Patent Publication Number: US-2023157150-A1

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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(a) to Korean Patent Application No. 10-2021-0159515, filed in the Republic of Korea on Nov. 18, 2021, the entire contents of which are incorporated herein by reference into the present application. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an organic light emitting diode, and more specifically, to an organic light emitting diode having excellent luminous properties and an organic light emitting device having the diode. 
     Discussion of the Related Art 
     As display devices have become larger, there exists a need for a flat display device with lower spacing occupation. Among the flat display devices, a display device using an organic light emitting diode (OLED) has come into the spotlight as a luminous display device replacing rapidly a liquid crystal display device (LCD). 
     The OLED can be formed as a thin film having a thickness less than 2000 Å and can be implement unidirectional or bidirectional images as electrode configurations. Also, the OLED can be formed on a flexible transparent substrate such as a plastic substrate so that OLED can implement a flexible or foldable display with ease. In addition, the OLED has advantages over LCD (liquid crystal display device), for example, the OLED can be driven at a lower voltage and has very high color purity. 
     In the OLED, when electrical charges are injected into an emitting material layer between an electron injection electrode (i.e., cathode) and a hole injection electrode (i.e., anode), electrical charges are recombined to form excitons, and then emit light as the recombined excitons are shifted to a stable ground state. 
     Fluorescent materials of the related art have shown low luminous efficiency because only the singlet excitons are involved in the luminescence process thereof. The phosphorescent materials in which triplet excitons as well as the singlet excitons are involved in the luminescence process have relatively high luminous efficiency compared to the fluorescent material. However, the metal complex as the representative phosphorescent material has too short luminous lifespan to be applicable into commercial devices. 
     SUMMARY OF THE DISCLOSURE 
     Accordingly, embodiments of the present disclosure are directed to an OLED and an organic light emitting device including the OLED that substantially obviates one or more of the problems due to the limitations and disadvantages of the related art. 
     An aspect of the present disclosure is to provide an OLED that can improve luminous efficiency, color purity and luminous lifespan and an organic light emitting device including the diode. 
     Additional features and aspects will be set forth in the description that follows, and in part will be apparent from the description, or can be learned by practice of the inventive concepts provided herein. Other features and aspects of the inventive concepts can be realized and attained by the structure particularly pointed out in the written description, or derivable therefrom, and the claims hereof as well as the appended drawings. 
     To achieve these and other aspects of the inventive concepts, as embodied and broadly described, an organic light emitting diode includes a first electrode; a second electrode facing the first electrode; and an emissive layer disposed between the first and second electrodes and including at least one emitting material layer, wherein the at least one emitting material layer includes a first compound, a second compound and a third compound, and wherein the first compound has the following structure of Formula 1 or a structure formed by linking two structures of Formula 1 via a direct or indirect bond, the second compound has the following structure of Formula 3 and the third compound has the following structure of Formula 5: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 1, 
     each of R 1  and R 2  is independently hydrogen, deuterium, tritium, unsubstituted or substituted C 1 -C 20  alkyl, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group; 
     each of R 3  and R 4  is independently an unsubstituted or substituted carbazolyl group; 
     Ar is an unsubstituted or substituted C 6 -C 30  aromatic ring or an unsubstituted or substituted C 3 -C 30  hetero aromatic group; 
     m is an integer of 1 to 4; and 
     n is an integer of 0 to 1, wherein m plus n is an integer of 1 to 4, 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 3, 
     each of R 5  and R 6  is independently hydrogen, deuterium, tritium, unsubstituted or substituted C 1 -C 20  alkyl, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group, wherein at least one of R 5  and R 6  is an unsubstituted or substituted C 1 -C 20  alkyl, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group; and 
     p is an integer of 1 to 4, 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 5, 
     each of R 11  to R 17  is independently hydrogen, deuterium, tritium, an unsubstituted or substituted C 1 -C 20  alkyl, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group; and 
     each of X 1  and X 2  is independently a halogen atom. 
     As an example, a Highest Occupied Molecular Orbital (HOMO) energy level of the first compound can be identical to or lower than a HOMO energy level of the second compound, and the HOMO energy level of the second compound can be identical to or lower than a HOMO energy level of the third compound. 
     For example, a HOMO energy level of the first compound and a HOMO energy level of the second compound can satisfy the following relationship in Equation (1), and the HOMO energy level of the second compound and a HOMO energy level of the third compound can satisfy the following relationship in Equation (2): 
       −0.3 eV≤HOMO DF1 −HOMO DF2 ≤0 eV  (1);
 
       −0.4 eV≤HOMO DF2 −HOMO FD ≤0 eV  (2),
 
     wherein HOMO DF1  indicates a HOMO energy level of the first compound, HOMO DF2  indicates a HOMO energy level of the second compound and HOMO FD  indicates a HOMO energy level of the third compound. 
     In another exemplar aspect, a Lowest Unoccupied Molecular Orbital (LUMO) energy level of the first compound can be identical to or higher than a LUMO energy level of the second compound, and the LUMO energy level of the second compound can be identical to or higher than a LUMO energy level of the third compound. 
     For example, a LUMO energy level of the first compound and a LUMO energy level of the second compound can satisfy the following relationship in Equation (3), and the LUMO energy level of the second compound and a LUMO energy level of the third compound can satisfy the following relationship in Equation (4): 
       0 eV≤LUMO DF1 −LUMO DF2 ≤0.3 eV  (3);
 
       0 eV≤LUMO DF2 −LUMO FD ≤0.3 eV  (4),
 
     wherein LUMO DF1  indicates a LUMO energy level of the first compound, LUMO DF2  indicates a LUMO energy level of the second compound and LUMO FD  indicates a LUMO energy level of the third compound. 
     In one exemplary aspect, the at least one emitting material layer can have a single-layered emitting material layer. The single-layered emitting material layer can further include a fourth compound. 
     Each of an excited singlet energy level and an excited triplet energy level of the fourth compound can be higher than each of an excited singlet energy level and an excited triplet energy level of the first compound, respectively. 
     Alternatively, the at least one emitting material layer includes a first emitting material layer disposed between the first and second electrodes and a second emitting material layer disposed between the first electrode and the first emitting material layer or between the first emitting material layer and the second electrode, and wherein the first emitting material layer includes the first compound and the second compound and the second emitting material layer includes the third compound. 
     The first emitting material layer can further include a fourth compound and the second emitting material layer can further include a fifth compound. 
     As an example, each of an excited singlet energy level and an excited triplet energy level of the fourth compound can be higher than each of an excited singlet energy level and an excited triplet energy level of the first compound, respectively, and an excited singlet energy level of the fifth compound can be higher than an excited singlet energy level of the third compound. 
     Optionally, when the at least one emitting material layer includes the first and second emitting material layers, the at least one emitting material layer can further include a third emitting material layer disposed oppositely to the second emitting material layer with respect to the first emitting material layer. 
     In one exemplary aspect, the emissive layer can include a single emitting unit. Alternatively, the emissive layer can include multiple emitting parts to form a tandem structure. 
     In another aspect, an organic light emitting device, such as an organic light emitting display device or an organic light emitting luminescent device comprises a substrate and the OLED disposed over the substrate, as described above. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the inventive concepts as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The accompanying drawings, which are included to provide a further understanding of the disclosure, are incorporated in and constitute a part of this application, illustrate embodiments of the disclosure and together with the description serve to explain principles of the disclosure. 
         FIG.  1    is a schematic circuit diagram of an organic light emitting display device in accordance with the preset disclosure. 
         FIG.  2    is a schematic cross-sectional view illustrating an organic light emitting display device in accordance with an exemplary aspect of the present disclosure. 
         FIG.  3    is a schematic cross-sectional view illustrating an organic light emitting diode (OLED) in accordance with an exemplary aspect of the present disclosure. 
         FIG.  4    is a schematic diagram illustrating a state in which excitons are transferred to efficiently to a third compound by adjusting HOMO and LUMO energy levels among first to fourth compounds of luminous materials in an EML in accordance with an exemplary aspect of the present disclosure. 
         FIG.  5    is a schematic diagram illustrating luminous mechanism by singlet and triplet energy levels among luminous material in an EML in accordance with an exemplary aspect of the present disclosure. 
         FIG.  6    is a schematic cross-sectional view illustrating an OLED in accordance with another exemplary aspect of the present disclosure. 
         FIG.  7    is a schematic diagram illustrating a state in which excitons are transferred to efficiently to a third compound by adjusting HOMO and LUMO energy levels among first to fifth compounds of luminous materials in an EML in accordance with an another exemplary aspect of the present disclosure. 
         FIG.  8    is a schematic diagram illustrating luminous mechanism by singlet and triplet energy levels among luminous materials in EMLs in accordance with another exemplary aspect of the present disclosure. 
         FIG.  9    is a schematic cross-sectional view illustrating an OLED in accordance with still another exemplary aspect of the present disclosure. 
         FIG.  10    is a schematic diagram illustrating a state in which excitons are transferred to efficiently to third and sixth compounds by adjusting HOMO and LUMO energy levels among first to seventh compounds of luminous materials in an EML in accordance with still another exemplary aspect of the present disclosure. 
         FIG.  11    is a schematic diagram illustrating luminous mechanism by singlet and triplet energy levels among luminous materials in EMLs in accordance with still another exemplary aspect of the present disclosure. 
         FIG.  12    is a schematic cross-sectional view illustrating an OLED in accordance with still another exemplary aspect of the present disclosure. 
         FIG.  13    is a schematic cross-sectional view illustrating an organic light emitting display device in accordance with another exemplary aspect of the present disclosure. 
         FIG.  14    is a schematic cross-sectional view illustrating an OLED in accordance with still another exemplary aspect of the present disclosure. 
         FIG.  15    is a schematic cross-sectional view illustrating an organic light emitting display device in accordance with still another exemplary aspect of the present disclosure. 
         FIG.  16    is a schematic cross-sectional view illustrating an organic light emitting display device in accordance with still another exemplary aspect of the present disclosure. 
         FIG.  17    is a schematic cross-sectional view illustrating an OLED in accordance with still another exemplary aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference and discussions will now be made below in detail to aspects, embodiments and examples of the disclosure, some examples of which are illustrated in the accompanying drawings. 
     The present disclosure relates to an organic light emitting diode (OLED) into which first to third compounds having adjusted energy levels are applied in an identical EML or adjacently disposed EMLs and an organic light emitting device having the OLED. The OLED can be applied into an organic light emitting device such as an organic light emitting display device and an organic light emitting luminescent device. As an example, a display device applying the OLED will be described. All the components of each display device according to all embodiments of the present disclosure are operatively coupled and configured. 
       FIG.  1    is a schematic circuit diagram of an organic light emitting display device in accordance with the present disclosure. As illustrated in  FIG.  1   , a gate line GL, a data line DL and power line PL, each of which cross each other to define a pixel region P, in an organic light emitting display device  100 . A switching thin film transistor Ts, a driving thin film transistor Td, a storage capacitor Cst and an organic light emitting diode D are formed within the pixel region P. The pixel region P can include a first pixel region P 1 , a second pixel region P 2  and a third pixel region P 3  ( FIG.  13   ). 
     The switching thin film transistor Ts is connected to the gate line GL and the data line DL, and the driving thin film transistor Td and the storage capacitor Cst are connected between the switching thin film transistor Ts and the power line PL. The organic light emitting diode D is connected to the driving thin film transistor Td. When the switching thin film transistor Ts is turned on by a gate signal applied into the gate line GL, a data signal applied into the data line DL is applied into a gate electrode of the driving thin film transistor Td and one electrode of the storage capacitor Cst through the switching thin film transistor Ts. 
     The driving thin film transistor Td is turned on by the data signal applied into the gate electrode so that currents proportional to the data signal are supplied from the power line PL to the organic light emitting diode D through the driving thin film transistor Td. And then, the organic light emitting diode D emits light with a luminance proportional to the currents flowing through the driving thin film transistor Td. In this case, the storage capacitor Cst is charged with voltages proportional to the data signal so that the voltage of the gate electrode in the driving thin film transistor Td is kept constant during one frame. Therefore, the organic light emitting display device  100  can display a desired image. 
       FIG.  2    is a schematic cross-sectional view of an organic light emitting display device  100  in accordance with an exemplary aspect of the present disclosure. All components of the organic light emitting device in accordance with all aspects of the present disclosure are operatively coupled and configured. 
     As illustrated in  FIG.  2   , the organic light emitting display device  100  includes a substrate  110 , a thin-film transistor Tr on the substrate  110 , and an organic light emitting diode (OLED) D over the substrate  110  and connected to the thin film transistor Tr. 
     The substrate  110  can include, but is not limited to, glass, thin flexible material and/or polymer plastics. For example, the flexible material can include, but is not limited to, polyimide (PI), polyethersulfone (PES), polyethylenenaphthalate (PEN), polyethylene terephthalate (PET), polycarbonate (PC) and combination thereof. The substrate  110 , over which the thin film transistor Tr and the OLED D are arranged, form an array substrate. 
     A buffer layer  122  can be disposed over the substrate  110 , and the thin film transistor Tr is disposed over the buffer layer  122 . The buffer layer  122  can be omitted. 
     A semiconductor layer  120  is disposed over the buffer layer  122 . In one exemplary aspect, the semiconductor layer  120  can include, but is not limited to, oxide semiconductor materials. In this case, a light-shield pattern can be disposed under the semiconductor layer  120 , and the light-shield pattern can prevent light from being incident toward the semiconductor layer  120 , and thereby, preventing the semiconductor layer  120  from being deteriorated by the light. Alternatively, the semiconductor layer  120  can include, but is not limited to, polycrystalline silicon. In this case, opposite edges of the semiconductor layer  120  can be doped with impurities. 
     A gate insulating layer  124  made of an insulating material is disposed on the semiconductor layer  120 . The gate insulating layer  124  can include, but is not limited to, an inorganic insulating material such as silicon oxide (SiO x ) or silicon nitride (SiN x ) (0&lt;x≤2). 
     A gate electrode  130  made of a conductive material such as metal is disposed over the gate insulating layer  124  so as to correspond to a center of the semiconductor layer  120 . While the gate insulating layer  124  is disposed over a whole area of the substrate  110  in  FIG.  2   , the gate insulating layer  124  can be patterned identically as the gate electrode  130 . 
     An interlayer insulating layer  132  made of an insulating material is disposed on the gate electrode  130  with covering over an entire surface of the substrate  110 . The interlayer insulating layer  132  can include, but is not limited to, an inorganic insulating material such as silicon oxide (SiO x ) or silicon nitride (SiN x ), or an organic insulating material such as benzocyclobutene or photo-acryl. 
     The interlayer insulating layer  132  has first and second semiconductor layer contact holes  134  and  136  that expose both sides of the semiconductor layer  120 . The first and second semiconductor layer contact holes  134  and  136  are disposed over opposite sides of the gate electrode  130  with spacing apart from the gate electrode  130 . The first and second semiconductor layer contact holes  134  and  136  are formed within the gate insulating layer  124  in  FIG.  2   . Alternatively, the first and second semiconductor layer contact holes  134  and  136  are formed only within the interlayer insulating layer  132  when the gate insulating layer  124  is patterned identically as the gate electrode  130 . 
     A source electrode  144  and a drain electrode  146 , which are made of conductive material such as a metal, are disposed 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 contact both sides of the semiconductor layer  120  through the first and second semiconductor layer contact holes  134  and  136 , respectively. 
     The semiconductor layer  120 , the gate electrode  130 , the source electrode  144  and the drain electrode  146  constitute the thin film transistor Tr, which acts as a driving element. The thin film transistor Tr in  FIG.  2    has a coplanar structure in which the gate electrode  130 , the source electrode  144  and the drain electrode  146  are disposed over the semiconductor layer  120 . Alternatively, the thin film transistor Tr can have an inverted staggered structure in which a gate electrode is disposed under a semiconductor layer and a source and drain electrodes are disposed over the semiconductor layer. In this case, the semiconductor layer can include amorphous silicon. 
     The gate line GL and the data line DL, which cross each other to define the pixel region P, and the switching element Ts, which is connected to the gate line GL and the data line DL, can be further formed in the pixel region P of  FIG.  1   . The switching element Ts is connected to the thin film transistor Tr, which is a driving element. Besides, the power line PL is spaced apart in parallel from the gate line GL or the data line DL, and the thin film transistor Tr can further include a storage capacitor Cst configured to constantly keep voltage of the gate electrode  130  for one frame. 
     A passivation layer  150  is disposed on the source and drain electrodes  144  and  146  over the whole substrate  110 . The passivation layer  150  has a flat top surface and a drain contact hole  152  that exposes the drain electrode  146  of the thin film transistor Tr. While the drain contact hole  152  is disposed on the second semiconductor layer contact hole  136 , it can be spaced apart from the second semiconductor layer contact hole  136 . 
     The OLED D includes a first electrode  210  for example disposed on the passivation layer  150  and connected to the drain electrode  146  of the thin film transistor Tr. The OLED D further includes an emissive layer  220  and a second electrode  230  each of which is disposed sequentially on the first electrode  210 . 
     The first electrode  210  is disposed in each pixel region. The first electrode  210  can be an anode and include a conductive material having a relatively high work function value. For example, the first electrode  210  can include, but is not limited to, a transparent conductive oxide (TCO). 
     In one exemplary aspect, when the organic light emitting display device  100  is a bottom-emission type, the first electrode  210  can have a single-layered structure of a transparent conductive material. Alternatively, when the organic light emitting display device  100  is a top-emission type, a reflective electrode or a reflective layer can be disposed under the first electrode  210 . For example, the reflective electrode or the reflective layer can include, but is not limited to, silver (Ag) or aluminum-palladium-copper (APC) alloy. In the OLED D of the top-emission type, the first electrode  210  can have a triple-layered structure of ITO/Ag/ITO or ITO/APC/ITO. 
     In addition, a bank layer  160  is disposed on the passivation layer  150  in order to cover edges of the first electrode  210 . The bank layer  160  exposes a center of the first electrode  210  corresponding to the pixel region P. 
     The emissive layer  220  is disposed on the first electrode  210 . In one exemplary aspect, the emissive layer  220  can have a single-layered structure of an emitting material layer (EML). Alternatively, the emissive layer  220  can have a multiple-layered structure of a hole injection layer (HIL), a hole transport layer (HTL), an electron blocking layer (EBL), an EML, a hole blocking layer (HBL), an electron transport layer (ETL) and/or an electron injection layer (EIL) (see,  FIGS.  3 ,  6 ,  9  and  12   ). In one aspect, the emissive layer  220  can have one emitting part. Alternatively, the emissive layer  220  can have multiple emitting parts to form a tandem structure. 
     The second electrode  230  is disposed over the substrate  110  above which the emissive layer  220  is disposed. The second electrode  230  can be disposed over a whole display area and can include a conductive material with a relatively low work function value compared to the first electrode  210 . The second electrode  230  can be a cathode. When the organic light emitting display device  100  is a top-emission type, the second electrode  230  is thin so as to have light-transmissive (semi-transmissive) property. 
     In addition, an encapsulation film  170  can be disposed over the second electrode  230  in order to prevent outer moisture from penetrating into the OLED D. The encapsulation film  170  can have, but is not limited to, a laminated structure of a first inorganic insulating film  172 , an organic insulating film  174  and a second inorganic insulating film  176 . 
     Moreover, the organic light emitting display device  100  can have a polarizer in order to decrease external light reflection. For example, the polarizer can be a circular polarizer. When the organic light emitting display device  100  is a bottom-emission type, the polarizer can be disposed under the substrate  110 . Alternatively, when the organic light emitting display device  100  is a top-emission type, the polarizer can be disposed over the encapsulation film  170 . In addition, a cover window can be attached to the encapsulation film  170  or the polarizer. In this case, the substrate  110  and the cover window can have a flexible property, thus the organic light emitting display device  100  can be a flexible display device. 
     Now, we will describe the OLED in more detail.  FIG.  3    is a schematic cross-sectional view illustrating an OLED in accordance with an exemplary aspect of the present disclosure. As illustrated in  FIG.  3   , the OLED D 1  comprises first and second electrodes  210  and  230  facing each other, and an emissive layer  220  having single emitting part disposed between the first and second electrodes  210  and  230 . The organic light emitting display device  100  includes a red pixel region, a green pixel region and a blue pixel region, and the OLED D 1  can be disposed in the red pixel region. 
     The emissive layer  220  includes an EML  240  disposed between the first and second electrodes  210  and  230 . Also, the emissive layer  220  can include at least one of an HTL  260  disposed between the first electrode  210  and the EML  240  and an ETL  270  disposed between the second electrode  230  and the EML  240 . In addition, the emissive layer  220  can further include at least one of an HIL  250  disposed between the first electrode  210  and the HTL  260  and an EIL  280  disposed between the second electrode  230  and the ETL  270 . Alternatively, the emissive layer  220  can further include an EBL  265  disposed between the HTL  260  and the EML  240  and/or an HBL  275  disposed between the EML  240  and the ETL  270 . 
     The first electrode  210  can be an anode that provides holes into the EML  240 . The first electrode  210  can include, but is not limited to, a conductive material having a relatively high work function value, for example, a transparent conductive oxide (TCO). In an exemplary aspect, the first electrode  210  can include, but is not limited to, indium tin oxide (ITO), indium zinc oxide (IZO), indium tin zinc oxide (ITZO), tin oxide (SnO), zinc oxide (ZnO), cerium doped indium oxide (ICO), aluminum doped zinc oxide (Al: ZnO, AZO), and the like. 
     The second electrode  230  can be a cathode that provides electrons into the EML  240 . The second electrode  230  can include, but is not limited to, a conductive material having a relatively low work function values, i.e., a highly reflective material such as Al, Mg, Ca, Ag, alloy thereof, combination thereof, and the like. 
     The EML  240  can include a first compound (Compound 1) DF 1 , a second compound (Compound 2) DF 2 , a third compound (Compound 3) FD and, optionally a fourth compound (Compound 4) H. For example, each of the first and second compounds DF 1  and DF 2  can be delayed fluorescent material, the third compound FD can be fluorescent material, and the fourth compound H can be host. 
     When holes and electrons meet each other to form excitons in the EML  240 , singlet exciton with a paired spin state and triplet exciton with an unpaired spin state are generated in a ratio of 1:3 by spin arrangement. Since the conventional fluorescent materials can utilize only the singlet excitons, they exhibit low luminous efficiency. The phosphorescent materials can utilize the triplet excitons as well as the singlet excitons, while they show too short luminous lifespan to be applicable to commercial devices. 
     Each of the first and second compounds DF 1  and DF 2  can be delayed fluorescent material having thermally activated delayed fluorescence (TADF) properties that can solve the problems accompanied by the conventional art fluorescent and/or phosphorescent materials. The delayed fluorescent material has very narrow energy level bandgap ΔE ST   DF1  or ΔE ST   DF2  between a singlet energy level S 1   DF1  or S 1   DF2  and a triplet energy level T 1   DF1  or T 1   DF2  ( FIG.  5   ). Accordingly, the excitons of singlet energy level S 1   DF1  or S 1   DF2  as well as the excitons of triplet energy level T 1   DF1  or S 1   DF2  in the first and second compounds DF 1  and DF 2  of the delayed fluorescent material can be transferred to an intermediate energy level state, i.e. ICT (intramolecular charge transfer) state (S 1 →ICT←T 1 ), and then the intermediate state excitons can be shifted to a ground state (ICT→S 0 ). 
     Since the delayed fluorescent material includes the electron acceptor moiety is spaced apart from the electron donor moiety within the molecule, dipole moment intra-molecular conformation exists in a highly polarized state. The dipole moment in a highly polarized state allows a Highest Occupied Molecular Orbital (HOMO) to interact little with a Lowest Unoccupied Molecular Orbital (LUMO) and to have ICT property in which triplet exciton energy and singlet exciton energy can be transferred therebetween. 
     The delayed fluorescent material must has an energy level bandgap ΔE ST   DF1  or ΔE ST   DF2  ( FIG.  5   ) equal to or less than about 0.3 eV, for example, from about 0.05 to about 0.3 eV, between the singlet energy level S 1   DF1  or S 1   DF2  and the triplet energy level T 1   DF1  or T 1   DF2  so that exciton energy in both the singlet energy level S 1   DF1  or S 1   DF2  and the triplet energy level T 1   DF1  or T 1   DF2  can be transferred to the ICT state. The material having little energy level bandgap ΔE ST  between the singlet energy level S 1  and the triplet energy level T 1  can exhibit common fluorescence with Inter system Crossing (ISC) in which the excitons of singlet energy level S 1  can be shifted to its ground state S 0 , as well as delayed fluorescence with Reverse Inter System Crossing (RISC) in which the excitons of triplet energy level T 1  can be converted upwardly to the excitons of singlet energy level S 1 , and then the exciton of singlet energy level S 1  transferred from the triplet energy level T 1  can be transferred to the ground state S 0 . 
     In other words, in the first and second compounds DF 1  and DF 2  of the delayed fluorescent material, exciton of 25% singlet energy level S 1   DF1  or S 1   DF2  as well as exciton of 75% triplet energy level T 1   DF1  or T 1   DF1  is transferred to the ICT state, and then excitons at ICT state drop to its ground state S 0  with light emission. The delayed fluorescent material exhibit an internal quantum efficiency of 100% in theory, it can implement luminous efficiency as the prior art phosphorescent material. 
     The first compound DF 1  included in the EML  240  can be delayed fluorescent material where a benzene ring is substituted with two cyano groups linked at meta-position and 1 to 4 carbazolyl groups. The first compound DF 1  having the delayed fluorescent property can have the following structure of Formula 1 or a structure formed by linking two structures of Formula 1 via a direct or indirect bond: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 1, 
     each of R 1  and R 2  is independently hydrogen, deuterium, tritium, unsubstituted or substituted C 1 -C 20  alkyl, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group; 
     each of R 3  and R 4  is independently an unsubstituted or substituted carbazolyl group; 
     Ar is an unsubstituted or substituted C 6 -C 30  aromatic ring or an unsubstituted or substituted C 3 -C 30  hetero aromatic group; 
     m is an integer of 1 to 4; and 
     n is an integer of 0 to 1, wherein m plus n is an integer of 1 to 4. 
     As used herein, substituent in the term “substituted” includes, but is not limited to, deuterium, tritium, unsubstituted or deuterium or halogen-substituted C 1 -C 20  alkyl, unsubstituted or deuterium or halogen-substituted C 1 -C 20  alkoxy, halogen, cyano, —CF 3 , a hydroxyl group, a carboxylic group, a carbonyl group, an amino group, a C 1 -C 10  alkyl amino group, a C 6 -C 30  aryl amino group, a C 3 -C 30  hetero aryl amino group, a C 6 -C 30  aryl group, a C 3 -C 30  hetero aryl group, a nitro group, a hydrazyl group, a sulfonate group, a C 1 -C 20  alkyl silyl group, a C 6 -C 30  aryl silyl group and a C 3 -C 30  hetero aryl silyl group. 
     For example, each of the C 6 -C 30  aromatic group, the C 3 -C 30  hetero aromatic group, the C 6 -C 20  aromatic ring and the C 3 -C 20  hetero aromatic ring of R 1 , R 2  or Ar in Formula 1 can be independently unsubstituted or substituted with, but is not limited to, at least one of C 1 -C 20  alkyl, cyano, C 6 -C 20  aryl and C 3 -C 20  hetero aryl. 
     As used herein, the term “aromatic” or “aryl” is well known in the art. The term includes monocyclic rings linked covalently or fused-ring polycyclic groups. As used herein, the term C 6 -C 30  aromatic group or the C 6 -C 20  aromatic ring can include independently, but is not limited to, C 6 -C 30  aryl, C 7 -C 30  aryl alkyl, C 6 -C 30  aryl oxy, C 6 -C 30  aryl amino, C 7 -C 30  aryl ester and C 8 -C 30  vinyl aryl. The aromatic group, the aromatic ring and/or the aryl can be unsubstituted or substituted with at least one of C 1 -C 20  alkyl, C 6 -C 30  aryl and/or C 3 -C 30  hetero aryl. 
     As an example, the C 6 -C 30  aromatic group and/or the C 6 -C 30  aryl group, which can constitute R 1  and/or R 2  in Formula 1, can include independently, but is not limited to, a non-fused or fused aryl group such as phenyl, biphenyl, terphenyl, naphthyl, anthracenyl, pentalenyl, indenyl, indeno-indenyl, heptalenyl, biphenylenyl, indacenyl, phenalenyl, phenanthrenyl, benzo-phenanthrenyl, dibenzo-phenanthrenyl, azulenyl, pyrenyl, fluoranthenyl, triphenylenyl, chrysenyl, tetraphenylenyl, tetracenyl, pleiadenyl, picenyl, pentaphenylenyl, pentacenyl, fluorenyl, indeno-fluorenyl and spiro-fluorenyl. 
     As used herein, the term “hetero aromatic” or “hetero aryl” refers to a heterocycles including hetero atoms selected from N, O and S in a ring where the ring system is an aromatic ring. The term includes monocyclic rings linked covalently or fused-ring polycyclic groups. As used herein, the term C 3 -C 30  hetero aromatic group or the C 3 -C 20  hetero aromatic ring can include independently, but is not limited to, C 3 -C 30  hetero aryl, C 4 -C 30  hetero aryl alkyl, C 3 -C 30  hetero aryl oxy, C 3 -C 30  hetero aryl amino, C 4 -C 30  hetero aryl ester and C 5 -C 30  hetero vinyl aryl. The hetero aromatic group, the hetero aromatic ring and/or the hetero aryl can be unsubstituted or substituted with at least one of C 1 -C 20  alkyl, C 6 -C 30  aryl and/or C 3 -C 30  hetero aryl. 
     As an example, the C 3 -C 30  hetero aromatic group and/or the C 3 -C 30  hetero aryl group, which can constitute R 1  and/or R 2  in Formula 1, can include independently, but is not limited to, an unfused or fused hetero aryl group such as pyrrolyl, pyridinyl, pyrimidinyl, pyrazinyl, pyridazinyl, triazinyl, tetrazinyl, imidazolyl, pyrazolyl, indolyl, iso-indolyl, indazolyl, indolizinyl, pyrrolizinyl, carbazolyl, benzo-carbazolyl, dibenzo-carbazolyl, indolo-carbazolyl, indeno-carbazolyl, benzo-furo-carbazolyl, benzo-thieno-carbazolyl, carbolinyl, quinolinyl, iso-quinolinyl, phthalazinyl, quinoxalinyl, cinnolinyl, quinazolinyl, quinolizinyl, purinyl, benzo-quinolinyl, benzo-iso-quinolinyl, benzo-quinazolinyl, benzo-quinoxalinyl, acridinyl, phenazinyl, phenoxazinyl, phenothiazinyl, phenanthrolinyl, perimidinyl, phenanthridinyl, pteridinyl, naphthyridinyl, furanyl, pyranyl, oxazinyl, oxazolyl, oxadiazolyl, triazolyl, dioxanyl, benzo-furanyl, dibenzo-furanyl, thiopyranyl, xanthenyl, chromenyl, iso-chromenyl, thiazinyl, thiophenyl, benzo-thiophenyl, dibenzo-thiophenyl, difuro-pyrazinyl, benzofuro-dibenzo-furanyl, benzothieno-benzo-thiophenyl, benzothieno-dibenzo-thiophenyl, benzothieno-benzo-furanyl, benzothieno-dibenzo-furanyl, N-substituted spiro-fluorenyl, spiro-fluoreno-acridinyl and spiro-fluoreno-xanthenyl. 
     In one exemplary aspect, m of the number of an electron donor moiety of the first compound DF 1  having the structure of Formula 1 can be, but is not limited to, an integer of 2 to 4, for example, 2 or 4, and n can be, but is not limited to, 0 or 1. As an example, each of R 1  and R 2  can be independently, but is not limited to, hydrogen, deuterium, tritium, C 1 -C 5  alkyl (e.g. methyl, ethyl, t-butyl) or C 6 -C 20  aryl (e.g. phenyl). Each of R 3  and R 4  can be independently, but is not limited to, carbazolyl unsubstituted or substituted with at least one, for example, at least two of C 1 -C 5  alkyl (e.g. methyl, ethyl, t-butyl) and/or C 6 -C 20  aryl (e.g. phenyl). Ar in Formula 1 can be, but is not limited to, a C 6 -C 15  aromatic ring (e.g. benzene ring) unsubstituted or substituted with at least one cyano group, for example, 1-3 cyano groups. 
     As an example, the first compound DF 1  can be selected from, but is not limited to, an organic compound having the following structure of Formula 2: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The second compound DF 2  can be delayed florescent material where a benzene ring is substituted with two cyano groups linked at para position and 1 to 4 carbazolyl groups. The second compound DF 2  having the delayed fluorescent property can have the following structure of Formula 3: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 3, 
     each of R 5  and R 6  is independently hydrogen, deuterium, tritium, unsubstituted or substituted C 1 -C 20  alkyl, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group, wherein at least one of R 5  and R 6  is an unsubstituted or substituted C 1 -C 20  alkyl, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group; and 
     p is an integer of 1 to 4. 
     As an example, each of the C 6 -C 30  aromatic group and the C 3 -C 30  hetero aromatic group of R 5  and R 6  can be independently unsubstituted or substituted with at least one of C 1 -C 20  alkyl, C 6 -C 20  aryl and C 3 -C 20  hetero aryl. 
     In one exemplary aspect, p of the number of an electron donor moiety of the second compound DF 2  having the structure of Formula 3 can be, but is not limited to, an integer of 2 to 4, for example, 2 or 4. As an example, each of R 5  and R 6  can be independently, but is not limited to, hydrogen, deuterium, tritium, C 1 -C 5  alkyl (e.g. methyl, ethyl, t-butyl) or C 6 -C 20  aryl (e.g. phenyl). For example, the second compound DF 2  can be selected from, but is not limited to, an organic compound having the following structure of Formula 4: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The organic compound having the structure of Formulae 1 to 4 has delayed fluorescent property as well as a singlet exciton energy level, a triplet exciton energy level, a HOMO energy level and a LUMO energy level appropriate for transferring efficiently exciton energies to the third compound FD, as described below. 
     The first and second compounds DF 1  and DF 2  of the delayed fluorescent material has little energy bandgap ΔE ST   DF1  or ΔE ST   DF2  between the excited singlet energy level S 1   DF1  or S 1   DF2  and the excited triplet energy level T 1   DF1  or T 1   DF2  of equal to or less than about 0.3 eV ( FIG.  7   ) and shows excellent quantum efficiency because the excited triplet exciton energies of the first and second compounds DF 1  and DF 2  are converted to the excited singlet exciton of the third compound FD by RISC. 
     Each of the first and second compounds DF 1  and DF 2  having the structure of Formulae 1 to 4 has a distorted chemical conformation due to the binding structure between the electron donor moiety and the electron acceptor moiety. Since the first and second compounds DF 1  and DF 2  utilize triplet excitons, addition charge transfer transition (CT transition) is induced in the first and second compounds DF 1  and DF 2 . Each of the first and second compounds DF 1  and DF 2  having the structure of Formulae 1 to 4 has wide full-width at half maximum (FWHM) so that they have limitation in color purity due to the luminous properties caused by the CT luminous mechanism. 
     The EML  240  includes the third compound FD of the fluorescent material in order to maximize the luminous property of the first and second compounds DF 1  and DF 2  of the delayed fluorescent material and to implement hyper-fluorescence. As described above, each of the first and second compounds DF 1  and DF 2  of the delayed fluorescent material can utilize both the singlet exciton energy and the triplet exciton energy. When the EML  240  includes the third compound FD of the fluorescent material having proper energy levels compared to each of the first and second compounds DF 1  and DF 2  of the delayed fluorescent material, the third compound FD can absorb exciton energies released from the first compound DF 1  via the second compound DF 2 , and then the third compound FD can generate 100% singlet excitons utilizing the absorbed exciton energies with maximizing its luminous efficiency. 
     The singlet exciton energy of the first and second compounds DF 1  and DF 2 , which includes the singlet exciton energy of the first and second compounds DF 1  and DF 2  converted from its own triplet exciton energy and initial singlet exciton energy of the first and second compounds DF 1  and DF 2  in the EML  240 , is transferred to the third compound FD of the fluorescent material in the same EML  240  via Forster resonance energy transfer (FRET) mechanism, and the ultimate emission is occurred at the third compound FD. Organic material having an absorption spectrum widely overlapped with a photoluminescence spectrum of the first and second compounds DF 1  and DF 2 , particularly, the second compound DF 2 , can be used as the third compound FD so that the exciton energy generated at the first and second compounds DF 1  and DF 2  can be efficiently transferred to the third compound FD. Since the third compound FD emits light with singlet excitons shifted from the excited state to the ground state, not CT luminous mechanism, it has narrow FWHM for improving color purity thereof. 
     The third compound FD in the EML  240  can be red fluorescent material. For example, the third compound FD can be an organic compound having a BODIPY (boron-dipyrromethene)-based fluorescent material with narrow FWHM. As an example, the third compound of BODIPY-based fluorescent material can have the following structure of Formula 5: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 5, 
     each of R 11  to R 17  is independently hydrogen, deuterium, tritium, an unsubstituted or substituted C 1 -C 20  alkyl, an unsubstituted or substituted C 6 -C 30  aromatic group or an unsubstituted or substituted C 3 -C 30  hetero aromatic group; and 
     each of X 1  and X 2  is independently a halogen atom. 
     As an example, each of the C 6 -C 30  aromatic group and the C 3 -C 30  hetero aromatic group of R 11  to R 17  can be independently unsubstituted or substituted with at least one of C 1 -C 20  alkyl, C 1 -C 20  alkoxy, C 6 -C 20  aryl and C 3 -C 20  hetero aryl. 
     In an exemplary aspect, at least two, for example, three or four, among R 11  to R 17  in Formula 5 can be the unsubstituted or substituted C 6 -C 30  aromatic group or the unsubstituted or substituted C 3 -C 30  hetero aromatic group as the third compound FD emitting red wavelength light. As an example, each of R 11 , R 13 , R 14 , R 15  and R 17  in Formula 5 can be independently unsubstituted or substituted C 6 -C 30  aryl or unsubstituted or substituted C 3 -C 30  hetero aryl, and each of R 12  and R 16  in Formula 5 can be independently hydrogen, deuterium, tritium or unsubstituted or substituted C 1 -C 10  alkyl. In an exemplary aspect, the third compound FD can be selected from, but is not limited to, an organic compound having the following structure of Formula 6: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The fourth compound H in the EML  240  can include any organic compound having wider energy level bandgap between a HOMO energy level and a LUMO energy level compared to the first and second compounds DF 1  and DF 2  and/or the third compound FD. As an example, when the EML  240  includes the fourth compound H of the host, the first compound DF 1  can be a first dopant, the second compound DF 2  can be a second dopant, and the third compound FD can be a third dopant. 
     In an exemplary aspect, the fourth compound H, which can be included in the EML  240 , can include, but is not limited to, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), 3,3′-bis(N-carbazolyl)-1,1′-biphenyl (mCBP), 1,3-Bis(carbazol-9-yl)benzene (mCP), 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile (mCP-CN), 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(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, 9-(6-(9H-carbazol-9-yl)pyridin-3-yl)-9H-3,9′-bicabazole and combination thereof. 
     For example, the fourth compound H can have the following structure of Formula 7: 
     
       
         
         
             
             
         
       
     
     wherein, in Formula 7, 
     each of Y 1  and Y 2  is independently CR 21 R 22 , N, O or S, when each of Y 1  and Y 2  is independently CR 21 R 22 , O or S, each of L 1  and L 2  is linked to a benzene ring of each fused ring, respectively, when each of Y 1  and Y 2  is independently N, each of L 1  and L 2  is linked to each of Y 1  and Y 2  of each fused ring, respectively; and 
     each of R 21  and R 22  is independently hydrogen, deuterium, tritium or unsubstituted or substituted C 1 -C 10  alkyl; and 
     each of L 1  and L 2  is independently a single bond, unsubstituted or substituted C 6 -C 20  arylene or unsubstituted or substituted C 3 -C 20  hetero arylene, wherein at least one of L 1  and L 2  is not a single bond. 
     As an example, each of Y 1  and Y 2  can be independently N, O or S and each of L 1  and L 2  can be independently a single bond, phenylene, dibenzofuranylene or dibenzothiophenylene. The fourth compound H having the structure of Formula 7 can be selected from, but is not limited to, an organic compound having the following structure of Formula 8: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In an exemplary aspect, when the EML  240  includes the first to fourth compounds DF 1 , DF 2 , FD and H, the contents of the fourth compound H in the EML  240  can be larger than the contents of the first and/or second compounds DF 1  and DF 2  in the EML  240 , and the contents of each of the first and second compounds DF 1  and DF 2  in the EML  240  can be larger than the contents of the third compound FD in the EML  240 . When the contents of the first and second compounds DF 1  and DF 2  is larger than the contents of the third compound FD, exciton energy can be effectively transferred from the first and second compounds DF 1  and DF 2  to the third compound FD via FRET mechanism. For example, the contents of the fourth compound H in the EML  240  can be about 45 wt % to about 60 wt %, for example, about 45 wt % to about 55 wt %, the contents of each of the first and second compounds DF 1  and DF 2  in the EML  240  can be about 10 wt % to about 40 wt %, and the contents of the third compound FD in the EML  240  can be about 0.1 wt % to about 5 wt %, for example, about 0.1 wt % to about 2 wt %, but is not limited thereto. 
     In one exemplary aspect, HOMO energy levels and/or LUMO energy levels among the fourth compound H of the host, the first and second compounds DF 1  and DF 2  of the delayed fluorescent material and the third compound FD of the fluorescent material must be properly adjusted. For example, the host must induce the triplet excitons generated at the delayed fluorescent material to be involved in the luminescence process without quenching as non-radiative recombination in order to implement hyper fluorescence. To this end, the energy levels among the fourth compound H of the host, the first and second compounds DF 1  and DF 2  of the delayed fluorescent material and the third compound FD of the fluorescent material should be adjusted. 
       FIG.  4    is a schematic diagram illustrating a state in which excitons are transferred to efficiently to a third compound by adjusting HOMO and LUMO energy levels among first to fourth compounds of luminous materials in an EML in accordance with an exemplary aspect of the present disclosure. As illustrated in  FIG.  4   , the HOMO energy level HOMO H  of the fourth compound H of the host can be lower than the HOMO energy level HOMO DF1  of the first compound DF 1  of the first delayed fluorescent material, and the LUMO energy level LUMO H  of the fourth compound H can be higher than the LUMO energy level LUMO DF1  of the first compound DF 1 . In other words, the energy level bandgap between the HOMO energy level HOMO H  and the LUMO energy level LUMO H  of the fourth compound H can be wider than the energy level bandgap between the HOMO energy level HOMO DF1  and the LUMO energy level LUMO DF1  of the first compound DF 1 . 
     As an example, an energy level bandgap (|HOMO H −HOMO DF1 |) between the HOMO energy level (HOMO H ) of the fourth compound H and the HOMO energy level (HOMO DF1 ) of the first compound DF 1 , or an energy level bandgap (|LUMO H −LUMO DF |) between the LUMO energy level (LUMO H ) of the fourth compound H and the LUMO energy level (LUMO DF1 ) of the first compound DF 1  can be equal to or less than about 0.5 eV, for example, between about 0.1 eV to about 0.5 eV. In this case, the charges can be transported efficiently from the fourth compound H to the first compound DF 1  and thereby enhancing the ultimate luminous efficiency in the OLED D 1 . 
     The HOMO energy level HOMO DF1  of the first compound DF 1  can be identical to or lower than the HOMO energy level HOMO DF2  of the second compound DF 2 . The HOMO energy level HOMO DF2  of the second compound DF 2  can be identical to or lower than the HOMO energy level HOMO FD  of the third compound FD. As an example, an energy level bandgap ΔHOMO- 1  between the HOMO energy level HOMO DF1  of the first compound DF 1  and the HOMO energy level HOMO DF2  of the second compound DF 2  can satisfy the following relationship in Equation (1), and an energy level bandgap ΔHOMO- 2  between the HOMO energy level HOMO DF2  of the second compound DF 2  and the HOMO energy level HOMO FD  of the third compound FD can satisfy the following relationship in Equation (2): 
       −0.3 eV≤HOMO DF1 −HOMO DF2 ≤0 eV  (1);
 
       −0.4 eV≤HOMO DF2 −HOMO FD ≤0 eV  (2).
 
     In another exemplary aspect, the LUMO energy level LUMO DF1  of the first compound DF 1  can be identical to or higher than the LUMO energy level LUMO DF2  of the second compound DF 2 . The LUMO energy level LUMO DF2  of the second compound DF 2  can be identical to or lower than the LUMO energy level LUMO FD  of the third compound FD. As an example, an energy level bandgap ΔLUMO- 1  between the LUOMO energy level LUMO DF1  of the first compound DF 1  and the LUMO energy level LUMO DF2  of the second compound DF 2  can satisfy the following relationship in Equation (3), and an energy level bandgap ΔLUMO- 2  between the LUMO energy level LUMO DF2  of the second compound DF 2  and the LUMO energy level LUMO FD  of the third compound FD can satisfy the following relationship in Equation (4): 
       0 eV≤LUMO DF1 −LUMO DF2 ≤0.3 eV  (3);
 
       0 eV≤LUMO DF2 −LUMO FD ≤0.3 eV  (4).
 
     For example, the HOMO energy levels HOMO DF1 , HOMO DF2  and HOMO FD  of the first to third compounds DF 1 , DF 2  and FD satisfy the relationship in Equation (1) and (2) and/or the LUMO energy levels LUMO DF1 , LUMO DF2  and LUMO FD  of the first to third compounds DF 1 , DF 2  and FD satisfy the relationship in Equation (3) and (4), holes and electrons injected into the EML  240  can be transferred to the first compound DF 1  having excellent luminous efficiency. Accordingly, the first compound DF 1 , which utilizes both initial singlet exciton energy and another singlet exciton energy transferred from its triplet exciton energy by RISC, can implement 100% internal quantum efficiency. The singlet exciton generated at the first compound DF 1  can be finally transferred efficiently to the third compound FD via the second compound DF 2 . 
     On the other hand, when the HOMO energy level HOMO DF1  of the first compound DF 1  is higher than the HOMO energy level HOMO DF2  of the second compound, and/or the HOMO energy level HOMO DF2  of the second compound DF 2  is higher than the HOMO energy level HOMO FD  of the third compound, holes injected into the EML  240  are trapped in the second compound DF 2  and/or the third compound FD, not the first compound DF 1 . In addition, when the LUMO energy level LUMO DF1  of the first compound DF 1  is lower than the LUMO energy level LUMO DF2  of the second compound, and/or the LUMO energy level LUMO DF2  of the second compound DF 2  is lower than the LUMO energy level LUMO FD  of the third compound, electrons injected into the EML  240  are trapped in the second compound DF 2  and/or the third compound FD, not the first compound DF 1 . The holes and electrons trapped in the third compound FD, which can utilizes only the singlet excitons, are recombined directly in the third compound FD with forming excitons and emitting light. In this case, the triplet exciton energies do not contribute the ultimate light emission with quenching so that the luminous efficiency in the EML  240  is deteriorated. 
     In addition, the holes trapped in the second compound DF 2  and the electrons trapped in the third compound FD, or the holes trapped in the third compound FD and the electrons trapped in the second compound DF 2  can form an exciplex. In this case, since the triplet exciton energies are quenching, the luminous efficiency in the EML  240  can be reduced. In addition, as the energy bandgap between the LUMO energy level and the HOMO energy level forming the exciplex becomes too narrow, light of longer wavelength is emitted. As the second compound DF 2  and the third compound FD emit light simultaneously, the FWHM of the emitted light becomes wider, and therefore, the color purity of the emitted light becomes deteriorated. 
     An energy level bandgap between the HOMO energy level HOMO DF1  and the LUMO energy level LUMO DF1  of the first compound DF 1  can be wider than an energy level bandgap between the HOMO energy level HOMO DF2  and the LUMO energy level LUMO DF2  of the second compound DF 2 . The energy level bandgap between the HOMO energy level HOMO DF2  and the LUMO energy level LUMO DF2  of the second compound DF 2  can be wider than an energy level bandgap between the HOMO energy level HOMO FD  and the LUMO energy level LUMO FD  of the third compound FD. In this case, the exciton energies generated in the first compound DF 1  can be transferred to the third compound FD via the second compound DF 2  so that the OLED D 1  in which the third compound FD emits ultimately can enhance its luminous efficiency and luminous lifespan with great. 
     Now, we will describe the luminous mechanism in the EML  240 .  FIG.  5    is a schematic diagram illustrating luminous mechanism by singlet and triplet energy levels among luminous materials in an EML in accordance with one exemplary aspect of the present disclosure. As schematically illustrated in  FIG.  5   , the singlet energy level S 1   H  of the fourth compound H, which can be the host in the EML  240 , is higher than the singlet energy level S 1   DF1  of the first compound DF having the delayed fluorescent property. In addition, the triplet energy level T 1   H  of the fourth compound H can be higher than the triplet energy level T 1   DF1  of the first compound DF 1 . As an example, the triplet energy level T 1   H  of the fourth compound H can be higher than the triplet energy level T 1   DF1  of the first compound DF 1  by at least about 0.2 eV, for example, at least about 0.3 eV such as at least about 0.5 eV. 
     When the triplet energy level T 1   H  and/or the singlet energy level S 1   H  of the fourth compound H is not high enough than the triplet energy level T 1   DF1  and/or the singlet energy level S 1   DF1  of the first compound DF 1 , the excitons at the triplet energy level T 1   DF1  of the first compound DF 1  can be reversely transferred to the triplet energy level T 1   H  of the fourth compound H. In this case, the triplet exciton reversely transferred to the fourth compound H where the triplet exciton cannot be emitted is quenched as non-emission so that the triplet exciton energy of the first compound DF 1  having the delayed fluorescent property cannot contribute to luminescence. 
     As an example, each of the first and second compounds DF 1  and DF 2  having the delayed fluorescent property can have the energy level bandgap ΔE ST   DF1  or ΔE ST   DF1  between the singlet energy level S 1   DF1  or S 1   DF2  and the triplet energy level T 1   DF1  or T 1   DF2  equal to or less than about 0.3 eV, for example between about 0.05 eV and about 0.3 eV. 
     In addition, the singlet exciton energy, which is generated at the first compound DF 1  of the first delayed fluorescent material for example converted to ICT complex by RISC in the EML  240 , should be efficiently transferred to the third compound FD of the fluorescent material via the second compound DF 2  of the second delayed fluorescent material so as to implement OLED D 1  having high luminous efficiency and high color purity. 
     To this end, the singlet energy level S 1   DF1  of the first compound DF 1  is higher than the singlet energy level S 1   DF2  of the second compound DF. Optionally, the triplet energy level T 1   DF1  of the first compound DF 1  can be higher than the triplet energy level T 1   DF2  of the second compound DF 2 . Also, the singlet energy level S 1   DF2  of the second compound DF 2  of the second delayed fluorescent material is higher than the singlet energy level S 1   FD  of the third compound FD of the fluorescent material. Optionally, the triplet energy level T 1   DF2  of the second compound DF 2  can be higher than the triplet energy level T 1   FD  of the third compound FD. 
     Returning to  FIG.  3   , the HIL  250  is disposed between the first electrode  210  and the HTL  260  and improves an interface property between the inorganic first electrode  210  and the organic HTL  260 . In one exemplary aspect, the HIL  250  can include, but is not limited to, 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), N-(biphenyl-4-yl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluoren-2-amine and combination thereof. The HIL  250  can be omitted in compliance with a structure of the OLED D 1 . 
     The HTL  260  is disposed between the HIL  250  and the EML  240 . In one exemplary aspect, the HTL  260  can include, but is not limited to, N,N′-Diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD), NPB, 4,4′-bis(carbazol-9-yl)biphenyl (CBP), Poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine] (Poly-TPD), Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine))] (TFB), Di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC), 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, N-(biphenyl-4-yl)-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)biphenyl-4-amine and combination thereof. 
     The ETL  270  and the EIL  280  can be laminated sequentially between the EML  240  and the second electrode  230 . The ETL  270  includes material having high electron mobility so as to provide electrons stably with the EML  240  by fast electron transportation. In one exemplary aspect, the ETL  270  can include, but is not limited to, any one of oxadiazole-containing compounds, triazole-containing compounds, phenanthroline-containing compounds, benzoxazole-containing compounds, benzothiazole-containing compounds, benzimidazole-containing compounds, triazine-containing compounds, and the like. 
     As an example, the ETL  270  can include, but is not limited to, 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-phenaathroline (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), diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide (TSPO1) and combination thereof. 
     The EIL  280  is disposed between the second electrode  230  and the ETL  270 , and can improve physical properties of the second electrode  230  and therefore, can enhance the luminous lifespan of the OLED D 1 . In one exemplary aspect, the EIL  280  can include, but is not limited to, an alkali metal halide or an alkaline earth metal halide such as LiF, CsF, NaF, BaF 2  and the like, and/or an organic metal compound such as lithium quinolate, lithium benzoate, sodium stearate, and the like. 
     When holes are transferred to the second electrode  230  via the EML  240  and/or electrons are transferred to the first electrode  210  via the EML  240 , the OLED D 1  can have short lifespan and reduced luminous efficiency. In order to prevent these phenomena, the OLED D 1  in accordance with this aspect of the present disclosure can have at least one exciton blocking layer adjacent to the EML  240 . 
     For example, the OLED D 1  of the exemplary aspect includes the EBL  265  between the HTL  260  and the EML  240  so as to control and prevent electron transfers. In one exemplary aspect, the EBL  265  can comprise, but is not limited to, TCTA, Tris[4-(diethylamino)phenyl]amine, N-(biphenyl-4-yl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol yl)phenyl)-9H-fluorene-2-amine, TAPC, MTDATA, mCP, mCBP, CuPc, N,N′-bis[4-(bis(3-methylphenyl)amino)phenyl]-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (DNTPD), TDAPB, 3,6-bis(N-carbazolyl)-N-phenyl-carbazole and combination thereof. 
     In addition, the OLED D 1  can further include the HBL  275  as a second exciton blocking layer between the EML  240  and the ETL  270  so that holes cannot be transferred from the EML  240  to the ETL  270 . In one exemplary aspect, the HBL  275  can comprise, but is not limited to, any one of oxadiazole-containing compounds, triazole-containing compounds, phenanthroline-containing compounds, benzoxazole-containing compounds, benzothiazole-containing compounds, benzimidazole-containing compounds, and triazine-containing compounds each of which can be used in the ETL  270 . 
     For example, the HBL  275  can include a compound having a relatively low HOMO energy level compared to the HOMO energy level of the luminescent materials in EML  240 . The HBL  275  can include, but is not limited to, BCP, BAlq, Alq 3 , PBD, spiro-PBD, Liq, Bis-4,5-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM), bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO), 9-(6-(9H-carbazol-9-yl)pyridine-3-yl)-9H-3,9′-bicarbazole and combination thereof. 
     In the above aspect, the first and second compounds DF 1  and DF 2  having the delayed fluorescent property and the third compound FD having the fluorescent property are included within the same EML. Unlike that aspect, the first compound and the second compound are included in separate EMLs. 
       FIG.  6    is a schematic cross-sectional view illustrating an OLED in accordance with another exemplary aspect of the present disclosure.  FIG.  7    is a schematic diagram illustrating a state in which excitons are transferred to efficiently to a third compound by adjusting HOMO and LUMO energy levels among first to fifth compounds of luminous materials in an EML in accordance with an another exemplary aspect of the present disclosure.  FIG.  8    is a schematic diagram illustrating luminous mechanism by singlet and triplet energy levels among luminous materials in EMLs in accordance with another exemplary aspect of the present disclosure. 
     As illustrated in  FIG.  6   , the OLED D 2  includes first and second electrodes  310  and  330  facing each other and an emissive layer  320  having single emitting part disposed between the first and second electrodes  310  and  330 . The organic light emitting display device  100  ( FIG.  2   ) includes a red pixel region, a green pixel region and a blue pixel region, and the OLED D 2  can be disposed in the red pixel region. 
     In one exemplary aspect, the emissive layer  320  includes an EML  340 . Also, the emissive layer  320  can include at least one of an HTL  360  disposed between the first electrode  310  and the EML  340  and an ETL  370  disposed between the second electrode  330  and the EML  340 . Also, the emissive layer  320  can further comprise at least one of an HIL  350  disposed between the first electrode  310  and the HTL  360  and an EIL  380  disposed between the second electrode  330  and the ETL  370 . Alternatively, the emissive layer  320  can further comprise an EBL  365  disposed between the HTL  360  and the EML  340  and/or an HBL  375  disposed between the EML  340  and the ETL  370 . The configuration of the first and second electrodes  310  and  330  as well as other layers except the EML  340  in the emissive layer  320  can be substantially identical to the corresponding electrodes and layers in the OLED D 1 . 
     The EML  340  includes a first EML (EML 1 , lower EML, first layer)  342  disposed between the EBL  365  and the HBL  375  and a second EML (EML 2 , upper EML, second layer)  344  disposed between the EML 1   342  and the HBL  375 . Alternatively, the EML 2   344  can be disposed between the EBL  365  and the EML 1   342 . 
     One of the EML 1   342  and the EML 2   344  includes the first and second compounds (first and second dopants) DF 1  and DF 2  of the delayed fluorescent material, and the other of the EML 1   342  and the EML 2   344  includes the third compound (third dopant) FD of the fluorescent material. Also, each of the EML 1   342  and the EML 2   344  includes a fourth compound (Compound 4) H 1  of a first host and a fifth compound (Compound 5) H 2  of a second host, respectively. As an example, the EML 1   342  can include the first and second compounds DF 1  and DF 2  and the EML 2   344  can include the third compound FD. 
     The first compound DF 1  in the EML 1   242  can include delayed fluorescent material having the structure of Formulae 1 to 2. The second compound DF 2  can include delayed fluorescent material having the structure of Formulae 3 to 4. The triplet exciton energy of each of the first and second compounds DF 1  and DF 2  having delayed fluorescent property can be converted upwardly to its own singlet exciton energy via RISC mechanism. While each of the first and second compounds DF 1  and DF 2  has high internal quantum efficiency, but it has poor color purity due to its wide FWHM. 
     The EML 2   344  includes the third compound FD of the florescent material. The third compound FD can include the BODIPY-based organic compound having the structure of Formulae 5 to 6. The third compound FD of the fluorescent material having the structure of Formulae 5 to 6 has relatively narrow FWHM (e.g. equal to or less than about 35 nm) compared to the FWHM of the first and second compounds DF 1  and DF 2 . While the third compound FD has an advantage in terms of color purity, but its quantum efficiency is limited since its triplet exciton cannot be involved in participate in the luminescence process. 
     However, in this exemplary aspect, the singlet exciton energy as well as the triplet exciton energy of the first and second compounds DF 1  and DF 2  having the delayed fluorescent property in the EML 1   342  can be transferred to the third compound FD in the EML 2   344  disposed adjacently to the EML 1   342  by FRET mechanism, and the ultimate light emission occurs in the third compound FD within the EML 2   344 . 
     In other words, the triplet exciton energy of the first compound DF 1  is converted upwardly to its own singlet exciton energy in the EML 1   342  by RISC mechanism. Then, both the initial singlet exciton energy and the converted singlet exciton energy of the first compound DF 1  is transferred to the singlet exciton energy of the third compound FD in the EML 2   344  through the second compound DF 2 . The third compound FD in the EML 2   344  can emit light using the triplet exciton energy as well as the singlet exciton energy. 
     As the singlet exciton energies generated at the first and second compounds DF 1  and DF 2  included in the EML 1   342  is efficiently transferred to the third compound FD in the EML 2   244 , the OLED D 2  can implement hyper fluorescence. In this case, while the first and second compounds DF 1  and DF 2  having the delayed fluorescent property only act as transferring exciton energy to the third compound FD, substantial light emission is occurred in the EML 2   344  including the third compound FD. The OLED D 2  can enhance its luminous efficiency as well as its color purity owing to narrow FWHM. 
     The fourth compound H 1  can be identical to or different from the fifth compound H 2 . For example, each of the fourth compound H 1  and the fifth compound H 2  can include, but is not limited to, the organic compound having the structure of Formulae 7 to 8. 
     Similar to the first aspect, the HOMO energy level HOMO DF1  of the first compound DF 1  can be identical to or lower than the HOMO energy level HOMO DF2  of the second compound DF 2 . The HOMO energy level HOMO DF2  of the second compound DF 2  can be identical to or lower than the HOMO energy level HOMO FD  of the third compound FD. Alternatively, the LUMO energy level LUMO DF1  of the first compound DF 1  can be identical to or higher than the LUMO energy level LUMO DF2  of the second compound DF 2 . The LUMO energy level LUMO DF2  of the second compound DF 2  can be identical to or higher than the LUMO energy level HOMO FD  of the third compound FD. 
     As an example, the energy bandgap ΔHOMO- 1  between the HOMO energy level HOMO DF1  of the first compound and the HOMO energy level HOMO DF2  of the second compound DF 2  can satisfy the relationship in Equation (1), and/or the energy level bandgap ΔHOMO- 2  between the HOMO energy level HOMO DF2  of the second compound DF 2  and the HOMO energy level HOMO FD  of the third compound FD can satisfy the relationship in Equation (2). Alternatively, the energy level bandgap ΔLUMO- 1  between the LUOMO energy level LUMO DF1  of the first compound DF 1  and the LUMO energy level LUMO DF2  of the second compound DF 2  can satisfy the relationship in Equation (3), and/or the energy level bandgap ΔLUMO- 2  between the LUMO energy level LUMO DF2  of the second compound DF 2  and the LUMO energy level LUMO FD  of the third compound FD can satisfy the relationship in Equation (4). Accordingly, holes and electrons injected into the EML  340  are transferred to the first compound DF 1 , and then, the first compound DF 1  utilizing both the singlet and triplet exciton energies can transfer exciton energies to the third compound FD via the second compound DF 2 . 
     Also, an energy level bandgap (|HOMO H −HOMO DF1 |) between the HOMO energy levels (HOMO H1  and HOMO H2 ) of the fourth and fifth compounds H 1  and H 2  and the HOMO energy level (HOMO DF1 ) of the first compound DF 1 , or an energy level bandgap (|LUMO H −LUMO DF1 |) between the LUMO energy levels (LUMO H1  and LUMO H2 ) of the fourth and fifth compounds H 1  and H 2  and the LUMO energy level (LUMO DF1 ) of the first compound DF 1  can be equal to or less than about 0.5 eV. 
     Also, each of the exciton energies generated in each of the fourth compound H 1  in the EML 1   342  and the fifth compound H 2  in the EML 2   344  should be transferred primarily to the first compound DF 1  of the first delayed florescent material and then to the third compound FD of the fluorescent material in order to realize efficient light emission. As illustrated in  FIG.  8   , each of the singlet energy levels S 1   H1  and S 1   H2  of the fourth and fifth compounds H 1  and H 2  is higher than the singlet energy level S 1   DF1  of the first compound DF 1 . In addition, each of the triplet energy levels T 1   H1  and T 1   H2  of the fourth and fifth compounds H 1  and H 2  can be higher than the triplet energy level T 1   DF1  of the first compound DF 1 . For example, the triplet energy levels T 1   H1  and T 1   H2  of the fourth and fifth compound H 1  and H 2  can be higher than the triplet energy level T 1   DF1  of the first compound DF 1  by at least about 0.2 eV, for example, by at least 0.3 eV such as by at least 0.5 eV. 
     Also, the singlet energy level S 1   H2  of the fifth compound H 2  of the second host is higher than the singlet energy level S 1   FD  of the third compound FD of the fluorescent material. Optionally, the triplet energy level T 1   H2  of the fifth compound H 2  can be higher than the triplet energy level T 1   FD  of the third compound FD. In this case, the singlet exciton energy generated at the fifth compound H 2  can be transferred to the singlet energy of the third compound FD. 
     In addition, the exciton energy, which is generated at the first compound DF 1  having the delayed fluorescent property for example converted to ICT complex by RISC in the EML 1   342 , should be efficiently transferred to the third compound FD of the fluorescent material in the EML 2   344 . To this end, the singlet energy level S 1   DF1  and/or the triplet energy level T 1   DF1  of the first compound DF 1  in the EML 1   342  is higher than the singlet energy level S 1   DF2  and/or the triplet energy level T 1   DF2 . In addition, the singlet energy level S 1   DF2  of the second compound DF 2  is higher than the singlet energy level S 1   FD  of the third compound FD in the EML 2   344 . Optionally, the triplet energy level T 1   DF2  of the second compound DF 2  can be higher than the triplet energy level T 1   FD  of the third compound FD. 
     Each of the contents of the fourth and fifth compounds H 1  and H 2  in the EML 1   342  and the EML 2   344  can be larger than or identical to each of the contents of the first and second compounds DF 1  and DF 2  and the third compound FD in the same layer, respectively. Also, each of the contents of the first and second compounds DF 1  and DF 2  in the EML 1   342  can be larger than the contents of the third compound FD in the EML 2   344 . In this case, exciton energy is efficiently transferred from the first and second compound DF 1  and DF 2  in the EML 1   342  to the third compound FD in the EML 2   344  via FRET mechanism. 
     As an example, the EML 1   342  can include each of the first and second compounds DF 1  and DF 2  between about 10 wt % and about 40 wt %. The EML 2   344  can include the third compound FD between about 1 wt % and about 10 wt %, for example, about 1 wt % and 5 wt %. 
     In one exemplary aspect, when the EML 2   344  is disposed adjacently to the HBL  375 , the fifth compound H 2  in the EML 2   344  can be the same material as the HBL  375 . In this case, the EML 2   344  can have a hole blocking function as well as an emission function. In other words, the EML 2   344  can act as a buffer layer for blocking holes. In one aspect, the HBL  375  can be omitted where the EML 2   344  can be a hole blocking layer as well as an emitting material layer. 
     In another exemplary aspect, when the EML 2   344  is disposed adjacently to the EBL  365 , the fifth compound H 2  in the EML 2   344  can be the same as the EBL  365 . In this case, the EML 2   344  can have an electron blocking function as well as an emission function. In other words, the EML 2   344  can act as a buffer layer for blocking electrons. In one aspect, the EBL  365  can be omitted where the EML 2   344  can be an electron blocking layer as well as an emitting material layer. 
     An OLED having a triple-layered EML will be explained.  FIG.  9    is a schematic cross-sectional view illustrating an OLED in accordance with still another exemplary aspect of the present disclosure.  FIG.  10    is a schematic diagram illustrating a state in which excitons are transferred to efficiently to third and sixth compounds by adjusting HOMO and LUMO energy levels among first to seventh compounds of luminous materials in an EML in accordance with still another exemplary aspect of the present disclosure.  FIG.  11    is a schematic diagram illustrating luminous mechanism by singlet and triplet energy levels among luminous materials in EMLs in accordance with still another exemplary aspect of the present disclosure. 
     As illustrated in  FIG.  9   , the OLED D 3  includes first and second electrodes  410  and  430  facing each other and an emissive layer  420  disposed between the first and second electrodes  410  and  430 . The organic light emitting display device  100  ( FIG.  2   ) includes a red pixel region, a green pixel region and a blue pixel region, and the OLED D 3  can be disposed in the red pixel region. 
     In one exemplary aspect, the emissive layer  420  having single emitting part includes a triple-layered EML  440 . The emissive layer  420  can include at least one of an HTL  460  disposed between the first electrode  410  and the EML  440  and an ETL  470  disposed between the second electrode  430  and the EML  440 . Also, the emissive layer  420  can further include at least one of an HIL  450  disposed between the first electrode  410  and the HTL  460  and an EIL  480  disposed between the second electrode  430  and the ETL  470 . Alternatively, the emissive layer  420  can further include an EBL  465  disposed between the HTL  460  and the EML  440  and/or an HBL  475  disposed between the EML  440  and the ETL  470 . The configurations of the first and second electrodes  410  and  430  as well as other layers except the EML  440  in the emissive layer  420  is substantially identical to the corresponding electrodes and layers in the OLEDs D 1  and D 2 . 
     The EML  440  includes a first EML (EML 1 , middle EML, first layer)  442 , a second EML (EML 2 , lower EML, second layer)  444  and a third EML (EML 3 , upper EML, third layer)  446 . The EML 1   442  is disposed between the EBL  465  and the HBL  475 , the EML 2   444  is disposed between the EBL  465  and the EML 1   442  and the EML 3   446  is disposed between the EML 1   442  and the HBL  475 . 
     The EML 1   442  includes the first and second compounds (first and second dopants) DF 1  and DF 2  of the delayed fluorescent material. Each of the EML 2   444  and the EML 3   446  includes the third compound (third dopant) FD 1  and a sixth compound (Compound 6, fourth dopant) FD 2  each of which is the fluorescent material, respectively. Also, each of the EML 1   442 , the EML 2   444  and the EML 3   446  includes the fourth compound H 1  of the first host, the fifth compound H 2  of the second host and a seventh compound (Compound 7) H 3  of a third host, respectively. 
     In accordance with this aspect, both the singlet energy as well as the triplet energy of the first and second compounds DF 1  and DF 2  of the delayed fluorescent material in the EML 1   442  can be transferred to the third and sixth compounds FD 1  and FD 2  of the fluorescent materials each of which is included in the EML 2   444  and EML 3   446  disposed adjacently to the EML 1   442  by FRET energy transfer mechanism. Accordingly, the ultimate emission occurs in the third and sixth compounds FD 1  and FD 2  in the EML 2   444  and the EML 3   446 . 
     In other words, the triplet exciton energy of the first compound DF 1  having the delayed fluorescent property in the EML 1   442  is converted upwardly to its own singlet exciton energy by RISC mechanism, then the singlet exciton energy including the initial and converted singlet exciton energy of the first and second compounds DF 1  and DF 2  included in the EML 1   442  is transferred to the singlet exciton energy of the third and sixth compounds FD 1  and FD 2  in the EML 2   444  and the EML 3   446  because each of the first and second compounds DF 1  and DF 2  has the singlet energy level S 1   DF1  or S 1   DF2  higher than each of the singlet energy levels S 1   FD1  and S 1   FD2  of the second and fifth compounds FD 1  and FD 2  ( FIG.  11   ). 
     Both the third and sixth compounds FD 1  and FD 2  included in the EML 2   444  and EML 3   446  can emit light using the singlet exciton energy as well as the triplet exciton energy transferred from the first and second compounds DF 1  and DF 2 . Each of the third and sixth compounds FD 1  and FD 2  has relatively narrow FWHM (e.g. equal to or less than about 35 nm) compared to the FWHM of the first and second compounds DF 1  and DF 2 . In this aspect, the OLED D 3  can improve its quantum efficiency as well as its color purity due to narrow FWHM. The ultimate emission occurs in the EML 2   444  and the EML 3   446  each of which includes the third compound FD 1  and the sixth compound FD 2 , respectively. 
     The first compound DF 1  includes the organic compound having the structure of Formulae 1 to 2, and the second compound DF 2  includes the organic compound having the structure of Formulae 3 to 4. Each of the third and sixth compounds FD 1  and FD 2  of the fluorescent material includes independently the organic compound having the structure of Formulae 5 to 6. The fourth compound H 1 , the fifth compound H 2  and the seventh compound H 3  can be identical to or different from each other. For example, each of the fourth compound H 1 , the fifth compound H 2  and the seventh compound H 3  can independently include, but is not limited to, the organic compound having the structure of Formulae 7 to 8, respectively. 
     Similar to the first and second aspects, the HOMO energy level HOMO DF1  of the first compound DF 1  can be identical to or lower than the HOMO energy level HOMO DF2  of the second compound DF 2 . The HOMO energy level HOMO DF2  of the second compound DF 2  can be identical to or lower than the HOMO energy levels HOMO FD1  and HOMO FD2  of the third and sixth compounds FD 1  and FD 2 . Alternatively, the LUMO energy level LUMO DF1  of the first compound DF 1  can be identical to or higher than the LUMO energy level LUMO DF2  of the second compound DF 2 . The LUMO energy level LUMO DF2  of the second compound DF 2  can be identical to or higher than the LUMO energy level HOMO FD1  and LUMO FD2  of the third and sixth compounds FD 1  and FD 2 . 
     As an example, the energy bandgap ΔHOMO- 1  between the HOMO energy level HOMO DF1  of the first compound and the HOMO energy level HOMO DF2  of the second compound DF 2  can satisfy the relationship in Equation (1), and/or the energy level bandgap ΔHOMO- 2  between the HOMO energy level HOMO DF2  of the second compound DF 2  and the HOMO energy level HOMO FD1  and HOMO FD2  of the third and sixth compounds FD 1  and FD 2  can satisfy the relationship in Equation (2). Alternatively, the energy level bandgap ΔLUMO- 1  between the LUOMO energy level LUMO DF1  of the first compound DF 1  and the LUMO energy level LUMO DF2  of the second compound DF 2  can satisfy the relationship in Equation (3), and/or the energy level bandgap ΔLUMO- 2  between the LUMO energy level LUMO DF2  of the second compound DF 2  and the LUMO energy level LUMO FD1  and LUMO FD2  of the third and sixth compounds FD 1  and FD 2  can satisfy the relationship in Equation (4). 
     Also, an energy level bandgap (|HOMO H −HOMO DF1 |) between the HOMO energy levels (HOMO H1 , HOMO H2  and HOMO H3 ) of the fourth, fifth and seventh compounds H 1 , H 2  and H 3  and the HOMO energy level (HOMO DF1 ) of the first compound DF 1 , or an energy level bandgap (|LUMO H −LUMO DF |) between the LUMO energy levels (LUMO H1 , LUMO H2  and LUMO H3 ) of the fourth, fifth and seventh compounds H 1 , H 2  and H 3  and the LUMO energy level (LUMO′) of the first compound DF 1  can be equal to or less than about 0.5 eV. 
     The singlet and triplet energy levels among the luminous materials should be properly adjusted in order to implement efficient luminescence. Referring to  FIG.  11   , each of the singlet energy levels S 1   H1 , S 1   H2  and S 1   H3  of the fourth fifth and seventh compounds H 1 , H 2  and H 3  of the first to third hosts is higher than the singlet energy level S 1   DF1  of the first compound DF 1 . Also, each of the triplet energy levels T 1   H1 , T 1   H2  and T 1   H  of the fourth, fifth and seventh compounds H 1 , H 2  and H 3  can be higher than the triplet energy level T 1   DF1  of the first compound DF 1 . 
     In addition, the singlet exciton energy, which is generated at the first compound DF 1  having the delayed fluorescent property for example converted to ICT complex by RISC in the EML 1   442 , should be efficiently transferred to each of third and sixth compounds FD 1  and FD 2  of the fluorescent material in the EML 2   444  and the EML 3   446  via the second compound DF 2 . To this end, the singlet energy level S 1   DF1  and/or the triplet energy level T 1   DF1  of the first compound DF 1  in the EML 1   442  is higher than the singlet energy level S 1   DF2  and/or the triplet energy level T 1   DF2  of the second compound DF 2 . In addition, the singlet energy level S 1   DF2  of the second compound DF 2  is higher than the singlet energy levels S 1   FD1  and S 1   FD2  of the third and sixth compounds FD 1  and FD 2  in the EML 2   444  and the EML  446 . Optionally, the triplet energy level T 1   DF2  of the second compound DF 2  can be higher than the triplet energy levels T 1   FD1  and T 1   FD2  of the third and sixth compounds FD 1  and FD 2 . 
     In addition, exciton energy transferred to each of the third and sixth compounds FD 1  and FD 2  from the first and second compounds DF 1  and DF 2  should not be transferred to each of the fifth and seventh compounds H 2  and H 3  in order to realize efficient luminescence. To this end, each of the singlet energy levels S 1   H2  and S 1   H3  of the fifth and seventh compounds H 2  and H 3 , each of which can be the second host and the third host, is higher than each of the singlet energy levels S 1   FD1  and S 1   FD2  of the third and sixth compounds FD 1  and FD 2  of the fluorescent material, respectively. Optionally, each of the triplet energy levels T 1   H2  and T 1   H3  of the fifth and seventh compounds H 2  and H 3  is higher than each of the triplet energy levels T 1   FD1  and T 1   FD2  of the third and sixth compounds FD 1  and FD 2 , respectively. 
     The contents of the first and second compounds DF 1  and DF 2  in the EML 1   442  can be larger than each of the contents of the third and sixth compounds FD 1  and FD 2  in the EML 2   444  or the EML 3   446 . In this case, exciton energy can be transferred sufficiently from the first and second compounds DF 1  and DF 2  in the EML 1   442  to each of the second and fifth compounds FD 1  and FD 2  in the EML 2   444  and the EML 3   446  via FRET mechanism. As an example, the EML 1   442  can include the first and second compounds DF 1  and DF 2  between about 1 wt % and about 50 wt %, for example, about 10 wt % and about 40 wt %. Each of the EML 2   444  and the EML 3   446  can include the third and sixth compound FD 1  and FD 2  between about 1 wt % and about 10 wt %, for example, about 1 wt % and 5 wt %. 
     In one exemplary aspect, when the EML 2   444  is disposed adjacently to the EBL  465 , the fifth compound H 2  in the EML 2   444  can be the same material as the EBL  465 . In this case, the EML 2   444  can have an electron blocking function as well as an emission function. In other words, the EML 2   444  can act as a buffer layer for blocking electrons. In one aspect, the EBL  465  can be omitted where the EML 2   444  can be an electron blocking layer as well as an emitting material layer. 
     When the EML 3   446  is disposed adjacently to the HBL  475 , the seventh compound H 3  in the EML 3   446  can be the same material as the HBL  475 . In this case, the EML 3   446  can have a hole blocking function as well as an emission function. In other words, the EML 3   446  can act as a buffer layer for blocking holes. In one aspect, the HBL  475  can be omitted where the EML 3   446  can be a hole blocking layer as well as an emitting material layer. 
     In still another exemplary aspect, the fifth compound H 2  in the EML 2   444  can be the same material as the EBL  465  and the seventh compound H 3  in the EML 3   446  can be the same material as the HBL  475 . In this aspect, the EML 2   444  can have an electron blocking function as well as an emission function, and the EML 3   446  can have a hole blocking function as well as an emission function. In other words, each of the EML 2   444  and the EML 3   446  can act as a buffer layer for blocking electrons or hole, respectively. In one aspect, the EBL  465  and the HBL  475  can be omitted where the EML 2   444  can be an electron blocking layer as well as an emitting material layer and the EML 3   446  can be a hole blocking layer as well as an emitting material layer. 
     In an alternative aspect, an OLED can include multiple emitting parts.  FIG.  12    is a schematic cross-sectional view illustrating an OLED in accordance with still another exemplary aspect of the present disclosure. 
     As illustrated in  FIG.  12   , the OLED D 4  includes first and second electrodes  510  and  530  facing each other and an emissive layer  520  with two emitting parts disposed between the first and second electrodes  510  and  530 . The organic light emitting display device  100  ( FIG.  1   ) includes a red pixel region, a green pixel region and a blue pixel region, and the OLED D 4  can be disposed in the red and/or green pixel regions. The first electrode  510  can be an anode and the second electrode  530  can be a cathode. 
     The emissive layer  520  includes a first emitting part  620  including a first EML (EML 1 )  640 , and a second emitting part  720  including a second EML (EML 2 )  740 . Also, the emissive layer  520  can further include a charge generation layer (CGL)  680  disposed between the first emitting part  620  and the second emitting part  720 . 
     The CGL  680  is disposed between the first and second emitting parts  620  and  720  so that the first emitting part  620 , the CGL  680  and the second emitting part  720  are sequentially disposed on the first electrode  510 . In other words, the first emitting part  620  is disposed between the first electrode  510  and the CGL  680  and the second emitting part  720  is disposed between the second electrode  530  and the CGL  680 . 
     The first emitting part  620  includes the EML 1   640 . The first emitting part  620  can further includes at least one of an HIL  650  disposed between the first electrode  510  and the EML 1   640 , a first HTL (HTL 1 )  660  disposed between the HIL  650  and the EML 1   640  and a first ETL (ETL 1 )  670  disposed between the EML 1   640  and the CGL  680 . Alternatively, the first emitting part  620  can further include a first EBL (EBL 1 )  665  disposed between the HTL 1   660  and the EML 1   640  and/or a first HBL (HBL 1 )  675  disposed between the EML 1   640  and the ETL 1   670 . 
     The second emitting part  720  includes the EML 2   740 . The second emitting part  720  can further include at least one of a second HTL (HTL 2 )  760  disposed between the CGL  680  and the EML 2   740 , a second ETL (ETL 2 )  770  disposed between the EML 2   740  and the second electrode  530  and an EIL  780  disposed between the ETL 2   770  and the second electrode  530 . Alternatively, the second emitting part  720  can further include a second EBL (EBL 2 )  765  disposed between the HTL 2   760  and the EML 2   740  and/or a second HBL (HBL 2 )  775  disposed between the EML 2   740  and the ETL 2   770 . 
     The CGL  680  is disposed between the first emitting part  620  and the second emitting part  720 . The first emitting part  620  and the second emitting part  720  are connected via the CGL  680 . The CGL  680  can be a PN-junction CGL that junctions an N-type CGL (N-CGL)  682  with a P-type CGL (P-CGL)  684 . 
     The N-CGL  682  is disposed between the ETL 1   670  and the HTL 2   760  and the P-CGL  684  is disposed between the N-CGL  682  and the HTL 2   760 . The N-CGL  682  transports electrons to the EML 1   640  of the first emitting part  620  and the P-CGL  684  transport holes to the EML 2   740  of the second emitting part  720 . 
     In this aspect, each of the EML 1   640  and the EML 2   740  can be a red emitting material layer. For example, at least one of the EML 1   640  and the EML 2   740  can include the first and second compounds DF 1  and DF 2  of the delayed fluorescent material, the third compound FD of the fluorescent material, and optionally the fourth compound H of the host. 
     As an example, when the EML 1   640  and/or the EML 2   740  includes the first to fourth compounds DF 1 , DF 2 , FD and H, the contents of the fourth compound H can be larger than each of the contents of the first and second compounds DF 1  and DF 2 , and each of the contents of the first and second compounds DF 1  and DF 2  can be larger than the contents of the third compound FD. In this case, exciton energy can be transferred efficiently from the first and second compounds DF 1  and DF 2  to the third compound FD. As an example, the contents of the fourth compound H in the EML 1   640  and/or the EML 2   740  can be between about 45 wt % and about 60 wt %, for example, about 45 wt % and about 55 wt %, each of the content of the first and second compounds DF 1  and DF 2  in the EML 1   640  and/or the EML 2   740  can be between about 10 wt % and about 40 wt %, and the contents of the third compound FD in the EML 1   640  and/or the EML 2   740  can be between about 0.1 wt % and about 5 wt %, for example, about 0.1 wt % and about 2 wt %, but is not limited thereto. 
     In one exemplary aspect, the EML 2   740  can include the first to third compounds DF 1 , DF 2  and FD, and optionally the first compound H as the same as the EML 1   640 . Alternatively, the EML 2   740  can include another compound for example different from at least one of the first to third compounds DF 1 , DF 2  and FD in the EML 1   640 , and thus the EML 2   740  can emit light different from the light emitted from the EML 1   640  or can have different luminous efficiency different from the luminous efficiency of the EML 1   640 . 
     In  FIG.  12   , each of the EML 1   640  and the EML 2   740  has a single-layered structure. Alternatively, each of the EML 1   640  and the EML 2   740 , each of which can include the first to third compounds, can have a double-layered structure ( FIG.  6   ) or a triple-layered structure ( FIG.  9   ), respectively. 
     In the OLED D 4 , the singlet exciton energy of the first and second compounds DF 1  and DF 2  of the delayed fluorescent material is transferred to the third compound FD of fluorescent material, and the ultimate emission is occurred at the third compound FD. Accordingly, the OLED D 4  can improve its luminous efficiency and color purity. In addition, the first compound DF 1  having the structure of Formulae 1 to 2, the second compound DF 2  having the structure of Formulae 3 to 4 and the third compound FD having the structure of Formulae 5 to 6 are included in the at least EML 1   640 , the luminous efficiency and color purity of the OLED D 4  can be further enhanced. Moreover, since the OLED D 4  has a double stack structure of a red emitting material layer, the OLE 4  D 4  can further improve its color sense or optimize its luminous efficiency. 
       FIG.  13    is a schematic cross-sectional view illustrating an organic light emitting display device in accordance with another exemplary aspect of the present disclosure. As illustrated in  FIG.  13   , an organic light emitting display device  800  includes a substrate  810  that defines first to third pixel regions P 1 , P 2  and P 3 , a thin film transistor Tr disposed over the substrate  810  and an OLED D disposed over the thin film transistor Tr and connected to the thin film transistor Tr. As an example, the first pixel region P 1  can be a green pixel region, the second pixel region P 2  can be a red pixel region and the third pixel region P 3  can be a red pixel region. 
     The substrate  810  can be a glass substrate or a flexible substrate. For example, the flexible substrate can be any one of a PI substrate, a PES substrate, a PEN substrate, a PET substrate and a PC substrate. A buffer layer  812  is disposed over the substrate  810  and the thin film transistor Tr is disposed over the buffer layer  812 . The buffer layer  812  can be omitted. As illustrated in  FIG.  2   , the thin film transistor Tr includes a semiconductor layer, a gate electrode, a source electrode and a drain electrode and acts as a driving element. 
     A passivation layer  850  is disposed over the thin film transistor Tr. The passivation layer  850  has a flat top surface and includes a drain contact hole  852  that exposes a drain electrode of the thin film transistor Tr. 
     The OLED D is disposed over the passivation layer  850 , and includes a first electrode  910  for example connected to the drain electrode of the thin film transistor Tr, and an emissive layer  920  and a second electrode  930  each of which is disposed sequentially on the first electrode  910 . The OLED D is disposed in each of the first to third pixel regions P 1 , P 2  and P 3  and emits different light in each pixel region. For example, the OLED D in the first pixel region P 1  can emit blue light, the OLED D in the second pixel region P 2  can emit green light and the OLED D in the third pixel region P 3  can emit red light. 
     The first electrode  910  is separately formed for each of the first to third pixel regions P 1 , P 2  and P 3 , and the second electrode  930  corresponds to the first to third pixel regions P 1 , P 2  and P 3  and is formed integrally. 
     The first electrode  910  can be one of an anode and a cathode, and the second electrode  930  can be the other of the anode and the cathode. In addition, one of the first electrode  910  and the second electrode  930  can be a transmissive (or semi-transmissive) electrode and the other of the first electrode  910  and the second electrode  930  can be a reflective electrode. 
     For example, the first electrode  910  can be an anode and can include conductive material having a relatively high work function value, i.e., a transparent conductive oxide layer of transparent conductive oxide (TCO). The second electrode  930  can be a cathode and can include conductive material having relatively low work function value, i.e., a metal material layer of low-resistant metal. For example, the first electrode  910  can include any one of ITO, IZO, ITZO, SnO, ZnO, ICO and AZO, and the second electrode  930  can include Al, Mg, Ca, Ag, alloy thereof (e.g. Mg—Ag) or combination thereof. 
     When the organic light emitting display device  800  is a bottom-emission type, the first electrode  910  can have a single-layered structure of a transparent conductive oxide layer. Alternatively, when the organic light emitting display device  900  is a top-emission type, a reflective electrode or a reflective layer can be disposed under the first electrode  910 . For example, the reflective electrode or the reflective layer can include, but is not limited to, Ag or APC alloy. In the OLED D of the top-emission type, the first electrode  910  can have a triple-layered structure of ITO/Ag/ITO or ITO/APC/ITO. Also, the second electrode  930  is thin so as to have light-transmissive (or semi-transmissive) property. 
     A bank layer  860  is disposed on the passivation layer  850  in order to cover edges of the first electrode  910 . The bank layer  860  corresponds to each of the first to third pixel regions P 1 , P 2  and P 3  and exposes a center of the first electrode  910 . 
     An emissive layer  920  is disposed on the first electrode  910 . In one exemplary aspect, the emissive layer  920  can have a single-layered structure of an EML. Alternatively, the emissive layer  920  can include at least one of an HIL, an HTL, and an EBL disposed sequentially between the first electrode  910  and the EML and/or an HBL, an ETL and an EIL disposed sequentially between the EML and the second electrode  930 . 
     In one exemplary aspect, the EML of the emissive layer  930  in the third pixel region P 3  of the red pixel region can include the first compound DF 1  of the first delayed fluorescent material having the structure of Formulae 1 to 2, the second compound DF 2  of the second delayed fluorescent material having the structure of Formulae 3 to 4, the third compound FD of the fluorescent material having the structure of Formula 5 to 6, and optionally the fourth compound H of the host having the structure of Formulae 7 to 8. 
     An encapsulation film  870  is disposed over the second electrode  930  in order to prevent outer moisture from penetrating into the OLED D. The encapsulation film  970  can have, but is not limited to, a triple-layered structure of a first inorganic insulating film, an organic insulating film and a second inorganic insulating film. 
     The organic light emitting display device  800  can have a polarizer in order to decrease external light reflection. For example, the polarizer can be a circular polarizer. When the organic light emitting display device  800  is a bottom-emission type, the polarizer can be disposed under the substrate  810 . Alternatively, when the organic light emitting display device  800  is a top emission type, the polarizer can be disposed over the encapsulation film  870 . 
       FIG.  14    is a schematic cross-sectional view illustrating an OLED in accordance with still another exemplary aspect of the present disclosure. As illustrated in  FIG.  14   , the OLED D 5  includes a first electrode  910 , a second electrode  930  facing the first electrode  910  and an emissive layer  920  disposed between the first and second electrodes  910  and  930 . 
     The first electrode  910  can be an anode and the second electrode  930  can be a cathode. As an example, the first electrode  910  can be a reflective electrode and the second electrode  930  can be a transmissive (or semi-transmissive) electrode. 
     The emissive layer  920  includes an EML  940 . The emissive layer  920  can include at least one of an HTL  960  disposed between the first electrode  910  and the EML  940  and an ETL  970  disposed between the EML  940  and the second electrode  930 . Also, the emissive layer  920  can further include at least one of an HIL  950  disposed between the first electrode  910  and the HTL  960  and an EIL  980  disposed between the ETL  970  and the second electrode  930 . In addition, the emissive layer  920  can further include at least one of an EBL  965  disposed between the HTL  960  and the EML  940  and an HBL  975  disposed between the EML  940  and the ETL  970 . 
     In addition, the emissive layer  920  can further include an auxiliary hole transport layer (auxiliary HTL)  962  disposed between the HTL  960  and the EBL  965 . The auxiliary HTL  962  can include a first auxiliary HTL  962   a  located in the first pixel region P 1 , a second auxiliary HTL  962   b  located in the second pixel region P 2  and a third auxiliary HTL  962   c  located in the third pixel region P 3 . 
     The first auxiliary HTL  962   a  has a first thickness, the second auxiliary HTL  962   b  has a second thickness and the third auxiliary HTL  962   c  has a third thickness. The first thickness is less than the second thickness and the second thickness is less than the third thickness. Accordingly, the OLED D 5  has a micro-cavity structure. 
     Owing to the first to third auxiliary HTLs  962   a ,  962   b  and  962   c  having different thickness to each other, the distance between the first electrode  910  and the second electrode  930  in the first pixel region P 1  emitting light in the first wavelength range (blue light) is smaller than the distance between the first electrode  910  and the second electrode  930  in the second pixel region P 2  emitting light in the second wavelength range (green light), which is longer than the first wavelength range. The distance between the first electrode  910  and the second electrode  930  in the second pixel region P 2  is smaller than the distance between the first electrode  910  and the second electrode  932  in the third pixel region P 3  emitting light in the third wavelength range (red light), which is longer than the second wavelength range. Accordingly, the OLED D 5  has improved luminous efficiency. 
     In  FIG.  14   , the first auxiliary HTL  962   a  is located in the first pixel region P 1 . Alternatively, the OLED D 5  can implement the micro-cavity structure without the first auxiliary HTL  962   c . In addition, a capping layer can be disposed over the second electrode  930  in order to improve out-coupling of the light emitted from the OLED D 5 . 
     The EML  940  includes a first EML (EML 1 )  942  located in the first pixel region P 1 , a second EML (EML 2 )  944  located in the second pixel region P 2  and a third EML (EML 3 )  946  located in the third pixel region P 3 . Each of the EML 1   942 , the EML 2   944  and the EML 3   946  can be a blue EML, a green EML and a red EML, respectively. 
     In one exemplary aspect, the EML 3   946  in the third pixel region P 3  can include the first compound DF 1  of the first delayed fluorescent material having the structure of Formulae 1 to 2, the second compound DF 2  of the second delayed fluorescent material having the structure of Formulae 3 to 4, the third compound FD of the fluorescent material having the structure of Formulae 5 to 6, and optionally, the fourth compound H of the host. In this case, the EML 3   946  can have a single-layered structure, a double-layered structure ( FIG.  6   ) or a triple-layered structure ( FIG.  6   ). In the EML 3   946 , the contents of the fourth compound H can be larger than each of the first and second compounds DF 1  and DF 2 , and each of the contents of the first and second compounds DF 1  and DF 2  can be larger than the contents of the third compound FD. 
     The EML 1   942  in the first pixel region P 1  can include host and blue dopant. For example, the host in the EML 1   942  can include the fourth compound H, and the blue dopant can include at least one of blue phosphorescent material, blue fluorescent material and blue delayed fluorescent material. 
     The EML 2   944  in the second pixel region P 2  can include host and green dopant. For example, the host in the EML 1   944  can include the fourth compound H, and the green dopant can include at least one of green phosphorescent material, green fluorescent material and green delayed fluorescent material. 
     The OLED D 5  emits blue light, green light and red light in each of the first to third pixel regions P 1 , P 2  and P 3  so that the organic light emitting display device  800  ( FIG.  13   ) can implement a full-color image. 
     The organic light emitting display device  800  can further include a color filter layer corresponding to the first to third pixel regions P 1 , P 2  and P 3  for improving color purity of the light emitted from the OLED D. As an example, the color filter layer can comprise a first color filter layer (blue color filter layer) corresponding to the first pixel region P 1 , the second color filter layer (green color filter layer) corresponding to the second pixel region P 2  and the third color filter layer (red color filter layer) corresponding to the third pixel region P 3 . 
     When the organic light emitting display device  800  is a bottom-emission type, the color filter layer can be disposed between the OLED D and the substrate  810 . Alternatively, when the organic light emitting display device  800  is a top-emission type, the color filter layer can be disposed over the OLED D. 
       FIG.  15    is a schematic cross-sectional view illustrating an organic light emitting display device in accordance with still another exemplary aspect of the present disclosure. As illustrated in  FIG.  15   , the organic light emitting display device  1000  includes a substrate  1010  defining a first pixel region P 1 , a second pixel region P 2  and a third pixel region P 3 , a thin film transistor Tr disposed over the substrate  1010 , an OLED D disposed over the thin film transistor Tr and connected to the thin film transistor Tr and a color filter layer  1020  corresponding to the first to third pixel regions P 1 , P 2  and P 3 . As an example, the first pixel region P 1  can be a blue pixel region, the second pixel region P 2  can be a green pixel region and the third pixel region P 3  can be a red pixel region. 
     The substrate  1010  can be a glass substrate or a flexible substrate. For example, the flexible substrate can be any one of a PI substrate, a PES substrate, a PEN substrate, a PET substrate and a PC substrate. The thin film transistor Tr is located over the substrate  1010 . Alternatively, a buffer layer can be disposed over the substrate  1010  and the thin film transistor Tr can be disposed over the buffer layer. As illustrated in  FIG.  2   , the thin film transistor Tr includes a semiconductor layer, a gate electrode, a source electrode and a drain electrode and acts as a driving element. 
     The color filter layer  1020  is located over the substrate  1010 . As an example, the color filter layer  1020  can include a first color filter pattern  1022  corresponding to the first pixel region P 1 , a second color filter pattern  1024  corresponding to the second pixel region P 2  and a third color filter pattern  1026  corresponding to the third pixel region P 3 . The first color filter pattern  1022  can be a blue color filter pattern, the second color filter pattern  1024  can be a green color filter pattern and the third color filter pattern  1026  can be a red color filter pattern. For example, the first color filter pattern  1022  can include at least one of blue dye or green pigment, the second color filter pattern  1024  can include at least one of green dye or red pigment and the third color filter pattern  1026  can include at least one of red dye or blue pigment. 
     A passivation layer  1050  is disposed over the thin film transistor Tr and the color filter layer  1020 . The passivation layer  1050  has a flat top surface and includes a drain contact hole  1052  that exposes a drain electrode of the thin film transistor Tr. 
     The OLED D is disposed over the passivation layer  1050  and corresponds to the color filter layer  1020 . The OLED D includes a first electrode  1110  for example connected to the drain electrode of the thin film transistor Tr, and an emissive layer  1120  and a second electrode  1130  each of which is disposed sequentially on the first electrode  1110 . The OLED D emits white light in the first to third pixel regions P 1 , P 2  and P 3 . 
     The first electrode  1110  is separately formed for each of the first to third pixel regions P 1 , P 2  and P 3 , and the second electrode  1130  corresponds to the first to third pixel regions P 1 , P 2  and P 3  and is formed integrally. The first electrode  1110  can be one of an anode and a cathode, and the second electrode  1130  can be the other of the anode and the cathode. In addition, the first electrode  1110  can be a transmissive (or semi-transmissive) electrode and the second electrode  1130  can be a reflective electrode. 
     For example, the first electrode  1110  can be an anode and can include conductive material having a relatively high work function value, i.e., a transparent conductive oxide layer of transparent conductive oxide (TCO). The second electrode  1130  can be a cathode and can include conductive material having relatively low work function value, i.e., a metal material layer of low-resistant metal. For example, the transparent conductive oxide layer of the first electrode  1110  can include any one of ITO, IZO, ITZO, SnO, ZnO, ICO and AZO, and the second electrode  1130  can include Al, Mg, Ca, Ag, alloy thereof (e.g., Mg—Ag) or combination thereof. 
     The emissive layer  1120  is disposed on the first electrode  1110 . The emissive layer  1120  includes at least two emitting parts emitting different colors. Each of the emitting part can have a single-layered structure of an EML. Alternatively, each of the emitting parts can include at least one of an HIL, an HTL, an EBL, an HBL, an ETL and an EIL. In addition, the emissive layer  1120  can further comprise a CGL disposed between the emitting parts. 
     At least one of the at least two emitting parts can include the first compound DF 1  of the first delayed fluorescent material having the structure of Formulae 1 to 2, the second compound DF 2  of the second delayed fluorescent material having the structure of Formulae 3 to 4, the third compound FD of the BODIPY-based fluorescent material having the structure of Formulae 5 to 6, and optionally the fourth compound H of the host having the structure of Formulae 7 to 8. 
     A bank layer  1060  is disposed on passivation layer  1050  in order to cover edges of the first electrode  1110 . The bank layer  1060  corresponds to each of the first to third pixel regions P 1 , P 2  and P 3  and exposes a center of the first electrode  1110 . As described above, since the OLED D emits white light in the first to third pixel regions P 1 , P 2  and P 3 , the emissive layer  1120  can be formed as a common layer without being separated in the first to third pixel regions P 1 , P 2  and P 3 . The bank layer  1060  is formed to prevent current leakage from the edges of the first electrode  1110 , and the bank layer  1060  can be omitted. 
     Moreover, the organic light emitting display device  1000  can further include an encapsulation film disposed on the second electrode  1130  in order to prevent outer moisture from penetrating into the OLED D. In addition, the organic light emitting display device  1000  can further comprise a polarizer disposed under the substrate  1010  in order to decrease external light reflection. 
     In the organic light emitting display device  1000  in  FIG.  15   , the first electrode  1110  is a transmissive electrode, the second electrode  1130  is a reflective electrode, and the color filter layer  1020  is disposed between the substrate  1010  and the OLED D. For example, the organic light emitting display device  1000  is a bottom-emission type. Alternatively, the first electrode  1110  can be a reflective electrode, the second electrode  1120  can be a transmissive electrode (or semi-transmissive electrode) and the color filter layer  1020  can be disposed over the OLED D in the organic light emitting display device  1000 . 
     In the organic light emitting display device  1000 , the OLED D located in the first to third pixel regions P 1 , P 2  and P 3  emits white light, and the white light passes through each of the first to third pixel regions P 1 , P 2  and P 3  so that each of a blue color, a green color and a red color is displayed in the first to third pixel regions P 1 , P 2  and P 3 , respectively. 
     A color conversion film can be disposed between the OLED D and the color filter layer  1020 . The color conversion film corresponds to the first to third pixel regions P 1 , P 2  and P 3 , and includes a blue color conversion film, a green color conversion film and a red color conversion film each of which can convert the white light emitted from the OLED D into blue light, green light and red light, respectively. For example, the color conversion film can include quantum dots. Accordingly, the organic light emitting display device  1000  can further enhance its color purity. Alternatively, the color conversion film can displace the color filter layer  1020 . 
       FIG.  16    is a schematic cross-sectional view illustrating an OLED in accordance with still another exemplary aspect of the present disclosure. As illustrated in  FIG.  16   , the OLED D 6  includes first and second electrodes  1110  and  1130  facing each other and an emissive layer  1120  disposed between the first and second electrodes  1110  and  1130 . The first electrode  1110  can be an anode and the second electrode  1130  can be a cathode. For example, the first electrode  1100  can be a transmissive electrode and the second electrode  1130  can be a reflective electrode. 
     The emissive layer  1120  includes a first emitting part  1220  including a first EML (lower EML, EML 1 )  1240 , a second emitting part  1320  comprising a second EML (middle EML, EML 2 )  1340  and a third emitting part  1420  comprising a third EML (upper EML, EML 3 )  1440 . In addition, the emissive layer  1120  can further includes a first charge generation layer (CGL 1 )  1280  disposed between the first emitting part  1220  and the second emitting part  1320  and a second charge generation layer (CGL 2 )  1380  disposed between the second emitting part  1320  and the third emitting part  1420 . Accordingly, the first emitting part  1220 , the CGL 1   1280 , the second emitting part  1320 , the CGL 2   1380  and the third emitting part  1420  are disposed sequentially on the first electrode  1110 . 
     The first emitting part  1220  can further include at least one of an HIL  1250  disposed between the first electrode  1110  and the EML 1   1240 , a first HTL (HTL 1 )  1260  disposed between the EML 1   1240  and the HIL  1250  and a first ETL (ETL 1 )  1270  disposed between the EML 1   1240  and the CGL 1   1280 . Alternatively, the first emitting part  1220  can further include at least one of a first EBL (EBL 1 )  1265  disposed between the HTL 1   1260  and the EML 1   1240  and a first HBL (HBL 1 )  1275  disposed between the EML 1   1240  and the ETL 1   1270 . 
     The second emitting part  1320  can further include at least one of a second HTL (HTL 2 )  1360  disposed between the CGL 1   1280  and the EML 2   1340 , a second ETL (ETL 2 )  1370  disposed between the EML 2   1340  and the CGL 2   1380 . Alternatively, the second emitting part  1320  can further include a second EBL (EBL 2 )  1365  disposed between the HTL 2   1360  and the EML 2   1340  and/or a second HBL (HBL 2 )  1375  disposed between the EML 2   1340  and the ETL 2   1370 . 
     The third emitting part  1420  can further include at least one of a third HTL (HTL 3 )  1460  disposed between the CGL 2   1380  and the EML 3   1440 , a third ETL (ETL 3 )  1470  disposed between the EML 3   1440  and the second electrode  1130  and an EIL  1480  disposed between the ETL 3   1470  and the second electrode  1130 . Alternatively, the third emitting part  1420  can further comprise a third EBL (EBL 3 )  1465  disposed between the HTL 3   1460  and the EML 3   1440  and/or a third HBL (HBL 3 )  1475  disposed between the EML 3   1440  and the ETL 3   1470 . 
     The CGL 1   1280  is disposed between the first emitting part  1220  and the second emitting part  1320 . For example, the first emitting part  1220  and the second emitting part  1320  are connected via the CGL 1   1280 . The CGL 1   1280  can be a PN-junction CGL that junctions a first N-type CGL (N-CGL 1 )  1282  with a first P-type CGL (P-CGL 1 )  1284 . 
     The N-CGL 1   1282  is disposed between the ETL 1   1270  and the HTL 2   1360  and the P-CGL 1   1284  is disposed between the N-CGL 1   1282  and the HTL 2   1360 . The N-CGL 1   1282  transports electrons to the EML 1   1240  of the first emitting part  1220  and the P-CGL 1   1284  transport holes to the EML 2   1340  of the second emitting part  1320 . 
     The CGL 2   1380  is disposed between the second emitting part  1320  and the third emitting part  1420 . For example, the second emitting part  1320  and the third emitting part  1420  are connected via the CGL 2   1380 . The CGL 2   1380  can be a PN-junction CGL that junctions a second N-type CGL (N-CGL 2 )  1382  with a second P-type CGL (P-CGL 2 )  1384 . 
     The N-CGL 2   1382  is disposed between the ETL 2   1370  and the HTL 3   1460  and the P-CGL 2   1384  is disposed between the N-CGL 2   1382  and the HTL 3   1460 . The N-CGL 2   1382  transports electrons to the EML 2   1340  of the second emitting part  1320  and the P-CGL 2   1384  transport holes to the EML 3   1440  of the third emitting part  1420 . 
     In this aspect, one of the first to third EMLs  1240 ,  1340  and  1440  can be a blue EML, another of the first to third EMLs  1240 ,  1340  and  1440  can be a green EML and the third of the first to third EMLs  1240 ,  1340  and  1440  can be a red EML. 
     As an example, the EML 1   1240  can be a blue EML, the EML 2   1340  can be a green EML and the EML 3   1440  can be a red EML. Alternatively, the EML 1   1240  can be a red EML, the EML 2   1340  can be a green EML and the EML 3   1440  can be a blue EML. Hereinafter, the OLED D 3  where the EML 1   1240  is a blue EML, the EML 2   1340  is a green EML and the EML 3  is a red EML will be described. 
     The EML 1   1240  can include a host and a blue dopant. For example, the host in the EML 1   1240  can include the fourth compound H, and the blue dopant can include at least one of the blue phosphorescent material, the blue fluorescent material and the blue delayed fluorescent material. 
     The EML 2   1340  can include a host and a green dopant. For example, the host in the EML 2   1340  can include the fourth compound H, and the green dopant can include at least one of the green phosphorescent material, the green fluorescent material and the green delayed fluorescent material. 
     The EML 3   1440  can include the first compound DF 1  of the first delayed fluorescent material having the structure of Formulae 1 to 2, the second compound DF 2  of the second delayed fluorescent material having the structure of Formulae 3 to 4, the third compound FD of the BODIPY-based fluorescent material having the structure of Formulae 5 to 6, and optionally, the fourth compound H of the host. The EML 3   1440  including the first to fourth compounds DF 1 , DF 2 , FD and H can have a single-layered structure, a double-layered structure ( FIG.  6   ) or a triple-layered structure ( FIG.  9   ). In the EML 3   1440 , the contents of the fourth compound H can be larger than each of the contents of the first and second compounds DF 1  and DF 2 , and each of the contents of the first and second compound DF 1  and DF 2  can be larger than the contents of the third compound FD. 
     The OLED D 6  emits white light in each of the first to third pixel regions P 1 , P 2  and P 3  and the white light passes though the color filter layer  1020  ( FIG.  15   ) correspondingly disposed in the first to third pixel regions P 1 , P 2  and P 3 . Accordingly, the organic light emitting display device  1000  ( FIG.  15   ) can implement a full-color image. 
       FIG.  17    is a schematic cross-sectional view illustrating an OLED in accordance with still another exemplary aspect of the present disclosure. As illustrated in  FIG.  17   , the OLED D 7  includes first and second electrodes  1110  and  1130  facing each other and an emissive layer  1120 A disposed between the first and second electrodes  1110  and  1130 . The first electrode  1110  can be an anode and the second electrode  1130  can be a cathode. For example, the first electrode  1100  can be a transmissive electrode and the second electrode  1130  can be a reflective electrode. 
     The emissive layer  1120 A includes a first emitting part  1520  comprising an EML 1  (lower EML)  1540 , a second emitting part  1620  comprising an EML 2  (middle EML)  1640  and a third emitting part  1720  comprising an EML 3  (upper EML)  1740 . In addition, the emissive layer  1120 A can further include a CGL 1   1580  disposed between the first emitting part  1520  and the second emitting part  1620  and a CGL 2   1680  disposed between the second emitting part  1620  and the third emitting part  1720 . Accordingly, the first emitting part  1520 , the CGL 1   1580 , the second emitting part  1620 , the CGL 2   1680  and the third emitting part  1720  are disposed sequentially on the first electrode  1110 . 
     The first emitting part  1520  can further include at least one of an HIL  1550  disposed between the first electrode  1110  and the EML 1   1540 , an HTL 1   1560  disposed between the EML 1   1540  and the HIL  1550  and an ETL 1   1570  disposed between the EML 1   1540  and the CGL 1   1580 . Alternatively, the first emitting part  1520  can further comprise an EBL 1   1565  disposed between the HTL 1   1560  and the EML 1   1540  and/or an HBL 1   1575  disposed between the EML 1   1540  and the ETL 1   1570 . 
     The EML 2   1640  of the second emitting part  1620  includes a middle lower EML (first layer)  1642  and a middle upper EML (second layer)  1644 . The middle lower EML  1642  is located adjacently to the first electrode  1110  and the upper middle EML  1644  is located adjacently to the second electrode  1130 . In addition, the second emitting part  1620  can further include at least one of an HTL 2   1660  disposed between the CGL 1   1580  and the EML 2   1640 , an ETL 2   1670  disposed between the EML 2   1640  and the CGL 2   1680 . Alternatively, the second emitting part  1620  can further comprise at least one of an EBL 2   1665  disposed between the HTL 2   1660  and the EML 2   1640  and an HBL 2   1675  disposed between the EML 2   1640  and the ETL 2   1670 . 
     The third emitting part  1720  can further include at least one of an HTL 3   1760  disposed between the CGL 2   1680  and the EML 3   1740 , an ETL 3   1770  disposed between the EML 3   1740  and the second electrode  1130  and an EIL  1780  disposed between the ETL 3   1770  and the second electrode  1130 . Alternatively, the third emitting part  1720  can further include an EBL 3   1765  disposed between the HTL 3   1760  and the EML 3   1740  and/or an HBL 3   1775  disposed between the EML 3   1740  and the ETL 3   1770 . 
     The CGL 1   1580  is disposed between the first emitting part  1520  and the second emitting part  1620 . For example, the first emitting part  1520  and the second emitting part  1620  are connected via the CGL 1   1580 . The CGL 1   1580  can be a PN-junction CGL that junctions an N-CGL 1   1582  with a P-CGL 1   1584 . The N-CGL 1   1582  is disposed between the ETL 1   1570  and the HTL 2   1660  and the P-CGL 1   1584  is disposed between the N-CGL 1   1582  and the HTL 2   1560 . 
     The CGL 2   1680  is disposed between the second emitting part  1620  and the third emitting part  1720 . For example, the second emitting part  1620  and the third emitting part  1720  are connected via the CGL 2   1680 . The CGL 2   1680  can be a PN-junction CGL that junctions an N-CGL 2   1682  with a P-CGL 2   1684 . The N-CGL 2   1682  is disposed between the ETL 2   1570  and the HTL 3   1760  and the P-CGL 2   1684  is disposed between the N-CGL 2   1682  and the HTL 3   1760 . 
     In this aspect, each of the EML 1   1540  and the EML 3   1740  can be a blue EML. In an exemplary aspect, each of the EML 1   1540  and the EML 3   1740  can include a host and a blue dopant. The host in each of the EML 1   1540  and the EML 3   1740  can include the fourth compound H, and the blue dopant in each of the EML 1   1540  and the EMI 3   1740  can include at least one of the blue phosphorescent material, the blue fluorescent material and the blue delayed fluorescent material. The host and/or the blue dopant in the EML 1   1540  can be identical to or different from the host and/or the blue dopant in the EML 3   1740 . As an example, the blue dopant in the EML 1   1540  can have different luminous efficiency and/or emission peak different from the luminous efficiency and/or emission peak of the blue dopant in the EML 3   1740 . 
     One of the middle lower EML  1642  and the middle upper EML  1644  of the EML 2   1640  can be a green EML and the other of the middle lower EML  1642  and the middle upper EML  1644  of the EML 2   1640  can be a red EML. The green EML and the red EML are sequentially disposed to form the EML 2   1640 . Hereinafter, the EML 2   1640  where the middle lower EML  1642  is a red EML and the middle upper EML  1644  is a green EML will be described 
     As an example, the middle lower EML  1642  of the red EML can include the first compound of DF 1  of the first delayed fluorescent material having the structure of Formulae 1 to 2, the second compound DF 2  of the second delayed fluorescent material having the structure of Formulae 3 to 4, the third compound FD of the fluorescent material having the structure of Formulae 5 to 6, and optionally the fourth compound H of the host having the structure of Formulae 7 to 8. The middle lower EML  1642  can have a single-layered structure, a double-layered structure ( FIG.  6   ) or a triple-layered structure ( FIG.  9   ). 
     In the middle lower EML  1642 , the contents of the fourth compound H can be larger than each of the contents of the first and second compounds DF 1  and DF 2 , and each of the contents of the first and second compound DF 1  and DF 2  can be larger than the contents of the third compound FD. 
     The middle upper EML  1644  of the green EML can include a host and a green dopant. The host in the middle upper EML  1644  can include the fourth compound H, and the green dopant in the middle upper EML  1644  can include at least one of the green phosphorescent materials, the green fluorescent materials and the green delayed fluorescent materials as described above. 
     The OLED D 7  emits white light in each of the first to third pixel regions P 1 , P 2  and P 3  and the white light passes though the color filter layer  1020  ( FIG.  15   ) correspondingly disposed in the first to third pixel regions P 1 , P 2  and P 3 . Accordingly, the organic light emitting display device  1000  ( FIG.  15   ) can implement a full-color image. 
     In  FIG.  17   , the OLED D 7  has a three-stack structure including the first to three emitting parts  1520 ,  1620  and  1720  which includes the EML 1   1540  and the EML 3   1740  as a blue EML. Alternatively, the OLED D 7  can have a two-stack structure where one of the first emitting part  1520  and the third emitting part  1720  each of which includes the EML 1   1540  and the EML 3   1740  as a blue EML is omitted. 
     Example 1 (Ex. 1): Fabrication of OLED 
     An OLED in which an EML includes Compound 1-1 of Formula 2 (HOMO: −5.9 eV, LUMO: −3.3 eV) as the first compound DF 1 , Compound 2-1 of Formula 4 (HOMO: −5.8 eV, LUMO: −3.4 eV) as the second compound DF 2 , Compound 3-1 of Formula 6 (HOMO: −5.5 eV, LUMO: −3.5 eV) as the third compound FD and Compound 4-1 (CBP) of Formula 8 (HOMO: −6.0 eV, LUMO: −2.4 eV) was fabricated. An ITO substrate was washed by UV-treated Ozone before using, and was transferred to a vacuum chamber for depositing emission layer. Subsequently, an anode, an emission layer and a cathode were deposited by evaporation from a heating boat under 10 −7  torr vacuum condition with setting deposition rate of 1 Å/s in the following order: 
     An anode (ITO, 50 nm); an HIL (HAT-CN, 5 nm); an HTL (NPB, 80 nm); an EBL (TAPC, 10 nm), an EML (Compound 4-1 (49 wt %), Compound 1-1 (25 wt %), Compound 2-1 (25 wt %), Compound 3-1 (1 wt %), 35 nm); an HBL (B3PYMPM, 10 nm); an ETL (TPBi, 35 nm), an EIL (LiF); and a cathode (Al). 
     The charge injection or transport materials used in the HIL, HTL, EBL, HBL and ETL are indicated below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Examples 2-3 (Ex. 2-3): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that each of the contents of the Compound 1-1 and Compound 2-1 in the EML was changed to 10 wt % and 40 wt %, respectively (Ex. 2), or each of the contents of the Compound 1-1 and Compound 2-1 in the EML was changed to 40 wt % and 10 wt %, respectively (Ex. 3). 
     Example 4 (Ex. 4): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 3-2 of Formula 6 (HOMO: −5.5 eV, LUMO: −3.5 eV) as the third compound instead of the Compound 3-1 in the EML was used. 
     Examples 5-6 (Ex. 5-6): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 4, except that each of the contents of the Compound 1-1 and Compound 2-1 in the EML was changed to 10 wt % and 40 wt %, respectively (Ex. 5), or each of the contents of the Compound 1-1 and Compound 2-1 in the EML was changed to 40 wt % and 10 wt %, respectively (Ex. 6). 
     Example 7 (Ex. 7): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 3-3 of Formula 6 (HOMO: −5.5 eV, LUMO: −3.5 eV) as the third compound instead of the Compound 3-1 in the EML was used. 
     Examples 8-9 (Ex. 8-9): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 7, except that each of the contents of the Compound 1-1 and Compound 2-1 in the EML was changed to 10 wt % and 40 wt %, respectively (Ex. 8), or each of the contents of the Compound 1-1 and Compound 2-1 in the EML was changed to 40 wt % and 10 wt %, respectively (Ex. 9). 
     Example 10 (Ex. 10): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 3-4 of Formula 6 (HOMO: −5.6 eV, LUMO: −3.6 eV) as the third compound instead of the Compound 3-1 in the EML was used. 
     Examples 11-12 (Ex. 11-12): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 10, except that each of the contents of the Compound 1-1 and Compound 2-1 in the EML was changed to 10 wt % and 40 wt %, respectively (Ex. 11), or each of the contents of the Compound 1-1 and Compound 2-1 in the EML was changed to 40 wt % and 10 wt %, respectively (Ex. 12). 
     Example 13 (Ex. 13): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 2-2 of Formula 4 (HOMO: −5.9 eV, LUMO: −3.5 eV) as the second compound instead of the Compound 2-1 in the EML was used. 
     Examples 14-15 (Ex. 14-15): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 13, except that each of the contents of the Compound 1-1 and Compound 2-1 in the EML was changed to 10 wt % and 40 wt %, respectively (Ex. 14), or each of the contents of the Compound 1-1 and Compound 2-1 in the EML was changed to 40 wt % and 10 wt %, respectively (Ex. 15). 
     Example 16 (Ex. 16): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 1-7 of Formula 2 (HOMO: −6.0 eV, LUMO: −3.4 eV, 10 wt %) as the first compound instead of the Compound 1-1, Compound 2-2 (40 wt %) of Formula 4 as the second compound instead of the Compound 2-1 and Compound 3-4 (1 wt %) as the third compound instead of the Compound 3-1 in the EML were used. 
     Example 17 (Ex. 17): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 1-8 of Formula 2 (HOMO: −5.9 eV, LUMO: −3.2 eV, 10 wt %) as the first compound instead of the Compound 1-1, Compound 2-1 (40 wt %) as the second compound and Compound 3-4 (1 wt %) as the third compound instead of the Compound 3-1 in the EML were used. 
     Example 18 (Ex. 18): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 17, except that the Compound 2-2 as the second compound instead of the Compound 2-1 in the EML was used. 
     Example 19 (Ex. 19): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 17, except that the Compound 2-5 of Formula 4 as the second compound instead of the Compound 2-1 and the Compound 3-1 of Formula 6 as the third compound instead of the Compound 3-4 in the EML were used. 
     Example 20 (Ex. 20): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 17, except that Compound 1-11 of Formula 2 (HOMO: −5.8 eV, LUMO: −3.2 eV) as the first compound instead of the Compound 1-8 in the EML was used. 
     Example 21 (Ex. 21): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 17, except that the Compound 1-11 of Formula 2 as the first compound instead of the Compound 1-8, the Compound 2-5 of Formula 4 as the second compound instead of the Compound 2-1 and the Compound 3-1 as the third compound instead of the Compound 3-4 in the EML were used. 
     Example 22 (Ex. 22): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 17, except that the Compound 2-2 of Formula 4 as the second compound instead of the Compound 2-1 and the Compound 3-1 of Formula 6 as the third compound instead of the Compound 3-4 were used. 
     Example 23 (Ex. 23): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 17, except that the Compound 2-4 as the second compound instead of the Compound 2-1 and the Compound 3-1 of Formula 6 as the third compound instead of the Compound 3-4 were used. 
     The following table 1 indicates the first to third compounds and HOMO energy levels and LUMO energy levels of the first to third compounds each of which was used in the EML of the OLEDs fabricated in Examples 1 to 23. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 First, Second, Third Compounds in EML 
               
            
           
           
               
               
               
               
            
               
                   
                 First Compound 
                 Second Compound 
                 Third Compound 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Sample 
                   
                 HOMO/LUMO(eV) 
                   
                 HOMO/LUMO(eV) 
                   
                 HOMO/LUMO(eV) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Ex. 1 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ex. 2 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ex. 3 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ex. 4 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-2 
                 −5.5/−3.5 
               
               
                 Ex. 5 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-2 
                 −5.5/−3.5 
               
               
                 Ex. 6 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-2 
                 −5.5/−3.5 
               
               
                 Ex. 7 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-3 
                 −5.5/−3.5 
               
               
                 Ex. 8 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-3 
                 −5.5/−3.5 
               
               
                 Ex. 9 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-3 
                 −5.5/−3.5 
               
               
                 Ex. 10 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-4 
                 −5.6/−3.6 
               
               
                 Ex. 11 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-4 
                 −5.6/−3.6 
               
               
                 Ex. 12 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 3-4 
                 −5.6/−3.6 
               
               
                 Ex. 13 
                 1-1 
                 −5.9/−3.3 
                 2-2 
                 −5.9/−3.5 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ex. 14 
                 1-1 
                 −5.9/−3.3 
                 2-2 
                 −5.9/−3.5 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ex. 15 
                 1-1 
                 −5.9/−3.3 
                 2-2 
                 −5.9/−3.5 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ex. 16 
                 1-7 
                 −6.0/−3.4 
                 2-2 
                 −5.9/−3.5 
                 3-4 
                 −5.6/−3.6 
               
               
                 Ex. 17 
                 1-8 
                 −5.9/−3.2 
                 2-1 
                 −5.8/−3.4 
                 3-4 
                 −5.6/−3.6 
               
               
                 Ex. 18 
                 1-8 
                 −5.9/−3.2 
                 2-2 
                 −5.9/−3.5 
                 3-4 
                 −5.6/−3.6 
               
               
                 Ex. 19 
                 1-8 
                 −5.9/−3.2 
                 2-5 
                 −5.7/−3.3 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ex. 20 
                 1-11 
                 −5.8/−3.2 
                 2-1 
                 −5.8/−3.4 
                 3-4 
                 −5.6/−3.6 
               
               
                 Ex. 21 
                 1-11 
                 −5.8/−3.2 
                 2-5 
                 −5.7/−3.3 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ex. 22 
                 1-8 
                 −5.9/−3.2 
                 2-2 
                 −5.9/−3.5 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ex. 23 
                 1-8 
                 −5.9/−3.2 
                 2-4 
                 −5.6/−3.2 
                 3-1 
                 −5.5/−3.5 
               
               
                   
               
            
           
         
       
     
     Comparative Example 1 (Ref. 1): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 4-1 of Formula 8 (69 wt %) as the fourth compound, Compound 1-1 of Formula 2 (30 wt %) as the first compound and Compound 3-1 of Formula 6 (1 wt %) as the third compound in the EML without using the second compound were used. 
     Comparative Example 2 (Ref. 2): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 4-1 of Formula 8 (49 wt %) as the fourth compound, Compound 1-1 of Formula 2 (50 wt %) as the first compound and Compound 3-1 of Formula 6 (1 wt %) as the third compound in the EML without using the second compound were used. 
     Comparative Example 3 (Ref. 3): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 4-1 of Formula 8 (29 wt %) as the fourth compound, Compound 1-1 of Formula 2 (70 wt %) as the first compound and Compound 3-1 of Formula 6 (1 wt %) as the third compound in the EML without using the second compound were used. 
     Comparative Example 4 (Ref. 4): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 4-1 of Formula 8 (69 wt %) as the fourth compound, the following Ref. 1 Compound (30 wt %) as the first compound and the following Ref. 4 Compound (1 wt %) as the third compound in the EML without using the second compound were used. 
     Comparative Example 5 (Ref. 5): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 4-1 of Formula 8 (49 wt %) as the fourth compound, the following Ref. 1 Compound (50 wt %) as the first compound and the following Ref. 4 Compound (1 wt %) as the third compound in the EML without using the second compound were used. 
     Comparative Example 6 (Ref. 6): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that Compound 4-1 of Formula 8 (29 wt %) as the fourth compound, the following Ref. 1 Compound (70 wt %) as the first compound and the following Ref. 4 Compound (1 wt %) as the third compound in the EML without using the second compound were used. 
     Comparative Example 7 (Ref. 7): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that the following Ref 2 Compound as the second compound instead of the Compound 2-1 in the EML was used. 
     Comparative Examples 8-9 (Ref 8-9): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 7, except that each of the contents of the Compound 1-1 and the Ref. 2 Compound in the EML was changed to 10 wt % and 40 wt %, respectively (Ref. 8), or each of the contents of the Compound 1-1 and the Ref. 2 Compound in the EML was changed to 40 wt % and 10 wt %, respectively (Ref. 9). 
     Comparative Example 10 (Ref. 10): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that the following Ref. 1 Compound as the first compound instead of the Compound 1-1 in the EML was used. 
     Comparative Examples 11-13 (Ref. 11-13): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 10, except that the Compound 2-2 of Formula 4 (Ref 11), the following Ref. 2 Compound (Ref. 12) or the following Ref. 3 Compound (Ref. 3) as the second compound instead of the Compound 2-1 in the EML was used. 
     Comparative Example 14 (Ref. 10): Fabrication of OLED 
     An OLED was fabricated using the same materials as Example 1, except that the following Ref 4 Compound as the third compound instead of the Compound 3-1 in the EML was used. 
     Comparative Examples 15-17 (Ref. 15-17): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 14, except that the Compound 2-2 of Formula 4 (Ref 15), the following Ref. 2 Compound (Ref. 16) or the following Ref 3 Compound (Ref. 17) as the second compound instead of the Compound 2-1 in the EML was used. 
     Comparative Example 18 (Ref. 18): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 14, except that the Compound 1-7 of Formula 2 as the first compound instead of the Compound 1-7 in the EML was used. 
     Comparative Examples 19-20 (Ref. 19-20): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 18, except that each of the contents of the Compound 1-7 and the Compound 2-1 in the EML was changed to 10 wt % and 40 wt %, respectively (Ref. 19), or each of the contents of the Compound 1-7 and the Compound 2-1 in the EML was changed to 40 wt % and 10 wt %, respectively (Ref. 20). 
     Comparative Example 21 (Ref. 21): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 18, except that the Compound 2-2 of Formula 4 as the second compound instead of the Compound 2-1 in the EML was used. 
     Comparative Examples 22-23 (Ref. 22-23): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 21, except that each of the contents of the Compound 1-7 and the Compound 2-2 in the EML was changed to 10 wt % and 40 wt %, respectively (Ref. 22), or each of the contents of the Compound 1-7 and the Compound 2-2 in the EML was changed to 40 wt % and 10 wt %, respectively (Ref. 23). 
     Comparative Example 24 (Ref. 24): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 18, except that the following Ref 2 Compound as the second compound instead of the Compound 2-1 in the EML was used. 
     Comparative Examples 25-26 (Ref. 25-26): Fabrication of OLED 
     An OLED was fabricated using the same materials as Comparative Example 24, except that each of the contents of the Compound 1-7 and the Ref. 2 Compound in the EML was changed to 10 wt % and 40 wt %, respectively (Ref. 25), or each of the contents of the Compound 1-7 and the Ref. 2 Compound in the EML was changed to 40 wt % and 10 wt %, respectively (Ref. 26). 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The following table 2 indicates the first to third compounds and HOMO energy levels and LUMO energy levels of the first to third compounds each of which was used in the EML of the OLEDs fabricated in Comparative Examples 1 to 26. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 First, Second, Third Compounds in EML 
               
            
           
           
               
               
               
               
            
               
                   
                 First Compound 
                 Second Compound 
                 Third Compound 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Sample 
                   
                 HOMO/LUMO(eV) 
                   
                 HOMO/LUMO(eV) 
                   
                 HOMO/LUMO 
               
               
                   
               
               
                 Ref. 1 
                 1-1 
                 −5.9/−3.3 
                 — 
                 — 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ref. 2 
                 1-1 
                 −5.9/−3.3 
                 — 
                 — 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ref. 3 
                 1-1 
                 −5.9/−3.3 
                 — 
                 — 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ref. 4 
                 Ref. 1 
                 −5.4/−2.8 
                 — 
                 — 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 5 
                 Ref. 1 
                 −5.4/−2.8 
                 — 
                 — 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 6 
                 Ref. 1 
                 −5.4/−2.8 
                 — 
                 — 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 7 
                 1-1 
                 −5.9/−3.3 
                 Ref. 2 
                 −6.0/−3.8 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ref. 8 
                 1-1 
                 −5.9/−3.3 
                 Ref. 2 
                 −6.0/−3.8 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ref. 9 
                 1-1 
                 −5.9/−3.3 
                 Ref. 2 
                 −6.0/−3.8 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ref. 10 
                 Ref. 1 
                 −5.4/−2.8 
                 2-1 
                 −5.8/−3.4 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ref. 11 
                 Ref. 1 
                 −5.4/−2.8 
                 2-2 
                 −5.9/−3.5 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ref. 12 
                 Ref. 1 
                 −5.4/−2.8 
                 Ref. 2 
                 −6.0/−3.8 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ref. 13 
                 Ref. 1 
                 −5.4/−2.8 
                 Ref. 3 
                 −5.8/−3.0 
                 3-1 
                 −5.5/−3.5 
               
               
                 Ref. 14 
                 1-1 
                 −5.9/−3.3 
                 2-1 
                 −5.8/−3.4 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 15 
                 1-1 
                 −5.9/−3.3 
                 2-2 
                 −5.9/−3.5 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 16 
                 1-1 
                 −5.9/−3.3 
                 Ref. 2 
                 −6.0/−3.8 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 17 
                 1-1 
                 −5.9/−3.3 
                 Ref. 3 
                 −5.8/−3.0 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 18 
                 1-7 
                 −6.0/−3.4 
                 2-1 
                 −5.8/−3.4 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 19 
                 1-7 
                 −6.0/−3.4 
                 2-1 
                 −5.8/−3.4 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 20 
                 1-7 
                 −6.0/−3.4 
                 2-1 
                 −5.8/−3.4 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 21 
                 1-7 
                 −6.0/−3.4 
                 2-2 
                 −5.9/−3.5 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 22 
                 1-7 
                 −6.0/−3.4 
                 2-2 
                 −5.9/−3.5 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 23 
                 1-7 
                 −6.0/−3.4 
                 2-2 
                 −5.9/−3.5 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 24 
                 1-7 
                 −6.0/−3.4 
                 Ref. 2 
                 −6.0/−3.8 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 25 
                 1-7 
                 −6.0/−3.4 
                 Ref. 2 
                 −6.0/−3.8 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                 Ref. 26 
                 1-7 
                 −6.0/−3.4 
                 Ref. 2 
                 −6.0/−3.8 
                 Ref. 4 
                 −4.9/−2.8 
               
               
                   
               
            
           
         
       
     
     Experimental Example 1: Measurement of Luminous Properties of OLED 
     Each of the OLED fabricated in Ex. 1-23 and Ref. 1-26 was connected to an external power source and then luminous properties for all the diodes were evaluated using a constant current source (KEITHLEY) and a photometer PR650 at room temperature. In particular, hole trap, electron trap, driving voltage (V), external quantum efficiency (EQE, %) and lifespan (LT 95 ) at 15.4 mA/cm 2  current density of the OLEDs were measured. The measurement results for the OLEDs fabricated in Examples are shown in the following Table 3 and the measurement results for the OLEDs fabricated in Comparative Examples are shown in the following Table 4: 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Luminous Properties of OLED 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Hole 
                 Electron  
                 @ 15.4 mA/cm 2   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Sample 
                 trap 
                 trap 
                 V 
                 EQE (%) 
                 T 95 Chr) 
               
               
                   
                   
               
               
                   
                 Ex. 1 
                 N 
                 N 
                 4.08 
                 17.6 
                 472 
               
               
                   
                 Ex. 2 
                 N 
                 N 
                 4.05 
                 18.0 
                 488 
               
               
                   
                 Ex. 3 
                 N 
                 N 
                 4.26 
                 16.9 
                 435 
               
               
                   
                 Ex. 4 
                 N 
                 N 
                 4.07 
                 16.8 
                 367 
               
               
                   
                 Ex. 5 
                 N 
                 N 
                 4.01 
                 17.0 
                 379 
               
               
                   
                 Ex. 6 
                 N 
                 N 
                 4.19 
                 16.5 
                 315 
               
               
                   
                 Ex. 7 
                 N 
                 N 
                 4.25 
                 16.0 
                 350 
               
               
                   
                 Ex. 8 
                 N 
                 N 
                 4.11 
                 16.3 
                 368 
               
               
                   
                 Ex. 9 
                 N 
                 N 
                 4.93 
                 15.1 
                 312 
               
               
                   
                 Ex. 10 
                 N 
                 N 
                 4.32 
                 15.4 
                 322 
               
               
                   
                 Ex. 11 
                 N 
                 N 
                 4.24 
                 15.9 
                 339 
               
               
                   
                 Ex. 12 
                 N 
                 N 
                 4.98 
                 14.7 
                 305 
               
               
                   
                 Ex. 13 
                 N 
                 N 
                 4.49 
                 13.1 
                 113 
               
               
                   
                 Ex. 14 
                 N 
                 N 
                 4.38 
                 13.4 
                 101 
               
               
                   
                 Ex. 15 
                 N 
                 N 
                 4.43 
                 12.6 
                 142 
               
               
                   
                 Ex. 16 
                 N 
                 N 
                 4.13 
                 16.0 
                 303 
               
               
                   
                 Ex. 17 
                 N 
                 N 
                 4.09 
                 16.4 
                 333 
               
               
                   
                 Ex. 18 
                 N 
                 N 
                 4.39 
                 15.2 
                 297 
               
               
                   
                 Ex. 19 
                 N 
                 N 
                 4.27 
                 17.1 
                 390 
               
               
                   
                 Ex. 20 
                 N 
                 N 
                 4.08 
                 15.7 
                 371 
               
               
                   
                 Ex. 21 
                 N 
                 N 
                 4.10 
                 17.3 
                 399 
               
               
                   
                 Ex. 22 
                 N 
                 N 
                 4.14 
                 15.6 
                 350 
               
               
                   
                 Ex. 23 
                 N 
                 N 
                 4.40 
                 14.5 
                 300 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Luminous Properties of OLED 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Hole 
                 Electron  
                 @ 15.4 mA/cm 2   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Sample 
                 trap 
                 trap 
                 V 
                 EQE (%) 
                 T 95 Chr) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Ref. 1 
                 N 
                 N 
                 4.12 
                 13.5 
                 33 
               
               
                   
                 Ref. 2 
                 N 
                 N 
                 4.26 
                 13.3 
                 95 
               
               
                   
                 Ref. 3 
                 N 
                 N 
                 4.40 
                 13.1 
                 237 
               
               
                   
                 Ref. 4 
                 Y 
                 N 
                 4.43 
                 11.6 
                 150 
               
               
                   
                 Ref. 5 
                 Y 
                 N 
                 4.49 
                 10.2 
                 82 
               
               
                   
                 Ref. 6 
                 Y 
                 N 
                 4.67 
                 10.1 
                 150 
               
               
                   
                 Ref. 7 
                 N 
                 Y 
                 4.85 
                 12.2 
                 80 
               
               
                   
                 Ref. 8 
                 N 
                 Y 
                 4.80 
                 12.6 
                 77 
               
               
                   
                 Ref. 9 
                 N 
                 Y 
                 4.89 
                 11.6 
                 95 
               
               
                   
                 Ref. 10 
                 Y 
                 Y 
                 4.88 
                 9.3 
                 53 
               
               
                   
                 Ref. 11 
                 Y 
                 Y 
                 4.80 
                 8.4 
                 67 
               
               
                   
                 Ref. 12 
                 Y 
                 Y 
                 4.65 
                 8.5 
                 95 
               
               
                   
                 Ref. 13 
                 Y 
                 N 
                 4.35 
                 11.5 
                 107 
               
               
                   
                 Ref. 14 
                 N 
                 Y 
                 4.87 
                 10.6 
                 52 
               
               
                   
                 Ref. 15 
                 N 
                 Y 
                 4.54 
                 10.9 
                 61 
               
               
                   
                 Ref. 16 
                 N 
                 Y 
                 4.60 
                 9.4 
                 70 
               
               
                   
                 Ref. 17 
                 Y 
                 N 
                 4.55 
                 10.0 
                 68 
               
               
                   
                 Ref. 18 
                 Y 
                 N 
                 4.42 
                 12.9 
                 151 
               
               
                   
                 Ref. 19 
                 Y 
                 N 
                 4.40 
                 13.5 
                 133 
               
               
                   
                 Ref. 20 
                 Y 
                 N 
                 4.69 
                 11.0 
                 162 
               
               
                   
                 Ref. 21 
                 Y 
                 N 
                 4.61 
                 12.4 
                 147 
               
               
                   
                 Ref. 22 
                 Y 
                 N 
                 4.45 
                 13.1 
                 140 
               
               
                   
                 Ref. 23 
                 Y 
                 N 
                 4.70 
                 11.4 
                 155 
               
               
                   
                 Ref. 24 
                 Y 
                 N 
                 4.58 
                 11.9 
                 120 
               
               
                   
                 Ref. 25 
                 Y 
                 N 
                 4.97 
                 9.7 
                 89 
               
               
                   
                 Ref. 26 
                 Y 
                 N 
                 4.36 
                 12.4 
                 112 
               
               
                   
                   
               
            
           
         
       
     
     As indicated in Tables 3 and 4, in the OLEDs fabricated in Examples where the HOMO energy level of the first compound of the first delayed fluorescent material was designed to be identical to or lower than the HOMO energy level of the second compound of the second delayed fluorescent material, the HOMO energy level of the second compound was designed to be identical to or lower than the HOMO energy level of the third compound of the fluorescent material, the LUMO energy level of the first compound was designed to be identical or higher than the LUMO energy level of the second compound, and the LUMO energy level of the second compound was designed to be identical or higher than the LUMO energy level of the third compound, optical properties were improved extremely. 
     On the contrary, in the OLEDs fabricated in Ref 7-26 wherein the HOMO and/or LUMO energy levels among the delayed fluorescent materials and fluorescent materials are not regulated with suing two delayed fluorescent materials, optical properties were reduced owing to hole traps and/or electron traps. 
     More particularly, compared to the OLEDs fabricated in Ref. 1-6 where one delayed fluorescent material and one fluorescent material were applied in the EML, the OLED fabricated in Ex. 1-26 reduced its driving voltage by maximally 14.1% and improved its EQE and lifespan (LT 95 ) by maximally 78.2% and 13.8 times, respectively. In addition, compared to the OLEDs fabricated in Ref. 7-26, the OLED fabricated in Ex. 1-26 reduced its driving voltage by maximally 19.3% and improved its EQE and lifespan by maximally 114.3% and 9.4 times. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the OLED and the organic light emitting device including the OLED of the present disclosure without departing from the technical idea or scope of the disclosure. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.