Abstract:
A planar organic light emitting diode (OLED) light source is processed on one of the substrates of a liquid crystal display (LCD) and sealed pin-hole free such that LCD processes, including internal polarizer, can be carried out on OLED without affecting the integrity of OLED and LCD. Both devices are held in alignment and hermetaically sealed between two substrates thus forming an integrated device and on application of suitable voltages to these devices OLED generates light and efficiently couples the light to LCD to function efficiently as a full color display.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     Benefit of Provisional patent Application No. 60/631,040 filed Nov. 24, 2004 
    U.S. patent application # 20040170861—Culligan Sean et.al—“Organic Light Emitting Diode for production of polarized light”    U.S. patent application # 20050007517 A1—Munisamy Anandan—“Organic Light Emitting Diode Backlight Integrated LCD”    U.S. patent application Publication # US2003/0063231 A1—Yuan-Tung Dai et.al—“LCD panel integrated with OLED”.   
 
     
    
     OTHER PUBLICATIONS  
       [0000]    
       
          SID&#39;04 Digest—Book I, p. 695 Si Nitride passivation layer, Ar plasma assisted&lt;120 C 30 A/sec, “A thin film encapsulation stack for PLED and OLED displays” F. J. H. Van Assche et.al  
          SID&#39;04 Digest—p. 1170-1173“Current Status and future prospect of in-cell polarizer technology”—Y. Ukai et.al  
          SID&#39;04 Digest—p. 1384-1387—“Thin film encapsulation-silicon nitride-silicon oxide-silicon nitride-silicon oxide-silicon nitride (NONON)”—H. Lifka et.al—plasma enhanced CVD.  
          Balu Pathangey and Raj Solanki—“Atomic layer deposition for nanoscale thin film coatings”—Vacuum Technology &amp; Coating, May 2000.  
       
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0009]     Not Applicable  
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX  
       [0010]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0011]     Liquid Crystal Displays (LCDs) invariably employ a backlight source for reading the information on the display screen. The efficiency of the display system is a primary consideration for battery powered display systems like the ones used in digital cameras, cell phones, personal digital assistants (PDAs), lap tops and so on. In conventional display systems, the backlight is a separate device and the LCD is a separate device. The light that is coupled to LCD from backlight device undergoes losses through optical elements like the light guide, diffuser sheet and reflectors. In fact only 50-60% of the light is transmitted to LCD. In order to increase the coupling efficiency the optical elements are redesigned and molded in to one single piece. Further, in manufacturing, the assembly cost is high to assemble all the optical components with light sources like Light Emitting Diodes (LEDs) and fluorescent lamps (FLs). If flat light sources are employed, they still require a diffuser sheet and prism sheets. The present invention dispenses with the need for all these, simplifying the assembly and decreasing the manufacturing cost.  
         [0012]     Prior inventions dealt with integrating Organic Light Emitting Diode (OLED) backlight to LCD through the substrate integration of OLED and LCD. For example one prior invention (U.S. patent application # 20050007517 A1—Munisamy Anandan—“Organic Light Emitting Diode Backlight Integrated LCD”) described the use of three substrates for integrating OLED backlight to LCD. In this invention, one substrate is shared by OLED and LCD and two hermetic seals were employed. The light from OLED needs to pass through the shared substrate of LCD and hence the coupling of light from OLED to LCD was not greatly enhanced but had better coupling than conventional two discrete OLED and LCD assembly. In another invention by Yuan-Tung Dai et.al (U.S. patent application Publication # US2003/0063231 A1—Yuan-Tung Dai et.al—“LCD panel integrated with OLED”) described red, blue and green OLED pixels fabricated on the upper portion of LCD substrate inside LCD. One major drawback of this invention is that the device can not work because OLED is sensitive to moisture and other chemicals inside LCD and needs to be sealed ‘pin-hole free’ prior to any subsequent process on OLEDs. The invention did not have any such pin-hole free sealing layer over OLED. Another drawback of this invention is the polarizing layer obtained through evaporation on OLED will not work because no polarizer through evaporation process can function as polarizer for LCD. A third drawback is the complexity of the structure. If red, blue and green OLEDs can be formed as pixels, there is no need for LCD to be built on them because OLED can itself be a light emitting display. A careful examination of this invention will distinctly reveal that the invention can never work.  
         [0013]     Unlike the prior inventions the current invention integrates OLED in the form of sheet source of light inside LCD through correct processes to preserve the integrity of the functioning of OLED and LCD. The device is simple in its structure and couples the light from OLED efficiently in to LCD.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     According to the present invention, a planar OLED is processed inside LCD, with OLED functioning as a continuous sheet of light source to backlight LCD. To accomplish this, the top surface of the bottom substrate of LCD is processed with OLED followed by a passivation layer which is a key layer whose process technique plays a vital role for this invention to work. Following the passivation layer is a transparent conductive layer, such as Indium Tin Oxide (ITO) followed by silicon dioxide (SiO 2 ) layer. Over SiO 2  layer is coated the internal polarizing layer of thin crystal film TCF-N015 made Optiva Inc. A plyimide layer, which serves as alignment layer for liquid crystal, is coated over TCF-N015. The bottom surface of top substrate of LCD contains color filter, flanked by a black matrix layer, over which is coated a passivation layer followed by ITO connected to thin film transistor (TFT). A thin film of SiO 2  is formed over ITO layer followed by TCF-N015 layer. A polyimide film over TCF-N015 serves as alignment layer for liquid crystal molecules. Between top and bottom substrate is sandwiched a thin liquid crystal film.  
         [0015]     It is an object of this invention to provide an internal backlight to LCD for better light coupling efficiency and thus maximize the optical efficiency of LCD.  
         [0016]     It is another object of this invention to integrate OLED backlight inside LCD making use of only two substrates and thus obtain a single compact unit that contains both LCD and backlight thus reduce the manufacturing cost.  
         [0017]     It is yet another object of this invention to provide a ‘pin-hole free’ passivation layer over OLED to preserve its integrity of operation and ease of introduction of ‘internal polarizer’, ITO and liquid crystal aligning layer for successful functioning of the integrated device.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is an isometric view of traditional backlight for LCD employing optical components according to one prior art.  
         [0019]      FIG. 2  is an isometric view of OLED integrated to LCD according to another prior art.  
         [0020]      FIG. 3A  is the cross section of OLED backlight inside LCD with active matrix TFTs at the top substrate of LCD, according to present invention.  
         [0021]      FIG. 3B  is the cross section of OLED device inside  FIG. 3A   
         [0022]      FIG. 4  is the cross section of another embodiment of OLED backlight inside a passive matrix LCD.  
     
    
     DETAILED DESCRIPTION  
       [0023]      FIG. 1  shows the isometric view  100  of the traditional backlight for LCD employing optical components according to a prior art. The light source  01  is either Light Emitting Diode (LED) or Cold Cathode Fluorescent Lamp (CCFL). A reflector  04  is placed behind the light source  01  to reflect the light forward to a light guide  03  which has a patterned reflector  02  to send uniform sheet of reflected light  09  towards the back surface of LCD  08 . A reflector  05  at far end of light guide  03  prevents light loss in the lateral direction. A diffuser sheet  07  above the wedge light guide makes the light uniform and the two prism sheets  06  over the diffuser sheet  07  collimates the light in to the useful viewing angle of LCD  08 .  
         [0024]      FIG. 2  shows the isometric view  200  of a direct backlight according to another prior art. The backlight box  21  contains linear fluorescent lamps  22  and a diffuser  23  assembled over the fluorescent lamps  22 . The diffused light  24  uniformly backlights the LCD  25 . For the sake of simplicity prism sheets are not shown in  FIG. 2 .  
         [0025]      FIG. 3A  illustrates the present invention through the cross section  300 , for a single pixel configuration, comprising stack of layers starting from OLED up to the top substrate of LCD. Bottom substrate  301  of LCD contains OLED  302  whose layer details are given in  FIG. 3B . OLED device  302  is sealed by a ‘pin-hole free’ passivation’ layer  303 . This layer is the most critical layer for functioning of the structure of the present invention. The thickness of this layer is in the range of 100 nm to 500 nm. The ‘passivation’ layer,  303  on OLED to protect OLED from all subsequent processing of the bottom substrate of LCD, can be deposited through many techniques and methods. The most critical nature of the process is the low temperature (&lt;130 C) aspect. Method (1): to employ ‘Atomic Layer Deposition’ (ALD) of thin films of SiO 2  , or Al 2 O 3 , or TiO 2 , or Ta 2 O 5 , or Y 2 O 3 , or HfO 2 , or Nb 2 O 5 , or MgO, or SiNx, or AlN in single layer or alternate layers of two films of different materials. Since this ‘passivation’ layer is critical for the operation of OLED, the porosity and the stress of the resulting film is very important. An illustration of forming a ‘passivation’ layer of TiO 2  through ALD process is as follows: The substrate carrying OLED, with ITO layer on top of OLED, is loaded inside an ALD chamber. During transport of OLED substrate to the ALD chamber it is important that no exposure of OLED processed substrate to ambient air takes place. The precursors for TiO 2  are injected in to the deposition chamber. The precursors for deposition are: (1) Titanium tetrachloride and (2) Ozone. The temperature of the chamber is set around 100° C. Report has appeared on the formation of TiO 2  even at room temperature through ALD process The chemical reaction that takes place at this temperature is: 
 TiCl 4 +O 3 →2TiO 2 +2ClO 2↑   
 2ClO 2  is flushed out of the chamber by a pulse of N 2  gas injected in to the chamber and pumping it out. As the growth rate of ALD is a maximum of 10 nm/min, a thickness of only 500° A of TiO 2  is sufficient. Method 2: In between the films of these oxides or nitrides, an organic film such as polyimide can also be spin coated for relieving the stress of the resultant film. However, an ITO film needs to be deposited on polyimide film prior to the deposition of ALD film. Method (3): Since ALD is a slow process with minimum porosity known in the area of thin films the manufacturing process time will be long. Hence, the first ‘passivation’ film, directly in contact with OLED transparent electrode of ITO (shown in  FIG. 3B ), can be one of the oxides or nitrides films through ALD process to a thickness of around 250-500 A followed by plasma enhanced chemical vapor deposition (CVD) of Silicon nitride layer with high rate of deposition of 10-30 A/sec to a thickness of 1 micron. Method (4): An alternate stack of films is to deposit the first oxide or nitride film through ALD process, to a thickness of 250-500 A, on the ITO electrode of OLED and then deposit subsequent alternate layers of silicon oxide and nitride through plasma CVD process to a thickness of around 1 micron. Method (5): Still another method of film stack is to deposit the first thin film layer of one of the oxides or nitrides, mentioned above, through ALD to a thickness range of 250-500 A and then spin coat an organic-inorganic hard-coat solution such as Desolite 4D5-15 or Desolite 4D5-221 made by DSM Desotech Inc., on the ALD film that can be dried and UV cured to obtain a thickness of 2-5 micron. The thickness of the film can be optimized depending upon the transmission required and at the same time with minimum permeation of other solvents that come in to contact due to subsequent LCD process steps. Over the ‘passivation’ layer  303  is a counter electrode  304  of LCD made of ITO, through sputtering, followed by SiO 2  film  305  obtained through vacuum evaporation to a thickness of approximately 100 nm. A polarizing film  306  is coated by using TCF-N015 material of Optiva Inc through ‘doctor-blading’ or rod shearing technique to a thickness of 400 nm. A polyimide layer  307  is spin coated over the polarizing film to a thickness of 75 nm and surface treated for alignment of LC molecules. Top substrate of LCD  318  contains color filter layer  316  surrounded by black matrix  317 . Over color filter layer  316  is deposited a ‘passivation’ layer  315 , different from layer  303 , that enables a good adhesion and conductivity for the subsequent ITO pixel electrode  313 . For active matrix LCDs the ITO electrode  313  is connected to Thin Film Transistor (TFT)  314 . SiO 2  film  312  to a thickness of 100 nm is vacuum evaporated on pixel electrode  313  followed by TCF-N015 layer  311  and polyimide layer  310 , as on the bottom substrate  301 . 
 
         [0026]      FIG. 3B  shows the layers of OLED  302  in detail. OLED serves as a continuous sheet source of backlight for the entire LCD. A reflective metal cathode around 300 nm made of Mg:Ag or any one of the metals or metal alloy of Li, Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, Ce, La, Nd, Sm, In, LiAl, is vacuum evaporated on the bottom substrate of LCD followed by an organic layer  321 , that functions as electron transport medium, through vacuum evaporation to a thickness of approximately 40 nm. This layer is followed by another organic layer called light generation layer  322  with or without doping, also vacuum deposited to a thickness of approximately 30 nm. Over the light generation layer are organic layers  323  and  324 . Layer  323  is called hole-transport layer and layer  324  is called hole injection layer and both these layers are vacuum deposited to approximately 30 nm. Final layer is anode layer  325  which is usually transparent ITO that is sputtered on to the hole-injection layer  324 . OLED process is a low temperature process involving predominantly vacuum evaporation process that is employed in small molecule OLED technology. However, other low temperature processes involving polymer OLED and phosphorescent OLED are equally compatible for processing the OLED device inside LCD. The light that is generated due to electron-hole recombination at the light generation layer  322  escapes through these transparent layers upward as depicted by  326 . These are the rays that will backlight LCD.  
         [0027]      FIG. 4  is the cross section of single pixel configuration  400  of another embodiment of the present invention illustrating OLED sheet source of backlight inside a passive matrix liquid crystal display. OLED backlight device  402  is processed on the inside surface of bottom substrate  401  of LCD followed by a critical pin-hole free‘passivation’ layer  403  as described under  FIG. 3A . Over this ‘passivation’ layer is sputtered a transparent conductive layer  404  of ITO that serves as one electrode of LCD. A SiO2 layer  405  an internal polarizing layer  406 , using TCF-N015 of Optiva Inc, and polyimide liquid crystal alignment layer  407  are laid sequentially on ITO layer  404  as described under  FIG. 3A . On the inside surface of the top substrate  417  of LCD, color filter layer  416  is coated with a surrounding black matrix layer  415 . A ‘passivation’ layer  414 , different from ‘passivation’ layer  403  on the bottom substrate, is deposited on the color filter layer for the subsequent ITO layer  413  to have a good adhesion and electrical conductivity to function as one of the electrodes of LCD. ITO layer  413 , followed by SiO2 layer  412 , polarizing layer  411  and alignment layer  410  are deposited in sequence as described in  FIG. 3A . A liquid crystal film  409  with limiting spacers  408  is sandwiched between top substrate  417  and bottom substrate  401  of LCD. Light rays  418  from OLED emerge through all the layers when the LC pixel is optically open.  
         [0028]     It will be apparent to those skilled in the art that various modifications and variations can be made in the construction, processing, configuration and/or operation and application of the present invention without departing from the scope or spirit of the invention. For example, in the embodiment described above in  FIG. 3A  TFT is located on the substrate containing color filter. This can be changed to have TFT on the substrate not containing the color filter. The illustration shown in  FIG. 3A  is for a Twisted Nematic (TN) LCD and hence LC alignment layers were coated on both substrates of LCD. For other modes of LCDs like the in-plane switching mode there may not be a need for alignment layer on both the substrates. Instead only one substrate needs to have LC alignment layer. There are other modes of LCD having only one polarizer. In the illustration shown in  FIG. 3A  there are two internal polarizing layers. This can be simplified to have only one polarizing layer on any one of the substrates. Similarly the illustration under  FIG. 4  is for a passive matrix LCD in general. The LC molecules can have 90°, 180° or 270° twist or homogeneous orientation or homeotropic orientation. The structure is applicable for all passive matrix family of LCDs. The OLED device illustrated is a single OLED device. This device can be a series processed OLED device or series and parallel processed OLED device containing several OLEDs with different wavelengths. The description of OLED given above, is for small molecule technology. But it is equally applicable for polymer LED and phosphorescent OLED. Thus it is intended that the present invention covers the modifications and variations of the invention provided they come within the scope of the appended claims and their equivalents.