Abstract:
An organic light-emitting device includes a substrate; a first electrode layer and a second electrode layer on the substrate, in parallel to the substrate, and facing each other; an emission layer between the first electrode layer and the second electrode layer, where the emission layer includes a first emission region, a second emission region, and a third emission region, where the emission layer includes a first common emission layer in the first emission region, the second emission region, and the third emission region; a second emission layer in the second emission region between the first common emission layer and the second electrode layer; and a third emission layer in the third emission region between the first common emission layer and the second electrode layer, and where the first common emission layer includes a first host, a first dopant, and a p-type dopant.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2012-0001552, filed on Jan. 5, 2012 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an organic light-emitting device and a method of manufacturing the same, and more particularly, to an organic light-emitting device including a common emission layer and a method of manufacturing the organic light-emitting device. 
     2. Description of the Related Art 
     Organic light-emitting devices (OLEDs) are self-light emitting devices that emit light when a voltage is applied thereto. OLEDs have high luminance, good contrast, wide viewing angles, high response speeds, and low driving voltages. OLEDs can also render multi-colored images. 
     An organic light-emitting device has a structure including an organic emission layer disposed between an anode and a cathode. When a voltage is applied across the electrodes, holes are injected from the anode and electrons are injected from the cathode into the organic emission layer. The injected holes and electrons undergo electron exchanges in adjacent molecules in the organic emission layer, thereby migrating to opposite electrodes. An electron-hole pair recombined in a certain molecule forms a molecular exciton in a high-energy excited state. The molecular excitons emit unique wavelengths of light upon returning to a low-energy ground state. 
     An organic light-emitting device includes a plurality of pixels and each pixel includes a red light-emitting region, a green light-emitting region, and a blue light-emitting region. In this case, a patterning process may be simplified by forming the blue light-emitting layer as a common layer. By forming an emission layer to have host and dopant structures, external quantum efficiency is increased and the emission wavelength is controlled. 
     SUMMARY 
     When a blue emission layer as a common layer includes a host and a dopant, hole transporting efficiency is reduced. Thus, embodiments of the present invention include an organic light-emitting device including a common emission layer, thereby increasing hole transporting efficiency. 
     According to an aspect of the present invention, an organic light-emitting device includes a substrate; a first electrode layer and a second electrode layer formed on the substrate and parallel to the substrate such that the first and second electrode layers face each other; an emission layer between the first electrode layer and the second electrode layer, wherein the emission layer includes a first emission region, a second emission region, and a third emission region, wherein the emission layer includes a first common emission layer in the first emission region, the second emission region, and the third emission region; a second emission layer in the second emission region between the first common emission layer and the second electrode layer; and a third emission layer in the third emission region between the first common emission layer and the second electrode layer, and wherein the first common emission layer includes a first host, a first dopant, and a p-type dopant. 
     A difference between the lowest unoccupied molecular orbital (LUMO) energy level of the p-type dopant and the highest occupied molecular orbital (HOMO) energy level of the first host may range from about 0.2 to about 1.0 eV. 
     The p-type dopant may have a LUMO energy level that is equal to or less than 5.5 eV. 
     The p-type dopant may include 2,3,5,6-tetrafluoro-7,7′,8,8′-tetracyanoquinodimethane (F4TCNQ), 7,7′,8,8′-tetracyanoquinodimethane (TCNQ), a perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA), 1,3,2-dioxaborine derivative, FeCl 3 , FeF 3 , or SbCl 5 . 
     An amount of the p-type dopant may be about 0.5 to about 3 wt % based on a total weight of the first common emission layer. 
     The first emission region may be a blue emission region, the second emission region may be a red emission region, and the third emission region may be a green emission region. The first common emission layer may be a blue common emission layer. 
     The first common emission layer may include a red common emission layer or a green common emission layer. 
     The organic light-emitting device may further include a hole injection layer or a hole transfer layer between the first electrode layer and the emission layer. The organic light-emitting device may further include an electron injection layer or an electron transfer layer between the second electrode layer and the emission layer. 
     The organic light-emitting device may further include a resonance layer between the emission layer and the second electrode layer. The resonance layer may have a thickness that varies in the first emission region, the second emission region, and the third emission region according to resonance distances of the first emission region, the second emission region, and the third emission region. The resonance layer may include an electron transfer layer. 
     The first electrode layer may be an anode, and the second electrode layer may be a cathode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by reference to the following detailed description when considered in conjunction with the attached drawings in which: 
         FIG. 1  is a schematic cross-sectional view of an organic light-emitting device according to an embodiment of the present invention; 
         FIG. 2  is a diagram of the energy levels of a host, an emission dopant, and a p-type dopant in a blue common emission layer according to an embodiment of the present invention; and 
         FIG. 3  is a graph comparing hole current density vs. voltage of the organic light-emitting devices of Examples 1 and 2 and Comparative Examples 1 through 3. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described in detail by explaining embodiments of the invention with reference to the attached drawings. 
       FIG. 1  is a schematic cross-sectional view of an organic light-emitting device  100  according to an embodiment of the present invention. The organic light-emitting device  100  includes sub-pixel regions including a red light-emitting region R, a green light-emitting region G, and a blue light-emitting region B. Reference numeral  112  indicates an insulating layer that defines a sub-pixel region. 
     The organic light-emitting device  100  includes a substrate  101 , an anode  111 , a hole injection layer  121 , a hole transfer layer  122 , an emission layer  125 , an electron transfer layer  127 , an electron injection layer  128 , and a cathode  131 . 
     The substrate  101  may be any substrate that is used in existing organic light emitting devices. In some embodiments, the substrate may be a glass substrate or a transparent plastic substrate with good mechanical strength, thermal stability, transparency, surface smoothness, ease of handling, and water resistance. The substrate  101  may be formed of an opaque material such as silicon, stainless steel, or the like. 
     The anode  111  may be formed of a material with a relatively high work function. The anode  111  may be formed of, but is not limited to, a transparent conductive oxide, for example, indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), Al-doped zinc oxide (AZO), indium oxide (In 2 O 3 ) or tin oxide (SnO 2 ). The anode  111  may be formed by vapor deposition or sputtering. 
     The hole injection layer  121  may be formed of, for example, a phthalocyanine compound, such as copper phthalocyanine, N,N′-diphenyl-N,N′-bis-[4-(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,4′-diamine (DNTPD), 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA), 4,4′4″-tris(N,N-diphenylamino)triphenylamine (TDATA), 4,4′,4″-tris{N,-(2-naphthyl)-N-phenylamino}-triphenylamine (2T-NATA), poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), polyaniline/dodecylbenzenesulfonic acid (Pani/DBSA), polyaniline/Camphor sulfonicacid (Pani/CSA), polyaniline/poly(4-styrenesulfonate) (PANI/PSS), or the like, but is not limited thereto. 
     
       
                 
         
             
             
         
      
       
                 
         
             
             
         
       
     
     The hole injection layer  121  may be formed by any of a variety of methods, for example, vacuum deposition, spin coating, casting, Langmuir-Blodgett (LB) deposition, or the like. 
     When the hole injection layer  121  is formed by vacuum deposition, the deposition conditions may vary according to the compound used to form the hole injection layer  121 , and the structure and thermal characteristics of the desired hole injection layer  121 . For example, the deposition conditions may include a deposition temperature of about 100° C. to about 500° C., a vacuum pressure of about 10 −8  Torr to about 10 −3  Torr, and a deposition rate of about 0.01 Å/sec to about 100 Å/sec. 
     When the hole injection layer  121  is formed by spin coating, the coating conditions may vary according to the compound used to form the hole injection layer  121 , and the structure and thermal characteristics of the desired hole injection layer  121 . For example, the deposition conditions may include a coating speed of about 2,000 rpm to about 5,000 rpm, and a thermal treatment temperature of about 80° C. to about 200° C. at which the solvent remaining after coating may be removed. 
     The hole injection layer  121  may have a thickness of about 100 Å to about 10000 Å, and in some embodiments, may have a thickness of about 100 Å to about 1000 Å. When the thickness of the hole injection layer  121  is within these ranges, the hole injection layer  121  may achieve satisfactory hole injecting ability without substantially decreasing driving voltage. 
     The hole transfer layer  122  may include, for example, a carbazole derivative, such as N-phenylcarbazole or polyvinyl carbazole, N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1-biphenyl]-4,4′-diamine (TPD), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB), 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), or the like, but is not limited thereto. 
     
       
                 
         
             
             
         
      
     
     The hole transfer layer  122  may be formed by any of a variety of methods, for example, vacuum deposition, spin coating, casting, Langmuir-Blodgett (LB) deposition, or the like. When the hole transfer layer  122  is formed by vacuum deposition or spin coating, the deposition conditions or the coating conditions may vary according to the compound used to form the hole transfer layer  122 , but in some embodiments may be substantially the same as those used to form the hole injection layer  121 . 
     The hole transport layer  122  may have a thickness of about 50 Å to about 1000 Å, and in some embodiments, the hole transport layer  122  may have a thickness of about 100 Å to about 800 Å. When the thickness of the hole transport layer  122  is within these ranges, the hole transport layer  122  may have satisfactory hole transporting ability without substantially decreasing driving voltage. 
     Optionally, the hole injection layer  121  and the hole transfer layer  122  may be replaced by a functional layer having both hole injection and hole transfer capabilities. 
     The emission layer  125  may include a red emission layer  125 R, a green emission layer  125 G, and a blue common emission layer  125 B. According to the present embodiment, the blue common emission layer  125 B is formed as a common layer on the hole transfer layer  122  across the red light-emitting region R, the green light-emitting region G, and the blue light-emitting region B. The red emission layer  125 R is patterned in the red emission region. The green emission layer  125 G is patterned in the green emission region. The red emission layer  125 R and the green emission layer  125 G are disposed on the blue common emission layer  125 B. 
     The blue common emission layer  125 B may include a blue host, a blue dopant, and a p-type dopant. 
     Non-limiting examples of the blue host include Alq 3 , 4,4′-N,N′-dicarbazole-biphenyl (CBP), poly(n-vinylcarbazole) (PVK), 9,10-di(naphthalene-2-yl)anthracene (ADN), TCTA, 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI), 3-tert-butyl-9,10-di(naphth-2-yl) anthracene (TBADN), E3, distyrylarylene (DSA), or the like. 
     
       
                 
         
             
             
         
      
       
                 
         
             
             
         
       
     
     Non-limiting examples of the blue dopant include F 2 Irpic, (F 2 ppy) 2 Ir(tmd), Ir(dfppz) 3 , ter-fluorene, 4,4′-bis(4-diphenylaminostyryl)biphenyl (DPAVBi), and 2,5,8,11-tetra-t-butylperylene (TBPe), which are represented by the compounds below: 
     
       
                 
         
             
             
         
      
       
                 
         
             
             
         
       
       
                 
         
             
             
         
       
     
     The p-type dopant may be a material that generates holes in a blue host so as to increase the hole transporting efficiency of the blue host. The p-type dopant that generates holes in the blue host may be a p-type dopant having a lowest unoccupied molecular orbital (LUMO) energy level that is similar to the highest occupied molecular orbital (HOMO) energy level of the blue host. For example, a difference between the LUMO energy level of the p-type dopant and the HOMO energy level of the blue host may range from about 0.2 to about 1.0 eV. The LUMO energy level of the p-type dopant is similar to the HOMO energy level of the blue dopant or is equal to or lower than 5.5 eV (which is lower than the HOMO energy level of the blue dopant), and thus may have good electron accepting capacity. 
     Non-limiting examples of the p-type dopant include organic compounds such as 2,3,5,6-tetrafluoro-7,7′,8,8′-tetracyanoquinodimethane (F4TCNQ), 7,7′,8,8′-tetracyanoquinodimethane (TCNQ), perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA), or 1,3,2-dioxaborine derivatives, and inorganic compounds such as iodine, FeCl 3 , FeF 3 , SbCl 5 , metal chloride, or metal fluoride. 
     
       
                 
         
             
             
         
      
     
       FIG. 2  is a diagram of the energy levels of a host, an emission dopant, and a p-type dopant included in the blue common emission layer  125 B according to an embodiment of the present invention. Referring to  FIG. 2 , the HOMO energy level of the host is lower than the HOMO energy level of the emission dopant (first dopant), and thus holes with a HOMO energy level of the host may be easily transferred to the HOMO energy level of the emission dopant. Thus, the emission dopant may serve as a trap-site for the holes of the host. However, due to the existence of the p-type dopant (second dopant) having a HOMO energy level that is much lower than the HOMO energy level of the host, and having a LUMO energy level that is similar to the HOMO energy level of the host, electrons with a HOMO energy level of the host may be transferred to a LUMO energy level of the p-type dopant. Since electrons with a HOMO energy level of the host can be transferred to a LUMO energy level of the p-type dopant, holes may be generated in a HOMO energy level of the host, thereby increasing the hole transporting efficiency of the host. 
     According to the present embodiment, since the p-type dopant increases the hole transporting efficiency of the blue host, holes that are injected from the anode  111  and pass through the hole injection layer  121  and the hole transfer layer  122  may reach the red emission layer  125 R or the green emission layer  125 G such that the speed of the holes may not be reduced in the blue common emission layer  125 B. 
     The red emission layer  125 R may include a red host and a red dopant. Like the blue host, non-limiting examples of the red host include Alq 3 , CBP, PVK, ADN, TCTA, TPBI, TBADN, E3, DSA, or the like. Non-limiting examples of the red dopant include PtOEP, Ir(piq) 3 , Btp 2 Ir(acac), Ir(piq) 2 (acac), Ir(2-phq) 2 (acac), Ir(2-phq) 3 , Ir(flq) 2 (acac), Ir(fliq) 2 (acac), DCM, DCJTB, and the like, which are represented by the compounds below. 
     
       
                 
         
             
             
         
      
       
                 
         
             
             
         
       
       
                 
         
             
             
         
       
     
     The green emission layer  125 G may include a green host and a green dopant. Like the blue host, non-limiting examples of the green host include Alq 3 , CBP, PVK, ADN, TCTA, TPBI, TBADN, E3, DSA, and the like. Non-limiting examples of the green dopant include Ir(ppy) 3 (tris(2-phenylpyridine)iridium, Ir(ppy) 2 (acac) (Bis(2-phenylpyridine)(Acetylacetonato)iridium(III), Ir(mppy) 3  tris(2-(4-tolyl)phenylpiridine)iridium, 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano[6,7,8-ij]-quinolizin-11-one (C545T), or the like. 
     
       
                 
         
             
             
         
      
     
     The red emission layer  125 R, the green emission layer  125 G, and the blue common emission layer  125 B may be formed by vacuum deposition, spin coating, casting, Langmuir-Blodgett (LB) deposition, or the like. When the red emission layer  125 R, the green emission layer  125 G, or the blue common emission layer  125 B are formed by vacuum deposition or spin coating, the deposition or coating conditions may vary according to the compound used to form the red emission layer  125 R, the green emission layer  125 G, or the blue common emission layer  125 B, but in some embodiments may be substantially the same as those used to form the hole injection layer. In order to form a layer including a host and a dopant, a code position method may be used. 
     The amount of the dopant in each of the red emission layer  125 R and the green emission layer  125 G, and the amount of the blue dopant in the blue common emission layer  125 B may range, but is not limited to, from about 0.01 wt % to about 15 wt % based on the total weight of each emission layer. The amount of the p-type dopant in the blue common emission layer  125 B may range from about 0.5 wt % to about 3 wt % based on the total weight of the blue common emission layer  125 B. When included within this range, the p-type dopant may effectively generate holes in the blue host, thereby increasing the hole transporting efficiency of the blue host. 
     The red emission layer  125 R, the green emission layer  125 G, and the blue common emission layer  125 B may each have a thickness of about 100 Å to about 1,000 Å. 
     The electron transfer layer  127  may be a layer that transfers holes injected from the cathode  131  to each emission layer and may include a known material, for example, Alq 3 , 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) 4,7-Diphenyl-1,10-phenanthroline (Bphen), 3-(4-Biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ), 4-(Naphthalen-1-yl)-3,5-diphenyl-4H-1,2,4-triazole (NTAZ), 2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD), BAlq (refer to the following formula), beryllium bis(benzoquinolin-10-olate) (Bebq 2 ), 9,10-di(naphthalene-2-yl)anthracene (AND), Compound 101 below, Compound 102 below, or the like, but is not limited thereto. 
     
       
                 
         
             
             
         
      
       
                 
         
             
             
         
       
     
     The electron transfer layer  127  may be formed by any of a variety of methods, for example, vacuum deposition, spin coating, casting, or the like. When the electron transfer layer  127  is formed by vacuum deposition or spin coating, the deposition or coating conditions may vary according to the compound used to form the electron transfer layer  127 , but in some embodiments may be substantially the same as those used to form the hole injection layer  121 . 
     The electron transfer layer  127  may have a thickness of about 100 Å to about 1,000 Å, for example, about 150 Å to about 500 Å. When the thickness of the electron transfer layer  127  is within these ranges, the electron transfer layer  127  may have satisfactory hole transporting ability without substantially increasing driving voltage. 
     The electron transfer layer  127  may include an electron transporting organic compound and a metal-containing material. The metal-containing material may include a lithium (Li) complex. Non-limiting examples of the Li complex include lithium quinolate (LiQ), Formula 103 below, or the like. 
     
       
                 
         
             
             
         
      
     
     The electron injection layer  128  for facilitating injection of electrons from the cathode  131  may be stacked on the electron transfer layer  127 . The electron injection layer  128  may be formed of a known material for forming an electron injection layer, such as LiF, NaCl, CsF, Li 2 O, BaO, or the like, but is not limited thereto. The deposition conditions of the electron injection layer  128  may vary according to the compound used to form the electron injection layer  128 , but in some embodiments, may be substantially the same as those used to form the hole injection layer  121 . 
     The electron injection layer  128  may have a thickness of about 1 Å to about 100 Å, for example, about 3 Å to about 90 Å. When the thickness of the electron injection layer  128  is within these ranges, the electron injection layer  128  may have satisfactory electron injection ability without substantially increasing driving voltage. 
     Optionally, the electron transfer layer  127  and the electron injection layer  128  may be replaced by a functional layer having both electron transfer and electron injection capabilities. 
     The cathode  131  may be formed of a metal, an alloy, an electrically conductive compound, or a mixture thereof, each of which has a low work function. The cathode  131  may be formed as a transmissive electrode by forming a thin film of, for example, lithium (Li), magnesium (Mg), aluminum (Al), aluminium-lithium (Al—Li), calcium (Ca), magnesium-indium (Mg—In), magnesium-silver (Mg—Ag), or the like. Various changes may be made, for example, in order to obtain a top emission-type organic light-emitting device, the transmissive electrode is formed of ITO, IZO, or the like. 
     Optionally, the organic light-emitting device  100  may have a resonance structure. In order to match resonance distances of the red, green, and blue emission regions R, G, and B, a resonance layer (not shown) may be used. The resonance layer may be formed by forming the electron transfer layer  127  or the hole transfer layer  122  to have different thicknesses for the respective red, green, and blue emission regions R, G, and B. Alternatively, the resonance layer may be formed as a separate layer between the anode  111  and the cathode  131 . 
     Thus far, embodiments in which the blue emission layer is used as a common emission layer have been described. Alternatively, the red emission layer or the green emission layer may be used as a common emission layer. That is, the red emission layer may be formed as a common emission layer on the hole transfer layer, and the green emission layer and blue emission layer may be patterned on the red common emission layer. Alternatively, the green emission layer may be formed as a common emission layer on the hole transfer layer, and the red emission layer and the blue emission layer may be patterned on the green common emission layer. 
     When the red emission layer is used as a common emission layer, the p-type dopant used in the blue emission layer in the above-described embodiment may be used as a second dopant in the red emission layer. Similarly, when the green emission layer is used as a common emission layer, the p-type dopant may be used as a second dopant in the green emission layer. Since the p-type dopant is used in a common emission layer, holes may be generated in the host, thereby increasing the hole transporting efficiency of the host. 
     An example of an organic light emitting device according to an embodiment of the present invention has been described with reference to the organic light-emitting device  100  shown in  FIG. 1 . However, if necessary, various changes may be made, and for example, if necessary, any one of the red emission layer  125 R and the green emission layer  125 G may be formed, a hole blocking layer may be formed between the emission layer  125  and the electron transfer layer  127 , an electron blocking layer may be formed between the emission layer  125  and the hole transfer layer  122 , or layers may be formed in order from the cathode on a substrate. 
     The following Examples are presented for illustrative purposes only, and no not limit the scope of the present invention. 
     Example 1 
     As an anode, a 15 Ω/cm 2  (500 Å) Corning ITO glass substrate was cut to a size of 50 mm×50 mm×0.7 mm, washed with ultrasonic waves in isopropyl alcohol and pure water for 5 minutes each, and then cleaned with UV and ozone for 30 minutes. MTDATA was vacuum-deposited on the ITO glass substrate to form a hole injection layer with a thickness of 100 Å. Then, an emission layer was formed on the hole injection layer to have a thickness of 400 Å using 94 wt % of TBADN as a blue host, 5 wt % of DPAVBI as a blue dopant, and 1 wt % of F4TCNQ as a p-type dopant. Al was vacuum-deposited on the emission layer to form a cathode with a thickness of 1,200 Å, thereby completing the manufacture of an organic light-emitting device. 
     Example 2 
     An organic light-emitting device was manufactured in the same manner as in Example 1, except that the weight ratio of TBADN:DPAVBI:F4TCNQ in the emission layer was 93:5:2 instead of 94:5:1. 
     Comparative Example 1 
     An organic light-emitting device was manufactured in the same manner as in Example 1, except that the weight ratio of TBADN:DPAVBI:F4TCNQ in the emission layer was 95:5:0 instead of 94:5:1. 
     Comparative Example 2 
     An organic light-emitting device was manufactured in the same manner as in Example 1, except that the weight ratio of TBADN:DPAVBI:F4TCNQ in the emission layer was 92:5:3 instead of 94:5:1. 
     Comparative Example 3 
     An organic light-emitting device was manufactured in the same manner as in Example 1, except that the weight ratio of TBADN:DPAVBI:F4TCNQ in the emission layer was 90:5:5 instead of 94:5:1. 
     Table 1 summarizes the structures of the organic light-emitting devices of Examples 1 and 2, and Comparative Examples 1 through 3. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Example 1 
                 ITO(500 Å)/MTDATA(100 Å)/TBADN(94%): 
               
               
                   
                   
                 DPAVBI(5%):F4TCNQ(1%)(400 Å)/AI(1200 Å) 
               
               
                   
                 Example 2 
                 ITO(500 Å)/MTDATA(100 Å)/TBADN(93%): 
               
               
                   
                   
                 DPAVBI(5%):F4TCNQ(2%)(400 Å)/AI(1200 Å) 
               
               
                   
                 Comparative 
                 ITO(500 Å)/MTDATA(100 Å)/TBADN(95%): 
               
               
                   
                 Example 1 
                 DPAVBI(5%):F4TCNQ(0%)(400 Å)/AI(1200 Å) 
               
               
                   
                 Comparative 
                 ITO(500 Å)/MTDATA(100 Å)/TBADN(92%): 
               
               
                   
                 Example 2 
                 DPAVBI(5%):F4TCNQ(3%)(400 Å)/AI(1200 Å) 
               
               
                   
                 Comparative 
                 ITO(500 Å)/MTDATA(100 Å)/TBADN(90%): 
               
               
                   
                 Example 3 
                 DPAVBI(5%):F4TCNQ(5%)(400 Å)/AI(1200 Å) 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 3  is a graph of hole current density vs. voltage of the organic light-emitting devices of Examples 1 and 2, and Comparative Examples 1 through 3. The hole current density was measured using a PR650 spectrometer. 
     Referring to  FIG. 3 , Example 1 had the highest current density, followed (in descending order) by Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3. That is, when F4TCNQ is doped at 2 wt % (Examples 1 and 2), the current density is increased compared to when F4TCNQ is not doped (Comparative Example 1). When F4TCNQ is doped at 3 wt % (Comparative Example 2), the current density is reduced compared to when F4TCNQ is not doped (Comparative Example 1). When F4TCNQ is doped at 5 wt % (Comparative Example 3), the current density is seriously reduced. It is deemed that when the amount of F4TCNQ is increased, electrons with the HOMO energy level of DPAVBI as an emission dopant are more likely to be transferred to the LUMO energy level of F4TCNQ such that the HOMO energy level of F4TCNQ serves as a new trap-site for holes of TBADN. 
     The hole transporting efficiency of a common emission layer may be increased by using a p-type dopant with a LUMO energy level that is close to a HOMO energy level of the host. 
     While the present invention has been illustrated and described with reference to certain exemplary embodiments, it will be understood by those of ordinary skill in the art that various changes can be made to the described embodiments without departing from the spirit and scope of the present invention as defined by the following claims.