Abstract:
An organic LED device comprises a substrate, a first driver TFT on the substrate, a second driver TFT on said substrate, and an insulating film on the substrate, the first driver TFT and the second driver TFT. There is a common anode on the insulating film. A first organic LED element is on a first portion of the anode and configured as a top emission struction, and a second organic LED element is on a second portion of the anode and configured as a top emission structure. A first cathode extends into the insulating film and electrically connects the first LED element with the first driver TFT. A second cathode extends into the insulating film and electrically connects the second LED element with the second driver TFT.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to an organic LED device, and more specifically to a top emission organic LED device suitable for significant screen size.  
           [0002]    Organic LEDs are well known today. When used in a planar display device, they can be driven by an active matrix drive method such as previously used for a liquid crystal display device. The active matrix deive can be used for a top emission structure or a bottom emission structure. FIG. 9 is a cross-sectional view of an organic LED device using the top emission structure according to the Prior Art. The organic LED device shown in FIG. 9 comprises a thin film transistor (TFT) structure  82  formed of p-type doped polycrystalline silicon (poly-Si) on a glass substrate. The TFT structure  82  is insulated from an upper structure by an insulating film  84 . A reflective metal anode  86  (such as molybdenum (Mo), nickel (Ni) and platinum (Pt)) is formed on an upper portion of the insulating film  84 . A hole injection layer  88  is formed on an adjacent and upper layer of the reflective anode  86 . A hole transport layer  90  and an electron transport layer  92  are formed on an upper layer of the hole injection layer. A translucent cathode  94  is formed on an upper layer of the electron transport layer  92 . This cathode  94  transmits a light beam generated by the organic LED therethrough and also supplies electrons. For example, the cathode  94  can be formed of a material having a small work function, such as aluminum (Al), sodium (Na), calcium (Ca), magnesium-silver (MgAg), barium (Ba) and strontium (Sr). A buffer layer  96  and a glass protective layer  98  are formed on the cathode  94 . Thus, the top emission structure is formed.  
           [0003]    The top emission type organic LED device shown in FIG. 9 is more efficient than the bottom emission type in that an aperture ratio can be improved without depending on the dimension of the TFT. However, the top emission type requires the very thin cathode  94  (about 10 nm) film in order to impart a transparency thereto. Therefore, the cathode  94  has has a disadvantage of being inevitably high in resistance. Because cathode  94  is high in resistance there is significant deop in cathode voltage. This increases from an end portion of a screen to a center portion thereof. Therefore, as the area of the organic LED device becomes larger, it is difficult to apply a sufficient voltage for driving the TFT from the end portion of the screen to the center portion of the screen. In order to reduce the voltage drop through the above-described cathode  94 , it is possible to add a low-resistance layer such as ITO,IZO,SnO x , and InO x  on the cathode  94 . Nevertheless, the ITO has some resistance. Therefore, when a large screen, for example ten inches, uses top emission organic LEDs it is difficult to provide an even level of intensity across the screen.  
           [0004]    [0004]FIG. 10 shows a Prior Art driver circuit  100  of a cathode-common mode, which uses a p-type driver TFT  102  and is used for driving the top emission organic LEDs. A drain electrode  102   d  of the driver TFT  102  is connected to an organic LED element  104 , a source electrode  102   s  is set at a common potential, and the driver circuit  100  is driven in the cathode-common mode. A gate electrode  102   g  of the driver TFT is connected to a switching TFT  108  to permit selective driving of the organic LED element  104 . The Ids current between the source and drain of the driver TFT  102  in a saturation region thereof is approximately proportional to (Vgs-Vth) 2  in the top emission structure shown in FIG. 10. “Vgs” is a voltage between the gate and the source, and “Vth” is a threshold voltage. Because Ids is given by a function only of the Vgs in the conventional top emission structure, the cathode-common mode is adopted. Variation of the Vgs of the TFT is accomodated by characteristic variation of the organic LED.  
           [0005]    The following Table 1 lists the types of TFTs that can be used for preventing the change of the Vgs following the characteristic variation of the organic LED. In Table 1, a reference symbol “circle” denotes types that can accomodate the characteristic variation of the organic LED element, and a reference symbol “cross” denotes types that are not capable of accomodating the characteristic variation of the organic LED element.  
                           TABLE 1                                   Anode-common   Cathode-common                           n-type TFT “circle”   “cross”           p-type TFT “cross”   “circle”                      
 
           [0006]    Even if any of the n-type TFT or the p-type TFT are used, the characteristic variation of the organic LED element can be accomodated by any of the anode-common mode and the cathode-common mode, respectively, when consideration is made only for that characteristic variation as described above. However, another disadvantage (as described below) will occur in the case of forming an anode-common structure by use of the n-type TFT as the driver TFT.  
           [0007]    [0007]FIG. 11 shows a cross-sectional structure of the driver circuit of FIG. 10 where the anode-common structure is formed by the n-type driver TFT  102 . The pixel also comprises switching TFT  108 , an anode  110 , a cathode  106  and LED element  104 . In the conventional top emission structure, the resistive cathode cannot be arranged as a lower electrode because the injection efficiency and light emission efficiency are significantly lowered. Therefore, in the case of forming the top emission structure by adopting the anode-common structure using the n-type TFT, as shown in FIG. 11, it becomes necessary to form a contact hole for anode  110  and cathode  106  in each pixel. This lowers the aperture ratio in the pixel of the organic LED element  108  which is undesirable. Such contact holes are not efficienct or productive and add to the cost. On the other hand, the cathode-common mode using the n-type TFT cannot restrict the variation of the Vgs following the characteristic variation of the organic LED and is inferior in display characteristics.  
           [0008]    Accordingly, an object of the present invention is to provide a top emission organic LED device with a less expensive construction than prior art devices.  
           [0009]    Another object of the present invention is to provide a top emission organic LED device for a wide screen.  
           [0010]    Another object of the present invention is to provide a to emission organic LED device of the foregoing type which has a high aperature ration.  
         SUMMARY OF THE INVENTION  
         [0011]    The invention resides in an organic LED device comprising a substrate, a first driver TFT on the substrate, a second driver TFT on said substrate, and an insulating film on the substrate, the first driver TFT and the second driver TFT. There is a common anode on the insulating film. A first organic LED element is on a first portion of the anode and configured as a top emission struction, and a second organic LED element is on a second portion of the anode and configured as a top emission structure. A first cathode extends into the insulating film and electrically connects the first LED element with the first driver TFT. A second cathode extends into the insulating film and electrically connects the second LED element with the second driver TFT.  
           [0012]    There are other features of the present invention. For example, N-type driver TFTs are used in the top emission structure. It was found that by adopting the anode-common structure in the organic LED device using the n-type driver TFT, an influence of the characteristic variation of the organic LED element to the Vgs can be minimized, and the characteristics can be stabilized. Also, the anode is planar and formed of a low-resistance material such as Al, Ni and Co. By use of this type of anode the common electrode connected to the plurality of pixels is lowered in resistance, thus making it possible to provide the organic LED device of a large area. The anode is formed as lines or a plane, thus making it possible to use the anode as the common electrode. The common anode configuration simplifies the manufacturing process.  
           [0013]    It is preferable that the driver TFT include any of n-type amorphous silicon and n-type polycrystalline silicon as an active layer. It is also preferable that the organic LED device include at least a light emitting portion and an electron transport portion, a part of each being formed self-consistently. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a driver circuit of an organic LED device of the present invention.  
         [0015]    [0015]FIG. 2 is a cross-sectional view of the driver circuit of the present invention.  
         [0016]    [0016]FIG. 3 ( a - e ) are cross-sectional views showing a process for manufacturing the organic LED device of the present invention.  
         [0017]    [0017]FIG. 4 ( a - c ) are cross-sectional views showing a process for manufacturing the organic LED device of the present invention.  
         [0018]    [0018]FIG. 5 is a cross-sectional view showing a process for manufacturing the organic LED device of the present invention.  
         [0019]    [0019]FIG. 6 is a cross-sectional view showing a process for manufacturing the organic LED device of the present invention.  
         [0020]    [0020]FIG. 7 is a cross-sectional view showing a process for manufacturing the organic LED device of the present invention.  
         [0021]    [0021]FIG. 8 is a plan view of the organic LED device manufactured according to the present invention.  
         [0022]    [0022]FIG. 9 is a cross-sectional view of a top emission organic LED device according to the Prior Art.  
         [0023]    [0023]FIG. 10 is a driver circuit of an organic LED device of a cathode-common structure according to the Prior Art.  
         [0024]    [0024]FIG. 11 is a cross-sectional view of a semiconductor structure of a driver circuit for a conventional organic LED device having an anode-common structure using an n-type doped TFT, according to the Prior Art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    Although the present invention is described below in detail based on an embodiment shown in the drawings, the present invention is not limited to this embodiment. FIG. 1 shows a driver circuit of an organic LED device  10  adopting an anode-common structure, according to the present invention. In the preferred embodiment, both the driver TFT  12  and the switching TFT  14  are made as n-type doped. The n-type driver TFT  12  and an n-type switching TFT  14  are connected to drive an organic LED element  16 . However, it is also possible to form the driver TFT  12  and the switching TFT  14  from different dope types (i.e. one p-type and the other n-type). As illustrated in FIG. 1, a gate electrode  12   g  of the driver TFT  12  is connected to a common electrode  20  through a capacitor  18 . A drain electrode  12   d  of the driver TFT  12  is connected to a cathode of the organic LED element  16 . A source electrode  12   s  of the driver TFT  12  is grounded. The anode of LED  16  is common to the anodes of the other, similar LEDs (not shown) in the screen. Consequently, an anode-common structure is formed.  
         [0026]    The gate electrode  12   g  of the driver TFT  12  is also connected to a drain electrode  14   d  of the switching TFT  14 . A source electrode  14   s  thereof is connected to a data line  22 . A gate electrode  14   g  thereof is connected to a selection line  24 . Thus, the organic LED element  16  is driven by TFTs  12  and  14 . The driver circuit shown in FIG. 1 forms one pixel of the organic LED device. A plurality of such pixels are arranged in a plane for an active matrix type drive.  
         [0027]    [0027]FIG. 2 shows the LED device  10  of FIG. 1 in semiconductor form. The driver circuit of LED device  10  includes the n-type TFTs  12  and  14  shown in FIG. 1. A TFT having any structure that has been known heretofore can be used for the present invention. However, in the present invention, it is necessary to use a TFT including an n-type active layer to implement an anode-common structure. Moreover, it is preferable to form the driver TFT  12  and the switching TFT  14  from the same dope type for the convenience of manufacturing and to maximize productivity. However, functionally, the driver TFT  12  and the switching TFT  14  can be made of different doping types, and the switching TFT  14  can include a p-type active layer. Moreover, an n-type poly-Si or an n-type amorphous silicon (a-Si) can be used for the active layer. However, to restrict characteristic variation associated with the organic LED element  16 , a-Si can be effectively used as the n-type active layer.  
         [0028]    As shown in FIG. 2, the switching TFT  14  and the driver TFT  12  are formed on a substrate  26 . The substrate  26  can be composed of various materials, such as SiOx, SiOxNy, Si and metal oxide. A conductive line  28  on substrate  26  connects the TFTs to each other. Another conductive line  30  on substrate  26  connects the TFTs to a cathode  36 . TFTs  12  and  14  are insulated from an upper structure thereof by an insulating film  32  such as a polymer film. Lines are formed on the insulating film  32  by any of a variety of patterning technologies known heretofore. For example, an anode  34  comprising a conductive material such as Al, Mo, Ni and ITO is patterned on the insulating film as lines or a plane. The anode  34  lines or plane reside in the same level as a common electrode (not shown). Moreover, anode  34  is connected to another anode of another pixel (similar to LED element  16  but not shown) and drives the organic LED element  16  in the anode-common mode. The cathode  36  is insulated from the anode  34  by the organic LED element  16 , and allows the organic LED element  16  to emit light. Moreover, the cathode  36  is connected through a via hole  38  to the line  30  formed on a lower layer side thereof and connected to the drain electrode  12   d  of the driver TFT  12 .  
         [0029]    As a result of the design illustrated in FIG. 2, an aperture ratio of the organic LED device is increased because no contact holes are formed in the cathode  36  and the anode  34 , respectively. Moreover, the anode  34  is connected through the common electrode to other pixels easily. These other pixels have the same-construction as pixel  10 . Moreover, because the anode  34  can be formed from a metal plane or lines, the anode  34  can be low in resistance. Therefore, the present invention does not cause a significant voltage drop from an end portion of a screen to a center portion thereof, thus making it possible to enlarge the screen.  
         [0030]    [0030]FIG. 3 ( a - e ) shows a method of manufacturing the organic LED device of the present invention. As shown in FIG. 3( a ), a gate electrode  44  and a line (not shown) for sending a data signal are patterned on an insulating substrate  42 . Next, as shown in FIG. 3( b ), a gate insulating film  48  composed of a material such as SiNx, SiOy and SiOxNy and an active layer  50  composed of poly-Si or a-Si are deposited, and a channel protective layer (etching stopper)  52  is patterned. Next, as shown in FIG. 3( c ), a source electrode  54  and a drain electrode  56 , each comprising Mo/Al/Mo, are patterned. Next, as shown in FIG. 3( d ), an insulating film  58  such as SiNx is deposited, and a contact hole  60  is formed in the insulating film  58 . Next, as shown in FIG. 3( e ), a connection element  61  composed of a conducting film such as ITO is formed, which is connected to upper wiring to be described later. Although this connection element  61  can be-omitted, formation thereof is desirable in order to obtain a good electric connection between the driver TFT on the lower layer side and the organic LED element on the upper layer side.  
         [0031]    [0031]FIG. 4 shows manufacturing process steps subsequent to those shown in FIG. 3. As shown in FIG. 4( a ), a polymer insulating film  62  is deposited on the structure formed in the process shown in FIG. 3( e ), and an aperture  64  corresponding to the contact hole  60  is formed. Next, as shown in FIG. 4( b ), a layer of a conductive material such as ITO, Mo and ITO/Mo is formed. This layer of the conductive material is patterned, and-thus an anode  66  for the organic LED element, which is shown in FIG. 4( b ), is formed. Also, a connection element  68  for stabilizing electric connectivity of the cathode to the driver TFT formed on the lower layer side is simultaneously formed on the inner side surfaces of the contact hole  60  and the aperture  64 . Although this connection element  68  can also be omitted, it is desirable to form the connection element  68  for the same reason as described above. Next, as shown in FIG. 4( c ), an organic or inorganic insulating film  67  for insulating the organic LED element and the other structures from each other is deposited and patterned, and thus a region for forming the organic LED element is formed. A portion  67 ′ that is not related to demarcation of the organic LED element can be removed. However, it is not necessary to remove the portion  67 ′ as long as it does not affect the function of the organic LED device.  
         [0032]    [0032]FIG. 5 shows a preprocessing process for forming the organic LED device. A polymer masking film such as photoresist is utilized to pattern a protruding structure  69  adjacent to a region where the organic LED element is formed. Preferrably, protruding structure  69  has an overhang as shown in FIG. 5. However, as long as the organic LED element of the present invention is obtained efficiently, the protruding structure  69  can be shaped in any form. Protruding structure  69  is used for forming at least three sides of the respective layers such as a light emitting portion and an electron transport portion, which constitute the organic LED element, together with a shadow mask “M” in a process to be described later. Moreover, the protruding structure  69  prevents the shadow mask from being applied with excessive heat during a deposition process such as evaporation of the organic LED element, and thus can enhance reusability of the shadow mask.  
         [0033]    Next, as shown in FIG. 6, the organic LED element  16  is deposited by use of a suitable deposition technology such as evaporation while protecting the other regions by use of a shadow mask M. This organic LED element is constituted by including layers such as a hole injection layer, a light emitting layer and an electron transport layer on the exposed anode electrode  66 . In this case, the thickness of the organic LED element can be set appropriately, for example in a range from 100 nm to 200 nm. Various dopants, organic or inorganic, such as ruburen and coumarin, can be added to the above-described respective layers in order to improve light emission efficiency.  
         [0034]    The shadow mask M shown in FIG. 6 can form end portions at least in three directions of the organic LED element  16  together with the protruding structure  69  while protecting the lower structure thereof. When forming a color display device, patterning is required by use of shadow masks corresponding to the respective colors of R, G and B. The pixels are shifted for each color. Next, in the manufacturing process, as shown in FIG. 7, the cathode  76  is patterned from a material having a smaller work function, such as MgAg, AlLi, so as to coat the organic LED element and the other structures, which are formed as shown in FIG. 6. As described above, the cathode is formed as a very thin film in order to impart a transparency thereto. To prevent the thin cathode from becoming discontinuous and unstable, a transparent conductive film such as ITO is adhered onto the cathode for the purpose of supplementing conductivity as the cathode and protecting such an unstable material having a small work function. Subsequently, a passivation film  78  formed of a material such as SiNx is further deposited for protecting the respective structures. Thus the organic LED device  10  according to the present invention is formed.  
         [0035]    Protruding structure  69  surrounds the organic LED element. Therefore, it becomes possible to form such elements as the cathode  76  and the ITO film on the organic LED element by use of the protruding structure  69  after forming the organic LED element. Moreover, because the protruding structure  69  includes the overhang, the adjacent pixels can be securely insulated from each other simultaneously. Consequently, it is unnecessary to form the pattern by use of the shadow mask when depositing the cathode  76 , thus making it possible to improve the efficiency of the manufacturing processes significantly. Thereafter, the passivation film is deposited.  
         [0036]    [0036]FIG. 8 is a plan view of a TFT substrate  80  according to the present invention. In the TFT substrate shown in FIG. 8, a plurality of pixels  81  are formed adjacent to one another. One pixel is formed in a region surrounded by the protruding structure  69 . The organic LED element  16  and the contact hole  38  shown in FIG. 2 are formed in the inside of the region surrounded by the protruding structure  69 . (Within the contact hole  38 , the aperture  64  is coated with the connection element  68 .) The cathode and the passivation film, which are formed on the upper portion of the organic LED element  16 , are formed by use of the protruding structure  69  in the region in the inside of the protruding structure  69 .  
         [0037]    As shown in FIG. 6, the upstream side of the overhang of the protruding structure  69  in the deposition process is wider than the downstream side. Therefore, the end portion of the organic LED element  16  on the side adjacent to the protruding structure  69  can be formed self-consistently. Moreover, the manufacturing cost is lowered because the upper structure can also be formed self-consistently by the protruding structure  69 .  
         [0038]    It is preferable to remove the protruding structure  69  after the process illustrated in FIG. 7. However, if desired, the protruding structure  69  can be left in provided there is no impediment in the subsequent manufacturing processes and resultant device characteristics. The passivation film may be formed after removing the protruding structure  69 .  
         [0039]    Although description has been made above for the present invention based on the embodiment shown in the drawings, the present invention is not limited to the embodiment shown in the drawings. The structure, material, order of the manufacturing processes and the like of the organic LED element, can be varied as long as a similar structure is obtained.