Abstract:
A light emitting device with optical enhancement structure. The light emitting device includes a light emitting element and an optical enhancement structure. Some of the light from the light emitting element is emitted in a diverging manner. The optical enhancement structure is optically coupled to the light emitting element, said optical enhancement structure having a light emerging surface that includes a central surface that is orthogonal to the normal and corner surfaces having profiles that are not orthogonal to the normal. The optical enhancement structure is a single structure for changing the normal angle of the first light emerging surface to increase light output efficiency. The optical enhancement structure have an optical characteristic that directs diverging light from the light emitting element along a path within the optical enhancement structure in a direction towards a normal of the pixel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority and is a continuation-in-part of pending U.S. patent application Ser. No. 10/939,017, filed on Sep. 9, 2004 and pending U.S. patent application Ser. No. 10/938,928, filed on Sep. 9, 2004. These applications are fully incorporated by reference as if fully set forth herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to light emitting devices, and more particularly to an organic light emitting device including an optical enhancement structure.  
         [0004]     2. Description of the Related Art  
         [0005]     Light output efficiency in conventional organic light emitting devices (OLED) and polymer light emitting devices (PLED) is insufficient due to total internal reflection (TIR) and the waveguide effect. Therefore, the actual light output efficiency is still very low although the internal quantum efficiency is near 100%.  
         [0006]      FIG. 2  is a cross-section of a conventional organic light emitting device, only showing a pixel P 1  region for simplicity. The pixel P 1  includes a substrate  10 , a reflective anode  20  formed on the substrate  10 , an organic light emitting layer  22  formed on the reflective anode  20 , a transparent cathode  24  formed on the organic light emitting layer  22 , and a passivation layer  900  formed on the transparent cathode  24 . As shown in  FIG. 2 , light beam L 1 , emitted from the edge of the organic light emitting layer  22 , reaches the boundary  901  of the pixel P 1  and cannot successfully emerge from the front of the device as useful light output.  
         [0007]      FIG. 3  is a cross-section of a conventional organic light emitting device, only showing a pixel region for simplicity. The pixel includes a substrate  10 , a reflective anode  20  formed on the substrate  10 , an organic light emitting layer.  22  formed on the reflective anode  20 , a transparent cathode  24  formed on the organic light emitting layer  22 , and a passivation layer  900  formed on the transparent cathode  24 . This conventional pixel does not add an optical enhancement structure as in the present invention. As shown in  FIG. 3 , when the light beam L 1 , emitted from the edge of the organic light emitting layer  22 , reaches the interface between the passivation layer  900  and the air, if the incident angle θ 1  exceeds the critical angle, total reflection occurs and the totally-reflected light beam is referred to as L t .  
         [0008]     Möller et al. use hemispherical micro-lens arrays to enhance light output efficiency of an OLED (J. of Appl. Phys., Vol. 91, No. 5, pp.3324-3327, 2002).  FIG. 1  shows a cross-section of a pixel of an OLED designed by Möller. Label  100  indicates a glass substrate,  200  a transparent anode,  220  an organic light emitting layer,  240  an opaque cathode, and  300  a hemispherical micro-lens array. The light output efficiency, however, is still not adequate with this more complex structure. By means of the complicated interface of the hemispherical micro-lens, light output angle changes. Thus, while light output efficiency is enhanced, the enhancement is not high enough.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention solves the above-mentioned problems and provides a light emitting device with high light output efficiency. The present invention places a specially-designed optical enhancement structure along light output pathway of the light emitting device. By means of the special profile of the optical enhancement structure, the total internal reflection effect is reduced, thus enhancing light output efficiency of the organic light emitting device.  
         [0010]     In one aspect of the present-invention, the light emerging surface of the optical enhancement structure is provided with a surface profile that reduces internal reflection. In one embodiment, the light emerging surface includes a central surface that is orthogonal to the normal and corner surfaces having profiles that are not orthogonal to the normal. The corner surface profiles may be at least one of an arcuate profile, a faceted profile, and a beveled profile. In another embodiment, light emerging surface include a convex surface, which may be an arcuate profile extending across the light emerging surface.  
         [0011]     The optical enhancement structure may also serve additional functions, such as passivation of underlying layers, in addition to enhancing the light output. In other words, the optical enhancement layer could be combined with other layers such as the passivation layer, cathode layers, etc.  
         [0012]     In one embodiment of the present invention, the light emitting device includes a plurality of pixels, each including a first electrode; an organic light emitting layer formed on the first electrode; a second electrode formed on the organic light emitting layer; and a first optical enhancement structure formed on the second electrode, such that light emitted from the organic light emitting layer can pass through the first optical enhancement structure and emerge from a first light emerging surface of the first optical enhancement structure. The first optical enhancement structure is a single structure for changing the normal angle of the first light emerging surface to increase light output efficiency.  
         [0013]     According to another embodiment of the present invention, the organic light emitting device includes a plurality of pixels, each including a first electrode; an organic light emitting layer formed on the first electrode; a second electrode formed on the organic light emitting layer; and a first optical enhancement structure formed on the second electrode, such that light emitted from the organic light emitting layer can pass through the first optical enhancement structure and emerge from a first light emerging surface of the first optical enhancement structure. The first light emerging surface is an arced surface, a surface composed of a plurality of connecting slanted surfaces with gradually changed slopes, or a combination thereof.  
         [0014]     According to a further embodiment of the present invention, the organic light emitting device includes a plurality of pixels, each including a first electrode; an organic light emitting layer formed on the first electrode; a second electrode formed on the organic light emitting layer; and a first optical enhancement structure formed on the second electrode, such that light emitted from the organic light emitting layer can pass through the first optical enhancement structure and emerge from a first light emerging surface of the first optical enhancement structure. The first light emerging surface includes a first surface and a second surface. The first surface has a flat or arced profile, and the second surface is on the sides of the first surface and is an arced surface, a slanted surface, or a surface composed of a plurality of connecting slanted surfaces with gradually changed slopes.  
         [0015]     In another aspect, the present invention uses an optical enhancement structure that directs diverging light from the light emitting layer along a path within the optical enhancement structure towards closer to the normal to emerge more light from the pixel to improve light output efficiency. The optical enhancement structure bends diverging light from the light emitting layer along a path within the optical enhancement structure, towards the normal of the pixel. In one embodiment, this may be accomplished with an optical enhancement structure having a refractive index profile that bends the diverging light from the light emitting layer towards the normal of the pixel. In another embodiment, the optical enhancement structure is structured with gradually changing refractive indices along the light output pathway of the organic light emitting device. The refractive indices decrease through the optical enhancement structure, towards the direction in which light emerges. The optical enhancement structure may be a single monolithic layer having a refractive index gradient, or comprise several layers of materials having different refractive indices. The optical enhancement structure may also serve additional functions, such as passivation of underlying layers, in addition to enhancing the light output. In other words, the optical enhancement structure could be combined with other layers such as passivation layer, cathode layers, etc. Thus, by diffracting light towards the normal of the pixel, more of the diverging light from the light emitting layer is directed to emerge from the pixel, thus enhancing light output efficiency.  
         [0016]     In a particular embodiment, the light emitting device includes a plurality of pixels, each pixel including a first electrode; a light emitting layer formed on the first electrode; and a second electrode formed on the organic light emitting layer. In one embodiment of the present invention, the light emitting device includes an optical enhancement structure formed on the second electrode, such that light emitted from the organic light emitting layer can pass through and emerge from the optical enhancement structure. In one embodiment of the present invention, the optical enhancement structure includes at least two optical enhancement layers consecutively disposed on the passivation layer and having different refractive indices than a refractive index of the passivation layer. In another embodiment, each consecutively disposed optical enhancement layer has a lower refractive index than the passivation layer and a preceding optical enhancement layer.  
         [0017]     In another aspect of the present invention, the optical enhancement layer further includes a light emerging surface having features for minimizing total reflection of light. In one embodiment, the light emerging surface includes an arcuate profile, a faceted profile, or a beveled profile. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a cross-section of a pixel of a conventional organic light emitting device with hemispherical micro-lens arrays.  
         [0019]      FIG. 2  is a cross-section of a conventional organic light emitting device without an optical enhancement structure.  
         [0020]      FIG. 3  is a cross-section of a pixel of another conventional organic light emitting device without an optical enhancement structure.  
         [0021]      FIG. 4  is a cross-section of a pixel of an organic light emitting device according to a first embodiment of a first aspect of the present invention.  
         [0022]      FIG. 5  is a cross-section of a pixel of an organic light emitting device according to a second embodiment of the present invention.  
         [0023]      FIG. 6  is a cross-section of a pixel of an organic light emitting device according to a third embodiment of the present invention.  
         [0024]      FIG. 7  is a cross-section of a pixel of an organic light emitting device according to a fourth embodiment of the present invention.  
         [0025]      FIG. 8  is a cross-section of a pixel of an organic light emitting device according to a fifth embodiment of the present invention.  
         [0026]      FIG. 9  is a cross-section of a pixel of an organic light emitting device according to a sixth embodiment of the present invention.  
         [0027]      FIG. 10  is a cross-section of a pixel of an organic light emitting device according to a seventh embodiment of the present invention.  
         [0028]      FIG. 11  is a cross-section of a pixel of an organic light emitting device according to an eighth embodiment of the present invention.  
         [0029]      FIG. 12  is a cross-section of a pixel of an organic light emitting device according to a ninth embodiment of the present invention.  
         [0030]      FIG. 13  is a cross-section of a pixel of an organic light emitting device according to a tenth embodiment of the present invention.  
         [0031]      FIG. 14  is a cross-section of a pixel of an organic light emitting device according to an eleventh embodiment of the present invention.  
         [0032]      FIG. 15  is a cross-section of a pixel of an organic light emitting device according to a twelfth embodiment of the present invention.  
         [0033]      FIG. 16  is a cross-section of a pixel of conventional organic light emitting device with hemispherical micro-lens arrays.  
         [0034]      FIG. 17  is a schematic diagram illustrating a light emitting display device of the present invention, incorporating a controller.  
         [0035]      FIG. 18  is a schematic diagram illustrating an electronic device, incorporating the light emitting display device of the present invention.  
         [0036]      FIG. 19  is a cross-section of an organic light emitting device according to a first embodiment of another aspect of the present invention.  
         [0037]      FIG. 20  is a cross-section of an organic light emitting device according to a second embodiment of the present invention.  
         [0038]      FIG. 21  is a cross-section of an organic light emitting device according to a third embodiment of the present invention.  
         [0039]      FIG. 22  is a cross-section of an organic light emitting device according to a fourth embodiment of the present invention.  
         [0040]      FIG. 23  is a cross-section of an organic light emitting device according to a fifth embodiment of the present invention.  
         [0041]      FIG. 24  is a cross-section of an organic light emitting device according to a sixth embodiment of the present invention.  
         [0042]      FIG. 25  is a cross-section of an organic light emitting device according to a seventh embodiment of the present invention.  
         [0043]      FIG. 26  shows a basic structure of a pixel of an organic light emitting device of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0044]     The present invention will be described below in connection with organic light emitting devices, to illustrate the general principle of the present invention. However, it is understood that the present invention is not limited to organic light emitting devices. Other types of light emitting devices can also take advantage of the present invention within the scope and spirit of the present invention.  
         [0045]      FIG. 26  is a cross-section view illustrating an organic light emitting device according to a first embodiment of the present invention. For the sake of simplicity,  FIG. 26  only shows a pixel region of the organic light emitting device. Further, there may be additional elements or components that are not shown in  FIG. 26  but which may be present in the organic light emitting device.  
         [0046]     Referring to  FIG. 26 , the pixel of the organic light emitting device includes a first electrode  11  formed on a glass substrate (not shown), an organic light emitting layer  12  formed on the first electrode  11 , a second electrode  13  formed on the organic light emitting layer  12 , and a first optical enhancement structure  14  formed on the second electrode  13 . Light emitted from the organic light emitting layer  12  can pass through the first optical enhancement structure  14  and emerge from a first light emerging surface  14  a of the first optical enhancement structure  14 . The first optical enhancement structure  14  is a single structure and can reduce total internal reflection effect.  
         [0047]      FIG. 4  is a cross-section of a pixel of an organic light emitting device according to a first embodiment of a first aspect of the present invention. The pixel includes a substrate  10 , a reflective anode  20  formed on the substrate  10 , an organic light emitting layer  22  formed on the reflective anode  20 , a transparent cathode  24  formed on the organic light emitting layer  22 , a passivation layer  26  formed on the transparent cathode  24 , and a first optical enhancement structure  30  formed on the passivation layer  26 .  
         [0048]     The first optical enhancement structure  30  includes a first light emerging surface  31  and a bottom surface  32 . The first light emerging surface  31  further includes a first surface  311  and a second surface  312 . The first surface  311  has a flat profile. The second surface  312  is on the sides of the first surface  311  to connect with the bottom surface  32  and has an arced profile. Alternatively, the second surface  312  can be composed of a plurality of connecting slanted surfaces with gradually changed slopes.  
         [0049]     As shown in  FIG. 4 , when the light beam L n3  (the same location as in conventional  FIG. 3 ), emitted from the edge of the organic light emitting, layer  22 , converges by the first optical enhancement layer  30  and successfully through the first optical enhancement layer  30  as the light beam L n2 . Light beam reaches the second surface  312  of the first optical enhancement layer  30 . Since the second surface  312  has an arcuate profile, the incident angle of the light beam is decreased to not exceed the critical angle. Thus, light beam will not be totally reflected but refract and emerge as the light beam L r . That is to say, the light beam that is otherwise totally reflected can refract and emerge by means of the second surface  312  of the first optical enhancement layer  30  of the present invention, thus further increasing light output efficiency.  
         [0050]      FIG. 5  is a cross-section of an organic light emitting device according to a second embodiment of the present invention. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , and a passivation layer  26 . Corresponding elements are the same as in  FIG. 4  and detailed descriptions are thus omitted here.  FIG. 5  differs from  FIG. 4  in the first optical enhancement structure. In  FIG. 5 , the first optical enhancement structure  35  is formed on the passivation layer  26  and has a first light emerging surface  31 , a bottom surface  32 , and a sidewall  33 . The first light emerging surface  31  includes a first surface  311  and a second surface  312 . The first surface  311  has a flat profile, and the second surface  312  has an arcuate profile and is on the sides of the first surface  311 . The second surface  312  connects the bottom surface  32  with the sidewall  33 .  
         [0051]     Similar to  FIG. 4 , the light beam totally reflected in the conventional OLED can refract and emerge by means of the optical enhancement structure  35  in  FIG. 5  of the present invention, thus increasing light output efficiency.  
         [0052]      FIG. 6  is a cross-section of an organic light emitting device according to a third embodiment of the present invention. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , and a passivation layer  26 . Corresponding elements are the same as in  FIG. 4  and detailed descriptions are thus omitted here.  FIG. 6  differs from  FIG. 4  in the first optical enhancement structure. In  FIG. 6 , the first optical enhancement structure  40  is formed on the passivation layer  26  and has a first light emerging surface  41  and a bottom surface  42 . The first light emerging surface  41  includes a first surface  411  and a second surface  412 . The first surface  411  has a flat profile, and the second surface  412  has a slanted or faceted profile and is on the sides of the first surface  411  to connect the bottom surface  42 .  
         [0053]     Similar to  FIG. 4 , the light beam on the edge that is totally reflected originally no longer satisfies the total reflection condition by means of the slanted profile of the second surface  412 . Thus, the light beam on the edge refracts and emerges, such that light output efficiency is enhanced.  
         [0054]      FIG. 7  is a cross-section of organic light emitting device according to a fourth embodiment of the present invention. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , and a passivation layer  26 . Corresponding elements are the same as in  FIG. 4  and detailed descriptions are thus omitted here.  FIG. 7  differs from  FIG. 4  in the first optical enhancement structure. In  FIG. 7 , the first optical enhancement structure  45  is formed on the passivation layer  26  and has a first light emerging surface  41 , a bottom surface  42 , and a sidewall  43 . The first light emerging surface  41  includes a first surface  411  and a second surface  412 . The first surface  411  has a flat profile, and the second surface  412  has a slanted or faceted profile and is on the sides of the first surface  411 . The second surface  412  connects the bottom surface  42  with the sidewall  43 .  
         [0055]     Similar to  FIG. 6 , since the second surface  412  has a slanted profile, the light beam on the edge that is totally reflected originally no longer satisfies the total reflection condition because of the presence of the first optical enhancement structure  45 . Thus, the light beam on the edge refracts and emerges, such that light output efficiency is enhanced.  
         [0056]      FIG. 8  is a cross-section of an organic light emitting device according to a fifth embodiment of the present invention. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , and a passivation layer  26 . Corresponding elements are the same as in  FIG. 4  and detailed descriptions are thus omitted here.  FIG. 8  differs from  FIG. 4  in the first optical enhancement structure. In  FIG. 8 , the first optical enhancement structure  50  has a first light emerging surface  51  and a bottom surface  52 . The first light emerging surface  51  has an arced profile.  
         [0057]     Similar to  FIG. 4 , the light beam on the edge that is totally reflected originally no longer satisfies the total reflection condition because of the presence of the arced profile of the first light emerging surface  51 . Thus, the light beam on the edge refracts and emerges, such that light output efficiency is enhanced.  
         [0058]      FIG. 9  is a cross-section of an organic light emitting device according to a sixth embodiment of the present invention. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , and a passivation layer  26 . Corresponding elements are the same as in  FIG. 4  and detailed descriptions are thus omitted here.  FIG. 9  differs from  FIG. 4  in the first optical enhancement structure. In  FIG. 9 , the first optical enhancement structure  55  is formed on the passivation layer  26  and has a first light emerging surface  51 , a bottom surface  52 , and a sidewall  53 . The first light emerging surface  51  has an arced profile and connects the bottom surface  52  with the sidewall  53 .  
         [0059]     Similar to  FIG. 8 , the light beam on the edge that is totally reflected originally no longer satisfies the total reflection condition because of the presence of the arced profile of the first light emerging surface  51 . Thus, the light beam on the edge refracts and emerges, such that light output efficiency is enhanced.  
         [0060]      FIG. 10  is a cross-section of a pixel of an organic light emitting device according to a seventh embodiment of the present invention. Referring to  FIGS. 4 and 10 ,  FIG. 10  differs from  FIG. 4  in that a second optical enhancement structure  61  is additionally disposed on the first optical enhancement structure  30 . The second optical enhancement structure  61  adheres to the first optical enhancement structure  30  to constitute a doublet lens. Thus, light emitted from the organic light emitting layer  22  can sequentially pass through the first and second optical enhancement structures  30  and  61  and emerge from a second light emerging surface  612  of the second optical enhancement structure  61 . The refractive index sequence (from large to small) is the passivation layer  26 , the first optical enhancement structure  30 , and the second optical enhancement structure  61 .  
         [0061]     Similar to  FIG. 4 , the light beam on the edge that is totally reflected originally no longer satisfies the total reflection condition because of the presence of the arced profile of the second surface  312  of the first optical enhancement structure  30 . Thus, the light beam on the edge refracts and emerges, such that light output efficiency is enhanced.  
         [0062]      FIG. 11  is a cross-section of an organic light emitting device according to an eighth embodiment of the present invention. Referring to  FIGS. 5 and 11 ,  FIG. 11  differs from  FIG. 5  in that a second optical enhancement structure  62  is additionally disposed on the first optical enhancement structure  35 . Thus, light emitted from the organic light emitting layer  22  can sequentially pass through the first and second optical enhancement structures  35  and  62  and emerge from a second light emerging surface  622  of the second optical enhancement structure  62 . The refractive index sequence (from large to small) is the passivation layer  26 , the first optical enhancement structure  35 , and the second optical enhancement structure  62 .  
         [0063]     Similar to  FIG. 5 , the light beam on the edge that is totally reflected originally no longer satisfies the total reflection condition because of the presence of the arced profile of the second surface  312  of the first optical enhancement structure  35 . Thus, the light beam on the edge refracts and emerges, such that light output efficiency is enhanced.  
         [0064]      FIG. 12  is a cross-section of an organic light emitting device according to a ninth embodiment of the present invention. Referring to  FIGS. 6 and 12 ,  FIG. 12  differs from  FIG. 6  in that a second optical enhancement structure  63  is additionally disposed on the first optical enhancement structure  40 . Thus, light emitted from the organic light emitting layer  22  can sequentially pass through the first and second optical enhancement structures  40  and  63  and emerge from a second light emerging surface  632  of the second optical enhancement structure  63 . The refractive index sequence (from large to small) is the passivation layer  26 , the first optical enhancement structure  40 , and the second optical enhancement structure  63 .  
         [0065]     Similar to  FIG. 6 , the light beam on the edge that is totally reflected originally no longer satisfies the total reflection condition because of the presence of the slanted profile of the second surface  412  of the first optical enhancement structure  40 . Thus, the light beam on the edge refracts and emerges, such that light output efficiency is enhanced.  
         [0066]      FIG. 13  is a cross-section of an organic light emitting device according to a tenth embodiment of the present invention. Referring to  FIGS. 7 and 13 ,  FIG. 13  differs from FIG;  7  in that a second optical enhancement structure  64  is additionally disposed on the first optical enhancement structure  45 . Thus, light emitted from the organic light emitting layer  22  can sequentially pass through the first and second optical enhancement structures  45  and  64  and emerge from a second light emerging surface  642  of the second optical enhancement structure  64 . The refractive index sequence (from large to small) is the passivation layer  26 , the first optical enhancement structure  45 , and the second optical enhancement structure  64 .  
         [0067]     Similar to  FIG. 7 , the light beam on the edge that is totally reflected originally no longer satisfies the total reflection condition because of the presence of the slanted profile of the second surface  412  of the first optical enhancement structure  45 . Thus, the light beam on the edge refracts and emerges, such that light output efficiency is enhanced.  
         [0068]      FIG. 14  is a cross-section of an organic light emitting device according to an eleventh embodiment of the present invention. Referring to  FIGS. 8 and 14 ,  FIG. 14  differs from  FIG. 8  in that a second optical enhancement structure  65  is additionally disposed on the first optical enhancement structure  50 . Thus, light emitted from the organic light emitting layer  22  can sequentially pass through the first and second optical enhancement structures  50  and  65  and emerge from a second light emerging surface  652  of the second optical enhancement structure  65 . The refractive index sequence (from large to small) is the passivation layer  26 , the first optical enhancement structure  50 , and the second optical enhancement structure  65 .  
         [0069]     Similar to  FIG. 8 , the light beam on the edge that is totally reflected originally no longer satisfies the total reflection condition because of the presence of the arced profile of the first light emerging surface  51  of the first optical enhancement structure  50 . Thus, the light beam on the edge refracts and emerges, such that light output efficiency is enhanced.  
         [0070]      FIG. 15  is a cross-section of an organic light emitting device according to a twelfth embodiment of the present invention. Referring to  FIGS. 9 and 15 ,  FIG. 15  differs from  FIG. 9  in that a second optical enhancement structure  66  is additionally disposed on the first optical enhancement structure  55 . Thus, light emitted from the organic light emitting layer  22  can sequentially pass through the first and second optical enhancement structures  55  and  66  and emerge from a second light emerging surface  662  of the second optical enhancement structure  66 . The refractive index sequence (from large to small) is the passivation layer  26 , the first optical enhancement structure  55 , and the second optical enhancement structure  66 .  
         [0071]     Similar to  FIG. 9 , the light beam on the edge that is totally reflected originally no longer satisfies the total reflection condition because of the presence of the arced profile of the first light emerging surface  51  of the first optical enhancement structure  55 . Thus, the light beam on the edge refracts and emerges, such that light output efficiency is enhanced.  
         [0072]     The reflective anode  20  suitable for use in the present invention can be ITO (indium-tin-oxide) or IZO (indium-zinc-oxide) combined with a reflective film or a high work function metal film. The organic light emitting layer  22  can include a hole transport layer (HTL), an emitting layer (EML) and an electron transport layer (ETL) The transparent cathode  24  can be formed by coating a transparent metal film. The passivation layer  26  can be a polymer.  
         [0073]     The first and second optical enhancement structures can be a polymer and function to reduce total internal reflection. The first and second optical enhancement structures can be formed by coating, photolithography, and etching applied in the semiconductor process; or can be a thermoplastic formed in a mold.  
         [0000]     Computer Simulation:  
         [0074]     The models disclosed, of  FIG. 3  (conventional),  FIG. 4  (the present invention),  FIG. 6  (the present invention),  FIG. 8  (the present invention),  FIG. 11  (the present invention), and  FIG. 16  (conventional) were created by computer simulation.  
         [0075]     The following parameters were established: reflectivity of the reflective anode  20  at 100%, organic light emitting layer  22  thickness of 0.15 μm with average refractive index of 1.75, transmittance of the transparent cathode at 100%, and pixel width 2000 μm.  
         [0076]      FIG. 3  (conventional): the thickness of the passivation layer 900 is 1000 μm and n=1.4.  
         [0077]      FIG. 4  (the present invention, single mesa type): the thickness of the passivation layer  26  is 1000 μm and n=1.46. The thickness of the first optical enhancement structure  30  is 275 μm and n=1.4. The first surface  311  has a width of 550 μm, and the second surface  312  has a curvature radius of 1500 μm.  
         [0078]      FIG. 6  (the present invention, single mesa type): the thickness of the passivation layer  26  is 1000 μm and n=1.46. The thickness of the first optical enhancement structure  40  is 200 μm and n=1.4. The first surface  411  has a width of 1000 μm.  
         [0079]      FIG. 8  (the present invention, single hemispherical type): the thickness of the passivation layer  26  is 1000 μm and n=1.46. The thickness of the first optical enhancement structure  50  is 200 μm and n=1.4. The first light emerging surface  51  has a curvature radius of 1500 μm.  
         [0080]      FIG. 11  (the present invention, doublet lens type): the thickness of the passivation layer  26  is 700 μm and n=1.46. The thickness of the first optical enhancement structure  50  is 575 μm and n=1.4. The first surface  311  has a width of 1750 μm, and the second surface  312  has a curvature radius of 1800 μm. The second optical enhancement structure  62  has a thickness of 10 μm and n=1.3.  
         [0081]      FIG. 16  (conventional, micro-lens type): the thickness of the passivation layer  920  is 1000 μm and n=1.4. The micro-lens array has a curvature radius of 10 μm.  
         [0082]     The computer simulation results are shown in Table 1. It can be seen that the OLED pixel structure of the present invention greatly enhances light output efficiency.  
                               TABLE 1                                       Optical   Light               enhancement   output           FIG.   structure   efficiency                               None   10%           (Conventional)               Micro-lens type   13%           (Conventional)               Single mesa type   19%           (The present invention)               Single mesa type   19%           (The present invention)               Single   18%           (The present invention)   hemispherical               type               Doublet lens   23%           (The present invention)   Type                      
 
         [0083]     In conclusion, the first aspect of the present invention disposes an optical enhancement structure with special design in light output pathway of the organic light emitting device. Thus, the total reflection effect is reduced and light output efficiency is greatly enhanced.  
         [0084]      FIG. 19  is a cross-section view illustrating an organic light emitting device according to a first embodiment of a second aspect of the present invention. For the sake of simplicity,  FIG. 19  only shows a pixel region P 10  of the organic light emitting device. Further, there may be additional elements or components that are not shown in  FIG. 19  but which may be present in the organic light emitting device.  
         [0085]     Referring to  FIG. 19 , the pixel P 10  of the organic light emitting device includes a substrate  10 , a reflective anode  20  formed on the substrate  10 , an organic light emitting layer  22  formed on the reflective anode  20 , a transparent cathode  24  formed on the organic light emitting layer  22 , a passivation layer  26  formed on the transparent cathode  24 , and an optical enhancement structure S formed on the passivation layer  26 . The passivation layer  26  is optically coupled to the light emitting layer  22  and has a refractive index. The optical enhancement structure S includes a plurality of optical enhancement layers. For example, the optical enhancement structure S includes a first or inner optical enhancement layer  30  disposed on the passivation layer  26 , and a second or outer optical enhancement layer  140  disposed on the first optical enhancement layer  130 . The plurality of optical enhancement layers are optically coupled to the light emitting layer. The passivation layer and the plurality of optical enhancement layers are configured such that the refractive indices of these layers are in decreasing order from the passivation layer to the outer optical enhancement layer. In other words, the refractive index n 2  of the first optical enhancement layer  130  is lower than the refractive index n 3  of the passivation layer  26 , while the refractive index n 1  of the second optical enhancement layer  140  is lower than the refractive index n 2  of the first optical enhancement layer  130 .  
         [0086]     The light emitting device of this embodiment of the present invention is configured such that the passivation layer  26  ( FIG. 19 ) is thinner than the conventional passivation layer  900  ( FIG. 2 ), and the total thickness of the passivation layer  26  and the optical enhancement structure S of the present invention can be kept approximately equal to or less than the thickness of the conventional passivation layer  900 . Thus the present invention does not necessarily increase the thickness of the overall structure.  
         [0087]     For example, the conventional passivation layer  900  is about 1000 μm thick, the passivation layer  26  of the present invention is about 700 μm thick, and the first and second optical enhancement layers  130  and  140  are about 150 μm each. Thus, light beam L n3  (as shown in  FIG. 19 .), emitted from the organic light emitting layer  22 , converges by the first and second optical enhancement layers  130  and  140  and successfully through the first and the second optical enhancement layers  130  and  140  as the light beam L n2  and L n1 . That is, by passing through different media (i.e., layers  26 ,  130  and  140  with differing refractive indices n 3 , n 2 , and n 1  to form a refractive index gradient in the optical enhancement structure S), light is converged and intensified, therefore, a portion of light that was originally blocked can emerge, thus enhancing light output efficiency.  
         [0088]     As is illustrated by the foregoing embodiment, the present invention uses an optical enhancement structure that directs diverging light from the light emitting layer along a path within the optical enhancement structure towards closer to the normal to emerge more light from the pixel to improve light output efficiency. In one aspect of the invention, the optical enhancement structure bends diverging light from the light emitting layer along a path within the optical enhancement structure towards the normal of the pixel. In one embodiment, this may be accomplished with an optical enhancement structure having a refractive index profile that bends the diverging light from the light emitting layer towards the normal of the pixel. In another embodiment, the optical enhancement structure is structured with gradually changing refractive indices along the light output pathway of the organic light emitting device. The refractive indices decrease through the optical enhancement structure, towards the direction in which light emerges. The optical enhancement structure may be a single monolithic layer having a refractive index gradient, or as illustrated in the embodiment of  FIG. 19 , comprises several layers of materials having different refractive indices. The optical enhancement structure may also serve additional functions, such as passivation of underlying layers, in addition to enhancing the light output. In other words, the optical enhancement structure could be combined with other layers such as the passivation layer, cathode layers, or others.  
         [0089]     As shown in  FIG. 19 , the light emerging surface  41  of the second optical enhancement layer  140  has a substantially flat or planar profile, but it is not limited to this. The light emerging surface  141  can also have a non-flat surface, such that the second optical enhancement layer  140  functions to reduce total reflection of light.  
         [0090]      FIG. 20  is a cross-section of an organic light emitting device according to a second embodiment of the present invention, showing a non-flat light emerging surface. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , a passivation layer  26 , and a first optical enhancement layer  130 . Corresponding elements are the same as in  FIG. 3  and detailed descriptions are thus omitted here.  FIG. 20  differs from  FIG. 19  in the second optical enhancement layer  40 . In  FIG. 20 , the second optical enhancement layer  140  is a total reflection-reducing layer and includes a light emerging surface  141  and a bottom surface  142 . The light emerging surface  141  includes first and second surfaces  1411  and  1412 . The first surface  1411  has a substantially flat profile, and the second surface  1412  has an arcuate profile on each boundary of the first surface  1411  to connect with the bottom surface  142 . The arcuate profile of the second surface  1412  can be smooth or be composed of a plurality of connecting faceted surfaces with gradually changed slopes.  
         [0091]     As shown in  FIG. 20 , light beam L n3  is emitted from the organic light emitting layer  22  through the first and the second optical enhancement layers  130  and  140  as the light beam L n2  and L n1  and light beam L n2  reaches the second surface  1412  of the second optical enhancement layer  140 . Since the second surface  1412  has an arcuate profile, the incident angle of the light beam L n2  is decreased to not exceed the critical angle. Thus, light beam L n2  will not be totally reflected but refract and emerge as the light beam L n3 . That is to say, the light beam that is otherwise totally reflected can refract and emerge by means of the second surface  1412  of the second optical enhancement layer  140  of the present invention, thus further increasing light output efficiency.  
         [0092]     The configuration of the light emitting device shown in  FIG. 4  facilitates light output efficiency. The passivation layer  26  is made thinner than the conventional passivation layer, and the refractive indices of the passivation layer and the two optical enhancement layers  130  and  140  are in gradual decreasing order. As a result, a light beam originally blocked by the boundary of the passivation layer can successfully emerge by the changed light pathway. Since the second surface  1412  has an arcuate profile, light beam is no longer totally reflected and is able to emerge.  
         [0093]      FIG. 21  is a cross-section of an organic light emitting device according to a third embodiment of the present invention. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , and a passivation layer  26 . Corresponding elements are the same as in  FIG. 19  and detailed descriptions are thus omitted here.  FIG. 21  differs from  FIG. 19  in the second optical enhancement layer  140 . In  FIG. 21 , the second optical enhancement layer  140  is a total reflection-reducing layer and has a light emerging surface  141 , a bottom surface  142 , and a boundary  143 . The light emerging surface  141  includes first and second surfaces  1411  and  1412 . The first surface  1411  has a substantially flat profile, and the second surface  1412  has an arcuate profile on each side of the first surface  1411 . The second surface  1412  connects the bottom surface  142  with the boundary  143 .  
         [0094]     Similar to  FIG. 4 , the passivation layer  26  is made thinner than the conventional passivation layer, and the refractive indices of the passivation layer and the two optical enhancement layers  130  and  140  are in gradual decreasing order. As a result, a light beam originally blocked by the boundary of the passivation layer can successfully emerge by the changed light pathway. Since the second surface  1412  has an arcuate profile, light beam is no longer totally reflected and is able to emerge.  
         [0095]      FIG. 22  is a cross-section of an organic light emitting device according to a fourth embodiment of the present invention. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , and a passivation layer  26 . Corresponding elements are the same as in  FIG. 19  and detailed descriptions are thus omitted here.  FIG. 22  differs from  FIG. 19  in the second optical enhancement layer. In  FIG. 22 , the optical enhancement structure S includes first and second optical enhancement layers  130  and  150 . The second optical layer  150  is a total reflection-reducing layer and includes a light emerging surface  151  and a bottom surface  152 . The light emerging surface  151  includes first and second surfaces  1511  and  1512 . The first surface  1511  has a substantially flat profile, and the second surface  1512  has a slanted or faceted profile and is disposed on the sides of the first surface  1511  to connect with the bottom surface  152 .  
         [0096]      FIG. 23  is a cross-section of an organic light emitting device according to a fifth embodiment of the present invention. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , and a passivation layer  26 . Corresponding elements are the same as in  FIG. 19  and detailed descriptions are thus omitted here.  FIG. 23  differs from  FIG. 19  in the second optical enhancement layer. In  FIG. 23 , the optical enhancement structure S includes a first optical enhancement layer  130  and a second optical enhancement layer  150 . The second optical enhancement layer  150  is a total reflection-reducing layer and includes a light emerging surface  151 , a bottom surface  152 , and a boundary  153 . The light emerging surface  151  includes a first surface  1511  and a second surface  1512 . The first surface  1511  has a substantially flat profile, and the second surface  1512  has a slanted or faceted profile and is on the sides of the first surface  1511 . The second surface  1512  connects the bottom surface  152 .  
         [0097]      FIG. 24  is a cross-section of an organic light emitting device according to a sixth embodiment of the present invention. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , and a passivation layer  26 . Corresponding elements are the same as in  FIG. 19  and detailed descriptions are thus omitted here.  FIG. 24  differs from  FIG. 19  in the second optical enhancement layer. In  FIG. 24 , the optical enhancement structure S includes a first optical enhancement layer  130  and a second optical enhancement layer  160 . The second optical enhancement layer  160  is a total reflection-reducing layer and has a light emerging surface  161  and a bottom surface  162 .  FIG. 24  shows that the light emerging surface  161  has an arcuate profile.  
         [0098]      FIG. 25  is a cross-section of an organic light emitting device according to a seventh embodiment of the present invention. The pixel includes a substrate  10 , a reflective anode  20 , an organic light emitting layer  22 , a transparent cathode  24 , and a passivation layer  26 . Corresponding elements are the same as in  FIG. 19  and detailed descriptions are thus omitted here.  FIG. 25  differs from  FIG. 19  in the second optical enhancement layer. In  FIG. 25 , the optical enhancement structure S includes a first optical enhancement layer  130  and a second optical enhancement layer  160 . The second optical enhancement layer  160  is a total reflection-reducing layer and has a light emerging surface  161 , a bottom surface  162 , and a boundary  163 . The light emerging surface  161  connects the bottom surface  162  with the boundary  163 . The light emerging surface  161  has an arcuate profile.  
         [0099]     In all the above embodiments of the present invention, the optical enhancement structure includes two optical enhancement layers as an example, but it is not limited to this. The optical enhancement structure of the present invention can include multiple optical enhancement layers, and such as 2 to 10, preferably 2 to 5 optical enhancement layers. The optical enhancement structure of the present invention includes multiple optical enhancement layers consecutively disposed on the passivation layer, with the most inner enhancement layer having a refractive index lower than the refractive index of the passivation layer and each successively disposed enhancement layer having a lower refractive index than the refractive index of the preceding layer. That is, the optical enhancement layer closest to the passivation layer  26  has the largest refractive index, and that farthest from the passivation layer  26  has the smallest refractive index.  
         [0100]     The reflective anode suitable for use in the present invention can be ITO (indium-tin-oxide) or IZO (indium-zinc-oxide) combined with a reflective film or a high work function metal film. The light emitting layer can be an organic light emitting layer that includes a hole transport layer (HTL), an emitting layer (EML) and an electron transport layer (ETL). The transparent cathode can be formed by coating a transparent metal film. The passivation layer can be a polymer.  
         [0101]     Each optical enhancement layer can be a polymer and can be formed by coating, photolithography, and etching applied in the semiconductor process; or can be a thermoplastic formed in a mold.  
         [0000]     Computer Simulation:  
         [0102]     The models disclosed, of  FIG. 2  (conventional),  FIG. 19  (the present invention), and  FIG. 16  (conventional) were created by computer simulation.  
         [0103]     The following parameters were established: reflectivity of the reflective anode  20  at 100%, organic light emitting layer  22  with thickness of 0.15 μm and average refractive index of 1.75, transmittance of the transparent cathode  24  at 100%, and pixel width of 2000 μm.  
         [0104]     Embodiment of  FIG. 2  (conventional): the thickness of the passivation layer  900  is 1000 μm and n=1.4.  
         [0105]     Embodiment of  FIG. 19  (the present invention): the thickness of the passivation layer  26  is 700 μm and n=1.46. The thickness of the first optical enhancement layer  130  is 150 μm and n=1.4. The thickness of the second optical enhancement layer  140  is 150 μm and n=1.3.  
         [0106]     Embodiment of  FIG. 16  (conventional, micro-lens type): the thickness of the passivation layer  920  is 1000 μm and n=1.4. The micro-lens array has a curvature radius of 10 μm.  
         [0107]     The computer simulation results are shown in Table 2. It can be seen that the OLED pixel structure of the present invention improves light output efficiency.  
                               TABLE 2                                       Optical   Light               enhancement   output           FIG.   structure   efficiency                               None   10%           (Conventional)               Micro-lens type   13%           (Conventional)               Two optical   14%-16%           (The present invention)   enhancement layers                      
 
         [0108]     In conclusion, the light emitting device of the present invention has improved light output efficiency due to a thinner passivation layer and incorporation of at least two optical enhancement layers disposed on the passivation layer, with each successive layer from the passivation layer to the outer enhancement layer having a lower refractive index than the preceding layer. The pathway of the light beam is changed by the different media layers, allowing that light beam to emerge.  
         [0109]     The light emitting device of the present can be coupled to a controller to form a light emitting display device. For example, the organic light emitting devices shown in  FIG. 4  and  FIG. 19  can be coupled to a controller  2 , forming a light emitting display device  3  as shown in  FIG. 17 . The controller  2  can comprise a source and gate driving circuits (not shown) to control the light emitting device  1  to render image in accordance with an input. The light emitting display device  3  and associated controller  2  may be directed to an OLED type display device.  
         [0110]      FIG. 18  is a schematic diagram illustrating an electronic device  5  incorporating the light emitting display device  3  shown in  FIG. 17 . An input device  4  is coupled to the controller  2  of the light emitting display device  3  shown in  FIG. 17  to form an electronic device  5 . The input device  4  can include a processor or the like to input data to the controller  2  to render an image. The electronic device  5  may be a portable device such as a PDA, notebook computer, tablet computer, cellular phone, or a display monitor device, or non-portable device such as a desktop computer.  
         [0111]     Other types of light emitting devices may include PLED, plasma display panel (PDP), chemiluminescent display devices, backlit liquid crystal display devices, or the likes.  
         [0112]     The foregoing description of the preferred embodiments of this invention has been presented for purposes of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments chosen and described provide an excellent illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. For example, while the invention is illustrated by way of example of the optical enhancement layer being on side of the passivation layer away from the light emitting layer, the optical enhancement layer may be deployed above the light emitting layer, either below the passivation layer, or completely omitting the passivation layer. In other words, the optical enhancement layer may also function as a passivation layer. Also the optical enhancement layer may be a single layer of material having a refractive index gradient. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.