Abstract:
An organic light emitting diode display includes a substrate; a plurality of OLEDs formed on the substrate, the OLEDs emitting polarized light wherein the OLEDs comprise a layer defining a periodic grating structure; a first electrode layer conforming to the grating structure; an OLED material layer formed over the first electrode layer and conforming to the grating structure; and a second electrode layer formed over the OLED material layer and conforming to the grating structure, wherein the first and/or second electrode layers are metallic layers, whereby the periodic grating structure induces surface plasmon cross coupling in the metallic electrode layer(s) to emit polarized light; and a polarizer located over the substrate through which the polarized light is emitted.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to organic light emitting diode displays, and more particularly to increasing the light output from the emissive layers.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is known to use polarizers with flat panel displays such as LCD and OLED displays to reduce the reflection of ambient light on the front of the flat panel displays. Circular polarizers are known to improve contrast in light emitting displays, for example, as disclosed in U.S. Pat. No. 4,100,455 issued Jul. 11, 1978 to DuBois; JP 03-222287; and U.S. Pat. No. 6,549,335 B1 issued Apr. 15, 2003 to Trapani et al. U.S. Pat. No. 6,392,727 B1 issued May 21, 2002 to Larson et al. describes the use of circular polarizers with LCD flat-panel displays.  
         [0003]     However, in an OLED display, circular polarizers while absorbing more than 99% of the ambient light incident on the polarizer, also absorb up to 60% of the light emitted from the OLED display. Moreover, much of the light output from the emissive elements in the OLED is absorbed within the device. Because the light emission from the OLED is unpolarized and Lambertian, light is emitted equally in all directions so that some of the light is emitted forward to a viewer, some is emitted to the back of the device and is either reflected forward to a viewer or absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. If a polarizer is used to enhance contrast, the polarizer also absorbs a substantial portion of the light. Thus in an OLED display with a polarizer, over 90% of the emitted light may be lost.  
         [0004]     It has been proposed to use a periodic, corrugated, grating structure to induce surface plasmon coupling for the light emitting layer in an organic luminescent device, thereby inhibiting lateral transmission and wave guiding of emitted light while increasing the efficiency and the light output of the structure. See Extraordinary transmission of organic photoluminescence through an otherwise opaque metal layer via surface plasmon cross coupling by Gifford et al.,  Applied Physics Letters , Vol. 80, No.  20 , May 20, 2002, pp. 3679-3681. Using this technique, it is theoretically possible to outcouple up to 93% of the light emitted by the organic luminescent materials in an organic luminescent device, however this technique does not reduce the reflectance of ambient light from the surface of the display.  
         [0005]     There is a need therefore for an improved organic light emitting diode display structure that avoids the problems noted above and improves the efficiency of the display for practical devices.  
       SUMMARY OF THE INVENTION  
       [0006]     The need is met by providing an organic light emitting diode display that includes a substrate; a plurality of OLEDs formed on the substrate, the OLEDs emitting polarized light wherein the OLEDs comprise a layer defining a periodic grating structure; a first electrode layer conforming to the grating structure; an OLED material layer formed over the first electrode layer and conforming to the grating structure; and a second electrode layer formed over the OLED material layer and conforming to the grating structure, wherein the first and/or second electrode layers are metallic layers, whereby the periodic grating structure induces surface plasmon cross coupling in the metallic electrode layer(s) to emit polarized light; and a polarizer located over the substrate through which the polarized light is emitted.  
       Advantages  
       [0007]     The display of the present invention has the advantage of providing a higher contrast and having a higher efficiency. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic cross sectional diagram of a top emitting OLED display according to the present invention;  
         [0009]      FIG. 2  is a schematic cross sectional diagram of a prior art top emitting OLED display;  
         [0010]      FIG. 3  is a schematic cross sectional diagram of a prior art bottom emitting OLED display; and  
         [0011]      FIG. 4  is a schematic cross sectional diagram of a bottom emitting OLED display according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     Referring to  FIG. 2 , a prior art top emitting OLED display device  10  includes a substrate  12 , and a thin film transistor (TFT) active matrix layer  14  comprising an array of TFTs that provides power to OLED elements. A patterned and planarized first insulating layer  16  is provided over the TFT active matrix layer, and an array of first electrodes  18  are provided over the planarized insulating layer  16  and in electrical contact with the TFT active matrix layer. A patterned second insulating layer  17  is provided over the array of first electrodes  18  such that at least a portion of the each of the first electrodes  18  is exposed.  
         [0013]     Over the first electrodes and insulating layers are provided red, green, and blue-emitting organic OLED elements,  19 R,  19 G, and  19 B, respectively. These elements are composed of further layers as described in more detail below. Herein, the collection of OLED elements, including hole injection, hole transport, and electron transport layers may also be referred to as the OLED layer  19 . The light emitting area is generally defined by the area of the first electrode  18  in contact with the OLED elements. Over the OLED layer  19  is provided a transparent, common second electrode  30  that has sufficient optical transparency to allow transmission of the generated red, green, and blue light. An optional second electrode protection layer  32  may be provided to protect the electrode and underlying layers. Each first electrode in combination with its associated OLED element and second electrode is herein referred to as an OLED. A typical top emitting OLED display device comprises an array of OLEDs wherein each OLED emits red, green or blue. A gap, generally filled with inert gas or a transmissive polymer material separates the electrode protection layer from an encapsulating cover  36 . The encapsulating cover  36  may also be a layer deposited directly on the common second electrode  30  or the optional second electrode protection layer  32 .  
         [0014]     In operation, the thin film transistors in TFT layer  14  control current between the first electrodes  18 , each of which can be selectively addressed, and the common second electrode  30 . Holes and electrons recombine within the OLED elements to emit light  24 R, G and B from the light emitting elements  19 R, G and B respectively. Because the layers are so thin, typically several hundred angstroms, they are largely transparent.  
         [0015]     Referring to  FIG. 1 a  top emitter embodiment of the present invention includes a substrate  12 , TFT layer  14 , an insulating layer  16 , first patterned electrode  18 , and second insulating layer  17 . Conventional OLED layers  19  are deposited upon the insulating layer  17  and first patterned metal electrodes  18 . A second, common electrode  30  and protection layer  32  are deposited above the OLED layers  19 . The display  10  is encapsulated with an encapsulating cover or layer  36 . A polarizer  40  is affixed to the encapsulating cover or layer  36  either on the outside (as shown) or inside the encapsulating cover or layer  36  (not shown) where it may be protected by the encapsulating cover or layer  36 . Preferably the polarizer  40  is a circular polarizer conventionally comprising a linear polarizer in combination with a quarter wave plate.  
         [0016]     The insulating layer  16  is made of conventional materials but is not a conventional planarization layer as in the prior art but rather has a periodic physical grating structure that makes the layer thicker in some locations and thinner in others. The size and period of the grating structure is selected to be effective to cause surface plasmon cross coupling in overlying metallic layers that conform to the grating structure. In particular, the first patterned metal electrode  18  has a similar periodic structure, as do the OLED layers  19 . The second electrode layer  30  is likewise conformable to the grating structure, but the top surface of the second electrode layer  30  or layers above the second electrode  30  may, or may not, conform to the periodic grating structure.  
         [0017]     In a preferred embodiment, the periodic grating structure of the insulating layer  16  differs for each of the red, green, and blue OLED light emitting areas  19 R,  19 G, and  19 B respectively. The period of the grating structure is centered on the frequency of light emitted by the OLED materials. For example, the periodic structure of the insulating layer  16  can have a period ranging from 200 to 1000 nm. The height of the physical structure is about 100 nm although larger or smaller heights are possible; the minimum thickness of the insulating layer must be sufficient to provide good insulation between the first patterned metal electrode  18  and the thin film electronics devices  14 . The period and heights of the periodic grating structure affect the frequency of optimum cross coupling and angular dependence. In general, the OLED element layer should be as thin as possible to cause as much energy as possible to undergo surface plasmon cross coupling in the metallic layers. The insulating layer  16  may be reflective or transmissive, or may be opaque to increase the contrast of the device. The insulating layer  16  is made by conventional means, for example photolithography.  
         [0018]     In operation, current is passed via the electrodes  18  and  30  through the light emitting elements  19  causing light to be emitted both upward through second electrode  30  and downward toward the substrate. The periodic structure of the first patterned metal electrode  18  and the OLED layer  19  causes surface plasmon cross coupling between the layers. The surface plasmon effect has the additional benefit of reducing the absorption of light in the electrode, further increasing the light output from the device. The emission from the OLED device is no longer Lambertian, but is highly directional along an axis perpendicular to the display and includes polarized emission. The light emitted forward is seen by a viewer. The light emitted backward is either absorbed or reflected by the insulating layer. The polarizer  40  is oriented such that the emitted polarized light  24  passes through the polarizer  40  without being substantially absorbed. As known in the prior art, approximately one half of the non-polarized light emission is absorbed by the polarizer  40 . Ambient light incident on the polarizer  40  is absorbed as known in the prior art. Hence, the present invention provides an improvement in light output and contrast.  
         [0019]     The present invention may be applied to both a top emitter (wherein light is emitted through the cover as described above) or a bottom emitter (wherein light is emitted through the substrate). In the bottom emitter case, the periodic grating structure may be created directly upon the substrate  12 , or to insulating or conducting layers applied to the substrate. Referring to  FIG. 3 ., a prior art bottom emitter device uses a patterned conductive layer  13  of indium tin oxide (ITO) deposited on the substrate to conduct current to the light emitting areas.  
         [0020]     Referring to  FIG. 4 , in a bottom emitter OLED display according to the present invention, the ITO is provided with a periodic grating pattern similar to that of the insulating layer  16  of the top emitter in the areas where light is emitted. The grating pattern is created in the ITO layer using well known photolithography techniques. A thin metal electrode layer  15  is deposited on the corrugated ITO, the organic materials are conformably deposited on the metal layer, and the remainder of the depositions are as described earlier. The thin metal electrode  15  may be omitted, but surface plasmon coupling will not be supported in the ITO layer alone. A polarizer  40  is located over the substrate  12  and arranged so that emitted, polarized light  24  passes through the substrate  12  without being substantially absorbed.  
         [0021]     Because the emitted light  24  is polarized and has an angular dependence on frequency, a diffuser may be included in the display  10  to mitigate the effect of color aberrations. This diffuser may be applied to the exterior of the device, for example, or the diffuser may be incorporated into the cover (for a top emitter) or the substrate (for a bottom emitter).  
         [0022]     In another embodiment of the present invention, the period of the structure of the insulating layer  16  and conformable layers deposited upon it may be constant across the device rather than different for each individual color  19 R, G, and B. This simplifies the construction of the device with some loss in efficiency of the light output and angular dependence of frequency.  
         [0023]     The present invention can be employed in most top or bottom emitting OLED device configurations. These include simple structures comprising a separate anode and cathode per OLED and more complex structures, such as passive matrix displays having orthogonal arrays of anodes and cathodes to form pixels, and active matrix displays where each pixel is controlled independently, for example, with a thin film transistor (TFT). As is well known in the art, OLED devices and light emitting layers include multiple organic layers, including hole and electron transporting and injecting layers, and emissive layers. Such configurations are included within this invention.  
         [0024]     In a preferred embodiment, the invention is employed in a device that includes Organic Light Emitting Diodes (OLEDs) which are composed of small molecule or polymeric OLEDs as disclosed in but not limited to U.S. Pat. No. 4,769,292, issued Sep. 6, 1988 to Tang et al. and U.S. Pat. No. 5,061,569, issued Oct. 29, 1991 to VanSlyke et al. Many combinations and variations of organic light emitting displays can be used to fabricate such a device.  
         [0025]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.  
       Parts List  
       [0000]    
       
           10  OLED display device  
           12  substrate  
           13  ITO layer  
           14  TFT layer  
           15  metal electrode layer  
           16  insulating layer  
           17  second insulating layer  
           18  first electrodes  
           19  OLED layer  
           19 R red-emitting organic materials layer  
           19 G green-emitting organic materials layer  
           19 B blue-emitting organic materials layer  
           24  emitted light  
           24 R red light  
           24 G green light  
           24 B blue light  
           30  second electrode  
           32  second electrode protection layer  
           36  encapsulating cover  
           40  circular polarizer