Patent Publication Number: US-2021167289-A1

Title: Organic light emitting diode and organic light emitting device including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the priority benefit of the Korean Patent Application No. 10-2019-0155531 filed in the Republic of Korea on Nov. 28, 2019 and the Korean Patent Application No. 10-2020-0128779 filed in the Republic of Korea on Oct. 6, 2020, the entire contents of all these applications are hereby expressly incorporated by reference as if fully set forth herein into the present application. 
     BACKGROUND 
     Field of Technology 
     The present disclosure relates to an organic light emitting diode, and more particularly, to an organic light emitting diode having excellent emitting property and an organic light emitting device including the organic light emitting diode. 
     Discussion of the Related Art 
     Recently, a demand for flat panel display devices having small occupied areas is increased. Among the flat panel display devices, a technology of an organic light emitting display device, which includes an organic light emitting diode (OLED) and can be referred to as an organic electroluminescent device, is rapidly developed. 
     The OLED emits light by injecting electrons from a cathode as an electron injection electrode and holes from an anode as a hole injection electrode into an emitting material layer, combining the electrons with the holes, generating an exciton, and transforming the exciton from an excited state to a ground state. In the fluorescent material, only singlet exciton is involved in the emission such that the related art fluorescent material has low emitting efficiency. In the phosphorescent material, both the singlet exciton and the triplet exciton are involved in the emission such that the phosphorescent material has higher emitting efficiency than the fluorescent material. However, the metal complex compound, which is a typical phosphorescent material, has a short emitting lifespan and thus has a limitation in commercialization. 
     SUMMARY 
     The present disclosure is directed to an OLED and an organic light emitting device that substantially obviate one or more of the problems associated with the limitations and disadvantages of the related conventional art. 
     Additional features and advantages of the present disclosure are set forth in the description which follows, and will be apparent from the description, or evident by practice of the present disclosure. The objectives and other advantages of the present disclosure are realized and attained by the features described herein as well as in the appended drawings. 
     To achieve these and other advantages in accordance with the purpose of the embodiments of the present disclosure, as described herein, an aspect of the present disclosure is an organic light emitting diode including a first electrode; a second electrode facing the first electrode; and an emitting material layer including a first compound and a second compound and positioned between the first and second electrodes, wherein the first compound includes a hexagonal-ring moiety including one boron atom, one oxygen atom and four carbon atoms, and the second compound includes a hexagonal-ring moiety including one boron atom, one nitrogen atom and four carbon atoms. 
     Another aspect of the present disclosure is an organic light emitting device including a substrate; and an organic light emitting diode disposed on or over the substrate, the organic light emitting diode including: a first electrode; a second electrode facing the first electrode; and an emitting material layer including a first compound and a second compound and positioned between the first and second electrodes, wherein the first compound includes a hexagonal-ring moiety including one boron atom, one oxygen atom and four carbon atoms, and the second compound includes a hexagonal-ring moiety including one boron atom, one nitrogen atom and four carbon atoms. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to further explain the present disclosure as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and together with the description serve to explain the principles of the present disclosure. 
         FIG. 1  is a schematic circuit diagram of an organic light emitting display device of the present disclosure. 
         FIG. 2  is a schematic cross-sectional view of an organic light emitting display device according to a first embodiment of the present disclosure. 
         FIG. 3  is a schematic cross-sectional view of an OLED according to a second embodiment of the present disclosure. 
         FIGS. 4A to 4D  are schematic views illustrating an energy level relation of a first compound and a second compound in the OLED according to one or more embodiments of the present disclosure. 
         FIG. 5  is a view illustrating an emission mechanism of an OLED according to the second embodiment of the present disclosure. 
         FIG. 6  is a schematic cross-sectional view of an OLED according to a third embodiment of the present disclosure. 
         FIG. 7  is a schematic cross-sectional view of an OLED according to a fourth embodiment of the present disclosure. 
         FIG. 8  is a schematic cross-sectional view of an OLED according to a fifth embodiment of the present disclosure. 
         FIG. 9  is a schematic cross-sectional view of an organic light emitting display device according to a sixth embodiment of the present disclosure. 
         FIG. 10  is a schematic cross-sectional view of an organic light emitting display device according to a seventh embodiment of the present disclosure. 
         FIG. 11  is a schematic cross-sectional view of an OLED according to an eighth embodiment of the present disclosure. 
         FIG. 12  is a schematic cross-sectional view of an organic light emitting display device according to a ninth embodiment of the present disclosure. 
         FIG. 13  is a schematic cross-sectional view of an OLED according to a tenth embodiment of the present disclosure. 
         FIG. 14  is a schematic cross-sectional view of an OLED according to an eleventh embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to some of the examples and preferred embodiments, which are illustrated in the accompanying drawings. 
     The present disclosure relates to an OLED, in which a delayed fluorescent material and a fluorescent material are applied in a single emitting material layer or adjacent emitting material layers, and an organic light emitting device including the OLED. For example, the organic light emitting device can be an organic light emitting display device or an organic lighting device. As an example, an organic light emitting display device, which is a display device including one or more OLEDs of the present disclosure, will be mainly described. 
       FIG. 1  is a schematic circuit diagram of an organic light emitting display device of the present disclosure. All the components of the organic light emitting display device according to all embodiments of the present disclosure are operatively coupled and configured. 
     As shown in  FIG. 1 , an organic light emitting display device includes a gate line GL, a data line DL, a power line PL, a switching thin film transistor TFT Ts, a driving TFT Td, a storage capacitor Cst, and an OLED D. The gate line GL and the data line DL cross each other to define a pixel region P. The pixel region can include a red pixel region, a green pixel region and a blue pixel region. Although a pixel region is exemplified, the organic light emitting display device can include a plurality of such pixel regions having a plurality of gate lines, data lines, TFTs, OLEDs, etc. 
     The switching TFT Ts is connected to the gate line GL and the data line DL, and the driving TFT Td and the storage capacitor Cst are connected to the switching TFT Ts and the power line PL. The OLED D is connected to the driving TFT Td. 
     In the organic light emitting display device, when the switching TFT Ts is turned on by a gate signal applied through the gate line GL, a data signal from the data line DL is applied to the gate electrode of the driving TFT Td and an electrode of the storage capacitor Cst. 
     When the driving TFT Td is turned on by the data signal, an electric current is supplied to the OLED D from the power line PL. As a result, the OLED D emits light. In this case, when the driving TFT Td is turned on, a level of an electric current applied from the power line PL to the OLED D is determined such that the OLED D can produce a gray scale. 
     The storage capacitor Cst serves to maintain the voltage of the gate electrode of the driving TFT Td when the switching TFT Ts is turned off. Accordingly, even if the switching TFT Ts is turned off, a level of an electric current applied from the power line PL to the OLED D is maintained to next frame. 
     As a result, the organic light emitting display device displays a desired image. 
       FIG. 2  is a schematic cross-sectional view of an organic light emitting display device according to a first embodiment of the present disclosure. 
     As shown in  FIG. 2 , an organic light emitting display device  100  includes a substrate  110 , a TFT Tr and an OLED D connected to the TFT Tr. 
     The substrate  110  can be a glass substrate or a plastic substrate. For example, the substrate  110  can be a polyimide substrate. 
     A buffer layer  122  is formed on the substrate, and the TFT Tr is formed on the buffer layer  122 . The buffer layer  122  can be omitted. 
     A semiconductor layer  120  is formed on the buffer layer  122 . The semiconductor layer  120  can include an oxide semiconductor material or polycrystalline silicon. 
     When the semiconductor layer  120  includes the oxide semiconductor material, a light-shielding pattern can be formed under the semiconductor layer  120 . The light to the semiconductor layer  120  is shielded or blocked by the light-shielding pattern such that thermal degradation of the semiconductor layer  120  can be prevented. On the other hand, when the semiconductor layer  120  includes polycrystalline silicon, impurities can be doped into both sides of the semiconductor layer  120 . 
     A gate insulating layer  124  is formed on the semiconductor layer  120 . The gate insulating layer  124  can be formed of an inorganic insulating material such as silicon oxide or silicon nitride. 
     A gate electrode  130 , which is formed of a conductive material, e.g., metal, is formed on the gate insulating layer  124  to correspond to a center of the semiconductor layer  120 . In  FIG. 2 , the gate insulating layer  124  is formed on an entire surface of the substrate  110 . Alternatively, the gate insulating layer  124  can be patterned to have the same shape as the gate electrode  130 . 
     An interlayer insulating layer  132 , which is formed of an insulating material, is formed on the gate electrode  130 . The interlayer insulating layer  132  can be formed of an inorganic insulating material, e.g., silicon oxide or silicon nitride, or an organic insulating material, e.g., benzocyclobutene or photo-acryl. 
     The interlayer insulating layer  132  includes first and second contact holes  134  and  136  exposing both sides of the semiconductor layer  120 . The first and second contact holes  134  and  136  are positioned at both sides of the gate electrode  130  to be spaced apart from the gate electrode  130 . In  FIG. 2 , the first and second contact holes  134  and  136  are formed through the interlayer insulating layer  132  and the gate insulating layer  124 . Alternatively, when the gate insulating layer  124  is patterned to have the same shape as the gate electrode  130 , the first and second contact holes  134  and  136  is formed only through the interlayer insulating layer  132 . 
     A source electrode  144  and a drain electrode  146 , which are formed of a conductive material, e.g., metal, are formed on the interlayer insulating layer  132 . The source electrode  144  and the drain electrode  146  are spaced apart from each other with respect to the gate electrode  130  and respectively contact both sides of the semiconductor layer  120  through the first and second contact holes  134  and  136 . 
     The semiconductor layer  120 , the gate electrode  130 , the source electrode  144  and the drain electrode  146  constitute the TFT Tr. The TFT Tr serves as a driving element. Namely, the TFT Tr is the driving TFT Td (of  FIG. 1 ). 
     In the TFT Tr, the gate electrode  130 , the source electrode  144 , and the drain electrode  146  are positioned over the semiconductor layer  120 . Namely, the TFT Tr has a coplanar structure. 
     Alternatively, in the TFT Tr, the gate electrode can be positioned under the semiconductor layer, and the source and drain electrodes can be positioned over the semiconductor layer such that the TFT Tr can have an inverted staggered structure. In this instance, the semiconductor layer can include amorphous silicon. 
     The gate line and the data line cross each other to define the pixel region, and the switching TFT is formed to be connected to the gate and data lines. The switching TFT is connected to the TFT Tr as the driving element. In addition, the power line, which can be formed to be parallel to and spaced apart from one of the gate and data lines, and the storage capacitor for maintaining the voltage of the gate electrode of the TFT Tr in one frame can be further formed. 
     A planarization layer  150  is formed on an entire surface of the substrate  110  to cover the source and drain electrodes  144  and  146 . The planarization layer  150  provides a flat top surface and has a drain contact hole  152  exposing the drain electrode  146  of the TFT Tr. 
     The OLED D is disposed on the planarization layer  150  and includes a first electrode  210 , which is connected to the drain electrode  146  of the TFT Tr, a light emitting layer  220  and a second electrode  230 . The light emitting layer  220  and the second electrode  230  are sequentially stacked on the first electrode  210 . The OLED D is positioned in each of the red, green and blue pixel regions and respectively emits the red, green and blue light. 
     The first electrode  210  is separately formed in each pixel region. The first electrode  210  can be an anode and can be formed of a conductive material, e.g., a transparent conductive oxide (TCO), having a relatively high work function. For example, the first electrode  210  can be formed of indium-tin-oxide (ITO), indium-zinc-oxide (IZO), indium-tin-zinc-oxide (ITZO), tin oxide (SnO), zinc oxide (ZnO), indium-copper-oxide (ICO) or aluminum-zinc-oxide (Al:ZnO, AZO). 
     When the organic light emitting display device  100  is operated in a bottom-emission type, the first electrode  210  may have a single-layered structure of the transparent conductive material layer. When the organic light emitting display device  100  of the present disclosure is operated in a top-emission type, a reflection electrode or a reflection layer can be formed under the first electrode  210 . For example, the reflection electrode or the reflection layer can be formed of silver (Ag) or aluminum-palladium-copper (APC) alloy. In this instance, the first electrode  210  may have a triple-layered structure of ITO/Ag/ITO or ITO/APC/ITO. 
     In addition, a bank layer  160  is formed on the planarization layer  150  to cover an edge of the first electrode  210 . Namely, the bank layer  160  is positioned at a boundary of the pixel region and exposes a center of the first electrode  210  in the pixel region. 
     The light emitting layer  220  as an emitting unit is formed on the first electrode  210 . The light emitting layer  220  can have a single-layered structure of an emitting material layer (EML) including an emitting material. To increase an emitting efficiency of the organic light emitting display device, the light emitting layer  220  can have a multi-layered structure. For example, the light emitting layer  220  can further include a hole injection layer (HIL), a hole transporting layer (HTL), an electron blocking layer (EBL), a hole blocking layer (HBL), an electron transporting layer (ETL) and an electron injection layer (EIL). The HIL, the HTL and the EBL are sequentially disposed between the first electrode ( 210 ) and the EML, and the HBL, the ETL and the EIL are sequentially disposed between the EML and the second electrode  230 . In addition, the EML can has a single-layered structure or a multi-layered structure. Moreover, two or more light emitting layers can be disposed to be spaced apart from each other such that the OLED D can have a tandem structure. 
     The second electrode  230  is formed over the substrate  110  where the light emitting layer  220  is formed. The second electrode  230  covers an entire surface of the display area and can be formed of a conductive material having a relatively low work function to serve as a cathode. For example, the second electrode  230  can be formed of aluminum (Al), magnesium (Mg), calcium (Ca), silver (Ag) or their alloy or combination. In the top-emission type organic light emitting display device  100 , the second electrode  230  may have a thin profile (small thickness) to provide a light transmittance property (or a semi-transmittance property). 
     Further, the organic light emitting display device  100  can include a color filter corresponding to the red, green and blue pixel regions. For example, when the OLED D, which has the tandem structure and emits the white light, is formed to all of the red, green and blue pixel regions, a red color filter pattern, a green color filter pattern and a blue color filter pattern can be formed in the red, green and blue pixel regions, respectively, such that a full-color display is provided. 
     When the organic light emitting display device  100  is operated in a bottom-emission type, the color filter can be disposed between the OLED D and the substrate  110 , e.g., between the interlayer insulating layer  132  and the planarization layer  150 . Alternatively, when the organic light emitting display device  100  is operated in a top-emission type, the color filter can be disposed over the OLED D, e.g., over the second electrode  230 . 
     An encapsulation film  170  is formed on the second electrode  230  to prevent penetration of moisture into the OLED D. The encapsulation film  170  includes a first inorganic insulating layer  172 , an organic insulating layer  174  and a second inorganic insulating layer  176  sequentially stacked, but it is not limited thereto. 
     The Organic light emitting display device  100  may further include a polarization plate (not shown) for reducing an ambient light reflection. For example, the polarization plate can be a circular polarization plate. In the bottom-emission type organic light emitting display device  100 , the polarization plate may be disposed under the substrate  110 . In the top-emission type organic light emitting display device  100 , the polarization plate may be disposed on or over the encapsulation film  170 . 
     In addition, in the top-emission type organic light emitting display device  100 , a cover window can be attached to the encapsulation film  170  or the polarization plate. In this instance, the substrate  110  and the cover window have a flexible property such that a flexible organic light emitting display device can be provided. 
       FIG. 3  is a schematic cross-sectional view of an OLED according to a second embodiment of the present disclosure. 
     As shown in  FIG. 3 , the OLED D 1  includes the first and second electrodes  210  and  230 , which face each other, and the light emitting layer  220  therebetween. The light emitting layer  220  includes an emitting material layer (EML)  240 . The organic light emitting display device  100  (of  FIG. 2 ) may include a red pixel region, a green pixel region and a blue pixel region, and the OLED D 1  may be positioned in the blue pixel region. 
     The first electrode  210  can be an anode, and the second electrode  230  can be a cathode. 
     The light emitting layer  220  further include at least one of a hole transporting layer (HTL)  260  between the first electrode  210  and the EML  240  and an electron transporting layer (ETL)  270  between the second electrode  230  and the EML  240 . 
     In addition, the light emitting layer  220  can further include at least one of a hole injection layer (HIL)  250  between the first electrode  210  and the HTL  260  and an electron injection layer (EIL)  280  between the second electrode  230  and the ETL  270 . 
     Moreover, the light emitting layer  220  can further include at least one of an electron blocking layer (EBL)  265  between the HTL  260  and the EML  240  and a hole blocking layer (HBL)  275  between the EML  240  and the ETL  270 . 
     For example, the HIL  250  may include at least one compound selected from the group consisting of 4,4′,4″-tris(3-methylphenylamino)triphenylamine (MTDATA), 4,4′,4″-tris(N,N-diphenyl-amino)triphenylamine(NATA), 4,4′,4″-tris(N-(naphthalene-1-yl)-N-phenyl-amino)triphenylamine(1T-NATA), 4,4′,4″-tris(N-(naphthalene-2-yl)-N-phenyl-amino)triphenylamine(2T-NATA), copper phthalocyanine(CuPc), tris(4-carbazoyl-9-yl-phenyl)amine(TCTA), N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine(NPB; NPD), 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile(dipyrazino[2,3-f:2′3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile; HAT-CN), 1,3,5-tris[4-(diphenylamino)phenyl]benzene(TDAPB), poly(3,4-ethylenedioxythiphene)polystyrene sulfonate(PEDOT/PSS), and N-(biphenyl-4-yl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluoren-2-amine, but it is not limited thereto. 
     The HTL  260  may include at least one compound selected from the group consisting of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine; TPD), NPB(NPD), 4,4′-bis(N-carbazolyl)-1,1′-biphenyl(CBP), poly[N,N′-bis(4-butylpnehyl)-N,N′-bis(phenyl)-benzidine](Poly-TPD), (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyediphenylamine))] (TFB), di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane(TAPC), 3,5-di(9H-carbazol-9-yl)-N,N-diphenylaniline(DCDPA), N-(biphenyl-4-yl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluoren-2-amine, and N-(biphenyl-4-yl)-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)biphenyl-4-amine, but it is not limited thereto. 
     The ETL  270  may include at least one of an oxadiazole-based compound, a triazole-based compound, a phenanthroline-based compound, a benzoxazole-based compound, a benzothiazole-based compound, a benzimidazole-based compound, and a triazine-based compound. For example, the ETL  270  may include at least one compound selected from the group consisting of tris-(8-hydroxyquinoline aluminum(Alq 3 ), 2-biphenyl-4-yl-5-(4-t-butylphenyl)-1,3,4-oxadiazole(PBD), spiro-PBD, lithium quinolate(Liq), 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene(TPBi), bis(2-methyl-8-quinolinolato-N1,O8)-(1,1′-biphenyl-4-olato)aluminum(BAlq), 4,7-diphenyl-1,10-phenanthroline(Bphen), 2,9-bis(naphthalene-2-yl)4,7-diphenyl-1,10-phenanthroline(NBphen), 2,9-dimethyl-4,7-diphenyl-1,10-phenathroline(BCP), 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole(TAZ), 4-(naphthalen-1-yl)-3,5-diphenyl-4H-1,2,4-triazole(NTAZ), 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene(TpPyPB), 2,4,6-tris(3′-(pyridin-3-yl)biphenyl-3-yl)1,3,5-triazine(TmPPPyTz), Poly[9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene]-alt-2,7-(9,9-dioctylfluorene)](PFNBr), tris(phenylquinoxaline(TPQ), and diphenyl-4-triphenylsilyl-phenylphosphine oxide(TSPO1), but it is not limited thereto. 
     The EIL  280  may include at least one of an alkali halide compound, such as LiF, CsF, NaF, or BaF 2 , and an organo-metallic compound, such as Liq, lithium benzoate, or sodium stearate, but it is not limited thereto. 
     The EBL  265 , which is positioned between the HTL  260  and the EML  240  to block the electron transfer from the EML  240  into the HTL  260 , may include at least one compound selected from the group consisting of TCTA, tris[4-(diethylamino)phenyl]amine, N-(biphenyl-4-yl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluoren-2-amine, TAPC, MTDATA, 1,3-bis(carbazol-9-yl)benzene(mCP), 3,3′-bis(N-carbazolyl)-1,1′-biphenyl(mCBP), CuPc, N,N′-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(DNTPD), TDAPB, DCDPA, and 2,8-bis(9-phenyl-9H-carbazol-3-yl)dibenzo[b,d]thiophene), but it is not limited thereto. 
     The HBL  275 , which is positioned between the EML  240  and the ETL  270  to block the hole transfer from the EML  240  into the ETL  270 , may include the above material of the ETL  270 . For example, the material of the HBL  275  has a HOMO energy level being lower than a material of the EML  240  and may be at least one compound selected from the group consisting of BCP, BAlq, Alq3, PBD, spiro-PBD, Liq, bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine(B3PYMPM), bis[2-(diphenylphosphino)phenyl]teeth oxide(DPEPO), 9-(6-9H-carbazol-9-yl)pyridine-3-yl)-9H-3,9′-bicarbazole, and TSPO1, but it is not limited thereto. 
     The EML  240  includes a first compound of a delayed fluorescent material (compound) and a second compound of a fluorescent material (compound). The EML can further include a third compound as a host. The EML  240  including the first and second compounds emits blue light, and the OLED D 1  is positioned in the blue pixel region. 
     For example, the third compound as the host can be one of 9-(3-(9H-carbazol-9-yephenyl)-9H-carbazole-3-carbonitrile (mCP-CN), CBP, 3,3′-bis(N-carbazolyl)-1,1′-biphenyl (mCBP), 1,3-bis(carbazol-9-yebenzene (mCP), oxybis(2,1-phenylene))bis(diphenylphosphine oxide) (DPEPO), 2,8-bis(diphenylphosphoryl)dibenzothiophene (PPT), 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB), 2,6-di(9H-carbazol-9-yl)pyridine (PYD-2Cz), 2,8-di(9H-carbazol-9-yl)dibenzothiophene (DCzDBT), 3′,5′-di(carbazol-9-yl)-[1,1′-bipheyl]-3,5-dicarbonitrile (DCzTPA), 4′-(9H-carbazol-9-yl)biphenyl-3,5-dicarbonitrile (pCzB-2CN), 3′-(9H-carbazol-9-yl)biphenyl-3,5-dicarbonitrile (mCzB-2CN), diphenyl-4-triphenylsilylphenyl-phosphine oxide (TSPO1), 9-(9-phenyl-9H-carbazol-6-yl)-9H-carbazole (CCP), 4-(3-(triphenylen-2-yl)phenyl)dibenzo[b,d]thiophene), 9-(4-(9H-carbazol-9-yl)phenyl)-9H-3,9′-bicarbazole, 9-(3-(9H-carbazol-9-yl)phenyl)-9H-3,9′-bicarbazole, or 9-(6-(9H-carbazol-9-yl)pyridin-3-yl)-9H-3,9′-bicabazole, but it is not limited thereto. 
     The delayed fluorescent material as the first compound in the EML  240  is represented by Formula 1. 
     
       
         
         
             
             
         
       
     
     In Formula 1, each of R1 and R2 is independently selected from the group consisting of hydrogen, deuterium, tritium, halogen, C1 to C20 alkyl, C6 to C30 aryl, C5 to C30 heteroaryl, C1 to C20 alkylamine, and C6 to C30 arylamine, and R3 is C5 to C60 heteroaryl including a nitrogen atom. 
     For example, each of R1 and R2 can be independently selected from the group consisting of hydrogen, C1 to C20 alky, e.g., tert-butyl, and C6 to C30 aryl, e.g., phenyl. R3 can be heteroaryl having an electron donor property. For example, R3 can be selected from the group consisting of indolocarbazolyl, diindolocarbazolyl, bis-carbazolyl, acridinyl, spiroacridinyl, phenoxazinyl, phenothiazinyl and their derivatives. 
     The term of “alkyl”, “aryl” and “heteroaryl” can include that substituted one and non-substituted one. When they are substituted, the substituent can be at least one of C1 to C20 alkyl, C6 to C30 aryl and C5 to C30 heteroaryl. 
     For example, the first compound can be one of compounds of Formula 2. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     A difference between a singlet energy level and a triplet energy level of the delayed fluorescent material is very small (is less than about 0.3 eV). The energy of the triplet exciton of the delayed fluorescent material is converted into the singlet exciton by a reverse intersystem crossing (RISC) such that the delayed fluorescent material has high quantum efficiency. However, since the delayed fluorescent material has wide full width at half maximum (FWHM), the delayed fluorescent material has a disadvantage in a color purity. 
     To overcome the problem of the color purity of the delayed fluorescent material, the EML  240  further includes the second compound of the fluorescent material to provide a hyper fluorescence. 
     The second compound of the fluorescent material is represented by Formula 3. 
     
       
         
         
             
             
         
       
     
     In Formula 3, each of R11 to R14 is independently selected from the group consisting of hydrogen, deuterium, tritium, boron, nitrogen, C1 to C20 alkyl, C6 to C30 aryl, C5 to C30 heteroaryl, C1 to C20 alkylamine, and C6 to C30 arylamine, or adjacent two of R11 to R14 are combined to form a fused ring including boron and nitrogen. Each of R15 and R18 is independently selected from the group consisting of hydrogen, deuterium, tritium, boron and nitrogen, C1 to C20 alkyl, C6 to C30 aryl, C5 to C30 heteroaryl, C1 to C20 alkylamine, and C6 to C30 arylamine. 
     For example, R12 and R13 can be combined to form a fused ring including boron and nitrogen. 
     The term of “alkyl”, “aryl” and “heteroaryl” can include that substituted one and non-substituted one. When they are substituted, the substituent can be at least one of C1 to C20 alkyl, C6 to C30 aryl and C5 to C30 heteroaryl. 
     The Formula 3 as the second compound can be represented by Formula 4-1 or Formula 4-2. 
     
       
         
         
             
             
         
       
     
     In Formula 4-1, each of R15 to R18 and R21 to R24 is independently selected from the group consisting of hydrogen, deuterium, tritium, boron and nitrogen, C1 to C20 alkyl, C6 to C30 aryl, C5 to C30 heteroaryl, C1 to C20 alkylamine, and C6 to C30 arylamine. In Formula 4-2, each of R15 to R18 and R31 to R34 is independently selected from the group consisting of hydrogen, deuterium, tritium, boron and nitrogen, C1 to C20 alkyl, C6 to C30 aryl, C5 to C30 heteroaryl, C1 to C20 alkylamine, and C6 to C30 arylamine. 
     For example, the second compound can be one of compounds of Formula 5. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The EML  240  in the OLED D of the present disclosure includes the first compound and the second compound, and the exciton of the first compound is transported into the second compound. As a result, the OLED D provide the emission with narrow FWHM and high emitting efficiency. 
     In the OLED D of the present disclosure, an energy level of the first compound and an energy level of the second compound satisfy a pre-determined condition such that the transporting efficiency of the exciton from the first compound into the second compound is increased. Accordingly, the emitting efficiency and the color purity of the OLED D and the organic light emitting display device are improved. 
     In addition, the energy level of the first compound, the energy level of the second compound and an energy level of the third compound satisfy a pre-determined condition such that the emitting efficiency and the color purity of the OLED D and the organic light emitting display device can be further improved. 
     Referring to  FIGS. 4A and 4B , which are a schematic view illustrating an energy level relation of a first compound and a second compound in the OLED of the present disclosure, a difference “ΔHOMO” between a highest occupied molecular orbital (HOMO) energy level “HOMO1” of the first compound and a HOMO energy level “HOMO2” of the second compound is less than 0.3 eV. 
     Namely, the HOMO energy level “HOMO1” of the first compound can be higher than the HOMO energy level “HOMO2” of the second compound as shown in  FIG. 4A , or the HOMO energy level “HOMO1” of the first compound can be lower than the HOMO energy level “HOMO2” of the second compound as shown in  FIG. 4B . In this instance, by satisfying the condition of “ΔHOMO&lt;0.3 eV”, the exciton generated in the host is efficiently transferred into the second compound through the first compound. For example, the difference “ΔHOMO” between the HOMO energy level “HOMO1” of the first compound and the HOMO energy level “HOMO2” of the second compound can be about 0.2 eV or less. 
     In addition, a difference “ΔLUMO” between a lowest unoccupied molecular orbital (LUMO) energy level “LUMO1” of the first compound and the LUMO energy level “LUMO2” of the second compound is about 0.3 eV or less. The LUMO energy level “LUMO1” of the first compound can be higher than the LUMO energy level “LUMO2” of the second compound. 
     As mentioned above, the first compound of the delayed fluorescent material uses the singlet exciton energy and the triplet exciton energy for emission. Accordingly, in the EML  240  including the first and second compounds, the energy of the first compound is transported into the second compound, and the light is emitted from the second compound. As a result, the emitting efficiency and the color purity of the OLED are improved. 
     On the other hand, if a delayed fluorescent material and a fluorescent material in the EML do not satisfy the above energy level relation, there is a limitation in the emitting efficiency and/or the color purity. Namely, referring to  FIG. 4C , when a difference “ΔHOMO” between a HOMO energy level “HOMO1” of the delayed fluorescent material and a HOMO energy level “HOMO2” of the fluorescent material is greater than or equal to 0.3 eV, a hole can be directly transferred from the host into the fluorescent material such that the emission can be directly generated from the fluorescent material. As a result, the emitting efficiency can be reduced. In addition, as shown in  FIG. 4D , when a difference “ΔHOMO” between a HOMO energy level “HOMO1” of the delayed fluorescent material and a HOMO energy level “HOMO2” of the fluorescent material is above 0.5 eV and a LUMO energy level “LUMO1” of the delayed fluorescent material is lower than a LUMO energy level “LUMO2” of the fluorescent material, an exciplex can be generated between a hole trapped in the fluorescent material and the LUMO energy level of the delayed fluorescent material such that the exciplex emission can be generated. As a result, the emitting wavelength range can be shifted. 
     In the EML  240 , the singlet energy level of the first compound is smaller than that of the third compound as the host and greater than that of the second compound. In addition, the triplet energy level of the first compound is smaller than that of the third compound as the host and greater than that of the second compound. 
     In the EML  240 , a weight ratio (weight %) of the first compound can be greater than that of the second compound and can be smaller than that of the third compound. When the weight ratio of the first compound is greater than that of the second compound, the energy of the first compound is sufficiently transferred into the second compound. For example, in the EML  240 , the first compound can have a weight % of about 20 to 40, and the second compound can have a weight % of about 0.1 to 5. However, it is not limited thereto. 
     Referring to  FIG. 5 , which is a view illustrating an emission mechanism of an OLED according to the second embodiment of the present disclosure, the single level “S1” and the triplet level “T1” generated in the third compound as a host is respectively transferred into the singlet energy “S1” and the triplet level “T1” of the first compound as a delayed fluorescent material. Since a difference between the singlet energy level of the first compound and the triplet energy level of the first compound is relatively small, the triplet energy level “T1” of the first compound is converted into the singlet energy level “S1” of the first compound by the RISC. For example, the difference (ΔE ST ) between the singlet energy level of the first compound and the triplet energy level of the first compound can be about 0.3 eV or less. Then, the singlet energy level “S1” of the first compound is transferred into the singlet energy level “S1” of the second compound such that the second compound provide the emission. 
     As mentioned above, the first compound having a delayed fluorescent property has high quantum efficiency. However, since the first compound has wide FWHM, the first compound has a disadvantage in a color purity. On the other hand, the second compound having a fluorescent property has narrow FWHM. However, since the triplet exciton of the second compound is not involved in the emission, the second compound has a disadvantage in an emitting efficiency. 
     However, in the OLED D 1  of the present disclosure, the singlet energy level of the first compound as the delayed fluorescent material is transferred into the second compound as the fluorescent dopant, and the emission is generated from the second compound. Accordingly, the emitting efficiency and the color purity of the OLED D 1  are improved. In addition, since the first compound of Formulas 1 and 2 and the second compound of Formulas 3 to 5 are included in the EML  240 , the emitting efficiency and the color purity of the OLED D 1  are further improved. [OLED] 
     An anode (ITO, 50 nm), an HIL (HAT-CN (Formula 6-1), 7 nm), an HTL (NPB (Formula 6-2), 45 nm), an EBL (TAPC (Formula 6-3), 10 nm), an EML (30 nm), an HBL (B3PYMPM (Formula 6-4), 10 nm), an ETL (TPBi (Formula 6-5), 30 nm), an EIL (LiF) and a cathode are sequentially deposited to form an OLED. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     (1) Comparative Example 1 (Ref1) 
     A host (m-CBP (Formula 7), 69 wt %), a compound 1-1 of Formula 2 (30 wt %) and the compound 8-1 of Formula 8 (1 wt %) are used to form the EML. 
     (2) Comparative Example 2 (Ref2) 
     A host (m-CBP (Formula 7), 69 wt %), a compound 1-1 of Formula 2 (30 wt %) and the compound 8-2 of Formula 8 (1 wt %) are used to form the EML. 
     (3) Comparative Example 3 (Ref3) 
     A host (m-CBP (Formula 7), 69 wt %), a compound 1-6 of Formula 2 (30 wt %) and the compound 8-1 of Formula 8 (1 wt %) are used to form the EML. 
     (4) Comparative Example 4 (Ref4) 
     A host (m-CBP (Formula 7), 69 wt %), a compound 1-6 (30 wt %) and the compound 8-2 of Formula 8 (1 wt %) are used to form the EML. 
     (5) Comparative Example 5 (Ref5) 
     A host (m-CBP (Formula 7), 69 wt %), a compound 9-1 of Formula 9 (30 wt %) and the compound 2-1 of Formula 5 (1 wt %) are used to form the EML. 
     (6) Comparative Example 6 (Ref6) 
     A host (m-CBP (Formula 7), 69 wt %), a compound 9-1 of Formula 9 (30 wt %) and the compound 2-10 of Formula 5 (1 wt %) are used to form the EML. 
     (7) Comparative Example 7 (Ref7) 
     A host (m-CBP (Formula 7), 69 wt %), a compound 9-1 of Formula 9 (30 wt %) and the compound 8-1 of Formula 8 (1 wt %) are used to form the EML. 
     (8) Comparative Example 8 (Ref8) 
     A host (m-CBP (Formula 7), 69 wt %), a compound 9-1 of Formula 9 (30 wt %) and the compound 8-2 of Formula 8 (1 wt %) are used to form the EML. 
     (9) Comparative Example 9 (Ref9) 
     A host (m-CBP (Formula 7), 69 wt %), a compound 9-2 of Formula 9 (30 wt %) and the compound 2-1 of Formula 5 (1 wt %) are used to form the EML. 
     (10) Comparative Example 10 (Ref10) 
     A host (m-CBP (Formula 7), 69 wt %), a compound 9-2 of Formula 9 (30 wt %) and the compound 8-1 of Formula 8 (1 wt %) are used to form the EML. 
     (11) Example 1 (Ex1) 
     A host (m-CBP (Formula 7), 69 wt %), the compound 1-1 of Formula 2 (30 wt %) and the compound 2-1 of Formula 5 (1 wt %) are used to form the EML. 
     (12) Example 2 (Ex2) 
     A host (m-CBP (Formula 7), 69 wt %), the compound 1-6 of Formula 2 (30 wt %) and the compound 2-1 of Formula 5 (1 wt %) are used to form the EML. 
     (13) Example 3 (Ex3) 
     A host (m-CBP (Formula 7), 69 wt %), the compound 1-1 of Formula 2 (30 wt %) and the compound 2-10 of Formula 5 (1 wt %) are used to form the EML. 
     (14) Example 4 (Ex4) 
     A host (m-CBP (Formula 7), 69 wt %), the compound 1-6 of Formula 2 (30 wt %) and the compound 2-10 of Formula 5 (1 wt %) are used to form the EML. 
     (15) Example 5 (Ex5) 
     A host (m-CBP (Formula 7), 69 wt %), the compound 1-1 of Formula 2 (30 wt %) and the compound 2-6 of Formula 5 (1 wt %) are used to form the EML. 
     (16) Example 6 (Ex6) 
     A host (m-CBP (Formula 7), 69 wt %), the compound 1-6 of Formula 2 (30 wt %) and the compound 2-6 of Formula 5 (1 wt %) are used to form the EML. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The emitting properties of the OLED in Comparative Examples 1 to 10 and Examples 1 to 6 are measured and listed in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Dopant1 
                 Dopant2 
                 V 
                 cd/A 
                 lm/W 
                 CIE(y) 
                 EQE [%] 
                 Δ HOMO [eV] 
                 Exciplex 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Ref1 
                 1-1 
                 8-1 
                 4.98 
                 12.1 
                 7.6 
                 0.184 
                 9.4 
                 0.3 
                 X 
               
               
                 Ref2 
                 1-1 
                 8-2 
                 4.82 
                 13.7 
                 8.9 
                 0.191 
                 10.4 
                 0.3 
                 X 
               
               
                 Ref3 
                 1-6 
                 8-1 
                 5.22 
                 7.9 
                 4.7 
                 0.154 
                 7.0 
                 0.4 
                 X 
               
               
                 Ref4 
                 1-6 
                 8-2 
                 6.14 
                 5.6 
                 2.9 
                 0.163 
                 5.3 
                 0.4 
                 X 
               
               
                 Ref5 
                 9-1 
                 2-1 
                 4.98 
                 12.1 
                 7.6 
                 0.208 
                 8.5 
                 0.3 
                 X 
               
               
                 Ref6 
                 9-1 
                  2-10 
                 4.87 
                 12.3 
                 8.4 
                 0.216 
                 10.8 
                 0.5 
                 X 
               
               
                 Ref7 
                 9-1 
                 8-1 
                 3.53 
                 16.4 
                 10.7 
                 0.417 
                 9.4 
                 0.7 
                 ◯ 
               
               
                 Ref8 
                 9-1 
                 8-2 
                 3.57 
                 24.5 
                 21.0 
                 0.336 
                 10.7 
                 0.7 
                 ◯ 
               
               
                 Ref9 
                 9-2 
                 2-1 
                 5.05 
                 10.2 
                 6.7 
                 0.212 
                 6.9 
                 0.4 
                 X 
               
               
                 Ref10 
                 9-2 
                 8-1 
                 3.35 
                 17.1 
                 13.5 
                 0.405 
                 8.3 
                 0.6 
                 ◯ 
               
               
                 Ex1 
                 1-1 
                 2-1 
                 3.72 
                 46.1 
                 38.9 
                 0.207 
                 21.2 
                 0.1 
                 X 
               
               
                 Ex2 
                 1-6 
                 2-1 
                 3.71 
                 47.2 
                 41.7 
                 0.185 
                 20.3 
                 0 
                 X 
               
               
                 Ex3 
                 1-1 
                  2-10 
                 3.78 
                 40.2 
                 33.7 
                 0.201 
                 20.6 
                 0.1 
                 X 
               
               
                 Ex4 
                 1-6 
                  2-10 
                 3.83 
                 41.4 
                 33.9 
                 0.149 
                 19.8 
                 0.2 
                 X 
               
               
                 Ex5 
                 1-1 
                 2-6 
                 3.47 
                 38.3 
                 30.4 
                 0.192 
                 18.5 
                 0.0 
                 X 
               
               
                 Ex6 
                 1-6 
                 2-6 
                 3.58 
                 37.1 
                 31.5 
                 0.183 
                 17.8 
                 0.1 
                 X 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, in comparison to the OLED of Comparative Examples 1 to 10, the emitting properties of the OLED in Examples 1 to 6 using the first compound of Formulas 1 and 2 and the second compound of Formulas 3 to 5 are improved. 
     The HOMO energy level and the LUMO energy level of the compounds 1-1 and 1-6 of Formula 2 as the first compound of the present disclosure, the compounds 2-1, 2-6 and 2-10 of Formula 5 as the second compound of the present disclosure, the compounds 8-1 and 8-2 of Formula 8, and the compounds 9-1 and 9-2 of Formula 9 are measured and listed Table 2. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 HOMO 
                 LUMO 
               
               
                   
                 compound 
                 [eV] 
                 [eV] 
               
               
                   
                   
               
             
            
               
                   
                 1-1 
                 −5.5 
                 −2.6 
               
               
                   
                 1-6 
                 −5.6 
                 −2.7 
               
               
                   
                 2-1 
                 −5.6 
                 −2.9 
               
               
                   
                 2-6 
                 −5.4 
                 −2.8 
               
               
                   
                 2-10 
                 −5.5 
                 −2.9 
               
               
                   
                 8-1 
                 −5.2 
                 −2.7 
               
               
                   
                 8-2 
                 −5.2 
                 −2.6 
               
               
                   
                 9-1 
                 −5.9 
                 −2.8 
               
               
                   
                 9-2 
                 −6.0 
                 −3.0 
               
               
                   
                   
               
            
           
         
       
     
     In the OLED of Examples 1 to 6, the difference between the HOMO energy level of the first compound and the HOMO energy level of the second compound is less than 0.3 eV such that the emitting efficiency of the OLED is improved. On the other hand, in the OLED of Comparative Examples 1 to 10, the difference between the HOMO energy level of the first compound and the HOMO energy level of the second compound is greater than or equal to 0.3 eV such that the emitting efficiency of the OLED is lowered and/or the emitting wavelength range is shifted. 
       FIG. 6  is a schematic cross-sectional view of an OLED according to a third embodiment of the present disclosure. 
     As shown in  FIG. 6 , an OLED D 2  according to the third embodiment of the present disclosure includes the first and second electrodes  310  and  330 , which face each other, and the light emitting layer  320  therebetween. The light emitting layer  320  includes an EML  340 . The organic light emitting display device  100  (of  FIG. 2 ) may include a red pixel region, a green pixel region and a blue pixel region, and the OLED D 2  may be positioned in the blue pixel region. 
     The first electrode  310  can be an anode, and the second electrode  330  can be a cathode. 
     The light emitting layer  320  can further includes at least one of the HTL  360  between the first electrode  310  and the EML  340  and the ETL  370  between the second electrode  330  and the EML  340 . 
     In addition, the light emitting layer  320  can further include at least one of the HIL  350  between the first electrode  310  and the HTL  360  and the EIL  380  between the second electrode  330  and the ETL  370 . 
     Moreover, the light emitting layer  320  can further include at least one of the EBL  365  between the HTL  360  and the EML  340  and the HBL  375  between the EML  340  and the ETL  370 . 
     The EML  340  includes a first EML (a first layer or a lower emitting material layer)  342  and a second EML (a second layer or an upper emitting material layer)  344  sequentially stacked over the first electrode  310 . Namely, the second EML  344  is positioned between the first EML  342  and the second electrode  330 . 
     In the EML  340 , one of the first and second EMLs  342  and  344  includes a first compound including a hexagonal-ring moiety (e.g., six-membered ring), which includes one boron atom, one oxygen atom and four carbon atoms, and the other one of the first and second EMLs  342  and  344  includes a second compound including a hexagonal-ring moiety, which includes one boron atom, one nitrogen atom and four carbon atoms. 
     For example, the first compound can be represented by Formula 1 and can be one of the compounds in Formula 2. The first compound has a delayed fluorescent property. The second compound can be represented by one of Formulas 3, 4-1 and 4-2 and can be one of the compounds in Formula 5. The second compound has a fluorescent property. 
     In addition, the first and second EMLs  342  and  344  further include a fourth compound and a fifth compound as a host, respectively. The fourth compound in the first EML  342  and the fifth compound in the second EML  344  can be same or different. For example, each of the host of the first EML  342 , i.e., the fourth compound, and the host of the second EML  344 , i.e., the fifth compound, can be the above third compound. 
     The OLED, where the first EML  342  includes the first compound of the delayed fluorescent material, will be explained. 
     As mentioned above, the first compound having a delayed fluorescent property has high quantum efficiency. However, since the first compound has wide FWHM, the first compound has a disadvantage in a color purity. On other hand, the second compound having a fluorescent property has narrow FWHM. However, the triplet exciton of the second compound is not involved in the emission, the second compound has a disadvantage in an emitting efficiency. 
     In the OLED D 2 , since the triplet exciton energy of the first compound in the first EML  342  is converted into the singlet exciton energy of the first compound and the singlet exciton energy of the first compound is transferred into the singlet exciton energy of the second compound in the second EML  344  by the RISC, the second compound provides the emission. Accordingly, both the singlet exciton energy and the triplet exciton energy are involved in the emission such that the emitting efficiency is improved. In addition, since the emission is provided from the second compound of the fluorescent material, the emission having narrow FWHM is provided. 
     As mentioned above, the difference between the HOMO energy level of the first compound and the HOMO energy level of the second compound is less than 0.3 eV such that the emitting efficiency of the OLED D 2  is further improved. 
     In the first EML  342 , the weight ratio of the fourth compound can be equal to or greater than that of the first compound. In the second EML  344 , the weight ratio of the fifth compound can be equal to or less than that of the second compound. 
     In addition, a weight ratio of the first compound in the first EML  342  can be greater than that of the second compound in the second EML  344 . As a result, the energy is sufficiently transferred from the first compound in the first EML  342  into the second compound in the second EML  344  by a fluorescence resonance energy transfer (FRET). For example, the first compound can have a weight % of about 1 to 50 in the first EML  342 , preferably about 10 to 40, more preferably about 20 to 40. The second compound can have a weight % of about 0.1 to 10 in the second EML  344 , preferably about 0.1 to 5. 
     When the HBL  375  is positioned between the second EML  344  and the ETL  370 , the fifth compound as the host of the second EML  344  can be same as a material of the HBL  375 . In this instance, the second EML  344  can have a hole blocking function with an emission function. Namely, the second EML  344  can serve as a buffer layer for blocking the hole. When the HBL  375  is omitted, the second EML  344  can serve as an emitting material layer and a hole blocking layer. 
     When the first EML  342  includes the second compound of the fluorescent material and the EBL  365  is positioned between the HTL  360  and the first EML  342 , the host of the first EML  342  can be same as a material of the EBL  365 . In this instance, the first EML  342  can have an electron blocking function with an emission function. Namely, the first EML  342  can serve as a buffer layer for blocking the electron. When the EBL  365  is omitted, the first EML  342  can serve as an emitting material layer and an electron blocking layer. 
       FIG. 7  is a schematic cross-sectional view of an OLED according to a fourth embodiment of the present disclosure. 
     As shown in  FIG. 7 , an OLED D 3  according to the fourth embodiment of the present disclosure includes the first and second electrodes  410  and  430 , which face each other, and the light emitting layer  420  therebetween. The light emitting layer  420  includes an EML  440 . The organic light emitting display device  100  (of  FIG. 2 ) may include a red pixel region, a green pixel region and a blue pixel region, and the OLED D 3  may be positioned in the blue pixel region. 
     The first electrode  410  can be an anode, and the second electrode  430  can be a cathode. 
     The light emitting layer  420  can further includes at least one of the HTL  460  between the first electrode  410  and the EML  440  and the ETL  470  between the second electrode  430  and the EML  440 . 
     In addition, the light emitting layer  420  can further include at least one of the HIL  450  between the first electrode  410  and the HTL  460  and the EIL  480  between the second electrode  430  and the ETL  470 . 
     Moreover, the light emitting layer  420  can further include at least one of the EBL  465  between the HTL  460  and the EML  440  and the HBL  475  between the EML  440  and the ETL  470 . 
     The EML  440  includes a first EML (a first layer, an intermediate emitting material layer)  442 , a second EML (a second layer, a lower emitting material layer)  444  between the first EML  442  and the first electrode  410 , and a third EML (a third layer, an upper emitting material layer)  446  between the first EML  442  and the second electrode  430 . Namely, the EML  440  has a triple-layered structure of the second EML  444 , the first EML  442  and the third EML  446  sequentially stacked. 
     For example, the first EML  442  can be positioned between the EBL  465  and the HBL  475 , the second EML  444  can be positioned between the EBL  465  and the first EML  442 , and the third EML  446  can be positioned between the HBL  475  and the first EML  442 . 
     In the EML  440 , the first EML  442  includes a first compound including a hexagonal-ring moiety, which includes one boron atom, one oxygen atom and four carbon atoms, and each of the second and third EMLs  444  and  446  includes a second compound including a hexagonal-ring moiety, which includes one boron atom, one nitrogen atom and four carbon atoms. 
     For example, the first compound can be represented by Formula 1 and can be one of the compounds in Formula 2. The first compound has a delayed fluorescent property. The second compound can be represented by one of Formulas 3, 4-1 and 4-2 and can be one of the compounds in Formula 5. The second compound has a fluorescent property. The second compound of the second EML  444  and the second compound of the third EML  446  can be same or different. 
     In addition, the first to third EMLs  442 ,  444  and  446  further include a sixth compound, a seventh compound and an eighth compound as a host, respectively. The sixth compound in the first EML  442 , the seventh compound in the second EML  444  and the eighth compound in the third EML  446  can be same or different. For example, each of the host of the first EML  442 , i.e., the sixth compound, the host of the second EML  444 , i.e., the seventh compound, and the host of the third EML  446 , i.e., the eighth compound can be the above third compound. 
     In the OLED D 3 , since the triplet exciton energy of the first compound in the first EML  442  is converted into the singlet exciton energy of the first compound and the singlet exciton energy of the first compound is transferred into the singlet exciton energy of the second compound in the second EML  444  and into the singlet exciton energy of the second compound in the third EML  446  by the RISC, the second compound in the second and third EMLs  444  and  446  provides the emission. Accordingly, both the singlet exciton energy and the triplet exciton energy are involved in the emission such that the emitting efficiency is improved. In addition, since the emission is provided from the second compound of the fluorescent material, the emission having narrow FWHM is provided. 
     As mentioned above, the difference between the HOMO energy level of the first compound and the HOMO energy level of the second compound is less than 0.3 eV such that the emitting efficiency of the OLED D 2  is further improved. 
     In the first EML  442 , the weight ratio of the sixth compound can be equal to or greater than that of the first compound. In the second EML  444 , the weight ratio of the seventh compound can be equal to or less than that of the second compound. In the third EML  446 , the weight ratio of the eighth compound can be equal to or less than that of the second compound. 
     In addition, a weight ratio of the first compound in the first EML  442  can be greater than each of that of the second compound in the second EML  444  and that of the second compound in the third EML  446 . As a result, the energy is sufficiently transferred from the first compound in the first EML  442  into the second compound in the second EML  444  and the second compound in the third EML  446  by a fluorescence resonance energy transfer (FRET). For example, the first compound can have a weight % of about 1 to 50 in the first EML  442 , preferably about 10 to 40, more preferably about 20 to 40. The second compound can have a weight % of about 0.1 to 10 in each of the second EML  444  and the third EML  446 , preferably about 0.1 to 5. 
     The seventh compound as the host of the second EML  444  can be same as a material of the EBL  465 . In this instance, the second EML  444  can have an electron blocking function with an emission function. Namely, the second EML  444  can serve as a buffer layer for blocking the electron. When the EBL  465  is omitted, the second EML  444  can serve as an emitting layer and an electron blocking layer. 
     The eighth compound as the host of the third EML  446  can be same as a material of the HBL  475 . In this instance, the third EML  446  can have a hole blocking function with an emission function. Namely, the third EML  446  can serve as a buffer layer for blocking the hole. When the HBL  475  is omitted, the third EML  446  can serve as an emitting layer and a hole blocking layer. 
     The seventh compound in the second EML  444  can be same as a material of the EBL  465 , and the eighth compound in the third EML  446  can be same as a material of the HBL  475 . In this instance, the second EML  444  can have an electron blocking function with an emission function, and the third EML  446  can have a hole blocking function with an emission function. Namely, the second EML  444  can serve as a buffer layer for blocking the electron, and the third EML  446  can serve as a buffer layer for blocking the hole. When the EBL  465  and the HBL  475  are omitted, the second EML  444  can serve as an emitting material layer and an electron blocking layer and the third EML  446  serves as an emitting material layer and a hole blocking layer. 
       FIG. 8  is a schematic cross-sectional view of an OLED according to a fifth embodiment of the present disclosure. 
     As shown in  FIG. 8 , the OLED D 4  includes the first and second electrodes  510  and  530 , which face each other, and the emitting layer  520  therebetween. The organic light emitting display device  100  (of  FIG. 2 ) may include a red pixel region, a green pixel region and a blue pixel region, and the OLED D 4  may be positioned in the blue pixel region. 
     The first electrode  510  may be an anode, and the second electrode  530  may be a cathode. 
     The emitting layer  520  includes a first emitting part  540  including a first EML  550  and a second emitting part  560  including a second EML  570 . In addition, the emitting layer  520  may further include a charge generation layer (CGL)  580  between the first and second emitting parts  540  and  560 . 
     The CGL  580  is positioned between the first and second emitting parts  540  and  560  such that the first emitting part  540 , the CGL  580  and the second emitting part  560  are sequentially stacked on the first electrode  510 . Namely, the first emitting part  540  is positioned between the first electrode  510  and the CGL  580 , and the second emitting part  580  is positioned between the second electrode  530  and the CGL  580 . 
     The first emitting part  540  includes the first EML  550 . 
     In addition, the first emitting part  540  may further include at least one of a first HTL  540   b  between the first electrode  510  and the first EML  550 , an HIL  540   a  between the first electrode  510  and the first HTL  540   b , and a first ETL  540   e  between the first EML  550  and the CGL  580 . 
     Moreover, the first emitting part  540  may further include at least one of a first EBL  540   c  between the first HTL  540   b  and the first EML  550  and a first HBL  540   d  between the first EML  550  and the first ETL  540   e.    
     The second emitting part  560  includes the second EML  570 . 
     In addition, the second emitting part  560  may further include at least one of a second HTL  560   a  between the CGL  580  and the second EML  570 , a second ETL  560   d  between the second EML  570  and the second electrode  164 , and an EIL  560   e  between the second ETL  560   d  and the second electrode  530 . 
     Moreover, the second emitting part  560  may further include at least one of a second EBL  560   b  between the second HTL  560   a  and the second EML  570  and a second HBL  560   c  between the second EML  570  and the second ETL  560   d.    
     The CGL  580  is positioned between the first and second emitting parts  540  and  560 . Namely, the first and second emitting parts  540  and  560  are connected to each other through the CGL  580 . The CGL  580  may be a P-N junction type CGL of an N-type CGL  582  and a P-type CGL  584 . 
     The N-type CGL  582  is positioned between the first ETL  540   e  and the second HTL  560   a , and the P-type CGL  584  is positioned between the N-type CGL  582  and the second HTL  560   a . The N-type CGL  582  provides an electron into the first EML  550  of the first emitting part  540 , and the P-type CGL  584  provides a hole into the second EML  570  of the second emitting part  560 . 
     The first and second EMLs  550  and  570  are a blue EML. At least one of the first and second EMLs  550  and  570  includes the first compound of Formula 1 and the second compound of Formula 3. For example, the first EML  550  may include the first being the delayed fluorescent compound and the second compound being the fluorescent compound. The first EML  550  may further include a third compound being a host. 
     In the first EML  550 , the weight ratio of the first compound may be greater than that of the second compound and smaller than that of the third compound. When the weight ratio of the first compound is greater than that of the second compound, the energy transfer from the first compound into the second compound is sufficiently generated. For example, in the first EML  550 , the first compound may have a weight % of about 20 to 40, the second compound may have a weight % of about 0.1 to 10, and the third compound may have a weight % of about 50 to 80. However, it is not limited thereto. 
     The second EML  570  may include the first compound of Formula 1 and the second compound of Formula 3. Namely, the second EML  570  may have the same organic compound as the first EML  550 . Alternatively, the second EML  570  may include a compound being different from at least one of the first compound and the second compound in the first EML  550  such that the first and second EMLs  550  and  570  have a different in an emitted-light wavelength or an emitting efficiency. 
     In the OLED D 4  of the present disclosure, the singlet energy level of the first compound as the delayed fluorescent material is transferred into the second compound as the fluorescent dopant, and the emission is generated from the second compound. Accordingly, the emitting efficiency and the color purity of the OLED D 4  are improved. In addition, since the first compound of Formulas 1 and 2 and the second compound of Formulas 3 to 5 are included in the first EML  550 , the emitting efficiency and the color purity of the OLED D 1  are further improved. Moreover, since the OLED D 4  has a two-stack structure (double-stack structure) with two green EMLs, the color sense of the OLED D 4  is improved and/or the emitting efficiency of the OLED D 4  is optimized. 
       FIG. 9  is a schematic cross-sectional view of an organic light emitting display device according to a sixth embodiment of the present disclosure. 
     As shown in  FIG. 9 , the organic light emitting display device  600  includes a first substrate  610 , where a first pixel region P 1 , a second pixel region P 2  and a third pixel region P 3  are defined, a second substrate  670  facing the first substrate  610 , an OLED D, which is positioned between the first and second substrates  610  and  670  and providing blue emission, and a color conversion layer  680  between the OLED D and the second substrate  670 . For example, the first pixel region P 1  may be a blue pixel region, the second pixel region P 2  may be a red pixel region, and the third pixel region P 3  may be a green pixel region. 
     Each of the first and second substrates  610  and  670  may be a glass substrate or a flexible substrate. For example, the flexible substrate may be a polyimide (PI) substrate, a polyethersulfone (PES) substrate, a polyethylenenaphthalate (PEN) substrate, a polyethylene terephthalate (PET) substrate or a polycarbonate (PC) substrate. 
     A TFT Tr, which corresponding to each of the red, green and blue pixels RP, GP and BP, is formed on the first substrate  610 . Alternatively, a buffer layer (not shown) may be formed on the first substrate  610 , and the TFT Tr may be formed on the buffer layer. 
     As explained with  FIG. 2 , the TFT Tr may include a semiconductor layer, a gate electrode, a source electrode and a drain electrode and may serve as a driving element. 
     A planarization layer (or passivation layer)  650  is formed on the TFT Tr. The planarization layer  1050  has a flat top surface and includes a drain contact hole  652  exposing the drain electrode of the TFT Tr. 
     The OLED D is disposed on the planarization layer  650  and includes a first electrode  660 , an emitting layer  662  and a second electrode  664 . The first electrode  660  is connected to the drain electrode of the TFT Tr, and the emitting layer  662  and the second electrode  664  are sequentially stacked on the first electrode  660 . 
     The first electrode  660  is formed to be separate in the first to third pixel regions P 1  to P 3 , and the second electrode  664  is formed as one-body to cover the first to third pixel regions P 1  to P 3 . 
     The first electrode  660  is one of an anode and a cathode, and the second electrode  664  is the other one of the anode and the cathode. In addition, the first electrode  660  may be a reflecting electrode, and the second electrode  664  may be a light transmitting electrode (or a semi-transmitting electrode). 
     For example, the first electrode  660  may be the anode and may be formed of a conductive material having a relatively high work function. The first electrode  660  may include a transparent conductive oxide material layer formed of a transparent conductive oxide (TCO) material and a reflection layer (or a reflection electrode layer). The second electrode  664  may be the cathode and may include a metallic material layer formed of a low resistance metallic material having a relatively low work function. For example, the first electrode  660  may have a structure of ITO/Ag/ITO or ITO/APC/ITO, but it is not limited thereto. The second electrode  664  may include Al, Mg, Ca, Ag or Mg—Ag alloy and may have a thin profile to be transparent (or semi-transparent). 
     A bank layer  666  is formed on the planarization layer  650  to cover an edge of the first electrode  660 . Namely, the bank layer  666  is positioned at a boundary of the first to third pixel regions P 1  to P 3  and exposes a center of the first electrode  660  in the first to third pixel regions P 1  to P 3 . Since the OLED D emits the blue light in the first to third pixel regions P 1  to P 3 , the emitting layer  662  may be formed as a common layer in the first to third pixel regions P 1  to P 3  without separation in the first to third pixel regions P 1  to P 3 . The bank layer  666  may be formed to prevent the current leakage at an edge of the first electrode  660  and may be omitted. 
     The emitting layer  662  as an emitting unit is formed on the first electrode  660 . For example, the emitting layer  662  may have a structure of  FIG. 3 ,  FIG. 6  or  FIG. 8  and may provide a blue light. Namely, the OLED D in the first to third pixel regions P 1  to P 3  emits the blue light. 
     The color conversion layer  680  includes a first color conversion layer  682  corresponding to the second pixel region P 2  and a second color conversion layer  684  corresponding to the third pixel region P 3 . The first color conversion layer  682  may be a red color conversion layer, and the second color conversion layer  684  may be a green color conversion layer. For example, the color conversion layer  680  may include an inorganic color conversion material such as a quantum dot. 
     The blue light from the OLED D is converted into the red light by the first color conversion layer  682  in the second pixel region P 2 , and the blue light from the OLED D is converted into the green light by the second color conversion layer  684  in the third pixel region P 3 . 
     Accordingly, the organic light emitting display device  600  can display a full-color image. 
     On the other hand, in the bottom emission type organic light emitting display device  600 , the color conversion layer  680  is disposed between the OLED D and the first substrate  610 . 
     Although not shown, the organic light emitting display device  600  may further include a color filter layer between the second substrate  670  and the color conversion layer  680 . In this instance, the color purity of the organic light emitting display device  600  may be further improved. On the other hand, in the bottom emission type organic light emitting display device  600 , the color filter layer may be disposed between the first substrate  610  and the color conversion layer  680 . 
       FIG. 10  is a schematic cross-sectional view of an organic light emitting display device according to a seventh embodiment of the present disclosure. 
     As shown in  FIG. 10 , the organic light emitting display device  1000  includes a substrate  1010 , wherein first to third pixel regions P 1 , P 2  and P 3  are defined, a TFT Tr over the substrate  1010  and an OLED D 5 . The OLED D 5  is disposed over the TFT Tr and is connected to the TFT Tr. For example, the first to third pixel regions P 1 , P 2  and P 3  may be a blue pixel region, a red pixel region and a green pixel region, respectively. 
     The substrate  1010  may be a glass substrate or a flexible substrate. For example, the flexible substrate may be a polyimide (PI) substrate, a polyethersulfone (PES) substrate, a polyethylenenaphthalate (PEN) substrate, a polyethylene terephthalate (PET) substrate or a polycarbonate (PC) substrate. 
     A buffer layer  1012  is formed on the substrate  1010 , and the TFT Tr is formed on the buffer layer  1012 . The buffer layer  1012  may be omitted. 
     As explained with  FIG. 2 , the TFT Tr may include a semiconductor layer, a gate electrode, a source electrode and a drain electrode and may serve as a driving element. 
     A planarization layer (or passivation layer)  1050  is formed on the TFT Tr. The planarization layer  1050  has a flat top surface and includes a drain contact hole  1052  exposing the drain electrode of the TFT Tr. 
     The OLED D 5  is disposed on the planarization layer  1050  and includes a first electrode  1060 , an emitting layer  1062  and a second electrode  1064 . The first electrode  1060  is connected to the drain electrode of the TFT Tr, and the emitting layer  1062  and the second electrode  1064  are sequentially stacked on the first electrode  1060 . The OLED D 5  is disposed in each of the first to third pixel regions P 1  to P 3  and emits different color light in the first to third pixel regions P 1  to P 3 . For example, the OLED D 5  in the first pixel region P 1  may emit the blue light, the OLED D 5  in the second pixel region P 2  may emit the red light, and the OLED D 5  in the third pixel region P 3  may emit the green light. 
     The first electrode  1060  is formed to be separate in the first to third pixel regions P 1  to P 3 , and the second electrode  1064  is formed as one-body to cover the first to third pixel regions P 1  to P 3 . 
     The first electrode  1060  is one of an anode and a cathode, and the second electrode  1064  is the other one of the anode and the cathode. In addition, one of the first and second electrodes  1060  and  1064  may be a light transmitting electrode (or a semi-transmitting electrode), and the other one of the first and second electrodes  1060  and  1064  may be a reflecting electrode. 
     For example, the first electrode  1060  may be the anode and may include a transparent conductive oxide material layer formed of a transparent conductive oxide (TCO) material having a relatively high work function. The second electrode  1064  may be the cathode and may include a metallic material layer formed of a low resistance metallic material having a relatively low work function. For example, the transparent conductive oxide material layer of the first electrode  1060  include at least one of indium-tin-oxide (ITO), indium-zinc-oxide (IZO), indium-tin-zinc oxide (ITZO), tin oxide (SnO), zinc oxide (ZnO), indium-copper-oxide (ICO) and aluminum-zinc oxide alloy (Al:ZnO), and the second electrode  1064  may include Al, Mg, Ca, Ag, their alloy, e.g., Mg—Ag alloy, or their combination. 
     In the bottom-emission type organic light emitting display device  1000 , the first electrode  1060  may have a single-layered structure of the transparent conductive oxide material layer. 
     On the other hand, in the top-emission type organic light emitting display device  1000 , a reflection electrode or a reflection layer may be formed under the first electrode  1060 . For example, the reflection electrode or the reflection layer may be formed of Ag or aluminum-palladium-copper (APC) alloy. In this instance, the first electrode  1060  may have a triple-layered structure of ITO/Ag/ITO or ITO/APC/ITO. In addition, the second electrode  1064  may have a thin profile (small thickness) to provide a light transmittance property (or a semi-transmittance property). 
     A bank layer  1066  is formed on the planarization layer  1050  to cover an edge of the first electrode  1060 . Namely, the bank layer  1066  is positioned at a boundary of the first to third pixel regions P 1  to P 3  and exposes a center of the first electrode  1060  in the first to third pixel regions P 1  to P 3 . 
     The emitting layer  1062  as an emitting unit is formed on the first electrode  1060 . The emitting layer  1062  may have a single-layered structure of an EML. Alternatively, the emitting layer  1062  may further include at least one of an HIL, an HTL, an EBL, which are sequentially stacked between the first electrode  1060  and the EML, an HBL, an ETL and an EIL, which are sequentially stacked between the EML and the second electrode  1064 . 
     In the first pixel region P 1  being the blue pixel region, the EML of the emitting layer  1062  includes the first compound being the delayed fluorescent compound and the second compound being the fluorescent compound. The EML of the emitting layer  1062  may further include a third compound being a host. The first compound is represented by Formula 1, and the second compound is represented by Formula 3. 
     An encapsulation film  1070  is formed on the second electrode  1064  to prevent penetration of moisture into the OLED D 5 . The encapsulation film  1070  may have a triple-layered structure including a first inorganic insulating layer, an organic insulating layer and a second inorganic insulating layer, but it is not limited thereto. 
     The organic light emitting display device  1000  may further include a polarization plate (not shown) for reducing an ambient light reflection. For example, the polarization plate may be a circular polarization plate. In the bottom-emission type organic light emitting display device  1000 , the polarization plate may be disposed under the substrate  1010 . In the top-emission type organic light emitting display device  1000 , the polarization plate may be disposed on or over the encapsulation film  1070 . 
       FIG. 11  is a schematic cross-sectional view of an OLED according to an eighth embodiment of the present disclosure. 
     Referring to  FIG. 11  with  FIG. 10 , the OLED D 5  is positioned in each of first to third pixel regions P 1  to P 3  and includes the first and second electrodes  1060  and  1064 , which face each other, and the emitting layer  1062  therebetween. The emitting layer  1062  includes an EML  1090 . 
     The first electrode  1060  may be an anode, and the second electrode  1064  may be a cathode. For example, the first electrode  1060  may be a reflective electrode, and the second electrode  1064  may be a transmitting electrode (or a semi-transmitting electrode). 
     The emitting layer  1062  may further include an HTL  1082  between the first electrode  1060  and the EML  1090  and an ETL  1094  between the EML  1090  and the second electrode  1064 . 
     In addition, the emitting layer  1062  may further include an HIL  1080  between the first electrode  1060  and the HTL  1082  and an EIL  1096  between the ETL  1094  and the second electrode  1064 . 
     Moreover, the emitting layer  1062  may further include an EBL  1086  between the EML  1090  and the HTL  1082  and an HBL  1092  between the EML  1090  and the ETL  1094 . 
     Furthermore, the emitting layer  1062  may further include an auxiliary HTL  1084  between the HTL  1082  and the EBL  1086 . The auxiliary HTL  1084  may include a first auxiliary HTL  1084   a  in the first pixel region P 1 , a second auxiliary HTL  1084   b  in the second pixel region P 2  and a third auxiliary HTL  1084   c  in the third pixel region P 3 . 
     The first auxiliary HTL  1084   a  has a first thickness, the second auxiliary HTL  1084   b  has a second thickness, and the third auxiliary HTL  1084   c  has a third thickness. The third thickness is smaller than the second thickness and greater than the first thickness such that the OLED D 5  provides a micro-cavity structure. 
     Namely, by the first to third auxiliary HTLs  1084   a ,  1084   b  and  1084   c  having a difference in a thickness, a distance between the first and second electrodes  1060  and  1064  in the third pixel region P 3 , in which a first wavelength range light, e.g., green light, is emitted, is smaller than a distance between the first and second electrodes  1060  and  1064  in the second pixel region P 2 , in which a second wavelength range light, e.g., red light, being greater than the first wavelength range is emitted, and is greater than a distance between the first and second electrodes  1060  and  1064  in the first pixel region P 1 , in which a third wavelength range light, e.g., blue light, being smaller than the first wavelength range is emitted. Accordingly, the emitting efficiency of the OLED D 5  is improved. 
     In  FIG. 11 , the third auxiliary HTL  1084   c  is formed in the first pixel region P 1 . Alternatively, a micro-cavity structure may be provided without the third auxiliary HTL  1084   c.    
     A capping layer (not shown) for improving a light-extracting property may be further formed on the second electrode  1084 . 
     The EML  1090  includes a first EML  1090   a  in the first pixel region P 1 , a second EML  1090   b  in the second pixel region P 2  and a third EML  1090   c  in the third pixel region P 3 . The first to third EMLs  1090   a ,  1090   b  and  1090   c  may be a blue EML, a red EML and a green EML, respectively. 
     The first EML  1090   a  in the first pixel region P 1  includes the first compound being the delayed fluorescent compound and the second compound being the fluorescent compound. The first EML  1090   a  in the first pixel region P 1  may further include a third compound being a host. The first compound is represented by Formula 1, and the second compound is represented by Formula 3. 
     In the first EML  1090   a  in the first pixel region P 1 , the weight ratio of the first compound may be greater than that of the second compound and smaller than that of the third compound. When the weight ratio of the first compound is greater than that of the second compound, the energy transfer from the first compound into the second compound is sufficiently generated. For example, in the first EML  1090   a  in the first pixel region P 1 , the first compound may have a weight % of about 20 to 40, the second compound may have a weight % of about 0.1 to 10, and the third compound may have a weight % of about 50 to 80. However, it is not limited thereto. 
     Each of the second EML  1090   b  in the second pixel region P 2  and the third EML  1090   c  in the third pixel region P 3  may include a host and a dopant. For example, in each of the second EML  1090   b  in the second pixel region P 2  and the third EML  1090   c  in the third pixel region P 3 , the dopant may include at least one of a phosphorescent compound, a fluorescent compound and a delayed fluorescent compound. 
     The OLED D 5  in  FIG. 11  respectively emits the blue light, the red light and the green light in the first to third pixel regions P 1  to P 3  such that the organic light emitting display device  1000  (of  FIG. 10 ) can provide a full-color image. 
     The organic light emitting display device  1000  may further include a color filter layer corresponding to the first to third pixel regions P 1  to P 3  to improve a color purity. For example, the color filter layer may include a first color filter layer, e.g., a blue color filter layer, corresponding to the first pixel region P 1 , a second color filter layer, e.g., a red color filter layer, corresponding to the second pixel region P 2 , and a third color filter layer, e.g., a green color filter layer, corresponding to the third pixel region P 3 . 
     In the bottom-emission type organic light emitting display device  1000 , the color filter layer may be disposed between the OLED D 5  and the substrate  1010 . On the other hand, in the top-emission type organic light emitting display device  1000 , the color filter layer may be disposed on or over the OLED D 5 . 
       FIG. 12  is a schematic cross-sectional view of an organic light emitting display device according to a ninth embodiment of the present disclosure. 
     As shown in  FIG. 12 , the organic light emitting display device  1100  includes a substrate  1110 , wherein first to third pixel regions P 1 , P 2  and P 3  are defined, a TFT Tr over the substrate  1110 , an OLED D, which is disposed over the TFT Tr and is connected to the TFT Tr, and a color filter layer  1120  corresponding to the first to third pixel regions P 1  to P 3 . For example, the first to third pixel regions P 1 , P 2  and P 3  may be a blue pixel region, a red pixel region and a green pixel region, respectively. 
     The substrate  1110  may be a glass substrate or a flexible substrate. For example, the flexible substrate may be a polyimide (PI) substrate, a polyethersulfone (PES) substrate, a polyethylenenaphthalate (PEN) substrate, a polyethylene terephthalate (PET) substrate or a polycarbonate (PC) substrate. 
     The TFT Tr is formed on the substrate  1110 . Alternatively, a buffer layer (not shown) may be formed on the substarte  1110 , and the TFT Tr may be formed on the buffer layer. 
     As explained with  FIG. 2 , the TFT Tr may include a semiconductor layer, a gate electrode, a source electrode and a drain electrode and may serve as a driving element. 
     In addition, the color filter layer  1120  is disposed on the substrate  1110 . For example, the color filter layer  1120  may include a first color filter layer  1122  corresponding to the first pixel region P 1 , a second color filter layer  1124  corresponding to the second pixel region P 2 , and a third color filter layer  1126  corresponding to the third pixel region P 3 . The first to third color filter layers  1122 ,  1124  and  1126  may be a blue color filter layer, a red color filter layer and a green color filter layer, respectively. For example, the first color filter layer  1122  may include at least one of a blue dye and a blue pigment, and the second color filter layer  1124  may include at least one of a red dye and a red pigment. The third color filter layer  1126  may include at least one of a green dye and a green pigment. 
     A planarization layer (or passivation layer)  1150  is formed on the TFT Tr and the color filter layer  1120 . The planarization layer  1150  has a flat top surface and includes a drain contact hole  1152  exposing the drain electrode of the TFT Tr. 
     The OLED D is disposed on the planarization layer  1150  and corresponds to the color filter layer  1120 . The OLED D includes a first electrode  1160 , an emitting layer  1162  and a second electrode  1164 . The first electrode  1160  is connected to the drain electrode of the TFT Tr, and the emitting layer  1162  and the second electrode  1164  are sequentially stacked on the first electrode  1160 . The OLED D emits the white light in each of the first to third pixel regions P 1  to P 3 . 
     The first electrode  1160  is formed to be separate in the first to third pixel regions P 1  to P 3 , and the second electrode  1164  is formed as one-body to cover the first to third pixel regions P 1  to P 3 . 
     The first electrode  1160  is one of an anode and a cathode, and the second electrode  1164  is the other one of the anode and the cathode. In addition, the first electrode  1160  may be a light transmitting electrode (or a semi-transmitting electrode), and the second electrode  1164  may be a reflecting electrode. 
     For example, the first electrode  1160  may be the anode and may include a transparent conductive oxide material layer formed of a transparent conductive oxide (TCO) material having a relatively high work function. The second electrode  1164  may be the cathode and may include a metallic material layer formed of a low resistance metallic material having a relatively low work function. For example, the transparent conductive oxide material layer of the first electrode  1160  include at least one of indium-tin-oxide (ITO), indium-zinc-oxide (IZO), indium-tin-zinc oxide (ITZO), tin oxide (SnO), zinc oxide (ZnO), indium-copper-oxide (ICO) and aluminum-zinc oxide alloy (Al:ZnO), and the second electrode  1164  may include Al, Mg, Ca, Ag, their alloy, e.g., Mg—Ag alloy, or their combination. 
     The emitting layer  1162  as an emitting unit is formed on the first electrode  1160 . The emitting layer  1162  includes at least two emitting parts emitting different color light. Each emitting part may have a single-layered structure of an EML. Alternatively, each emitting part may further include at least one of an HIL, an HTL, an EBL, an HBL, an ETL and an EIL. In addition, the emitting layer  1162  may further include a charge generation layer (CGL) between the emitting parts. 
     The EML of one of the emitting parts includes the first compound of Formula 1 and the second compound of Formula 3. For example, the EML of one of the emitting parts may include the first compound being the delayed fluorescent compound and the second compound being the fluorescent compound. The EML of one of the emitting parts may further include a third compound being a host. 
     A bank layer  1166  is formed on the planarization layer  1150  to cover an edge of the first electrode  1160 . Namely, the bank layer  1166  is positioned at a boundary of the first to third pixel regions P 1  to P 3  and exposes a center of the first electrode  1160  in the first to third pixel regions P 1  to P 3 . As mentioned above, since the OLED D emits the white light in the first to third pixel regions P 1  to P 3 , the emitting layer  1162  may be formed as a common layer in the first to third pixel regions P 1  to P 3  without separation in the first to third pixel regions P 1  to P 3 . The bank layer  1166  may be formed to prevent the current leakage at an edge of the first electrode  1160  and may be omitted. 
     Although not shown, the organic light emitting display device  1100  may further include an encapsulation film is formed on the second electrode  1164  to prevent penetration of moisture into the OLED D. In addition, the organic light emitting display device  1100  may further include a polarization plate under the substrate  1110  for reducing an ambient light reflection. 
     In the organic light emitting display device  1100  of  FIG. 12 , the first electrode  1160  is a transparent electrode (light transmitting electrode), and the second electrode  1164  is a reflecting electrode. In addition, the color filter layer  1120  is positioned between the substrate  1110  and the OLED D. Namely, the organic light emitting display device  11000  is a bottom-emission type. 
     Alternatively, in the organic light emitting display device  1100 , the first electrode  1160  may be a reflecting electrode, and the second electrode  1154  may be a transparent electrode (or a semi-transparent electrode). In this case, the color filter layer  1120  is positioned on or over the OLED D. 
     In the organic light emitting display device  1100 , the OLED D in the first to third pixel regions P 1  to P 3  emits the white light, and the white light passes through the first to third color filter layers  1122 ,  1124  and  1126 . Accordingly, the blue light, the red light and the green light are displayed in the first to third pixel regions P 1  to P 3 , respectively. 
     Although not shown, a color conversion layer may be formed between the OLED D and the color filter layer  1120 . The color conversion layer may include a blue color conversion layer, a red color conversion layer and a green color conversion layer respectively corresponding to the first to third pixel regions P 1  to P 3 , and the white light from the OLED D can be converted into the blue light, the red light and the green light. The color conversion layer may include a quantum dot. Accordingly, the color purity of the OLED D may be further improved. 
     The color conversion layer may be included instead of the color filter layer  1120 . 
       FIG. 13  is a schematic cross-sectional view of an OLED according to a tenth embodiment of the present disclosure. 
     As shown in  FIG. 13 , the OLED D 6  includes the first and second electrodes  1160  and  1164 , which face each other, and the emitting layer  1162  therebetween. 
     The first electrode  1160  may be an anode, and the second electrode  1164  may be a cathode. The first electrode  1160  is a transparent electrode (a light transmitting electrode), and the second electrode  1164  is a reflecting electrode. 
     The emitting layer  1162  includes a first emitting part  1210  including a first EML  1220 , a second emitting part  1230  including a second EML  1240  and a third emitting part  1250  including a third EML  1260 . In addition, the emitting layer  1162  may further include a first CGL  1270  between the first and second emitting parts  1210  and  1230  and a second CGL  1280  between the first emitting part  1210  and the third emitting part  1250 . 
     The first CGL  1270  is positioned between the first and second emitting parts  1210  and  1230 , and the second CGL  1280  is positioned between the second and third emitting parts  1230  and  1250 . Namely, the third emitting part  1250 , the second CGL  1280 , the second emitting part  1230 , the first CGL  1270  and the first emitting part  1230  are sequentially stacked on the first electrode  1160 . In other words, the first emitting part  1210  is positioned between the second electrode and the first and second CGL  1270 , and the second emitting part  1230  is positioned between the first and second CGLs  1270  and  1280 . The third emitting part  1250  is positioned between the second CGL  1280  and the first electrode  1160 . 
     The first emitting part  1210  may further include a first HTL  1210   a  under the first EML  1220  and a first ETL  1210   b  over the first EML  1220 . Namely, the first HTL  1210   a  may positioned between the first EML  1220  and the second CGL  1270 , and the first ETL  1210   b  may be positioned between the first EML  1220  and the first CGL  1270 . 
     In addition, the first emitting part  1210  may further include a first HTL  1210   a  under the first EML  1220 , a first ETL  1210   b  over the first EML  1220  and an EIL  1210   c  over the first ETL  1210   b . Namely, the first HTL  1210   a  is positioned between the first EML  1220  and the first CGL  1270 , and the first ETL  1210   b  and the EIL  1210   c  are positioned between the first EML  1220  and the second electrode  1164 . 
     In addition, the first emitting part  1210  may further include an EBL (not shown) between the first HTL  1210   a  and the first EML  1220  and an HBL (not shown) between the first ETL  1210   b  and the first EML  1220 . 
     The second emitting part  1230  may further include a second HTL  1230   a  under the second EML  1240  and a second ETL  1230   b  over the second EML  1240 . Namely, the second HTL  1230   a  is positioned between the second EML  1240  and the second CGL  1280 , and the second ETL  1230   b  is positioned between the second EML  1240  and the first CGL  1270 . 
     In addition, the second emitting part  1230  may further include an EBL (not shown) between the second HTL  1230   a  and the second EML  1240  and an HBL (not shown) between the second ETL  1230   b  and the second EML  1240 . 
     The third emitting part  1250  may further include an HIL  1250   a  and a third HTL  1250   b  under the third EML  1260  and a third ETL  1250   c  over the third EML  1260 . Namely, the HIL  1250   a  and the third HTL  1250   b  are positioned between the third EML  1260  and the first electrode  1160 , and the third ETL  1250   b  is positioned between the third EML  1260  and the second CGL  1280 . 
     In addition, the third emitting part  1250  may further include an EBL (not shown) between the third HTL  1250   b  and the third EML  1260  and an HBL (not shown) between the third ETL  1250   c  and the third EML  1260 . 
     One of the first to third EMLs  1220 ,  1240  and  1260  may be a blue EML, another one of the first to third EMLs  1220 ,  1240  and  1260  may be a green EML, and the other one of the first to third EMLs  1220 ,  1240  and  1260  may be a red EML. 
     For example, the first EML  1220  may be a blue EML, the second EML  1240  may be a green EML, and the third EML  1260  may be a red EML. Alternatively, the first EML  1220  may be a blue EML, the second EML  1240  may be a red EML, and the third EML  1260  may be a green EML. 
     The first EML  1220  includes the first compound being the delayed fluorescent compound and the second compound being the fluorescent compound. The first EML  1220  may further include a third compound being a host. The first compound is represented by Formula 1, and the second compound is represented by Formula 3. 
     In the first EML  1220 , the weight ratio of the first compound may be greater than that of the second compound and smaller than that of the third compound. When the weight ratio of the first compound is greater than that of the second compound, the energy transfer from the first compound into the second compound is sufficiently generated. For example, in the first EML  1220 , the first compound may have a weight % of about 20 to 40, the second compound may have a weight % of about 0.1 to 10, and the third compound may have a weight % of about 50 to 80. However, it is not limited thereto. 
     The second EML  1240  includes a host and a green dopant (or a red dopant), and the third EML  1260  includes a host and a red dopant (or a green dopant). For example, in each of the second and third EMLs  1240  and  1260 , the dopant may include at least one of a phosphorescent compound, a fluorescent compound and a delayed fluorescent compound. 
     The OLED D 6  in the first to third pixel regions P 1  to P 3  (of  FIG. 12 ) emits the white light, and the white light passes through the color filter layer  1120  (of  FIG. 12 ) in the first to third pixel regions P 1  to P 3 . Accordingly, the organic light emitting display device  1100  (of  FIG. 12 ) can provide a full-color image. 
       FIG. 14  is a schematic cross-sectional view of an OLED according to an eleventh embodiment of the present disclosure. 
     As shown in  FIG. 14 , the OLED D 7  includes the first and second electrodes  1360  and  1364 , which face each other, and the emitting layer  1362  therebetween. 
     The first electrode  1360  may be an anode, and the second electrode  1364  may be a cathode. The first electrode  1360  is a transparent electrode (a light transmitting electrode), and the second electrode  1364  is a reflecting electrode. 
     The emitting layer  1362  includes a first emitting part  1410  including a first EML  1420 , a second emitting part  1430  including a second EML  1440  and a third emitting part  1450  including a third EML  1460 . In addition, the emitting layer  1362  may further include a first CGL  1470  between the first and third emitting parts  1410  and  1450  and a second CGL  1480  between the second emitting part  1430  and the third emitting part  1450 . 
     The first CGL  1470  is positioned between the first and third emitting parts  1410  and  1450 , and the second CGL  1480  is positioned between the second and third emitting parts  1430  and  1450 . Namely, the second emitting part  1430 , the second CGL  1480 , the third emitting part  1450 , the first CGL  1470  and the first emitting part  1410  are sequentially stacked on the first electrode  1360 . In other words, the first emitting part  1410  is positioned between the second electrode  1364  and the first CGL  1470 , and the second emitting part  1430  is positioned between the second CGL  1480  and the first electrode  1360 . The third emitting part  1450  is positioned between the first and second CGLs  1470  and  1480 . 
     The first emitting part  1410  may further include a first HTL  1410   a  under the first EML  1420  and a first ETL  1410   b  and an EIL  1410   c  over the first EML  1420 . Namely, the first HTL  1410   a  may positioned between the first EML  1420  and the first CGL  1470 , and the first ETL  1410   b  and the EIL  1410   c  may be positioned between the first EML  1420  and the second electrode  1364 . 
     In addition, the first emitting part  1410  may further include an EBL (not shown) between the first HTL  1410   a  and the first EML  1420  and an HBL (not shown) between the first ETL  1410   b  and the first EML  1420 . 
     The second emitting part  1430  may further include an HIL  1430   a , a second HTL  1430   b  under the second EML  1440  and a second ETL  1430   c  over the second EML  1440 . Namely, the HIL  1430   a , the second HTL  1430   b  may be positioned between the first electrode  1360  and the second EML  1440 , and the second ETL  1430   c  may be positioned between the second EML  1440  and the second CGL  1480 . 
     In addition, the second emitting part  1430  may further include an EBL (not shown) between the second HTL  1430   b  and the second EML  1440  and an HBL (not shown) between the second ETL  1430   c  and the second EML  1440 . 
     The third emitting part  1450  may further include a third HTL  1450   a  under the third EML  1460  and a third ETL  1450   b  over the third EML  1460 . Namely, the third HTL  1450   a  may be positioned between the second CGL  1480  and the third EML  1460 , and the third ETL  1450   b  may be positioned between the third EML  1460  and the first CGL  1470 . 
     In addition, the third emitting part  1450  may further include an EBL (not shown) between the third HTL  1450   a  and the third EML  1460  and an HBL (not shown) between the third ETL  1450   b  and the third EML  1460 . 
     Each of the first and second EMLs  1420  and  1440  is a blue EML. At least one of the first and second EMLs  1420  and  1440 , e.g., the first EML  1420 , includes the first compound of Formula 1 and the second compound of Formula 3. In addition, the first EML  1420  may further include a third compound as a host. 
     In the first EML  1220 , the weight ratio of the first compound may be greater than that of the second compound and smaller than that of the third compound. When the weight ratio of the first compound is greater than that of the second compound, the energy transfer from the first compound into the second compound is sufficiently generated. 
     For example, in the first EML  1420 , the first compound may have a weight % of about 20 to 40, the second compound may have a weight % of about 0.1 to 10, and the third compound may have a weight % of about 50 to 80. However, it is not limited thereto. 
     The second EML  1440  may include the first compound of Formula 1 and the second compound of Formula 3. Namely, the second EML  1440  may have the same organic compound as the first EML  1420 . Alternatively, the second EML  1440  may include a compound being different from at least one of the first compound and the second compound in the first EML  1420  such that the first and second EMLs  1420  and  1440  have a different in an emitted-light wavelength or an emitting efficiency. 
     The third EML  1460  includes a lower EML  1460   a  and an upper EML  1420   b . The lower EML  1460   a  is closer to the first electrode  1360 , and the upper EML  1460   b  is closer to the second electrode  1364 . 
     One of the lower and upper EMLs  1460   a  and  1460   b  of the third EML  1460  is a green EML, and the other one of the lower and upper EMLs  1460   a  and  1460   b  of the third EML  1460  may be a red EML. Namely, the green EML (or the red EML) and the red EML (or the green EML) are sequentially stacked to form the third EML  1460 . 
     Each of the lower EML  1460   a  and the upper EML  1460   b  may include a host and a dopant. In each of the lower EML  1460   a  and the upper EML  1460   b , the dopant may include at least one of a phosphorescent compound, a fluorescent compound and a delayed fluorescent compound. 
     Alternatively, the third EML  1460  may have a single-layered structure of a yellow-green EML. 
     The OLED D 7  in the first to third pixel regions P 1  to P 3  (of  FIG. 12 ) emits the white light, and the white light passes through the color filter layer  1120  (of  FIG. 12 ) in the first to third pixel regions P 1  to P 3 . Accordingly, the organic light emitting display device  1100  (of  FIG. 12 ) can provide a full-color image. 
     In  FIG. 14 , the OLED D 7  has a three-stack (triple-stack) structure including the first and second EMLs  1420  and  1440  being the blue EML with the third EML  1460 . Alternatively, one of the first and second EMLs  1420  and  1440  may be omitted such that the OLED D 7  may have a two-stack (double-stack) structure. 
     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 invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.