Patent Publication Number: US-2005122053-A1

Title: Organic electroluminescent display

Description:
BACKGROUND OF THE INVENTION  
      1.Field of the Invention  
      The present invention relates to an organic electroluminescent display (OLED) technology and more particularly to an integrated OLED with a brightness regulating film for light transmission to achieve high contrast under different ambient light intensities.  
      2. Description of the Related Art  
      Among the new generation of flat panel techniques, organic electroluminescent display (OLED) has the advantages of self-luminescence, wide-viewing angle, thin profile, light weight, low driving voltage and simple fabrication process. Generally, a laminated structure over a glass substrate of a unit of the OLED includes a cathode, a hole injection layer, a hole transport layer, an organic electroluminescent layer, an electron transport layer, an electron injection layer and a cathode. Thus, when electrons and holes are combined as excitons in the organic luminescent layer, light radiates from the transparent faceplate.  
      When operating the OLED under brighter lighting conditions, the internal reflective electrodes thereof, however, reflect the external ambient light and contrast thereof is thus lowered. A conventional method for improving contrast in an OLED is the use of black light-absorption material to fabricate the electrode thereof, thus preventing reflection. Another conventional method is the use of a polarizer to regulate incident light and reflective light through the principle of interference, thereby eliminating internal reflection by destructive interference. In U.S. Pat. No. 6,211,613, an OLED structure for improving contrast and lowering power consumption is disclosed, as shown by the schematic cross-section diagram in  FIG. 1 . In  FIG. 1 , a conventional OLED device  10  includes a transparent substrate  12 , an anode  14 , a light emitting layer  16 , a cathode  18 , an insulating layer  20  and a reflective layer  22 , and a circular polarizer  24  is further disposed on one side of the transparent substrate  12 . The anode layer  14  material can be a transparent conductive material such as indium tin oxide (ITO), the insulating layer  20  material can be an aluminum oxide layer, and the reflective layer  22  material can be an aluminum layer, for example. In addition, the arrow symbol L t  illustrates an exterior light radiating into the OLED device  10 , the arrow L r  illustrates a reflection of the exterior light by the OLED device  10  being blocked by the circular polarizer  24 , and the arrow L e  illustrates a light penetrating the circular polarizer  24  illuminated by the OLED device  10 . Although internal reflection caused by the external ambient light can be reduced in the described manner, about 50% of light intensity illuminated by the OLED  10  is also reduced.  
     SUMMARY OF THE INVENTION  
      Accordingly, an object of the invention is to provide a integrated OLED having a transparent display panel with a bright regulating film for light transmission capable of achieving high contrast under different ambient light intensities.  
      The integrated OLED of the invention has a transparent display substrate, a reflective sheet and an optical film, wherein the optical film is able to adjust transmitted-light intensity.  
      A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:  
       FIG. 1  is cross section of a conventional OLED as referenced in the related art;  
       FIG. 2  is a cross section of an OLED of the invention;  
       FIG. 3  illustrates controlled light transmission via a optical slit in a brighter environment;  
       FIG. 4  illustrates controlled light transmission via a optical slit in a darker environment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In  FIG. 2 , a cross section of an organic electroluminescent display (OLED) of the invention is illustrated. The OLED has a transparent display panel  30 , a brightness regulating film  40  for adjusting light transmission, a reflective sheet  42  and a photo sensor  44 , wherein the brightness regulating film  40  is disposed between the transparent display panel  40  and the reflective sheet  42 . In addition, the transparent display panel  30  includes a transparent substrate  32 , a first transparent electrode layer  34 , a light-emitting layer  36  and a second transparent electrode layer  38 . Preferably, the transparent substrate  32  is a glass substrate, the first transparent electrode layer  34  is an indium tin oxide (ITO) layer, the light-emitting layer  36  is an organic electroluminescent layer and the second transparent electrode layer  36  is an ITO layer. Further, a hole injection layer, a hole transport layer, an electron transport layer and an electron injection layer can be fabricated between the first transparent electrode layer  34  and the second transparent electrode layer  38 .  
      The brightness regulating film  40  is a film such as a optical slit for controlling light intensity transmitted from the environment into the transparent display panel  30  and passing through the brightness regulating film  40  according to a light intensity detected by the photo sensor  44 . Preferably, the brightness regulating film  40  is formed of electrochromic material or liquid crystal capable for controlling light transmission by adjusting a driving current applied thereon. For example, the electrochromic material is an intrinsic transparent material and can be changed into opaque or colored material after applying predetermined currents thereon.  
      A key feature of the invention is the use of the transparent display panel  30  with the brightness regulating film  40  disposed between the transparent display panel  30  and the reflective sheet  42  for adjusting light transmission as a main display panel to optimize contrast and brightness in an OLED.  
      In  FIG. 3 , light transmission intensity through the brightness regulating film  40  can be controlled through applying different levels of current thereon and a portion of the external light  46  is absorbed by the brightness regulating film  40  while the photo sensor  44  senses light from an brighter environment, even the brightness regulating film  40  can be changed into an opaque mode to entirely block the external light  46  and absorb all the light without any reflection when the environment brighter, thus providing high contrast over the transparent display panel  30 .  
      In  FIG. 4 , light transmission intensity of the brightness regulating film  40  can be controlled through applying different levels of current thereon and only a small portion of the external light  46  is absorbed by the optical film  40 , while the majority of the light  52  is reflected to enhance contrast over the transparent display panel  30 , even the brightness regulating film  40  can be changed to transparent mode to allow a portion of the light  54  emitted from the transparent display panel  30  reflected by the reflective sheet  42  to enhance brightness when the environment is darker. Thus, the transparent display panel  30  can achieve the same brightness when the driving current applied to the OLED is reduced.  
      Accordingly, the internal reflection in an OLED can be controlled by the brightness regulating film  40  of the invention and optimized contrast of the transparent display panel  30  can be maintained under environments with different light intensity without requiring the additional use of a polarizer or fabrication of light-absorbing electrodes. Insufficient display brightness can be prevented and the driving current required by the OLED can be also reduced by the use of the brightness regulating film of the invention, thus achieving merits such as lower energy consumption and high brightness.  
      While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.