Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0023646, filed in the Korean Intellectual Property Office, on Mar. 19, 2009, the entire content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The following description relates to an organic light emitting diode (OLED) display. 
     (b) Description of the Related Art 
     Display quality of an OLED display is greatly influenced by external light. That is, when external light is transmitted into the OLED display that includes an OLED and a thin film transistor, reflection of the external light occurs in layers that form the OLED and the thin film transistor. For example, a metal layer used as an electrode of the OLED has high light reflectivity so that most of the external light can be reflected. The reflected external light is mixed with light emitted from an organic emission layer so that display visibility of the OLED display is deteriorated. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     An aspect of an embodiment of the present invention is directed toward an organic light emitting diode (OLED) display capable of reducing or minimizing reflection of external light to improve display visibility of the OLED display. 
     An OLED display according to an exemplary embodiment of the present invention includes a pixel having a plurality of sub-pixels. Each of the plurality of sub-pixels includes a first sub-pixel having a first anode and a first organic emission layer, a second sub-pixel having a second anode and a second organic emission layer, and a third sub-pixel having a third anode and a third organic emission layer. The first, second, and third anodes satisfy the following condition: 
                 W   ⁢           ⁢   1     +     W   ⁢           ⁢   2       &lt;     2   ⁢           ⁢   W   ⁢           ⁢   3     &lt;       2   3     ⁢   P           
where W 1 , W 2 , and W 3  respectively denote a width of the first anode, a width of the second anode, and a width of the third anode measured along a direction traversing the first sub-pixel, the second sub-pixel, and the third sub-pixel and where P denotes a width of the pixel measured along the direction traversing the first sub-pixel, the second sub-pixel, and the third sub-pixel.
 
     The OLED display may further include a pixel defining layer disposed on edge portions of the first, second, and third anodes. The pixel defining layer may form a first opening exposing a portion of the first anode, a second opening exposing a portion of the second anode, and a third opening exposing a portion of the third anode. 
     The first anode, the second anode, the third anode, and the pixel defining layer may satisfy the following condition:
 
 w 1 +w 2+12 μm&lt; W 1+ W 2
 
where w 1  and w 2  respectively denote a width of the first opening and a width of the second opening measured along the direction traversing the first, second, and third sub-pixels.
 
     The widths of the first, second, and third anodes may be gradually decreased in the order of the third anode, the first anode, and the second anode. The widths of the first, second, and third openings may be gradually decreased in the order of the third opening, the first opening, and the second opening. 
     The widths of the first, second, and third openings may be respectively smaller than those of the first, second, and third anodes. The edge portions of the first, second, and third anodes that are overlapped with the pixel defining layer may have a constant width. The constant width of each of the edge portions of the first, second, and third anodes that are overlapped with the pixel defining layer may be greater than 3 μm. 
     The second anode may include an externally extended via hole region, and the second organic emission layer may be formed on the second anode, excluding the via hole region. 
     The OLED display may further include a black-colored planarization layer disposed in a lower portion of the first, second, and third anodes. In one embodiment, the first, second, and third anodes is between the black-colored planarization layer and the first, second, and third organic emission layers. 
     The first organic emission layer may be an organic emission layer for a red color. The second organic emission layer may be an organic emission layer for a green color. The third organic emission layer may be an organic emission layer for a blue color. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective schematic view of an OLED display according to a first exemplary embodiment of the present invention. 
         FIG. 2  is a cross-sectional schematic view of the OLED display according to the first exemplary embodiment of the present invention. 
         FIG. 3  is a circuit schematic diagram of a sub-pixel of a panel assembly of  FIG. 1 . 
         FIG. 4  and  FIG. 5  show partially exploded cross-sectional schematic views of the panel assembly of  FIG. 1 . 
         FIG. 6  is a top plan schematic view of an anode and a pixel defining layer in a configuration of the panel assembly of  FIG. 5 . 
         FIG. 7  is a top plan schematic view of the anode and an organic emission layer in the configuration of the panel assembly of  FIG. 5 . 
         FIG. 8  is a partially exploded cross-sectional schematic view of a panel assembly in a configuration of a light emitting device according to a second exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. 
       FIG. 1  and  FIG. 2  respectively show a perspective schematic view and a cross-sectional schematic view of an organic light emitting diode (OLED) display according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 1  and  FIG. 2 , an OLED display  100  according to the present exemplary embodiment includes a panel assembly  12 , a flexible circuit board  14 , and a printed circuit board  16 . The panel assembly  12  includes a display area A 10  and a pad area A 20 , and displays an image in the display area A 10 . The flexible circuit board  14  is fixed to the pad area A 20 , and the printed circuit board  16  is electrically connected with the panel assembly  12  through the flexible circuit board  14 . 
     The panel assembly  12  includes a first substrate  18  and a second substrate  22 . The second substrate  22  is smaller than the first substrate  18  and of which an edge portion is attached to the first substrate  18  by a sealant  20  ( FIG. 2 ). The display area A 10  is located in an area where the first and second substrates  18  and  22  are overlapped at a region defined by an interior side or sides of the sealant  20 , and the pad area A 20  is located on the first substrate  18  at an external side (i.e., a side facing away or oppositely away from the interior side) of the sealant  20 . 
     A plurality of sub-pixels are disposed in a matrix pattern in a display region on the display area A 10  of the first substrate  18 , and a scan driver and a data driver are located between the display area A 10  and the sealant  20  or at the external side of the sealant  20  for driving the sub-pixels. In the pad area A 20  of the first substrate  18 , pad electrodes for transmitting electrical signals to the scan and data drivers are located. 
     An integrated circuit chip  24  and the flexible circuit board  14  are mounted on the pad area A 20  of the first substrate  18 . A protective layer  26  is formed around the integrated circuit chip  24  and the flexible circuit board  14  to protect pad electrodes formed in the pad area A 20  by covering them. Surface Mounted Devices are mounted on the printed circuit board  16  for passing driving signals, and a connector  28  is installed on the printed circuit board  16  for transmitting an external signal thereto. 
     In a rear side of the panel assembly  12 , a bezel for increasing bending strength of the panel assembly  12  or a buffering tape for increasing impact resistance of the panel assembly  12  may be formed. The flexible circuit board  14  fixed to the pad area A 20  is bent toward the rear side of the panel assembly  12  to make the printed circuit board  16  face the rear side of the panel assembly  12  as shown in  FIG. 2 . 
       FIG. 3  is a circuit schematic diagram of a sub-pixel of the panel assembly of  FIG. 1 , and  FIG. 4  is a cross-sectional schematic view of the panel assembly of  FIG. 1 . 
     Referring to  FIG. 3  and  FIG. 4 , a sub-pixel of the panel assembly  12  is formed of an OLED L 1  and a driving circuit unit. The OLED L 1  includes an anode (hole injection electrode)  30 , an organic emission layer  32 , and a cathode (electron injection electrode)  34 . Here, the driving circuit unit includes at least two thin film transistors T 1  and T 2  and at least one storage capacitor C 1 . Here, in one embodiment, the at least two thin film transistors include a switching transistor T 1 , and a driving transistor T 2 . 
     The switching transistor T 1  is connected to a scan line SL 1  and a data line DL 1 , and transmits a data voltage from the data line DL 1  according to a switching voltage from the scan line SL 1  to the driving transistor T 2 . The storage capacitor C 1  is connected to the switching transistor T 1  and a power source line VDD, and stores a voltage that corresponds to a difference of a voltage transmitted from the switching transistor T 1  and a voltage supplied to the power source line VDD. 
     The driving transistor T 2  is connected to the power source line VDD and the storage capacitor C 1  and supplies an output current I OLED  that is proportional to a square of a difference between the voltage stored in the storage capacitor C 1  and a threshold voltage to the OLED L 1 , and the OLED L 1  emits light in accordance with the output current I OLED . As shown in  FIG. 4 , the driving transistor T 2  includes a source electrode  36 , a drain electrode  38 , and a gate electrode  40 , and the anode  30  of the OLED L 1  may be connected to the drain electrode  38  of the driving transistor T 2 . 
     A planarization layer  42  is disposed on the source electrode  36  and the drain electrode  38  of the driving transistor T 2 , and the anode  30  is formed on the planarization layer  42 . A via hole  421  is formed in the planarization layer  42 , and the anode  30  is connected to the drain electrode  38  through the via hole  421 . A pixel defining layer  44  is formed on the anode  30  and the planarization layer  42 . The pixel defining layer  44  forms an opening  46  in a portion that overlaps the anode  30  in each sub-pixel to expose the anode  30 . In addition, the organic emission layer  32  is filled in the opening of the pixel defining layer  44  and thus contacts the anode  30 . 
     The organic emission layer  32  may be formed to be the same size as or larger than the opening  46  of the pixel defining layer  44 . That is, if the size (e.g., a surface area) of the organic emission layer  32  is larger than that of the opening  46  of the pixel defining layer  44 , the organic emission layer  32  may be formed over a side wall of the fixed defining layer  44  at where the opening  46  is formed and an upper surface of the pixel defining layer  44 . In  FIG. 4 , as an example, the size of the organic emission layer  32  is substantially the same as that of the opening  46  of the pixel defining layer  44 . The configuration of the sub-pixel is not limited thereto, and may be suitably modified. 
     In the previously described OLED L 1 , the anode  30  is formed as a metal layer having a light reflection characteristic, and the cathode  34  is formed as a transparent conductive layer. Therefore, light emitted from the organic emission layer  32  is emitted out of the panel assembly  12  through the cathode  34  and the second substrate  22 , and the anode  30  reflects light emitted toward the first substrate  18  among the light emitted from the organic emission layer  32  to increase luminous efficiency. In this case, the anode  30  reflects not only light emitted from the organic emission layer  32 , but also light transmitted from the outside (e.g., sunlight) and into the panel assembly  12 . 
     According to the following configuration of the anode  30 , the OLED display  100  of an exemplary embodiment can provide luminous efficiency while reducing or minimizing reflection of external light due to the anode  30 . 
       FIG. 5  is a partially enlarged cross-sectional schematic view of the panel assembly of  FIG. 1 ,  FIG. 6  is a top plan schematic view of the anode and the pixel defining layer of  FIG. 5 , and  FIG. 7  is a top plan schematic view of the anode and the organic emission layer of  FIG. 5 . 
     Referring to  FIG. 5  to  FIG. 7 , a pixel of the OLED display  100  includes a first sub-pixel SP 1  including a first anode  301  and a first organic emission layer  321 , a second sub-pixel SP 2  including a second anode  302  and a second organic emission layer  322 , and a third sub-pixel SP 3  including a third anode  303  and a third organic emission layer  323 . The cathode  34  is formed over the first to third sub-pixels SP 1 , SP 2 , and SP 3  rather than being divided for each sub-pixel. 
     In the first exemplary embodiment, the OLED display  100  has the first sub-pixel SP 1  corresponding to a red color, the second sub-pixel SP 2  corresponding to a green color, and the third sub-pixel SP 3  corresponding to a blue color in order to realize a full-colored image. Accordingly, the first organic emission layer  321  is formed as an emission layer for the red color, the second organic emission layer  322  is formed as an emission layer for the green color, and the third organic emission layer  323  is formed as an emission layer for the blue color. However, each color realized by each of the plurality of organic emission layers is not limited thereto, and it may be appropriately modified according to the realization purpose or configuration of the OLED display and a choice of a person having ordinary skill in the art. 
     The pixel defining layer  44  includes a first opening  461  disposed in the first sub-pixel SP 1 , a second opening  462  disposed in the second sub-pixel SP 2 , and a third opening  463  disposed in the third sub-pixel SP 3 . The first to third anodes  301 ,  302 , and  303  respectively have different widths, and the first to third openings  461 ,  462 , and  463  also have different widths. 
     In  FIG. 6 , the widths of the first to third anodes  301 ,  302 , and  303  disposed along a direction (the x-axis in the drawing) traversing the first to third sub-pixels SP 1 , SP 2 , and SP 3  are respectively denoted as W 1 , W 2 , and W 3 . In addition, the widths of the first to third openings  461 ,  462 , and  462  disposed along a direction that is the same as that of the first to third anodes  301 ,  302 , and  303  (i.e., along the direction (the y-axis in the drawing) traversing the first to third sub-pixels SP 1 , SP 2 , and SP 3 ) are respectively denoted as w 1 , w 2 , and w 3 . 
     The widths of the first to third anodes  301 ,  302 , and  303  and the widths of the first to third openings  461 ,  462 , and  463  may be inversely proportional to luminance efficiency of the first to third organic emission layers  321 ,  322 , and  323  respectively disposed in the corresponding sub-pixels. That is, an organic emission layer having the lowest luminous efficiency among the first to third organic emission layers  321 ,  322 , and  323  may have a larger area in the pixel by increasing the width of the anode and the width of the opening than the widths of the corresponding anode and opening of organic emission layers having relatively higher luminous efficiency. 
     The widths of the first to third anodes  301 ,  302 , and  303  may be gradually decreased in the order of the third anode  303 , the first anode  301 , and the second anode  302 , and the widths of the openings  461 ,  462 , and  463  may also be gradually decreased in the order of the third opening  463 , the first opening  461 , and the second opening  462 . To put it another way, the width W 3  of the third anode  303  is larger than the width W 1  of the first anode  301 , and the width W 1  of the first anode  301  is larger than the width W 2  of the second anode  302 . In addition, the width w 3  of the third opening  463  is larger that the width w 1  of the first opening  461 , and the width w 1  of the first opening  461  is larger than the width w 2  of the second opening  462 . Therefore, the size of the first, second, and third organic emission layers  321 ,  322 , and  323  in one pixel may be gradually decreased in the order of the third organic emission layer  323 , the first organic emission layer  321 , and the second organic emission layer  322 . 
     In addition, the first to third anodes  301 ,  302 , and  303  are formed wider than the openings  461 ,  462 , and  463  of the pixel defining layer  44  of the corresponding sub-pixel so that the edge portions of the first to third anodes  301 ,  302 , and  303  partially overlap the pixel defining layer  44 . In this case, excluding a portion where the via hole  421  is formed, the edges of the openings  461 ,  462 , and  463  of the pixel defining layer  44  are separated by a set or predetermined distance from the edges of the anodes  301 ,  302 , and  303 . Therefore, excluding a portion where the via hole  421  is formed, the overlapped portions w 4  (refer to  FIG. 6 ) of the first to third anodes  301 ,  302 , and  303  and the pixel defining layer  44  may have a constant width. 
     The first, second, and third organic emission layers  321 ,  322 , and  323  may be formed to have the same or a greater width than the openings  461 ,  462 , and  463  of a pixel defining layer  44  of the corresponding sub-pixel. In  FIG. 7 , the first to third organic emission layers  321 ,  322 , and  323  are formed to be the same (or substantially the same) in size as the openings  461 ,  462 , and  463  of the pixel defining layer of the corresponding sub-pixel. 
     In the OLED display  100  according to the present exemplary embodiment, the first to third anodes  301 ,  302 , and  303  satisfy the following condition 1. 
     
       
         
           
             
               
                 
                   
                     
                       W 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                     + 
                     
                       W 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                   &lt; 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     W 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   &lt; 
                   
                     
                       2 
                       3 
                     
                     ⁢ 
                     P 
                   
                 
               
               
                 1 
               
             
           
         
       
     
     Here, P denotes the width of a pixel. 
     In condition 1, if the sum of W 1  and W 2  is more than two times W 3 , the third anode  303  and the third organic emission layer  323  does not have sufficient areas in the pixel so that luminance efficiency of the third organic emission layer  323  cannot be increased. In addition, if twice W 3  is more than two thirds the pixel width P in condition 1, the areas of the first and second anodes  301  and  302  and the areas of the first and second organic emission layers  321  and  322  are excessively decreased such that luminance efficiency of the first and second organic emission layers  321  and  322  may not be enough. 
     In addition, in the OLED display  100  according to the present exemplary embodiment, the first to third anodes  301 ,  302 , and  303  and the pixel defining layer  44  satisfy the following condition 2.
 
 w 1+ w 2+12 μm&lt; W 1+ W 2  2
 
     Condition 2 implies that the width w 4  of the overlapped portion of the first to third anodes  301 ,  302 , and  303  and the pixel defining layer  44  is greater than 3 μm. 
     If condition 2 is not satisfied, the pattern quality of the pixel defining layer  44  is deteriorated when the openings  461 ,  462 , and  463  of the pixel defining layer  44  are formed through the photolithography process so that the pixel defining layer  44  at the edge portions of the anodes  301 ,  302 , and  303  may be damaged. In this case, the anodes  301 ,  302 , and  303  may contact the cathode  34  at the edge portions of the anode  301 ,  302 , and  303  such that an electrical short circuit may occur, thereby causing damage to the organic emission layers  321 ,  322 , and  323 . 
     Since the OLED display  100  according to the present exemplary embodiment satisfies condition 2, the electrical short circuit between the anodes  301 ,  302 , and  303  and the cathode  34  and the damage to the organic emission layers  321 ,  322 , and  323  can be suppressed. 
     As described, in the OLED display  100  according to the present exemplary embodiment, the width of each of the anodes  301 ,  302 , and  303  is set to be different from each other for each sub-pixel according to efficiency of the organic emission layers  321 ,  322 , and  323  while reducing or minimizing the width w 4  of the overlapped portion of the anodes  301 ,  302 , and  303  with the pixel defining layer  44 . 
     Therefore, when light is emitted from the organic emission layers  321 ,  322 , and  323  of the corresponding sub-pixel, the first to third anodes  301 ,  302 , and  303  can reduce or minimize unexpected reflection of external light by reducing or minimizing the width w 4  of the portion overlapped with the pixel defining layer  44  while increasing efficiency of reflection toward the second substrate  22 . Accordingly, the OLED display  100  according to the present exemplary embodiment can improve visibility by decreasing reflection of external light (by decreasing or minimizing width w 4 ). 
     In the above-described sub-pixel configuration, the first to third organic emission layers  321 ,  322 , and  323  may be formed in such a configuration that one or more of these layers  321 ,  322 , and  323  do not overlap with their corresponding via holes  421  by being patterned so that the planarity of the organic emission layers  321 ,  322 , and  323  and the cathode  34  can be increased. 
     For example, the first and third anodes  301  to  303  that have relatively large widths and areas among the first to third anodes  301 ,  302 , and  303  may have a rectangular shape and the via hole  421  may be formed inside thereof. In addition, the first and third organic emission layers  321  and  323  may have a partially concave portion so as to maintain a constant distance with their corresponding via holes  421 . 
     On the other hand, the second anode  302  having the smallest width and area among the first to third anodes  301 ,  302 , and  303  may have an externally extended via hole region  50  (refer to  FIG. 7 ). The via hole  421  here is disposed in the via hole region  50 , and the second organic emission layer  322  may be formed in a rectangular shape on the second anode electrode  302 , excluding the via hole region  50 . Therefore, sufficient patterning space can be easily obtained in the second sub-pixel having the smallest width and area. To put it another way, in the embodiment as shown, each of the first and third anodes  301  and  303  has a polygonal shape region (e.g., a rectangular shape region) and a via hole region therein, and each of the first and third organic emission layers has the partially concave portion at the via hole region (or at a position corresponding to the via hole  421 ). By contrast, the second anode  302  has a polygonal shape region and the externally extended via hole region extending out of the polygonal shape region, and the second organic emission layer  322  is formed only on the polygonal shape region of the second anode  302 . 
       FIG. 8  is a partially enlarged cross-sectional view of a panel assembly in a configuration of a light emitting device according to a second exemplary embodiment of the present invention. 
     Referring to  FIG. 8 , an OLED display  101  according to the present exemplary embodiment is the same as the OLED display of the first exemplary embodiment, excluding that a planarization layer  42 ′ is formed of a black-colored material that absorbs light. Like reference numerals are used for like elements of the first exemplary embodiment. 
     The planarization layer  42 ′ may be formed as a black-colored acryl-based material. The black planarization layer  42 ′ absorbs external light transmitted to portions between anodes  301 ,  302 , and  303  to improve outdoor visibility. Therefore, the OLED display  101  of the second exemplary embodiment can further decrease reflection of external light compared to the OLED  100  of the first exemplary embodiment so that outdoor display visibility can be improved. 
     The following Table 1 shows reflectance of external light of an OLED display of a comparative example, the OLED display of the first exemplary embodiment, and the OLED display of the second exemplary embodiment, measured through computer simulation. The OLED display of the comparative example includes anodes each having the same width measured along the direction traversing the first to third sub-pixels, and is the same (or substantially the same) as the OLED display of the first exemplary embodiment, excluding the shape of the anodes. 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Reflectance of external light 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Comparative example 
                 40.8% 
               
               
                   
                 First exemplary embodiment 
                 37.2% 
               
               
                   
                 Second exemplary embodiment 
                 36.4% 
               
               
                   
                   
               
             
          
         
       
     
     Simulation measurement of the reflectance of external light is performed through a comparative experiment of reflectance of external light of a standard reflection plate and reflectance of external light of the experiment subject under a condition of using the same standard light source. The reflectance of external light in the table is a calculated value of a comparative value to the reflectance of external light of the standard reflection plate under an assumption that the reflectance of external light of the standard reflection plate is 100. 
     As shown in the table, the reflectance of the external light can be decreased more in the OLED display of the first and second exemplary embodiments where the widths of the anodes is varied than in the OLED display of the comparative example where the widths of the anodes are the same in first to third sub-pixels, and particularly, the reflectance of the external light is decreased by a maximum of 4.4% in the OLED display of the second exemplary embodiment where the black planarization layer is formed. 
     As such and in view of the foregoing, the OLED display according to an embodiment of the present invention can improve outdoor visibility by reducing or minimizing reflection of external light due to anodes. In addition, the planarization layer according to an embodiment of the present invention is formed as a black-colored material so that external light transmitted to portions between the anodes can be absorbed by the black planarization layer, thereby further improving the outdoor visibility. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Technology Category: 5