Abstract:
An organic light emitting diode display capable of reducing the shortening of image stacking lifetime caused by the residue of the barrier ribs produced during the forming of the barrier ribs is provided. The display includes: a substrate; a first pixel electrode formed on the substrate; barrier ribs formed on the substrate, and having an opening exposing the first pixel electrode; a second pixel electrode formed on the first pixel electrode; an organic light emitting member formed on the second pixel electrode; an organic light emitting member formed on the second pixel electrode; a common electrode formed on the organic light emitting member; and a thin film encapsulation member covering the common electrode. The width of the second pixel electrode is greater than the exposure width of the first pixel electrode exposed through the opening of the barrier ribs.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2011-0029518, filed in the Korean Intellectual Property Office on Mar. 31, 2011, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Aspects of embodiments of the present invention are directed toward an organic light emitting diode display and a manufacturing method thereof. 
     2. Description of Related Art 
     An organic light emitting diode display includes two electrodes and an organic light-emitting layer interposed between the two electrodes. Electrons injected from one of the electrodes and holes injected from the other electrode are combined in the organic light emitting layer to form excitons. The excitons release energy, thereby causing light to be emitted. 
     A pixel electrode, which is one of the electrodes of the organic light emitting diode display, may be formed of three layers, such as indium tin oxide (ITO)/Ag alloy/indium tin oxide (ITO). In addition, barrier ribs having openings exposing the greater part of the pixel electrode are formed thereon to surround the edge of the pixel electrode. 
     Since the barrier ribs are formed by photolithography, residue of the barrier ribs remains on the pixel electrode. As a result, current does not uniformly inject into the pixel electrode of the organic light emitting diode display, thereby leading to a reduction in image stacking lifetime. The image stacking lifetime refers to a time period during which images can keep the same picture quality. 
     Although plasma treatment may be performed to remove by-products of the barrier ribs, plasma treatment using oxygen may cause the Ag alloy constituting the pixel electrode to be deformed. In addition, it is difficult to completely remove the residue of the barrier ribs by plasma treatment using nitrogen. Moreover, the luminous efficiency decreases because the Ag alloy constituting the pixel electrode has a lower reflectivity than pure Ag. 
     The above information disclosed in this Background section is only for enhancement of understanding of the described embodiments. 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 
     Aspects of embodiment of the present invention are directed toward an organic light emitting diode display and, in particular, to an organic light emitting diode display that improves image stacking lifetime and improves luminous efficiency, and a manufacturing method thereof. 
     In an exemplary embodiment according to the present invention, an organic light emitting diode display is provided. The organic light emitting diode display includes a substrate, a first pixel electrode on the substrate, barrier ribs on the substrate, a second pixel electrode on the first pixel electrode, an organic light emitting member on the second pixel electrode, a common electrode on the organic light emitting member, and a thin film encapsulation member covering the common electrode. The barrier ribs have an opening exposing the first pixel electrode. A width of the second pixel electrode is greater than an exposure width of the first pixel electrode exposed through the opening of the barrier ribs. 
     The first pixel electrode may include a first translucent layer, a first metal layer on the first translucent layer, and a second translucent layer on the first metal layer. 
     The second pixel electrode may include a second metal layer, and a third translucent layer on the second metal layer. 
     The first metal layer may include silver (Ag), aluminum (Al), platinum (Pt), or a combination thereof. The second metal layer may include silver (Ag). 
     The first translucent layer, the second translucent layer, and the third translucent layer may include a conductive oxide. 
     The conductive oxide may include indium tin oxide (ITO), zinc oxide (ZnO), tin oxide (SnO 2 ), ruthenium oxide (RuOx), iridium oxide (IrOx), or a combination thereof. 
     A thickness of the second metal layer may be between 300 Å and 20000 Å. 
     A thickness of the third translucent layer may be between 20 Å and 300 Å. 
     The organic light emitting diode display may further include an adjacent second pixel electrode on an adjacent first pixel electrode. The adjacent second pixel electrode is adjacent to the second pixel electrode. A difference between the width of the second pixel electrode and the exposure width of the first pixel electrode may be less than a distance between the second pixel electrode and the adjacent second pixel electrode. 
     The second pixel electrode may overlap with edges of the barrier ribs. 
     According to another exemplary embodiment of the present invention, a manufacturing method of an organic light emitting diode display is provided. The method includes forming a first pixel electrode on a substrate, forming barrier ribs having an opening exposing the first pixel electrode on the substrate, forming a second pixel electrode on the first pixel electrode, forming an organic light emitting member on the second pixel electrode, forming a common electrode on the organic light emitting member, and forming a thin film encapsulation member covering the common electrode. A width of the second pixel electrode is greater than an exposure width of the first pixel electrode exposed through the opening of the barrier ribs. 
     The forming of the first pixel electrode may include sequentially laminating a first translucent layer, a first metal layer, and a second translucent layer. 
     The forming of the second pixel electrode may include sequentially laminating a second metal layer and a third translucent layer. 
     The method may further include forming an adjacent second pixel electrode on an adjacent first pixel electrode. The adjacent second pixel electrode is adjacent to the second pixel electrode. A difference between the width of the second pixel electrode and the exposure width of the first pixel electrode may be less than a distance between the second pixel electrode and the adjacent second pixel electrode. 
     The forming of the second pixel electrode may include overlapping the second pixel electrode with edges of the barrier ribs. 
     According to an exemplary embodiment, it is possible to prevent image stacking lifetime from being shortened due to residue of the barrier ribs produced during the formation of the barrier ribs by forming the second pixel electrode with a larger width than the exposure width of the first pixel electrode. Further, the luminous efficiency can be improved because the second metal layer of the second pixel electrode is formed from pure silver (Ag) having a high reflectivity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an equivalent circuit diagram of an organic light emitting diode display according to an exemplary embodiment. 
         FIG. 2  is a cross-sectional view of an organic light emitting diode display according to an exemplary embodiment. 
         FIG. 3  is a graph of current versus driving voltage, which shows a comparison between a comparative example and an exemplary embodiment. 
         FIG. 4  is a graph of image stacking lifetime, which shows a comparison between a comparative example and an exemplary embodiment. 
         FIG. 5  to  FIG. 7  are cross-sectional views sequentially illustrating a manufacturing method of an organic light emitting diode display according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     In order to clarify the description of the disclosed embodiments, elements extrinsic to their description are omitted. Further, like reference numerals refer to like elements throughout the application. In addition, the sizes and thicknesses of the elements in the drawings are not necessarily to scale, but rather for better understanding and ease of description. The present invention is not limited thereto. 
     An organic light emitting diode display according to an exemplary embodiment will now be described in detail with reference to  FIG. 1  and  FIG. 2 . 
       FIG. 1  is an equivalent circuit diagram of an organic light emitting diode display according to an exemplary embodiment.  FIG. 2  is a cross-sectional view of an organic light emitting diode display according to an exemplary embodiment. 
     As shown in  FIG. 1 , the organic light emitting diode display includes a plurality of signal lines  121 ,  171 , and  172  and a plurality of pixels PX connected thereto and arranged substantially in a matrix. The signal lines  121 ,  171 , and  172  include a plurality of gate lines  121  for transmitting gate signals (or scan signals), a plurality of data lines  171  for transmitting data signals, and a plurality of driving voltage lines  172  for transmitting driving voltages. The gate lines  121  extend substantially in a row direction and are substantially parallel to each other, while the data lines  171  and the driving voltage lines  172  extend substantially in a column direction and are substantially parallel to each other. 
     Each pixel PX includes a switching thin film transistor Qs, a driving thin film transistor Qd, a storage capacitor Cst, and an organic light emitting diode (OLED) LD. The switching transistor Qs has a control terminal coupled to one of the gate lines  121 , an input terminal coupled to one of the data lines  171 , and an output terminal coupled to the driving transistor Qd. The switching transistor Qs transmits the data signals applied to the data line  171  to the driving transistor Qd in response to the scan signal applied to the gate line  121 . 
     The driving transistor Qd has a control terminal coupled to the switching transistor Qs, an input terminal coupled to the driving signal line  172 , and an output terminal coupled to the organic light emitting diode LD. The driving transistor Qd drives an output current I LD  having a magnitude that varies according to the voltage between the control terminal and the input terminal. 
     The capacitor Cst is coupled between the control terminal and the input terminal of the driving transistor Qd. The capacitor Cst stores the data signal applied to the control terminal of the driving transistor Qd and sustains it after the switching transistor Qs is turned off. 
     The organic light emitting diode LD includes an anode connected to the output terminal of the driving transistor Qd and a cathode connected to a common voltage Vss. The organic light emitting diode LD displays images by emitting light with different intensities according to the output current I LD  of the driving transistor Qd. 
     In  FIG. 1 , the switching transistor Qs and the driving transistor Qd are n-channel field effect transistors FETs. However, in other embodiments, at least one of the switching transistor Qs or the driving transistor Qd may be a p-channel field effect transistor. In addition, in other embodiments, the interconnection between the transistors Qs and Qd, the storage capacitor Cst, and the organic light emitting diode LD may be different from what is shown. 
     The detailed structure of the organic light emitting diode display shown in  FIG. 1  will now be described with reference to  FIG. 2 . 
     As shown in  FIG. 2 , a thin film transistor layer  120  having thin film transistors including the switching thin film transistor Qs and the driving thin film transistor Qd is formed on a substrate  110 . A first pixel electrode  710  corresponding to an anode is formed on the thin film transistor layer  120 . The output terminal of the driving thin film transistor Qd is connected to the first pixel electrode  710 . The first pixel electrode  710  includes a first translucent layer  711 , a first metal layer  712 , and a second translucent layer  713 . 
     The first translucent layer  711  may be made of a transparent conductive oxide. The transparent conductive oxide may be made of, for example, indium tin oxide (ITO), zinc oxide (ZnO), tin oxide (SnO 2 ), ruthenium oxide (RuOx), iridium oxide (IrOx), or a combination thereof. 
     The first metal layer  712  may be formed from a metal having reflective properties. For example, the first metal layer  712  may be made of a low resistance metal such as silver (Ag), aluminum (Al), platinum (Pt), or a combination thereof, with a thickness of about 20 Å to 250 Å. 
     The second translucent layer  713  may be made of a transparent conductive oxide. The transparent conductive oxide may be made of, for example, indium tin oxide (ITO), zinc oxide (ZnO), tin oxide (SnO 2 ), ruthenium oxide (RuOx), iridium oxide (IrOx), or a combination thereof. 
     Barrier ribs  350  are formed on the thin film transistor layer  120 , surrounding the edges of the first pixel electrode  710 . The barrier ribs  350  define openings  350   a  exposing the greater part of the first pixel electrode  710  by surrounding the edges of the first pixel electrode  710 . 
     A second pixel electrode  720  is formed on the edges of the first pixel electrode  710  and the barrier ribs  350 . The second pixel electrode  720  overlaps with the edges of the barrier ribs  350 . 
     The second pixel electrode  720  includes a second metal layer  721  and a third translucent layer  722  which are sequentially laminated. The second metal layer  721  may be formed from silver (Ag) having a high reflectivity. Thus, higher reflectivity leads to higher luminous efficiency. 
     The second metal layer  721  may be formed with a thickness of 300 Å to 20000 Å. If the thickness of the second metal layer  721  is less than 300 Å, the reflectivity may lower below acceptable levels. If the thickness of the second metal layer  721  is greater than 20000 Å, the processing time and the manufacturing costs may increase above acceptable levels. The third translucent layer  722  may be made of a transparent conductive oxide. The transparent conductive oxide may be made of, for example, Indium tin oxide (ITO), zinc oxide (ZnO), tin oxide (SnO 2 ), ruthenium oxide (RuOx), iridium oxide (IrOx), or a combination thereof. 
     The third translucent layer  722  may be formed with a thickness of 20 Å to 300 Å. If the thickness of the third translucent layer  722  is less than 20 Å, it may be difficult to protect the second metal layer  721  from the outside. If the thickness of the third translucent layer  722  is greater than 300 Å, the third translucent layer  722  may absorb excessive outside light. 
     In  FIG. 2 , the width d 2  of the second pixel electrode  720  is greater than the exposure width d 1  of the first pixel electrode  710  exposed through the openings  350   a  of the barrier ribs  350 . By forming the second pixel electrode  720  with a larger width d 2  than the exposure width d 1  of the first pixel electrode  710 , it is possible to prevent image stacking lifetime from being shortened due to residue of the barrier ribs  350  produced during the formation of the barrier ribs  350 . Accordingly, current uniformly injects into the second pixel electrode  720  formed on the first pixel electrode  710 , and the second pixel electrode  720  having a larger width d 2  than the exposure width d 1  of the first pixel electrode  710  allows for a higher aperture ratio, resulting in a lower current density. Thus, the image stacking lifetime is not reduced. 
     Moreover, in  FIG. 2 , half of the difference between the width d 2  of the second pixel electrode  720  and the exposure width d 1  of the first pixel electrode  710  is less than half of the distance w between two adjacent second pixel electrodes  720  (that is, the difference between the width d 2  of the second pixel electrode  720  and the exposure width d 1  of the first pixel electrode  710  is less than the distance w between two adjacent second pixel electrodes  720 ). If half of the difference between the width d 2  of the second pixel electrode  720  and the exposure width d 1  of the first pixel electrode  710  is greater than or equal to half of the distance w between two adjacent second pixel electrodes  720 , the two adjacent second pixel electrodes  720  may short circuit with each other. 
     An organic light emitting member  760  is formed on the second pixel electrode  720 . The organic light emitting member  760  may include an organic light emitting layer (EML)  740  for emitting light and supplementary layers  730  and  750  for improving the luminous efficiency of the organic light emitting layer  740 . The additional layers  730  and  750  may include a hole supplementary layer  730  including a hole injection layer (HIL) and a hole transporting layer (HTL) and an electron supplementary layer  750  including an electron transport layer (ETL) and an electron injection layer (EIL). In this case, the hole supplementary layer  730 , the organic light emitting layer  740 , and the electron supplementary layer  750  are sequentially laminated on the second pixel electrode  720 . 
     A common electrode  800  corresponding to a cathode is formed on the organic light emitting member  760 . The common electrode  800  is made of an MgAg alloy. The common electrode  800  is formed on the entire surface of the substrate, and together with the first pixel electrode  710  and the second pixel electrode  720  causes the current to flow to the organic light emitting member  760 . 
     The first pixel electrode  710  and the second pixel electrode  720  may form a microcavity structure together with the common electrode  800 . The microcavity structure refers to a structure in which light is repeatedly reflected between a reflective layer and a translucent layer that are spaced apart by an optical length to amplify light of a particular wavelength through constructive interference. In the present exemplary embodiment, the first pixel electrode  710  and the second pixel electrode  720  may serve as reflective layers, and the common electrode  800  may serve as a translucent layer. The optical length of each pixel may be controlled by changing the distance between the first pixel electrode  710 , the second pixel electrode  720 , and the common electrode  800 . 
     The first pixel electrode  710  and the second pixel electrode  720  considerably modify the illumination characteristics of light emitted by the organic light emitting member  760 . Of the modified light, light near the wavelength corresponding to a resonance wavelength of the microcavity is strengthened by the common electrode  800  and emitted toward the common electrode  800 , and light of other wavelengths is suppressed. 
     A thin film encapsulation member  900  for covering and encapsulating the common electrode  800  is formed on the common electrode  800 . 
       FIG. 3  is a graph of current versus driving voltage, which shows a comparison between a comparative example and an exemplary embodiment.  FIG. 4  is a graph of image stacking lifetime, which shows a comparison between a comparative example and the exemplary embodiment. 
     As shown in  FIG. 3 , a driving current curve B changing with the driving voltage of the organic light emitting diode display according to an exemplary embodiment, which includes the first pixel electrode  710  and the second pixel electrode  720 , does not decline substantially. On the contrary, curve B is almost similar to a curve A of the driving current I changing with the driving voltage Vg of the organic light emitting diode display of a comparative example, which only includes the first pixel electrode  710 . Accordingly, the exemplary embodiment has comparable current versus driving voltage performance when compared to that of a comparative embodiment. 
     Moreover, as shown in  FIG. 4 , the image stacking (IS) lifetime is about 25 hours on average when lifetime curves C 1 , C 2 , and C 3  for the organic light emitting diode display of a comparative example reach 97%. However, in an exemplary embodiment, the image stacking (IS) lifetime increases to 104 hours and 200 hours, respectively, when lifetime curves D 1  and D 2  for the organic light emitting diode display according to the exemplary embodiment reach 97%. 
     A manufacturing method of an organic light emitting diode display according to an exemplary embodiment will be described in detail hereinafter with reference to  FIGS. 5 to 7 . 
       FIG. 5  to  FIG. 7  are cross-sectional views sequentially illustrating a manufacturing method of an organic light emitting diode display according to an exemplary embodiment. 
     As shown in  FIG. 5 , a thin film transistor layer  120  including a switching thin film transistor Qs and a driving thin film transistor Qd is first formed on a substrate  110 . A first pixel electrode  710  is formed on the thin film transistor layer  120 . The first pixel electrode  710  is formed by sequentially laminating a first translucent layer  711 , a first metal layer  712 , and a second translucent layer  713 . The first translucent layer  711  may be made of a transparent conductive oxide, the first metal layer  712  may be formed from a metal having reflective properties, and the second translucent layer  713  may be made of a transparent conductive oxide. In addition, barrier ribs  350  having openings  350   a  exposing the greater part of the first pixel electrode  710  are formed on the thin film transistor layer  120  to surround the edges (or periphery) of the first pixel electrode  710 . 
     Next, as shown in  FIG. 6 , the second pixel electrode  720  is formed on the edges of the first pixel electrode  710  and the barrier ribs  350 . The second pixel electrode  720  is formed by sequentially laminating the second metal layer  721  and the third translucent layer  722 . In this case, the second metal layer  721  may be formed from pure silver (Ag) having a high reflectivity in order to improve luminous efficiency, and the third translucent layer  722  may be made of a transparent conductive oxide. The second metal layer  721  may be formed by thermal evaporation, and the third translucent layer  722  may be formed by sputtering. 
     In addition, the width d 2  of the second pixel electrode  720  is greater than the exposure width d 1  of the first pixel electrode  710  exposed through the openings  350   a  of the barrier ribs  350 . Accordingly, it is possible to prevent image stacking lifetime from being shortened due to residue of the barrier ribs  350  produced during the formation of the barrier ribs  350 . 
     Moreover, half of the difference between the width d 2  of the second pixel electrode  720  and the exposure width d 1  of the first pixel electrode  710  is less than half of the distance w between two adjacent second pixel electrodes  720  (that is, the difference between the width d 2  of the second pixel electrode  720  and the exposure width d 1  of the first pixel electrode  710  is less than the distance w between two adjacent second pixel electrodes  720 ). If half of the difference between the width d 2  of the second pixel electrode  720  and the exposure width d 1  of the first pixel electrode  710  is greater than or equal to half of the distance w between two adjacent second pixel electrodes  720 , the two adjacent second pixel electrodes  720  may short circuit with each other. 
     Next, as shown in  FIG. 7 , an organic light emitting member  760  is formed on the second pixel electrode  720 . The organic light emitting member  760  is formed by sequentially laminating a hole supplementary layer  730 , an organic light emitting layer  740 , and an electron supplementary layer  750 . Continuing with  FIG. 2 , a common electrode  800  is formed on the organic light emitting member  760 , and a thin film encapsulation member  900  is formed on the common electrode  800 . 
     While this disclosure 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, and equivalents thereof. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 Description of selected symbols 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 110: substrate 
                 120: thin film transistor layer 
               
               
                 350: barrier ribs 
                 350a: openings (in barrier ribs) 
               
               
                 710: first pixel electrode 
                 720: second pixel electrode 
               
               
                 760: organic light emitting member 
                 800: common electrode 
               
               
                 900: thin film encapsulation member