Abstract:
In particular, one embodiment of the present invention provides an organic electroluminescent display which comprises a light emitting device formed on a lower insulating substrate, and an upper substrate mounted to encapsulate the light emitting device, wherein the upper substrate is provided with a desiccant, the desiccant being formed with at least one hole passing through the desiccant.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 2003-81209 filed on Nov. 17, 2003, which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the present invention relates to organic electroluminescent displays (OLEDs) generally, and, more particularly, to an OLED containing a desiccant formed with holes therein which prevent bubbles from forming when the desiccant is adhered to a substrate or other substantially planar surface. 
     2. Description of the Related Art 
     OLEDs offer a thinner dimension, wider viewing angle, lighter weight, smaller size, quicker response time, and lower power consumption, as compared to a cathode ray tube (CRT) display or liquid crystal display (LCD). Moreover, OLEDs can be easily fabricated using simple manufacturing processes because its simple structure includes only three major elements, namely an anode electrode, an organic material layer, and a cathode electrode. Because of these and other advantages OLEDs are emerging as the next generation of flat panel displays. 
     In an OLED configured in the conventional manner, an organic light emitting device is formed on a lower insulating substrate, with an upper substrate disposed above the organic light emitting device. 
     A significant disadvantage associated with this configuration is that materials used to form the organic light emitting layer and cathode electrode have insufficient resistance against moisture and/or oxidation. This weakness deteriorates the reliability of the display over time. In particular, internal oxidation and/or accumulation of moisture causes areas of the display to become non-emissive. Such non-emissive areas are called “dark spots”. As time goes by, the dark spots spread to neighboring areas and eventually destroy the entire display. 
     In an attempt to solve the foregoing problem, a conventional OLED encapsulates the organic light emitting layer and the cathode electrode to protect them against moisture and oxygen. The process involves forming a desiccant placing recess in an encapsulation substrate made of a glass material; adhering a desiccant in the recess; and bonding the encapsulation substrate to an insulating substrate with a sealant. The bonding process is performed in an inert gas such as nitrogen (N 2 ), argon (Ar) or the like, to prevent the encapsulation of atmospheric moisture and oxygen. 
     Conventionally, the desiccant material is formed on a layer of adhesive material and to a thickness of about 200 μm. The desiccant material is made of barium oxide (BaO), calcium carbonate (CaCO 3 ), calcium oxide (CaO), phosphor oxide (P 2 O 5 ), Zeolite, Silicagel, Alumina, and like elements and compounds which eliminate moisture by physisorption or chemisorption. 
     A significant disadvantage of using the method described above is that a gas trapped between the desiccant and a substrate will form bubbles as the desiccant is adhered to the substrate. The bubbles place portions of the desiccant into contact with the organic light emitting device, which is problematic because the moisture accumulated by the desiccant eventually penetrates the organic light emitting device and makes it non-emissive. 
     Conventional attempts to solve the bubble problem included forming a deep desiccant placing recess to a depth of about 350 μm. This depth prevented protruding and non-protruding areas of the bubble-deformed desiccant from contacting the light-emitting device. However, because the thickness of the substrate is proportional to the depth of the desiccant placing recess, the deep desiccant placing recess necessitates a thick substrate. This, in turn, creates a heavier OLED. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present invention, the OLED includes a desiccant formed with at least one hole therein. The hole prevents bubbles from forming when the desiccant is adhered to a substrate or other substantially planar surface by permitting gas trapped between the desiccant and the substrate to escape. 
     In particular, one embodiment of the present invention provides an organic electro luminescence display, comprising: a light emitting device formed on a lower insulating substrate; and an upper substrate mounted to encapsulate the light emitting device, wherein the upper substrate is provided with a desiccant, the desiccant being formed with at least one hole passing through the desiccant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating an organic electroluminescent display configured in accordance with an embodiment of the present invention. 
         FIG. 2A  is a plan view illustrating a desiccant used in an organic electroluminescent display configured in accordance with an embodiment of the present invention. 
         FIG. 2B  is a cross-sectional view taken along line I-I′ of  FIG. 2A . 
     
    
    
     In the drawings, the thickness of layers and regions are exaggerated for clarity. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a cross-sectional view of an organic electroluminescent display configured in accordance with an embodiment of the present invention. 
     As shown in  FIG. 1 , the organic electroluminescent display includes a lower insulating substrate  100 , an organic light emitting device  110  formed on the lower insulating substrate  100 , and an upper substrate  120  having a desiccant placing recess  137  to which a desiccant  130  formed with at least one hole  135  is adhered to an inner surface of the desiccant placing recess  137 . The hole  135  of the desiccant  130  acts to prevent bubbles from forming as the desiccant  130  is adhered to the upper substrate  120 . 
     As shown, the upper substrate  100  further includes sealing recesses  150 A,  150 B, which bond to a sealant  140 A,  140 B, such that the upper substrate  120  is bonded to the lower substrate and encapsulates the organic, light-emitting device  110 . 
     In one embodiment, the organic light emitting device  110  includes a first electrode, an organic emission layer, and a second electrode, wherein when the first electrode is used as an anode electrode, the second electrode is used as a cathode electrode, and vice versa. 
     The organic emission layer can be composed of various layers according to its function. In one exemplary embodiment, the organic emission layer is of a multiple-layer structure that includes an emission layer and at least one of a hole injecting layer (HIL), a hole transporting layer (HTL), a hole blocking layer (HBL), an electron transporting layer (ETL) and an electron injecting layer (EIL). 
     The layer of desiccant  130  is made of at least one material selected from a group consisting of barium oxide (BaO), calcium carbonate (CaCO 3 ), calcium oxide (CaO), phosphor oxide (P 2 O 5 ), Zeolite, Silicagel, Alumina, and the like. A layer of adhesive material such as a sticker or adhesive thin film, may be used to adhere the desiccant  130  to the upper substrate  120 . In one embodiment, the desiccant  130  is a strip of preformed material. 
       FIG. 2A  is a plan view illustrating a desiccant used in an OLED configured in accordance with an embodiment of the present invention.  FIG. 2B  is a cross-sectional view taken along line I-I′ of  FIG. 2A . 
     Referring to  FIGS. 2A and 2B , desiccant  200  may include a layer of desiccant material  220  formed on a layer of adhesive material  210 . A plurality of holes  230  formed in the desiccant  200  pass through the desiccant  200  from side to side. The layer of desiccant material  220  absorbs moisture, and the layer of adhesive material  210  affixes the layer of desiccant material  200  to the upper substrate  120 . In one embodiment, the layer of desiccant material  220  has a thickness of less than about 200 μm. 
     The holes  230  prevent gas bubbles from forming when the desiccant  200  is adhered to the upper substrate  120 , and may have various shapes such as, but not limited to, circular, rectangular, and triangular shapes. To effectively prevent the formation of the bubbles, each hole  230  should have a circular shape of diameter of more than about 1.0 mm. In one embodiment the diameter of each hole is in the range of about 1.0 mm to about 2.0 mm. If the diameter of the hole  230  is too small, bubbles will form as the desiccant  200  is adhered. Oppositely, if the diameter of the hole  230  is too large, it is difficult to keep the desiccant  200  in a film form, and there are problems adhering the desiccant  200  to the substrate  120 . 
     In an exemplary embodiment, the holes  230  formed through the desiccant  200  occupy an area of less than 6% of the total area of the desiccant  200 . In particular, the holes  230  may occupy an area in the range of about 1% to about 6% of the total area of the desiccant  200 . If the area occupied by the holes exceeds more than about 6%, the desiccant&#39;s moisture absorbing qualities tend to decrease. 
     On the other hand, the number of the holes  230  formed in the desiccant  200  may increase as the area of the desiccant  200  increases. Thus a large desiccant  200  may include more holes than a smaller desiccant. In one embodiment, the desiccant holes number between about 13,000/m 2  to 20,000/m 2 , and are formed to extend through the desiccant material from a top surface thereof to a bottom surface thereof. 
     To prevent the formation of bubbles the desiccant  200  may be formed with at least one hole per 1 cm 2 . 
     In an OLED configured as described above, gas bubbles are prevented from forming between the desiccant  130 ,  200  and the upper substrate ( 120 ) because a gas normally trapped between the desiccant and the substrate is released through the holes  230 . Releasing the trapped gas causes the desiccant  130 ,  200  to lie flat without contacting the organic light-emitting device. Two benefits incur from this. First, the formation of dark spots is prevented. Second, the desiccant placing recess can be formed in depths of about 350 μm or less, as measured from an inner surface  160  of the recess on which the desiccant is adhered to an outer surface  170  of the substrate  100  on which the light-emitting device is formed. 
     In this manner, the thickness of the upper substrate  120  can be reduced to create a lightweight OLED suitable for mobile transport. 
     While the present invention has been described with reference to embodiments, it is understood that the disclosure has been made for purpose of illustrating the invention not to limit the scope of the invention. The embodiments herein described may be modified and changed without departing from the scope and spirit of the invention.