Abstract:
Organic light emitting devices including an electron transport-emission layer, and methods of preparing the same are included. The electron transport-emission layer may be an electron transport-red emission layer, an electron transport-green emission layer or an electron transport-blue emission layer. The methods produce high yields of the organic light emitting devices and are less expensive than conventional methods.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2007-0053414, filed on May 31, 2007 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to organic light emitting devices and methods of preparing the same. More particularly, the present invention relates to an organic light emitting device including an electron transporting-red emission layer for emitting red light, an electron transporting-green emission layer for emitting green light, or an electron transporting-blue emission layer for emitting blue light. 
         [0004]    2. Description of the Related Art 
         [0005]    Organic light emitting devices (“OLEDs”) are self emissive devices, have wide viewing angles, excellent contrast characteristics, and quick response times. Due to these advantages, OLEDs have drawn much attention. In addition, OLEDs have excellent driving voltage characteristics and excellent response speed characteristics, and can produce various colors. Thus, a lot of research is being carried out into OLEDs. 
         [0006]    An OLED generally includes an anode/emission layer/cathode stacked structure. The OLED may also include a hole injection layer, a hole transport layer, and/or an electron injection layer stacked between the anode and the emission layer or between the emission layer and the cathode. Therefore, OLEDs can have an anode/hole transport layer/emission layer/cathode structure, or an anode/hole transport layer/emission layer/electron injection layer/cathode structure. However, conventional OLEDs are manufactured by complex process and materials are lost during deposition, making conventional OLEDs unsuitable for mass production. 
       SUMMARY OF THE INVENTION 
       [0007]    In one embodiment of the present invention, an OLED includes an electron transporting-red emission layer for emitting red light, an electron transporting-green emission layer for emitting green light, or an electron transporting-blue emission layer for emitting blue light. 
         [0008]    In another embodiment of the present invention, a method of preparing the OLED is provided. 
         [0009]    According to one embodiment of the present invention, an OLED includes a substrate, a first electrode, a second electrode, and an organic layer between the first electrode and the second electrode. The organic layer includes: a) a green emission layer patterned in a green subpixel, a blue emission layer patterned in a blue subpixel, and an electron transporting-red emission layer which covers the red, green, and blue subpixels; b) a red emission layer patterned in a red subpixel, a blue emission layer patterned in a blue subpixel, and an electron transporting-green emission layer which covers the red, green, and blue subpixels; or c) a red emission layer patterned in a red subpixel, a green emission layer patterned in a green subpixel, and an electron transporting-blue emission layer which covers the red, green, and blue subpixels. 
         [0010]    The organic light emitting device further includes at least one layer selected from hole injection layers, hole transport layers, hole blocking layers, and electron injection layers. 
         [0011]    When the organic layer further includes a hole injection layer, the thickness of the hole injection layer of the red subpixel is greater than or equal to the thickness of the hole injection layer of the green subpixel, which is greater than or equal to the thickness of the hole injection layer of the blue subpixel. 
         [0012]    According to another embodiment of the present invention, a method of preparing an organic light emitting device includes forming a first electrode on a substrate, forming an organic layer on the first electrode, and forming a second electrode on the organic layer. In one embodiment, the step of forming an organic layer includes forming a green emission layer in a green subpixel, forming a blue emission layer in a blue subpixel, and then forming an electron transporting-red emission layer over the red, green, and blue subpixels. The green emission layer is formed using a green emission layer deposition mask and the blue emission layer is formed using a blue emission layer deposition mask. The electron transporting-red emission layer is formed using an open mask. 
         [0013]    In an alternative embodiment, a red emission layer is formed in a red subpixel, a blue emission layer is formed in a blue subpixel, and then an electron transporting-green emission layer is formed over the red, green, and blue subpixels. The red emission layer is formed using a red emission layer deposition mask and the blue emission layer is formed using a blue emission layer deposition mask. The electron transporting-green emission layer is formed using an open mask. 
         [0014]    In another embodiment, a red emission layer is formed in a red subpixel, a green emission layer is formed in a green subpixel, and then an electron transporting-blue emission layer is formed over the red, green, and blue subpixels. The red emission layer is formed using a red emission layer deposition mask and the green emission layer is formed using a green emission layer deposition mask. The electron transporting-blue emission layer is formed using an open mask. 
         [0015]    The step of forming an organic layer may further include at least one process selected from forming a hole injection layer, forming a hole transport layer, forming a hole blocking layer, and forming an electron injection layer. 
         [0016]    The methods of the present invention can produce high yields of OLEDs at low costs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    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: 
           [0018]      FIG. 1  is a schematic cross-sectional view of an organic light emitting device according to one embodiment of the present invention; 
           [0019]      FIG. 2  is a schematic cross-sectional view of an organic light emitting device according to another embodiment of the present invention; 
           [0020]      FIG. 3  is a schematic cross-sectional view of an organic light emitting device according to yet another embodiment of the present invention; 
           [0021]      FIG. 4  is a schematic cross-sectional view of an organic light emitting device according to still another embodiment of the present invention; 
           [0022]      FIG. 5  is a schematic cross-sectional view of an organic light emitting device according to still yet another embodiment of the present invention; 
           [0023]      FIG. 6  is a schematic cross-sectional view of an organic light emitting device according to yet another embodiment of the present invention; 
           [0024]      FIG. 7  is a graph comparing the luminance efficiency of the OLEDs prepared according to Example 1 and Comparative Example 1; 
           [0025]      FIG. 8  is a graph comparing the lifetime characteristics of the OLEDs prepared according to Example 1 and Comparative Example 1; 
           [0026]      FIG. 9  is a graph comparing the UV spectra of the OLEDs prepared according to Example 1 and Comparative Example 1; 
           [0027]      FIG. 10  is a graph comparing the luminance efficiency of the OLEDs prepared according to Example 2 and Comparative Example 2; and 
           [0028]      FIG. 11  is a graph comparing the UV spectra of the OLEDs prepared according to Example 2 and Comparative Example 2. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]      FIG. 1  is a schematic cross-sectional view of an OLED  10  according to one embodiment of the present invention. Referring to  FIG. 1 , the OLED  10  includes a substrate  11 , first electrodes  12 , a hole injection layer  16 , and a hole transport layer  18 . A green emission layer  22 G is patterned on the portion of the hole transport layer  18  corresponding to a green subpixel (G), and a blue emission layer  22 B is patterned on the portion of the hole transport layer  18  corresponding to a blue subpixel (B). An electron transporting-red emission layer  24 R, an electron injection layer  26 , and a second electrode  28  are then formed on the resulting structure. 
         [0030]    The substrate  11  illustrated in  FIG. 1  can be any substrate used in OLEDs. Nonlimiting examples of suitable substrates include glass substrates and transparent plastic substrates, each of which have good mechanical strength, thermal stability, transparency, and surface planarization, and each of which are waterproof and easily treatable. Although not illustrated in  FIG. 1 , a planarization layer, an insulating layer, or the like can also be formed on the substrate  11 . 
         [0031]    The first electrodes  12  are formed on the substrate  11 . The first electrodes  12  can be patterned to correspond to red, green or blue subpixels, as illustrated in  FIG. 1 , but are not limited thereto. The first electrodes  12  can be anodes or cathodes. The first electrodes  12  can be transparent electrodes, semi-transparent electrodes, or reflective electrodes. Nonlimiting examples of suitable materials for the first electrodes  12  include ITO, IZO, SnO 2 , ZnO, Al, Ag, Mg, or the like. In addition, the first electrodes  12  can have a layered structure including two or more layers formed of at least two kinds of materials. 
         [0032]    The spaces between the first electrodes  12  may be filled with an insulating layer  14 . The insulating layer  14  can be formed of any known insulating material. Nonlimiting examples of suitable materials for the insulating layer include inorganic materials, such as SiO 2 , SiN x , and the like, and organic materials, such as polyimide-based resins, acryl-based resins, and the like. 
         [0033]    The hole injection layer  16  is formed on the first electrodes  12 . The hole injection layer  16  can cover the red, green, and blue subpixels, as illustrated in  FIG. 1 . The hole injection layer  16  may have various structures. For example, unlike the embodiment illustrated in  FIG. 1 , the hole injection layer  16  can be patterned such that each of the red, green and blue subpixels is covered by an individual hole injection layer. 
         [0034]    The hole injection layer  16  can be formed using various methods, such as vacuum deposition, spin coating, casting, Langmuir-Blodgett (LB), and the like. 
         [0035]    When the hole injection layer  16  is formed by vacuum deposition, the deposition conditions may differ according to the compound used to form the hole injection layer, the structure and thermal characteristics of the hole injection layer to be formed, and the like. However, in general, the deposition conditions may include a deposition temperature ranging from about 100 to about 500° C., a vacuum pressure ranging from about 10 −8  to about 10 −3  torr, and a deposition speed ranging from about 0.01 to about 100 Å/sec. 
         [0036]    When the hole injection layer  16  is formed by spin coating, the spin coating conditions may differ according to the compound used to form the hole injection layer, the structure and thermal characteristics of the hole injection layer to be formed, and the like. However, in general, the spin coating conditions may include a coating speed ranging from about 2000 rpm to about 5000 rpm, and a heat treatment temperature (at which the used solvent is removed after the spin coating) ranging from about 80° C. to about 200° C. 
         [0037]    The hole injection layer  16  can be formed of any known hole injecting material. Nonlimiting examples of suitable materials for the hole injection layer  16  include phthalocyanine compounds (such as copper phthalocyanine), starburst-type amine derivatives (such as TCTA, m-MTDATA, or m-MTDAPB), and conductive polymers (such as polyaniline/dodecylbenzenesulfonic acid (Pani/DBSA), poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), polyaniline/camphor sulfonic acid (Pani/CSA), and (polyaniline)/poly(4-styrene-sulfonate) (PANI/PSS). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0038]    The hole injection layer  16  may have a thickness ranging from about 10 nm to about 200 nm. In one embodiment, for example, the hole injection layer  16  has a thickness ranging from about 60 nm to about 150 nm. The thickness of the hole injection layer  16  may be determined in consideration of process time and manufacturing costs. 
         [0039]    The hole transport layer  18  is disposed on the hole injection layer  16 . The hole transport layer  18  can be formed using various methods, such as vacuum deposition, spin coating, casting, LB, and the like. When the hole transport layer  18  is formed by deposition or spin coating, the deposition or coating conditions may differ according to the compound used to form the hole transport layer. However, in general, the deposition or coating conditions may be similar to the deposition or coating conditions used to form the hole injection layer  16 . 
         [0040]    Nonlimiting examples of suitable materials for the hole transport layer  18  include 1,3,5-tricarbazolylbenzene, 4,4′-biscarbazolylbiphenyl, polyvinylcarbazole, m-biscarbazolylbenzene, 4,4′-biscarbazolyl-2,2′-dimethylbiphenyl, 4,4′,4″-tri(N-carbazolyl)triphenylamine, 1,3,5-tri(2-carbazolylphenyl)benzene, 1,3,5-tris(2-carbazolyl-5-methoxyphenyl)benzene, bis(4-carbazolylphenyl)silane, N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1-biphenyl]-4,4′diamine (TPD), N,N′-di(naphthalene-1-yl)-N,N′-diphenyl benzidine (α-NPD), N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB), IDE320 (produced by Idemitz Co.), (poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB), (poly(9,9-dioctylfluorene-co-bis-(4-butylphenyl-bis-N,N-phenyl-1,4-phenylenediamine) (PFB), or the like. 
         [0041]    The thickness of the hole transport layer  18  may range from about 10 nm to about 200 nm. In one embodiment, for example, the thickness ranges from about 20 nm to about 150 nm. The thickness of the hole transport layer  18  may be determined in consideration of process time and manufacturing costs. 
         [0042]    As illustrated in  FIG. 1 , the hole transport layer can cover the red, green, and blue subpixels. The hole transport layer  18  may have various structures. For example, unlike that illustrated in  FIG. 1 , the hole transport layer  18  can be patterned such that each of the red, green and blue subpixels is covered by an individual hole transport layer. 
         [0043]    The green emission layer  22 G is patterned on the portion of the hole transport layer  18  corresponding to the green subpixel, and the blue emission layer  22 B is patterned on the portion of the hole transport layer  18  corresponding to the blue subpixel. 
         [0044]    The green emission layer  22 G and the blue emission layer  22 B can be formed of various known emissive materials, such as known host and dopant materials. Specifically, the dopant can be a known fluorescent dopant or a known phosphorescent dopant. Nonlimiting examples of suitable host materials include Alq 3 , 4,4′-N,N′-dicarbazole-biphenyl (CBP), poly n-vinylcarbazole (PVK), distyrylarylene (DSA), and the like. 
         [0045]    Nonlimiting examples of suitable green dopants include Ir(ppy) 3  where ppy denotes phenylpyridine, Ir(ppy) 2 (acac), Ir(mpyp) 3 , C545T, and the like. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0046]    Nonlimiting examples of suitable blue dopants include F 2 Irpic, (F 2 ppy) 2 Ir(tmd), Ir(dfppz) 3 , ter-fluorene, and the like. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0047]    In one embodiment, the green host can be a compound represented by Formula 1, below. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    In Formula 1, R 1  and R 2  are each independently selected from hydrogen and C 6 -C 30  aryl groups. Nonlimiting examples of suitable C 6 -C 30  aryl groups include pentalenyl groups, indenyl groups, phenyl groups, naphthyl groups, azulenyl groups, heptalenyl groups, biphenylenyl groups, acenaphthylenyl groups, fluorenyl groups, phenalenyl groups, phenanthrenyl groups, anthracenyl groups, fluoranthenyl groups, triphenylenyl groups, pyrenyl groups, chrysenyl groups, naphthacenyl groups, picenyl groups, perylenyl groups, pentaphenyl groups, and hexacenyl groups. In one embodiment, at least hydrogen in the C 6 -C 30 aryl group is substituted with a hydroxyl group, cyano group, a C 1 -C 10  alkyl group (such as a methyl group, ethyl group, or the like), or a C 1 -C 10  alkoxy group (such as a methoxy group, an ethoxy group, or the like). 
         [0048]    Nonlimiting examples of suitable green host materials include Compounds 1 through 3 illustrated below. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0049]    In one embodiment, the green dopant can be a compound represented by Formula 2 below. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    In Formula 2, R 3 , R 4 , R 5 , R 6 , R 7 , and R 8  are each independently selected from hydrogen, C 1 -C 20  alkyl groups, C 1 -C 20  alkoxy groups, and C 6 -C 30  aryl groups. Nonlimiting examples of suitable C 1 -C 20  alkyl groups include methyl groups, ethyl groups, propyl groups, butyl groups, and the like. Nonlimiting examples of suitable C 1 -C 20  alkoxy groups include methoxy groups, ethoxy groups, propoxy groups, butoxy groups, and the like. The C 6 -C 30  aryl group may be the same as that described above with respect to the green host material. 
         [0050]    Nonlimiting examples of suitable green dopant materials include Compounds 4, 5, and 6 illustrated below. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0051]    The dopant may be present in an amount ranging from about 0.1 to about 50 parts by weight based on 100 parts by weight of the total amount of host and dopant. In one embodiment, for example, the dopant may be present in an amount ranging from about 0.5 to about 35 parts by weight based on 100 parts by weight of the total amount of host and dopant. When the dopant is present within these ranges, concentration quenching may be prevented. 
         [0052]    The thickness of each of the green emission layer  22 G and the blue emission layer  22 B may range from about 10 nm to about 100 nm. In one embodiment, for example, the thickness may range from about 10 nm to about 60 nm. When the thickness of the emission layers is within these ranges, excellent emission characteristics can be obtained. 
         [0053]    When the green emission layer  22 G and the blue emission layer  22 B are formed by deposition, the green emission layer  22 G can be formed using a green emission layer deposition mask, followed by formation of the blue emission layer  22 B using a blue emission layer deposition mask. 
         [0054]    An electron transporting-red emission layer  24 R may be formed on the green emission layer  22 G and the blue emission layer  22 B. The electron transporting-red emission layer  24 R can cover the red, green, and blue subpixels as illustrated in  FIG. 1 . 
         [0055]    The electron transporting-red emission layer  24 R can transport electrons and emit red light. Referring to  FIG. 1 , in the red subpixel, the electron transporting-red emission layer  24 R is formed on the hole transport layer  18 . The organic light emitting device  10  illustrated in  FIG. 1  can emit red light through the electron transporting-red emission layer  24 R. 
         [0056]    The electron transporting-red emission layer  24 R may include at least one material selected from organic metal complexes represented by Formula 3. 
         [0000]      [M L 2 ] a    Formula 3 
         [0000]    In Formula 3, L is an anionic ligand, M is a metal capable of making a 5- or 6-coordinate bond with L, and a is an integer ranging from 2 to 4. Since a is an integer ranging from 2 to 4, the organic metal complex is a form of oligomer, and not a monomer. 
         [0057]    In Formula 3, M is a metal capable of making a 5- or 6-coordinate bond with L. Accordingly, the organic metal complex represented by Formula 3 may be an oligomer having two or more metals, M. The two or more metals (M) can be bound to a single ligand at the same time. Nonlimiting examples of suitable metals for M include Zn, Co, Ni, and Fe. 
         [0058]    In Formula 3, L is an anionic ligand and can be coordinated to one or more central metal, M. Nonlimiting examples of suitable ligands (L) include those represented by Formulae 4a, 4b, 4c, and 4d. 
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         [0059]    The ligands represented by Formulae 4b and 4d have three sites for binding with M (represented by *) and can bond to two metals (M) (refer to Formulae 6, 7, 8, and 9). Therefore, an organic metal complex having an oligomer shape represented by Formula 3 can be obtained. 
         [0060]    In Formulae 4a, 4b, 4c, and 4d, A, B, C, D, E, and F are each independently selected from aromatic rings and heteroaromatic rings. For example, A, B, C, and D may each independently be a 5-20 member aromatic ring or a 5-20 member heteroaromatic ring. 
         [0061]    Nonlimiting examples of suitable rings for A and C include benzene and naphthalene. Nonlimiting examples of suitable rings for B and D include pyridine, benzothiazole, benzooxazole, quinoline, and benzoimidazole. Nonlimiting examples of suitable rings for E and F include quinoline and benzoquinoline. 
         [0062]    A and B can be combined using various methods. For example, A and B can be bound to each other through a single bond, or can be fused to each other. C and D can also be combined using various methods. For example, C and D can be bound to each other through a single bond, or can be fused to each other. 
         [0063]    In Formulae 4a and 4b, X 1 , X 2 , X 3  and X 4  are each independently selected from C, N, O, S and P. In Formulae 4c and 4d, X 5  and X 6  are each independently selected from C 1 -C 10  alkylenes and C 2 -C 10  alkenylenes. 
         [0064]    In Formulae 4a, 4b, 4c, and 4d, * denotes a site for bonding with M. 
         [0065]    Nonlimiting examples of suitable ligands for L include those represented by Formula 5, below. 
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         [0066]    Nonlimiting examples of suitable compounds satisfying Formula 3 include compounds represented by Formulae 6, 7, 8 and 9, below. 
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         [0067]    in addition to the organic metal complex represented by Formula 3, the electron transporting-red emission layer  24 R may further include a phosphorescent dopant, in which case the organic metal complex represented by Formula 3 is used as a host. The phosphorescent dopant can be any phosphorescent dopant known in the art. Nonlimiting examples of suitable phosphorescent dopants include organic metal complexes containing Ir, Pt, Os, Re, Ti, Zr, or Hf. For example, the phosphorescent dopant may be an organic metal complex containing Ir or Pt. When the phosphorescent dopant is a 4-coordinate Pt-containing organic metal complex, the emission layer can be formed at relatively low temperatures, thereby obtaining high yield, excellent efficiency, and excellent lifetime characteristics. 
         [0068]    Specific nonlimiting examples of suitable phosphorescent dopants include bisthienylpyridine acetylacetonate Iridium, bis(benzothienylpyridine)acetylacetonate Iridium, bis(2-phenylbenzothiazole)acetylacetonate Iridium, bis(1-phenylisoquinoline)acetylacetonate Iridium, tris(1-phenylisoquinoline)Iridium, tris(phenylpyridine)Iridium, tris(2-biphenylpyridine)Iridium, tris(3-biphenyl pyridine)Iridium, tris(4-biphenyl pyridine)Iridium, Ir(pq) 2 (acac) (where pq denotes 2-phenylquinoline and acac denotes acetylacetone (refer to Formula 10)), Platinum(II)octaethylporphyrin (PtOEP), Ir(piq) 3  (where piq denotes phenylisoquinoline (refer to Formula 11)), Ir(piq) 2 acac (refer to Formula 12), compounds represented by Formula 13, compounds represented by Formula 14, compounds represented by Formula 15, compounds represented by Formula 16, and mixtures thereof. 
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         [0069]    The electron transporting-red emission layer  24 R can be formed using an open mask, and can cover the red, green, and blue subpixels. As used herein, the term “open mask” refers to a mask for forming a layer covering an emission unit including red, green, and blue subpixels. For example, the open mask may have a structure such that the portions corresponding to the emission unit including red, green, and blue subpixels are open. Accordingly, by using the open mask, a common layer covering the red, green, and blue subpixels can be formed. 
         [0070]    The electron injection layer  26  is formed of a material that enables easy injection of electrons from an anode, and is formed on the electron transporting-red emission layer  24 R. The electron injection layer  26  can be formed using various methods, such as vacuum deposition, spin coating, and casting. When the electron injection layer  26  is formed by vacuum deposition or spin coating, the vacuum deposition or spin coating conditions may differ according to the compound used. However, the vacuum deposition or spin coating conditions may be similar to the vacuum deposition or spin coating conditions discussed above under which the hole injection layer  16  is formed. The electron injection layer  26  can cover the red, green, and blue subpixels, as illustrated in  FIG. 1 . 
         [0071]    The electron injection layer  26  can be formed of any electron injection layer material known in the art, nonlimiting examples of which include LiQ, NaQ, CsQ, LiF, NaCl, CsF, Li 2 O, and BaO. The thickness of the electron injection layer  26  may range from about 0.1 nm to about 10 nm. In one embodiment, for example, the thickness ranges from about 0.5 nm to about 9 nm. When the thickness of the electron injection layer  26  is within these range, excellent electron injecting characteristics and driving voltage characteristics can be obtained. 
         [0072]    A second electrode  28  is disposed on the electron injection layer  26 . The second electrode  28  can be formed using vacuum deposition or sputtering. The second electrode  28  can be a cathode or an anode. The second electrode  28  can be formed of a metal, an alloy, an electroconductive compound, or a blend thereof, each of which has a low work function. Nonlimiting examples of suitable materials for the second electrode  28  include Li, Mg, Al, Al—Li, Ca, Mg—In, Mg—Ag, and the like. The second electrode  28  can have other structures. For example, the second electrode  28  can have a layered structure including two or more layers formed of at least two kinds of materials. 
         [0073]    In an OLED according to the embodiment of the present invention illustrated in  FIG. 1 , each red subpixel includes the first electrodes  12 , the hole injection layer  16 , the hole transport layer  18 , the electron transporting-red emission layer  24 R, the electron injection layer  26 , and the second electrode  28 . Each green subpixel includes the first electrodes  12 , the hole injection layer  16 , the hole transport layer  18 , the green emission layer  22 G, the electron transporting-red emission layer  24 R, the electron injection layer  26 , and the second electrode  28 . Each blue subpixel includes the first electrodes  12 , the hole injection layer  16 , the hole transport layer  18 , the blue emission layer  22 B, the electron transporting-red emission layer  24 R, the electron injection layer  26 , and the second electrode  28 . In the red subpixel, the electron transporting-red emission layer  24 R can emit red light and transport electrons. 
         [0074]    The OLED having the structure described above is more easily prepared than a conventional OLED. According to a conventional method of preparing an OLED, a red emission layer, a green emission layer, and a blue emission layer are separately formed, and then an electron transport layer is formed over the red, green, and blue subpixels. Specifically, a red emission layer is formed using a red emission layer deposition mask, a green emission layer is formed using a green emission layer deposition mask, and a blue emission layer is formed using a blue emission layer deposition mask. Then, an electron transport layer is formed over the red, green, and blue subpixels using an open mask. Therefore, four masking processes are required. 
         [0075]    However, according to one embodiment of the present invention, a method of preparing an OLED (such as that illustrated in  FIG. 1 ) includes forming a green emission layer  22 G using a green emission layer deposition mask, forming a blue emission layer  22 B using a blue emission layer deposition mask, and then forming an electron transporting-red emission layer  24 R using an open mask. Therefore, only three masking processes are performed. 
         [0076]    Accordingly, a method of preparing an OLED according to one embodiment the present invention is simpler and uses fewer materials than the conventional method. Thus, costs of preparing an OLED can be reduced. 
         [0077]      FIG. 2  is a cross-sectional view of an organic light emitting device  20  according to another embodiment of the present invention. The OLED  20  illustrated in  FIG. 2  has the same layer structure as the OLED illustrated in  FIG. 1 , except for the thickness of the hole injection layer  16 . In the embodiment depicted in  FIG. 2 , the portions of the hole injection layer  16  covering the red, green and blue subpixels may have different thicknesses. Specifically, the thickness of the portion of the hole injection layer covering the red subpixel is greater than the thickness of the portion covering the green subpixel, which is greater than the thickness of the portion covering the blue subpixel. Such variation in the thickness of the hole injection layer  16  maximizes light extraction efficiency in consideration of the resonance cycles of red light formed in the red subpixel, green light formed in the green subpixel, and blue light formed in the blue subpixel when the OLED  20  operates. 
         [0078]    In another embodiment, although not illustrated in  FIG. 2 , the thickness of the hole transport layer  18  can also differ in the red, green, and blue subpixels. Specifically, the thickness of the portion of the hole transport layer covering the red subpixel may be greater than the thickness of the portion covering the green subpixel, which may be greater than the thickness of the portion covering the blue subpixel. Such variation in the thickness of the hole transport layer  18  maximizes light extraction efficiency in consideration of the resonance cycles of red light formed in the red subpixel, green light formed in the green subpixel, and blue light formed in the blue subpixel. 
         [0079]      FIG. 3  is a cross-sectional view of an OLED  30  according to another embodiment of the present invention. Referring to  FIG. 3 , the OLED  30  includes a substrate  31 , first electrodes  32 , a hole injection layer  36 , and a hole transport layer  38 . A red emission layer  42 R is patterned on the portion of the hole transport layer corresponding to the red subpixel (R), and a blue emission layer  42 B is patterned on the portion of the hole transport layer  38  corresponding to the blue subpixel (B). An electron transporting-green emission layer  44 G, an electron injection layer  46 , and a second electrode  48  are disposed on the resulting structure. 
         [0080]    The substrate  31 , the first electrodes  32 , the hole injection layer  36 , the hole transport layer  38 , the blue emission layer  42 B, the electron injection layer  46 , and the second electrode  48  are the same as in the previous embodiment described with reference to  FIG. 1 . 
         [0081]    In  FIG. 3 , the red emission layer  42 R can be formed of various emissive materials known in the art. For example, a known fluorescent host and known fluorescent dopant may be used. Alternatively, a known phosphorescent host and known phosphorescent dopant can be used. 
         [0082]    Nonlimiting examples of suitable red hosts include Alq 3 , 4,4′-N,N′-dicarbazole-biphenyl (CBP), poly n-vinylcarbazole (PVK), and distyrylarylene (DSA). 
         [0083]    Nonlimiting examples of suitable red dopants include PtOEP, Ir(piq) 3 , Btp 2 Ir(acac), and DCJTB. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0084]    In one embodiment, the red emission layer ( 42 R) may include an organic metal complex represented by Formula 3, as described above. In addition to the organic metal complex represented by Formula 3, in some embodiments, the red emission layer ( 42 R) may further include a phosphorescent dopant including Ir, Pt, Os, Re, Ti, Zr or Hr, as described above. 
         [0085]    The dopant may be present in an amount ranging from about 0.1 to about 50 parts by weight based on 100 parts by weight of the total amount of host and dopant. IN one embodiment, for example, the dopant may be present in an amount ranging from about 0.5 to about 35 parts by weight based on 100 parts by weight of the total amount of host and dopant. When the dopant is present in an amount within these ranges, concentration quenching may be prevented. 
         [0086]    The thickness of the red emission layer  42 R may range from about 10 nm to about 100 nm. In one embodiment, for example, the thickness ranges from about 10 nm to about 60 nm. When the thickness of the red emission layer  42 R is within these range, excellent emission characteristics can be obtained. When the red emission layer  42 R and the blue emission layer  42 B illustrated in  FIG. 3  are formed by deposition, the red emission layer  42 R can be formed using a red emission layer deposition mask, and the blue emission layer  42 B can be formed using a blue emission layer deposition mask. 
         [0087]    An electron transporting-green emission layer  44 G is formed on the red emission layer  42 R and blue emission layer  42 B. The electron transporting-green emission layer  44 G can cover the red, green, and blue subpixels, as illustrated in  FIG. 3 . The electron transporting-green emission layer  44 G can transport electrons and emit green light. Referring to  FIG. 3 , in the green subpixel, the electron transporting-green emission layer  44 G is formed on the hole transport layer  38 . The OLED  30  illustrated in  FIG. 3  can emit green light through the electron transporting-green emission layer  44 G. 
         [0088]    The electron transporting-green emission layer  44 G can include, for example, a green host represented by Formula 1 above and a green dopant represented by Formula 2 above. Specifically, the electron transporting-green emission layer  44 G can include a green host selected from Compounds 1, 2, and 3 (listed above) and a green dopant selected from Compounds 4, 5, and 6 (listed above). 
         [0089]    The electron transporting-green emission layer  44 G can be formed using an open mask, and can cover the red, green, and blue subpixels. 
         [0090]    In the OLED illustrated in  FIG. 3 , each red subpixel includes the first electrodes  32 , the hole injection layer  36 , the hole transport layer  38 , the red emission layer  42 R, the electron transporting-green emission layer  44 G, the electron injection layer  46 , and the second electrode  48 . Each green subpixel includes the first electrodes  32 , the hole injection layer  36 , the hole transport layer  38 , the electron transporting-green emission layer  44 G, the electron injection layer  46 , and the second electrode  48 . Each blue subpixel includes the first electrodes  32 , the hole injection layer  36 , the hole transport layer  38 , the blue emission layer  42 B, the electron transporting-green emission layer  44 G, the electron injection layer  46 , and the second electrode  48 . In the green subpixel, the electron transporting-green emission layer  44 G can emit green light and transport electrons. 
         [0091]    According to one embodiment of the present invention, a method of preparing the OLED illustrated in  FIG. 3  includes forming a red emission layer  42 R using a red emission layer deposition mask, forming a blue emission layer  42 B using a blue emission layer deposition mask, and then forming an electron transporting-green emission layer  44 G using an open mask. Accordingly, only three masking processes are performed. 
         [0092]    The method of preparing an OLED according to the current embodiment is simpler and uses fewer materials than the conventional method of preparing an OLED. Thus, costs for preparing the OLED can be reduced. 
         [0093]      FIG. 4  is a cross-sectional view of an OLED  40  according to another embodiment of the present invention. The OLED  40  illustrated in  FIG. 4  has the same layered structure as the OLED illustrated in  FIG. 3 , except for the thickness of the hole injection layer  36 . In the embodiment depicted in  FIG. 4 , the portions of the hole injection layer  36  covering the red, green and blue subpixels may have different thicknesses. Specifically, the thickness of the portion of the hole injection layer covering the red subpixel is greater than the thickness of the portion covering the green subpixel, which is greater than the thickness of the portion covering the blue subpixel. Such variations in the thickness of the hole injection layer  36  maximizes light extraction efficiency in consideration of the resonance cycles of red light formed in the red subpixel, green light formed in the green subpixel, and blue light formed in the blue subpixel when the OLED  40  operates. 
         [0094]    In another embodiment, although not illustrated in  FIG. 4 , the thickness of the hole transport layer  38  can also differ in the red, green, and blue subpixels. Specifically, the thickness of the portion of the hole transport layer covering the red subpixel is greater than the thickness of the portion covering the green subpixel, which is greater than thickness of the portion covering the blue subpixel. Such variations in the thickness of the hole transport layer  38  maximize light extraction efficiency in consideration of the resonance cycles of red light formed in the red subpixel, green light formed in the green subpixel, and blue light formed in the blue subpixel when the OLED  40  operates. 
         [0095]      FIG. 5  is a cross-sectional view of an OLED  50  according to another embodiment of the present invention. Referring to  FIG. 5 , the OLED  50  includes a substrate  51 , first electrodes  52 , a hole injection layer  56 , and a hole transport layer  58 . A red emission layer  62 R is patterned on the portion of the hole transport layer corresponding to the red subpixel (R), and a green emission layer  62 G is patterned on the portion of the hole transport layer  58  corresponding to the green subpixel (G). An electron transporting-blue emission layer  64 B for emitting blue light, an electron injection layer  66 , and a second electrode  68  are formed on the resulting structure. 
         [0096]    The substrate  51 , the first electrodes  52 , the hole injection layer  56 , the hole transport layer  58 , the red emission layer  62 R, the green emission layer  62 G, the electron injection layer  66 , and the second electrode  68  are the same as described above with respect to the OLED depicted in  FIGS. 1 and 3 . 
         [0097]    When the red emission layer  62 R and the green emission layer  62 G illustrated in  FIG. 5  are formed by deposition, the red emission layer  62 R can be formed using a red emission layer deposition mask, and the green emission layer  62 G can be formed using a green emission layer deposition mask. 
         [0098]    The electron transporting-blue emission layer  64 B for emitting blue light is formed on the red emission layer  62 R and the green emission layer  62 G. The electron transporting-blue emission layer  64 B can cover the red, green, and blue subpixels, as illustrated in  FIG. 5 . 
         [0099]    The electron transporting-blue emission layer  64 B can transport electrons and emit blue light. Referring to  FIG. 5 , in the blue subpixel (B), the electron transporting-blue emission layer  64 B is formed on the hole transport layer  58 . The OLED  50  illustrated in  FIG. 5  can emit blue light through the electron transporting-blue emission layer  64 B. 
         [0100]    The electron transporting-blue emission layer  64 B can include a blue host selected from Compounds 7, 8, 9 and 10, below. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0101]    The electron transporting-blue emission layer  64 B can further include a blue dopant, such as GBD32, GBD132, or GBD69, each available from Gracel Co. 
         [0102]    The electron transporting-blue emission layer  64 B can be formed using an open mask and cover the red, green, and blue subpixels. 
         [0103]    In the OLED illustrated in  FIG. 5 , each red subpixel includes first electrodes  52 , the hole injection layer  56 , the hole transport layer  58 , the red emission layer  62 R, the electron transporting-blue emission layer  64 B, the electron injection layer  66 , and the second electrode  68 . Each green subpixel includes first electrodes  52 , the hole injection layer  56 , the hole transport layer  58 , the green emission layer  62 G, the electron transporting-blue emission layer  64 B, the electron injection layer  66 , and the second electrode  68 . Each blue subpixel includes first electrodes  52 , the hole injection layer  56 , the hole transport layer  58 , the electron transporting-blue emission layer  64 B, the electron injection layer  66 , and the second electrode  68 . That is, in the blue subpixel, the electron transporting-blue emission layer  64 B can emit blue light and transport electrons. 
         [0104]    According to one embodiment of the present invention, a method of preparing the OLED illustrated in  FIG. 5  includes forming a red emission layer  62 R using a red emission layer deposition mask, forming a green emission layer  62 G using a green emission layer deposition mask, and then forming an electron transporting-blue emission layer  64 B using an open mask. Accordingly, only three masking processes are performed. 
         [0105]    The method of preparing an OLED according to the current embodiment is simpler and uses fewer materials than the conventional method of preparing an OLED. Thus, costs for preparing the OLED can be reduced. 
         [0106]      FIG. 6  is a cross-sectional view of an OLED  60  according to another embodiment of the present invention. The OLED  60  illustrated in  FIG. 6  has the same layered structure as the OLED illustrated in  FIG. 5 , except for the thickness of the hole injection layer  56 . In the embodiment depicted in  FIG. 6 , the portions of the hole injection layer  56  covering the red, green and blue subpixels may have different thicknesses. Specifically, the thickness of the portion of the hole injection layer covering the red subpixel (R) is greater than the thickness of the portion covering the green subpixel (G), which is greater than the thickness of the portion covering the blue subpixel (B). Such variations in the thickness of the hole injection layer  56  maximize light extraction efficiency in consideration of the resonance cycles of red light formed in the red subpixel (R), green light formed in the green subpixel (G), and blue light formed in the blue subpixel (B) when the OLED  40  operates. 
         [0107]    In another embodiment, although not illustrated in  FIG. 6 , the thickness of the hole transport layer  58  can also differ in the red, green, and blue subpixels. Specifically, the thickness of the portion of the hole transport layer covering the red subpixel is greater than the thickness of the portion covering the green subpixel, which is greater than the thickness of the portion covering the blue subpixel. Such variations in the thickness of the hole injection layer  58  maximize light extraction efficiency in consideration of the resonance cycles of red light formed in the red subpixel (R), green light formed in the green subpixel (G), and blue light formed in the blue subpixel (B), when the OLED  40  operates. 
         [0108]    Exemplary OLEDs according to the present invention have been described with reference to  FIGS. 1 through 6 . However, it is understood that the OLEDs can have other structures. For example, an OLED according to an embodiment of the present invention can include a hole blocking layer (HBL) formed on an emission layer. The HBL prevents diffusion of triplet excitons or holes to the electron transport layer when the emission layer includes a phosphorescent dopant. 
         [0109]    The present invention will now be described with reference to the following examples. These examples are presented for illustrative purposes only and are not intended to limit the scope of the present invention. 
       EXAMPLES 
     Example 1 
       [0110]    A 100 nm-thick Ag and ITO substrate (produced by SDI Co., Ltd) was cut to a size of 50 mm×50 mm×0.7 mm. The cut substrate was washed using ultrasonic waves in isopropyl alcohol, washed using ultrasonic waves in pure water for five minutes, washed using UV for 30 minutes, and then washed using ozone. 
         [0111]    A m-TDATA hole injecting material was deposited on the anode to form a m-TDATA layer having a thickness of 60 nm such that the m-TDATA layer covered the green and blue subpixels. Then, a further m-TDATA layer having a thickness of 20 nm was formed over the green subpixel so that the hole injection layer over the green subpixel had a thickness of 80 nm, and the hole injection layer over the blue subpixel had a thickness of 60 nm. Then, NPB as a hole transporting material was deposited on the hole injection layer to form a hole transport layer having a thickness of 20 nm which covered the green and blue subpixels 
         [0112]    GGH01 (produced by Gracel Co.) as a green host and GGD01 (also produced by Gracel Co.) in a weight ratio of 97:3 as a green dopant were vacuum-deposited on the portion of the hole transport layer corresponding to the green subpixel to form a green emission layer having a thickness of 20 nm. Then, GBH 02 and GBD 32 (both produced by Gracel Co.) in a weight ratio of 97:3 were deposited on the green emission layer and the hole transport layer to form an electron transporting-blue emission layer having a thickness of 45 nm. That is, in the green subpixel, the electron transporting-blue emission layer was deposited on the green emission layer, and in the blue subpixel, the electron transporting-blue emission layer was deposited on the hole transport layer. 
         [0113]    LiQ was deposited on the electron transporting-blue emission layer to form an electron injection layer having a thickness of 0.5 nm. Then, MgAg was deposited to a thickness of 150 nm to form a cathode. As a result, an OLED was prepared. 
       Comparative Example 1 
       [0114]    An OLED was prepared as in Example 1, except that Alq 3  was deposited on the green emission layer and hole transport layer instead of GBH 02 and GBD 32. 
       Measurement Example 1 
       [0115]    The efficiency and lifetime of the OLEDs prepared according to Example 1 and Comparative Example 1 were measured using a PR650 (Spectroscan) Source Measurement Unit and McScience Polaronix M6000. The results are shown in  FIGS. 7 and 8 . 
         [0116]    Referring to  FIG. 7 , the OLED prepared according to Example 1 showed higher efficiency than the OLED prepared according to Comparative Example 1. 
         [0117]    Referring to  FIG. 8 , the OLED prepared according to Example 1 showed a longer lifetime than the OLED prepared according to Comparative Example 1. 
         [0118]    Electroluminescence (EL) spectrums of the OLEDs prepared according to Example 1 and Comparative Example 1 were measured using a Spectra Radiometer (PR650, Photo Researcher Inc.). The results are shown in  FIG. 9 . Referring to  FIG. 9 , the OLEDs prepared according to Example 1 and Comparative Example 1 showed maximum emission at about 520 nm, which indicates green emission. 
       Example 2 
       [0119]    A 100 nm-thick Ag and ITO substrate (produced by SDI Co., Ltd) was cut to a size of 50 mm×50 mm×0.7 mm. The cut substrate was washed using ultrasonic waves in isopropyl alcohol, washed using ultrasonic waves in pure water for five minutes, washed using UV for 30 minutes, and then washed using ozone. 
         [0120]    A m-TDATA hole injecting material was deposited on the anode to form a m-TDATA layer having a thickness of 60 nm covering the red and blue subpixels. Then, a further m-TDATA layer having a thickness of 40 nm was formed over the red subpixel so that the hole injection layer over the red subpixel had a thickness of 100 nm, and the hole injection layer over the blue subpixel had a thickness of 60 nm. Then, NPB as a hole transporting material was deposited on the hole injection layer to form a hole transport layer having a thickness of 20 nm covering the red and blue subpixels 
         [0121]    GDI1403 (produced by Gracel Co.) as a red host and RD25 (produced by UDC) as a red dopant in a weight ratio of 90:10 were vacuum-deposited on the portion of the hole transport layer corresponding to the red subpixel to form a red emission layer having a thickness of 40 nm. Then, GBH 02 and GBD 32 (both produced by Gracel Co.) in a weight ratio of 97:3 were deposited on the red emission layer and the hole transport layer to form an electron transporting-blue emission layer having a thickness of 45 nm. That is, in the red subpixel, the electron transporting-blue emission layer was deposited on the red emission layer; and in the blue subpixel, the electron transporting-blue emission layer is deposited on the hole transport layer. 
         [0122]    LiQ was deposited on the electron transporting-blue emission layer to form an electron injection layer having a thickness of 0.5 nm. Then, MgAg was deposited to a thickness of 150 nm to form a cathode. As a result, an OLED was prepared. 
       Comparative Example 2 
       [0123]    An OLED was prepared as in Example 2, except that Alq 3  was deposited on the red emission layer and the hole transport layer instead of GBH 02 and GBD 32. 
       Measurement Example 2 
       [0124]    The efficiency of the OLEDs prepared according to Example 2 and Comparative Example 2 were measured using a PR650 (Spectroscan) Source Measurement Unit and a McScience Polaronix M6000. The results are shown in  FIG. 10 . 
         [0125]    The efficiency results for the OLED prepared according to Example 2, shown in  FIG. 10 , depict the errors which can occur from the resonance structure. It is expected that if the color coordinates of the two OLEDs are tuned, the efficiency of the OLED prepared according to Example 2 will be higher than that of the OLED prepared according to Comparative Example 2. 
         [0126]    EL spectrums of the OLEDs prepared according to Example 2 and Comparative Example 2 were measured using a Spectra Radiometer (PR650, Photo Researcher Inc.). The results are shown in  FIG. 11 . Referring to  FIG. 11 , the OLEDs prepared according to Example 2 and Comparative Example 2 showed maximum emission at about 630 nm, which indicates red emission. 
         [0127]    As described above, an OLED according to the present invention uses an electron transport layer to emit red, green, or blue light. A method of preparing the OLED is simple. Accordingly, the method of preparing the OLEDs according to the present invention are inexpensive and achieve high yield. 
         [0128]    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 modifications and changes to the described embodiments may be made without departing from the spirit and scope of the present invention as defined by the following claims.