Abstract:
An organic light emitting display device includes a first electrode formed on a substrate and being a reflective electrode, a second electrode facing the first electrode and being a semi-transparent electrode, and red, green and blue emission layers formed between the first and second electrodes, wherein a maximum electroluminescent peak of the redemission layer and a maximum photoluminescence peak of a host included in the red emission layer satisfy Equation 1 below:
 
RED ELλmax −RH PLλmax ≧120 nm  &lt;Equation 1&gt;
 
wherein RED ELλmax  is a maximum electroluminescent peak of the red emission layer, and RH PLλmax  is a maximum photoluminescence peak of a red host included in the red emission layer.

Description:
This application claims the benefit of Korean Patent Application No. 10-2012-0148822, filed on Dec. 18, 2012, which is hereby incorporated by reference as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an organic light emitting display device with enhanced color purity according to viewing angle. 
     Discussion of the Related Art 
     Organic light emitting display devices, which one form of flat panel display device, are self-emissive devices and have faster response speed, higher luminous efficacy, higher luminance, and wider viewing angle than other flat panel display devices. An organic light emitting display device includes an anode, a cathode facing the anode, and an organic emission layer (EML) disposed therebetween. Holes injected from the anode and electrons injected from the cathode are recombined in the organic EML, forming excitons, which are electron-hole pairs and the excitons return to the ground state, thus releasing energy, whereby light is emitted. 
     As a method of enhancing optical efficiency by effectively extracting light emitted from an organic EML, microcavity is used. In a top emission structure, microcavity is based on a principle in which light is repeatedly reflected by a reflective electrode (e.g., an anode) and a semi-transparent electrode (e.g., a cathode) spaced a set distance apart (i.e., an optical path length), and strong interference effects between these light beams occurs and thus light having a particular wavelength is amplified and light having other wavelengths excluding the particular wavelength is eliminated. 
     In a microcavity structure, however, an optical path at the front and an optical path at the side differ and thus a wavelength of light that causes resonance is changed. Accordingly, the optical path at the side with a greater viewing angle than that of the front is relatively long and resonance light emitted is shifted towards short wavelengths. That is, as illustrated in  FIG. 1A , a maximum electroluminescent peak of a red light emitting cell is shifted towards short wavelengths by about 75 to 85 nm at a viewing angle of 60° as compared to a viewing angle of 0°, as illustrated in  FIG. 1B , a maximum electroluminescent peak of a green light emitting cell is shifted towards short wavelengths by about 36 to 50 nm at a viewing angle of 60° as compared to a viewing angle of 0°, and as illustrated in  FIG. 1C , a maximum electroluminescent peak of a blue light emitting cell is shifted towards short wavelengths by about 10 to 14 nm at a viewing angle of 60° as compared to a viewing angle of 0°. This is because, when a non-uniform doping region is formed in an EML, light is emitted by an undoped host. Accordingly, a color shift phenomenon according to viewing angle occurs and thus a conventional organic light emitting display device has reduced reliability in color purity. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an organic light emitting display device that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an organic light emitting display device with enhanced color purity according to viewing angle. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an organic light emitting display device includes a first electrode formed on a substrate and being a reflective electrode, a second electrode facing the first electrode and being a semi-transparent electrode, and red, green and blue emission layers formed between the first and second electrodes, wherein a maximum electroluminescent peak of each of the red, green and blue emission layers and a maximum photoluminescence peak of a host included in each emission layer satisfy Equations 1 to 3 below.
 
RED ELλmax −RH PLλmax ≧120 nm,  &lt;Equation 1&gt;
 
wherein RED ELλmax  is a maximum electroluminescent peak of the red emission layer, and RH PLλmax  is a maximum photoluminescence peak of a red host included in the red emission layer.
 
Green ELλmax −GH PLλmax ≧20 nm  &lt;Equation 2&gt;
 
wherein GREEN ELλmax  is a maximum electroluminescent peak of the green emission layer, and GH PLλmax  is a maximum photoluminescence peak of a green host included in the green emission layer.
 
BLUE ELλmax −BH PLλmax ≧20 nm,  &lt;Equation 3&gt;
 
wherein BLUE ELλmax  is a maximum electroluminescent peak of the blue emission layer, and BH PLλmax  is a maximum photoluminescence peak of a blue host included in the blue emission layer.
 
     The second electrode may have a transmittance of 30 to 60% at a wavelength of 430 nm, a transmittance of 20 to 50% at a wavelength of 550 nm, and a transmittance of 15 to 40% at a wavelength of 650 nm, and the second electrode may have a sheet resistance of 1Ω/□ to 15Ω/□ and a work function of 3.7 to 4.7 eV. 
     The maximum photoluminescence peak of a red host may be between 450 and 485 nm, the maximum photoluminescence peak of a green host may be between 450 and 530 nm, and the maximum photoluminescence peak of a blue host may be between 400 and 435 nm. 
     The red host may be formed of BAlq 3  series such as a compound represented by Formula 1 below, a material represented by Formula 2 below, or Be complexes such as BeBq 2  represented by Formula 3 below. 
     
       
                 
         
             
             
         
      
     
     The green host may be formed of a material represented by Formula 4 below, BCP series such as a compound represented by Formula 5 below, CBP series such as a compound represented by Formula 6 below, CDBP series, or a material represented by Formula 7 below. 
     
       
                 
         
             
             
         
      
     
     The blue host may be formed of a material represented by Formula 8 below or an anthracene derivative. 
     
       
                 
         
             
             
         
      
     
     The organic light emitting display device may further include a front sealing layer formed on the second electrode, the front sealing layer including organic layers and inorganic layers alternately formed several times. 
     The second electrode may have a single layer structure or a multilayer structure and has a total thickness of 100 to 400 Å, and each of the layers of the second electrode may be formed of a metal, an inorganic material, a mixture of metals, a mixture of a metal and an inorganic material, or a mixture thereof. When each layer is formed of the mixture of a metal and an inorganic material, a mix ratio of the metal to the inorganic material may be between 10:1 and 1:10. When each layer is formed of the mixture of metals, a mix ratio of the metals may be between 10:1 and 1:10. 
     The metal may be Ag, Mg, Yb, Li, or Ca, and the inorganic material may be LiO 2 , CaO, LiF, or MgF 2 . 
     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
         FIG. 1A  is a graph showing luminescence spectrums according to viewing angle of a red light emitting cell of a conventional organic light emitting display device; 
         FIG. 1B  is a graph showing luminescence spectrums according to viewing angle of a green light emitting cell of a conventional organic light emitting display device; 
         FIG. 1C  is a graph showing luminescence spectrums according to viewing angle of a blue light emitting cell of a conventional organic light emitting display device; 
         FIG. 2  is a sectional view of an organic light emitting display device according to the present invention; 
         FIGS. 3A to 3C  are graphs showing luminescence spectrums according to a maximum photoluminescence peak of a red host of a red emission layer of each of the organic light emitting display devices according to comparative examples and example when a second electrode of each thereof is formed as a transparent electrode and has a thickness of 500 Å; 
         FIG. 4  is an enlarged view of a region of a wavelength of 450 to 575 nm in luminescence spectrums of each of the organic light emitting display devices of comparative examples and example illustrated in  FIGS. 3A to 3C ; and 
         FIGS. 5A and 5B  are images and graphs showing luminescence spectrums according to viewing angle of organic light emitting display devices according to Comparative Example and Example when a second electrode of each organic light emitting display device is a semi-transparent electrode. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings and the following examples. 
       FIG. 2  is a sectional view of an organic light emitting display device according to the present invention. 
     As illustrated in  FIG. 2 , the organic light emitting display device includes red, green and blue light emitting cells formed on a substrate  101 . 
     Each of the red, green and blue light emitting cells includes a first electrode  102 , a hole injection layer (HIL)  112 , a first hole transport layer (HTL)  114 , an emission layer (EML)  110 , an electron transport layer (ETL)  116 , a second electrode  104 , and a front sealing layer  120  that are sequentially formed on the substrate  101 . In addition, each of the red and green light emitting cells further includes a second HTL  118  formed between the first HTL  114  and the EML  110 . 
     Any one of the first and second electrodes  102  and  104  is formed as a semi-transparent electrode and the other thereof is formed as a reflective electrode. When the first electrode  102  is a semi-transparent electrode and the second electrode  104  is a reflective electrode, the organic light emitting display device is of a bottom emission type emitting light downward. When the second electrode  104  is a semi-transparent electrode and the first electrode  102  is a reflective electrode, the organic light emitting display device is of a top emission type emitting light upward. In the present invention, a case in which the first electrode  102  as an anode is formed as a reflective electrode and the second electrode  104  as a cathode is a semi-transparent electrode will be described by way of example. 
     The first electrode  102  has a multilayer structure including a metal layer formed of aluminum (Al) or an Al alloy (e.g., AlNd) and a transparent layer formed of indium tin oxide (ITO), indium zinc oxide (IZO), or the like and serves as a reflective electrode. 
     The second electrode  104  is formed of a single layer or multiple layers, and each layer constituting the second electrode  104  is formed of a metal, an inorganic material, a mixture of metals, a mixture of a metal and an inorganic material, or a mixture thereof. In this regard, when each layer is formed of a metal and an inorganic material, a mix ratio of metal to inorganic material is between 10:1 and 1:10. When each layer is formed of a mixture of metals, a mix ratio of the metals is between 10:1 and 1:10. The metal constituting the second electrode  104  may be Ag, Mg, Yb, Li, or Ca and the inorganic material thereof may be Li 2 O, CaO, LiF, or MgF 2 , which assist movement of electrons and thus enable many electrons to be supplied to the EML  110 . 
     The second electrode  104  has a thickness of 100 to 400 Å and a sheet resistance of 15Ω or less, and the second electrode  104  has a work function of 3.7 to 4.7 eV that is lower than that of the first electrode  102 . 
     In addition, the second electrode  104  has a light transmittance of 30 to 60% at a wavelength of 430 nm, 20 to 50% at a wavelength of 550 nm, and 15 to 40% at a wavelength of 650 nm and thus serves as a semi-transparent electrode. 
     The HIL  112  supplies holes from the first electrode  102  to the first and second HTLs  114  and  118 . The first and second HTLs  114  and  118  supply holes from the HIL  112  to the EML  110  of each light emitting cell. The second HTL  118  is not formed in the blue light emitting cell, and the thickness of the second HTL  118  is greater in the red light emitting cell than in the green light emitting cell. An efficiency of each light emitting cell in a vertical direction may be optimized through constructive interference of light emitted by adjusting the thickness of the second HTL  118  of each light emitting cell. The ETL  116  supplies electrons from the second electrode  104  to the EML  110  of each light emitting cell. 
     In each of the red (R), green (G) and blue (B) EMLs  110 , the holes supplied via the first and second HTLs  114  and  118  and the electrons supplied via the ETL  116  are recombined, thereby generating light. In this regard, the R EML  110  has the greatest thickness, the B EML  110  has the smallest thickness, and the G EML  110  has a thickness within the range of the thicknesses of the R and B EMLs  110 . The efficiency of each light emitting cell in a vertical direction may be optimized through constructive interference of light emitted by adjusting the thickness of the EML  110  of each light emitting cell. 
     The front sealing layer  120  prevents permeation of external moisture or oxygen and thus enhances reliability. For this operation, the front sealing layer  120  has a structure in which organic layers and inorganic layers are alternately formed several times. The inorganic layers are formed of at least one of aluminum oxide (Al x O x ), silicon oxide (SiO x ), SiN x , SiON, and LiF so as to primarily prevent permeation of external moisture or oxygen. The organic layers secondarily prevent permeation of external moisture or oxygen. In addition, the organic layers alleviate stress between the layers according to bending of the organic light emitting display device and enhance planarization performance. The organic layers are formed of an acryl-based resin, an epoxy-based resion, or a polymer material such as polyimide or polyethylene. 
     In the present invention, the R, G and B EMLs  110  are formed so as to satisfy conditions as shown in Equations 1 to 3 below.
 
RED ELλmax −RH PLλmax ≧120 nm  [Equation 1]
 
Green ELλmax −GH PLλmax ≧20 nm  [Equation 2]
 
BLUE ELλmax −BH PLλmax ≧20 nm  [Equation 3]
 
     As shown in Equation 1, a difference between a maximum electroluminescent peak (REDELλmax) of the R EML  110  and a maximum photoluminescence peak (RHPLλmax) of a red host included in the R EML  110  is 120 nm or greater. As shown in Equation 2, a difference between a maximum electroluminescent peak (GREENELλmax) of the G EML  110  and a maximum photoluminescence peak (GHPLλmax) of a green host included in the G EML  110  is 20 nm or greater. As shown in Equation 3, a difference between a maximum electroluminescent peak (BLUEELλmax) of the B EML  110  and a maximum photoluminescence peak (BHPLλmax) of a blue host included in the B EML  110  is 20 nm or greater. A case in which each of the R, G and B EMLs  110  includes a single host has been described by way of example using Equations 1 to 3. In another embodiment, each of the R, G and B EMLs  110  includes at least two hosts and, in this case, one of the at least two hosts is formed so as to satisfy the conditions of Equations 1 to 3. In addition, in Equations 1 to 3, the electroluminescent peaks refer to maximum values of light emitted when a voltage is applied to the manufactured organic electroluminescent device, and the photoluminescence peaks refer to maximum values of light that represent a characteristic color of each EML. 
     In Equation 1, the red host has a maximum photoluminescence peak (RHPLλmax) of 450 to 485 nm and is formed of BAlq 3  series such as a compound represented by Formula 1 below, a material represented by Formula 2 below, or Be complexes such as BeBq 2  represented by Formula 3 below. 
     
       
                 
         
             
             
         
      
     
     In Equation 2, the green host of the G EML  110  has a maximum photoluminescence peak (RHPLλmax) of 450 to 530 nm and is formed of a material represented by Formula 4 below, BCP series such as a compound represented by Formula 5 below, CBP series such as a compound represented by Formula 6 below, CDBP series, or a material represented by Formula 7 below. 
     
       
                 
         
             
             
         
      
     
     In Equation 3, the blue host of the B EML  110  has a maximum photoluminescence peak (RHPLλmax) of about 400 to 435 nm and is formed of a material represented by Formula 8 below or an anthracene derivative. 
     
       
                 
         
             
             
         
      
     
       FIGS. 3A to 3C  are graphs showing luminescence spectrums according to a maximum photoluminescence peak of a red host of a red EML of each of the organic light emitting display devices according to comparative examples and example when a second electrode of each thereof is formed as a transparent electrode and has a thickness of 500 Å. In this regard, formation of the second electrode to a thickness of 500 Å is because an out-coupling curve is present around 525 nm, which is the same wavelength as that at a viewing angle of 60 degrees when the second electrode is a semi-transparent electrode and thus the host of the corresponding color emits light around 525 nm. 
     In Comparative Example 1 illustrated in  FIG. 3A , a red host included in a red EML is formed of a carbazole derivative having a maximum photoluminescence peak of 490 nm and, in Comparative Example 2 illustrated in  FIG. 3B , a red host included in a red EML is formed of a carbazole derivative having a maximum photoluminescence peak of 510 nm. In this case, an overlapping area between the photoluminescence peak of the red host and a maximum luminescence peak (OC curve) of a red light emitting cell is wide. As the overlapping area increases, amplification between the two peaks increases and thus luminescent strength increases, leading to the greatest change in color according to viewing angle. 
     By contrast, in example illustrated in  FIG. 3C , when a red host is formed of a Be complex derivative having a maximum photoluminescence peak of 468 nm, an overlapping area between the photoluminescence peak of the red host and a maximum luminescence peak (OC curve) of a red light emitting cell is small. As the overlapping area decreases, amplification between the two peaks decreases, which leads to the smallest change in color according to viewing angle. 
     In addition, in Comparative Example 1 in which the red EML includes the red host formed of a carbazole derivative having a maximum photoluminescence peak of 510 nm and a red dopant, a shoulder peak as illustrated in  FIG. 4  is present at a wavelength of 500 to 550 nm and thus color purity is reduced. By contrast, in example in which an EML includes a red host formed of Be complex series having a maximum photoluminescence peak of 468 nm and a red dopant, a shoulder peak is not present at a wavelength of 500 to 550 nm and thus color purity reliability is enhanced. 
       FIGS. 5A and 5B  are images and graphs showing luminescence spectrums according to viewing angles of organic light emitting display devices according to Comparative Example and Example when a second electrode of each organic light emitting display device is a semi-transparent electrode. 
     In Comparative Example illustrated in  FIG. 5A , when a red host included in a red EML is formed of a carbazole derivative having a maximum photoluminescence peak of 510 nm, a maximum electroluminescent peak (REDELλmax) of a red EML is 620 nm and thus a difference between the maximum electroluminescent peak (REDELλmax) of a red EML and a maximum photoluminescence peak (RHPLλmax) of the red host included in the red EML is 110 nm. In this case, an overlapping area between the electroluminescent peak of the red EML and the photoluminescence peak of the red host at a viewing angle of 0 degrees is relatively wide, and an overlapping area between the electroluminescent peak of the red EML and the photoluminescence peak of the red host at a viewing angle of 60 degrees is relatively wide. As the overlapping area between the electroluminescent peak of the red EML and the photoluminescence peak of the red host increases, amplification of the electroluminescent peak of the red EML and the photoluminescence peak of the red host increases and thus changes in color occur. Accordingly, there is a difference between colors at viewing angles of 0° and 60°. 
     By contrast, in Example illustrated in  FIG. 5B , when a red host included in a red EML is formed of Be complex series having a maximum photoluminescence peak of 468 nm, a maximum electroluminescent peak (REDELλmax) of the red EML  110  is 620 nm and thus a difference between the maximum electroluminescent peak (REDELλmax) of the red EML  110  and a maximum photoluminescence peak (RHPLλmax) of the red host included in the red EML  110  is 152 nm, which is greater than 120 nm. In this case, an overlapping area between the electroluminescent peak of the red EML  110  and the photoluminescence peak of the red host at a viewing angle of 0° is smaller than that in Comparative Example, and an overlapping area between the electroluminescent peak of the red EML  110  and the photoluminescence peak of the red host at a viewing angle of 60° is smaller than that in Comparative Example. As the overlapping area between the electroluminescent peak of the red EML  110  and the photoluminescence peak of the red host decreases, amplification of the electroluminescent peak of the red EML  110  and the photoluminescence peak of the red host is relatively small and thus changes in color hardly occur. Accordingly, a difference between colors at viewing angles of 0° and 60° is small and thus color purity according to viewing angle is kept maintained and color purity reliability is enhanced. 
     As is apparent from the foregoing description, in an organic light emitting display device according to the present invention, a difference between a maximum electroluminescent peak (REDELλmax) of a red EML and a maximum photoluminescence peak (RHPLλmax) of a red host included in the red EML is 120 nm or greater, a difference between a maximum electroluminescent peak (GREENELλmax) of a green EML and a maximum photoluminescence peak (GHPLλmax) of a green host included in the green EML is 20 nm or greater, and a difference between a maximum electroluminescent peak (BLUEELλmax) of a blue EML and a maximum photoluminescence peak (BHPLλmax) of a blue host included in the blue EML is 20 nm or greater. Accordingly, the organic light emitting display device may undergo no reduction in color purity according to viewing angle and suppress host emission, whereby color reliability according to viewing angle may be enhanced. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.