Abstract:
An organic light emitting diode (OLED) backlight integrated LCD comprising three substrates of which one substrate is shared by LCD and OLED backlight. The shared substrate has two surfaces of which one surface contains, among other layers, a transparent electrode, an internal polarizer and an aligning layer for liquid crystal and faces the top substrate of LCD that also contains identical layers. A thin liquid crystal film is sandwiched between the two substrates containing the said layers. The second surface of said shared substrate contains active organic layers that include an anode and a cathode and faces the bottom substrate of OLED backlight. The two surfaces of the shared substrate are hermetically sealed to the other two substrates of LCD and OLED. When electrically activated the OLED backlight device substantially couples and transmit its generated light in to the LCD.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     Benefit of Provisional patent Application No. 60/485,571 filed Jul. 8, 2003, Docket No. OLIT1140  
         [0002]     Provisional Patent Application No. 60/461,098 filed Apr. 8, 2003; also filed as full patent on Mar. 31, 2004  
         [0003]     U.S. Pat. No. 6,208,391—Fukushima, et al. Mar. 27, 2001  
         [0004]     U.S. Pat. No. 5,754,159—Wood et. al, May 19, 1998  
         [0005]     U.S. Pat. No. 6,542,145—Reisinger et. al, Apr. 1, 2003  
         [0006]     U.S. Pat. No. 6,639,349—Bahadur, Oct. 28, 2003 
     
    
     OTHER PUBLICATIONS  
       [0007]     Y. Bobrov, L. Fennell, P. Lazarev, M. Paukshto, S. Remizov “Manufacturing of a Thin-Film LCD”  Journal of the SID,  10/4, 317-321, 2002.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0008]     Not Applicable  
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX  
       [0009]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0010]     Transmissive Liquid Crystal Displays (LCDs) employed in display systems for various applications require a backlight device. Backlights are based on different technologies like, fluorescent lamp (FL) technology, electro-luminescent (EL) technology, Light emitting diode (LED) technology and organic light emitting diode (OLED) technology. As the backlight device is placed behind the planar LCD in intimate contact, it is preferable to have a flat geometry for the backlight device. Except LED technology all other technologies offer this benefit. In addition, the light coupling from the backlight to the LCD needs to be high to reduce light losses and hence increase in efficiency. It has also been observed that the variation in gap between the backlight and LCD causes optical defects like, appearance of fringes and rings. This invention relates to the integration of OLED backlight to LCD to increase the light coupling efficiency and reduction of optical defects together with the compactness of LCD-OLED assembly. This invention particularly lays emphasis on the use of internal polarizers to the LCD, which enables, the integration of OLED backlight device more robust and reliable against internal and external moisture and oxygen permeation.  
         [0011]     Prior arts dealt with the integration of backlight devices to LCD using different technologies that included OLED backlight device as well. In one prior art of 1998, (U.S. Pat. No. 5,754,159) Wood et. al described the integration of a fluorescent backlight to LCD through a diffuser attached to the back substrate of LCD. The bottom surface of the diffuser, not facing the LCD, was used as one of the cover plate for the fluorescent light source. The other substrate with fluorescent cavity was sealed to the diffuser plate. For fluorescent light source to function in this manner, the diffuser plate ought to be of glass. There were two electrodes for the functioning of the fluorescent light source. One electrode was laid on the diffuser plate and the other electrode was laid on the substrate carrying the fluorescent cavity. The phosphor was coated on the electrode laid on the fluorescent cavity. An inert gas with mercury was filled in the fluorescent cavity. An application of sufficient voltage to the electrodes result in gas discharge that produces UV that falls on the phosphor to generate visible light. This invention is not strictly an integration of light source to LCD because the back surface of the back substrate of LCD was not utilized for sharing the function of one of the substrates of the backlight device. Further, the phosphor coated on one of the electrodes will erode away due to sputtering during operation and render the device useless.  
         [0012]     In another invention by Bahadur et. al, (U.S. Pat. No. 6,639,349), OLED backlight device was appended to the external surface of a fluorescent backlight device by using the external surface of the fluorescent backlight device as one of the substrates of OLED backlight device. The purpose of the integration of OLED to the fluorescent backlight device was to operate OLED backlight device during night and turn off fluorescent backlight device. The operation was for ‘dual mode’ viewing to take advantage of bright fluorescent light during day and low intensity light from OLED during night. This invention did not integrate OLED backlight device directly to the LCD. Another art of 2001 described the integration of EL backlight device to LCD. Fukushima et. al (U.S. Pat. No. 6,208,391) described the integration of EL lamp making use of the external surface of the bottom polarizer of LCD. The external surface of the bottom polarizer had a transparent electrode of EL lamp and the other electrode was laid on the bottom substrate of EL lamp. Phosphor was sandwiched between the two electrodes. This invention did not integrate OLED backlight to LCD.  
         [0013]     A very recent art by Reisinger et. al (U.S. Pat. No. 6,542,145) in 2003, describes the integration of OLED backlight to LCD. In this description, the external surface of the bottom polarizer of LCD is utilized for forming one electrode of OLED upon which organic layers are formed. The third substrate contains the second electrode of OLED. One drawback of this invention is the use of Indium Tin Oxide layer for both the electrodes of OLED. It is well known in the art of OLED technology that one of the electrodes needs to be cathode with material of low work function and the other electrode needs to be anode with material of high work function. In this invention both the electrodes are of the same material having the same work function. Hence the OLED device, according to this invention will either be inoperative or inefficient. Another drawback of this invention is the use of bottom surface of bottom polarizer of LCD, which is external to the bottom substrate. The polarizer is forming part of OLED substrate. Typically the polarizer sheets are laminated to the glass substrate of LCD. Because these sheets are plastics based, they contain lot of moisture. The moisture will de-sorb during the operation of OLED and migrate to OLED to degrade the life of OLED. It is well known that moisture creates ‘black spots’ in OLED and these spots expand both during operation and storage of OLED, thus rendering the device useless.  
         [0014]     Still another invention by Anandan et. al (U.S. Patent under examination—Ser. No. 60461098 with filing date Apr. 08, 2003) describes the integration of three flat panel devices. Each flat panel device in this disclosure shares the substrate of each device, starting from LCD going through a guest-host cell and finally to the backlight device. Unlike all the inventions of prior art, this invention integrates three flat panel devices through sharing of substrates.  
       BRIEF SUMMARY OF THE INVENTION  
       [0015]     According to the present invention, LCD is integrated to OLED backlight device through series of process steps. Most LCDs used in majority of applications like, note book computers, palm pilot, digital clock, digital camera, both direct view and projection television, auto dash board, color cell phone and the like, incorporate two polarizer films. These LCDs invariably come under the family of ‘twisted nematic’ or ‘super twisted nematic’ or ‘ferro-electric LCD. Polarizer films are not incorporated in certain other families that include polymer dispersed liquid crystal display’ (PDLCD) and ‘dichroic liquid crystal display’ (DLCD). For LCDs employing polarizer films, according to this invention, LCD comprises a top glass plate, the inside surface of which has a transparent Indium Tin Oxide (ITO) coating. The layer following this is a planarization layer followed by ‘thin crystal film’ (TCF) layer made by Optiva Incorporated. Following this layer is a polyimide alignment layer, followed by the main liquid crystal film, followed by a polyimide alignment layer, followed by TCF film followed by a planarization layer and finally followed by another ITO on the inner surface of the bottom glass substrate. The completed LCD, with its perimeter seal, is sputtered with ITO on the external surface of the bottom glass substrate of LCD, through a suitable mask, to serve as anode, with high work function, of the subsequent OLED stack. Alternatively, this ITO layer can be pre-formed prior to the fabrication of LCD. After sputtering the ITO layer, other organic layers like, the hole-injection layer, hole transport layer, light generation layer and electron transport layer and a cathode layer of low work function are deposited. Finally a glass cover, with suitable dessicant, is used for perimeter sealing of the OLED stack, at the border of the external surface of the bottom glass substrate of LCD. It is important that the perimeter seal of LCD be vacuum tight to undergo all OLED processes in vacuum of the order of 10 −7  torr. Thus the integrated structure consists of three glass substrates instead of the conventional four substrates. The same integrated structure holds, even if LCD is made of flexible substrate and OLED is made of flexible substrate, except that additional moisture barrier multi-layers have to be laid on flexible substrates both for LCD and OLED, prior to the active layers.  
         [0016]     As the Optiva&#39;s TCF polarizer layer is internal to the LCD, the integrated structure is more durable because the external surface of LCD is glass instead of plastic lamination. Additionally, the life of OLED is enhanced because the glass surface de-sorbs less moisture compared to plastic. Due to the elimination of one substrate and inclusion of internal polarizer, the integrated structure is 50% thinner than the conventional structure and reduces the manufacturing costs. In the integrated structure, the light coupling, from backlight device to LCD, is maximum. For LCDs that do not incorporate polarizers, LCD fabrication is simpler in that planarization layers and TCF layers can be eliminated. In an alternate sequence of fabrication, for LCDs that do not employ active matrix substrates, OLED backlight device can be fabricated first and the LCD fabrication can be done next on the external surface of the light emitting side of glass substrate of OLED. It is preferable to start the fabrication of low yielding device first and the high yielding device next for LCDs that do not employ active matrix substrates. This is necessary to have the compatibility in process temperature of both the devices.  
         [0017]     It is an object of the invention to provide an integrated OLED backlight device to the LCD to increase the light coupling to LCD and thus reduce the light losses.  
         [0018]     It is another object of this invention to employ internal polarizers to LCD, and thus eliminate plastic lamination of traditional polarizers, to enhance the life performance of the integrated OLED backlight device.  
         [0019]     It is yet another object of this invention to provide low weight and thinner integrated device by using three substrates instead of four substrates.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is an isometric view of an OLED backlight device.  
         [0021]      FIG. 1B  is a cross sectional view of OLED backlight device taken from  FIG. 1   
         [0022]      FIG. 2  is an isometric view of a traditional LCD.  
         [0023]      FIG. 2B  is a cross sectional view of the traditional LCD taken from  FIG. 2 .  
         [0024]      FIG. 3  is an isometric view of conventional assembly of LCD and OLED backlight device.  
         [0025]      FIG. 4  is the plan view of the external surface of the bottom glass substrate of LCD illustrating the transparent anode pattern of OLED.  
         [0026]      FIG. 5  is the plan view of the external surface of the bottom substrate of LCD, illustrating the anode pattern and stack of multi-layer thin films of OLED.  
         [0027]      FIG. 6  is an isometric view of OLED backlight device integrated to LCD.  
         [0028]      FIG. 7  is the cross section of the LCD-OLED backlight integrated.  
         [0029]      FIG. 8  is the cross section of LCD alone that is shown as integrated to OLED backlight device in  FIG. 6 . 
     
    
     DETAILED DESCRIPTION  
       [0030]      FIG. 1  shows the isometric view of an OLED backlight device  100 . An organic multi-layer stack  4  is enclosed between top glass substrate  2  and bottom glass substrate  1 , more precisely cover glass. The thickness of the organic layer is denoted by t and is around 125 nm to 200 nm. The stack  4  is sealed all around by a perimeter seal  3 , which is hermetic to prevent moisture and Oxygen entering the device. When a voltage of 5V is applied across the stack  4 , through the reflective cathode  5  and the transparent anode, not shown in  FIG. 1 , the organic stack emits visible light  6 . Also not shown in  FIG. 1  is a gap between cathode  5  and bottom substrate  1 .  FIG. 1B  is the cross section taken from  FIG. 1 . The OLED stack, enclosed between the top glass substrate  2  and bottom substrate  1  and sealed at the perimeter by the seal  3 , contains a cathode  5  and an anode  7  that inject electrons and holes respectively to the organic stack  4 . The anode  7  is usually of Indium Tin Oxide (ITO). It can be noticed that there is a gap between cathode  5  and glass cover  1 . The cathode is of low work function material and is usually from one of the materials of Lithium, Cesium, Barium, Strontium, Calcium, Aluminum, Magnesium or combination of these materials or combination of oxides or fluorides of these materials. Widely known cathode materials are Mg:Ag; Li:Al; LiF:Al; CsF:Al; BaO; Ca. In addition to the requirement of low work function, the cathode material should form efficient electron injection interface with the adjacent electron transport layer.  
         [0031]     There are two well-known structures in OLED namely, ‘up-emitting’ and ‘down-emitting’. In an ‘up-emitting’ structure, the generated light comes out through the cathode and in a ‘down-emitting’ structure, the generated light comes through the anode. Most ‘down-emitting’ structures employ transparent ITO, to a thickness of 40 nm, to transmit light with a transmission of greater than 90%. In up-emitting structure, the cathode material has to be transparent like ITO. All the known cathode materials do not have optical transmission as good as ITO. Hence the cathode layer thickness is greatly reduced during the process to make it transparent. Too thin a layer results in low electrical conductivity. Hence an optimum thickness in the range of 7 nm to 12 nm is conventionally employed for best results.  
         [0032]     The organic layers consist of, starting from the cathode side, electron transport layer, light generation layer, hole transport layer and hole injection layer. Sometimes electron transport layer and light generation, usually doped with another organic material to increase the light generation efficiency, will be of the same organic material. Similarly, hole injection layer and hole transport layer will be of the same material. The thickness of individual layer varies between 15 nm to 75 nm.  
         [0033]      FIG. 2  is an isometric view of a traditional twisted nematic LCD,  200 . A thin, around 5 micron, liquid crystal film  24  is sandwiched between top glass substrate  22  and bottom glass substrate  21  and sealed all around by a perimeter seal  23  to serve as a hermetic seal for the display.  FIG. 2B  is a cross sectional view of LCD  200  taken from  FIG. 2 . Thin liquid crystal film  24  is sandwiched between the top glass substrate  22  and bottom glass substrate  21  and sealed at the perimeter by the seal  23 . The external surface of the top glass substrate  22 , is plastic laminated with a polarizer  25  and similarly the external surface of the bottom glass substrate  21  is plastic laminated with a polarizer  26 . The polarizers  25  and  26  are arranged optically parallel for the display functioning of LCD. There are certain applications wherein the polarizers are arranged optically crossed. The transparent electrode  27 , on the inner surface of the top glass substrate  22 , and the transparent electrode  28 , on the inner surface of the bottom substrate  21 , serve for applying voltage across the liquid crystal film  24 .  
         [0034]      FIG. 3  shows the isometric view of the conventional assembly  300  of the backlight device with the LCD. LCD  31  is assembled over the OLED backlight device  32  and a protective film  33  is placed between LCD  31  and OLED backlight  32 . It is worth noting that the assembly has four substrates  34 ,  35 ,  36  and  37  with a protective layer  33  in between. Light losses take place at the protective layer  33  and in the substrate  36 .  
         [0035]      FIG. 4  is a plan view  400  of the external surface of bottom glass plate  41  of completed LCD, according to this invention. This surface faces the OLED backlight device. The completed LCD, with the external surface of substrate  41 , is placed in a vacuum chamber, that is back-filled with inert gas, and is sputtered with ITO  42  to a thickness of 40 nm, through a shadow mask, to obtain a cathode lead-out  44  and anode lead-out  43 . Subsequently the glass plate  41  and the surface of ITO becomes the substrate for OLED. LCD, whose external surface of glass substrate  41 , processed in this manner, faces sequence of OLED processes without coming out of the vacuum chamber and finally undergoes a hermetic sealing process inside Nitrogen controlled glove-box by directly being transported from vacuum chamber to the glove box.  FIG. 5  is a plan view  500  of the external surface of bottom glass plate  51  of LCD, with full LCD in tact, that has undergone all the OLED processes, including multi-layer organic layer deposition. Multi-layer organic layer  54  is over ITO  52  and reflective cathode layer  55  is over multi-layer organic layer  54 . Portion of cathode lead-out  56  and portion of anode lead-out  53  are clear of organic layers with the help of appropriate shadow masks. A cover glass, not shown in  FIG. 5 , is sealed over the glass plate  51  at the periphery. This could be clearly seen in  FIG. 6  and  FIG. 7 .  
         [0036]      FIG. 6  is an isometric view of the assembly of the LCD-OLED backlight integrated structure  600  according to the present invention. The uniqueness of the present invention lies in the use of internal polarizer, not shown in  FIG. 6  but shown in  FIG. 7 , for LCD that is integrated to the OLED backlight device. The integrated structure shown in  FIG. 6  illustrates the LCD  61 , with its top substrate  63 , sharing its bottom substrate  64  with OLED backlight device  62 , whose top substrate is  64 . For OLED backlight device  62 , the glass plate  65  is just a cover glass plate used for hermetic sealing with the shared substrate  64  at the periphery. Plate  65  does not contain any active elements of OLED except a dessicant, not shown in  FIG. 6 . It is significant to note that there is no protective sheet between OLED and LCD and the light coupling to LCD from OLED is maximum. To have this maximum light coupling condition, It is important that a fraction of light that is likely to be wave-guided laterally through the thickness of substrate  64  is minimized by a careful well known standard techniques.  
         [0037]      FIG. 7  is the cross sectional view of the integrated LCD-OLED assembly  700 . The LCD comprises a top glass substrate  76  followed by a stack of layers  79  consisting of transparent electrode pattern, a planarization layer, internal polarizer, such as TCF material from Optiva, and a plyimide alignment layer. This stack is best shown in  FIG. 8 . Following the stack  79  is a thin liquid crystal film  77 , which is followed by another stack of  79 . The shared substrate  72  has active elements of LCD on the surface opposite to OLED and active elements of OLED on the surface opposite to LCD. Organic stack  73 , which includes transparent anode, and reflective cathode  74 , of OLED, are on the shared substrate  72 . Bottom Glass substrate  71  of OLED merely serves as a cover glass for hermetic sealing. It can be noted that there is a gap between the cathode  74  and cover glass  71 . The shared substrate  72  is sealed at its periphery, on both surfaces, to LCD via the seal  78  and to OLED via the seal  75 . It should be recognized that liquid crystal is filled after the perimeter sealing to the top LCD substrate  76 . The exclusion of plastic laminated conventional polarizer inside OLED enhances the life performance of this LCD-OLED integrated assembly.  
         [0038]      FIG. 8  is the cross section of LCD  800  that is integrated, according to this invention, in  FIG. 7 . The shared substrate  82 , OLED side is not shown in  FIG. 8 , is sealed to the top glass substrate  81  of LCD through the seal  83 , which is usually an epoxy seal. Transparent electrodes  84 , usually ITO, on both the substrates are followed by a planarization layer  85 , such as SiO 2 , which, in turn, is followed by an internal polarizer  86 , such as TCF layer of Optiva Incorporated. Following the internal polarizer  86  is a liquid crystal molecule aligning plyimide layer  87  contacting which is the thin liquid crystal film  88 .  
         [0039]     The embodiment shown in  FIG. 6  can be readily manufactured. The key to the robust and reliable integrated structure of the present invention is the use of ‘internal polarizers’ for LCDs using polarizers. For LCDs that do not employ active matrix substrates, the process temperatures of LCD and OLED are compatible. Under this condition two sequences of manufacturing of this integrated assembly are possible. The first sequence lays emphasis on the low yielding device to be fabricated first. The high yielding device is fabricated next integral to the low yielding device. For example, if the OLED has lower yield than LCD, because of matured manufacturing nature of LCD, OLED is fabricated first. Using the external surface of the light emitting side of OLED, the processes for LCD should commence next. As the OLED is a solid state device and is filled with Nitrogen at atmospheric pressure, the shared substrate that is subjected to LCD&#39;s low temperature process conditions will be stable through all the process steps of LCD. For LCDs employing active matrix substrates, LCD is fabricated first and then OLED process should commence on the external surface of the bottom substrate, shared substrate, of LCD. Since OLED processes are done in vacuum, it is important that the sealing of LCD is vacuum tight to keep the liquid crystal film stable.  
         [0040]     It will be apparent to those skilled in the art that various modifications and variations can be made in the construction, configuration and/or operation of the present invention without departing from the scope or spirit of the invention. For example, OLED backlight device in the foregoing illustration is for the fabrication of a single diode. In fact, any number of diodes, one top of each other, can be integrally processed on the shared substrate to have a series operation or parallel operation of OLEDs. In the same manner, different colors of OLEDs can be integrally processed on the shared substrate. In the foregoing illustration, ‘down-emitting’ structure of OLED and its processing sequence are described. In this respect one example of variation is to change to ‘up-emitting’ structure. In this case, the shared substrate merely serves a cover lid for OLED and the bottom substrate of OLED becomes the active substrate that starts with a reflective anode layer, followed by organic stack and finally a thin transparent cathode layer. Another example of the variation is the use of flexible substrates both for LCD and OLED instead of glass substrates. Variation in sealing of OLED through encapsulation of the OLED stack followed by perimeter metal seal or epoxy seal can also be done. Variation in the profile of the shared substrate for minimizing the light loss due to wave guiding can also be done. Finally, this integrated OLED backlight to LCD can be done for any type and family of LCD. 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.