Patent Publication Number: US-7915817-B2

Title: Double-sided light emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/849,193, filed May 20, 2004 now U.S. Pat. No. 7,492,095, which claims the benefit of Korean Patent Application No. 2003-34179, filed on May 28, 2003 and No. 2003-86116, filed on Nov. 29, 2003, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flat panel display and, more particularly, to a double-sided organic light emitting device capable of preventing definition of image quality from being deteriorated by external light. 
     2. Description of the Related Art 
     Flat panel displays such as organic light emitting devices, field emission displays (FEDs) and so forth are subjected to considerable decrease in contrast according to intensity of external light. To prevent this phenomenon, for example, a black matrix for blocking external light has been used. In spite of using such a black matrix, it is very difficult to completely block the external light on an emission region to make a black state. 
     Meanwhile, such an organic light emitting device for blocking the external light using a circular-polarizing plate is disclosed in U.S. Pat. No. 5,596,246. The conventional organic light emitting device using the circular-polarizing plate is provided with an organic electroluminescent (EL) element consisting of a transparent electrode, an organic emission layer and a reflective electrode formed on an insulating substrate. The insulating substrate is encapsulated with an encapsulating substrate using a sealant (not shown in the drawing), and a circular-polarizing plate consisting of a linear-polarizing plate and a λ/4 compensating plate disposed on an outer surface of the insulating substrate. 
     The conventional organic light emitting device constructed as set forth above is designed so that an angle between a retardation axis of the λ/4 compensating plate and a polarization axis of the linear-polarizing plate becomes 45 degrees. Thus, the external light passes through the linear-polarizing plate to become linear-polarized light, and the linear-polarized light passes through the λ/4 compensating plate to become circular-polarized light. The circular-polarized light is reflected through the reflective electrode, and become linear-polarized light through the λ/4 compensating plate. The linear-polarized light is absorbed and blocked through the linear-polarizing plate. The conventional organic light emitting device as above-mentioned has an advantage in that it can improve contrast by blocking the external light using the circular-polarizing plate, but has a disadvantage in that it requires a separate reflective plate in order to block the external light. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an aspect of the present invention to provide a double-sided organic light emitting device capable of improving a contrast by blocking external light. 
     It is another aspect of the invention to provide a double-sided organic light emitting device capable of blocking reflected external light as well as bottom transmitted light. 
     It is yet another aspect of the invention to provide a double-sided organic light emitting device capable of increasing a definition of image quality. 
     It is yet another aspect of the invention to provide a double-sided organic light emitting device capable of increasing a definition of image quality by blocking external light both at an image viewing position and at its opposite position. 
     In order to accomplish these and/or other aspects there is provided a double-sided light emitting device comprising lower and upper substrates, an emission element formed between an inner surface of the upper substrate and an inner surface of the lower substrate and emitting predetermined light, an upper layer of polarizing material disposed on at least one of inner or outer surfaces of the upper substrate, and a lower layer of polarizing material disposed on at least one of inner or outer surfaces of the lower substrate. 
     The lower and upper layers of polarizing material are coating layers coated on the outer surfaces of the lower and upper substrates respectively, or coating layers coated on the inner surfaces of the lower and upper substrates respectively. The upper layer of polarizing material is a coating layer coated on the inner surface of the upper substrate, and the lower layer of polarizing material is a coating layer coated on the outer surface of the lower substrate, and alternatively, the upper layer of polarizing material is a coating layer coated on the outer surface of the upper substrate, and the lower layer of polarizing material is a coating layer coated on the inner surface of the lower substrate. Furthermore, the lower and upper layers of polarizing material are disposed so that their polarization axes are perpendicular to each other, and each are a coating layer having a thickness from about 0.1 μm to 50.0 μm. 
     According to another aspect of the present invention, there is provided a double-sided light emitting device, which comprises lower and upper substrates, an emission element formed between an inner surface of the upper substrate and an inner surface of the lower substrate and emitting predetermined light, an upper polarizing plate disposed on any one of inner or outer surfaces of the upper substrate, and a lower polarizing plate disposed on any one of inner or outer surfaces of the lower substrate. 
     The lower and upper polarizing plates are polarizing films bonded on the inner surfaces of the lower and upper substrates respectively. The upper polarizing plate may be a polarizing film bonded on the inner surface of the upper substrate, and the lower polarizing plate may be a polarizing film bonded on the outer surface of the lower substrate. Further, the upper polarizing plate may be a polarizing film bonded on the outer surface of the upper substrate, and the lower polarizing plate may be a polarizing film bonded on the inner surface of the lower substrate. Otherwise, the lower and upper polarizing plates are polarizing films bonded on the outer surfaces of the lower and upper substrates, respectively. Further, the lower and upper polarizing plates are disposed so that their polarization axes are perpendicular to each other, and each is a polarizing film having a thickness from about 50 μm to 300 μm. 
     According to yet another aspect of the present invention, there is provided a double-sided light emitting device, which comprises lower and upper substrates, an emission element formed between an inner surface of the upper substrate and an inner surface of the lower substrate and emitting predetermined light, an upper polarizing element disposed on any one of inner or outer surfaces of the upper substrate, a lower polarizing element disposed on any one of inner or outer surfaces of the lower substrate, an upper compensating plate disposed between the upper polarizing element and the emission element, and a lower compensating plate disposed between the lower polarizing element and the emission element, wherein a retardation value of each of the lower and upper compensating plates is λ/4, and each angle between the lower and upper compensating plates and the lower and upper polarizing plates. 
     Here, a crossing angle between a retardation axis of the lower compensating plate disposed between the lower polarizing element and the emission element and a polarization axis of the lower polarizing element has a direction opposite to a crossing angle between a retardation axis of the upper compensating plate disposed between the upper polarizing element and the emission element a polarization axis of the upper polarizing element. 
     The lower polarizing element is disposed on the outer surface of the lower substrate, and the lower compensating plate is disposed between the lower polarizing element and the outer surface of the lower substrate, and the upper polarizing element is disposed on the outer surface of the upper substrate, and the upper compensating plate is disposed between the upper polarizing element and the outer surface of the upper substrate. Otherwise, the lower polarizing element is disposed on the outer surface of the lower substrate, and the lower compensating plate is disposed between the lower polarizing element and the outer surface of the lower substrate, and the upper polarizing element is disposed on the inner surface of the upper substrate, and the upper compensating plate is disposed between the upper polarizing element and the inner surface of the upper substrate. 
     Alternatively, the lower polarizing element may be disposed on the inner surface of the lower substrate, and the lower compensating plate may be disposed between the lower polarizing element and the inner surface of the lower substrate, and the upper polarizing element may be disposed on the inner surface of the upper substrate, and the upper compensating plate may be disposed between the upper polarizing element and the inner surface of the upper substrate. Otherwise, the lower polarizing element may be disposed on the inner surface of the lower substrate, and the lower compensating plate may be disposed between the lower polarizing element and the inner surface of the lower substrate, and the upper polarizing element may be disposed on the outer surface of the upper substrate, and the upper compensating plate may be disposed between the upper polarizing element and the outer surface of the upper substrate. 
     Furthermore, the lower and upper compensating plates include at least one compensating film having a predetermined phase difference retardation axis. When a phase difference retardation axis of each of the lower and upper compensating plates is λ/4 and angles between retardation axes of the lower and upper compensating plates and polarization axes of the lower and upper polarizing elements are opposite to each other, external light which is incident and transmitted from a position opposite to an observing position of light emitted from the emission element is no longer transmitted toward an observer regardless of not only an angle between polarization axis of the upper polarizing element and the phase difference retardation axes of the lower and upper compensating plates but also an angle between polarization axis of the lower polarizing element and the phase difference retardation axes of the lower and upper compensating plates. 
     According to yet still another aspect of the present invention, there is provided a double-sided light emitting device, which comprises lower and upper substrates, an emission element formed between an inner surface of the upper substrate and an inner surface of the lower substrate and emitting predetermined light, an upper polarizing element disposed on any one of inner and outer surfaces of the upper substrate, a lower polarizing element disposed on any one of inner and outer surfaces of the lower substrate, and an upper compensating plate disposed between the upper polarizing element and the emission element, and a lower compensating plate disposed between the lower polarizing element and the emission element, wherein angle between phase difference retardation axis of the lower compensating plate and polarization axis of the lower polarizing element and angle between phase difference retardation axis of the upper compensating plate and polarization axis of the upper polarizing element are opposite to each other, and wherein at a position where light emitted from the emission element is observed, light emitted from the emission element is transmitted, and all external light incident at the observed position of the light and at a position opposite to the observed position of the light are blocked, and external light reflected within the emission element is blocked. 
     According to yet still another aspect of the present invention, there is provided a double-sided light emitting device comprising lower and upper substrates, an emission element formed between an inner surface of the upper substrate and an inner surface of the lower substrate and emitting predetermined light, an upper polarizing element disposed on any one of inner and outer surfaces of the upper substrate, and a lower polarizing element disposed on any one of inner and outer surfaces of the lower substrate, wherein the lower and upper polarizing elements are disposed so that polarization axes of the lower and upper polarizing elements perpendicular to each other, and wherein at a position where light emitted from the emission element is observed, light emitted from the emission element is transmitted and all external light incident at the observed position of the light and at a position opposite to the observed position of the light are blocked. 
     The lower and upper polarizing elements each are a coating layer of polarizing material having a thickness from about 0.1 μm to 50.0 μm, or a polarizing film having a thickness from about 50 μm to 300 μm. The lower and upper polarizing elements are disposed any one of on the inner surfaces of the lower and upper substrates respectively and on the outer surfaces of the lower and upper substrates respectively, and otherwise the lower and upper polarizing elements are disposed on the inner surface of the upper substrate and the outer surface of the lower substrate respectively, or on the outer surface of the upper substrate and the inner surface of the lower substrate respectively. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  shows a cross-sectional structure of a double-sided organic light emitting device according to a first embodiment of the invention; 
         FIG. 2  shows a cross-sectional structure of a double-sided organic light emitting device according to a second embodiment of the invention; 
         FIG. 3  shows a cross-sectional structure of a double-sided organic light emitting device according to a third embodiment of the invention; 
         FIG. 4  shows a cross-sectional structure of a double-sided organic light emitting device according to a fourth embodiment of the invention; 
         FIG. 5  shows a cross-sectional structure of a double-sided organic light emitting device according to a fifth embodiment of the invention; 
         FIG. 6  shows a cross-sectional structure of a double-sided organic light emitting device according to a sixth embodiment of the invention; 
         FIG. 7  shows a cross-sectional structure of a double-sided organic light emitting device according to a seventh embodiment of the invention; 
         FIG. 8  shows a cross-sectional structure of a double-sided organic light emitting device according to an eighth embodiment of the invention; 
         FIGS. 9A and 9B  are views explaining a principle of blocking external light in the double-sided organic light emitting device according to the first embodiment of the invention; 
         FIG. 10  shows a cross-sectional structure of a double-sided organic light emitting device according to a ninth embodiment of the invention; 
         FIG. 11  shows a cross-sectional structure of a double-sided organic light emitting device according to a tenth embodiment of the invention; 
         FIG. 12  shows a cross-sectional structure of a double-sided organic light emitting device according to an eleventh embodiment of the invention; 
         FIG. 13  shows a cross-sectional structure of a double-sided organic light emitting device according to a twelfth embodiment of the invention; 
         FIGS. 14A and 14B  are views explaining a principle of blocking external light in the double-sided organic light emitting device according to the ninth embodiment of the invention; 
         FIG. 15  shows a cross-sectional structure of a double-sided organic light emitting device according to a thirteenth embodiment of the invention; 
         FIG. 16  shows a cross-sectional structure of a double-sided organic light emitting device according to a fourteenth embodiment of the invention; 
         FIG. 17  shows a cross-sectional structure of a double-sided organic light emitting device according to a fifteenth embodiment of the invention; 
         FIG. 18  shows a cross-sectional structure of a double-sided organic light emitting device according to a sixteenth embodiment of the invention; 
         FIG. 19  shows a cross-sectional structure of a double-sided organic light emitting device according to a seventeenth embodiment of the invention; and 
         FIGS. 20A and 20B  are views explaining a principle of blocking external light in the double-sided organic light emitting device according to the thirteenth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
       FIG. 1  shows a cross-sectional structure of a double-sided organic light emitting device according to a first embodiment of the invention. 
     Referring to  FIG. 1 , there is an insulating substrate as a lower substrate  110 , on which an anode electrode  120  is formed. An organic thin layer  130  is formed on the anode electrode  120 . A cathode electrode  140  is formed on the organic thin layer  130 . A passivation layer  150  is formed on the cathode electrode  140 . An encapsulating substrate  160  as an upper substrate is bonded and encapsulated to the lower substrate  110  using a sealant (not shown). 
     The lower and upper substrates  110  and  160  may make use of a transparent substrate such as a glass substrate. The anode electrode  120  is a transparent electrode, which is formed by depositing and patterning transparent conductive layer of ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide) or so forth on an inner surface of the lower substrate  110 . The organic thin layer  130  includes at least one of a hole injecting layer (HIL), a hole transporting layer (HTL), an emission layer, a hole blocking layer (HBL), an electron transporting layer (ETL) and an electron injecting layer (EIL). The cathode electrode  140  is also a transparent electrode, which is formed by depositing a metal layer of Ca, LiF or so forth which has a low work function. The passivation layer  150  is formed using a transparent sealant, so that it is possible not only to guarantee a lifetime of the organic light emitting device exposed in the air but also prevent oxidation of the cathode electrode  140  or the anode electrode  120 . 
     Lower and upper polarizing elements  170 A and  180 A are arranged on outer surfaces of the lower and upper substrates  110  and  160 , respectively. The polarizing elements  170 A and  180 A are layers of a polarizing material, which are coated on the outer surfaces of the substrates  110  and  160 , respectively. Further, the layer of the polarizing material is formed by coating a polarizing solution available from OPTIVA INC. at a thickness from about 0.1 μm to 50.0 μm. 
     Here, preferably, both the lower polarizing element  170 A arranged on the outer surface of the lower substrate  110  and the upper polarizing element  180 A arranged on the outer surface of the upper substrate  160  have their polarization axes perpendicular to each other. Hence, the lower and upper polarizing elements  170 A and  180 A may be integrally formed with the lower and upper substrates  110  and  160 . 
     In the first embodiment, the lower substrate  110  on which an electroluminescent (EL) element  100  is formed is encapsulated with the upper substrate  160 , and then the lower and upper polarizing elements are formed by coating the layers  170 A and  180 A of the polarizing material on the outer surfaces of the lower and upper substrates  110  and  160 . Thereby, the organic light emitting device can be fabricated. 
     Alternatively, the lower and upper polarizing elements may be formed by coating the layers  170 A and  180 A of the polarizing material on the outer surfaces of the lower and upper substrates  110  and  160 , and then the EL element  100  may be formed on the lower substrate  110 , and finally the lower substrate may be encapsulated with the upper substrate. Thereby, the organic light emitting device may be fabricated. 
       FIG. 2  shows a cross-sectional structure of a double-sided organic light emitting device according to a second embodiment of the invention. Similarly to that of the first embodiment, the organic light emitting device according to the second embodiment has a structure where polarizing elements  170 B and  180 B are formed on the outer surfaces of the lower and upper substrates  110  and  160 , respectively. 
     Here, the lower polarizing element  170 B and the upper polarizing element  180 B are polarizing plates bonded on the outer surfaces of the substrates  110  and  160 , respectively. Each of the polarizing plates  170 B and  180 B is formed by bonding a polarizing film between about 50 to 300 μm in thickness on each outer surface of the lower and upper substrates  110  and  160 . In this case, the lower polarizing element  170 B bonded on the outer surface of the lower substrate  110  and the upper polarizing element  180 B bonded on the outer surface of the upper substrate  160  are preferably bonded in such a manner that their polarization axes are perpendicular to each other. 
     In the second embodiment, the lower substrate  110  on which the emission element  100  is formed is encapsulated with the upper substrate  160 , and then the lower and upper polarizing elements are formed by bonding the lower and upper polarizing films  170 B and  180 B on the outer surfaces of the lower and upper substrates  110  and  160 . Thereby, the organic light emitting device can be fabricated. 
     Alternatively, the lower and upper polarizing elements may be formed by bonding the lower and upper polarizing films  170 B and  180 B on the outer surfaces of the lower and upper substrates  110  and  160 , and then the EL element  100  may be formed on the lower substrate  110 , and finally the lower substrate  110  may be encapsulated with the upper substrate  160 . Thereby, the organic light emitting device may be fabricated. 
       FIG. 3  shows a cross-sectional structure of a double-sided organic light emitting device according to a third embodiment of the invention. The organic light emitting device according to the third embodiment has a structure where a lower polarizing element  270 A is arranged on an outer surface of a lower substrate  210 , and an upper polarizing element  280 A is arranged on an inner surface of an upper substrate  260 . 
     Referring to  FIG. 3 , in the first embodiment, an inner surface of the lower substrate  210  is provided with an emission element  200 , which includes an anode electrode  220  as a lower electrode, an organic thin layer  230  and a cathode electrode  240  as an upper electrode. A passivation layer  250  is formed on the cathode electrode  240 . The upper substrate  260  is bonded and encapsulated to the lower substrate  210  using a sealant (not shown). 
     The polarizing elements  270 A and  280 A are arranged on an outer surface of the lower substrate  210  and an inner surface of the upper substrate  260 , respectively. The polarizing elements  270 A and  280 A are layers of a polarizing material which are coated on an outer surface of the lower substrate  210  and an inner surface of the upper substrate  260 , respectively. Further, the layer of the polarizing material is formed by coating a polarizing solution available from OPTIVA INC. at a thickness from about 0.1 μm to 50.0 μm. 
     Here, both the lower polarizing element  270 A arranged on the outer surface of the lower substrate  210  and the upper polarizing element  280 A arranged on the inner surface of the upper substrate  260  are preferably formed in such a manner that their polarization axes are perpendicular to each other. Hence, the lower and upper polarizing elements  270 A and  280 A whose polarization axes are perpendicular to each other may be integrally formed with the lower and upper substrates  210  and  260 . 
     In the third embodiment, the emission element  200  is formed on the lower substrate  210 , and then the lower and upper polarizing elements are formed by coating the layers  270 A and  280 A of the polarizing material on the outer surface of the lower substrate  210  and the inner surface of the upper substrate  260 . Thereby, the organic light emitting device can be fabricated. 
     Alternatively, the lower and upper polarizing elements are formed by coating the layers  270 A and  280 A of the polarizing material on the outer surface of the lower substrate  210  and the inner surface of the upper substrate  260 , and then the emission element  200  may be formed on the lower substrate  210 , and finally the lower substrate  210  may be encapsulated with the upper substrate  260 . Thereby, the organic light emitting device may be fabricated. 
       FIG. 4  shows a cross-sectional structure of a double-sided organic light emitting device according to a fourth embodiment of the invention. As in the third embodiment, the organic light emitting device according to the fourth embodiment has a structure where a lower polarizing element  270 B is arranged on the outer surface of the lower substrate  210  and an upper polarizing element  280 B is arranged on the inner surface of the upper substrate  260 . 
     Here, the lower polarizing element  270 B and the upper polarizing element  280 B are polarizing plates, one polarizing element  270 B bonded on the outer surfaces of the lower substrate  210  and the other  280 B is bonded on the inner surface of the upper substrate  260 . The polarizing plates  270 B and  280 B are formed by bonding a polarizing film between about 50 to 300 μm in thickness on the outer surfaces of the lower substrate  210  and on the inner surface of the upper substrate  260 . In this case, the lower polarizing element  270 B bonded on the outer surface of the lower substrate  210  and the upper polarizing element  280 B bonded on the inner surface of the upper substrate  260  are preferably bonded in such a manner that their polarization axes are perpendicular to each other. 
     In the fourth embodiment, the emission element  200  is formed on the lower substrate  210 , and then the lower and upper polarizing elements are formed by bonding the lower and upper polarizing films  270 B and  280 B on the outer surfaces of the lower substrate  210  and on the inner surface of the upper substrate  260 . Thereby, the organic light emitting device can be fabricated. 
     Alternatively, the lower and upper polarizing elements may be formed by bonding the lower and upper polarizing films  270 B and  280 B on the outer surfaces of the lower substrate  210  and on the inner surface of the upper substrate  260 , and then the emission element  200  may be formed on the lower substrate  210 , and finally the lower substrate  210  may be encapsulated with the upper substrate  260 . Thereby, the organic light emitting device may be fabricated. 
       FIG. 5  shows a cross-sectional structure of a double-sided organic light emitting device according to a fifth embodiment of the invention. The organic light emitting device according to the fifth embodiment has a structure where a lower polarizing element  370 A is arranged on an inner surface of a lower substrate  310 , and an upper polarizing element  380 A is arranged on an outer surface of an upper substrate  360 . 
     Referring to  FIG. 5 , as in the first embodiment, an inner surface of the lower substrate  310  is provided with an emission element  300 , which includes an anode electrode  320  as a lower electrode, an organic thin layer  330  and a cathode electrode  340  as an upper electrode. A passivation layer  350  is formed on the cathode electrode  340 . The upper substrate  360  is bonded and encapsulated to the lower substrate  310  using a sealant (not shown). 
     The polarizing elements  370 A and  380 A are arranged on an inner surface of the lower substrate  310  and an outer surface of the upper substrate  360 , respectively. The polarizing elements  370 A and  380 A are layers of a polarizing material which are coated on an inner surface of the lower substrate  310  and an outer surface of the upper substrate  360 , respectively. Further, the layer of the polarizing material is formed by coating a polarizing solution available from OPTIVA INC. at a thickness from about 0.1 μm to 50.0 μm. 
     Here, both the lower polarizing element  370 A arranged on the inner surface of the lower substrate  310  and the upper polarizing element  380 A arranged on the outer surface of the upper substrate  260  are preferably formed in such a manner that their polarization axes are perpendicular to each other. Hence, the lower and upper polarizing elements  370 A and  380 A whose polarization axes are perpendicular to each other may be integrally formed with the lower and upper substrates  310  and  360 . 
     In the fifth embodiment, the lower and upper polarizing elements  370 A and  380 A are formed by coating the layers of the polarizing material on the inner surface of the lower substrate  310  and the outer surface of the upper substrate  360 , and then the emission element  300  is formed on the layer  370 A of the polarizing material and is encapsulated with the upper substrate which is provided with the layer  380 A of the polarizing material on the outer surface thereof. Thereby, the organic light emitting device can be fabricated. 
       FIG. 6  shows a cross-sectional structure of a double-sided organic light emitting device according to a sixth embodiment of the invention. As in the fifth embodiment, the organic light emitting device according to the sixth embodiment has a structure where a lower polarizing element  370 B is arranged on an inner surface of a lower substrate  310 , and an upper polarizing element  380 A is arranged on an outer surface of an upper substrate  360 . 
     Here, the lower polarizing element  370 B and the upper polarizing element  380 B are polarizing plates, one polarizing element  370 B is bonded on the inner surface of the lower substrate  310  and the other polarizing element  380 B is bonded on the outer surface of the upper substrate  360 . The polarizing plates  370 B and  380 B are formed by bonding a polarizing film between about 50 to 300 μm in thickness on the inner surfaces of the lower substrate  310  and on the outer surface of the upper substrate  360 . In this case, the lower polarizing element  370 B bonded on the inner surface of the lower substrate  310  and the upper polarizing element  380 B bonded on the outer surface of the upper substrate  360  are preferably bonded in such a manner that their polarization axes are perpendicular to each other. 
     In the sixth embodiment, the lower and upper polarizing elements are formed by bonding the lower and upper polarizing films  370 B and  380 B on the inner surfaces of the lower substrate  310  and on the outer surface of the upper substrate  360 , and then the emission element  300  is formed on the lower polarizing film  370 B, and finally the lower substrate  310  is encapsulated with the upper substrate  360 . Thereby, the organic light emitting device can be fabricated. 
       FIG. 7  shows a cross-sectional structure of a double-sided organic light emitting device according to a seventh embodiment of the invention. The organic light emitting device according to the seventh embodiment has a structure where lower and upper polarizing elements  470 A and  480 A are arranged on inner surfaces of lower and upper substrates  410  and  460 , respectively. 
     Referring to  FIG. 7 , as in the first embodiment, the inner surface of the lower substrate  410  is provided with an emission element  400 , which includes an anode electrode  420  as a lower electrode, an organic thin layer  430  and a cathode electrode  440  as an upper electrode. A passivation layer  450  is formed on the cathode electrode  440 . The upper substrate  460  is encapsulated to the lower substrate  410  using a sealant (not shown). 
     The polarizing elements  470 A and  480 A are arranged on the inner surfaces of the lower and upper substrates  410  and  460 , respectively. The polarizing elements  470 A and  480 A are layers of a polarizing material which are coated on the inner surfaces of the lower and upper substrates  410  and  460 , respectively. Further, the layer of the polarizing material is formed by coating a polarizing solution available from OPTIVA INC. at a thickness from about 0.1 μm to 50.0 μm. 
     Here, both the lower polarizing element  470 A arranged on the inner surface of the lower substrate  410  and the upper polarizing element  480 A arranged on the inner surface of the upper substrate  460  are preferably formed in such a manner that their polarization axes are perpendicular to each other. Hence, the lower and upper polarizing elements  470 A and  480 A whose polarization axes are perpendicular to each other may be integrally formed with the lower and upper substrates  410  and  460 . 
     In the seventh embodiment, the lower and upper polarizing elements  470 A and  480 A are formed by coating the layers of the polarizing material on the inner surfaces of the lower and upper substrates  410  and  460 , and then the emission element  400  is formed on the lower polarizing element  470 A and is encapsulated with the upper substrate  460 . Thereby, the organic light emitting device can be fabricated. 
       FIG. 8  shows a cross-sectional structure of a double-sided organic light emitting device according to an eighth embodiment of the invention. As in the seventh embodiment, the organic light emitting device according to the eighth embodiment has a structure where lower and upper polarizing elements  470 B and  480 B are arranged on the inner surfaces of lower and the upper substrates  410  and  460 , respectively. 
     Here, the lower polarizing element  470 B and the upper polarizing element  480 B are polarizing plates, which are bonded on the inner surfaces of the lower and upper substrates  410  and  460 , respectively. The polarizing plates  470 B and  480 B are formed by bonding a polarizing film between about 50 to 300 μm in thickness on each inner surface of the lower and upper substrates  410  and  460 . In this case, both the lower polarizing element  470 B bonded on the inner surface of the lower substrate  410  and the upper polarizing element  480 B bonded on the inner surface of the upper substrate  460  are preferably bonded in such a manner that their polarization axes are perpendicular to each other. 
     In the eighth embodiment, the lower and upper polarizing elements are formed by bonding the lower and upper polarizing films  470 B and  480 B on the inner surfaces of the lower and upper substrates  410  and  460 , and then the emission element  400  is formed on the lower polarizing element  470 B and is encapsulated with the upper substrate. Thereby, the organic light emitting device can be fabricated. 
     In the double-sided organic light emitting device according to the first embodiment of the invention, a principle of blocking external light will be described below with reference to  FIGS. 9A and 9B . 
     As shown in  FIG. 9A , when an observer  190  looks on the side of the encapsulating substrate  160  as the upper substrate, internal light  191  emitted from the EL layer  130  is linearly polarized through the upper polarizing element  180 A to travel in an arrow direction  192 , so that the observer  190  sees the light through the upper substrate  160 . The linearly polarized internal light oscillates in the same direction as the polarization axis of the polarizing element  180 A. 
     Meanwhile, external light  195  which is incident from the observer  190  to the encapsulating substrate  160  is linearly polarized through the upper polarizing element  180 A to travel in an arrow direction  196 . Internal transmitted light which is linearly polarized through the upper polarizing element  180 A is reflected by a layer structure of the EL element  100 , and the reflected external light is linearly polarized in a different direction to cross 90 degrees with an incident angle of the light which is incident through the encapsulating substrate  160 , thus failing in transmission. 
     Further, in the case of another external light which is incident at a position opposite to the observer  190 , namely, transmitted external light which is incident on and transmitted through the insulating substrate  110 , it is linearly polarized through the lower polarizing element  170 A. Here, the polarization axes of the lower and upper polarizing elements  170 A and  180 A are arranged perpendicular to each other, so that the transmitted external light which has been linearly polarized does not pass through the upper polarized element  180 A. In other words, in the case of the transmitted external light, its polarization axis when it has been linearly polarized is perpendicular to its polarization axis when it has been incident on the insulating substrate  110 . As a result, the transmitted external light passing through the lower substrate  110  at the position opposite to the observer  190  is blocked without being emitted through the upper polarizing element  180 A. 
     As shown in  FIG. 9B , when the observer  190  looks on the side of the insulating substrate  110  as the lower substrate, internal light  191  emitted from the EL layer  130  is linearly polarized through the lower polarizing element  170 A to travel in the arrow direction  192 , so that the observer  190  sees the light through the lower substrate  110 . The linearly polarized internal light oscillates in the same direction as the polarization axis of the lower polarizing element  170 A. 
     Meanwhile, external light  195  that is incident from the observer  190  to the insulating substrate  110  is linearly polarized through the lower polarizing element  170 A to travel in an arrow direction  196 . Internal transmitted light which is linearly polarized through the lower polarizing element  170 A is reflected by the layer structure of the EL element  100 , and the reflected external light is linearly polarized in a different direction to cross 90 degrees with an incident angle of the light which is incident through the insulating substrate  110 , thus failing in transmission. 
     Further, in the case of another external light which is incident at a position opposite to the observer  190 , namely, transmitted external light which is incident on and transmitted through the encapsulating substrate  160 , it is linearly polarized through the upper polarizing element  180 A. Here, the polarization axes of the lower and upper polarizing elements  170 A and  180 A are arranged perpendicular to each other, so that the transmitted external light which has been linearly polarized does not pass through the lower polarized element  170 A. In other words, in the case of the transmitted external light, its polarization axis when it has been linearly polarized is perpendicular to its polarization axis when it has been incident on the encapsulating substrate  160 . As a result, the transmitted external light passing through the encapsulating substrate  160  at the position opposite to the observer  190  is blocked without being emitted through the lower polarizing element  170 A. 
     Here, the reflected external light refers to light which is incident on the encapsulating substrate  160  to travel toward the insulating substrate  110  and is reflected through the internal emission element  100  to travel toward the encapsulating substrate  160  again, or which is incident on the insulating substrate  110  to travel toward the encapsulating substrate  160  and is reflected through the internal emission element  100  to travel toward the insulating substrate  110  again. Further, the transmitted external light refers to light which is incident through the encapsulating substrate  160  to travel toward the insulating substrate  110 , or which is incident through the insulating substrate  110  to travel toward the encapsulating substrate  160 . 
     As set forth above, on whichever side of the lower and upper substrates the observer  190  looks, only the light emitted from the emission layer  120  is allowed to pass through the lower or upper substrate  110  or  160 , but the reflected or transmitted external light is not allowed to pass through the lower or upper substrate  110  or  160 , thus being dissipated. Consequently, it is possible to realize a double-sided light emitting structure in which a contrast is prevented from being deteriorated by the external light. 
     The principle of blocking the external light has been described in connection with the double-sided organic light emitting device of the first embodiment with reference to  FIGS. 9A and 9B , but is not limited to it. Therefore, as in the first to eighth embodiments, in the case where the lower polarizing element is arranged on any one of inner and outer surfaces of the lower substrate and the lower and the upper polarizing elements are arranged on any one of inner and outer surfaces of the upper substrates so that the polarization axis of the lower polarizing element is perpendicular to that of the upper polarizing element, it is possible to accomplish the above-mentioned effects of blocking the external light. Furthermore, the principle of blocking external light according to the first to eighteen embodiments of the invention may be applicable to display devices including other emission elements. 
       FIG. 10  shows a cross-sectional structure of a double-sided organic light emitting device according to a ninth embodiment of the invention. 
     Referring to  FIG. 10 , there is a transparent lower insulating substrate  510 , such as a glass substrate, on which a lower electrode  520  as an anode electrode is formed. An organic thin layer  530  and an upper electrode  540  as a cathode electrode are formed on the lower electrode  520 . The lower electrode  520  functions as the anode electrode, which forms a transmission electrode consisting of a transparent conductive layer of ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide) or so forth. The organic thin layer  530  includes at least one selected from a hole injecting layer (HIL), a hole transporting layer (HTL), an emission layer, a hole blocking layer (HBL), an electron transporting layer (ETL) and an electron injecting layer (EIL). The upper electrode  540  functions as the cathode electrode, which is formed by a metal layer of Ca, LiF or so forth which has a low work function. In this manner, the lower electrode  520 , the organic thin layer  530  and the upper electrode  540  constitute an EL element  500 . 
     A transparent encapsulating substrate  560  such as the glass substrate is bonded and encapsulated to the lower substrate  510  using a sealant (not shown). Lower and upper circular-polarizing plates  570  and  580  are disposed on outer surfaces of the lower and encapsulating substrates  510  and  560 , respectively. The lower circular-polarizing plate  570  includes a lower linear-polarizing plate  575  and a lower compensating plate  571 . The lower compensating plate  571  makes use of a λ/4 compensating plate. Similarly, the upper circular-polarizing plate  580  includes an upper linear-polarizing plate  585  and an upper compensating plate  581 . The upper compensating plate  581  makes use of a λ/4 compensating plate. 
     In the double-sided organic light emitting device having a configuration as set forth above, when each phase difference retardation value, of the lower and upper compensating plates  571  and  581  is denoted by x, the phase difference retardation value, x, satisfies the following expression.
 
 nλ/ 2 ≦x ≦( n+ 1)λ/2, where  n  is an integer number.
 
     Thus, the double-sided organic light emitting device of the present invention can block the external light regardless of the direction in which the observer looks. Further, when the external light is reflected inside the emission element  500 , the reflected external light can be blocked. Thus, the double-sided organic light emitting device of the present invention has a high contrast. 
     A principle of blocking the external light in the double-sided organic light emitting device according to the ninth embodiment of the invention will be described below with reference to  FIGS. 14A and 14B . 
     First, in the case where an observer  590  looks on the side of the encapsulating substrate  560 , internal light emitted from the emission layer  530  is seen through the upper circular-polarizing plate  580 . External light  595  which is incident from the observer  590  to the encapsulating substrate  560  is circularly polarized through the linear-polarizing plate  585  and the compensating plate  581  to travel in an arrow direction  596 . 
     In this manner, the external light circularly polarized through the upper circular-polarizing plate  580  is reflected by a layer structure of the EL element  500  and is circularly polarized in a different direction. Here, the left circularly polarized light is converted into a right circularly polarized light. Then, the right circularly polarized light is converted into a linear-polarized light by the upper compensating plate  581 . Here, the linear-polarized light converted by the upper compensating plate  581  crosses 90 degrees with an incident angle of the light which is initially incident through the encapsulating substrate  560 , thus failing in transmission. 
     Meanwhile, in the case of another external light which is incident at a position opposite to the observer  590 , namely, transmitted external light which is incident on and transmitted through the insulating substrate  510 , if each of the lower and upper compensating plates  571  and  581  has a phase difference retardation axis of λ/4, and if a crossing angle between the phase difference retardation axis of the upper compensating plate  581  and a polarization axis of the upper polarizing plate  585  is opposite to a crossing angle between the phase difference retardation axis of the lower compensating plate  571  and a polarization axis of the lower polarizing plate  575 , the transmitted external light which passes through the insulating substrate  510  fails to be transmitted to the observer  590  regardless of angles between the upper polarizing plate  585  and the lower and upper compensating plates  571  and  581  and between the lower polarizing plate  575  and the lower and upper compensating plates  571  and  581 . 
     For example, as in  FIG. 14A , when the linear-polarizing plate  585  of the upper circular-polarizing plate  580  and the linear-polarizing plate  575  of the lower circular-polarizing plate  570  are arranged so that their polarization axes are parallel to each other, the transmitted external light  596  incident on and transmitted through the insulating substrate  510  is circularly polarized through the linear-polarizing plate  575  and the compensating plate  571  which constitute the lower circular-polarizing plate  570 , and then travels toward the encapsulating substrate  560 . 
     In this case, because the phase difference delay axes of the lower and upper compensating plates  571  and  581  are equal to each other, the transmitted external light is shifted twice by λ/4 in the same direction, consequently by a total of λ/2, and is transformed into linearly polarized light. In the case of the transmitted external light, its polarization axis after it is linearly polarized is perpendicular to its polarization axis before it is linearly polarized, i.e., when it is incident on the insulating substrate  510 . As a result, the transmitted external light passing through the insulating substrate  510  at the position opposite to the observer  590  is blocked without being emitted through the upper circular-polarizing plate  580 . 
     As in  FIG. 14B , even when the linear-polarizing plate  585  of the upper circular-polarizing plate  580  and the linear-polarizing plate  575  of the lower circular-polarizing plate  570  are arranged so that their polarization axes are perpendicular to each other, external light which is incident through the encapsulating substrate  560  and is reflected through the EL element  500 , i.e., reflected external light is blocked as well according to the same principle as in  FIG. 14A . 
     Further, the external light incident on the side of the insulating substrate  510  passes through the insulating substrate  510 , i.e., the transmitted external light is circularly polarized through the linear-polarizing plate  575  and the compensating plate  571  which constitute the lower circular-polarizing plate  570 , and then travels toward the encapsulating substrate  560 . In this case, because the phase difference retardation axes of the lower and upper compensating plates  571  and  581  are perpendicular to each other, the transmitted incident light is linearly polarized in the same direction as the direction when passing through the insulating substrate  510 , the upper linear-polarizing plate  585  on the side of the encapsulating substrate is perpendicular to the lower linear-polarizing plate  575 , so that the transmitted external light passing through the insulating substrate  510  is blocked without being emitted toward the observer  590  through the upper circular-polarizing plate  580 . 
     Here, the reflected external light refers to light which is incident on the encapsulating substrate  560  to travel toward the insulating substrate  510  and is reflected through the internal EL element  500  to travel toward the encapsulating substrate  560  again, or which is incident on the insulating substrate  510  to travel toward the encapsulating substrate  560  and is reflected through the internal EL element  500  to travel toward the insulating substrate  510  again. Further, the transmitted external light refers to light which is incident through the encapsulating substrate  560  to travel toward the insulating substrate  510 , or which is incident through the insulating substrate  510  to travel toward the encapsulating substrate  560 . 
     As a result, only the light  591  emitted from the emission layer  530  is seen to the observer  590 , but the external light incident on the side of the encapsulating substrate is blocked. Therefore, although the light is emitted from the emission layer  530  in both opposite directions, a background on the side of the insulating substrate is not projected, so that the observer  590  can recognize only the light emitted from the emission layer  530 . This allows a definition of image quality to be improved. 
     For this reason, in the double-sided organic light emitting device of the invention, with regard to the transmitted external light which is incident from the direction opposite to the observer  590 , it is preferable that the crossing angles between the retardation axes of the lower and upper compensating plates  571  and  581  and the polarization axes of the lower and upper linear-polarized plates  575  and  585  become rotational angles opposite to each other at the lower and upper substrates. 
       FIG. 11  shows a cross-sectional structure of a double-sided organic light emitting device according to a tenth embodiment of the invention. 
     Referring to  FIG. 11 , the double-sided organic light emitting device of the tenth embodiment is similar to that of the ninth embodiment except for a compensating plate. Specifically, the compensating plate is configured of one λ/4 compensating plate in the first embodiment, but a plurality of compensating films in the tenth embodiment, each of which has a phase difference retardation axis and functions as the λ/4 compensating plate. 
     In the double-sided organic light emitting device according to the tenth embodiment, there is a lower insulating substrate  610 , on which a lower electrode  620  is formed. An organic thin layer  630  and an upper electrode  640  are formed on the lower electrode  620 . An encapsulating substrate  660  is bonded and encapsulated to the lower substrate  610  using a sealant. Lower and upper circular-polarizing plates  670  and  680  are disposed on outer surfaces of the lower and encapsulating substrates  610  and  660 , respectively. The lower circular-polarizing plate  670  includes a lower linear-polarizing plate  675 A and a lower compensating plate  671 A. The lower compensating plate  671 A makes use of the λ/4 compensating plate. Similarly, the upper circular-polarizing plate  680  includes an upper linear-polarizing plate  685 A and an upper compensating plate  681 A. The upper compensating plate  681 A makes use of the plurality of compensating films  682 - 684  so as to function as the λ/4 compensating plate. Here, the compensating films  682  through  684  have their phase difference retardation axes which are equal to or different from each other. 
       FIG. 12  shows a cross-sectional structure of a double-sided organic light emitting device according to an eleventh embodiment of the invention. 
     Referring to  FIG. 12 , the double-sided organic light emitting device of the eleventh embodiment is similar to that of the ninth embodiment except for a compensating plate  671 B of the lower circular-polarizing plate  670 . Specifically, the compensating plate  671 B is configured of one λ/4 compensating plate in the ninth embodiment, but a plurality of compensating films  672  through  674  in the eleventh embodiment, each of which has a phase difference retardation axis and functions as the λ/4 compensating plate. 
       FIG. 13  shows a cross-sectional structure of a double-sided organic light emitting device according to a twelfth embodiment of the invention. 
     Referring to  FIG. 13 , the double-sided organic light emitting device of the twelfth embodiment is similar to that of the ninth embodiment except for compensating plates  671 C and  681 C of the lower and upper circular-polarizing plates  670  and  680 , respectively. Specifically, each of the compensating plates  671 C and  681 C is configured of one λ/4 compensating plate in the ninth embodiment, but the plurality of compensating films  672  through  674 ; and  682  through  684  in the twelfth embodiment, each of which has a phase difference retardation axis and functions as the λ/4 compensating plate. 
     In the double-sided organic light emitting devices according to the tenth to twelfth embodiments shown in  FIGS. 11 through 13  as in the ninth embodiment, when each phase difference retardation value, of the lower and upper compensating plates  671  and  681  is denoted by x, each phase difference retardation value x satisfies the following expression:
 
 nλ/ 2 ≦x ≦( n+ 1)λ/2, where  n  is an integer number.
 
     Thus, as in the ninth embodiment, both transmitted external light and reflected external light are blocked based on the principle as shown in  FIGS. 14A and 14B , so that it is possible to improve a definition of image quality. 
       FIG. 15  shows a cross-sectional structure of a double-sided organic light emitting device according to a thirteenth embodiment of the invention. 
     Referring to  FIG. 15 , the double-sided organic light emitting device of the thirteenth embodiment is similar to that of the ninth embodiment, but is applicable in the case that it is intended to improve effects of blocking light on the side of the encapsulating substrate rather than the lower substrate by providing the circular-polarizing plate only on the side of the encapsulating substrate. 
     In the double-sided organic light emitting device according to the thirteenth embodiment, there is a lower insulating substrate  710 , on which a lower electrode  720  is formed. An organic thin layer  730  and an upper electrode  740  are formed on the lower electrode  720 . An encapsulating substrate  760  is bonded and encapsulated to the lower substrate  710  using a sealant. Upper circular-polarizing plate  780  is disposed on outer surfaces of the encapsulating substrate  760 , and lower linear-polarizing plate  776  is disposed on outer surfaces of the lower substrate  710 . The circular-polarizing plate  780  includes a linear-polarizing plate  785  and a compensating plate  781 . The compensating plate  781  makes use of the λ/4 compensating plate. 
     In the thirteenth embodiment shown in  FIG. 15 , as shown in  FIGS. 20A and 20B , it is possible to accomplish effects of blocking not only external light incident only at a position of an observer  790 , i.e. on the side of the encapsulating substrate but also reflected light of this external light. 
       FIG. 16  shows a cross-sectional structure of a double-sided organic light emitting device according to a fourteenth embodiment of the invention. 
     Referring to  FIG. 16 , the double-sided organic light emitting device of the fourteenth embodiment is similar to that of the thirteenth embodiment, but is different in that it is intended to improve effects of blocking light on the side of the lower substrate rather than the encapsulating substrate by providing the circular-polarizing plate only on the side of the lower substrate. A lower circular-polarizing plate  770  is configured of a linear-polarizing plate  775  and a compensating plate  771 . The compensating plate  771  makes use of the λ/4 compensating plate. Thus, in the case where the observer  790  looks on the side of the lower substrate  710 , it is possible to obtain effects of blocking not only external light incident only on the side of the lower substrate  710  but also reflected light of this external light. 
       FIG. 17  shows a cross-sectional structure of a double-sided organic light emitting device according to a fifteenth embodiment of the invention. 
     Referring to  FIG. 17 , the double-sided organic light emitting device of the fifteenth embodiment is similar to that of the thirteenth embodiment except that a compensating plate  881 B of an upper circular-polarizing plate  880  is configured using a plurality of compensating films  882  through  884 , each of which has a phase difference retardation axis. 
       FIG. 18  shows a cross-sectional structure of a double-sided organic light emitting device according to a sixteenth embodiment of the invention. 
     Referring to  FIG. 18 , the double-sided organic light emitting device of the sixteenth embodiment is similar to that of the fourteenth embodiment except that a compensating plate  871  of a lower circular-polarizing plate  870  is configured using a plurality of compensating films  872  through  874 , each of which has a phase difference retardation axis. 
       FIG. 19  shows a cross-sectional structure of a double-sided organic light emitting device according to a seventeenth embodiment of the invention. 
     Referring to  FIG. 19 , the double-sided organic light emitting device of the seventeenth embodiment is similar to that of the ninth embodiment. However, in the case where polarizing plates  971  and  981  and compensating plates  975  and  985  which constitute circular-polarizing plates  970  and  980  are formed like a film or coating layer as the first or second embodiment, the circular-polarizing plates  970  and  980  are arranged on inner surfaces of lower and encapsulating substrates  910  and  960 , respectively. Alternatively, among the polarizing plates  971  and  981  and the compensating plates  975  and  985  which constitute the circular-polarizing plates  970  and  980 , the polarizing plates  971  and  981  may be arranged on outer surfaces of lower and encapsulating substrates  910  and  960  respectively, while the compensating plates  975  and  985  may be arranged on inner surfaces of lower and encapsulating substrates  910  and  960 , respectively. 
     Further, alternatively, any one of the lower and upper linear-polarizing plates  971  and  981  may be formed on the inner surface of any one of the lower and encapsulating substrates  910  and  960 , and the other polarizing plate may be formed on the outer surface of the other substrate. In addition, the lower and upper compensating plates may be disposed between the inner and outer surfaces of the substrates and emission layer. 
     The ninth to seventeenth embodiments of the invention has been illustrated regarding the double-sided organic light emitting device in which the circular-polarizing plates, each of which includes the linear-polarizing plate and the λ/4 compensating plate, are bonded on one side surfaces of the lower and upper substrates, thereby blocking both the transmitted external light and the reflected external light. However, this principle may be true of other emission elements. Further, the passivation layer may be additionally formed on the cathode electrode of the emission element. 
     As can seen from the foregoing, according to the invention, by bonding the polarizing plates on both opposite sides of the double-sided organic light emitting device to block the external light, it is possible to realize the double-sided organic light emitting device having a high contrast. Further, in the case that it is applied to a folder-type double-sided display device, the polarizing plates bonded on both surfaces of the glass substrate can not only block the external light but also function to protect the lower and upper insulating substrates, i.e., to resist a shock. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.