 20 when it is measured according to JIS C 2123. Further, it is desirable for the black silicone resin to have a dielectric strength equal to or larger than 1 x 10 5 V/cm in order to avoid damage to electrodes and pixels due to discharges among electrodes during action of the EL panel. In Fig. 5, there is disclosed a thin film EL panel having no moisture absorber therein. If it is used in a well-conditioned atmosphere, a reasonably long life will be guaranteed even in this case. Fig. 7 shows a sixth preferred embodiment of the present invention. The thin film EL panel shown in Fig. 7 has a substantially same structure as that shown in Fig. 4 except for a hole 21 of a small diameter, for example of about 6mm, pierced in the moistureproof sheet 18. This small hole 21 is provided for dehydrating the inner space 19 of the EL panel to remove the moisture introduced thereinto during the fabrication process of the EL panel. Upon dehydrating the inner space 19, the EL panel is charged into a vacuum chamber (not shown) and is vacuum-dehydrated therein by raising the inner temperature up to 100°C or more. Then, dry N 2 gas or air is filled into the vacuum chamber and the hole 21 is sealed quickly by adhering a small strip 22 having the same layered structure as that of the moistureproof sheet 18 from the outside thereof. By heating and hardening epoxy resin 9' applied to the under side of the small strip 22, the hole 21 is sealed surely and, thus, the fabrication process of the EL panel is completed. The number of small holes to be pierced may be two or more. Further, it is desirable to pierce the small hole in the moistureproof sheet 18 beforehand for smooth and quick dehydrating operation. However, it is also possible to pierce the small hole upon dehydrating the EL panel. According to the sixth preferred embodiment of the present invention, the moisture having penetrated into the inner space of the EL panel during the fabrication process thereof is surely dehydrated resulting in a much higher reliability against degradation due to moisture. Fig. 8 shows experimental data related to the life-time of sealing obtained under three sealing conditions. The life-time of sealing at room temperature and humidity was estimated from an acceleration life-time test wherein thin film EL panels were operated to repeat light emission in an atmosphere at a temperature of 85°C and a relative humidity of 85%. As is apparent from Fig. 8, the life-time of sealing is greatly affected by an ambient humidity upon sealing the EL panel but the dehydration operation can remove undesirable effects based on a high ambient humidity perfectly. The life-time of sealing demanded for actual use is about 5 x 10 4 hours and this criterion can be easily satisfied by the present preferred embodiment of the present invention, as indicated by the case C in Fig. 8. Since the EL panel 1 is backed up by the black silicone resin 13, the background of the display is black. This enhances the contrast of display, the quality of display and the appearance of the panel. The sealing process of the EL panel is simplified since the moistureproof sheet is adhered to the substrate in a dry atmosphere in the present invention. Further, the EL panel according to the present invention can be made thinner and lighter when compared with the conventional one wherein the backup glass plate is used. It is to be noted that the present invention can be applied to both dot matrix type and segment type EL devices.

Claims: A thin film electroluminescent panel comprising a transparent substrate (2), a thin film electroluminescent device (1) formed on said substrate, and a flexible moistureproof sheet (18) which covers said device and is sealed to said substrate around the periphery of said device, characterised in that a coating of black silicone resin (13) of addition reaction type envelops the outer surface of said device (1) covered by said sheet (18). A thin film electroluminescent panel according to claim 1, wherein a sheet-like moisture absorber (14 - Figs. 4, 7) is arranged between said coating (13) and said moistureproof sheet (18). A thin film electroluminescent panel according to claim 1, wherein powder-like or pellet-like moisture absorber (14 - Fig. 1) is dispersed within said coating (13). A thin film electroluminescent panel according to claim 1, wherein powder-like or pellet-like moisture absorber (14 - Fig. 2) is arranged on the surface of said coating (13). A thin film electroluminescent panel according to claim 1, wherein a gas-permeable sack (20) containing powder-like or pellet-like moisture absorber (14 - Fig. 3) is arranged between said coating (13) and said moistureproof sheet (18). A thin film electroluminescent panel according to any one of claims 1 to 5, wherein said moistureproof sheet (18) has at least one small hole (21) and a dehydrated inner space (19) of said panel is sealed by a sealing means (22) over said at least one small hole.

Inventors: OGURA TAKASHI, YAMASHITA TAKUO, YOSHIDA MASARU, OGURA, TAKASHI, YAMASHITA, TAKUO, YOSHIDA, MASARU
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: H05B33
IPC codes: H05B33